(Digital Presentation) Electrochemical Hydrogen Production and Charge Storage Mechanisms on Mxenes Via in-Situ/Operando Raman Spectroelectrochemistry

Abstract

The production of hydrogen fuel from water splitting through the hydrogen evolution reaction (HER) has been investigated for more than a century; yet, there exists no commercial water splitting device because of the lack of an efficient and cost-effective electrocatalyst. This scientific gap can be filled by the new class of two-dimensional carbides and nitrides, known as MXenes, if their mechanisms of electrocatalysis can be understood. Herein, we report on the electrocatalytic properties of MXenes and elucidate their mechanisms of electrocatalysis and charge storage. More specifically, we report on the detailed mechanisms of HER electrocatalysis and charge storage on the benchmark Ti3C2 carbide MXene in acidic (HCl) and neutral (Na2SO4) electrolytes using an in-situ/operando Raman spectroelectrochemical approach. This approach allows us to monitor, in real time, the structural behavior of the materials under operating conditions. The correlation between the Raman shifts and the current density provides insights into the mechanism of HER. In acidic electrolyte, we found that the HER mechanism proceeds through a surface protonation mechanism due to termination group changes, whereas neutral electrolyte HER undergoes an overcharging mechanism prior to splitting water to form hydrogen gas. For capacitive charge storage, we found that acidic electrolytes allow for pseudocapacitive behavior to take place in a similar surface protonation mechanism as for HER. In neutral electrolytes, the charge storage mechanism follows a purely double layer capacitance mechanism with sodium ions adsorbing and desorbing from the surface. The new knowledge from this work can be applied to a broad class of materials and systems to advance the field of energy conversion and storage.