Covalent Attachment of Cobalt Bis(Benzylaminedithiolate) to Reduced Graphene Oxide as a Thin-Film Electrocatalyst for Hydrogen Production with Remarkable Dioxygen Tolerance

Abstract

Hydrogen (H2) 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 H2. 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 1H 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 × 104 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.

Structural and Spectroscopic Characterization of Co(II) Bis(Benzenedichlorodithiolate): An Intermediate in Hydrogen Evolution Catalysis

“…Previously, others and ourselves have shown that Co bis(benzenedithiolate) type complexes are highly active catalysts for the Hydrogen Evolution Reaction (HER). This includes homogenous electrocatalytic HER [1], homogeneous photocatalytic HER [1,2], and heterogeneous HER when either physisorbed to electrode surfaces [3][4][5], as active sites in MOFs [6][7][8][9], or, as recently reported, when covalently attached to an electrode surface [10]. These catalysts perform well in HER compared to other heterogenized earth-abundant transition-metal molecular complexes that have been reported in the literature [10][11][12].…”

“…This includes homogenous electrocatalytic HER [1], homogeneous photocatalytic HER [1,2], and heterogeneous HER when either physisorbed to electrode surfaces [3][4][5], as active sites in MOFs [6][7][8][9], or, as recently reported, when covalently attached to an electrode surface [10]. These catalysts perform well in HER compared to other heterogenized earth-abundant transition-metal molecular complexes that have been reported in the literature [10][11][12]. However, Co bis(benzenedithiolate) type complexes are often shown to decline in activity over time [4,5,10].…”

“…These catalysts perform well in HER compared to other heterogenized earth-abundant transition-metal molecular complexes that have been reported in the literature [10][11][12]. However, Co bis(benzenedithiolate) type complexes are often shown to decline in activity over time [4,5,10]. Therefore, it is desirable to better understand the molecular mechanism of these catalysts to elucidate the reason(s) for catalyst failure and to obtain insight into how these catalysts function in HER, both homogenously and heterogeneously.…”

“…As catalysts for HER [1][2][3][4][5]10], some information has been obtained about the putative HER mechanism of these complexes. However, how the non-innocent benzenedithiolate ligands impact HER catalysis exactly is not well understood.…”

“…In this paper, however, we show that the Co(II) form of the complex is in fact in the doublet spin state with a square-planar geometric structure, providing further experimental insight into the HER mechanism of Co bis(benzenedithiolate) type complexes. As catalysts for HER [1][2][3][4][5]10], some information has been obtained about the putative HER mechanism of these complexes. However, how the non-innocent benzenedithiolate ligands impact HER catalysis exactly is not well understood.…”