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.

The oxygen evolution reaction at multi-cycled iron oxy-hydroxide films in aqueous alkaline solution is discussed. Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slopes of ca. 60 mV dec À1 and 40 mV dec À1 are found at low overpotentials depending on the oxide growth conditions, with an apparent Tafel slope of ca. 120 mV dec À1 at high overpotentials. Reaction orders of ca. 0.5 and 1.0 are observed at low and high overpotentials, again depending on the oxide growth conditions. A mechanistic scheme involving the active participation of octahedrally coordinated anionic iron oxyhydroxide surfaquo complexes, which form the porous hydrous layer, is proposed. The latter structure contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. This work brings together current research in heterogeneous electrocatalysis and homogeneous molecular catalysis for water oxidation.

The oxygen evolution reaction at multi-cycled iron oxy-hydroxide films in aqueous alkaline solution is discussed. The factors affecting the growth and electrocatalytic properties of the latter are described. In particular, steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slopes of ca. 60 mV dec−1 and 40 mV dec−1 are found at low overpotentials depending on the oxide growth conditions, with an apparent Tafel slope of ca. 120 mV dec−1 at high overpotentials. Reaction orders of ca. 0.5, 1.0 and 1.5 are observed at low and high overpotentials, again depending on the oxide growth conditions. A mechanistic scheme involving the active participation of octahedrally coordinated anionic iron oxyhydroxide surfaquo complexes, which form the porous hydrous layer, is proposed. The latter structure contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. This work brings together current research in both heterogeneous electrocatalysis and homogeneous molecular catalysis for water oxidation.

The redox switching reaction and the oxygen evolution reaction at multi-cycled nickel oxy-hydroxide films in aqueous alkaline solution are discussed. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution. The dynamics of redox switching has been quantified in terms of a diffusive frequency using the Laviron-Aoki diffusion model. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution with slopes of ca. 60 mVdec −1 and ca. 120 mVdec −1 at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity remains invariant at ca. 1.0 as the potential is increased. These observations are rationalized in terms of a kinetic scheme involving surfaquo groups.

An environmentally benign and scalable route for the production of gram scale quantities of nitrogen-doped graphene using a downstream microwave plasma source is reported. Simultaneous reduction and doping of graphene oxide is achieved and the process negates the need for high temperatures and toxic solvents associated with existing methods. This gas-phase low temperature process is completely dry and, thus, minimises re-aggregation of graphene flakes which is typically associated with liquid phase reduction methods. The resulting material has many potential uses, particularly in electrochemical energy.

No abstract

Outstanding issues regarding the film formation, redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been revisited. The oxide is grown using a repetitive potential multicycling technique, and the mechanism of the latter hydrous oxide formation process has been discussed. A duplex layer model of the oxide/solution interphase region is proposed. The acid/base behaviour of the hydrous oxide and the microdispersed nature of the latter material has been emphasised. The hydrous oxide is considered as a porous assembly of interlinked octahedrally coordinated anionic metal oxyhydroxide surfaquo complexes which form an open network structure. The latter contains considerable quantities of water molecules which facilitate hydroxide ion discharge at the metal site during active oxygen evolution, and also charge compensating cations. The dynamics of redox switching has been quantified via analysis of the cyclic voltammetry response as a function of potential sweep rate using the Laviron-Aoki electron hopping diffusion model by analogy with redox polymer modified electrodes. Steady state Tafel plot analysis has been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

No abstract