Fabrication of a Molybdenum Dioxide/Multi-Walled Carbon Nanotubes Nanocomposite as an Anodic Modification Material for High-Performance Microbial Fuel Cells

A nanocomposite of multi-walled carbon nanotubes (MWCNTs) decorated with molybdenum dioxide (MoO2) nanoparticles is fabricated through the reduction of phosphomolybdic acid hydrate on functionalized MWCNTs in a hydrogen–argon (10%) atmosphere in a tube furnace. The MoO2/MWCNTs composite is proposed as an anodic modification material for microbial fuel cells (MFCs). MWCNTs have outstanding physical and chemical peculiarities, with functionalized MWCNTs having substantially large electroactive areas. In addition, combined with the exceptional properties of MoO2 nanoparticles, the synergistic advantages of functionalized MWCNTs and MoO2 nanoparticles give a MoO2/MWCNTs anode a large electroactive area, excellent electronic conductivity, enhanced extracellular electron transfer capacity, and improved nutrient transfer capability. Finally, the power harvesting of an MFC with the MoO2/MWCNTs anode is improved, with the MFC showing long-term repeatability of voltage and current density outputs. This exploratory research advances the fundamental application of anodic modification to MFCs, simultaneously providing valuable guidance for the use of carbon-based transition metal oxide nanomaterials in high-performance MFCs.


