Understanding the Oxygen Evolution Reaction Mechanism on CoO_x using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

Abstract

ABSTRACT:Photoelectrochemical water splitting is a promising approach for renewable production of hydrogen from solar energy and requires interfacing advanced water splitting catalysts with semiconductors. Understanding the mechanism of function of such electrocatalysts at the atomic scale and under realistic working conditions is a challenging, yet important, task for advancing efficient and stable function. This is particularly true for the case of oxygen evolution catalysts and, here, we study a highly active Co 3 O 4 /Co(OH) 2 biphasic electrocatalyst on Si by means of operando ambient pressure X-ray photoelectron spectroscopy performed at the solid/liquid electrified interface. Spectral simulation and multiplet fitting reveal that the catalyst undergoes chemical-structural transformations as a function of the applied anodic potential, with complete conversion of the Co(OH) 2 and partial conversion of the spinel Co 3 O 4 phases to CoO(OH) under pre-catalytic electrochemical conditions. Furthermore, we observe new spectral features in both Co 2p and O 1s core level regions to emerge under oxygen evolution reaction conditions on CoO(OH). The operando photoelectron spectra support assignment of these newly observed features to highly active Co 4+ centers under catalytic conditions. Comparison of these results to those from a pure phase spinel Co 3 O 4 catalyst supports this interpretation and reveals that the presence of Co(OH) 2 enhances catalytic activity by promoting transformations to CoO(OH). The direct investigation of electrified interfaces presented in this work can be extended to different materials under realistic catalytic conditions, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design. Introduction Sustainable solutions are required to mitigate the impact of increasing world energy demand on the environment and avoid depletion of natural energy sources (1). While transduction of solar energy to electricity has already made a significant impact on the renewable energy sector, the intermittent nature of sunlight imposes critical storage challenges (2,3,4,5,6). Furthermore, addressing broader energy needs, particularly in transportation, requires new technologies for the next generation of renewable fuels. Within this context, (photo)electrochemical conversion of sunlight to hydrogen -or other chemical fuels -represents an appealing approach to both solar energy conversion and high energy density storage. An essential step in such artificial photosystems is the oxygen evolution reaction (OER), in which the protons and electrons required for the fuel formation reaction are harnessed from water (2-7, 8, 9). However, OER pathways can be complex and impose significant kinetic bottlenecks. To address this challenge, increasing efforts have been devoted to developing OER catalysts possessing high activity, long-term durability, and low cost, a combination of attributes that is most commonly obtained with first row transition metal oxides (10,11,12,13,14,15...

Observing the Electrochemical Oxidation of Co Metal at the Solid/Liquid Interface Using Ambient Pressure X-ray Photoelectron Spectroscopy

“…2,28,29 Therefore, the formation of cobalt oxyhydroxide on the WE surface at +0.4 V holding potential is anticipated in our operando experiment. As shown in Figure 2a, there is an additional component which 19,21,24 We conclude that the new species formed at +0.4 V is CoOOH. Hence, three surface species are used to deconvolute the XPS spectrum.…”

“…18 Investigations show the Co 3 O 4 /Co(OH) 2 biphasic electrocatalyst converts to CoOOH, which was determined to promote the enhanced electrocatalytic activity. 19 In this work, the same tender X-ray AP-XPS was used to characterize cobalt foil as the working electrode at different applied potentials in a 0.1 M KOH electrolyte. The evolution of the Co metal surface species was observed by acquiring Co 2p and O 1s spectra at various fixed potentials.…”

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…Thew orking electrode was then embedded into apolypropylene container that served as the electrochemical cell. [1, [22][23][24] Thed eposition solution was then exchanged for 1m KOHf or electrochemical testing and XAS data collection. solutions of Co(NO 3 ) 2 and FeCl 2 , [15] leading to highly porous films where nearly every Co/Fec ation is exposed to the electrolyte.…”

“…[25] ForF e-free CoO x H y ,t he Co K-edge energy at 0.1 Vv s. E O 2 =OH À (anodic of the Co oxidation wave but cathodic of the OER current onset) shows mostly Co 3+ (Supporting Information, Figure S1A). [22,[26][27][28][29] It is possible that these oxidized Co sites represent those performing the OER, perhaps at reactive edge sites. Based on the higher K-edge energy shown in Figure 2A, as mall amount of oxidation occurs (0.5 eV increase in edge energy) when the catalyst potential is stepped from 0.1 Vt o 0.4 Vvs.…”

Zn0.35Co0.65O – A Stable and Highly Active Oxygen Evolution Catalyst Formed by Zinc Leaching and Tetrahedral Coordinated Cobalt in Wurtzite Structure

“…[10,11,52] The leaching of Zn 2+ in alkaline media leads to increased metal vacancies in the material, as proven by the EXAFS analysis of the sample after electrochemical reaction. For all those samples, it is agreed that the OER activity arises from adjacent Co (III-IV) O(H) groups connected by µ 2 -O-bridges.…”