Haloarchaeal conductive hydrogel bioelectrodes for enhanced hydrogen production in hypersaline microbial electrolysis cells.

Microbial electrolysis cells (MECs) for hydrogen (H2) production are typically limited to low-salinity environments, restricting their application to saline wastewaters and industrial brines. Here we developed a conductive hydrogel-based haloarchaeal bioelectrode for H2 production under hypersaline conditions (up to 14 % NaCl). The halophilic archaeon Haloferax volcanii was encapsulated within a self-assembling fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF)/Ti3C2Tx MXene conductive hydrogel and immobilized on carbon felt, providing a ready-to-use biocatalytic interface. The hydrogel-immobilized biocathode produced H2 at 28.89 μmol L⁻1h⁻1 at 7 % NaCl, compared with 12.80 μmol L⁻1h⁻1 for an abiotic control. Electrochemical impedance spectroscopy revealed reduced electrode impedance after encapsulation, and scanning electron microscopy confirmed cell immobilization within the hydrogel matrix. H2 production increased with the concentration of viable H. volcanii cells, whereas equal or higher loadings of non-viable cells did not enhance it; cells recovered from the hydrogel resumed growth, confirming that encapsulation preserved viability. Adding cell lysate to the electrolyte also enhanced H2 production. H. volcanii-encapsulated bioanodes likewise increased H2 production in dual-chamber MECs at an applied cathode potential of - 0.7 V vs. Ag/AgCl. Notably, these results were obtained with an archaeon in which no hydrogenases have been identified, pointing to unexplored mechanisms of electron transfer and H2 production. To our knowledge, this is the first integration of a halophilic archaeon with a conductive Fmoc-FF/MXene hydrogel for bioelectrochemical H2 production in media containing 7 and 14 % NaCl. These findings establish conductive hydrogel-immobilized haloarchaea as a proof-of-concept platform for extending microbial electrolysis to high-salinity environments.