Development and operation of gold and cobalt oxide nanoparticles
containing polypropylene based enzymatic fuel cell for renewable fuels
Muhammet Samet Kilic
a, Seyda Korkut
b,n, Baki Hazer
a, Elif Erhan
ca
Department of Chemistry, Bulent Ecevit University, 67100 Zonguldak, Turkey
bDepartment of Environmental Engineering, Bulent Ecevit University, 67100 Zonguldak, Turkey c
Department of Environmental Engineering, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey
a r t i c l e i n f o
Article history: Received 4 March 2014 Received in revised form 9 May 2014
Accepted 24 May 2014 Available online 9 June 2014 Keywords:
Enzymatic fuel cell Polypropylene Nanoparticles Glucose oxidase Bilirubin oxidase
a b s t r a c t
Newly synthesized gold and cobalt oxide nanoparticle embedded Polypropylene-g-Polyethylene glycol was used for a compartment-less enzymatic fuel cell. Glucose oxidase and bilirubin oxidase were selected as anodic and cathodic enzymes, respectively. Electrode fabrication and EFC operation parameters were optimized to achieve high power output. Maximum power density of 23.5mW cm2
was generated at a cell voltage of þ560 mV vs Ag/AgCl, in 100 mM PBS pH 7.4 with the addition of 20 mM of synthetic glucose solution. 20mg of polymer amount with 185 mg of glucose oxidase and 356 mg of bilirubin oxidase was sufficient to get maximum performance. The working electrodes could harvest glucose, produced during photosynthesis reaction of Carpobrotus Acinaciformis plant, and readily found in real domestic wastewater of Zonguldak City in Turkey.
& 2014 Elsevier B.V. All rights reserved.
1. Introduction
An enzymatic fuel cell (EFC) uses enzymes to convert chemical energy to electrical energy. Fuels such as hydrogen, alcohols and sugars, with oxygen as oxidant, can be employed in research-based EFC prototypes due to enzyme diversity (Palmore et al., 1998;Palmore and Kim, 1999;Heller, 2004;Barriere et al., 2006). Of these, the glucose/O2 EFC has received considerable attention
due to the relatively sufficient concentration of glucose in blood, plant and organic wastes leading to potential application of EFCs to in vivo power production for low energy demanding biomedical devices (Heller, 2004). The advantage of EFCs is to exploit the selectivity of enzymes in order to build a fuel cell that does not require the separation of the anode from the cathode. Both electrodes can be immersed in one compartment containing fuel and oxidant at the same time. By this way, EFC can be miniaturized in principle down to micrometer scale, in order to be used, e.g., for implantable devices and in vivo mediums (Brunel et al., 2007; Kendall, 2002;Service, 2002).
Although EFCs can theoretically meet the power demands of some biomedical devices, low power output of EFCs remains an issue. For development of EFCs with high power, an efficient
electrical enzyme–electrode communication related to working electrode types and EFC operational parameters is required. There-fore, it is necessary to optimize electrode fabrication steps and EFC operational parameters. Polymeric film structure, one of these parameters, is effective to get maximum performance from EFCs since it mainly controls the electronflow between electrode and enzyme. Polypropylene (PP) is one of the most important poly-olefines due to its wide industrial production, low cost, good mechanical properties, easy processing, and excellent recyclability (Hazer, 1990,2010 Eroğlu et al., 1996; Hazer et al., 1992; Yıldız et al., 2012;Ozturk et al., 2013;Koike and Cakmak, 2009;Lee et al., 2009). Furthermore, it is a very versatile, hydrophobic polymer that has medical and industrial applications due to its goodfilm andfiber properties (Kilic et al., 2014). The direct charge transfer from the enzyme redox center to the electrode cannot be usually seen, so sometimes adsorption or immobilization of artificial electron relays, such as some metallic complexes into the polymer, is proposed to obtain fast electron transfer especially for the operation of compartment-less EFCs.
