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Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells
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Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells Hengduo Xu, Xiangchun Quan* Key Laboratory of Water and Sediment Sciences of Ministry of Education, State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, PR China
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abstract
Article history:
A novel anode modification method was established based on immobilizing a redox
Received 17 September 2015
mediator riboflavin (RF) onto carbon cloth using bio-inspired and self-assembled peptide
Received in revised form
nanotubes (PNTs). The cyclic voltammetry (CV) and electrochemical impedance spectros-
20 November 2015
solution showed that the RF/PNT electrode demonstrated a higher copy (EIS) in Fe(CN)3/4 6
Accepted 24 November 2015
electrocatalytic activity and faradic charge capacity than the untreated electrode. The
Available online xxx
chronamperometry analysis showed that RF/PNT electrode in the presence of Shewanella oneidensis MR-1 generated a current density of 28.9 mA cm2, increased by 263.3% compared
Keywords:
to the bare electrode. The MFC operated with the RF/PNT anode generated a maximum
Peptide nanotubes
power density of 767 mW m2, which was 2.9 times larger than control. Besides, RF/PNT
Riboflavin
modification significantly reduced anode total internal resistance of MFCs. The self-
Electron transfer
organized PNTs could increase specific surface and electrical conductivity of anodes and
Microbial fuel cell
provide a good carrier for redox mediator immobilization, which may contribute to the improved performance of MFCs. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microbial fuel cell (MFC) is an emerging and promising technology for energy recovery from wastes and wastewater [1e4]. Exoelectrogenic bacteria attached on the anode of MFCs can biocatalyze and convert organic matters into electricity through transferring electrons from anode to cathode via external circuit. Various factors may affect power generation in MFCs, such as reactor configuration, electrode materials, proton exchange membrane, electrolyte compositions, exoelectrogenic bacteria and microbial electron transfer [5e8].
Among these factors, the electron transfer efficiency between bacteria and anodes is particularly important in increasing current density and boosting power generation in MFCs [7,9]. Over the years, a great effort has been made to improve electron transfer between bacteria and anodes by adding redox mediators, or increasing anode specific surface area, electrical conductivity and biocompatibility [10e12]. Exoelectrogenic bacteria attached on anode are diverse. Some bacteria possess direct electron transfer (DET) ability, some bacteria can only transfer electron from outer membrane to electrode in the presence of redox mediator. Therefore, a variety of electron
* Corresponding author. Tel./fax: þ86 10 58802374. E-mail address: [email protected] (X. Quan). http://dx.doi.org/10.1016/j.ijhydene.2015.11.124 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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redox mediators, such as neutral red, anthraquinone1,6-disulfonic acid (AQDS), methylene blue and 1,4naphthoquinone (NQ), have been used to promote electron transfer and boost power generation in MFCs [13e15]. Generally, these redox mediators are added directly into anolyte or immobilized onto electrodes with carbon composites or polymers [10,16,17]. As the immobilized redox mediators may release from anodes and lose with anolyte changing, it is hard to maintain a stable and persistent effectiveness in electron transfer [18]. Therefore, it is necessary to develop a more effective and stable method to immobilize redox mediators onto electrodes. Recently, peptide nanotubes (PNTs), as a unique class of bio-inspired nanomaterial, have drawn great attention due to their unique properties such as excellent electrical conductivity and biocompatibility [19e21]. A peptide consisting of several to dozens of amino acids can self-assemble into well ordered nanostructures such as PNTs. These nanotubes have extraordinary similarity to carbon nanotubes (CNTs) in morphology and aspect ratio although they do not have the same electron transfer mechanism [22]. PNTs have been regarded as a good carrier for enzyme immobilization [23]. It has been reported that enzyme encapsulated inside PNTs could retain a stable activity for an extended period even under unfavorable conditions [21]. As PNTs can be readily formed under benign conditions and modified with biological and chemical elements, they have become an ideal candidate for stabilizing labile units in biosensors and bionanodevices [24]. Although PNTs have been regarded as an attractive material for enzyme immobilization and biosensor elements, PNTs have not been used for electrode modification in MFCs up to date. In this study, to improve electron transfer from bacteria to anode in MFCs, riboflavin (RF), as a redox mediator, was immobilized onto the anode of carbon cloth (CC) using PNTs and the performance of MFCs with the RF/PNT anode was investigated. Scanning Electron Microscopy (SEM) was used to characterize electrode surface morphology. Cyclic Voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) were used to characterize the electrochemical properties of the RF/PNT modified electrodes. The performance of the MFCs with the modified anode was also examined by measuring power density and EIS. To the best of our knowledge, this work for the first time investigated the anode modification using PNTs encapsulating RF.
Materials and methods Electrode preparation Carbon cloth (HeSen Co. Ltd, ShangHai, China) was used as the anode supporting material and it was pretreated before use by immersing in acetone overnight, rinsing with ultra-pure water and followed by heating in a muffle furnace at 370 C for 30 min. The riboflavin/PNT (RF/PNT) modified anode electrode was prepared via layer-by-layer (LBL) assembly. Specifically, a carbon cloth (2.5 cm 2.5 cm) was first positively charged through immersing in 3 g L1 polyethyleneimine (PEI) (Sigma)
solution for 10 min and then drying under nitrogen atmosphere. The positively charged PEI layer on the carbon cloth favors the process of integration with negatively charged PNTs by electrostatic interaction [25]. A diphenylalanine (L-Phe-L-Phe, FF) peptide (GL Biochem Ltd. China) stock solution (100 mg mL1) was freshly prepared by dissolving it in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP). To encapsulate riboflavin with PNTs, a mixed solution of FF and riboflavin was prepared through diluting the above stock FF solution with 50 mM riboflavin aqueous solution (final FF concentration 2 mg mL1), as PNTs could form through self-assembly at low concentrations [26]. The surface of PEI-treated carbon cloth was then spread with 200 mL of the mixed solution of riboflavin and FF and dried at room temperature overnight. As Nafion is often used as the binding agent to load redox mediators onto electrode [27,28], immobilization of riboflavin with Nafion was also investigated for the purpose of comparison to PNT immobilization method. Riboflavin was immobilized directly onto the electrode of carbon cloth via Nafion binding according to the following procedure [29]: 300 mL riboflavin solution (50 mM) was mixed with 300 mL Nafion (5%) solution and 150 mL isopropanol; the mixture was then supplemented with carbon power (25 mg) to form a paste; the paste was spread to every side of the carbon cloth and dried at room temperature for 24 h. Both the RF/PNT and riboflavin/Nafion (RF/NF) modified electrodes were loaded with a total amount of 5 mmol riboflavin.
