Ultrathin carbon layer-coated mesoporous core-shell-type FeP/Fe₂O₃/C for the hydrogen evolution reaction

Due to their low cost and high abundance, iron-based electrocatalysts are considered a promising alternative to platinum for the hydrogen evolution reaction (HER). Herein, we synthesized and evaluated a mesoporous core-shell-type iron phosphide/iron oxide (FeP/Fe₃O₄) coated with few ultrathin carbon layers as an electrocatalyst for the HER. FeP/Fe₃O₄ was produced through the partial phosphidation of Fe₃O₄ mesoporous microspheres. Our findings indicate that even partial phosphidation activates the surface of Fe₃O₄ for the HER and that FeP/Fe₃O₄ outperforms pure FeP. Although FeP/Fe₃O₄ exhibited higher electrochemical impedance and charge-transfer resistance compared to FeP, the FeP/Fe₃O₄ electrode demonstrated superior performance in both acidic and basic electrolytes. In acidic solution, the η₁₀ values for FeP/Fe₃O₄/C and FeP/C were approximately 90 and 135 mV_RHE, respectively, while in basic medium, they were approximately 303 and 261 mV_RHE. In addition, the specific activity of the FeP/Fe₃O₄ electrode, normalized to the electrochemically active surface area, surpassed that of the FeP electrode. The superior performance of FeP/Fe₃O₄ was linked to its active centers and turnover frequency (TOF). Specifically, the number of active sites in FeP/Fe₃O₄ was 1.58 × 10⁻⁸ mol, whereas in FeP, it was 1.2 × 10⁻⁸ mol. At η = 90 mV_RHE, the TOF of the FeP/Fe₃O₄ electrode was estimated to be 0.47 s⁻¹, approximately 2-fold higher than that of FeP (0.47 s⁻¹). Estimation of the exchange current density (i_o) and Tafel slopes indicated faster HER kinetics at the catalytic interface of FeP/Fe₃O₄ (0.18 mA cm⁻², 62 mV dec⁻¹) compared to FeP (0.12 mA cm⁻², 89 mV dec⁻¹). In addition, the FeP/Fe₃O₄ electrode maintained a stable current density (20 mA cm⁻²) for 24 h of continuous operation. Two spin-polarized DFT models were used to obtain information on the Gibbs free energy (ΔG_H) and the corresponding adsorption energy (ΔE_H). These models included a FeP surface with and without carbon layers, as well as a surface consisting of FeP and Fe₃O₄. In addition, the calculations offered insights into the stability of the phosphide surface, both with and without carbon layers.