Table of contents

Among 2D materials, graphene is by far the most attractive one due to its distinguished electronic, chemical and mechanical properties. It has been used in a large number of applications covering electrical devices, reinforced composite materials and energy storage to only cite a few. Besides its intrinsic properties, the applications of graphene can be extended to other fields by functionalizing it using different molecules and/or particles. It is even more interesting when the production and functionalization of graphene can be achieved in a single-step process. In this work, we report on a single-step synthesis of functionalized graphene using electrochemical exfoliation of graphite sheets in an acidic medium containing metallic complexes. The resulting materials were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, electrochemical techniques, Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy.

Mesoporous carbons (MC) are attractive catalyst supports for polymer electrolyte membrane fuel cells (PEMFCs) applications because of their low cost, high surface area, good electrical conductivity, and high chemical stability under normal operating conditions.^[1] However, carbon corrosion in the cathode compartment can result in a significant reduction in the catalyst electrochemical active surface area (ECSA), that it will negatively impact in the fuel cell performance.^[2] Hence, alternative catalyst substrates with a greater chemical stability under acidic and oxidizing conditions have captured significant attention in recent years. ^[3,4]

Among the most promising materials for PEMFCs are carbon nanotubes (CNTs), graphene, and hybrid core/shell metal oxide/mesoporous carbons (MO_x/MC). Unfortunately, CNTs and graphene are still too expensive for this application, while studies on hybrid supports, in the case of metal oxides, were limited to low metal oxide contents due to the reduction on surface area observed after the deposition of metal oxide nanoparticles.^[5,6]

For that reason, the use of ordered mesoporous carbons obtained by carbonization of polymeric materials synthesized in the presence of soft-templates such as surfactants or polyelectrolytes^[7] or silica nanoparticles as hard-templates^[1] is quite attractive. It can contribute to produce chemically stable materials with an adequate surface area and pore size distribution, and these properties can be modified by merely varying the synthesis and carbonization conditions.^[1,4,7]

In this study, the effort was oriented to develop mesoporous carbons (MC), and hybrid TiO₂/MC catalyst supports with high surface area and tailored micro-mesoporous structure. Carbon materials were obtained by carbonization of a resorcinol-formaldehyde resin synthesized in the presence of silica nanoparticles and poly(diallyldimethylammonium) chloride, a polyelectrolyte as soft template. The deposition of TiO₂ nanoparticles was carried out using a sol-gel method that has shown to result in the formation of core/shell TiO₂/carbon nanocomposite structures with low TiO₂ segregation.^[5,6] Both, MC and TiO₂/MC substrates were decorated with Pt or PtRu catalyst nanoparticles using standard impregnation methods.

The materials were characterized using standard methods (XRD, XPS, TEM, SEM, TGA, ICP-OES, and Raman spectroscopy). The catalytic activity of Pt and PtRu nanoparticles toward the oxygen reduction reaction (ORR), and the oxidation of methanol (MOR) was investigated using a number of electrochemical techniques in a three-electrode cell configuration.

The improvement in the catalytic activity of Pt toward the ORR and the chemical stability were both assigned to the high BET surface area of the starting carbon materials (> 500 m²/g) that allowed working with high TiO₂ loadings while still keeping a reasonable electrical conductivity. The high TiO₂ content seems to improve the catalyst distribution on the support, and the contact between the TiO₂ and Pt nanoparticles. Preliminary results for the oxidation of methanol on PtRu/TiO₂/MC also show a significant improvement when compared to Pt/MC and Pt/TiO₂/MC, but the changes are less significant when compared with PtRu/MC, however more studies are underway to confirm these findings.

REFERENCES

• Forouzandeh F.; Banham D.W.; Feng F.; Li X.; Ye S.; Birss V., ECS Transactions58 (1) 1739-1749 (2013).

• Shao Y.Y, Yin G.P., Zang , Gao Y.Z., Electrochim. Acta51, 5853 (2006).

• Wang Y-J, Wilkinson D.P., Zhang J., Chem. Rev.111 (12) 7625-7651 (2011).

• Bruno M., Viva F.A., Carbon Materials for Fuel Cells BT - Direct Alcohol Fuel Cells: Materials, Performance, Durability and Applications, in: H.R. Corti, E.R. Gonzalez (Eds.), Springer Netherlands, Dordrecht, pp. 231–270 (2014).

• Odetola C.; Trevani L.; Easton E., Power Sources294, 254–263 (2015).

• Odetola C., Easton E.B., Trevani L., J. Hydrogen Energy 41, 8199–8208 (2016)

• Bruno M.; Viva F.; Petruccelli M.; Corti H., Power Sources278, 458–463 (2015).

Structural energy storage is a field associated with the combination of important functions such as structural reliability, energy storage, and energy delivery. The combination of said functions ensures savings in the overall volume and mass of the system. Reduced graphene oxide (rGO) and aramid nanofibers (ANF) composite film electrodes were developed for the purpose of structural energy storage. Graphene is studied due to its remarkable mechanical properties, electrical conductivity and electrochemical surface area. Furthermore, Kevlar^® aramid nanofibers enhance the structural properties of the composite due to their excellent mechanical properties. However, external strains are expected on the composite film due to its structural function, and the response of the electrical properties to external strain needs to be evaluated. In this work, the change in electrical resistance was measured under bending-induced strain, using a home-built instrument that allows for accurate strain measurements using image analysis. We show that the resistance of the film decreases under bending via a mechanism we called "mechanical annealing." This mechanism may be the effect of improved stacking of rGO sheets. The degree of mechanical annealing was also correlated to the amount of bending strain, as well as the amount of ANF loading. Our results demonstrate an improvement, rather than deterioration of the electrical resistance of rGO/ANF composite films under strain, making them suitable for structural energy storage applications.

In this work, an one step green electrochemical synthesis of a supercapacitor with high performance and stability based on carbon nitride and polypyrrole is described. The new composite was easily obtained through the electrodeposition of polypyrrole on the surface of carbon nitride using an aqueous suspension. This material was fully characterized using both theoretical studies and experimental techniques. The sample showed a capacitance value of 810 F g^-1 (at 0.2 A g^-1), retaining 92% of its initial value after 6000 charge-discharge cycles. Furthermore, the composite material retains 75% of the capacitance compared to the data measured at 0.2 A g^-1 ( 810 F g^-1) and 9.0 A g^-1 (610 F g^-1). Finally, normalizing the capacitance considering only the polypyrrole mass it was found a capacitance value of 3029 F g^-1. To explain these results, it was used impedance spectroscopy and DFT calculations. The computational experiments showed the formation of hydrogen bonds between the N-H of the polypyrrole and the π-system of the carbon nitride which stabilized the structures. Besides, a planarization effect could also be observed over the polypyrrole chains. Both results are supported by the experimental data collected from the impedance experiments.

The search for cleaner energy sources is required to shift focus to the generation of energy from electrochemical systems, batteries, fuel cells, solar cells, and super capacitors, among others. In this study we report a simple approach for preparation of the graphene (GR) and carbon (C) supported manganese(IV) and cobalt(II/III) oxides using microwave-assisted synthesis. The X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) methods were used for the characterization of morphology, structure and composition of the prepared the graphene and carbon supported MnO₂-Co₃O₄ nanocomposites. Electrochemical performance of the MnO₂-Co₃O₄/GR and MnO₂-Co₃O₄/C nanocomposites was evaluated using long life cyclic voltammetry (CV) and galvanostatic charge-discharge.

The MnO₂-Co₃O₄/GR and MnO₂-Co₃O₄/C nanocomposites which have different morphologies, have been prepared by microwave synthesis. It was found that the graphene supported MnO₂-Co₃O₄ nanoparticles has significant influence on the electrochemical performance of composites electrodes compared with that of the carbon supported MnO₂-Co₃O₄ nanoparticles. The maximum specific capacitance was achieved at the MnO₂-Co₃O₄/GR electrode in the 1 M Na₂SO₄ aqueous solution at the scan rate of 10 mV s^-1. After 1000 charge/discharge cycles at the scan rate 100 mV s^-1 the capacitance of all the materials synthesized in this study remains over 94 %, suggesting great potential for supercapacitors.

Supercapacitors (SCs) have received great attention due to their rapid charging-discharging rate and long life cyclic stability. Transition metal oxides (TMOs) have been widely investigated as electrode materials for their theoretically higher specific capacitance. Here, a facile and effective synthesis route of metal oxide embedded in porous carbon nanofiber support has been developed. First, the corresponding metal salts were dissolved in a homogenous polyacrylonitrile (PAN) and cellulose diacetate (CDA) polymer solution. Fresh fibers were obtained by electrospinning the polymer solution. Then it was annealed at high temperature in N₂ atmosphere to obtain the carbonized samples. During the thermal treating process, metal salts are converted to less oxidation states or even only metal, while PAN forms carbon backbone and CDA as the sacrificial polymer is burned to form pores. For samples with only metal formed, the hydrothermal treatment is subsequently performed for further oxidation. When tested for supercapacitors, the 3D connected porous nanofiber structure can provide transport channel for electrolyte during charging-discharging process. In detail, porous TMOs/carbon fibers including cheap and high performance TMOs, such as NiO, Co₃O₄, MnO₂ and ZnO based materials, or even multiple metal TMOs will be systematically investigated. The effects of different annealing temperature on the compositions, porous structure and electrochemical performance will also be investigated. The preliminary results show that TMOs supported by the porous carbon fiber demonstrated greatly improved electrochemical performance, indicating that this facile synthesis method can be a promising alternative to prepare TMOs electrode materials with high performance.

In this research study, we have presented a simple two-step synthesis path to producing a cost-effective high porosity carbon material via acidic dehydration of white sugar. The electrochemical behaviour of the activated sugar-based carbon material (ASC), activated at 400°C (ASC 400) and adopted as a supercapacitor electrode in a symmetric device demonstrated a limit specific capacitance of 242.67 F g^-1 at 1 A g^-1. The device also demonstrated a good efficacy as an established material for supercapacitors suitable for high power applications with a satisfactory energy density of 19 Wh Kg^-1 and power density, 750 W kg^-1 at a gravimetric specific current of 1 A g^-1. The results obtained provide a potential route to converting cheap refined biomass sources into highly porous nanostructured materials for energy storage device applications.

