Differential pulse voltammetry was conducted in accordance with section 4.12, with the exception of running the potential from -0.2 V to 0.4 V or -0.2 to 1.2 V for the oxidation peaks. The potentials were then reversed for the reduction peaks. Scan rates varied from 50 mV.s -1 to 10 mV.s -1 . A voltage amplitude of 25 mV was used for all samples.