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Abstract

In microbial fuel cell (MFC), the anode is the carrier of microbial attachment and growth, and its material and surface structure play a vital role in MFC electricity generation. Therefore, anode surface optimization is an effective way to improve MFC performance. Although the power generation of bacteria has been confirmed and studied as early as the beginning of the 20th century, up to now, MFC still has the extremely challenging problem of low current and low power output in practical application. To improve the performance of MFC, several strategies have been applied to enhance the bacterial extracellular electron transfer. One promising technology is the genetic engineering approach, and some outstanding research results have been obtained. Another effective strategy is to design and fabricate a high-performance electrode because anode material is the essential factor affecting MFC performance, which provides surface active sites for microbial adhesion, reproduction and interfacial electron transfer. At present, the MFC anodes mainly include carbon-based electrodes and a variety of metal electrodes, but untreated anodes have always been unable to overcome the obstacle of low power output. Anode modification, a common and effective method, is employed for improving the power output of MFC. For this reason, this review is primarily focused on the applications of various anode materials and its nanoscale modification in the field of MFC, including the influence of different anode materials on the power output of MFC, and analyzes the reasons why anode modification enhances output performance. Furthermore, the influence of anode research on the practical application of MFC in the future is prospected.

Keywords

microbial fuel cells anode nanoscale modification extracellular electron transfer electricity generation performance

Introduction

Microbial fuel cell (MFC) anode is the carrier of bacterial attachment, which plays a vital role in microbial growth and electron transfer efficiency.¹ Therefore, it is of great significance to improve the electricity production capacity of MFC to study the electron transfer mechanism of electricity-producing bacteria, select anode materials with excellent performance, analyze the effects of anode materials and surface characteristics on microbial growth and MFC output performance, and accelerate the interfacial electron transfer rate between bacterial outer membrane cytochrome c protein (OMCs) and the electrode. Due to its excellent conductivity, biocompatibility, chemical stability and low price,² carbon electrode has become the most widely used anode material in the field of MFC, mainly including carbon paper (CP),^3–7 carbon felt (CF),^8–11 carbon cloth (CC),^12–15 graphite rod,^16–19 graphite fiber brush,^20–23 reticulated vitreous carbon^24–27 and so on. Microbial fuel cell anode equipped with CC was inoculated with anaerobic activated sludge to produce electricity. The maximum output power density of 610 mW/m² was obtained by starting the battery with sodium acetate as an electron donor.²⁸ Desulfovibrio desulfuricans was inoculated into CC anode to generate electricity. The air cathode MFC was successfully started with sodium sulfate and magnesium sulfate as electron donors, respectively, and the power output was 0.51 mW/cm².²⁹ Wang et al.³⁰ fabricated an effective three-dimensional N-doped macroporous carbon foam (NMCF) bioanode, a power density of MFC has been dramatically improved. Such remarkable performances would be likely attributed to two major aspects. On the one hand, the unique 3D macroporous structure of NMCFs favored the bacterial proliferation and the enrichment of electroactive Geobacter genus on the electrode surface. On the other hand, the anodic N doping especially pyrrolic N promoted the extracellular electron transfer (EET) process of Geobacter. However, due to the high surface energy state of carbon element, it is easy to lose electrons to make the surface manifest reducibility. The electron transition generated by the metabolism of electrogenic microorganisms to the carbon electrode has to consume higher energy, resulting in a larger loss of anodic activation overpotential. Therefore, reducing the energy state of carbon-based anode surface and activation overpotential of anode reaction is the key to improve anode performance. Surface modification and nano-modification are the main technologies to significantly reduce the anode activation loss.

In order to obtain high performance anode and improve the power generation of MFC, noble metals such as Au, Pt and Pd are often used in MFC due to their excellent conductivity, wide potential range and high surface area. In addition, there are common metals such as Fe, Cu, Ni and other transition metals such as Mo, Ru, Mn, Ti and so on. Lovley research group³¹ used Au as anode and Geobacter sulfurreducens as electricity-producing bacteria to study the performance of MFC using acetate as substrate. The results indicated that the current output of MFC could be maintained at 0.4–0.7 mA after 6–10 days of operation, which was due to the effective electron transfer from bacteria to gold electrode. Generally speaking, the smooth surface of metal is not conducive to the attachment and growth of bacteria, and the high cost of precious metals limits its wide use, while copper dissolves in the anode solution during battery discharge, which has a great toxic effect on microorganisms and should be avoided. Therefore, the use of metal electrodes in MFC is not common, and the current reports mainly focus on stainless steel and titanium electrodes.^32–34

