Precisely controlled electronic state of Fe-Ni3P electrocatalyst via moderate phosphorization for urea-assisted overall water splitting

Phosphorization process is one of the most practical and efficient way for electronic modification of transition metals. However, achieving precise control on the electronic states of transition metal catalysts is difficult during phosphorization process for remarkable catalytic capacities in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we provided a novel method of precisely adjusting the electronic states of Ni and Fe via moderate phosphorization process, which exerts the synergistic effect between the electroactive elemental Fe and Ni3P species. The transformation of Ni to Ni3P and preservation of elemental Fe via moderate phosphorization were of significant essence for the electrocatalytic advances towards HER and OER processes. The density of theory (DFT) calculations showed that the total density of states (TDOS) of Fe/Ni3P (moderate phosphorization) is more than 2.5 times higher than that of FeP/Ni2P (complete phosphorization). Simultaneously, the Fe contribution to the TDOS in the form of FeP is 6 times less than that in the form of Fe element and the Ni contribution in the form of Ni2P is also not competitive with that in the form of Ni3P. The resulting FeNi3P@NF-2P electrocatalyst exhibits 208 and 239 mV OER overpotentials at 10 and 100 mA cm-2, as well as 137 mV for HER at 10 mA cm-2. A 1.567 V cell voltage was required to electrochemically decompose water at 10 mA cm-2. Surprisingly, by adding 0.33 M urea in the electrolyte, the cell voltage was reduced as low as 1.499 V. We demonstrate a novel approach of preparing electrocatalysts by precisely tuning the electronic properties of metals via moderate phosphorization.