Figure 5: The performance of the cathodes before and after operation (a: cathode 1 treated in citrate buffer (pH 3.0), b: cathode 2 treated in acetate buffer (pH 4.0), c: cathode 3 treated in MES buffer (pH 5.5), d: cathode 4 not treated, e: cathode 5 not treated, f: cathode 6 not treated).