Table of contents

Interest in hydrogen solar production from water splitting is motivated by the need to find a green, renewable and environmentally safe fuel. Hydrogen is considered as a viable option sustainable development beyond fossil fuels, especially when it is produced from water by using only renewable energy sources. The identification of low cost materials for the photogeneration of hydrogen is recently a growing field of research. In the last years, pyrite (FeS₂) has received increasing attention as a material for photoelectrochemical applications^1,2 and hydrogen photogeneration. Due to its low cost and not toxicity, high optical absorption coefficient, and an adequate electronic properties (E_gap=1.1±0.1eV)³, FeS₂ shows some advantages compared with other semiconductor materials to be used as a photoelectrode in a PEC. Previous works^3,4 about the Ti doping of pyrite thin films have been reported and their properties investigated. In this work the photogeneration of hydrogen in a photoelectrochemical cell with n-type Ti doped FeS₂ thin film as photoanode will be demonstrated.

Ti-FeS₂ thin films have been grown by sulphuration of Fe layers (100 nm) thermally evaporated on discs in a vacuum sealed ampoule at 300 ºC. Film morphology and crystallinity were investigated by Scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Cubic FeS₂ was identified as the unique phase at 300ºC (Fig1).

Electro- and photoelectrochemical measurements have been carried out using a conventional three-electrode glass cell with a quartz window (PEC) to illuminate the photoanode, i.e. the Ti-FeS₂ samples used as working electrode. As counter electrode a platinum sheet was used and the reference one was a Ag/AgCl reference electrode. 0.5M Na₂SO₃ aqueous solution was used as electrolyte, which serves both, to increase the conductivity and as sacrificial agent. An halogen lamp (200 W) was utilized as white light source. Photocurrents show positive values, as expected from the doping with titanium, because as far as we know, pure pyrite thin films always present p-type conductivity³, but Ti doped pyrite show n-type behavior^4,5.

A mass spectrometer connected to the PEC is used to measure the evolved H₂ from the photoelectrochemical water splitting. Results show that Ti-FeS₂ exhibit a high flow of photogenerated hydrogen from bias potential such low as 0.5V (vs. Ag/AgCl). In fact, strong bubbling hydrogen was experimentally observed in our photoelectrochemical cell and quantified with the mass spectrometer (Fig. 2).

In conclusion, the hydrogen photogeneration by using Ti-FeS₂in a PEC under bias potential has been demonstrated. Quantitative results will be exposed in the oral communication.

Acknowledgements

Financial support from MINECO (MAT2011-22780) is acknowledged. Authors thank to F. Moreno for his technical assistance.

References

¹ J. Jiao, L. Chen, D. Kuang, W. Gao, H. Feng, J. Xia, RSC Advances 1, 255-261 (2011).

² H. Huang, T. Ling, S.Z. Qiao, X.W. Du, J. Mat. Chem. A 1, 11828 (2013).

³ I. J. Ferrer, D. M. Nevskaia, C. de las Heras and C. Sánchez. Solid State Commun. 74, (9) 913-916 (1990).

4 I. J. Ferrer, J. R. Ares, C. Sánchez, Solid State Phenom.80-81, 281-286 (2001).

⁵A. Pascual, P. Díaz-Chao, I. J. Ferrer, C. Sánchez, J. R. Ares. Solar Energy Materials & Solar Cells 87 575-582 (2005).

Photoelectrochemistry (PEC) is an attractive method for producing hydrogen as an alternative fuel to oil. Limitations arising from device efficiencies, cost, and durability demonstrated in laboratories prevent the implementation of this technology on a larger scale. The chalcopyrite material class, exemplified by its most popular alloy Cu(In,Ga)Se₂, encompasses some of the most promising candidates to meet the criteria for cheap, sustainable solar fuel production. As we recently reported[i], co-evaporated 1.6 eV CuGaSe₂ offers very high-saturated photocurrent densities (20 mA.cm^-2 in pH 0 under AM1.5G illumination), long durability (up to 400 hours), and relatively high Faradaic efficiency (>85% for non-catalyzed systems). Although CuGaSe₂ has the highest bandgap of the copper chalcopyrite class, its optical characteristics are still too close to that of amorphous silicon (a-Si), a low-cost material our research team has identified as an ideal photovoltaic driver in a monolithic hybrid photoelectrode device. Nevertheless, a solar-to-hydrogen efficiency of 3.7% was achieved using a co-planar integration scheme, where CuGaSe₂ was connected in series with three a-Si solar cells. In order to increase the water-splitting efficiency, novel chalcopyrite alloys with bandgaps greater than 1.6 eV must be developed.

In the present communication, we report on our efforts to synthesize 1.8-2.2 eV band-gap chalcopyrite materials for PEC water splitting. Specifically, we investigated the effect of sulfur on the optical and photoelectrochemical characteristics of the copper chalcopyrite material class. Using co-evaporated 1 μm-thick CuGaSe₂ as baseline system, we demonstrate that the substitution of selenium with sulfur is accomplished through a simple annealing step. As a result, a dramatic change in optical properties was observed, with a bandgap increase from 1.6 eV (CuGaSe₂) to 2.4 eV (CuGaS₂), in good agreement with theoretical predictions[ii]. Then, by simply adjusting the indium content in the film during the initial growth process, the bandgap of sulfurized copper chalcopyrite was decreased from 2.4 eV [GGI=Ga/(Ga+In)=1] to 2.2 eV (GGI»0.8) and finally to 2.0 eV (GGI»0.7), as presented in Fig. 1. X-ray diffraction data (Fig. 2) indicated successful bulk sulfurization by the shift of the prominent (112), (220), and (312) reflections to higher angles. Preliminary PEC analyses revealed an anodic shift of the flatband potential with increasing bandgap, when compared to CuGaSe₂. This suggests that the bandgap modification in sulfurized films primarily stems from a downward shift of the valence band, an ideal situation for p-type PEC systems. Saturated photocurrent densities greater than -5 mA/cm² were achieved with 2.0 eV red CuIn_0.3Ga_0.7S₂ photocathodes in 0.5M H₂SO₄ under AM1.5_G simulated illumination.

[i] N. Gaillard, D. Prasher, J. Kaneshiro, S. Mallory, and M. Chong, MRS Spring Meeting, Z2.07 (2013).

[ii] M. Bär, W. Bohne, J. Rohrich, E. Strub et al., Appl. Phys. Lett. 96, 3857 (2004).

In this work large band gap bowing of dilute antimonide alloys of gallium nitride, Ga(Sb_x)N_1-x has been investigated. Our computational calculations using first principles density functional theory² revealed that a small amount of Sb incorporation is sufficient to achieve a significant band gap reduction in GaN from 3.4eV to 2eV. Theoretical calculations predicted that Ga(Sb)_xN_1-x alloys with 2 eV band gap straddle the electrochemical potentials of the hydrogen and oxygen evolution reactions. Theoretical computations with Sb composition beyond 7% change the electronic band gap from direct to indirect.

Synthesis of crystalline GaSb_xN_1-x alloys were carried out using metal organic chemical vapor deposition using trimethyl gallium (TMGa) and Trimethyl Antimony (TMSb) and ammonia at x values ranging from 0-8%. Synthesis was carried out on different planar substrates and GaN nanowires. X-ray diffraction measurements showed a monotonic increase in the lattice with increase in antimonide composition which corroborates with theoretical calculations. Optical measurements like UV-Vis spectroscopy and photocurrent spectroscopy suggested a rapid decrease in band gap from 3.4 to 2 eV with small concentration of antimonide incorporation. Experimental data from optical measurements indicated direct band gap transition for alloys less than 7at% and an indirect band gap transition for alloys beyond 7% as shown in fig. 1. In addition Mott Schottky measurements showed that Ga(Sb)_xN_1-x alloys ranging from 0-8% straddle the water oxidation and reduction potentials in agreement to computational calculations. Moreover, the photo-electrochemical data on activity and stability suggest that these alloys are highly suitable for solar water splitting under visible light irradiation.

