- Email: [email protected]
chitosan electrodeposition
G Model JIEC 4755 No. of Pages 7
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition Dong Sup Kima , Han Suk Choia , Xiaoguang Yanga , Ji Hyun Yanga , Ja Hyun Leeb , Hah Young Yooc , Jinyoung Leed, Chulhwan Parke,* , Seung Wook Kima,f,* a
Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea Department of Food Science and Engineering, Dongyang Mirae University, 445, Gyeongin-ro, Guro-gu, Seoul, Republic of Korea Department of Biotechnology, Sangmyung University, Seoul 03016, Republic of Korea d Gyedang College of General Education, Sangmyung University, Cheonan, Chungnam 31066, Republic of Korea e Department of Chemical Engineering, Kwangwoon University, Seoul 01897, Republic of Korea f Department of Chemistry, Faculty of Science and Technology, Universitas Airlangga, Surabaya, 60115, Indonesia b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 April 2019 Received in revised form 23 August 2019 Accepted 26 August 2019 Available online xxx
An enzyme fuel cell (EFC) using composite graphite oxide/cobalt/chitosan (GO/Co/chitosan) mediator is prepared to convert chemical energy into electrical energy. The degree of chitosan deacetylation affects physicochemical and electrochemical properties of the EFC. The soluble molecular weight of chitosan polymer influences the viscosity of chitosan solution. Also, the solubility of chitosan influences the electrochemical properties of the EFC assembled by electrodeposition of GO/Co/chitosan mediator particles on electrodes. EFC performance could be improved by addition of acetic acid during deacetylation. Acetic acid (5% (v/v)) showed an efficient electron transfer between the mediator and electrolyte in the EFC. The EFC produced a potential voltage of 0.548 V vs. Ag/AgCl with power density of 1198.09 mW/cm2. The power generation was the highest at slightly acidic condition and without addition of cofactor. This study shows the efficient participation of chitosan on electron transfer in EFC application. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Enzyme fuel cell Chitosan Acetic acid Viscosity Glucose oxidase Laccase
Introduction Biofuel cells, used for generation of electrical energy from abundant biological substrates, can be constructed using a variety of methods [1]. Biofuel cells can operate under physiological conditions. They can be used with biological systems including humans. Thus, they have received much interest in use in a variety of biomedical applications as power source implants [2]. Enzyme fuel cell (EFC), a type of biofuel cell, is an attractive alternative energy supply for nano-microelectronic devices in biosensor applications [3]. In bioelectronics system, EFCs can utilize a set of redox enzymes such as glucose oxidase and laccase to catalyze the conversion of chemical energy into electrical energy. Therefore, EFCs have potential to achieve excellent electron transfer while relying on environmentally friendly and biocompatible materials. Graphite based materials provide good advantages such as large surface areas and high electronic conductivity. Additionally, chitosan displays excellent stability, reproducibility, sensitivity
* Corresponding authors. E-mail addresses: [email protected] (C. Park), [email protected] (S.W. Kim).
and selectivity. These materials could be applied to various fields [4]. Their applications include communication devices, implantable devices, and miniaturized biosensors that require physically small power sources. Recent efforts have focused on the use of glucose as a fuel for EFCs with in vivo applications. Therefore, many researchers are interested in biomedical applications such as pacemakers, implantable devices, and healthcare related power sources [4,5]. Relying on the same principles as conventional fuel cells, EFCs can generate electricity based on the release of electrons from oxidation at the anode. Considering the high cost of membranes used to provide selectivity, enzymes are more advantageous due to their specificity. Membrane separator that is necessary for conventional fuel cells is unnecessary for EFCs [6]. The development of new materials and new mediators is therefore important to improve electron transfer kinetics at the electrode and to enhance EFC performance. Chitosan can be obtained from deacetylation of chitin by enzymatic methods or alkaline hydrolysis. As a naturally abundant biopolymer, chitosan has been used as a bio-functional material in many fuel cell applications, including membrane polymers and electrodes [7,8]. It is a suitable functional material because this natural polymer adsorbs readily with excellent biocompatibility
https://doi.org/10.1016/j.jiec.2019.08.060 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
2
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
and biodegradability properties. Amino groups of chitosan can become polyelectrolytic by carrying a positive charge (R-NH3+), resulting in a cathodic biopolymer [9]. A large number of hydroxyl and amino groups on chitosan can act as coordination sites for metal ions in the immobilization process of EFC electrode assembly. Furthermore, chitosan is a natural biopolymer that aids enzyme immobilization on electrodes, which is a crucial process in the establishment of an EFC. Electron transfer, mechanical, and immobilization properties of chitosan are influenced by its solubility, which is improved in acidic solutions [10]. Under certain conditions, chitosan can even exhibit hydrogel formation and viscoelastic behavior. Thus, slightly acidic pH of chitosan solution can positively influence its rheological and morphological properties as well as its electrochemical performance in EFC systems [11]. Although electrolytes such as sodium phosphate are known to improve properties of chitosan in solution, the addition of acids during chitosan dissolution has only been recently explored [4,12]. Several organic and inorganic acids (i.e., hydrochloric acid, phosphoric acid, succinic acid, acetic acid, citric acid, and formic acid) have been investigated [4,6]. Considering the possible bio-applications of EFCs, using acetic acid (a weak organic acid, pH 3–6) in combination with chitosan is attractive. Viscosity is another important factor to consider because it influences penetration of chitosan into fabrics or biopolymers. Under acidic conditions, chitosan solutions are viscous. Thus, they could be quantitatively evaluated using viscosity parameters. Furthermore, it is worth exploring the effect of different concentrations of acetic acid on chitosan with respect to power generation of EFCs. Properties of the polymer solution (e.g., molecular weight, viscosity, and conductivity) and surface electrodeposition conditions (e.g., applied electrical voltage and power density) should be evaluated as functions of acetic acid concentration to understand any improved behavior through addition of chitosan solution in the EFC [13]. We have previously developed a mediator using graphite oxide (GO) and cobalt composite (GO/Co) and determined optimal conditions for its use in EFCs [14,27]. Based on the high density of amino groups present in chitosan, chitosan was chosen as part of the mediator composite to offer electrostatic interactions with redox enzyme. However, the performance of EFCs using GO/Co/chitosan has been limited due to chitosan viscosity [22]. It is necessary to overcome the viscosity limitation. Thus, it was investigated in the present study at various acetic acid concentrations. Changes in morphology at different acetic acid concentrations were compared with the previous mediator. We found that the electron transfer performance was improved using the optimized acetic acid concentration with GO/Co/chitosan relative to GO/Co. The EFC system could also be improved using this optimized acetic acid concentration with the GO/Co/chitosan mediator based on cyclic voltammetry (CV) measurements and power density. This study used chitosan for adsorption in an EFC system. Effects of acetic acid and pH in aqueous solution on dissolution of chitosan were determined. A new mediator was established and characterized by scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy. In addition, rheological properties were measured and optimized in chitosan/acetic acid solutions at various pH, in order to determine the parameters that could lead to improvements. Electron transfer in the EFC is confirmed using CV, power density, and cell potential. Results showed improved performance based on power density. In addition, rheological properties were measured and optimized in the dissolved chitosan solution with an additional acetic acid at various pH parameters that led to important improvement.
