Tailored Electrodes for Efficient Electrochemical-Biomethanogenic CO 2 Reduction Using Wastewater-Derived Catholytes

An electrolyzer-bioreactor assembly using real wastewater-streams as catholyte allowed for performing bioelectrochemical methanogenesis closer than ever to practical implementation. Hereto, we developed a novel zero-gap cell with a porous transport layer at the cathode specifically concepted for operating in-line with a wastewater-fed methanogenic reactor. In-depth analysis of the electrode composition indicated an up to 4-fold variation of the current density depending on the binder and carbon black contents. This electrolyzer configuration and electrode selection along with a robust pentlandite type cathode catalyst, Fe3Ni3Co3S8, allowed for an efficient supply of hydrogen (H2) into the catholyte that recirculated through the bioreactor, which was dominated by Methanobacterium and Methanobrevibacter archaea. This hybrid system produced methane over 220 days with weekly average rates up to 669 LNm-2cathode d-1 with current densities of 30 mA cm-2 at only 2.2 V corresponding to a 45% energy efficiency, which is a landmark performance for those systems.