General and scalable preparation of Prussian blue analogues on arbitrary conductive substrates and their derived metal phosphides as highly efficient and ultra-long-life bifunctional electrocatalysts for overall water splitting

Prussian blue analogs (PBA) and their derivatives show great application potentials in energy and environmental fields. However, the preparation of PBA and their derivatives on specific substrates are still far from practical use. Herein, we propose an innovative general and scalable strategy to grow PBA coatings on any conductive substrates including Ni, Co, stainless steel, Mo, W, Ti, Ta, Ni/carbon cloth (CC), and FTO with large sizes and high curvatures through spontaneous growth or electrodeposited seeds assisted epitaxial growth in solution. The potential difference between Fe³⁺/Fe²⁺ redox pair in the solution and metal ions/metal redox pair of the metal substrate may promote the release of metal ions from the substrate and induce the spontaneous growth. As a demonstration, PBA/Ni/CC samples prepared on Ni/CC substrates with this approach were transformed into nickel iron phosphides (NiFeP/CC) via a simple phosphidation process, which show excellent bifunctional electrocatalytic activities towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) due to the unique structures and desirable chemical compositions. It only requires very low overpotentials of 260 mV to achieve 200 mA cm⁻² for OER and 129 mV to achieve 10 mA cm⁻² for HER, respectively. Furthermore, the NiFeP/CC|| NiFeP/CC couple requires a low cell voltage of 1.57 V to reach 10 mA cm⁻² and shows an ultralong lifetime with no obvious degradation after electrolyzing for 300 h, which is much superior to many other reported electrocatalysts. This work provides a facile strategy for designing efficient bifunctional electrocatalysts derived from spontaneously grown PBAs.