Iridium-Cluster-Implanted Ruthenium Phosphide Electrocatalyst for Hydrogen Evolution Reaction

Introduction

Water electrolysis for hydrogen evolution reaction (HER) is a promising hydrogen production method. However, this process typically relies on Pt catalysts, which are expensive despite exhibiting high intrinsic activity [1]. Development of non-Pt HER catalysts is highly important for the implementation of green hydrogen energy [2]. Ruthenium phosphides are promising candidates in this regard because Ru is cheaper and more earth-abundant than Pt [3, 4]. The phosphidation of transition metals is known to afford positively charged metal sites and negatively charged phosphorus sites, thereby altering the Gibbs free energy of hydrogen adsorption (ΔG_H*) in favor of hydrogen production [5, 6]. In addition, ruthenium phosphides are more stable and resistant to cathodic corrosion than Ru-metal catalysts under HER conditions [7–9].

The HER mechanism depends on the pH of the electrolyte. In acidic media, HER activity depends only on ΔG_H* [10], whereas in alkaline media, it is influenced by both the free energy of hydrogen adsorption and the energy barrier of water dissociation (H₂O + e⁻ + * → H* + OH⁻) [11]. Thus, achieving satisfactory HER performance is difficult without considering pH conditions. For example, the energy barrier for water dissociation on the Pt (111) surface is + 0.89 eV [12], while the corresponding Gibbs free energy of hydrogen adsorption is − 0.20 eV [13]. These results theoretically support the low HER activity of Pt crystals in alkaline media, which often requires extrinsic modifications to achieve pH-universal activity.

Tingting et al. investigated the HER activity of different ruthenium phosphides (Ru₂P, RuP, and RuP₂) through experiments and theoretical calculations. The results demonstrated that Ru₂P exhibits the highest intrinsic HER activity under acidic conditions [8]. Moreover, the formation of a heterostructure between Ru and Ru₂P resulted in charge redistribution, leading to enhanced HER performance due to a higher electronic state at the Fermi level [14]. The tensile strain induced in ruthenium phosphides by heteroatom doping has been reported to shift the d-orbital level, promoting hydrogen adsorption and water dissociation [15]. However, the strategy of regulating electronic structure to weaken the binding of hydrogen intermediates in ruthenium phosphides remains challenging.

The incorporation of cationic or anionic external elements has been widely employed as an effective strategy to modulate the...