In the present study, gold and cobalt oxide nanoparticle embedded Polypropylene-g-Polyethylene glycol (PP-g-PEG) was newly synthesized and first time used for an EFC. Electrode fabrication parameters and EFC operational conditions were care-fully optimized to maximize the system performance. The elec-trical generation ability of the optimized EFC was investigated by Contents lists available atScienceDirect
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Biosensors and Bioelectronics
http://dx.doi.org/10.1016/j.bios.2014.05.068
0956-5663/& 2014 Elsevier B.V. All rights reserved.
nCorresponding author. Tel.:þ90 372 2574010; fax: þ90 372 2574023.
using glucose produced in a photosynthetic plant and found in domestic wastewater.
2. Material and methods 2.1. Reagents
Chlorinated polypropylene (PP) (Mw 150,000 Da, three repeat-ing units have 1 Cl in average), polyethylene glycol with Mn¼8000 Da (PEG8000), Tetrahydrofuran (THF, Z99% GC grade), Glucose oxidase from Aspergillus niger (GOx) (10 KU), Bilirubin oxidase from Myrothecium verrucaria (BOD) (25 U) and Laccase from Trametes versicolor were obtained from Sigma-Aldrich. Sodium hydroxide, hydrochloric acid, potassium di-hydrogen phos-phate and di-potassium hydrogen phosphos-phate were purchased from Merck. Glucose monohydrate was supplied from Riedel. Stock solutions of enzymes and glucose were daily prepared in 100 mM pH 7.4 phosphate buffers.
2.2. Apparatus and electrochemical measurements
Fuel cell experiments were performed by using a CHI 1040B Model electrochemical analyzer. Glassy carbon (ؼ3 mm) and gold (ؼ2 mm) working electrodes, Platinum wire counter-elec-trode, Ag/AgCl (3 M NaCl) reference eleccounter-elec-trode, and a conventional electrochemical cell obtained from the samefirm. Electrochemical measurements were carried out in aerated 100 mM, pH 7.4 phos-phate buffer solution with an applied potential of 200 mV for anode and þ360 mV for cathode under continuous stirring at 100 rpm by using four-electrode compartment-less cell. Four-electrode system was immersed into the cell; anodic and cathodic potentials were applied and waited for reaching to a steady-state amperometric current value under the constant stirring at 100 rpm. Then, various concentrations of glucose ranging between 10 and 300 mM were added successively to the electrochemical cell to produce current–time recordings for both anode and cathode sides of the EFC system.
2.3. Synthesis of gold and cobalt oxide nanoparticles embedded PP-g-PEG
PP-g-PEG was synthesized by modifying a previously reported procedure (Kalaycı et al., 2010,2013;Balcı et al., 2010). Typically, chlorinated polypropylene, 1.5 g (10 mmol Cl) was dissolved in 50 mL of freshly distilled THF. 20 mL of a THF solution containing PEG (10 mmol) and 2 mL of an aqueous NaOH solution (50 wt%) were added drop wise to the chlorinated PP solution within 20 min. The reaction mixture was poured into 500 mL of water containing 1 mL of concentrated HCl after stirring for 3 days at room temperature. The polymer wasfiltered, washed with water and dried under vacuum overnight. For the purification, it was re-dissolved in THF and re-precipitated in 200 mL of distilled water and then dried under vacuum overnight at 50°C. After the purification step gold and cobalt oxide nanoparticles were embedded into the polymer by the following procedure: solutions of metal salts 0.58 g of HAuCl4in 20 mL THF, 0.35 g of CoCl2 6H2O
in 6.4 mL pure water and the reducing agent 65 mg of NaBH4in
3.2 g of pure water were prepared. PP-g-PEG (0.2 g) was dissolved in 10 mL of THF. 4 drops of metal salt solutions were added into the polymer solution and vigorously stirred at room temperature for an hour. Then, 2 drops of the reducing agent were added to this mixture and stirred again for 2 h. Final solution was poured into a Petri dish (ؼ5 cm) to achieve film formation via solvent casting. Composite polymerfilm was peeled away from the Petri dish after 24 h, washed with methanol and dried under vacuum at room temperature for one day. 10 mg mL1 of polymer solution was prepared in toluene for readily use in fuel cell system.
