Investigation into the Preparation and Electrochemical Energy Storage Performance of Nickel Cobalt Oxide-Based Composite Anode Materials

Abstract

Microbial fuel cells (MFCs) are a novel bioenergy technology that utilizes microorganisms to catalyze the conversion of fuels into electricity. However, traditional MFCs are constrained by the low electricity generation capacity of microorganisms, resulting in relatively low power output. Additionally, the inability of traditional MFCs to store electricity significantly limits their practical applications. In this study, we fabricate a novel oxide graphite/nickel cobalt oxide (GO/NiCo₂O₄) capacitive composite bioanode material supported on stainless-steel fiber felt (SSFF). This composite material combines the excellent biocompatibility of graphite oxide and the energy storage capacity of nickel cobalt oxide. Consequently, the prepared anode exhibits significant advantages, including high specific capacitance, efficient electron transport, and enhanced biocompatibility. The MFC with the SSFF/GO/NiCo₂O₄ anode demonstrated a significantly enhanced power density, achieving a maximum of 1267.5 mW/m²—1.38-fold and 2.23-fold higher than those of the SSFF/GO and SSFF anodes, respectively. Moreover, the modified anode (SSFF/GO/NiCo₂O₄) exhibited a stored charge (Q_s) of 1405.35 C/m², representing 2.61-fold and 35.79-fold increases compared to the SSFF/GO and SSFF anodes, respectively. High-throughput analysis revealed that SSFF/GO/NiCo₂O₄-modified anode achieved an electrogenic bacterial efficiency exceeding 81%, which was significantly higher than that of the SSFF/GO and SSFF anodes. The results of this study not only provide valuable insights and theoretical guidance for the development of MFCs using capacitive composite anode materials, they also present sustainable power solutions for low-power electronic systems, such as miniaturized sensors and IoT devices.

1. Introduction

Microbial fuel cells (MFCs) are bioelectrochemical systems that utilize microorganisms to catalyze fuel oxidation and generate electricity. The microorganisms in the anode chamber oxidize organic matter to produce electrons and protons (H⁺). The electrons are transported to the anode surface directly or indirectly, and then are further transmitted to the cathode through the external circuit, while the proton hydrogen is transferred to the cathode through the proton exchange membrane (PEM), thus forming a circuit to generate current [1]. MFCs have many advantages compared with traditional fuel cells. First, MFCs have a relatively high energy conversion rate and, as a low-cost and long-cycle power system, have development potential. Second, while purifying the environment, MFCs can utilize waste liquid and waste materials as fuel to generate electricity, causing no carbon dioxide emissions or pollution [2,3,4,5]. Although MFCs have advantages, such as high material adaptability, no secondary pollution, high energy conversion efficiency, and ease of operation, their low output power and inability to store energy significantly limit practical applications. The anode material of microbial fuel cells serves as a carrier for the attachment of electricity-producing microorganisms. It not only affects the electricity-producing bacteria adhering to the anode surface, but also influences the transfer of electrons from the bacteria to the anode material surface, playing a crucial role in enhancing the electrical output of microbial fuel cells [6]. The ideal anode material should possess excellent biocompatibility, high specific surface area, high electron transfer rate, strong stability, high porosity, low cost, and ease of preparation, among other characteristics [6]. The traditional MFC anode materials are mostly based on carbon materials. Although carbon materials have good biocompatibility, their poor electrical conductivity, low electron transfer rate, low strength, propensity for deformation, and high cost limit their power generation performance and practical applications. Compared with traditional carbon materials (carbon cloth), stainless steel fiber felt (SSFF) has advantages such as high electrical conductivity, three-dimensional macroporous solid structure, good corrosion resistance, and low cost, making it an ideal substrate material [7,8]. Guo et al. [7] obtained a continuous current density for the SSFF after heat treatment that was seven times that of the untreated SSFF through the heat treatment method. Hou et al. [8] prepared polyaniline/SSFF anodes by impregnation and electro-polymerization methods and applied them in MFC anodes. Among them, the polyaniline/SSFF anode prepared by impregnation method achieved a maximum power density of 347 mW/m² at a current density of 1.15 A/m², while the polyaniline/SSFF anode prepared by the electro-polymerization method obtained a maximum power density of 360 mW/m² at a current density of 1.49 A/m². J.M. Sonawane et al. [9] explored the potential of using polyaniline-modified stainless steel plates as a low-cost anode for MFCs, which have the ability to effectively promote microbial growth and maintain long-term stability. This indicates that SSFF has the potential to serve as the anode substrate material for MFCs. However, the poor biocompatibility and high overpotential of SSFF limit its application as an anode in bioelectrochemical systems [10]. Therefore, surface modification of SSFF electrodes is necessary to enhance their biocompatibility and increase the electron transfer rate.

