Subsurface oxygen reduction reaction activity on Ti2N MXene revealed by in situ Raman spectroelectrochemistry

Abstract

Pt-based catalysts are generally used for the oxygen reduction reaction (ORR) in batteries and fuel cells but due to their high price, low abundance, and poor stability, alternative materials are needed. Herein, we investigate the electrocatalytic ORR activity of a Ti₂N nitride MXene in alkaline medium. First, we synthesize the Ti₂N MXene via oxygen-assisted molten salt etching and delamination via tetramethylammonium hydroxide (TMAOH). We then perform characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD analysis shows clear etching of the Al from the parent MAX phase with a shift in the (002) peak to lower 2θ values. SEM images show visible accordion-like morphology in the clay and lateral flake sizes of roughly 5 μm. The MXene catalyst undergoes a four-electron (4e⁻) ORR catalysis and follows a reaction mechanism different from Pt/C (Tafel slope of 69.3 mV dec⁻¹vs. 94.8 mV dec⁻¹). The new ORR catalyst has an onset potential of 0.70 V vs. RHE and better ORR stability compared to Pt/C and Ti₃C₂ carbide MXene with 81.97% current retention over 15 hours of chronoamperometry. We use in situ Raman spectroelectrochemistry to uncover the mechanisms of ORR in real-time. We find that the subsurface played a significant role in the ORR catalysis and that the adsorption of O₂ on the basal plane was the rate-determining step. These results expand the applications of MXenes in electrocatalysis, which in turn will accelerate the discovery of cost-effective and efficient catalysts for metal-air batteries and fuel cells.