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β-MnO2 nanocomposites as cathode electrocatalyst for oxygen reduction reaction in microbial fuel cells
Accepted Manuscript Polyaniline/β-MnO2 nanocomposites as cathode electrocatalyst for oxygen reduction reaction in microbial fuel cells Xinxing Zhou, Yunzhi Xu, Xiaojie Mei, Ningjie Du, Rongmao Jv, Zhaoxia Hu, Shouwen Chen PII:
S0045-6535(18)30066-3
DOI:
10.1016/j.chemosphere.2018.01.058
Reference:
CHEM 20639
To appear in:
ECSN
Received Date: 19 September 2017 Revised Date:
12 January 2018
Accepted Date: 13 January 2018
Please cite this article as: Zhou, X., Xu, Y., Mei, X., Du, N., Jv, R., Hu, Z., Chen, S., Polyaniline/βMnO2 nanocomposites as cathode electrocatalyst for oxygen reduction reaction in microbial fuel cells, Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.01.058. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT Polyaniline/β-MnO2 nanocomposites as cathode electrocatalyst for oxygen
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reduction reaction in microbial fuel cells
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Xinxing Zhou, Yunzhi Xu, Xiaojie Mei, Ningjie Du, Rongmao Jv, Zhaoxia Hu*,
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Shouwen Chen*
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Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School
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of environmental and biological engineering, Nanjing University of Science and
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Technology, Nanjing 210018, China
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*Corresponding author information:
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Tel.: +86-25-84315532;
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Fax: +86-25-84315518.
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E-mail address: [email protected]; [email protected]
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Abstract An efficient and inexpensive catalyst for oxygen reduction reaction (ORR),
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polyaniline (PANI) and β-MnO2 nanocomposites (PANI/β-MnO2), was developed for
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air-cathode microbial fuel cells (MFCs). The PANI/β-MnO2, β-MnO2, PANI and
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β-MnO2 mixture modified graphite felt electrodes were fabricated as air-cathodes in
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double-chambered MFCs and their cell performances were compared. At a dosage of
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6 mg cm-2, the maximum power densities of MFCs with PANI/β-MnO2, β-MnO2,
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PANI and β-MnO2 mixture cathodes reached 248, 183 and 204 mW m-2, respectively,
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while the cathode resistances were 38.4, 45.5 and 42.3 Ω, respectively, according to
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impedance analysis. Weak interaction existed between the rod-like β-MnO2 and
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surficial growth granular PANI, this together with the larger specific surface area and
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PANI electric conducting nature enhanced the electrochemical activity for ORR and
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improved the power generation. The PANI/β-MnO2 nanocomposites are a promising
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cathode catalyst for practical application of MFCs.
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Keywords: Cathode catalyst; PANI/β-MnO2 nanocomposites; Oxygen reduction
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reaction; Microbial fuel cell
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1. Introduction
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Microbial fuel cell (MFC) is a green energy technology that converts chemical
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energy harvested in organic compounds whether pollutant or not into electrical energy
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utilizing the power of microorganisms. It has the characteristics of wide range of raw
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materials and good biocompatibility (Cheng et al., 2006; Anu Prathap et al., 2013).
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Oxygen has superiority as electron acceptor for MFCs due to its high oxidation 2
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sustainability (Dong et al., 2012b). However, the sluggish kinetics of the oxygen
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reduction reaction (ORR) in a near neutral medium seriously limits the power density
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production of MFCs (Wen et al., 2012). To solve this problem, platinum (Pt) and
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Pt-based materials are widely used as catalyst of ORR in MFC cathodes. Nevertheless,
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noble metal platinum is expensive, unrenewable, and sensitive to catalyst poisoning
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which attenuates the performance of MFCs and limits its large-scale application.
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Because of this, it is necessary to pour a great deal of efforts on developing alternative
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catalysts which are easy to produce, low consumption, and have the similar or better
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electrocatalytic activity than Pt for ORR.
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Manganese dioxide (MnO2) materials with different crystal structure (α-MnO2,
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β-MnO2, and γ-MnO2) have been investigated extensively as the cathode catalysts for
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ORR in MFCs due to its rich resources, low cost, environment friendliness, and good
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catalytic activity. Zhang et al. (Zhang et al., 2009) compared the three crystal types (α,
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β and γ) of MnO2 as cathode catalyst in MFCs, they found that β-MnO2 showed the
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best effective ORR catalytic activity owing to its highest BET surface area and
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average oxidation state of manganese (AOS: 3.59), which was higher than those of
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α-MnO2 (3.55) and γ-MnO2 (3.48), since AOS was an indicator of the manganese
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state in the structures of manganese dioxides and the higher AOS stands for the higher
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oxidative activity (Zhang et al., 2009). Therefore, β-MnO2 is chosen as the object of
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this study. However, the poor electronic conductivity of MnO2 remains a major
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challenge and limits the rate capability for MFC power performance. To address this
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such as polypyrrole (PPy) (He et al., 2007; Tran et al., 2007), polythiophene (PTh) (Li
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et al., 2009) or polyaniline (PANI) (Li et al., 2008a; Li et al., 2008b) materials. These
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polymers have attracted more and more attention because of their low cost,
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convenient synthesis, strong energy-storage capacity and high electrical conductivity.
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Among the various electroconductive polymers studied to date, PANI has been
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considered as the most attractive polymer due to its excellent conductivity and
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environmental stability (Wang et al., 2017). More importantly, PANI also has
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electrocatalytic activity for ORR (Khomenko et al., 2005). In previous reports,
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polyaniline/inorganic composites have been proved to have better electrical
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conductivity (Yuan et al., 2008; Lai et al., 2011). The PANI-MnO2 composites have
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been reported in the application of supercapacitors (Jiang et al., 2012; Chen et al.,
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2013). However, the reports of these kinds of PANI-MnO2 especially β-MnO2
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composites function as air-cathode catalyst in application of MFCs are only few.
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In this paper, we successfully loaded PANI on the surface of β-MnO2 nanorods
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by in situ chemical oxidative polymerization method and investigated the possibility
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of PANI/β-MnO2 nanocomposites as cathode catalyst for ORR in an MFC. The
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performances of MFCs were compared with three kinds of materials (β-MnO2,
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PANI+β-MnO2 mixture and PANI/β-MnO2 nanocomposites) used as cathode catalysts
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in MFC. Furthermore, the cathode with different loading amount of PANI/β-MnO2
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nanocomposites as catalyst were also investigated.
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2. Experimental
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2.1 Materials Aniline was purchased from Shanghai Bai Lingwei Chemical Technology Co.,
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Ltd. Potassium permanganate (KMnO4) was obtained from Sinopharm Chemical
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Reagent Co., Ltd. Aniline sulfate was purchased from Aladdin Industrial Corporation.
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The graphite felt with a thickness of 5 mm was bought from Zibo Jinpeng Carbon Co.,
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Ltd. Cation exchange membrane (CEM) was purchased from Hangzhou El
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Environmental Protection Technology Co., Ltd. (IEC = 2.4 mmol g-1). All other
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reagents used in this work were of analytical grade and were used without further
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purification. Deionized water was produced by an EPED pure water system and used
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in all solution.
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2.2 Synthesis of PANI
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To a three-necked flask, 100 mL of hydrochloric acid solution (1 M) and aniline
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(0.05 mol, 4.65 g) were added sequentially. The solution was stirred and cooled to 0-5
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o
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solution dropwise, and the transparent solution gradually turned into mazarine. After
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the continuous stirring for 3 h, the solution was kept still overnight. The precipitate
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was collected by filtration, washed with deionized water and dried under vacuum
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oven at 65 oC and ground into powder (Ma et al., 2007).
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2.3 Synthesis of β-MnO2 nanorods
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C in an ice bath. Then ammonium peroxydisulfate (1 M, 50 mL) was added to the
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β-MnO2 was prepared by a previously reported method (Zhang et al., 2009). In a
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typical procedure, 1.2 g of KMnO4, 2.7 mL of ethanol and 51.3 mL of deionized water
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were added into a 100 mL beaker. After the dissolution of KMnO4, the solution was 5
ACCEPTED MANUSCRIPT transferred into a Teflon-lined stainless steel autoclave (300 mL) and heated at 125 oC
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for 24 h. The obtained precipitate was collected by filtration, washed with deionized
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water and ethanol several times, and dried at 100 oC in a vacuum oven. Finally, the
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precipitates were annealed at 300 oC in air for 5 h at a ramping rate of 3.5 oC min-1.
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2.4 Synthesis of PANI/β-MnO2 nanocomposites
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In a 500 mL beaker, aniline sulfate (0.143 g, 0.5 mmol) was dissolved in 100 mL
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of sulfuric acid, and the solution was cooled to 0-5 oC in an ice bath. Under vigorous
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stirring, 0.279 g of as-prepared β-MnO2 was added and kept for 2 h. The obtained
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precipitate was collected by filtration, washed with deionized water and ethanol
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several times, respectively, then dried in a vacuum oven at 60 oC.
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2.5 Pretreatment of graphite felt and cation exchange membrane
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First, the graphite felt was cut into small piece with the size of 2 × 2 × 0.5 cm
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and put into a 250 mL beaker. Next, ethanol (150 mL) was added to immerse the
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graphite felt completely and ultrasonic for 10 min. Then the graphite felt was rinsed
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by deionized water and the operation was repeated until the deionized water was
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transparent. Finally, the graphite felt was dried in a vacuum oven at 60 oC for 24 h.
