Morphological modulation of iron carbide embedded nitrogen-doped hierarchically porous carbon by manganese doping as highly efficient bifunctional electrocatalysts for overall water splitting

In the development of water splitting, the sluggish electrocatalytic kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have restricted their energy conversion efficiencies. Along with the continuous rise in the prices of noble metals and transition metals (such as cobalt and nickel), constructing high-efficiency HER/OER catalysts based on low cost transition metals, such as iron and manganese, is becoming more meaningful in developing industrialized water splitting devices. In this paper, in the absence of a template or active agent, three-dimensional, hierarchically porous FexMny nanoparticles (NPs) were embedded and nitrogen-doped carbon materials (denoted as FexMny@NC; x:y, representing the molar ratio of Fe:Mn) were successfully prepared via pyrolysis of corresponding precursors containing different metallic salt components. Various morphological, structural, and chemical characterization analysis demonstrate that at an Fe:Mn molar ratio of 3:1, the optimal Fe3Mn1@NC material shows high graphitization degree, rich mesoporous structures, a large surface area, and abundant carbon defects/edges, which promote the uniform dispersion of pyridinic-N (pyridinic-N-metal), graphitic-N, carbon oxygen bonds (CO), manganese oxide (MnO) nanocrystals, and Fe3C NPs-embedded, N-doped carbon sheet (Fe3C@NC) active sites. In alkaline conditions, the HER onset potentials (Eonset) and potentials recorded at 10 mA cm-2 (E10) of the optimal Fe3Mn1@NC are just 84.8 and 156 mV more negative than those of 20 wt% platinum carbon (Pt/C). Meanwhile, the OER Eonset and E10 values of the optimal Fe3Mn1@NC are just 8 and 18.7 mV more positive than those of RuO2. Furthermore, optimized Fe3Mn1@NC catalysts were assembled into a water splitting cell, where the catalytic current density achieves 10 mA cm-2 at a low voltage of 1.6287 V (with superior catalytic stability), which is just 24.9 mV higher than that of the (-) 20 wt% Pt/C||RuO2 (+) benchmark (1.6038 V) under the same conditions. This work describes the regulating efficiency of Mn toward growing mesopores and opens new possibilities for the development of novel carbonaceous catalysts with excellent hydroxide catalytic efficiencies based on low cost Mn/Fe elements.