Figure 1

Even though room temperature ionic liquids (RTILs) have been widely used as electrolytes in electrochemical applications including supercapacitors and batteries, the behavior of RTILs at charged interfaces is still not well understood. The cations are expected to have a significant effect on physical properties (i.e., electrolyte viscosity and conductivity) as well as the electrochemical performance.^{1, 2} For a fully understanding of the cation-effect, herein, we provide a systematic study of the influence of the physical properties of RTIL electrolytes with different structures of cations on the electrochemical performance of carbon-based supercapacitors along with a molecular dynamics (MD) simulation. N-doped reduced graphene oxide aerogel (N-rGO) supercapacitors are incorporated with RTIL electrolytes consisting of cyclic nitrogen-based cations i.e., imidazolium, which are connected with a variable length of alkyl chains (i.e., ethyl, butyl and hexyl). Changing these cations do not widen the potential window of the ionic liquid; however, they do affect the electrochemical performance in term of specific capacitances and specific energies of the supercapacitor. We found that the heterocycles of RTILs affect ionic conductivity while the alkyl chain plays a role in ion diffusion. In this perspective, we hope that this fundamental study may pave the way to design the structure of RTIL electrolyte for high energy storage performance.

The nature of solar radiation is not steady, therefore having a solar-capacitor device that can simultaneously capture, convert and store solar energy is a considerable device. A self-charging capacitor is designed and fabricated as a functional solar energy storage device, which is accomplished in situ self-assembled storage technique. The solar-capacitor device is assembled on a multilayered photoelectrode combining different concentration of cationic dye (i.e. methylene blue, MB) and conducting polymer (i.e. polyaniline, PANI) in contact with H₂SO₄-Polyvinyl alcohol (PVA) gel electrolyte. The mechanism of storage the photo-generated charges takes place at the changing of redox oxidation states. Due to the high porosity of the multi-walled carbon nanotube (MWCNT) based counter electrode and PANI at the working electrode the device is working as photocapacitor. But, because of the presence of MB molecules that embed in the PANI can absorb the light and excite the electron from the highest occupied molecular orbital (HOMO) level to the lowest unoccupied molecular orbital (LUMO) level. Therefore, the supercapacitor can be charged when the working electrode is illuminated. In this study, a photo-electrochemical device based on two-terminals that combines H₂SO₄-PVA, MWCNT, and fluorine doped tin oxide conductive substrate (FTO) as working electrode is successfully designed and fabricated. This device can work independently as solar cell, supercapacitor, or a solar-capacitor device. Different concentrations of the cationic dye, MB, composite with PANI have been used as a thin film coated electrochemically on the surface of the FTO. The experiments are carried out in the dark at the beginning and then repeated when the working electrode is under the effect of solar irradiation. After illumination, the experiments repeat in dark again to see the effect of the light on the performance of the device. When the device is exposed to the light, the redox oxidation peaks of PANI/MB change. This clearly shows the photo-electrochemical reaction of the PANI/MB. The open circuit voltage across the cell in the dark is 0.92 mV. Then, the cell voltage is increased gradually from 0.92 mV to 4.9 mV in 400 s of illumination. After turning off the light, the voltage is dropped to the dark value immediately. The results of electrochemical and electrical characterization techniques such as Cyclic Voltammetery (CV), Electrochemical Impedance Spectroscopy (EIS), Open Circuit Voltage (OSV), and Short Circuit Current (SCC) on the solar-capacitor device have been addressed in this study. The results are encouraging for application of PANI/dye composite film for solar-capacitor devices to harvest solar energy and store charges in a single device with two-terminals.

Figure 1

In this work, a simple/scalable microwave-facilitated hydrothermal route is used to produce nitrogen self-doped graphene quantum dots (NGQDs) from a sole glucosamine precursor. These NGQDs with average sizes of ~6nm show bright/stable fluorescence both in the visible and near-IR. The structural and optical properties of as-prepared NGQDs are further altered to provide control for optoelectronic applications by using ozone, thermal and laser treatment. Both laser and thermal processing serve as controllable avenues to decrease GQD emission via anticipated reduction processes. Oxidative ozone treatment results in the decrease of GQD average size down to 5.23 nm and a more disordered structure due to the introduction of the new functional groups. Structural and optical characterization was performed utilizing TEM, AFM, SEM, FTIR, EDX, Raman, fluorescence and absorbance. FTIR, EDX and Raman data suggest that this processing introduces oxygen-containing functional groups, enhancing the atomic percentage of oxygen and increasing I_D/I_G ratio. Ozone treatment shows enhancement of visible emission is observed from 0 to 16 min ozone processing with following over oxidation-induced defect-related quenching. On the other hand, progressive increase in defect-related NIR emission is observed up to 45 min. Such alteration of optoelectronic properties of GQDs is used to enhance their performance in photovoltaic devices.

Untreated NGQDs (Un-NGQDs) and ozone-treated NGQDs (Oz-NGQDs) are utilized as a photoactive layer to fabricate a variety of solar cells. Although devices with untreated NGQDs show performances similar to existing reports, Oz-NGQDs exhibit significant improvement with maximum PCE of 2.64% and a short circuit current density of 4.8 mA/cm² and an open circuit voltage of ~0.65V with a fill factor of ~83.4%. This enhancement can be potentially attributed to the increased/broadened visible absorption feature in device state due to the efficient charge transfer between the hole-blocking layer of TiO₂ and Oz-NGQD having enhanced concentration of functional groups. This work suggests ozone treatment as an easy and powerful technique to alter the optoelectronic properties of versatile and scalably produced NGQDs which can be successfully utilized as an eco-friendly photoactive layer to boost the photovoltaic performance of solar cells.

Supercapacitors are also known as electrochemical capacitors (ECs) attracted considerable attention because of properties like high power density, rapid charge-discharge rate (few seconds), long cycle life (>100000 cycles) and low maintenance cost. These properties make EC an excellent candidate for applications like hybrid electric vehicles, frequency regulations in smart grids etc. However, designing freestanding binder-free electrodes with superior mechanical strength and suitable capacitance is still in the development stage. Graphene, a 2D form of carbon is one of the best candidates for supercapacitor electrode material, because of its properties, like, high surface area (~2630 m2 g-1), high intrinsic electrical conductivity, and chemical stability. Restacking and aggregation of graphene sheets during electrode fabrication process leads to ineffective use of its high surface area. Vertically oriented graphene (VOG) sheets on a conductive substrate can be a useful electrode for supercapacitor application. Exposed graphene sheets on the surface of graphite, increases the surface area of the electrode by order of magnitude and reduces the contact resistance since graphene sheets remain attached to the substrate. Here, we demonstrate an electrochemical approach for engineering graphite surface in such a way that we can directly grow vertically oriented graphene on the graphite substrate itself. Electrochemical exfoliation process increases interlayer spacing causing more and easy access to the surface area by electrolyte ions, thus reducing diffusion resistance. Anodization of graphite in 1 M H2SO4 leads to the growth of VOGs on a graphite substrate. Exfoliated graphite (EG) not only act as a current collector but also stores enough charges in the interlayer spaces. EG shows excellent capacitive performance in 1 M sodium sulphate electrolyte. The capacitance of graphite, 0.05 F cm-2 increases to 0.75 F cm-2 (scan rate of 5 mV s-1) in case of EG.

The CeO₂, known as one of the typical rare earth oxides, is seen to have considerable potential for being used as novel anode materials for secondary batteries because it shows excellent electrochemical redox properties. When it is considered as an anode material, the foremost advantages are that its volumetric and morphological changes are hardly observed, so it could provide good stability and longevity of secondary batteries. It is well explained by the fully reversible phase transformation between fluorite structured CeO₂ and cubic structured Ce₂O₃ during the electrochemical reaction.

Nonetheless, it is essential to improve the conductivity problem of the CeO₂ which has intrinsically low electron conductivity in order to apply it to secondary batteries. Meanwhile, there used to be a widely used method called carbon composite method to support nonconductive metal oxides.

In this study, we have synthesized the CeO₂-Carbon composite sphere in the form of rice balls to improve the conductivity problems of the rare earth oxide during their electrochemical performance. A rice ball structure is expected to provide more advanced electronic conductive networks than a core shell structure does. Indeed, high stability was observed during the charging and discharging process of LIB due to the nanostructures in the form of rice balls.

Supercapacitors are energy conversion devices with higher power density compared to batteries and higher energy density compared to common capacitors.[1] These particular properties have attracted considerable attention with regard to numerous applications in such diverse fields as power electronics, military equipment, and hybrid electric vehicles (i.e. HEVs, in order to help the stop and go function and to provide peak power for improved acceleration).[2] In addition, supercapacitors can play an important role in complementing the energy storage functions of batteries and fuel cells by providing back-up power supplies to protect against power disruptions.[3,4]

Supercapacitors have four major components, namely, current collectors, electrodes, electrolytes and separators.[3] However, although electrodes have been actively studied, there are far fewer studies of current collectors than the other there components. Typical current collectors are nickel (Ni), platinum (Pt), gold (Au), aluminum (Al), silver (Ag) and copper (Cu). Although Ag compared with the other materials offers many advantages including high current-carrying capability (i.e., lowest resistivity at 1.63 x 10-8 Ωm)[5] and good chemical and thermal stability,[6] Ag current collectors are rarely used in supercapacitors. In our previous study, which demonstrated the usefulness of a solution processed Ag current collector in supercapacitors, we used a 2-dimensional (2D) Ag plated polymer film as the current collectors.[7] In this study, we propose a method of enhancing the usefulness of a solution processed Ag current collector by making a 3-dimensional (3D) porous Ag nonwoven mat current collector from the cellulosic template in order to maximize the efficiency of the Ag current collector. This may have the result of maximizing the contact area between the electrode and the electrolyte, such as Ni foam.[8] This 3D porous Ag nonwoven mat would be also useful in such applications as the cathodes of alkaline fuel cells owing to its good chemical and thermal stability,[9] and might also be used in a variety of filtration applications where the antimicrobial and antibacterial properties of silver make silver membranes a very efficient filtration system.[10]

In this study, we propose a simple method of making a 3D porous silver nonwoven mat as the current collector of supercapacitors, and investigate their super-capacitive properties using cyclic voltammetry in 1 M Na2SO4. For the purpose of comparison, the electrochemical properties of the 2D Ag plated current collector were also investigated. Cellulosic templates were used to make a 3D porous Ag nonwoven mat as the current collector of a supercapacitor with an Ag nanoparticle dispersed solution and simple spray equipment.

References

[1] X. Dong, W. Shen, J. Gu, L. Xiong, Y. Zhu, H. Li, J. Shi, J. Phys. Chem. B 110 (2006) 6015-6019.

[2] C. Portet, P. L. Taberna, P. Simon, E. Elahaut, C. Laberty-Robert, Electrochim. Acta 50 (2005) 4174-4181.

[3] G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797-828.

[4] Y. Jang, J. Jo, H. Jang, I. Kim, D. Kang, K. –Y. Kim, , Appl. Phys. Lett. 104 (2014) 243901.

[5] K. C. R. D. Silva, B. J. Kaseman, D. J. Bayless, Int. J. Hydr. Energy 36 (2011) 779-786.

[6] W. A. Meulenberg, O. Teller, U. Flesch, H. P. Buchkremer, D. Stöver, J. Mater. Sci. 36 (2001) 3189-3195.

