As a new pollutant treatment technology, microbial fuel cell (MFC) has a broad prospect. In this article, the devices assembled using walnut shells are named biochar-microbial fuel cell (B-MFC), and the devices assembled using graphene are named graphene-microbial fuel cell (G-MFC). Under the condition of an external resistance of 1,000 Ω, the B-MFC with biochar as the electrode plate can generate a voltage of up to 75.26 mV. The maximum power density is 76.61 mW/m2, and the total internal resistance is 3,117.09 Ω. The removal efficiency of B-MFC for ammonia nitrogen (NH3-N), chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) was higher than that of G-MFC. The results of microbial analysis showed that there was more operational taxonomic unit (OTU) on the walnut shell biochar electrode plate. The final analysis of the two electrode materials using BET specific surface area testing method (BET) and scanning electron microscope (SEM) showed that the pore size of walnut shell biochar was smaller, the specific surface area was larger, and the pore distribution was smoother. The results show that using walnut shells to make electrode plates is an optional waste recycling method and an electrode plate with excellent development prospects.

  • Preparation of electrode plates for microbial fuel cells using waste walnut shells.

  • The sewage treatment performance of the walnut shell biochar electrode plate has been improved.

  • Walnut shell biochar electrode plate is more suitable for microbial attachment and growth.

With the development and progress of society, humans have excessively exploited and utilized existing resources, causing waste of resources and environmental pollution (Zhang et al. 2022b). Waste utilization is an important way to solve energy and environmental crises. Wastewater contains a large amount of chemical energy, and the amount of discharge is large, there are many types of pollutants, which will become a major problem for us to deal with wastewater (Jiang et al. 2023). Microbial fuel cells (MFCs) are currently a new type of sewage treatment technology that integrates sewage treatment and microbial electricity generation. MFCs contain electrogenic bacteria on the anode as the main body, and the electrode plate is the attachment carrier and growth place for microorganisms. Therefore, it is crucial to find a low-cost electrode plate that can allow a large number of microorganisms to attach and grow. At present, most MFC electrodes use graphene and carbon cloth (Borja-Maldonado & Zavala 2022). Among them, graphene has better mechanical strength, but its surface is smooth, which is not conducive to the attachment and growth of microorganisms. The surface of carbon cloth is rough, which is conducive to the attachment and growth of microorganisms, but its mechanical strength is too low and it is easy to be damaged. Compared with traditional electrodes, biochar materials have the advantages of a rough surface, wide sources, and low price. In recent years, research and application on biochar have become increasingly widespread (Dhanda et al. 2023). For example, Wang et al. (2020) used biochar derived from peanut shells to adsorb Cr(VI) from aqueous solutions. Removal of phenol pollutants in water using biochar by Guo et al. (2022a). Li et al. (2018) used biochar made from corn cobs to be applied to the cathode of a MFC. The results showed that the maximum output voltage and power density were 0.221 V and 458.85 mW/m3, respectively. Numerous studies have shown that biochar, as a good medium for promoting the separation of photo generated charges in photocatalysts, has high carbon content, larger specific capacitance, and better electron transfer efficiency (Ning et al. 2024). Although biochar prepared from wood and bamboo has advantages such as porous structure and rich specific surface area, it has drawbacks such as high production cost and short service life (Ning et al. 2024). Walnut shells themselves have a natural porous structure, and the biochar produced usually has a higher porosity, which is beneficial for adsorbing and storing harmful substances. The porosity of walnut shell biochar can reach up to 86.64% under the condition of an adsorbent of 250 μg/L (Tulun et al. 2021). The advantage of having a high porosity leads to a higher specific surface area. As an electrode plate for MFCs, it can increase the contact area and improve electron transfer efficiency. Moreover, walnut shells have a wide range of sources, low cost, and are easy to modify due to their porous structure, making them a very promising precursor for biochar (Guo et al. 2022b).

Chemical oxygen demand (COD), ammonia nitrogen (NH3-N), total nitrogen (TN), and total phosphorus (TP) are common pollutants in domestic wastewater, so their removal is of great significance. From previous studies, it can be seen that biochar has a good adsorption effect on heavy metals in the environment and also has advantages such as good electron transfer efficiency. It has been found that walnut shells have high porosity and good adsorption properties. However, there is currently limited research on the application of walnut shell biochar in MFCs. Therefore, we speculate that its application in wastewater treatment and improving the electrochemical performance of MFC also has good effects. Therefore, we designed the following experiments to verify whether it can improve the performance of MFC, in order to find a new electrode material that can replace traditional graphite electrodes. Finally, we characterized the two electrode materials and analyzed their differences in depth. In this study, the electrochemical performance and wastewater treatment capacity of B-MFC assembled with walnut shell biochar were tested against G-MFC assembled with traditional graphene. The treatment capacity of sewage mainly includes the ability to remove COD, NH3-N, TN, and TP.

Materials and chemicals

Na2HPO4, KCl, ZnCl2, and HCl were purchased from Sinopharm Chemical Reagent Co (Shanghai, China), and all reagents were of analytical grade.

MFC construction

Two 500 mL organic glass bottles were used to construct a double-chamber MFC anode plate and cathode plate with the same volume of graphite plate (4 × 5 × 0.5 cm). The distance between the two electrode plates is approximately 10 cm. The cathode chamber is a semi-saturated potassium chloride solution, and a volume fraction of 20% of activated sludge taken from the secondary sedimentation tank of the sewage treatment plant is inoculated in the anode chamber. The inlet water in the anode chamber is taken from the Nibu Bay sewage treatment plant in Huangdao District, and its water quality characteristics are shown in Table 1. The wastewater flows through the anode chamber through a peristaltic pump and is discharged. Proton is transferred between the anode chamber and the cathode chamber through a proton exchange membrane, and an external resistance of 1,000 Ω is connected by a wire to form a circuit. MFC runs on an 8-day cycle. Experimental setup: Three parallel experiments. Throughout the experiment, the MFC was placed in an environment with a temperature of 25 °C and a pH value of approximately 7–8. The device with graphene as the electrode plate is named G-MFC, and the device with walnut shell biochar as the electrode plate is named B-MFC. Experimental data were analyzed using IBM SPSS Statistics 26. The images in this article were drawn with 3D Max 2021 and Origin 2022 (Figure 1).
Table 1

Characteristics of influent water quality

NameInflow water quality (mg/L)
COD 300 
NH3-N 35 
TN 65 
TP 12 
NameInflow water quality (mg/L)
COD 300 
NH3-N 35 
TN 65 
TP 12 
Figure 1

Device diagram.

Preparation of walnut shell biochar

The walnuts purchased in the market were peeled and crushed, passed through a 40-mesh molecular sieve, placed in a quartz boat, and then put into a tube furnace for carbonization at 400 °C for 90 min in a vacuum to obtain a black product (Zhang et al. 2021). A certain amount of black product and zinc chloride solid at a mass ratio of 5:3, respectively, was weighed and placed in a beaker, and then completely submerged in deionized water. After stirring, the samples were placed in a 105 °C oven to dry for 24 h. The dried black product was placed in the center of the tube furnace and calcined under vacuum at 800 °C for 2 h. After the reaction was completed, it was cooled to room temperature under vacuum protection. The calcined samples were first washed with 10% HCl solution, then deionized water until neutral, and finally dried in an oven at 105 °C for 24 h to obtain the carbonized product of the walnut shell. The prepared biochar sample was mixed with polyaniline and hot melt adhesive at a mass ratio of 5:1:4, then the mixed material was placed in a corundum boat mold for compaction and then placed into a 200 °C tube furnace for vacuum hot melting. After heat-melting for 30 min, the sample was removed and cooled down to room temperature naturally. The size of the prepared electrode material sample was 4 × 5 × 0.5 cm.

Data collection and analysis methods

Effluent water samples were collected every 24 h and stored in a refrigerator at 0–4 °C. The wastewater treatment capacity of MFC was evaluated by measuring NH3-N, TN, COD, and TP. NH3-N was measured using the nano reagent colorimetric method, TP was measured using the molybdate spectrophotometric method, COD was measured using the rapid digestion method, and TN was measured using the alkaline potassium persulfate UV spectrophotometric method. COD was determined using a digester (XJY-II, Qingdao Kaiyue Environmental Protection Equipment Co, Ltd, China) and a portable COD water quality analyzer (KY-200, Qingdao Kaiyue Environmental Protection Equipment Co, Ltd, China). Measuring absorbance was carried out using an ultraviolet spectrophotometer (UV-1200S, Shanghai, China). The data acquisition module (DAQM-4202, Xian Zhouzheng Electronic Technology Co, Ltd, China) collected the voltage data of the MFC every 1 min.