Fig. 1 Tauc plots for direct transition and indirect transition for 2 % Sb (a,b) and 8 % Sb (c,d).

Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).

References:

1. S. Sunkara, V. Vendra, J. Jasinski, T. Deutsch, A.N. Andriotis, K. Rajan, M. Menon and M.K. Sunkara, "New Visible Light Absorbing Materials for Solar Fuels, Ga(Sb_x)N_1-x", In Review (2013)

2. R.M. Sheetz, E. Richter, A.N. Andriotis, C. Pendyala, M.K. Sunkara and M. Menon, "Visible light absorption and large band gap bowing in dilute alloys of gallium nitride with antimony", Phys. Rev. B, 84, 075304 (2011)

Semiconductors have been extensively used as photocatalysts and photoelectrodes for solar fuel generation. Currently single semiconducting material cannot suffer from low efficiency of conversion of solar energy to chemical energy. An alternative to overcome this barrier is to the development of hetero-structured photocatalysts.

This presentation shows that integration of semiconductors with graphene can significantly improve the photocatalytic activity. First, hematite (a-Fe₂O₃) nanocrystals have been deposited on the reduced graphene oxide (rGO) sheets to form the hematite/rGO heterogeneous photocatalyst, which shows much higher photocatalytic activity toward water oxidation.¹ The photocatalysis enhancement is due to the fact that facilitates the charge migration and reduces the charge recombination rate.

In addition, we have developed a heterogeneous photocatalyst based on the nanoscale p-n junction of p-type molybdenum disulfide/n-type nitrogen-doped reduced graphene oxide.² It is interesting that MoS₂ alone exhibits negligible photocatalytic activity. In contrast, the MoS₂/rGO p-n junction photocatalyst show significant photocatalytic activity toward hydrogen generation.

References

[1] F. Meng, J. Li, S. K. Cushing, J. Bright, M. Zhi, J. Rowley, Z. Hong, A. Manivannan, A. D. Bristow, N. Q. Wu, ACS Catalysis, 3 (2013), 746–751.

[2] F. Meng, J. Li, S. K. Cushing, M. Zhi, N. Q. Wu, J. Am. Chem. Soc., 135 (2013), 10286-10289

Acknowledgement:

The resource and facilities used were partially supported by NSF (EPS 1003907), the West Virginia University Research Corporation and the West Virginia EPSCoR Office. The use of WVU Shared Facility was appreciated.

Abstract

With the global energy demand constantly rising, the need for developing new abundant and environmentally benign sources of energy is ever increasing.¹ Consequently, solar energy is expected to play an increasingly important role in the future. One of the major strategies for solar energy conversion that is currently under development is the light-driven splitting of water into its constituent elements. Inspired by nature's extensive use of metalloporphyrins as solar energy harvesters and electron transfer agents, artificial porphyrins have found prominent use as photosensitizers in hydrogen producing schemes.²The photocatalytic production of hydrogen can be accomplished by systems containing a photosensitizer, an electron relay, a sacrificial electron donor and a catalyst. The great challenges that remain in the field include the development of systems, which employ earth-abundant materials, and the improvement of the systems activity and durability.

Here, we report two noble metal free bioinspired photocatalytic systems, which use porphyrins or a corrole as photosensitizers and the cobaloxime as a catalyst (Figure 1). In the first one a water soluble Zn porphyrin was used as the photosensitizer and [Co^III(dmgH)₂(py)Cl)] as the catalyst (Figure 1 left part). This system is effective in photoinduced H₂ production in MeCN/water 1:1 with TEOA as a sacrificial donor.³ In the second one the photosensitizer is directly coordinated to the cobaloxime catalyst (Figure 1 right part). From transient absorption studies we observed an electron transfer from the chromophore to the cobalt catalyst, whereas the photocatalytic H₂ production was low.⁴

REFERENCES

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T. Lazarides, M. Delor, I. V. Sazanovich, T. M. McCormick, I. Georgakaki, G. Charalambidis, J. A. Weinstein, A. G. Coutsolelos, Chem. Commun. accepted, DOI: 10.1039/C3CC45025B.

K. Peuntinger, T. Lazarides, D. Dafnomili, G. Charalambidis, G. Landrou, A. Kahnt, R. P. Sabatini, D. W. McCamant, D. T. Gryko, A. G. Coutsolelos, D. M. Guldi, J. Phys. Chem. C, 2013, 117, 1647.

Branched nanowire (NW) heterostructures have recently been attracted considerable attention for solar water splitting and clean hydrogen production due to their unique properties such as nanoscale integration of different functional materials, greatly enhanced junction and surface area, enhanced gas evolution efficiency, broadband light absorption, etc. Moreover, branched NWs can be fabricated using facile and scalable fabrication methods such as hydrothermal or solvothermal growth methods. In this presentation, we show branched NWs of different compositions for core (or trunk) and branch NWs which were fabricated with facile and low-cost synthesis methods using cheap, non-toxic, and earth abundant materials including Si, CuO, Cu₂O, ZnO, TiO₂, and Fe₂O₃. The branched NW structures and the heterostructures' interfaces are investigated in detail using different characterization techniques such as SEM/HRSEM, TEM/HRTEM, STEM/HRSETM, etc. The photoelectrochemical (PEC) performances including photocurrent turn-on potential, photocurrent, solar conversion efficiency, and incident photon-to-current efficiency (IPCE) are studied systematically and optimized, based on different core and branch NW dimensions, for each specific branched NW heterostructure to provide efficient water splitting in a neutral medium. The electrode stability of different branched NWs is also investigated and long-term stability of over one day or several hours using a thin passivation layer or robust branched NWs are presented. The achieved results pave the way for accomplishing spontaneous overall solar water splitting for clean, efficient, cost-effective and durable solar hydrogen generation at large scales.

Metal-oxide semiconductors have received a wide attention as photo anodes since the demonstration of water splitting by TiO₂ electrode under UV illumination [1]. Significant research efforts were focused on developing various semiconductors with band-gap engineering for efficient photo water splitting. Bismuth oxide (Bi₂O₃) with a band gap of 2.8eV is a promising photo anode which exhibits good electrical conductivity, oxygen ion conductivity, and high dielectric permittivity. Due to its distinctive properties, Bi₂O₃ finds applications in a wide range of areas like gas sensors, optical coatings, microelectronics, photocatalysts, solid state electrolytes, superconductors etc. In spite being a non-toxic material with appropriate band gap and valence band edge position (+3.13 V vs. NHE), Bi₂O₃ demonstrates a poor hydrogen evolution due to its lower conduction band edge position (+0.33 V vs. NHE). The general strategies employed to overcome these limitations were simultaneous doping and nano-sizing of the material.

In the present investigation, Bi₂O₃ nanoporous films were synthesized by electrochemical anodization in the electrolyte solutions containing citric acid, ethylene glycol and glycerol. Nanoporous bismuth oxide films were formed by anodizing bismuth circular discs of 3 mm thick and 12.7 mm diameter. Anodization was carried out at various potentials ranging from 3 V to 60 V for different time durations ranging from 0.5 to 2 h. After anodization, the samples were thermally annealed at 200 °C for 2h. The influences of anodization time, electrolyte concentration and applied voltages on morphology have been investigated in this study. Potentiodynamic, potentiostatic, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis studies were carried out with and without illuminated conditions.

