Biochar cathodes for power-to-protein: a circular approach for anaerobic digestion side-product valorisation

Power-to-Protein is considered an innovative approach to store renewable energy by producing Single-Cell Protein as an alternative protein source to feed the growing global population, while lowering production impacts and recovering industrial side-streams such as CO2 and NH3. Nevertheless, a win-win approach should balance protein yield with cost reduction. Three different biochar-functionalized cathodes were tested in Microbial Electrosynthesis Cells at a cathode potential of −1.2 V vs. Ag/AgCl for the recovery of CO2 and NH3 from anaerobic digestion. One biochar was activated with CO2, while the other two underwent imidazole impregnation and subsequent pyrolysis. The biochar application on the cathode led to improved performance compared to bare carbon cloth control in terms of protein productivity, protein content on dry matter, and nitrogen capture efficiency. However, the biochar post-treatments (i.e., nitrogen doping with imidazole and second-round pyrolysis) did not provide any additional benefit with respect to the key performance indicators considered. In particular, the untreated biochar achieved 47% and 59% higher protein space-time yield compared to the two post-treated biochars. Regarding relative protein content and amino acid profile, no significant differences were observed among the biochar-functionalized cathodes, all of which exhibited a satisfactory proportion of essential amino acids (39 - 41% of total amino acids). The protein-rich biomass obtained in this study shows protein contents and essential amino acid proportions comparable to those reported for soybean, highlighting the potential of Power-to-Protein as a sustainable biotechnological route for future feed and food applications supporting energy transition and applying circular economy approach.