Microbial fuel cell power output depends on the substrate degradation rate of microorganisms, the transfer rate of metabolic electrons from intima to outer membrane, and then from outer membrane protein to anode, as well as the performance of the electrode itself. With substrate metabolism and EET, protons are also released into the anode solution and affect the power generation process. The anode affects the interaction and effective contact distance between microorganism and electrode, and then affects the adhesion, growth, EET pathway and electron transfer efficiency of microorganism. For example, the pretreatment of anode materials can shorten the start-up time of MFC and maintain the stability of biofilm, and good stability is helpful for the long-term operation of MFC. In addition, the anode material is also an important factor in determining the anode potential, which determines the final redox potential of intracellular electrons and thus determines the metabolic pathway of microorganisms. The physicochemical properties of different electrode materials are also quite different, and the electrode resistance is an important factor affecting the power output. Although reducing the electrode resistance does not always increase the power output in practical applications, high conductivity is still one of the important issues to be investigated when selecting electrode materials. In short, anode material and its surface structure affect microbial growth, electron transfer and Coulombic efficiency, so it is of great significance to select and optimize electrode materials to enhance MFC performance. Herein, this review mainly summarizes the effect of nano-scale modified anodes on MFC power output, analyzes the reasons for the enhancement of MFC performance, and discusses possible ways to further optimize anodes, as well as prospects for potential promising anode materials.

Surface modification of anode

To prepare high performance anodes, one is to select the appropriate materials and develop processing technology directly to obtain new anodes that are beneficial to microbial growth and electricity production, such as three-dimensional anodes with micron pores.³⁵ Another effective and common method is to modify the electrode surface. Modification usually includes the change of morphology and structure, as well as using functional materials for surface modification. The property improvement for MFC equipped with modified anode is visually illustrated in Figure 1.

Figure 1.

A schematic diagram of the decorated anode to improve microbial fuel cell power output.

Modification of Conductive Nanomaterials

Conductive polymer modification

The conductive polymer modified electrode has good biocompatibility and is easy to be processed into a variety of complex shapes and sizes, light weight, good stability and adjustable resistivity.³⁶ Therefore, it has been widely used to improve the performance of anode materials in the field of MFC, among which Polyaniline (PANI), polypyrrole (PPy) and poly neutral red film modified electrodes have been more studied. Ding et al.³⁷ synthesized polypyrrole nanowire arrays (PPy-NA) on the surface of Au sheet by galvanostatic deposition method. The morphology was shown in Figure 2(a)–(d). PPy-NA exhibited excellent performance in mediating bacterial EET. The combined effect of the inherent electrochemical nature of PPy and the porous structured bacterial network that was generated on the PPy-NA enabled long-term stability. The high efficiency was attributed to the enhanced electron transfer rate between PPy-NA and microorganisms. Moreover, different nanostructure of conductive polymers can affect bacterial EET. The fiber PPy modified anode was more suitable for high power output of MFC.³⁸ PPy and adhesive polydopamine (PDA) in situ modified electrogenic bacteria can improve the conductivity and adhesion of anode to improve the hydrophilicity of conductive polymer. PPy-PDA modified cells as anode not only greatly improve the direct EET, but also riboflavin secretion that is conducive to indirect EET is enhanced, thereby improving the power density output of MFC.³⁹ The hybride of TiO₂ and PANI can remarkably raise the output performance of MFC (Figure 2(e)). This is due to the synergistic effect between TiO₂ and PANI, which makes the charge transfer resistance at the anode interface significantly reduced, the transient charge storage capacity greatly improved, and the surface biomass also increased, thus promoting bacterial EET and maintaining high power density output during the long-term operation of MFC.⁴⁰ In 2012, Ding et al.⁴¹ deposited PANI on the surface of gold electrode to form PANI nanowire (Polyaniline nanowire array), which proved that ordered PANI can act as a solid polymer mediator in microbial EET process, and can significantly enhance EET current in an adjustable manner at a specific potential. Additionally, the power output of CP anode modified by poly (3,4-ethylenedioxythiophene) (PEDOT) was increased compared with the unmodified anode. Cyclic voltammetry and electrochemical impedance spectroscopy analysis showed that PEDOT modification increased the effective redox active sites and reduced the interfacial electron transfer impedance of the anode.⁴² In brief, the conductive polymer modified electrode enhanced the conductivity, reduced the electron transfer resistance, and improved the electrode capacitance. The surface of the modified electrode became rougher and expanded the surface area of bacteria attachment. In order to further improve the output performance of MFC, conductive polymers are usually prepared into a three-dimensional porous structure to facilitate larger biofilm formation, or to improve the hydrophilicity of conductive polymers. The anode modified with conductive polymer is given in Table 1.