All the depositions were examined under a FEI Quanta 200F scanning electron microscope. Figure 1 shows the nanoporous morphology of the oxide layer formed at 3V for 30minutes in citric acid electrolyte. The diameters of the pores were in the range of 20 -50 nm and total thickness of the film was about 500nm. It was observed that pore diameter and film thickness changed with the change in applied potential, time and electrolyte concentration. Photo electrochemical studies were carried out using a potentiostat (Gamry, Reference 600) with platinum as the counter electrode. Potential Vs current plots were constructed by scanning the potential of the sample from the open circuit potential to 0.5 V at a scan rate of 5mV/s in 1 M KOH solution. Potentiostatic measurements were carried out in 1 M KOH solution at 0.2 V and 0.5 V_Ag/AgCl. A solar simulator with an AM 1.5 filter was used for illuminating the samples. The thickness of the oxide layer increased with the increase in the anodization potential. The photo current density of the nanoporous bismuth oxide increased with increase in the thickness of the oxide. The dark current density decreased with the increase in thickness of the oxide layer. The maximum photo current density (I_illuminated-I_dark) recorded at an applied potential of 0.5 V_Ag/AgCl was 1 mA/cm² for the sample anodized at 20 V. The sample anodized at 60 V showed about 20 µA/cm² dark current density and 0.8 mA/cm² photo current density at 0.5 V_Ag/AgCl. The photo activity of the nanoporous bismuth oxide is comparable to that of nanotubular TiO₂ oxide photo anodes. The nanoporous Bi₂O₃ contained a defect concentration in the range of 7x10¹⁶ – 4x10¹⁸ cm^-3 under dark condition for various anodized conditions. Upon illumination, the defect density increased to values in the range of 4x10¹⁷ to 2x10¹⁹ cm^-3. A detailed discussion will be provided in the final presentation on the photo electrochemical behavior of the Bi₂O₃ nanoporous structure as a function of morphology, and defect and electronic structures.

References

• A. Fujishima and K. Honda, Nature 238 (1972) 37

Photoelectrochemical splitting of water is one of the cleanest ways of producing hydrogen. However, the efficiency of this process is less than 1% with the currently used materials. Improvement in the electrode performance is limited by either chemical instability or poor light absorption properties of these materials. Although transition metal oxides are relatively stable during gas evolution, they have wide band gaps and thus absorb only UV part of the solar spectrum. Hence there is an intense search for newer materials with improved chemically stability and optical properties. In this work, we performed post synthesis nitridation of oxide nanowire arrays to form a completely new nitrided phase that has a lower band gap. The photoelectrochemical properties of these nanowire electrodes were characterized by UV-Vis, impedance, and photocurrent spectroscopy. The results are discussed in terms of their water splitting efficiency.

Tungsten oxide nanowire arrays were synthesized by hot filament CVD on both FTO and quartz substrates. Figure 1 shows the SEM image of WO₃ array on FTO substrates. Post synthesis nitridation in ammonia resulted in the complete phase transformation to W₂N. X-ray diffraction and transmission electron microscopy confirmed their high degree of crystallinity.

Figure 2 shows the Tauc plot obtained from UV-Vis spectroscopy. Oxidized WO₃ has a band gap of 3 eV. On the other hand nitridation leads to a reduction in the band gap from 3 eV to 2.5 eV. The photoelectrochemical measurements were done in a three electrode configuration. Figure 3 shows a representative I-V characteristics of WO₃ in dark and under simulated AM 1.5 solar light. The electrodes were further characterized by impedance, and photocurrent spectroscopy. The results are discussed in terms of their water splitting efficiency.

Photocatalytic water splitting is considered one of the most promising approaches for reducing both the reliance on fossil fuels and the emission of greenhouse gases such CO2 in the atmosphere. A working photocatalytic water splitting device must provide the voltage required for splitting water into hydrogen and oxygen (1.23 V) without external applied bias. It is therefore desirable that the photon absorbers utilized in such device provide the highest photovoltage possible together with a significant current density.

GaP is a semiconductor material having 2.25 eV indirect bandgap and a theoretical maximum photocurrent density of about 12.5 mA/cm². The best solar cells made of GaP show an open-circuit voltage of approximately 1.5 V and a maximum photocurrent density close to 2 mA/cm². p-GaP utilized as a photocathode for hydrogen evolution shows significantly lower open-circuit voltage (+0.35 V RHE, with Pt cocatalyst), mainly because of inefficient charge separation at the semiconductor/electrolyte junction. Furthermore, this semiconductor suffers from corrosion in acidic conditions, thus requiring appropriate protection.

One approach for improving charge separation and open-circuit voltage consists of forming a p-n heterojunction on GaP. We deposit different n-type metal oxides (TiO₂, Nb₂O₅, ...) thus forming an heterojunction which significantly enhances charge separation upon light irradiation by forming a built-in potential at the junction interface. This built-in potential effectively drives electrons towards the surface of the photoelectrode with the hydrogen evolution reaction occurring at more positive potential compared to the bare p-GaP under the same operating conditions.

The observed open-circuit voltage for the modified photocathodes is +0.70 V RHE, representing an increase of more than 300 mV compared to the pristine p-GaP semiconductor and marking an unprecedented value of open-circuit voltage for GaP-based photocathodes for hydrogen production. It is found that the high carrier density of the n-type oxides shifts the distribution of the built-in potential almost entirely towards the lightly doped p-type substrate and forms an asymmetric charge depletion region at the junction, as depicted in Figure 1. Moreover, TiO₂shows excellent stability over long-time operation, unveiling its double role of brilliant material for both heterojunction formation and protection against corrosion of the substrate.

Further improvement of the aforementioned system and a favorable coupling with an efficient photoanode could lead to a scenario where photocatalytic water splitting is carried out without any external applied bias under solar light irradiation.

References:

Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473

Grätzel, M. Photoelectrochemical cells. Nature 2001, 414, 338-344

Seger, B.; Pedersen, T.; Laursen, A. B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. Using TiO₂ as a Conductive Protective Layer for Photocathodic H₂ Evolution. J. Am. Chem. Soc. 2013, 135 (3), 1057-1064

Lu, X.; Huang, S.; Diaz, M.; Kotulak, N.; Hao, R.; Opila, R. & Barnett, A. Wide Band Gap Gallium Phosphide Solar Cells. IEEE J. Photovolt., 2012, 2, 214-220

Kaiser, B.; Fertig, D.; Ziegler, J.; Klett, J.; Hoch, S.; Jaegermann, W. Solar Hydrogen Generation with Wide-Band-Gap Semiconductors: GaP(100) Photoelectrodes and Surface Modification. ChemPhysChem 2012, 13, 3053-3060

Butler, M. A.; Ginley, D. S. P-Type GaP as a Semiconducting Photoelectrode. J. Electrochem. Soc. 1980, 127 (6), 1273-1278

High temperature water electrolysis has a tremendous potential in enabling a cost-effective and sustainable production of synthetic fuels like hydrogen by using renewable heat and electricity from concentrating solar thermal (CSP) plants [1]. Most efforts in this research area are currently directed towards investigating solid oxide electrolysis cells (SOECs) based on fuel cell technology [2,3], although this approach is not optimal for integration with CSP plants because of electrolysis temperatures (i.e., above 800°C) significantly exceeding those of most solar heat storage systems. In order to improve thermal matching with current solar thermal fluids, development of electrolysis processes within the 500-600°C range would be a desirable option. In this context, molten salts could be seen as an ideal medium to lower process temperatures with respect to solid oxide electrolyzers since overall ionic conductivity and transport numbers of liquid salts are usually higher than solid-type electrolytes. Recently, alkali molten carbonate salts have gained a return of attention as a promising and attractive electrolyte for electrochemical conversion processes of mineral ores and CO₂ gas at moderate temperatures [4,5].

