Study of voltage reversal of stacked sediment microbial fuel cells

Sediment microbial fuel cell (SMFC) is a renewable energy source with a wide range of raw materials. They have a simple structure, low cost, and broad application prospects in the field of self-power supply, attracting the attention of many researchers. However, the output current of SMFC is weak and the output voltage is low, which does not meet the power supply requirements of general electronic devices. Therefore, it is necessary to connect multiple SMFCs to obtain the required voltage and current. Series operation can increase the output voltage, while parallel operation can increase the output current. However, voltage reversal occurs in series operation, which can affect the output performance of the battery. In this work, voltage reversal was studied by two SMFCs respectively connected in series and in parallel after their output voltage stabilized. External load of the series and parallel batteries were connected to 330 ohms, 680 ohms, 1000 ohms, 2000 ohms, and 380K ohms, respectively. It was found that batteries with poor performance in series battery packs experienced voltage reversal, which occurred at 330 ohms, 680 ohms, and 1000 ohms. When the resistances became 2000 ohms and 380K ohms, parallel battery packs did not experience similar phenomena. The reason for voltage reversal was provided in this article. In addition, voltage reversal occurred in the stage of low resistances, which indicating that the greater the current, the greater the possibility of voltage reversal.


Introduction
Energy is an indispensable driving force for human production and life.However, in the process of rapid economic development, the supply relationship of energy has become increasingly tense, and at the same time, the adverse consequences caused by traditional fossil energy consumption have also increased.In order to alleviate this situation, people have proposed the development of sustainable green energy.Microbial fuel cell (MFC) is a kind of clean energy source that has received widespread attention due to their advantages of low environmental pollution and mild operating conditions [1][2][3][4].Sediment microbial fuel cell (SMFC) is a type of microbial fuel cell, utilizing the metabolism of microorganisms to convert the biomass energy of underwater sediments into chemical energy [5].Sediment microbial fuel cell is a type of membrane free MFC, typically arranged in natural water bodies.The basic structure is that the anode is buried in the sediment, and the cathode is placed in the water above the sediment.The sediment naturally separates the anode and cathode, maintaining the anaerobic environment of the anode.SMFC uses the sediment as the fuel of the battery, and the dissolved oxygen in water as the electron acceptor.There are a large number of catalytic microorganisms acting as biological catalysts in the sediment near the anode surface [6,7].
SMFC has a simple structure and low construction cost without any use of proton exchange membranes.However, it is necessary in ordinary MFC to separate the anode and cathode.In addition, the dissolved oxygen in water and sediment required for SMFC are both natural and come from a wide range of sources.If these sediments can be fully utilized, it will provide a huge energy source for human society.Such advantages of SMFC are considered as one of the most promising MFCs in practical environments.
Although SMFC is an ideal clean energy source, the power generation performance of a single SMFC is limited.The open circuit voltage (OCV) of a single SMFC is between 0.20 V and 1.16 V, and the output power density is only 100 to 2000 m W/m 2 [8].This is due to its smaller cathode and anode potential difference and lower electron transfer efficiency [9].Improving the performance of SMFCs can be achieved through catalysts, substrates, and electrode materials [10,11,12], but it also incurs issues such as high cost and reduced practicality.Serializing or paralleling SMFCs to obtain sufficient electrical energy is an effective solution [13].However, in practical applications, voltage reversal occurs in SMFCs connected in series, resulting in lower voltage after series operation.
Voltage reversal refers to the phenomenon where the anode potential exceeds the cathode potential, causing the output voltage to become negative.This phenomenon not only reduces the output voltage, but also causes damage to the biological anode, reducing the battery life.Oh and Logan [14] connected batteries with different substrate amounts in series, in which batteries with fewer substrates produced a voltage of -0.6 V. Adding substrates can restore voltage to the battery.Therefore, they believed that insufficient substrates reduce the number of electrons produced by microbial metabolism, leading to voltage reversal.Taeyoung Kim et al. [15] believed that the lack of substrates is not the fundamental cause of voltage reversal, but rather the imbalance between electrodes.Any parameter that affects electrode performance may lead to voltage reversal consequences.From the perspective of microscopic particles, the series operation of SMFCs causes electrons to move across individual cells, but protons cannot move like this.Chen Xi et al. [16] demonstrated this conclusion through diode forward and reverse series experiments.Understanding voltage reversal and understanding the factors that cause voltage reversal will help improve the practicality of SMFC stacks.
It is worth noting that although voltage reversal occurs in SMFCs operated in series, voltage reversal may not necessarily occur in series operation.Many researchers overlook the important factor of current when observing voltage reversal.Voltage reversal only occurs when the current reaches a certain strength [9].The aim of this work is to further explores the current situation when voltage reversal occurs by changing the external resistance of the SMFC group during operation, providing a certain theoretical basis for avoiding the occurrence of voltage reversal phenomenon.