MFC construction and operation Dual-chambered and cubic-shaped MFCs were constructed using plexiglass. Each chamber had an effective volume of 27 mL (3 cm 3 cm 3 cm, length width height). A Nafion 117 proton exchange membrane (DuPont, Wilmington, DE) was used as the separator between the two chambers, which was pretreated in boiling H2O2 (30%) and 0.5 M H2SO4 before use. Four different anode electrodes, i.e. RF/PNT electrode, RF/ NF electrode, PNT electrode and bare carbon cloth electrode, were used in MFCs systems. Anaerobic sludge from a municipal wastewater treatment plant (Beijing, China) was inoculated into the anode chamber to enrich electrochemical activity bacteria on anode. The anolyte contained: CH3COONa 1.64 g L1, NH4Cl 0.31 g L1, KCl 0.13 g L1, NaH2PO4$2H2O 3.32 g L1, Na2HPO4$12H2O 10.32 g L1, a trace mineral solution 12.5 mL L1 and vitamin solution 5 mL L1 [30]. The cathode electrode was a carbon paper containing 0.5 mg Pt cm2 (2.5 cm 2.5 cm). Phosphate buffer solution (PBS) (50 mM) served as catholyte. Oxygen served as electron acceptor, which was provided by an air diffuser settled at the bottom of the cathode chamber. The cathode and anode were connected by a titanium wire and fixed with an external resistance of 1000 U. All the MFCs were operated at a constant temperature (30 ± 1 C).
Electrodes characterization: electrochemical property and morphology The voltage across an external resistor in a MFC was recorded every 10 min using a data acquisition system (USB8253, RBH Co., China). Power density and current density of MFCs were
Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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determined through measuring polarization curve when external resistor changed from 50 to 10,000 U. The Coulumbic efficiency (CE) was calculated as E ¼ (CEx/CTh) 100, where CEx is the total coulombs determined by integrating the current over time; CTh is the theoretical amount of coulombs from oxidizing acetate completely. Power density and current density were normalized by the projected surface area of one side of the anode electrode. CV, EIS and chronamperometry (CA) were performed in a three-electrode conventional cell with an electrochemical workstation (ChenHua CH660, Shanghai Chenhua Apparatus Corporation, China). The tested anode electrode acted as the working electrode, and a Ag/AgCl electrode and a Pt plate served as the reference electrode and counter electrode, respectively. CV was measured at a scanning rate of 10 mV s1 between 0 V and 1.0 V. For EIS test, a frequency range from 100 KHz to 0.01 Hz was applied with a perturbation signal of 5 mV. The CV and EIS were conducted in a 10 mM K3Fe(CN)6/ K4Fe(CN)6 (1:1) with 0.1 M KCl as the supporting electrolyte. CA measurement was conducted in PBS at a constant potential of 0.4 V under a stirring condition (100 rpm), in the presence of substrate of sodium lactate and strain Shewanella oneidensis MR-1. A nitrogen environment was maintained during the period of measurements. The morphology of RF/PNT electrode was observed under a field-emission scanning electron microscope (SEM, Hitachi S4800, Japan) after splitting Pt. X-ray diffraction (XRD) analysis was performed using a Ttrax thetaetheta diffractometer (Panalytical) with copper anode, parallel beam optics and generator power of 12 kW.
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Results Characterization of the RF/PNT electrode: structure and morphology The structure of the RF/PNT modified electrode is schematically illustrated in Fig. 1(A). The carbon cloth was first deposited by a PEI layer, which is positively charged and favors the integration with negatively charged PNTs. Riboflavin incorporated PNTs self-assembled and attached to the PEI layer of carbon cloth via layer-by-layer assembly by electrostatic interaction [25]. The structure and morphology of the RF/PNT electrode was also characterized using SEM (Fig. 1(B, C)). Fig. 1(B) shows that the surface of carbon cloth was bound with multilayer of PNTs which interconnected and formed a three-dimensional (3D) network structure, and the PNTs showed a diameter of 50e150 nm. From the section of some erected PNTs, we can see the nanotube structure (Fig. 1(C)). It has been reported that the peptide nanotube structure is assembled via head-to-tail hydrogen bonding and the peptide main chains are closely held by water clusters [31]. Such 3D network structure favors bacterial colonization and electron transfer from anode bacteria to the electrode, and therefore showed better biocompatibility than the bare carbon cloth. The XRD patterns of peptide nanotubes were shown in Fig. 1(D). The result demonstrated that the PNTs layers exhibited numerous diffraction peaks, indicating the highly crystalline nature of the PNTs arrays. The six periodic diffraction peaks (labeled 1e6) indicated that PNTs had a six
Fig. 1 e Schematic illustration of the structure of RF/PNT modified electrode (A); SEM image showing multilayer network structure of PNTs on anode electrode and nanotube structure of some vertically aligned PNTs (B, C); XRD showing a peptide array on the surface of RF/PNT modified electrode (D). Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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fold symmetrical crystal structure which is in agreement with the single crystal of diphenylalanine [26]. The XRD result further confirmed the presence of PNTs on the modified carbon cloth.
