Covalent Attachment of Cobalt Bis(Benzylaminedithiolate) to Reduced Graphene Oxide as a Thin-Film Electrocatalyst for Hydrogen Production with Remarkable Dioxygen Tolerance
Virginia A. Larson, Nicolai Lehnert
Abstract
Hydrogen (H 2 ) can be produced in the water splitting reaction, specifically on the cathodic side through the hydrogen evolution reaction (HER), where two protons and two electrons are combined to make H 2 . Herein, we report a molecular catalyst, [cobalt bis(benzylammoniumdithiolate)] +, covalently attached to graphene oxide (GO) as a thin-film catalyst for HER. A member of the acclaimed cobalt dithiolene family of HER catalysts, this complex was characterized by UV–vis and paramagnetic 1 H NMR spectroscopy, cyclic voltammetry, and mass spectrometry, showing properties similar to those of known cobalt bis(benzenedithiolate)-type complexes. The amine-modified complex is then covalently attached to GO through reaction with epoxide groups, and the resulting GO–Co suspension is drop-cast onto glassy carbon electrodes to give thin films. These films were characterized by atomic force and scanning electron microscopy, which show wrinkled films with a thickness of 330 ± 120 nm. When reduced, the reduced graphene oxide (RGO)-[cobalt bis(benzylammoniumdithiolate)] + films ( RGO-1 ) show high activity for electrocatalytic hydrogen production in acidic aqueous conditions with turnover frequencies of up to 1000 s –1 at pH 0, an overpotential of 273 ± 5 mV at pH 3, and a Faradaic efficiency (FE) of 97 ± 4%. Excitingly, with atmospheric levels of dioxygen, RGO-1 remains completely stable and delivers a 79 ± 3% FE for the HER. Kinetic and thermodynamic electrocatalysis parameters are further provided, including analysis of the onset potentials, foot-of-the-wave analysis, Tafel slopes, and plateau currents. The latter gives a rate constant of 1.5 × 10 4 M –1 s –1 for HER for RGO-1 . Controlled potential electrolysis for multiple hours shows improved activity and durability over those of the analogous physisorbed systems. This study of a HER molecular catalyst immobilized in thin RGO films continues to develop our understanding of thin-film electrocatalysis for the advancement of both clean hydrogen production and other electrocatalytic reactions related to clean energy and chemical syntheses.