[7] S. M. Yoon, J. S. Go, J. –S. Yu, D. W. Kim, Y. Jang, S. –H. Lee, J. Jo, J. Nanosci. Nanotechnol. 13 (2013) 7844-7849.

[8] R. Shi, L. Jiang, C. Pan, Soft Nanosci. Lett. 1 (2011) 11-15.

[9] F. Bidault, A. Kucernak, J. Pow. Sour. 195 (2010) 2549-2556.

[10] Y. Chen, F. Wang, D. Chen, F. Dong, H. J. Park, C. Kwak, Z. Shao, J. Pow. Sour. 210 (2012) 146-153.

Acknowledgments

This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant NRF-2016M1A2A2940915/ 10052802/ 10067668/ CAP-15-04-KITECH/ NK210D/ N0002310).

In an effort to create a paintable/printable thermoelectric materials, comprised exclusively of organic components, polyaniline (PANi), graphene, and double-walled carbon nanotubes (DWNT) were alternately deposited from aqueous solutions using the layer-by-layer assembly technique. Graphene and DWNT are stabilized with an intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). A 1 µm thick film, composed of 80 PANi/graphene-PEDOT:PSS/PANi/DWNT-PEDOT:PSS quadlayers (QL) exhibits electrical conductivity (σ) of 1.88 X 10⁵ S/m and a Seebeck coefficient (S) of 120 µV/K, producing a thermoelectric power factor (S²σ) of 2710 µW/(mK²). This is the highest value ever reported for a completely organic material measured at room temperature. Furthermore, this performance matches or exceeds that of commercial bismuth telluride. Air-stable n-type organic thermoelectric nanocomposites were achieved by depositing layers of double-walled carbon nanotubes (DWNT), stabilized with polyethylenimine (PEI), and graphene oxide (GO) in a layer-by-layer fashion from aqueous solutions. A 30 bilayer film (~ 610 nm thick), comprised of this DWNT-PEI/GO sequence, exhibits electrical conductivity of 27.3 S/cm and Seebeck coefficient of -30 µV/K, producing a power factor of 2.5 µW/(mK²). Low temperature thermal reduction (150 °C for 30 min) of this composite thin film significantly improves its thermoelectric performance. An electrical conductivity of 460 S/cm and Seebeck coefficient of -93 µV/K are achieved. A 30 BL DWNT-PEI/reduced graphene oxide (rGO) film (~480 nm thick) exhibits a power factor as large as 400 µW/(mK²), which is one of the highest values reported for an organic n-type material. The combination of water-based processing, air stability and high power factor is a major step toward producing efficient thermoelectric devices on flexible substrates (e.g. textiles for clothing). For the first time, there is a real opportunity to harness waste heat from unconventional sources, such as body heat to power devices in an environmentally-benign way.

Figure 1

Single wall carbon nanotubes (SWCNTs) are one-dimensional materials with sharp van Hove singularities. Preparation of SWCNTs with a selected electronic structure, fabrication of their aligned thin films[1] and tuning their Fermi level allow us to reveal unique phenomena, which cannot be observed or correctly understood in conventional SWCNT random network thin films. For example, we observed anomalously large optical absorption for perpendicular polarization to the tube axis in highly doped aligned SWCNTs, which is related to intersubband transition plasmon phenomena in SWCNTs.[2] Here in this talk, we would like to discuss the thermoelectric phenomena in fermi level tuned and aligned SWCNTs.

In our society, a large amount of heat at relatively low temperature, which is emitted from factories, houses, human bodies, and so on, is left unused. Development of high-performance flexible thermoelectric devices is crucial to efficiently convert such unused waste heat into electric power. Understanding of relationships between electrical conductivity, Seebeck coefficient, and thermal conductivity in thin films of flexible materials and tuning of these parameters is crucial to improve the performance. Since the seminal study by Hicks and Dresselhaus,[3] the thermoelectric properties of low dimensional materials have been intensively studied, but it still has been of great importance to experimentally clarify how the one dimensional electronic structures can influence and enhance its thermoelectric properties, although there are several important theoretical predictions for enhancement of thermoelectric properties in one-dimensional electronic systems.[3,4] SWCNTs are a model of one-dimensional flexible materials, and will play important roles for realization of high-performance flexible thermoelectric devices.

Previously, we revealed how the location of Fermi-level influences the thermoelectric properties of semiconducting SWCNTs with diameter of 1.4 nm using electrolyte gating approaches.[5] However, in the previous study, the purity of chirality was not enough to clarify the intrinsic characteristics of semiconducting SWCNTs. In this talk, we will discuss the relationships between electronic structures, Fermi-level and thermoelectric properties using single chirality SWCNTs. In addition, we will discuss how the thermoelectric properties and conductivities exhibit isotropic or an-isotropic properties in aligned SWCNT thin films.[6]

Acknowledgment: K.Y. acknowledges support by JST CREST through Grant Number JPMJCR17I5, Japan

The ability to selectively extract semiconducting single-walled carbon nanotube (s-SWCNT) species from the raw material with high fidelity is leading to their incorporation as elements in a variety of optical and electronic applications. We have recently employed conjugated polymers based on the fluorene chemical moiety to produce tailored s-SWCNT samples that can be incorporated into photovoltaic, thermoelectric, and transistor architectures.

By controlling both the extent of carbon nanotube bundling and/or removing the insulating polymer that wraps the individual carbon nanotubes, we explore the complex effects of these morphological modifications on the transport of energy by excitonic species and charge carriers in enriched s-SWCNT networks. We show that removal of the insulating polymer, which is often strongly bound to the carbon nanotubes through van de Waals forces between the π-electron systems of the two components, results in a significant improvement in charge carrier transport. In contrast, exciton transport is subtly dependent on the specific nanotube-nanotube interactions in the network. For instance, exciton energy transfer to minority low-bandgap species within a bundle is extremely efficient, but this often traps the exciton in an individual bundle, thereby inhibiting long-range transport within the network.

As alluded to above, the performance of carbon nanotubes in these applications is sometimes strongly dependent on the interplay between the optical and electronic properties of the s-SWCNT networks, which can be controlled by tuning the charge carrier density. We have developed a number of processes that allow us to exert fine control over the charge carrier density injected by redox-active chemical dopants, including the ability to controllably dope the s-SWCNT network p-type and n-type. We will discuss how these strategies can be exploited to exert fine control over charge-carrier transport in the doped s-SWCNT networks, allowing us to target the optimum doping level for the desired application.

Finally, we will discuss the implications of our findings within the context of the incorporation of enriched s-SWCNT networks in energy harvesting applications.

Recently, perovskite solar cells (PSCs) have got much attention since the big jump of the power conversion efficiency (PCE) from about 3.9% [1] to 23.3% [2] in less than a decade that is comparable to the developed Si and CIGS-based counterparts in more than 40 years. Accordingly, tremendous efforts have been devoted on increasing durability of the produced devices at low cost. PSC commonly invented by sandwiching the perovskite layers between two charge selective transport layers. Then, the two conducting electrodes are placed outside of charge selective layers, one is facing the light usually transparent conducting oxide, and the other is a noble metal like gold or silver. The metal electrode typically prepared by thermal evaporation at high vacuum level that's substantially increase the cost as well for the cost of noble metal. Moreover, these metals either react with the PSC components or migrate into the device at environmental temperature and operational conditions that is subsequently reduces the overall performance and stability [3].

Single-walled carbon nanotubes (SWCNTs) displayed superb physical, properties in terms of conductivity, light absorption, thermal, mechanical, and chemical ones. SWCNTs could be incorporated into the PSC system [4]. SWCNTs is a favorable electrode material, owing to its abundance, hydrophobicity, and mechanical robustness [5]. The application of SWCNT as the top electrode expressively enhances the stability of PSCs by removing the ion migration, and considerably reduces the fabrication cost as it can be easily deposited onto devices by a simple mechanical transfer [6]. We recently showed that the trifillic acid-treated SWCNTs can have a great applicability with 2D/3D FACsPbI₃ system reaching PCE of 17.6% with enhanced stability [7]. The conductivity of SWCNT film could be increased as the work function is tuned by using a vapor-assisted doping of trifluoromethanesulfonic acid (TFMS).

We extend this work on the MAPbI₃ system showing better performance. In this presentation, we will present our latest results on doping of SWCNT electrodes with diluted solution of TFMS having a new record of PCE for the SWCNT-based PSCs. Our devices showed higher PCE of more than 18 % with a J_SC of 22.67 mA cm^-2, V_OC of 1.104 V, and FF of 0.724 through the optimization of hole transport layer concentration as shown in figure 1. Additionally, we will show the possibility of introducing our new 1D heterostructures by wrapping of various SWCNT with BN and TMD layers using chemical vapor deposition into the solar cells and devices field. These SWCNT-based heterostructures are expected to have a broad interest and impact in fabricating BN-protected or gated SWNT devices and building more sophisticated 1D material systems [8]. These new structures are expected to give extra functionality, durability, and great applicability in electronic, optoelectronic, and energy devices.

References:

[1] A. Kojima et al., J. Am. Chem. Soc., (2009), 6050.

[2] NREL's best research efficiency chart., (2018).

[3] K. Domanski et al., ACS Nano, 10 (2016), 6306.

[4] I. Jeon et al., Topics Curr. Chem., (2018), 376:4.

[5] I. Jeon et al., Adv. Energy Mater., (2018) online [DOI:10.1002/aenm.201801312].

[6] I. Jeon et. al., J. Phys. Chem. C, (2017), 121, 25743.

[7] J.-W. Lee et. al. Nano Lett., (2018), ASAP [DOI: 10.1021/acs.nanolett.8b04190].

[8] R. Xiang et al., (2018), submitted. [Cite: https://arxiv.org/abs/1807.06154].

Figure 1

A donor-acceptor-donor (D-A-D) dye consisting of ferrocene units (donors) and a benzothiadiazole unit (acceptor) was synthesized to use the photosensitizer for hydrogen evolution reaction (HER) from water within semiconducting single-walled carbon nanotube (s-SWCNT). The dye was easily encapsulated into s-SWCNT by a solution method to form dye-encapsulated s-SWCNTs (dye@SWCNTs). Subsequently, physical modification of dye@SWCNTs by a fullerodendron gave dye@SWCNT/fullerodendron nanohybrids having dye/s-SWCNT/C₆₀ coaxial heterojunction. Interestingly, dye@SWCNT/fullerodendron nanohybrids act as the photosensitizer for HER in the presence of sacrificial donor, electron relay, and co-catalyst due to the mobile carrier generation upon photoexcitation of the dye molecules.