Electrochemical characterization of the modified electrodes The electrocatalytic behavior of RF/PNT, PNT, RF/NF and plain carbon cloth electrode were investigated through measuring CV solution (Fig. 2). As revealed in Fig. 2(A), all and EIS in Fe(CN)3/4 6 the four electrodes showed obvious redox peaks in the CVs graph due to the redox of potassium hexacyanoferrate. The RF/PNT electrode showed a cathodic peak current (Ipc) of 1.63 mA cm2 at 232 mV and an anodic peak current (Ipa) of 1.60 mA cm2 at 714 mV, much higher than the PNT modified electrode (Ipc: 0.78 mA cm2; Ipa: 1.27 mA cm2), the RF/NF modified electrode (Ipc: 0.57 mA cm2; Ipa: 0.52 mA cm2) and the unmodified electrode (Ipc: 0.43 mA cm2; Ipa: 0.47 mA cm2). For the RF/PNT electrode, the ratio of anodic peak current to cathodic peak current jIpa/Ipcj (0.98) approaches 1, suggesting electron transfer reaction at the electrode is nearly reversible or quasireversible. As RF has a standard redox potential of 0.203 V (vs. normal hydrogen electrode, NHE), much more negative than the standard redox potential of Fe3þ/Fe2þ (0.2 V vs. NHE), and therefore RF can serve as the redox mediator to promote electron
transfer from RF to hexacyanoferrate. The RF/PNT electrode showed higher redox peaks and larger CV graph area than the PNT or RF/NF modified electrode and the bare electrode, indicating that it had a higher electrocatalytic activity and faradic charge capacity. This could be attributed to the increased electroactive surface area and accelerated mass transfer on the electrode. EIS is an effective method to probe the interfacial properties of modified electrodes and electron-transfer kinetics [32]. EIS was performed for the three modified electrodes and bare electrode in a three-electrode system by applying small amplitude perturbations (5 mV amplitude) from higher frequency (100 KHz) to lower frequency (0.01 Hz). A Randle equivalent circuit was used to model the complex impedance (Fig. 2(B)) in a Nyquist plot. Ohmic resistance (Rohm) can be calculated from the intercept at real part (Z0 ) at a very high frequency, which represents intrinsic resistance of electrode materials, the contact resistance at the active material/current collector interface, and ionic resistance of electrolyte [33]. The charge transfer resistance (Rct) at an electrode/electrolyte interface can be estimated from the semicircle of the Nyquist plot, which represents the resistance of electrochemical reactions on the electrode [33]. The RF/PNT electrode had a Rct of 5.9 U, much smaller than that of the PNT (7.3 U), RF/NF (15.5 U) and plain carbon cloth electrode (18.5 U). These data indicated that encapsulation of RF by PNTs further increased interfacial charge transfer compared to the electrode modified by RF/NF. This result is in complete agreement with the CV data. All the four electrodes exhibited practically uniform Rohm of 21 U. The possible mechanisms for the RF/PNT electrode in reducing electron transfer resistance were as follows: firstly, the PNTs with tubular nanostructures offered more electrochemically active sites for potassium hexacyanoferrate entering into the reaction centers; secondly, the PNTs composed of spatially aligned aromatic systems were well adhered to the surface of carbon cloth, which may contribute to direct electron transfer [22]; last but not least, the redox mediator RF encapsulated into the tubular structure of PNTs may promote electron transfer.
Bioelectrocatalysis behavior of RF/PNT electrode
Fig. 2 e Cyclic voltammograms (A) and Nyquist plots (B) in 10 mM K3Fe(CN)6/K4Fe(CN)6 with 0.1 M KCl as the supporting electrolyte.
To explore the microbial current generation during bioelectrocatalysis process, CA was conducted for the RF/PNT modified and unmodified control electrodes in PBS solution (pH 7.0) containing sodium lactate and strain S. oneidensis MR1 at a constant potential (0.4 V versus an Ag/AgCl reference electrode). The obtained current density (I) versus time (t) plots was presented in Fig. 3. After successive addition of 100 mM sodium lactate under a continuous stirring condition, the two electrodes exhibited obvious faraday current response but the RF/PNT electrode had a higher amperometric response against the unmodified one. The current density at the RF/PNT electrode started to increase after running for 1 h and reached a plateau of 29 mA cm2, while the current density at the control electrode increased after 2 h and attained a plateau of 8 mA cm2. These data further indicated that RF/PNT modification increased microbial current generation compared to the control electrode possibly due to the combined effects of
Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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Fig. 3 e Representative amperometric (I to t) response to successive addition of 100 mM sodium lactate in PBS (pH 7.0) at ¡0.4 V with RF/PNT electrode and plain carbon cloth electrode (control).
redox mediator riboflavin and PNTs which promoted electron transfer from bacteria to electrode.
MFC performances with different anode electrodes
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Polarization and power density curves are used as common methods to evaluate MFC performances. Fig. 5(A) showed that the power density first increased with the increase of current density, and then fell down after reaching a maximum value. This is a typical relationship between power density and current density. The MFC with the RF/PNT anode generated a maximum power density of 767 mW m2, which was 2.9-fold, 2.2-fold and 2.0-fold higher than the power density generated by the MFCs with the control anode (258 mW m2), PNT anode (346 mW m2) and RF/NF anode (382 mW m2), respectively. The internal resistance of MFCs is an important factor determining the efficiency of power recovery from organic matters. The high maximum power densities are based on the low internal resistances. Therefore, the internal resistance of the MFCs with the four different anode electrodes was further investigated through EIS analysis. The Nyquist graph showed that the MFC with the RF/PNT anode had the lowest Rohm of 31.7 U, followed by the RF/NF anode of 57.7 U and the PNT anode of 72.9 U, while the plain carbon cloth showed the largest Rohm of 142.9 U (Fig. 5(B)). This result suggests that RF/ PNT immobilization on the anode electrode reduced intrinsic resistance of active materials, contact resistance and ionic resistance of electrolyte. The MFC with the RF/PNT anode also showed a relatively low Rct of 56.4 U, smaller than that with the RF/NF anode (91.4 U), PNT anode (117.9 U) or plain carbon cloth (203.7 U), suggesting that encapsulation of RF by PNTs further increased interfacial charge transfer compared to the electrode modified by RF/NF, PNT alone or the unmodified
Anode surface properties have a great effect on bioelectricity generation in MFCs. The performance of the MFCs with the RF/ PNT, RF/NF, PNT or plain carbon cloth anode were evaluated from voltage output and power density. Fig. 4 showed typical voltage profiles for the MFCs with different anodes after achieving a steady state. The MFC with the RF/PNT anode generated a maximum voltage of 522 ± 43 mV, higher than that with the RF/NF (460 ± 57 mV) anode, the PNT anode (440 ± 16 mV) or the plain carbon cloth anode (430 ± 35 mV). Correspondingly, the MFC with the RF/ PNT anode attained a CE of 38.3 ± 1.8%, higher than the MFC with the RF/NF anode (33.9 ± 2.4%), the PNT anode (32.7 ± 1.6%) and with the control anode (29.5 ± 5.1%).
