Conductive polypyrrole hydrogels and carbon nanotubes composite as an anode for microbial fuel cells

Abstract

Conducting polymer hydrogels, a unique class of materials having the advantageous features of both hydrogels and organic conductors, possess excellent electrochemical properties due to their intrinsic porous structure. Herein, we report a facile and scalable method for synthesizing conductive polypyrrole hydrogels/carbon nanotubes (CPHs/CNTs) using phytic acid as both gelator and dopant, and this composite was used as an anode in a dual-chamber microbial fuel cell (MFC). The high electrocatalytic activity of this material significantly reduced the interfacial charge transfer resistance and facilitated the extracellular electron transfer on the anode surface. The three dimensional porous structure and hydrophilicity of this composite enhanced the biofilm formation on the anode surface. CPHs/CNTs anode increased the maximum power density from 871 ± 33 mW m⁻² to 1898 ± 46 mW m⁻² and exhibited high stability in the two-chambered MFC. These results demonstrated that the synthesis of the CPHs/CNTs composite offered an effective approach towards enhancing the power production in MFCs.

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1. Introduction

A microbial fuel cell (MFC) is a bioelectrochemical system that converts chemical energy in organic matter into electrical energy by the catalysis of microorganisms.^1,2 As a promising environmental biotechnology for wastewater treatment and renewable energy production, MFCs have drawn much attention in the past decade.^3,4 Great progress has been made in this field and this technology has demonstrated its potential for use in applications such as wastewater treatment, biochemical oxygen demand detection, energy production, chemical and fuel synthesis and bioremediation.^5–10 However, the practical applications of MFCs are still limited due to their relatively small power density. The extracellular electron transfer from bacteria to electrode, a key parameter that defines the theoretical limits of energy conversion, is currently a limiting factor in enhancing the power density of MFCs.^11,12 Therefore, great efforts should be made to facilitate the extracellular electron transfer in order to boost power generation by MFCs.

The anode material plays a key role in determining the power production of MFCs, because it not only influences the biofilm formation on the electrode surface, but also affects the extracellular electron transfer between the microorganisms and the electrode. To date, carbon materials, such as carbon paper, carbon cloth and graphite felt, are the most commonly used anodes in MFCs due to their excellent biocompatibility, high specific surface area, good conductivity and high stability in a microbial inoculum mixture.¹³ However, these carbon materials demonstrate very low electrocatalytic activity for the anode microbial reactions and the electron transfer efficiency needs to be improved.¹⁴ As a result, the development of an anode material with high electrocatalytic activity and excellent biocompatibility is an effective approach towards accelerating the electron transfer and enhancing power production in MFCs. Conducting polymers have attracted great interest recently.¹⁴ Some conducting polymers were prepared for application in MFCs for increasing the power density due to their high electrocatalytic activity, conductivity and chemical stability.^15–17 Carbon nanotubes were also coated on carbon cloth, carbon paper and sponge to improve the power production in MFCs because of their high specific surface area and excellent conductivity.^18–20 Moreover, anode materials with porous structures allowed internal colonization, which enhanced the interaction between electrode and biofilms and consequently accelerated the electron transfer.²¹ However, these materials were typically physically adsorbed on the electrode surfaces through weak interactions, which meant that they could easily be desorbed under normal experimental conditions, or they were coated on the electrode surfaces by binders such as Nafion and polytetrafluoroethylene, which were prohibitively expensive.^14,16,20

As a subclass of conducting polymers, conducting polymer hydrogels have the advantageous features of both hydrogels and organic conductors and possess excellent electrochemical properties due to their intrinsic porous structure and high conductivity.²² Conducting polymer hydrogels demonstrated excellent electrochemical activity and superior electrode performance in different electrochemical devices.^23,24 When CNTs were embedded into conducting polymer hydrogels as conductive backbones, the composite possessed higher conductivity and electrochemical activity as an anode in a lithium ion battery.²⁵ However, CNTs-doped conducting polymer hydrogels without any binders for enhanced electrocatalytic activity in MFCs have so far not been reported.

In this study, we use a facile and scalable method to synthesize a conductive polypyrrole hydrogels and carbon nanotubes composite (CPHs/CNTs), which could be directly coated on graphite felt without any binders. This composite was used as an anode in two-chambered MFCs and its electrocatalytic activity and biocompatibility were investigated.

