Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes

Abstract

This paper presents a review of the redox and electrocatalytic properties of transition metal oxide electrodes, paying particular attention to the oxygen evolution reaction. Metal oxide materials may be prepared using a variety of methods, resulting in a diverse range of redox and electrocatalytic properties. Here we describe the most common synthetic routes and the important factors relevant to their preparation. The redox and electrocatalytic properties of the resulting oxide layers are ascribed to the presence of extended networks of hydrated surface bound oxymetal complexes termed surfaquo groups. This interpretation presents a possible unifying concept in water oxidation catalysis -bridging the fields of heterogeneous electrocatalysis and homogeneous molecular catalysis. In contextOver the past 50 years considerable research efforts have been devoted to the realisation of efficient, economical and renewable energy sources. Metal oxide materials have played a large part in this drive with demonstrated applications at both the research and commercial level. Their use in areas such as batteries, fuel cells and water electrolysis has resulted in the development of materials with a diverse range of structural and chemical properties. In all cases, understanding the fundamental electrochemistry of the material can be invaluable for rational design and optimisation. This review focuses on the redox, charge transport and electrocatalytic properties of transition metal oxide electrodes as they pertain to the electrolytic splitting of water. Particular emphasis is placed on the nature of the active surface which is interpreted in terms of hydrated interlinked oxymetal complexes termed surfaquo groups. In this way, the review seeks to bridge the gap between heterogeneous electrocatalysis and homogeneous molecular catalysis for water oxidation, areas of considerable modern interest and activity.

Mechanistic Parameters of Electrocatalytic Water Oxidation on LiMn2O4 in Comparison to Natural Photosynthesis

“…Disk Tafel slopes between 70 and 90 mV dec −1 are typical for composite electrodes containing carbon; Tafel slopes of 88, 70, and 85 mV dec −1 were reported for MnO, Mn 2 O 3 , and MnO 2 , respectively 80. The significance of these values is unknown and they do not correspond to commonly discussed mechanisms of the OER 27, 30, 74. Aside from intermediate surface coverage on these oxides (i.e., not low coverage or full coverage),27, 30 it is conceivable that the Tafel slope is influenced by Mn comproportionation,81 which would affect the number of electrons transferred prior to the RLS.…”

“…Historically, the three mechanistic parameters are mainly discussed in the context of four mechanistic sequences27, 30 that are based on the works of Bockris,29 Krasil'shikov,82 O'Grady,83 and Kobussen 84, 85. The latter mechanistic sequence is also used in contemporary theoretical work86, 87 and has received much attention lately.…”

“…Electrochemical analysis of mechanistic parameters is very mature 25, 26, 27, 28, 29, 30. Recently, Minguzzi et al 31.…”

Chemical Recognition of Active Oxygen Species on the Surface of Oxygen Evolution Reaction Electrocatalysts

“…For the Fe‐containing films, the potential–pH dependence becomes close to −90 mV/pH for pH >12.5. This value, characteristic of a super‐Nernstian potential–pH shift corresponding to the exchange of 2 electrons and 3 protons (ratio of H + /e − involved equals to 1.5),17, 25 indicate a different deprotonation mechanism for Fe‐containing film. Hence, depending on the deprotonation process, different oxygen species are formed on the surface of the films that show, or not, specific interaction with TMA + cation, as visualize by Raman spectroscopy (Supporting Information, Figure S8).…”

Cover Feature: Mechanistic Parameters of Electrocatalytic Water Oxidation on LiMn2O4 in Comparison to Natural Photosynthesis (ChemSusChem 22/2017)

“…[25][26][27][28][29][30] Recently,M inguzzi et al [31] introducedd ynamic Ta rgeted improvement of the low efficiencyo fw ater oxidation during the oxygen evolution reaction (OER) is severelyh indered by insufficient knowledge of the electrocatalytic mechanism on heterogeneouss urfaces. We chose LiMn 2 O 4 as a model system for mechanistic investigations as it shares the cubane structure with the active site of photosystem II and the valenceo fM n 3.5 + with the dark-stable S1 state in the mechanism of natural photosynthesis.…”