Experimental scheme
Two same single SMFCs were made as follows.Firstly, a plastic measuring cylinder with a base diameter of 165 mm and a height of 265 mm was as the container for a single SMFC.The anode and cathode were rectangular activated carbon felt with a bottom area of 100 cm2 and a thickness of 5 mm.The produced carbon felt was soaked in 1 mol/L dilute hydrochloric acid for a sufficient time.After soaking, carbon felt rinsed multiple times with deionized water until the pH returned to neutral and dried in air naturally.Secondly, the sedimentary substrates as the anode environment need to be collected and filtered, which from the bottom of Xuanwu Lake.
Thirdly, a single SMFC was set up and connected it to an external circuit until the output voltage stabilized.Due to the attachment of microorganisms to the anode of SMFCs, it took a period of time for the microbial community to stabilize.This stage was called the domestication process of the SMFC or the start-up process of the SMFC.The start-up process of each SMFC was carried out separately, with an external resistor of 1000 ohms connected until the voltage on its resistor remains stable, indicating that the SMFC start-up is complete.
After the voltage of the SMFC stabilized, we can perform series and parallel operations on above two SMFCs.As shown in Figure 1, these two SMFCs, named as SMFC1 and SMFC2, were in series operation of the SMFCs by connecting the anode of SMFC1 to the cathode of SMFC2, and the cathode of SMFC1 was connected to the anode of SMFC2 through an external resistor.During the startup process, the external resistance was also selected as 1000 ohms.After the output voltage of the SMFCs in series stabilized, the resistances of 330 ohms, 680 ohms, 1000 ohms, 2000 ohms, and 380K ohms were respectively connected to the circuit and record the voltage changes of the series SMFCs.After that, the external resistance was changed in order to observe the voltage changes of the series and parallel battery packs.All experiments were conducted at 25 degrees centigrade.

Figure 1. Series connection of SMFCs
Finally, parallel operation was performed on two SMFCs.As shown in Figure 2, the anode of SMFC1 was connected to the anode of SMFC2 as the anode of the SMFCs stack.Similarly, the cathode of SMFC1 is connected to the cathode of SMFC2 as the cathode of the SMFCs stack.After the connection is completed, the 1000 ohms resistor was connected to the circuit and the SMFCs proceeded to the startup phase.The subsequent operations were consistent with the SMFCs stack.

Output voltage of two single SMFCs
After building the two batteries, a 1000 ohms resistor was connected externally for a certain period of domestication. Figure 3 shows the variation of output voltage of SMFC1 and SMFC2 with the time.It was found that both of these two SMFCs showed similar variation trend of output voltage.At the beginning, both of these two curves exhibited the relative higher output voltage, indicating the startup process of two SMFCs.As increasing the time of domestication, both these two SMFCs obtained a stable voltage as shown in Figure 3, in which the stable output voltages of SMFC1 and SMFC2 are 0.106 V and 0.244 V, respectively.