2. Materials and methods

2.1. Preparation of the CPHs/CNTs

Phytic acid (an abundant natural product from plants) was used as both a gelator and dopant to synthesize the CPHs/CNTs composite. The CNTs were pretreated by sonication in a 1 : 3 mixture of 70% nitric acid and 97% sulfuric acid for 4 h, followed by precipitation and rinsing with water.¹⁴ Graphite felt was pretreated by immersing in 1 M NaOH for 12 h and then 1 M H₂SO₄ for 12 h, followed by rinsing with deionized (DI) water.

The composite was synthesized by a liquid phase reaction in a solution-based method modified from a previously described approach.²⁵ The preparation mechanism and chemical structure of the polypyrrole hydrogel are illustrated in Fig. 1. In particular, the synthesis process of CPHs/CNTs was carried out via the following steps. First, 0.084 mL pyrrole monomer (98%, Sigma-Aldrich) and 0.184 mL phytic acid solution (50%, wt% in water, Sigma-Aldrich) were mixed with 2 mL isopropanol alcohol, taken as solution A. Solution B was prepared by dissolving 0.274 g ammonium persulfate (98%, Sigma-Aldrich) in 2 mL DI water with 5 mg dispersed CNTs. Then, these two solutions were thoroughly mixed and sonicated for 5 min to form a CPHs/CNTs liquid product. Finally, the CPHs/CNTs liquid product was coated onto a graphite felt to obtain the CPHs/CNTs anode. The anode was dried in a vacuum oven and then immersed in DI water and isopropanol alcohol for 12 h to thoroughly remove excess phytic acid and inorganic salts from this anode surface.

Fig. 1 The preparation mechanism and chemical structure of the polypyrrole hydrogel, wherein phytic acid served as a dopant and a crosslinker.

2.2. Characterization of the CPHs/CNTs

When the CPHs/CNTs anode was dried in air, its morphology was examined by field emission scanning electron microscopy (FESEM) (JSM-6701F, JEOL). Fourier transform infrared spectroscopy (FT-IR) was used to confirm the chemical structure of the CPHs/CNTs.

The electrochemical properties of the CPHs/CNTs anode were analyzed by cyclic voltammetry (CV) in a conventional three-electrode system (CHI660D, Chenhua Instrument Co., China). The CPHs/CNTs anode served as the working electrode, whereas a Pt foil electrode was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The CV test was performed in 0.5 mM K₃[Fe(CN)₆] with 0.2 M KCl in MilliQ water, at a scanning rate of 50 mV s⁻¹ and in a range from −0.2 V to 0.7 V (vs. SCE) at room temperature.

Electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range from 0.1 Hz to 100 kHz at the working potential with a perturbation signal of 10 mV when the MFCs were operated with an external resistance of 200 Ω. The MFC anode served as the working electrode, whereas the cathode was the counter electrode and an SCE placed in the anode chamber was used as the reference electrode.

2.3. MFC setup and operation

Two-chambered MFCs (Fig. 2), separated by proton exchange membranes (Nafion, 117, Dupont, 4 cm × 4 cm), were constructed as described in our previous report.²⁶ The membrane was sequentially boiled in H₂O₂ (30%), DI water, 0.5 M H₂SO₄, and DI water (each time for 1 h). Each chamber (4 cm × 2 cm × 4 cm) had a total volume of about 32 mL. CPHs/CNTs and bare graphite felt (4 cm × 0.3 cm × 4 cm) were placed in the anode chamber while graphite felt (4 cm × 0.3 cm × 4 cm) coated by Pt/C was used for the cathodes. Titanium wire was used to connect the anodes and cathodes.

Fig. 2 Image of the two-chambered MFC reactor.

Cathodes used for MFCs were prepared by coating Pt/C catalysts onto the graphite felt surface (0.5 mg Pt per cm²) using 5% Nafion solution as the binder.²⁷ In brief, catalysts were firstly mixed and dispersed well in the Nafion and ethanol solution by ultrasonication. Then, the dispersion was coated onto the graphite felt followed by drying at room temperature.

