The role of iron in Ni-Fe binary catalysts for electrochemical glycerol oxidation.

NiFe binary compounds have shown great potential in the electrocatalytic oxidation of biomass-derived glycerol towards valuable products. However, understanding the role of Fe on the glycerol oxidation process of Ni-based catalysts remains challenging due to the complex multi-alcohol-group oxidation. Herein, our systematic study identifies that Fe can improve both the dynamic process of Ni(OH)2 transformation into active NiOOH and the kinetic process of redox reaction between NiOOH and glycerol. Specifically, the introduction of Fe can significantly reduce the onset potential of glycerol oxidation by > 100 mV while maintaining > 95 % selectivity of HCOOH. The in situ Raman results show that the Ni2+/Ni3+ conversion potential is exactly the same as the onset potential of glycerol oxidation, indicating that NiOOH is the most important active structure, and Fe doping can optimize the dynamic process of NiOOH generation. The electrochemical dynamic analysis reveals that the oxidation of glycerol on Ni-based catalysts follows an electrochemical-chemical process, and the chemical reaction constant (k) value is greatly improved by 5 times with the presence of Fe. Density functional theory (DFT) calculation reveals that the electron transfer from Fe to Ni can help the extraction of electrons from adsorbed glycerol, thus lowering the reaction barrier and accelerating the reaction rate. Based on this, we constructed an efficient photoanode with Metal-Insulator-Semiconductor (MIS) junction structure (n-Si/SiOx/Ni3Fe1) that demonstrated the highest glycerol oxidation photocurrent and excellent selectivity for formate.