Experimental Materials Chitosan (200–800 cp viscosity, 75–85% deacetylated, 190–310 kDa), glucose oxidase (GOx; EC1.1.3.4, derived from Aspergillus niger), laccase (Lac; EC 1.10.3.2, derived from Trametes versicolor), N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), cobalt(II) chloride hexahydrate (CoCl26H2O), graphite powder, and potassium ferricyanide were purchased from Sigma-Aldrich, USA. Anhydrous D-glucose, (98.0%, w/v) used as GOx substrate, ammonium hydroxide (NH4OH), anhydrous dextrose (98.0% w/v), and potassium permanganate were obtained from Samchun Pure Chemical Company (Korea). MnO4 and sulfuric acid (H2SO4, 98.0%, v/v) were purchased from Daejung Chemical Co. (Korea). Glacial acetic acid and 25 Tris-acetate-EDTA (TAE) buffer were obtained from Bio Basic (Canada). All percentages of acetic acid are represented as % w/v unless otherwise indicated. Preparation of mediator with graphite oxide pretreatment and cobalt coating The acid treatment of graphite was carried out using a H2SO4/ H3PO4 solution prepared in a 9:1 volume ratio, with the ratio of graphite powder and H2SO4/H3PO4 solution being 4 g to 300 mL. This process is exothermic, which can reach 120 C [14,15]. Oxidation of graphite was performed by slowly adding KMnO4 to the acidic graphite solution (6:1 w/w KMnO4 to graphite powder) with stirring in strong acid solution overnight and cooling from a water jacket. Afterward, the oxidized graphite particles (graphite oxide, GO) were washed with distilled water, centrifuged at 4500 rpm for 20 min, and dried in a vacuum oven at 60 C. GO was then added to a 2.0 M CoCl26H2O solution to coat the cobalt and NH4OH (30% v/v) was added as needed to adjust the pH to 9.0 [16,17]. After centrifugation of the mixed solution, GO/Co composites were obtained after washing with ethanol and drying at room temperature. Preparation of chitosan solution with a response mediator Chitosan solutions (1–7%, w/v) were prepared by dissolving chitosan powder in 25 TAE buffer. To achieve better dissolution, 5% (v/v) acetic acid was mixed with the chitosan solution (1%, v/v) followed by autoclaving at 121 C for 20 min and subsequently cooling with continuous stirring using a magnetic stir bar for 1 h. Undissolved chitosan shells and foams were removed by vacuum filtration with nylon net filters (11 mm). Rheological measurements of prepared chitosan solutions were performed using a Brookfield digital viscometer (DV-II + Pro). The viscosity was measured using 20 mL of chitosan solution at a spinning time of 5 min. All experiments were carried out in duplicate with <5% variation in results. The GO/Co composites were embedded in filtered chitosan solution overnight while the color of the particle solution changed to dark red [18]. Dissolved particles were weighed using an electronic scale (HR-200 AND, Co., Korea) [19]. Electrodeposition of GO/Co/chitosan on the gold electrode CHI101 gold electrodes were used as the working electrode and the reference electrode (CHI Instruments, USA). A power supply (EV215; Carl Roth, Belgium) was used for the deposition of GO/Co/ chitosan on the gold electrode over a wide voltage range [19]. The process was conducted between 0 V and 1200 V at 500 mA, 300 W. Electrophoretic deposition of gold electrodes was complete at 19 V and charged at 200 mA.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Enzyme immobilization Redox enzyme concentrations of 1 g/L GOx and 0.5 g/L Lac were immobilized on deposited electrodes in sodium phosphate buffer (0.1 M, pH 6.3) to generate redox electrodes for EFCs. After acid pretreatment, 20 mM EDC and 5 mM NHS were used for coupling the redox enzyme to the mediator prepared in Section “Preparation of chitosan solution with a response mediator”. Enzymes were selfassembled onto GO/Co/chitosan-based electrodes [16]. To generate amine groups on the mediator, coupling reaction of this immobilized system was carried out with sodium phosphate buffer (0.05 M, pH 6.3) for 8 h at 4 C. Power curve and electrochemical properties Power curve of the developed EFC was measured using a potentiostat/galvanostat with an optional frequency response analyzer. Electrochemical properties of the GO/Co/chitosan mediator were determined using the obtained CV response and chronopotentiometry (CP). Fig. 1 illustrates the 3-electrode system developed with a working electrode, reference (Ag/AgCl) electrode, and counter electrode. Electrochemical responses of the enzyme electrode for EFC capacity were measured with CP using a VersaSTAT 3 (AMETEK, Princeton Applied Research, USA). The effect of acetic acid concentration on the modified chitosanmediator to act as an EFC was also evaluated with CV and CP. EFCs based on D-glucose employ redox enzymes that consist of a protein component and cofactors that are immobilized on the bioelectrode of the EFC through self-assembly. In this study, the redox enzymes were immobilized by covalent bonds on GO/Co/ chitosan using EDC and NHS (Fig. 1). Specimen characterization Microstructures of the GO/Co/chitosan surface on the mediator particles were characterized using SEM (Inspect F50, FEI, USA). SEM images of samples of graphite dissolved in chitosan solution with various concentrations (1, 3, 5, and 7%) of acetic acid were analyzed. FTIR spectroscopy (FTIR-4600, Jasco, Japan) was used to
3
identify the functional groups of the modified mediators that were coated with CoCl2 and deposited in chitosan solution containing acetic acid at different concentrations [16]. Results and discussion Effect of acidic conditions on surface structure of GO/Co/chitosan Effect of acetic acid concentration on surface structure of GO/ Co/chitosan was observed using SEM. Compared with SEM images of other adsorption materials, structural changes including morphology and crystalline phases of GO/Co/chitosan were analyzed at different concentrations of acetic acid. SEM images (Fig. 2) were obtained to show surface structure of the modified mediator with acetic acid at concentrations of 1, 3, 5, and 7%. Aggregation of GO/Co/chitosan particles, a key mechanical property of Co-modified composites, improved with increasing concentration of acetic acid added during the dissolution of chitosan. The GO/Co/chitosan mediator possessed a smooth surface at lower concentrations of acetic acid (Fig. 2a and b). After increasing acetic acid concentration to 5% (Fig. 2c), more obvious aggregation of Go/Co/chitosan mediator was observed. This aggregation could be due to increasing electrochemical power density of EFC performance (Fig. 5a). Thus, the addition of acetic acid has a positive influence on the dispersion of the GO/Co/chitosan mediator. According to the miscibility of acetic acid added into chitosan, the formation and dispersion of GO/Co/chitosan composite could be influenced by additional concentration with the same quantity of acetic acid [20]. For a good dispersion, the GO/Co/chitosan composition should be uniform and aggregation of mediator particles should be prevented. SEM images shown in Fig. 2 indicate that 5% of additional acetic acid is the optimal concentration considering that surface morphology of mediator particles contains homogeneously tiny structures that are suitable for EFCs. In addition, SEM images of GO/Co/chitosan mediator samples are shown at two different scales (5.0 mm and 2.0 mm, Fig. 2). Magnified images showed that the dispersion of mediator particles was insufficient when lower concentrations of acetic acid (1% and 3% v/v) were added, resulting in a layer structure (Fig. 2a and b), indicating the lack of chitosan dispersion. This insufficient dispersion negatively influenced electron transfer when the composite was applied as a mediator on the electrode of an EFC. The aggregation of mediator particles with nonuniform size when 7% acetic acid was added (Fig. 2d) resulted in over dispersion. Chitosan normally aggregates in aqueous solution and when additional acetic acid was added at higher concentrations (5% and 7%). The aggregation was enhanced in solution. The crystalline structure was also considered a factor that can affect physicochemical properties of electron transport at the composite surface. Overall, 5% acetic acid provided the best crystallinity and uniform particles. Therefore, this concentration was deemed optimal and employed for the improved GO/Co/ chitosan mediator.
Effect of acetic acid on solution properties of chitosan Chitosan contains amino groups, unlike chitin, and exhibits gel characteristics. When chitosan is dissolved in a solution containing acetic acid, the following reactions occur:
Fig. 1. Illustration of an enzyme fuel cell (EFC) and the developed electron transfer mediators. The anode contains glucose oxidase (GOx) and the cathode contains laccase as biocatalysts.