Lead halide perovskite solar cells (PSCs) are a promising alternative energy source that has received a considerable attention in recent year. Their certified power conversion efficiencies (PCEs) now exceed 20% and only a few challenges, such as long-term stability and cost of fabrication, remain before commercialization. PSCs in general have a structure in which a photo-active material and two charge-selective materials are sandwiched by a transparent bottom electrode and a metal top electrode. Typically, the metal electrodes are thermally evaporated in vacuum, which incur substantial increase in fabrication costs and material costs. Further, using these metal electrodes induces the ion-migration, which is detrimental to the long-term stability of the perovskite layer. Thus, it is crucial that we find an alternative to the metal electrodes to bring PSCs to the next level.

Having high conductivity and facile processability, aerosol synthesized single-walled carbon nanotubes (CNTs) have showcased promising potential as the top electrode in PSCs. Using CNTs as the top electrode drastically improves the PSC stability by both removing the ion migration and functioning as an effective moisture barrier. Further, CNTs are mechanically and chemically robust which contribute to the durability of the PSCs. Moreover, the fact that CNTs are made up of earth-abundant carbon atoms only and the aerosol-synthesized CNT films are direct-transferable means the low-cost production. The only shortcoming of employing the CNTs as the top electrode in PSCs, this far, has been the limited PCE. This is because CNTs do not reflect light unlike metals and doping the CNT top electrodes without damaging the layers underneath is extremely difficult. Recently, we reported a vapor-assisted ex-situdoping method using trifluoromethanesulfonic acid (TFMS), to dope the CNT top electrode in PSCs with no damage to the materials underneath and reported the record-high PCE of 17.56%. However, the vapor doping was too weak as a long exposure to the TFMS vapor resulted in a reaction between TFMS and 4-tert-butylpyridine (t-BP). Also, it was difficult to control the exposure time exactly everytime. Therefore, it is necessary to find a more controlled and effective doping technology. The key here is to develop a new doping method that leads to the minimal interaction with t-BP while demonstrating the maximum doping effect.

Herein, we report CNT-laminated PSCs in which optimized concentration of TFMS dopant in an apolar solvent was applied by a drop-casting method to increase the conductivity of the CNTs while avoiding the reaction with t-BP and the perovskite layer underneath. The choice of apolar solvent was chlorobenzene which exhibited superior doping effect and stability compared with other solvents. Furthermore, higher concentration of spiro-MeOTAD was used in the CNT-laminated PSCs by exploiting the high mobility of the porous CNT network. The combination of those two led to a PCE of 18.5%, which is comparable to 18.3% of the metal electrode-based PSCs. The CNT-laminated devices demonstrated a stability time of more than 1000 operating hours, which is by far greater than that of the reference devices.

Figure 1

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been thoroughly investigated as the components of various photovoltaic cells due to several advantages such as spectral tunability, absence of charge-transfer (CT) states, giant aspect ratio, chemical robustness, and hydrophobicity. In the previous study, it was reported that the heterojunctions between s-SWCNTs and perylene diimide (PDI)-based electron acceptors yield long-lived charge separated states whose lifetimes are more than 1.5 µs. Besides the potential of PDI-based electron acceptors to substitute fullerene-based electron acceptors, this work noted the significance of the molecular geometries of PDI-based electron acceptors which result in molecular aggregation and the associated charge delocalization in the acceptor phase. However, the true characteristics of the long-lived charge carriers for these heterojunctions have not been revealed yet. For instance, the degree to which the charge carriers generated at these heterojunctions are free or trapped was not yet clear. Moreover, it was not determined whether the charge recombination process from these heterojunctions is monomolecular-like or bimolecular-like. In this study, we explored the nature of charge carriers for the heterojunctions between (6,5) s-SWCNTs and two different PDI-based electron acceptors by combining two effective spectroscopic techniques: transient absorption (TA) and time-resolved microwave conductivity (TRMC).

Two PDI-based electron acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, were synthesized and coated on (6,5) s-SWCNT films to form donor-acceptor heterojunctions. TA and TRMC studies reveal that the dynamics of the charge-separated states across the (6,5) s-SWCNT/PDI-based acceptor heterojunctions remain similar over three orders of magnitude in absorbed photon flux. This fluence independence of the heterojunctions indicates that the charge carriers recombine 'pseudo'-monomolecularly. Moreover, the charge recombination kinetics from TA and TRMC studies are well-matched, indicating that most of the generated charge carriers are free, not trapped. The unconventionally strong suppression of bimolecular charge recombination from these heterojunctions, supported by fluence independence of charge recombination dynamics, may be attributed to the high carrier mobility and good charge delocalization in both (6,5) s-SWCNTs and PDI-based acceptors. These factors can also be regarded as the origin of high free charge carrier generation in these heterojunctions.

These photophysical studies provide the fundamental understandings of the charge generation process in s-SWCNT-based heterojunctions and how different electron acceptor materials can impact the nature of charge generation with respect to the heterojunction energetics and molecular orientations. The results can inform rational design strategies for s-SWCNT-based optoelectronic applications.

Carbon quantum dots (CQDs) have emerged as promising materials for optoelectronic applications on account of carbon's intrinsic merits of high stability, low cost and environment-friendliness. However, the CQDs usually give broad emission with full width at half maximum exceeding 80 nm, which fundamentally limit its display applications. We demonstrate multicolored narrow bandwidth emission (full width at half maximum of 29 to 30 nm) from triangular CQDs with a quantum yield up to 54 to 72%. Detailed structural and optical characterizations together with theoretical calculations revealed that the molecular purity and crystalline perfection of the triangular CQDs are key to the high color-purity. Moreover, multicolored light-emitting diodes based on these CQDs displayed good stability, high color-purity and high-performance with maximum luminance of 1882 to 4762 cd m^-2 and current efficiency of 1.22 to 5.11 cd A^-1. This work will set the stage for developing next-generation high-performance CQDs-based light-emitting diodes.

Capacitive mixing is a newly emerging technique for the production of renewable energy from differences in salinity. The method is based on the controlled mixing of two streams with different salt concentrations which are alternatingly brought into contact with pre-charged porous electrodes, taking advantage of the fact that modification of the electrical double layer of the electrodes results in changes in the solution salinity. In most research, the renewable energy resources are seawater and river water. Here, we demonstrate that energy extraction by capacitive mixing can take place with acidic wastewater and seawater as energy resources. This concept is proved by means of the fabrication of a proton-selective carbon cathode (the negatively polarized electrode), achieved by carbonation of cellulose filter paper, followed by mild activation in concentrated nitric acid. Considerable energy extraction was demonstrated even when the concentration of the saline solution was tenfold that of the acidic solution.

Figure 1

Lithium metal-sulfur (Li-S) batteries have gained wide attention due to extremely high energy density, but their application was hindered by the challenges in both Li anode and sulfur cathode. Herein, we designed a dual functional, highly ordered, and interconnected mesoporous carbon (MPC) to address the challenges in cathode and anode simultaneously. On the anode, a thin layer of MPC annealed at 500 ºC (MPC-500) was introduced to mechanically suppress Li dendrite, which provided interfacial protection on the Li metal and stabilized the Li plating/stripping. This novel anode showed excellent cycling stability with more than 300 hours at a high current density of 3 mA cm^-2. On the cathode, MPC annealed at 1300 ºC (MPC-1300) was utilized to confine sulfur. The large surface area and nanopores significantly alleviated the shuttle effect, which contributes to the cycling stability of the Li-S battery. Meanwhile, the conductive carbon promote electron transfer. When a Li-S cell was assembled with MPC-500 and MPC-1300 as anode interface layer and cathode additive, respectively, an exceptional reversible capacity of 607 mA h g^-1 was achieved at a current density of 1 C (=1675 mA g^-1) and maintained at high capacity retention of 90% after 100 cycles.

Previously, carbon was regarded as a support material for nanoparticle utilization in electrocatalysis fields, however various physicochemical tenability via doping, functionalization and morphology engineering opens its functional role in various electrocatalytic applications. Furthermore, carbon materials are highly accentuated as a promising alternative to state-of-art systems based on precious metals due to its cost-effectiveness and natural abundance. In this talk, we discuss simple and straightforward approach to prepare carbon-shell-protected nanoparticles (NPs) that have both high catalytic activity and long-term durability for electrochemical energy applications. A single-step thermal treatment of polydopamine-coated NPs leads to the formation of thin-layer carbon shell, providing physical and chemical protections preventing nanoparticle degradation. Moreover, carbon shell can successfully protect chemical properties of NPs during electrochemical operation condition, verifying superior stability. Finally, we also suggest several promising strategies to enhance utilization of porous nanostructure for electrocatalysis, focusing on pore structure engineering and bridging functional links between electrochemical performance and its pore structure.

With the development of the industry, the need for better energy storage devices that can be used in various devices continues to grow, but the pace at which practical alternatives are developed is very slow. Many prior studies have succeeded in increasing the energy density of lithium-ion batteries to a significant level, but they have not been able to lead to actual production due to battery instability, life problems, production and price issues. Many battery manufacturers currently faced the limit capacity of the cathode and are aiming to improve the energy density by developing an anode with a capacity of 430 mAh / g by mixing graphite with a high capacity active materials. However, in such a method, it is difficult to satisfy the high power density due to the difference in charge/discharge speed between the graphite and the high capacity active material. So, in reality, current access and pricing policies are not able to create a significant gap in the lithium-ion battery platform. For this reason, the study of lithium-ion capacitors satisfying both energy density and power density has received much attention due to its high versatility. Lithium-ion capacitors can use conventional lithium-ion anodes and use carbon materials instead of heavy metal alloys as cathodes. Initial lithium-ion capacitor research was concentrated on an anode capable of high capacity such as a lithium-ion battery. In fact, the main factor that determines the capacity per mass/volume of lithium-ion capacitors is the capacity of the cathode. As the cathode of the lithium-ion capacitor, a porous carbon material which has been used as the electrode material of the electric double layer capacitor has been widely used. However, the electrode of the EDLC stores OH- ions, while the cathode of the lithium-ion capacitor should adsorb and store much larger ions such as PF6- ions. Thus, the electrode material originally used in the EDLC may be unsuitable for storing larger ions due to the small pore size. Here we have found improvements that can improve the capacity of lithium-ion capacitors. We intend to develop a cathode material for lithium-ion capacitors that can store larger ions such as PF6- ions in this work. We have developed seven cathode materials that composed of concentrate pores with a certain size using graphene and a template that is vaporized below 100 degrees of Celsius. The size of the pores ranges from less than 5 nm to 150 nm. Although the effects of the number of Angstrom pores have been addressed in the meantime, the impact of macro size pores has not been addressed in the lithium-ion capacitor research. We evaluated the capacity and characteristics of the developed cathode using PF6-, TFSI-, BF4-, ClO4-, and so on. In the experimental results, we confirmed that macro-sized pores have a profound effect on the adsorption and storage of ions and their obvious tendency. To put it briefly, the TFSI- and PF6- ions are significantly different in size, and thus the cathode with the highest capacity is different from each other, which is revealed by the tendency. Additionally, the full cell lithium-ion capacitor with developed cathodes and a commercial MCMB anode recorded the highest energy density of up to 145 Wh/kg and an energy density over 70 Wh/kg at a power density of 7200 W/kg. We have demonstrated the effect of macro-sized pores on the cathode of a lithium-ion capacitor and succeeded in developing a cathode material for an excellent lithium ion capacitor.

