Table of contents

One of the grand challenges facing humanity today is the development of an alternative energy system that is safe, clean, and sustainable and where combustion of fossil fuels no longer dominates. A distributed renewable electrochemical energy and mobility system (DREEMS) could meet this challenge. At the foundation of this new energy system are a number of electrochemical devices including fuel cells, electrolyzers, and flow batteries. For all these devices electrocatalysis plays a critical role in controlling their performance and cost, and thus their commercial viability. In this presentation, I will focus on our recent work on non-precious metal based oxygen and hydrogen evolution catalysts in base.

The identification of materials needed for efficient energy conversion and fuel production in electrochemical systems must be guided by two equally important fundamental properties: optimization of their catalytic behavior and their long-term stability in hostile electrochemical environments. This is especially true for the oxygen evolution reaction (OER), the anodic-half cell reaction that takes place at high overpotentials on oxidized metal surfaces in hydrogen-oxygen electrolyzers, metal air batteries and electrometallurgy. The kinetics of the OER have, for the most part, been closely tied to the concept of the volcano plot, which generally expresses the rate of the OER as a function of more fundamental properties of the oxide materials, known as descriptors. These analyses showed that the interaction between the substrate and the reactants, intermediates, and the products has to be optimized for the reaction to proceed efficiently. Although concepts resulting from volcano plot analyses have led to the establishment of important catalytic trends, many fundamental questions still remain open. One key question is what relationship exists between the kinetics of the OER and the stability of oxide materials. The lack of understanding of such stability-activity relationships derives mainly from the fact that research directed to the development of anode materials for the OER has been strongly "activity-centric", while almost completely ignoring the stability of active components during the OER.

Here, we show that this disparity in focus has masked the inherently close ties that exist between the stability and activity of monometallic and complex oxide catalysts. By studying the stability-activity relationships of well-characterized oxide surfaces, we demonstrate that there is a fundamental link between the stability of catalysts and their reactivity for the OER. This trend is observed for many oxide catalysts in both acidic and alkaline electrolytes, indicating that the stability-activity relationship is independent of the specific ionic/molecular species mediating the OER (i.e. OH^- vs. H₂O). We found that the degree of stability is always inversely proportional to activity and that stable surfaces are, in fact, not reactive.

The development of sustainable energy is one of the most important scientific challenges in the 21st century. A critical element for sustainable energy implementation is to have efficient energy conversion and storage. Oxygen electrocatalysis is central to enable photoelectrochemical and electrolytic water-splitting, fuel cells, and metal-air batteries.  Probing a fundamental catalyst "design" principle" that links surface structure and chemistry to the catalytic activity can guide the search for highly active catalysts that are cost effective and abundant in nature.  While such a design concept exists for metal catalysts, little is known about the design principles for oxygen electrocatalysis on oxides. Recent advances in identifying the design principles and activity descriptors of transition metal oxides will be presented. We will show that these fundamental concepts can be used to tune transition metal oxide surfaces with much enhanced catalytic activities. Moreover, we will discuss how oxide bulk electronic structures can influence the catalytic activities of oxides, from which two different reaction mechanisms are proposed. Lastly, connecting bulk to surface electronic structures is challenging but much needed to provide mechanistic insights, and some in-situ synchrotron X-ray measurements to this end will be discussed.

Oxygen evolution reaction (OER) catalysis limits the efficiency of H₂ production through water electrolysis/photoelectrolysis, a route to large-scale energy storage. The factors governing the activity of OER catalysts are not well understood. We found Ni-Fe oxyhydroxides are the fastest known water oxidation catalysts under basic conditions when compared to others using a quantitative electrochemical-microbalance approach. In-situ electrical, photoelectron spectroscopy, x-ray diffraction, and electrochemical analyses on Ni_1–xFe_xOOH films show that the reaction mechanism relies on the local electronic structure of a Ni-O(H)-Fe active site. Experiments with rigorous exclusion of Fe electrolyte impurities reveal that, contrary to common belief, pure NiOOH is a poor OER catalyst. FeOOH was found to be a poor electrical conductor, explaining its low apparent OER activity. Measurements on Co_1–xFe_xOOH support the hypothesis that NiOOH and CoOOH provide a conductive host for the Fe-related active sites. In sum, these results establish a new activity trend for OER catalysts, opposite to previous ones, that informs catalyst design.

(1) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-cast metal oxide thin film electrocatalysts for oxygen evolution. J. Am. Chem. Soc.2012, 134, 17253.

(2) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel–iron oxyhydroxide oxygen-evolution electrocatalysts: The role of intentional and incidental iron incorporation. J. Am. Chem. Soc.2014, 136, 6744.

(3) Burke, M. S.; Kast, M.; Trotochaud, L.; Smith, A.; Boettcher, S. W. Cobalt-iron (oxy)hydroxide oxygen evolution electrocatalysts: The role of structure and composition on activity, stability and catalytic mechanism. To be submitted 2014.

Recent proliferation of environmentally conscious applications that acquire energy from renewable sources brings with them an interesting challenge of grid integration. Due to their unpredictability and intermittent nature renewable energy sources require substantial overcapacity to guarantee certain levels of energy production to satisfy targeted demand at any given time. This creates opportunities to capture output of that excess capacity during periods of high production/low demand, when the grid cannot accept such excesses. Such situations create opportunities to obtain electrical power at very low cost. One use of such inexpensive energy could be the electrolytic production of hydrogen for the purpose of energy storage (to be used in fuel cells at a later time), as chemical feedstock, or fuel (to supplement natural gas). Source of H₂ is also necessary for deployment of fuel cells vehicles, and PEM electrolysis is one of the most promising environmentally clean sources. If done using renewable electricity it would also help meet energy independence goals for countries presently importing fossil fuels.

A PEM water electrolyzer essentially works like a fuel cell in reverse. While some aspects and challenges of its operation are common / similar to fuel cells, some are different. In general it could be said that thanks to the somewhat higher kinetics of oxygen evolution as opposed to oxygen reduction the performance of electrolyzers has to date slightly exceed the performance of related fuel cells, but the difference is not dramatic. We intend to shatter this common perception and present some preliminary data showing operation of electrolyzer at previously unheard of power levels. This new level of performance was made possible by adapting techniques, procedures, and processes 3M already developed for making and testing fuel cells to electrolyzers. Our 3M proprietary NSTF catalyst was found to be an extremely good fit to water electrolysis (carbonless, durable, low mass transport losses, high conductivity, good heat dissipation, strongly hydrophilic – weakness in fuel cells, but strength in electrolysis). Recent advancements in fuel cell PEM membranes were equally as adaptable to water electrolysis and as important as the catalyst to enable such high power operation. What's perhaps of even more interest is that while the new high power operation mode is enabled, at the same time the efficiency of hydrogen production at low power levels, should one choose to run the electrolyzer that way, is not negatively affected. The presentation will cover progress achieved to date on the development of tools for characterization of PEM electrolyzers and performance. One of the highlights will be a description of >10 A/cm² electrolyzer cell operation using propriety 3M MEAs with very low PGM loading (much less than present commercial electrolyzer MEAs). The associated challenges (durability, etc.), being a good fit to the intermittent nature of renewable energy sources, and potentially changing the basis of competition will be discussed.

