Application of advanced anodes in microbial fuel cells for power generation: A review

Journal Pre-proof Application of advanced anodes in microbial fuel cells for power generation: A review Teng Cai, Lijun Meng, Gang Chen, Yu Xi, Nan Jiang, Jialing Song, Shengyang Zheng, Yanbiao Liu, Guangyin Zhen, Manhong Huang PII:

S0045-6535(20)30177-6

DOI:

https://doi.org/10.1016/j.chemosphere.2020.125985

Reference:

CHEM 125985

To appear in:

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Received Date: 26 June 2019 Revised Date:

22 December 2019

Accepted Date: 20 January 2020

Please cite this article as: Cai, T., Meng, L., Chen, G., Xi, Y., Jiang, N., Song, J., Zheng, S., Liu, Y., Zhen, G., Huang, M., Application of advanced anodes in microbial fuel cells for power generation: A review, Chemosphere (2020), doi: https://doi.org/10.1016/j.chemosphere.2020.125985. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Can the power generation and wastewater treatment like “Tai Chi” is complementary

1

Application of advanced anodes in microbial fuel cells for power

2

generation: A review

3

Teng Cai

4

Shengyang Zheng a, Yanbiao Liu a,b, Guangyin Zhen b,c, Manhong Huang a,b,*

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a College of Environmental Science and Engineering, State Environmental Protection

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Engineering Center for Pollution Treatment and Control in Textile Industry, Donghua

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University, Shanghai 201620, China.

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b Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092,

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China.

a,c,1

, Lijun Meng

a,1

, Gang Chen a, Yu Xi a, Nan Jiang a, Jialing Song a,

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c Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of

11

Ecological and Environmental Sciences, East China Normal University, Shanghai,

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200241, China.

13

*

Corresponding author. Tel/Fax: 0086-21-6779 2546. E-mail address: [email protected] (Manhong Huang) 1 Two authors have contributed equally to this paper. 1

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Abstract

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Microbial fuel cells (MFCs) the most extensively described bioelectrochemical

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systems (BES), have been made remarkable progress in the past few decades.

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Although the energy and environment benefits of MFCs have been recognized in

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bioconversion process, there are still several challenges for practical applications on

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large-scale, particularly for relatively low power output by high ohmic resistance and

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long period of start-up time. Anodes serving as an attachment carrier of

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microorganisms plays a vital role on bioelectricity production and extracellular

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electron transfer (EET) between the electroactive bacteria (EAB) and solid electrode

23

surface in MFCs. Therefore, there has been a surge of interest in developing advanced

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anodes to enhance electrode electrical properties of MFCs. In this review, different

25

properties of advanced materials for decorating anode have been comprehensively

26

elucidated regarding to the principle of well-designed electrode, power output and

27

electrochemical properties. In particular, the mechanism of these materials to enhance

28

bioelectricity generation and the synergistic action between the EAB and solid

29

electrode were clarified in detail. Furthermore, development of next generation anode

30

materials and the potential modification methods were also prospected.

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Keywords: Microbial fuel cells; Anode modification; Power generation; Biofilm;

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Extracellular electron transfer

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2

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Contents

35 36 37 38 39 40 41 42 43 44 45 46 47 48

1 Introduction .................................................................................................................................... 5 2 Anode electrodes of MFCs ............................................................................................................. 7 2.1 Traditional anode electrodes ............................................................................................... 7 2.2 Strategies of modified anode electrodes.............................................................................. 9 2.2.1 3D natural resource anode electrodes....................................................................... 9 2.2.2 Metal/metal oxides-based anode electrodes ........................................................... 11 2.2.3 Conductive polymers decoration ............................................................................ 15 2.2.4 Nitrogen-enriched anode electrodes ....................................................................... 18 2.2.5 CNTs-based anode electrodes ................................................................................ 20 2.2.6 Graphene-based anode electrodes .......................................................................... 22 3 Conclusions and outlooks ............................................................................................................ 25 Conflicts of interest ......................................................................................................................... 26 Acknowledgements ......................................................................................................................... 26 Reference ........................................................................................................................................ 27

49 50

3

51

Abbreviations BES CB CC CE CF CNFs CNTs CP CPs CPH CPM

Bioelectrochemical systems Carbon brush Carbon cloth Coulombic efficiency Carbon felt Carbon nanofibers Carbon nanotubes Carbon paper Conductive polymers Conductive polypyrrole hydrogel Chemical polymerizd

DC Dual chamber DET Direct electron transfer ECPM Electrochemical polymerized ECR Electrochemical reduction ED Electrodeposition EET Extracellular electron transfer ETR Electron transfer rate GF Graphite felt Gr Graphene GO Graphene oxide GP Graphite plate GR Graphite rod

HNQ Imax MET MFCs MWCNT Pmax PAN PANI PBS PDA

2-hydroxy-1,4-naphthoquinone Maximum current density Mediator electron transfer Microbial fuel cells Multi-walled carbon nanotube Maximum power density Polyacrylonitrile Polyaniline Phosphate buffer solution Polydopamine

PEDOT PPy Rct rGO

Poly (3,4-ethylenedioxythiophene) Polypyrrole Charge electron transfer Reduced graphene oxide

SC SPM SS

Single chamber Self-polymerization Stainless steel

SSM SSP SSW SWCNT 3D

Stainless steel mesh Stainless steel plate Stainless steel wool Single walled carbon nanotube Three-dimensional

4

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1 Introduction

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Over the past century, fossil fuel has supported the developments of industry and

54

economy. The resulting global energy crisis and environmental pollution have

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accelerated the research on clean and renewable energy (Qiao et al., 2010; Elimelech

56

and Phillip, 2011; Sun et al., 2016; Slate et al., 2019). Biofuel cells (BFCs), an

57

emerging and sustainable technology, has attracted much attention in recent years for

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its ability to convert directly chemical energy from the organic matters into electricity

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power using renewable catalyst (Liu et al., 2014c; Zhao et al., 2017; Chatterjee et al.,

60

2019). Microbial fuel cells (MFCs) is an important branch of BFCs which accounts

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for over 80% of bioelectrochemical systems (BES) (Fig. 1a). MFCs often utilize

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microorganisms as catalysts to capture bioelectricity, fuels, chemicals and metals from

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the abandoned waste and/or wastewater (Ren, 2017; Chatterjee et al., 2019; Guan et

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al., 2019a; Guan et al., 2019b), such as domestic wastewater, industrial wastewater,

65

animal waste, plant waste, food waste, landfill leachate as well as sewage sludge (Fig.

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2a, b) (Sun et al., 2016; Feng et al., 2017; Santoro et al., 2017; Asefi et al., 2019; Yun

67

et al., 2019). Furthermore, the number of papers published based on MFCs in different

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research fields in recent five years have been summarized in Fig. 1b, including anode

69

performance, extracellular electron transfer (EET), catalysts preparation to speed up

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oxygen reduction reaction in cathode, different types of wastewater as well as

71

contaminants (e.g., heavy metal, nitrate) both in anode/cathode chamber. This

72

technology ingratiates the trend of waste recycling and it is expected to solve the

73

intractable problem of water shortage and renewable energy recovery (Logan and

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Elimelech, 2012).