Figure 2.

(a) Scanning electron microscopy image of the polypyrrole nanowire arrays film showing the evenly distributed of polypyrrole nanowires on a Au plate. Inset: high magnification scanning electron microscopy image of the polypyrrole nanowires. (b) Scanning electron microscopy image of the cross-section of polypyrrole nanowire arrays. (c) A porous bacterial network formed on polypyrrole nanowire arrays surface after 60 h of electrochemical culture at 0.2 V. The microbes were marked in red. (d) Enlarged scanning electron microscopy image of microbes on the polypyrrole nanowire arrays electrode, where several microbes are bundled tightly together and are connected with neighboring microbe bundles, forming a conduit to facilitate the electron transfer. (e) Polyaniline-coated ordered TiO₂ nanosheets and biofilm formed on their surface exhibiting excellent biocompatibility, the reduction of internal resistance is beneficial to the output of high power density.

Table 1.

Power density output of MFC equipped with conductive polymer modified anodes.

Anode materials	Modification of anode	The maximum power density (mW·m⁻²)	Bacteria	MFC Configuration	Ref
Carbon cloth (CC)	VA-PANI/CC	853	Shewanella oneidensis MR-1	Dual-chamber	⁶⁶
Stainless steel (SS)	PPy/SS	1190.9	Anaerobic granular sludge	Single-chamber	⁶⁷
Stainless steel	PEDOT/SS	608.6	Anaerobic granular sludge	Single-chamber	⁶⁸
Carbon paper (CP)	CP/CNT(W)	2015.6	Shewanella loihica PV-4	Dual-chamber	⁴⁸
Ni	Graphene/Ni	468.2	Shewanella putrefaciens	Dual-chamber	⁶⁹
Nickel foam sheet	NiO-graphene/Ni	3632	Shewanella putrefaciens CN32	Dual-chamber	⁷⁰
Carbon felt (CF)	GO/CF	184.9	Mixed anaerobic sludge	Single-chamber	⁷¹
Titanium mesh	Graphene aerogel/Ti	583.8*	Anaerobic sludge	Dual-chamber	⁷²
Graphite felt (GF)	Ppy-NP/GF PTh-NP/CF	1,220 800	Shewanella putrefaciens	Single-chamber	⁷³
Carbon nanofibers (CNFs)	CNTs/CNFs	362	Freshwater wetland sediment	Dual-chamber	⁷⁴
Carbon paper (CP)	Fe/CP Co/CP	117.8 165.6	Sewage wastewater	Single-chamber	⁷⁵

Note: vertically aligned polyaniline (VA-PANI), polythiophene nanoparticles (PTh-NPs), graphene oxide (GO), GO-zeolite modified carbon felt anode (GZMA), the asterisk after 583.8 represents W·m⁻³.

Carbon nanotube modification

Carbon nanotube (CNT) has good electrical conductivity, large specific surface area, high thermal stability and chemical inertness, and can form a mesoporous spatial structure.⁴³ Therefore, CNT is a very ideal electrode material and is widely used in the anode research of MFC. Sun et al.⁴⁴ used layer-by-layer self-assembly technology to modify multi-walled carbon nanotube (MWCNT) on CP electrode for the first time, and studied the performance of composite electrode as MFC anode. The results showed that MFC equipped with the modified anode produced a higher power density by inoculating anaerobic granular sludge, which was 20% enhancement comparing to that of the bare CP anode. Impedance spectrum test proved that the charge transfer resistance of composite electrode decreased from 1163 Ω to 258 Ω. Especially, cytochrome C protein attached to MWCNTs modified electrode can significantly promote the direct electron transfer (DET) between cytochrome c and electrode.⁴⁵ As shown in Figure 3(a), vertically grown MWCNTs were modified on the surface of nickel silicide for miniature (1.25 μL) MFC anode. The modified electrode has good biocompatibility and conductivity. Modify multi-walled carbon nanotubes increased the anode surface-to-volume ratio, which improved the ability of microbial coupling and electron transfer to the anode. These factors contributed to the power output of MFC.⁴⁶ Liang⁴⁷ proved that the combination of CNT and G. sulfurreducens biofilm can shorten the start-up time of MFC and reduce the anode impedance. The results indicate that during the 40-days operation the anode impedance and voltage output of MFC with CNT powder are 180 Ω and 650 mV, respectively. The anode impedance without CNT powder increases from the initial 250 Ω–540 Ω, and the voltage decreases from 630 mV to 540 mV. Therefore, the addition of CNT powder is beneficial to stabilize the anode resistance and keep the internal resistance and power output of MFC running for a long time constant. From Figure 3(b)–(g), welding and assembling three-dimensional interconnected CNT on carbon electrodes to prepare high-performance anodes is a novel method. CNTs and polyelectrolytes form an integral network on CP surface by layer-by-layer assembly and welding (CP/CNT(W)). Due to the minimization of ohmic loss at the interface, MFCs assembled with CP/CNT(W) electrodes have excellent electrochemical performance to facilitate the formation of larger biofilm and electron transport.⁴⁸ In addition, The composite modified anodes between several conductive polymers for MFC can greatly enhance power output, which is due to their synergistic effect and complementary advantages, jointly promoting the bacterial EET.^49–51