Figure 1

Hydrogen production from proton exchange Hydrogen production from proton exchange membrane (PEM) water electrolysis is attractive due to its simple and clean nature.  However, the technology is still expensive due to high materials cost for the catalysts, membrane, and bipolar plate. In most commercial processes, iridium (Ir) black is used as an oxygen evolution reaction (OER) catalyst at the anode. A high precious group metal (PGM) loading (> 4mg/cm²) has been used due to significant anode activation loss caused by sluggish kinetics of the OER.

Instead of using commercial Ir black, we have developed two types of high-performance OER catalysts for PEM water electrolysis. One is Ir dispersed on some supports with high oxidation resistance; the other is extended continuous nanostructures including Ir nanowires and Ir nanotubes (see Figure 1).   

First, Ir nanoparticles have been dispersed on tungsten-doped titanium oxide (Ir/W-TiO₂). Doping of TiO₂ with an element W can substantially enhance the electronic conductivity of TiO₂ while maintaining its high corrosion resistance at high electrolyzer operating voltages. A series of Ir/ W-TiO₂ catalysts with decreasing Ir loading from 70 wt. % to 25 wt. % has been synthesized. TEM images illustrates that Ir nanoparticles vary from 1 nm to 5 nm. The electrochemical surface area (ECSA) has been measured via mercury underpotential deposition (UPD).

In addition to Ir/W-TiO_2, Ir nanotubes have been synthesized via a porous alumina template.  The template was coated with an iridium precursor by subliming iridium precursor at an appropriate temperature. The template support was removed via chemical   etchings. The average length of the Ir nanotubes is 5mm and the diameter ranges from 250 – 400 nm with a wall thickness of ≤50 nm. The template pore size and process conditions are being varied to change the dimension of the Ir nanotubes.  

The activity and durability of these catalysts have been evaluated in rotating disk electrodes (RDEs) and electrolyzers. Selected catalysts have reduced PGM loading of electrodes by a factor of 10 while maintaining equivalent electrolyzer performance. These catalysts will help to lower the PEM electrolyzer capital cost making PEM water electrolysis more viable for a variety of applications.

Figure 1

Understanding the oxygen evolution reaction (OER) is crucial for improving the performance of water electrolysis. Delafossite oxides (ABO₂) were investigated for their potential as OER catalysts using density functional theory (DFT) calculations. The relationships between the calculated e_g or t_2g occupancy of the B site and the experimentally-determined OER activity were examined. Our examination suggests that t_2g electrons of the B site can be significant for the OER process. The calculated t_2g occupancy is approximately linearly related to OER activity. Thus, we propose that t_2g occupancy can be employed as a simple descriptor of the OER activity of delafossite oxide catalysts.

Electrocatalytic water oxidation plays the key role as efficient and stable platform in some of the promising renewable energy conversion and storage devices, such as water electrolyzers, solar water-splitting cells, and lithium-air batteries. The electrocatalytic oxygen evolution reaction (OER) at the anode often couples with electron or photoelectron CO₂ reductionand, H₂ evolution reactions at the cathode for the above devices but limits the performance of the entire system because of the sluggish OER kinetics, insufficient reaction sites, or low stability of precious metal catalysts. Series of different transition metallic catalysts were investigated in order to significantly increase the activity of OER catalysts and efficiently improve the performance of energy conversion and storage systems. Ni-Fe system is found to be the most promising bimetallic catalytic system with robust stability, much lower cost, and considerably higher activity than the IrO₂, the well-known best OER catalyst. Nano scaled catalysts with high electrochemical surface area (ECSA) not only exhibits a promising feasibility to effectively increase the performance of OER catalysts, but also to be simply utilized in renewable energy conversion and storage devices Herein, we reported 4 nm Ni−Fe nanoparticles (Ni_yFe_1−yO_x/C) featuring amorphous structures prepared via a solution-phase nanocapsule method for active and durable OER electrocatalysts in alkaline electrolyte. The Ni−Fe nanoparticle catalyst containing 31% Fe (Ni_0.69Fe_0.31O_x/C) shows the highest activity, exhibiting a 280 mV overpotential at 10 mA cm⁻² (equivalent to 10% efficiency of solar-to-fuel conversion) and a Tafel slope of 30 mV dec⁻¹ in 1.0 M KOH solution. The achieved OER activity outperforms NiO_x/C and commercial Ir/ C catalysts and is close to the highest performance of crystalline Ni−Fe thin films reported in the literature. In addition, a Faradaic efficiency of 97% measured on Ni_0.69Fe_0.31Ox/C suggests that carbon support corrosion and further oxidation of nanoparticle catalysts are negligible during the electrocatalytic OER tests. Ni_0.69Fe_0.31O_x/C further demonstrates high stability as there is no apparent OER activity loss (based on a chronoamperometry test) or particle aggregation (based on TEM image observation) after a 6 h anodization test. The high efficiency and durability make these supported amorphous Ni−Fe nanoparticles potentially applicable in the (photo)electrochemical cells for water splitting to make H₂ fuel or CO₂ reduction to produce usable fuels and chemicals.

Oxides of manganese and cobalt have proved to be effective (electro)catalysts for the water oxidation half-reaction in water splitting. Recently, the research focus has mostly been on developing high surface area morphologies of catalysts with high specific activity that are potentially easily adaptable to photoelectrochemical cells for water splitting. However, it is still not completely understood why these catalysts show such high intrinsic activity, and many have amorphous structures, making it difficult to elucidate structure-function relationships. Using crystalline oxides as an experimental framework, we have elucidated the effects of structure on the activity and stability of cobalt and manganese oxides for the oxygen evolution reaction (OER). For instance, in two distinct polymorphs of lithium cobalt oxide (LiCoO₂), we have found the cubic structure (Fd-3m, spinel-like), with a Co₄O₄ oxo-metallic cube motif, is highly active for water oxidation, whereas the layered polymorph (R-3m) with slabs of CoO₂ (used for rechargeable Li ion batteries) is not. In fact, upon electrochemical processing, the layered polymorph undergoes a surface restructuring that we can monitor using high-resolution transmission electron microscopy (HRTEM). This has been shown to occur in both aqueous and non-aqueous electrolyte. After the transformation occurs, which can also be monitored by the electrochemical profile and corrosion of lithium, the OER activity of the layered material mirrors that of the cubic LiCoO₂. However, it is unstable and eventually diminishes with repeated electrochemical cycling or prolonged electrolysis. In this way, we show that the underlying bulk crystal structure has influence over not just activity, but also stability, both equally important for commercial applications. Parallel to this work, we isolated discrete Co₂O₂, Co₃O₃, and Co₄O₄ molecular clusters in the same ligand sets, and found only the complete Co₄O₄ cubanes were catalytically active for water oxidation. This difference in activity correlates to the accessibility of a formal Co⁴⁺ oxidation state, which occurs at modest potentials only for the cubane, which compensates by delocalizing the hole across all cobalt and bridging oxos. The singly oxidized cubane is sufficient to produce O₂ just in reaction with hydroxide. We believe this phenomenon relates back to the solid-state, and is in part the reason for the high activity in the cubic LiCoO₂.