At the same time, water electrolysis in alkali molten carbonates has been recently mentioned as a feasible process [2], although no systematic studies on this process are available in literature. The overall electrolysis reactions can be written as:

cathode, H₂O+CO₂ + 2e = H₂+CO₃^2- (1)

anode, CO₃^2- = CO₂ +0.5O₂ +2e (2)

The anode gas mixture made of 33 % O₂ + 67% CO₂ is ideal for oxy-combustion processes. Thus, a zero CO₂ emission system could be realized by integrating a molten carbonate electrolysis with an oxycombustion process. The combined process could enable a CO₂ closed-loop recycling scheme with CO₂ capture. Part of the post-combustion CO₂ could be, in fact, re-injected to the cathode, whereas the excess CO₂ could be easily captured.

From a mechanistic point of view, the exact cathode reduction reaction is not well understood. Although the cathode reaction can be interpreted in terms of a conventional water reduction process (eq. (1)), there are indications in literature that bicarbonate anions are likely the reactive species rather than dissolved water according to [6]:

2HCO₃^- + 2e =H₂+2CO₃^2- (3)

The presence of bicarbonate is possible because, in presence of a water-containing atmosphere, molten carbonates cannot be considered as a pure solvent, but rather as a mixture of three anionic constituents, namely carbonate, bicarbonate and hydroxide ions, according to the following chemical equilibrium:

H₂O+CO₃^2- = HCO₃^- + OH^- (4)

Thermodynamic calculations indicate that bicarbonate concentration is lower than hydroxide, but it is not negligible below 600°C. Initial lab-scale work has confirmed the effective presence of hydrogen as main cathode reaction product in a electrolysis conducted in ternary alkali carbonate eutectic mixture Li₂CO₃-Na₂CO₃-K₂CO₃ (43.5-31.5-25.0 mol %) despite the relatively low values of steady-state current density observed within the 525-600°C range (see Fig.1). Cyclic voltammetry experiments are also underway to study in more detail kinetics and mechanisms of the cathodic reaction, which is an essential aspect for interpreting and modeling the overall electrolysis process. Detailed results of these exploratory investigations will be shown at the time of Conference.

Previous studies on the electrochemical reduction of CO₂ at metallic electrodes suggest that the adsorption of hydrogen species is structure-sensitive.^[1] Surface roughening is likely to introduce defects favorable for the reaction of adsorbed hydrogen atoms, an important intermediate step in the electro-reduction of CO₂ in protic media.^{[1, 2]} Furthermore, roughened surfaces have been shown to be more selective toward hydrocarbon products, which was attributed to a higher number of uncoordinated metal sites.^[2] These finding were confirmed by DFT calculations, which suggest that CO₂ activation and reduction occurs at these sites. ^[3]

To further gauge the effect of morphology on the faradaic efficiency and distribution of products obtained during the electroreduction of CO₂, Cu and Sn foams were electrosynthesized on Cu substrates using a recently reported process.^[4] The metal foams were found to be mechanically stable during their preparation, handling, and use in the electrocatalytic reduction of CO₂. Both Cu and Sn foams are attractive metals for the electrocatalytic reduction of CO₂because of their low cost and non-toxic nature.

Electroreduction of CO₂ was performed in a typical H-cell under potentiostatic conditions. The faradaic efficiency of producing formate from CO₂ at the metal foams and the equivalent planar metal electrode are compared in Figure 1a and 1b. The faradaic efficiencies obtained with the Cu foam electrode were found to be higher at all potentials with a maximum efficiency of 37% at -1.5V for HCOOH. Previously reported data is included for comparison.^[5] Likewise, the faradaic efficiencies for HCOOH generation at the Sn foam electrode were found to be higher at all potentials with a maximum efficiency of 89.5% at -1.7V. Previously reported data with slightly different conditions (Sn gas diffusion cell in 0.5 M NaHCO₃) is included for comparison.^[6]

XRD analysis of the Cu and Sn foams did not reveal major differences in the relative ratio of dominant crystal facets when compared to the equivalent planar samples of high purity metal. The hierarchical nature of the pore architecture along with high surface area of the metal foams could affect the reaction kinetics and contribute to the observed increase in faradaic efficiency. For example, the porous nature of the metal foams may promote interactions between incoming CO₂, adsorbed surface species, electrolyte, and reducing equivalents that are novel relative to the same reaction at a planar electrode. The effect of varying the electrodeposition time and resulting foam architecture on the electroreduction of CO₂will be the focus of this presentation.

References

• Hori, Y., A. Murata, and R. Takahashi, Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1989. 85(8): p. 2309-2326.

• Tang, W., A.A. Peterson, A.S. Varela, Z.P. Jovanov, L. Bech, W.J. Durand, S. Dahl, J.K. Norskov, and I. Chorkendorff, The importance of surface morphology in controlling the selectivity of polycrystalline copper for CO2 electroreduction. Physical Chemistry Chemical Physics, 2012. 14(1): p. 76-81.

• Durand, W.J., A.A. Peterson, F. Studt, F. Abild-Pedersen, and J.K. Nørskov, Structure effects on the energetics of the electrochemical reduction of CO2 by copper surfaces. Surface Science, 2011. 605(15–16): p. 1354-1359.

• Shin, H.C., J. Dong, and M. Liu, Nanoporous Structures Prepared by an Electrochemical Deposition Process. Advanced Materials, 2003. 15(19): p. 1610-1614.

• Kuhl, K.P., E.R. Cave, D.N. Abram, and T.F. Jaramillo, New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy & Environmental Science, 2012. 5(5): p. 7050-7059.

• G.K.S. Prakash, F.A. Viva, G.A. Olah, Electrochemical reduction of CO2 over Sn-Nafion® coated electrode for a fuel-cell-like device, Journal of Power Sources, 223 (2013) 68-73.

A combined experimental and computational study of catalytic particle size effects during CO₂ electroreduction on size-controlled Cu nanoparticles is presented. Cu nanoparticle catalysts in the 2 – 15 nm mean size range were prepared, and their catalytic activity and selectivity during CO₂ electroreduction were analyzed and compared to bulk Cu electrode. An increase in the catalytic activity as well as the H₂ and CO selectivity was observed with decreasing Cu particle size, in particular for particles below around 2 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed with smaller particle size. A spherical Cu particle model was used to analyze the size dependence of the surface atomic coordination, which was correlated to experimental results.

In photocatalytic processes, it is not only need to generate an electron-hole pair, but the electronic transfer must be efficient in the reaction process, involving multi-electronic redox reactions, occurring at the near vicinity at the surface of the semiconductor. All these factors need the surface engineering of the semiconductor to increase the overall efficiency of the process. The surface properties of TiO₂ can be modified by the addition of some elements which give rise to a local lattice modification, introduce new potential catalysts issues and alter or vary the composition and bond ending of the last two nanometers layer being it essential and outstanding for the final photocatalytic properties.

In this contribution, we will present a comparative analysis among different potential additives such as magnesium, indium. More detailed surface analysis techniques have revealed significant changes at the outer part of the TiO₂ material. XPS spectra, especially at the energy zone of oxygen and titanium, and corresponding to the last two nanometers, show significant evolution of the different detected bonds as the concentration of the additives is increased.

All these data have been correlated with the photocatalytic behavior for CO₂ reduction as a function of the additive concentration below the limit of their solubility for avoiding segregation effects. A correlation between Ti⁺³ and adsorbed molecular oxygen concentration found in these outer layers will be discussed as well as their correlation with the total productivity rate, considering all the effective photogenerated electrons for the reduction of CO₂ from the GC analysis of the products obtained, including CO, CH₄ and C₂ hydrocarbons. These findings confirm that the benefit introduced by the catalytic additive is mostly related to the "surface quality" and how it is lost as the concentration of additive is increased. Likewise, it is determined that the overall productivity is kinetically limited by a four-hole chemistry of the water oxidation reaction, like in the photoactivated water splitting process. Nevertheless, in spite of these limitative issues, significant increases, a factor of 3 o 4, can be achieved in the productivity of reduced CO₂ if the concentration do not overcome some limits whereas the selectivity of the products becomes determined by the nature of the additive.

