Modified electrode surfaces with hydrogen evolution reaction catalysts derived from electropolymerized complexes with redox active ligands.

The demand for energy is growing exponentially, and to keep up with these demands new technologies for renewable energy have received increased attention. Hydrogen is one of the most promising energy sources for the future and plays a vital role in water electrolysis and fuel cells, as the hydrogen evolution reaction (HER) is the main step in the water splitting process. To increase the reaction rate and improve efficiency for the water electrolysis, catalysts are used to minimize the overpotential. Most of the current electrocatalysts for HER are heterogeneous in nature and are dominated by platinum and other precious metals due to their high current density and small Tafel slope; however, they are extremely costly and have rare-earth abundance. For this reason, cost-effective catalysts must be developed. Previously, many have seen the best success by employing the use of earth-abundant transition metal chalcogenides to use as homogeneous molecular electrocatalysts, the most promising of which is molybdenum sulfide. These electrocatalysts do display low overpotentials and high HER activity; however, they contain a low number of active sites. Many have worked to address these issues. The biggest challenge with heterogeneous catalysts as a whole is the inability to do detailed mechanism investigations. Homogeneous catalysts, alternatively, have attractive properties of activity and selectivity. The main issues with homogeneous catalysts are recycling and separation from product. To combine the benefits of both heterogeneous and homogeneous catalysts, immobilization of characterized catalysts onto the solid electrode surface to allow them to work under heterogeneous conditions is proposed. This heterogenization of a homogeneous catalyst onto the electrode surface is an ideal way to study a catalyst. The goal of this work was to develop and engineer new carbon materials, while heterogenizing new and existing homogeneous thiomesicarbazone (TSC) compounds, supplied by the Grapperhaus/Buchanan Research Group, as electrocatalysis of HER. Thermodynamics, kinetics, and transport were the driving forces in the study. A series of metal complexes based on inexpensive bis-thiomesicarbazone ligands including bis-thiophenepyrrolebutylamine(BTP4A), diacetyl-bis(N-4-methyl-3-thiosemicarbazide) (ATSM), and ATSM with pyrrole attached (ATSMpy) were synthesized and characterized by NMR, IR, cyclic voltammetry, and square wave voltammetry. Modified electrodes were prepared with films deposited on glassy carbon, standard pencils, and carbon paste electrodes, and evaluated as potential HER catalysts using cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy. HER studies in 0.5 M aqueous H2SO4 (10 mA cm−2) revealed that modified electrode surfaces of glassy carbon, carbon paste, and standard pencils with TSCs gave promising electrochemical activity to be used for HER catalysis application. Pencil electrodes have shown to report improved activity due to increased surface interactions. Specifically, the blank pencil with C15 (Ni-ATSM) reported the lowest overpotential, Tafel slope, and charge transfer resistance of any sample, with overpotential values of 0.214-0.328 V. This sample combination will be further studied to prove its viability to be used as electrocatalysts with modified electrodes for HER