Non noble metal "earth abundant" electrocatalysts will significantly reduce the costs and make it possible to use electrochemical water splitting (electrolysis) on a very large scale. Non noble metal catalysts can be an effective replacement even if their electrochemical activities are a fraction of their noble metal counterparts. If these catalysts can be developed to an extent where their operational lifetime would become comparable to their noble metal counterparts, a large part of the problem would be solved. MnO₂ is earth abundant and well known for catalyzing oxygen evolution. A form of MnO_x cluster is known to catalyze water oxidation in photosynthesis. These catalysts although with immense potential remain a long way from successful implementation within electrolysers. In order to understand the limitations of this catalyst under the operational conditions, it is important to study this material in situ. Setup for in situ Surface Enhanced Raman Spectroscopy is used for tracking the structural changes of hydrous MnO_x. Electrodeposition of MnO_x on the surface of Au was achieved by passing an anodic current of 50 mC/cm² at 1,6V.

The resulting MnO₂ phase could be termed as the alpha/MnO₂ phase. The structural integrity of the MnO_x was studied by following the Mn-O-Mn stretching various potential ranges in various electrolytes. Onset of phase changes and corrosion could be clearly established. Beside this, other methods such as XRD, SEM, TEM were applied to study these thin films. Depending upon the nature of electrolyte the catalyst acquired a specific structure.

Figure 1

Designing electrode materials for oxygen evolution reaction (OER) and for oxygen reduction reaction (ORR) is crucial for the development of renewable energy technology devices, such as water electrolyzers and fuel cells [1]. Alkaline water electrolysis is seen as an eco-friendly process for large-scale production of hydrogen required for hydrogen economy. However, the efficiency of water electrolysis in practice is limited by the large overpotential necessary to drive OER, resulting in high cost of the process and produced hydrogen.

There are few literature reports on OER at perovskite-based electrodes where different carbon supporting materials were often used for increase of active surface area and conductivity [1-3]. The aim of this study was to examine the intrinsic activity of perovskite materials.

Therefore, six different perovskite oxides were used for preparation of electrodes with no carbon support, to be studied for OER in alkaline media. These were La_0.8Sr_0.2Fe_0.8Co_0.2O₃, La_0.7Sr_0.3MnO₃, La₂NiO₄, La_1.9Pr_0.1CuO₄, La_1.8Pr_0.2NiO₄ and La_1.9Sr_0.1NiO₄. OER studies were performed using cyclic voltammetry (CV) and linear scan voltammetry (LSV) in potassium hydroxide (KOH) solution. Main reaction parameters, namely Tafel slopes, charge transfer coefficients, exchange current densities, and activation energies were determined from the LSV data. Influence of operating parameters, specifically electrolyte composition and temperature, was also investigated.

First results indicated the highest activity of La_1.9Sr_0.1NiO₄ for the OER in the studied temperature range (25 – 85 ºC).

References

[1] M. Risch, K. A. Stoerzinger, S. Maruyama, W. T. Hong, I. Takeuchi, Y. Shao-Horn, J. Am. Chem. Soc. 136 (2014) 5229 − 5232.

[2] C. Jin, X. Cao, L. Zhang, C. Zhang, R. Yang, J. Power Sources 241 (2013) 225 – 230.

[3] R. A. Rinc, E. Ventosa, F. Tietz, J. Masa, S. Seisel, V. Kuznetsov, W. Schuhmann, ChemPhysChem 15 (2014) 2810 – 2816.

A bifunctional oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER) catalyst is essential for rechargeable metal-air batteries and regenerative fuel cells. Platinum (Pt) and iridium oxide (IrO₂) are the state-of-the-art ORR and OER catalysts, respectively. However the high price and scarcity of these platinum group metals (PGMs) has been an obstacle for wide spread application of these catalysts. Recently, in alkaline media, carbon based ORR and perovskite OER catalysts have demonstrated similar or even better catalytic activities compared to the counterpart PGM catalysts [1, 2]. Therefore, if we combine these two non-PGM catalysts, a non-PGM bifunctional ORR/OER catalyst can be obtained. A hindrance in this approach is the vulnerability of carbon-based ORR catalysts to oxidation in the OER potential range, i.e., potentials > 1.5 V vs. RHE. Thus development of robust ORR catalysts under practical OER conditions is a key to realize this kind of bifunctional catalysts.

 The carbon support used in our ORR catalysts was black pearl (BP) 2000 [1]. In preliminary tests, however, we found that BP 2000 undergoes oxidization at potentials around ca. 1.2 V vs. RHE and above (data not shown). In this work, we used reduced graphene oxide (rGO) as an alternative support to synthesize oxidation resistant ORR catalysts. The OER catalyst we chose was a perovskite (La_1-xSr_x)CoO_3-δ (LSC). Pre-synthesized LSC was added into the initial solution of the rGO based ORR catalyst synthesis process, and after drying and heat-treatment, bifunctional (LSC + rGO) catalysts were obtained. In measuring the OER activity of the LSC catalyst, acetylene black (AB) carbon was added to the LSC (LSC + AB) to increase the electrical conductivity. Fig. 1 shows the comparison of ORR/OER activities between (LSC + AB) and (LSC + rGO). As expected, the ORR activity of (LSC + rGO) is greatly improved by ca. 200 mV in terms of E_½, in comparison to that of (LSC + AB). Interestingly even the OER activity of (LSC + rGO) becomes higher than that of (LSC + AB). Thanks to the enhancement of both ORR and OER activities with (LSC + rGO), highly active bifunctional catalysts are obtained. In this talk, material analysis results and diverse electrochemical performances of the (LSC + rGO) catalysts will be presented.

Acknowledgements

Support from the Directed Research of the Los Alamos National Laboratory's Laboratory Directed Research & Development (LDRD-DR) is greatly acknowledged.

References

• Chung et al., Nat. Commun. 4, 1922 (2013).

• Suntivich et al., Science, 334, 1383 (2011).

