Evaluating Oxygen Evolution Activity in Molybdenum-Derived Nitrides, Sulfides, and Oxides

INTRODUCTIONIn the future, hydrogen production will be performed by direct electrolysis of seawater. However, every time hydrogen is produced, it is necessary to avoid generating chlorine because it has a negative impact on the ecosystem. We have been developing an oxygen evolution anode for seawater electrolysis that produces only oxygen without chlorine. It has been found that anodically deposited g-MnO2-type Mn1-xMoxSnyO2+xanodes showed the oxygen evolution efficiency for more than 4000 hours in the electrolysis of 0.5 M NaCl [1]. The anodic deposition method has required many steps and further cleaning of the electrolytic diaphragm. For these reasons, in order to commercialize oxygen evolution anodes for seawater electrolysis, it is essential to establish a simple manufacturing method. We are considering fabricating Mn oxide as an active material on an IrO2 intermediate layer formed on a titanium substrate using calcination method.In this study, Mn oxide electrodes were fabricated by coating various concentrations of manganese nitrate butanol solutions on the IrO2intermediate layer and at various calcination temperatures. We have investigated the crystal structure and Mn oxide weight that would have the highest activity for oxygen evolution efficiency.EXPERIMENTALTitanium metal substrates were immersed to roughen enhancing the anchor effect of the substrate on the electrocatalyst layer in concentrated H2SO4. IrO2 intermediate layer was formed by coating on one side of the titanium substrate in 0.26 M Chloroiridic acid with a brush, dried at 363 K for 10 min in air. The other side was coated in the same procedure and dried. And then these substrates was calcined at 723 K for 10 min in air. This procedure was repeated three times but the final calcination of the specimen was continued for 60 min at 723 K in air for calcination. Mn oxide electrocatalyst for oxygen evolution was formed by coating on the IrO2 / Ti in 0.073～0.52 M Mn(NO3)2 butanolsolution with a brush and drying at 353 K for 90 min in the same way as when forming the IrO2 intermediate layer. Calcination was performed at 473, 573, and 723 K. Impurities formed without becoming manganese oxides were electrolytically cleaned in 0.5 M NaCl until no dissolution of the impurities was observed.The performance of the electrode was examined by electrolysis of 0.5 M NaCl solution at 1000Am-2. The oxygen evolution efficiency was estimated by the difference between the total charge passed during electrolysis and the formation charge of chlorine analyzed by iodometric titration. Polarization curves were measured galvanostatically. Correction for IR drop was made with the electrochemical impedance spectroscopy (EIS) method.The characterization of the electrode was carried out by XRD and EPMA.RESULTS AND DISCUSSIONXRD diffraction clarified that Mn₂O₃ were consisted at 723 K, Ramsdellite-type MnO₂ at 673 K, and IrO2 at 573 K.Figure 1 shows the relationship between Mn oxide weight and oxygen evolution efficiency. The oxygen evolution efficiency of the anode consisted of only IrO2 was about 8%. It found that the oxygen evolution efficiency was different due to the difference in calcination temperature, that is, the difference in crystal structure to Mn oxide weight of about 2.5 mgcm-2. The oxygen evolution efficiency of the anode with Ramsdellite-type MnO₂ formed at 573 K was about 57 % at 1.8 mgcm-2. The oxygen evolution efficiency of anodes formed at 473 and 723 K with the equivalent weight was about 52 and 32 % respectively. Previous research has shown that anodes containing the γ-MnO2 formed by anodic deposition have high catalytic activity for oxygen evolution [2]. The γ-MnO2 contains stacking faults in which β-MnO2 is mixed into the ramsdellite-MnO2[3]. The electrode with 4.5 mgcm-2 of Mn₂O₃ at 723 K shows the maximum oxygen generation efficiency, which is about 83%.A future challenge is to add substances such as molybdenum as additional elements to obtain higher oxygen evolution efficiency. In conventional electrode fabrication using anodic deposition, the addition of molybdenum significantly has improved oxygen generation efficiency.REFERENSE[1]Z. Kato, J. Bhattarai , N. Kumagai, K. Izumiya, and K. Hashimoto, Applied. Surface. Science. 257 (2011) 8230.[2] A. A. El-Moneim, N. Kumagai, K. Asami, and K. Hashimoto , Materials Transactions, Vol. 46, No. 2 (2005) pp. 309[3] Jian-Bao LI, K. Koumoto and H. Yanagida, Journal of the Ceramic Society of Japan, 96 [1] (1988)74 Figure 1