2.4. Preparation of EFC electrodes
The electrode fabrication step was preceded by a cleaning phase of the glassy carbon electrode surface using gamma alumina powder, then rinsing with distilled water. 2mL of the nanoparticles embedded polymer solution (10 mg mL1) was directly spread onto the surface of the cleaned anode and cathode. The electrodes were then allowed to dry for solvent evaporation at room temperature. Working electrodes were then washed with distilled water. 20mL of GOx (20 mg mL1) and 20mL of BOD (20 mg mL1)
were dropped onto the polymerfilm coated anode and cathode, respectively. The electrodes were allowed to dry and wait for enzyme immobilization at room temperature for 2 h. Then the electrodes were washed in 2 mL of 100 mM, pH 7.4 phosphate buffer solution to remove the unbound enzyme from the surface of the electrodes.
3. Results and discussion 3.1. Characterization studies
FT-IR, FTIR-ATR and GPC characterizations of thefinal polymer were previously reported by our chemistry group (Kalaycı et al., 2010,2013;Balcı et al., 2010).1H-NMR spectra of the samples in
CDCl3 as solvent and tetra methylsilane as the internal standard
were recorded using Bruker mq 20 Minispec model Pulsed NMR Spectrometer (Fig. S1, Supporting information). In the 1H NMR
spectrum of the polymer, a sitrict signal between 3 and 4 ppm is assigned to the PEG, the peaks between 0.9 and 2 ppm are attributed to the PP, and the peak observed between 3.5 and 4 ppm presents–CH2–Cl group of the chlorinated PP chain,
respec-tively. SEM images of PP-g-PEG (Fig. 1A), GOx immobilized PP-g-PEG (Fig. 1B) and BOD immobilized PP-g-PEG (Fig. 1C) coated electrode surface were taken with Quanta FEG 450 model SEM. Several pores were clearly observed from the SEM of pure PP-g-PEG coated electrode surface. Different surface morphologies were observed from the SEM images of GOx and BOD immobilized polymer layer; in addition, the pores were not recognized easily for the enzyme bound polymer layers. X-ray diffraction showed that cobalt was oxidized to cobalt oxide nanoparticle in the polymer chain (not presented).
The electrochemical characteristics of the nanoparticles embedded PP-g-PEG and pure PP-g-PEG (without nanoparticle) film coated electrodes were evaluated through cyclic voltammetry (CV) (Fig. 2). The CVs were obtained in 10 mL of 100 mM pH 7.4 phosphate buffer at a potential scan between 600 and þ600 mV at a scan rate of 100 mV s1. The pure polymer did not
show any electroactivity (Fig. 2). In the case of metallic nanopar-ticles present in polymer, CV waves were shaped since the polymer showed electroactivity and did not block the electron transport between the electrode materials and the enzymes.
