Fig. 4: Transient ETS quantifications of H sorption processes and CO₂RR performances during potentiostatic electrolysis.

a Phase transition response of Pd and Pd₄Ag under potentiostatic conditions in different electrolytes (0.1 M). The potential was first kept at 0.5 V_RHE for 10 s and then shifted to −0.38 V_RHE for 100 s. Film thickness is ~25 nm. b Schematic illustration of M−H states and CO₂RR performances of Pd and Pd₄Ag in near neutral conditions. “PD” in (b) represents proton donor. The yellow and brown cycles represent the adsorbed H on the surface (H_ads) and absorbed H in the bulk (H_abs), respectively. c Summary of equilibrium H/M ratios obtained in different electrolytes at 100 s. The H/M ratios are obtained by first normalizing the responses in (a) with reference to the maximum phase transition responses (Pd: 45%, Pd₄Ag: 13%) obtained in 0.1 M HClO₄, and then referring to the published quantitative relationship between the resistivity and H/M ratios of pure Pd and Pd₈₀Ag₂₀ alloy (Fig. S14). On the top shows the pH and pK_a values of the proton donors with relatively high proton-donating capacities. The pK_a of 0, 10.33, 7.21, and 6.35 corresponds to H₃O⁺, HCO₃⁻, H₂PO₄⁻, and H₂CO₃, respectively. d Summary of the time for 50% and 90% level of maximum phase transition in Pd and Pd₄Ag. The error bars in (c, d) present the standard errors in two repeated ETS experiments. e Proposed mechanism for proton donation, H sorption and CO₂RR on Pd-based materials in near neutral conditions. “M−int” in (e) represents the surface adsorbed intermediates on metals during CO₂RR. Source data are provided as a Source data file.