Ligand-Tuned Energetics for the Selective Synthesis of Ni 2 P and Ni 12 P 5 Possessing Bifunctional Electrocatalytic Activity toward Hydrogen Evolution and Hydrazine Oxidation Reactions.

The occurrence of many phases and stoichiometries of nickel phosphides calls for the development of synthetic levers to selectively produce phases with purity. Herein, thiol (-SH) and carboxylate (-COO - ) functional groups in ligands were found to effectively tune the energetics of nickel phosphide phases during hydrothermal synthesis. The initial kinetic product Ni 2 P transforms into thermodynamically stable Ni 12 P 5 at longer reaction times. The binding of carboxylate onto Ni 2 P promotes this phase transformation to produce pure-phase Ni 12 P 5 within 5 h compared to previous reports (∼48 h). Thiol-containing ligands inhibit this transformation process by providing higher stability to the Ni 2 P phase. Cysteine-capped Ni 2 P showed excellent geometric and intrinsic electrocatalytic activity toward both hydrogen evolution and hydrazine oxidation reactions under alkaline conditions. This bifunctional electrocatalytic nature enables cysteine-capped Ni 2 P to promote hydrazine-assisted hydrogen generation that requires lower energy (0.46 V to achieve 10 mA/cm geo 2 ) compared to the conventional overall water splitting (1.81 V to achieve 10 mA/cm geo 2 ) for hydrogen generation.