Bifunctional manganese oxide–silver nanocomposites anchored on graphitic mesoporous carbon to promote oxygen reduction and inhibit cathodic biofilm growth for long-term operation of microbial fuel cells fed with sewage

Abstract

In this article, we prepared manganese oxide–silver nanocomposites anchored on graphitic mesoporous carbon (MnO_x–Ag/GMC) with different Ag nanoparticle (AgNPs) contents as cathodic catalysts to not only improve the oxygen reduction reaction (ORR) kinetics but also inhibit the biofilm growth on the cathode surface of air-cathode microbial fuel cells (MFCs) fed with sewage. The MnO_x–Ag/GMC-3 electrocatalyst, which was prepared with an Ag : MnO_x weight ratio of 1 : 1, exhibited the best performance for the ORR in an abiotic neutral media electrolyte with an onset potential and half-wave potential of 0.33 V and 0.023 V (vs. SHE), respectively, which is 1.3–1.6-fold higher than those of other prepared electrocatalysts and are comparable to the Pt/C electrocatalyst (i.e., 0.39 V and 0.119 V vs. SHE). Rotating disk electrode analysis revealed that the ORR for MnO_x–Ag/GMC nanocomposites proceeds via a mixed pathway (4-electron and 2-electron ORR), suggesting that the main mechanism of bacterial-growth inhibition was via the in situ H₂O₂ production that was subsequently decomposed to generate hydroxyl radicals, making MnO_x–Ag/GMC a potential ORR electrocatalyst for the practical application of MFCs. Notably, MnO_x–Ag/GMC nanocomposites showed high antibacterial activity in MFCs, suppressing biofilm growth on the cathode. Consequently, MFCs with MnO_x–Ag/GMC nanocomposites had a much higher maximum power density (160 mW m⁻²) compared to Pt/C cathode-based MFCs (60 mW m⁻²) with a much lower closed circuit potential decay during continuous operation for 5 months.