Metal–Polypyridyl Catalysts for Electro- and Photochemical Reduction of Water to Hydrogen

David Z. Zee †#, Teera Chantarojsiri †, Jeffrey R. Long *†⊥#, and Christopher J. Chang *†‡§∥

Departments of †Chemistry and ‡Molecular and Cell Biology and the §Howard Hughes Medical Institute,University of California, Berkeley, Berkeley, California 94720, United States

∥Chemical Sciences Division and ⊥Materials Sciences Division and the #Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

Acc. Chem. Res., Article ASAP

DOI: 10.1021/acs.accounts.5b00082

Publication Date (Web): June 23, 2015

Copyright © 2015 American Chemical Society

*(J.R.L.) E-mail: jrlong@berkeley.edu., *(C.J.C.) E-mail: chrischang@berkeley.edu.

 Special Issue

Published as part of the Accounts of Chemical Research special issue “Earth Abundant Metals in Homogeneous Catalysis”.

Biography

David Z. Zee received a B.A. degree in Chemistry and Economics from Swarthmore College in 2007. He joined the laboratory of Prof. Jeffrey Long at UC Berkeley and was awarded a NSF Graduate Research Fellowship. His research interests lie in water and CO2 reduction catalysis and the activation of other small molecule substrates relevant for sustainable energy purposes.

Biography

Teera Chantarojsiri was awarded a Development and Promotion of Science and Technology scholarship (DPST) from the Thai government in 2005 to study Chemistry. She received her B.S. (honors) in Chemistry from Stanford University in 2010, where she conducted research under the supervision of Prof. Dan Stack. She is currently completing her Ph.D. at UC Berkeley in the laboratory of Prof. Christopher Chang working on inorganic synthesis and small molecule activations.

Biography

Jeffrey R. Long received a B.A. degree from Cornell University in 1991 and earned his Ph.D. from Harvard University in 1995. Following postdoctoral work at Harvard and the University of California, Berkeley, he joined the faculty at Berkeley in 1997, where he is currently Professor of Chemistry. He is also a Senior Faculty Scientist in the Materials Sciences Division of Lawrence Berkeley National Laboratory. In addition, he is the lead-PI for the Berkeley Hydrogen Storage Program, Director of the Center for Gas Separations, and a cofounder of Mosaic Materials, Inc. His research areas include the synthesis of inorganic clusters and solids with unusual electronic and magnetic properties, generation of microporous metal–organic frameworks for applications in gas storage, separations, and catalysis, and the development of molecular catalysts for electro- and photochemical water splitting.

Biography

Christopher J. Chang is the Class of 1942 Chair Professor in the Departments of Chemistry and Molecular and Cell Biology at UC Berkeley, Howard Hughes Medical Institute Investigator, and Faculty Scientist in the Chemical Sciences Division of Lawrence Berkeley National Laboratory. He is a Senior Editor of ACS Central Science. Chris received his B.S. and M.S. degrees from Caltech in 1997, studied as a Fulbright scholar in Strasbourg, France, and received his Ph.D. from MIT in 2002 with Dan Nocera. After postdoctoral studies with Steve Lippard, Chris joined the UC Berkeley faculty in 2004. Research in the Chang lab is focused on chemical biology and inorganic chemistry, with particular interests in molecular imaging and catalysis applied to neuroscience and sustainable energy. His group’s work has been honored by awards from the Dreyfus, Beckman, Sloan, and Packard Foundations, Amgen, AstraZeneca, and Novartis, Technology Review, ACS (Cope Scholar, Eli Lilly, Nobel Laureate Signature, Baekeland), RSC (Transition Metal Chemistry), and SBIC.

Abstract

Conspectus

Climate change, rising global energy demand, and energy security concerns motivate research into alternative, sustainable energy sources. In principle, solar energy can meet the world’s energy needs, but the intermittent nature of solar illumination means that it is temporally and spatially separated from its consumption. Developing systems that promote solar-to-fuel conversion, such as via reduction of protons to hydrogen, could bridge this production–consumption gap, but this effort requires invention of catalysts that are cheap, robust, and efficient and that use earth-abundant elements. In this context, catalysts that utilize water as both an earth-abundant, environmentally benign substrate and a solvent for proton reduction are highly desirable. This Account summarizes our studies of molecular metal–polypyridyl catalysts for electrochemical and photochemical reduction of protons to hydrogen. Inspired by concept transfer from biological and materials catalysts, these scaffolds are remarkably resistant to decomposition in water, with fast and selective electrocatalytic and photocatalytic conversions that are sustainable for several days. Their modular nature offers a broad range of opportunities for tuning reactivity by molecular design, including altering ancillary ligand electronics, denticity, and/or incorporating redox-active elements. Our first-generation complex, [(PY4)Co(CH3CN)2]2+, catalyzes the reduction of protons from a strong organic acid to hydrogen in 50% water. Subsequent investigations with the pentapyridyl ligand PY5Me2furnished molybdenum and cobalt complexes capable of catalyzing the reduction of water in fully aqueous electrolyte with 100% Faradaic efficiency. Of particular note, the complex [(PY5Me2)MoO]2+ possesses extremely high activity and durability in neutral water, with turnover frequencies at least 8500 mol of H2 per mole of catalyst per hour and turnover numbers over 600 000 mol of H2 per mole of catalyst over 3 days at an overpotential of 1.0 V, without apparent loss in activity. Replacing the oxo moiety with a disulfide affords [(PY5Me2)MoS2]2+, which bears a molecular MoS2 triangle that structurally and functionally mimics bulk molybdenum disulfide, improving the catalytic activity for water reduction. In water buffered to pH 3, catalysis by [(PY5Me2)MoS2]2+ onsets at 400 mV of overpotential, whereas [(PY5Me2)MoO]2+ requires an additional 300 mV of driving force to operate at the same current density. Metalation of the PY5Me2 ligand with an appropriate Co(ii) source also furnishes electrocatalysts that are active in water. Importantly, the onset of catalysis by the [(PY5Me2)Co(H2O)]2+ series is anodically shifted by introducing electron-withdrawing functional groups on the ligand. With the [(bpy2PYMe)Co(CF3SO3)]1+ system, we showed that introducing a redox-active moiety can facilitate the electro- and photochemical reduction of protons from weak acids such as acetic acid or water. Using a high-throughput photochemical reactor, we examined the structure–reactivity relationship of a series of cobalt(ii) complexes. Taken together, these findings set the stage for the broader application of polypyridyl systems to catalysis under environmentally benign aqueous conditions.