Unraveling the Trade-Off Effect of Pyrogenic Carbons Between Biopseudocapacitors and Bioconductors During Anaerobic Methanogenesis

Pyrogenic carbons (PCs), with varying structures depending on the materials and thermal treatment conditions, have been extensively used to enhance anaerobic digestion by mediating electron transfer. However, the underlying mechanism has yet to be explored. Herein, the redirection and enhancement of the direct interspecies electron transfer (DIET) pathway were evidenced, along with the upregulated electrochemical properties and structural proteins in the methanogenic consortia. Further, we found that PCs featured trade-off properties of "biopseudocapacitor" and "bioconductor" during thermal treatment, as endowed by the evolution of oxygen-containing functional groups (for charging and discharging) and graphitic structure (for conductivity). Correspondingly, their trade-off effect on mediating syntrophic methanogenesis (SM) was realized between the generally acknowledged bioconductor role and the pseudocapacitive effect, as highlighted by the enhanced SM of reduced PCs from more balanced electron exchange capacities. Consequently, a performance comparison of PCs obtained at 450, 650, and 850 °C in SM resulted in an optimized sample at 650 °C, where a 61.3 ± 1.8% increase in methane production rate and a 33.4 ± 1.1% decrease in lag time were observed. Microbiologically, DIET-active Methanothrix and Geobacteraceae flourished with the intra- and extracellular electron transport channels established. These findings provide new insights into the mediating mechanism and renewable potential of PCs in regulating energy-harvesting biochemical processes toward carbon neutrality.