CH3COOH + H2O , CH3COO + H3O+
(1)
Chit-NH2 + H3O+ , Chit-NH3+ + H2O
(2)
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
4
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Fig. 2. SEM images of different GO/Co/chitosan morphologies depending on various concentrations of acetic acid. (a) 1%, (b) 3%, (c) 5%, and (d) 7%.
An interesting behavior of chitosan in acetic acid is its dissolution at low concentrations of acid. With a higher degree of deacetylation with acetic acid, the more positively charged amine group is formed when chitosan is dissolved in solution.
Therefore, the chitosan molecule becomes positively charged in aqueous media under acidic conditions. Chitosan is insoluble in water, alkali, and aqueous solutions above pH 7.0 due to its stable semi-crystalline structure in common organic solvents [21]. The
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
5
Fig. 3. Variation in relative viscosity and pH depending on the concentration of acetic acid and weight of dissolved chitosan.
pH of chitosan solution depends on the concentration of acetic acid which in turn influences its viscosity (Fig. 3). With increasing concentration of acetic acid, the solubility of chitosan and viscosity of the solution also increased due to protonation of amino groups in the chitosan polymer. Addition of acetic acid results in lower pH of the chitosan solution. Therefore, it is important to investigate the relationships of solution pH with viscosity and solubility. Chitosan viscosity increased with decreasing pH. Especially when pH decreased from 7.0 to 5.0, a rapid increase of solution viscosity was observed (Fig. 3). The viscosity of the solution increased as the concentration of acetic acid required to dissolve chitosan particles increased. The decrease was sharper when the pH was adjusted to above 7.0 or below 5.0. This supports the fact that weakly acidic conditions can positively influence the mobility of chitosan solution. In addition, a rapid increase of dissolved chitosan occurs when solution pH in the acidic range is adjusted with at least 3% v/v acetic acid. The weight of dissolved chitosan was >0.05 g when acetic acid concentration was 3%, 5%, or 7%, while the pH corresponded to 7.0, 5.6, or 4.8, respectively. With pH > 7.0, chitosan particles exhibited higher viscosity in accordance with molecular weight, the degree of deacetylation, ion strength, and pH [22]. In related work, weak acid, like acetic acid, was employed for chitosan dissolution to decrease solution pH and to raise solution viscosity [23]. These results showed that a chitosan solution containing at least 5% v/v acetic acid exhibited the strongest relationship between viscosity and pH [11]. The positive influence of additional acetic acid was not significant when acetic acid concentration increased from 5% to 7%, indicating that 5% acetic acid was the optimal amount of acid to improve chitosan solution. Furthermore, the effect of acetic acid on the dissolution of a mediator based on chitosan was investigated. Weights of GO/Co/ Chitosan samples were measured after dissolution in 1, 3, 5, and 7% acetic acid, with dissolution of 9.6, 52.8, 65.2, and 70.8% w/w, respectively, of chitosan-based mediator. Based on the structure of GO/Co/chitosan, a higher concentration of acetic acid could yield more dispersed chitosan and formation of porous structures which could result in higher concentration of immobilized cobalt on the electrode and improved electron transfer of the EFC. High solubility of chitosan with higher concentration of acetic acid is unsuitable for improving electron transfer of the mediator. FTIR spectra of GO and GO/Co/chitosan confirmed the presence of functional groups (Fig. 4). Significant deoxygenation of GO/Co/ chitosan was indicated by hydroxyl group stretching vibrations of
GO at 3280 cm1. The spectrum of GO/Co/chitosan contained prominent peaks for carboxyl and epoxy groups observed at 955– 1100 cm1, 1600–1760 cm1, and 1601 cm1. The pattern of crosslinked chitosan was indicated by a strong peak at 3400 cm1 due to OH stretching. Peaks on the GO/Co/chitosan composite increased at 3289.96 cm1 compared to the spectrum for GO [19]. The GO/Co/chitosan FTIR spectrum showed carboxyl group of GO with amino group of chitosan which was linked to oxygen and carboxyl group of GO. Cobalt hydroxo species gave rise to the peak found at 683 cm1 from decomposition of the GO/Co/chitosan composite. Effect of acetic acid on EFC power density with chitosan mediator The performance of the improved mediator should be evaluated using an EFC system. The effect of acetic acid on EFC power density was investigated to determine the performance of the modified chitosan mediator. The most common reactions used in glucose EFCs involved oxidation of D-glucose to gluconolactone and reduction of oxygen to water as follows: C6H12O6 → C6H12O7 + 2H+ + 2e
(3)
Fig. 4. FTIR spectra of GO and GO/Co/chitosan composites with 5% acetic acid.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
6
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Fig. 5. Effect of acetic acid concentration. (a) Power density of the EFC (The error bars are based on measurements in quintuplicate). (b) Cyclic voltammograms of EFC.
O2 + 2H+ + 2e → H2O
(4)
This redox reaction provides two electrons per mole of glucose with a maximum reversible cell potential (0.6 V) in EFCs [23]. Chronopotentiometry was performed to optimize the concentration of acetic acid in the development of GO/Co/chitosan mediators by using cyclic voltammetry to determine the highest power density of the EFC system (Fig. 5). The power density of EFCs was determined with the electron transfer mediators generated using various concentrations of acetic acid (1, 3, 5, and 7%). Operation of the EFC with 5% acetic acid resulted in the best performance. The power density slowly increased when acetic acid concentration increased from 1 to 3% v/ v. It rapidly increases at 5% acetic acid, then rapidly decreases with
7% acetic acid (Fig. 5a). The EFC with 5% acetic acid generated the maximum power density (1198.09 mW/cm2). Low concentrations (i.e., 1 and 3%) of acetic acid were insufficient for appropriate generation of composite materials, leading to poor electron transfer. Reaction condition was one of the determining factors for effective dispersion and metallization of cobalt. The high solubility of chitosan was related to the addition of higher concentrations of acetic acid. A mediator that is highly soluble with uniform aggregation is preferable to partial aggregation which negatively influences the dispersion and metallization of cobalt. Thus, the power density of EFC rapidly decreased when 7% acetic acid was employed. These results indicated that 5% acetic acid was appropriate to form desired Co(OH)2 layer on GO/Co/ chitosan as demonstrated by SEM images (Fig. 2) and supported by chronopotentiometry (Fig. 5a). The EFC process with 5% acetic acid was expected to generate the maximum amount of electrical power due to active electron transfer of the mediator with optimal concentration of GO/Co/ chitosan on the EFC electrode. The prepared electrodes consisted of redox enzymes immobilized on GO/Co/chitosan that were submerged in a potassium ferricyanide solution. The CVs of EFCs containing the GO/Co/chitosan mediator were measured with different concentrations of acetic acid (Fig. 5b), exhibited reversible redox peaks similar to the performance of a previously reported related system [24]. A scan rate of 100 mV/s and a potential range from 0.6 V to +0.6 V (vs. Ag/AgCl) were used for CV measurements with a galvanostat/potentiostat and different concentrations of acetic acid (Fig. 5b). The best overall performance of the EFC occurred when 5% acetic acid was used, so this concentration was used to achieve uniform coating thickness. The effect of additional acid and the accurate differences of its concentrations were confirmed by electrochemical analysis based on both power density (Fig. 5a) and cyclic voltammetry (Fig. 5b). Table 1 shows various glucose-based EFCs using different electrode materials and mediators. It can be seen that the EFC using GO/Co/chitosan exhibits a comparable power density performance. When the Copper(II) sulfate/CNT was used as mediator electrode [26], the power density (220 mW/cm2) was much lower than GO/Co/chitosan electrode (1198 mW/cm2). Comparing with the results of Nafion1 based electrodes [27,28], the power density obtained in this study was slightly improved. However, this result proves that GO/Co/chitosan is much more efficient than Nafion1 based mediator when considering the cost. As mentioned earlier, each bioelectrode of the EFC was attached to redox enzymes. Glucose oxidase was immobilized on the anode and laccase was immobilized on the cathode. For an EFC system, it is highly critical to optimize conditions of enzyme oxidation to improve electron transfer using gold electrodes [29]. To solve this problem, the mediator should be designed to increase affinity, enhance capacitance, and reduce the loss of electron potential. Meanwhile, immobilization of redox enzyme produces electron transfer through the mediator, while electron flux is disrupted by the medium or material capacitance. The mediator modified by Co has the property of capacitance and flux of the electron, which can
Table 1 Glucose-based EFCs using various electrode materials and mediators. EFCs Anode
Cathode
GOx/copper(II) sulfate/CNT GOx/Nafion1/CNT GOx/Nafion1/chitosan/carbon cloth GOx/GO/Co/chitosan
Lac/copper Lac/Nafion1/CNT K3Fe(CN)6/carbon cloth Lac/GO/Co/chitosan
Fuel (mM)
Power density (mW/cm2)
Ref.