Although electrochemical water splitting currently represents a small amount of the US hydrogen supply, its role is expected to grow with the emergence of a hydrogen-based infrastructure.[1] Electrolysis is reasonably cost-competitive with a variety of technologies, including batteries (solid state and flow), pumped hydro, and compressed air.[2] When coupled with renewable energy sources, such as wind and solar, electrochemical water splitting can produce hydrogen using a nearly carbon free pathway.[3] The commercial progress of electrolyzers is limited in part by the cost of the catalyst layer. The oxygen evolution reaction is the focus of electrocatalyst development due to slow kinetics relative to hydrogen evolution; iridium nanoparticles are commonly used to catalyse oxygen evolution, with platinum added to ensure conductivity throughout the catalyst layer. Extended surface nanostructures have previously been developed as electrocatalysts in hydrogen fuel cells, where they have produced an order of magnitude higher specific activity than nanoparticles in oxygen reduction and demonstrated a potential benefit in accelerated stress tests.[4] Extended nanostructures have recently been developed for the oxygen evolution reaction, where a similar specific activity improvement was observed in rotating disk electrode (RDE) half-cells. Catalysts formed by spontaneous galvanic displacement are ideally situated, being able to take advantage of the specific activities generally associated with the catalyst type, while significantly improving upon the traditionally low surface areas of extended surfaces. This approach has been used in forming iridium-nickel and iridium-cobalt nanowires.[5] While nanoparticles have modest site-specific activity in oxygen reduction and evolution, nanoparticle evolution catalysts are at a particular disadvantage with regards to surface area, since carbon supports cannot be used at high potentials. Iridium-cobalt and iridium-nickel nanowires have been synthesized with surface areas of 70 m2 gIr ‒ 1, more than double the nanoparticles. In RDE half-cells, the extended nanostructures produce an oxygen evolution mass activity 7 to 8 times greater throughout the kinetic region. Accelerated stress tests were completed by potential holds and cycling over a variety of conditions to study catalyst degradation and establish standardized testing protocols. Acid treatment has been used to remove excess nickel and cobalt template, improving catalyst durability. Following accelerated stress tests in RDE half-cells, iridium-cobalt and iridium-nickel nanowires exceed the mass activity of nanoparticles by more than an order of magnitude. Membrane electrode assembly (MEA) fabrication of these materials has produced electrolyzers with higher performance than the benchmark device. Recent improvements in MEA performance suggest that the promise of extended surface catalysts observed in half-cells can be realized in the application. [1] R. Forgie, G. Bugosh, K.C. Neyerlin, Z. Liu, P. Strasser, Electrochemical and Solid-State Letters, 13 (2010) B36-B39. [2] K. Harrison, M. Peters, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review13/pd031_harrison_2013_o.pdf, 2013. [3] K. Harrison, M. Peters, C. Ainscough, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/progress13/ii_a_2_harrison_2013.pdf, 2013. [4] S.M. Alia, Y.S. Yan, B.S. Pivovar, Catalysis Science & Technology, 4 (2014) 3589-3600. [5] H. Xu, High-Performance, Long-Lifetime Catalysts for Proton Exchange Membrane Electrolysis, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review14/pd103_xu_2014_o.pdf, 2014. Figure 1. Surface areas (x-axis) and site-specific oxygen evolution activities (y-axis) of iridium-cobalt nanowires, iridium-nickel nanoparticles, and iridium nanoparticles prior to (red, blue, black) and following (dark red, dark blue, grey) durability testing in RDE half cells. Arbitrary mass activities (1, 4, 7 A mgIr ‒ 1) are included as solid black lines. Figure 1

The oxygen evolution and reduction properties of La0.8Sr0.2CoO3 are characterized using two-dimensional model electrodes with different reaction planes, synthesized on SrTiO3 single crystal substrates by pulsed laser deposition. Thin-film X-ray diffraction and reflectivity measurements confirm the epitaxial growth of 29-nm-thick La0.8Sr0.2CoO3 (001), (110), and (111) films on SrTiO3 (001), (110), and (111) substrates, respectively. Cyclic voltammetry curves in 1-M KOH aqueous solution indicate that the (110) film has the highest activity for oxygen evolution and reduction reactions. An expansion of the La0.8Sr0.2CoO3 lattice is observed after the oxygen reduction process, indicating the formation of oxygen defects, with the highest number of defects being produced in the (110) film. X-ray reflectivity analysis demonstrates the formation of a surface layer on the La0.8Sr0.2CoO3 films during electrochemical cycling due to the decomposition of La0.8Sr0.2CoO3. The surface structure constructed at the electrode/electrolyte interface is a crucial factor influencing oxygen evolution and reduction activity of La0.8Sr0.2CoO3.