Figure 3.

(a) Scanning electron microscopy image of bacterial growth after operation. Image shows excellent compatibility and stability of modify multi-walled carbon nanotube. Scanning electron microscopy images of the biofilm adhered on carbon paper (b), carbon paper/carbon nanotube (d) and carbon paper/carbon nanotube(W) (f) anodes, and schematic electrons transfer on the surface of PV-4 and anodes after inoculation (c, e and g).

Graphene modification

Graphene is a new member of the carbon family, which has a special two-dimensional crystal structure of single atomic layer, and the characteristics of large specific surface area, good mechanical strength, excellent electrical conductivity and high electrocatalytic performance, so it has become one of the research hotspots of anode modification in the field of MFC in recent years. Stainless steel mesh (SSM) modified with graphene (GMS) as MFC anode, the maximum output power density reached 2668 mW/m², which was 18 times and 17 times higher than that of SSM and polytetrafluoroethylene modified stainless steel mesh (PMS) anode, respectively. This result was attributed to the large specific surface area of GMS and the attachment of a large number of microorganisms to the anode surface, thus improving the power generation performance.⁵² Graphene was decorated onto CC anode by electrochemical deposition method, inoculated Pseudomonas aeruginosa, and started dual-chamber MFC. The experimental results show that the power output and energy conversion rate of graphene modified MFC anode are 2.7 times and 3 times higher than those of unmodified CC, respectively. This is mainly due to the fact that graphene promotes the growth of bacteria on the electrode surface, leads to the increase of DET active sites and stimulates bacteria to secrete mediators with higher electron transfer efficiency.⁵³ Huang et al.⁵⁴ firstly modified graphene oxide nanoribbons (GONRs) network on CP surface by electrochemical deposition method as MFC anode. The results confirmed that the GONRs modified electrode had a larger electrochemical active surface area, which could promote the transfer of extracellular electrons to the electrode. The current density and power density were about 4 times higher than those of CP electrode. Graphene oxide nanoribbons played a role similar to nanowires in the EET process, which increased the distance of electron transfer. A three-dimensional reduced graphene (RGO) modified nickel foam anode was prepared by controllable deposition of reduced graphene sheets on nickel foam substrate. This electrode not only provides a higher specific surface area for bacterial growth and attachment of more electron mediators, but also uniform microporous scaffolds are conducive to the effective diffusion of culture medium. Based on the volume calculation of anode materials, the flexible reduced graphene composite foam nickel electrode inoculated with Shewanella oneidensis MR-1 has a power output of 661 W/m³, which is much higher than that of nickel foam electrode and traditional carbon electrode under the same experimental conditions.⁵⁵ Xiao et al.⁵⁶ researched two kinds of graphene materials with different morphologies, and modified MFC anodes with flake graphene and folded graphene respectively. The results showed that anode modified with folded graphene particles had higher current output, which was attributed to its higher conductivity, larger surface area, oxygen reduction catalytic activity and the open structure formed by stacking to promote mass transfer. Therefore, this kind of electrode has higher power output, up to 3.6 W/m³, which was twice as high as that of activated carbon modified electrode, while the power output of unmodified CC anode was only 0.3 W/m³.

