Synergistically well-mixed MOFs grown on nickel foam as highly efficient durable bifunctional electrocatalysts for overall water splitting at high current densities

Highlights: • Well-mixed Fe- and Ni-MOFs in-situ grown on Ni foam via one-step hydrothermal process. • Achieve ultralow η₅₀₀ of 234 and 294 mV for HER and OER, respectively. • Require only 1.495 V@10 mA cm⁻² and 1.80 V@500 mA cm⁻² for full water splitting. • Exhibit ultrastability at 500 mA cm⁻² for overall water-splitting over 100 h. -- Abstract: Metal-organic framework (MOF), possessing versatile catalytic activities, remarkable structural diversity, high surface areas, and tunable pore sizes, was recently demonstrated an outstanding catalyst for electrolytic water splitting. The electrolytic performances can be much enhanced with a synergistic design of the MOFs, which was realized as uniformly well-mixed and dispersed Fe- and Ni-MOFs (termed as MFN-MOFs) in-situ grown on backbones of nickel foam (NF). The electrocatalyst thus developed exhibited ultra-high activities at high current densities for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH, delivering 10 and 500 mA cm⁻² at ultralow overpotentials of 79 and 234 mV, respectively with a small Tafel slope of 30.1 mV dec⁻¹ for the HER and achieving ultralow overpotentials of 235 and 294 mV for the OER at 50 and 500 mA cm⁻², respectively with a small Tafel slope of 55.4 mV dec⁻¹. The inter-molecular synergistic interactions between the well-mixed and dispersed Fe- and Ni-MOFs not only facilitate the critical charge transfer for the redox reactions but also disperse the active metal ion sites to enhance their degree of utilization to achieve exceptional OER and HER performances. The MFN-MOFs/NF//MFN-MOFs/NF couple exhibited ultralow cell voltages of 1.495 V@10 mA cm⁻² and 1.80 V@500 mA cm⁻² with a Tafel slope of only 79.5 mV dec⁻¹ in 1 M KOH, outperforming most of the state-of-the-art bifunctional electrode couples and benchmark couple of Pt-C/NF//IrO₂/NF. More importantly, the MFN-MOFs/NF electrode exhibited ultrastability at high current densities, with a minor decay of 3.7% in a chronopotentiometric stability test conducted at a commercially viable high current density of 500 mA cm⁻² for overall water-splitting over 100 h. This synergistic effect boosting design concept for electrocatalysts was successfully demonstrated, opening up a new way for catalyst design toward large-scale H₂ production.