Activating Ru in the pyramidal sites of Ru2P-type structures with earth-abundant transition metals for achieving extremely high HER activity while minimizing noble metal content

Abstract

Rational design of efficient pH-universal hydrogen evolution reaction catalysts to enable large-scale hydrogen production via electrochemical water splitting is of great significance, yet a challenging task. Herein, Ru atoms in the Ru₂P structure were replaced with M = Co, Ni, or Mo to produce M_2−xRu_xP nanocrystals. The metals show strong site preference, with Co and Ni occupying the tetrahedral sites and Ru the square pyramidal sites of the CoRuP and NiRuP Ru₂P-type structures. The presence of Co or Ni in the tetrahedral sites leads to charge redistribution for Ru and, according to density functional theory calculations, a significant increase in the Ru d-band centers. As a result, the intrinsic activity of CoRuP and NiRuP increases considerably compared to Ru₂P in both acidic and alkaline media. The effect is not observed for MoRuP, in which Mo prefers to occupy the pyramidal sites. In particular, CoRuP shows state-of-the-art activity, outperforming Ru₂P with Pt-like activity in 0.5 M H₂SO₄ (η₁₀ = 12.3 mV; η₁₀₀ = 52 mV; turnover frequency (TOF) = 4.7 s⁻¹). It remains extraordinarily active in alkaline conditions (η₁₀ = 12.9 mV; η₁₀₀ = 43.5 mV) with a TOF of 4.5 s⁻¹, which is 4x higher than that of Ru₂P and 10x that of Pt/C. Further increase in the Co content does not lead to drastic loss of activity, especially in alkaline medium, where, for example, the TOF of Co_1.9Ru_0.1P remains comparable to that of Ru₂P and higher than that of Pt/C, highlighting the viability of the adopted approach to prepare cost-efficient catalysts.