Zr as Effective Modulator of Electrocatalytic Properties of M₂P (M=Ni, Co, Fe)

Transition metal phosphides (TMP) have recently attracted attention as promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) due to their superior activity and high stability compared to other alternative electrocatalysts. However, further improvements are needed to bridge the gap in HER activity with that of the standard material, Pt. The most suggested routes are to mix or dope a second metal into the representative binary TMPs, namely, Ni₂P, Co₂P, and Fe₂P, but examples of doped metals are generally limited in later 3d metals, leading to open questions about the effects of other early transition metals. Also, the randomness of doped atom distribution over the crystal structure generally obscures the analysis of accurate surface and electronic structures. Herein, we utilized Zr, an electropositive early-transition metal, to modify the crystal and electronic structure of binary M₂P (M = Ni, Co, Fe). We have successfully synthesized single-phase ternary Zr₂M₁₂P₇ (M = Ni, Co, Fe) phosphides using the metal-flux method. By sharing the same ordered crystal structure of the Zr₂Fe₁₂P₇ type, it allowed the effects of different 3d transition metals to be accurately compared. Interestingly, the inclusion of Zr enhanced the HER activity in all three M₂P, but the highest enhancement was observed in the Ni phase, with an η₁₀ reduction of 64% compared to that for Ni₂P. It is hypothesized that the Zr₂M₁₂P₇ structure has more advantageous active sites for HER compared to those of binary M₂P phases and the electronic structure is favorably modified to hydrogen adsorption. DFT calculations were performed to validate the correlation of the metal d-band center and density of states at E_F to HER activity. This result showed that other electropositive metals could be potential sources to enhance the HER activity of TMP electrocatalysts.