Graphite oxide, as a carbon nanomaterial, features a large specific surface area, high electrical conductivity, and good biocompatibility, and has long been a major focus in material modification research [11,12,13,14]. Pan et al. [15] successfully prepared polypyrrole/graphite oxide (PPy/GO) composites by in situ chemical polymerization, varying the feed ratio of pyrrole (Py) to graphite oxide. The results show that the PPy/GO composite material exhibits a very high specific capacitance and a relatively low charge transfer resistance. Shul et al. [16] combined graphite oxide with titanium dioxide, which increased the specific surface area of titanium dioxide and extended its absorption spectrum to the visible and infrared light regions, significantly enhancing the catalytic efficiency in photocatalytic reactions. Li et al. [17] successfully prepared a graphite/graphite oxide composite electrode for vanadium redox flow batteries and investigated the redox reactions of [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ by cyclic voltammetry and electrochemical impedance spectroscopy. The results show that after adding 3 wt% graphite oxide to the graphite electrode, the electrochemical performance of the electrode was greatly improved. The ratio of the redox peak currents of the [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ couples on the composite electrode increased by nearly twofold compared to that on the graphite electrode, and the charge transfer resistance of the redox couples also decreased significantly. Xu et al. [18] synthesized nanostructured MoO₂/graphite oxide (GO) composites via a simple solvothermal method. The MoO₂/GO composite with 10 wt% GO exhibited a reversible capacity of 720 mAh/g at a current density of 100 mA/g and maintained a reversible capacity of 560 mAh/g after 30 cycles at a high current density of 800 mA/g. The aforementioned research has fully demonstrated that the composite electrode modified with graphite oxide has achieved improvements in terms of increased specific surface area and enhanced electrochemical performance. However, the specific capacitance of carbon materials is relatively low, and they still lack the ability to store energy. Therefore, further modification is necessary.

The best way to solve these problems is to store the energy generated by MFCs effectively and supply it as needed. Efficient energy storage and utilization in MFCs represent a current research priority and are critical for achieving their practical application. At present, the capacitive electrodes in MFCs mainly employ metal oxide materials [19]. For instance, Cao et al. [20] fabricated a low-cost binder-free air cathode by in-situ electrodeposition of nickel cobalt oxide nanosheets on carbon cloth, which exhibited a 12.96% higher catalytic activity than commercial Pt/C. Ge et al. [21] synthesized a nano-urchin-like nickel cobalt oxide modified activated carbon cathode via hydrothermal method, and achieved the maximum power density in MFCs (2.28 times that of bare activated carbon), which was comparable to the commercial Pt/C electrode. O P et al. [22] successfully prepared three kinds of spinel-type NiCo₂O₄ nanostructures with different NCO (nickel cobalt oxide) loading amounts and rGO-loaded hybrid catalysts by adjusting the nickel cobalt oxide precursor through the hydrothermal method. The hybrid catalyst exhibited exceptional electrochemical performance, including a high specific capacitance (1305 F g⁻¹ at 5 mV s⁻¹), outstanding rate performance, and superior cycling stability. Even after 3000 cycles in a supercapacitor application, its cycling stability remains at 89%. Li et al. [23] synthesized nickel cobalt oxide-embedded nitrogen-doped carbon (NC) composites (nickel cobalt oxide@NC) by a novel strategy combining electrospinning and thermal treatment. The asymmetric battery based on nickel cobalt oxide @ NC exhibits excellent energy storage capacity, showing a high specific energy of 61.2 Wh kg⁻¹ at 548.6 W kg⁻¹. This indicates that the modification of the anode material with cobalt and nickel can significantly enhance the electrochemical performance of the anode.

In this study, a graphite oxide (GO)/nickel cobalt oxide(NiCo₂O₄) modified capacitive composite anode material was synthesized on stainless steel fiber felt (SSFF). Considering the conductivity of SSFF, the biocompatibility of GO, and the pseudocapacitive property of nickel cobalt oxide, this anode has significant advantages, including a large specific capacitance, efficient electron transfer, and excellent biocompatibility. The composite properties of these three electrode materials leveraged their individual advantages and compensated for their deficiencies [24]. Therefore, modifying SSFF with GO and NiCo₂O₄ is expected to improve the biocompatibility and energy storage performance of SSFF while retaining its high conductivity and corrosion resistance. Applying the modified material to the MFC’s anode is expected to achieve simultaneous power generation and efficient energy storage. This research provided a novel contribution to the field as the improved MFC simultaneously combined power generation and energy storage functions. When the power supply and electrical energy do not match, the electrical energy produced by microorganisms is first stored in the capacitive anode; when electrical energy is needed, it can rapidly release a large current (the sum of the stored and generated parts), significantly enhancing the output current and providing an alternative power supply option for small sensors and communication devices [24,25].

