Selective structure transformation for NiFe/NiFe2O4 embedded porous nitrogen-doped carbon nanosphere with improved oxygen evolution reaction activity

A bottleneck is currently met on the very promising NiFe2O4-based catalyst for oxygen evolution reaction (OER) in water splitting. Herein a significantly improved OER catalytic performance was found by selective phosphating of NiFe/NiFe2O4 embedded into porous nitrogen-doped carbon nanosphere (FeNi-FeNiO/CNS), that resulted in structure evolution by transferring the low active sites of FeNi alloy to more active amorphous metal phosphate/NiFe2O4 in the hybrid catalyst (P-FeNiO/CNS). The spectroscopic probing and microscopic observation clearly showed the structure evolution from the hybrid NiFe/NiFe2O4 to metal phosphate/NiFe2O4, and a greatly improved catalytic performance for oxygen evolution was thus obtained even a very low amount of metal was present in the system. Specifically, a quite low overpotential of 220 mV was required on P-FeNiO/CNS loaded on an inert glassy carbon electrode to drive 10 mA cm−2, about 70 mV less than that of pristine FeNi-FeNiO/CNS and 92 mV less than that of the commercial benchmark IrO2 catalyst. The catalytic activity and efficiency also outperformed most of the analogous Fe-Ni based catalysts. The theory analysis demonstrated FeNi alloy had the lowest activity, NiFe2O4 was inferior and metal phosphate/NiFe2O4 had the superior performance for OER. The highly improved catalytic performance on P-FeNiO/CNS could be attributed to the phosphating process resulting in electronic structure modulation, more active sites exposure on the surface and multi-components synergistic effect for the catalytic process. This finding has important implications for OER performance improvement of NiFe2O4 catalysts system and opens a novel way to boost the performance of other NiFe based catalysts for water oxidation.