Introduction
The ever-increasing global energy consumption requirements, involving the excessive depletion of unsustainable fossil fuel and consequent severe environmental pollution problems, are intensively driving the development of green, renewable carbon-neutral energy technologies [1][2][3][4][5][6].Microbial fuel cells (MFCs), utilizing effectual electroactive bacteria as anodic electricigens, can directly transform the chemical energy of inexhaustible organic matter or biomass into available clean electrical energy [7][8][9][10][11][12].MFCs can effectively remove environmental waste, such as wastewater, while producing bioelectricity [13].This feature is considerably attractive to scientific researchers in the MFC field worldwide.
Despite the excellent bioelectricity generation and breakthrough of MFCs in the past few decades [14 -17], their low power harvesting and high manufacturing cost inputs immensely restrict their practical use [18][19][20][21][22].Many factors can influence the power generation performance of MFCs, such as cell configurations, anodic or cathodic modification materials, proton exchange membranes, anolytes, and electron media [4,[23][24][25][26].Anodic modification plays an essential role in MFC performance, and diversifying anodic modification materials through exploration is a valuable research direction in the MFC field.
Molecules 2024, 29, 2541 2 of 14 Carbon nanotubes (CNTs), which are one-dimensional carbon nanomaterials, have excellent physical and chemical peculiarities due to their special structural characteristics.CNTs have been utilized as anodic modification materials of MFCs due to their excellent electronic conductivity, mechanical properties, and biocompatibility [27].Additionally, numerous CNTs can efficaciously constitute three-dimensional network structures; such structures can form large holes and are conducive to the attachment and habitation of active microorganisms.Sun et al. [28] fabricated three-dimensional CNT networks on an MFC anode through layer-by-layer self-assembly.The modified anode with CNTs had a large specific surface area.The interface electron transfer impedance of MFCs with this anode reduced from 1163 to 258 Ω, and the power density increased by 20% compared to the bare carbon paper anode.Peng et al. [29] modified a glassy carbon electrode using CNT materials and discovered that CNTs can accelerate the extracellular electron transfer (EET) between the active microorganism Shewanella oneidensis and an anode.The current density output of the MFC was 9.70 ± 0.40 µA cm −2 , which was 82-fold higher than that of an MFC with an unmodified anode.Liang et al. [30] directly mixed CNTs and the bacterium Geobacter sulfurreducens in a composite biofilm architecture and applied it as an MFC anode.The experimental results exhibit markedly reduced startup time and anode resistance of the MFC.The voltage output and electricity production performance of the MFC were also heightened.Moreover, the maximum power density of the MFC based on the CNT hydrogel biological anode was 65% higher than that of the control group [31].However, for unfunctionalized CNTs with exiguous surface defects, some functional groups in the end caps of nanotubes, such as hydroxyls, cannot be completely exposed; these reduce the dispersibility of CNTs.As anodic modification materials, unfunctionalized CNTs have small active areas and are not conducive to the heavy exposure of catalytic sites [32].CNT surface functionalization is usually performed using a strong oxidizer or acid corrosion, and various functional groups can be introduced into purified CNT surfaces.Through synergistic interaction with intercalation of concentrated sulfuric acid and oxidation of concentrated nitric acid, mixed-acid treatment can purify CNTs by removing their impurities; it can also connect large numbers of hydroxyl, carboxyl, and other oxygen-containing groups on CNT surfaces.This can enhance CNT dispersion and the binding of CNTs with other substances, enhancing the efficiency of decorating CNTs with metal nanoparticles or oxide particles [33].
Molybdenum minerals are abundant in nature and cost-effective.In recent years, transition metal compounds containing molybdenum have also garnered considerable attention.Molybdenum carbide (Mo 2 C), a transition metal carbide with a high melting point and hardness, has outstanding thermal stability, corrosion resistance, electronic structure, and catalytic activity, similar to precious metals [34][35][36].Mo 2 C has been utilized as an anodic modification material for MFCs.Zou et al. [2] used Mo 2 C nanoparticles with small grain sizes and good crystallinity to modify porous graphene nanocomposites via electrostatic assembly combined with high-temperature carburization, greatly enhancing the adhesion of active bacteria and the allegro formation of stable biofilms.These active bacteria secreted adequate electrochemical biomolecules, such as flavin, around the anode, thus increasing the EET rate from the bacterial cells to the anode.Finally, the MFC proposed by Zou et al. achieved a satisfactory power density of 1697 mW m −2 , which was twice and 13 times those of MFCs with graphene and bare carbon cloth (CC) anodes, respectively.MoO 2 is also a transition metal oxide with a high melting point.MoO 2 has superior conductivity, chemical stability, and charge transmission properties; thus, it has potential applications in catalysts, chemical sensors, supercapacitors, lithium-ion batteries, electrochromic displays, and field-emission materials [37].The valence band of MoO 2 has many high-density free electrons, which can effectively accelerate the catalytic activity of Mo and improve the catalytic performance of MoO 2 ; therefore, MoO 2 has been diffusely applied in the field of catalysis.Because of its outstanding conductivity and charge carrier transfer efficiency and the tunnel-like voids in its crystal structure, MoO 2 is beneficial for embedding and extricating charged particles at high speeds; hence, MoO 2 is a candidate material in the field of supercapacitors [38].MoO 2 has been utilized as an anodic modification material for MFCs.Zeng et al. [39] synthesized polydopamine-modified Mo 2 C/MoO 2 nanoparticles through thermal reduction incorporating in situ polydopamine modification and recommended them for MFC anodic modification.The maximum power density of their MFC was 1.64 ± 0.09 W m −2 .Li et al. [40] prepared Co-modified MoO 2 nanoparticles dispersed on nitrogen-doped carbon nanorods and utilized them as MFC anode electrocatalysts.The high biocompatibility of MoO 2 could enrich electroactive bacteria on the anode, and modification with Co augmented the electrocatalytic activity; simultaneously, N doping improved the electronic conductivity of the carbon nanorods.Their experimental results also indicate good electrocatalytic activity during charge transfer of the anode, and the maximum power density of the MFC is 2.06 ± 0.05 W m −2 .A previous study discussed nitrogen-doped CC grafting with MoO 2 microspheres (N@MoO 2 /CC) prepared via in situ polymerization and high-temperature carburization for the development of high-performance anodes of MFCs [41].The N@MoO 2 /CC anode exhibited remarkable bioelectricity generation due to its dual function of promoting bacterial colonization and enriching electroactive bacteria, thereby improving the electrocatalytic activity of the anode.An MFC with the N@MoO 2 /CC anode obtained a maximum power density of 3.01 ± 0.23 W m −2 .Furthermore, MWCNTs composites modified with MoO 2 nanoparticles have been used for methanol oxidation [42].MWCNTs@MoO 2 -C nanocable composites have outstanding electrochemical performance in lithium-ion battery anodes [43].To the best of our knowledge, relevant systematic reports about MoO 2 /MWCNTs nanocomposites as anodes of Escherichia coli (E.coli)-inoculated MFCs are scarce.
According to the above studies, multi-walled CNTs (MWCNTs) have superior electronic conductivity and MoO 2 nanoparticles have been confirmed to have excellent biocompatibility and electrocatalytic activity.We consider and propose the effective recombination of functionalized MWCNTs and MoO 2 nanoparticles to construct anodic modification materials for MFCs.This can give full play to the advantages of functionalized MWCNTs and MoO 2 nanoparticles.Accordingly, in this paper, MoO 2 -nanoparticle-decorated functionalized MWCNTs (MoO 2 /MWCNTs) nanocomposites are fabricated by mixing functionalized MWCNTs and phosphomolybdic acid in a tube furnace in a hydrogen-argon (10%) atmosphere.These nanocomposites are then utilized as anodic modification materials for E. coli-inoculated MFCs.Electrochemical cyclic voltammetry (CV) measurements show that the electroactive areas of the functionalized MWCNTs and MoO 2 /MWCNTs electrodes are notably enhanced compared with those of nonfunctionalized MWCNTs and bare CC electrodes.This is beneficial for the mass attachment of active bacteria and the rapid formation of stable biofilms.Moreover, the MoO 2 /MWCNTs anode has enhanced EET efficiency and nutrient transfer capability.Therefore, compared with the bare CC, MWCNT, and functionalized MWCNTs anodes, the MFC with the MoO 2 /MWCNTs nanocomposites bioanode has higher power density outputs and long-term voltage and current density stability.Herein, a carbon-based transition metal oxide nanocomposite (MoO 2 /MWCNTs) is successfully prepared and utilized as an MFC anode.This establishes a solid theoretical and experimental foundation for searching for anodic modification materials for high-performance MFCs.

