Exploring a bimetallic catalyst family for hydrogen oxidation with insights into superior activity and durability

Abstract

Anion exchange membrane fuel cells are limited by the slow kinetics of the alkaline hydrogen oxidation reaction (HOR). Aided by density functional theory combined with fine-tuned machine learning interatomic potential, we establish a family of bimetallic catalysts with controlled surface atomic arrangements to identify the optimal catalysts for HOR. Our theoretical analysis successfully predicts the HOR activity rankings of these catalysts, consistent with the experimental results. RuIr exhibits the highest activity, followed by PtRu, AuIr, PtRh, PtIr, PtAu, RhIr, RuRh, AuRu, and AuRh. These trends correlate with the electron-accepting tendencies and the adsorption strengths of H₂ and OH* on the catalysts. Among all candidates, RuIr emerges as the most active and durable bimetallic catalyst. Furthermore, operando X-ray absorption spectroscopy and electrochemical measurements reveal a strong synergistic effect of RuIr, where Ir exhibits superior electron-accepting tendency and strong H₂ adsorption, while Ru demonstrates strong OH* adsorption, accelerating the alkaline HOR process.

Fig. 1. Integrating DFT calculations and MLIP model for surface and adsorption analysis.

a Depiction of the optimized iridium (Ir) metal surface (slab model) simulating iridium grown on palladium (Pd) cubic seeds ($\text{Fm}\overline{3}\text{m}$) and (001) surface. This panel includes the calculated electrostatic potential across the slab used to determine the work function ($\varphi =V_{vac}-E_F$), where $V_{vac}$ is the vacuum level, $E_F$ is the calculated Fermi level, and $V_{avg}$ is the average electrostatic potential across the slab. b Adsorption of H₂ and OH* on metal surfaces, illustrating various molecule configurations and adsorption sites identified via the Delaunay triangulation algorithm. A pre-trained MLIP model (CHGNet) was fine-tuned with additional DFT calculations to estimate total energies and to rapidly screen configurations of adsorbates and adsorption sites. Lower energy structures were re-evaluated with precise DFT calculations to identify the most favorable adsorbate-adsorbent configurations. c Comparative analysis of the relative Fermi levels and adsorption energies of H₂ and OH* on various metal surfaces, all conforming to Pd (001) symmetry. d Summary of the features of various metals and their bimetallic alloys based on DFT-calculated electron-giving tendencies and H₂/OH* adsorption strengths. More negative Fermi levels and adsorption energies indicate stronger electron accepting and adsorption capabilities, respectively. Features of the superior metal in bimetallic catalysts were chosen to represent the combined properties. e Predicted HOR activity sequence derived from DFT calculations for single metals and bimetals, prioritizing electron acceptance followed by H₂ and OH* adsorption strength.

Fig. 2. Synthetic design, growth kinetics, and structural and compositional characterizations of Pd@Ru_0.47Ir_0.53 core-shell nanocubes.

a Schematic of epitaxial growth to obtain bimetallic atomic layers on Pd cubic seeds enclosed by {100} facets. b–f TEM, HAADF-STEM, FFT, and EDS-mapping analysis of Pd@Ru_0.47Ir_0.53 core-shell nanocubes. g ICP-OES analysis of Ru and Ir elements in the RuIr atomic layers. h XPS spectrum of Pd@Ru_0.47Ir_0.53 core-shell nanocubes for Ru 3p. i XPS spectrum of Pd@Ru_0.47Ir_0.53 core-shell nanocubes for Ir 4f. Source data for Fig. 2 are provided as a Source Data file.

Fig. 3. Electronic interaction and coordination environment of Pd@RuIr core-shell nanocubes.

a XANES spectra of Pd@RuIr core-shell nanocubes and their corresponding metallic foils and oxidation states at the Ru K-edge. b XANES spectra of Pd@RuIr core-shell nanocubes and their corresponding metallic foils and oxidation states at the Ir L₃-edge. c FT-EXAFS spectra of Pd@RuIr core-shell nanocubes and their corresponding metallic foils and oxidation states at the Ru K-edge. d FT-EXAFS spectra of Pd@RuIr core-shell nanocubes and their corresponding metallic foils and oxidation states at the Ir L₃-edge. e FT-EXAFS spectra (lines) and curve fits (points) of Pd@RuIr core-shell nanocubes at the Ru K-edge. f FT-EXAFS spectra (lines) and curve fits (points) of Pd@RuIr core-shell nanocubes at the Ir L₃-edge. g–i WT-EXAFS contour maps of Pd@RuIr core-shell nanocubes, Ru foil, RuO₂ at Ru K-edge. j–l WT-EXAFS contour maps of Pd@RuIr core-shell nanocubes, Ir foil, IrO₂ at Ir L₃-edge. Source data for Fig. 3 are provided as a Source Data file.

Fig. 4. HOR activity of Pd@M core-shell nanocubes (M = Ir, Pt, Ru, Rh, and Au), Pd@bimetallic alloy core-shell nanocubes with different bimetallic combinations of shells and commercial Pt/C.

a Polarization curves of Pd@M core-shell nanocubes (M = Ir, Pt, Ru, Rh, and Au) and commercial Pt/C in H₂-saturated 0.1 M KOH at a scan rate of 5 mV s⁻¹ and rotating speed of 1600 rpm. b Polarization curves of Pd@bimetallic alloy core-shell nanocubes in H₂-saturated 0.1 M KOH at a rotating speed of 1600 rpm with a scan rate of 5 mV s⁻¹. c Comparison of the specific activity at 0.05 V_RHE. These data are obtained with no iR correction. Source data for this Figure are provided as a Source Data file.

Fig. 5. HOR durability.

CV curves in H₂-saturated 0.1 M KOH and rotating speed of 1600 rpm of a Pd@Ru_0.47Ir_0.53, b Pd@Pt_0.51Ir_0.49, c Pd@Au_0.45Rh_0.55 core-shell nanocubes, d Commercial Pt/C at 1st, 750th, 1500th, and 3000th cycles. e HAADF-STEM and EDS-mapping analysis of Pd@Ru_0.47Ir_0.53 core-shell nanocubes after 3000 CV cycles. These data are obtained with no iR correction. Source data for this Figure are provided as a Source Data file.

Fig. 6. Catalytic mechanism revealed by operando XAS and electrochemical characterization.

a, b Operando XANES spectra and FT-EXAFS spectra of RuIr atomic layers measured under air, OCP and applied potentials of 0.65 V_RHE, 0.90 V_RHE, and 1.05 V_RHE during alkaline HOR at the Ru K-edge. c, d Operando XANES spectra and FT-EXAFS spectra of RuIr atomic layers measured under air, OCP and applied potentials of 0.65 V_RHE, 0.90 V_RHE, and 1.05 V_RHE during alkaline HOR at the Ir L₃-edge. e CV curves of Pd@Ru_0.47Ir_0.53 core-shell nanocubes, Pd@Ru core-shell nanocubes, Pd@Ir core-shell nanocubes and commercial Pt/C in N₂-saturated 0.1 M KOH. f CO stripping curves of Pd@Ru_0.47Ir_0.53 core-shell nanocubes, Pd@Ru core-shell nanocubes, Pd@Ir core-shell nanocubes and commercial Pt/C. g Adsorption of OH* on Ru, Ir, and RuIr surfaces, shown in both top and cross-sectional views, along with calculated OH* adsorption energies. h Schematic of the alkaline HOR catalytic mechanism. These data are obtained with no iR correction. Source data for this Figure are provided as a Source Data file.