Nano-metal modification

The internal resistance of MFC affects its power generation performance. Hence, reducing the internal resistance is beneficial to the output of high power density. Nano-metal materials used as catalysts have a variety of excellent properties, such as small size effect, high catalytic activity, good stability and biocompatibility. It can be dispersed and fixed on various carbon-based carriers such as activated carbon and graphite by physical or chemical methods. As an anode, it can effectively improve the electrical performance of MFC. The Pt is a common electrocatalyst in MFC, some studies have shown that Pt modified titanium electrode has good electrical properties, and pure titanium electrode is not suitable for MFC anode material.⁵⁷ It can be seen from Figure 4 that a three-dimensional CF electrode decorated with Pt nanoarrays (CF@Pt) as bioanode for MFC. Pt nanoarrays act as excellent electron relays, reducing significantly the charge transfer resistance. CF@Pt anode equipped MFC can run stably for a long time. Having a large surface area and excellent electrical conductivity, this binder-free nanoscale electrode was found to effectively promote the growth of electrochemically active bacteria and facilitate EET from bacteria to their host.⁵⁸ Sun et al.⁵⁹ prepared gold-coated CP electrode for MFC, inoculated with S. oneidensis MR-1, the electricity production of MFC increased by 47% compared with naked CP electrode. Electrochemical cyclic voltammetry analysis and scanning electron microscopy observation revealed that the gold decorated CP electrode facilitated the formation of MR-1 biofilm. Consequently, gold composite carbon material anode can obviously reinforce the output performance of MFC. Nevertheless, the current output performance of MFC with pure gold anode is poor and cannot output continuously, which is due to the low compatibility between gold and bacteria.⁶⁰ Transition metal Pt and tantalum (Ta) modified titanium electrodes have higher current output in long-term use. The biofilm formation by adding ferric iron and acetate indicates that there are many kinds of ferric reducing bacteria in the biofilm, which are in the dominant group on the electrode surface, while biofilm formation on the surface of the pure titanium electrode is poor and insufficient for further analyses.⁶¹ Ouitrakul et al.⁶² have studied the effects of silver, stainless steel, aluminum, nickel and other materials as anodes on the properties of MFC. It is found that nickel, aluminum and stainless steel electrodes can obtain higher open circuit voltage than carbon electrodes, but the power output is lower than that of carbon electrodes. In order to explain this phenomenon, impedance tests demonstrate that nickel and stainless steel electrodes have higher electrode/solution interface impedance, resulting in greater activation loss. Fan et al.⁶³ modified graphite anode with Au and Pd nanoparticles and inoculated S. oneidensis MR-1 as electricity-producing bacteria. It was found that the electricity output of Au modified anode was 20 times higher than that of graphite anode, and Pd modified anode was 50–150% higher than that of the graphite anode, indicating that Au and Pd modified electrodes could facilitate bacterial EET and reinforce the electricity production performance of MFC. The anodes modified by nano-metal materials have a wide range of variety. In addition to precious metals such as Au, Pt, Pd, Rh, Ir, Ag, etc., other metals such as Ni and Cu^64,65 are also used for anode decoration of MFC. Nevertheless, the cost of precious metals is too high to be widely used. The cost of stainless steel, titanium, aluminum and other electrodes is low, but the adhesion and growth ability of microorganisms on their surfaces is weak, which limits their widespread use in the field of MFC.

Figure 4.

(a) Schematic diagram of three-dimensional carbon felt anode with biofilm: without Pt nanoarrays (left side) and with Pt nanoarrays (right side). (b) Carbon felt@Pt without biofilm. (c) Carbon felt@Pt with biofilm.

Modification of nano-metal oxide materials

Iron oxide modification

Metal oxides such as iron oxide, as electron acceptors of bacteria in natural environment, can also improve the performance of MFC. For Fe oxide, EET is mediated by OMCs and self-secretion flavin molecules. It has been reported that α-FeOOH nanowires with a diameter of 30–50 nm and a length of 500–800 nm form a porous structure on the surface of CP by hydrothermal method. This composite electrode is beneficial to microbial growth and mass transfer, and promotes EET.⁷⁶ Nakamura et al.⁷⁷ found that the addition of nano-Fe₂O₃ colloid into MFC could facilitate the cross-linking of Shewanella loihica PV-4 cells to form a network structure conducive to long-distance electron transport. The results of CV test and analysis indicated that the current of MFC increased by more than 300 times compared with that without Fe₂O₃. The enhanced redox peak current proved the formation of long-distance EET channel in the colloid network. Nano-iron oxide modification increased the hydrophilicity and roughness of electrode surface that favored the attachment and growth of bacteria. The needle-like Fe₃O₄/Fe₂O₃ nanorods were formed on the surface of 304 stainless steel plates (SS) by 70 cycles of cyclic voltammetry treatment after oxidation for 8 min (CV-SS-8). The treated electrode drastically decreased the internal resistance and enhanced the current output. The needle-like Fe₃O₄ and larger Fe₂O₃ nanorods decreased the contact angle of the stainless steel surface (Figure 5), which increased the thickness of the attached electroactive bacteria biofilm.⁷⁸ In 2012, Peng⁷⁹ also studied the effect of nano-Fe₃O₄ and activated carbon (AC) powder mixed then pressing on SSM as an anode for electrical properties of MFC with air cathode. The results demonstrated that the anode with Fe₃O₄ composite had the maximum power output of 664 ± 17 mW/m², which was 22% higher than that of AC modified (AcM) and 56 times higher than that of SSM electrode. Tafel curve proved that the anode of composite Fe₃O₄ had the best dynamic activity. The charge-discharge test of MFC for the first time revealed that the addition of Fe₃O₄ reinforced the anode capacitance. The net charge reserve of the AcFeM electrode achieved 389 ± 18 C/m² after 10 min of open circuit, which was 32% higher than that of AcM (294 ± 30 C/m²). Accordingly, the enhancement of anode capacitance contributes to the improvement of performance. According to the above literature reports, the anode modified by iron-containing compound nanoparticles improved the capacitance of the anode, increased the electron transfer distance, promoted the formation of biofilm, and effectively shortened the start-up time of MFC. Thus, the power output of MFC was reinforced.