Photocatalytic or photoelectrochemical processes that couple water oxidation and CO₂ reduction are currently under intense investigation for solar fuel production. As these so called "artificial photosynthesis (AP)" systems are expected to remediate the issue of ever increasing CO₂ level in the atmosphere, it is necessary to construct them using cheap and efficient materials for large scale deployment. The importance of water oxidation and CO₂ reduction catalysis for AP has been well recognized in recent years, yet significant challenges still remain to find low cost materials that are both efficient and stable [1]. Towards the search for catalyst based on earth abundant materials, we previously investigated transition metal tungstates as water oxidation catalysts [2] and nickel macrocycles for CO₂ reduction [3]. In this work, perovskites-type of metal oxides is further studied to improve catalyst performance at neutral pH as oxygen evolution catalyst. We also report a new strategy of ligand modification to silver surface, which allows the control of product selectivity for electrochemical CO₂ reduction. Details about materials synthesis, characterization of physiochemical properties and evaluation of catalytic performance will be discussed in this presentation, as well as studies on catalyst stability and the influences of electrolytes.

(1) J. R. McKone, N. S. Lewis and H. B. Gray, Chem. Mater., 2013 ASAP

(2) H. Jia, J. Stark, L. Q. Zhou, C. Ling, T. Sekito, Z. Markin. RSC Adv. 2012, 2(29), 10874-10881

(3) J. Schneider, H. Jia, K. Kobiro, et al. Energy Environ. Sci. 2012, 5, 9502-9510.

Photovoltaic modules are under active consideration as a major contributor to future energy requirements. Coupled with an electrolyzer, this energy system converts energy harvested from the sun into chemical power. As the demand for a sustainable yet efficient and cost effective approach of producing hydrogen increases, researchers are seeking ways to improve the technology of forming solar fuel. Mimicking the idea of how nature collects and stores solar energy in chemicals bonds through photosynthesis, economically viable water splitting cells capable of splitting water directly at the semiconductor surface are being developed. The catalytic semiconductor is designed to be both a light absorber and an energy converter to store solar energy in the simplest chemical bond, H₂, thereby eradicating significant fabrication and system costs involved with the use of separate electrolyzers wired to photovoltaic cells.

In this work, water splitting cells have been designed to consist of multi-component nanorods of titanium dioxide and platinum with well-defined nanostructures to function as photocatalytic cell for hydrogen production. As the TiO₂-Pt nanorods are irradiated with light in the presence of a water source, oxygen and hydrogen are evolved at the anode TiO₂ and cathode Pt segments of the nanorods respectively. The alternating segments of TiO₂ semiconductor and Pt metal enable the control of the direction of charge movement and light absorption pathways in the material, thereby presenting a solution to improving the overall efficiency of photocatalytic hydrogen production.

By employing templated electrodeposition, homogeneous multi-segmented TiO₂/Pt nanorods have been successfully fabricated. This simple method of synthesis permits an easier control of the position and composition of TiO₂ and Pt along the length of the nanorods, which allows for a customizable and highly reproducible method of obtaining segmented rods with uniformly distributed active sites for efficient catalytic activity. The UV absorption properties of these multi-segmented TiO₂/Pt nanorods are then compared to single segmented nanorods consisting of only TiO_2.

Abstract: Recently, we showed that the deposition of crystalline WO₃ layers on copper oxide nanowire arrays to result in a five-fold improvement of photocurrent density (~ 1.7 mA/cm² at 0 V 10 vs. RHE) over titania coated copper oxide NW arrays. Most importantly, the deposition of WO₃ or CuWO₄ reduced CuO phase impurity to Cu₂O. Improvement in the phase purity of nanowire arrays, led to the enhancement in photoactivity.¹ The origin of degradation in photoelectrochemical performance with time is currently being investigated.

Introduction: Photoelectrochemical (PEC) water splitting using sunlight is an attractive approach for producing clean hydrogen towards reducing carbon dioxide emissions and meeting the growing global energy demand. The biggest obstacle with PEC water splitting is the unavailability of a suitable semiconductor with proper band gap and band edge energetics and durability. Cu₂O is a promising semiconducting material for photo-electro-chemical water splitting due to its band gap (Eg = 2.0 eV) and its band edge 25 positions straddling the water oxidation and reduction reactions. Major drawbacks with Cu₂O include its poor stability in aqueous solutions, short carrier diffusion lengths (20-100 nm) and high absorption depth (10 microns). A semiconductor with a 2 eV band gap is expected to yield a photocurrent density of about 15 mA/cm². The photoactivity performance of Cu₂O has been a major concern, in addition to its rapid dissolution in aqueous solution.

One-dimensional nanostructures such as nanowires have the potential to produce high photoactivity due to fast charge transport properties and reduced length scales for minority carrier diffusion expected in single crystalline nanowires. In our previous work, we developed a rapid two-step process for the scalable synthesis of copper oxide nanowire arrays on copper foils.² This involves a wet chemical oxidation of copper foils followed by oxidation in atmospheric microwave plasma. The nanowire arrays were subsequently coated with titania using atomic layer deposition to improve the aqueous stability. The photocurrents obtained with this method are much lower (<0.4 mA/cm²) but better than the data obtained with polycrystalline thin films to that date. The presence of a mixed phase of Cu₂O and CuO has been identified as a possible cause for the low photoactivity. CuO has an indirect band gap of 1.4 eV leading to inefficient photon absorption and also cannot generate sufficient energy required to split water. Hence, phase purity of Cu₂O is an important factor that needs to be addressed to improve the photoactivity of the Cu₂O NW arrays made through atmospheric plasma oxidation. In this contribution, we investigated the use of n-type crystalline WO₃ conformal layers on Cu2O nanowire arrays using hot wire chemical vapor deposition and studied their impact on photoactivity and durability.

Experimental: Copper oxide nanowire arrays were synthesized by wet chemical oxidation followed by short exposure to atmospheric plasma. Copper foils were immersed in an aqueous solution of sodium hydroxide and ammonium persulfate. The copper hydroxide nanowires produced were converted into copper oxide nanowires by means of exposure to an atmospheric plasma discharge for 2 minutes. The details of the process are described elsewhere. The deposition of WO3 or CuWO4 was performed in a HWCVD reactor using oxygen flow over hot tungsten and copper filaments used as the sources for tungsten oxide and copper oxide vapors.

Results and Discussion: The copper oxide nanowires are coated with titania layer using atomic layer deposition and tungsten oxide layers and copper tungstate layers using hot-wire chemical vapor deposition. The PEC characterization studies show 3-4 times enhancement in photoactivity for copper tungstate coated copper oxide nanowire arrays compared to that titania coated copper oxide nanowire arrays. See Figure 1.

Acknowledgements: Financial support from US DOE (DE-FG02-07ER46375) is acknowledged.

References:

• A. Martinez-Garcia et al., J. Mater. Chem. A., In Press (2013).

• S. Sunkara et al., Catalysis Today, 199, 27-35 (2013)

Fig. 1 PEC performance of copper tungstate coated copper oxide nanowires compared to titania coated copper oxide nanowire arrays in pH 5, 0.5M aqueous solution of Na₂SO₄.

An effective solar light-driven water splitting in photo-electrochemical devices requires use of materials which combine both high photo-conversion efficiency and long term stability goals. For the latter reason the choice of the suitable systems is restricted to the semiconducting oxides that, in most cases, do not undergo photo-corrosion in aqueous solutions but which are also able to absorb efficiently the visible light. The low optical absorption coefficients near the fundamental band edge, which determine the extent of solar light absorption by many photo-anode materials, are the critical factors for a number of semiconductors characterized by an indirect optical transition. This is also the case of n-type semiconducting WO₃ or Fe₂O₃ responding to the blue, respectively, green part of the solar spectrum for which the optical absorption depths exceed the minority charge carriers collection distances.Therefore, incoupling of light into semiconductor films by scattering from plasmonic nanostructures and/or resonant coupling of the plasmonic near field to the semiconductor, have a potential to improve the effectiveness of the photocurrent spectral response of the employed photoanode. The results regarding the improved light absorption and charge collection in doped and/or mixed semiconducting oxide photo-anodes will be presented.