Robust conjugated two- (2D) and three-dimensional (3D) non-metallic conductive structures have attracted immense interest due to their unusual electronic, optoelectronic, magnetic and electrocatalytic properties. Their tunable structures and properties promise to offer many opportunities in various applications. Nevertheless, methods developed for the synthesis of non-metallic conductive materials, which are capable of producing fused-aromatic based stable frameworks with uniformly decorated heteroatoms with/without holes, remain limited, even after decades of intensive exploration in science and technology. To overcome these issues, stable organic materials have been designed and synthesized. They have uniformly distributed heteroatoms,¹ holes with heteroatoms² and transition metal nanoparticles in the holes.³ The structures were confirmed using various characterization techniques, including scanning tunneling microscopy (STM). Based on the stoichiometry of 2D layered structures, they were, respectively, designated C₂N, C₃N, C₄N, and M@C₂N (M = Co, Ni, Pd, Pt, Ru). Their electronic and electrical properties were evaluated by electrooptical and electrochemical measurements along with density-functional theory (DFT) calculations. Furthermore, robust three-dimensional (3D) cage-like organic materials have also been constructed and they show high sorption properties.^4,5 The results suggest that these newly-developed 2D and 3D non-metallic conductive materials offer greater opportunities, from wet-chemistry to device applications.

References:

[1] Mahmood, et al. Two-dimensional polyaniline (C₃N) from carbonized organic single crystals. Proceedings of National Academy of Sciences, USA 2016, 113, 7414.

[2] Mahmood, et al. Nitrogenated holey two-dimensional structure. Nature Communications 2015, 6, 6486.

[3] Mahmood, et al. An efficient and pH-universal ruthenium-based catalyst for the hydrogen evolution reaction. Nature Nanotechnology 2017, 12, 441.

[4] Bae, et al. Forming a three dimensional porous organic network via explosion of organic single crystals in solid-state. Nature Communications 2017, 8, 159-Highlighted in Nature Nanotechnology 2018, 13, 4.

[5] Mahmood, et al. A robust 3D cage-like ultramicroporous network structure with high gas uptake capacities. Angewandte Chemie International Edition 2018, DOI: 10.1002/ange.201800218.

The less activity and durability of non-noble transition metal based ORR catalysts are bringing the metal-free catalysts derived from carbon allotropes on high demand. The pristine graphene is an electroneutral material and inactive towards the catalytic ORR reaction however the alteration of its electronic property with heteroatom doping makes it active towards electrocatalysis.^{[1, 2]} In addition, nitrogen doping is reported to be the most effective way to enhance the catalytic properties of nitrogen-doped graphene (NGr) which creates ORR active center at a nearby carbon atom. Previously reported experimental and theoretical results prove that the N-doped graphene with 3D morphology and doped pyridinic-N are the primary reason for the higher activity of N-doped graphene.^{[1, 3]} Taking into account the importance of nitrogen doping in graphene, here we have prepared the 3D N-doped reduced graphene oxide by using NaCl crystals as a structure directing agent. In addition, the fundamental understanding of the ORR at the catalyst surface is a major confront which is hindering the improvement of catalyst performance under operating conditions of fuel cell.^[4] Moreover, the catalyst surface wettability is considered to be the most significant factor, which directly reflects from the O₂ storage and activation by the active biological molecules such as hemoglobin, laccase, etc.

Here, we introduced the role of catalysts surface hydrophobicity created by induced defects towards the ORR reaction in acidic medium. We control the hydrophobicity of the catalyst surface by using NaCl crystals as a spacer and defect creator during the preparation of 3D N-doped porous graphene. The physical and electrochemical characterizations of the prepared catalysts revealed the role of surface hydrophobicity towards the triggering of ORR reaction at low overpotential by capturing O₂ near to the reaction centers. The catalyst with underwater Wenzel-Cassie (UWC) coexistent state of surface hydrophobicity has started ORR at lower overpotential (onset potential 0.85 V_RHE) than the catalyst with underwater Wenzel (UW) state (0.57 V_RHE) in acidic medium. The adopted method for the synthesis of 3D porous N-doped graphene catalyst involves NaCl crystals acts as a structure directing agent to get 3D NrGO morphology (Fig. 1a). The used NaCl crystals also help in the introduction of localized defects in the graphene framework during the synthesis process. The prepared catalyst NrGO/NaCl shows a low overpotential and high current density compared to the NrGO, mainly due to the induced surface hydrophobicity involved in O₂ storage, high concentration of doped pyridinic-N and the 3D graphene morphology. Fig. 1b shows the comparative Tafel analysis of NrGO and NrGO-NaCl samples displaying comparable ORR activity of the sample prepared by using NaCl as a template with the state-of-the-art (Pt/C) catalyst. Furthermore, we are now trying to demonstrate the effect of catalyst surface hydrophobicity of metal-free N-doped graphene in the real time PEMFCs application. It is believed that the induced catalyst surface hydrophobicity will be involved in prevention of water flooding problem faced in the PEMFCs operation.

References: [1] T. Kondo, S. Casolo, T. Suzuki, T. Shikano, M. Sakurai, Y. Harada, M. Saito, M. Oshima, M. I. Trioni, G. F. Tantardini, J. Nakamura, Physical Review B2012, 86, 035436.

[2] Z. Hou, X. Wang, T. Ikeda, K. Terakura, M. Oshima, M.-a. Kakimoto, Physical Review B2013, 87, 165401.

[3] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science2016, 351, 361.

[4] S. K. Singh, K. Takeyasu, J. Nakamura, Advanced Materials, 2018, 1804297, DOI: 10.1002/adma.201804297.

Figure 1

For both proton exchange membrane and direct methanol fuel cells, oxygen reduction reaction (ORR) is a performance-determining step. While Pt has been established as a standard bearer in ORR catalyst development, practical long-term applications require continuing effort to pursue active, stable, as well as affordable materials as alternatives to precious metals.

In this talk, carbon-based vertically aligned carbon nanofibers (VACNFs), essentially a stack of conical graphitic structures, will be discussed as highly promising ORR catalysts. In an alkaline medium, at -0.4 V applied potential, the current density measured on VACNF cathode is twice more than that measured with commercial Pt catalyst (Pt/C). Molecular insights have been gained through the use of Density functional theory (DFT) calculations to: (1) reveal interactions of ORR intermediate species at the catalytically active sites; and (2) elucidate the origin of catalytic activities of VACNF under reaction conditions. Specifically, semi-periodic fishbone-like stacked graphene sheets, bare or terminated with H and OH, have been employed to understand the impact on ORR mechanism and corresponding reactivities. Bindings of O₂, O, and OH at various configurations obtained from DFT are then used as descriptors to assess catalyst performance against simple Pt (111) and Pt (211) surfaces.

In addition, Pt nanocatalysts supported on VACNF (Pt/VACNF), which can push the ORR current density even higher, will be discussed as well. Again, with DFT, a model with Pt anchored at the VACNF edges was proposed to investigate the functionalities of Pt/VACNF for alkaline ORR reactions. In this talk, both associative and dissociative ORR pathways will be analyzed based on the established models. In particular, the overall computed ORR thermodynamics are shown to be favored toward Pt/VACNF than Pt (111), consistent with experimental results. Nevertheless, the unique interactions of ORR intermediates such as O*, OOH*, and OH* with catalytic active VACNF edge sites, revealed by DFT calculations, suggest intriguing but also complex electrochemistries.

Direct electricity-powdered production of value-added carbon organic compoundsfrom CO₂ and H₂O, a process that mimics natural Wood–Ljungdahl pathway (WLP), is of fundamental and practical interestfor renewable energy applications. The process depends on electrotrophy, the ability of some microorganisms to use electrons derived from an electrode as the sole energy source to reduce carbon dioxide. In this work, an acetogenic microorganism Clostridium ljungdahliiis immobilizedon carbon felt (CF) andhighly enriched semiconducting single-walled carbon nanotube (SWCNT) scaffolds under different conditions (physical soaking and growth under electrochemical bias). As a proof of principle, we demonstrate that this conductive carbon-bacteria system can utilize electrical current to fix CO₂ under to multiple chemical targets, such as acetate and ethanol. The porous structure of the hybrid electrode provides the large surface area needed for high bacteria loading and excellent diffusion kinetics, while the s-SWCNTs appear to facilitate unique modes of bacterial adsorption and strong interfacial interactions. As such, the hybrid system enables low overpotential (η < 200 mV) and high Faradaic efficiency (> 90%). With the SWCNT modification and bacteria electrochemical growth, the hybrid electrode exhibited much higher performance on acetate production due to the enhanced bacteria-electrode interface quality for charge transfer. I will also discuss our efforts to understand the biochemicalpathways and possible underlying charge transfer mechanisms of CO₂reduction of this SWCNT-Clostridium ljungdahliisystem. These results suggest a potential route for using SWCNTs to facilitate efficient microbial electrochemical biofilm formation and energy storage, furthering the potential applicability in bioelectrochemical systems.

To replace scarce and expensive Pt-based oxygen reduction reaction (ORR) catalysts in acidic conditions, non-precious metal catalysts (NPMCs) based on third-row transition metals and N-doped carbon (M/N/C) have been intensively studied to date. Here, we present a novel versatile strategy to control and enhance the activity of the single Fe-N₄ site by incorporating electron-withdrawing/donating functionalities on the carbon plane. These incorporated functionalities change the strength of the electronic effect, which is derived from the delocalized pi-band of carbon plane to the d-orbital of the Fe ion in Fe-N₄ site; therefore, the adsorption strength of ORR intermediates at the Fe-N₄ site was controlled by incorporated functionalities. Electrochemical CO₂ reduction reaction (CO₂RR) has attracted a lot of interest as a highly potential CO₂ utilization system. Due to the high overpotential in CO₂RR, the development of an effective catalyst is highly required for CO₂ based long-term energy storage system. We present a metal-organic hybrid catalyst (Co-PPy-C), which consists of Co and polypyrrole, as a highly active electrocatalyst for CO₂RR. Co-PPy-C exhibited high Faradaic efficiency and metal mass activity for CO production at low overpotential region.