Physical Characterization of Materials
X-ray diffraction (XRD) patterns can characterize the phase and crystallinity of materials.Figure 1 shows the XRD patterns of MWCNTs, functionalized MWCNTs, and the MoO 2 /MWCNTs nanocomposites.All three materials display strong diffraction peaks at approximately 2θ = 26 approximately 2θ = 26°, which is a typical characteristic (002) diffraction peak of MWCNTs.Compared with the diffraction peak of the nonfunctionalized MWCNTs, that of the functionalized MWCNTs is slightly wider, and no other special diffraction peaks appear.Nevertheless, the MoO2/MWCNTs nanocomposites show MoO2 characteristic peaks at 2θ = 36.979°,37.344°, 53.293°, 53.578°, 53.938°, 60.251°, and 66.653° (JCPDS-73-1807).This indicates the successful preparation of the MoO2/MWCNTs nanocomposite.The surface morphologies of the MWCNTs, functionalized MWCNTs, and MoO2/MWCNTs are characterized using scanning electron microscopy (SEM).Figure 2af shows the SEM images of the MWCNTs, functionalized MWCNTs, and MoO2/MWCNTs magnified 50,000 and 100,000 times, respectively.As exhibited in Figure 2a, the MWCNTs have evident tubular structures.Figure 2b shows that the nanotubes form interlaced spatial network frameworks.The functionalized MWCNTs are short (Figure 2c) because ultrasound in mixed acids can separate nanotubes and generate oxygen-containing groups on MWCNTs surfaces [44].As seen in Figure 2d, the wall surfaces of the functionalized MWCNTs are slightly rough, which may be due to the presence of surface oxygen-containing groups.Figure 2e,f depicts numerous small particles loaded on the MWCNTs Combined with the XRD diagram of MoO2/MWCNTs, these figures indicate that these small particles are MoO2 nanoparticles. Figure 2f shows the formation of a spatial grid construction and large holes.These are subsequently verified using the SEM-EDS elemental analysis spectra of the MWCNTs, functionalized MWCNTs, and MoO2/MWCNTs in Figure S1.The pristine MWCNTs (Figure S1a) and functionalized MWCNTs (Figure S1b) have no Mo elements, whereas MoO2/MWCNTs have Mo elements (Figure S1c).
The morphologies of the functionalized MWCNTs and the MoO2/MWCNTs nanocomposites are further characterized using transmission electron spectroscopy (TEM).As shown in Figure 3a,b, the functionalized MWCNTs (Figure 3a) have clear tubular configurations, and the nanotube surfaces have no granular materials.In contrast, numerous MoO2 nanoparticles are anchored on the functionalized MWCNTs in MoO2/MWCNTs nanocomposites (Figure 3b).The sizes of the MoO2 nanoparticles are not distributed homogeneously; they range from 20 to 60 nm, and a few nanoparticles are agglomerated.The surface morphologies of the MWCNTs, functionalized MWCNTs, and MoO 2 / MWCNTs are characterized using scanning electron microscopy (SEM).Figure 2a-f shows the SEM images of the MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs magnified 50,000 and 100,000 times, respectively.As exhibited in Figure 2a, the MWCNTs have evident tubular structures.Figure 2b shows that the nanotubes form interlaced spatial network frameworks.The functionalized MWCNTs are short (Figure 2c) because ultrasound in mixed acids can separate nanotubes and generate oxygen-containing groups on MWCNTs surfaces [44].As seen in Figure 2d, the wall surfaces of the functionalized MWCNTs are slightly rough, which may be due to the presence of surface oxygen-containing groups.Figure 2e,f depicts numerous small particles loaded on the MWCNTs.Combined with the XRD diagram of MoO 2 /MWCNTs, these figures indicate that these small particles are MoO 2 nanoparticles.Figure 2f shows the formation of a spatial grid construction and large holes.These are subsequently verified using the SEM-EDS elemental analysis spectra of the MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs in Figure S1.The pristine MWCNTs (Figure S1a) and functionalized MWCNTs (Figure S1b) have no Mo elements, whereas MoO 2 /MWCNTs have Mo elements (Figure S1c).
The morphologies of the functionalized MWCNTs and the MoO 2 /MWCNTs nanocomposites are further characterized using transmission electron spectroscopy (TEM).As shown in Figure 3a,b, the functionalized MWCNTs (Figure 3a) have clear tubular configurations, and the nanotube surfaces have no granular materials.In contrast, numerous MoO 2 nanoparticles are anchored on the functionalized MWCNTs in MoO 2 /MWCNTs nanocomposites (Figure 3b).The sizes of the MoO 2 nanoparticles are not distributed homogeneously; they range from 20 to 60 nm, and a few nanoparticles are agglomerated.