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The purified graphite felt was acidic oxidized in a sealed 300 mL Teflon-lined
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stainless steel autoclave containing 120 mL mixed acid solution (VH SO :VHNO3 = 3:1) at
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80 oC for 8 h (Zhang et al., 2013). The acidic oxidized graphite felt was washed with
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deionized water, and sonicated for 30 min in the deionized water until the pH of the
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rinsed water became neutral. Then they were dried in a vacuum oven at 100 oC for 5 h
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and saved in a sealed bag.
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thoroughly with deionized water and then immersed into deionized water at least 24 h
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before use.
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2.6 Electrode fabrication
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Graphite felt was used as the supporting carbon/electrode material, and several
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air-cathodes were prepared as parallel control. The air-electrode was fabricated by
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ultrasonic-load method. The as-prepared β-MnO2, PANI+β-MnO2 mixture or
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PANI/β-MnO2, the binder of polyvinylidene fluorine were mixed by a weight ratio of
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95:5 in 20 mL of N-methyl-2-pyrrolidone in a ultrasonic bath for 15 min. A piece of
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acid treated graphite felt (2 × 2 × 0.5 cm) was immersed into the solution and placed
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in the ultrasonic bath for another 30 min. Then the graphite felt was taken out and
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dried in a vacuum oven at 60 oC for 10 h to remove the solvent. The above step was
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repeated several times until gained the settled catalyst loading. The prepared
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air-cathodes were named as GF, GF-PANI+β-MnO2 and GF-PANI/β-MnO2. We also
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studied the effect of PANI/β-MnO2 loading amount on the performance of the MFC.
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The prepared air-electrodes were named as GF-PANI/β-MnO2-x, where x refers to the
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catalyst loading amount (mg cm-2).
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2.7 MFC configuration and operation
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The double-chamber air-cathode MFC was constructed with two inner
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cylindrical Plexiglas chambers (d × H = 8 × 5.5 cm, 280 mL), each chamber consists
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of two ports for feed inlet/outlet, gas inlet/outlet and electrode connection parts. They
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were separated by the CEM. The graphite felt-based electrodes were placed parallel to 7
ACCEPTED MANUSCRIPT the CEM facing each other with a distance of 3 cm each. The anode and cathode were
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connected by titanium wire as current collector with conductive paste. The anodic
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chamber was filled with medium containing Na2HPO4·12H2O (11.1 g L-1),
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NaH2PO4·2H2O (2.96 g L-1), NaAc·3H2O (2.72 g L-1), NH4Cl (0.31 g L-1),
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MgCl2·6H2O (0.21 g L-1), NaCl (1.0 g L-1), CaCl2 (0.02 g L-1), KCl (0.13 g L-1) and
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trace element (1 mL L-1), whereas the medium was sterilized at 121 oC for 15 min
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before use. The culture was inoculated from anaerobic digester sludge collected from
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Nanjing East sewage treatment plant. The catholyte was comprised of a phosphate
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buffer solution (PBS, pH=7), which consisted of Na2HPO4·12H2O (11.1 g L-1),
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NaH2PO4·2H2O (2.96 g L-1) and NaCl (1.0 g L-1). Air was continuously fed into the
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cathode chamber using an air pump at a rate of 90 mL min-1 to maintain a sustained
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concentration of oxygen during the MFC operation. All the reactors were controlled at
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30 oC in a water bath incubator. At the start-up period, an external resistor (1000 Ω)
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was connected in the circuit. The schematic figure of the MFC is shown in Fig. SM-1.
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2.8 Characterizations and measurements
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X-ray diffraction (XRD) was conducted on D8 Advance (Bruker) using Cu Kα
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radiation at 40 kV and with 2θ of 10°-70°. Scanning electron microscope (SEM) was
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performed on a Quant 250FEG (FEI) machine. Transmission electron microscopy
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(TEM) analysis was conducted on a TECNAI G2 20 LaB6 (FEI) electron microscope
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operated at 200 kV. Thermal gravimetric analysis (TGA) was carried out on a
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SDTA851E instrument (TA-instruments-waters LLC) at a heating rate of 10 oC min-1
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under flowing air from 50 to 800 oC. Fourier transform infrared (FT-IR) spectra was
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specific surface area and pore size distribution of the catalyst were analyzed using the
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Brunauer–Emmet–Teller (BET, ASAP-2020, Micromeritics, America) method by the
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adsorption and desorption of N2. X-ray photoelectron spectroscopy (XPS) analysis
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was conducted on a PHI Quantera II ESCA System with Al Ka radiation at 1486.8 V.
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2.9 Electrochemical analysis
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All electrochemical analysis of the MFC was conducted on an electrochemical
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work station (CHI 604E, China). Polarization curves and power density curves were
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obtained by varying the external resistance from 5000 to 82 Ω and the voltage was
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recorded until the value was stable for 15 min by an exquisite multimeter. Current
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density and power density were calculated by normalizing current and power to the
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anode surface area (8 cm2) according to the following equation (Karra et al., 2013),
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Power density =
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where V is the voltage across the external resistor, R is the external resistance, and A
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is the anode surface area.
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Cyclic voltammetry (CV) measurement was performed using a three-electrode
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system, where the prepared air-cathode, an Ag/AgCl electrode and a platinum wire
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were used as the working, reference and counter electrode, respectively. The CV
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measurement was recorded from -0.8 to 0.4 V at a scan rate of 50 mV s-1 in PBS (pH
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= 7) saturated by air. Tafel plots and the exchange current density (i0) were recorded
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by sweeping the overpotential from 0 to 100 mV at 10 mV s-1, calculated by using 9
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the following empirical equation (Huang et al., 2017a). lgi = lgi0 -
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βnFη
(3)
2.303RT
Electrochemical impedance spectroscopy (EIS) measurement was carried out at
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the open circuit potential (OCV) with an AC perturbation of 5 mV over a frequency
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range from 0.1 Hz to 10 kHz. The anode was used as the working electrode, while the
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cathode as the counter and reference electrode. The resistance data were fitted with
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Zsimpwin software (Echem), where the ohmic resistance (Rs) was obtained from the
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Nyquist impedance plots at the point where -Z” was equal to zero at high frequency,
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the charge-transfer resistance (Rct) was calculated from a semicircular fit of the
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charge-transfer impedance in the Nyquist plots (Yuan et al., 2015).
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3. Results and discussion
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3.1 Synthesis and characterization of the PANI/β-MnO2 composites
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Fig. 1 shows the synthetic diagram of the PANI/β-MnO2 nanocomposites. The
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nanocomposites were prepared by in-situ PANI polymerization in the presence of
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β-MnO2 nanorods.
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Fig. 2 shows XRD pattern of the PANI, β-MnO2 and PANI/β-MnO2
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nanocomposites. The XRD pattern of PANI showed two broad peaks at 2θ of 20.5°
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and 25.3°, which represented the periodicities parallel (100) and perpendicular (110)
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to the PANI chain, respectively (Fig. 2a) (Ryu et al., 2007; Zhu et al., 2007). As
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shown in Fig. 2b, all of the diffraction peaks for the as-prepared sample were indexed
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with the standard XRD pattern of β-MnO2 (JCPDS No. 24-0735, tetragonal symmetry
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with P42/mnm space group and lattice constants of a = 4.399 nm and c = 2.874 nm)
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PANI/β-MnO2 nanocomposites (Fig. 2c) and pristine β-MnO2 except the diffraction
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peaks at 2θ = 20.5° and 25.3° which resulted from the PANI on the β-MnO2 surface,
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confirming the presence of β-MnO2 in the nanocomposites after in-situ polymerization.
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No characteristic impurity peak was observed, indicating that high purity
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PANI/β-MnO2 nanocomposites were produced by the simple polymerization.
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Fig. 3 shows the SEM and TEM images of β-MnO2 nanorods and PANI/β-MnO2
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nanocomposites. As clearly observed in Fig. 3a and b, the as-prepared β-MnO2 shown
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1-D nanostructured crystals with smooth surface. TEM images in Fig. 3c and d further
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confirmed the formation of β-MnO2 nanorods with average length of 10-20 µm and
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average diameter of 200-300 nm. The length and diameter of the as-prepared β-MnO2
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in this paper were larger than those reported in the literature (Liu et al., 2017), but
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more orderly (Liu et al., 2005) and scattered (Zang et al., 2011). Compared with the
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β-MnO2 nanorods, the synthesized PANI/β-MnO2 nanocomposites in Fig. 3e and f
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exhibited shorter average length about 8-12 µm and had some granules and cracks
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(red dashed frame) on the surface. The reason of this phenomenon was that β-MnO2
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worked as oxidant and template during the aniline monomer polymerization to form
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PANI on the nanorods, and partial β-MnO2 was consumed (Yao et al., 2013). This
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would lead to an intimate interface, facilitating donor–acceptor interactions between
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PANI and β-MnO2 (Zhou et al., 2017). TEM images also proved that some small
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black granules grown on the nanorods (Fig. 3g and h). We composed that these
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granules were PANI nanogranules which were confirmed by FT-IR and XPS analysis.
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nanocomposites was two times more than that of β-MnO2 nanorods. Introducing the
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PANI granules and cracks increased the specific surface area and will bring about
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more active sites for ORR of the PANI/β-MnO2 nanocomposites.
