Controlled direct electrodeposition of crystalline NiFe/amorphous NiFe-(oxy)hydroxide on NiMo alloy as a highly efficient bifunctional electrocatalyst for overall water splitting

• Crystalline NiFe/amorphous NiFe-(oxy)hydroxide on NiMo (NM@ c NF/ a NFO) is prepared. • Precise control of electrodeposition voltage tunes the c NF/ a NFO ratio and yield. • NM@ c NF/ a NFO achieves low overpotentials and small Tafel slopes for both OER and HER. • NM@ c NF/ a NFO enables water splitting at small voltages with competitive stability. • The synergistic effects enhance the electrocatalytic performances. The synthesis of an electrocatalyst with high activity, long lifetimes, low cost, bifunctional ability, and facile fabrication strategy simultaneously for overall water splitting is urgent and remains a grand challenge. Herein, a controlled direct electrodeposition strategy is developed to deposit the heterogeneous crystalline NiFe ( c NF)/amorphous NiFe-(oxy)hydroxide ( a NFO) standing nanosheets on the NiMo modified substrate (NM@ c NF/ a NFO) as a highly efficient bifunctional electrocatalyst for overall water splitting. The precise control of electrodeposition voltage can tune the component ratio of c NF and a NFO and the yield of c NF/ a NFO nanosheets, and −3.5 V is the optimal voltage. The ultrafine NiFe nanocrystals with good electronic conductivity and abundant edges, the amorphous NiFe-(oxy)hydroxide support with rich nanosized pores and defects, and the firm grasp of the NiMo alloy toward the standing nanosheets generate complex synergistic effects, rendering abundant active sites, fast charge and mass transfer, and excellent electrochemical stability. NM@ c NF/ a NFO can achieve 20 mA cm −2 at a low overpotential of 133.2 mV with a small Tafel slope of 19.9 mV dec −1 for oxygen evolution reaction and 91.9 mV with 59.9 mV dec −1 for hydrogen evolution reaction. Accordingly, NM@ c NF/ a NFO as both anode and cathode delivers 10 mA cm −2 at a cell voltage of 1.45 V and prominent stability for over 100 h even at 100 mA cm −2 . Our work demonstrates a facile voltage-controlled electrodeposition strategy for the design of highly active, cost-effective, stable, and bifunctional electrocatalysts for water splitting.