Heteroatom-doped carbon-encapsulated FeP nanostructure: A multifunctional electrocatalyst for zinc–air battery and water electrolyzer

Abstract

The rational design and synthesis of efficient multifunctional electrocatalysts for renewable energy technologies is of significant interest. Herein, we demonstrate a novel approach for the synthesis of a nitrogen and phosphorus dual-doped mesoporous carbon-encapsulated iron phosphide (FeP@NPC) nanostructure and its multifunctional electrocatalytic activity toward an oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction for zinc–air battery (ZAB) and alkaline water-splitting applications. FeP@NPC is obtained by the carbothermal reduction of the precursor complex [Fe(bpy)₃](PF₆)₂ in the presence of melamine without any traditional phosphidating agent. The PF₆^- counteranion is used for the phosphidation of Fe. FeP@NPC obtained at 900 °C (FeP@NPC-900) exhibits excellent bifunctional oxygen electrocatalytic performance with a very low potential gap (ΔE = E_1/2^ORR – E_j10^ORR ) of 670 mV. The ZAB device delivers a peak power density of 190.15 mW cm^-2 (iR-corrected), specific capacity of 785 mA h gZn^-1, and energy density of 706.5 Wh gZn^-1 at 50 mA cm^-2 . The ZAB exhibits excellent charge–discharge cycling stability for over 35 h with negligible voltaic efficiency loss (0.9%). Three CR2032 coin-cell-based ZABs made of an FeP@NPC-900 air cathode connected in series power 81 LEDs for 15 min. FeP@NPC-900 also has promising electrocatalytic activity toward water splitting in acidic as well as in alkaline pH. The benchmark current density of 10 mA cm^-2 is achieved with a two-electrode alkaline water electrolyzer at a cell voltage of 1.65 V. ZAB-powered water electrolyzer is made by integrating two rechargeable ZABs connected in series with the two-electrode water electrolyzer. The ZAB powers the electrolyzer for 24 h without a significant loss in the open-circuit voltage. The catalyst retains its initial structural integrity even after continuous water electrolysis for 24 h.