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Fig. SM-2 shows the TGA curves of PANI, β-MnO2 and PANI/β-MnO2. PANI
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showed continuous weight loss till 660 oC, where the weight loss below 300 oC was
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due to the removal of physic-adsorbed water, interstitial water or dopant anions (Ni et
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al., 2010; Sen et al., 2013), and the sharp weight loss to 660 oC was ascribed to the
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large-scale thermal degradation of the PANI chains (Nirmalesh Naveen et al., 2015),
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while the small weight reserved at 800 oC was due to the final carbonization of
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intermediate chemicals (Bhadra et al., 2007). Only a small weight loss (3%) was
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observed for the β-MnO2 nanorods up to 800 oC. However, a small weight loss
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between 500 and 650 oC, which was probably owing to the reduction of manganese
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ions going from tetravalent to trivalent state accompanied with oxygen evolution
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(Zhang et al., 2007b). As observed in Fig. SM-2, weight loss of the PANI/β-MnO2
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nanocomposites majorly occurred up to 500 oC, which was attributed to the loss of
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adsorbed water and the decomposition of the PANI (Ni et al., 2010). Obviously,
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comparing with pure PANI, the thermal decomposition temperature of PANI in the
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PANI/β-MnO2 nanocomposites was lower. This can be explained by the fact that a
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coordination bond between manganese and nitrogen atoms in PANI weakened the
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interactive forces of the PANI inter-chains (Pham et al., 2009; Anu Prathap et al.,
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2013). The weight was stable at about 80% when the temperature was above 600 oC
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and the residues were mainly composed of β-MnO2. It is considered that the weight
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ratio of PANI and β-MnO2 in the prepared PANI/β-MnO2 was about 1:4. The structural information and chemical component of PANI/β-MnO2
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nanocomposites were also identified by the FT-IR spectroscopy, as shown in Fig.
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SM-3. Typical absorption bands at about 1110, 721, and 474 cm−1 attributed to the
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Mn-O vibrations of MnO6 octahedra in MnO2 (Wang et al., 2008) (Fig. SM-3a) and
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characteristic absorption peaks at 807 (C-C and C-H in the benzenoid ring), 1122 (the
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=N+–H -stretching), 1220 (the C–N+ stretching), 1294 (the C–N stretching of the
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secondary aromatic amine), 1485 (the aromatic C=C stretching of the benzenoid ring),
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and 1558 cm−1 (the aromatic C=C stretching of the quinonoid ring) (Fig. SM-3b)
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(Zhang et al., 2013; Yang et al., 2017) for PANI were clearly observed in the spectra
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of β-MnO2 and PANI, respectively. As for the nanocomposites, Fig. SM-3c shows all
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the characteristic absorption bands of PANI and β-MnO2. However, the characteristic
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peaks of Mn-O vibrations at 721 and 474 cm-1 shifted to 712 and 484 cm-1,
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respectively, reflecting a mutual interaction between PANI and β-MnO2 that, most
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likely, was a hydrogen bond formed between oxygen atom of Mn-O and N-H in PANI
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(Yao et al., 2013). In addition, our observation of both the difference in peak position
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and intensity between the PANI/β-MnO2 and the pure PANI (Fig. SM-3b), further
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indicating that there was a mutual interaction between PANI and β-MnO2, which
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verified PANI has been grown on the surface of β-MnO2 nanorods (Sun et al., 2015).
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Fig. SM-4 shows the nitrogen adsorption-desorption isotherm curves of the
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β-MnO2 nanorods and PANI/β-MnO2 nanocomposites, and the inset graph exhibits 13
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the pore size distribution for the two materials. Both materials showed type IV
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adsorption-desorption isotherm curves with a hysteresis loop, indicating a
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well-ordered mesoporous structure.
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volumes of β-MnO2 nanorods were 19.9 m2 g-1 and 0.023 cm3 g-1. The BET surface
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areas of β-MnO2 nanorods was larger than those of the commercial β-MnO2 powder
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(0.11 m2 g-1) (Zhang et al., 2006), pure phase of tetragonal MnO2 (12.4 m2 g-1) (Xu et
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al., 2016) and MnO2 nanorods ( 16.17 m2 g-1) (Zhang et al., 2007a). However, the
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as-prepared PANI/β-MnO2 nanocomposites possessed a larger BET surface areas
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(39.7 m2 g-1) and total pore volumes (0.042 cm3 g-1) than those of pure β-MnO2
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nanorods (19.9 m2 g-1) in this paper, PPy/β-MnO2 (10.3 m2 g-1) (Chen et al., 2017),
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np-RuO2/nr-MnO2 (31.9 m2 g-1) (Xu et al., 2016) and Ag/MnO2 rods (19.5 m2 g-1),
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tubes (25.6 m2 g-1), wires (34.7 m2 g-1) (Li et al., 2016). That was mainly ascribed to
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introducing PANI granules onto the β-MnO2 nanorods surface. The inset in the figure
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shows the pore size distribution curves calculated using the Barrett–Joyner–Halenda
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equation from the desorption branch of the isotherms (Chen et al., 2013). The BJH
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analysis exhibited that PANI/β-MnO2 nanocomposites had an intensive pore size
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distribution of ~3.87 nm, which was classified as a mesopore distribution, with an
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adsorption average pore diameter of 6.63 nm, that was smaller than that of β-MnO2
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nanorods (6.90 nm). The high surface area and high mesoporous nature of
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PANI/β-MnO2 nanocomposites may result in more exposure of catalyst active sites
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diffusing O2 and electrolyte, leading to enhancement during the ORR process.
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The measured BET surface areas and total pore
Fig. 4 displays the XPS spectra of PANI/β-MnO2 nanocomposites. Four elements 14
ACCEPTED MANUSCRIPT including C, N, O, and Mn were detected in Fig. 4a, demonstrating the presence of
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PANI and β-MnO2 in the nanocomposites. Furthermore, to examine the oxidation
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state of Mn in the PANI/β-MnO2 nanocomposites (Pan et al., 2016), XPS spectrum of
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Mn 2p is plotted in Fig. 4b. Two peaks located at 654.4 and 642.7 eV corresponded to
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Mn 2p1/2 and Mn 2p3/2, respectively, which was in agreement with the energy
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separation (11.7 eV) between Mn 2p1/2 and Mn 2p3/2 reported previously (Zhou et al.,
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2017). As reported previously, the Mn oxidation state in manganese oxides can be
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determined from the separation of peak energies (∆Eb) between the two peaks of the
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Mn 3s components (Luan et al., 2016). The as-prepared PANI/β-MnO2 showed a
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separation energy of 4.83 eV for the Mn 3s doublet (Fig. 4c), which indicated that the
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dominant Mn4+ ions existed in the nanocomposites and the chemical composition was
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β-MnO2. The existence of β-MnO2 can be further varied by O1s peaks, which was
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further deconvoluted into three major bond components in Fig. 4d (Asif et al., 2016).
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The three peaks centered at binding energy 530.21, 531.41, and 532.32 eV were
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corresponding to Mn-O-Mn bond for the tetravalent oxide, Mn-O-H bond for
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hydroxide and H-O-H bond for residual structural water, respectively (Li et al., 2011).
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3.2 Electrochemical activity of PANI/β-MnO2 for ORR
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The activity of various cathodes for ORR was investigated with both CV and
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Tafel curves in PBS (pH = 7). Fig. 5a shows the CV curves of four prepared
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air-cathodes, GF, GF-β-MnO2-6.0, GF-PANI+β-MnO2-6.0 (β-MnO2:PANI = 4:1
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according to the results of TGA results of PANI/β-MnO2) and GF-PANI/β-MnO2-6.0
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in air-saturated PBS. Except for GF electrode, all the electrodes exhibited ORR peaks 15
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-0.1
and
-0.3
V.
Among
them,
obvious
ORR
peak
of
the
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GF-PANI/β-MnO2-6.0 electrode was clearly observed at -0.17 V, which was more
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positive than GF-PANI+β-MnO2-6.0 electrode (-0.21 V) and GF-β-MnO2-6.0
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electrode (-0.24 V). Remarkably, the ORR peak current of GF-PANI/β-MnO2-6.0
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electrode was bigger than other three electrodes. Compared with β-MnO2 and
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PANI+β-MnO2 mixture catalysts, PANI/β-MnO2 nanocomposites demonstrated higher
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catalytic activity towards ORR (Zhang et al., 2012), that is because (1) PANI itself
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had the ability to catalyze ORR (Khomenko et al., 2005); (2) introducing PANI
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granules onto the β-MnO2 nanorods surface increased the surface areas and total pore
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volumes of catalyst, which might facilitate the diffusion, adsorption, and transport of
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O2 (Cheng et al., 2010); (3) the specific interaction between β-MnO2 and PANI, which
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was confirmed by FT-IR analysis, could enhance the electron delocalization and
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increase the electrical conductivity, resulting in a higher catalytic activity (Ding, 2009;
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Zhou et al., 2017).
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Exchange current density (i0) is an important factor to evaluate the ORR
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performance which is also the main factor to estimate the electrogenic activity and
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charge transfer coefficient of an electrode (Huang et al., 2017b). The Tafel plots and
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fitting results are shown in Fig. 5b and i0 calculated for all the cathodes were listed in
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Table SM-1. The valid linear Tafel regression (R2 > 0.99) was determined by the
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overpotential ranging from 60 to 80 mV. The i0 value of GF-PANI/β-MnO2-6.0
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cathode (0.66 × 10-4 A cm-2) was higher than those of GF-β-MnO2-6.0 cathode (0.44 ×
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10-4 A cm-2) and GF-PANI+β-MnO2-6.0 cathode (0.57 × 10-4 A cm-2), which was also
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activation energy barrier of forward ORR on the GF-PANI/β-MnO2-6.0 electrode.
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These results indicated that the catalytic activity for ORR was evidently enhanced
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after generating PANI granules onto the external surface of β-MnO2 nanorods (Zhao
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et al., 2009). This trend also consisted with the following power density, polarization
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curves and EIS results, which further illustrated that PANI/β-MnO2 nanocomposites
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accelerated the kinetics of catalytic activity, consequently, enhancing the power output
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of MFCs.
