Fig. 4: Finite element simulation, DFT calculations and in situ synchrotron X-ray absorption spectroscopic study.

a Color maps of electron density distribution on the surfaces of electrodes with different curvature radii of 2, 4, and 6 nm. b Simulated surface density distribution of K⁺ on a tip electrode with a curvature radius of 2 nm. c The applied potential-dependent local K⁺ concentration on different curved tips. d Free energy diagrams of H₂O adsorption/dissociation on different sites without K⁺ (denoted as E-field) and with K⁺ (denoted as K⁺/E-field). e PDOS of Ru and Ni sites with K⁺ on RuNi/NC surface toward H₂O adsorption. f Free energy diagrams of H* adsorption on different sites without K⁺ (denoted as E-field) and with K⁺ (denoted as K⁺/E-field). g Comparison of H₂O dissociation and H* adsorption free energies on different sites. h Schematic illustration of the proposed HER catalytic mechanism on RuNi/NC. i XANES and (j) EXAFS spectra at the Ru K-edge, and (k) XANES and (l) EXAFS spectra at the Ni K-edge of RuNi/NC recorded at different applied voltages during electrocatalytic HER process.