Porous Cobalt-Based Thin Film as a Bifunctional Catalyst for Hydrogen Generation and Oxygen Generation

Photosystem II (PSII) is the water splitting enzyme of photosynthesis. Its appearance during evolution dramatically changed the chemical composition of our planet and set in motion an unprecedented explosion in biological activity. Powered by sunlight, PSII supplies biology with the 'hydrogen' needed to convert carbon dioxide into organic molecules. The questions now are can we continue to exploit this photosynthetic process through increased use of biomass as an energy source and, more importantly, can we address the energy/CO2 problem by developing new photochemical technologies which mimic the natural system? (Critical review, 82 references).

The mechanism of the oxygen evolution reaction (OER) by catalysts prepared by electrodepositions from Co(2+) solutions in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and (18)O isotope experiments. Low-potential electrodepositions enabled the controlled preparation of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approximately equal to 2.3 × RT/F for the production of oxygen from water in neutral solutions. The electrochemical rate law exhibits an inverse first order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] ≥ 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased ∼3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equilibrium between Co(III)-OH and Co(IV)-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling.