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Article

Validity and Reproducibility of Counter Electrodes for Linear Sweep Voltammetry Test in Microbial Electrolysis Cells

by
Hyungwon Chai
1,†,
Bonyoung Koo
1,†,
Sunghoon Son
2 and
Sokhee Philemon Jung
1,*
1
Department of Environment and Energy Engineering, Chonnam National University, Gwangju 61186, Republic of Korea
2
Korean Institute of Energy Research, Jeju 63357, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors are the co-first authors.
Energies 2024, 17(11), 2674; https://doi.org/10.3390/en17112674
Submission received: 23 April 2024 / Revised: 17 May 2024 / Accepted: 29 May 2024 / Published: 31 May 2024
(This article belongs to the Section A: Sustainable Energy)
Browse Figures
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Abstract

:
The electrode is a key component in a microbial electrolysis cell (MEC) that needs significant improvement for practical implementation. Accurate and reproducible analytical methods are substantial for the effective development of electrode technology. Linear sweep voltammetry (LSV) is an essential analytical method for evaluating electrode performance. In this study, inoculated carbon brush (IB), abiotic brush (AB), Pt wire (PtW), stainless steel wire (SSW), and mesh (SSM) were tested to find the most suitable counter electrode under different medium conditions. The coefficient of variation (Cv) of maximum current (Imax) was the most decisive indicator of the reproducibility test. This study shows that (i) the electrode used in operation is an appropriate counter electrode in an acetate-added condition, (ii) the anode LSV test should avoid the use of Pt wire as counter electrodes, and (iii) PtW is an appropriate counter electrode in cathode LSV in all conditions.

1. Introduction

Wastewater treatment is an energy-demanding process [1,2,3,4,5]. According to the United States Environmental Protection Agency (EPA), the annual amount of wastewater treated by wastewater treatment facilities is about 46.9 billion tons. Organic wastewater treatment consumes 15 GW, which is 3% of the total electricity production in the United States. Other developed countries show similar trends [6].
Microbial electrochemical systems (MESs) are novel systems employing microbial electrochemical phenomena to produce bioenergy or value-added chemicals [7,8,9,10,11,12,13,14,15,16]. These include microbial fuel cell (MFC), sediment microbial fuel cell (SMFC), microbial electrolysis cell (MEC), microbial desalination cell (MDC), microbial reverse electrodialysis cell (MRC), microbial electrosynthesis cell (MESC) technologies, etc. [17,18,19]. Among MESs, MFCs are the system that generates electricity by degrading organic matter in wastewater using a microbial catalyst in the anode [20,21,22,23,24,25,26,27,28,29,30]. This technology has the potential to prevent direct fuel combustion and convert the chemical energy of organic waste into bioelectricity [31]. MECs are a technology that, like MFCs, removes organic matter from wastewater, but it produces green hydrogen gas at the cathode [32,33].
The electrode is one of the most important parts of MESs, because most biotic or abiotic electrochemical reactions occur in anode and cathode electrodes [34,35]. In MES experiments, electrode performance can be evaluated using linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Among them, LSV is very easy to perform. In addition, it produces reliable data [36]. Thus, it is commonly used in many areas such as MESs [37,38,39,40,41,42,43,44], fuel cells [45,46], and hydrogen generation technologies [47,48]. In general, electrochemical tests involve the analysis of chemical response when an electrical stimulus is applied to the system. In an LSV test, an electrical stimulus is a change of potential, and the chemical response is converted by the current signal. In an LSV test, to change the potential of the electrode in a solution, two electrodes (working electrode and reference electrode) are needed to measure the potential. The current at a working electrode is measured, and the potential between the working electrode and a reference electrode is swept linearly in time [49].
Electrode LSV tests in MESs are mainly performed using a three-electrode cell [37,38,50], consisting of a working electrode, a reference electrode, and a counter electrode. The counter electrode has a process opposite from the working electrode; it is not monitored. Various counter electrodes (platinum [38,51,52,53,54,55,56,57], dimensionally stable anodes [58,59]) have been used in MESs. However, no research has been conducted on the effect of counter electrodes on LSV data validity or reproducibility. In this study, it was hypothesized that experiment reproducibility would be different depending on the counter electrode used for the LSV test. The structure, electrical activity, and reactivity of the material constituting the counter electrode will show differences in the experimental results.
Therefore, the purpose of this study was to evaluate the reproducibility of using different counter electrodes for anode and cathode LSV tests in a microbial electrolysis cell (MEC). The anode and cathode used in the experiment were carbon fiber brush and stainless steel mesh commonly used in MECs [60,61]. To compare reproducibility, the counter electrodes used in this study were Pt wire, stainless steel wire, and mesh in anode LSV tests. In cathode LSV tests, inoculated carbon brush, abiotic carbon brush, Pt wire, stainless steel wire, and mesh were used as counter electrodes. The reproducibility of the cathode LSV test with different electrode performances and different experimental conditions was then evaluated and compared. This study presents data that can be used to set counter electrodes according to electrode LSV test conditions.

2. Materials and Methods

2.1. Electrode Fabrication

In this study, two kinds of cathodes were fabricated to compare cathodes with different performances. A stainless steel mesh (SSM) cathode (surface area of 7 cm2, # 60, SUS 304) was used after treating it with flame oxidation [62]. An activated carbon cathode with nickel powder (Ni-AC-SSM cathode) was fabricated for the cathode LSV test [63], which had a high catalytic activity for hydrogen evolution reactions (HER). The Ni-AC-SSM cathode was prepared by attaching a catalyst layer, comprising 96.8 mg (8.8 mg/cm2) of Ni powder, 300 mg (26.5 mg/cm2) of AC powder, and 1 mL of PVDF binder, onto the surface of the SSM cut to a size of 7 cm2 [38]. A carbon brush anode was used as an anode electrode. Carbon fibers were twisted with two titanium rods (length of 70 mm, 17 gauge; grade 2, Seoul Titanium) [40] and then treated at 450 °C for 30 min in a furnace with anaerobic conditions (FX/FHX, DAIHAN Scientific, Wonju city, Gangwon-do, Republic of Korea) before inoculation [64]. The reason carbon fiber is twisted with titanium rods is because titanium has excellent electrical conductivity, so it can function as a current collector.

2.2. MEC Configuration and Operation

A single-chamber MEC reactor was fabricated with polycarbonate, having 28 mL of cylindrical bed volume (4 cm in length and 3 cm in diameter), as described in a previous study [43].
The anode and cathode horizontally faced each other and were spaced at about 15 mm to prevent short circuits. The brush anode was inoculated with domestic wastewater (Wastewater Treatment Plant, Gwangju, Republic of Korea) having effluents of primary sedimentation (22.4 mL) and activated sludge (5.6 mL) in the MFC with an external resistance of 1000 Ω [65]. When the current of the MFC was stably produced, the experiment was performed by switching to MEC mode. The voltage applied for the anode and cathode was +0.9 V using a power supply (2230, Keithely Instrument, Cleveland, OH, USA). The MEC was operated in 100 mM carbonate buffer solution (CBS) in a fed-batch mode with an external resistance of 10 Ω at 30 °C. The CBS contained NaH2PO4 0.026 g/L, Na2HPO4 0.05 g/L, NH4Cl 0.31 g/L, KCl 0.13 g/L, NaHCO3 8.4 g/L, trace minerals 1 mL/L, vitamins 1 mL/L (Table S1), and sodium acetate 20 mM as a substrate [42].

2.3. Electrochemical Test

Linear sweep voltammetry (LSV) was performed using different counter electrodes to compare electrochemical test reproducibility of anode and cathode performances. LSV was performed in a three-electrode system. To evaluate the validity and reproducibility of LSV tests, the maximum current (Imax), inflection potential (Ef), and steep slope (m) of LSV curves were comparatively evaluated. Imax is the maximum absolute value of the current in LSV.
In the LSV test, the reproducibility was compared by calculating the Ef and m at which current began to be generated. An electrode potential was measured with the Ag/AgCl reference electrode. In the cathode LSV test, based on the voltage required for hydrogen production, the slope in the voltammogram at which current is produced was calculated to compare the reproducibility [66]. The small value of Ef lowers the overpotential, making it more effective for catalysts to promote hydrogen production. The catalyst with a steeper slope (m) is the more effective producer of hydrogen gas, allowing for a higher current under an applied voltage [67]. Therefore, to compare the reproducibility of each value, LSV curves were analyzed by linear regression to obtain the values of Ef and m [68].
The open-circuit voltage (OCV) time of all anode LSV tests was 90 min. LSV tests were performed using a potentiostat (SP1, WonATecH, Seoul, Republic of Korea). Platinum wire (PtW, length 5 cm, diameter 0.5 mm), stainless steel wire (SSW, length 5 cm, diameter 0.5 mm), and stainless steel mesh (SSM, surface area 7 cm2) were used as counter electrodes for anode LSV tests. A scan rate of 1.0 mV/s was applied to the anode over the potential range from open circuit potential (OCP) to −0.2 V [44]. The acetate oxidation potential determines the scan range for the anode LSV and should encompass 0.3 V. We initiated with OCP to ensure the stability of the anode biofilm.
Inoculated carbon brush (IB, length of 25 mm, diameter of 25 mm), meaning carbon brush inoculated with electroactive bacteria, abiotic carbon brush (AB, length of 25 mm, diameter of 25 mm), PtW, SSW, and SSM were used as counter electrodes for cathode LSV tests. Cathode LSV tests were conducted in two experiments to compare reproducibility with only CBS (pH 8.33, conductivity 8.42 mS/cm) and CBS with sodium acetate (pH 8.31, conductivity 9.75 mS/cm) as the electrolyte. The pH and conductivity were measured using a pH/conductivity meter (S213, METTLER TOLEDO (SEOUL), Seoul, Republic of Korea). A scan rate of 1.0 mV/s was applied to the cathode over the potential range from −0.3 V to −1.2 V. The reason for setting this scan range is related to the hydrogen production potential of the cathode. It included the hydrogen production potential of −0.414 V and was set from −0.3 V to −1.2 V considering overpotential [33]. For all LSV tests, the Ag/AgCl reference electrode (RE-1B, ALS, Japan; −0.209 V vs. SHE) was inserted in the medium through a rubber gasket. It was located in the middle of the MEC. LSV tests were performed in duplicate to reduce experimental error.

2.4. Statistics

To evaluate the reproducibility of LSV test results, average values and standard deviations of duplicate experiments were calculated. The standard deviation represents an absolute degree of value difference. A larger average of values had a higher standard deviation. However, the standard deviation is not suitable for comparing samples with different average values. Therefore, the coefficient of variation (Cv) was used for adjusting the standard deviation that varied due to differences in average values.
The coefficient of variation was calculated with the following formulation:
C v = σ x ¯ × 100   ( % )
where σ was the standard deviation, and x ¯ was the average value. A relatively small standard deviation compared to the average meant that the degree of dispersion was low, indicating a high reproducibility of experiments. Cv was calculated about the maximum current (Imax), inflection potential (Ef) at which inflection occurs, and steep slope (m) after passing the inflection potential in LSV tests (n = 2).

3. Results

3.1. Anode Linear Sweep Voltammetry Test

A LSV test is performed to measure the maximum current, which is an indicator characteristic of electrode performance in MESs. The generated current value is an important indicator of the electrochemical activity of the anode biofilm. The reproducibility of the anode LSV test may differ depending on the materials of the counter electrode. To compare the reproducibility of the anode (inoculated carbon brush), LSV tests were performed, and PtW, SSW, and SSM were used as counter electrodes (Figure 1 and Figure S1).
The maximum current value of PtW using Pt wire as the counter electrode was 11.05 ± 0.24 mA. Maximum current values of SSW and SSM using stainless steel wire and mesh as the counter electrode were 11.11 ± 0.05 mA and 12.59 ± 0.06 mA, respectively (Table 1). This indicates that the activity of the anode biofilm was highest when SSM was employed as the counter electrode. This is attributed to the larger surface area of SSM compared to SSW and PtW, thereby providing a broader range of active sites available for the reaction.
The coefficient of variation (Cv) of the maximum current was the lowest for SSW (0.45%), followed by that for SSM (0.48%) and PtW (2.17%) (Figure 2). SSW and SSM had 79.2% and 77.8% lower Cv than PtW, respectively. The reproducibility of the anode LSV test was high for SSW and SSM. It is presumed that this is because stainless steel, the material of the cathode used during MEC operation, was used as the counter electrode in the anode LSV test. In addition, research has recently been conducted confirming that platinum performs antibacterial reactions in platinum nanoparticles [69]. Therefore, in the biotic anode LSV test, PtW may not be an appropriate counter electrode.
As can be seen in Table 1, the slope (m) of the LSV curve was also confirmed to have high reproducibility in SSW and SSM. However, the reproducibility of the Ef at which current production starts did not follow the trend of the above results. Slope (m) represents a factor linked to reaction rate, and Ef pertains to overpotential. The trend in reproducibility seems to align more closely with reaction current and rate.

3.2. Stainless Steel Mesh (SSM) Cathode Linear Sweep Voltammetry Test

The current value generated in the cathode LSV test is a clear indicator of the electrochemical activity of the cathode in hydrogen production. Therefore, the cathode LSV test is performed to confirm the hydrogen production performance, and the Pt electrode is mainly used as a counter electrode. However, this study was performed to identify the reproducibility of the cathode LSV test when using various counter electrodes. Therefore, to compare the reproducibility of cathode LSV tests, IB, AB, PtW, SSW, and SSM were used as counter electrodes.
In general, the materials of each counter electrode, such as Pt, stainless steel, and carbon fiber, are stable materials. Pt wire is widely used as an electrode material in electrochemical experiments and applications, being chemically stable and resistant to corrosion. Stainless steel is cost-effective, has high mechanical strength, and offers stability, making it widely used. Carbon fiber is lightweight, strong, and chemically stable, exhibiting high electrochemical stability and suitability for various chemical reactions.
A cathode LSV test was practically performed without acetate filled, unlike an anode LSV. However, the reproducibility of LSV tests with acetate filled was evaluated in this study (Figure 3 and Figure S2). The maximum current values of IB and AB in the cathode LSV test with acetate filled were −8.79 ± 0.15 and −7.12 ± 0.25 mA, respectively. The maximum current value of PtW using Pt wire as the counter electrode was −7.96 ± 0.16 mA. Maximum current values of SSW and SSM using stainless steel wire and mesh as counter electrodes were −7.15 ± 0.19 and −6.53 ± 0.23 mA, respectively. The highest maximum current observed in IB can be attributed to its function as a catalyst in the biofilm, unlike other counter electrodes. This leads to increased electrochemical activity, as IB’s biofilm demonstrates enhanced capability to oxidize acetate and release electrons, particularly under a negative cathode potential in the presence of acetate.
The Cv of maximum currents was the lowest for IB (1.76%), followed by that for PtW (2.01%), SSW (2.72%), AB (3.43%), and SSM (3.44%) (Figure 4). IB had 12.4% lower Cv than PtW. In addition, it showed 48.6% lower Cv than AB without biofilm. With acetate filled, cathode LSV test results using an inoculated carbon brush as a counter electrode showed that both maximum current and reproducibility were high. This is because an inoculated carbon brush, an anode material used during MEC operation, was used as a counter electrode. In addition, since it was filled with acetate, it is assumed that the reproducibility is high due to the biofilm activity of the inoculated carbon brush. However, the reproducibility of Ef and slope (m) was high when stainless steel was used as a counter electrode.
To compare reproducibility from each counter electrode more accurately, the cathode LSV tests were performed without acetate filled [37]. The maximum current value of IB and AB was −8.87 ± 0.15 and −6.19 ± 0.17 mA, respectively. The maximum current of PtW was −6.07 ± 0.07 mA. The maximum current of IB was higher than that for any other counter electrode, similar to that in the previous cathode LSV test with acetate filled. However, the Cv of the maximum currents was the lowest for PtW (1.24%), followed by that for IB (1.71%), AB (2.73%), SSM (3.64%), and SSW (4.17%) (Figure 4). PtW had 27.4% lower Cv than IB. Without acetate filled, the reproducibility was high when using Pt wire as the counter electrode. Pt had high reproducibility because it has a high electrochemical reactivity [70]. To identify the difference in reproducibility more clearly, cathode LSV tests with excellent performance were conducted in the next section.

3.3. Ni-AC-SSM Cathode Linear Sweep Voltammetry Test

The addition of nickel powder as a conductive material of the cathode is one of the good ways to improve hydrogen production in the MEC [38]. Therefore, the reproducibility of the cathode LSV test was evaluated using a Ni-AC-SSM cathode with higher activity for hydrogen evolution reaction (HER) than stainless steel (Figure 5 and Figure S3). The maximum current values of IB and AB with acetate filled were −16.01 ± 1.41 and −16.99 ± 3.73 mA, respectively. The maximum current value of PtW was −13.26 ± 0.47 mA. The maximum current values of SSW and SSM were −13.96 ± 1.27 and −13.59 ± 2.02 mA, respectively (Table 1). The maximum current of AB was higher than that of any other counter electrode. However, the Cv of the maximum current was the lowest for PtW (3.54%), followed by that for IB (8.81%), SSW (9.13%), SSM (14.82%), and AB (21.95%) (Figure 6). The Ef could not be obtained, because there was no bent part in the cathode LSV curve of Ni-AC-SSM. This demonstrates the same phenomenon as observed in previous studies [33]. Ni-AC-SSM exhibits the characteristic of higher current generation compared to plain SSM. However, identifying the point at which overpotential can be confirmed remains elusive. The reproducibility of slope (m) was high when stainless steel was used as a counter electrode, followed by PtW.
The maximum current values of IB and AB without acetate filled were −15.73 ± 1.15 and −12.44 ± 0.30 mA, respectively. The maximum current value of PtW was −13.11 ± 0.08 mA. The maximum current values of SSW and SSM were −11.95 ± 0.15 and −13.57 ± 0.70 mA, respectively. The maximum current of IB was higher than that for other counter electrodes. However, the Cv of the maximum currents was the lowest for PtW (0.61%), followed by that for SSW (1.21%), AB (2.43%), SSM (5.15%), and IB (7.31%) (Figure 6). The slope (m) had high reproducibility when Pt wire was used as a counter electrode. In the cathode LSV test of Ni-AC-SSM, reproducibility using Pt wire as a counter electrode was the best regardless of the substrate.