75

Fig. 1

76

Currently, the low power output and high operation costs of MFCs have 5

77

significantly limited the large-scale practical applications (Logan and Regan, 2006;

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Logan and Rabaey, 2012; Choudhury et al., 2017; Cai et al., 2019b; Zhang et al.,

79

2019). The main factors affecting power generation of MFCs systems include reactor

80

configuration, inoculum sources, substrates, ions strength of solution, ion exchange

81

membrane (if used), internal/external resistance of cell, electrode materials and the

82

electrodes spacing between the anode and cathode (Logan and Regan, 2006; Logan,

83

2009; Butler and Nerenberg, 2010; Nam et al., 2010; Qiao et al., 2010; Zhang et al.,

84

2011; Wang et al., 2013; Winfield et al., 2016; Santoro et al., 2017). Among these

85

factors, fabrication of a high performance and low costs electrode is the key to

86

improve power output (Fan et al., 2017). In BES, the rate of redox reaction has been

87

substantial improved by using the elaborate-designed electrode materials (Chen et al.,

88

2011). The anode of MFCs served as an attachment carrier of microorganisms plays a

89

vital role on the EET between the electroactive bacteria (EAB) and solid electrode

90

surface (Fig. 2c) and bioelectricity production in MFCs (Yuan et al., 2013). In this

91

regard, it is of crucial importance to develop a novel anode material with potential

92

application values and the synergistic effects between the properties of anode surface

93

and microorganisms. A desirable anode should have the following properties (Wei et

94

al., 2011; Zhou et al., 2011; Zhu et al., 2011; Hindatu et al., 2017; Sonawane et al.,

95

2017; Cai et al., 2019a; Palanisamy et al., 2019):1) good conductivity to speed up the

96

process of electron transfer rate (ETR); 2) excellent biocompatibility and low

97

biotoxicity for microbes; 3) higher specific surface area to provide more attachment

98

sites of microbes and catalytic activity sites; 4) chemical stability and anti-corrosion

99

resistance; 5) flexibility and durability; 6) low economic cost and convenient to

100 101

commercial application. Fig. 2 6

102

To sum up, in this present review, a comprehensively overview focusing on the

103

advanced anode modification strategies in terms of different properties were provided,

104

including pristine biochar and its composite materials, metal or metal oxide modified

105

materials, conductive polymers, heteroatom doping, carbon nanotubes (CNTs) as well

106

as graphene-based modified materials. Moreover, the reason of those materials to

107

enhance bioelectricity generation and the synergistic effects between the EAB and

108

solid electrodes were further clarified. At the end of this review, some potential

109

research to develop the next generation advanced anode were also expected. An

110

intended purpose of this review is to summarize some of the crucial findings in the

111

related research fields and to try to extract possible fundamental principles from

112

diverse research topics.

113

2 Anode electrodes of MFCs

114

2.1 Traditional anode electrodes

115

Among these candidates, carbonaceous materials are still the most widely used in

116

MFCs due to their biocompatibility, conductivity and chemical stability. Traditionally

117

carbon-based electrodes include carbon cloth (CC), carbon paper (CP), carbon felt

118

(CF), carbon brush (CB), graphite plate (GP), graphite sheet (GS), graphite rod (GR),

119

granular activated carbon (GAC), reticulated glass carbon (RGC) and stainless steel

120

mesh (SSM) (Heijne et al., 2008; Wang et al., 2009; Liu et al., 2010; Wei et al., 2011;

121

Cai et al., 2019a). The plane structure, packed structure as well as brush structure of

122

traditional anode electrodes are reviewed in details by Wei et al (2011). It is an

123

undisputed fact that most of these materials mentioned above in MFCs studies are

124

based on the projected area power density, which has some limitations to evaluate

125

MFCs performance. For instance, it is difficult to calculate the real surface area when

126

the porous electrodes are used in MFCs, which leads to a significant difference in 7

127

maximizing power density of different porous electrodes but with the same projected

128

surface area (He, 2017). In this case, it is of practical significance to develop a proper

129

benchmark to accurately and quantitatively evaluate MFCs performance of different

130

electrode materials. Some reasons in previous studies have demonstrated that most

131

anode materials are not suitable for amplification in practical engineering. Graphite

132

electrodes have a certain mechanical strength, but the relative low biofilm

133

accumulation on the surface seriously affects the power output of MFCs resulting

134

from its smooth surface. CC/CP and RGC have a rough surface but poor mechanical

135

strength. Thus, it is difficult to meet the requirements for real-world application. CF

136

electrodes possess good flexibility and conducive to the adhesion of microorganisms.

137

However, due to its relatively high thickness, the formation of biofilm on the surface

138

hinders the substrates diffusion from outside to inside, resulting in the inside anode

139

surface unfavorable for microbial colonization. Although SSM electrodes can meet

140

the most requirements of anode electrodes, the enriched biofilm is easy to fall off after

141

using for a long period of time due to gravity, so it is difficult to achieve a good

142

bioelectricity production. In contrast, CB has a better performance for power

143

generation, however, metal (usually titanium wire) is required as a joint in the process

144

of CB electrodes fabrication, which greatly increases the economic cost. Therefore,

145

researchers are usually choosing CC/CF or graphite as the control rather than carbon

146

brush when developing novel anode electrodes. Various factors should be considered

147

comprehensively when choosing appropriate materials as MFCs anode. Specifically,

148

the availability of cost effective anodes are capable of achieving higher performance

149

for long term operation, while also involving easy maintenance or, where possible, to

150

be completely free of maintenance (Sonawane et al., 2017).

8

151

In order to achieve these goals mentioned above and further improve the energy

152

harvest of MFCs system, versatile modification methods have been developed, such

153

as, physical, chemical as well as electrochemical methods, in order to increase the

154

surface area, reduce the internal impedance of anode electrodes and accelerate the

155

ETR between EAB and anode.

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2.2 Strategies of modified anode electrodes

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2.2.1 3D natural resource anode electrodes

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The unique 3D macroporous scaffold of biochar has received substantial attention

159

of researchers. This structure retains intact even after high temperature calcination.

160

Notably, more mesoporous or micropores could be formed in the process of calcining,

161

which greatly increases the specific surface area of the materials and provides more

162

attachment sites for microorganisms, resulting in a faster mass transfer of substrate

163

and higher bioelectricity generation in MFCs (Guzman et al., 2017). Some

164

breakthroughs have been made by Chen et al. (2012a) using natural resource to

165

fabricate MFCs anode electrodes. For instance, the kenaf (Fig. 3a~c), pomelo peel

166

(Fig. 3d, e) (Chen et al., 2012c) and packing materials (Fig. 3f, g) (Chen et al., 2012b)

167

were used to prepare 3D macroporous carbon-based scaffold in MFCs by simple

168

carbonization

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bioelectrocatalytic activity and the maximum current density of the pomelo peel

170

electrode was 40.2 A⋅m-2 (electrode thickness 2.20 mm, projected area 0.98 × 0.96

171

cm2), which was 19% higher than that of kenaf electrode (32.5 A⋅m-2). It was mainly

172

due to its reticular macropore structure (porosity 97%, pore size >100 µm), which

173

provides more opening channels for EAB attachment and substrates diffusion.

174

Additionally, hydrophilic anode electrodes surface is also beneficial to the growth and

175

colonization of microorganisms, greatly facilitating ETR between the solid electrodes

process.

Interestingly,

these

9

electrodes

exhibit

superior

176

and EAB. However, the volume current density decreases with anode electrodes

177

thickness increasing, which also indicates that the biofilm formed by microbial

178

propagation still causes the blockage of the internal pore-like structure of the

179

electrode and reduces the power output in MFCs. After that, a novel conception based

180

on the layered corrugated carbon (LCC) structure of anode electrodes has been put

181

forward by Chen et al. (2012b). The protogenic material with a natural structure is

182

derived from the most abundant packing materials in our society: corrugated

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cardboard (Fig. 3f, g). It is noteworthy that the power output was up to 390 A⋅m-2 at

184

six corrugated layers, which was much higher than that of kenaf and pomelo peel

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carbon-based electrodes. Compared with them, there is no barrier for substrate

186

diffusion because the unique "corrugated" structure of anode electrode forms a

187

millimeter level channel. Meanwhile, the importance of macrostructure morphology

188

of anode electrodes for power generation in MFCs is also demonstrated, which

189

provides theoretical foundation for the development of microbial electrocatalytic

190

anode architecture.

