Engineering built-in electric fields through p-n heterojunction construction for enhanced dual-functional water electrolysis in Co2P/Fe2P systems

The rational design of heterostructured electrocatalysts with efficient charge transfer pathways is crucial for advancing sustainable water-splitting technology. In this work, a bimetallic Co 2 P/Fe 2 P heterojunction catalyst is successfully synthesized on nickel foam (NF) through a hydrothermal method followed by controlled phosphorization. The heterointerface between Co 2 P and Fe 2 P induces a strong built-in electric field (BEF) due to their distinct Fermi level alignment, as confirmed by Mott-Schottky analysis and Kelvin probe force microscopy (KPFM). This BEF gives rise to electron-deficient Co regions and electron-enriched Fe regions near the heterointerface, which is expected to modulate the local electronic structure and facilitate interfacial charge transfer, thereby benefiting the overall HER/OER kinetics. The as-prepared Co 2 P/Fe 2 P@NF exhibits exceptional bifunctional activity in alkaline media, achieving low overpotentials of 109 mV for the hydrogen evolution reaction (HER) and 165 mV for the oxygen evolution reaction (OER) at 20 mA cm −2 , with Tafel slopes of 54.8 mV dec −1 and 31.5 mV dec −1 , respectively. A two-electrode electrolyzer assembled with Co 2 P/Fe 2 P@NF as both anode and cathode requires only 1.50 V to deliver 20 mA cm −2 , surpassing most reported non-noble metal catalysts. Systematic characterizations (XPS, TEM, EIS) reveal that the hierarchical nanoplate structure with abundant heterointerfaces provides a high electrochemical surface area (17.4 mF cm −2 ) and facilitates electrolyte penetration. Moreover, the catalyst exhibits outstanding long-term durability, retaining at least 98% of its initial performance after 20 h of continuous operation at 20 mA cm −2 under both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) conditions. This work highlights the pivotal role of interfacial BEF engineering in developing high-performance electrocatalysts for practical water-splitting applications.