Hydrogen modified titanium oxide (HM-n-TiO₂) thin films were synthesized by thermal oxidation of Ti metal substrate. The hydrogenation of n-TiO₂ was carried out electrochemically under cathodic polarization at −1.6 V vs SCE in 2.5 KOH electrolyte under dark condition. The photoactivity of HM-n-TiO₂ thin films was assessed by measuring the rate of the water splitting reaction under solar simulated light in terms of photocurrent density, J_p (mA cm^-2). The HM-n-TiO₂ photoelectrodes exhibited a fourfold enhancement in their photocatalytic activity compared to n-TiO₂ prior to hydrogenation. The HM-n-TiO₂ photoelectrodes were characterized using X-ray diffractions and the change in the valence band induced by the hydrogenation of n-TiO₂ was examined by XPS measurements.

One of the great challenges in artificial photosynthesis is the development of efficient catalysts for oxidation of water to molecular oxygen. To date, the most extensively investigated catalysts for O₂ evolution are nanostructured IrO₂ and RuO₂.¹ They are robust and efficient water oxidation catalysts that exhibit high turnover frequencies under mild conditions. However, the high cost and scarcity of noble metals severely limit the widespread use of these catalysts for solar fuel production. Thus, considerable effort has been devoted for development of efficient catalysts based on more abundant materials. Recently, we demonstrated that stabilization of the Mn³⁺ species relative to charge disproportionation (2Mn³⁺ → Mn²⁺ + Mn⁴⁺) is an effective strategy to lower the overpotential under neutral pH.^2,3 In this study, we examined O₂ evolution activity of Sr₃Fe₂O_7-δ at various temperature conditions, as Sr₃Fe₂O_7-δ is known to show the charge disproportionation (2Fe⁴⁺ → Fe³⁺ + Fe⁵⁺) below 70 ^oC.⁴

A Sr₃Fe₂O_7-δ film electrode was prepared by a spray drying method. Sr₃Fe₂O_7-δ polycrystalline was synthesized by solid state reaction of SrCO₃ and Fe₂O₃ and then suspended in ethanol. The suspension was deposited on a fluorine-doped tin oxide (FTO) electrode held on a 170 ^oC hotplate. The concentration of dissolved oxygen was monitored simultaneously with current density versus potential measurements using a needle-type oxygen microsensor.

Figure 1 shows polarization curves of a Sr₃Fe₂O_7-δ electrode at 30 ^oC and 70 ^oC. Upon sweeping the electrode potential at 30 ^oC, an increase in both anodic current and dissolved O₂ concentration was observed at an onset potential of approximately 1.1 V. This result indicates that the observed anodic current was attributed to water oxidation. It should be noted that when the solution temperature was raised to 70 ^oC, the onset potentials for O₂ production showed a negative shift from 1.1 V to 0.9 V. In this presentation, we will discuss the origin of the observed difference in O₂ evolution activity at between 30 ^oC and 70 ^oC as well as temperature dependent efficiency of charge disproportionation of Fe⁴⁺.

References

1. Y. Zhao, E. A. Hernandez-Pagan, N. M. Vargas-Barbosa, J. L. Dysart, and T. E. Mallouk, J. Phys. Chem. Lett., 2, 402 (2011).

2. T. Takashima, K. Hashimoto, and R. Nakamura, J. Am. Chem. Soc., 134, 18153 (2012).

3. T. Takashima, K. Hashimoto, and R. Nakamura, J. Am. Chem. Soc., 134, 1519 (2012).

4. K. Kuzushita, S. Morimoto, S. Nasu, and S. Nakamura, J. Phys. Soc. Jpn., 69, 2767 (2000).

Abstract

We report a novel approach for increasing the rate of organic material decomposition photo-catalyzed by visible light activated tungsten oxide (WO₃). It was confirmed that photo-catalysis by WO₃ nano-particles fabricated by gas phase method and annealed at 600 °C for 1 hour achieves 30 times higher decomposition rate compared to photo-catalysis by commonly reported nitrogen doped titanium dioxides (TiO₂). 1) The high decomposition rate in the presence of WO₃ nano-particles could be attributed to their high crystalline quality and large surface area. It was also found that the decomposition rate can further be doubled by addition of metal oxides such as zirconium oxide (ZrO₂) or TiO₂. Increased adsorbability of organic material by the WO₃ nano-particles as a result of ZrO₂ or TiO₂ addition could explain the observed enhancement in the decomposition rate. Furthermore, it is shown that immersing WO3 nano-particles mixed with ZrO₂ or TiO₂ in Pt colloid or Ru ionic solutions achieve complete decomposition of organic material to CO₂ and H₂O. This is made possible by the catalytic effect of Ru and Pt, which decreases the activation energy required for breaking down C-C bonds. These novel methods are already in the mass production stage.

Experiment and Result

It is well known that photo-catalyst activity improves with higher specific surface area and/or better crystallinity.In order to achieve higher surface area and improved crystallinity we fabricated nano-sized WO₃ particles from vapor phase, followed by annealing at atmospheric conditions. A TEM image of the WO₃ nano particles obtained from the ammonium paratungstate vapor is shown in Figure 1. The grain size was about 10 nm before annealing and about 20 nm after one hour 600°C annealing. The Raman spectra are shown in Figure 2. Before annealing the WO₃ peaks at 700 and 800 cm-1 are broad, and the W=O terminating group peak appears in 930 cm^-1. After annealing and at higher annealing temperatures the former two peaks become sharper, while the latter almost disappears. The peaks at 140, 270 and 320 cm^-1 belong to monoclinic WO₃ crystals and indicate improved cristallinity with monoclinic structure at higher annealing temperatures.2)3) With these WO₃ nano particles organic material decomposed at a 30 times higher rate as compared to decomposition catalyzed by commonly reported nitrogen-doped titanium dioxides.(Fig. 3) Comparing the measured decrease of organic material concentration in the presence of WO₃ with the reaction rate equations for the processes involved indicates that the decomposition rate is controlled by the speed of organic adsorption at the photocatalyst surface. To increase the adsorbability of the organic materials, metallic oxides such as ZrO₂ and TiO₂were added. As a result, the decomposition rate can further be doubled by addition of metal oxides.

The remaining issue is the weaker oxidizing ability of WO₃ as compared to TiO₂. This diminishes WO₃'s capability for decomposition of some organic molecules, e.g. acetic acids. As a solution to this problem it is proposed to add ruthenium or platinum by immersing the WO₃ nano-particles mixed with ZrO₂ or TiO₂ in Pt colloid or Ru ionic solutions. These results suggest that complete decomposition of acetic acid with WO₃photocatalyst may be possible, too. (Fig. 4)

It is believed that the synthesized photocatalyst reported here, can be used as indoor deodorant.

1) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. taga, Science,293, 269 (2001)

2) M. Boulova and G. Lucazeau, J. Solid State Chem. 167, 425(2002)

3) A. G. Souza-Filho, V. N. Freire, J. M. Sasaki, J. Mendes Filho, J. F. Juliao, U.U. Gomes, J. Raman Spectrosc.31, 451(2000)

Hydrogen generation is a significant area of energy research, with the aim to deliver low-cost, renewable energy sources which are environmentally friendly. One possible method is solar water splitting via photo-electrochemical reaction. Photo-electrochemical (PEC) hydrogen generation is a promising technology and alternative to photovoltaic (PV)-electrolyser combined systems. Although the PEC technology is promising, the efficiency of this technology is limited by thermodynamics and technical issues. Recent research has focused on aspects of materials development to improve efficiency. With multiple band-gap electrodes, the thermodynamic efficiency, and so the overall generated hydrogen quantity, can be increased. In the case of applications where there is a heating requirement beyond the need to generate hydrogen, there are further options for extracting energy from the solar resource by utilizing the longer wavelength radiation.

