Construction of an Experimental System

The experimental system was mainly composed of a single-chamber PMFC and an external load and data acquisition system, as shown in Fig. 1. Among them, the single-chamber PMFC was composed of a potted Scindapsus Aureus, a cathode, and an anode. The plant pot, with a diameter of 16 cm and a height of 10 cm, actually acted as the PMFC reactor. Bacteria-rich activated sludge mixed with cadmium contaminated soil was used as the planting soil. The bottom of the flowerpot was covered with 2~3 cm thick planting soil, and a square of carbon felt of 5 cm×5 cm was placed above it as the anode. Scindapsus Aureus was planted in the pot, and another part of the planting soil was used to fill the roots. The soil layer height was about 8 cm to ensure that the plant roots were located near the anode. The cathode is a 5 cm×5 cm square of carbon felt and was placed on the surface of the pot soil to make it fully in contact with oxygen in the air. The cathode and anode were connected through copper wires and externally connected with a 1000 Ω resistor. Organic matter in soil is degraded by electrogenic microorganisms to produce electrons. The output voltage generated by PMFC was collected in real-time through a data acquisition card (MPS-010602; Beijing Qichuang Mofei Electronic Technology Co., Ltd., China) and transmitted to a computer through a USB interface for storage, processing, and display.

Microbial Culture and Planting Soil Preparation

Electrogenic bacteria are indispensable for soil remediation and power generation processes in PMFC. Soil is rich in a variety of microbial species and is a common source of inoculants used in biological treatment systems, which can directly supply some of the microorganisms needed for PMFC. In order to further enhance the number and activity of exoelectrogens in the pot soil, some soil was acclimated to obtain microbial strains and added to the pot soil. The soil used for microbial cultivation was taken from the roots of a certain tree on campus. 0.01 g Cd(NO₃)₂, 100 g soil, 400 mL deionized water, and 3 mL trace element solution (SL-6, Shandong Tuopu Biol-engineering CO., LTD., China) were added to the culture bottle, which was kept in an anaerobic state. The culture bottle was then placed in a biochemical incubator and cultured at 30ºC for about three days. When there are obvious suspended flocculents and gas expansion in the mixed solution in the cultivation bottle, it can be considered that microbial cultivation has been successful.

Fig. 1.

Experimental system of PMFC.

Cadmium contaminated soil was obtained through manual addition. The soil collected from the campus was naturally air dried, then ground and passed through a stainless steel sieve with evenly distributed 2 mm holes to obtain impure and pollution-free loess. 0.1 g of Cd(NO₃)₂ was mixed with 1000 g and left at room temperature for one day, then used as pot soil.

Preparation of Electrodes

Firstly, the carbon felt of 5 cm×5 cm×0.8 cm was soaked in acetone for 2 h, then washed repeatedly with deionized water until neutral, and then dried at 60ºC in a blast drying oven for 3 h to obtain the unmodified conventional carbon felt (CF) electrode.

The CeO₂ solution was obtained by dissolving 0.5 g nanoscale cerium oxide powder in 200 mL deionized water and stirring for 2 h. Then, the carbon felt electrode prepared in the previous step was immersed in CeO₂ solution for 2 h, and then dried in an oven at 60ºC for 3 h to obtain the nanoscale CeO₂ modified carbon felt anode (CeO₂/CF).

Dissolve 2.28 g ammonium persulfate in 100 mL deionized water, 50 mL of which was polymerized with 4 mL aniline monomer to form polyaniline, then dissolved in 200 ml deionized water and stirred for 2 h to obtain a PANI solution. The prepared conventional carbon felt anode was soaked in PANI solution for 2 h, and then dried in a 60ºC oven for 3 h to obtain a polyaniline modified carbon felt anode (PANI/CF).

54 mL PANI solution and 0.5 g CeO₂ were dissolved in 200 mL deionized water and stirred for 2 h to obtain PANI-CeO₂ solution. The conventional CF electrode was soaked in PANI- CeO₂ solution for 2 h, and then dried in an oven at 60ºC for 3 h to obtain a polyaniline and cerium oxide modified carbon felt anode (PANI-CeO₂/CF).