4. Discussion

4.1. Reproducibility of Anode Linear Sweep Voltammetry Test

The reproducibility of inoculated carbon brush anode LSV tests was evaluated by changing the counter electrode. The maximum current of the SSM was 13.9% higher than that for PtW and 13.3% higher than that of the SSW, because the stainless steel mesh had the largest surface area among these three counter electrodes [66,71].
The slope of the anode LSV curve is an insignificant indicator. However, the oxidation potential (Ef) to start current production can often be identified in other studies [72,73]. The reproducibility of the Ef value was the best in PtW, because Pt has better electrical reactivity than stainless steel [70]. The onset potential of current production appears to be determined by the material stability and reactivity of the counter electrode. Compared with other previously reported studies, the reproducibility of the maximum current was also higher, as the Pt material of the counter electrode was supported. The counter electrode was made of carbon cloth coated by 0.5 mg/cm2 Pt/60% on carbon support with a geometric area of 6.25 cm2; its Cv of maximum current was 1.93% in the anode LSV test [74].
In the case of the maximum current, anode LSV tests showed excellent reproducibility when stainless steel (SS) was used versus when Pt was used as a counter electrode (Figure 2). The Cv of the maximum current was 79.2% lower for SSW and 77.8% lower for SSM than for PtW. In the operation of the MEC, the cathode was used as a stainless steel mesh. Therefore, it was found that the reproducibility of the maximum current is high when a counter electrode, such as in the MEC operating condition, is used.
In previous studies, the antimicrobial activity was evaluated by synthesizing Pt nanoparticles [69]. Bare nanoparticles exhibited antibacterial effects against most pathogens tested. This may have been less reproducible due to the expression of the antimicrobial activity of Pt wire in the anode bacteria of the MEC.

4.2. Reproducibility of Cathode Linear Sweep Voltammetry Test

Unlike an anode LSV test, a cathode LSV test is almost always conducted without a substrate [52,53,63,75]. However, in the present study, cathode LSV tests were conducted to compare the reproducibility using various counter electrodes, including inoculated and abiotic electrodes. Therefore, the comparison was performed with and without acetate filled as a substrate.
In cathode LSV tests with acetate filled, IB had the highest maximum current value, because the biofilm of the carbon brush used as a counter electrode had high electrochemical activity (Table 1). This result is consistent with previous studies [76,77,78] that show improved electrochemical performance of MECs using bio-cathodes as counter electrodes.
The reproducibility of the maximum current of the SSM cathode LSV test was high in IB. The maximum current Cv of IB was 12.4% lower than PtW and 48.7% lower than SSM. It was also 48.6% lower than AB without biofilm. This result showed enhanced reproducibility for an electrochemically active biofilm. In addition, like the anode LSV test, cathode LSV tests showed high reproducibility in the counter electrode (inoculated carbon brush anode) used during MEC operation.
The substrate affected the electro-activity of the counter electrode. Thus, the reproducibility of the SSM cathode LSV test was evaluated without acetate filled. The maximum current was high in IB. However, the Cv of the maximum current was the lowest in PtW, followed by that in IB (Figure 4). These results showed that under LSV test conditions without acetate filled, the use of Pt wire as a counter electrode had excellent reproducibility.
On the occasion of Ef and slope (m), the reproducibility was high in SSM and SSW. This appears to have high reproducibility of the slope and onset potential that initiates hydrogen and current production when a material such as a working electrode is used as a counter electrode, but this is not clear. This needs to be interpreted more clearly in future studies.
To compare the tendency of results, a Ni-AC-SSM cathode fabricated by a blended powder of AC and Ni was also tested. There was a difference in cathode performance. The maximum current value of the Ni-AC-SSM cathode was generally higher than that of the SSM cathode because of the high activity of HER. The Ni-AC-SSM cathode has a larger surface area than SSM [33]. It also has higher electrical conductivity due to mixed ACs [38]. However, it has low durability and reproducibility, because cathode catalysts are attached to the current collector [79]. Therefore, in cathode LSV tests, the Ni-AC-SSM cathode had higher Cv than SSM. The reproducibility was overall not good (Table 1).
The Cv value of the maximum current was lower without acetate filled than that with acetate filled (Figure 6). This result demonstrates that reproducibility without acetate filled is better when conducting LSV tests of the cathode. The Cv of the maximum current with acetate filled was the lowest for PtW, followed by that for IB. However, the Cv of the maximum current without acetate filled was exclusively the lowest for PtW. The results of all experiments confirmed that the reproducibility of the maximum current was high when electrode LSV tests were performed under the same conditions (counter electrode, substrate) of MEC operation. However, it is recommended to use a Pt electrode as a counter electrode without a substrate for cathode LSV tests. These results indicate that it is more important to make sure that the counter electrode is electrochemically activated during LSV tests without acetate filled.
In other studies, cathode LSV tests were conducted using various counter electrodes other than those used in this study. The counter electrodes mainly used were Pt wire and Pt plate. When the Pt plate was used as a counter electrode, the Cv of the maximum current was 4.16% in an area of 1 cm2 [56] and 5.61% in an area of 6 cm2 [54]. The area did not significantly affect the reproducibility of the maximum current. Pt foil [57] and Pt-Ti mesh [80] were also used as counter electrodes in other previous studies. When Pt-Ti mesh was used as a counter electrode, Cv was only 1.82%. However, the Pt wire has a lower Cv value than all other counter electrodes in this study as well as other studies (Table 2). This is believed to be highly reproducible in electrochemical reactions due to the higher electrode stability and lower roughness of the Pt electrode surface compared to stainless steel or carbon-based electrodes. Therefore, even compared with other studies, using Pt wire as a counter electrode in the cathode LSV test is a way to improve the experiment’s reproducibility.