Microbial community analysis

The analysis of microbial community structure in the anode chamber by Sangon Biotechnology includes DNA extraction, database construction and sequencing, and data analysis. High throughput analysis was conducted by the Shanghai Biotechnology Sequencing Department (Shanghai, China) to analyze the relative abundance and diversity of colonies, while microbial function prediction analysis was conducted using PICRUSt (v1.14) software.

ACE and Chao 1

The ACE index is an index used to estimate the number of unobserved species in a community. Based on species richness data, it takes into account the impact of species that occur less frequently on the estimated results, allowing for more accurate estimates of species richness in communities. The ACE index is often used to assess the number of species not found in a sample and has important implications for studying species diversity and community structure. The Chao 1 index is also an index used to estimate the number of undiscovered species and is commonly used to evaluate the potential species richness in a sample. The Chao 1 index considers the relationship between species occurrence frequency and species richness, which can better estimate the diversity of species in a community. The Chao 1 index is commonly used to compare species diversity between different samples.

Characterization analysis of electrode materials

The surface morphology, porosity, roughness, and chemical elements of electrode materials of two kinds of MFCs were analyzed by a scanning electron microscope (SEM; Zeiss 500 manufacturer Carl Zeiss, Germany), an energy-dispersive spectrometer (EDS; Zeiss 500 manufacturer Carl Zeiss, Germany), and a specific surface area test method (BET; Micromeritics ASAP 2460, McMuritic Company, USA).

Analysis of the electrochemical performance of MFCs

Power generation is one of the important indicators to evaluate the performance of MFC, and the internal resistance of MFC is an important way to improve the performance of MFC (Luo et al. 2013; Caizán-Juanarena et al. 2020; Tian et al. 2022). As can be seen from Figure 2, the voltage generated by walnut shell biochar can reach up to 75.63 mV, and the maximum voltage generated by G-MFC is 72.45 mV. After the power production is stable, the voltage generated by B-MFC is slightly smaller than that of G-MFC, and the voltage fluctuation of B-MFC is larger than that of G-MFC. The maximum output powers of B-MFC and G-MFC are 76.61 and 80.74 mW/m2, respectively. It can be seen that the electrochemical performance of B-MFC is slightly lower. As shown in Table 2, the internal resistance of B-MFC is 233.77 Ω higher than that of carbon plates. Reducing the internal resistance of MFC is an important way to improve their electrochemical performance (Pasternak et al. 2018). Therefore, it is necessary to analyze the internal resistance composition of MFC (Zhang et al. 2022a). Ohm internal resistance (RΩ) refers to the inherent resistance of the component itself composed of wires and resistors in the circuit. Electrochemical internal resistance refers to an internal resistance in electrochemical cells (such as lithium batteries and MFC), mainly caused by electrolyte, electrode material, and other factors. In general, the ohmic internal resistance and electrochemical internal resistance are internal resistance existing in the circuit, but the ohmic internal resistance mainly refers to the inherent resistance of the component itself in the traditional circuit, and the electrochemical internal resistance is a special internal resistance in the electrochemical cell. The internal resistance of MFC is composed of ohmic internal resistance and non-ohmic internal resistance. The ohmic internal resistance is mainly related to the distance between the two electrode plates and the ionic strength of the electrolyte solution. Non-ohmic internal resistance consists of mass transfer internal resistance and electrochemical internal resistance. Ohmic internal resistance was measured by the current interruption method. When G-MFC is in a stable discharge condition with 1,000 Ω, the circuit is interrupted suddenly, the voltage jumps from 68.4 to 123.17 mV, and the step voltage ΔU is 54.77 mV. The main reason for the voltage jump is that the ohm loss in the battery drops to 0 in a very short time after the current is suddenly interrupted. It is calculated that the current intensity before power failure is 0.07 mA. RΩ is equal to ΔU/0.07, which is 782.45 Ω. In the same way, the RΩ of B-MFC is 826.56 Ω. The internal resistance composition of MFC is shown in Table 3. The internal resistance composition of B-MFC is higher than that of G-MFC. However, the difference in ohmic internal resistance between the two is 44.11 Ω, while the difference in non-ohmic internal resistance is about 190 Ω. The internal resistance of both MFC is mainly composed of non-ohmic internal resistance, so it is very critical to reduce the non-ohmic internal resistance of MFC (Elmekawy et al. 2013).
Table 2