Figure 5.

Images of the anode surfaces. Laser scanning confocal microscopy imaging of CV-SS-8 with biofilm for displaying the morphological characteristic of the biofilm (a). Water contact angle of CV-SS-8, hydrophilic surfaces facilitate microbial adhesion and proliferation (b). Three-dimensional atomic force microscopy image of CV-SS-8 (c), the surface roughness increased markedly after CV treatment.

Titanium dioxide modification

TiO₂ is an n-type semiconductor and one of the most attractive metal oxides, which has good chemical stability,⁸⁰ biocompatibility⁸¹ and environmental friendliness.⁸² Therefore, TiO₂ has been widely used in capacitors,⁸³ sensors,⁸⁴ photocatalysis,⁸⁵ solar cells⁸⁶ and other fields. However, the relatively low conductivity limits the use of TiO₂ alone as anode modifier in MFC, which is mainly in the form of composite electrode in the literature. Zou et al.⁸⁷ studied the CC electrode modified by small-size TiO₂ nanocrystals and reduced graphene composite (TiO₂/rGO), and inoculated Shewanella putrefaciens CN32 to produce electricity. The results showed that the TiO₂/rGO@CC anode produced a power output of 540 mW/m², which was 1.4, 1.9 and 3.4 times higher than that of rGO@CC (395 mW/m²), TiO₂@CC (288 mW/m²) and CC (160 mW/m²) anodes, respectively. This is because the good biocompatibility and large specific surface area of TiO₂ nanocrystals are conducive to the large-scale growth of bacteria. The high conductivity of reduced graphene promotes the rapid transfer of electrons from extracellular membrane proteins to electrodes. The synergistic effect of the two greatly improves the power output of MFC. Maurer-Jones et al.⁸⁸ revealed the effects of the type of TiO₂ nanoparticles on biofilm formation, specific function and flavin secretion of S. oneidensis. The results showed that the formation of S. oneidensis biofilm slowed down in the presence of TiO₂ nanoparticles, but the flavin secretion and metal reduction ability were significantly enhanced. This change is not the result of bacterial oxidative stress, but the change in microbial gene expression caused by adjacent nanoparticles. As shown in Figure 6(a) and (b), TiO₂ nanoparticles combined with loofah sponge (LS), then carbonized them at 900°C in nitrogen atmosphere to successfully prepare carbon-covered TiO₂ nanoparticles (LSC-TiO₂) with three-dimensional core-shell structure. the maximum power output of MFC reached 2.59 ± 0.12 W/m² when inoculated with active anaerobic sludge for power generation, which was 63% and 201% higher than that of LSC and graphite anode, respectively. This was due to the interaction between TiO₂ and carbon enhanced the electrochemical capacitance of the three-dimensional electrode.⁸⁹ Wen et al.⁹⁰ synthesized anatase TiO₂ nanoparticles composite CNT modified CC anode. It is found that the anode potential of CNTs@TiO₂ is the most negative at the same current density, which is very beneficial to the generation of MFC bioelectricity. The maximum output power is 1.12 W/m², which is 1.5 and 1.7 times higher than that of CNT (0.73 W/m²) and TiO₂ nanoparticles (0.67 W/m²) modified anode, respectively. One of the reasons for the enhanced power output of the composite electrode is the synergy of CNT and TiO₂ nanoparticles, which makes the electrode have good biocompatibility and conductivity, and promotes the formation of larger biofilm and the transfer of interfacial electrons. In order to improve the electron transport of TiO₂, different nanostructures and morphologies are synthesized, in which ordered nanostructures are an effective method. Vertically oriented TiO₂ nanosheets modified CP electrodes (TiO₂-NSs/CP) were used for MFC anode without any composite conductive materials, and the biological electricity production was significantly better than that of CP electrodes (Figure 6(c)). Its unique surface structure played a key role in enhancing MFC power output. Vertically grown TiO₂-NSs on a CP surface formed vertically penetrating pores that offer a larger contact area to OMCs for DET, high biocompatibility and facilitating solution diffusion. The results demonstrated that the electrode surface modification with appropriate TiO₂ nanostructures was helpful for EET of exoelectrogens.⁹¹ In addition, in continuous and stable operation of MFCs, the electron transfer rate constant (k_et) of OMCs on TiO₂-NSs/CP anode is three times higher than that on CP anode due to the more favorable conformational change of OMCs on TiO₂-NSs and the synergistic energy level shift of electron donors and acceptors. These results revealed that TiO₂-NSs had intrinsic electrocatalytic activity in the microbial EET process.⁹² Nanoscale TiO₂ can be used as a suitable material to regulate the redox state of OMCs. Due to the inherent electrocatalytic activity, high biocompatibility and stability, and variable morphology and tunable surface properties of nano-TiO₂, it can be expected that the properly designed TiO₂ nanostructures can become excellent solid catalysts for microbial EET and redox protein DET, and more applications can be explored in the field of bioelectronic devices in the future. The performance of MFC equipped with the modification of anode via nano-metal oxide materials is listed in Table 2.