5. Conclusions

Anode and cathode LSV tests with various counter electrodes were conducted to assess reproducibility. The choice of counter electrode may vary depending on the electrode reaction type. When selecting a counter electrode for anode LSV tests, the cathode material used during MEC operation should be considered. Using stainless steel as a counter electrode resulted in higher reproducibility of the maximum current compared to Pt. Substrate presence should also be considered when choosing a counter electrode for cathode LSV tests. Cathode LSV tests with acetate showed high reproducibility when using an inoculated carbon brush as the anode during MEC operation. Conversely, Pt wire yielded high reproducibility in cathode LSV tests without acetate. For LSV tests with high reproducibility, it is recommended to use the same counter electrode as in MEC operating conditions (with substrate). In the absence of a substrate, a Pt electrode should be used. These findings suggest that counter electrodes can be tailored to specific conditions for anode and cathode LSV tests.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en17112674/s1, Figure S1: LSV curves (second set) of an inoculated carbon brush anode. Platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. An amount of 100 mM CBS with 20 mM acetate was used as a medium. Figure S2: LSV curves (second set) of a stainless steel mesh (SSM) cathode. Inoculated carbon brush (IB), abiotic carbon brush (AB), platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in 20 mM acetate (A,C) and non-acetate (B,D) in 100 mM CBS. Figure S3: LSV curves (second set) of Ni-AC-SSM cathode. Inoculated carbon brush (IB), abiotic carbon brush (AB), platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in 20 mM acetate (A,C) and non-acetate (B,D) in 100 mM CBS. Figure S4: Picture of MEC used in the present study. Table S1: Components of vitamins and minerals in the carbonate buffer solution used in this study.

Author Contributions

Conceptualization, H.C. and S.P.J.; Investigation, H.C.; Methodology, H.C. and S.P.J.; Validation, S.P.J.; Project administration, S.P.J.; Writing—original draft preparation, H.C., B.K. and S.P.J.; Writing—review and editing, B.K., S.S. and S.P.J.; Supervision, S.P.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2021R1A2C1013989), the Research Development Project of Gwangju Green Environment Center (No. 23-03-10-16-12) in 2023, and the Regional Innovation Strategy (RIS) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education(MOE) (2021RIS-002).