Polarization curve fitting data

Equationy = a + b × x
Drawing G-MFC B-MFC 
Intercept 229.47 ± 1.62 231.42 ± 1.19 
Slope −2,883.32 ± 44.26 −3,117.0883 ± 38.67266 
Residual sum of squares 10.71 9.49 
R2 0.998 0.998 
Equationy = a + b × x
Drawing G-MFC B-MFC 
Intercept 229.47 ± 1.62 231.42 ± 1.19 
Slope −2,883.32 ± 44.26 −3,117.0883 ± 38.67266 
Residual sum of squares 10.71 9.49 
R2 0.998 0.998 
Table 3

MFC internal resistance composition

Internal resistance compositionG-MFCB-MFC
Apparent internal resistance (Ω) 2,883.32 3,117.09 
Ohmic internal resistance (Ω) 782.45 826.56 
Non-ohmic internal resistance (Ω) 2,100.86 2,290.53 
Internal resistance compositionG-MFCB-MFC
Apparent internal resistance (Ω) 2,883.32 3,117.09 
Ohmic internal resistance (Ω) 782.45 826.56 
Non-ohmic internal resistance (Ω) 2,100.86 2,290.53 
Figure 2

Electrochemical performance of MFC: (a) Voltage change curve; (b) Power density curve; (c) Polarization curve.

Figure 2

Electrochemical performance of MFC: (a) Voltage change curve; (b) Power density curve; (c) Polarization curve.

Close modal

Sewage treatment capacity of MFC

The anode microorganisms of MFC can degrade organic pollutants in water while releasing electrons and protons. The removal rate of pollutants in sewage is also an important indicator to measure the performance of MFC (Roy et al. 2023). As can be seen in Figure 3, the COD content of B-MFC effluent decreased from 312.54 mg/L to about 62 mg/L, NH3-N content decreased from 37.85 mg/L to about 13.5 mg/L, TN content decreased from 65.87 mg/L to about 47.43 mg/L, and TP content decreased from 12.52 mg/L to about 9.83 mg/L. The removal rates of COD, NH3-N, TN, and TP in the B-MFC anode chamber were 79.98, 64.33, 30, and 21.49%, respectively. The removal rates of COD, NH3-N, TN, and TP were 79.98, 64.33, 30, and 21.49%, respectively. The COD content of the G-MFC effluent was reduced from 312.54 mg/L to about 66.56 mg/L, the NH3-N content was reduced from 37.85 mg/L to about 14.4 mg/L, the TN content was reduced from 65.87 mg/L to about 46.65 mg/L, and the TP content was reduced from 12.52 mg/L to about 9.83 mg/L, respectively. The removal efficiencies of COD, NH3-N, TN, and TP in the anode chamber of G-MFC were about 78.7, 61.96, 29.18, and 15.81%, respectively. B-MFC has higher removal efficiency than G-MFC for NH3-N, COD, TN, and TP.
Figure 3

MFC wastewater treatment performance: (a) COD change curve; (b) NH3-N change curve; (c) TN change curve; (d) TP change curve.

Figure 3

MFC wastewater treatment performance: (a) COD change curve; (b) NH3-N change curve; (c) TN change curve; (d) TP change curve.

Close modal

Most MFC designs have COD removal efficiencies of 80–95% when treating various types of wastewater. This proves the effectiveness of B-MFC as wastewater treatment and is of great significance for the preparation of biochar electrode plates from walnut shells (Roy et al. 2023).

α-diversity analysis

To further analyze the microbial diversity in the anode chamber, the microbial community structure in the anode chamber of B-MFC was analyzed using G-MFC as a control. α-diversity is a comprehensive index reflecting species richness (Bergsten et al. 2021). Alpha differences mainly include ACE, Chao 1, Shannon, Simpson, and other indicators, where ACE and Chao 1 are abundance-based non-parametric estimates of species richness. This article selects two indicators, ACE and Chao 1, for analysis. The results are shown in Figure 4. Both indicators of B-MFC are greater than G-MFC, indicating that compared with G-MFC, the diversity of microorganisms on the electrode plate of the anode chamber of B-MFC is richer. OTU (operational taxonomic unit) is a taxonomic unit used in microbial diversity analysis to cluster sequences based on their similarity. The more OTU, the higher the microbial diversity is. After analyzing the number of OTUs on the electrode plate, the results are shown in Figure 5, it was found that B-MFC had 252 more OTUs than G-MFC. The above results show that compared with the control group, there are more types and numbers of microorganisms on the B-MFC electrode plate.
Figure 4

Alpha diversity analysis: (a) Chao index; (b) Ace index.

