CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402325

Visible-Light-Enhanced Electrocatalysis and Bioelectrocatalysis Coupled in a Miniature Glucose/Air Biofuel Cell Lingling Zhang,[a, b] Zhikun Xu,[a] Baohua Lou,[a, b] Lei Han,[a, b] Xiaowei Zhang,[a, b] and Shaojun Dong*[a, b] A glucose/air biofuel cell (BFC) that can convert both chemical and light energy into electricity is described. Polyterthiophene (pTTh), a photoresponsive conducting polymer, serves as cathode and catalyzes the reduction of oxygen. Taking advantage of the good environmental stability and exceptional optical properties of pTTh, the assembled BFC exhibits excellent stability and a fast photoresponse with an open-circuit voltage (Voc) of 0.50 V and a maximum power output density (Pmax) of 23.65 mW cm 2 upon illumination by visible light of 10 mW cm 2, which is an enhancement of ca. 22 times as compared to Pmax in the dark. Additionally, we propose a possible mechanism for this enhancement. Fabricating a BFC in this manner provides an energy conversion model that offers high efficiency at low cost, paving an avenue for practical solar energy conversion on a large scale.

In the past several decades, traditional nonrenewable resources have been consumed rapidly, and these traditional resources can not satisfy the growing worldwide demand in the future. Academic and industrial research activity towards finding alternative energy sources or improving the efficiency of energy use has increased dramatically.[1] Biofuel cells (BFCs) comprise an efficient energy conversion technology, transforming renewable resources (chemical energy) into electricity by employing biocatalysts (i.e., enzymes, organelles, and micro-organisms).[2, 3] However, the power output of BFCs is still limited to one-dimensional energy conversion (chemical to electrical energy), while integrating multiple conversion routes, such as converting both chemical and light energy into electricity efficiently, in a well-organized BFC is desirable to achieve improvements in their power output. In 2003, Gust et al. assembled a hybrid BFC that substituted a dye-sensitized nanoparticulate semiconductor photoanode for the regular bioanode, coupling light energy and chemical energy in a dual conversion process.

[a] Dr. L. L. Zhang, Dr. Z. K. Xu, Dr. B. H. Lou, Dr. L. Han, Dr. X. W. Zhang, Prof. S. J. Dong State Key Laboratory of Electroanalytical Chemistry Changchun Institute of Applied Chemistry, Chinese Academy of Sciences Changchun Jilin,130022 (PR China) E-mail: [email protected] [b] Dr. L. L. Zhang, Dr. B. H. Lou, Dr. L. Han, Dr. X. W. Zhang, Prof. S. J. Dong University of Chinese Academy of Sciences Beijing, 100049 (PR China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402325.

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In principle, this system could provide more power than either process working independently.[4] The oxygen reduction reaction (ORR), the routine cathodic reaction of a biofuel cell, is always (electro)catalyzed by redox enzymes (blue multicopper oxidases such as laccase and billirubin oxidase)[5, 6] or noble metals or alloys (Pt, Pd, and their alloys or nanocomposites).[7–9] Nonetheless, laccase is inactive above pH 6.0 and readily inhibited by halogenides.[5, 10] Meanwhile, problems of high cost and poor stability are inevitable in billirubin oxidase and platinum-based nanomaterials. These phenomena hinder their widespread adoption in fuel-cell cathodes. In recent years, a series of alternative ORR catalysts have been studied, such as metal-free or metal oxide nanomaterials and their composites.[8, 11–13] Their electrocatalytic behaviors are always studied in either acidic or alkaline solution, whereas BFCs prefer to operate in neutral environment or physiological fluid. Conducting polymers are good choices for oxygen reduction. In 2005, Khomenko et al. performed a systematic investigation on various conducting polymers concerning their electrocatalytic activity towards oxygen reduction and explained that the catalytic activity resulted from the unique electronic structure. Both the oxygen atoms were activated by forming bonds to the carbon atoms at the polymeric surface, and then were reduced.[14] Winther-Jensen et al. reported a vapor phasepolymerized PEDOT electrode that showed higher catalytic activity towards O2 reduction than platinum-based electrodes.[15] On the other hand, conducting polymers, especially polythiophenes and their derivations, are a kind of p-type organic semiconductors. Their photovoltaic effect has been widely used in polymer solar cells and dye-sensitized solar cells.[16–22] By combining their electrocatalytic activity and photophysical properties, conducting polymers have become efficient light-enhanced catalysts for ORR in more recent research works.[23–25] Winther-Jensen and co-workers demonstrated that an “alloy” of PEDOT and polythiophenes shows electrocatalytic activity towards ORR. This activity could be enhanced significantly under visible-light illumination. In a simple fuel-cell, the lightenhanced current could be maintained over long periods, attributed to the durability of the heterojunction of light-harvesting polythiophene and hole-conducting PEDOT.[25] Despite their remarkable performance in polymer solar cells, dye-sensitized solar cells, and dissolved oxygen sensors, the use of polythiophenes for photoenhanced cathodes in BFCs has rarely been reported up to now. Actually, they are promising candidate materials for cathodic catalysts because of their ChemSusChem 0000, 00, 1 – 5

