and Supplementary Note 2 for details). To simplify the model, we utilized a monoprotic organic acid (AH) with a pKa = 2, equivalent to that of the first deprotonation reaction for maleic acid. One final component of the model that was necessary to match simulated pH to experiments for each electrode spacing was a sheet resistor in the electrochemical cell. The sheet resistance could be due to formation of a thin nonconductive polymer film on the anode surface that causes ohmic drop. The wider electrode spacing (640 𝜇m) required a larger sheet resistance to match the simulated to the experimental pH values, which could be due to the larger applied steady potentials (See Supplementary Note 2 for details).