Figure 4

Alpha diversity analysis: (a) Chao index; (b) Ace index.

Close modal
Figure 5

Quantitative analysis of OTUs.

Figure 5

Quantitative analysis of OTUs.

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Species composition analysis

In order to visually analyze the microbial communities on the electrode plates of the two MFCs, visual string plots were used to reveal the changes in the microbial community composition of the MFC anode at the genus level. For both MFC with different electrode materials, the thick-walled phylum was dominant. As can be seen in Figure 6, the species of dominant bacteria were similar for both MFC at the genus level, with Klebsiella and Acidophilus xyloglucosus dominating. There was a large difference in the percentage of acid-eating bacteria genera in the microbial composition of the anode chambers of the two MFCs (Garbini et al. 2023), with acid-eating bacteria genera accounting for about 6.08% of the total in the B-MFC and about 13.52% in the G-MFC. Gluconacetobacter accounted for 14.54 and 16.16% in B-MFC and C-MFC, respectively, and Klebsiella accounted for 26.21 and 23.91% in B-MFC and G-MFC, respectively. This also verifies that it is closely related to the power generation capacity of MFC. Studies have shown that Klebsiella belongs to electrogenic bacteria (Guo et al. 2020), but Gluconacetobacter's ability to produce electricity has not yet been proven.
Figure 6

Microbial community structure.

Figure 6

Microbial community structure.

Close modal

Functional prediction analysis

After analyzing alpha diversity and species composition, it is necessary to analyze the different functions of microbial communities under different conditions. Therefore, 16srRNA was used to conduct functional prediction analysis of the microbial communities of B-MFC and C-MFC anode electrode plates. PICRUS 1 (phylogenetic investigation of communities by reconstruction of unobserved states) was used to obtain the COG function classification diagram. In B-MFC and G-MFC, transcriptional regulators, arabinose efflux permease, arabinose efflux permease, and response regulators consisting of a CheY-like receiver domain and a winged-helix DNA-binding domain belong to both and have a high abundance gene. It can be seen from Figure 7 that a large number of genes related to transport and metabolism in the anode chamber and the rapid synthesis of related proteins indicate that the microorganisms are using the available factors in the sewage to grow and reproduce (Sen Thapa et al. 2022; Li et al. 2023). Moreover, the abundance of genes expressed in B-MFC was higher. For example, methyl-accepting chemotaxis protein and response regulator containing CheY-like receiver, AAA-type ATPase, and DNA-binding domains are significantly more abundant in B-MFC than in G-MFC. It may be due to the richer microbial diversity in B-MFC, which is consistent with previous findings. Based on these results, it can be further speculated that during the operation of MFC, microorganisms related to organic pollutant degradation and energy production are enriched in the anode microbial community (Guo et al. 2023).
Figure 7

Feature prediction heatmap.

Figure 7

Feature prediction heatmap.

Close modal

SEM and EDS of different electrode materials

Many studies have shown that the performance of MFCs is closely related to the physical properties of electrode materials, especially the surface roughness of electrode materials. In order to better explain the reasons for these differences, the scanning electron microscopy images of the two electrode materials are shown in Figure 8. From Figure 8(a) and 8(b), it can be seen that the electrode material surface of G-MFC is smooth, while the walnut shell biochar electrode material surface in B-MFC is relatively rough. From Figure 8(c) and 8(d), it can be observed that the electrode material in G-MFC has no pores, while pore structures are found in walnut shell biochar. This may be due to the activation effect of added zinc chloride on biochar.
Figure 8

SEM of different electrode materials: (a) and (c) are G-MFC; (b) and (d) are B-MFC.

Figure 8

SEM of different electrode materials: (a) and (c) are G-MFC; (b) and (d) are B-MFC.

Close modal
From Figures 9 and 10, it can be seen that carbon is the main element in both electrode plate materials, and both contain oxygen and zinc elements. However, the carbon, oxygen, and zinc elements in the electrode materials of B-MFC are significantly higher than those of G-MFC, especially with the highest carbon content. From the above results, it can be seen that the conductivity of electrode materials is closely related to carbon elements. Studies have shown that the increase in conductivity is attributed to the higher carbon content and the formation of biochar matrix within graphite nanocrystals (Hassanpour et al. 2020). The addition of zinc chloride is also one of the main reasons for the differences between the two electrode materials. Walnut shell biochar has a rougher surface and more pore structures than graphene electrode plates.
Figure 9

EDS: (a) B-MFC; (b) G-MFC.