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CHEMSUSCHEM COMMUNICATIONS high stability, low costs, and ease of preparation. Herein, we assemble a photoenhanced glucose/air BFC in which in situ electropolymerized polyterthiophene (pTTh) acts as cathodic catalyst and glucose oxidase (GOD) confined onto tetrathiafulvalene (TTF–ordered mesoporous carbon (OMC) composite as the anode biocatalyst. This is the first example of the integration of a photoresponsive polymer into a BFC and opens up a new approach to efficient multi-dimension energy conversion, promoting the development of power-generating devices. Visible light avoids the probability of damage to biomolecules caused by UV light; an impediment for the widespread application of BFCs. Additionally, both the bioanode and photocathode modification proceed using a matrix of screen-printed carbon electrodes (SPCEs) on monolithic polymethyl methacrylate (PMMA). SPCEs can be mass-produced at low cost and the use of PMMA makes it possible to fabricate miniature and portable BFCs in the future. Screen-printing technology for electrode manufacture has been widely applied in electrochemical sensors owing to the simplicity of preparation (even for mass production and commercialization) and low cost. In our previous work, a biofuel cell based on paper-based SPCEs was been assembled.[26] In this work, a more durable and reproducible matrix, PMMA, is chosen to replace the flexible and disposable paper. Briefly, SPCEs were prepared by printing conductive carbon ink onto 0.8 cm  2.0 cm PMMA plates through the patterned screen. The as-prepared SPCEs are tight enough to resist fluid flow even in long-term immersion. Scheme 1 A is an illustration of the photoenhanced BFC. The glucose oxidase confined onto the tetrathiafulvalene–ordered mesoporous carbon composite (i.e., GOD/TTF-OMC) bioanode catalyzes the oxidation of glucose, and the pTTh cathode catalyzes oxygen reduction under

Scheme 1. A) Schematic illustration of the miniature photoenhanced glucose/air biofuel cell on PMMA. B) Top-view image of the photoenhanced BFC microchip. C) Digital photo of a BFC operating under illumination.

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Scheme 2. Principle of the photoenhanced glucose/air biofuel cell.

visible light illumination (Scheme 1 B and C). A portable homemade light source was placed just above the system and its light intensity was measured as 10 mW cm 2 at the electrode surface. The electrolyte containing biofuel was loaded into the perforated polydimethylsiloxane (PDMS) layer covering the electrodes. A proposed mechanism of operation for the light-enhanced enzymatic biofuel cell is shown in Scheme 2. Glucose oxidation occurs at the GOD-based bioanode, and the released electrons travel to the photocathode via the external circuit. In the dark, the electrons move to the pTTh surface and are incorporated into molecular oxygen, after which oxygen reduction occurs. Given that the energy level of the electrons from the external circuit is relatively low, oxygen reduction commences at a more negative potential at a slow rate. Upon light illumination, holes and electrons are generated in the pTTh. The external electrons combine with the holes in the valence bands, while the photoexcited electrons in the pTTh conduction band, with a high energy level, participate in the oxygen reduction reaction, leading to an increasing current and a decreasing overpotential. Conducting polymers have been employed for electrocatalytic ORR in the past several decades because they offer high electrical conductivity, good chemical stability, and are relatively easy to synthesize. In addition polythiophenes and their derivates have intrinsic photovoltaic properties, which is favorable when striving for photoinduced enhancement of the ORR. Terthiophene (TTh) possesses better light-harvesting characteristics and a lower oxidation potential than other thiophenes, and we hence selected TTh as monomer for in situ polymerization. We performed the polymerization by electrochemical potentiostatic deposition, which is a method superior to chemical polymerization because it is free of oxidants, leads to easier preparation of materials, and allows fine control over thickness and homogeneity. We determined, by cyclic voltammetric electrodeposition (Supporting Information, Figure S1), that oxidation of TTh to the polymer pTTh started at 0.85 V (vs. Ag/AgCl). We hence adopted a relatively positive constant ChemSusChem 0000, 00, 1 – 5

