FeCoCrNiMo High-Entropy Alloys Enable Extracellular Electron Transfer for Enhanced CO2 Biomethanogenesis

To simultaneously promote both direct (via electron carriers) and indirect electron transfer (via hydrogen carriers) in the microbial electroreduction of CO2 to CH4, FeCoCrNiMo high-entropy alloys (HEAs) combined with methanogens were proposed herein. Experimental results showed that spherical HEA heterostructures significantly enhanced the electrical conductivity and catalytic activity. FeCoCrNiMo HEAs increased the abundance of hydrogenotrophic methanogens (e.g., Methanobacterium) and electroactive bacteria (e.g., Ureibacillus), as well as the expression of conductive pilus-associated gene (pilA) to promote electron transfer. FeCoCrNiMo HEAs also increased the content of aromatic proteins and tryptophan in extracellular polymeric substances, enhancing biofilm conductivity, while the uniform dispersion of the five metal elements reduced both biofilm resistance and charge transfer resistance. Therefore, HEAs enhanced the maximum methane production rate by 6.58 times (2.17 mmol L–1 day–1) with 90.6% faradaic efficiency. This study demonstrated that FeCoCrNiMo HEAs promoted electron shuttling efficiency, biofilm electroactivity, and stability, highlighting their potential as multifunctional promoters for driving CO2 reduction to methane.