Given the grim situation of global warming and energy crisis, replacing traditional energy conversions based on carbon cycle with water cycle is a sustainable development trend. The synergistic electrocatalysis for value-added chemical production through oxygen species (O_ads: OH*, O*, and OOH*) and the active hydrogen species (H_ads) derived from water splitting powered by “green” electricity from renewable energy resource (wind, solar, etc.) is a promising manner, because of its reduced energy consumption and emission and high Faradaic efficiency. The study and summarization of catalytic mechanism of synergistic electrocatalysis are particularly significant, but are rarely involved. In this review, recent progress of various synergistic electrocatalysis systems for generating valuable products based on water cycle is systematically summarized. Importantly, the catalytic mechanism of synergistic electrocatalysis and the positive effect of O_ads and H_ads species produced by water splitting during the synergistic electrocatalytsis are detailedly elucidated. Furthermore, the regulation of water-derived O_ads and H_ads species for achieving efficient matchability of synergistic electrocatalysis is emphatically discussed. Finally, we propose the limitations and future goals of this synergistic system based on water cycle. This review is guidance for design of synergistic electrocatalysis architectures for producing valuable substances based on water cycle.

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.