MFCs were inoculated using a mixed bacterial culture from another MFC, which originally started up with a primary clarifier overflow from Gaobeidian Wastewater Treatment Plant in Beijing and has been running for over one year. A medium containing 10 mM sodium acetate, 50 mM phosphate buffer solution (Na₂HPO₄, 4.09 g L⁻¹ and NaH₂PO₄·H₂O, 2.93 g L⁻¹), NH₄Cl (0.31 g L⁻¹), KCl (0.13 g L⁻¹), metal salt (12.5 mL L⁻¹) and vitamin (5 mL L⁻¹) solutions was used as an anolyte to feed the MFCs.²⁸ The catholyte was composed of 50 mM phosphate buffer solution, NH₄Cl (0.31 g L⁻¹) and KCl (0.13 g L⁻¹).

The feeding solution had been sparged with high purity nitrogen gas for 30 min to remove oxygen, which is harmful to electrochemically active bacteria in MFCs. The anode chamber was maintained under anaerobic conditions, while the cathode chamber was purged with sterile air (10 mL min⁻¹). MFCs were operated in a temperature-controlled incubator at 30 °C in a fed batch mode and the feeding solutions were refreshed when the voltage dropped below 30 mV.

Cell voltages (U) across an external resistance of 200 Ω were measured and recorded using a data acquisition system (AD8201H, Ribohua Co., Ltd). Polarization and power density curves were obtained by varying the external resistance (R) from 10 Ω to 1000 Ω. Polarization was tested when the MFCs became stable and reproducible in power production. For each test, it took about 10 min to reach a stable voltage output. Current (I) was calculated by I = U/R and power density (P) was normalized to the projected area of the anode surface, calculated as P = U × I/S, where S was the projected area of the anode surface.

Biomass on the electrodes was quantified by measuring bacterial protein.²⁹ At the end of the test, the anodes were removed from the MFCs and the washed biomass was treated in 1 M NaOH for 10 min at 100 °C to solubilize the attached proteins. Then, the protein was measured using the Bradford protein assay at 595 nm. All the experiments were carried out in duplicate and the average values with standard deviations were obtained.

3. Results and discussion

3.1. FT-IR and morphology characterization of the CPHs/CNTs composite

The chemical structure of the as-synthesized CPHs/CNTs was analyzed by FT-IR spectroscopy (Fig. 3). The absorption peak at 1552 cm⁻¹ was due to the in-ring stretching of C=C bonds in the pyrrole rings. The peaks at 1307 cm⁻¹ and 1045 cm⁻¹ were assigned to the C–H in-plane vibration and the peak at 1189 cm⁻¹ was attributed to the C–N stretching vibration. These characteristic peaks confirmed that polypyrrole hydrogels were formed by the chemical oxidation method.^23,30

Fig. 3 FT-IR spectrum of the CPHs/CNTs composite with labeled peaks.

SEM images of the as-synthesized CPHs/CNTs shown in Fig. 4 demonstrated that the composite had a three-dimensional porous nanostructure with CNTs as the fortifier. The nanostructured polypyrrole hydrogels provided nanoscale and micrometer sized pores, which significantly enhanced the interface between the electrochemically active microorganisms and the electrodes. The highly conductive CNTs were embedded and intertwined within the hydrogel bulk, which effectively improved the electronic conductivity of the composite.

Fig. 4 SEM images of the CPHs/CNTs composite (a and b), bare graphite felt (c), graphite felt coated by CPHs/CNTs (d), biofilm formation on bare graphite felt (e) and graphite felt coated by CPHs/CNTs (f) after MFC operation.

The key for the synthesis of the CPHs/CNTs composite was phytic acid, which served as both the dopant and the gelator in the formation of the porous structure.²⁴ Phytic acid crosslinked the polypyrrole by protonating the nitrogens on the polypyrrole chains. Each phytic acid molecule could interact with more than one polypyrrole chain and consequently a highly crosslinked structure was formed. The polypyrrole hydrogels were hydrophilic because there was an excess of phosphorous groups in the phytic acid, which would enhance the biofilm formation because it was demonstrated that hydrophilic surface was beneficial for the electroactive biofilm formation.³¹ Therefore, the three dimensional porous structure and the hydrophilicity of this CPHs/CNTs composite was expected to exhibit excellent biocompatibility.