3.2. Optimization of working electrodes fabrication step 3.2.1. Selection of the cathodic enzyme and electrode material
It is known that both BOD and GOx show efficient activity at the same pH level around 7 (Mano et al., 2003a,2003b). This is highly advantageous for our compartment-less EFC system where anode and cathode are immersed into the same solution. However, BOD activity can be lower in comparison to laccase activity especially at in vivo mediums such as plants generally have a pH level ranging between 5 and 6. (Osman et al., 2011). Therefore, laccase was commonly selected as cathodic enzyme in EFC studies (Smolander et al., 2008; Gupta et al., 2011; Karaskiewicz et al., 2012;Stolarczyk et al., 2011;Barriere et al., 2006;Lee et al., 2013; Wang et al., 2009). From this viewpoint, laccase and BOD were tested at cathode side of the EFC. Two EFCs including laccase or BOD were fabricated with the same fabrication procedure as follows: 2mL of nanoparticles embedded PP-g-PEG solution was directly spread onto the surface of the cleaned glassy carbon anode and cathode. The electrodes were then allowed to dry for solvent evaporation at room temperature. 20mL of GOx (10 mg mL1) was
dropped onto the anode, and 20mL of BOD (10 mg mL1) and
20mL of laccase (10 mg mL1) were dropped onto the cathode,
respectively. The working electrodes were allowed to dry and waited for enzyme immobilization at room temperature for 2 h. Electrochemical measurements were carried out in aerated 100 mM, pH 7.4 phosphate buffer solution for the EFC system including BOD and 100 mM, pH 5 citrate buffer for the EFC system including laccase, with an applied potential of200 mV for anode and þ360 mV for BOD immobilized cathode, and 200 mV for anode and þ590 mV for laccase immobilized cathode under continuous stirring at 100 rpm by using a four-electrode cell. Increasing concentrations of glucose were added to both reaction mediums to produce amperometric current–time recordings.Fig. S2(A) and (B), Supporting information show oxygen reduction of BOD and laccase cathode of EFC, respectively. It is clearly shown from Fig. S2(A) that BOD is proper cathodic enzyme for our compartment-less EFC system due to its stable and proportional signals towards increased concentrations of glucose additions. The signals of the laccase cathode were noisy, unstable and dispropor-tional vs increasing glucose concentration (Fig. S2(B), Supporting information). Lower glucose oxidation current was observed at the anode of the laccase containing EFC system than the BOD contain-ing one due to the fact that GOx activity might be blocked at pH 5. Biofuel cells generally involve complex arrangements of immo-bilization polymers, redox mediators, and enzymes that must easily interact and conform to electrode material. The feature of different electrode material results in different activation polariza-tion losses (Wang et al., 2013). EFC systems including GOx anode and BOD cathode were prepared by using both glassy carbon and gold electrode material, and successive addition of various glucose concentrations was conducted according to the procedure given above. Cathodic current generation of the BOD cathode prepared with gold electrode (Fig. S2(C), Supporting information) was compared with glassy carbon electrode result shown inFig. S2(A), Supporting information. It was clearly seen that current genera-tion was higher for glassy carbon. Glassy carbon electrodes have been widely used and compared with metal electrodes due to its biocompatibility with tissue, having low residual current over a wide potential range and minimal propensity to show a deterio-rated response as a result of electrode fouling (Korkut Ozoner et al., 2010; Jin et al., 2008; Lane and Blaha, 1990; Ewing et al., 1981;Mattson and Jones, 1976).
3.2.2. Amount of polymer and immobilized enzyme
Increasing film density may lead to a hindrance effect for electron transfer between electrode and enzyme since it creates a
Fig. 2. Cyclic voltammogram of nanoparticle embedded PP-g-PEG/GOx anode, PP-g-PEG/BOD cathode and PP-g-PEG coated electrode (without nanoparticle) in 10 mL of 100 mM phosphate buffer solution (pH 7.4) at a potential scan ranging between600 and þ600 mV (vs Ag/AgCl) with a scan rate of 100 mV s1.
barrier for mass transfer. Therefore, the amount of the polymeric film should be optimized to acquire maximum power from the system: EFC systems were prepared in a series by coating the electrodes with various amounts of nanoparticles embedded PP-g-PEG (2.5–5–10–20–40 mg). 20 mL of GOx (10 mg mL1) and 20mL
of BOD (10 mg mL1) were dropped onto the anode and cathode for each EFC system. The electrodes prepared with different polymer amounts were tested by adding 10 mM glucose into the cell compartment including 10 mL of 100 mM, pH 7.4 aerated phosphate buffer solution with an applied potential of 200 mV for anode and þ360 mV for cathode. The cathodic current den-sities obtained from EFC series were presented in Fig. S3(A), Supporting information. Bradford Protein Assay (Bradford, 1976) was used to calculate immobilized enzyme amount on the electrode surface coated with various polymer quantities (Table S1,Supporting information). Immobilized GOx and BOD quantity increased by increasing polymer amount. However, the highest current density was generated from the EFC working electrodes coated with 20mg of polymer. It is clearly shown that current generation performance of EFC system depends mainly on electron transfer mechanism through the electrochemical process rather than the amount of immobilized enzyme. Even though both enzyme quantity on electrode surface and as a result biochemical reaction rate increased with increasing polymer weight, after a definite polymer weight a hindrance effect was created for electron transport occurring between enzymes and electrode material.