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3.3 MFC performance
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To evaluate the performance of as-prepared ORR catalysts, double-chamber
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MFC were constructed and operated in batch mode. The constructed MFC shared one
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anode chamber, but used different cathodes after the MFC entered stable period.
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Fig. 6 shows the power density curves, polarization curves, anode and cathode
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potentials, and Nyquist curves of the MFC fabricated with the prepared electrodes.
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The data were also listed in Table 1. Open circuit voltage (OCV) is a useful indicator
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of the electrode performance, usually closely related to the electrode activity. As
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shown in Fig. 6a, the OCVs of the MFC with different air-electrode were in the order
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of: GF-PANI/β-MnO2-6.0 (770 mV) > GF-PANI+β-MnO2-6.0 (739 mV) >
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GF-β-MnO2-6.0 (723 mV) > GF (680 mV). In addition the MFC cathode modified by
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PANI/β-MnO2 had maximum power density (Pmax) of 248 mW m-2, which was 1.22,
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1.36 and 1.85 times higher than that of PANI+β-MnO2 mixture (204 mW m-2),
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β-MnO2 (183 mW m-2) and bare (134 mW m-2), higher than CNT-β-MnO2 (98.7 mW
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ACCEPTED MANUSCRIPT m-2) (Lu et al., 2011), MnOx/C (161 mW m-2) (Roche et al., 2009), 0.4 wt% Pt/MnO2
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(165 mW m-2) (Khan et al., 2015), MnO2/CNTs (210 mW m-2) (Zhang et al., 2011)
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and graphite/β-MnO2 (172 mW m-2) (Zhang et al., 2009). The result suggested that the
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PANI/β-MnO2 nanocomposites catalyst performed higher catalytic activity and
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improved the power output. Power density and polarization curves were also
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measured to determine the performances of the MFC with different loading amount of
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PANI/β-MnO2 nanocomposites (0, 2.2, 4.1 and 6.0 mg cm-2) as cathode catalyst in Fig.
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6b. Remarkably, the MFC equipped with the GF-PANI/β-MnO2-6.0 electrode
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generated the largest Pmax, which was 1.29 and 1.63 times higher than that of
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GF-PANI/β-MnO2-4.1 electrode (191 mW m-2) and GF-PANI/β-MnO2-2.2 electrode
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(152 mW m-2). Both OCV and power density increased with the increase of loading
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amount of PANI/β-MnO2 catalyst, which was owing to increasing active sites and
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accelerating kinetics towards ORR with the increase of catalyst amount.
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The curves of potential versus current density for the individual cathodes and
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anodes are shown in Fig. 6c. It was obvious that there was an insignificant difference
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in the anode potentials, but the cathode potentials were considerably different. This
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phenomenon also occurred in Fig. 6d, which exhibited the potentials of individual
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cathodes and anodes of different loading amount of PANI/β-MnO2 nanocomposites as
393
cathode catalyst. These observations implied that the differences in power output
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resulted from cathodes rather than anodes (Huang et al., 2017a) and the differences in
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the MFC performances mainly were attribute to variations of the cathode catalysts and
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the loading amount of catalysts in the catalytic activities.
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must have high conductivity (Hao et al., 2017). All the Nyquist plots of the electrodes
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with different catalysts and different loading amount of PANI/β-MnO2 catalyst were
400
tested using AC impedance measurements. The Nyquist plots and the equivalent
401
circuit model are shown in Fig. 6e and f. The results indicated that there was a
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remarkable decrease in the resistance of MFC system after the addition of cathode
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catalyst. Both the ohmic resistance (Rs) and charge transfer resistance (Rct) of the GF
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electrode
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GF-PANI/β-MnO2-6.0 electrode in Fig. 6e. The values of Rs for GF, GF-β-MnO2-6.0,
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GF-PANI+β-MnO2-6.0 and GF-PANI/β-MnO2-6.0 electrode were estimated to be
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50.4, 45.5, 42.3 and 38.4 Ω, respectively. Besides, there was an obvious reduction in
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Rct for catalyst coated cathodes. The GF-PANI/β-MnO2-6.0 electrode exhibited Rct of
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9.2 Ω, which was lower than GF (93.3 Ω), GF-β-MnO2-6.0 (19.8 Ω), and
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GF-PANI+β-MnO2-6.0 (14.4 Ω) electrode. The lowest Rs and Rct obtained for
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GF-PANI/β-MnO2-6.0 electrode might be explained by pronounced synergistic effect
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between PANI and β-MnO2 of the PANI/β-MnO2 catalyst, which increased the
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conductivity of the electrode surface and accelerated electron transfer throughout the
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electrode (Wang et al., 2012). Fig. 6f shows the Nyquist plots of the cathodes with
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different loading amount of PANI/β-MnO2 catalyst. The Rs of GF-PANI/β-MnO2-6.0
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electrode was ~1.15, 1.23 and 1.31 times lower than GF-PANI/β-MnO2-4.1,
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GF-PANI/β-MnO2-2.2 and bare electrode. In addition, there was also a reduction
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trend in Rct. The Rct of GF-PANI/β-MnO2-6.0 electrode was lower than bare (93.3 Ω),
higher
than
GF-β-MnO2-6.0,
GF-PANI+β-MnO2-6.0
and
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19
ACCEPTED MANUSCRIPT GF-PANI/β-MnO2-2.2 (23.9 Ω) and GF-PANI/β-MnO2-4.1 (15.6 Ω) electrode. This
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reduction in Rs and Rct could result from the more PANI/β-MnO2 catalyst on the
421
cathode surface, which were deemed to provide excess active sites for oxygen
422
molecules and improve more electrocatalytic activity of the cathode for ORR. The
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total internal resistance of the MFC was estimated by polarization slope method,
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which was denoted as Rint and listed in Table 1. It was also in good agreement with
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power density results, which further indicated that the loading of PANI/β-MnO2
426
catalyst onto graphite felt surface decreased the resistance and accelerated the kinetics
427
activity. Therefore, it will be an efficient ORR catalyst for MFC applications to
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increase power output.
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4. Conclusion
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Novel PANI/β-MnO2 nanocomposites prepared by hydrothermal and in-situ
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chemical oxidative polymerization were used as cathode catalyst in MFCs. The
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PANI/β-MnO2 exhibited higher catalytic activity for ORR than β-MnO2 and
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PANI+β-MnO2 mixture, owing to the interaction between β-MnO2 and PANI,
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increased specific surface area and electric conductivity, which facilitated the electron
435
transfer process. Several tests results indicated that the PANI/β-MnO2 cathode did
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promote the power generation of the MFC, the maximum power density was 1.2-1.4
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times as high as those by controlled cathodes, while cathode resistance was 1.1-1.2
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times lower, indicating the promising potential as ORR catalyst in MFC applications.
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Acknowledgements
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We gratefully thank the National Natural Science Foundation of China 20
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(21276128), the Basic Research Program of Jiangsu Province of China (BK20141398)
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for the financial support.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Schematic of the preparation of the PANI/β-MnO2 nanocomposites. Fig. 2. XRD patterns of (a) PANI powders, (b) β-MnO2 nanorods and (c)
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PANI/β-MnO2 nanocomposites. Fig. 3. SEM images of β-MnO2 (a and b) and PANI/β-MnO2 (e and f), and TEM images of β-MnO2 (c and d) and PANI/β-MnO2 (g and h).
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Fig. 4. X-ray photoelectron spectroscopy (XPS) analysis of the PANI/β-MnO2
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Fig. 5. (a) CV curves for ORR in air-saturated PBS (pH=7) at the scan rate of 50 mV s-1 with different catalyst cathodes and (b) Tafel plots of all cathodes at 1 mV s-1 and
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Fig. 6. Power density and polarization curves as function of current density of MFC with cathode loading different types of catalyst (a), and with cathode loading different
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amount of PANI/β-MnO2 (b). Individual potential (vs. Ag/AgCl) as function of current
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density of MFC with cathode loading different types of catalyst (c), and with cathode loading different amount of PANI/β-MnO2 (d). Nyquist curves of the EIS data of MFC with cathode loading different types of catalyst (e), and with cathode loading different amount of PANI/β-MnO2 (f).
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OCP
Pmax
Rint (Ω)
Cathode (mV)
(cathode/mV)
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(mW m )
(Ω)
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Rs
Rct
680
244
134
243.7
50.4
93.3
GF-β-MnO2-6.0
723
276
183
175.3
45.5
19.8
GF-PANI+β-MnO2-6.0
739
281
204
164.7
42.3
14.4
GF-PANI/β-MnO2-6.0
770
307
248
155.6
38.4
9.2
GF-PANI/β-MnO2-4.1
730
GF-PANI/β-MnO2-2.2
705
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GF
280
191
170.2
44.2
15.6
258
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182.1
47.2
23.9
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ACCEPTED MANUSCRIPT Highlights The PANI/β-MnO2 nanocomposites are synthesized successfully.
The nanocomposites serve as electrocatalysts for the air cathode of MFCs.
The performance of PANI/β-MnO2 nanocomposites modified MFC is enhanced.
The nanocomposites are potential electrocatalysts for air-cathode MFCs.