Figure 6.

(a) Current density of microbial fuel cells over three charge/discharge cycles with 10 min of charging and 30 min of discharging. (b) Relationship between the power densities of the microbial fuel cells and the areal capacitance values of the anodes. (c) TiO₂-NSs/carbon paper electrode, power output and polarization curves of the microbial fuel cells equipped using different anodes.

Table 2.

Power density output of MFC equipped with the modification of anodes via nano-metal oxide materials.

Anode materials	Modification of anode	The maximum power density (mW·m⁻²)	Bacteria	MFC Configuration	Ref
Carbon felt (CF)	Bio-FeOx/CF	797	Shewanella loihica PV-4	Dual-chamber	⁹⁷
Carbon felt (CF)	Fe₂O₃-PDHC/CF	3184.4	Electrogenic bacteria	Dual-chamber	⁹⁸
Carbon felt (CF)	graphene/Fe₂O₃	334	Anaerobically fermented biomass sludge	Dual-chamber	⁹⁹
Carbon foam (CF)	NiO/CF	928	Wastewater	Dual-chamber	¹⁰⁰
Carbon paper (CP)	NTiO₂-NSs/CP	747	Shewanella loihica PV-4	Dual-chamber	¹⁰¹
Stainless-steel mesh (SS)	MgFe₂O₄/SS	430.3	Anaerobic sludge	Single-chamber	¹⁰²
Carbon felt (CF)	MnCo₂O₄@CF	945	Anaerobic sludge	Dual-chamber	¹⁰³
Carbon paper (CP)	Fe₃O₄/CNT/CP	865	Escherichia coli K12	Dual-chamber	¹⁰⁴
Carbon paper (CP)	α-FeOOH-NWs/CP	122.6	Shewanella loihica PV-4	Dual-chamber	¹⁰⁵
Carbon felt (CF)	MnFe₂O₄	3836	Anaerobic sludge	Dual-chamber	¹⁰⁶
Carbon felt (CF)	NiFe₂O₄-MXene@CF	1385	Anaerobic sludge	Dual-chamber	¹⁰⁷

Note: Fe₂O₃-PDHC/CF (Fe₂O₃-polyaniline-dopamine hybrid composite modified carbon felt), MXene (Ti₃C₂T_x: titanium carbide), NTiO₂-NSs/CP (nitrogen doped TiO₂ nanosheets decorated carbon paper).