EFCs systems including different amounts of enzyme were prepared by dropping solution containing 100–200–400–600– 1000mg enzyme onto the 20 mg of polymer coated anode and cathode, respectively. EFC systems were tested by adding 10 mM glucose into the cell including 100 mM, pH 7.4 aerated phosphate buffer solution at an applied potential of200 mV for anode and þ360 mV for cathode. The cathodic current densities obtained from EFC series were shown inFig. S3(B), Supporting information. Physically adsorbed enzyme amounts into the polymericfilm were calculated using the Bradford method (Table S2, Supporting information). The results showed that the cathodic current increased with increasing enzyme loading and reached the max-imum when the amount of immobilized BOD was 356mg. How-ever, both GOx and BOD loading continuously increased; the current remained unchanged, which was due to the saturation of enzyme active centers with glucose at tested glucose concentration.
3.3. Optimization of operational condition: cell voltage, pH, and salt parameters
The redox reaction of GOx and BOD is stated below (Flexer and Mano, 2010): ⟹ + ++ − Anode: 2C H O6 12 6 2C H O 4H 4e (1) GOx 6 10 6 + ++ −⟹ Cathode: O2 4H 4e 2H O (2) BOD 2 + → +
Enzymatic fuel cell: 2C H O6 12 6 O2 2C H O6 10 6 2H O2 (3)
EFCs system are worked by applying a proper potential to anode and cathode to supply electronflow through the circuit. Glucose is oxidized to gluconolactone by the anodic enzyme GOx, and electrons are released (Eq. (1)). The released electrons are transferred to the polymer from the enzyme and then the anode electrode material. The electrons are flowed through to the cathode due to the driving force of the applied anodic potential. They are cathed by cathode with the aid of cathodic potential, then
easily by PP-g-PEG due to its metallic nanoparticles embedded structure. BOD reduces oxygen to water by using these electrons (Eq. (2)). The electron flows through cathode is observed as electrical current with applied cell voltage (Ecell¼Ecathode–Eanode).
The electrical power density P of a biofuel cell is defined as the product of the cell potential and the generated current density P¼I Vcell. It can be calculated by normalizing the power with
respect to the electrode cross-sectional area or the electrode volume (Osman et al., 2011). Fig. S4(A), Supporting information shows the power density of the EFC operated at various voltages (Anode voltage/Cathode voltage: 100/þ100, 200/þ200, 200/þ360, 300/þ400, 400/þ400, and 500/þ500 mV, respectively) including 10 mM glucose. The redox potential of GOx at pH 7.2 is 360 mV (Stankovich et al., 1978), and that of BOD is about þ360 mV vs Ag/AgCl (Hirose et al., 1998). When the anode was poised at200 mV and the cathode at þ360 mV vs Ag/AgCl, the cell operating atþ560 mV, the maximum power was obtained as 14mW cm2. Thus, when the cell operates at þ560 mV, the
potentials of GOx and BOD differ by 720 mV. This value, repre-senting the limit of the operating voltage of the cell, is close to the difference between the observed threshold potentials for electro-oxidation of glucose (200 mV vs Ag/AgCl) and electroreduction of oxygen (þ480 mV vs Ag/AgCl) (Mano et al., 2003a, 2003b). Although the potential difference between the GOx redox centers and the redox polymer is only 160 mV, the polymer effectively wired the enzyme redox centers due to its gold and cobalt oxide nanoparticle content. It should be noted that the power density is within the range of those (5–10 mW cm2) already described for
