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

Abstract

Because of low cost and high abundance, iron-based electrocatalysts are deemed as a potential substitute of platinum for the hydrogen evolution reaction (HER). Herein, mesoporous core-shell-type iron phosphide/iron oxide (FeP/Fe3O4) coated with few ultrathin carbon layers is synthesized and evaluated for the HER. The FeP/Fe3O4 was obtained through the partial phosphidation of Fe3O4 mesoporous microspheres. Findings suggest that even partial phosphidation activates the surface of Fe3O4 for the HER, and FeP/Fe3O4 outperforms pure FeP. Although FeP/Fe3O4 has higher electrochemical impedance and charge transfer resistance compared to FeP, the FeP/Fe3O4 electrode exhibits superior performance in both acidic and basic electrolytes. In acidic solution, the η10 values for FeP/Fe3O4/C and FeP/C are approximately 90 and 135 mVRHE, respectively, while in basic medium, they are approximately 303 and 261 mVRHE. In addition, the specific activity of the FeP/Fe3O4, normalized to the electrochemically active surface area, is better than that of the FeP electrode. The superior performance of the FeP/Fe3O4 is correlated to its active centers and turnover frequency (TOF). The number of active sites in the FeP/Fe3O4 is 1.58 × 10−8 mol, while in the FeP it is 1.2 × 10−8 mol. At η = 90 mVRHE, the TOF of the FeP/Fe3O4 electrode is estimated to be 0.47 s−1, which is ~2 fold higher than that of FeP (0.47 s−1). Estimation of exchange current density (io) and Tafel slopes indicate a faster HER kinetics at the catalytic interface of the FeP/Fe3O4 (0.18 mA cm–2, 62 mV dec–1) than that of FeP (0.12 mA cm–2, 89 mV dec–1). In addition, the FeP/Fe3O4 electrode shows stable current density (20 mA cm–2) for 24 h of continuous operation. Two spin-polarized DFT models were used to obtain information on Gibbs free energy (ΔGH) and the corresponding adsorption energy (ΔEH). 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 provide insight into the stability of the phosphide surface, both with and without carbon layers.