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S0045-6535(18)30066-3
DOI:
10.1016/j.chemosphere.2018.01.058
Reference:
CHEM 20639
To appear in:
ECSN
Received Date: 19 September 2017 Revised Date:
12 January 2018
Accepted Date: 13 January 2018
Please cite this article as: Zhou, X., Xu, Y., Mei, X., Du, N., Jv, R., Hu, Z., Chen, S., Polyaniline/βMnO2 nanocomposites as cathode electrocatalyst for oxygen reduction reaction in microbial fuel cells, Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.01.058. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT Polyaniline/β-MnO2 nanocomposites as cathode electrocatalyst for oxygen
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reduction reaction in microbial fuel cells
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Xinxing Zhou, Yunzhi Xu, Xiaojie Mei, Ningjie Du, Rongmao Jv, Zhaoxia Hu*,
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Shouwen Chen*
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Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School
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of environmental and biological engineering, Nanjing University of Science and
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Technology, Nanjing 210018, China
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*Corresponding author information:
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Tel.: +86-25-84315532;
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Fax: +86-25-84315518.
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E-mail address: [email protected]; [email protected]
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Abstract An efficient and inexpensive catalyst for oxygen reduction reaction (ORR),
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polyaniline (PANI) and β-MnO2 nanocomposites (PANI/β-MnO2), was developed for
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air-cathode microbial fuel cells (MFCs). The PANI/β-MnO2, β-MnO2, PANI and
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β-MnO2 mixture modified graphite felt electrodes were fabricated as air-cathodes in
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double-chambered MFCs and their cell performances were compared. At a dosage of
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6 mg cm-2, the maximum power densities of MFCs with PANI/β-MnO2, β-MnO2,
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PANI and β-MnO2 mixture cathodes reached 248, 183 and 204 mW m-2, respectively,
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while the cathode resistances were 38.4, 45.5 and 42.3 Ω, respectively, according to
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impedance analysis. Weak interaction existed between the rod-like β-MnO2 and
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surficial growth granular PANI, this together with the larger specific surface area and
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PANI electric conducting nature enhanced the electrochemical activity for ORR and
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improved the power generation. The PANI/β-MnO2 nanocomposites are a promising
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cathode catalyst for practical application of MFCs.
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Keywords: Cathode catalyst; PANI/β-MnO2 nanocomposites; Oxygen reduction
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reaction; Microbial fuel cell
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1. Introduction
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Microbial fuel cell (MFC) is a green energy technology that converts chemical
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energy harvested in organic compounds whether pollutant or not into electrical energy
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utilizing the power of microorganisms. It has the characteristics of wide range of raw
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materials and good biocompatibility (Cheng et al., 2006; Anu Prathap et al., 2013).
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Oxygen has superiority as electron acceptor for MFCs due to its high oxidation 2
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sustainability (Dong et al., 2012b). However, the sluggish kinetics of the oxygen
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reduction reaction (ORR) in a near neutral medium seriously limits the power density
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production of MFCs (Wen et al., 2012). To solve this problem, platinum (Pt) and
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Pt-based materials are widely used as catalyst of ORR in MFC cathodes. Nevertheless,
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noble metal platinum is expensive, unrenewable, and sensitive to catalyst poisoning
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which attenuates the performance of MFCs and limits its large-scale application.
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Because of this, it is necessary to pour a great deal of efforts on developing alternative
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catalysts which are easy to produce, low consumption, and have the similar or better
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electrocatalytic activity than Pt for ORR.
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Manganese dioxide (MnO2) materials with different crystal structure (α-MnO2,
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β-MnO2, and γ-MnO2) have been investigated extensively as the cathode catalysts for
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ORR in MFCs due to its rich resources, low cost, environment friendliness, and good
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catalytic activity. Zhang et al. (Zhang et al., 2009) compared the three crystal types (α,
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β and γ) of MnO2 as cathode catalyst in MFCs, they found that β-MnO2 showed the
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best effective ORR catalytic activity owing to its highest BET surface area and
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average oxidation state of manganese (AOS: 3.59), which was higher than those of
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α-MnO2 (3.55) and γ-MnO2 (3.48), since AOS was an indicator of the manganese
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state in the structures of manganese dioxides and the higher AOS stands for the higher
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oxidative activity (Zhang et al., 2009). Therefore, β-MnO2 is chosen as the object of
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this study. However, the poor electronic conductivity of MnO2 remains a major
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challenge and limits the rate capability for MFC power performance. To address this
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such as polypyrrole (PPy) (He et al., 2007; Tran et al., 2007), polythiophene (PTh) (Li
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et al., 2009) or polyaniline (PANI) (Li et al., 2008a; Li et al., 2008b) materials. These
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polymers have attracted more and more attention because of their low cost,
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convenient synthesis, strong energy-storage capacity and high electrical conductivity.
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Among the various electroconductive polymers studied to date, PANI has been
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considered as the most attractive polymer due to its excellent conductivity and
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environmental stability (Wang et al., 2017). More importantly, PANI also has
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electrocatalytic activity for ORR (Khomenko et al., 2005). In previous reports,
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polyaniline/inorganic composites have been proved to have better electrical
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conductivity (Yuan et al., 2008; Lai et al., 2011). The PANI-MnO2 composites have
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been reported in the application of supercapacitors (Jiang et al., 2012; Chen et al.,
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2013). However, the reports of these kinds of PANI-MnO2 especially β-MnO2
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composites function as air-cathode catalyst in application of MFCs are only few.
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In this paper, we successfully loaded PANI on the surface of β-MnO2 nanorods
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by in situ chemical oxidative polymerization method and investigated the possibility
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of PANI/β-MnO2 nanocomposites as cathode catalyst for ORR in an MFC. The
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performances of MFCs were compared with three kinds of materials (β-MnO2,
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PANI+β-MnO2 mixture and PANI/β-MnO2 nanocomposites) used as cathode catalysts
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in MFC. Furthermore, the cathode with different loading amount of PANI/β-MnO2
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nanocomposites as catalyst were also investigated.
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2. Experimental
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2.1 Materials Aniline was purchased from Shanghai Bai Lingwei Chemical Technology Co.,
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Ltd. Potassium permanganate (KMnO4) was obtained from Sinopharm Chemical
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Reagent Co., Ltd. Aniline sulfate was purchased from Aladdin Industrial Corporation.
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The graphite felt with a thickness of 5 mm was bought from Zibo Jinpeng Carbon Co.,
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Ltd. Cation exchange membrane (CEM) was purchased from Hangzhou El
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Environmental Protection Technology Co., Ltd. (IEC = 2.4 mmol g-1). All other
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reagents used in this work were of analytical grade and were used without further
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purification. Deionized water was produced by an EPED pure water system and used
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in all solution.
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2.2 Synthesis of PANI
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To a three-necked flask, 100 mL of hydrochloric acid solution (1 M) and aniline
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(0.05 mol, 4.65 g) were added sequentially. The solution was stirred and cooled to 0-5
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o
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solution dropwise, and the transparent solution gradually turned into mazarine. After
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the continuous stirring for 3 h, the solution was kept still overnight. The precipitate
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was collected by filtration, washed with deionized water and dried under vacuum
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oven at 65 oC and ground into powder (Ma et al., 2007).
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2.3 Synthesis of β-MnO2 nanorods
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β-MnO2 was prepared by a previously reported method (Zhang et al., 2009). In a
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typical procedure, 1.2 g of KMnO4, 2.7 mL of ethanol and 51.3 mL of deionized water
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were added into a 100 mL beaker. After the dissolution of KMnO4, the solution was 5
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for 24 h. The obtained precipitate was collected by filtration, washed with deionized
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water and ethanol several times, and dried at 100 oC in a vacuum oven. Finally, the
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precipitates were annealed at 300 oC in air for 5 h at a ramping rate of 3.5 oC min-1.
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2.4 Synthesis of PANI/β-MnO2 nanocomposites
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In a 500 mL beaker, aniline sulfate (0.143 g, 0.5 mmol) was dissolved in 100 mL
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of sulfuric acid, and the solution was cooled to 0-5 oC in an ice bath. Under vigorous
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stirring, 0.279 g of as-prepared β-MnO2 was added and kept for 2 h. The obtained
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precipitate was collected by filtration, washed with deionized water and ethanol
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several times, respectively, then dried in a vacuum oven at 60 oC.
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2.5 Pretreatment of graphite felt and cation exchange membrane
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First, the graphite felt was cut into small piece with the size of 2 × 2 × 0.5 cm
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and put into a 250 mL beaker. Next, ethanol (150 mL) was added to immerse the
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graphite felt completely and ultrasonic for 10 min. Then the graphite felt was rinsed
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by deionized water and the operation was repeated until the deionized water was
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transparent. Finally, the graphite felt was dried in a vacuum oven at 60 oC for 24 h.
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The purified graphite felt was acidic oxidized in a sealed 300 mL Teflon-lined
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stainless steel autoclave containing 120 mL mixed acid solution (VH SO :VHNO3 = 3:1) at
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80 oC for 8 h (Zhang et al., 2013). The acidic oxidized graphite felt was washed with
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deionized water, and sonicated for 30 min in the deionized water until the pH of the
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rinsed water became neutral. Then they were dried in a vacuum oven at 100 oC for 5 h
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and saved in a sealed bag.
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thoroughly with deionized water and then immersed into deionized water at least 24 h
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before use.
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2.6 Electrode fabrication
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Graphite felt was used as the supporting carbon/electrode material, and several
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air-cathodes were prepared as parallel control. The air-electrode was fabricated by
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ultrasonic-load method. The as-prepared β-MnO2, PANI+β-MnO2 mixture or
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PANI/β-MnO2, the binder of polyvinylidene fluorine were mixed by a weight ratio of
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95:5 in 20 mL of N-methyl-2-pyrrolidone in a ultrasonic bath for 15 min. A piece of
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acid treated graphite felt (2 × 2 × 0.5 cm) was immersed into the solution and placed
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in the ultrasonic bath for another 30 min. Then the graphite felt was taken out and
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dried in a vacuum oven at 60 oC for 10 h to remove the solvent. The above step was
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repeated several times until gained the settled catalyst loading. The prepared
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air-cathodes were named as GF, GF-PANI+β-MnO2 and GF-PANI/β-MnO2. We also
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studied the effect of PANI/β-MnO2 loading amount on the performance of the MFC.
