Amine-ligand modulated ruthenium nanoclusters as a superior bi-functional hydrogen electrocatalyst in alkaline media

Abstract

An active and stable bi-functional catalyst for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is highly desirable for clean energy conversion devices, such as anion exchange membrane fuel cells (AEMFCs) and magnesium (Mg)–seawater batteries. Herein, DFT analysis predicates that the amine-ligand modulation manipulates the d-band center of ruthenium (Ru) downshift from the Fermi level, resulting in optimized hydrogen and hydroxide bonding energies (HBE and OHBE), and thus lowering the limiting energy of the HOR/HER. We elaborately fabricate NH₂-ligand modulated Ru nanoclusters (Ru/PEI-XC), taking advantage of the abundant –NH₂ groups of polyethylene imine (PEI). The optimal Ru/PEI-XC catalyst exhibits a superior HOR activity of 423.3 A g_metal⁻¹ at an overpotential (η) of 50 mV and a specific exchange current density of 687.1 μA cm_metal⁻², which are about 1.7 and 3.6 fold those of the commercial Pt/C catalyst. Ru/PEI-XC presents an overwhelming advantage for the HER in a wide pH range, especially in alkaline electrolyte, with η = 13 mV at −10 mA cm⁻², which is 20 mV lower than that of Pt/C. Both the potentiostatic and accelerated degradation tests manifest the excellent long-term stability of Ru/PEI-XC in catalyzing the HOR and HER. Furthermore, Ru/PEI-XC as the anode of an AEMFC delivers a peak power density of 1.4 W mg_PGM⁻¹, comparable with that of its Pt/C-based counterpart. A proof-of-concept rechargeable Mg–seawater battery could also be driven by the Ru/PEI-XC cathode, delivering a maximum discharging power density of 18.9 mW cm⁻², with low charging voltage and good cycling properties. This study reveals that ligand modulation is an effective strategy to manipulate the d-band center of metals and tune the reactivity, which paves an avenue for designing advanced hydrogen electrocatalysts.