191

A recent tendency has been demonstrated that electrodes prepared by nitrogen

192

doping or nitrogen-rich biomass could effectively improve bioelectricity production

193

(further discussion in section 2.2.4) (Yuan et al., 2013; Zhang et al., 2014; Yuan et al.,

194

2015; Ma et al., 2016; Lu et al., 2017; Wang et al., 2017a; Chen et al., 2018; Li et al.,

195

2018). It should be attributed to a high nitrogen content anode beneficial for

196

improving biocompatibility. It results in a suitable micro-environment for

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microorganisms attachment and colonization, thus helping to reduce the charge

198

transfer resistance between the biofilm and solid electrodes (Wang et al., 2017a; Chen

199

et al., 2018). The high-performance anode electrodes fabricated by natural resource in

200

the last five years were summarized in Table 1, including chestnut shells (Chen et al., 10

201

2016a; Chen et al., 2018), corn straw (Karthikeyan et al., 2015), bamboo (Zhang et al.,

202

2014), wood-derived (Huggins et al., 2016), loofah sponge (Yuan et al., 2013),

203

forestry residue/compressed milling residue (Huggins et al., 2014), silk cocoon (Lu et

204

al., 2017) and almond shell (Li et al., 2019a) etc. The common features of these

205

materials include, 3D mesoporous/microporous structure, roughness surface and good

206

conductivity to achieve fast ETR and microorganism growth dynamics (Karthikeyan

207

et al., 2015). Furthermore, these carbon-based natural materials are cheap, easy to

208

obtain, which is in line with the concept of green and sustainable development (Chen

209

et al., 2012b).

210

Fig. 3

211

Table 1

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2.2.2 Metal/metal oxides-based anode electrodes

213

Generally, most of the metal or metal oxides are not intended to be used directly

214

for MFCs anode electrode due to their corrosion-prone properties (Sonawane et al.,

215

2017). In the past few years, many nanoparticles of metals, metal oxides as well as

216

their alloys have been used as MFCs anodes, such as, Pd (Quan et al., 2015; Quan et

217

al., 2018), Au (Guo et al., 2012; Wu et al., 2013b; Kim et al., 2015; Jarosz et al., 2019),

218

NiO (Gao et al., 2018; Wu et al., 2018a; Zhong et al., 2018), MnO2 (Zhang et al.,

219

2015; Chen et al., 2016b; Zhang et al., 2016; Phonsa et al., 2018; Xu et al., 2018),

220

CoxOy (Mohamed et al., 2017; Gao et al., 2018), TiO2 (Wu et al., 2013b;

221

Garcia-Gomez et al., 2015; Tang et al., 2015b; Yin et al., 2015; Feng et al., 2016; Jia

222

et al., 2016; Liu et al., 2016; Yin et al., 2016; Zhou et al., 2016; Xu et al., 2018; Ying

223

et al., 2018), WO3 (Wang et al., 2013; Varanasi et al., 2016), Ni0.1Mn0.9O1.45 (Zeng et

224

al., 2018a), NiTi (Taşkan et al., 2019), Co/Ni (Lan et al., 2018), NiWO4 (Geetanjali et

225

al., 2019) and Co-MoO2 (Li et al., 2019b). Precious metals can be widely used in 11

226

electrochemistry, mainly due to their high conductivity, wide potential window and

227

high catalytic activity (Kumar et al., 2013). However, it is difficult to popularize in

228

MFCs anode on a large scale considering their high cost. The catalytic activity of

229

non-noble metal and its oxide nanoparticles is almost equivalent to that of precious

230

metals, which can greatly reduce the ohmic resistance and increase the attachment of

231

EAB on the surface of the electrode (Wang et al., 2013; Hindatu et al., 2017).

232

Therefore, it has a broad research prospect.

233

Ti or TiO2 is an ideal metal oxide for modifying MFCs anode electrode in terms

234

of good biocompatibility, anti-corrosion properties and the optical and dielectric

235

properties. A considerable part of researches use titanium wire/mesh as the anode

236

collector to collect the electrons generated by substrate oxidation (Qiao et al., 2008a;

237

Ying et al., 2019). Actually, the energy harvest in MFCs modified by pure TiO2

238

nanoparticles may be lead to a lower efficiency than other nanoparticles, such as

239

carbon nanotubes (Wen et al., 2013). One of the reasons for this phenomenon should

240

be the large resistance of TiO2 nanoparticle film due to the electron transport

241

obstruction at grain boundaries. Nevertheless, the process of electron transfer could be

242

enhanced by different kinds of structural design (Talapin et al., 2010), like, TiO2

243

nanowires (Jia et al., 2016), TiO2 hierarchical microspheres (Wu et al., 2013a),

244

vertically oriented TiO2 nanosheets (Yin et al., 2015), TiO2 nanotube arrays (Feng et

245

al., 2016), etc. One-dimensional TiO2 nanostructures with a relatively small amount of

246

grain boundaries are able to provide fast electron transport and have been extensively

247

used in other fuel cells (Macaira et al., 2013). A novel porous carbon anode electrode

248

with 3D ordered honeycomb structure was designed by TiO2 nanoparticles (Fig. 4a, b)

249

(Liu et al., 2016). After being modified, the uniform macroporous distribution and

250

high specific surface area was obtained, greatly improving the roughness of anode and 12

251

increasing biocompatibility simultaneously. Results showed that the maximum power

252

density (Pmax) of modified electrode was 973 mW⋅m-2, which was 2.3 times higher

253

than that of traditional carbon cloth anode. Ying et al. (2018) fabricated a TiO2 thin

254

film coated stainless steel mesh anode using electrochemical method (Fig. 4c, d).

255

After MFCs equipped with composite anode running for 20 d, the total protein density

256

of modified electrode surface was 12 times higher than that of the control anode,

257

implying that the modified anode has superior biocompatibility. Moreover, Rct (charge

258

transfer resistance) is only 3.55 Ω, which significantly improves the efficiency of

259

electron transfer and the maximum current density (Imax) is up to 70 A⋅m-2.

260

As an important non-precious metal catalyst, transition metal oxides or their

261

hybrid nanomaterials are also widely used in the modification of anode in MFCs.

262

They have the advantages of environmentally friendly, low-cost, as well as

263

high-powered energy storage. Wu et al. (2018a) developed a 3D layered porous anode

264

electrodes by one-step hydrothermal synthesis with pectin as initiator to tailor the

265

porous structure. Intriguingly, NiO microspheres assembled by nanoflakes were

266

inserted in the graphene layers rather than just deposited on the surface of graphene

267

nanosheets (Fig. 4e). This unique microstructure enables MFCs with the composite

268

anode to exhibit better electrocatalytic activity and deliver maximum power density as

269

high as 3632 mW⋅m-2. Notably, different micro-morphology of NiO nanoparticles

270

display different electrochemical properties. Inspired by a high capacitive of NiO and

271

high conductivity of polyaniline (PANI), Zhong et al. (2018) combined the

272

performance of NiO and PANI and prepared a petaloid shaped NiO@PANI-CF

273

composite electrode through in-situ polymerization reaction (Fig. 4f). MFCs with this

274

composite anode has the advantage of low Rct (10.4 Ω), low polarization performance,

275

high specific surface area, super-hydrophilicity and good biocompatibility, resulting in 13

276

a Pmax of 1078 mW⋅m-2. Furthermore, the MFCs system was used for dying

277

wastewater treatment, the removal efficiency of chemical oxygen demand (COD) and

278

dye decolorization reached 95.94% and 64.24%, respectively. Cobalt (Co) and their

279

hybrids could improve the hydrophobicity, roughness and interface impedance

280

between biofilms and solid electrodes by electrodeposition on different carbon-based

281

electrodes. The modified electrodes was used in MFCs to treat food wastewater, the

282

power output, COD removal and coulombic efficiency (CE) of this system were

283

enhanced (Mohamed et al., 2017). The bi-component composites of transition metal

284

oxides are also developed in MFCs anode modification. Zeng et al. (2018a)

285

synthesized

286

microellipsoids as high performance electrocatalyst for the anodic oxidation reaction

287

via a simple co-precipitation reaction (Fig. 4g, h). The power output reached 1390

288

mW⋅m-2, which might be attributed to the bi-component synergistic effect facilitating

289

the EET process between the anode and EAB.

a

porous

self-assembled

micro-/nanostructured

Ni0.1Mn0.9O1.45

290

The performance of MFCs with different metal/metal oxide nanoparticles or their

291

hybrids as anode electrocatalyst were summarized in Table 2. In summary, the anode

292

electrodes were modified via various synthesis methods, such as electrodeposition,

293

in-situ hydrothermal reaction or co-precipitation reaction. The modifications can

294

improve the conductivity of anode electrodes and reduce the internal resistance of Rct,

295

thus increasing the electrode surface roughness and specific surface area to make it

296

easier for bacteria attachment and reproduction. Therefore, the electrocatalytic activity

297

of microorganisms could be enhanced to accelerate the ETR between biofilm and

298

solid electrodes.