EFCs combined with various mediators (Nazaruk et al., 2008;Tan et al., 2008;Deng et al., 2008). However, these systems are based on more expensive and complex configurations involving for instance carbon nanotubes and ionic liquid (Liu and Dong, 2007) or multi-walled carbon nanotubes and electrogenerated polymer (Tan et al., 2008) as immobilization matrices. A power density of 1.38mW cm2was reported for a membrane-less EFC comprised
of carbon nanotubes deposited on porous silicon and GOx–laccase enzyme couple with 4 mM glucose (Wang et al., 2009). Okuda et al. (2007) stated 64 nW cm2power using a compartmentless EFC with only glucose dehydrogenase enzyme and carbon paste matrix as a result of 20 mM glucose addition. A power density range of 0.11–0.17 mW cm2 was reported at a cell voltage of
þ240 mV for GOx–laccase EFC system including osmium which was commonly used material in recent years to obtain excellent signals (Jenkins et al., 2012).Fig. S5,Supporting information shows current signals of pure, gold nanoparticle, and gold/cobalt oxide nanoparticle embedded PP-g-PEG coated electrodes. EFC system without nanoparticles did not give amperometric response to the fuel. When the polymer was synthesized with only gold nanopar-ticle, observable currents were formed by addition of increasing glucose concentrations. However, these signals are not sufficient to generate utilizable power in comparison to gold and cobalt oxide nanoparticle including EFC system. It is clearly seen fromFig. S5 (C) that the highest cathodic current and therefore the most powerful energy were generated when gold and cobalt oxide nanoparticles were together in the polymer chain.
Fig. S4(B), Supporting information depicts the power densities of the EFC system in citrate and phosphate buffers at various pH values of 4.4–5.4–6.4–7.4 and 8.4 at a cell voltage of þ560 mV obtained from the addition of 20 mM glucose. Power density increased with pH changes from 4.4 to 7.4, then sharply decreased. The maximum power density (23.5mW cm2) was obtained at pH
7.4. This result can be explained by the pH activity profile of GOx and BOD that presents the highest activity at the same pH level around 7 (Mano et al., 2003a,2003b;Rengaraj et al., 2011).
Higher working potential might be required to observe redox mechanism on the working electrode surface in the absence of a
supporting electrolyte (Can et al., 2012). Fig. S4(C), Supporting information depicts the current generated from the anode and cathode by the addition of 20 mM glucose to reaction mediums including 0.3–0.6–1–2 mg mL1of NaCl as supporting electrolyte
at a cell potential ofþ560 mV. It was observed that NaCl addition resulted in current decrease for working electrodes. This can be attributed to the inhibition effect of excessive salt concentration on catalysis by binding directly to an enzymic active site or disruption of the local structure of enzyme active site (Park and Raines, 2001). In the experiments, phosphate quantity (as a salt) in the working buffer was sufficient to acquire maximum current generation. Thus the EFC system can be easily operated in in vivo mediums such as plants without extra salt addition to accelerate electron transport.