3.2. Electrochemical analysis of the CPHs/CNTs composite

Conductivity was a key factor for the anode material in the MFCs. The bare graphite felt had a high conductivity of 138 S m⁻¹ and the graphite felt coated by CPHs/CNTs had a conductivity of 130 S m⁻¹. This result indicated that CPHs/CNTs composite possessed excellent conductivity. The electron transfer ability of the CPHs/CNTs surface was investigated by CV using K₃[Fe(CN)₆] as a redox probe. CV in ferrocyanide presented features of a typical quasi-reversible electron transfer process (Fig. 5). The peak current of the CPHs/CNTs (3.29 ± 0.16 mA cm⁻²) was almost three times as high as the peak current of the bare graphite felt (1.12 ± 0.08 mA cm⁻²). The peak potential separation of the CPHs/CNTs was about 159 mV, while a much larger peak potential separation of about 278 mV was recorded for the bare graphite felt. The higher peak current and the smaller peak potential separation indicated a smaller resistance of the chemical reaction on the CPHs/CNTs surface. These results demonstrated that CPHs/CNTs significantly enhanced the electron transfer on the electrode surface.

Fig. 5 CVs of CPHs/CNTs and bare graphite felt in 0.5 mM K₃[Fe(CN)₆] with 0.2 M KCl at a scanning rate of 50 mV s⁻¹.

EIS of the electrode provided key information that allowed the analysis of the electrochemical reactions on the electrode surface and the bacterial metabolism in the MFCs. In this study, EIS experiments were conducted on the anodes to investigate the anode resistance of MFCs under working cell conditions. The Nyquist plots were recorded when the steady-state current was generated (Fig. 6). The plots showed semicircles over the high-frequency range followed by straight lines in the lower-frequency range. The solution resistance, resulting from the ionic resistance of the electrolyte, was found at the high frequency, wherein the plot intersected the Z′ axis. The diameter of the semicircle represented the charge transfer resistance, originating from the resistance of the electrochemical reactions on the electrode surface, while the straight line stood for the diffusion resistance.

Fig. 6 EIS (Nyquist plots) of the CPHs/CNTs anode and the bare graphite felt anode at working potential with a perturbation signal of 10 mV in the MFCs.

The measured solution resistance and the diffusion resistance of both the CPHs/CNTs and the bare graphite felt were very similar. However, the charge transfer resistance of the two materials was quite different. The charge transfer resistance of CPHs/CNTs was about 5.3 Ω, while the value was 19.2 Ω for the bare graphite felt. The much smaller charge transfer resistance of the CPHs/CNTs indicated that the resistance of the electrochemical reactions was reduced and extracellular electron transfer was facilitated on the anode surface. This result demonstrated that CPHs/CNTs had high electrocatalytic activity and greatly enhanced the microbial electron transfer in the MFC.

3.3. CPHs/CNTs anode performance in MFCs

To investigate the CPHs/CNTs composite with respect to microbial electricity production, it was used as an anode in a two-chambered MFC. Bare graphite felt was also studied in an MFC for the control experiment. Direct anodic degradation of acetate was a form of respiration, wherein the terminal electron acceptor was the electrode. The oxidation of acetate availed itself of the tricarboxylic acid (TCA) cycle and the membrane-bound electron transfer chain. Acetate entered the TCA cycle through acetyl-CoA to generate NADH, NADPH and FADH₂, which were the starting points of the electron transfer chain, with electrons travelling through flavoproteins, iron–sulfur proteins, a quinone pool and a series of cytochromes.^32,33 When the power generation was stable and reproducible with an external resistance of 200 Ω, a current density of about 0.16 mA cm⁻² in the MFC with the CPHs/CNTs anode and a current density of about 0.12 mA cm⁻² in the MFC with the bare anode were obtained. The higher current density in this study demonstrated that the CPHs/CNTs anode enhanced the current production in the MFC.

Polarization and power density curves (Fig. 7) were obtained by varying the external resistance. The polarization curve indicated that the CPHs/CNTs anode reduced the internal resistance of the MFC from 48 Ω to 30 Ω, which was consistent with the EIS study. The MFCs with the CPHs/CNTs anode had a maximum power density of 1898 ± 46 mW m⁻², while the MFCs with bare graphite felt only exhibited a maximum power density of 871 ± 33 mW m⁻². The considerably enhanced power density demonstrated that the CPHs/CNTs material was an ideal anode to facilitate electron transfer from bacteria to electrode in MFCs.

Fig. 7 Polarization and power density curves of the MFCs with the CPHs/CNTs anode and the bare graphite felt anode.

