According to our wet experiment needs, the hardware part designed an electric energy harvesting circuit for microbial fuel cells. Considering that the output power of the microbial fuel cell is small, which is insufficient to directly drive the load and boost circuit, the charge pump is used to collect the power of the battery as the starting voltage of the subsequent DC conversion circuit. The supercapacitor is used as the energy storage element, and finally the DC is converted into the voltage required by the load.
As shown in Figure 1[1], the MFC is composed of an anode, a cathode, and a proton exchange membrane. At the anode of the battery, the organic matter is catalyzed by microorganisms to decompose electrons and hydrogen ions, the electrons are transferred to the cathode through the external wires, and the hydrogen ions are transferred to the cathode through the proton exchange membrane in the middle. At the cathode, electrons, hydrogen ions, and electron acceptor oxygen react to form water.
Figure 1: Schematic diagram of microbial fuel cell
According to Figure 2 Figure 3 Figure 4, the REDOX activity of engineered strains was higher. The engineered strain of MFC cell has lower internal resistance. The maximum output power of the engineered strain was 243.77±25.2mW/m2. Although the strain used in our experiment has a good performance in the output power density, we still need to use a charge pump to collect battery power to start the subsequent DC conversion circuit.
Figure 2: Cyclic voltammetry(CV) curve
Figure 3: Linear sweep voltammetry(LSV) curve
Figure 4: Output power density curve
As shown in Figure 5[2], the MFC uses a charge pump to charge the supercapacitor. Once the voltage of the capacitor reaches a threshold level, the charge pump sends a signal that triggers the switch to close, allowing the supercapacitor to start discharging and convert the power to the voltage level required for the load through the DC converter. When the voltage of the capacitor drops to the preset lower limit threshold, the switch is automatically disconnected, so as to realize the automatic cycle charging and discharging process of the supercapacitor.
Figure 5: Circuit flow chart
The S-882Z Series is a charge pump IC for step-up DC-DC converter startup. Its features include:[3]
As shown in Figure 6 and Figure 7,
Figure 6: block diagram of S-882Z series
Figure 7: Operation Diagram
Through DC-DC converter, the input voltage is from 1.6V to 2.2V and the output voltage is 3V, which makes the LED light on. Figure 8 shows the details of the circuit The chip we chose is TPS61023.
Figure 8: The DC-DC converter
When the capacitor voltage is charged to 2.2V, the OUT pin of the charge pump outputs a high level of 2.2V, making the N-type FET on, and the drain pole of the N-type is low, that is, the gate pole of the P-type is low, so that the P-type is turned on, which is equivalent to the switch in Figure 5.
The circuit schematic diagram is shown in Figure 9. The PCB layout is shown in Figure 10. The 3D model is shown in Figure 11.
Figure 9: Circuit schematic diagram
Figure 10: The PCB layout
Figure 11: The 3D model
The physical circuit after welding is shown in Figure 12.
Figure 12: Physical circuit
When the open circuit output voltage of the microbial battery is above 0.3V, our circuit start to work. When the battery stable output voltage is 0.5V, it is connected to the circuit, and the LED light can be observed shining once about every fifteen seconds. Figure 13, Figure 14 and Video 1 illustrate this result for us. Although our circuits are already working very well.
However, due to the series LED resistance in the load circuit, the effective output efficiency of the circuit is greatly reduced.
If the load is replaced by other appliances with lower power consumption, such as the stm32u0, the efficiency of the circuit can be significantly improved.
Since the effect is not very intuitive, we will not show it.
Figure 13
Figure 14
MFC stores energy for supercapacitors through charge pumps, and the electronic switches are closed and disconnected between 2.2~1.6 V, so as to realize the automatic cyclic charging and discharging of energy storage capacitors, the charging speed is proportional to the input voltage, and inversely proportional to the size of the energy storage capacitors, and the final voltage DC is converted to 3.3V to provide electric energy for the LED. This circuit is suitable for the collection of electrical energy from microbial fuel cells, which can collect the energy generated by microorganisms for subsequent use.
[1]: LIU Yuan-feng, ZHANG Xiu-ling, LI Cong-ju. Advances in carbon-based anode materials for microbial fuel cells[J]. Chinese Journal of Engineering, 2020, 42(3): 270-277. DOI: 10.13374/j.issn2095-9389.2019.09.27.008
[2]: Electric energy harvester for microbial fuel cells[J]. Editorial Office of Optics and Precision Engineering, 2013,21(7): 1707-1712 DOI: 10.3788/OPE.20132107.1707.
[3]: Seiko Instruments,” ULTRA-LOW VOLTAGE OPERATION CHARGE PUMP IC FOR STEP-UP DC-DC CONVERTER STARTUP, ” S-882Z Series datasheet, 2005-2010 [Rev.2.0_00]