The U.S. Energy Information Administration has projected a 27% growth in energy consumption by the year 2050.¹ The continuous increase in the energy demands worldwide and the environmental concerns reaching critical levels, has required researchers to make the development of clean and sustainable energy conversion devices their paramount challenge. Fuel cells and metal-air batteries with high efficiency and low cost are the prime candidates. Platinum loaded carbon substrate is the catalyst of choice in fuel cells for anodic oxidation of H₂ and cathodic reduction of O₂ but large-scale commercial production is restricted by its prohibitive cost, limited supply, and weak durability.² Platinum's vulnerability to potential drift over time and easy loss of activity over methanol poisoning, questions its competitive catalytic activity in comparison to Pt or Pt-alloy based catalyst and are easily synthesized, have low cost and are earth-abundant is still highly desired.

Heteroatom doped carbon electrocatalysts are highly considered and well researched because of their high abundance and low cost, with transition metal and nitrogen doped carbon material showing competitive result towards the oxygen reduction reaction (ORR).^3,4 Among those heteroatoms phosphorus doped carbon is an emerging catalyst with high potential. The attraction towards phosphorus arises from the fact that it has a larger atomic radius- hence larger covalent radius- and higher electron-donating ability than nitrogen, making it a promising dopant with an ability to modify the electron transport properties. The Introduction of phosphorus, an atom with lower electronegativity, changes the charge distribution and electronic properties of high surface area carbon structure. Phosphorus doping increases the affinity towards an acceptor molecule such as Oxygen⁵, hence a better interaction with the oxygen intermediate and enhances their electrocatalytic activities towards ORR. Such heteroatoms are typically covalently bonded within a carbon framework ensuring long-term stability. The advantages of a heteroatom doped porous carbon are twofold, stabilization of the non-precious metal in the alkaline medium and generation of active sites for facilitating the oxygen reduction reaction. Electrocatalytic activity of phosphorus doped and Iron treated phosphorus doped carbon has only been covered by few groups and needs more improvements.^6,7 The purpose of the study of electron transfer processes is to understand the reactivity of a relatively new, doped porous carbon complexes incorporating the Phosphorus as a heteroatom bonded to a metal center. According to DFT calculations, O₂ and other ORR intermediates can be stably adsorbed on to Fe sites⁸, making phosphorus and iron doped carbon a prime candidate for ORR.

Using a single precursor as a source of phosphorus, we propose a simple one-step chemical activation, with anhydrous ZnCl₂ and anhydrous FeCl₂ as the agents of carbonization, yielding P and Fe-doped porous carbon (PFeC). The electrocatalytic activity towards ORR was studied using a rotating ring-disk electrode (RRDE) technique and rotating disk electrode connected to the CHI 600 potentiostat. The electrochemical activity of PFeC was compared with the commercially available 20 wt% Pt/C. PFeC showed high activity towards ORR, with comparable onset potential and a small negative shift in half-wave potential. PFeC selectively catalyzes oxygen to water via a direct four-electron pathway. PFeC showed to be inert toward alcohol oxidation and had superior long-term stability.

References

(1) U.S. EIA. Annual Energy Outlook 2018 with Projections to 2050. Annu. Energy Outlook 2018 with Proj. to 20502018, 44 (8), 1–64.

(2) Steele, B. C.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature2001, 414 (November), 345–352.

(3) Lin, L.; Zhu, Q.; Xu, A. W. Noble-Metal-Free Fe-N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc.2014, 136 (31), 11027–11033.

(4) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electroctalytic Activity for Oxygen Reduction. Science (80-. ).2009, 323 (FEBRUARY), 760–764.

(5) Cruz-Silva, E.; Lopez-Urias, F.; Munoz-Sandoval, E.; Sumpter, B. G.; Terrones, H.; Charlier, J. C.; Meunier, V.; Terrones, M. Phosphorus and Phosphorus-Nitrogen Doped Carbon Nanotubes for Ultrasensitive and Selective Molecular Detection. Nanoscale2011, 3 (3), 1008–1013.

(6) Razmjooei, F.; Singh, K. P.; Bae, E. J.; Yu, J.-S. A New Class of Electroactive Fe- and P-Functionalized Graphene for Oxygen Reduction. J. Mater. Chem. A2015, 3 (20), 11031–11039.

(7) Singh, K. P.; Bae, E. J.; Yu, J. S. Fe-P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc.2015, 137 (9), 3165–3168.

(8) Lee, D. H.; Lee, W. J.; Lee, W. J.; Kim, S. O.; Kim, Y. H. Theory, Synthesis, and Oxygen Reduction Catalysis of Fe-Porphyrin-like Carbon Nanotube. Phys. Rev. Lett.2011, 106 (17), 8–11.

The concept of a core–shell metallic structures, with well defined atomically-thin "shell" material delineated from the "core" support with atomic-scale sharpness opens the door to a multitude of surface-driven materials properties that can be precisely controlled. However, in practice, such architectures are difficult to preserve due to the entropic cost of a segregated near-surface architecture, and the core and surface atoms inevitably mix through interdiffusion over time or under processing conditions (e.g. in Proton Exchange Membrane Fuel Cells (PEMFC)).

Herein, We present a systematic study of interdiffusion in a Au@Pt Core@Shell architectures and the role of an interrupting monolayer graphene sandwiched in between. The physical and chemical structure of the near-surface is probed via mean-free-path tuned X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HRTEM), and probing the oxygen reduction reaction (ORR) as an electrochemical performance indicator. We find that at operating temperatures above 100 °C, there is a potential for interdiffusion to occur between the shell and support metals, which can diminish the catalyst activity toward ORR. The introduction of a single-layer graphene, at the interface between the core and shell metals, acts as a barrier that prevents unwanted core leaching and/or surface-alloying between the layered metals. STEM imaging shows that fully wetted Pt monolayers can be grown on stand-alone single graphene template, allowing a high level of surface utilization of the catalyst material. We present how the use of graphene as an interdiffusion barrier mitigates the loss of surface catalytic sites, showing much improved retention of Pt monolayer surface at elevated temperatures.

It has been postulated that non-platinum group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR) must meet four criteria: metal, carbon, nitrogen, and pyrolysis. The precise nature of the catalytic active site remains under debate. Many permutations of metal, nitrogen, and carbon are formed during pyrolysis, only a few of which are catalytically active. Results from multiple studies indicates that metals surrounded by four pyridinic nitrogen atoms form at least part of the active site. It stands to reason that the greater the number of metal-nitrogen-carbon (M-N_x-C_y) sites formed, the greater the active site density will be. However, nanoparticle formation is likely to compete at very high densities. Other groups have demonstrated enhanced catalytic activity in bimetallic catalysts for both ORR and hydrogen evolution reaction (HER) using first and second row non-PGM transition metals in M-N₄-C_y centers. The present research focuses on the use of metal organic frameworks (MOFs), primarily zinc imidazolate frameworks (ZIFs), as precursors for bimetallic catalysts. ZIFs are of interest because they are highly porous, contain a high density of M-N₄-C_y centers, and are simple to prepare. Metal choice in ZIF precursors (Zn or Co) affects the porosity and graphitization of the heat-treated material. Cobalt catalyzes the formation of carbon nanotubes upon pyrolysis. This research is an investigation of the effects of MOF precursor type, heat treatment, and non-PGM metal pairs (Fe, Co, Mn or Mo) on catalytic activity. It is hypothesized that carbon nanotubes and other graphitic structures formed during pyrolysis of Co-containing ZIFs facilitate the cooperation of proximal metal sites in electron transfer. This will be demonstrated by evaluation of bimetallic Co-containing catalysts in HER or ORR by varying the degree of graphitization while holding the weight percent ratio of Co and another metal (e.g. Fe) constant. Comparison of this data with catalysts prepared from precursors with optimal graphitization and varying the ratio of Co to the other metal will elucidate the relative importance of local carbon structure and electron transfer between two different metal centers.

This work was supported by the U.S. Department of Energy under award number DE-EE0008416. The authors declare no competing financial interests.

Green, efficient energy storage and conversion systems are an important means of addressing global environmental and energy crisis. The current battery systems have been difficult to meet the increasing market demand due to their relatively low energy densities, therefore it is urgent to develop new energy storage technologies with high energy density and low cost. To this end, Professor Zhongwei Chen and his research group adhere to a bottom-up "material-electrode-battery-system" strategy to develop a variety of advanced energy storage technologies, including lithium-ion batteries, zinc-air batteries, fuel cells, for a wide range of energy storage applications. Through the rational material design and engineering, various high-performance electrodes with hierarchical micro- and nanostructures have been developed. Moreover, the electrode structures are controlled and optimized toward high recharge efficiency and energy density. Together, these innovate materials and electrode architectures practically allow industrial assembly and production of more efficient, affordable, and safer battery systems for clean and sustainable energy storage.

The rational design and fabrication of nanostructured materials with desired electrochemical performance is highly demanded for energy storage applications. Functionalization of graphene based materials using chemical moieties not only alter the electronic structure of the underlying graphene but also enable in only limited enhancement of targeted properties. Surface modification of graphene based materials using other nanostructures enhances the effective properties by minimally modifying the properties of pristine graphene backbone. In this pursuit we have fabricated bio-inspired hierarchical nanostructures based on Ni-Co layered double hydroxide on the reduced graphene oxide core shells using template based wet chemical approach. The as-obtained materials with tuned morphology have been characterized structurally and electrochemically. The results show that the charge storage capability of the synthesized material is 3 fold higher than that of pristine materials. These characterizations confirm that the obtained material has large effective surface and has promising applications in catalysis and energy storage. This surface modification strategy is not limited to only transition metal based hydroxides but also may pave a way for the fabrication of many other nanostructured materials.

Luminol exhibits electrochemical redox activity when electropolymerized on various substrates, and has been used for electrocatalysis and biosensors [1, 2]. This electrochemical behavior is due to the presence of different amine functional groups within the polymer structure. Recent studies have shown that introducing amine groups on a carbon surface improves the charge storage capacity and the conductivity of the obtained material [3]. Our work aims to leverage the electrochemical properties of polyluminol (Plum) for electrochemical capacitors (ECs). Firstly, the Plum was synthesized using a chemical approach and characterized using spectroscopic methods and electrochemical analyses. Secondly, the polymer was deposited on carbon nanotubes (CNT) to enhance the charge storage capacity for ECs.

The chemical composition of the polymer was found to be a mixture of benzoid and quinoid segments, bonded via secondary (NH) or tertiary amine (=N-) groups, respectively. Such functionalities facilitated chemical, thermal, and electrochemical stability of the polymer. The composite electrode material was obtained by in-situ chemical polymerization of Plum on CNT. Morphological analyses of CNT confirmed the increase of the Plum thickness on CNT with polymerization time, reaching a saturation point of 6.5 nm. In addition, a 4x increase of charge storage capacity with good electrode stability was observed compared to bare CNT over a potential window of 1.2 V as seen in figure 1. Study of the redox reaction kinetics revealed a mostly capacitive contribution of the polymer on CNT. This work showed the successful surface modification and engineering of CNT using a simple and effective fabrication method, which is suitable for large-scale fabrication of composite electrode materials for ECs.

