In-situ fabrication of bimetallic FeCoO-FeCoS heterostructure for high-efficient alkaline freshwater/seawater electrolysis.

Rational construction of bifunctional electrocatalysts with long-term stability and high electrocatalytic activity is of great importance, but it is challenging to obtain highly efficient non-precious metal-based catalysts for overall seawater electrolysis. Herein, a nickel foam (NF) self-supporting CoFe-layered double hydroxide (CoFe-LDH/NF) was directly converted into FeCoO-FeCoS heterostructure via hydrothermal method in 50 mM NaS solution, instead of FeCoO@FeCoS core-shell structure. The FeCoO-FeCoS heterojunction shows nanosheets structure with rough surface (the thickness of ∼ 198.9 nm), which provides rich oxide/sulfide interfaces, high electrochemical active area, a large number of active sites, as well as fast charge and mass transfer. In 1.0 M KOH solution, 1.0 M KOH + 0.5 M NaCl, and alkaline natural seawater, the FeCoO-FeCoS heterojunction exhibits eminently electrocatalytic performance, with overpotentials of η = 225 mV, η = 233 mV, and η = 238 mV for OER, as well as η = 271 mV, η = 273 mV, and η = 277 mV for HER, respectively. Furthermore, self-supporting FeCoO-FeCoS electrode (FeCoO-FeCoS/NF) as the cathode and anode of an electrolyzer exhibits a lower cell voltage of E = 1.75 V in alkaline seawater than those of FeCoS/NF (1.77 V), CoFe-LDH/NF (1.87 V), and FeCoO/NF (1.91 V). Specifically, FeCoO-FeCoS electrolyzer can stably produce hydrogen for over 48 h in alkaline freshwater/seawater electrolyte. These outstanding electrocatalytic performances and corrosion resistance to salty-water can be attributed to the surface reconstruction behavior of the FeCoO-FeCoS/NF catalyst during OER, which leads to the in-situ formation of metal oxyhydroxides. In particular, the FeCoO-FeCoS heterojunction is also very competitive among most state-of-the-art non-noble metal-based catalysts, whether in KOH or alkaline salty-water electrolytes.