Comparison of in situ and ex situ electrolytic hydrogen supply for electricity-driven single-cell protein production

Both in situ and ex situ hydrogen supply modes have been used to produce single-cell protein from CO₂ with hydrogen-oxidizing bacteria (HOB), but a systematic comparison is lacking. This study evaluates hydrogen supply modes using a gas-lift reactor with internal and external electrolyzers. The in situ mode had a significantly higher hydrogen mass-transfer coefficient (1.33 min⁻¹) than the ex situ mode (0.13 min⁻¹), resulting in 18% greater biomass productivity (0.26 vs. 0.22 g/L/day). HOB grew as biofilm and suspension in the in situ mode, while in suspension only under the ex situ mode. Protein content was higher in the in situ mode (57.3%) than in the ex situ mode (47.7%). Although electrolysis energy consumption was 70% higher in the in situ mode, total energy consumption was lower due to reduced reactor heating needs. These findings indicate the in situ mode outperforms the ex situ mode in biomass productivity, protein quality, and energy efficiency.