2. Experiment

2.1. Preparation of SSFF/GO/NiCo₂O₄ Electrode Materials

First, a mixture of 10 mL of nitric acid (chromatographic grade, 70%, Harbin Kaimesi Technology Co., Ltd., Harbin, China) and 20 mL of sulfuric acid (chromatographic grade, 70%, Harbin Kaimesi Technology Co., Ltd., China) was cooled to 0 °C in an ice bath. Next, 1 g of graphite powder (99.95% metals basis, Harbin Kaimesi Technology Co., Ltd., Harbin, China) was added to this acidic solution. After 1 h of reaction, 11 g of potassium chlorate (AR, 99.5%, Harbin Kaimesi Technology Co., Ltd., China) was gradually introduced to ensure that the reaction temperature remained relatively low. Then the mixture was stirred at room temperature for three days, washed with deionized water until neutral, and a black slurry was obtained. Finally, the prepared graphite oxide residue was dried in an oven at 60 °C [26]. The prepared graphite oxide was ground into powder, and a 4 g·L⁻¹ graphite oxide solution was prepared. After ultrasonic dispersion of the graphite oxide solution, the SSFF was added to the mixed solution and ultrasonicated for 5 min. The SSFF was then taken out, and a pipette was used to draw the mixed solution and evenly drop it on the surface of the SSFF, and it was then placed on a 60 °C oven to dry for 12 h to obtain the SSFF/GO electrode. Refer to Figures S4–S7 in the Supporting Information for photographic documentation of the associated reaction vessels and apparatus.

Preparation of manganese cobalt oxide powder was as follows. First, 10 mL of 0.4 mol/L cobalt nitrate (AR, 99%, Harbin Kaimesi Technology Co., Ltd., China) solution, 5 mL of 0.4 mol/L nickel nitrate (AR, 98%, Harbin Kaimesi Technology Co., Ltd., China) solution, and 10 mL of 1.2 mol/L urea (AR, ≥99%, Harbin Kaimesi Technology Co., Ltd., China) solution were added successively to a beaker containing 25 mL of distilled water, then stirred for 30 min. The above solution was transferred to a reaction vessel and subjected to a hydrothermal reaction at 140 °C for 20 h. The reaction vessel was then cooled to room temperature. The post-reaction mixture was rinsed with distilled water and anhydrous ethanol, respectively, and this process was repeated three times. A pink NiCo₂O₄ powder precursor was obtained. The precursor was placed in a vacuum drying oven and dried at 60 °C for 12 h. The prepared precursor was calcined at 350 °C for 4 h in a muffle furnace to obtain NiCo₂O₄ powder. According to the ratio of 8:1:1, 0.08 g of NiCo₂O₄ powder, 0.01 g of acetylene black (99.9%, Dongguan Kelude Experimental Equipment Technology Co., Ltd., Dongguan, China), and 0.01 g of polyvinylidene fluoride (average Mw~400,000, Harbin Kaimesi Technology Co., Ltd., China) were weighed and dispersed in N-methylpyrrolidone (anhydrous grade, ≥99.5%, Harbin Kaimesi Technology Co., Ltd., China) to form a uniform slurry. The slurry was evenly coated onto the SSFF/GO electrode using a pipette, and then placed in a forced-draft drying oven at 80 °C for 12 h to fabricate the SSFF/GO/NiCo₂O₄ electrode.

2.2. MFC Construction and Operation

The two-chamber MFC in this article was composed of a cathode chamber, an anode chamber, and a proton exchange membrane. The physical diagram of the two-chamber MFC is shown in Figure S1 (Supporting Information). Among them, the cathode chamber and the anode chamber were separated by a Nafion membrane (Nafion 117, DuPont, Wilmington, DE, USA), and were fixed in the middle with clips. The processed carbon rods (Xi’an Carbon Materials Co., Ltd., Xi’an, China) were used as the cathode, and 250 mL of 10 g/L potassium ferricyanide (A.R. 99.5%; Shanghai Aolai Biochemical Technology Co., Ltd., Shanghai, China) solution was poured into the cathode chamber. Next, 250 mL of nutrient solution (the formula of the nutrient solution is shown in the Supporting Information Table S1) was added to the anode chamber, and a certain amount of carbon source (sodium acetate with a concentration of 2.5 g/L) was added. The anode electrode was inserted, and the chamber was kept sealed to provide an anaerobic environment for the electrogenic bacteria. All the microbial strains in this article were from sewage treatment plants. Subsequently, the microbial strains were acclimated by adjusting the temperature and external resistance. After the MFC reactor was assembled, an 8000 Ω resistor was connected externally. After the output voltage of the MFC stabilized, electrochemical tests are conducted. Before the tests, the anolyte in the anode chamber of the MFC device was regularly replaced to ensure a stable supply of nutrients in the anode chamber, which promoted the growth and metabolism of microorganisms.