Figure 1

A rapid screening technique based on scanning electrochemical microscopy (SECM) in a tip-generation–substrate-collection (TG–SC) mode was applied to identify potential electrocatalysts for both oxidation reduction reaction (ORR) and oxidation evolution reaction (OER). Ternary ruthenium based chalcogenide and ruthenium based oxide electrocatalyst arrays were fabricated and screened for ORR and OER, respectively. The potential electrocatalysts were then characterized by SEM, EDX, XRD and XPS. The catalytic activity of potential electrocatalysts were further examined during cyclic voltammetric scans of the substrate with a tip close to the substrate. The quantitative rate of ORR on the candidate substrates was determined for different substrate potentials from SECM approach curves by fitting to a theoretical model. The electron transfer number of OER was calculated rotating ring-disk electrode (RRDE) technique. The best ruthenium based electrocatalysts for ORR and OER would be identified in this work.

Introduction

The conversion of chemical potential energy into kinetic energy from different gaseous and liquid fuels is quite low. Fuel cell technology has been extensively researched to combat these issues. Fundamentally, these fuel cells are driven by electrochemical kinetics and intrinsic kinetics at the catalyst layer. Recently, interest to catalysts which can electroreduce oxygen during the cathodic and evolute oxygen during anodic cycles (bi-functional catalyst) was significantly grown from manufactures of metai-air batteries, electrolyzers and regenerative fuel cells. Presently, the most conventional bi-functional catalysts are made from Pt-group elements (Pt, Ir, Ru etc). It has also been discovered that by placing these on commercial carbon supports that have high surface area, it enhances their activity [1]. The main drawback and need for more catalyst research is the incredibly high cost and low abundance of Pt-group elements.

The goal of this research has been to create a catalyst that is stable in alkaline media, have high surface area, is easy to synthesize, can be produced at lower cost and still have high bifunctional electrocatalytic activity. The materials of choice was a spinel-based unsupported CuCo₂O₄catalysts. The Sacrificial Support Method (SSM) was one of four synthetic approaches [2-6].

Experimental

Synthesis.       Preparation of CuCo₂O₄catalyst by SSM method

High surface area silica (Cab-O-Sil™ EH-5, surface area: ~400 m² g^-1) was dispersed in water on the ultrasonic bath. Then, the calculated amounts of Cu(NO₃)₂*xH₂O and Co(NO₃)₂*6H₂O (Sigma-Aldrich) were added to the silica colloidal suspension. Total loading of metals on silica was calculated to be 23wt%. Silica and precursors mixture was allowed to dry in an oven overnight at T=85ºC. The dry mixture of silica and nitrates was calcined in air at T=550ºC for 3h. The silica support was etched by means of 7M KOH overnight. The obtained wet powder was washed with DI water until a neutral pH was achieved. After drying at T=85ºC, the powder was used for physico-chemical and electrochemical characterizations

.

Characterization. The catalysts were characterized by BET, SEM, TEM and XRD methods. Catalytic activity for ORR and OER were measured in alkaline media using the RRDE method [7]. The counter electrode used was a Pt wire and the reference electrode was a Hg/HgO reference electrode corresponding to an RHE of 0.926 at 25°C. The inks for the RDE experiments were prepared by mixing the catalyst powder with an optimized amount of the ionomer Nafion™ in an IPA/H₂O solution. The Nafion™ is used as a binder for the catalyst as well as a proton pathway in the ORR and OER. Homogeneity of the inks was achieved by means of sonication using an ultrasound probe.

Results and Discussion

As it can be seen from the XRD images in Figure 1the spinel phase pure structure was formed only in case on usage of SSM.

Figure 1. XRD data on CuCo₂O₄electrocatalysts.

Further in this research we have correlated amount of CuO with decrease of activity in both ORR and OER. DFT calculations confirmed our hypothesis.

Conclusion

Synthesis of spinel-based catalysts by SSM was investigated. Upon examination of the physical characteristics, it can be assumed that further modification is necessary to design of the electrocatalysts. The electrochemical characteristics from the RDE experimentation proved that the catalysts are bifunctionally active and can act as a replacement for conventional Pt-group catalysts in the appropriate conditions.

References

(1)     Antolini, E. Appl. Catal., B. 2009, 88, 1

(2)     N. I. Andersen, A. Serov, P. Atanassov, Appl. Cat. B  (2014), DOI: 10.1016/j.apcatb.2014.08.033

(3)     A. Serov, M. Padilla, A. J. Roy, P. Atanassov, T. Sakamoto, K. Asazawa, H. Tanaka ", Angewandte Chemie Int. Ed. (2014), DOI: 10.1002/anie.201404734

(4)     Ulises Martinez, A. Serov, Monica Padilla Plamen Atanassov, ChemSusChem, (2014), DOI: 10.1002/cssc.201402062

(5)     A. Serov, K. Artyushkova, P. Atanassov, , Adv. Energy Mater., 4: 1301735 (2014). doi: 10.1002/aenm.201301735.

(6)     U. Tylus, Q. Jia, K. Strickland, N. Ramaswamy, A. Serov, P. Atanassov, S. Mukerjee, J. Phys. Chem. C, 118 (17) (2014) pp 8999–9008.

(7)     Gasteiger, H.A.; Kocha, S.S.; Sompalli, B.; Wagner, F.T. Appl. Catal., B. 2005, 56, 9

Figure 1

The growth and/or deposition of uniformly dispersed nanoclusters is a significant challenge for the field of electrocatalysis science. Here we employ ALD technique to obtain ultrafine nickel oxide nanoclusters uniformly dispersed on flat electrodes. Atomic layer deposition (ALD) is a versatile technique to the precise growth of a variety of materials. Specifically, the key point in ALD process is that precursor molecules evaporated at an appropriate temperature react only with chemically reactive surface sites (e.g. hydroxyl groups on surfaces), and these reactions are self-limiting, i.e. controllable at the atomic level. Thus, we utilize a self-limiting nature of ALD for nanocluster formation. Meso-tetra(4-carboxyphenyl)porphyrin (TCPP) monolayer is adsorbed parallel to the electrode surface, and subsequently metallated with Mn (or Fe)-OH (denoted by MnTCPP or FeTCPP) as reactive sites for nickel oxide growth. The nickel oxide nanoclusters, then, are grown on the -OH functional groups of MnTCPPs (or FeTCPPs) via ALD. The growth of nickel oxide nanoclusters is demonstrated in detail by transmission electron microscopy images and grazing incidence small angle X-ray scattering analysis, as well as by in-situ monitoring mass gain during deposition. Additionally, to evaluate the electrocatalytic water oxidation activity of nanoclusters, TCPP molecules are removed from nanocluster films through post-ozone-treatments at a relatively low temperature. Based on electrochemical analysis, 1-2 nm sized-nanocluster films show a significantly higher water oxidation activity (i.e. higher water oxidation currents for unit number of nickel atoms deposited on films) compared with that of a very thin film. Consequently, this shows that the uniformly dispersed nanoclusters should be very useful for many applications including electrocatalysis, catalysis, and supercapacitors.

