Synergistic effect of the bifunctional polydopamine–Mn3O4 composite electrocatalyst for vanadium redox flow batteries

Abstract

In this work, a novel catalyst based on the polydopamine (PDA)–Mn₃O₄ composite was synthesized and optimized by varying PDA and Mn₃O₄ loadings. PDA was used to immobilize Mn₃O₄ nanoparticles due to its abundant functional groups and self-polymerization properties. Bifunctional PDA acts as not only a bridge between Mn₃O₄ nanoparticles and graphite felt (GF) but also a catalyst for the VO²⁺/VO₂⁺ redox reaction. The composite electrocatalyst significantly improves the electrochemical performance of GF on which the VO²⁺/VO₂⁺ redox reaction tends to be sluggish. Interestingly, it was found that there was a synergistic effect in the PDA–Mn₃O₄ composite which resulted in better electrochemical catalytic activity than pure PDA or pure Mn₃O₄. The electrochemical kinetics of the optimum PDA–Mn₃O₄ GF with 4 h PDA polymerization time and 50 ppm Mn₃O₄ was investigated by cyclic voltammetry and the results showed that the diffusion coefficient of PDA–Mn₃O₄ GF was approximately twice that of blank GF. As expected, the optimum composite GF exhibited substantially improved performance in the vanadium redox flow battery (VRFB) single cell test. The electrolyte utilization of the VRFB with PDA–Mn₃O₄ GF was 94% while that of the VRFB with blank GF was 85%. The voltage efficiency (VE) and energy efficiency (EE) of the VRFB with PDA–Mn₃O₄ GF were increased by 5.9% and 5.2%, respectively at high current density (150 mA cm⁻²) compared to those of blank GF due to the synergistic effect of PDA–Mn₃O₄ GF. Moreover, PDA–Mn₃O₄ GF showed outstanding stability in both the stability test and VRFB single cell operation due to the strong interaction between PDA and Mn₃O₄. Compared with previously reported electrode materials based on modified GF, the novel catalyst described in this work exhibited higher energy efficiency at comparable or higher current density. Furthermore, the superior stability of Mn₃O₄ on the electrode with the use of bifunctional PDA could improve the lifetime of VRFBs by hindering catalyst loss and dissolution during cycles.