Reversing the Rate versus Overpotential Scaling in Electrocatalytic Hydrogen Evolution Induced by Electrostatic Field Engineering at Remote Sites

Abstract

The rate versus overpotential relationship arising from molecular scaling effects is a key challenge in developing efficient electrocatalysts. Circumventing these unyielding scaling relationships is a persistent challenge in the field of electrocatalysis. Escaping this iron law of rate versus overpotential requires judicious modulation, which can balance the electrophilic and nucleophilic/basic characteristics of the electrocatalyst, which is rarely straightforward. Here, we report local electrostatic field-persuaded inverse scaling relationships of the electrocatalytic hydrogen evolution reaction (HER). For this purpose, we report six mononuclear NiII complexes 1–6, where 4, 5, and 6 contain positively charged quaternary ammonium moieties at remote sites. Without perturbing the primary coordination sphere, this molecular scaffold allows the installation of a local positive charge, which would increase the electrophilic character, favoring electron transfer processes. This is reflected by the anodic shift in the reductive redox potentials from neutral to charge-appended complexes (1 to 6). Since the catalytically relevant anionic/reduced species are achieved easily via electrostatic stabilization by the local positive charge, this leads to a lower overpotential. Density functional theory (DFT) calculations reveal that the appended local positive charge facilitates the r.d.s. step by ground-state destabilization of the starting intermediate, which leads to a lower activation barrier and facilitates the HER. Thus, the present study demonstrates the optimum balance between the electrophilic character and basicity of the electrocatalyst, assisted by the local positive charge, leading to inverse scaling relationships. To the best of our knowledge, this is the first demonstration of a positive electrostatic field-assisted enhanced HER activity with a gradual decrease in overpotential.