Water splitting by electrolysis is an effective and renewable way to produce hydrogen fuel from acid or alkaline electrolytes.  Presently, alkaline water electrolyzers are being the most widely employed, largely due to the use of low-cost catalysts.  Yet, there is strong interest for expanded use of acid water electrolyzers in future because of better device performance than the alkaline.  In this work, we explore and develop new fabrication techniques that could minimize the use of the Pt catalyst that is highly active, but too expensive to be used for hydrogen evolution reaction (HER) in acid water electrolyzers.  Of our particular interest is to grow Pt monolayer films and nanostructures on tungsten carbide (WC) supports by electrodeposition. 

Fig. 1 shows Pt deposition as a function of applied potential from a NaCl-supporting electrolyte.  An interesting finding is that the deposition of Pt is prevented at potentials below -0.5 V, in spite of high driving forces for Pt clusters to form and grow.  This talk will present ways in which this anomalous deposition behavior can be exploited for preparing novel Pt catalysts. A brief summary will be given for Pt deposition on Au [[1]] and Ni [[2]] electrodes where a monolayer-thick Pt film is deposited at -0.8 V but additional growth is quenched.  For WC substrates, we will discuss challenges associated with the lack of Pt depositing at -0.80 V, before introducing to a new approach that is developed to facilitate the formation of Pt nuclei.   

The proposed method involves cycling the potential between -0.8 V and -0.45 V at a fast rate of 50 mV/s.  As shown in Fig. 2, Pt nanoclusters with high particle density could be grown on WC, and their average size is about 5 nm. To gain mechanistic insights on Pt nanostructure nucleation and growth, the deposits were analyzed using scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and underpotentially deposited (UPD) hydrogen and copper.  Correlations between deposition potential cycle and particle growth and particle density will be discussed.  

Finally, Pt deposits were examined as catalysts for HER in 0.5 M sulfuric acid.  The dependence of HER activity on the number of deposition cycles is shown in Fig. 3 where the HER activity approaches that of bulk platinum as the deposition cycles increase even though the expected total amount of platinum is far less than a complete coverage. This work illustrates that electrodeposition provides a simple and scalable way for preparing Pt nanocluster/WC catalysts, which exhibit high activity and durability toward HER in acid media.    

[1] Y. Liu, D. Gokcen, U. Bertocci, T. P. Moffat, Science, 338, 1327 (2012).

[2] Y. Liu, C. M. Hangarter, D. Garcia, T. P. Moffat, Surf. Sci. (2014)  dx.doi.org/10.1016/j.susc.2014.06.002

Figure 1

Advanced catalysts with element-abundance, economic-cost and stability are desired for electrochemical hydrogen evolution reaction (HER) to scale-up the promising clean energy of hydrogen.[1] The discoveries in the (de)hydrogenation of petroleum chemicals have inspired several efficient and economic catalysts of metal non-oxides towards HER, such as Mo-based sulfides and carbides.[2] MoS₂ possesses a layered structure exposing Mo- and S- edges, which are highly active for HER due to the moderate hydrogen bonding energy (ΔG_H*).[3] However, its performance is still limited by the poor conductivity and the uncontrolled properties of active-sites.

In our recent work, efforts have been made to synergically enhance the conductivity and active-site abundance of MoS₂, achieving the high current densities and low onset overpotentials for HER.[4-6] For example, we proposed a new route to confine MoS₂ growth within an in-situ formed polysaccharide matrix from glucose condensation during hydrothermal processes, and after the following carbonization, MoS₂/C nanocomposites evenly integrating ultrathin MoS₂ nanosheets (2 ~ 4 nm) with conducting carbon were successfully harvested (Fig. 1a).[6] The MoS₂/C exhibited an excellent HER activity characterized by higher current densities and lower onset overpotentials than the conventional MoS₂. In particular, the MoS₂/C with a suitable MoS₂ content of 14.8% showed a small onset overpotential of ~80 mV, a high current density of 88 mA cm^-2 at η = 200 mV, which are ascribed to the abundant rim-sites on ultrathin MoS₂ nanosheets and the improved conductivity by carbon. On the other hand, a facile microwave-assisted hydrothermal method was further introduced to fabricated active-site enriched MoS₂ nanosheets employing reactant self-shelter, in which the excessive S source (e.g., thiourea) would cover the highly-active Mo-sites during MoS₂ formation, and then be removed by following treatment with H₂SO₄, resulting in abundant active sites on MoS₂. The electrocatalytic test well-confirmed the significantly improved active-sites (~3 orders) and conductivity as compared with the traditionally prepared MoS₂. Thanks to the facile synthesis and outstanding electrochemical behaviors, our effort is expected to pave the way for earth-abundant, economic and efficient electrocatalysts used in energy conversion and storage.

References:

[1] A. B. Laursen, S. Kegnaes, S. Dahl, I. Chorkendorff, Energy Environ. Sci., 2012, 5, 5577.

[2] C. G. Morales-Guio, L. Stern and X. L. Hu, Chem. Soc. Rev., 2014, 43, 6555.

[3] T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science, 2007 317, 100-102.

[4] L. C. Yang, S. N. Wang, J. J. Mao, J. W. Deng, Q. S. Gao, Y. Tang, O. G. Schmidt , Adv. Mater., 2013, 25, 1180.

[5] Q. S. Gao, N. Liu, S. N. Wang, Y. Tang, Nanoscale, 2014, 6, 14106.

[6] N. Liu, L. C. Yang, S. N. Wang, Z. W. Zhong, S. N. He, X. Y. Yang, Q. S. Gao, Y. Tang, J. Power Source, 2015, 275, 588.

Figure 1

Direct electrochemical splitting of water is an attractive method to generate the renewable energy carrier hydrogen [1].  Low-cost catalyst for hydrogen evolution reaction (HER) is one key step in this process [2].  So far, there are no other pure metals except for noble elements really showing high activities for HER, and many efforts have been made to look for highly catalytic activated transition metal based alloys for HER [3,4].

In this work, HER was investigated on Ni-Co alloy electrode and Ni-Co-Sm₂O₃ composite electrode in 0.50 M Na₂SO₄ + 0.10 M H₂SO₄ solution.  It was indicated that Sm₂O₃ particles promoted the formation of an amorphous structure of Ni-Co alloy coating, which was benefit for HER.  The surface of Ni-Co-Sm₂O₃ composites was more coarse and grainy compared with that of Ni-Co alloy layer (Fig. 1).  Steady-state polarization curves (Fig. 2) showed that the hydrogen evolution potential of Ni-Co-Sm₂O₃ composite electrode positively moved by about 240 mV at 8.0 mA cm^-2 compared with that of Ni-Co alloy electrode, indicating that the embedded Sm₂O₃ particles enhanced the electrocatalytic activity of Ni-Co coating for HER.  Furthermore, Ni-Co-Sm₂O₃ composite electrode has considerably lower charge transfer resistance and higher exchange current density.