Data Availability Statement

Data is contained within the article (and Supplementary Materials).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Inoculated carbon brush (IB) anode LSV curves (the 1st set). Platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in 100 mM CBS medium with 20 mM acetate.
Figure 1. Inoculated carbon brush (IB) anode LSV curves (the 1st set). Platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in 100 mM CBS medium with 20 mM acetate.
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Figure 2. Coefficient of variation (Cv) and maximum current (Imax) of the IB-anode LSV tests.
Figure 2. Coefficient of variation (Cv) and maximum current (Imax) of the IB-anode LSV tests.
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Figure 3. Stainless steel mesh (SSM) cathode LSV curves (the 1st set). Inoculated carbon brush (IB), abiotic carbon brush (AB), platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in an acetate-filled condition (20 mM) (A,C) or a non-acetate condition (B,D) in 100 mM CBS medium.
Figure 3. Stainless steel mesh (SSM) cathode LSV curves (the 1st set). Inoculated carbon brush (IB), abiotic carbon brush (AB), platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in an acetate-filled condition (20 mM) (A,C) or a non-acetate condition (B,D) in 100 mM CBS medium.
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Figure 4. Coefficient of variation (Cv) and maximum current (Imax) of the SSM-cathode LSV tests. Cv in an acetate-filled condition (Cv-A), Cv in a non-acetate condition (Cv-N), Imax in an acetate-filled condition (Imax-A), Imax in a non-acetate condition (Imax-N).
Figure 4. Coefficient of variation (Cv) and maximum current (Imax) of the SSM-cathode LSV tests. Cv in an acetate-filled condition (Cv-A), Cv in a non-acetate condition (Cv-N), Imax in an acetate-filled condition (Imax-A), Imax in a non-acetate condition (Imax-N).
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Figure 5. Ni-AC-SSM cathode LSV curves (the 1st set). Inoculated carbon brush (IB), abiotic carbon brush (AB), platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in an acetate-filled condition (20 mM) (A,C) or a non-acetate condition (B,D) in 100 mM CBS medium.
Figure 5. Ni-AC-SSM cathode LSV curves (the 1st set). Inoculated carbon brush (IB), abiotic carbon brush (AB), platinum wire (PtW), stainless steel wire (SSW), or stainless steel mesh (SSM) was used as a counter electrode. LSV was performed in an acetate-filled condition (20 mM) (A,C) or a non-acetate condition (B,D) in 100 mM CBS medium.
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Figure 6. Coefficient of variation (Cv) and maximum current (Imax) from the Ni-AC-SSM cathode LSV tests. Cv in an acetate-filled condition (Cv-A), Cv in a non-acetate condition (Cv-N), Imax in an acetate-filled condition (Imax-A), Imax in a non-acetate condition (Imax-N).
Figure 6. Coefficient of variation (Cv) and maximum current (Imax) from the Ni-AC-SSM cathode LSV tests. Cv in an acetate-filled condition (Cv-A), Cv in a non-acetate condition (Cv-N), Imax in an acetate-filled condition (Imax-A), Imax in a non-acetate condition (Imax-N).
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Table 1. Maximum current (Imax), coefficients of variation (Cv) of Imax, inflection potential (Ef) at which inflection occurs, and steep slope (m) after passing the inflection potential of LSV curves (n = 2). Electrode potentials were measured using the Ag/AgCl reference electrode.
Table 1. Maximum current (Imax), coefficients of variation (Cv) of Imax, inflection potential (Ef) at which inflection occurs, and steep slope (m) after passing the inflection potential of LSV curves (n = 2). Electrode potentials were measured using the Ag/AgCl reference electrode.
IB Anode LSV Test (20 mM Acetate)
Counter ElectrodeImax (mA)Cv (%)Ef (V)Cv (%)m (mA/V)Cv (%)
PtW11.05 ± 0.242.17−0.45 ± 0.000.0339.86 ± 0.992.49
SSW11.11 ± 0.050.45−0.49 ± 0.000.4235.27 ± 0.120.34
SSM12.59 ± 0.060.48−0.45 ± 0.000.4747.09 ± 0.050.09
SSM cathode LSV test (20 mM acetate)
Counter electrodeImax (mA)Cv (%)Ef (V)Cv (%)m (mA/V)Cv (%)
IB−8.79 ± 0.151.76−0.87 ± 0.101.2425.10 ± 0.903.58