50 100 100 100
220 1,120 960 1198
[26] [27] [28] This study
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
be buffered for electron transfer on the electrode surface [30]. Furthermore, the suitable porosity of the layer induced by GO/Co/ chitosan could facilitate mediator capacitance because the adsorption of stratified material increase the surface area of GO/ Co/chitosan. The reactivity of the mediator is also improved by using a porous material because electrons generated from the reaction are more easily held by the porous surface. Additionally, the effect of capacitance on the EFC system can be identified by the immediate change of the current after the potential is reversed in cyclic voltammetry. Identification of the capacitance effect is also necessary to confirm the change of CV data, which has already been checked in our previous work [25]. Thus, the capacitance effect on the EFC system established in this study can be ignored. Conclusions A GO/Co/chitosan mediator-modified electrode was developed to observe the electrochemical behavior of chitosan solutions containing different concentrations of acetic acid. Power density, conductivity, and viscosity were determined at different acetic acid concentrations (1, 3, 5, and 7%) in chitosan solutions. It was clear that the conductivity in the presence of chitosan increased due to dissociation of the weak acid. With optimum concentration of acetic acid (5%) determined for modification of GO/Co/chitosan mediator, the highest power density of the EFC (1198.08 mW/cm2) was detected at a cell voltage of 0.42 V vs Ag/AgCl. Increased viscosity positively affects the formulation of GO/Co/chitosan mediator particles, which improves the power density of EFCs. These results indicate that electron transfer by the GO/Co/chitosan mediator developed here in this study with 5% acetic acid can improve the electrical power density in EFCs. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MSIP). (No. 2014R1A2A2A01007321 and NRF-2019R1A2C1006793) and the Industrial Strategic Technology Development Program (10051513) funded by the Ministry of Trade, Industry, and Energy (MI, Korea).
7
References [1] S.G. Hong, J.H. Kim, R.E. Kim, S.J. Kwon, D.W. Kim, H.T. Jung, J.S. Dordick, J. Kim, Biotechnol. Bioprocess Eng. 21 (2016) 573. [2] Q. Xu, F. Zhang, L. Xu, P. Leung, C. Yang, H. Li, Renew. Sustain. Energy Rev. 67 (2017) 574. [3] T. Kuretake, S. Kawahara, M. Motooka, S. Uno, Sensors 17 (2017) 281. [4] T. Jayaraman, A.P. Murthy, V. Elakkiya, S. Chandrasekaran, P. Nithyadharseni, Z. Khan, R.A. Senthil, R. Shanker, M. Raghavender, P. Kuppusami, M. Jagannathan, M. Ashokkumar, J. Ind. Eng. Chem. 64 (2018) 16. [5] N.H. Nguyen, J.R. Kim, S. Park, Biotechnol. Bioprocess Eng. 23 (2018) 564. [6] M. Moumene, D. Rochefort, M. Mohamedi, Int. J. Electrochem. Sci. 8 (2013) 2009. [7] H. Junoh, J. Jaafar, M.N.A.M. Norddin, A.F. Ismail, M.H.D. Othman, M.A. Rahman, N. Yusof, W.N.W. Salleh, H. Ilbeygi, J. Nanomater. 690965 (2015) 1. [8] J. Ma, Y. Sahai, Carbohydr. Polym. 92 (2013) 955. [9] V. Zargar, M. Asghari, A. Dashti, Chem. Eng. Technol. 2 (3) (2015) 204. [10] J. Sun, L. Yang, M. Jiang, Y. Shi, B. Xu, H.L. Ma, J. Chromatogr. B 1054 (2017) 57. [11] C. Drosou, M. Krokida, C.G. Biliaderis, Food Hydrocolloids 77 (2018) 726. [12] F. Ardeshiri, A. Akbari, M. Peyravi, M. Jahanshahi, Korean J. Chem. Eng. 36 (2019) 255. [13] S. Iqbal, S. Ahmad, J. Ind. Eng. Chem. 60 (2018) 53. [14] J.C. An, E.J. Lee, I. Hong, J. Ind. Eng. Chem. 47 (2017) 56. [15] C.M.M. Santos, M.B.Q. Garcia, A.M.S. Silva, R. Santus, P. Morlière, E. Fernandes, Tetrahedron Lett. 54 (2013) 85. [16] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, ACS Nano 4 (2010) 4806. [17] X. Wang, W.S. Liu, X. Lu, P.S. Lee, J. Mater. Chem. 22 (2012) 23114. [18] W. Yu, S. Zhan, Z. Shen, Q. Zhou, Chem. Eng. J. 345 (2018) 462. [19] H.U. Lee, H.Y. Yoo, T. Lkhagvasuren, Y.S. Song, C. Park, J. Kim, S.W. Kim, Biosens. Bioelectron. 42 (2013) 342. [20] O. Mabayoje, M. Seredych, T.J. Bandosz, J. Colloid. Interf. Sci. 378 (2012) 1. [21] I. Younes, M. Rinaudo, Mar. Drugs 13 (2015) 1133. [22] C.I. Cámara, M.V.C. Quiroga, N. Wilke, A. Jimenez-Kairuz, L.M. Yudi, Electrochim. Acta 94 (2013) 124. [23] S.M.L. Silva, C.R.C. Braga, M.V.L. Fook, C.M.O. Raposo, L.H. Carvalho, E.L. Canedo, Theophile Thephanides (Eds.), Engineering and Technology, InTech, Europe, 2012, pp. 43–63. [24] L. Magerusan, F. Pogacean, M. Coros, C. Socaci, S. Pruneanu, C. Leostean, I.O. Pana, Electrochim. Acta 283 (2018) 578. [25] D.S. Kim, S.B. Kim, X. Yang, J.H. Lee, H.Y. Yoo, Y. Chun, J. Cho, C. Park, J. Lee, S.W. Kim, J. Electrochem. Soc. 162 (2015) G113. [26] M. Chung, T.L. Nguyen, T.Q.N. Tran, H.H. Yoon, I.T. Ki, Appl. Surf. Sci. 429 (2018) 203. [27] Z. Kang, K. Jiao, J. Cheng, R. Peng, S. Jiao, Z. Hu, Biosens. Bioelectron. 101 (2018) 60. [28] N.B. Duong, C.L. Wang, L.Z. Huang, W.T. Fang, H. Yang, J. Power Sources 429 (2019) 111. [29] W. Putzbach, N.J. Ronkainen, Sensors 13 (2013) 4811. [30] C. Cannes, F. Kanoufi, A.J. Bard, J. Electroanal. Chem. 547 (2003) 83.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition Dong Sup Kima , Han Suk Choia , Xiaoguang Yanga , Ji Hyun Yanga , Ja Hyun Leeb , Hah Young Yooc , Jinyoung Leed, Chulhwan Parke,* , Seung Wook Kima,f,* a
Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea Department of Food Science and Engineering, Dongyang Mirae University, 445, Gyeongin-ro, Guro-gu, Seoul, Republic of Korea Department of Biotechnology, Sangmyung University, Seoul 03016, Republic of Korea d Gyedang College of General Education, Sangmyung University, Cheonan, Chungnam 31066, Republic of Korea e Department of Chemical Engineering, Kwangwoon University, Seoul 01897, Republic of Korea f Department of Chemistry, Faculty of Science and Technology, Universitas Airlangga, Surabaya, 60115, Indonesia b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 April 2019 Received in revised form 23 August 2019 Accepted 26 August 2019 Available online xxx
An enzyme fuel cell (EFC) using composite graphite oxide/cobalt/chitosan (GO/Co/chitosan) mediator is prepared to convert chemical energy into electrical energy. The degree of chitosan deacetylation affects physicochemical and electrochemical properties of the EFC. The soluble molecular weight of chitosan polymer influences the viscosity of chitosan solution. Also, the solubility of chitosan influences the electrochemical properties of the EFC assembled by electrodeposition of GO/Co/chitosan mediator particles on electrodes. EFC performance could be improved by addition of acetic acid during deacetylation. Acetic acid (5% (v/v)) showed an efficient electron transfer between the mediator and electrolyte in the EFC. The EFC produced a potential voltage of 0.548 V vs. Ag/AgCl with power density of 1198.09 mW/cm2. The power generation was the highest at slightly acidic condition and without addition of cofactor. This study shows the efficient participation of chitosan on electron transfer in EFC application. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Enzyme fuel cell Chitosan Acetic acid Viscosity Glucose oxidase Laccase