References

[1] C. Lupi, A. Dell'Era, M. Pasquali. In situ activation with Mo of Ni-Co alloys for hydrogen evolution reaction. International Journal of Hydrogen Energy, 2014, 39: 1932-1940

[2] Wei Cui, Chenjiao Ge, Zhicai Xing, Abdullah M. Asiri, Xuping Sun. Ni_xS_y-MoS₂ hybrid microspheres: One-pot hydrothermal  synthesis and  their application as a  novel hydrogen evolution reaction electrocatalyst  with enhanced activity. Electrochimica Acta, 2014, 137: 504–510

[3] Zhengwei Xie, Ping He, Licheng Du, Faqin Dong, Ke Dai, Tinghong Zhang. Comparison of four nickel-based electrodes for hydrogen evolution reaction. Electrochimica Acta, 2013, 88: 390-394

[4] Ping He, Xiaofang Yi, Yongjun Ma, Wei Wang, Faqin Dong, Licheng Du, Hongtao Liu. Effect of Gd₂O₃ on the hydrogen evolution property of nickel-cobalt coatings electrodeposited on titanium substrate, Journal of Physics and Chemistry of Solids, 2011, 72: 1261-1264

Figure 1

In a recent publication Pt deposition at negative potentials revealed an unanticipated self-terminating characteristic that enables controlled deposition of Pt monolayer films from a K₂PtCl₄-NaCl electrolyte. Herein we explore the generality of such reactions by examining a variety of other transition metal systems. In the case of Pt, the deposition reaction is quenched at potentials just negative of proton reduction by alteration of the double layer structure and adsorption properties induced by a saturated surface coverage of underpotential deposited hydrogen, (H_upd). The surface may be reactivated for Pt deposition by stepping the potential to more positive values where H_upd is oxidized and fresh sites for adsorption of PtCl₄^2- become available. Periodic pulsing of the potential enables sequential deposition of additional Pt layers to fabricate films of desired thickness relevant to a range of advanced technologies in manner that is tantamount to wet atomic layer deposition (ALD).  This talk will present an update on the exploration and use of self-terminating electrodeposition reactions to form electrocatalysts for use in water electrolysis and related reactions.

We have recently introduced corrole metal complexes (metallocorroles) as catalysts for various reactions, including those that are of prime importance for water splitting into its elements- the hydrogen and oxygen evolution reactions. A careful choice of substituents on the corrole ligand and the kind of metal ion chelated by them allows for lowering of the electrocatalytic overpotential and increasing reactivity.

 An arsenal of electrochemical and spectroscopic methodologies, which in the case of metallocorroles is very rich, was applied for acquiring highly valuable information about the mechanisms of action (see upper part of the Figure for the hydrogen evolution reaction). Preliminary results regarding water oxidation with the aid of immobilized catalysts (see lower part of the Figure) will be presented as well.

 REFERENCES

 "Cobalt Corrole Catalyst for Efficient Hydrogen Evolution Reaction from H₂O under Ambient Conditions: Reactivity, Spectroscopy and DFT Calculations", Inorg. Chem.2013, 52, 3381-3387.

"The Cobalt Corrole Catalyzed Hydrogen Evolution Reaction: Surprising Electronic Effects and Characterization of Key Reaction Intermediates" Chem. Commun. 2014, 50, 2725-2727.

Figure 1

Over the last decades, the ABO₃ perovskites have been extensively studied as active catalyst materials for various low and high temperature redox reactions; e.g. hydrogen or oxygen evolution reactions and internal and biomass reforming. However, the mechanisms of their catalytic/electrochemical activity and stability vs. A- and B-site doping especially in aqueous solutions have not been clarified yet. Furthermore, it is not clear how the oxygen deficiency created in doped perovskites (e.g. AA'BB'O_3-δ) by partial substitution of less valent cations on A- and/or B-sites contributes to their oxygen exchange redox behavior at ambient conditions. In this regard, a comparative study of the PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ catalytic behavior was performed considering possible applications related to electrochemical and thermochemical hydrogen/oxygen evolution

PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ, where x= 0.1, 0.5, 0.9, were synthesized using a modified nitrate-glycine Pechini method (1) followed by a heat-treatment in air at 700^oC, 900°C and 1200°C. X-ray diffraction patterns reveal that the formation and chemical stability of the perovskite phase depends on the nature of the lanthanide element (A), relative composition of Ni to Co, and the heat-treatment temperature. For example, praseodymium based perovskites are more stable at higher heat-treatment temperatures (1200^oC) than SmNi_xCo_1-xO_3-δ. Furthermore, formation of the perovskite-phase (Fig. 1) is favorable at low Ni/Co ratios (x=0.1and 0.5) resulting in rhombohedral single-phase crystal structure. On contrary, high Ni/Co ratio (x=0.9) do not yield the perovskites phase.

The SEM and BET single point specific surface area (SSA) approaches were applied to determine the materials morphology. The HER and OER electrochemical performance was evaluated from cyclic voltammetry. A combination of Temperature Programmed Reduction/Oxidation (TPRO) and Thermo-Gravimetric (TG) analysis was further employed to understand their catalytic activity towards thermochemical hydrogen evolution.

In order to improve the electronic conductivity, the perovskite-graphene composites were synthesized by mixing the perovskites with graphene platelets (90 wt. %). The HER and OER electrochemical performance of PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ was studied in three-electrode configuration in alkaline medium. The structure-dependent electrochemical behavior of these nanocomposites as a function of nickel to cobalt molar ratio (x= 0.1, 0.5) and the heat-treatment temperature will be discussed in terms of Tafel plots, chemical stability, and potential applications in comparison to the state-of-the-art catalysts.

Acknoledgements: The authors gratefully acknowledge the financial support from the ACS Petroleum Research Fund (Project number 53614-UR10) and the NSF EPSCoR support (Awards IIA-1330840 and IIA-1330842).

References: 1.         M. C. Schrandt, P. Kolla and A. Smirnova, MRS Online Proceedings Library, 1542 (2013).

Figure 1

In the present study, on the basis of detailed density functional theory (DFT) calculations, and using Ni hydroxy(oxide) films on Pt(111) and Au(111) electrodes as model systems, we describe a detailed structural and electrocatalytic analysis of hydrogen evolution (HER) at three-phase boundaries under alkaline electrochemical conditions. We demonstrate that the structure and oxidation state of the films can be systematically tuned by changing the applied electrode potential and/or the nature of substrates. Structural features determined from the theoretical calculations provide a wealth of information that is inaccessible by purely experimental means, and these structures, in turn, strongly suggest that a bifunctional reaction mechanism for alkaline HER will be operative at the interface between the films, the metal substrates, and the surrounding aqueous medium. This bifunctionality produces important changes in the calculated barriers of key elementary reaction steps, including water activation and dissociation, as compared to traditional monofunctional Pt surfaces.

