Hydrogen adsorption engineering by intramolecular proton transfer on 2D nanosheets

Abstract

Proton transfer has been intensively researched in the catalysis of reactions involving hydrogen, such as the hydrogen evolution reaction (HER), oxygen evolution reaction, and carbon dioxide reduction. Recently, two-dimensional (2D) materials have gained attention as catalysts for these reactions, and their catalytic effect upon changing the size, shape, thickness, and phase has been studied. However, there are no reports on the role of proton transfer in catalysis by 2D materials. Here, a novel way to enhance the catalytic effect of 2D MoS2 was demonstrated via functionalization with four different organic moieties: phenyl–Me, phenyl–OMe, phenyl–OH, and phenyl–COOH groups. The role of proton transfer in 2D catalysis was carefully investigated via electrochemical kinetic analysis and electrical measurement. The best HER performance was observed with proton-donating COOH-functionalized active materials due to intramolecular proton transfer, which shows potential in hydrogen adsorption engineering using proton transfer. In addition, other molecularly functionalized 2D catalysts, including MoTe2 and graphene, also show proton transfer due to the incorporation of organic moieties, providing enhanced HER performance. Adding organic molecules to two-dimensional materials can reduce the Gibbs free energy of energy-releasing chemical reactions. Hydrogen is a renewable and environmentally-friendly source of energy. Fuel cells release this potential energy and create electricity when a chemical reaction at an electrode oxidizes the hydrogen and leaves just protons. These then generate a current as they cross the cell to a second electrode. Catalysts that increase the rate of this reaction thus improve the performance of the fuel cell. Using electrochemical kinetic analysis and electrical measurement, Hyoyoung Lee from Sungkyunkwan University, Suwon, South Korea and colleagues investigated proton transfer in two-dimensional catalysts to which they had added various phenyl derivatives. They showed that these organic molecules both reduced the energy at which proton transfer begins and improved device stability. In this study, we investigate the effect of surface functional group of 2D materials on the hydrogen evolution reaction (HER) and it shows both band state (ΔGH) and the wettability of 2D catalyst influence on the onset potential. In particular, the COOH functionalized 2D materials demonstrate good catalytic effect and good stability during HER because the COOH moiety increases the polarization of the electrode related to wettability as well as reduces the hydrogen absorption energy of the Mo atom and S atom through proton transfer.