Modification of anode electrode in microbial fuel cell for electrochemical recovery of energy and copper metal
Graphical abstract
Introduction
Microbial fuel cell (MFC) is a diversifying technology that has drawn researchers' attention with direct conversion of chemical energy from organic substrate to electrical energy by exo-electrogenesis of microorganisms [1,2]. The energy demand for electrochemical reaction in MFC is fulfilled by organic waste present in the anode chamber [3]. In spite of innumerous efforts have been made to promote the performance of MFCs [4,5], poor power density, low electron transfer efficiency, high membrane and construction cost still limit the performance of MFCs [6,7]. The performance of MFC is highly dependent on the anodic modification, as anodic material is crucial in the design of an MFC [8,9]. The mandatory properties of anode include large surface area, high electronic conductivity, good biocompatibility and good stability [1,10]. To enhance the performance of an anode, ammonia treatment [11], Fe3O4 [12], polyaniline modification [13], electrochemical oxidation [14], graphene/polyaniline [15] have been employed.
The precious metal like copper is extracted from lithosphere at an increasing rate. Almost 60% of copper has been lost in waste repositories and only 40% is available to cater current demand [16]. The MFC is a sustainable technology to recover copper from waste with simultaneous energy generation [17,18]. The few researchers have used dual chamber MFC for bivalent copper removal with high rate of copper recovery and simultaneous electricity generation [[19], [20], [21], [22]]. Few of them have used single chamber MFC for copper recovery and studied the effect of copper on microbial community [23,24].
The cathodic reduction of Cu2+ is incorporated with simultaneous oxidation of organics in an anode chamber. The proposed reduction of Cu2+ to Cu(s) and Cu2+ to Cu2O was depends on favorable pH and Cu2+ concentration [18]. The electrons are generated in an anode chamber and flow through external circuit towards cathode chamber, where reduction of copper metal takes place [25].
Unfortunately the electron transfer between bacteria and anode electrode is difficult due to low bacteria loading capacity [26]. The anode material should provide space for bioaccumulation, effective immobilization, growth and cultivation and should also offer good biocompatibility facile substrate movement to microorganisms. Anode should also be a good conductor to facilitate electron transfer and capacitor to collect current from all electrode regions [27,28].
Graphene and its derivative have been proved to be the best material because of their unique electrical, thermal, mechanical properties and have wide applications in energy related devices [29,30]. It also provides extra ordinary properties like good biocompatibility and good mechanical strength [31,32]. Graphene possess good electron transfer properties, but have poor electron accepting capacities [33]. Graphene oxide (GO) possesses poor stability with long term use and low electronic conductivity [34,35].
Fullerene (C60) is also one of the high specific surface materials entirely composed of carbon in the form of spherical shape and have diverse application in building electrodes, electronics, batteries, supercapacitor, fuel cell, electrochemical sensors and biosensors [36]. The C60 shows improved electrical conductivity, structural stability and thermal stability when formulated and processed in optimized manner. The C60 is allotropic carbon structure and possesses remarkable physico-chemical properties, particularly high electron accepting in organic photovoltaic cells [37]. The remarkable properties of C60 have facilitated its use in electrochemical activities. Partially reduced fullerene (rC60) modified electrodes have been proved as an excellent working electrode having properties such as high electro-active surface area, excellent electronic conductivity and good biocompatibility [38,39].
The conducting polymers like polyaniline (PANI), PPy, thiophene have been extensively used in MFC [40,41] to improve the bacteria adhesion and electron transfer [42,43]. The carbon material possesses poor electrical conductivity and deprived extracellular efficiency compared to noble metals limits its use in MFCs [44].
Therefore by considering the wide versatility of GO and C60, in this study we have demonstrated the synthesis of prepared rGO/PPy composite graphite anode by electro-polymerization and rC60/PPy composite graphite anode by simple electrochemical methods respectively. The prepared rGO/PPy and rC60/PPy composite exhibited the better electrochemical properties compare to bare graphite and PPy alone. The performances of modified rGO/PPy and rC60/PPy composites were investigated and compared with bare graphite and PPy modified anodes as an effective electrochemical platform for the recovery of copper metal by using MFC. The effect of rGO and rC60 content on the performance of MFC based on rGO/PPy and rC60/PPy composite modified electrode is presented and discussed for power generation and metal copper recovery. This study involves the characterization of composites by SEM, FT-IR, XRD, CV and EIS method. This novel route facilitates the electrochemical remediation of precious copper metal.
Section snippets
Materials
The experimental chemicals were purchased from viable sources and were of analytical grade. Pyrrole (Py) was distilled before use. The GO was prepared by modified Hummers method [44]. Distilled water was produced by Milli-Q filter (Millipore, USA). Fullerene (C60) and dimethyl formadide (DMF) were obtained from Sigma Aldrich (USA). The bare graphite electrodes (area = 8 cm2) were washed with DI water three times, rinsed with H2O2 to remove impurity and then dried at 60 OC for 2 h. A copper wire
Scanning electron microscopy (SEM)
The morphological structure of modified anode electrodes is given in Fig. 1. It clearly ascertained different structure for different modification. Fig. 1a shows the pure graphite electrode having smooth and multilayer configuration without any modification. The white, blistering structure was exhibited by polymerized PPy over the plain graphite surface as shown in Fig. 1b. Fig. 1c shows the morphological configuration of rGO/PPy composite over the plain graphite structure. The rGO sheets were
Conclusion
Microbial fuel cell electrodes were effectively modified by PPy, rGO/PPy and rC60/PPy. The resulting electrodes were successfully explored for the removal of Cu2+ from synthetic copper ion solution. The MFCs with modified rGO/PPy yields the maximum power density of 835.21 mW/m3, which is 2 times larger than bare graphite electrode with simultaneous 82.8% Cu2+ removal. The lower Rct value and improved oxidation-reduction current of rGO/PPy electrode by EIS and CV analysis show the improved
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