2.5. Cyclic voltammetry

The cyclic voltammetry of sulfite solution has been performed with many electrodes [20,23,24], and we attempt to further investigate the sulfur species evolution on Ti/MMO electrode in this study. Both 0-10 mM sulfite and 100 mM sodium sulfate electrolyte were prepared in 200 mL solution. To run cyclic voltammetry for the sulfite solution, a Ti/MMO electrode (2 cm²) was used as the working electrode, a Pt electrode (4 cm²) as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. Scans were performed from −0.6 V to 1.7 V (vs SHE, standard hydrogen electrode) at a rate of 50 mV/s with SP-300 electrochemical workstation (BioLogic, France). Stable and closed cyclic voltammetry curves, which represent the steady-state electrocatalytic reactions, were obtained typically after two runs. 0.1 mM EDTA solution was used when stated, to exclude the effect of metal catalysis for sulfite oxidation on Ti/MMO anode. All cyclic voltammetry assays were performed at pH 7.