299

Fig. 4

300

Table 2 14

301

2.2.3 Conductive polymers decoration

302

Conductive polymers (CPs) have been received extensive attention in MFCs

303

anode modification (Hindatu et al., 2017; Zhao et al., 2017). The main purpose

304

focused on increasing the surfaceness and speeding up the ETR. To obtain high

305

bioelectricity production, CPs are commonly co-doped with other nanomaterials to

306

decorate anode in MFCs. Some representative works using different CPs and their

307

hybrids in MFCs were provided in Table 3.

308

There are three types of CPs that are frequently used in MFCs, including PANI

309

(Wang et al., 2013; Yuan et al., 2013; Liao et al., 2015; Hidalgo et al., 2016; Zhang et

310

al., 2017b; Sonawane et al., 2018a; Sonawane et al., 2018b; Yellappa et al., 2019),

311

Poly (3,4-ethylenedioxythiophene) (PEDOT) (Kang et al., 2015; Liu et al., 2015;

312

Kang et al., 2016; Guzman et al., 2017) and polypyrrole (PPy) (Pu et al., 2018;

313

Sonawane et al., 2018b; Kisieliute et al., 2019). All of the three polymers are obtained

314

from monomers by electrodeposition, chemical polymerization or electrochemical

315

polymerization etc. PANI, a positively charged conductive polymer, could generate

316

electrostatic attraction with negatively cells (Zhao et al., 2017) and has been

317

extensively studied for MFCs anode decoration. For example, Sonawane et al. (2018a)

318

applied the electro-polymerization to coat PANI film on the surface of a stainless steel

319

plate by elaborate and selective choice of the synthesis conditions with respect to

320

acids and aniline monomers concentration. Highly uniform and adherent conductive

321

PANI-SS anode electrode was obtained, resulting in a Pmax of 780 mW⋅m-2 (Imax 1.4

322

A⋅m-2). Subsequently, the stainless steel wool (SSW) coated by PANI (PANI-SSW)

323

and PPy (PPy-SSW) were also developed (Fig. 5a~c). Electrochemical experiments

324

indicated that the Rct of pristine SSW (240 Ω⋅cm-2) was much high than that of

325

PANI-SSW (67 Ω⋅cm-2) and PPy-SSW (142 Ω⋅cm-2), with a result of Pmax of 1.27, 15

326

1.87 and 2.88 W⋅m-2 , respectively, implying that PANI coating is more suitable than

327

PPy for the anode modification in MFCs (Sonawane et al., 2018b). Intriguingly, the

328

morphology design for electrode surface was also substantiated by using different

329

types acids doped PANI. Liao & Su et al. (2015) investigated three acids of HCl,

330

H2SO4 and tartaric acid (TA) doped PANI by in-situ chemical polymerization. Results

331

showed that the synthetic TA/PANI composite electrode exhibited the most superior

332

performance with a Pmax of 490 mW⋅m-2, which was higher than that of MFCs with

333

inorganic acids doped PNAI modified anode. These results should be ascribed to the

334

mesoporous networks structure (Fig.5 d) of TA/PANI anode. Actually, most of the

335

previous works based on PANI modified electrode are considered to be co-doped with

336

other nanomaterials due to the synergistic effect between PANI and nanomaterials,

337

such as metal oxides, graphene, CNTs or functionalized CNTs, etc.

338

A considerable amount of works have focused on PPy anode modification in

339

MFCs due to its excellent electrocatalytic properties, good environmental stability,

340

facile synthesis and low economic cost (Phonsa et al., 2018; Pu et al., 2018). A

341

PPy-stainless

342

electrochemical depositing PPy onto SS (Pu et al., 2018). The as-prepared anode

343

exhibited a superior corrosion resistance with a Pmax of 1191 mW⋅m-2, which was

344

about 29 times higher than that of bare SS anode. Such an enhancement is due to the

345

PPy deposited on SS surface increasing specific capacitance and reducing internal

346

resistance. It has been reported that the redox mediator immobilizing on the surface of

347

anode could strengthen bioelectricity production (Pan and Zhou, 2015; Xu and Quan,

348

2016; Martinez et al., 2017). Nevertheless, the mediator was easily to be desorbed

349

under regular experimental circumstances and results in the performance reduction

350

over time. Thus, Tang and Ng et al. (2017) developed a facile and scalable method to

steel

(SS)

composite

anode

16

was

synthesized

using

in-situ

351

prepare a conductive polypyrrole hydrogel (CPH)/ anthraquinone-2-sulfonate (AQS)

352

hybrid anode via electrochemical reduction of the in-situ generated AQS diazonium

353

salts, which was covalently bound to CPH. The CPH/AQS composite bioanode can

354

reduce Rct from 28.3 Ω to 4.1 Ω, and increase the hydrophilicity of the porous

355

structural materials, leading to biofilm formation rapidly. These merits of good

356

biocompatibility, high electrocatalytic activity, long term stability and scalability

357

provide a direction for the preparation of high performance MFCs anodes. It is highly

358

desirable to coat the individual bacterial cells with a conjugated polymer to endow

359

them more functionality. Song et al. (2017) developed an in-situ polymerization to

360

coat PPy on the surface of individual exoelectrogenic bacteria, such as Shewanella

361

oneidensis MR-1, Escherichia coli, Streptococcus thermophilus, Ochrobacterium

362

anthropic (Fig.5 e, f). The internal resistance of electron transfer was only 45.6 Ω

363

between exoelectrogenic bacteria coated by PPy thin film, which was 23 times less

364

than that of control group, resulting in a 14.1 times higher power output than that of

365

untreated bacteria. This study provides a good starting point for the application of

366

cell-surface modification in microbial electrochemical technologies. However, further

367

studies are still needed to investigate whether the modified exoelectrogenic bacteria

368

has the same functional characteristics with its progeny cell surface.

369

PEDOT has the advantage of simple molecular structure, good conductivity, good

370

biocompatibility and stability (Green and Abidian, 2015), which

371

used in solar cells (Sun et al., 2015), biosensor (Mantione et al., 2017), organic

372

electrochemical semiconductor materials (Noh, 2016), electrode material (Zhao et al.,

373

2016), etc. The modified anode with a porous PEDOT layer significantly increased

374

the availability of redox active sites, while reduced the interface resistance between

375

biofilm and anode, resulting in 43% power output enhancement (Liu et al., 2015). 17

has been widely

376

Kang et al. (2015) loaded PEDOT on different anode configuration (graphite plate,

377

carbon cloth, graphite felt), and demonstrated that the graphite felt anode have a

378

synergetic effect of PEDOT coating due to higher surface area and open structure,

379

which promotes power output and CE of MFCs. Then, the optimized PEDOT load

380

capacity of 2.5 mg⋅cm-2 was determined (Kang et al., 2016). Meanwhile, microbial

381

diversity of anode biofilm indicated that the microbial community structure varied

382

greatly from the inoculum and the abundance of EAB accounted for 83% after

383

PEDOT modifying, which proved that the PEDOT coated anode was conducive to

384

microbial attachment and propagation. Guzman et al. (2017) fabricated a high

385

porosity and specific surface area polyacrylonitrile (PAN) nanofiber mat by

386

electrospinning. The as-obtained carbon nanofibers (CNFs) electrode coated by

387

PEDOT significantly improved their capacitance and conductivity. However, the

388

nascent PAN mat is a non-conductive material without capacitance. This result

389

verified that the multi-functional CNFs electrode can be realized through simple

390

electrodeposition PEDOT film.