Fig. 4 e Typical voltage output profiles for the MFCs with the RF/PNT, RF/NF, PNT and control anode.
Fig. 5 e Polarization curves and power output (A) and Nyquist plots (B) for MFCs with RF/PNT, RF/NF, PNT and plain carbon cloth anode.
Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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electrode. On the whole, the MFC with the RF/PNT electrode showed a anode total internal resistance of 88.1 U, which was reduced by 40.9%, 53.8% and 74.5% compared to the MFCs with the RF/NF modified electrode, PNT modified electrode and unmodified electrode, respectively. This may be an important reason for the enhanced power generation in the MFCs with the RF/PNT anode.
Comparative investigation of RF release from RF/PNT and RF/NF anodes The stability and persistence of electron mediator on an electrode is an important factor influencing its availability and activity. The RF release from Nafion or PNTs immobilized electrode was compared for a continuous 14 days (Fig. 6). For both electrodes, RF was released fast during the first 5 days and then stabilized at a low level. Much more RF was released from the RF/NF electrode compared to the RF/PNT electrode from the very beginning to the end of experiment. An accumulative RF release of 1.7 mg and 0.9 mg was obtained on Day 15 for the RF/NF and RF/PNT electrode, respectively, accounting for 71% and 40% of the total loaded amounts of RF (2.4 mg), respectively. After 30 days of operation, RF on the RF/ NF electrode was exhausted, 45% of RF remained on the RF/ PNT electrode. These data indicated that the RF/PNT electrode retained RF more effectively than the RF/NF electrode at the same loading amount of RF, which may contribute to more electron transfer through a mediated electron transfer (MET) mechanism.
Discussion Redox mediators play an important role in mediating electrons from bacteria to extracellular substrates and therefore determine the performance of MFCs. As electrons derived from NADH are transmitted to the terminal membrane enzyme and then released outside of cells, the redox potential of mediators should be as low as possible while being higher than that of NADH (0.325 V vs. NHE) [10]. In practice, the
Fig. 6 e Riboflavin release profiles for RF/PNT and RF/NF electrode in PBS.
redox potential of RF (0.203 V vs. NHE) is higher than NADþ/ NADH, suggesting it can serve as a redox mediator for electron transfer. Previous study indicated that addition of riboflavin into anolyte as electron mediators contributed to 2e5 folds increase in power density for MFCs with Shewanella sp. HN-41 as the exoelectrogenic bacteria [34]. On the contrary, removal of riboflavin from biofilms led to more than 70% reduction in the rate of electron transfer to the electrodes [35]. Although redox mediator can be supplemented into anolyte to boost current generation in MFCs [17], it is hard to maintain durable effectiveness as it will lose with system running [34]. In addition, regular addition of mediators may increase operation cost and difficulty and result in environmental risks. To overcome these drawbacks, a variety of methods have been tried to immobilize redox mediators onto electrodes in order to increase the stability and availability of redox mediators to attached exoelectrogenic bacteria. For example, a redox mediator AQDS was immobilized onto carbon felts using a conductive polymer polypyrrole [13]. In this study, although RF release was observed for both the RF/PNT and RF/NF electrode (Fig. 6), the RF/PNT electrode showed a much lower RF release rate than the RF/NF electrode, which greatly maintained the quantity and activity of RF to the anode. Peptide nanotubes have been regarded as a promising material for biosensors fabrication [23,24]. It has been reported that enzyme immobilized by PNTs could maintain a good and persistent catalytic activity. For example, Park et al. used bio-inspired PNTs as an encapsulation template to immobilize horseradish peroxidase and glucose oxidase onto electrode to establish a glucose biosensor [19]. Results showed that PNTs modified electrode provided the enzyme with a biocompatible nanoenvironment as it sustained enzyme activity for an extended time and promoted possible direct electron transfer through the PNTs to the electrode. In this study, a self-assembled peptide nanomaterial was for the first time used to encapsulate RF onto anode electrodes of MFCs due to its good biocompatibility, excellent electrical conductivity and hollow nanotube structure. PNTs attached to the electrode showed a three dimension network structure, which may increase specific surface area of the electrode and provide a better physical contact to bacteria cell surface and promote bacterial adhesion. PNTs show an excellent electric conductivity which favors electron transfer from exoelectrogenic bacteria to anode electrodes [1]. It has been reported that electrons transfer in peptide nanostructures may occur by a hopping mechanism [20]. Hydrogen bonds, which play an important role in supporting cyclic peptide molecules structure, could lower the lowest unoccupied molecular orbital of peptide bridges and thus increase the super-exchange electron transfer [36]. In addition, the highly ordered peptide nanotube structures with aromatic groups on top of each other could also contribute to high hopping efficiency due to small value of the decay constant [37]. Furthermore, the hollow tubular structure of PNTs provides a mild environment for the immobilized redox mediator and maintains its activity and stability for an extended period, which can promote electron transfer from bacteria to anode electrode through a MET mechanism [21,35]. All these factors may contribute to the enhanced performance of the MFCs with the RF/PNT
Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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electrode. On the whole, this study provides a novel and easy way to immobilize redox mediator on the anode of MFCs by self-assembled PNTs. As PNTs could readily form on the surface of electrode and possess unique nanotube structure and good electric conductivity, it also offers the potential application in cathode modification and serving versatile template for functional nanomaterials fabrication on electrodes. These works deserve further study.
Conclusions This study proposed a novel anode modification method based on encapsulating RF using self-assembled hollow tubular PNTs via layer-by-layer assembly. This modified anode exhibited good electrochemical activity and biocompatibility. The RF/PNT anode greatly promoted power generation in MFCs, accelerated electron transfer from bacteria to electrode and reduced internal resistance when compared to the untreated anode or the RF/NF anode. The RF/PNT electrode effectively retained RF on electrodes and presented excellent stability in a long-term operation. Anode modification through RF immobilization with PNTs is a promising strategy to enhance the performance of MFCs.
Acknowledgments This research was supported by “Specialized Research Fund for the Doctoral Program of Higher Education of China” (20130003110028).
