Self-stratified and self-powered micro-supercapacitor integrated into a microbial fuel cell operating in human urine

Abstract

A self-stratified microbial fuel cell fed with human urine with a total internal volume of 0.55 ml was investigated as an internal supercapacitor, for the first time. The internal self-stratification allowed the development of two zones within the cell volume. The oxidation reaction occurred on the bottom electrode (anode) and the reduction reaction on the top electrode (cathode). The electrodes were discharged galvanostatically at different currents and the two electrodes were able to recover their initial voltage value due to their red-ox reactions. Anode and cathode apparent capacitance was increased after introducing high surface area activated carbon embedded within the electrodes. Peak power produced was 1.20 ± 0.04 mW (2.19 ± 0.06 mW ml^-1) for a pulse time of 0.01 s that decreased to 0.65 ± 0.02 mW (1.18 ± 0.04 mW ml^-1) for longer pulse periods (5 s). Durability tests were conducted over 44 h with ≈2600 discharge/recharge cycles. In this relatively long-term test, the equivalent series resistance increased only by 10% and the apparent capacitance decreased by 18%.

Keywords: Discharge; High power density; Microbial fuel cell; Self-powered; Supercapacitor; Urine.

Graphical abstract

Fig. 1

Schematic of the SC-MFC and an image of the SC-MFC.

Fig. 2

Overall (a), positive electrode (b) and negative electrode (c) complete discharges for the SC-MFC-control at different current pulses.

Fig. 3

Overall (a), positive electrode (b) and negative electrode (c) complete discharges for the SC-MFC-CapNE with improved anode at different current pulses.

Fig. 4

Overall (a), positive electrode (b) and negative electrode (c) complete discharges for the SC with improved anode and double cathode at different current pulses.

Fig. 5

Ragone plots of SC-MFC-control, SC-MFC-CapNE and SC-MFC-CapPE. Dash lines indicate characteristic discharge time.

Fig. 6

P_pulse for SC-MFC-control (a), SC-MFC-CapNE (b) and SC-MFC-CapPE (c) at t_pulse of 0.01 s, 0.1 s, 0.5 s, 1 s and 5 s.

Fig. 7

2600 cycles of discharges (2 s) and self-recharge (60 s) of SC-MFC-CapPE. Overall cell discharge (a) and single electrode discharge (b). Zoom of overall cell discharge (c) and single electrode discharge (d).

Fig. 8

5 s rest and 2 s discharge at 2 mA after 0, 5, 11, 22, 33 and 44 h. Overall cell voltage (a) and single electrode potential (b) profiles. V_max (c), resistance (d) and apparent capacitance (e) trend over long terms operations.