Electrochemical Behaviors of Electrodes
Large electroactive areas in MFC anodes are favorable for the massive adherence of active bacteria; the electroactive areas of anodes are estimated via electrochemical doublelayer capacitance [45,46].The capacitance properties of MWCNTs, functionalized MWCNTs, and MoO2/MWCNTs electrodes with the same geometric areas are evaluated using their electrochemical CV behaviors.Figure 5a-d

Electrochemical Behaviors of Electrodes
Large electroactive areas in MFC anodes are favorable for the massive adherence of active bacteria; the electroactive areas of anodes are estimated via electrochemical double-layer capacitance [45,46].The capacitance properties of MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs electrodes with the same geometric areas are evaluated using their electrochemical CV behaviors.Figure 5a-d [49].However, anodes with large electroactive areas do not necessarily have high power densities.The power density of an MFC is also highly related to the electron conductivity of the anode, the EET rate between the anode and active microorganisms, the efficiency of nutrient transfer to the biofilm surface, and other factors [2,50,51].The functionalized MWCNTs and MoO 2 /MWCNTs electrodes have large electroactive areas, indicating that the increase in the electroactive area of the MoO 2 /MWCNTs nanocomposite is mainly attributed to MWCNTs functionalization.Large numbers of components with oxygen-containing functional groups attach to the surfaces of functionalized MWCNTs; consequently, functionalized MWCNTs have little aggregation and easily disperse due to electrostatic interactions, and their active groups are easily exposed [52].In addition, a large internal capacitance of an anode material can enhance its instantaneous charge storage, which is also important for improving MFC performance [46].The CV curves of the MoO 2 /MWCNTs electrode form almost rectangular shapes in Figure 5d, suggesting that the MoO 2 /MWCNTs nanocomposite has high electronic conductivity [45]; this is due to the excellent electronic conductivity and charge transmission of the MoO 2 nanoparticles in the MoO 2 /MWCNTs electrode [37,38].
Molecules 2024, 29, x FOR PEER REVIEW 7 of 14 electroactive areas help improve MFC performance [49].However, anodes with large electroactive areas do not necessarily have high power densities.The power density of an MFC is also highly related to the electron conductivity of the anode, the EET rate between the anode and active microorganisms, the efficiency of nutrient transfer to the biofilm surface, and other factors [2,50,51].The functionalized MWCNTs and MoO2/MWCNTs electrodes have large electroactive areas, indicating that the increase in the electroactive area of the MoO2/MWCNTs nanocomposite is mainly attributed to MWCNTs functionalization.Large numbers of components with oxygen-containing functional groups attach to the surfaces of functionalized MWCNTs; consequently, functionalized MWCNTs have little aggregation and easily disperse due to electrostatic interactions, and their active groups are easily exposed [52].In addition, a large internal capacitance of an anode material can enhance its instantaneous charge storage, which is also important for improving MFC performance [46].The CV curves of the MoO2/MWCNTs electrode form almost rectangular shapes in Figure 5d, suggesting that the MoO2/MWCNTs nanocomposite has high electronic conductivity [45]; this is due to the excellent electronic conductivity and charge transmission of the MoO2 nanoparticles in the MoO2/MWCNTs electrode [37,38].

