Development of Molecular Electrocatalysts for CO2 Reduction and H2 Production/Oxidation

Abstract

The conversion of solar energy to fuels in both natural and artificial photosynthesis requires components for both light-harvesting and catalysis. The light-harvesting component generates the electrochemical potentials required to drive fuel-generating reactions that would otherwise be thermodynamically uphill. This Account focuses on work from our laboratories on developing molecular electrocatalysts for CO₂ reduction and for hydrogen production. A true analog of natural photosynthesis will require the ability to capture CO₂ from the atmosphere and reduce it to a useful fuel. Work in our laboratories has focused on both aspects of this problem. Organic compounds such as quinones and inorganic metal complexes can serve as redox-active CO₂ carriers for concentrating CO₂. We have developed catalysts for CO₂ reduction to form CO based on a [Pd(triphosphine)(solvent)]²⁺ platform. Catalytic activity requires the presence of a weakly coordinating solvent molecule that can dissociate during the catalytic cycle and provide a vacant coordination site for binding water and assisting C−O bond cleavage. Structures of [NiFe] CO dehydrogenase enzymes and the results of studies on complexes containing two [Pd(triphosphine)(solvent)]²⁺ units suggest that participation of a second metal in CO₂ binding may also be required for achieving very active catalysts. We also describe molecular electrocatalysts for H₂ production and oxidation based on [Ni(diphosphine)₂]²⁺ complexes. Similar to palladium CO₂ reduction catalysts, these species require the optimization of both first and second coordination spheres. In this case, we use structural features of the first coordination sphere to optimize the hydride acceptor ability of nickel needed to achieve heterolytic cleavage of H₂. We use the second coordination sphere to incorporate pendant bases that assist in a number of important functions including H₂ binding, H₂ cleavage, and the transfer of protons between nickel and solution. These pendant bases, or proton relays, are likely to be important in the design of catalysts for a wide range of fuel production and fuel utilization reactions involving multiple electron and proton transfer steps. The generation of fuels from abundant substrates such as CO₂ and water remains a daunting research challenge, requiring significant advances in new inexpensive materials for light harvesting and the development of fast, stable, and efficient electrocatalysts. Although we describe progress in the development of redox-active carriers capable of concentrating CO₂ and molecular electrocatalysts for CO₂ reduction, hydrogen production, and hydrogen oxidation, much more remains to be done.