2.3. Characterizations and Measurements

The surface morphology and elemental composition of the electrode were analyzed using a Hitachi SU5000 scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan) from Japan and an Oxford Ultim Max40 energy dispersive spectrometer (Oxford Instruments, Abingdon, UK) from the UK, with an acceleration voltage set at 20 kV. The phase analysis of the material was conducted using the SMARTLAB 9Kw X-ray diffractometer (XRD) (Rigaku Corporation, Tokyo, Japan) from Rigaku, Japan. The diffraction conditions were as follows: scanning rate of 10°/min and scanning range from 5° to 90°. The phases were identified based on the diffraction data from the standard patterns.

All the electrochemical experiments in this paper were conducted in the MFC reactor system. In this study, the prepared MFC anode was used as the working electrode, the cathode (carbon rod) as the counter electrode, and the Ag/AgCl electrode as the reference electrode. By varying the size of the external resistance, different voltages of the MFC at its optimal state were measured. The corresponding current values were calculated based on the voltages, and the MFC power density curve and polarization curve based on voltage and current were constructed. A constant resistance was applied to the anode to generate a chronopotentiogram, and the changes in current over time are recorded. The internal resistance of the MFC was obtained through AC impedance measurement, and the electrode process kinetics, double-layer effect, and diffusion were analyzed by EIS. The frequency range of the EIS test is 1 Hz to 100 kHz, and the potential perturbation is 5 mV. This experiment was conducted using a Shanghai Chenhua CHI760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) (Figure S2, Supporting Information) in a two-chamber MFC reactor.

2.4. Microbial Characterization Technique

High-throughput sequencing technology was carried out in accordance with the standardized measurement protocol provided by Shanghai Sangon Biotech Co., Ltd. (Shanghai, China). The samples were pre-treated, centrifuged, and DNA was extracted using the EZNATM Mag-Bind Soil DNA Kit (OMEGA) (OMEGA Bio-tek, Norcross, GA, USA). After quality inspection, the first round of PCR amplification was carried out. After accurately quantifying the genomic DNA using the Qubit^® 4.0 DNA Assay Kit, the amount of DNA to be added to the PCR reaction was determined. The primers used for PCR were fused with the 16SV3-V4 primers of the sequencing platform. The second round of amplification introduced Illumina bridge PCR compatible primers. After amplification, the library size was determined and quality checked. Once the quality check results met the on-machine requirements, the library concentration was measured using the Qubit^® 4.0 Fluorometer. Finally, the products were purified, and the microbial community and proportion were determined through microbial classification and sequencing.

3. Results and Discussion

3.1. Preparation and Morphology Characteristics of SSFF/GO/NiCo₂O₄ Anode

The SEM images in Figure 1 provide valuable insights into the morphologies of the SSFF, SSFF/GO and SSFF/GO/NiCo₂O₄ electrodes. According to the SEM images, the morphological features of the pre-treated SSFF electrode fibers, shown in Figure 1a–c and magnified 500, 1000, and 5000 times, respectively, are very smooth and clean. This indicates that the pre-treated SSFF provides suitable conditions for loading GO in the next step. The morphological features of the SSFF/GO electrode fibers magnified 500, 1000 and 5000 times are shown in Figure 1d–f. It can be seen from the figures that the GO is in a sheet-like form and firmly wraps around the SSFF fibers, providing a vast specific surface area for the loading of NiCo₂O₄. The morphological features of the SSFF/GO/NiCo₂O₄ electrode fibers magnified 500, 1000, and 5000 times are shown in Figure 1g, Figure 1h, and Figure 1i, respectively. It can be seen from Figure 1g–i that NiCo₂O₄ spherical nanoparticles are uniformly dispersed on the SSFF loaded with GO. Figure 1j shows the spherical nickel cobalt oxide nanoparticles magnified 10,000 times. It can be clearly seen that the nickel cobalt oxide has a sea urchin-like morphology, which indicates that loading nickel cobalt oxide further increases the specific surface area of the electrode. Figure 1k shows the morphology of the NiCo₂O₄-loaded fibers magnified 10,000 times. It can be seen that the fibers of the SSFF/GO electrode are all tightly wrapped by NiCo₂O₄, providing more active sites for the attachment of microorganisms. The results further indicated that loading nickel cobaltite increased the specific surface area of the electrode, which was conducive to the adhesion of more microorganisms to form biofilms.

Figure 2 shows the elemental mapping images of the SSFF, SSFF/GO and SSFF/GO/NiCo₂O₄ electrodes. The data of the element proportion in

Table 1. The parameter of EDS spectra with (a) SSFF, (b) SSFF/GO, and (c) SSFF/GO/NiCo₂O₄ electrodes.

