This paper reviews work carried out over a number of years to try to elucidate the mechanism of oxygen reduction and methanol oxidation in alkaline solution. We have sought to achieve this by combining electrochemical, spectroscopic and solid-state chemical approaches, bringing together as wide a variety of techniques as possible both to shed light on the mechanisms and to point the way to more effective and efficient fuel cells. This work has become considerably more topical in recent years with the development of anion-exchange electrolyte membranes that can operate in alkaline environments, an important advance since it permits both the use of non-noble-metal catalysts and organic fuels at the anode, the latter precluded in aqueous alkaline electrolyte due to precipitation of inorganic carbonates at the electrode surface.

The oxidation of methanol at a polycrystalline Pt anode was investigated with in situ external reflectance Fourier transform infrared (FTIR) spectroscopy in both nitrogen- and oxygen-saturated 0.1 M KOH. The data strongly suggest that adsorbed formate is either the, or at least an important, intermediate in the direct pathway, with attack on this intermediate (rather than its formation) being the rate-determining step. The HCOOads is formed via an Eley−Rideal-type reaction between solution methanol and OHads. Linearly adsorbed CO (COL) appears to be a spectator rather than a participant in the methanol oxidation reaction, at least up to relatively high potentials, i.e., 0.1 V. The primary products of the oxidation are formate up to −0.5 V, then bicarbonate as well as formate and finally only CO2 at potentials greater than or equal to −0.3 V. Clearly, the pH in the thin layer drops below 10.25 at potentials greater than −0.5 V and below 6.37 in the latter potential region, these being pK a,2 and pK a,1, respectively, of carbonic acid. This indicates the disadvantage of the external reflectance approach, where the thin layer causes very restricted diffusion, compared to those techniques such as surface-enhanced infrared spectroscopy (SEIRAS) that employ attenuated total reflectance. However, the external reflectance approach has a major advantage over SEIRAS of detecting solution-borne intermediates and products, which are inaccessible to the latter technique because of its lack of sensitivity beyond the near-electrode region. At potentials greater than −0.1 V, the CO2 solution band remained constant as excess CO2 was forced into bubbles, pushing electrolyte out of the thin layer. In oxygen-saturated solution, no COL was observed. The O2-reduction products clearly modified the Pt surface and altered the characteristics of the Pt oxide film formed at higher potentials, which, among other effects, removed the nucleation sites for CO2 bubble formation. Because of the bubble formation in N2-saturated solution, it was difficult to judge whether more CO2 was formed in O2-saturated KOH, but the amounts of solution formate and bicarbonate formed in the latter were significantly higher than those in N2-saturated solution.

This paper reports in situ FTIR studies on the oxidation of formate at polycrystalline Pt in aqueous KOH. Data are presented which show that hydroxyl species play a major role in the electro-oxidation of small organic molecules under alkaline conditions at polycrystalline Pt, and that a number of possible mechanistic pathways are possible. Small changes in experimental conditions appear to be able to cause the reaction to flick between these pathways; for example, the presence of oxygen has a marked effect upon the observed electrochemistry. In contrast to acid solution, our postulated model includes the formation of intermediates bonded through O atoms, rather than C, as being an important option in alkaline solution. Finally, the pH distribution across the reflective electrode in external reflectance IR is modelled and significant variations in pH across the electrode surface in FTIR cells predicted and confirmed experimentally.

This paper describes, for the first time, the electrochemical characteristics of a novel, composite electrode, comprising a thin TiO2 layer sandwiched between a silicon wafer and a metal grid. Holes thermally and/or photochemically generated near the Si/TiO2 interface in the Si are able to reach the surface of the TiO2 and oxidize water and/or species in solution. Hole transport across the TiO2 is facilitated by the application of a bias voltage across the silicon and metal grid. At low bias voltages, oxidation of water takes place at the metal grid; at higher voltages, the oxidation takes place directly at the TiO2 as surface states are accessed. The generation of holes is enhanced significantly if the TiO2 surface is irradiated with visible light. A theoretical model is presented to explain the observed data. The anodes represent a completely new area of oxidative electrochemistry with potential application across a wide range of technology, from fuel cells-on-a-chip to electroorganic chemistry.