Investigation of MFC Performance
When the voltage outputs of the MFCs reach stable plateaus on a voltage test card, electrochemical linear sweep voltammetry (LSV) measurements are performed to collect the polarization curves of MFCs with bare CC, MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs bioanodes.Their power outputs are recorded and evaluated using power (P) proportional to the geometrical area (A) of the anode (P A = P/A).The p values are calculated using voltages and homologous biocurrent values (P = UI) on the MFC polarization curves.The polarization and power density curves are used to assess the bioelectricity generation performance of E. coli-catalyzed MFCs. Figure 6

Investigation of MFC Performance
When the voltage outputs of the MFCs reach stable plateaus on a voltage test card electrochemical linear sweep voltammetry (LSV) measurements are performed to collec the polarization curves of MFCs with bare CC, MWCNTs, functionalized MWCNTs, and MoO2/MWCNTs bioanodes.Their power outputs are recorded and evaluated using power (P) proportional to the geometrical area (A) of the anode (PA = P/A).The p values are calculated using voltages and homologous biocurrent values (P = UI) on the MFC polarization curves.The polarization and power density curves are used to assess the bioelectricity generation performance of E. coli-catalyzed MFCs. Figure 6    The electrochemical impedance spectroscopy (EIS) spectra of the bare CC, MWC-NTs, functionalized MWCNTs, and MoO 2 /MWCNTs bioanodes are shown in Figure 7.The semicircle in the high-frequency region of the EIS spectra shows the charge transfer impedance (R ct ) from the electroactive exoelectrogens to the anodic electrocatalyst [45].The bare CC bioanode has the largest R ct , confirming the poor EET rate and high cell internal resistance of the MFC with the bare CC bioanode.The MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs bioanodes have relatively smaller R ct values, reflecting the accelerated EET from the electroactive microbes to anodic modification and the smaller cell internal resistance of the MFCs [51,53].In Figure 7, the first intersection point of the EIS curves and the x axis is the impedance of the anodic solution (R s ).A smaller R s value indicates faster interfacial mass transfer between the anodic solution and the bioanode, indicating that more nutrients in the anodic solution are used for active E. coli growth.Nutrient transfer to the biofilm surface is the primary driver of MFC energy production [51].The MoO 2 /MWCNTs bioanode has the lowest R s value, suggesting better interface mass transfer efficiency between the MoO 2 /MWCNTs bioanode and the anodic solution, which is essential for improving MFC performance.The MoO 2 /MWCNTs nanocomposite integrates the synergistic advantages of the functionalized MWCNTs and MoO 2 nanoparticles, facilitating nutrient transfer to the biofilm surface on the MoO 2 /MWCNTs anode and achieving superior power density.
Molecules 2024, 29, x FOR PEER REVIEW the accelerated EET from the electroactive microbes to anodic modification smaller cell internal resistance of the MFCs [51,53].In Figure 7, the first intersectio of the EIS curves and the x axis is the impedance of the anodic solution (Rs).A sm value indicates faster interfacial mass transfer between the anodic solution and anode, indicating that more nutrients in the anodic solution are used for activ growth.Nutrient transfer to the biofilm surface is the primary driver of MFC ener duction [51].The MoO2/MWCNTs bioanode has the lowest Rs value, suggesting b terface mass transfer efficiency between the MoO2/MWCNTs bioanode and the an lution, which is essential for improving MFC performance.The MoO2/MWCNT composite integrates the synergistic advantages of the functionalized MWCN MoO2 nanoparticles, facilitating nutrient transfer to the biofilm surface MoO2/MWCNTs anode and achieving superior power density.

Fabrication and Characterization of Materials
The fabrication procedures of the MoO2/MWCNTs nanocomposites are as follows: first, MWCNTs are functionalized through mixed-acid treatment.Then, 1.0 g of MWCNTs powder is placed in a 200 mL beaker and a 40 mL mixture of concentrated nitric acid and sulfuric acid (volume ratio: 1:3) is slowly poured into the beaker.The beaker is sealed using a sealing film, placed under ultrasound for 1 h, and left to sit at room temperature for 24 h.The mixture is pumped and filtered, washed with ultrapure water (ρ = 18.25 MΩ cm) several times until the pH value is close to neutral, and vacuum-dried at 60 °C for later use.Second, 100 mg of functionalized MWCNTs are added to a 50 mL phosphomolybdic acid aqueous solution with a concentration of 20 mg mL −1 .After 12 h of magnetic stirring, the suspension is centrifuged at 10,000 rpm min −1 and then washed with ultrapure water three times following cryogenic desiccation.The aforementioned samples are placed in a tube furnace, where a hydrogen-argon (10%) mixture is exhausted for 30 min in advance.The samples are then subjected to reduction at 900 °C for 3 h.MoO2/MWCNTs nanocomposite powder is acquired after the mixture is allowed to naturally cool to normal temperature.
The characterization details of the materials are in the Supplementary Materials.
Next, 2.0 mg of the MoO2/MWCNTs powder is mixed with 400 µL of Nafion-alcohol solution (0.1 wt%), and a uniform suspension forms after ultrasound exposure for 1 h.The suspension is added drop by drop to the prepared CC surface (1.0 cm × 1.0 cm), dried under infrared light, and used as an MFC anode.The MWCNTs, functionalized MWCNTs, and modified CC electrodes are prepared following the above process.MWCNTs, functionalized MWCNTs, and bare CC electrodes (1.0 cm × 1.0 cm) are the contrast anodes.
Bare pretreated CP electrodes (2.0 cm × 2.0 cm) are the cathodes of all MFCs.