Anodes	Element	Welght %	Atomic %
SSFF	Fe	61.36	47.56
	Cr	17.09	14.22
	Ni	9.38	6.92
SSFF/GO	C	31.32	57.95
	O	14.21	19.74
	Fe	36.68	14.59
	Cr	10.36	4.43
	Ni	5.36	2.03
SSFF/GO/NiCo2O4	C	7.15	20.13
	O	16.64	35.15
	Co	37.10	21.28
	Ni	19.21	11.06
	Fe	14.75	8.93
	Cr	4.42	2.88

was statistically obtained from the EDS spectrogram in Figure S3 of the Supporting Information. The SSFF electrode was predominantly composed of iron (Fe, 61.36 wt%), chromium (Cr, 17.07 wt%), and nickel (Ni, 9.38 wt%) (Table 1), consistent with the elemental profile of the stainless steel fiber felt substrate. The SSFF/GO electrode is mainly composed of carbon, oxygen, iron, chromium and nickel, among which the contents of carbon and oxygen are significantly increased, indicating that the graphite oxide has enhanced the active substances of the substrate. Their mass percentages (Table 1) are 31.32%, 14.21%, 36.68%, 10.36%, and 5.36% respectively. For the SSFF/GO/NiCo₂O₄ electrode, the contents of oxygen, cobalt, and nickel have significantly increased, with mass percentages (Table 1) of 16.64%, 37.10%, and 19.21% respectively. Moreover, the molar ratio of cobalt to nickel is two to one, which is consistent with the fact that nickel cobalt oxide is the main active material. The EDS data validate the successful synthesis and uniform deposition of NiCo₂O₄ on the SSFF/GO substrate, corroborating the expected elemental distribution.

X-ray diffraction (XRD) is a powerful technique for analyzing the crystal structure and composition of materials. Figure 3 shows the XRD patterns of SSFF, SSFF/GO and SSFF/GO/NiCo₂O₄. For the SSFF electrode (Figure 3a), high-intensity diffraction peaks were observed at 2θ = 43.6°, 50.8°, and 74.7°. Through XRD analysis, these diffraction peaks matched well with the (111), (200), and (220) crystal planes of the standard austenitic stainless steel (PDF#33-0397), once again confirming that the basic composition of SSFF is Fe, Cr, and Ni. For the SSFF/GO electrode, Figure 3b shows that a diffraction peak appears at 2θ = 10.5°, which is not present in Figure 3a. This is due to the addition of graphite oxide. This peak corresponds to the (001) plane of GO, confirming the successful synthesis and uniform loading of GO onto the SSFF substrate. This diffraction peak is related to the (001) crystal plane of graphite oxide, and no other impurity peaks appeared. The prepared graphite oxide has extremely high purity, which can fully exert the good electrical conductivity of graphite oxide, promote charge transfer, and increase the specific surface area of the electrode, providing a broader active site for the attachment of microorganisms. For the SSFF/GO/NiCo₂O₄ electrode, as shown in Figure 3c, the characteristic peaks of nickel cobalt oxide at 2θ = 31.1°, 36.7°, 38.4°, 44.6°, 55.4°, 59.1°, and 65.0° correspond respectively to the (220), (311), (222), (400), (422), (511), and (440) crystal planes of nickel cobalt oxide (PDF#20-0781). The prepared nickel cobalt oxide is a cubic structure with a crystal group of F∗3(202), indicating that the product is nickel cobalt oxide particles. The integration of NiCo₂O₄ fully leverages its bimetallic oxide properties, enhancing and increasing the pseudocapacitance characteristics, which can further enhance the power density of the electrode, optimize the energy storage performance, and provide greater output power.

