Keeping semiconductors safe from harm Solar cells harvest the energy of sunlight to create electricity, but electricity is hard to store. Solar cells could also be used to make hydrogen from water, which can be stored as a fuel. Separating water into hydrogen and oxygen, however, presents challenges, especially if this is done directly by illuminating the anode that oxides water. Under the acidic or alkaline conditions needed for practical devices, semiconducting anode materials corrode during operation. Hu et al. now show that amorphous titanium dioxide coatings can protect semiconductors from alkaline corrosion while still allowing light through. Science , this issue p. 1005

All Rights Reserved iii For my mom iv ACKNOWLEDGEMENTSI would first like to thank Harry for his guidance over the last five years. Harry has supported my growth as a scientist beyond my wildest expectations-in addition to being the most sincere and caring mentor I could have hoped for, he has given me countless opportunities to define myself beyond the confines of the laboratory and gain exposure in the chemistry community. I could not have made it through these projects without his insight and encouragement, and for that I am truly blessed by this graduate experience.Second, I would like to thank Jay for his support and advice, teaching me everything I know about kinetics and spectroscopy, building me a diode array spectrometer, and routinely reminding me of the value of fundamental research. Like the best of mentors, he has challenged me to be an assertive researcher and calmly watched me freak out more than once. This thesis would not be nearly as comprehensive without his guidance.I have had some of the most rewarding scientific discussions of my graduate career with my thesis committee members, Mitchio Okumura, Nate Lewis, and Tom Miller, and I want to thank them for their support over the last five years. I would especially like to acknowledge Nate for giving me the opportunity to cochair the 2009 Renewable Energy: Solar Fuels Gordon-Kenan Graduate Research Seminar. I also want to thank Bruce Brunschwig and Jonas Peters who provided a good deal of guidance on this project.My undergraduate research advisor, Dan Nocera, has continued to support, encourage, and advise me over the five and a half years since I graduated from MIT. It is from Dan that I gained my love for group theory (5.04) and inorganic spectroscopy, and I am grateful for his continued guidance. I have enjoyed collaborating with a great many researchers during my graduate work.Etsuko Fujita, Dmitry Polyanskiy, and Jim Muckerman at Brookhaven National Laboratory have been enthusiastic about my research for many years and welcomed me into their labs for two visits. Etsuko in particular has been a great friend and role model and I look forward to maintaining our relationship for years to come. It has been a pleasure to make progress towards powering the planet with researchers in both the Lewis and Peters groups.Leslie O'Leary, Judy Lattimer, and Emily Warren have all made contributions towards anchoring cobaloxime catalysts to silicon electrodes, and Xile Hu, Louise Berben, Nate Szymczak, and Charles McCrory have all taught me a great deal about electrochemistry, synthesis, and catalytic hydrogen evolution.I have had the opportunity to work with a number of talented undergraduates at Caltech.Carolyn Valdez has been a great friend and I have been truly honored to watch her grow as a scientist over four years at Caltech. She spearheaded the binuclear cobaloxime catalyst project (Chapter 5), helped lead the Blair High School SHArK program, and is hands-down one of the best all-around undergraduates I ever met at Caltech. I am excited that we will b...

Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF2)2(CH3CN)2 and Co(dpgBF2)2(CH3CN)2 (where dmgBF2 is difluoroboryl-dimethylglyoxime and dpgBF2 is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with [14]-tetraene-N4 (Tim) ligands of the type [Co(TimR)X2] n + (R = methyl or phenyl, X = Br or CH3CN; n = 1 with X = Br and n = 3 with X = CH3CN), have been observed to evolve H2 electrocatalytically at potentials between −0.55 V and −0.20 V vs SCE in CH3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H2 production. For the complexes Co(dmgBF2)2(CH3CN)2, Co(dpgBF2)2(CH3CN)2, [Co(TimMe)Br2]Br, and [Co(TimMe)(CH3CN)2](BPh4)3, bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H2 production. In contrast, the complexes [Co(TimPh/Me)Br2]Br and [Co(TimPh/Me)(CH3CN)2](BPh4)3 were less stable, and bulk electrolysis only produced faradaic yields for H2 production of 20−25%. Cyclic voltammetry of Co(dmgBF2)2(CH3CN)2, [Co(TimMe)Br2]+, and [Co(TimMe)(CH3CN)2]3+ in the presence of acid revealed redox waves consistent with the Co(III)−H/Co(II)−H couple, suggesting the presence of Co(III) hydride intermediates in the catalytic system. The potentials at which these Co complexes catalyzed H2 evolution were close to the reported thermodynamic potentials for the production of H2 from protons in CH3CN, with the smallest overpotential being 40 mV for Co(dmgBF2)2(CH3CN)2 determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF2)2(CH3CN)2 was also able to oxidize H2 in the presence of a suitable conjugate base. Digital simulations of the electrochemical data were used to study the mechanism of H2 evolution catalysis, and these studies are discussed.

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

Splitting water to hydrogen and oxygen is a promising approach for storing energy from intermittent renewables, such as solar power. Efficient, scalable solar-driven electrolysis devices require active electrocatalysts made from earth-abundant elements. In this mini-review, we discuss recent investigations of homogeneous and heterogeneous hydrogen evolution electrocatalysts, with emphasis on our own work on cobalt and iron complexes and nickel-molybdenum alloys.