Introduction Biofuel cells, used for generation of electrical energy from abundant biological substrates, can be constructed using a variety of methods [1]. Biofuel cells can operate under physiological conditions. They can be used with biological systems including humans. Thus, they have received much interest in use in a variety of biomedical applications as power source implants [2]. Enzyme fuel cell (EFC), a type of biofuel cell, is an attractive alternative energy supply for nano-microelectronic devices in biosensor applications [3]. In bioelectronics system, EFCs can utilize a set of redox enzymes such as glucose oxidase and laccase to catalyze the conversion of chemical energy into electrical energy. Therefore, EFCs have potential to achieve excellent electron transfer while relying on environmentally friendly and biocompatible materials. Graphite based materials provide good advantages such as large surface areas and high electronic conductivity. Additionally, chitosan displays excellent stability, reproducibility, sensitivity
* Corresponding authors. E-mail addresses: [email protected] (C. Park), [email protected] (S.W. Kim).
and selectivity. These materials could be applied to various fields [4]. Their applications include communication devices, implantable devices, and miniaturized biosensors that require physically small power sources. Recent efforts have focused on the use of glucose as a fuel for EFCs with in vivo applications. Therefore, many researchers are interested in biomedical applications such as pacemakers, implantable devices, and healthcare related power sources [4,5]. Relying on the same principles as conventional fuel cells, EFCs can generate electricity based on the release of electrons from oxidation at the anode. Considering the high cost of membranes used to provide selectivity, enzymes are more advantageous due to their specificity. Membrane separator that is necessary for conventional fuel cells is unnecessary for EFCs [6]. The development of new materials and new mediators is therefore important to improve electron transfer kinetics at the electrode and to enhance EFC performance. Chitosan can be obtained from deacetylation of chitin by enzymatic methods or alkaline hydrolysis. As a naturally abundant biopolymer, chitosan has been used as a bio-functional material in many fuel cell applications, including membrane polymers and electrodes [7,8]. It is a suitable functional material because this natural polymer adsorbs readily with excellent biocompatibility
https://doi.org/10.1016/j.jiec.2019.08.060 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
2
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
and biodegradability properties. Amino groups of chitosan can become polyelectrolytic by carrying a positive charge (R-NH3+), resulting in a cathodic biopolymer [9]. A large number of hydroxyl and amino groups on chitosan can act as coordination sites for metal ions in the immobilization process of EFC electrode assembly. Furthermore, chitosan is a natural biopolymer that aids enzyme immobilization on electrodes, which is a crucial process in the establishment of an EFC. Electron transfer, mechanical, and immobilization properties of chitosan are influenced by its solubility, which is improved in acidic solutions [10]. Under certain conditions, chitosan can even exhibit hydrogel formation and viscoelastic behavior. Thus, slightly acidic pH of chitosan solution can positively influence its rheological and morphological properties as well as its electrochemical performance in EFC systems [11]. Although electrolytes such as sodium phosphate are known to improve properties of chitosan in solution, the addition of acids during chitosan dissolution has only been recently explored [4,12]. Several organic and inorganic acids (i.e., hydrochloric acid, phosphoric acid, succinic acid, acetic acid, citric acid, and formic acid) have been investigated [4,6]. Considering the possible bio-applications of EFCs, using acetic acid (a weak organic acid, pH 3–6) in combination with chitosan is attractive. Viscosity is another important factor to consider because it influences penetration of chitosan into fabrics or biopolymers. Under acidic conditions, chitosan solutions are viscous. Thus, they could be quantitatively evaluated using viscosity parameters. Furthermore, it is worth exploring the effect of different concentrations of acetic acid on chitosan with respect to power generation of EFCs. Properties of the polymer solution (e.g., molecular weight, viscosity, and conductivity) and surface electrodeposition conditions (e.g., applied electrical voltage and power density) should be evaluated as functions of acetic acid concentration to understand any improved behavior through addition of chitosan solution in the EFC [13]. We have previously developed a mediator using graphite oxide (GO) and cobalt composite (GO/Co) and determined optimal conditions for its use in EFCs [14,27]. Based on the high density of amino groups present in chitosan, chitosan was chosen as part of the mediator composite to offer electrostatic interactions with redox enzyme. However, the performance of EFCs using GO/Co/chitosan has been limited due to chitosan viscosity [22]. It is necessary to overcome the viscosity limitation. Thus, it was investigated in the present study at various acetic acid concentrations. Changes in morphology at different acetic acid concentrations were compared with the previous mediator. We found that the electron transfer performance was improved using the optimized acetic acid concentration with GO/Co/chitosan relative to GO/Co. The EFC system could also be improved using this optimized acetic acid concentration with the GO/Co/chitosan mediator based on cyclic voltammetry (CV) measurements and power density. This study used chitosan for adsorption in an EFC system. Effects of acetic acid and pH in aqueous solution on dissolution of chitosan were determined. A new mediator was established and characterized by scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy. In addition, rheological properties were measured and optimized in chitosan/acetic acid solutions at various pH, in order to determine the parameters that could lead to improvements. Electron transfer in the EFC is confirmed using CV, power density, and cell potential. Results showed improved performance based on power density. In addition, rheological properties were measured and optimized in the dissolved chitosan solution with an additional acetic acid at various pH parameters that led to important improvement.
