(10a) In Situ Studies of the Composition and Structure of Co and Ni Oxide Catalysts for the Electrochemical Oxidation of Water
One of the grand challenges of the 21st century is the development of means for converting solar energy into gaseous and liquid fuels. This challenge is currently being pursued within the Joint Center for Artificial Synthesis (JCAP), a DOE funded solar Hub collocated at Caltech and the Lawrence Berkeley National Laboratory (LBNL). An important part of the problem is the discovery and development of catalysts for the oxidation of water and the generation of hydrogen that operate with a low overpotential that are based on low cost, earth abundant elements. Past research that of the two half reactions, the one with the higher overpotential is oxidation of water, and consequently, we have focused attention on this process. To this end we have developed a method for carrying out in situ confocal Raman spectroscopy and in situ resonant inelastic x-ray scattering (RIXS) using specially designed electrochemical cells. The latter effort involves collaboration with Anders Nilsson at the Stanford Linear Accelerator (SLAC).
The transformation in the compositional and structural changes occurring in Co and Ni oxides during the electrochemical oxidation of water in KOH and NaOH have shown that the working state of the catalyst is highly dependent on the applied potential. Raman spectroscopy has shown that electrodeposited Co3O4 is a spinel, but under potentials approaching those required for the evolution of O2 oxidizes to CoOOH. This transformation has been confirmed by RIXS, which shows additionally evidence for the intercollation of alkali metal cations from the electrolyte and the consequent increase in the oxidation state of Co above 3+. Density functional theory calculation conducted in collaboration with Jen Norskov have revealed that the experimentally observed changes lead to a progressive reduction in the overpotential for the oxygen evolution reaction. More recent studies with Ni have shown that in a basic electrolyte, Ni is immediately oxidized to Ni(OH)2 and under anodic potential is converted to b-NiOOH. Addition of Fe to the electrodeposited Ni results in a mixed metal oxyhydroxide that is up to 102 more active than NiOOH. Taken together, these observations provide guidance to the discovery of catalysts exhibiting low overpotentials for the oxidation of water, a key electrochemical process involved in the splitting of water to H2 and O2 using solar energy.