In-operando XAS to unravel the surface modifications of Nickel based electrocatalysts in the hydrogen evolution reaction. A comparison of pu

Nickel and its alloys have extraordinary electrocatalytic activity towards hydrogen evolution reaction (HER), being suitable substitute of platinum group metal (PGM) to cut water electrolyzers cost. Still, the identification of the oxidation state of the metal active sites and its reversibility during operation is lacking for these materials; it is crucial to understand and improve the electrocatalyst reactivity and durability. The oxidation state of Ni and NiM alloys (M=Fe,Mo), when subject to polarization in an electrochemical (EC) cell will be evaluated through X-ray Adsorption Spectroscopy (XAS). The primary goal will be to probe variations in metal speciation, via XANES and EXAFS, both ex-situ and in-operando. This study will: i) reveal the dynamics of variation of Ni active sites oxidation state during constant voltage polarization; ii) identify the reversibility of the process; iii) identify the effect of the exposure of the electrocatalyst to the air (ex situ vs in operando).