3.2. The Output of MFC

The power generation performance curves of MFCs with three different anodes are shown in Figure 4. The MFC with the SSFF/GO anode achieved a maximum power density of 921.6 mW/m² (Figure 4a), which was 1.62 times higher than that of the MFC with the SSFF anode (567.7 mW/m²). This improvement is attributed to the high specific surface area of GO, which provided abundant reaction sites and facilitated rapid ion transport, thereby enhancing electrode efficiency. The MFC with SSFF/GO/NiCo₂O₄ anode achieved a maximum power density of 1267.5 mW/m², which was 1.38 times and 2.23 times higher than that of the MFC with SSFF/GO anode (921.6 mW/m²) and the MFC with SSFF anode (567.7 mW/m²), respectively. The incorporation of NiCo₂O₄ enhanced the electrode’s pseudocapacitive properties, improving energy storage capacity and power density. This is mainly due to the fact that the composite of nickel cobalt oxide enhances the capacitance of the electrode. The pseudocapacitance property of nickel cobalt oxide improves the energy storage performance of MFC, thereby increasing the power density. According to relevant research, the power density of an MFC is directly related to the performance of its anode capacitance [27]. Polarization curves within a current density range of 0–3.5 A/m² revealed voltage drops of 305 mV, 387 mV, and 469 mV for the SSFF/GO/NiCo₂O₄, SSFF/GO, and SSFF MFCs, respectively, indicating that the battery modified with SSFF/GO/NiCo₂O₄ had the lowest polarization (Figure 4b). Under the same current density, the SSFF/GO/NiCo₂O₄ battery exhibits a larger output voltage, which is consistent with the trend of the power density curve. According to the anode polarization curve, the open-circuit potential of the SSFF/GO/NiCo₂O₄ anode is much higher than that of the SSFF/GO (−495 mV) and SSFF (−461 mV) anodes, reaching −536 mV. The anode potential of SSFF/GO/NiCo₂O₄ drops at a slower rate, and its polarization degree is relatively low, while the anode polarization degrees of SSFF/GO and SSFF increase successively. The results show that the utilization of GO/NiCo₂O₄ composite material enhances the redox reaction rate, reduces the electron transfer impedance, promotes the electrode process, and is conducive to improving the overall performance of the MFC. The bimetallic oxide modification promoted microbial adhesion, expanded reactive sites, and increased output voltage while reducing polarization, offering a viable power solution for low-energy sensors and communication devices. This also provides an alternative power supply option for future small sensors and communication devices.

Table 2. The power density of MFCs modified with metal oxides on the anode was documented in the literature.

	References	Anode	Power Density
1	This study	SSFF/GO/NiCo2O4	1267.5 mW/m2
2	Hou et al. [8]	SSFF/PANI	360 mW/m2
3	Mehdinia et al. [29]	MWCNT/SnO2/GC	1421 mW/m2
4	Park et al. [30]	CP/CNT/Fe3O4	830 mW/m2
5	Wang et al. [31]	S/N-CNT/PANI/MnO2	1019.5 mW/m2
6	Wang et al. [28]	CF/MnO2/PANI/MnO2	1124.8 mW/m2

shows the comparison of the power density results of MFCs in this study with those in multiple other studies. It can be concluded that, through the composite of GO and NiCo₂O₄ on the SSFF substrate and the dual metal oxide of graphite oxide and nickel cobalt oxide, this study achieves a synergistic effect. The spinel structure of nickel cobalt oxide provides abundant redox active sites, promoting extracellular electron transfer. Graphite oxide acts as a conductive bridge to connect SSFF and NiCo₂O₄, reducing the interface resistance. This design breaks through the limitations of traditional single modification strategies (gold-based metal oxides or conductive polymers). Moreover, although the power density studied by Mehdinia et al. [28] is slightly higher than that in this study, they used expensive glass carbon (GC) and MWCNT substrates, with costs much higher than those of SSFF in this study. In terms of substrate selection, SSFF is cheaper compared to other substrates and is suitable for large-scale applications, ensuring a balance between power density increase and reduced research costs. Thus, the SSFF/GO/NiCo₂O₄ novel composite anode studied in this study achieves a balance among performance, cost, and stability, providing new ideas for the practical application of MFCs.

Figure 5 presents the AC impedance plots of the three electrodes. As shown in the figure, the Nyquist plots for all three electrodes comprised two distinct regions: a semicircle in the high-frequency range and a linear segment in the low-frequency range. The high-frequency region represents the electrochemical reaction process of the electrodes. The smaller the diameter of the semi-circle, the smaller the Faraday charge transfer resistance Rct between the electrode and the electrolyte, and the smaller the degree of electrochemical polarization. The left intersection point of the semi-circle in the high-frequency region with the X-axis is the solution for the impedance R Ω. The transfer impedances Rct and R Ω of the SSFF electrode solution are 7.18 and 3.31 Ω respectively, and those of the SSFF/GO electrode solution are 4.19 and 3.18 Ω respectively, both of which are higher than those of the SSFF/GO/NiCo₂O₄ electrode (3.21 and 1.85 Ω). These results demonstrate that the incorporation of graphite oxide (GO) enhanced interfacial interactions between the electrode and biofilm, thereby lowering electron transfer impedance. The synergistic effect of nickel cobalt oxide (NiCo₂O₄) further promoted electron transfer, leading to significant electrode performance enhancement. Impedance fitting analysis confirmed that the GO/NiCo₂O₄ composite facilitated extracellular electron transfer (EET) at the biofilm-electrode interface. This improvement was attributed to the composite’s ability to expand the electrode’s specific surface area, which supported stable biofilm formation and efficient electron transport, ultimately minimizing impedance and optimizing electrochemical performance.

