The current was confirmed to be from proton reduction by visible gas bubble formation at slower scan rates (Figure ) and a hydrogen test from our previous work at pH 4 . The drop-off correlates well with the concentration of any acidic proton donor. At pH 4 and 6 the current looks to be dominated by free protons and the remaining current drops away at the pK a value of dihydrogen phosphate at a pH of 7.2 where it is a higher concentration than the free H + ions. At higher pH values the protons would come from H 2 O and the catalysis would have to swap from a water dissociation mechanism, which it seems unable to perform.