Output voltage characteristics in series operation Style and spacing
The voltage of each SMFC and the total voltage in series connection are shown in Figure 4.There are five stages could be observed for three curves (V(SMFC1), V(SMFC2), V(Series)) in the figure corresponding to the five different connection conditions with five resistances (330 ohms, 680 ohms, 1000 ohms, 2000 ohms, and 380K ohms).As time goes on, the resistance value of the connection increases, and the voltage also increases.From the figure, it can be seen that when the resistance is 330 ohms, 680 ohms, or 1000 ohms, SMFC1 undergoes voltage reversal, with voltage values of -0.072 V, -0.046 V, and -0.022 V, respectively.When the resistance is 2000 ohms and 380K ohms, the voltage reversal phenomenon disappears, and the voltage of SMFC1 returns to a positive voltage.However, when the resistance is 2000 ohms, the voltage value of SMFC1 is still very small compared to SMFC2, indicating that the negative impact of voltage reversal has not disappeared, and a larger resistance is needed to restore its original voltage.The result of the series experiment is consistent with the findings of Oh and Logan [14].They believe that in the series system, batteries with weaker output voltage are more likely to experience voltage reversal due to substrate deficiency.In this experiment, the output performance of SMFC1 is intentionally set to be worse than SMFC2, and voltage reversal occurred unexpectedly on SMFC1.In addition, as the external resistance decreases, equivalent to the current flowing through the circuit increases, voltage reversal is more likely to occur, and the degree of voltage reversal is deeper.

Output voltage characteristics in parallel operation
The voltage of each SMFC and the total voltage during parallel connection are shown in Figure 5.
Although the external resistance has changed, the voltage of each SMFC at different stages is consistent with the total voltage.Three voltage curves perfectly match.It can be observed that there is no voltage reversal phenomenon during series connection.So, we have reason to believe that, from the perspective of increasing power alone, the parallel connection system is superior to the series connection system.Abazarian et al [17].simulated and studied the performance of SMFC in series and parallel under water flowing conditions.They found that the output voltage of 3 or 4 SMFCs in series is only 53% -61% of the theoretical value, while parallel connection does not result in significant energy loss.The main reason is that the substrate of the upstream SMFCs rushes into the downstream SMFCs with the water flowing, causing a difference in substrate concentration between SMFCs.As a result, voltage reversal occurred in the upstream SMFCs.This phenomenon also matches our experimental results very well.

Explanation of voltage reversal phenomenon in series operation
For independent sediment microbial fuel cells, microorganisms oxidize organic matter in an anaerobic environment, producing electrons and protons.Bacteria near the anode transfer electrons to the anode electrode.Due to the existence of electrode potential difference, protons migrate to the cathode through the interface between substrate and water, and electrons flow from the anode to the cathode through external resistance, where they react with the final electron acceptor (oxygen) and protons.The electrons and protons generated by an independent SMFC are conserved and will not produce excessive amounts.However, in the case of series operation, electrons can move across the battery through an external circuit, but protons cannot.This will result in the migration of electrons and protons to a certain battery cathode that are not generated in the same battery.Due to differences in battery performance, the number of generated electrons and protons will also be different, which will lead to the accumulation of electrons or protons, thereby changing the potential of the cathode or anode.Voltage reversal is the result of the anode potential exceeding the cathode potential.When a SMFC undergoes voltage reversal, other SMFCs connected in series will charge it, resulting in a decrease in the overall output voltage.In parallel connection, the electrons generated by the SMFC can reach its cathode through an external circuit and react with the proton, so there will be no electron flow across the SMFC, so voltage reversal will not occur.
When the current changes, the degree of voltage reversal also changes accordingly.The larger the current, the easier it is for voltage reversal to occur.When the current is large enough, the movement speed of electrons accelerates.The SMFC with good performance produces more electrons and protons than the SMFC with poor performance.When more electrons pass through the external circuit and reach the cathode of the SMFC with poor performance, the number of electrons is more than that of protons, leading to electron aggregation; When fewer electrons pass through the external circuit to reach the cathode of a battery with better performance, the electrons are not enough to react with the proton.Due to the high current, the anode of a poorer battery will produce more electrons to fill the gap, resulting in proton aggregation at the anode.So, when the current is high, SMFCs with poor performance will experience an increase in anode potential and a decrease in cathode potential.When the anode potential exceeds the cathode potential, voltage reversal occurs.When the current is low, the speed of proton movement can keep up with the speed of electron migration, and excess electrons can react with excess protons in time, and the voltage reversal phenomenon disappears.

Conclusion
The voltage reversal occurs in series operation of SMFCs, which exists in the battery with poor properties.With the increase of current, the voltage reversal is easier to occur.
Voltage reversal is caused by an imbalance in the number of protons generated by a single SMFC and electrons moving across individual cells.The greater the current, the more pronounced the imbalance.

Figure 4 .
Figure 4. Different output voltages in series system

Figure 5 .
Figure 5. Different output voltages in parallel systems