• G.-F. Zhang and H.-Y. Chen, Analytica Chimica Acta, 419, 25 (2000).

• A. Sassolas, L. J. Blum and B. D. Leca-Bouvier, Sensors and Actuators B: Chemical, 139, 214 (2009).

• N. Phattharasupakun, J. Wutthiprom, P. Suktha, N. Ma and M. Sawangphruk, Journal of The Electrochemical Society, 165, A609 (2018).

Figure 1

Nanostructured carbon materials, such as graphene and nanoporous carbon, have become increasingly prominent for use as electrodes in supercapacitors, owing to their high specific and volumetric surface area and good electrical conductivity. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer, and the low energy to limited electrode surface area (SA). As such, low-dimensional carbon nanostructures have been extensively explored. However, anomalous and nonlinear relationships between observed capacitances and SAs have prompted a need to revise our mechanistic understanding of charge storage. In this talk, we present our recent findings regarding charge storage mechanisms, along with possible faradaic reactions, in carbon-based supercapacitors from combined quantum mechanical and classical molecular dynamics simulations. Our first-principles studies suggest that the total interfacial capacitance (C_T) of graphene-based supercapacitors is determined from both electrode (C_Q) and electric double layer (C_D) capacitances which are in series. It has also been demonstrated that the C_Q, and thereby C_T, can be significantly enhanced as a result of changes to the electronic structure through chemical functionalization, doping, and/or structural deformation. Furthermore, our voltammetric molecular dynamics simulations have revealed the complex non-equilibrium processes associated with extreme confinement of ionic liquid electrolytes within subnanometer pores. Based on these results, we will also discuss a mechanistic perspective centered around ion reorganization kinetics, particularly the dependence of ion migration efficiency, and relatedly, the capacitance, on pore morphology. This talk will also briefly touch on possible faradaic reactions that may cause self-discharge.

Recent developments in the field of flexible and wearable electronic devices necessitate wearable supercapacitor technologies to power them. Electrode nanostructuring is a viable strategy to prepare high performance supercapacitor electrodes. Carbon nanomaterials such as carbon nanotubes, carbon nanopetals, graphene, etc are invariably used as supercapacitor electrodes due to their salient features such as good electronic conductivity, low density, large surface area to volume ratio, good chemical and electrochemical stabilities, high capacitance, bendability, etc. Herein, we report the development of all-solid-state flexible supercapacitor using graphene (Gr)/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) hybrid electrodes. The Gr/PEDOT:PSS hybrid electrodes are prepared by electrochemical deposition followed by dip-coating procedures. The morphology and texture of the Gr/PEDOT:PSS hybrids are examined by scanning electron microscopy and atomic force microscopy. The structure of the hybrids is determined by X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. The surface chemical analysis is performed using X-ray photoelectron spectroscopy. The Gr/PEDOT:PSS hybrid electrodes are tested by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge-discharge measurements. The all-solid-state Gr/PEDOT:PSS hybrid supercapacitor exhibits excellent electrochemical performance with high flexibility and is a potential candidate to integrate with textile fabrics for application in wearable electronic devices.

Silicon has been extensively studied as an anode material in lithium-ion batteries due to its extremely high theoretical specific capacity of 3578 mAh/g (Li₁₅Si₄). However, the huge volume fluctuations associated with this high cycling capacity create great challenges with regard to electrode mechanical integrity and continual electrolyte decomposition. Common strategies to alleviate this volume expansion are to study silicon alloys, silicon oxide, and to blend silicon materials with graphite active materials.

Herein we study graphite-SiO blends in an attempt to achieve stable cycling performance. We study a wide variety of laminate formulations by varying the graphite/SiO ratio, the binder type, the conductive additive type, and the weight percent of each of these components. We find that the use of single-walled carbon nanotubes (SWCNT) as conductive additive is crucial to prevent rapid capacity fade during initial cycling. The use of SWCNT also enables the use of significantly less conductive additive and binder, likely providing both enhanced conductivity and mechanical stability to the electrode during volume fluctuation.

Acknowledgement:

We gratefully acknowledge the support from the U.S. Department of Energy's Vehicle Technologies Office. This work is conducted under the Cell Analysis, Modeling, and Prototyping (CAMP) Facility at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357.

The encapsulation of sulfur within carbon matrices is widely utilized in the cathode of a rechargeable lithium-sulfur battery, whose energy density largely depends on the design of the carbon structure. Here we report an advanced graphene nanocage structure with the capability of hosting both cyclo-S₈ and smaller sulfur molecules (S_2-4). The cage inner cavity is partially filled with S₈ to form a yolk-shell structure that enables free volumetric variation of S₈ during (de)lithiation. In the graphene shell of the cage, S₈ are downsized to S_2-4 to activate extra sulfur loading sites within graphene layers. Importantly, the graphene shell exhibits inward volumetric variation upon (de)lithiation of the loaded S_2-4, and the overall electrode strain is thus minimized. This prototyped design promises an ultimate solution to maximize sulfur loading in carbon matrices as well as to circumvent the polysulfide dissolution problem and boost the commercialization of lithium-sulfur batteries in the future.

Through first-principles computational methods, the general electrochemistry of carbon materials is presented for potential applications. In developing a systematic approach for electrochemical properties, various structures of carbon materials including quinone derivatives, conjugated carbon molecules, and DNA are investigated. As one of the key insights into electrochemistry, reduction potentials are pursued, via density functional theory (DFT) modeling with PBE0/6-31G**+ calculations. Furthermore, given that solvents are often involved in electrochemical applications, the extent of their interactions with carbon materials is considered using PBF calculations. Results are analyzed using machine learning. From this study, new insights are offered on the relationship between structures and properties in carbon materials through DFT, achieving a full understanding of its electrochemistry for design of new materials.

Graphene is an atomically thick sheet consisting of a graphitic carbon network with excellent mechanical strength, electrical and thermal conductivity, and chemical stability. Holey graphene is a structural derivative of graphene with arrays of through-thickness holes across the lateral surface of the nanosheet. The presence of holes not only has minimal detrimental effect to the graphene properties, but also leads to enhanced performance in applications such as electronics, sensors, and energy storage. For example, the holes allow more facile cross-plane ion transport than intact graphene, making holey graphene an ideal electrode material for electrochemical energy storage. The presentation will focus on another property of holey graphene that may greatly extend its application space; the ability to be compression molded into robust articles or architectures under solvent-free conditions without the need for parasitic binders. The unique dry compressibility of holey graphene has enabled facile fabrication of high mass loading, high performance electrodes for supercapacitors and high-energy battery systems such as lithium-oxygen and lithium-sulfur batteries. In addition, the versatility of the dry compression process has enabled facile tuning of homogeneous and composite electrode architectures to further improve the device performance.

The lithium sulfur battery is the one of the promising alternative rechargeable battery system to replace the conventional lithium ion battery system. It has a high capacity and energy density of 1,675 mA h g−1 and 2,500 Wh kg−1, respectively. The energy density is about five times higher compared to lithium ion batteries. Furthermore, sulfur is cheap, naturally abundant and safe . One of the main constraints is related to the poor electrical conductivity of sulfur which needs addition of high amount of conductive carbon, Another main problem is the high solubility of the intermediate polysulfides in the organic solvent electrolytes during discharge/charge process. Dissolved polysulfide components are moving back and forward between Li Anodes and Sulfur Cathodes, causing a rapid capacity loss and a low coulombic efficiency. The common approach to solve the problem was using the concept of sulfur encapsulation in the porous carbon structure. This strategy is the most popular study for cathodes in lithium sulfur battery in the last few years. It has been shown to solve the problems of low conductivity and dissolution of polysulfides simultaneously. As the results, the electrochemical performance of sulfur cathodes has been improved due the better electronic conductivity and the inhibation of polysulfide dissoltion. In this current works, biomass based porous carbons have been synthesized by using chemical activation of tropical salacca fruit peel with potassium carbonate and used as sulfur cathode component for lithium-sulfur batteries. The motivation was to obtain carbon with specific morphology and structures used for the application in composite sulfur cathodes for lithium sulfur battery. As shown by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption isotherms, the porous carbon material has microporous-mesoporous structures with honeycomb morphology. Composite sulfur cathode has shown a large initial discharge capacity of 802 mAh g−1 at current density of 0.1 C , which remained stable during 50 charge-discharge cycles . This stable and reversible cycleability of this composite cathode has been contributed by a unique combination of micro and mesoporosity, as well assurface functional groups, which could allow the retention of the intermediate polysulfides within the carbon porous structures, without enormous dissolution in the electrolyte.

With rising energy concerns, efficient energy conversion and storage devices are urgently required to provide a sustainable, green energy supply. Electrochemical energy storage devices, such as supercapacitors and batteries, have been proven to be the most effective energy conversion and storage technologies for practical application. Currently, carbon materials hold the key for the development of high-performance electrochemical energy storage devices. However, the widely used carbon materials, such as graphite and activated carbon are often derived from non-renewable resources under relatively harsh environments, which hinders the sustainable development of electrochemical energy storage systems. In this context, biomass demonstrates many desired properties to derive renewable carbon materials for both electrochemical energy storage applications, because of its natural abundance and unlimited availability. Here, natural biomass, such as cotton textile, wheat flour, and corncobs, have been explored to produce renewable carbon materials via a low-cost and high throughput manufacturing process for energy storage systems design. Excitingly, the biomass-derived renewable activated carbon scaffolds not only demonstrated hierarchically porous structures but also excellent flexibility, making them ideal backbones for next-generation energy storage design. Specifically, activated cotton textile (ACT) with excellent flexibility and conductivity has been successfully derived from cotton textile for flexible energy storage systems design, such as flexible supercapacitors, flexible lithium-ion batteries, and flexible lithium-sulfur batteries. Besides flexible ACT, carbon nanotubes (CNTs) have also successfully derived from the natural yeast-fermented wheat dough without using any extra-catalysts or additional carbon sources. Yeast-derived carbon nanotubes from the fermented wheat dough not only provide an ideal sulfur host for lithium-sulfur batteries with a record lifespan of 1500 cycles but also expand our current understanding of the synthesis of carbon nanotubes. Biowastes-corncob, have also been explored to derive onion-like carbon materials for energy storage application. These research activities not only brought new insights on the deriving renewable carbon materials from natural abundant biomass resources but also boosted the design and fabrication of next-generation flexible energy-storage devices, which hold great promise for future wearable/flexible electronics.