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The prepared air-electrodes were named as GF-PANI/β-MnO2-x, where x refers to the
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catalyst loading amount (mg cm-2).
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2.7 MFC configuration and operation
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The double-chamber air-cathode MFC was constructed with two inner
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cylindrical Plexiglas chambers (d × H = 8 × 5.5 cm, 280 mL), each chamber consists
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of two ports for feed inlet/outlet, gas inlet/outlet and electrode connection parts. They
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were separated by the CEM. The graphite felt-based electrodes were placed parallel to 7
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connected by titanium wire as current collector with conductive paste. The anodic
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chamber was filled with medium containing Na2HPO4·12H2O (11.1 g L-1),
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NaH2PO4·2H2O (2.96 g L-1), NaAc·3H2O (2.72 g L-1), NH4Cl (0.31 g L-1),
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MgCl2·6H2O (0.21 g L-1), NaCl (1.0 g L-1), CaCl2 (0.02 g L-1), KCl (0.13 g L-1) and
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trace element (1 mL L-1), whereas the medium was sterilized at 121 oC for 15 min
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before use. The culture was inoculated from anaerobic digester sludge collected from
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Nanjing East sewage treatment plant. The catholyte was comprised of a phosphate
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buffer solution (PBS, pH=7), which consisted of Na2HPO4·12H2O (11.1 g L-1),
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NaH2PO4·2H2O (2.96 g L-1) and NaCl (1.0 g L-1). Air was continuously fed into the
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cathode chamber using an air pump at a rate of 90 mL min-1 to maintain a sustained
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concentration of oxygen during the MFC operation. All the reactors were controlled at
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30 oC in a water bath incubator. At the start-up period, an external resistor (1000 Ω)
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was connected in the circuit. The schematic figure of the MFC is shown in Fig. SM-1.
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2.8 Characterizations and measurements
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X-ray diffraction (XRD) was conducted on D8 Advance (Bruker) using Cu Kα
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radiation at 40 kV and with 2θ of 10°-70°. Scanning electron microscope (SEM) was
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performed on a Quant 250FEG (FEI) machine. Transmission electron microscopy
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(TEM) analysis was conducted on a TECNAI G2 20 LaB6 (FEI) electron microscope
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operated at 200 kV. Thermal gravimetric analysis (TGA) was carried out on a
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SDTA851E instrument (TA-instruments-waters LLC) at a heating rate of 10 oC min-1
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under flowing air from 50 to 800 oC. Fourier transform infrared (FT-IR) spectra was
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specific surface area and pore size distribution of the catalyst were analyzed using the
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Brunauer–Emmet–Teller (BET, ASAP-2020, Micromeritics, America) method by the
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adsorption and desorption of N2. X-ray photoelectron spectroscopy (XPS) analysis
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was conducted on a PHI Quantera II ESCA System with Al Ka radiation at 1486.8 V.
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2.9 Electrochemical analysis
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All electrochemical analysis of the MFC was conducted on an electrochemical
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work station (CHI 604E, China). Polarization curves and power density curves were
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obtained by varying the external resistance from 5000 to 82 Ω and the voltage was
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recorded until the value was stable for 15 min by an exquisite multimeter. Current
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density and power density were calculated by normalizing current and power to the
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anode surface area (8 cm2) according to the following equation (Karra et al., 2013),
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where V is the voltage across the external resistor, R is the external resistance, and A
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is the anode surface area.
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Cyclic voltammetry (CV) measurement was performed using a three-electrode
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system, where the prepared air-cathode, an Ag/AgCl electrode and a platinum wire
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were used as the working, reference and counter electrode, respectively. The CV
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measurement was recorded from -0.8 to 0.4 V at a scan rate of 50 mV s-1 in PBS (pH
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= 7) saturated by air. Tafel plots and the exchange current density (i0) were recorded
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by sweeping the overpotential from 0 to 100 mV at 10 mV s-1, calculated by using 9
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the following empirical equation (Huang et al., 2017a). lgi = lgi0 -
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βnFη
(3)
2.303RT
Electrochemical impedance spectroscopy (EIS) measurement was carried out at
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the open circuit potential (OCV) with an AC perturbation of 5 mV over a frequency
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range from 0.1 Hz to 10 kHz. The anode was used as the working electrode, while the
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cathode as the counter and reference electrode. The resistance data were fitted with
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Zsimpwin software (Echem), where the ohmic resistance (Rs) was obtained from the
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Nyquist impedance plots at the point where -Z” was equal to zero at high frequency,
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the charge-transfer resistance (Rct) was calculated from a semicircular fit of the
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charge-transfer impedance in the Nyquist plots (Yuan et al., 2015).
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3. Results and discussion
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3.1 Synthesis and characterization of the PANI/β-MnO2 composites
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Fig. 1 shows the synthetic diagram of the PANI/β-MnO2 nanocomposites. The
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nanocomposites were prepared by in-situ PANI polymerization in the presence of
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β-MnO2 nanorods.
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Fig. 2 shows XRD pattern of the PANI, β-MnO2 and PANI/β-MnO2
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nanocomposites. The XRD pattern of PANI showed two broad peaks at 2θ of 20.5°
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and 25.3°, which represented the periodicities parallel (100) and perpendicular (110)
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to the PANI chain, respectively (Fig. 2a) (Ryu et al., 2007; Zhu et al., 2007). As
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shown in Fig. 2b, all of the diffraction peaks for the as-prepared sample were indexed
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with the standard XRD pattern of β-MnO2 (JCPDS No. 24-0735, tetragonal symmetry
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with P42/mnm space group and lattice constants of a = 4.399 nm and c = 2.874 nm)
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PANI/β-MnO2 nanocomposites (Fig. 2c) and pristine β-MnO2 except the diffraction
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peaks at 2θ = 20.5° and 25.3° which resulted from the PANI on the β-MnO2 surface,
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confirming the presence of β-MnO2 in the nanocomposites after in-situ polymerization.
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No characteristic impurity peak was observed, indicating that high purity
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PANI/β-MnO2 nanocomposites were produced by the simple polymerization.
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Fig. 3 shows the SEM and TEM images of β-MnO2 nanorods and PANI/β-MnO2
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nanocomposites. As clearly observed in Fig. 3a and b, the as-prepared β-MnO2 shown
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1-D nanostructured crystals with smooth surface. TEM images in Fig. 3c and d further
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confirmed the formation of β-MnO2 nanorods with average length of 10-20 µm and
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average diameter of 200-300 nm. The length and diameter of the as-prepared β-MnO2
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in this paper were larger than those reported in the literature (Liu et al., 2017), but
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more orderly (Liu et al., 2005) and scattered (Zang et al., 2011). Compared with the
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β-MnO2 nanorods, the synthesized PANI/β-MnO2 nanocomposites in Fig. 3e and f
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exhibited shorter average length about 8-12 µm and had some granules and cracks
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(red dashed frame) on the surface. The reason of this phenomenon was that β-MnO2
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worked as oxidant and template during the aniline monomer polymerization to form
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PANI on the nanorods, and partial β-MnO2 was consumed (Yao et al., 2013). This
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would lead to an intimate interface, facilitating donor–acceptor interactions between
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PANI and β-MnO2 (Zhou et al., 2017). TEM images also proved that some small
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black granules grown on the nanorods (Fig. 3g and h). We composed that these
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granules were PANI nanogranules which were confirmed by FT-IR and XPS analysis.
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nanocomposites was two times more than that of β-MnO2 nanorods. Introducing the
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PANI granules and cracks increased the specific surface area and will bring about
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more active sites for ORR of the PANI/β-MnO2 nanocomposites.
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Fig. SM-2 shows the TGA curves of PANI, β-MnO2 and PANI/β-MnO2. PANI
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showed continuous weight loss till 660 oC, where the weight loss below 300 oC was
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due to the removal of physic-adsorbed water, interstitial water or dopant anions (Ni et
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al., 2010; Sen et al., 2013), and the sharp weight loss to 660 oC was ascribed to the
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large-scale thermal degradation of the PANI chains (Nirmalesh Naveen et al., 2015),
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while the small weight reserved at 800 oC was due to the final carbonization of
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intermediate chemicals (Bhadra et al., 2007). Only a small weight loss (3%) was
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observed for the β-MnO2 nanorods up to 800 oC. However, a small weight loss
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between 500 and 650 oC, which was probably owing to the reduction of manganese
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ions going from tetravalent to trivalent state accompanied with oxygen evolution
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(Zhang et al., 2007b). As observed in Fig. SM-2, weight loss of the PANI/β-MnO2
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nanocomposites majorly occurred up to 500 oC, which was attributed to the loss of
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adsorbed water and the decomposition of the PANI (Ni et al., 2010). Obviously,
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comparing with pure PANI, the thermal decomposition temperature of PANI in the
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PANI/β-MnO2 nanocomposites was lower. This can be explained by the fact that a
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coordination bond between manganese and nitrogen atoms in PANI weakened the
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interactive forces of the PANI inter-chains (Pham et al., 2009; Anu Prathap et al.,
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2013). The weight was stable at about 80% when the temperature was above 600 oC
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and the residues were mainly composed of β-MnO2. It is considered that the weight
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ratio of PANI and β-MnO2 in the prepared PANI/β-MnO2 was about 1:4. The structural information and chemical component of PANI/β-MnO2
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nanocomposites were also identified by the FT-IR spectroscopy, as shown in Fig.
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SM-3. Typical absorption bands at about 1110, 721, and 474 cm−1 attributed to the
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Mn-O vibrations of MnO6 octahedra in MnO2 (Wang et al., 2008) (Fig. SM-3a) and
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characteristic absorption peaks at 807 (C-C and C-H in the benzenoid ring), 1122 (the
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=N+–H -stretching), 1220 (the C–N+ stretching), 1294 (the C–N stretching of the
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secondary aromatic amine), 1485 (the aromatic C=C stretching of the benzenoid ring),
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and 1558 cm−1 (the aromatic C=C stretching of the quinonoid ring) (Fig. SM-3b)
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(Zhang et al., 2013; Yang et al., 2017) for PANI were clearly observed in the spectra
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of β-MnO2 and PANI, respectively. As for the nanocomposites, Fig. SM-3c shows all
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the characteristic absorption bands of PANI and β-MnO2. However, the characteristic
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peaks of Mn-O vibrations at 721 and 474 cm-1 shifted to 712 and 484 cm-1,
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respectively, reflecting a mutual interaction between PANI and β-MnO2 that, most
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likely, was a hydrogen bond formed between oxygen atom of Mn-O and N-H in PANI
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(Yao et al., 2013). In addition, our observation of both the difference in peak position
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and intensity between the PANI/β-MnO2 and the pure PANI (Fig. SM-3b), further
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indicating that there was a mutual interaction between PANI and β-MnO2, which
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verified PANI has been grown on the surface of β-MnO2 nanorods (Sun et al., 2015).
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Fig. SM-4 shows the nitrogen adsorption-desorption isotherm curves of the
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β-MnO2 nanorods and PANI/β-MnO2 nanocomposites, and the inset graph exhibits 13
ACCEPTED MANUSCRIPT 287
the pore size distribution for the two materials. Both materials showed type IV
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adsorption-desorption isotherm curves with a hysteresis loop, indicating a
289
well-ordered mesoporous structure.
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volumes of β-MnO2 nanorods were 19.9 m2 g-1 and 0.023 cm3 g-1. The BET surface
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areas of β-MnO2 nanorods was larger than those of the commercial β-MnO2 powder
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(0.11 m2 g-1) (Zhang et al., 2006), pure phase of tetragonal MnO2 (12.4 m2 g-1) (Xu et
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al., 2016) and MnO2 nanorods ( 16.17 m2 g-1) (Zhang et al., 2007a). However, the
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as-prepared PANI/β-MnO2 nanocomposites possessed a larger BET surface areas
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(39.7 m2 g-1) and total pore volumes (0.042 cm3 g-1) than those of pure β-MnO2
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nanorods (19.9 m2 g-1) in this paper, PPy/β-MnO2 (10.3 m2 g-1) (Chen et al., 2017),
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np-RuO2/nr-MnO2 (31.9 m2 g-1) (Xu et al., 2016) and Ag/MnO2 rods (19.5 m2 g-1),
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tubes (25.6 m2 g-1), wires (34.7 m2 g-1) (Li et al., 2016). That was mainly ascribed to
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introducing PANI granules onto the β-MnO2 nanorods surface. The inset in the figure
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shows the pore size distribution curves calculated using the Barrett–Joyner–Halenda
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equation from the desorption branch of the isotherms (Chen et al., 2013). The BJH
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analysis exhibited that PANI/β-MnO2 nanocomposites had an intensive pore size
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distribution of ~3.87 nm, which was classified as a mesopore distribution, with an
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adsorption average pore diameter of 6.63 nm, that was smaller than that of β-MnO2
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nanorods (6.90 nm). The high surface area and high mesoporous nature of
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PANI/β-MnO2 nanocomposites may result in more exposure of catalyst active sites
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diffusing O2 and electrolyte, leading to enhancement during the ORR process.
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The measured BET surface areas and total pore
Fig. 4 displays the XPS spectra of PANI/β-MnO2 nanocomposites. Four elements 14
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PANI and β-MnO2 in the nanocomposites. Furthermore, to examine the oxidation
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state of Mn in the PANI/β-MnO2 nanocomposites (Pan et al., 2016), XPS spectrum of
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Mn 2p is plotted in Fig. 4b. Two peaks located at 654.4 and 642.7 eV corresponded to
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Mn 2p1/2 and Mn 2p3/2, respectively, which was in agreement with the energy
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separation (11.7 eV) between Mn 2p1/2 and Mn 2p3/2 reported previously (Zhou et al.,
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2017). As reported previously, the Mn oxidation state in manganese oxides can be
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determined from the separation of peak energies (∆Eb) between the two peaks of the
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Mn 3s components (Luan et al., 2016). The as-prepared PANI/β-MnO2 showed a
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separation energy of 4.83 eV for the Mn 3s doublet (Fig. 4c), which indicated that the
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dominant Mn4+ ions existed in the nanocomposites and the chemical composition was
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β-MnO2. The existence of β-MnO2 can be further varied by O1s peaks, which was
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further deconvoluted into three major bond components in Fig. 4d (Asif et al., 2016).
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The three peaks centered at binding energy 530.21, 531.41, and 532.32 eV were
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corresponding to Mn-O-Mn bond for the tetravalent oxide, Mn-O-H bond for
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hydroxide and H-O-H bond for residual structural water, respectively (Li et al., 2011).
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3.2 Electrochemical activity of PANI/β-MnO2 for ORR
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The activity of various cathodes for ORR was investigated with both CV and
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Tafel curves in PBS (pH = 7). Fig. 5a shows the CV curves of four prepared
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air-cathodes, GF, GF-β-MnO2-6.0, GF-PANI+β-MnO2-6.0 (β-MnO2:PANI = 4:1
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according to the results of TGA results of PANI/β-MnO2) and GF-PANI/β-MnO2-6.0
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in air-saturated PBS. Except for GF electrode, all the electrodes exhibited ORR peaks 15
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-0.1
and
-0.3
V.
Among
them,
obvious
ORR
peak
of
the
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GF-PANI/β-MnO2-6.0 electrode was clearly observed at -0.17 V, which was more
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positive than GF-PANI+β-MnO2-6.0 electrode (-0.21 V) and GF-β-MnO2-6.0
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electrode (-0.24 V). Remarkably, the ORR peak current of GF-PANI/β-MnO2-6.0
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electrode was bigger than other three electrodes. Compared with β-MnO2 and
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PANI+β-MnO2 mixture catalysts, PANI/β-MnO2 nanocomposites demonstrated higher
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catalytic activity towards ORR (Zhang et al., 2012), that is because (1) PANI itself
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had the ability to catalyze ORR (Khomenko et al., 2005); (2) introducing PANI
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granules onto the β-MnO2 nanorods surface increased the surface areas and total pore
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volumes of catalyst, which might facilitate the diffusion, adsorption, and transport of
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O2 (Cheng et al., 2010); (3) the specific interaction between β-MnO2 and PANI, which
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was confirmed by FT-IR analysis, could enhance the electron delocalization and
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increase the electrical conductivity, resulting in a higher catalytic activity (Ding, 2009;
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Zhou et al., 2017).
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Exchange current density (i0) is an important factor to evaluate the ORR
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performance which is also the main factor to estimate the electrogenic activity and
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charge transfer coefficient of an electrode (Huang et al., 2017b). The Tafel plots and
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fitting results are shown in Fig. 5b and i0 calculated for all the cathodes were listed in
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Table SM-1. The valid linear Tafel regression (R2 > 0.99) was determined by the
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overpotential ranging from 60 to 80 mV. The i0 value of GF-PANI/β-MnO2-6.0
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cathode (0.66 × 10-4 A cm-2) was higher than those of GF-β-MnO2-6.0 cathode (0.44 ×
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10-4 A cm-2) and GF-PANI+β-MnO2-6.0 cathode (0.57 × 10-4 A cm-2), which was also
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activation energy barrier of forward ORR on the GF-PANI/β-MnO2-6.0 electrode.
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These results indicated that the catalytic activity for ORR was evidently enhanced
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after generating PANI granules onto the external surface of β-MnO2 nanorods (Zhao
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et al., 2009). This trend also consisted with the following power density, polarization
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curves and EIS results, which further illustrated that PANI/β-MnO2 nanocomposites
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accelerated the kinetics of catalytic activity, consequently, enhancing the power output
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of MFCs.
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3.3 MFC performance
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To evaluate the performance of as-prepared ORR catalysts, double-chamber
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MFC were constructed and operated in batch mode. The constructed MFC shared one
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anode chamber, but used different cathodes after the MFC entered stable period.
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Fig. 6 shows the power density curves, polarization curves, anode and cathode
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potentials, and Nyquist curves of the MFC fabricated with the prepared electrodes.
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The data were also listed in Table 1. Open circuit voltage (OCV) is a useful indicator
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of the electrode performance, usually closely related to the electrode activity. As
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shown in Fig. 6a, the OCVs of the MFC with different air-electrode were in the order
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of: GF-PANI/β-MnO2-6.0 (770 mV) > GF-PANI+β-MnO2-6.0 (739 mV) >
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GF-β-MnO2-6.0 (723 mV) > GF (680 mV). In addition the MFC cathode modified by
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PANI/β-MnO2 had maximum power density (Pmax) of 248 mW m-2, which was 1.22,
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1.36 and 1.85 times higher than that of PANI+β-MnO2 mixture (204 mW m-2),
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β-MnO2 (183 mW m-2) and bare (134 mW m-2), higher than CNT-β-MnO2 (98.7 mW
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(165 mW m-2) (Khan et al., 2015), MnO2/CNTs (210 mW m-2) (Zhang et al., 2011)
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and graphite/β-MnO2 (172 mW m-2) (Zhang et al., 2009). The result suggested that the
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PANI/β-MnO2 nanocomposites catalyst performed higher catalytic activity and
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improved the power output. Power density and polarization curves were also
380
measured to determine the performances of the MFC with different loading amount of
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PANI/β-MnO2 nanocomposites (0, 2.2, 4.1 and 6.0 mg cm-2) as cathode catalyst in Fig.
