Fig. 4: Spatially decoupled electrochemical strategy tackling surface passivation and securing Ca²⁺ supply.

a Schematic diagram of the passivation mitigation by anodic acidity and stable precipitation by cathodic alkalinity, with driving forces of Ca²⁺ and H₃₋ₓPO₄⁽ˣ⁻³⁾ ions labeled. b The evolution of Ca²⁺ concentration over electrolysis under various current densities. The SEM images of the pristine limestone (c) and post-use limestone (e). Element mapping of the pristine limestone (d) and post-use limestone (f). The Raman spectrum (g) and XRD pattern (h) of pristine and post-use limestone. Condition: 18 A m^–2; 1 L; 36 h. Data with error bars represented as mean ± s.d. (n = 3).