Just as it is possible to have a PV/T hybrid system, the PEC unit may also be used in a PEC/T hybrid mode, thus delivering both heat and power as a CHP system. Despite the promise of PEC technology, there is little research on it in terms of modelling and system simulation. According to the knowledge of the authors there is no published research on such hybrid systems.

In this work, a model of a hybrid PEC/T system has been implemented. Simulations with this model were carried out to compare PEC technology and its developments in terms of energy gain in a home environment where both heat and electricity demands are used. When there is heat demand, a buffer tank is implemented for heat storage.

Case studies were considered, consisting of a typical three-person household in the UK to investigate the present and the near-future capability of energy supplying and reduction of CO₂ emission according to the UK building energy regulation.

Results show that single band-gap photo-electrode materials are not able to cover completely the energy demands for the household if demand includes space and hot water heating. However, with multiple band-gap electrodes and with extra solar heat (or combined PEC-thermal) utilization the system efficiency can be significantly increased.

To date, most research on heterogeneous CO₂ electroreduction has been aimed at decreasing the large overpotential required for hydrocarbon synthesis. Specifically, most scientists have focused on reducing the external voltage to as low as 1.24 V, as this value represents the thermodynamic limit for producing organic compounds via CO₂ reduction coupled with water oxidation [CO₂ + H₂O to (CH₂O) + O₂, where (CH₂O) represents a carbohydrate].

As an alternative approach, it may be possible to exploit the highly active geoelectrochemical reactions mediated by microorganisms living in deep-sea hydrothermal environments. In such environments, chemolithoautotrophs, as opposed to photoautotrophs, primarily contribute to biomass production through the utilization of geochemical energy sources, such as H₂S and H₂, emitted from hydrothermal vents. Chemolithoautotrophic bacteria are able to utilize these energy-rich compounds for the production of hydrocarbons at rates that are one order of magnitude higher than that of photosynthesis, and several species can synthesize hydrocarbons using Fe²⁺ ions as a sole electron source for CO₂ reduction. It is worth noting that Fe-oxidizing bacteria utilize the proton-motive force (PMF) generated by the reduction of O₂ to H₂O to elevate the energy of electrons obtained from Fe²⁺ oxidation by ~ 1 eV. Thus, although the reversible potential of the Fe³⁺/Fe²⁺ redox couple (0.77 V vs SHE) is more positive than that of NAD(P)/NAD(P)H (−0.32 V at pH 7), Fe-oxidizing bacteria are capable of generating NAD(P)H as a source of reductive energy for CO₂fixation.

By harnessing the ability of Fe-oxidizing bacteria to elevate intracellular electrons into a higher energetic state, it may be possible to construct an integrated bioelectrochemical process in which the bacteria function not only as effective CO₂ reduction catalysts, but also as voltage-multiplier circuits. In this system, CO₂ reduction would occur at an external voltage that is one order of magnitude lower than that of conventional heterogeneous electrocatalytic systems. The thermodynamic limit of the external voltage required for the simultaneous reduction and oxidation of CO₂ and H₂O, respectively, is determined by the difference between the reversible potentials of E(O₂/H₂O) and E(Fe³⁺/Fe²⁺). Thus, the thermodynamic limit of the external voltage is estimated to be 0.035 V at pH 7 if 0.77 V is adopted as the redox potential of the Fe³⁺/Fe²⁺ couple. Based on these estimated values, the integrated bioelectrochemical process mediated by Fe-oxidizing bacteria can potentially proceed at an external voltage lower than 1.24 V, and thus provide an approach for utilizing the low-voltage electricity generated from hydro, solar, wind, and geothermal sources for the electrochemical conversion of CO₂ to chemical fuels.

Photovoltaic is one of the most fascinating ways for direct solar energy conversion. The demand for photovoltaic is mainly covered by crystalline silicon; substantial cost reductions are necessary for large scale applications. A big challenge for thin film PV is the development of large area semiconductor thin films. The main candidate semiconductor materials for thin film solar cells at present are amorphous silicon, CdTe and Cu(In,Ga)(Se,S)₂.

Generally, even though physical vapor deposition (PVD) methods have been widely used for making the CIGS thin film ^1-3, its high process cost because of the vacuum system and difficulty on the large scale up still remained to overcome for mass production.

Non-vacuum methods, such as hydrazine-based solution process, paste coating, and electrodeposition have been studied to achieve the low cost process with high efficiency ^4-7. Electrodeposition could substitute for PVD methods because it has several advantages in cost competitiveness, easily scaled up, convenience control in room temperature and atmosphere pressure. Recently, the efficiency of CIGS solar cell manufactured by electrodeposition method exceeds 11% ⁸.

In this study, the effects of sulfurization temperature on CuIn(Se,S)₂ (CISeS) thin film solar cell have been investigated. Unlike well-known CIGS absorber, we adjusted the bandgap by controlling the ratio of Se and S, which could simplify the deposition process for obtaining absorber layer. 1㎛-CuInSe₂ layer was deposited on Mo/SLG (soda lime glass) at room temperature by electrodeposition method. To form CuIn(Se,S)₂ thin film, thin films were annealed in 5% H₂S-95% Ar atmosphere at 425-600℃. As the sulfurization temperature increased, the grain growth of CISeS films was improved and the ratio of Se/S decreased, then the optical bandgap was close to ideal value for solar cell (~1.4eV). However, MoS₂layers appeared at high temperature (≥500℃) and the film morphology was getting more porous with increasing temperature. As a result, the conversion efficiency decreased as the sulfurization temperature increased, and the maximum value of the conversion efficiency was 2.32% at 425℃.

To improve the cell efficiency, the effects of sulfurization time on grain growth have been investigated. CuInSe₂ thin films were annealed in 5% H₂S-95% Ar atmosphere at 425℃ for 20-60min. When the sulfurization time increased, the crystallinity of CuIn(Se,S)₂ film improved. As a result, the conversion effieciency increased to 3.48% without anti-reflection coating layer.

The first results of the synthesis of mechanically stable, crystalline mesoporous films of titanium oxides with various stoichiometry, which will be applied either as an anode or a cathode for fuel cell electrodes, will be presented. The composition of TiO₂ and substoichiomteric Ti_nO_2n-1 Magneli phases which has been obtained in a high temperature and in reductive atmosphere is expected to have a potential to serve as an anode for enhanced oxidation of organic fuel and to be employed as a cathode when modified with the co-catalyst (NiO/Ni) to allow an effective oxygen reduction. These applications require high degrees of conductivity and a large contact area with the reaction media. In addition, highly crystalline oxides are desired in order to optimize the properties such as electroactivity or electronic conductivity. The synthesis approach of substoichiometric Ti_nO_2n-1 oxides and the structural characterization in view of their potential application in fuel cell systems will be the objective of the presentation.

Recently, the performance of organic solar cells has dramatically improved by incorporating a functional interfacial layer between active layer and metal electrode, forming bulk heterojunction (BHJ). Many interfacial materials, such as metal oxides, self-assembled monolayers (SAMs) and conjugated polyelectrolytes (CPEs), have successfully enhanced energy conversion efficiency of solar cells [4]. Among these interlayers, titanium oxide (TiO_x) was used for an optical spacer as well as a hole blocking layer which led to an increase in the power conversion efficiency (PCE). In addition TiOx is known to improve the stability of the organic solar cell devices [5].