The successful identification of the structures of thin metal films and three-phase boundary catalysts is not only an important step towards accurate identification and prediction of a variety of oxide/electrode interfacial structure-properties relationships, but also provides the foundation for rational design and control of 'targeted active phases' at catalytic interfaces. The successful design of bifunctional electrocatalysts that exploit these structures, in turn, could ultimately lead to advances in the development of alkaline fuel cells.

References

¹Surnev, S.; Fortunelli, A.; Netzer, F. P. Chem Rev 2012, 113, 4314.

²Shaikhutdinov, S.; Freund, H.-J. Annu. Rev. Phys. Chem. 2012, 63, 619.

³Subbaraman, R.; Stamenkovic, V.; Markovic, N. et al, Science 2011, 334, 1256.

⁴Subbaraman, R.; Greeley, J.; Stamenkovic, V.; Markovic, N. et al, Nat Mater2012, 11, 550

⁵Strmcnik, D; Stamenkovic, V. R.; Markovic, N. et al, Nat Chem 2013, 5, 300.

In this study, we demonstrated photoelectrochemical(PEC) hydrogen generation using InGaN-based semiconductors as the working electrode for water splitting under solar illumination. For a working electrode made of n-type semiconductor, the holes drive the oxidation of water on the photoelectrode, and the electrons transport within semiconductor and in turn move to the cathode electrode leading to the reduction reaction. Although GaN is potentially resistant to aqueous solution and its energy band is suitable for water photoeletrolysis, GaN only absorbs UV light which is about 5% of the solar spectrum. The bandgap energy of the In_xGa_1-xN can be tuned from 0.7eV to 3.4eV by changing the Indium content to fit the most of the terrestrial solar spectrum. Therefore, InGaN-based working electrodes for water splitting are expected to be more efficient than the GaN working electrodes.

　　The working electrodes used in this study featuring meshed metal contacts with SiO₂ protection layer were immersed in electrolyte to enhance the collection efficiency of photogenerated carriers. On the other hand, the generation rate of hydrogen could be improved by applying external bias onto the working electrodes to enhance the separation of photogenerated electron-hole pairs in the semiconductor and the charge transfer at the interface between semiconductor and electrolyte during the PEC water splitting process. However, the external bias on PEC water splitting requires extra input power except light illumination. In this study, InGaN-based semiconductors associated with meshed metal contacts were served as working electrodes without external bias to conduct the PEC water splitting process under the light illumination.

　　Instead of using an external bias provided by power supply, a solar cell was used to raise the driving force to increase the rate of hydrogen production. The solar cell was connected in series between the InGaN-based working electrode and the platinum (Pt) counter electrode to bias the PEC cell. In principle, the water splitting for generating hydrogen involves two mechanisms during the light illumination if the solar cell supplies bias voltage larger than 1.23 V. The mechanisms are photoelectrolysis and electrolysis of water splitting. To solely evaluate the efficiency of photoelectrolysis or electrolysis of water splitting, the output voltage of solar cell was varied through changing the type of solar cell to allow the PEC cell with bias less or larger than 1.23 V. The preliminary results indicated that the hybrid working electrodes exhibited a marked improvement in the efficiency of hydrogen generation.

　　The detailed results will be presented in the forthcoming conference.

Figure 1

In the last two decades, the development of nanoscience and technology developed several nanostructures at different forms, such as nanoparticles, nanowires, nanotubes and nanoribbons. These materials offer a high potential for improving properties and can be used in many fields such as microelectronics, batteries, photovoltaic and photocatalytic devices. Vanadium oxides belong to the important class of 3d transition metals, with various electric, magnetic and structural properties, making these materials attractive for many industrial applications. This study aimed to obtain and characterize vanadium oxide films on titanium from ammonium metavanadate and then modify it by impregnation with Bi (III). The vanadium pentoxide was synthesized on Ti substrates by painting the surface with a suspension of 250 mg ammonium metavanadate in 5 ml of polyethylene glycol (PEG-300). The coated Ti plate was then heated at 400, 500 or 600 °C for one hour. Subsequently, the film obtained was impregnated with Bi (III) for 120 minutes under constant stirring, using 50 mL of a solution 10 mmol L^-1 of bismuth nitrate at nitric acid. Then the samples were again annealed for one hour at the same temperature. The films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Visible spectroscopy and photoelectrochemical. From SEM analysis was possible to observe, after impregnation of Bi(III), the growing of long crystals on V₂O₅ films at 400 and 500 °C, while for the sample to 600 °C a globular structure was preferentially deposited on the tip of V₂O₅ crystals. Through EDS analysis it was observed that the sample obtained at 500 °C is that has the highest amount of Bi in the composition. The mapping by EDS showed that the elements were distributed over all surface. From XRD analysis the presence of V₂O₅ orthorhombic phase and monoclinic BiVO₄ was detected. Band-gap calculated for the samples before and after impregnation also proved that V₂O₅ was converted to BiVO₄, since the value before impregnation was 2.20 eV and after impregnation showed a value of 2.42 eV. The photocurrent value obtained for nano-V₂O₅ films impregnated with Bi³⁺ at 500 °C was 5 times higher than pure BiVO₄ films. The results showed that it has been possible obtain a nanostructured V₂O₅ / BiVO₄film on Ti substrate which is photoactive and could be used both in photoelectrocatalytic water as oxidation of organic molecules.

Acknowledgements: FAPESP 2012/23422-5 and CAPES.

It is important to develop alternative catalysts made of inexpensive and earth-abundant elements with high catalytic activity and stability to replace the noble metal catalysts in various environmental¹ and renewable energy applications.² Here we report earth-abundant metal pyrites MS₂ (M as Fe, Ni, and Co) as high performance electrocatalysts^3,4 for oxygen reduction reaction (ORR) that is selective for generating hydrogen peroxide. In neutral solution of 0.1 M sodium perchlorate and acidic solution of 0.1 M perchloric acid, all three MS₂ show high ORR catalytic activity with excellent Tafel slopes. Based on Koutecky-Levich (K-L) analysis, all MS₂ and noble metal show similar potential-dependent selectivity of the two-electron peroxide pathway in neutral condition, and CoS₂ is the best among metal pyrites in terms of the positive potential for 50% H₂O₂selectivity. The stability of metal pyrites is quite good in under ORR operation in both neutral and acidic solutions. These inexpensive metal pyrites are excellent ORR catalysts for selective peroxide generation and could find promising applications in environmental and renewable energy technologies.

1. X.-W. Liu; W.-W. Li; H.-Q. Yu; "Cathodic catalysts in bioelectrochemical systems for energy recovery from wastewater" Chem. Soc. Rev., 2014, 43, 7718-7745.

2. Matthew S. F.; Song Jin;  "Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications" Energy Environ. Sci., 2014, 7, 3519-3542.