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Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells Hengduo Xu, Xiangchun Quan* Key Laboratory of Water and Sediment Sciences of Ministry of Education, State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, PR China
article info
abstract
Article history:
A novel anode modification method was established based on immobilizing a redox
Received 17 September 2015
mediator riboflavin (RF) onto carbon cloth using bio-inspired and self-assembled peptide
Received in revised form
nanotubes (PNTs). The cyclic voltammetry (CV) and electrochemical impedance spectros-
20 November 2015
solution showed that the RF/PNT electrode demonstrated a higher copy (EIS) in Fe(CN)3/4 6
Accepted 24 November 2015
electrocatalytic activity and faradic charge capacity than the untreated electrode. The
Available online xxx
chronamperometry analysis showed that RF/PNT electrode in the presence of Shewanella oneidensis MR-1 generated a current density of 28.9 mA cm2, increased by 263.3% compared
Keywords:
to the bare electrode. The MFC operated with the RF/PNT anode generated a maximum
Peptide nanotubes
power density of 767 mW m2, which was 2.9 times larger than control. Besides, RF/PNT
Riboflavin
modification significantly reduced anode total internal resistance of MFCs. The self-
Electron transfer
organized PNTs could increase specific surface and electrical conductivity of anodes and
Microbial fuel cell
provide a good carrier for redox mediator immobilization, which may contribute to the improved performance of MFCs. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microbial fuel cell (MFC) is an emerging and promising technology for energy recovery from wastes and wastewater [1e4]. Exoelectrogenic bacteria attached on the anode of MFCs can biocatalyze and convert organic matters into electricity through transferring electrons from anode to cathode via external circuit. Various factors may affect power generation in MFCs, such as reactor configuration, electrode materials, proton exchange membrane, electrolyte compositions, exoelectrogenic bacteria and microbial electron transfer [5e8].
Among these factors, the electron transfer efficiency between bacteria and anodes is particularly important in increasing current density and boosting power generation in MFCs [7,9]. Over the years, a great effort has been made to improve electron transfer between bacteria and anodes by adding redox mediators, or increasing anode specific surface area, electrical conductivity and biocompatibility [10e12]. Exoelectrogenic bacteria attached on anode are diverse. Some bacteria possess direct electron transfer (DET) ability, some bacteria can only transfer electron from outer membrane to electrode in the presence of redox mediator. Therefore, a variety of electron
* Corresponding author. Tel./fax: þ86 10 58802374. E-mail address: [email protected] (X. Quan). http://dx.doi.org/10.1016/j.ijhydene.2015.11.124 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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redox mediators, such as neutral red, anthraquinone1,6-disulfonic acid (AQDS), methylene blue and 1,4naphthoquinone (NQ), have been used to promote electron transfer and boost power generation in MFCs [13e15]. Generally, these redox mediators are added directly into anolyte or immobilized onto electrodes with carbon composites or polymers [10,16,17]. As the immobilized redox mediators may release from anodes and lose with anolyte changing, it is hard to maintain a stable and persistent effectiveness in electron transfer [18]. Therefore, it is necessary to develop a more effective and stable method to immobilize redox mediators onto electrodes. Recently, peptide nanotubes (PNTs), as a unique class of bio-inspired nanomaterial, have drawn great attention due to their unique properties such as excellent electrical conductivity and biocompatibility [19e21]. A peptide consisting of several to dozens of amino acids can self-assemble into well ordered nanostructures such as PNTs. These nanotubes have extraordinary similarity to carbon nanotubes (CNTs) in morphology and aspect ratio although they do not have the same electron transfer mechanism [22]. PNTs have been regarded as a good carrier for enzyme immobilization [23]. It has been reported that enzyme encapsulated inside PNTs could retain a stable activity for an extended period even under unfavorable conditions [21]. As PNTs can be readily formed under benign conditions and modified with biological and chemical elements, they have become an ideal candidate for stabilizing labile units in biosensors and bionanodevices [24]. Although PNTs have been regarded as an attractive material for enzyme immobilization and biosensor elements, PNTs have not been used for electrode modification in MFCs up to date. In this study, to improve electron transfer from bacteria to anode in MFCs, riboflavin (RF), as a redox mediator, was immobilized onto the anode of carbon cloth (CC) using PNTs and the performance of MFCs with the RF/PNT anode was investigated. Scanning Electron Microscopy (SEM) was used to characterize electrode surface morphology. Cyclic Voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) were used to characterize the electrochemical properties of the RF/PNT modified electrodes. The performance of the MFCs with the modified anode was also examined by measuring power density and EIS. To the best of our knowledge, this work for the first time investigated the anode modification using PNTs encapsulating RF.
Materials and methods Electrode preparation Carbon cloth (HeSen Co. Ltd, ShangHai, China) was used as the anode supporting material and it was pretreated before use by immersing in acetone overnight, rinsing with ultra-pure water and followed by heating in a muffle furnace at 370 C for 30 min. The riboflavin/PNT (RF/PNT) modified anode electrode was prepared via layer-by-layer (LBL) assembly. Specifically, a carbon cloth (2.5 cm 2.5 cm) was first positively charged through immersing in 3 g L1 polyethyleneimine (PEI) (Sigma)
solution for 10 min and then drying under nitrogen atmosphere. The positively charged PEI layer on the carbon cloth favors the process of integration with negatively charged PNTs by electrostatic interaction [25]. A diphenylalanine (L-Phe-L-Phe, FF) peptide (GL Biochem Ltd. China) stock solution (100 mg mL1) was freshly prepared by dissolving it in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP). To encapsulate riboflavin with PNTs, a mixed solution of FF and riboflavin was prepared through diluting the above stock FF solution with 50 mM riboflavin aqueous solution (final FF concentration 2 mg mL1), as PNTs could form through self-assembly at low concentrations [26]. The surface of PEI-treated carbon cloth was then spread with 200 mL of the mixed solution of riboflavin and FF and dried at room temperature overnight. As Nafion is often used as the binding agent to load redox mediators onto electrode [27,28], immobilization of riboflavin with Nafion was also investigated for the purpose of comparison to PNT immobilization method. Riboflavin was immobilized directly onto the electrode of carbon cloth via Nafion binding according to the following procedure [29]: 300 mL riboflavin solution (50 mM) was mixed with 300 mL Nafion (5%) solution and 150 mL isopropanol; the mixture was then supplemented with carbon power (25 mg) to form a paste; the paste was spread to every side of the carbon cloth and dried at room temperature for 24 h. Both the RF/PNT and riboflavin/Nafion (RF/NF) modified electrodes were loaded with a total amount of 5 mmol riboflavin.
