Fig. 3: CO₂RR and H sorption processes in KHCO₃ and K₂HPO₄/KH₂PO₄ electrolytes.

a–c Potential-dependent selectivity for formate production (a), and current densities for formate production (b), and hydrogen evolution (c) with Pd (red) and Pd₄Ag (blue) catalysts in 0.1 M CO₂-saturated KHCO₃ (solid curves) and K₂HPO₄/KH₂PO₄ (dash curves). d Phase transition responses of Pd in 0.1 M Ar-saturated KHCO₃ and HClO₄ with varying scan rates. Film thickness is ~200 nm. e–h On-chip I_G−V_G (CV, dash, left y axis) and I_SD−V_G (ETS, solid, right y axis) curves of Pd (e, g) and Pd₄Ag (f, h) in 0.1 M Ar- and CO₂-saturated KHCO₃. g and h depict the enlarged negative-potential-sweeping ETS curves (0.2 to −0.5 V_RHE) in (e) and (f), respectively. i–l On-chip I_G−V_G (CV, dash, left y axis) and I_SD−V_G (ETS, solid, right y axis) curves of Pd (i, k) and Pd₄Ag (j, l) in 0.1 M Ar- and CO₂-saturated K₂HPO₄/KH₂PO₄. k and l depict the enlarged negative-potential-sweeping ETS curves (0.2 to −0.5 V_RHE) in (i) and (j), respectively. Solid arrows in (d–l) indicate the potential sweeping direction. Film thickness is ~200 nm. The gradient background (from red to blue color) in (a–c, g, h, k, l) represents the increased CO poisoning effect concluded from Figs. S15, 16. The rectangular shadings in (d, e, f, i, j) highlight the H sorption regions reflected on ETS in specific electrolytes (green in KHCO₃ and red in K₂HPO₄/KH₂PO₄). m Schematic illustration of different Pd−H states in KHCO₃ (left panel) and K₂HPO₄/KH₂PO₄ (right panel), and the corresponding CO₂RR processes at the interfaces. The yellow and brown cycles represent the adsorbed H on the surface (H_ads) and absorbed H in the bulk (H_abs), respectively. n Summary of phase transition potentials of Pd (blue) and Pd₄Ag (red) under CO₂RR conditions in KHCO₃ and K₂HPO₄/KH₂PO₄ obtained at 10 mV/s. The error bars in (n) present the standard error in two ETS experiments with film thicknesses of ~25 nm and ~200 nm. Source data are provided as a Source data file.