Unraveling the Synergistic Mechanism of Bi-Functional Nickel–Iron Phosphides Catalysts for Overall Water Splitting

Abstract

Ni–Fe bimetallic electrocatalysts are expected to replace existing precious metal catalysts for water splitting and achieve industrial applications due to their high intrinsic activity and low cost. However, the mechanism by which Ni and Fe species synergistically enhance catalytic activity remains obscure, which still needs further in-depth study. In this study, a highly active bi-functional electrocatalyst of Ni2P/FeP heterostructures is constructed on Fe foam (Ni2P/FeP-FF), clearly illustrating the effect of Ni on Fe species for oxygen evolution reaction (OER) and revealing the true catalytic active phase for hydrogen evolution reaction (HER). The Ni2P/FeP-FF only needs overpotentials of 217 and 42 mV to reach 10 mA cm−2 for OER and HER, respectively, exhibiting superior bi-functional activity for overall water splitting. The Ni can elevate the strength of Fe-O on Ni2P/FeP-FF surface and promote the formation of high-valence FeOOH phase during OER, thus enhancing OER performance. Based on first-principles calculations and Raman characterizations, the Ni2P/Ni(OH)2 heterojunction evolved from Ni2P/FeP is identified as the real high active phase for HER. This study not only builds a near-commercial bifunctional electrocatalyst for overall water splitting, but also provides a deep insight for synergistic catalytic mechanism of Ni and Fe species.