MFC construction and operation Dual-chambered and cubic-shaped MFCs were constructed using plexiglass. Each chamber had an effective volume of 27 mL (3 cm 3 cm 3 cm, length width height). A Nafion 117 proton exchange membrane (DuPont, Wilmington, DE) was used as the separator between the two chambers, which was pretreated in boiling H2O2 (30%) and 0.5 M H2SO4 before use. Four different anode electrodes, i.e. RF/PNT electrode, RF/ NF electrode, PNT electrode and bare carbon cloth electrode, were used in MFCs systems. Anaerobic sludge from a municipal wastewater treatment plant (Beijing, China) was inoculated into the anode chamber to enrich electrochemical activity bacteria on anode. The anolyte contained: CH3COONa 1.64 g L1, NH4Cl 0.31 g L1, KCl 0.13 g L1, NaH2PO4$2H2O 3.32 g L1, Na2HPO4$12H2O 10.32 g L1, a trace mineral solution 12.5 mL L1 and vitamin solution 5 mL L1 [30]. The cathode electrode was a carbon paper containing 0.5 mg Pt cm2 (2.5 cm 2.5 cm). Phosphate buffer solution (PBS) (50 mM) served as catholyte. Oxygen served as electron acceptor, which was provided by an air diffuser settled at the bottom of the cathode chamber. The cathode and anode were connected by a titanium wire and fixed with an external resistance of 1000 U. All the MFCs were operated at a constant temperature (30 ± 1 C).
Electrodes characterization: electrochemical property and morphology The voltage across an external resistor in a MFC was recorded every 10 min using a data acquisition system (USB8253, RBH Co., China). Power density and current density of MFCs were
Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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determined through measuring polarization curve when external resistor changed from 50 to 10,000 U. The Coulumbic efficiency (CE) was calculated as E ¼ (CEx/CTh) 100, where CEx is the total coulombs determined by integrating the current over time; CTh is the theoretical amount of coulombs from oxidizing acetate completely. Power density and current density were normalized by the projected surface area of one side of the anode electrode. CV, EIS and chronamperometry (CA) were performed in a three-electrode conventional cell with an electrochemical workstation (ChenHua CH660, Shanghai Chenhua Apparatus Corporation, China). The tested anode electrode acted as the working electrode, and a Ag/AgCl electrode and a Pt plate served as the reference electrode and counter electrode, respectively. CV was measured at a scanning rate of 10 mV s1 between 0 V and 1.0 V. For EIS test, a frequency range from 100 KHz to 0.01 Hz was applied with a perturbation signal of 5 mV. The CV and EIS were conducted in a 10 mM K3Fe(CN)6/ K4Fe(CN)6 (1:1) with 0.1 M KCl as the supporting electrolyte. CA measurement was conducted in PBS at a constant potential of 0.4 V under a stirring condition (100 rpm), in the presence of substrate of sodium lactate and strain Shewanella oneidensis MR-1. A nitrogen environment was maintained during the period of measurements. The morphology of RF/PNT electrode was observed under a field-emission scanning electron microscope (SEM, Hitachi S4800, Japan) after splitting Pt. X-ray diffraction (XRD) analysis was performed using a Ttrax thetaetheta diffractometer (Panalytical) with copper anode, parallel beam optics and generator power of 12 kW.
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Results Characterization of the RF/PNT electrode: structure and morphology The structure of the RF/PNT modified electrode is schematically illustrated in Fig. 1(A). The carbon cloth was first deposited by a PEI layer, which is positively charged and favors the integration with negatively charged PNTs. Riboflavin incorporated PNTs self-assembled and attached to the PEI layer of carbon cloth via layer-by-layer assembly by electrostatic interaction [25]. The structure and morphology of the RF/PNT electrode was also characterized using SEM (Fig. 1(B, C)). Fig. 1(B) shows that the surface of carbon cloth was bound with multilayer of PNTs which interconnected and formed a three-dimensional (3D) network structure, and the PNTs showed a diameter of 50e150 nm. From the section of some erected PNTs, we can see the nanotube structure (Fig. 1(C)). It has been reported that the peptide nanotube structure is assembled via head-to-tail hydrogen bonding and the peptide main chains are closely held by water clusters [31]. Such 3D network structure favors bacterial colonization and electron transfer from anode bacteria to the electrode, and therefore showed better biocompatibility than the bare carbon cloth. The XRD patterns of peptide nanotubes were shown in Fig. 1(D). The result demonstrated that the PNTs layers exhibited numerous diffraction peaks, indicating the highly crystalline nature of the PNTs arrays. The six periodic diffraction peaks (labeled 1e6) indicated that PNTs had a six
Fig. 1 e Schematic illustration of the structure of RF/PNT modified electrode (A); SEM image showing multilayer network structure of PNTs on anode electrode and nanotube structure of some vertically aligned PNTs (B, C); XRD showing a peptide array on the surface of RF/PNT modified electrode (D). Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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fold symmetrical crystal structure which is in agreement with the single crystal of diphenylalanine [26]. The XRD result further confirmed the presence of PNTs on the modified carbon cloth.
