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

Microbial electrolysis cells (MECs) for hydrogen (H 2 ) 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 H 2 production under hypersaline conditions (up to 14 % NaCl). The halophilic archaeon Haloferax volcanii was encapsulated within a self-assembling fluorenylmethoxycarbonyl-diphenylalanine (Fmoc-FF)/Ti 3 C 2 T x MXene conductive hydrogel and immobilized on carbon felt, providing a ready-to-use biocatalytic interface. The hydrogel-immobilized biocathode produced H 2 at 28.89 μmol L⁻ 1 h⁻ 1 at 7 % NaCl, compared with 12.80 μmol L⁻ 1 h⁻ 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. H 2 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 H 2 production. H. volcanii -encapsulated bioanodes likewise increased H 2 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 H 2 production. To our knowledge, this is the first integration of a halophilic archaeon with a conductive Fmoc-FF/MXene hydrogel for bioelectrochemical H 2 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.