391

Fig. 5

392

Table 3

393

2.2.4 Nitrogen-enriched anode electrodes

394

Recent investigations have reported that doping of heteroatoms into carbonaceous

395

materials is an effective strategy to enhance the electrocatalytic activity of MFCs due

396

to their unique electronic properties and good biocompatibility (Ci et al., 2012; Lu et

397

al., 2017; Bi et al., 2018; Li et al., 2018). Particularly introduction of N-containing

398

functional groups or increase in the N ratio in carbon materials would induce

399

absorption and growth of preferred microbes on the electrode to increase biocatalytic

400

activity and facilitate the EET (Yuan et al., 2013; Yu et al., 2015; Lu et al., 2017). The 18

401

reported methods include NH4HCO3 pretreatment (Liu et al., 2014b), ammonia and

402

nitric acid treatment (Cheng and Logan, 2007; Zhu et al., 2011; Mohamed et al.,

403

2018), heterocycle-polymer modification (Yuan et al., 2013), natural source

404

N-enriched materials (Lu et al., 2017), etc. In addition, N-dopant could create defects

405

that can break the chemical inertness in carbon material sands thus leading to rich

406

hydrophilic (N) defects, which is benefit to accelerate electron transfer to and from

407

proteins on cell membranes (Ci et al., 2012).

408

Yu et al. (2015) developed a nitrogen-doped carbon nanoparticles (NDCN) anode

409

and used S. oneidensis MR-1, one of the most well-established exoelectrogens in

410

MFCs, as inoculation. Subsequent cyclic voltammetry (CV) test found that three pairs

411

of redox peaks were appeared when MFCs with NDCN anode, and the catalytic

412

current caused by mediator electron transfer (MET) was much higher than that of

413

untreated anode. This phenomenon should mainly attribute to the use of NDCN

414

increasing anodic absorption of flavins and consequently improving MFCs

415

performance. Notably, it is the first demonstration of MET enhancement via electrode

416

modification. Bi et al. (2018) designed a 3D N-doped porous carbons through

417

one-step pyrolysis of sodium citrate and melamine and then coated on the surface of

418

CC. The as-prepared electrode was applied to the H type dual-chamber MFCs. Results

419

showed that Pmax was up to 2777 mW⋅cm-2, which was 2 times as large as commercial

420

CC electrodes. This is mainly because the porous carbon structure of nitrogen doping

421

increased the biocompatibility of the anode, facilitated the growth and reproduction of

422

microorganisms, and reduced the internal resistance of modified anode. Yuan et al.

423

(2019) synthesized a porous N-doped anode with high N/C ratio of 3.9% and large

424

electrochemically active surface area of 145.4 cm2, which enriched the bacteria

425

attachment and provided more active sites for EET. Intriguingly, the content of 19

426

riboflavin also increased on the surface of anode, implying that the N-doped

427

modification anode electrode is a viable strategy for enhancing the performance of

428

MFCs.

429

2.2.5 CNTs-based anode electrodes

430

Carbon nanotubes (CNTs) have received increasing attention from researchers

431

due to their unique mechanical, electrical and chemical properties, since they are

432

discovered in 1991. According to the characteristics of tubular structure, they can be

433

divided into Single walled carbon nanotube (SWCNT) and Multi-walled carbon

434

nanotube (MWCNT). Both of them have high elastic modulus, tensile strength and

435

thermal/electrical conductivity (Yazdi et al., 2016), which enable CNTs to be applied

436

in various fields, such as, biosensors (Hu et al., 2017), energy storage and conversion

437

(Jiang et al., 2013), tissue engineering (Scaffaro et al., 2017) microelectronics (Cai et

438

al., 2017), etc. Fig. 6 illustrates the attachment and colonization of bacteria cells on

439

the anode surface in MFCs with different CNTs modification anode materials.

440

Although previous studies have indicated that CNTs have a certain biotoxication

441

on cells, which might restrain the bacteria propagation and even kill cells (Magrez et

442

al., 2006; Yazdi et al., 2016). However, the applications of CNTs in MFCs anode

443

modification still emerge in endlessly where regulating the dosage of CNTs, coupling

444

with other conductive polymers or grafting with other functional groups are usually

445

used to reduce their biotoxicity. Strikingly, those modified methods have positive

446

effects on improving MFCs performance. For instance, Liang et al. (2011) and Zhang

447

et al. (2017a) proposed a concept of CNTs hybrid biofilm by mixing MWCNT with

448

anode biofilm. It not only increased the conductivity of composite biofilm, but also

449

enhanced the electron transfer and substrate diffusion of the biofilm. Moreover, the

450

start-up time was also shortened and the composite biofilm exhibited a long-term 20

451

stability and durability. All of these led to a better MFCs performance. Incontestably,

452

the MFCs performance could be further improved by doping conductivity polymer

453

with CNTs. Cui et al. (2015) fabricated a composite anode (CNTs/PANI/GF) through

454

electropolymerized PANI on the surface of graphite felt and followed by

455

electrophoretic deposition of carbon nanotubes. Consequently, both of the roughness,

456

hydrophilicity and electrical conductivity of the resulting material were significantly

457

enhanced. Then, the as-obtained electrode was installed in the MFCs anode chamber,

458

resulting in a Pmax of 257 mW⋅cm-2. It was 3.4 and 1.9-folds than that of pristine GF

459

and PANI/GF electrode, respectively.

460

CNTs modified anode can not only increase the conductivity and electrocatalytic

461

activity, but also greatly increase the specific surface area. So the anode surface can

462

provide more attachment sites for bacteria attachment, and facilitate the transfer

463

process of the substrate as well as the electron transfer pathway (Zhao et al., 2017). In

464

this regard, Xie and Cui (2011) have made a great contribution in terms of MFCs

465

anode surface modification with CNTs. They synthesized a CNTs-textile anode with

466

two scale porous structure, consisting of microscale porous CNTs layer and a

467

macroscale porous textile. Such structure realized an affinitive mechanical contact and

468

rapid ETR duo to the intertwined CNTs-textile fibers. The as-prepared anode

469

delivered a 10-fold-lower Rct and 141% higher energy recovery than that of traditional

470

CC anode. After that, a CNTs coated 3D macroporous sponge was developed.

471

Compared to the CNTs-textile anode, the former possessed a lower internal resistance,

472

greater stability, more tunable and uniform macroporous structure, and superior

473

mechanical properties. These properties make the 3D CNTs-sponge achieve a Pmax of

474

1240 mW⋅cm-2, which is the highest values obtained to date for MFCs fed with

475

domestic wastewater. Furthermore, to investigated the interaction between subcellular 21

476

structures and nanomaterials, a mesoscale morphology with ridge features was

477

induced by crinkling CNTs films on 3D substrate via solvent-induced swelling and

478

shrinkage of substrate (Xie et al., 2014). Delord et al. (2017) also developed a

479

CNTs-based fiber mats as MFCs anode by weaving fibers solely comprised of CNTs.

480

The anode has extremely high porosity and specific surface area. When the electrode

481

is applied to MFCs, delivering an Imax of 75 A⋅m-2. Additionally, the preparation of

482

electrode is simple, easy to scaled up and automatized, which makes it worthy for

483

further research. Fig. 6

484

485

2.2.6 Graphene-based anode electrodes

486

Graphene (Gr), a unique 2D planar honeycomb-like lattice nanomaterial, consist

487

of sp2 hybridization carbon atoms with similar features to CNTs in many aspects.

488

Unlike CNTs, Gr has good biocompatibility and high ETR (Liu et al., 2012; Lv et al.,

489

2016), which is an important factor to investigate anode electrodes performance in

490

MFCs. In addition, it is worth noting that the study of Gr has gradually inclined to the

491

applications in electrochemistry areas due to their special electron mobile rate and

492

electrocatalytic activity (ElMekawy et al., 2017). Especially, MFCs is considered to

493

be an important branch of fuel cells (Farooqui et al., 2018), thus developing a high

494

performance anode material used Gr in MFCs is also received greatly attention from

495

researchers (Sonawane et al., 2017; Zhao et al., 2017).