In this study, the organic photovoltaic with bulk heterojunction (BHJ), thieno(3,4-b)-thiophene/ benzodithiophene copolymer and (6,6)-phenyl C₇₁ butyric acid methyl ester (PTB7/PC₇₁BM) treated with titanium oxide (TiO_x) interlayer was fabricated and compared with the stability of organic solar cells based on poly (3- hexylthiophene) and [6,6]-phenylC₇₁ butyric acid methyl ester (P3HT/PC₇₁BM) with TiO_x interlayer. TiO_x was prepared by sol-gel chemistry method. The crystalline structure of TiO_x was described by X-ray diffraction (XRD) while the absorption spectrum, and surface morphology of the BHJ were studied by UV-Vis spectroscopy and atomic force microscopy (AFM) respectively.

Organic solar cells were fabricated by spin-coating PEDOT:PSS layer on the top of ITO glass substrate. After annealing, the substrates were then transferred into a nitrogen filled glove box for spin-casting photoactive layers. Then TiO_x layer was spin coated in air. Subsequently, aluminum (Al) electrode was deposited, producing the devices active area of ~ 10 mm². Finally after Al deposition, the P3HT/PC₇₁BM device was subjected to annealing treatment in glove box.

The electrical properties of the resultant devices were investigated by measuring the current density–voltage (J–V). Also the normalized efficiency of both devices as a function of time has been measured and stability was analyzed in terms of optical, structural and morphological degradation.

References

[1]K.-D. Kim, D. C. Lim, H. O. Seo, J. Y. Lee, B. Y. Seo, D. J. Lee, Y. Song, S. Cho, J.-H. Lim, Y. D. Kim, Applied Surface Science, 279, 2013, 380– 383.

[2] W. Zhang, B. Zhao, Z. He, X. Zhao, H. Wang, S. Yang, H. Wu and Y. Cao, Energy Environ. Sci., 6, 2013, 1956–1964.

[3] Y.-M. Chang and C.-Y. Leu, J. Mater. Chem. A, 1, 2013, 6446–6451.

[4] H. Zhou, Y. Zhang, J. Seifter , S. D. Collins, C. Luo, G. C. Bazan, T.-Q. Nguyen , and A. J. Heeger, Adv. Mater, 25, 2013, 1646–1652.

[5] S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nature Photonics, 3, 2009, 297-303.

1. Introduction

Photocatalytic decomposition of organic pollutants is one of the promising method for environmental purification. Most of the researchers focused on efficient decomposition of environmental organic contaminants under visible light irradiation or indoor fluorescent light.¹ Oxide-supported metal catalysts are widely acknowledged as important catalysts that are related to versatile key in photochromism, chemical sensing and photocatalytic activity. Tungsten trioxide (WO₃) acts as a ideal visible light photocatalyst owing to its small band gap energy (2.4-2.8 eV) and high oxidation power of vanlence band holes. But, WO₃ exhibits low photocatalytic activity due its high recombination rate of photoexcited electrons and holes. In order to suppress the recombination of photoexcited carriers and significantly increase the photocatalytic efficiency, most research has focused to prepare WO₃ doping constituent or WO₃ composites.^2,3 In the present study, we have synthesized PtPb/WO₃ and PtAu/WO₃ alloy catalysts, demonstrated to exhibit the efficiency for the decompostion of organic pollutants under visible light irradiation.

2. Experimental

A simple one-pot synthesis approach to obtain WO₃ supported PtAu catalyst is as follows. In the synthesis, 0.5 g of WO₃was suspended in 15 mL of anhyrous methanol for 15 min in a two neck round bottom flask. Pt (0.0075 g) and Au (0.0045 g) precursors were dissolved in 15 mL anhydrous methanol in the shielded vial uder Ar atmosphere and transferred to the suspensions. Then, the solution was stirred for 30 min to yield homogeneous color soluion. Required amount of NaBH₄ in mehtanol was injected into the suspensions. The product were washed and centrifuged with anhydrous methanol for three times and dried under vacuum. Similar procedure was followed to synthesis the PtPb/WO₃ catalyst.

3. Results and Discussion

Figure 1 shows the TEM images of (a) Pt/WO₃ (b) PtPb/WO₃ and (c) PtAu/WO₃ photocatalysts. Pt and PtPb and PtAu NPs are showed monodispersity and homogeneity on the WO₃ supports. The average particle size of the three catalysts can be ranked as PtPb/WO₃~PtAu/WO₃ > Pt/WO₃. The TEM results show that the addition of Pb and Au increases the size of NPs. Figure 2 depicts that the CO₂ formation profile over different photocatalysts under visible light irradiation time. PtPb/WO₃ photocatalyst (1 wt %) shows the higher rate of CO₂formation over other photocatalysts. The details about the characterization and photocatalytic activity of catalysts would be discussed in the presentation.

4. References

1. M.R. Hoffmann, S.T. Martin, W. Choi and D.W. Bahnemann, Chem. Rev., 95, 69, (1995).

2. R. Abe, H. Takami, N. Murakami and B. Otani, J. Am. Chem. Soc., 130, 7780, (2008).

3. M. Miyauchi, Phys. Chem. Chem. Phys., 10, 6258, (2008).

In the operational process of dye sensitized solar cell (DSSC) photo excitation of the sensitizer is followed by electron injection into the conduction band of the mesoporous oxide semiconductor. The dye molecule is regenerated by the redox system, which itself is restored at the counter electrode by electrons passed through the load. Some electrons in conduction band of the photo-electrode travel back to the electrolyte, which results in a loss of efficiency in the DSSC [1-2]. Therefore, it is important to suppress this recombination reaction and to improve the collection of photo-injected electrons for enhancing the cell performance [3]. Tungsten oxide (WO₃) is a semiconductor oxide material with a band-gap of 2.6–3.0 eV [3], and it is becoming the focus of research attention due to its unique electronic properties. Various WO₃ nanostructures (nanoparticles, nanoplatelets, nanorods, and nanowires) are of special interest as promising candidate [3, 4] as a photo-electrode.

It is well known that polyaniline (PANI) is one of the most promising conducting polymers. Due to its high electrochemical activity, environmental stability and low cost, PANI materials have been employed to fabricate efficient counter electrode in DSSCs [5].

Recently, there is a large interest to incorporate carbon nanotubes (CNTs) into organic solar cells because of the unique electrical properties of CNTs. For example, CNTs were used as electron acceptors in the photoactive layer of the solar cells [6]. Also, CNTs were used as transparent anodes in order to replace the prevailed wildly used indium tin oxide (ITO) [7].

In this work nanomaterials for photo- and counter-electrodes are synthesized and investigated. The morphological structures of WO₃ nanorods deposited onto TiO₂ nanoparticles and PANI-CNTs. The electro-catalytic activity of the counter electrode is investigated using cyclic voltammetry technique.

The CNTs blended PANI base counter electrode and ruthenium dye sensitized WO₃/TiO₂ based photo-electrode were assembled to form DSSC. The electrical properties of the fabricated solar cells were investigated by measuring the current density voltage (J–V) under both darkness and illumination conditions. It was found that there is a significant improvement in the performance of DSSC used WO₃ nanorods.

References

1 M. Grätzel, Journal of Photochemistry and PhotobiologyPhotochemistry Reviews 4, 145–153 (2003).

2 S. M. Yong, T. Nikolay, B. T. Ahn, D. K. Kim, Journal of Alloys and Compounds54, 113–117(2013).

3 F.G. Wang, C. Di Valentin, G. Pacchioni, Chemcatchem4 476–478(2012).

4 H.D. Zheng, Y. Tachibana, K. Kalantar-zadeh, Langmuir26 19148–19152 (2010).

5 G. Wang, W. Xing, S. Zhuo, Electrochimica Acta 66 , 151– 157. (2012).

6 S Berson., D. Bettignies., S. Bailly, S. Guillerez, B. Jousselme, Adv. Funct. Mater. 17, 3363–3370 (2007).

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