Other metal oxides modification

The anode modified by nano-metal oxide materials can increase the specific surface area, which affects the power generation efficiency of MFC. Therefore, the electrode composite nano-semiconductors applied in MFC should have low internal resistance, good electrocatalytic activity, low cost and non-toxic to cells. Mehdinia et al.⁹³ used SnO₂ nanoparticles and RGO composite (RGO/SnO₂) modified CC for MFC anode by hydrothermal method combined with microwave assistance. The maximum output power density of 1624 mW/m² was obtained, which was 2.8 and 4.8 times higher than that of single RGO modification and CC electrode, respectively. This is because the high conductivity and large specific surface area of the nanocomposite electrode promote the formation of biofilm and accelerate the electron transfer. The results show that RGO/SnO₂ composite is a dominant anode material, which enhances the electricity output of MFC. Nano-sized ruthenium oxide (RuO₂) particles exhibit fast and reversible redox behavior in a wide potential window, large surface area, high chemical stability and favorable metallic conductivity. Thus, it is also served as MFC anode. RuO₂ decorated CF as anode for dual-chamber MFC and inoculated with anaerobic activated sludge to generate electricity. The maximum output power of MFC was 17 times higher than that of the unmodified control electrode. Electrochemical tests showed that the electron transfer rate between bacteria and electrode was substantially improved, and it was also confirmed that the interaction between RuO₂ and cells could promote EET.⁹⁴ MnO₂ is a transition metal dioxide with high specific capacitance and environmentally benignity. It may be a potential pseudo-capacitive anode modifier. MnO₂ was modified on CF electrode by electrodeposition to enhance the performance of MFC. When the deposition time was 60 min, the anode capacitance increased by 46 times, and the maximum output power density reached 3580 ± 130 mW/m², which was 24.7% higher than that of pure CF anode (2870 mW/m²).⁹⁵ Tungsten trioxide (WO₃) has favorable biocompatibility and its rough surface favors bacterial colonization. Consequently, WO₃ is considered as an excellent candidate for anode materials in MFC. Polyaniline was decorated onto mesoporous tungsten trioxide (m-WO₃) by chemical oxidation. The composite electrode was applied in MFC and inoculated with Escherichia coli. The m-WO₃ electrode loaded with PANI showed a unique electrocatalytic activity with a maximum output power density of 0.98 W/m², while only 0.76 W/m² and 0.48 W/m² for the MFC using individual m-WO₃ and PANI electrocatalyst, respectively. The improved electrocatalytic activity was attributed to the synergistic effect of good biocompatibility of m-WO₃ and excellent conductivity of PANI, which led to the high output performance of MFC. Most importantly, the combination of m-WO₃ and PANI improved the electrochemical activity of proton gain and loss of PANI.⁹⁶ Metal oxide decorated anodes, with good biocompatibility, stability and large specific surface area, will become one of the hotspots in the field of MFC anode modification.

Conclusion and Outlook

Anode is the carrier and electron acceptor of microbial growth, and its performance greatly affects the power density of MFC. Therefore, improving anode performance is of great research value for improving MFC output. In this review, we focus on the effect of anode and its modification on MFC power generation, and analyze the reason of anode modification to improve MFC power output. The modified anode mainly includes the following advantages: (1) enhanced biocompatibility and increased specific surface area are beneficial to the large-scale growth of electricity-producing bacteria and promote the formation of more active biofilm on the anode surface, (2) good environmental friendliness, light weight and easy operation, (3) good physical and chemical stability is helpful to the long-term operation of MFC, (4) reducing the internal resistance is conducive to high power density output, (5) increasing the active area and promoting mass transfer.

Although the anode modification has received extensive attention and achieved certain research results, the overall performance of MFC has also been improved, the low power density output is still the key to restrict its practical application and the most urgent problem to be solved. Therefore, there is still much room for improvement of anode. Due to the excellent characteristics of carbon-based electrodes, it is still the most attractive material in the future. Besides, nano-semiconductor electrode is also an alternative electrode for practical application of MFC in the future due to its good biocompatibility, stability and relatively low price. This is because the rich structure of nano-semiconductor is expected to design a surface morphology with fast electron transfer. In addition, its conductivity can be changed by doping, which can effectively reduce the charge transfer resistance between bacterial OMCs and the electrode, and increase the transient charge storage capacity of the electrode. The preparation process is simple, and the MFC power generation is significantly improved. With the rapid development of anodes, further exploration of the mechanism is expected to design new high-performance anodes, thereby improving the output performance of MFC and further facilitating the practical application of MFC.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Shanxi Natural Science Foundation of China (No. 201901D111181).

ORCID iD

Tao Yin

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Performance improvement of microbial fuel cells through assembling anodes modified with nanoscale materials

Abstract

Keywords

Introduction

Surface modification of anode

Modification of Conductive Nanomaterials

Conductive polymer modification

Carbon nanotube modification

Graphene modification

Nano-metal modification

Modification of nano-metal oxide materials

Iron oxide modification

Titanium dioxide modification

Other metal oxides modification

Conclusion and Outlook

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

References