Microbial electrosynthesis (MES) can sustainably convert CO₂ to products and significant research is currently being conducted towards this end, mainly in laboratory-scale studies. The high-cost ion exchange membrane, however, is one of the main reasons hindering the industrialization of MES. This study investigates the conversion of CO₂ (as a sole external carbon source) to CH₄ using membraneless MES inoculated with anaerobic granular sludge. Three types of electrodes were tested: carbon cloth (CC) and CC functionalized with Cu NPs, where Cu NPs were deposited for 15 and 45 min, respectively. During the MES experiment, which lasted for 144 days (six cycles), methane was consistently higher in the serum bottles with CC electrodes and applied voltage. The highest CH₄ (around 46%) was found in the second cycle after 16 days. The system’s performance declined during the following cycles; nevertheless, the CH₄ composition was twice as high compared to the serum bottles without voltage. The MES with Cu NPs functionalized CC electrodes had a higher performance than the MES with plain CC electrodes. Microbial profile analysis showed that the Methanobacterium was the most dominant genus in all samples and it was found in higher abundance on the cathodes, followed by the anodes, and then in the suspended biomass. The genus Geobacter was identified only on the anodes regarding relative bacterial abundance at around 6–10%. Desulfovibrio was the most dominant genus in the cathodes; however, its relative abundance was significantly higher for the cathodes with Cu NPs.

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This work evaluates the performance of three configurations (V1, V2, and V3) of novel membraneless continuous-flow ultrasonic-assisted plasma-driven solution electrolysis (PDSE) reactors (CombiSonoPlasma^®) operated at near-ambient temperature and pressure, for the single-pass treatment of model hypersaline produced water (HPW) originating from the oil extraction industry, with simultaneous hydrogen co-production. The performance of the reactors was assessed based on (i) the formation of gaseous oil decomposition products (CO, CO₂, and CH₄) as indicators of crude oil destruction and (ii) hydrogen generation as a valuable energy co-product. The results demonstrate that coupling ultrasonication with PDSE provides a bidirectional synergistic effect: ultrasonication reduces the PDSE electric power demand, while PDSE operation simultaneously decreases the power consumption of the ultrasonic generator. Among the investigated configurations, along with improved oil decomposition, Reactor V2 delivered the most pronounced energy benefits, achieving reductions in electrical power consumption of 33.5% for PDSE and 44.5% for the ultrasonic generator. These findings highlight the potential of continuous-flow ultrasound-assisted PDSE as an energy-efficient process intensification strategy for oilfield HPW treatment combined with hydrogen co-generation.