Fabrication and Characterization of Materials
The fabrication procedures of the MoO 2 /MWCNTs nanocomposites are as follows: first, MWCNTs are functionalized through mixed-acid treatment.Then, 1.0 g of MWCNTs powder is placed in a 200 mL beaker and a 40 mL mixture of concentrated nitric acid and sulfuric acid (volume ratio: 1:3) is slowly poured into the beaker.The beaker is sealed using a sealing film, placed under ultrasound for 1 h, and left to sit at room temperature for 24 h.The mixture is pumped and filtered, washed with ultrapure water (ρ = 18.25 MΩ cm) several times until the pH value is close to neutral, and vacuum-dried at 60 • C for later use.Second, 100 mg of functionalized MWCNTs are added to a 50 mL phosphomolybdic acid aqueous solution with a concentration of 20 mg mL −1 .After 12 h of magnetic stirring, the suspension is centrifuged at 10,000 rpm min −1 and then washed with ultrapure water three times following cryogenic desiccation.The aforementioned samples are placed in a tube furnace, where a hydrogen-argon (10%) mixture is exhausted for 30 min in advance.The samples are then subjected to reduction at 900 • C for 3 h.MoO 2 /MWCNTs nanocomposite powder is acquired after the mixture is allowed to naturally cool to normal temperature.
The characterization details of the materials are in the Supplementary Materials.
Next, 2.0 mg of the MoO 2 /MWCNTs powder is mixed with 400 µL of Nafion-alcohol solution (0.1 wt%), and a uniform suspension forms after ultrasound exposure for 1 h.The suspension is added drop by drop to the prepared CC surface (1.0 cm × 1.0 cm), dried under infrared light, and used as an MFC anode.The MWCNTs, functionalized MWC-NTs, and modified CC electrodes are prepared following the above process.MWCNTs, functionalized MWCNTs, and bare CC electrodes (1.0 cm × 1.0 cm) are the contrast anodes.
Bare pretreated CP electrodes (2.0 cm × 2.0 cm) are the cathodes of all MFCs.High-purity nitrogen is pumped into the anode chambers of the MFCs for 50 min to remove the dissolved oxygen from the anolyte and domesticate E. coli in the anaerobic atmospheres.Resistors (1000 Ω) are connected to external circuits.Next, the MFCs are placed in a thermostatic water bath at 37 • C. The voltages across the resistor are collected using a NI6009 voltage test card (NI, USA).Electrochemical measurements are obtained using a CHI760e workstation (Chenhua, Shanghai, China).The polarization curves of the MFCs with the bare CC, MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs bioanodes are acquired via electrochemical LSV (scanning interval: open-circuit potential to 0 V).EIS is performed at the open-circuit potential (amplitude: 5 mV; frequency domain: 10 5 -10 −2 Hz).The MFC bioanodes are the working electrodes and the MFC cathodes are the reference and counter electrodes.