496

In fact, more than 70% of the total internal resistance of MFCs is ascribed to

497

diffusion resistance and activation loss (Ahn and Logan, 2013; ElMekawy et al.,

498

2017). Previous works have indicated that Gr modified electrode can stimulate

499

microbes to secrete the signaling molecules, which could accelerate the growth of

500

microbes and simultaneously serve as mediator molecule to boost the electron transfer 22

501

efficiency (Zhang et al., 2011; Jain et al., 2012; ElMekawy et al., 2017). The affinitive

502

physical contact could realize fast electron transfer between extracellular protein

503

secreted by bacteria or conductive pili and Gr modified electrode in a pseudo-direct

504

electrode transfer process via endogenous electron mediators (Qiao et al., 2008b;

505

Logan, 2009; Liu et al., 2012; ElMekawy et al., 2017; Zhu et al., 2019a). Additionally,

506

the ultrahigh specific surface area of Gr (∼2600 m2⋅g-1) provides more channels and

507

active sites for the substrate diffusion and interfacial reaction, which might be an

508

effective approach to reduce the diffusion resistance and activation loss (Li et al.,

509

2012; Liu et al., 2014a). To overcome the tedious fabrication procedure of Gr

510

composite electrode and clarify the ambiguous mechanism for its effect on BES, Sun

511

et al. fabricated a Gr/PANI hybrid electrode via one-step and in-situ method for

512

simultaneously Gr exfoliation and aniline polymerization (Sun et al., 2017). The

513

as-prepared electrode exhibited excellent energy harvest and the Pmax was 24 times

514

higher than that of untreated electrode. Moreover, further analysis revealed that the

515

Gr/PANI electrode has a synergistic effect on both DET and MET of S. oneidensis

516

MR-1. Cheng et al. (2018) developed a reduced graphene oxidation hybrid gold

517

nanoparticle (rGO/Au) anode via layer-by-layer assembly method. Coating on CB

518

surface and leaf aqueous extract was introduced to improve the hydrophilicity of

519

hybrid electrode, which not only shorten the start-up time of MFCs by 75%, but also

520

exhibited a high power output of 33.7 W⋅m-3 (Imax 69.4 A⋅m-3). Such

521

high-performance should be attributed to a high biocompatibility and rough surface,

522

which is beneficial to bacteria attachment and colonization. Meanwhile, Au

523

nanoparticles enhance the electrocatalytic activity and speed up the ETR. Surface

524

cytochrome was synthesized by exoelectrogenic bacteria, which also reduced the

525

interfacial resistance. 23

526

3D nano-structure electrodes applied in MFCs can not only increase the specific

527

surface area, but also furnish more open structure for substrate diffusion. Therefore,

528

researchers are dedicated to develop a novel 3D Gr anode electrode for MFCs (Yuan

529

et al., 2012; Yong et al., 2014; Yang et al., 2016; Chen et al., 2017; Yu et al., 2018;

530

Yadav and Verma, 2019). For example, Yang et al. (2016) used graphene oxide

531

aerogel (GOA) to modify the graphite fiber brush (GFB) and formed a 3D-3D

532

structure composite electrode as MFCs anode electrode for a long-terms steadily

533

operation (18 months). Unexpectedly, Power density was continuously increased with

534

time from 714 mW⋅cm-2 to 2520 mW⋅cm-2, which was contrary to most of the

535

previous works. Since the biofilm of anode would become denser over time, this

536

tremendously impeded the diffusion of substrate into internal of anode structure, thus

537

consequently obtaining a low energy harvest. This result implies that a 3D open

538

structure of anode is very crucial for continuously generation of bioelectricity in

539

MFCs. Recently, researchers found that GO could be in-situ reduced by EAB, which

540

makes anode modification by bio-reduced GO possible (Zhou et al., 2019; Zhu et al.,

541

2019b). For instance, a dual-bioelectrode modified with graphene oxide (GO) was

542

prepared by Chen et al. (2017), and applied in situ microbial-induced reduction of GO

543

and polarity reversion in MFCs. Both of the anode and cathode displayed a 3D

544

architecture, but the viability and thickness on the anode surface were reduced

545

compared with control. This may be due to the direct contact of bacteria with Gr,

546

resulting in the cell membrane damage and loss of cytoplasm (Hu et al., 2010). In

547

addition, previous studies have shown that the direct contact of bacteria with the

548

extremely sharp edges of nanowalls can cause a cell membrane damage of bacteria

549

(Akhavan and Ghaderi, 2010). Although the modified materials have slightly

550

biotoxicity to bacteria, the CE and power density were higher than that of control. 24

551

These studies mentioned above have shown that the conductivity, specific surface

552

area, 3D open-porous structure as well as electrocatalytic activity can be enhanced by

553

Gr and/or their hybrids, indicating a broad application prospect of Gr nano-materials

554

in MFCs. Table 4 summarized the application of Gr decorated anode electrodes in

555

recent years, and it is believed that more functional Gr modified electrodes will be

556

developed in MFCs in the future. Table 4

557

558

3 Conclusions and outlooks

559

MFCs are a novel bioelectrochemical device that integrates wastewater treatment

560

and bioelectricity generation. A tremendous breakthrough has been made regarding of

561

power output in MFCs from few mW⋅cm-2 in the bud to several W⋅cm-2 (based on

562

anode projected area) in current state, which was improved by three orders of

563

magnitude owing to continuously efforts of researchers. Notably, there are still a

564

considerable number of MFCs researchers working on the development of

565

high-performance electrode materials. Thus, these different anode modification

566

strategies were summarized in Table 5 aiming to provide design guidelines for the

567

next generation anode materials. In the context of wastewater treatment, however, it

568

has long been hypothesized that MFCs takes the advantage of energy self-powered

569

rather than energy consumption. It is reasonable to obtain a net-positive energy

570

harvest in practical waste treatment if the energy potential in waste could be efficient

571

exploited by MFCs. Therefore, it is of practical significant to explore more efficient

572

measures to reduce internal energy loss and extensively study the treatment efficiency

573

of different real wastewater sources in MFCs to boost a stability bioelectricity

574

generation and obtain a better waste treatment efficiency. Notably, the researches on

575

MFCs are interdisciplinary, such as environment, chemistry, energy, biology, sensing, 25

576

materials, etc. In this regard, multidisciplinary research on the application of MFCs

577

may spark unexpected results. Therefore, possible future research on anodes of MFCs

578

can be carried out in the following aspects.

579

I) Using natural waste materials to develop a low cost and high performance

580

anode electrode. The 3D macropore biomass electrodes like kenaf, pomelo peel and

581

loofah have exhibited a superior power output. Moreover, natural resources are

582

cost-effective and readily obtained , which can satisfy most of requirements for anode

583

electrodes. The pore size of electrode can be reasonably tuned through carbonization

584

process. Also, the conductivity and electrocatalytic activity can be further enhanced

585

through modification.

586

II) Currently, 3D printing technology is relatively mature, so the anode

587

architecture (like, honeycomb-like and spongy) can be designed through 3D printing

588

to increase the specific surface area and tune pore structure elaborately. Thus, it is

589

possible to realize more attachment and catalytic active sites of modified anode

590

without blocking the diffusion of substrate;

591

Ⅲ) At last, after obtaining superior anode electrodes, it is necessary to strengthen

592

the long-term performance study of electrode in the real wastewater treatment to

593

investigate the stability, durability, mechanical property and secondary pollution of

594

anode electrodes. Therefore, a considerable part of works need to be carried out in

595

depth to realize the large-scale practical applications of MFCs. Table 5

596 597 598 599 600

Conflicts of interest Declarations of interest: none. Acknowledgements This work was supported by the National Natural Science Foundation of China 26

601

(No. 21477018), the fund from Key Laboratory of Jiangxi Province for Persistant

602

Pollutants Control and Resources Recycle (No. ES201980203), National Key

603

Research Development Program of China (2016YFC0400502) and the fundamental

604

research funds for the central universities.

