Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Magnetite/zeolite nanocomposite-modified cathode for enhancing methane generation in microbial electrochemical systems Mung Thi Vu, Md Tabish Noori, Booki Min ⁎ Department of Environmental Science and Engineering, Kyung Hee University, Seocheon-dong, Yongin-si, Gyonggi-do 446-701, Republic of Korea HIGHLIGHTS • Magnetite/zeolite (MZ) was synthesized as novel cathode catalyst in MES. • Electrochemical analyses proved superior catalytic activity of MZ for methanation. • Current generation, substrate and VFAs removal rate were promoted in MES-MZ. • MZ catalyst facilitated electron transfer by lowering cathode overpotential losses. ARTICLE INFO Keywords: Microbial electrochemical system Magnetite/zeolite nanocomposites Direct electron transfer Electromethanogenesis Methane yield ABSTRACT In this study, magnetite/zeolite (MZ) was successfully synthesized to use as a feasible and cost-effective cathode catalyst for enhancing methane generation in a microbial electrochemical system (MES). The novel MZ catalyst consists of hydrophilic zeolite cores and conductive magnetite nanoparticles for enhanced electroactive biofilm development on the cathode by facilitating micro-channels for nutrient diffusion, increased surface area, and reduced charge transfer resistance. The MES using an MZ cathode (MES-MZ) achieved a maximum methane yield of 0.38 ± 0.010 LCH 4 /gCOD, which was significantly higher than that of the control operation without a catalyst (0.33 ± 0.008 LCH 4 /gCOD). The methane production rate was increased by almost 21% from 196 mL/ (L.d) in the control MES to 238 mL/(L.d) in the MES-MZ, along with an improvement in the methane percentage from 73% to 79%. In addition, the maximum current generation was recorded using the MES-MZ at 9.29 ± 0.16 mA, which was about 16% higher than that of 8.0 ± 0.13 mA observed in the control reactor and is consistent with about a 36.2% improvement of the Coulombic efficiency. The CV and EIS analyses revealed that MZ lowered the overpotential losses during the electron transfer process, and revealed a more positive cathode potential with the MES-MZ (-0.48 V vs. Ag/AgCl), which possibly suggests direct electron transfer for the dominant pathway for the conversion of carbon dioxide to methane. 1. Introduction The emerging microbial electrochemical system (MES) with anae- robic digestion (AD) is a cutting-edge technology that enables efficient methane production and simultaneously increases wastewater treat- ment efficiency [1]. Similar to the conventional AD counterpart, a conceptual MES-AD model comprises pure or mixed bacterial commu- nities that are responsible for degrading organic matter (the substrate) and generating methane as a valuable by-product [2]. In addition, the MES enriched with electroactive microbes (exoelectrogens) can oxidize recalcitrant compounds and transfer electrons to the anode, which aids the electrochemical reduction of CO 2 that occurs spontaneously at the cathode site under a minor applied voltage [3]. Because an external electrical circuit is used to connect two electrode compartments, the MES also offers the potential recovery of electron flow to produce electricity as a complementary benefit. Although immense breakthroughs have been achieved in lab-scale MES studies, finding an inexpensive and feasible cathode catalyst still remains one of the most critical challenges to bring this technology closer to practical applications [4,5]. While the electrode assembly alone accounts for almost 20% of the total cost of an electrochemical system, the cost of the catalyst is up to 70% of the electrode price [6,7]. Previously, a precious metal Pt-based catalyst was the prime choice in the bioelectrochemical systems like MES-AD due to its excellent hy- drogen evolution catalytic activity, which accelerates methane gen- eration through the indirect electron transfer (IET) route, known as https://doi.org/10.1016/j.cej.2020.124613 Received 30 October 2019; Received in revised form 19 February 2020; Accepted 28 February 2020 ⁎ Corresponding author. E-mail address: bkmin@khu.ac.kr (B. Min). Chemical Engineering Journal 393 (2020) 124613 Available online 29 February 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved. T