3. Matthew S. F.; Mark A. L.; Qi Ding; Nicolas S. K.; Song Jin; "Earth-Abundant Metal Pyrites (FeS₂, CoS₂, NiS₂, and Their Alloys) for Highly Efficient Hydrogen Evolution and Polysulfide Reduction" J. Phys. Chem. C, 2014, 118 (37), 21347-21356.

4. Matthew S. F.; Rafal D.; Mark A. L.; Nicholas S. K.; Qi Ding; Song Jin; "High-Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS₂) Micro- and Nanostructures" J. Am. Chem. Soc., 2014, 136 (28), 10053-100061.

Figure 1

Metal sulfide-based nanostructured materials have emerged as promising catalysts for hydrogen evolution reaction (HER), and significant progress has been achieved in enhancing their activity and durability for the HER. The understanding of nanoscale size-dependent catalytic activities can suggest critical information regarding catalytic reactivity, providing the scientific basis for the design of advanced catalysts. However, nanoscale size effects in metal sulfide-based HER catalysts have not yet been established fully, due to the synthetic difficulty in precisely size-controlled metal sulfide nanoparticles. In this work, we prepared molybdenum sulfide (MoS₂) nanoparticles with monolayer precision from one to four layers with the nearly constant basal plane size of 5 nm, and explored their size-dependent catalytic activity in the HER. Using density functional theory (DFT) calculations, we identified the most favorable single-, double-, and triple-layer MoS₂ model structures for the HER, and calculated elementary step energetics of the HER over these three model structures. Combining HER activity measurements and the DFT calculation results, we establish that the turnover frequency of MoS₂ nanoparticles in the HER increases in a quasi-linear manner with decreased layer numbers. Cobalt-promoted MoS₂ nanoparticles also exhibited similar HER activity trend. We attribute the higher HER activity of smaller metal sulfide nanoparticles to the higher degree of oxidation, higher sulfur coordination number, formation of the 1T phase, and lower activation energy at the rate-determining step. This insight into the nanoscale size-dependent HER activity trend will facilitate the design of advanced HER catalysts as well as other hydrotreating catalysts.

In a previous work, we demonstrated the performance of MoS2 synthesized on Reduced Graphene Oxide as a electrocatalyst to hydrogen evolution reaction (HER) and the role of graphene to avoid 3D growth of MoS₂.¹ The present work shows the improved performance as a electrocatalyst to HER of Molybdenum Disulfide (MoS₂) synthesized on Mechanical Exfoliated Graphene (MoS₂/GRA) instead of chemically Exfoliated Graphene. By X-ray Photoelectron Spectroscopy of the composites, we detected that the first layers of MoS₂ are bonded to oxygen of the RGO by chemically covalent bond (Mo-O-C) and absence of this bands to MoS₂ when synthesized on mechanical exfoliated graphene, as illustrated in figure 1a and 1b. The structure and morphology were characterized by Scanning Electron Microscopy and Transmission Electron Microscopy. The electrochemical characterizations shows that overpotential was significantly reduced when mechanically exfoliated graphene was used in the synthesis of composite (MoS₂/GRA), (see Figure 1c). A synergetic effect was observed, once pure materials were employed a poor performance was reported. Moreover, the interface structure is a key parameter on the acting of composite, taking account that the simple mix of both, chemically exfoliated graphene with MoS₂ and mechanically exfoliated Graphene with MoS₂, exhibited different on set values.

Figure 1

An ionic liquid (IL)-driven, facile, scalable route to new carbon nanostructures comprising pure carbon nanotube cores and heteroatom-doped carbon sheath layers (CNT/HDC) has been developed. The design of the CNT/HDC nanocomposites allows for combining electrical conductivity derived from the CNTs with the catalytic activity of the heteroatom-containing HDC sheath layers. The CNT/HDC nanostructures showed excellent electrocatalytic activity for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in an alkaline medium. The ORR activity of CNT/HDC nanostructures is one of the best performances among the heteroatom-doped nanocarbon catalysts in terms of half-wave potential and kinetic current density. The kinetic parameters of the CNT/HDC nanostructures, including 4-electron transfer selectivity and exchange current density, compared favorably with those of a Pt/C catalyst. The CNT/HDC nanostructures also exhibited superior long-term durability and poison-tolerance relative to Pt/C. Bifunctional oxygen electrocatalytic activity of CNT/HDC was comparable to that of Ir/C and better than that of Pt/C. In addition, the CNT/HDC nanostructures showed high current and power densities when employed as a cathode catalyst in alkaline fuel cell, which sheds light on their practical applicability.

The electrochemical splitting of water offers an attractive way to provide a carbon-free source of hydrogen, however the efficiency is limited primarily by the overpotential of the anodic oxygen evolution reaction (OER, 4OH^-→ 2H₂O + 4e^- + O₂).¹

Knowledge of oxide formation prior and during the OER is essential for understanding the mechanism and electrocatalysts. Extensive studies on the composition and morphology of metal and oxide electrodes in the OER by Raman,² XAS,³ and QCM⁴have been reported. However, there is no reported technique to directly interrogate the oxide film during potential cycling or its evolution over time and multiple electrochemical cycles.            

In this work, in situ electrochemical stress measurements are used to interrogate changes in oxide structure before and during the oxygen evolution reaction (OER) from Ir, Ni, Co, Au, and Pt electrodes in alkaline electrolyte. Stress evolution during potential cycling reports on changes in oxidation state and oxide forms. Hysteresis observed in the potential - dependent stress from Ir, Au, and Pt electrodes is associated with chemical irreversibility in electrode composition and roughness. Alternatively, Ni and Co exhibit reversible conversion between hydroxide and oxyhydroxide forms during cycling. From the experimentally determined stress, charge passed during electrode oxidation, and Young's modulus, the change in strain exhibited by Ni and Co electrodes during hydroxide-oxyhydroxide conversion is calculated to be 7.0% and 8.4%, respectively. We also show that the magnitude of change in stress is proportional to the amount of material that is further oxidized. The similarity between processes yielding higher oxides and those involved with the OER mechanism yields are a rough correlation between film thickness and OER onset.⁵

Finally, we report the effect of electrodeposition additives to modify electrodeposited OER-active films to achieve high electrocatalyst efficiency for this reaction.

References

(1) Matsumoto, Y.; Sato, E. Materials Chemistry and Physics1986, 14, 397-426.

(2) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J Electrochem Soc1988, 135, 885-892.

(3) Totir, D.; Mo, Y.; Kim, S.; Antonio, M. R.; Scherson, D. A. J Electrochem Soc2000, 147, 4594-4597.

(4) Mo, Y.; Hwang, E.; Scherson, D. A. J Electrochem Soc1996, 143, 37-43.

(5) Hoang, T. T. H.; Cohen, Y.; Gewirth, A. A. Analytical Chemistry2014, 86, 11290-11297.