Conclusions
Herein, we successfully construct a MoO 2 /MWCNTs nanocomposite by mixing functionalized MWCNTs and phosphomolybdic acid in a tube furnace in a hydrogen-argon (10%) atmosphere.Its composition and morphology are studied using XRD, SEM, TEM, and XPS.MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs materials are utilized as anodic modification materials for E. coli-inoculated MFCs.Abundant oxygen-containing groups attach to the surfaces of the functionalized MWCNTs, which can result in large electroactive areas and is conducive to the attachment of active bacteria.A larger internal capacitance of anode materials can enhance instantaneous charge storage, which is also important for improving MFC performance.Combining the large electroactive areas of the functionalized MWCNTs and the outstanding charge transmission of the MoO 2 nanoparticles, the MoO 2 /MWCNTs anode has a large electroactive area and excellent electronic conductivity.Additionally, the MFC with the MoO 2 /MWCNTs bioanode exhibits a small charge transfer impedance and anodic solution impedance, demonstrating the efficient EET and good nutrient transfer capability of the bioanode.Finally, the MFC with the MoO 2 /MWCNTs bioanode generates high power density (2.72-, 1.82-, and 1.18-fold higher than those of the MFCs with the bare CC, nonfunctionalized MWCNTs, and functionalized MWCNTs bioanodes, respectively).This paper can be a reference for further applications of carbon-based transition metal oxide nanocomposites in the field of high-performance MFCs.administration, J.M. and J.J.; funding acquisition, J.M. and J.J.All authors have read and agreed to the published version of the manuscript.

Figure 3 .
Figure 3. TEM images of (a) functionalized MWCNTs and (b) MoO2/MWCNTs.The wide XPS spectra of the MWCNTs and functionalized MWCNTs are shown in Figure 4a.The nonfunctionalized MWCNTs have C and trace O, which may be caused by small amounts of oxygen-containing functional groups on the nanotube end caps of the MWCNTs or oxygen from the atmosphere.The functionalized MWCNTs also have C and

Figure 3 .
Figure 3. TEM images of (a) functionalized MWCNTs and (b) MoO 2 /MWCNTs.The wide XPS spectra of the MWCNTs and functionalized MWCNTs are shown in Figure 4a.The nonfunctionalized MWCNTs have C and trace O, which may be caused by small amounts of oxygen-containing functional groups on the nanotube end caps of the MWCNTs or oxygen from the atmosphere.The functionalized MWCNTs also have C

Figure 5 .
Figure 5. CV responses of electrodes in PBS solution at various scan rates: (a) bare CC, (b) MWCNTs, (c) functionalized MWCNTs, and (d) MoO2/MWCNTs.The curve numbers are the millivolts per second of (ν1-ν12) 10-120; (e) Evaluation of Cdl by plotting mean capacitance current values against corresponding scan rates.
illustrates the power density and polarization curves of the MFCs with bare CC, MWCNT, functionalized MWCNTs, and MoO 2 /MWCNTs bioanodes at a scan rate of 1.0 mV s −1 .The maximum power densities of the four MFCs are 1541.36,2300.29,3556.89, and 4185.20 mW m −2 , respectively.The corresponding biocurrent densities are 3027.06,4851.80,7432.35, and 8466.50 mA m −2 , respectively.Evidently, the maximum power and current density of the MFC with the MoO 2 /MWCNTs bioanode are superior to those of the MFCs with the bare CC, MWCNTs, and functionalized MWCNTs bioanodes.The maximum power density of the MFC with the MoO 2 /MWCNTs bioanode is 2.72-, 1.82-, and 1.18-fold higher than those of the MFCs with the bare CC, MWCNTs, and functionalized MWCNTs bioanodes, respectively.Hence, as an anodic modification material, the MoO 2 /MWCNTs nanocomposite can markedly enhance the power generation performance of MFCs.The E. coli-inoculated MFC with the functionalized MWCNTs anode in our study reached a maximum power density of 3556.89mW m −2 at a scan rate of 1.0 mV s −1 , which is slightly lower than that of an E. coli-inoculated MFC with a CNTs anodic electrocatalyst based on a PBE binder at a scan rate 1.0 mV s −1 (3800 mW m −2 ) [26].Nevertheless, the MFC with the MoO 2 /MWCNTs anode yields a higher power density of 4185.20 mW m −2 .Thus, the introduction of MoO 2 nanoparticles into the MoO 2 /MWCNTs anode can enhance the power generation of MFCs.This may be attributed to the large electroactive area and high electronic conductivity of the MoO 2 /MWCNTs anode.Molecules 2024, 29, x FOR PEER REVIEW 8 of 14 illustrates the power density and polarization curves of the MFCs with bare CC, MWCNT, functionalized MWCNTs and MoO2/MWCNTs bioanodes at a scan rate of 1.0 mV s −1 .The maximum power densities of the four MFCs are 1541.36,2300.29,3556.89, and 4185.20 mW m −2 , respectively.The corresponding biocurrent densities are 3027.06,4851.80,7432.35, and 8466.50 mA m −2 , respec tively.Evidently, the maximum power and current density of the MFC with the MoO2/MWCNTs bioanode are superior to those of the MFCs with the bare CC, MWCNTs and functionalized MWCNTs bioanodes.The maximum power density of the MFC with the MoO2/MWCNTs bioanode is 2.72-, 1.82-, and 1.18-fold higher than those of the MFCs with the bare CC, MWCNTs, and functionalized MWCNTs bioanodes, respectively Hence, as an anodic modification material, the MoO2/MWCNTs nanocomposite can mark edly enhance the power generation performance of MFCs.The E. coli-inoculated MFC with the functionalized MWCNTs anode in our study reached a maximum power density of 3556.89mW m −2 at a scan rate of 1.0 mV s −1 , which is slightly lower than that of an E coli-inoculated MFC with a CNTs anodic electrocatalyst based on a PBE binder at a scan rate 1.0 mV s −1 (3800 mW m −2 )[26].Nevertheless, the MFC with the MoO2/MWCNTs anode yields a higher power density of 4185.20 mW m −2 .Thus, the introduction of MoO2 nanoparticles into the MoO2/MWCNTs anode can enhance the power generation of MFCs.This may be attributed to the large electroactive area and high electronic conductivity of the MoO2/MWCNTs anode.