Experimental Materials Chitosan (200–800 cp viscosity, 75–85% deacetylated, 190–310 kDa), glucose oxidase (GOx; EC1.1.3.4, derived from Aspergillus niger), laccase (Lac; EC 1.10.3.2, derived from Trametes versicolor), N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), cobalt(II) chloride hexahydrate (CoCl26H2O), graphite powder, and potassium ferricyanide were purchased from Sigma-Aldrich, USA. Anhydrous D-glucose, (98.0%, w/v) used as GOx substrate, ammonium hydroxide (NH4OH), anhydrous dextrose (98.0% w/v), and potassium permanganate were obtained from Samchun Pure Chemical Company (Korea). MnO4 and sulfuric acid (H2SO4, 98.0%, v/v) were purchased from Daejung Chemical Co. (Korea). Glacial acetic acid and 25 Tris-acetate-EDTA (TAE) buffer were obtained from Bio Basic (Canada). All percentages of acetic acid are represented as % w/v unless otherwise indicated. Preparation of mediator with graphite oxide pretreatment and cobalt coating The acid treatment of graphite was carried out using a H2SO4/ H3PO4 solution prepared in a 9:1 volume ratio, with the ratio of graphite powder and H2SO4/H3PO4 solution being 4 g to 300 mL. This process is exothermic, which can reach 120 C [14,15]. Oxidation of graphite was performed by slowly adding KMnO4 to the acidic graphite solution (6:1 w/w KMnO4 to graphite powder) with stirring in strong acid solution overnight and cooling from a water jacket. Afterward, the oxidized graphite particles (graphite oxide, GO) were washed with distilled water, centrifuged at 4500 rpm for 20 min, and dried in a vacuum oven at 60 C. GO was then added to a 2.0 M CoCl26H2O solution to coat the cobalt and NH4OH (30% v/v) was added as needed to adjust the pH to 9.0 [16,17]. After centrifugation of the mixed solution, GO/Co composites were obtained after washing with ethanol and drying at room temperature. Preparation of chitosan solution with a response mediator Chitosan solutions (1–7%, w/v) were prepared by dissolving chitosan powder in 25 TAE buffer. To achieve better dissolution, 5% (v/v) acetic acid was mixed with the chitosan solution (1%, v/v) followed by autoclaving at 121 C for 20 min and subsequently cooling with continuous stirring using a magnetic stir bar for 1 h. Undissolved chitosan shells and foams were removed by vacuum filtration with nylon net filters (11 mm). Rheological measurements of prepared chitosan solutions were performed using a Brookfield digital viscometer (DV-II + Pro). The viscosity was measured using 20 mL of chitosan solution at a spinning time of 5 min. All experiments were carried out in duplicate with <5% variation in results. The GO/Co composites were embedded in filtered chitosan solution overnight while the color of the particle solution changed to dark red [18]. Dissolved particles were weighed using an electronic scale (HR-200 AND, Co., Korea) [19]. Electrodeposition of GO/Co/chitosan on the gold electrode CHI101 gold electrodes were used as the working electrode and the reference electrode (CHI Instruments, USA). A power supply (EV215; Carl Roth, Belgium) was used for the deposition of GO/Co/ chitosan on the gold electrode over a wide voltage range [19]. The process was conducted between 0 V and 1200 V at 500 mA, 300 W. Electrophoretic deposition of gold electrodes was complete at 19 V and charged at 200 mA.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Enzyme immobilization Redox enzyme concentrations of 1 g/L GOx and 0.5 g/L Lac were immobilized on deposited electrodes in sodium phosphate buffer (0.1 M, pH 6.3) to generate redox electrodes for EFCs. After acid pretreatment, 20 mM EDC and 5 mM NHS were used for coupling the redox enzyme to the mediator prepared in Section “Preparation of chitosan solution with a response mediator”. Enzymes were selfassembled onto GO/Co/chitosan-based electrodes [16]. To generate amine groups on the mediator, coupling reaction of this immobilized system was carried out with sodium phosphate buffer (0.05 M, pH 6.3) for 8 h at 4 C. Power curve and electrochemical properties Power curve of the developed EFC was measured using a potentiostat/galvanostat with an optional frequency response analyzer. Electrochemical properties of the GO/Co/chitosan mediator were determined using the obtained CV response and chronopotentiometry (CP). Fig. 1 illustrates the 3-electrode system developed with a working electrode, reference (Ag/AgCl) electrode, and counter electrode. Electrochemical responses of the enzyme electrode for EFC capacity were measured with CP using a VersaSTAT 3 (AMETEK, Princeton Applied Research, USA). The effect of acetic acid concentration on the modified chitosanmediator to act as an EFC was also evaluated with CV and CP. EFCs based on D-glucose employ redox enzymes that consist of a protein component and cofactors that are immobilized on the bioelectrode of the EFC through self-assembly. In this study, the redox enzymes were immobilized by covalent bonds on GO/Co/ chitosan using EDC and NHS (Fig. 1). Specimen characterization Microstructures of the GO/Co/chitosan surface on the mediator particles were characterized using SEM (Inspect F50, FEI, USA). SEM images of samples of graphite dissolved in chitosan solution with various concentrations (1, 3, 5, and 7%) of acetic acid were analyzed. FTIR spectroscopy (FTIR-4600, Jasco, Japan) was used to
3
identify the functional groups of the modified mediators that were coated with CoCl2 and deposited in chitosan solution containing acetic acid at different concentrations [16]. Results and discussion Effect of acidic conditions on surface structure of GO/Co/chitosan Effect of acetic acid concentration on surface structure of GO/ Co/chitosan was observed using SEM. Compared with SEM images of other adsorption materials, structural changes including morphology and crystalline phases of GO/Co/chitosan were analyzed at different concentrations of acetic acid. SEM images (Fig. 2) were obtained to show surface structure of the modified mediator with acetic acid at concentrations of 1, 3, 5, and 7%. Aggregation of GO/Co/chitosan particles, a key mechanical property of Co-modified composites, improved with increasing concentration of acetic acid added during the dissolution of chitosan. The GO/Co/chitosan mediator possessed a smooth surface at lower concentrations of acetic acid (Fig. 2a and b). After increasing acetic acid concentration to 5% (Fig. 2c), more obvious aggregation of Go/Co/chitosan mediator was observed. This aggregation could be due to increasing electrochemical power density of EFC performance (Fig. 5a). Thus, the addition of acetic acid has a positive influence on the dispersion of the GO/Co/chitosan mediator. According to the miscibility of acetic acid added into chitosan, the formation and dispersion of GO/Co/chitosan composite could be influenced by additional concentration with the same quantity of acetic acid [20]. For a good dispersion, the GO/Co/chitosan composition should be uniform and aggregation of mediator particles should be prevented. SEM images shown in Fig. 2 indicate that 5% of additional acetic acid is the optimal concentration considering that surface morphology of mediator particles contains homogeneously tiny structures that are suitable for EFCs. In addition, SEM images of GO/Co/chitosan mediator samples are shown at two different scales (5.0 mm and 2.0 mm, Fig. 2). Magnified images showed that the dispersion of mediator particles was insufficient when lower concentrations of acetic acid (1% and 3% v/v) were added, resulting in a layer structure (Fig. 2a and b), indicating the lack of chitosan dispersion. This insufficient dispersion negatively influenced electron transfer when the composite was applied as a mediator on the electrode of an EFC. The aggregation of mediator particles with nonuniform size when 7% acetic acid was added (Fig. 2d) resulted in over dispersion. Chitosan normally aggregates in aqueous solution and when additional acetic acid was added at higher concentrations (5% and 7%). The aggregation was enhanced in solution. The crystalline structure was also considered a factor that can affect physicochemical properties of electron transport at the composite surface. Overall, 5% acetic acid provided the best crystallinity and uniform particles. Therefore, this concentration was deemed optimal and employed for the improved GO/Co/ chitosan mediator.
Effect of acetic acid on solution properties of chitosan Chitosan contains amino groups, unlike chitin, and exhibits gel characteristics. When chitosan is dissolved in a solution containing acetic acid, the following reactions occur:
Fig. 1. Illustration of an enzyme fuel cell (EFC) and the developed electron transfer mediators. The anode contains glucose oxidase (GOx) and the cathode contains laccase as biocatalysts.
