Nafion 117 membrane

Sodium sulfate anhydrous (Na₂SO₄, ≥99.0%) was purchased from Carlo Erba and used to prepare electrolytes with a concentration of 0.1 M. Sodium citrate dihydrate (≥99.0%), salicylic acid (99.5%), sodium nitroferricyanide de-hydrate (99.0%), ammonium chloride (NH₄Cl, 99.99%) and sodium hypochlorite solution (NaClO, 6−14%) were bought from Alfa Aesar and used to measure the ammonia in the electrolyte via the indophenol method. All the solutions were prepared with ultrapure water (MilliQ).
Nafion 117 membranes (N117), purchased from Dupont, were used for the absorption test, with a nominal thickness of 182 µm. The Zirfon Perl (ZP) separator was purchased from Agfa.

To evaluate the capacity of EET, the overnight culture suspension of S. oneidensis was inoculated into fresh Luria–Bertani (LB) broth at 1% inoculum and incubated at 30 °C, 200 rpm, until the OD₆₀₀ reached ∼2.0. The harvested cells were washed thrice with fresh M9 buffer and subsequently resuspended in 140 ml of electrolyte (M9 buffer was supplemented with 5% LB, 20 mM lactate, 0.5 mM isopropyl-β-d-thiogalactopyranoside, and 50 μg ml⁻¹ of kanamycin). Dual-chamber MFCs (140-ml working volume) were separated using Nafion 117 membranes (DuPont Inc., USA). Carbon cotton was used as the electrodes for the anode (1.0 cm × 1.0 cm) and carbon cloth for cathode (2.5 cm × 3 cm). The cathodic electrolyte was prepared with 50 mM K₂HPO₄, 50 mM KH₂PO₄, and 50 mM K₃[Fe(CN)₆]. The voltage was measured across a 2-kΩ external resistor in the external circuit and recorded using a multimeter (DT9205A).

To evaluate the efficiency of EET, the overnight S. oneidensis culture suspension (0.5 ml) was inoculated into 50 ml fresh LB broth and incubated at 30 °C with shaking (200×g) until the optical density (OD₆₀₀) of the cell culture reached ∼1.0. Then, the cells were harvested by centrifugation and washed three times with fresh M9 buffer (Supplementary Table 3). The cell pellets were subsequently re-suspended in 140 ml electrolyte (5% LB broth plus 95% M9 buffer supplemented with 20 mM lactate and 1 mM Na and Nm). Additionally, previous experiments in our lab demonstrated that IPTG had no effect on the cell physiology and EET of S. oneidensis. The culture medium was supplemented with 1.0 mM IPTG (optimized in Supplementary Fig. 2) and 50 μg ml⁻¹ kanamycin to ensure consistent culture conditions. Dual-chamber MFCs (140 ml working volume) separated by nafion 117 membranes (DuPont Inc., USA) were used. Carbon cloth was used as the electrodes for both the anode (2.5 cm × 2.5 cm, i.e., the geometric area is 6.25 cm²) and cathode (2.5 cm × 3 cm). The cathodic electrolyte was composed of 50 mM K₃[Fe(CN)₆] in 50 mM K₂HPO₄ and 50 mM KH₂PO₄ solution. To measure the voltage generated, a 2 kΩ external resistor was connected into the external circuit of MFCs, and the MFC potential was recorded using a digital multimeter (DT9205A).