Figure 8
Figure 8 shows the cell voltage and current density profiles over four continu charge cycles of the MFC with the MoO2/MWCNTs bioanode.The voltage pea MFC in the first discharge cycle emerges at approximately 400 mV, and the corresp current density peak is approximately 4000 mA m −2 .The voltage plateaus reach a mately 510, 450, and 480 mV from the second discharge cycle to the fourth, whe corresponding current densities reach approximately 5100, 4500, and 4800 mA spectively.This may be attributed to the stimulated electrochemical activation of t troactive bacteria (E.coli) on the anode surface during long-term MFC operation [5 sequently, enhanced voltage and current density plateaus appear in the second fourth discharge cycles of the MFC.These findings indicate the satisfactory electric eration capacity and long-term voltage-output repeatability of the MFC w MoO2/MWCNTs bioanode.

Figure 8
Figure8shows the cell voltage and current density profiles over four continuous discharge cycles of the MFC with the MoO 2 /MWCNTs bioanode.The voltage peak of the MFC in the first discharge cycle emerges at approximately 400 mV, and the corresponding current density peak is approximately 4000 mA m −2 .The voltage plateaus reach approximately 510, 450, and 480 mV from the second discharge cycle to the fourth, whereas the corresponding current densities reach approximately 5100, 4500, and 4800 mA m −2 , respectively.This may be attributed to the stimulated electrochemical activation of the electroactive bacteria (E.coli) on the anode surface during long-term MFC operation[54].Consequently, enhanced voltage and current density plateaus appear in the second to the fourth discharge cycles of the MFC.These findings indicate the satisfactory electricity generation capacity and long-term voltage-output repeatability of the MFC with the MoO 2 /MWCNTs bioanode.

Figure 8 .
Figure 8. Cell voltage and current density profiles over four consecutive discharge cycles for MFC with the MoO2/MWCNTs bioanode.

Figure 8 .
Figure 8. Cell voltage and current density profiles over four consecutive discharge cycles for MFC with the MoO 2 /MWCNTs bioanode.

3. 3 .
Establishment, Operation, and Evaluation of E. coli-Inoculated MFCs Cubic dual-chamber MFCs (volume of single chamber: 100 mL) with proton exchange membranes (Nafion 212, Dupont, Wilmington, DE, USA) are used to assess the performance of the MFCs with the bare CC, MWCNTs, functionalized MWCNTs, and MoO 2 /MWCNTs bioanodes.The microorganism E. coli (BNCC133264, Beijing, China) in a fluid nutrient medium (nutrient broth, Aobox, Beijing, China) is placed in a thermostatic incubator for 20 h at 37 • C. PBS solutions (pH = 7.0) are used as the base electrolyte of the anodes and cathodes; it comprises NaH 2 PO 4 •2H 2 O (11.05 g L −1 ) and NaHCO 3 (10.0g L −1 ).A 50 mL PBS solution containing 5.0 g L −1 yeast extract, 10.0 g L −1 glucose, and 5 mM 2-hydroxy-1, 4-naphthoquinone (HNQ, Sigma-Aldrich, St. Louis, MO, USA) is utilized as the MFC anolyte.Next, 20 mL of fluid nutrient media containing E. coli is inoculated into the anode chambers and utilized as active microflora biocatalysts of the MFCs.The catholyte is a 70 mL PBS solution including 0.1 M KCl and 50 mM K 3 [Fe(CN) 6 ].All items are sterilized at 121 • C in an autoclave for 15-20 min in advance.