Surface reconstruction in amorphous CoFe-based hydroxides/crystalline phosphide heterostructure for accelerated saline water electrolysis

Developing electrocatalysts with high activity and robust performance for large-scale seawater electrolysis to produce hydrogen holds immense significance. Herein, a highly active bifunctional electrode composed of amorphous cobalt-iron layered double hydroxides (CoFeLDH) and crystalline nickel phosphide (Ni₂P) (denoted as CoFeLDH@Ni₂P), is employed to boost hydrogen production through seawater electrolysis. The strong interface coupling effectively modifies the electronic structure at active sites, thereby accelerating the catalytic reaction kinetics. Impressively, in situ Raman and post-stability analyses demonstrate a unique reconstruction behavior on the CoFeLDH@Ni₂P electrode. Bimetal co-incorporated NiOOH (CoFe-NiOOH) and Ni(OH)₂ species are formed during the oxygen evolution reaction (OER), while CoFeLDH@Ni₂P can transform into Ni(OH)₂ species during the hydrogen evolution reaction (HER) process. Furthermore, the highly negatively charged surface selectively rejects Cl^- ions by formed PO₄^3-, endowing CoFeLDH@Ni₂P with excellent tolerance and promising durability in saline electrolytes. Consequently, the CoFeLDH@Ni₂P electrode exhibits an overpotential of 106 mV for HER at 10 mA cm^-2 and 308 mV for OER to achieve 100 mA cm^-2 in 1.0 M KOH solution. Additionally, the CoFeLDH@Ni₂P_(+,-) electrolyzer requires a low cell voltage of 1.56 V to deliver 10 mA cm^-2 in 1.0 M KOH + Seasalt. This work presents an appealing strategy for the rational design of advanced electrocatalysts with amorphous-crystalline interfaces, which reveals the source of the activity of transition-metal phosphating compounds in saline water electrolysis.