Main Analysis Method

The voltage and current density are used to reflect the electrical performance of the PMFC. The output voltage of PMFC is the potential difference between the anode and cathode. In the experiments, the voltage values were collected in real time by a data acquisition card. The current density was calculated based on the measured voltage, effective anode surface area, and load resistance, namely:

where I_A is the current density, U is the measured voltage, A is the surface area of the anode, and R is the load resistance. In this study, the surface area of the anode was 2.5⋅10^-3 m², and the value of load resistance was set to 1000 Ω.

In order to better understand the surface structure changes of the modified carbon felt, the surface morphology of anode samples before and after modification was characterized by scanning electron microscopy (SEM, JSM-6360LV, Tokyo JEOL Co., LTD., Japan).

In order to further analyze the conductivity of the electrodes, the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were performed on a chemical workstation (CHI660E, Shanghai Chenhua Instrument Co., LTD., China), with an MFC anode as the working electrode, a platinum electrode as the auxiliary counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. In the tests, the potential range is -0.6 V~0.6 V, and the scanning speed is 0.01 V/s.

In order to understand the polarization and power generation characteristics and analyze the effect of the anode modification method on the performance of PMFC, the polarization curve of PMFC was measured by the variable resistance method. Adjust the load resistance of the PMFC in the range of 9000~100 Ω (9000-7000-5000-3000-2000-1000-500-200-100 Ω) through the resistance box, record the steady-state output voltage under different load conditions, and calculate the corresponding current density, thus obtaining the polarization curve of the PMFC.

The Cd removal rate is an important indicator to characterize the effectiveness of soil remediation. The total concentration of Cd in the soil was determined by graphite furnace atomic absorption spectrophotometry (AA-6880) after digestion with hydrochloric acid-nitric acid-hydrofluoric acid-perchloric acid. By comparing the initial and final Cd concentrations in soil, the removal rate of Cd in soil by PMFC was calculated, and the remediation effect of different configurations of PMFC on Cd contaminated soil was analyzed. The formula for calculating the removal rate of Cd is as follows:

where C_cd0 and C_cdend represent the initial and final values of cadmium in the soil corresponding to the beginning and end of the experiment, respectively, and C_cdrr denotes the removal rate of cadmium by PMFC at the end of the operation.

Electricity Generation Performance

The output voltage curves of PMFCs with CF, CeO₂/CF, PANI/CF, and PANI-CeO₂/CF anodes are shown in Fig. 2. Due to the influence of microbial activity and other factors, the output voltage decreased significantly after running for 200 h, so the operating cycle was set to 200 h. It can be seen that the maximum output voltage values of PMFCs loaded with CF, CeO₂/CF, PANI/CF, and PANI-CeO₂/CF anodes were 2.46, 4.64, 6.42, and 10.01 mV, respectively. Compared with CF, CeO₂/CF and PANI/CF, the output voltage of PMFC using the PANI-CeO₂/CF anode increased by 307.2%, 115.8%, and 55.9%, respectively, indicating that the anodic carbon felt modification with PANI or CeO₂ has a certain effect on improving the generation capacity of PMFC. PANI and CeO₂ are effective electrode modification materials, especially when using a blend of PANI and CeO₂ for anode modification. The electricity generation capacity of PMFC was improved most significantly.

Further observation of Fig. 2 reveals voltage fluctuations within the operating cycle. This is mainly due to the complex biochemical reactions between microorganisms, soil, and plants. Random factors such as temperature, microbial activity, and light intensity can have impacts on the reaction process, leading to aperiodic fluctuations in the generating voltage.

Polarization Characteristics

Fig. 3 shows the polarization curves of four sets of PMFCs with different anodes. The polarization curve can directly reflect the total voltage loss of a fuel cell. The “higher” the polarization curve, the smaller the total voltage loss of the fuel cell. It can be seen from the polarization curves shown in Fig. 3 that, under the same current generation, the output voltage of PMFC using the PANI-CeO₂/CF anode was always higher than that of PMFCs using other three anodes, while the polarization curve of PMFC using the conventional unmodified anode was the lowest. These show that the voltage losses of PMFCs with three modified anodes were reduced to different degrees compared with PMFCs with an unmodified anode, and the voltage loss of PMFC with PANI-CeO₂/CF was the least, which presented a better effect on improving the power supply efficiency of PMFC.

