The electrochemical hydrogen evolution reaction (HER) is considered a sustainable energy approach to advance fuel-cell technologies. Although the development of HER electrocatalysts is enriching, homogeneous HER electrocatalysis that resembles the enzymatic HER pathway, such as the HER activity of [NiFe] hydrogenases, is highly desired. Several reported Ni-based HER electrocatalysts bearing flexible coordination spheres employ Ni0 and NiII-hydride species, yet they do not follow a biomimetic mechanism. Herein, we report electrochemical HER with turnover frequencies up to 500,000 s–1 using a bioinspired NiII complex, (NCHS2)Ni(OTf)2, bearing a 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane (NCHS2) ligand in the presence of CF3CO2H (≤0.043 M) and H2O (≥1 M) in MeCN electrolyte. (NCHS2)Ni(OTf)2 coverts into [(NCHS2)Ni(MeCN)2]2+ upon dissolution into MeCN, and, interestingly, the NCHS2 ligand limits the electroreduction of [(NCHS2)Ni(MeCN)2]2+ up to NiI oxidation state upon activating the C–H bond of the ligated NCHS2, and thus results in NiI and NiIII species formation, following a mechanism similar to that of [NiFe] hydrogenases. Electron paramagnetic resonance (EPR) spectroscopy was also employed to probe such NiI and NiIII intermediates. Furthermore, we present catalytic Tafel plots to benchmark the HER activity of our Ni complex versus other reported Ni-based HER electrocatalysts, revealing that our complex can achieve comparatively high turnover frequency at low acid concentration, without compromising the overpotential. Finally, the (NCHS2)Ni(OTf)2 complex sets an example of a “constrained geometry catalyst” that is biomimetic and heralds a new generation of HER electrocatalysts.

Molecular Ni(II) electrocatalysts resemble [NiFe] hydrogenases pathway for H2 evolution

ACS Fall 2021

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