3.4. Power generation from municipal wastewater and Carpobrotus Acinaciformis plant
The optimized EFC was operated using glucose as renewable fuel found in C. Acinaciformis plant and wastewater to generate power. A living plant ideally keeps on generating fuel during photosynthesis to make the biofuel cell work for as long as the anode and cathode are active. We selected C. Acinaciformis plant since it is suitable energy source for our work due to its high water content and glucose generating capacity. The plant was kept in darkness for at least 24 h before the experiments. The working electrodes were immersed into the aqueous leaf of the plant with reference electrode and counter-electrode. Electrochemical experi-ment started in the dark state (light-off) was carried out at an applied potential of 200 mV for anode and þ360 mV for cathode. The current reached nearly a stable baseline during time period of about 1 h, then the plant was illuminated.Fig. 3shows current responses upon illumination/darkness cycles of anode and cathode. Glucose oxidation current generated by GOx increased upon illumination and oxygen reduction by BOD was observed at
the cathode due to the electrons flowing through the circuit simultaneously. Currents observed inFig. 3can only be related to glucose oxidation and oxygen reduction since GOx is highly selective for glucose in comparison to other sugars, and BOD only accepts oxygen as electron acceptor (Flexer and Mano, 2010), However, the baseline was unstable due to other biological pro-cesses occurring within the leaf during photosynthesis. The obtained power density from the plant was calculated to be 1.3 mW cm2 at a cell potential ofþ560 mV. We demonstrated
that the optimized EFC could harvest glucose and oxygen pro-duced during photosynthesis to produce energy, transforming sunlight into electricity in a simple, renewable, and sustainable way. When a cell needs energy it breaks down glucose via glycolysis and cellular respiration and then energy in the carbon bonds eventually gets transferred to the ADP allowing it to form ATP. At the beginning phase of aerobic biological oxidation, complex biodegradable hydrocarbons are broken down to simple glucose form to get energy for bacteria in aeration tank. Therefore, it is possible to reach high glucose content atfirst stages of aerobic oxidation of domestic wastewaters. The wastewater was collected from the activated sludge reactor (near the inlet region) of the Wastewater Treatment Plant of Zonguldak City. Sample was filtered to remove bacteria and other particulate metabolites (0.2mm-diameter cellulose acetate Whatman membrane filter) prior to experiments. EFC was immersed into 10 mL of phosphate buffer solution and operated at a cell potential of þ560 mV. 500mL of wastewater sample was added into the reaction medium (sample was 20-fold diluted in 10 mL of operating buffer).Fig. 4 depicts anodic and cathodic currents of the system utilized glucose content of wastewater. Power density was calculated to be 16 mW cm2for undiluted wastewater sample. Ghangrekar and
Shinde (2007)obtained a power density of 0.673mW cm2from a
synthetic wastewater; Jang et al. (2004) reported a maximum
Fig. 3. Photosynthesis evolution: current signal of the EFC working electrodes dipped into the plant at the cell voltage ofþ560 mV vs Ag/AgCl.
Fig. 4. Current signals of the EFC working electrodes obtained from the addition of domestic wastewater sample into the working buffer at the cell voltage of þ560 mV vs Ag/AgCl.
power of 0.13mW cm2. Another group examined the possibility
of generating electricity from dairy-cow waste slurry using micro-bial fuel cell and reported a power density of 0.034mW cm2
(Yokoyama et al., 2006). Power densities from the various micro-bial fuel cell studies operated with domestic wastewaters were reported as 2.4, 2.8 and 14.6mW cm2 (Liu and Logan, 2004).
Although the EFC system we optimized was membrane-less and operated in one cell, it had reasonable and higher power density in comparison to these reported studies.
4. Conclusion
The polymer wired especially GOx redox centers effectively due to its metallic nanoparticle content. The optimized EFC generated a maximum power density of 14mW cm2 with 10 mM glucose,
23.5mW cm2with 20 mM glucose at a cell voltage of þ560 mV
at pH 7.4. Power output of 1.3 mW cm2was observed in the
plant experiment. The EFC was also tested for real domestic wastewater sample of Zonguldak City in Turkey and resulted in a power output of 16 mW cm2. We believe that the developed
EFC will play a significant role for energy conversion by using glucose readily found in wastewater and in vivo mediums.
Acknowledgment
This work was financially supported by The Scientific & Technological Research Council of Turkey (TUBITAK) (Grant nos. 112Y100 and 211T016) and the Bulent Ecevit University Research Fund (Grant nos. BEU-2012-10-03-13 and BEU-2013-77047330-01).
Appendix A. Supporting information
Supplementary data associated with this article can be found in the online version athttp://doi:10.1016/j.bios.2014.05.068.
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