Water Oxidation by Amorphous Cobalt‐Based Oxides: Volume Activity and Proton Transfer to Electrolyte Bases

Abstract

Water oxidation in the neutral pH regime catalyzed by amorphous transition-metal oxides is of high interest in energy science. Crucial determinants of electrocatalytic activity were investigated for a cobalt-based oxide film electrodeposited at various thicknesses on inert electrodes. For water oxidation at low current densities, the turnover frequency (TOF) per cobalt ion of the bulk material stayed fully constant for variation of the thickness of the oxide film by a factor of 100 (from about 15 nm to 1.5 μm). Thickness variation changed neither the nanostructure of the outer film surface nor the atomic structure of the oxide catalyst significantly. These findings imply catalytic activity of the bulk hydrated oxide material. Nonclassical dependence on pH was observed. For buffered electrolytes with pKa values of the buffer base ranging from 4.7 (acetate) to 10.3 (hydrogen carbonate), the catalytic activity reflected the protonation state of the buffer base in the electrolyte solution directly and not the intrinsic catalytic properties of the oxide itself. It is proposed that catalysis of water oxidation occurs within the bulk hydrated oxide film at the margins of cobalt oxide fragments of molecular dimensions. At high current densities, the availability of a proton-accepting base at the catalyst-electrolyte interface controls the rate of water oxidation. The reported findings may be of general relevance for water oxidation catalyzed at moderate pH by amorphous transition-metal oxides.

Towards Versatile and Sustainable Hydrogen Production through Electrocatalytic Water Splitting: Electrolyte Engineering

“…Particularly, the larger p K a resulted in an improved OER onset potential (Figure 13 b and c) 167. This observation indicates the involvement of the deprotonation/protonation process in the rate‐determining step,167 which was also observed for the Co OER catalysts 162…”

“…For more details about the Co–phosphate system, the readers are referred to the tutorial review by the group 160. Following this study, some groups attempted to elucidate and further improve the performance of Co‐based materials at near‐neutral pH conditions 161, 162, 163, 164. Dau and co‐workers investigated the OER performance of Co electrodes in various electrolytes (phosphate, carbonate, glycine, TRIS, acetate, HEPES, chloride, borate, and MES), which revealed a similarity between the OER–pH relationship and pH titration (Figure 12 a).…”

In Situ Observations of Oxygen Evolution Cocatalysts on Photoelectrodes by X-ray Absorption Spectroscopy: Comparison between Cobalt-Phosphate and Cobalt-Borate

“…In the present work, we focused on the differences in the reaction activities of Co-P i and Co-B i cocatalysts on photoelectrodes. In spite of many previous studies, [28][29][30][31][32][33][34][35][36][37][38][39][40][41] during photoelectrochemical reactions, using in situ Co-K edge XAFS, and subsequently discuss the functioning of these OER cocatalysts.…”

“…9,15,[21][22][23][24][25][26][27] Among these, cobalt-phosphate (Co-P i ) and cobalt-borate (Co-B i ) cocatalysts electrodeposited from a dilute Co 2+ solution containing phosphate and borate electrolytes, respectively, are known to generate highly active OER cocatalysts. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] As an example, Nocera et al reported that a Si photoelectrode modified with a Co-P i cocatalyst exhibited high activity, with a solar energy conversion efficiency of 4.7%. 21 Domen et al also discovered that a Co-P i -modified Ba-doped Ta 3 N 5 photoelectrode had an efficiency of 1.5% using single-photon photoanodes system.…”

Enhanced Kinetics of Hole Transfer and Electrocatalysis during Photocatalytic Oxygen Evolution by Cocatalyst Tuning

“…33 Other noble metals, such as Rh, Pt, Ru, Pd, and Ir, were also shown to have a similar impact, suggesting a common underlying mechanism governing the improved performance of M-CoOx in OER efficiency. [30][31][32] The second role of the cocatalyst is presumed to be the charge separation at the metal (oxide)-semiconductor interface. In fact, in photoelectrochemical OER using a CoOx/BaTaO2N system, the addition of RhOx has been found to significantly improve the photocurrent and lower the onset potential.…”