Elsevier

Chemosphere

Volume 193, February 2018, Pages 618-624
Chemosphere

In situ fabrication of green reduced graphene-based biocompatible anode for efficient energy recycle

https://doi.org/10.1016/j.chemosphere.2017.11.057Get rights and content

Highlights

  • Anode containing rGO/Au nanocomposite was synthesized by Eucalyptus extract.

  • A 3.2-fold higher power density of 33.7 W m−3 was achieved.

  • Anode affords larger surface roughness for microbial colonization.

  • RGO/Au nanocomposites facilitate electron transfer from electricigens to the anode.

Abstract

Improving the anode configuration to enhance biocompatibility and accelerate electron shuttling is critical for efficient energy recovery in microbial fuel cells (MFCs). In this paper, green reduced graphene nanocomposite was successfully coated using layer-by-layer assembly technique onto carbon brush anode. The modified anode achieved a 3.2-fold higher power density of 33.7 W m−3 at a current density of 69.4 A m−3 with a 75% shorter start period. As revealed in the characterization, the green synthesized nanocomposite film affords larger surface roughness for microbial colonization. Besides, gold nanoparticles, which anchored on graphene sheets, promise the relatively high electroactive sites and facilitate electron transfer from electricigens to the anode. The reduction-oxidation peaks in cyclic voltammograms indicated the mechanism of surface cytochromes facilitated current generation while the electrochemical impedance spectroscopy confirmed the enhanced electron transfer from surface cytochrome to electrode. The green synthesis process has the potential to generate a high performing anode in further applications of MFCs.

Introduction

Microbial fuel cells (MFCs) have received increasing attention over the past decade due to their potential applications to replace or minimize the use of carbon-based sources of energy (Rabaey, 2010). Simultaneous wastewater treatments and renewable power generation via electro-active microorganisms’ catalysis were achieved successfully in MFCs. However, the relatively low power output has limited the practical application of MFCs. Anode performance, which is directly affected by bacteria attachment and electron transfer, is the key limiting factor for high current output (Katuri et al., 2011). The widely used carbon paper and carbon cloth provide limited electron collecting capacities and lack for effective surface area for bacterial adhesion (Liu et al., 2012). Therefore, it is essential to develop effective anode materials with high biocompatibility, conductivity and large specific surface area.

Extensive efforts have been directed to enhancement of certain properties of anode such as the use of various graphite (Kardi et al., 2017) and reticulated vitreous carbons. The modification of electrodes by conducting polymers (Tao et al., 2016), carbon nanotubes (CNTs) (Zhao et al., 2012), graphene (Guo et al., 2014), and metal particles (Guo et al., 2012) have been studied as well. A recent study reported a 17-fold higher maximum power density of 2.420 W m−2 was achieved in a mediator-less MFC inoculated with Escherichia coli using polypyrrole/poly(vinyl alcohol-co-polyethylene) nanofibers/poly(ethylene terephthalate) (PPy/NFs/PET) anode (Tao et al., 2016). The MFCs with CNTs textile anode achieved high areal power densities of 1.098 W m−2 (Xie et al., 2011). It is reported that the modified anode using ionic liquid functionalized graphene enhanced the maximum power density output 4.2-fold at 0.601 W m−2 (Zhao et al., 2013). A two-chambered MFC equipped with the gold nanoparticles modified anode achieved a higher maximum power density of 0.346 W m−2 with a 36% shorter start-time than the unmodified group (Guo et al., 2012). The use of graphene aerogels as electrodes has been reported as well, in which the porous graphene aerogel anode achieved high areal power density (0. 680 W m−2) but limited volumetric power density (0.68 W m−3) (Wang et al., 2013).

Graphene has been a major focus of composite materials due to its unique 2D sp2-hybridized carbon network structure, which has the ability to facilitate the superior electrical and mechanical properties. It also retains a fine thermal/chemical stability and large surface area (Xu et al., 2008). However, the toxicological behavior of reduced graphene oxidation (rGO) has been widely documented and graphene-based materials have been utilized for antibacterial applications (Maas, 2016). The main mechanisms for cellular toxicity caused by rGO are based on severe insertion and cutting away of cell membranes and destructive extraction of lipid molecules (Tu et al., 2013). It is reported that the density of graphene edges determines the antibacterial activity of surface-deposited graphene (Pham et al., 2015), and the decoration or capping of proteins and biomolecules can deactivate the antibacterial activity of rGO (Hui et al., 2014).