CV Characteristics

The cyclic voltammetry (CV) curves of four different anodes are shown in Fig. 4. From the results in the figure, it can be seen that among the four types of anodes, the PANI-CeO₂/CF anode has the largest integrated area and the best symmetry in the CV closure curve. The integral area of the CV curve represents the discharge capacity of a fuel cell. The larger the area, the closer the mutual contact between the electrode and electrolyte, and the greater the charge storage capacity and charge transfer amount, resulting in a larger battery capacity [32, 33]. The better the symmetry of the CV curve, the better the reversibility of the electrochemical reaction of the electrode. Therefore, the CV test results fully demonstrate that the synergistic effect of PANI and CeO₂ can improve the electrochemical activity of the anode surface, thereby promoting the rapid formation of biofilm on the anode surface, accelerating the electron transfer rate, and intensifying the reaction process of organic matter in soil being oxidized by microbial metabolism in the anode and oxides being reduced in the cathode, thus significantly improving the abilities of PMFC in power generation and soil purification.

Fig. 2.

the output voltage of PMFCs.

Fig. 3.

Polarization curves.

Fig. 4.

CV curves.

SEM Images

The SEM images of the four different anodes measured before the experiments are shown in Fig. 5. It can be seen that the surface of a conventional CF anode without modification was smooth, so the specific surface area was small, and the number of microorganisms that attached to and grew on its surface was also small. The surface of the anode modified with nano-CeO₂ or PANI was loaded with a small amount of CeO₂ or PANI particles, but no obvious biofilm has been formed, which to some extent increased the specific surface area of the anode. Compared with the first three types of anodes, the carbon felt electrode co-modified by CeO₂ and PANI formed a more obvious biofilm on the surface, attached more conductive particles on the surface, further improved the surface roughness, and significantly increased the specific surface area and pore volume, which was more conducive to the adhesion and growth of microorganisms on the electrode surface and greatly improved the electron transfer rate, and thus, significantly improving the electricity generation performance and soil treatment ability of PMFC.

Fig. 7.

Randles equivalent circuit.

Fig. 6.

EIS graph.

EIS Tests

Internal resistance is an important factor affecting the efficiency of the power supply. The internal resistance of PMFC can be measured by EIS characteristics. The EIS characteristics were tested using an electrochemical workstation at a polarization potential of -0.1 V with a scanning frequency range of 1~1.0 × 10⁶ Hz. The Nyquist curves of the EIS of PMFCs with the four different anodes are shown in Fig. 6, and the Randles equivalent circuit is shown in Fig. 7, where R_s represents the solution internal resistance, R_CT represents the charge transfer resistance, CPE represents the constant phase angle element, and Z_W is the Warburg impedance.

In the EIS spectrum, the left intersection point between the semicircle and the horizontal axis of the EIS curve corresponds to the solution internal resistance R_s, and the diameter of the semicircle corresponds to the charge transfer impedance R_CT [34]. The impedance values of the four anodes obtained from the EIS test results are shown in Table 1, where the equivalent internal resistance of PMFC was approximately calculated by R_eq≈R_s+R_CT.

Fig. 5.

SEM images of anodes.

According to the above EIS test results, it can be seen that the use of PANI-CeO₂ modified anodes effectively reduced the internal resistance of PMFCs, which indicates that this anode modification method is very effective for improving the electric generation performance and power supply efficiency of PMFCs.

Cd Removal Rate

The removal rate of heavy metals reflects the remediation ability of PMFC on contaminated soil. The test data for soil cadmium content and cadmium removal rate before and after the experiment are shown in Table 2.

From the experimental data in Table 2, it can be seen that the removal rates of Cd in soil by PMFCs using CF, CeO₂/CF, PANI/CF, and PANI-CeO₂/CF anodes were 61.07%, 91.48%, 92.75%, and 93.91%, respectively. PMFC using PANI CeO₂/CF modified carbon felt had the highest Cd removal rate, which was 53.77%, 2.66%, and 1.25% higher than that using CF, CeO₂/CF, and PANI/CF modified carbon felt anodes, respectively. This indicates that PMFC not only has the ability to use plants and soil to generate electricity, but also has a good role in solving the problem of heavy metal pollution in soil. The anodic modification of PMFC with CeO₂ and PANI significantly increased the removal rate of heavy metals in soil, which is very effective in improving the soil remediation ability of PMFC.