We first probe the ability of TM foils to abstract protons from phosphate anions with cyclic voltammetry (CV, Figure and S1). Phosphate-mediated HER (Figure , black dashed lines) is observed as a mass-transfer limited peak at potentials less-cathodic than water-dissociation (Supplementary Figures and), where protons are easier to extract from phosphate anions than from water. The comparable mass-transfer limited current density (ca. 2 mA/cm 2 geo;