earned a B.S. degree in chemistry from the University of Dayton in 2001 and as an undergraduate worked on conductive polymer syntheses at the Air Force Research Laboratory at Wright Patterson Air Force Base. He completed an M.S. degree in 2004 and Ph.D. degree in 2008 at Portland State University and joined the Lewis group at Caltech in 2008. He is currently an NSF-ACCF postdoctoral fellow (2009) and has been studying the electrical characteristics of inorganic semiconductors in contact with conductive polymers. His research interests include molecular semiconductors for solar energy conversion, porphyrin macrocycles for optoelectronic applications, and catalyst materials for photoelectrolysis. Emily L. Warren received a B.S. in chemical engineering at Cornell University in 2005. She received an M.Phil in Engineering for Sustainable Development from Cambridge University in 2006. She is currently a graduate student in Chemical Engineering at the California Institute of Technology. Her research interests include semiconductor photoelectrochemistry, solar energy conversion, and semiconductor nanowires. She is currently a graduate student in Chemical Engineering at the California Institute of Technology, working under Nathan S. Lewis. James R. McKone is in his third year of graduate studies in the Division of Chemistry and Chemical Engineering at the California Institute of Technology, working under Nathan S. Lewis and Harry B. Gray. In 2008 he graduated from Saint Olaf College with a Bachelor of Arts degree, double-majoring in music and chemistry. His current research focuses on semiconductor-coupled heterogeneous catalysis of the hydrogen evolution reaction using mixtures of earth-abundant transition metals. Shannon W. Boettcher earned his B.A. degree in chemistry from the University of Oregon, Eugene (2003), and, working with Galen Stucky, his Ph.D. in Inorganic Chemistry from the University of California, Santa Barbara (2008). Following postdoctoral work with Nate Lewis and Harry Atwater at the California Institute of Technology (2008-2010), he returned to the University of Oregon to join the faculty as an Assistant Professor. His research interests span synthesis and physical measurement with the goal of designing and understanding solid-state inorganic material architectures for use in solar-energy conversion and storage.

Nanoparticles of nickel phosphide (Ni2P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solutions, under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni2P nanoparticles were hollow and faceted to expose a high density of the Ni2P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H2(g) with nearly quantitative faradaic yield, while also affording stability in aqueous acidic media.

Figure S1. Panel (a) shows the J-E data collected for the electrode fabricated with 'enhanced' absorption due to light-trapping elements, in 0.5 M aq. H 2 SO 4 under ELH-type W-halogen solar simulation. Panel (b) shows a cross-sectional SEM image of the same sample. Panel (c) compares the spectral response collected for the sample with light-trapping elements ('enhanced') versus the spectral response for the normal sample. The red response in the 'enhanced' cell is significantly improved. Panel (d) shows the increased J sc with reduced angle dependence, for the enhanced sample compared to the normal sample. Panel (e) shows a digital photograph of a normal Pt/n + p-Si wire-array electrode evolving hydrogen under ~ 1 sun illumination. Small bubbles can be seen nucleating on the wire-array surface. The larger bubbles are stuck on the epoxy, and are the result of the coalescence of many small bubbles. S1

Through decades of sustained effort, researchers have made substantial progress on developing technologies for solar-driven water splitting. Nevertheless, more basic research is needed before prototype devices with a chance for commercial success can be demonstrated. In this Perspective, we summarize the major design constraints that motivate continued research in the field of solar-driven water splitting. Additionally, we discuss key device components that are now available for use in demonstration systems and prototypes. Finally, we highlight research areas where breakthroughs will be critical for continued progress toward commercial viability for solar-driven water-splitting devices.

Earth-abundant metals are attractive alternatives to the noble metal composite catalysts that are used in water electrolyzers based on proton-exchange membrane technology. Ni−Mo alloys have been previously developed for the hydrogen evolution reaction (HER), but synthesis methods to date have been limited to formation of catalyst coatings directly on a substrate. We report a method for generating unsupported nanopowders of Ni−Mo, which can be suspended in common solvents and cast onto arbitrary substrates. The mass-specific catalytic activity under alkaline conditions approaches that of the most active reported non-noble HER catalysts, and the coatings display good stability under alkaline conditions. We have also estimated turnover frequencies per surface atom at various overpotentials and conclude that the activity enhancement for Ni−Mo relative to pure Ni is due to a combination of increased surface area and increased fundamental catalytic activity.