CH3COOH + H2O , CH3COO + H3O+
(1)
Chit-NH2 + H3O+ , Chit-NH3+ + H2O
(2)
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
4
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Fig. 2. SEM images of different GO/Co/chitosan morphologies depending on various concentrations of acetic acid. (a) 1%, (b) 3%, (c) 5%, and (d) 7%.
An interesting behavior of chitosan in acetic acid is its dissolution at low concentrations of acid. With a higher degree of deacetylation with acetic acid, the more positively charged amine group is formed when chitosan is dissolved in solution.
Therefore, the chitosan molecule becomes positively charged in aqueous media under acidic conditions. Chitosan is insoluble in water, alkali, and aqueous solutions above pH 7.0 due to its stable semi-crystalline structure in common organic solvents [21]. The
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
5
Fig. 3. Variation in relative viscosity and pH depending on the concentration of acetic acid and weight of dissolved chitosan.
pH of chitosan solution depends on the concentration of acetic acid which in turn influences its viscosity (Fig. 3). With increasing concentration of acetic acid, the solubility of chitosan and viscosity of the solution also increased due to protonation of amino groups in the chitosan polymer. Addition of acetic acid results in lower pH of the chitosan solution. Therefore, it is important to investigate the relationships of solution pH with viscosity and solubility. Chitosan viscosity increased with decreasing pH. Especially when pH decreased from 7.0 to 5.0, a rapid increase of solution viscosity was observed (Fig. 3). The viscosity of the solution increased as the concentration of acetic acid required to dissolve chitosan particles increased. The decrease was sharper when the pH was adjusted to above 7.0 or below 5.0. This supports the fact that weakly acidic conditions can positively influence the mobility of chitosan solution. In addition, a rapid increase of dissolved chitosan occurs when solution pH in the acidic range is adjusted with at least 3% v/v acetic acid. The weight of dissolved chitosan was >0.05 g when acetic acid concentration was 3%, 5%, or 7%, while the pH corresponded to 7.0, 5.6, or 4.8, respectively. With pH > 7.0, chitosan particles exhibited higher viscosity in accordance with molecular weight, the degree of deacetylation, ion strength, and pH [22]. In related work, weak acid, like acetic acid, was employed for chitosan dissolution to decrease solution pH and to raise solution viscosity [23]. These results showed that a chitosan solution containing at least 5% v/v acetic acid exhibited the strongest relationship between viscosity and pH [11]. The positive influence of additional acetic acid was not significant when acetic acid concentration increased from 5% to 7%, indicating that 5% acetic acid was the optimal amount of acid to improve chitosan solution. Furthermore, the effect of acetic acid on the dissolution of a mediator based on chitosan was investigated. Weights of GO/Co/ Chitosan samples were measured after dissolution in 1, 3, 5, and 7% acetic acid, with dissolution of 9.6, 52.8, 65.2, and 70.8% w/w, respectively, of chitosan-based mediator. Based on the structure of GO/Co/chitosan, a higher concentration of acetic acid could yield more dispersed chitosan and formation of porous structures which could result in higher concentration of immobilized cobalt on the electrode and improved electron transfer of the EFC. High solubility of chitosan with higher concentration of acetic acid is unsuitable for improving electron transfer of the mediator. FTIR spectra of GO and GO/Co/chitosan confirmed the presence of functional groups (Fig. 4). Significant deoxygenation of GO/Co/ chitosan was indicated by hydroxyl group stretching vibrations of
GO at 3280 cm1. The spectrum of GO/Co/chitosan contained prominent peaks for carboxyl and epoxy groups observed at 955– 1100 cm1, 1600–1760 cm1, and 1601 cm1. The pattern of crosslinked chitosan was indicated by a strong peak at 3400 cm1 due to OH stretching. Peaks on the GO/Co/chitosan composite increased at 3289.96 cm1 compared to the spectrum for GO [19]. The GO/Co/chitosan FTIR spectrum showed carboxyl group of GO with amino group of chitosan which was linked to oxygen and carboxyl group of GO. Cobalt hydroxo species gave rise to the peak found at 683 cm1 from decomposition of the GO/Co/chitosan composite. Effect of acetic acid on EFC power density with chitosan mediator The performance of the improved mediator should be evaluated using an EFC system. The effect of acetic acid on EFC power density was investigated to determine the performance of the modified chitosan mediator. The most common reactions used in glucose EFCs involved oxidation of D-glucose to gluconolactone and reduction of oxygen to water as follows: C6H12O6 → C6H12O7 + 2H+ + 2e
(3)
Fig. 4. FTIR spectra of GO and GO/Co/chitosan composites with 5% acetic acid.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
6
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Fig. 5. Effect of acetic acid concentration. (a) Power density of the EFC (The error bars are based on measurements in quintuplicate). (b) Cyclic voltammograms of EFC.
O2 + 2H+ + 2e → H2O
(4)
This redox reaction provides two electrons per mole of glucose with a maximum reversible cell potential (0.6 V) in EFCs [23]. Chronopotentiometry was performed to optimize the concentration of acetic acid in the development of GO/Co/chitosan mediators by using cyclic voltammetry to determine the highest power density of the EFC system (Fig. 5). The power density of EFCs was determined with the electron transfer mediators generated using various concentrations of acetic acid (1, 3, 5, and 7%). Operation of the EFC with 5% acetic acid resulted in the best performance. The power density slowly increased when acetic acid concentration increased from 1 to 3% v/ v. It rapidly increases at 5% acetic acid, then rapidly decreases with
7% acetic acid (Fig. 5a). The EFC with 5% acetic acid generated the maximum power density (1198.09 mW/cm2). Low concentrations (i.e., 1 and 3%) of acetic acid were insufficient for appropriate generation of composite materials, leading to poor electron transfer. Reaction condition was one of the determining factors for effective dispersion and metallization of cobalt. The high solubility of chitosan was related to the addition of higher concentrations of acetic acid. A mediator that is highly soluble with uniform aggregation is preferable to partial aggregation which negatively influences the dispersion and metallization of cobalt. Thus, the power density of EFC rapidly decreased when 7% acetic acid was employed. These results indicated that 5% acetic acid was appropriate to form desired Co(OH)2 layer on GO/Co/ chitosan as demonstrated by SEM images (Fig. 2) and supported by chronopotentiometry (Fig. 5a). The EFC process with 5% acetic acid was expected to generate the maximum amount of electrical power due to active electron transfer of the mediator with optimal concentration of GO/Co/ chitosan on the EFC electrode. The prepared electrodes consisted of redox enzymes immobilized on GO/Co/chitosan that were submerged in a potassium ferricyanide solution. The CVs of EFCs containing the GO/Co/chitosan mediator were measured with different concentrations of acetic acid (Fig. 5b), exhibited reversible redox peaks similar to the performance of a previously reported related system [24]. A scan rate of 100 mV/s and a potential range from 0.6 V to +0.6 V (vs. Ag/AgCl) were used for CV measurements with a galvanostat/potentiostat and different concentrations of acetic acid (Fig. 5b). The best overall performance of the EFC occurred when 5% acetic acid was used, so this concentration was used to achieve uniform coating thickness. The effect of additional acid and the accurate differences of its concentrations were confirmed by electrochemical analysis based on both power density (Fig. 5a) and cyclic voltammetry (Fig. 5b). Table 1 shows various glucose-based EFCs using different electrode materials and mediators. It can be seen that the EFC using GO/Co/chitosan exhibits a comparable power density performance. When the Copper(II) sulfate/CNT was used as mediator electrode [26], the power density (220 mW/cm2) was much lower than GO/Co/chitosan electrode (1198 mW/cm2). Comparing with the results of Nafion1 based electrodes [27,28], the power density obtained in this study was slightly improved. However, this result proves that GO/Co/chitosan is much more efficient than Nafion1 based mediator when considering the cost. As mentioned earlier, each bioelectrode of the EFC was attached to redox enzymes. Glucose oxidase was immobilized on the anode and laccase was immobilized on the cathode. For an EFC system, it is highly critical to optimize conditions of enzyme oxidation to improve electron transfer using gold electrodes [29]. To solve this problem, the mediator should be designed to increase affinity, enhance capacitance, and reduce the loss of electron potential. Meanwhile, immobilization of redox enzyme produces electron transfer through the mediator, while electron flux is disrupted by the medium or material capacitance. The mediator modified by Co has the property of capacitance and flux of the electron, which can
Table 1 Glucose-based EFCs using various electrode materials and mediators. EFCs Anode
Cathode
GOx/copper(II) sulfate/CNT GOx/Nafion1/CNT GOx/Nafion1/chitosan/carbon cloth GOx/GO/Co/chitosan
Lac/copper Lac/Nafion1/CNT K3Fe(CN)6/carbon cloth Lac/GO/Co/chitosan
Fuel (mM)
Power density (mW/cm2)
Ref.
