A dual-active Co-CoO heterojunction coupled with Ti₃C₂-MXene for highly-performance overall water splitting

Development of cost-effective and highly-efficient bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts is crucial for overall water splitting in practical utilization. Herein, we proposed a novel non-noble metal bifunctional HER/OER electrocatalyst by synergistically coupling a dual-active Co-based heterojunction (Co-CoO) with high conductive and stable two-dimensional Ti₃C₂-MXene (defined as Co-CoO/Ti₃C₂-MXene). A series of characterizations and theoretical calculations verify that the synergistic effect of metallic Co with HER activity and CoO with OER performance leads to superb bifunctional catalytic performance, and Ti₃C₂-MXene can enhance electrical conductivity and prevent the aggregation of the Co-based catalysts, thereby improving both the activity and stability. Co-CoO/Ti₃C₂-MXene presents low onset potential (η_onset) of 8 mV and Tafel slope of 47 mV·dec⁻¹ for HER (close to that of Pt/C) and η_onset of 196 mV and Tafel slope of 47 mV·dec⁻¹ for OER (superior to that of RuO₂). Assembled as an electrolyzer, Co-CoO/Ti₃C₂-MXene shows a low voltage of 1.55 V at 10 mA·cm⁻², high Faradaic efficiency and remarkable stability. It can be driven by a solar cell of ~ 1.55 V for consecutive production of hydrogen and oxygen gases.