605

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34

Table 1. Summary of properties of MFCs anode electrodes prepared by natural resource

Kenaf Pomelo peel Corrugated cardboard Loofah sponge Loofah sponge-PANI N/TiO2- loofah sponge Bamboo charcoal tube Sewage sludge -coconut shell Compressed milling residue

Performance Pmax Imax (mW m-2) (A m-2)

Substrate

Source of inoculation

Half-cell

PBS+Acetate

Domestic sewage

/

32.5

(Chen et al., 2012a)

Half-cell

PBS+Acetate

Domestic sewage

/

40.2

(Chen et al., 2012c)

Half-cell SC SC SC Tubular DC (cathode: CC 82cm2)

PBS+Acetate PBS+NaAc PBS+NaAc PBS+NaAc

Strain breeding Effluent of MFCs Effluent of MFCs Effluent of MFCs

/ 701 1090 2590

390 4.4 d 5.6 d /

(Chen et al., 2012b) (Yuan et al., 2013) (Yuan et al., 2013) (Tang et al., 2015b)

PBS+NaAc

Effluent of MFCs

1652

/

(Zhang et al., 2014)

0.5 cm×3.0 cm2

SC

PBS+NaAc

Effluent of MFCs

1069

/

(Yuan et al., 2015)

/

DC (cathode: CC 38cm2)

PBS+NaAc

Anaerobic sludge

532

/

(Huggins et al., 2014)

Anode materials

Size 0.23 cm×1.52 cm2 a 2.2 cm×0.94 cm2 1.8 cm×3.9 cm2 0.5 cm×3.0 cm2 0.5 cm×3.0 cm2 0.5 cm×3.0 cm2 2.4 cm×1.57 cm2 a

Reactor configuration

0.2 cm×0.25 Half-cell PBS+NaAc Anaerobic sludge / 31.2 cm2 King mushroom/ 0.2 cm×0.25 Half-cell PBS+NaAc Anaerobic sludge / 20.9/30.2 Mushroom cm2 0.3 cm×66.4 SC PBS+NaAc Anaerobic sludge 850 / Chestnut shells (powder) cm2 2.7 cm in Barbed chestnut shell SC PBS+NaAc Effluent of MFCs 759 9.6 diameter Silk cocoon 0.08 g b SC PBS+NaAc Anaerobic sludge 5.0 c / b Onion (powder) 0.05 g SC PBS+NaAc Effluent of MFCs 742 9.77 Coffee waste 0.01 g b SC Luria-bertani E. coli DH5α 2000 / a, external tubular surface area; b, based on the mass of biochar; c, mass power density, mW⋅g-1; d, volume current density, mA⋅cm-3. Corn straw

Reference

(Karthikeyan et al., 2015) (Karthikeyan et al., 2015) (Chen et al., 2018) (Chen et al., 2016a) (Lu et al., 2017) (Li et al., 2018) (Hung et al., 2019)

Table 2. Summary of the performance of different metal/metal oxides modified Performance Anode materials Size (cm ) Reactor configuration Substrate Source of inoculation Pmax Imax -2 (mW⋅m ) (A⋅m-2) TiO2-SSM 7 DC (cathode, CF 6cm2) M9+NaAc Effluent of MFCs 2870 70 CM a-TiM b 2 DC (cathode, CF 6cm2) M9+NaAc Effluent of MFCs / 12.2 FeO-CP 12 DC (cathode, CP 12cm2) Distillery wastewater Strain breeding 140.5 0.27 Fe3O4-CC 6.25 DC (cathode, CP) Simulated wastewater Active sludge 728 / Fe3C-PGC c 4 SC PBS+NaAc Effluent of MFCs 1856 / MnO2-CF 7.1 DC (cathode, CB) PBS+NaAc Effluent of MFCs 3580 18.73 WO3/PANI-CF 9 SC C6H12O6 Escherichia coli 980 3.71 NiO/PANI-CF 9 DC(cathode, CF) PBS+NaAc Anaerobic sludge 1079 / Co/CoxOy-Graphite 6.25 SC Food wastewater Food wastewater 576.3 / SnO2/rGO-CC 6 DC (cathode, Pt rod) PBS+ C6H12O6 Escherichia coli 1624 2.8 Mo2C-CF 2 DC (cathode, CFB d) Lactate S. putrefaciens CN32 1025 1.04 Pd-CC 10 DC (cathode, CP 10cm2) PBS+NaAc S. oneidensis MR-1 500 / Au-CP 25 DC (cathode, CP 25cm2) PBS+ C6H12O6 Anaerobic granular sludge 346 0.88 Au/MWCNT-CC 25 DC (cathode, GF 25 cm2) PBS+ C6H12O6 Anaerobic digester sludge 178 ~0.45 Ni0.1Mn0.9O1.45-CF 4 SC Acetate Effluent of MFCs 1390 2.38 NiWO4/GO-CC 1 SC Simulated wastewater Anaerobic sludge 1458 7.16 a, CM: Carbon microwiresa; b, TiM: Titanium mesh; c, PGC: porous graphitized carbon; d, CFB: Carbon fiber brush. 2

Reference (Ying et al., 2018) (Ying et al., 2019) (Harshiny et al., 2017) (Xu et al., 2018) (Hu et al., 2019) (Zhang et al., 2015) (Wang et al., 2013) (Zhong et al., 2018) (Mohamed et al., 2017) (Mehdinia et al., 2014) (Zou et al., 2017) (Quan et al., 2015) (Guo et al., 2012) (Wu et al., 2018b) (Zeng et al., 2018a) (Geetanjali et al., 2019)

Table 3 A comparison of the performance using conductive polymers modified electrodes in MFCs Anodes

Preparation of CPs

Size (cm2)

Reactor configuration

Substrate PBS + NaAc M9 + sodium lactate

Effluent of MFCs

The Performance Pmax Imax (mW⋅m-2) (A⋅m-2) 567 /

(Zhang et al., 2017b)

S. oneidensis MR-1

490

/

(Liao et al., 2015)

Resource of inoculation

Reference

PANI-CC

ECPM

4

DC

TAa/PANI-CC

CPM

2

DC

PANI-SSP

ECPM

3.5

SC

M9 + NaAc

4% LLe

780

1.4

PANI-SSW

ECPM

7.5

SC

M9 + NaAc

4% LL

2880

6.7

NFb/PANI-CC

CPM

2

388.6

0.201

(Liu et al., 2017)

CPM

7

M9 + sodium lactate PBS + NaAc

S. oneidensis MR-1

PANI/Gr-CC

DC (cathode CF, 6cm2) SC

Effluent of MFCs

884

/

PPy-SSW

ECPM

7.5

SC

M9 + NaAc

4% LL

1870

5.6

(Huang et al., 2016) (Sonawane et al., 2018b)

PPy-SS

ECPM

4

SC

PBS + NaAc

Anaerobic granular sludge

1191

1.37

(Pu et al., 2018)

PPy/MnO2-SS

ED

50

SC

Anaerobic sludge

440

/

(Phonsa et al., 2018)

PPy/VCNTsc-CFs

/

/

Wastewater

1877

15

(Zhao et al., 2019)

CPH/AQS-GF

ECR

16

Effluent of MFCs

1920

-

(Tang and Ng, 2017)

PEDOT-GF

CPM

16

1620

3.5

(Kang et al., 2016)

ECPM CPM

7.06 3.4

Organic wastewater PBS + lactic acid PBS + NaAc

Anaerobic sludge

PEDOT-CC PEDOT-CNFs

SC DC (cathode GF, 16cm2) DC (cathode CC, 16cm2) DC Half-cell

S. loihica PV-4 G. sulfurreducens

140 -

0.018 10.66

PTh-GF

CPM

3.5

SC

PBS + C6H12O6

S. putrefaciens

800

1.9

(Liu et al., 2015) (Guzman et al., 2017) (Sumisha and Haribabu, 2018) (Du et al., 2017) (Jiang et al., 2017) (Zeng et al., 2018b)

Simulated wastewater PBS + NaAc PBS + NaAc

PDA-SSM CPM 16 SC PBS + NaAc Domestic sewage 803 PDA-MGCF SPM 4 DC (cathode CF) M9 + lactic acid S. putrefaciens 1735 10.6 PDA-CCFd CPM 4 SC PBS + NaAc Effluent of MFCs 931 10.5 a, TA: Tartaric acid; b, NF: Nano-flower; c, VCNTs: Vertical carbon nanotubes; d, CCF: Carbonized cotton fiber; e, LL: Landfill leachate.