According to previous studies, gold nanoparticles (Au NPs) can serve as an ideal electrode surface modification material due to their unique high surface energy and biocompatibility (Guo et al., 2012). However, particle size and aggregation degree are two vital factors that decide the catalytic efficiency of nanoparticles (Liu et al., 2011b). Therefore, the construction of nanocomposite with high nanoparticle disperse ratio and good mechanical properties are desirable for anode modification.

This study aims to fabricate a biocompatible anode with high surface activity in order to create a high bacterial adhesion rate and efficient electron shuttling in bio-electrochemical systems. The layer-by-layer (LBL) assembly method coupled with plant-mediated green reduction was first applied for the dispersion and reduction of graphene and gold nanocomposite onto carbon brush (CB). Multilayered films formed by self-assembly, which based on electrostatic interactions, with precise control of film component and thickness. Aggregation can be avoided when Au NPs are uniformly deployed on graphene sheets, while the anchored Au NPs serve as spacers that increase the distance between the graphene sheets and double the size of the accessible surface (Goncalves et al., 2009). The green synthesized rGO films assembled on carbon fibers create folds and wrinkles on the anode surface. This enlarges the specific surface area for bacteria attachment and benefit the growth of biofilms. The reduced graphene and gold nanocomposites (rGO/Au) have more active sites for catalyst and fast electron transfer processes on the surface of rGO sheets and Au NPs(Su et al., 2017).

This work aims to determine the efficiency of rGO/Au nanocomposite coated carbon brush (rGO/Au CB) anode in promoting electricity generation and energy recycle. To achieve this, (1) the performance of the modified anode in MFCs was studied, (2) electrochemical catalytic activity of modified electrodes and electron transfer between bacteria and the anode were investigated by cyclic voltammetry (CV) and electrochemical impedance spectra (EIS), (3) surface of modified anode before using in MFCs was characterized to confirm the coat of rGO/Au on CB, (4) after inoculation, the surface of modified anode were also investigated by Scanning Electron Microscope (SEM) to evaluate the biocompatibility of rGO/Au CB anodes.

Section snippets

Chemicals and materials

Carbon brushes were firstly cleaned by soaking them in 1 M HCl and 1 M NaOH, and then they were washed by deionized water and dried in air. Polyethyleneimine (PEI) (M.W. 60,000, 50 wt% aq. solution, branched) and Gold (III) chloride trihydrate were purchased from Sigma. Graphene oxide (GO) was prepared from graphite powders according to the modified Hummers method (Hummers and Offeman, 1958). Dried Eucalyptus leaves were collected from a vicinity in Adelaide (South Australia). The ground

Characterization of green synthesized biocompatibility anode

UV–vis absorption spectra were measured following the construction of bilayers (Fig. 1a). The absorbance peak at 234 nm was found to increase linearly with the number of bilayers formed in the layer-by-layer structure. In the inset of Fig. 1a, the GO peaks disappeared after green reduction while new peaks at 269 and 579 nm were shown, which indicated the form of green synthesized rGO and gold nanoparticles. The peak at 269 nm is ascribed to π−π* transition of the Csingle bondC band and the shoulder peak

Conclusion

The layer-by-layer structure of green reduced rGO/Au NPs film creates a high bacteria loading capacity, promotes intimate contact between the electricigens and anode surface and facilitates cell-anode interaction. The small Rct of rGO/Au CB (42 Ω) indicates a faster electron transfer rate between electrodes and electrolytes in the process of electricity generation and power delivery. This green approach for designing biocompatible anode provides much potential for high-performance MFCs and

Acknowledgements

This research was supported by the CRC for Contamination Assessment and Remediation of Environment, Australia. Miss Ying Cheng thanks the University of South Australia, University of Newcastle and CRC CARE for the scholarship.

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