Figure 9

EDS: (a) B-MFC; (b) G-MFC.

Close modal
Figure 10

EDS mapping. G-MFC: (a) is the carbon content; (b) is the O content; (c) is the Zn content. B-MFC: (d) is the carbon content; (e) is the O content; (f) is the Zn content.

Figure 10

EDS mapping. G-MFC: (a) is the carbon content; (b) is the O content; (c) is the Zn content. B-MFC: (d) is the carbon content; (e) is the O content; (f) is the Zn content.

Close modal

BET testing

BET is an important means of analyzing indicators such as material surface area and porosity (Pan et al. 2005). The surface area and porosity of electrode materials in MFC are important factors affecting MFC. From Table 4, it can be seen that the surface area of walnut shell biochar is about 13 times, with B-MFC mainly having microporous area, while graphene has a very small proportion of microporous area. The adsorption and desorption pore sizes of G-MFC are larger than those of B-MFC, and the average pore radius of G-MFC is about 50.92 nm, while the average pore radius of B-MFC is only 2.12 nm. This indicates that the smaller the pore size, the larger the specific surface area, and the larger the pore volume.

Table 4

BET

NameG-MFCB-MFC
BET surface area 50.9216 m2/g 650.61 m2/g 
t-Plot micropore area 7.19 m2/g 571.72 m2/g 
Pore volume 0.06 cm3/g 0.34 cm3/g 
t-Plot micropore volume 0.004 cm3/g 0.295 cm3/g 
Mean pore size of adsorption (4 V/A, BET) 5.08 nm 2.12 nm 
BJH mean pore radius of adsorption (2 V/A) 4.07 nm 1.55 nm 
BJH desorption mean pore radius (2 V/A) 3.72 nm 1.59 nm 
NameG-MFCB-MFC
BET surface area 50.9216 m2/g 650.61 m2/g 
t-Plot micropore area 7.19 m2/g 571.72 m2/g 
Pore volume 0.06 cm3/g 0.34 cm3/g 
t-Plot micropore volume 0.004 cm3/g 0.295 cm3/g 
Mean pore size of adsorption (4 V/A, BET) 5.08 nm 2.12 nm 
BJH mean pore radius of adsorption (2 V/A) 4.07 nm 1.55 nm 
BJH desorption mean pore radius (2 V/A) 3.72 nm 1.59 nm 

The adsorption curve is generally divided into six types. As shown in Figure 11, B-MFC belongs to the Type I isotherm, where the adsorption capacity rapidly increases under relatively low relative pressure. After reaching a certain pressure, the adsorption reaches saturation, similar to the Langmuir isotherm. The Type I isotherm usually reflects the phenomenon of microporous adsorption on microporous adsorbents. The adsorption curve of G-MFC belongs to the H3 type in the IV type adsorption isotherm, and the hysteresis loop in the H3 type reflects that the adsorption curve has not reached saturation, indicating that the pore structure of G-MFC is irregular. From Figure 12, it can be seen that there are a large number of microporous structures distributed in B-MFC, while G-MFC has almost none. From this, it can be concluded that the surface area of walnut shell biochar is larger, the number of micropores is higher, and the pore size distribution is regular.
Figure 11

Adsorption curve.

Figure 11

Adsorption curve.

Close modal
Figure 12

Aperture distribution map.

Figure 12

Aperture distribution map.

Close modal

In summary, it is feasible to prepare MFC electrode plates using walnut shell biochar. The biochar prepared from walnut shells has a rich microporous structure, carbon elements, and a larger surface area. The electrochemical performance of B-MFC assembled with biochar was slightly lower than that of G-MFC, but the wastewater treatment performance of B-MFC assembled with biochar was improved. However, the growth of a large number of microorganisms attached to the walnut shell biochar indicated that the internal resistance of the electrochemical reaction was small. The reason for the poorer performance of the B-MFC in producing electricity may be due to the larger internal resistance to mass transfer. Therefore, the selection of suitable activator and optimal calcination temperature can reduce the mass transfer internal resistance of MFC and thus improve the performance of B-MFC.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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