(Sonawane et al., 2018a) (Sonawane et al., 2018b)

Table 4. A comparison of the performance by Gr-based modified electrodes in MFCs The Performance Size (projected Anode

aera, cm2)

Reactor configuration

Substrate

Source of inoculation

Imax

Pmax -2

(mW⋅m ) a

CGP -CC

2

Reference

-2

(A⋅m )

4.5

DC (cathode CC, 4.5cm )

PBS + NaAc

Anaerobic sludge

3600

2.89

(Xiao et al., 2012)

7

SC

PBS + NaAc

Anaerobic sludge

670

5.89

(Tang et al., 2015a)

16

DC (cathode)

PBSS + sodium lactate

S. oneidensis MR-1

2520

-

(Yang et al., 2016)

rGO/MnO2-CF

7.1

DC (cathode CB)

PBS + NaAc

Effluent of MFCs

2065

3.75

(Zhang et al., 2016)

GO/PANI-GP

2

DC (cathode GF, 6cm)2)

M9 + sodium lactate

S. oneidensis MR-1

381

0.8

(Sun et al., 2017)

rGO/PANI-CC

4.9

DC (cathode CFB)

PBS + NaAc

Domestic wastewater

306

~1.0

(Lin et al., 2019)

DGMBEe-CF

30

DC (biocathode 30cm2)

C6H12O6+NaHCO3

Activated sludge

112.4

4.3

(Chen et al., 2017)

Gr-CP

6.75

DC (cathode CP, 6.75cm2)

Simulated wastewater

Anaerobic sludge

2381

6.0

(Yu et al., 2018)

rGO/Au-CB

-

DC

PBS + NaAc

Anaerobic sludge

33.7 W⋅m-3

69.4 A⋅m-3

(Cheng et al., 2018)

36

SC

Simulated wastewater

Anaerobic sludge

280.6

1.1

(Paul et al., 2018)

b

GL -GP c

GOA -GFB

d

f

GOZ -SSM g

2

GRF -GP

2

DC (cathode GF, 6cm )

M9 + sodium lactate

S. oneidensis MR-1

257

0.8

(Wang et al., 2017b)

ELh-RGO/CFPi

4

DC (cathode CFP, 4cm2)

PBS+HNQ+ C6H12O6

Escherichia coli

1158

2.55

(Zhou et al., 2019)

a, CGP: Crumpled graphene particles; b, GL: Graphite layer; c, GOA Graphene oxide aerogel; d, GFB: Graphite fiber brush; e, DGMBE: Dual graphene modified bioelectrode; f, GOZ: Graphene oxide zeolite; g, GRF: Graphene riboflavin; h, EL: Eucalyptus leaves; i, CFP: Carbon fiber paper.

Table 5 A comprehensive comparison of different anode modification strategies Modified strategies 3D natural resource

Fabrication methods
Carbonization

Metal/metal oxides-based


Hydrothermal synthesis,
Electrodeposition,
Co-precipitation reaction

Conductive polymers


Electrodeposition,
Chemical polymerization
Electrochemical polymerization
Thermal reduction
Pyrolysis

Nitrogen-enriched

CNTs-based

Graphene-based


Chemical vapor deposition
Layer-by-layer assembly
Liquid-liquid interface transfer
Solution casting
Layer-by-layer assembly

Advantages
Facile synthesis, low economic cost
Good biocompatibility
Unique 3D macroporous scaffold to facilitate substrate diffusion in anode inside
Sustainable, no or less secondary pollution
High catalytic activity
Low electrical resistance
High-powered energy storage
Easy to elaborately design surface morphology of electrode
Environmental friendliness
High energy density, large specific surface area
Facile synthesis and low economic cost
Good biocompatibility, chemical stability and scalability
Unique electronic properties
Good biocompatibility
Beneficial to EAB attachment and reproduction
Accelerating cells metabolism and ETR
High elastic modulus, tensile strength
Superior electrical resistance and catalytic activity
Excellent energy density, large specific surface area
Improving hydrophilicity of electrode surface

Disadvantages
Lower mechanical strength
Relatively high electrical resistance


Superior electrical properties, mechanical strength
Ultrahigh specific surface area
Good biocompatibility and ETR


Relatively high economic cost
Extremely sharp edges of Gr nanowalls can cause cell membrane damage


Poor anti-corrosion and stability
Redundant synthesis procedure


Risk of shedding of polymers
Electric conductivity affecting by physical properties


Relatively poor structural stability


Certain biotoxication
Relatively high economic cost

Fig. 1 A bibliometric study on BES in Web of Science with the phrases “microbial fuel cell”, “microbial electrolysis cell”, “microbial electrosynthesis”, “microbial desalination cell” (a); Venn diagram statistics (b) disclosing different research field based on microbial fuel cell in recent 5 years. The number indicates published journal articles, e.g., 1205 indicates the number of papers according to search “microbial fuel cell” And power *generation Not wastewater treatment, 495 is the number of papers according to search “microbial fuel cell” And power *generation And anode performance Not wastewater treatment. Date:10/12/2019.

Fig. 2 (a) Bioelectricity generation from different sources waste in MFCs; (b) schematic of diagram of the dual-chamber MFCs and (c) the EET mechanisms by electrochemical active bacteria including DET and MET (i) DET by nanowires and/or conductive pili, (ii) DET by outer membrane cytochrome, (iii) MET by externally added mediators and self-secrete mediators.

Fig. 3 SEM images of biomass electrodes derived from natural resource: (a) overview image of a piece of cleaved kenaf; (b) transverse section from position b; (c) longitudinal section from position c (Chen et al., 2012a); (d) the 3D sponge-like pomelo peel electrode, (the insert is photograph of matured peeled pomelo); (e) high-resolution image of porous structure pomelo peel electrode (Chen et al., 2012c); (f) the single layered corrugated carbon structure electrode and (g) the different magnifications of a LCC biofilm electrode (Chen et al., 2012b).

Fig. 4 SEM images of 3D ordered honeycomb shaped porous carbon structure (a); (b) TiO2 nanoparticles modified electrode (Liu et al., 2016); (c) SSM - TiO2; (d) the enlarged image shown in (c) (Ying et al., 2018); (e) the morphology of NiO nanoparticles embedded between graphene layers (Wu et al., 2018a);(f) the petaloid shaped NiO@PANI-CF composite electrode (Zhong et al., 2018); (g) the Ni0.1Mn0.9O1.45 product and (h) TEM image of Ni0.1Mn0.9O1.45 product (Zeng et al., 2018a).

Fig. 5 The photograph of stainless steel wool anodes a) plain SSW, b) PANI/SSW, and c) PPy/SSW (Sonawane et al., 2018b); d) PANI nanowires doped with tartaric acid (Liao et al., 2015); e), f) TEM images of PPy-coated S. oneidensis MR-1 (Song et al., 2017).

Fig. 6 The characteristic of microbes adhesion in anode equipped with different CNTs materials in MFCs system (the external circle represents profitably for microbes adhesion, and the inner circle represents adverse)

Highlights


Elucidating difference nature advanced materials for anode decoration in MFCs.




Analyzing the reason of anode electrode to enhance bioelectricity generation.




Investigation the synergistic effects between microbes and solid electrode.




Summarizing fabrication technique for high performance anode electrodes.




Proposing possible modification methods for next-generation anode electrodes.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: