Iron phosphide encapsulated in P-doped graphitic carbon as efficient and stable electrocatalyst for hydrogen and oxygen evolution reactions.

The development of durable and efficient non-noble electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is highly desirable but challenging for the commercialization of renewable energy systems. Herein, a facile strategy is developed for the synthesis of iron phosphide (FeP) nanoparticles protected with an overcoat of "multifunctional" P-doped graphitic carbon as a cost-effective electrocatalyst. The key point is the utilization of MOF-derived iron nanoparticles embedded in graphitic carbon (Fe@GC), which are synthesized via the pyrolysis of the Fe-MIL-88 template and subsequent phosphorization of Fe and simultaneous doping of P in carbon. Compared to the direct phosphorization of Fe-MIL-88, resulting in Fe2P on amorphous carbon (Fe2P@APC), this strategy gives easier access to phosphorization and P doping through pyrolysis temperature regulation. High-temperature pyrolysis can also yield the graphitic carbon encapsulated nanoparticle structure (FeP@GPC), which increases conductivity and prevents agglomeration as well as dissolution under harsh operating conditions, and thus contributes to enhanced activity and long-time stability. The optimized FeP@GPC exhibits superior activity compared to Fe2P/FeP@GPC and Fe2P@APC, which is attributed to the modified electronic structure of FeP due to its greater P proportion than Fe2P together with the strong synergy between the nanoparticles and graphitic carbon. In detail, FeP@GPC exhibits an ultralow overpotential of 72 mV and 278 mV to achieve the current density of 10 mA cm-2 for the HER in acid and OER in alkaline media, respectively, together with negligible degradation after 20 h, which ranks among the best Fe-based electrocatalysts.