Nanomaterials are anticipated to be promising storage media owing to their high surface-to-mass ratio. Therefore, wide variety of natural and synthetic carbon nanoallotropes are of central of importance for energy storage applications, particularly in Li-ion batteries. Accordingly, for last two decades number of experimental studies have been reported on the synthesis of carbonaceous electrode materials for battery with Li storage capacity larger than theoretical limit for graphite (LiC₆). However, the origin of excess capacity and feasibility of battery performance remain controversial. Therefore, it is not clear whether these new structures could have a higher capacity than graphite. Our experimental and theoretical studies of wide range of various type of carbon allotropes for Li-ion battery electrodes help to clarify the fundamentals of Li storage in carbon nanostructures and shed light on the rational design of nanoarchitectures for energy storage.

Introduction

Olivine-type LiMnPO₄ which is a cathode material for lithium ion batteries exhibits a higher operating potential (4.1 V vs. Li⁺/Li) than LiFePO₄ thus has gathered more attention in recent years. However, due to the low electron conductivity of LiMnPO₄ and the Jahn-Teller effect of Mn³⁺, it is difficult to achieve satisfied cycle performance. Fe doping of 10-30 mol% is promising as a solution to this problem [1]. Graphene is a single atomic monolayer of graphite and has been found a variety of applications in energy conversion and storage devices, due to the excellent properties such as high carrier mobility (~10,000 cm² V⁻¹ s⁻¹ at room temperature), structural flexibility, chemical and thermal stability, mechanical strength and ultrahigh theoretical specific surface area (2630 m²g⁻¹) [2]. Previous studies have reported the improved performance of LiMPO₄ (M = Fe, Mn and Co by combining with graphene. In particular, it is attractive that the active material is supported on both sides of graphene to form a sandwich structure. One is that active material particles can be highly loaded and interconnected by the highly conductive matrix formed by graphene. The other one is that the sandwich structure can be beneficial for suppressing the graphene restacking and active material particles agglomeration. It is reported that such a graphene-metal oxide nanoparticle composite material can greatly improve the performances of the anode. However, there is still lack of a facial way to obtain ideally sandwich-structured composite material for cathode [3]. We will introduce a facial and high efficient way to synthesize carbon-coated LiMn_0.7Fe_0.3PO₄ (LMFP)/reduced graphene oxide (rGO) sandwich-structured composite for high power lithium ion batteries [4].

Results and discussion

The synthesis of sheet-like carbon-coated LMFP/rGO composite material is shown in Fig. 1. First, 0.3 g of LMFP powder (without carbon coating, average particle size 200 nm), 3 mL of oxidized graphite suspension and 0.04 g of sucrose are dispersed in 100 mL of water. Thereafter, by ultrasonic treatment for 2.5 hours, the oxidized graphite gradually peels off to form graphene oxide, and at the same time, the LMFP nanoparticles are supported on the surface of the GO by interaction with the GO functional group. The dissolved sucrose is coated with GO and LMFP particles to form a polymeric coating. After lyophilization, carbon coated LMFP/rGO composite material (LMFP/rGO@C) having a sandwich structure was obtained by heat treatment at 700 degrees under a reducing atmosphere. LMFP/rGO@C, acetylene black and binder were mixed at a mass ratio of 8:1:1 and coated onto Al foil to obtain a cathode electrode. Electrochemical characteristics were evaluated by preparing a 2032 type coin cell using Li foil as the counter electrode. The electrolyte was 1 mol dm^-3 LiPF₆/EC-DEC (1: 2).

SEM image of the synthesized LMFP/rGO@C composite material is shown in Fig. 2. It is clearly observed that all the LMFP particles are uniformly dispersed on the rGO surface at high density (50 to 100 particles mm^-2), forming a sandwich structure. The total content of rGO and carbon coating is estimated to be about 7 wt% from TG. Figure 3 shows the discharge rate performance of LMFP/rGO@C and carbon coated only LMFP (LMFP@C). It was confirmed that the discharge capacity of LMFP/rGO@C is greatly improved compared with LMFP@C. This should be due to the formation of a good three-dimensional conductive network by graphene.

Acknowledgement

We thank Taiheiyo Cement Co., Ltd for the providing of LiMn_0.7Fe_0.3PO₄ powders and NIMS for the TEM measurement.

Reference

1) Gong, et.al., Energy Environ. Sci. 4, 3223 (2011).

2) S. Han, et.al., Small. 9 1173–1187 (2013).

3) Z.-S. Wu, et.al. Nano Energy. 1 (2012).

4) D. Ding, et.al., submitted.

Figure 1

A facial force-driven reflux technique was used to develop fibre-like carbon material from freeze-dried reduced graphene oxide (RGO) firstly prepared by using a modified Hummers method. Extensive characterization of the as-synthesized nanofibres material was achieved by various analytical techniques. The carbon nanofibres displayed a high specific surface area with good pore size distributions, which could be beneficial for energy storage applications. Electrochemical measurements of the carbon nanofibre electrodes in a symmetric configuration with aqueous (1 M Na₂SO₄, 6 M KOH), and protic ionic liquid (1-ethylimidazolium bis (trifluoromethanesulfonly) imide) electrolytes (ILE) displayed excellent electrochemical performance with the dominant electric double layer capacitor (EDLC) behaviour. The fabricated device shows higher electrochemical performance in the ILE due to its larger cell operating potential (3.0 V) as compared with the aqueous electrolytes (0.8 V). The optimized electrochemical properties especially in terms of higher specific energy and superior stability, suggest the material's potential applications as electrode for supercapacitors.

This is the first report of a novel directional flow-aided sonochemistry (FAS) exfoliation method to synthesize graphene with nearly ideal structure. The FAS treatment allows for control of graphene structural order and chemical uniformity not possible through prior top-down wet methods. Graphite is exfoliated into single-nm scale thickness graphene that is nearly defect free (at-edge sonication graphene "AES-G") or is highly defective (in-plane sonication graphene "IPS-G"). The AES-G has a Raman G/D band intensity ratio of 14.3 and an XPS derived oxygen content of 1.3 at.%, while the IPS-G has I_G/D of 1.6 and oxygen content of 6.2 at.%. Graphene and related carbons are widely employed as templates and protection layers to improve metal plating behavior in sodium and lithium metal battery (SMBs, LMBs) anodes. We then use AES-G and IPS-G to examine the role of structure and chemistry of graphene supports in promoting efficient Na metal cycling. The graphene serves a dual role in stabilizing the Na metal anode, being a nucleation template and serving as a protective layer to keep the metal from severely reacting with the electrolyte. We are the first to demonstrate that graphene defects are actually quite deleterious for efficient Na plating and stripping. AES-G yields state-of-the-art Na performance, with stable cycling at 2 mA/cm² at 100% Coulombic efficiency (CE), and an areal capacity of 1 mAh/cm². Meanwhile IPS-G performs on-par with the baseline Cu support in terms of severe charging instability. The explanation is that the defective graphene demonstrates much more copious SEI formation due to its defects and oxygen groups being catalytic. A thicker SEI results in a larger overpotential and worse CE loss during subsequent Na plating/stripping, manifesting in severe mossy metal dendrite growth and periodic electrical shorts. We therefore propose the following design rule for next-generation supports for Na metal: An ideal architecture will not only possess a large surface area for copious preferred heterogenous nucleation, but itself will be non-catalytic for SEI formation.

Developing a simple, cost-efficient, and scalable process for the production of graphene attracted a considerable interest, since, the production of graphene-based devices still suffers from various challenges. At one hand, the graphene products are not optimal for practical applications because they contain a large distribution of size and numbers of layers. At another hand, the common processes for production and integration of graphene not only suffers from being costly, time-consuming, and complicated but also the quality, performance and surface condition of produced material are hard to control.

In this study, we used the bipolar concept to design a single step, eco-friendly, cost-efficient, and simple process to simultaneously fabricate and in-situ deposit high-quality graphene-based layers directly on conductive substrates. Formation of graphene oxide and reduced graphene oxide on the substrates are confirmed by different materials characterization methods. The symmetrical 2 electrode systems were assembled and electrochemical performance of produced electrodes was investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge tests. The excellent performance of the supercapacitor electrode was confirmed and promising results for AC/DC filter application were obtained. The results will be presented in detail at the conference.

Key words: bipolar electrochemistry, graphene, supercapacitor, energy storage, AC filter

A thick layer of self-supported electrode showing simultaneous high specific capacity and areal capacity is developed by wrapping nickel on the carbon nanofibers (CNFs) via an electrospinning technique. This superior property is established by enhancing the interconnection among the nanofibers. Polyvinylpyrrolidone (PVP), polyacrylonitrile (PAN), and nickel acetate are used to fabricate the nanofibers. Carbonization assists inter-bonded morphologies in nanofibers and PVP helps to bind the nanofibers during carbonization process. Thus well bonded CNFs wrapped by Nickel show better specific and areal capacity. Different percentages of nickel in the nanofibers show a variation in the battery performance. The optimized nickel wrapped CNFs based anode represents a favorable porous nanostructure and large mass loading, which exhibits an areal capacity as high as 7.04 mAh/cm² and excellent cycling stability.

Carbon materials are ubiquitous in catalysis, separation, and energy storage/conversion. The creation of well-defined carbon architectures is essential for a number of the aforementioned applications. Recently, we have developed several methods for the synthesis of carbon materials with controlled mesostructures and compositions. The mesostructures of these carbon materials are highly stable and can be further tailored via graphitization and surface functionalization for energy-storage and energy-conversion applications. This presentation will be focused on our recent development in (a) self-assembly approaches to the preparation of carbon composite materials for controlling mesostructures and morphologies and (b) surface modification techniques to control the interfacial chemistry of carbon materials for energy-storage and energy-conversion applications.

Immiscible polymer blends yield carbon materials with fine and controlled pore architectures for supercapacitor applications. Aromatic polyimides derived from 4,4'-hexafluoroisopropylidine diphthalic anhydride (6FDA) have attracted attention as high performance polymers due to their high free volume and thermal stability. In this study, 6FDA and DABA (3,5-diamino benzoic acid) were used as monomers to synthesize 6FDA-DABA polymer. The synthesized polymer was blended with polyacrylonitrile to prepare a series of immiscible polymer blend compositions which were electrospun, subsequently carbonized and tested for electrochemical performance. Both polymers afford carbons upon thermal treatment up to 1000 °C under an inert environment. 6FDA-DABA has a carboxylic moiety which acts as an in-situ porogen upon decarboxylation, creating pores in electrospun fibers which are accessible to electrolyte ions. Preliminary studies show promising results upon only carbonization as an electrode material for supercapacitors. Results obtained from further activation by CO₂ at 1000 °C were: specific capacitance of 137 F/g, with an energy density 66 Wh/kg, and a power density of 1.7 kW/kg with an excellent capacitance retention of 99% after 1500 cycles.