3.3. The Storage Ability of MFCs

Figure 6 presents the potential-time curves of the SSFF, SSFF/GO, and SSFF/GO/NiCo₂O₄ anodes. Due to the modification of the two anode materials with pseudocapacitive materials, the potential decreases slowly over time (Figure 6). In contrast, the final potential of the SSFF/GO/NiCo₂O₄ anode reached −0.058 V during the 15-min charging period, Both of which were higher than that of the unmodified SSFF anode (−0.363 V). This observation indicates that the modified anodes, owing to their enhanced specific capacitance, partially stored charges generated by electrogenic bacteria, thereby decelerating the potential drop. Since nickel cobalt oxide is a pseudocapacitive material, the addition of nickel cobalt oxide can increase the specific capacitance and store more charges. The storage of charges can slow down the decrease of potential [32,33,34,35,36]. In other words, the greater the electrode capacitance of the anode, the slower the rate of potential drop.

Figure 7 presents the discharge curves of MFCs equipped with three distinct anodes. It can be seen from the figure that the discharge curves of the three electrodes all exhibited an initial peak current, then dropped rapidly, and finally reached a stable state. The discharge curve parameters of the three different electrodes can be compared and viewed in

Table 3. The parameter of discharging curves with SSFF anode, SSFF/GO anode, and SSFF/GO/NiCo₂O₄ anode.

Anodes	Parameter	C15/D15
SSFF anode	ih (A/m2)	10.31
	is (A/m2)	0.24
	Qs (C/m2)	39.27
	Qt (C/m2)	427.08
SSFF/GO anode	ih (A/m2)	37.55
	is (A/m2)	3.31
	Qs (C/m2)	538.65
	Qt (C/m2)	4537.42
SSFF/GO/NiCo2O4 anode	ih (A/m2)	70.93
	is (A/m2)	8.67
	Qs (C/m2)	1405.35
	Qt (C/m2)	11,836.1

. During the 15 min charging/discharging test, the peak current density (i_h) and steady-state current density (i_s) of the SSFF/GO/NiCo₂O₄ anode were 70.925 A/m² and 8.6675 A/m², respectively, both of which were higher than those of the SSFF/GO and SSFF anodes. Additionally, the total charge (Q_t) of the SSFF/GO/NiCo₂O₄ anode during the 15-min discharge test reached 11,836.1 C/m², representing increases of 7298.7 C/m² and 11,409 C/m² compared to the SSFF/GO and SSFF anodes, respectively. The storage charge (Q_s) of the SSFF/GO/NiCo₂O₄ anode (1405.35 C/m²) was 2.61-fold and 35.79-fold higher than that of the SSFF/GO and SSFF anodes, respectively, indicating superior energy storage capacity in the modified anodes. Figure 7 shows that the two modified anodes exhibited significantly enhanced storage capacity. The graphite-oxide-modified anode (SSFF/GO) enhanced bacterial adhesion and electron storage by virtue of its increased specific surface area, thereby improving both power generation and energy storage efficiency. The addition of graphite oxide significantly enhanced the catalytic activity of the electrode and facilitated the effective transfer of electrons from the electrogenic bacteria to the anode surface. The anode modified by oxidized graphite shows a significantly larger storage capacity compared to the unmodified substrate. Therefore, graphite oxide has demonstrated excellent electrochemical performance as an electrode material. Further improvement was achieved with the addition of NiCo₂O₄. This is attributed to the excellent pseudocapacitive properties of NiCo₂O₄, which is conducive to the adhesion of electrogenic bacteria and promotes the electron transfer between bacteria and the electrode.

Therefore, the SSFF/GO/NiCo₂O₄ modified anode not only promotes the electron transfer of the electrogenic bacteria to the anode surface, but also facilitates the adhesion and growth of microorganisms and enhances the rate of electron production from the oxidation of organic matter by microorganisms, as well as the storage capacity. Due to the favorable environment provided by GO and NiCo₂O₄ for the metabolism of electrogenic bacteria, the SSFF/GO/NiCo₂O₄ anode demonstrated highly efficient energy storage performance. Thus, the catalytic activity of the electrogenic bacteria was enhanced. Of course, this is also attributed to the fact that the electrons produced by the electrogenic bacteria can be promptly stored in the electrode, enabling the modified anode to simultaneously generate and store electrons in an open state and release both parts of the electrons instantaneously in a closed state. This allows the SSFF/GO/NiCo₂O₄ bifunctional bioanode material to achieve simultaneous electricity generation and energy storage in the MFC system. This dual functionality enabled the SSFF/GO/NiCo₂O₄ bioanode to act as an initial capacitor for microbial electrical energy when external power demand was mismatched. When power is needed, it rapidly discharges to generate a higher current (the sum of the stored electricity and the generated electricity), significantly enhancing its current output and solving the problem of MFCs’ low output power [37,38,39,40].

