Powering the planet with solar fuel

Abstract

Mol. hydrogen has emerged as an attractive candidate for a clean, renewable fuel to meet the world's skyrocketing demand for energy. Hydrogenase enzymes that contain iron and nickel cofactors evolve H_2 catalytically from water with turnover frequencies as high as 9000 s^(-1) at 30 C. However, the relative instability of these enzymes under aerobic conditions has led to the search for robust inorg. catalysts that can produce hydrogen from water. Platinum is an excellent catalyst for proton redn. and hydrogen oxidn., but scarcity and high cost limit its widespread use. Our emphasis is on heterogeneous and homogeneous catalysts made from earth-abundant elements that could be part of scalable solar fuel devices. Promising heterogeneous catalysts include MoS_2 and Ni-Mo, which reduce protons in aq. solns. with catalytic efficiencies near that of platinum. While homogeneous catalysts typically degrade faster than their heterogeneous counterparts, mol. systems are much easier to study mechanistically. Cobalt complexes enable electrocatalytic prodn. of H_2 from solns. with high turnover frequencies, and kinetics investigations have established that the reactive intermediate is a Co(II)-hydride. The challenge of water oxidn. in many ways eclipses that of proton redn. The oxidn. reaction involves the rearrangement of more protons and electrons, and fewer good catalysts for the reaction exist that are made of earth-abundant materials. We have found that 3 to 5 nm metal-oxide particles made by pulsed laser ablation of precursors in water are very active water oxidn. catalysts. We hope to elucidate the electronic structures of these very small nanoparticles as part of a program with the goal of understanding their mechanisms.