Measurement and Analysis

From day 21 until day 156, anode potentials, cathode potentials, membrane potentials and cell potentials were logged with a field point (National Instruments FP-2000; FP-AI-112) similar to Helder et al. (2012) and Wetser et al. (2015b). Prior to mentioned period, the anode potentials, the cathode potentials and the cell potential were manually measured with a multimeter. Apart from data logger, during potentiostat control generated current was logged with IviumSoft of Ivium Technologies connected to a lab PC.

Every 1 or 2 weeks, liquid samples were taken from the anode and the cathode. Anolyte samples were taken using filtered syringe and catholyte samples were taken from cathode outlet before entering recirculating bottle. Samples were stored in -20°C for further analysis. Conductivity and pH were measured right after sample collections. Conductivity was measured using HQ440d multi pH/LDO/conductivity meter HACH and pH was measured using a PHM210 standard pH meter, MeterLab Radiometer analytical.

Acetate concentrations were determined by gas chromatography (Agilent 7890B, United States) as described earlier (Jourdin et al., 2018). An HP-FFAP Column was used (25 m × 0.32 mm × 0.50 μm). The detector (FID) and injection temperatures were 240 and 250°C, respectively. The oven temperature was 60°C for 3 min, 21°C min^-1 up to 140°C, 8°C min^-1 up to 150°C and constant for 1.5 min, 120°C min^-1 up to 200°C and constant for 1.25 min, and finally 120°C min^-1 up to 240°C and constant for 3.5 min. Helium was used as carrier gas at a flow of 1.25 mL min^-1 for the first 3.5 min and 2 mL min^-1 until the end of the run. 1 μL of sample was injected in the column. Acetate concentration result can be found in the Supplementary Figure 3.