USE OF GAS EMISSION BY BIOELECTROCHEMICAL APPROACHES FOR POWER-TO-X

The transition toward a climate-neutral energy system requires technologies capable of con-verting surplus renewable electricity into valuable products within a circular and sustainable framework. Among these, Power-to-X (PtX) is emerging as a promising solution for energy conversion and storage. Bioelectrochemical systems (BESs) offers the possibility to couple renewable electricity storage with microbially catalysed carbon and nitrogen capture. Their mild operating conditions, renewability and tolerance to impurities of the biocatalyst could enable the valorisation of industrial side-streams and CO2-rich emissions. This PhD project explored bioelectrochemical PtX pathways, focusing on Power-to-Methane (PtM) and Power-to-Protein (PtP), with the goal of improving their efficiency, cost-effectiveness, and feasibility. Both approaches were investigated as strategies for capturing natural and industrial CO2 emissions. Within the PtM framework, a comprehensive review was conducted to assess reactor config-urations, CO2 sources, key operational parameters, and current barriers limiting the scaling up of this system. Experimentally, low-cost terracotta bioreactors were developed to capture CO2 in natural degassing areas. Although methane productivity remained low (8.5 ± 4.9 mL L-1 d-1) due to gas leakage, salt precipitation, sulphate-reducing bacteria competition, and CO2 dosing inefficiencies, the systems achieved a Coulombic efficiency of 45 ± 13%. The results confirmed the suitability of terracotta as a porous material capable of maintaining an anoxic environment for methanogenesis paving its possible application directly in-situ after improvements. PtP was investigated as a route to couple renewable electricity storage with the valorisation of biogas-derived CO2 and NH3-rich digestate liquid fraction from anaerobic digestion, es-tablishing a circular economy loop. Single-cell protein production was tested in double-chamber microbial electrosynthesis cells (MES) at different cathode polarizations (−0.6 and −1.0 V vs Ag/AgCl), alongside an unpolarized control. Carbon-cloth electrodes functional-ized with wood-derived biochar were used as cathodes. More negative cathodic polarization enhanced CO2 and N capture (up to 39 ± 2% and 6.7 ± 0.8%, respectively) and protein con-tent (69.1 ± 1.0% at −1.0 V vs 43.2 ± 0.6% at −0.6 V and 33.1 ± 1.3% in the unpolarized control). No significant differences were observed between the amino acid profiles obtained using the two cathode polarizations. Microbial community analysis revealed a shift toward autotrophic and electrotrophic taxa at −1.0 V, whereas heterotrophs predominated in the con-trol. A second set of experiments assessed the effect of different biochar post-treatments under -1.2 V vs Ag/AgCl cathode polarization. Biochar-enriched electrodes outperformed bare car-bon cloth. CO2-activated biochar exhibited the best performance, attributed to its higher sur-face area, abundance of functional groups, and amorphous structure offering more suitable pore dimensions for microorganisms interactions and thus electron transfer. N-impregnated and post-pyrolyzed biochars led to slightly lower protein yields due to their smoother and more graphitic structures. Single-cell proteins produced in this experiment displayed protein contents of 42-49%, similar to or slightly higher than soybean meal, and essential amino ac-ids content representing 39–41% of total amino acids. Overall, the achieved nutritional value demonstrated the potential of PtP as a sustainable bi-otechnological pathway for feed and food production. Combined with insights from PtM, this work advances the development of bioelectrochemical PtX technologies as enabling tools for the energy transition within a circular and bio-based economy.