A hybrid bioelectrochemical system coupling a zero-gap cell and a methanogenic reactor for carbon dioxide reduction using a wastewater-derived catholyte

Hybrid electrochemical and biological systems hold promise for producing valuable products from abundant feedstocks. An electrolyzer-bioreactor assembly using wastewater streams as catholytes could bring bioelectrochemical methanogenesis closer to practical implementation. Herein, we develop a zero-gap cell with a porous transport layer in the cathode specifically conceptualized for operating in line with a wastewater-fed methanogenic reactor. In-depth analysis of the electrode composition indicates 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, allow for an efficient supply of hydrogen into the catholyte that recirculates through the bioreactor, which is dominated by Methanobacterium and Methanobrevibacter archaea. This hybrid system produces methane over 220 days with a maximal weekly average rate of 669 LN m−2cathode d−1. The corresponding average current density sustained by the entire hybrid system is 30 mA cm-2 at a 2.2 V set potential.