50 100 100 100
220 1,120 960 1198
[26] [27] [28] This study
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060
G Model JIEC 4755 No. of Pages 7
D.S. Kim et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
be buffered for electron transfer on the electrode surface [30]. Furthermore, the suitable porosity of the layer induced by GO/Co/ chitosan could facilitate mediator capacitance because the adsorption of stratified material increase the surface area of GO/ Co/chitosan. The reactivity of the mediator is also improved by using a porous material because electrons generated from the reaction are more easily held by the porous surface. Additionally, the effect of capacitance on the EFC system can be identified by the immediate change of the current after the potential is reversed in cyclic voltammetry. Identification of the capacitance effect is also necessary to confirm the change of CV data, which has already been checked in our previous work [25]. Thus, the capacitance effect on the EFC system established in this study can be ignored. Conclusions A GO/Co/chitosan mediator-modified electrode was developed to observe the electrochemical behavior of chitosan solutions containing different concentrations of acetic acid. Power density, conductivity, and viscosity were determined at different acetic acid concentrations (1, 3, 5, and 7%) in chitosan solutions. It was clear that the conductivity in the presence of chitosan increased due to dissociation of the weak acid. With optimum concentration of acetic acid (5%) determined for modification of GO/Co/chitosan mediator, the highest power density of the EFC (1198.08 mW/cm2) was detected at a cell voltage of 0.42 V vs Ag/AgCl. Increased viscosity positively affects the formulation of GO/Co/chitosan mediator particles, which improves the power density of EFCs. These results indicate that electron transfer by the GO/Co/chitosan mediator developed here in this study with 5% acetic acid can improve the electrical power density in EFCs. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MSIP). (No. 2014R1A2A2A01007321 and NRF-2019R1A2C1006793) and the Industrial Strategic Technology Development Program (10051513) funded by the Ministry of Trade, Industry, and Energy (MI, Korea).
7
References [1] S.G. Hong, J.H. Kim, R.E. Kim, S.J. Kwon, D.W. Kim, H.T. Jung, J.S. Dordick, J. Kim, Biotechnol. Bioprocess Eng. 21 (2016) 573. [2] Q. Xu, F. Zhang, L. Xu, P. Leung, C. Yang, H. Li, Renew. Sustain. Energy Rev. 67 (2017) 574. [3] T. Kuretake, S. Kawahara, M. Motooka, S. Uno, Sensors 17 (2017) 281. [4] T. Jayaraman, A.P. Murthy, V. Elakkiya, S. Chandrasekaran, P. Nithyadharseni, Z. Khan, R.A. Senthil, R. Shanker, M. Raghavender, P. Kuppusami, M. Jagannathan, M. Ashokkumar, J. Ind. Eng. Chem. 64 (2018) 16. [5] N.H. Nguyen, J.R. Kim, S. Park, Biotechnol. Bioprocess Eng. 23 (2018) 564. [6] M. Moumene, D. Rochefort, M. Mohamedi, Int. J. Electrochem. Sci. 8 (2013) 2009. [7] H. Junoh, J. Jaafar, M.N.A.M. Norddin, A.F. Ismail, M.H.D. Othman, M.A. Rahman, N. Yusof, W.N.W. Salleh, H. Ilbeygi, J. Nanomater. 690965 (2015) 1. [8] J. Ma, Y. Sahai, Carbohydr. Polym. 92 (2013) 955. [9] V. Zargar, M. Asghari, A. Dashti, Chem. Eng. Technol. 2 (3) (2015) 204. [10] J. Sun, L. Yang, M. Jiang, Y. Shi, B. Xu, H.L. Ma, J. Chromatogr. B 1054 (2017) 57. [11] C. Drosou, M. Krokida, C.G. Biliaderis, Food Hydrocolloids 77 (2018) 726. [12] F. Ardeshiri, A. Akbari, M. Peyravi, M. Jahanshahi, Korean J. Chem. Eng. 36 (2019) 255. [13] S. Iqbal, S. Ahmad, J. Ind. Eng. Chem. 60 (2018) 53. [14] J.C. An, E.J. Lee, I. Hong, J. Ind. Eng. Chem. 47 (2017) 56. [15] C.M.M. Santos, M.B.Q. Garcia, A.M.S. Silva, R. Santus, P. Morlière, E. Fernandes, Tetrahedron Lett. 54 (2013) 85. [16] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, ACS Nano 4 (2010) 4806. [17] X. Wang, W.S. Liu, X. Lu, P.S. Lee, J. Mater. Chem. 22 (2012) 23114. [18] W. Yu, S. Zhan, Z. Shen, Q. Zhou, Chem. Eng. J. 345 (2018) 462. [19] H.U. Lee, H.Y. Yoo, T. Lkhagvasuren, Y.S. Song, C. Park, J. Kim, S.W. Kim, Biosens. Bioelectron. 42 (2013) 342. [20] O. Mabayoje, M. Seredych, T.J. Bandosz, J. Colloid. Interf. Sci. 378 (2012) 1. [21] I. Younes, M. Rinaudo, Mar. Drugs 13 (2015) 1133. [22] C.I. Cámara, M.V.C. Quiroga, N. Wilke, A. Jimenez-Kairuz, L.M. Yudi, Electrochim. Acta 94 (2013) 124. [23] S.M.L. Silva, C.R.C. Braga, M.V.L. Fook, C.M.O. Raposo, L.H. Carvalho, E.L. Canedo, Theophile Thephanides (Eds.), Engineering and Technology, InTech, Europe, 2012, pp. 43–63. [24] L. Magerusan, F. Pogacean, M. Coros, C. Socaci, S. Pruneanu, C. Leostean, I.O. Pana, Electrochim. Acta 283 (2018) 578. [25] D.S. Kim, S.B. Kim, X. Yang, J.H. Lee, H.Y. Yoo, Y. Chun, J. Cho, C. Park, J. Lee, S.W. Kim, J. Electrochem. Soc. 162 (2015) G113. [26] M. Chung, T.L. Nguyen, T.Q.N. Tran, H.H. Yoon, I.T. Ki, Appl. Surf. Sci. 429 (2018) 203. [27] Z. Kang, K. Jiao, J. Cheng, R. Peng, S. Jiao, Z. Hu, Biosens. Bioelectron. 101 (2018) 60. [28] N.B. Duong, C.L. Wang, L.Z. Huang, W.T. Fang, H. Yang, J. Power Sources 429 (2019) 111. [29] W. Putzbach, N.J. Ronkainen, Sensors 13 (2013) 4811. [30] C. Cannes, F. Kanoufi, A.J. Bard, J. Electroanal. Chem. 547 (2003) 83.
Please cite this article in press as: D.S. Kim, et al., Improvement of power generation of enzyme fuel cell by novel GO/Co/chitosan electrodeposition, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.08.060