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6b. Remarkably, the MFC equipped with the GF-PANI/β-MnO2-6.0 electrode
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generated the largest Pmax, which was 1.29 and 1.63 times higher than that of
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GF-PANI/β-MnO2-4.1 electrode (191 mW m-2) and GF-PANI/β-MnO2-2.2 electrode
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(152 mW m-2). Both OCV and power density increased with the increase of loading
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amount of PANI/β-MnO2 catalyst, which was owing to increasing active sites and
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accelerating kinetics towards ORR with the increase of catalyst amount.
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The curves of potential versus current density for the individual cathodes and
389
anodes are shown in Fig. 6c. It was obvious that there was an insignificant difference
390
in the anode potentials, but the cathode potentials were considerably different. This
391
phenomenon also occurred in Fig. 6d, which exhibited the potentials of individual
392
cathodes and anodes of different loading amount of PANI/β-MnO2 nanocomposites as
393
cathode catalyst. These observations implied that the differences in power output
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resulted from cathodes rather than anodes (Huang et al., 2017a) and the differences in
395
the MFC performances mainly were attribute to variations of the cathode catalysts and
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the loading amount of catalysts in the catalytic activities.
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398
must have high conductivity (Hao et al., 2017). All the Nyquist plots of the electrodes
399
with different catalysts and different loading amount of PANI/β-MnO2 catalyst were
400
tested using AC impedance measurements. The Nyquist plots and the equivalent
401
circuit model are shown in Fig. 6e and f. The results indicated that there was a
402
remarkable decrease in the resistance of MFC system after the addition of cathode
403
catalyst. Both the ohmic resistance (Rs) and charge transfer resistance (Rct) of the GF
404
electrode
405
GF-PANI/β-MnO2-6.0 electrode in Fig. 6e. The values of Rs for GF, GF-β-MnO2-6.0,
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GF-PANI+β-MnO2-6.0 and GF-PANI/β-MnO2-6.0 electrode were estimated to be
407
50.4, 45.5, 42.3 and 38.4 Ω, respectively. Besides, there was an obvious reduction in
408
Rct for catalyst coated cathodes. The GF-PANI/β-MnO2-6.0 electrode exhibited Rct of
409
9.2 Ω, which was lower than GF (93.3 Ω), GF-β-MnO2-6.0 (19.8 Ω), and
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GF-PANI+β-MnO2-6.0 (14.4 Ω) electrode. The lowest Rs and Rct obtained for
411
GF-PANI/β-MnO2-6.0 electrode might be explained by pronounced synergistic effect
412
between PANI and β-MnO2 of the PANI/β-MnO2 catalyst, which increased the
413
conductivity of the electrode surface and accelerated electron transfer throughout the
414
electrode (Wang et al., 2012). Fig. 6f shows the Nyquist plots of the cathodes with
415
different loading amount of PANI/β-MnO2 catalyst. The Rs of GF-PANI/β-MnO2-6.0
416
electrode was ~1.15, 1.23 and 1.31 times lower than GF-PANI/β-MnO2-4.1,
417
GF-PANI/β-MnO2-2.2 and bare electrode. In addition, there was also a reduction
418
trend in Rct. The Rct of GF-PANI/β-MnO2-6.0 electrode was lower than bare (93.3 Ω),
higher
than
GF-β-MnO2-6.0,
GF-PANI+β-MnO2-6.0
and
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19
ACCEPTED MANUSCRIPT GF-PANI/β-MnO2-2.2 (23.9 Ω) and GF-PANI/β-MnO2-4.1 (15.6 Ω) electrode. This
420
reduction in Rs and Rct could result from the more PANI/β-MnO2 catalyst on the
421
cathode surface, which were deemed to provide excess active sites for oxygen
422
molecules and improve more electrocatalytic activity of the cathode for ORR. The
423
total internal resistance of the MFC was estimated by polarization slope method,
424
which was denoted as Rint and listed in Table 1. It was also in good agreement with
425
power density results, which further indicated that the loading of PANI/β-MnO2
426
catalyst onto graphite felt surface decreased the resistance and accelerated the kinetics
427
activity. Therefore, it will be an efficient ORR catalyst for MFC applications to
428
increase power output.
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4. Conclusion
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Novel PANI/β-MnO2 nanocomposites prepared by hydrothermal and in-situ
431
chemical oxidative polymerization were used as cathode catalyst in MFCs. The
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PANI/β-MnO2 exhibited higher catalytic activity for ORR than β-MnO2 and
433
PANI+β-MnO2 mixture, owing to the interaction between β-MnO2 and PANI,
434
increased specific surface area and electric conductivity, which facilitated the electron
435
transfer process. Several tests results indicated that the PANI/β-MnO2 cathode did
436
promote the power generation of the MFC, the maximum power density was 1.2-1.4
437
times as high as those by controlled cathodes, while cathode resistance was 1.1-1.2
438
times lower, indicating the promising potential as ORR catalyst in MFC applications.
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Acknowledgements
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We gratefully thank the National Natural Science Foundation of China 20
ACCEPTED MANUSCRIPT 441
(21276128), the Basic Research Program of Jiangsu Province of China (BK20141398)
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for the financial support.
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Yao, W., Zhou, H., Lu, Y., 2013. Synthesis and property of novel MnO2@polypyrrole coaxial nanotubes as electrode material for supercapacitors. J. Power Sources. 241, 359-366.
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Zhang, W.X., Wang, H., Yang, Z.H., Wang, F., 2007a. Promotion of H2O2 decomposition activity over β-MnO2 nanorod catalysts. Colloids Surf., A. 304, 60-66. Zhang, W.X., Yang, Z.H., Wang, X., Zhang, Y.C., Wen, X.G., Yang, S.H., 2006. Large-scale synthesis of β-MnO2 nanorods and their rapid and efficient catalytic oxidation of methylene blue dye. Catal. Commun. 7, 408-412.
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Zhang, Y.P., Hu, Y.Y., Li, S.Z., Sun, J., Hou, B., 2011. Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell. J. Power Sources. 196, 9284-9289.
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Zhao, F., Slade, R.C., Varcoe, J.R., 2009. Techniques for the study and development of microbial fuel cells: an electrochemical perspective. Chem. Soc. Rev. 38, 1926-1939. Zhou, D., Che, B., Lu, X., 2017. Rapid one-pot electrodeposition of polyaniline/manganese dioxide hybrids: a facile approach to stable high-performance anodic electrochromic materials. J. Mater. Chem. C. 5, 1758-1766.
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electrode with surface modification for H2/Br2 fuel cell. J. Power Sources. 242, 15-22.
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Zhang, L., Shao, Z. G., Wang, X., Yu, H., Liu, S., Yi, B., 2013. The characterization of graphite felt
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Schematic of the preparation of the PANI/β-MnO2 nanocomposites. Fig. 2. XRD patterns of (a) PANI powders, (b) β-MnO2 nanorods and (c)
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PANI/β-MnO2 nanocomposites. Fig. 3. SEM images of β-MnO2 (a and b) and PANI/β-MnO2 (e and f), and TEM images of β-MnO2 (c and d) and PANI/β-MnO2 (g and h).
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Fig. 4. X-ray photoelectron spectroscopy (XPS) analysis of the PANI/β-MnO2
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nanocomposites: (a) survey spectra, (b) Mn 2p spectra, (c) Mn 3s spectra, and d O 1s spectra.
Fig. 5. (a) CV curves for ORR in air-saturated PBS (pH=7) at the scan rate of 50 mV s-1 with different catalyst cathodes and (b) Tafel plots of all cathodes at 1 mV s-1 and
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the linear fit for the Tafel plots (inset).
Fig. 6. Power density and polarization curves as function of current density of MFC with cathode loading different types of catalyst (a), and with cathode loading different
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amount of PANI/β-MnO2 (b). Individual potential (vs. Ag/AgCl) as function of current
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density of MFC with cathode loading different types of catalyst (c), and with cathode loading different amount of PANI/β-MnO2 (d). Nyquist curves of the EIS data of MFC with cathode loading different types of catalyst (e), and with cathode loading different amount of PANI/β-MnO2 (f).
ACCEPTED MANUSCRIPT Table 1. Performance of the MFC using GF-PANI/β-MnO2 air-cathode. Cathode resistance OCV
OCP
Pmax
Rint (Ω)
Cathode (mV)
(cathode/mV)
-2
(mW m )
(Ω)
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Rs
Rct
680
244
134
243.7
50.4
93.3
GF-β-MnO2-6.0
723
276
183
175.3
45.5
19.8
GF-PANI+β-MnO2-6.0
739
281
204
164.7
42.3
14.4
GF-PANI/β-MnO2-6.0
770
307
248
155.6
38.4
9.2
GF-PANI/β-MnO2-4.1
730
GF-PANI/β-MnO2-2.2
705
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280
191
170.2
44.2
15.6
258
152
182.1
47.2
23.9
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ACCEPTED MANUSCRIPT Highlights The PANI/β-MnO2 nanocomposites are synthesized successfully.
The nanocomposites serve as electrocatalysts for the air cathode of MFCs.
The performance of PANI/β-MnO2 nanocomposites modified MFC is enhanced.
The nanocomposites are potential electrocatalysts for air-cathode MFCs.
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