AB−7.12 ± 0.253.43−0.93 ± 0.011.0222.82 ± 1.185.18
PtW−7.96 ± 0.162.01−0.88 ± 0.011.1322.35 ± 1.155.14
SSW−7.15 ± 0.192.72−0.87 ± 0.010.8119.50 ± 0.301.54
SSM−6.53 ± 0.233.44−0.89 ± 0.000.1918.70 ± 0.201.07
SSM cathode LSV test (non-acetate)
Counter electrodeImax (mA)Cv (%)Ef (V)Cv (%)m (mA/V)Cv (%)
IB−8.87 ± 0.151.71−0.91 ± 0.000.3816.85 ± 0.452.67
AB−6.19 ± 0.172.73−0.99 ± 0.041.0220.94 ± 1.316.25
PtW−6.07 ± 0.071.24−0.94 ± 0.010.5616.00 ± 0.805.00
SSW−6.49 ± 0.274.17−0.95 ± 0.000.1516.75 ± 0.251.49
SSM−6.44 ± 0.233.64−0.93 ± 0.000.0415.95 ± 0.050.31
Ni-AC-SSM cathode LSV test (20 mM acetate)
Counter electrodeImax (mA)Cv (%)Ef (V)Cv (%)m (mA/V)Cv (%)
IB−16.01 ± 1.418.81N/DN/D11.17 ± 2.0918.67
AB−16.99 ± 3.7321.95N/DN/D22.35 ± 8.3737.43
PtW−13.26 ± 0.473.54N/DN/D14.72 ± 1.026.94
SSW−13.96 ± 1.279.13N/DN/D15.62 ± 0.483.04
SSM−13.59 ± 2.0214.82N/DN/D15.50 ± 0.291.89
Ni-AC-SSM cathode LSV test (non-acetate)
Counter electrodeImax (mA)Cv (%)Ef (V)Cv (%)m (mA/V)Cv (%)
IB−15.73 ± 1.157.31N/DN/D14.03 ± 2.1115.09
AB−12.44 ± 0.302.43N/DN/D21.18 ± 7.5735.75
PtW−13.11 ± 0.080.61N/DN/D14.04 ± 0.443.16
SSW−11.95 ± 0.151.21N/DN/D15.77 ± 0.161.04
SSM−13.57 ± 0.705.15N/DN/D13.16 ± 1.3310.12
LSV was performed in 100 mM CBS medium. Inoculated carbon brush (IB), abiotic carbon brush (AB), platinum wire (PtW), stainless steel wire (SSW), stainless steel mesh (SSM), and activated carbon cathode with nickel powder (Ni-AC-SSM).
Table 2. Various anode and cathode test results from previous works. Maximum current (Imax), coefficients of variation (Cv) of Imax, inflection potential (Ef) at which inflection occurs, and steep slope (m) after passing the inflection potential of LSV curves (n = 2). Electrode potentials were measured using the Ag/AgCl reference electrode.
Table 2. Various anode and cathode test results from previous works. Maximum current (Imax), coefficients of variation (Cv) of Imax, inflection potential (Ef) at which inflection occurs, and steep slope (m) after passing the inflection potential of LSV curves (n = 2). Electrode potentials were measured using the Ag/AgCl reference electrode.
Anode LSV Tests
Counter ElectrodeImax (mA)Cv (%)Ef (V)Cv (%)m (mA/V)Cv (%)Reference
SSW11.11 ± 0.050.45−0.49 ± 0.000.4235.27 ± 0.120.34This study
Pt/C a16.36 ± 0.321.93N/DN/DN/DN/D[74]
Ni foam b2.80 ± 0.186.25−0.32N/DN/DN/D[72]
AB cN/DN/D−0.41 ± 0.012.44N/DN/D[73]
Cathode LSV tests (non-acetate)
Counter electrodeImax (mA)Cv (%)Ef (V)Cv (%)m (mA/V)Cv (%)Reference
PtW−13.11 ± 0.080.61N/DN/D14.04 ± 0.443.16This study
PtW d−2.17 ± 0.010.280.02 ± 0.009.52N/DN/D[55]
PtW eN/DN/D−0.40N/D5.44N/D[68]
Pt plate f−8.90 ± 0.505.61N/DN/DN/DN/D[54]
Pt plate g−2.26 ± 0.094.16N/DN/DN/DN/D[56]
Pt-Ti mesh h−74.25 ± 1.351.82−0.69 ± 0.011.1643.8 ± 1.73.88[80]
Pt foil i−5.95N/D0.08N/DN/DN/D[57]
a. Pt/C electrode (carbon cloth coated by 0.5 mg/cm2 Pt/60% on carbon support, area of 6.25 cm2); b. Ni foam (nickel foam electrode, area of 3.14 cm2); c. AB (abiotic brush, diameter of 25 mm, length of 100 mm); d. PtW (platinum wire, diameter of 0.5 mm, length of 5 mm); e. PtW (platinum wire, diameter of 0.5 mm, length of 5 mm); f. Pt plate (platinum plate, area of 6 cm2); g. Pt plate (platinum plate, area of 1 cm2); h. Pt-Ti mesh (platinum-titanium mesh electrode, area 10 cm2); i. Pt foil (platinum foil electrode, 8 cm2).
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MDPI and ACS Style

Chai, H.; Koo, B.; Son, S.; Jung, S.P. Validity and Reproducibility of Counter Electrodes for Linear Sweep Voltammetry Test in Microbial Electrolysis Cells. Energies 2024, 17, 2674. https://doi.org/10.3390/en17112674

AMA Style

Chai H, Koo B, Son S, Jung SP. Validity and Reproducibility of Counter Electrodes for Linear Sweep Voltammetry Test in Microbial Electrolysis Cells. Energies. 2024; 17(11):2674. https://doi.org/10.3390/en17112674

Chicago/Turabian Style

Chai, Hyungwon, Bonyoung Koo, Sunghoon Son, and Sokhee Philemon Jung. 2024. "Validity and Reproducibility of Counter Electrodes for Linear Sweep Voltammetry Test in Microbial Electrolysis Cells" Energies 17, no. 11: 2674. https://doi.org/10.3390/en17112674

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