Catalytic activity of 2D MXenes toward electroreduction processes: Oxygen reduction and hydrogen evolution reactions

In this work, 2D Mo and Ti MXenes (Mo2TiC2 and Ti3C2) were synthesised from the chemical etching of their MAX phases (Mo2TiAlC2 and Ti3AlC2). The physicochemical characterisation demonstrated the removal of Al from precursors obtaining layered structures enriched with metal carbides domains. Focused on potential clean energy applications of these materials, their catalytic activity was assessed in two reactions: the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). The ORR activity of MXenes is quite low: Mo2TiC2 developed a slightly higher activity towards ORR in acidic media than Ti3C2 MXene, with an onset potential for the reaction at 0.4 VRHE. However, 3-fold higher performances were achieved in alkaline media for all the studied materials, suggesting a strong dependence of their catalytic activity with the pH. On the other hand, Mo2TiC2 and Ti3C2 exhibited high catalytic activity toward the HER in acidic media in comparison to their commercial MAX phases and few commercial carbides. This catalytic activity was evidenced from the low onset potentials for HER, as well as low overpotentials at 10 mA cm-2, especially for Mo2TiC2 (0.17 and 0.379 VRHE, respectively). Differential electrochemical mass spectrometry (DEMS) experiments allowed to establish the onset potentials, avoiding the interference of other faradaic processes in the same potential region, and the rate determining steps (rds) for HER at the studied materials. Tafel plots from ionic currents for m/z = 2 indicate that the Heyrovsky step is the rds for HER at Mo2TiC2, whereas Volmer step is the rds at Ti3C2. Long-term stability tests for the MXenes demonstrated that these materials maintain its activity after 10 h under operation conditions. EIS experiments performed at onset and HER evolution potentials corroborated the relevance of the presence of oxide reduction processes competing with the HER, which explains the higher catalytic activity of Mo2TiC2 with respect to Ti3C2.