A self-powered supercapacitive microbial fuel cell (MFC) is here reported. Microbial fuel cell (MFC) is an interesting bio-electrochemical technology that is used for wastewater treatment with simultaneous organic removal and electricity generation. Due to the much lower current/power production compared to hydrogen and methanol fuel cells, a smart design is desired to increase performance up to the levels required for real applications. The most used method to store/deliver energy from an MFC at high rate is coupling it with an external supercapacitor with capacitances in the order of Farads¹. The main disadvantage of this approach is that high capacitance supercapacitors require long time to be recharged at the low MFC current regimes. This makes the MFC-Supercapacitor system working with long recharge stand-by time and low switch-on/off frequency.

Here, a microbial fuel cell has been integrated with internal supercapacitor (SC)². The MFC anode was the SC negative electrode. Its potential (≈-0.5 V vs Ag/AgCl) was related to the presence of bacteria that consumed the oxygen present into the solution and colonize the carbon brush electrode. The MFC cathode, an air-breathing cathode working at ≈+0.1 V vs Ag/AgCl, was the SC positive electrode. Single chamber MFC filled with 50% volume of 0.1M phosphate buffer saline (PBS) solution and 50% activated sludge was used during the experiments. Galvanostatic discharges were performed with current pulses up to 4 mA. An MFC with activated carbon (AC)-based cathode featured a maximum power of 2.98 Wm^-2 (5.36 Wm^-3) which was mainly affected by the cathode ohmic losses. In order to further enhance the output, two strategies were adopted. The first one was simply enhance the overall cell voltage by utilizing cathode catalysts with higher potential like non-platinum based catalyst (iron-aminoantipyrine (Fe-AAPyr))³ or enzymatic-based catalyst (bilirubin oxidase (BOx)⁴). The second option was to utilize an additional high capacitive electrode of low resistance, short-circuited with the cathode, coupled with the negative electrode (MFC-AdE). The AdE used was a carbon brush coated with AC. The first option brought to the increase of the voltage from 0.6V with AC to 0.68V with Fe-AAPyr and to 0.78V with BOx cathode. Figure 1 showed the discharge of the MFC using Fe-AAPyr cathode with and without the AdE. Consequently, maximum power (P_max) achieved 6.53 Wm^-2 (11.76 Wm^-3) and 4 Wm^-2 (7.2 Wm^-3) using BOx and Fe-AAPyr, respectively. The use of AdE significantly reduced the cell ohmic drop and led to a much important improvement (Figure 1). Pulses discharges up to 45 mA were performed. The cell resistance decreased by one order of magnitude allowing higher current pulses and power generated. In fact, P_max was 84.4 Wm^-2 (152 Wm^-3) with BOx cathode, 62.2 Wm^-2 (112 Wm^-3) with Fe-AAPyr and 26.7 Wm^-2 (49 Wm^-3) with AC cathode. The addition of the AdE is a smart and easy way to overcome the ohmic losses of the system.

This current/power output is in the same range of MFCs coupled with external supercapacitors. The main advantage of the system here reported is the much shorter recharging time (in the order of seconds/minutes) of interest for applications that have to be powered with more frequent pulses, like sensors. Also, utilization of the MFC electrodes as the supercapacitor electrodes makes the system simpler and more compact.

Acknowledments

CS was funded by the Electrochemical Society and Bill & Melinda Gates Foundation under initiative: “Applying Electrochemistry to Complex Global Challenges”. FS acknowledges financial support by Università di Bologna (Researcher Mobility Program).

References

1. G. Papaharalabos, J. Greenman, C. Melhuish, C. Santoro, P. Cristiani, B. Li, I. Ieropoulos. Int. J. Hydrogen Energy 38(26), 11552 (2013).

2. C. Santoro, F. Soavi, A. Serov, C. Arbizzani, P. Atanassov. Biosens. Bioelectron. 78, 229 (2016).

3. C. Santoro, A. Serov, C.W. Narvaez Villarrubia, S. Stariha, S. Babanova, K. Artyushkova, A.J. Schuler, P. Atanassov. Sci. Rep. 5, 16596 (2015).

4. C. Santoro, S. Babanova, P. Atanassov, B. Li, I. Ieropoulos, P. Cristiani. J. Electrochem. Soc. 160 (10), H720 (2013).

Figure 1. Cell voltage and electrode potentials of the MFC and MFC-AdE at 1mA discharge pulse. The cathode was Fe-AAPyr.