Mechanisms of Hydrogen Evolution Reaction in Two-Dimensional Nitride MXenes Using In Situ X‑Ray Absorption Spectroelectrochemistry
journal contributionposted on 24.02.2021, 17:34 authored by Abdoulaye Djire, Hanyu Zhang, Benjamin J. Reinhart, O. Charles Nwamba, Nathan R. Neale
Two-dimensional (2D) MXenes based on early transition-metal carbides and nitrides have highly tunable electrochemical properties including excellent catalytic activity for the hydrogen evolution reaction (HER). Compared to carbide MXenes, nitride MXenes feature metal–nitrogen bonds that are expected to impart unique electronic and structural characteristics for electrocatalysis. Recently, we showed that the HER activity can be enhanced by modifying a pristine exfoliated Ti4N3Tx MXene with various metal ions to produce mixed-metal nitride MXenes M-Ti4N3Tx (M = V, Cr, Mo, and Mn; Tx = O and/or OH). Here, we use in situ X-ray absorption spectroscopy (XAS) to elucidate the mechanisms of HER activity in these 2D M-Ti4N3Tx MXenes. X-ray absorption near-edge structure (XANES) results confirm the presence of multiple oxidation states in the Ti4N3Tx and M-Ti4N3Tx MXenes during electrochemical and HER activities. In most cases, metal-ion oxidation states are unaffected over the potential range studied except for the Cr-Ti4N3Tx MXene that undergoes a partial reduction of Cr3+ to Cr2+ during HER. The structural analysis of the M-Ti4N3Tx MXenes under electrochemical and HER conditions is provided by the extended X-ray absorption fine structure (EXAFS), which further reveals that the mechanism of HER catalysis involves the creation of oxygen vacancies on the basal planes. The XAS data are clear that these oxygen vacancies are coincident with metal-ion reduction, which combined generate active sites for HER. We find that the alloyed metal can either change the reduction potential of the Ti4+ ions (in the Mo-Ti4N3Tx MXene) or act as a single-atom catalyst (in the Cr-Ti4N3Tx MXene), both of which can be exploited to tune the activity in electrocatalysis and photocatalysis schemes.
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EXAFSMo-Ti 4 N 3 T x MXeneXAS2 D M-Ti 4 N 3 T x MXenesCr-Ti 4 N 3 T x MXeneHydrogen Evolution ReactionOHmixed-metal nitride MXenes M-Ti 4 N...HER activityTwo-Dimensional Nitride MXenesoxygen vacancieshydrogen evolution reactionmetal-ion oxidation statesM-Ti 4 N 3 T x MXenesX-ray absorption near-edge structureX-ray absorption spectroscopyXANESTi 4 N 3 T xtunable electrochemical propertiesexfoliated Ti 4 N 3 T x MXene