Selective electro-oxidation of phenol to benzoquinone/hydroquinone on polyaniline enhances capacitance and cycling stability of polyaniline electrodes

Polyaniline (PANI)-based conductive polymers are promising electrode candidates for energy storage, but they suffer the common hurdle of poor cycling stability due to structural rupture during the doping and de-doping processes. Electro-oxidation of phenol is an attractive method for removing phenol from contaminated wastewaters, but the method usually suffers from electrode fouling due to formation of non-conductive polymers on the anode. Here, we propose an environmentally benign strategy that addresses these two issues simultaneously, which can substantially enhance the cycling stability of PANI and prevent passivation of the electrode. We show that, in comparison with the bare graphite brush (GB) electrode, the PANI-GB electrode (aniline electro-polymerized on GB) enables high selectivity of phenol conversion to the redox-active benzoquinone (BQ)/hydroquinone (HQ). These species are largely immobilized on the electrode, as evidenced via chemical identification and physicochemical characterization. Meanwhile, the P/40-PANI-GB electrode (obtained from CV scans of phenol on PANI-GB over 40 cycles) achieves a larger specific capacitance and, more importantly, a remarkably higher capacitance retention rate compared to the PANI-GB electrode. Such improvements are demonstrated to be caused by the availability of a second BQ/HQ redox system and the strong affinity between PANI and BQ/HQ via the interactions of pi-pi stacking, hydrogen bonding, and chemical doping.