3.4. Analysis of Anode Biodiversity and Microbial Community Structure

High-throughput sequencing provides abundant genetic information by virtue of its high yield and resolution. In this study, the microbial composition attached to capacitive anodes was investigated. High-throughput testing was performed on microorganisms colonizing the SSFF, SSFF/GO, and SSFF/GO/NiCo₂O₄ anodes, and SSFF/GO/NiCo₂O₄ anodes, yielding data on electrogenic microbial flora composition and colony counts. The SSFF, SSFF/GO, and SSFF/GO/NiCo₂O₄ anodes were represented by samples S1, S2, and S3, respectively. Figure 8a–c shows the microbial species abundance of different anodes. The SSFF anode surface was supported by a variety of microbial flora, i.e., electrogenic bacteria including bacteroidia (9.16%), alphaproteobacteria (6.61%), and gammaproteobacteria (28.43%). For the SSFF/GO anode surface, major constituents included bacteroidia (30.40%), alphaproteobacteria (1.37%), and gammaproteobacteria (22.89%). For the SSFF/GO/NiCo₂O₄ anode surface, major constituents included bacteroidia (37.84%), gammaproteobacteria (30.06%), and alphaproteobacteria (13.14%). Most of the electrogenic microorganisms reported in the literature are distributed in these categories [41,42]. These microorganisms, serving as the primary electrogenic species across the anodes, decomposed organic matter in the anode chamber, generating charges that enhanced the MFC capacitance. The content of electrogenic bacteria on the SSFF, SSFF/GO, and SSFF/GO/NiCo₂O₄ anodes reached 44.20%, 54.67%, and 81.04% respectively. The content of electrogenic bacteria on the modified anodes was all higher than that on the SSFF anode, indicating that the modified composite anodes promoted electrogenic bacteria to become the dominant bacteria among the electrode microorganisms. Charges are mainly produced by electrogenic bacteria through the decomposition of organic substrates present in the nutrient solution. Comparative analysis of Figure 8a–c revealed that the graphite-oxide-modified anode (S2) exhibited a larger specific surface area, which is conducive to the adhesion of electrogenic bacteria, and the content of electrogenic bacteria has been increased. However, compared with the SSFF/GO anode, the content of electrogenic bacteria in the SSFF/GO/NiCo₂O₄ anode has increased by 1.48 times. This is because the nickel cobalt oxide in the SSFF/GO/NiCo₂O₄ anode tightly coats the fibers of the electrode, and the nickel cobalt oxide nanoparticles further increase the specific surface area, which is conducive to the adhesion of electrogenic bacteria and provides a new selective environment for the metabolism of bacteria. The low impedance of the SSFF/GO/NiCo₂O₄ anode promoted electron transfer for bacteria with limited electron transfer capacity [43,44], thereby amplifying electrogenic bacteria proliferation.

Figure 9 presents the Venn diagrams illustrating the total microbial species distribution across the three anodes.

As shown in Figure 9, the SSFF/GO/NiCo₂O₄ anode exhibited a higher number of species compared to the SSFF/GO and SSFF anodes, demonstrating that the graphite oxide/nickel cobalt oxide composite anode enhances biocompatibility for electrogenic bacteria. Meanwhile, the experimental results validated the accuracy of the proposed methodology. As Figure 8 and Figure 9 show, the increased diversity and abundance of electrogenic bacteria directly contributed to improved electrogenic capacity. Consequently, both the natural power generation capability and power density showed significant enhancement, aligning with prior research findings.

4. Conclusions

In this study, a stainless steel fiber felt (SSFF) was used as the substrate to fabricate a graphite oxide/nickel cobalt oxide (GO/NiCo₂O₄) electrode, which was then applied in a microbial fuel cell (MFC) to enhance the energy generation and storage capacity of the MFC. The experimental results showed that the maximum power density of the MFC with SSFF/GO/NiCo₂O₄ as the anode was 2.23 times and 1.38 times higher than that of the MFCs with SSFF and SSFF/GO as the anodes, respectively. Furthermore, during the 15-min discharge test, the Q_t of the SSFF/GO/NiCo₂O₄ anode (11,836.1 C/m²) was higher by 11,409 C/m² and 7298.7 C/m² compared with those of the SSFF anode and the SSFF/GO anode, respectively. High-throughput detection results showed that the anode modified by graphite oxide and nickel cobaltite contained more than 81% of electricity-producing microorganisms. In this paper, a capacitive composite bioanode of nickel cobalt oxide/graphite oxide was fabricated on a stainless steel fiber mat. By combining the characteristics of large specific surface area and good biocompatibility of graphite oxide with the energy storage advantages of nickel cobalt oxide pseudocapacitive materials. The prepared anode exhibited advantages such as large specific capacitance and surface area, excellent electron transfer ability, and good biocompatibility, and it achieved enhanced performance. The improved MFC demonstrated synchronous power generation and energy storage functions, which significantly enhanced its current output and provided stable power for small low-power electrical devices, solving the problem of MFCs’ low output power. This system represents a highly promising power supply for small sensors and communication devices.