ELECTROCHEMICAL HYDROGEN EVOLUTION BY COBALT AND NICKEL SELENOLATE COORDINATION POLYMERS

The ability to efficiently and cost-effectively convert solar energy into molecular hydrogen through water splitting as a method for storing solar energy in chemical bonds is necessary for meeting rising energy demands and to mitigate the adverse effects of carbon-based fuels on the environment. Earth-abundant homogeneous and hetereogeneous electrocatalysts for the hydrogen evolution reaction (HER) have been developed as alternatives to platinum, the state of the art HER catalyst whose high cost and scarcity limit the global scalability of platinum based solar-to-hydrogen converting devices. Hydrogenase enzymes, which efficiently catalyze reversible hydrogen production and oxidation in nature near the thermodynamic potential, have served as models for designing biomimetic catalysts for HER based on earth abundant materials. [NiFeSe] hydrogenases, a subclass of [NiFe] hydrogenase enzymes with a selenocysteine (Sec) replacing a cysteine (Cys) residue terminally bound to the Ni center, display higher activities for HER and greater O₂-tolerance than the conventional sulfur only [NiFe] hydrogenases. However, the role selenium plays in these [NiFeSe] hydrogenases is currently unclear. Molecular systems bearing the Ni-Se motif that are active catalysts for HER and are stable under strong acidic reducing conditions necessary for practical solar-to-fuel converting devices have yet to be developed. Recently, we have successfully synthesized cobalt and nickel selenolate coordination polymers based on benzene-1,2,4,5-tetraselenolate (BTSe), the first examples of molecular models of [NiFeSe] hydrogenase that efficiently catalyze HER in acidic aqueous media. Fourier-transform spectroscopy and x-ray photoelectron spectroscopy techniques were used to structurally characterize the metal selenolate coordination polymers. The ability to catalyze HER in acidic aqueous media was investigated using a variety of electrochemical techniques and the efficiency was determined using gas chromatography. To reach a current density of 10 mA/cm², the benchmarking metric for HER, both cobalt and nickel systems display overpotentials of only ̴ 350 mV, representing the first highly active molecular models of [NiFeSe] hydrogenase. In addition, the cobalt selenolate polymer, which operates with 100% Faradaic efficiency, displays a 217 mV improvement in the overpotential compared to the sulfur only analogue we have previously reported in our laboratory. This enhancement in activity upon replacement of sulfur with selenium is analogous to the increased activity of [NiFeSe] hydrogenase in comparison to [NiFe] hydrogenase allowing for further understanding and insight into the unique importance of selenium in improving HER activity and leading to the continued development of earth-abundant biomimetic catalysts for practical solar-to-hydrogen converting devices.