Function of Internal and External Fe in a Ni-Based Precatalyst System Toward Oxygen Evolution Reaction.

It is well known that the "iron" impurity will influence the oxygen evolution reaction (OER) in an alkaline electrolyte, especially for the Ni-based electrocatalyst. Many research studies have investigated the function of Fe in the OER active phase, such as M (OH) 2 / M OOH ( M = Ni and/or Fe), while, surprisingly, very few studies have examined the function of Fe in the "precatalyst" system. Accordingly, in this work, the Ni 3- x Fe x P ( x = 0, 0.5, 1) series as an Ni-based precatalyst was employed to inspect the function of internal and external Fe in the Ni-based precatalyst system. It was realized that the sample with internal Fe (i.e., Ni 2.5 Fe 0.5 P and Ni 2 FeP) exhibits efficient OER activity compared to that of the Fe-free one (i.e., Ni 3 P) owing to the large amount of active M (OH) 2 / M OOH formed on the surface. This indicates that the internal Fe in the present system may have the ability to facilitate the phase transformation; it was later rationalized from electronic structural calculations that the d band center of the internal Fe (middle transition metal) and Ni (late transition metal) holds the key for this observation. Adding excessive ferrous chloride tetrahydrate (FeCl 2 ·4H 2 O) as the external Fe in the electrolyte will greatly improve the OER performances for Ni 3 P; nevertheless, that the OER activity of Ni 2 FeP is still much superior than that of Ni 3 P corroborates the fact that the Fe impurity is not the only reason for the elevated OER activity of Ni 2 FeP and that internal Fe is also critical to the phase transformation as well as OER performance.