Hydrotalcite-wrapped Co–B alloy with enhanced oxygen evolution activity

Hydrogen production from water electrolysis is an important measure to achieve clean and sustainable energy in the future. Oxygen evolution reaction is a half-reaction of water electrolysis process, and its efficiency has a great influence on the overall reaction. The morphology of the catalyst has a great influence on the catalytic performance of the catalyst for oxygen evolution, but the mechanism of the effect of the morphology on the activity is still unclear. Therefore, the influence of the catalyst morphology on the catalytic activity needs to be studied urgently. Co3O4 is widely used in oxygen evolution reactions. Therefore, we took three Co3O4 with different morphologies as the research objects to study the effect of morphology on the oxygen evolution activity of Co3O4. The results show that among the three different morphologies of cobalt tetroxide, the Co3O4 with porous sheet morphology exhibits the best oxygen evolution activity (η10 = 1.54 V in 1 M KOH). The reason for the better activity of cobalt tetroxide with the morphology of the hollow sheet may be due to its larger specific surface area and more active sites, which reduces the reaction electron transfer barrier. Our work sheds new light on the design of novel oxygen evolution electrocatalysts.

Water splitting using a semiconductor photocatalyst has been extensively studied as a means of solar-to-hydrogen energy conversion. Powder-based semiconductor photocatalysts, in particular, have tremendous potential in cost mitigation due to system simplicity and scalability. The control and implementation of powder-based photocatalysts are, in reality, quite complex. The identification of the semiconductor–photocatalytic activity relationship and its limiting factor has not been fully solved in any powder-based semiconductor photocatalyst. In this work, we present systematic and quantitative evaluation of photocatalytic hydrogen and oxygen evolution using a model strontium titanate powder/aqueous solution interface in a half reaction. The electron density was controlled from 1016 to 1020 cm–3 throughout the strontium titanate powder by charge compensation with oxygen nonstoichiometry (the amount of oxygen vacancy) while maintaining its crystallinity, chemical composition, powder morphology, and the cryst...

A kinetic study of the electroformation and electroreduction of oxygen in KOH solutions has been carried out on a LaNiO3 electrode obtained through a malic acid precursor route. The new electrocatalyst was found to show greatly enhanced activity for both oxygen evolution and reduction. The apparent electrocatalytic activity of this electrode for oxygen reduction was more than 10 times higher than those reported for similar electrodes obtained by other methods. It was, however, observed to be less active electrocatalytically for oxygen evolution as compared to the active LaNiO3 electrode obtained by the Bockris–Otagawa coprecipitation method. The electrochemical reaction order with respect to OH− concentration was found to be approximately 1 in the case of oxygen evolution while that for oxygen reduction, was approximately −1. The Tafel slope values for the reactions were ∼2.3RT/F and ∼2×2.3RT/3F, respectively.

CoNi nanoparticles anchored inside carbon nanotube networks by transient heating: Low loading and high activity for oxygen reduction and evolution

Composition-adjustable spinel-type metal oxides, Mnx Co3-x O4-δ (x=0.8-1.4), were synthesized in ethanol solutions by a rapid inorganic self-templating mechanism using KCl nanocrystals as the structure-directing agent. The Mnx Co3x O4δ materials showed ultrahigh oxygen evolution activity and strong durability in alkaline solutions, and are capable of delivering a current density of 10 mA cm(-2) at 1.58 V versus the reversible hydrogen electrode in 0.1 M KOH solution, which is superior in comparison to IrO2 catalysts under identical experimental conditions, and comparable to the most active noble-metal and transition-metal oxygen evolution electrocatalysts reported so far. The high performance for catalytic oxygen evolution originates from both compositional and structural features of the synthesized materials. The moderate content of Mn doping into the spinel framework led to their improved electronic conductivity and strong oxidizing ability, and the well-developed porosity, accompanied with the high affinity between OH(-) reactants and catalyst surface, contributed to the smooth mass transport, thus endowing them with superior oxygen evolution activity.

Iron-Based nanoporous Metal-Organic frameworks with Side-Chain amino substituents for Efficiency-Regulated oxygen evolution reaction

Metal-free carbonaceous catalysts have potential applications for oxygen evolution reaction (OER) devices because of their low-cost and abundant supply. We report that fluorine-doped carbon black is an active catalyst for OER. Fluorine-doped carbon black (F-KB) is simply synthesized by the pyrolysis of KETJENBLACK (KB) as carbon substrate with Nafion as fluorine precursor. As a result, the OER activity of F-KB is significantly higher than that of pristine KB in alkaline media. The OER catalytic activity of F-KB is found to be dependent on the quantity and characteristics of carbon-fluorine bonding (C–F) which can be controlled by the pyrolysis temperature. It is further found that the OER activity depends on the quantity of semi-ionic C–F bonds, but not covalent C–F bonds. This result proves the importance of carbon atoms with semi-ionic C–F bonds as the active sites for OER.