Electrochemical characterization of the modified electrodes The electrocatalytic behavior of RF/PNT, PNT, RF/NF and plain carbon cloth electrode were investigated through measuring CV solution (Fig. 2). As revealed in Fig. 2(A), all and EIS in Fe(CN)3/4 6 the four electrodes showed obvious redox peaks in the CVs graph due to the redox of potassium hexacyanoferrate. The RF/PNT electrode showed a cathodic peak current (Ipc) of 1.63 mA cm2 at 232 mV and an anodic peak current (Ipa) of 1.60 mA cm2 at 714 mV, much higher than the PNT modified electrode (Ipc: 0.78 mA cm2; Ipa: 1.27 mA cm2), the RF/NF modified electrode (Ipc: 0.57 mA cm2; Ipa: 0.52 mA cm2) and the unmodified electrode (Ipc: 0.43 mA cm2; Ipa: 0.47 mA cm2). For the RF/PNT electrode, the ratio of anodic peak current to cathodic peak current jIpa/Ipcj (0.98) approaches 1, suggesting electron transfer reaction at the electrode is nearly reversible or quasireversible. As RF has a standard redox potential of 0.203 V (vs. normal hydrogen electrode, NHE), much more negative than the standard redox potential of Fe3þ/Fe2þ (0.2 V vs. NHE), and therefore RF can serve as the redox mediator to promote electron
transfer from RF to hexacyanoferrate. The RF/PNT electrode showed higher redox peaks and larger CV graph area than the PNT or RF/NF modified electrode and the bare electrode, indicating that it had a higher electrocatalytic activity and faradic charge capacity. This could be attributed to the increased electroactive surface area and accelerated mass transfer on the electrode. EIS is an effective method to probe the interfacial properties of modified electrodes and electron-transfer kinetics [32]. EIS was performed for the three modified electrodes and bare electrode in a three-electrode system by applying small amplitude perturbations (5 mV amplitude) from higher frequency (100 KHz) to lower frequency (0.01 Hz). A Randle equivalent circuit was used to model the complex impedance (Fig. 2(B)) in a Nyquist plot. Ohmic resistance (Rohm) can be calculated from the intercept at real part (Z0 ) at a very high frequency, which represents intrinsic resistance of electrode materials, the contact resistance at the active material/current collector interface, and ionic resistance of electrolyte [33]. The charge transfer resistance (Rct) at an electrode/electrolyte interface can be estimated from the semicircle of the Nyquist plot, which represents the resistance of electrochemical reactions on the electrode [33]. The RF/PNT electrode had a Rct of 5.9 U, much smaller than that of the PNT (7.3 U), RF/NF (15.5 U) and plain carbon cloth electrode (18.5 U). These data indicated that encapsulation of RF by PNTs further increased interfacial charge transfer compared to the electrode modified by RF/NF. This result is in complete agreement with the CV data. All the four electrodes exhibited practically uniform Rohm of 21 U. The possible mechanisms for the RF/PNT electrode in reducing electron transfer resistance were as follows: firstly, the PNTs with tubular nanostructures offered more electrochemically active sites for potassium hexacyanoferrate entering into the reaction centers; secondly, the PNTs composed of spatially aligned aromatic systems were well adhered to the surface of carbon cloth, which may contribute to direct electron transfer [22]; last but not least, the redox mediator RF encapsulated into the tubular structure of PNTs may promote electron transfer.
Bioelectrocatalysis behavior of RF/PNT electrode
Fig. 2 e Cyclic voltammograms (A) and Nyquist plots (B) in 10 mM K3Fe(CN)6/K4Fe(CN)6 with 0.1 M KCl as the supporting electrolyte.
To explore the microbial current generation during bioelectrocatalysis process, CA was conducted for the RF/PNT modified and unmodified control electrodes in PBS solution (pH 7.0) containing sodium lactate and strain S. oneidensis MR1 at a constant potential (0.4 V versus an Ag/AgCl reference electrode). The obtained current density (I) versus time (t) plots was presented in Fig. 3. After successive addition of 100 mM sodium lactate under a continuous stirring condition, the two electrodes exhibited obvious faraday current response but the RF/PNT electrode had a higher amperometric response against the unmodified one. The current density at the RF/PNT electrode started to increase after running for 1 h and reached a plateau of 29 mA cm2, while the current density at the control electrode increased after 2 h and attained a plateau of 8 mA cm2. These data further indicated that RF/PNT modification increased microbial current generation compared to the control electrode possibly due to the combined effects of
Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
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Fig. 3 e Representative amperometric (I to t) response to successive addition of 100 mM sodium lactate in PBS (pH 7.0) at ¡0.4 V with RF/PNT electrode and plain carbon cloth electrode (control).
redox mediator riboflavin and PNTs which promoted electron transfer from bacteria to electrode.
MFC performances with different anode electrodes
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Polarization and power density curves are used as common methods to evaluate MFC performances. Fig. 5(A) showed that the power density first increased with the increase of current density, and then fell down after reaching a maximum value. This is a typical relationship between power density and current density. The MFC with the RF/PNT anode generated a maximum power density of 767 mW m2, which was 2.9-fold, 2.2-fold and 2.0-fold higher than the power density generated by the MFCs with the control anode (258 mW m2), PNT anode (346 mW m2) and RF/NF anode (382 mW m2), respectively. The internal resistance of MFCs is an important factor determining the efficiency of power recovery from organic matters. The high maximum power densities are based on the low internal resistances. Therefore, the internal resistance of the MFCs with the four different anode electrodes was further investigated through EIS analysis. The Nyquist graph showed that the MFC with the RF/PNT anode had the lowest Rohm of 31.7 U, followed by the RF/NF anode of 57.7 U and the PNT anode of 72.9 U, while the plain carbon cloth showed the largest Rohm of 142.9 U (Fig. 5(B)). This result suggests that RF/ PNT immobilization on the anode electrode reduced intrinsic resistance of active materials, contact resistance and ionic resistance of electrolyte. The MFC with the RF/PNT anode also showed a relatively low Rct of 56.4 U, smaller than that with the RF/NF anode (91.4 U), PNT anode (117.9 U) or plain carbon cloth (203.7 U), suggesting that encapsulation of RF by PNTs further increased interfacial charge transfer compared to the electrode modified by RF/NF, PNT alone or the unmodified
Anode surface properties have a great effect on bioelectricity generation in MFCs. The performance of the MFCs with the RF/ PNT, RF/NF, PNT or plain carbon cloth anode were evaluated from voltage output and power density. Fig. 4 showed typical voltage profiles for the MFCs with different anodes after achieving a steady state. The MFC with the RF/PNT anode generated a maximum voltage of 522 ± 43 mV, higher than that with the RF/NF (460 ± 57 mV) anode, the PNT anode (440 ± 16 mV) or the plain carbon cloth anode (430 ± 35 mV). Correspondingly, the MFC with the RF/ PNT anode attained a CE of 38.3 ± 1.8%, higher than the MFC with the RF/NF anode (33.9 ± 2.4%), the PNT anode (32.7 ± 1.6%) and with the control anode (29.5 ± 5.1%).
