Hydrogen evolution reaction mechanism on Ti3C2 MXene revealed by in situ/operando Raman spectroelectrochemistry.

ABSTRACT

MXenes have shown great promise as electrocatalysts for the hydrogen evolution reaction (HER), but their mechanism is still poorly understood. Currently, the benchmark Ti3C2 MXene suffers from a large overpotential. In order to reduce this overpotential, modifications must be made to the structure to increase the reaction rate of the H+/e- coupled transfer steps. These modifications heavily depend on understanding the HER mechanism. To remedy this, in situ/operando Raman spectroelectrochemistry combined with density functional theory (DFT) calculations are utilized to probe the HER mechanism of the Ti3C2 MXene catalyst in aqueous media. In acidic electrolytes, the -O- termination groups are protonated to form Ti-OH bonds, followed by protonation of the adjacent Ti site, leading to H2 formation. DFT calculations show that the large overpotential is due to the lack of an optimum balance between O and Ti sites. In neutral electrolytes, H2O reduction occurs on the surface and leads to surface protonation, followed by H2 formation. This results in an overcharging of the structure that leads to the observed large HER overpotential. This study provides new insights into the HER mechanisms of MXene catalysts and a pathway forward to design efficient and cost-effective catalysts for HER and related electrochemical energy conversion systems.