Metal–air batteries and fuel cells show a great deal of promise in advancing low-cost, high-energy-density charge storage solutions for sustainable energy applications. To improve the activities and stabilities of electrocatalysts for the critical oxygen reduction and evolution reactions (ORR and OER, respectively), a greater understanding is needed of the catalyst/carbon interactions and carbon stability. Herein, we report how LaNiO3 (LNO) supported on nitrogen-doped carbon nanotubes (N-CNT) made from a high-yield synthesis lowers the overpotential for both the OER and ORR markedly to enable a low bifunctional window of 0.81 V at only a 51 μg cm–2 mass loading. Furthermore, the addition of LNO to the N-CNTs improves the galvanostatic stability for the OER by almost 2 orders of magnitude. The nanoscale geometries of the perovskites and the CNTs enhance the number of metal–support and charge transfer interactions and thus the activity. We use rotating ring disk electrodes (RRDEs) combined with Tafel slope ...

An attempt is made to replace the use of IrO2 by SnO2–Sb2O5 in the intermediate layer which is necessary to avoid the growth of insulating titanium oxide on the titanium substrate for oxygen evolution γ-MnO2 type Mn1-x-yMoxSnyO2+x anodes in seawater electrolysis. The manganese–molybdenum–tin triple oxides, Mn1-x-yMoxSnyO2+x, prepared by anodic deposition on the SnO2–Sb2O5-coated titanium substrate from MnSO4, Na2MoO4 and SnCl4 solutions showed around 98.6% initial oxygen evolution efficiency at a current density of 1000 Am-2 in 0.5 M NaCl of pH 1 at room temperature. In order to increase the stability of the anodes, coating at various times to form the intermediate SnO2–Sb2O5 layer with sufficient thickness on titanium substrate, was performed. The Mn1-x-yMoxSnyO2+x electrodes deposited on the intermediate layer formed from seven times coating showed about 98% oxygen evolution efficiency after 20 h electrolysis. A small addition of Sb2O5 to the intermediate layer (that is, Sb5+/Sn4+ = 0.124 in the coating solution) seems to be more effective to replace the use of IrO2 for high electronic conductivity and activity of oxygen evolution in seawater electrolysis. The formation of the double oxides of the intermediate SnO2–Sb2O5 layer after seven times coating seemed responsible for both high conductivity and stability of the Mn1-x-yMoxSnyO2+x anodes.Keywords: CO2 recycling, oxygen evolution electrode, intermediate SnO2–Sb2O5 layer, seawater electrolysis, titanium substrate.DOI: 10.3126/jncs.v23i0.2097J. Nepal Chem. Soc., Vol. 23, 2008/2009 Page: 54-64

As the most well-known electrocatalyst for cathodic hydrogen evolution in water splitting electrolyzers, platinum is unfortunately inefficient for anodic oxygen evolution due to its over-binding with oxygen species and excessive dissolution in oxidative environment. Herein we show that single Pt atoms dispersed in cobalt hydrogen phosphate with an unique Pt(OH)(O3)/Co(P) coordination can achieve remarkable catalytic activity and stability for oxygen evolution. The catalyst yields a high turnover frequency (35.1 ± 5.2 s−1) and mass activity (69.5 ± 10.3 A mg−1) at an overpotential of 300 mV and excellent stability. Mechanistic studies elucidate that the superior catalytic performance of isolated Pt atoms herein stems from optimal binding energies of oxygen intermediate and also their strong electronic coupling with neighboring Co atoms that suppresses the formation of soluble Ptx>4 species. Alkaline water electrolyzers assembled with an ultralow Pt loading realizes an industrial-level current density of 1 A cm−2 at 1.8 volts with a high durability.

Strontium ferrates and cobaltates with compositions SrFeO3-δ (0.06⩽δ⩽0.40) and SrCoO3-δ (0.04⩽δ⩽0.30) were synthesized. The dependence of the oxygen electrode properties on the δ value was examined in 1 mol dm−3 KOH solution. In the SrFeO3-δ series, the samples with 0.24<δ<0.29, showed the highest activity in both oxygen evolution and reduction reactions. In contrast, no strong dependence on the δ value was observed in SrCoO3-δ, which also showed a high catalytic activity for oxygen evolution.