Fig. 4 e Typical voltage output profiles for the MFCs with the RF/PNT, RF/NF, PNT and control anode.
Fig. 5 e Polarization curves and power output (A) and Nyquist plots (B) for MFCs with RF/PNT, RF/NF, PNT and plain carbon cloth anode.
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electrode. On the whole, the MFC with the RF/PNT electrode showed a anode total internal resistance of 88.1 U, which was reduced by 40.9%, 53.8% and 74.5% compared to the MFCs with the RF/NF modified electrode, PNT modified electrode and unmodified electrode, respectively. This may be an important reason for the enhanced power generation in the MFCs with the RF/PNT anode.
Comparative investigation of RF release from RF/PNT and RF/NF anodes The stability and persistence of electron mediator on an electrode is an important factor influencing its availability and activity. The RF release from Nafion or PNTs immobilized electrode was compared for a continuous 14 days (Fig. 6). For both electrodes, RF was released fast during the first 5 days and then stabilized at a low level. Much more RF was released from the RF/NF electrode compared to the RF/PNT electrode from the very beginning to the end of experiment. An accumulative RF release of 1.7 mg and 0.9 mg was obtained on Day 15 for the RF/NF and RF/PNT electrode, respectively, accounting for 71% and 40% of the total loaded amounts of RF (2.4 mg), respectively. After 30 days of operation, RF on the RF/ NF electrode was exhausted, 45% of RF remained on the RF/ PNT electrode. These data indicated that the RF/PNT electrode retained RF more effectively than the RF/NF electrode at the same loading amount of RF, which may contribute to more electron transfer through a mediated electron transfer (MET) mechanism.
Discussion Redox mediators play an important role in mediating electrons from bacteria to extracellular substrates and therefore determine the performance of MFCs. As electrons derived from NADH are transmitted to the terminal membrane enzyme and then released outside of cells, the redox potential of mediators should be as low as possible while being higher than that of NADH (0.325 V vs. NHE) [10]. In practice, the
Fig. 6 e Riboflavin release profiles for RF/PNT and RF/NF electrode in PBS.
redox potential of RF (0.203 V vs. NHE) is higher than NADþ/ NADH, suggesting it can serve as a redox mediator for electron transfer. Previous study indicated that addition of riboflavin into anolyte as electron mediators contributed to 2e5 folds increase in power density for MFCs with Shewanella sp. HN-41 as the exoelectrogenic bacteria [34]. On the contrary, removal of riboflavin from biofilms led to more than 70% reduction in the rate of electron transfer to the electrodes [35]. Although redox mediator can be supplemented into anolyte to boost current generation in MFCs [17], it is hard to maintain durable effectiveness as it will lose with system running [34]. In addition, regular addition of mediators may increase operation cost and difficulty and result in environmental risks. To overcome these drawbacks, a variety of methods have been tried to immobilize redox mediators onto electrodes in order to increase the stability and availability of redox mediators to attached exoelectrogenic bacteria. For example, a redox mediator AQDS was immobilized onto carbon felts using a conductive polymer polypyrrole [13]. In this study, although RF release was observed for both the RF/PNT and RF/NF electrode (Fig. 6), the RF/PNT electrode showed a much lower RF release rate than the RF/NF electrode, which greatly maintained the quantity and activity of RF to the anode. Peptide nanotubes have been regarded as a promising material for biosensors fabrication [23,24]. It has been reported that enzyme immobilized by PNTs could maintain a good and persistent catalytic activity. For example, Park et al. used bio-inspired PNTs as an encapsulation template to immobilize horseradish peroxidase and glucose oxidase onto electrode to establish a glucose biosensor [19]. Results showed that PNTs modified electrode provided the enzyme with a biocompatible nanoenvironment as it sustained enzyme activity for an extended time and promoted possible direct electron transfer through the PNTs to the electrode. In this study, a self-assembled peptide nanomaterial was for the first time used to encapsulate RF onto anode electrodes of MFCs due to its good biocompatibility, excellent electrical conductivity and hollow nanotube structure. PNTs attached to the electrode showed a three dimension network structure, which may increase specific surface area of the electrode and provide a better physical contact to bacteria cell surface and promote bacterial adhesion. PNTs show an excellent electric conductivity which favors electron transfer from exoelectrogenic bacteria to anode electrodes [1]. It has been reported that electrons transfer in peptide nanostructures may occur by a hopping mechanism [20]. Hydrogen bonds, which play an important role in supporting cyclic peptide molecules structure, could lower the lowest unoccupied molecular orbital of peptide bridges and thus increase the super-exchange electron transfer [36]. In addition, the highly ordered peptide nanotube structures with aromatic groups on top of each other could also contribute to high hopping efficiency due to small value of the decay constant [37]. Furthermore, the hollow tubular structure of PNTs provides a mild environment for the immobilized redox mediator and maintains its activity and stability for an extended period, which can promote electron transfer from bacteria to anode electrode through a MET mechanism [21,35]. All these factors may contribute to the enhanced performance of the MFCs with the RF/PNT
Please cite this article in press as: Xu H, Quan X, Anode modification with peptide nanotubes encapsulating riboflavin enhanced power generation in microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.124
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e8
electrode. On the whole, this study provides a novel and easy way to immobilize redox mediator on the anode of MFCs by self-assembled PNTs. As PNTs could readily form on the surface of electrode and possess unique nanotube structure and good electric conductivity, it also offers the potential application in cathode modification and serving versatile template for functional nanomaterials fabrication on electrodes. These works deserve further study.
Conclusions This study proposed a novel anode modification method based on encapsulating RF using self-assembled hollow tubular PNTs via layer-by-layer assembly. This modified anode exhibited good electrochemical activity and biocompatibility. The RF/PNT anode greatly promoted power generation in MFCs, accelerated electron transfer from bacteria to electrode and reduced internal resistance when compared to the untreated anode or the RF/NF anode. The RF/PNT electrode effectively retained RF on electrodes and presented excellent stability in a long-term operation. Anode modification through RF immobilization with PNTs is a promising strategy to enhance the performance of MFCs.
Acknowledgments This research was supported by “Specialized Research Fund for the Doctoral Program of Higher Education of China” (20130003110028).
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