Anode material was of great significance to the power production of MFCs because the anode surface properties influenced the electroactive biofilm formation. The biomass formed on the anode surface was measured using a protein assay, which indicated that the protein density on the CPHs/CNTs was about 376 μg cm⁻², therefore higher than the value of 324 μg cm⁻² on bare graphite felt. The enhanced biofilm formation was due to the CPHs/CNTs composite, which exhibited three-dimensional porous structures and hydrophilicity. The porous structures allowed internal colonization to enhance the interaction between the electrode and the biofilm, while the hydrophilicity was conducive to electroactive biofilm attachment.^21,31 As a result, the CPHs/CNTs enhanced the biocompatibility and helped to improve the power production of MFCs.

The anode not only influenced the electroactive biofilm formation, but also affected the extracellular electron transfer from bacteria to anode. The CPHs/CNTs showed high electrocatalytic activity in the CV and EIS studies. The high electrocatalytic activity reduced the resistance of the chemical reactions on the anode surface and facilitated the extracellular electron transfer. Therefore, CPHs/CNTs with high electrocatalytic activity enhanced the microbial electron transfer and contributed to greater power generation in the MFCs.

The stability of the CPHs/CNTs anode was tested to investigate its suitability for long term operation. The CPHs/CNTs anode was removed from the MFC reactor and autoclaved after a period of three months. Then, the MFC was reconstructed and operated following the procedures above. Polarization and power density curves of the reconstructed MFC were obtained when the power generation was reproducible and sustainable (Fig. 7). The maximum power density of the reconstructed MFC with the CPHs/CNTs anode reached 1733 ± 46 mW m⁻², which represented only an 8.7% decrease. This result indicated that the CPHs/CNTs had high stability and could be used for long term operation in MFCs.

Anode modification was an efficient method to enhance the performance of MFCs. Table 1 summarized some modification methods used in MFC studies. Heat treatment improved the electrochemically active surface area. However, the power density only increased by 3% (ref. 34). Quinone covalently grafted on the anode surface enhanced the electron transfer and consequently the power density, but the operation was complex and the quinine was expensive.²⁶ A CNT/polyaniline coating and polypyrrole coated CNTs required binders such as Nafion and polytetrafluoroethylene, which were prohibitively expensive.^14,30

Table 1 Anode modification methods in MFCs

Modification method	MFC performance	Reference
Heat treatment of carbon mesh	Increased power density from 893 mW m^−2 to 922 mW m^−2	34
Quinone grafted on graphite felt	Increased power density from 967 mW m^−2 to 1872 mW m^−2	26
CNT/polyaniline coating on nickel foam	The power density reached 42 mW m^−2	14
Polypyrrole coated CNTs	The maximum power density was 228 mW m^−2	30
CPHs/CNTs coating on graphite felt	Enhanced power density from 871 mW m^−2 to 1898 mW m^−2	This study

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The CPHs/CNTs composite met the requirements of an ideal anode for MFCs: high electrocatalytic activity, excellent biocompatibility, high chemical stability and high conductivity. Moreover, the synthesis method was simple and scalable. More importantly, the CPHs/CNTs composite had strong adhesion on carbon materials without the addition of any binders. Because of these advantages, the CPHs/CNTs composite held promising potential for application in MFCs to enhance power generation.

4. Conclusion

A facile and scalable approach was used to synthesize conductive polypyrrole hydrogels/carbon nanotubes (CPHs/CNTs) that synergized the advantageous features of hydrogels and organic conductors. CV and EIS studies demonstrated that this material exhibited high electrocatalytic activity, which accelerated the extracellular electron transfer on the anode surface. The porous structure and hydrophilicity of CPHs/CNTs enhanced biofilm formation on the anode surface. The CPHs/CNTs anode increased the maximum power density greatly and exhibited high stability in a two-chambered MFC. These results demonstrated that the synthesis of the CPHs/CNTs composite provided an effective approach towards enhancing the power production in MFCs.

Acknowledgements

This study was supported by a grant from the Environment & Water and Industry Development Council, Singapore (MEWR 651/06/159) and a grant from the Bill & Melinda Gates Foundation (OPP109475). We also acknowledge the financial support from the National Natural Science Foundation of China (no. 21176242 & no. 21176026). Tang Xinhua thanks NUS Graduate School for Integrative Sciences and Engineering for a research scholarship support.

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