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Figure 1. A) SEM image of pTTh, and a zoomed-in image (inset). B) Absorption spectra of (a) ITO, and (b) pTTh electrodeposited onto ITO. C) Polarization curves of the cathode in air-saturated 0.1 m pH 7.0 PBS (a) without and (b) with illumination. D) Polarization curves of the cathode in (a) N2-saturated, (b) air-saturated, and (c) O2-saturated 0.1 m pH 7.0 PBS under illumination. Scan rate: 1 mV s 1.

potential of 1.0 V in subsequent polymer formation and growth experiments. Scanning electron microscopy (SEM) images of as-prepared pTTh (Figure 1 A) revealed a continuous, rough film with many wrinkles, indicating the effective increase of the electrode area. The rough film facilitates reflection and refraction of light on the surface, and accordingly is beneficial for improving the efficiency of light utilization. Transparent indium tin oxide (ITO) glass was used as the support to obtain absorption spectra of electropolymerized pTTh. Figure 1 B shows the marked response of pTTh film to visible light in the range 400–600 nm. To evaluate the effect of light illumination on the ORR, we compared the catalytic activity of the pTTh cathode in dark and light conditions (Figure 1 C). Under light illumination the catalytic current was enhanced by a factor of ca. 25, reaching 171.3 mA cm 2 (RSD = 0.7 %, n = 5), and the onset potential was as high as 0.46  0.01 V (vs. Ag/AgCl). The electrocatalytic oxygen reduction crucially depends on the amount of electropolymerized pTTh, and therefore we optimized by investigating different polymerization charges and evaluating the behaviors of the obtained pTTh films towards ORR. The data in Figure S2 (Supporting Information) reveal that the catalytic current increases with pTTh film thickness, as expected, and then reaches a maximum value at a charge density of 0.6 C cm 2, which was selected for subsequent experiments. Figure 1 D illustrates the responses of the pTTh cathode towards environments with different oxygen contents. No catalytic current was obvious under anaerobic conditions, while in air-saturated electrolyte and electrolyte exposed to oxygen bubbling the cathode exhibited high electrocatalytic activity (with onset potentials of 0.46  0.01 V and 0.53  0.01 V (vs. Ag/AgCl), respectively). These findings confirm that electron communication between photoactivated pTTh and O2 are fast. OMC has a high surface area, well-defined pore size, and excellent intrinsic conductivity, which makes it suitable for elec 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 2. A) SEM image of bare OMC. B) SEM image of TTF-OMC composite. C) XRD profiles of (a) bare OMC and (b) TTF-OMC composite. D) Polarization curves of bioanode in air-saturated 0.1 m pH 7.0 PBS containing (a) 0, (b) 25, and (c) 50 mm glucose. Scan rate: 1 mV s 1.

trocatalysis and fuel cell applications.[27] TTF is an efficient redox mediator to facilitate electron transfer between flavoproteins (e.g., glucose oxidase) and the electrode surface. During the electron transfer process TTF undergoes two one-electron oxidations, resulting in the formation of TTF + and TTF2 + .[28] Because of the unique porous structure of OMC and p-stacking interactions, TTF can be incorporated into OMC to form a redox-active composite, which enhances the electron shuttling function and drastically reduces the reaction’s overpotential. SEM and X-ray diffraction (XRD) analysis were used to investigate the TTF-OMC composite. TTF interacted strongly with OMC, as can be determined by comparing Figure 2 A and Figure 2 B. Figure 2 C shows XRD curves of (a) bare OMC, and (b) the TTF-OMC composite. The well-resolved diffraction peaks at 2q values of 1.08, 1.78, and 2.08 (curve a) can be assigned to reflections characteristic of the (100), (110), and (200) planes, respectively, of hexagonal structures in OMC.[29] After TTF modification, a partial deterioration of the ordered mesoporous structure is observed except for the weak main diffraction peak (100) (curve b), implying that the basic hexagonal order is retained in the structure. We also evaluated the stability of the composite on a screen-printed carbon electrode by voltammetric scans for over 500 cycles (Supporting Information, Figure S4). There is a subtle decrease after long-term cycling, which is an encouraging basis for building a stable mediating bioelectrocatalytic system. Figure 2 D shows how the bioelectrocatalysis reaction at the GOD bioanode responds to different glucose concentrations. The catalytic potential commences at 0.05  0.01 V (vs. Ag/AgCl) and the current density reaches a peak value of 318 mA cm 2 at 50 mm glucose (RSD = 2.3 %, n = 5). Before assembling a photoenhanced glucose/air BFC, we studied crossover between GOD/TTF-OMC bioanode and photoresponsive pTTh cathode (Supporting Information, Figure S5). It is clear that neither the oxygen content nor light illumination affect bioelectrocatalysis at the bioanode. The presence of 50 mm glucose also hardly interferes with the photoChemSusChem 0000, 00, 1 – 5

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after storage at 4 8C for 1 week. Even after 30 days, the power output declined by only about 25 %, and this decline is likely due to partial loss of GOD activity. In addition, these results are superior to a microfabricated BFC with enzyme-modified bioelectrodes and a small-size enzymatic BFC on paper, and even to a UV-light-driven anatase-based photoelectrochemical fuel cell (Table 1).[26, 31, 33] Although there are still shortfalls in these light-enhanced devices compared to other photoelectrochemical BFCs,[32, 34, 35] further optimization and improvement (e.g, modification of the conducting polymer, modulation or doping of components, engineering improvements, fine miniaturization) are promising routes to solve problems and meet the requirements for real applications. In summary, we report a new type of photoenhanced glucose/air biofuel cell (BFC). Polyterthiophene (pTTH), a photoresponsive conducting polymer used in a BFC for the first time, serves as cathode for oxygen reduction enhanced by visible light. Coupled with the high electrocatalytic activity toward glucose oxidation at the GOD bioanode, the fabricated BFC exhibits a Voc of 0.50 V and a Pmax of 23.65 mW cm 2 under visible light irradiation of 10 mW cm 2, verifying the feasibility of using pTTh as photoenhanced cathodic catalyst. We also propose a mechanism to explain the enhancement of the photoinduced power output. Combined with inexpensive screenprinting technology suitable for mass production, we expect that the as-fabricated miniature, portable BFC devices can be produced on a large scale with high efficiency and at low cost. Furthermore, the BFCs exhibit excellent long-term stability and the ability to quickly respond to light; important properties for their practical application in Figure 3. A) Polarization curves and the dependence of the power density on the BFC voltage (a) in the dark, diverse energy conversion (b) without glucose, and (c) upon 10 mW cm 2 visible light illumination. B) The power output cycle of the photoenhanced BFC microchip with illumination on and off. schemes in the future.

enhanced oxygen reduction at the cathode. Such excellent performance characteristics make it possible to assemble a compartmentless BFC on monolithic PMMA. The positions of two patterned carbon ink electrodes printed onto PMMA are shown in Scheme 1. TTh electropolymerization at the cathode was carried out by masking the other electrode with tape and, after polymerization, removing the tape and preparing the bioanode. Considering that we aim for a simple practical BFC device, we did not anchor an oxygen bubbling device and used oxygen from air as the oxidant. Casting a PDMS “hole” exposed to the air onto the electrodes, we investigated the performance of a BFC in 0.1 m pH 7.0 PBS containing 50 mm glucose in dark and light conditions, respectively. The open-circuit voltage (Voc) values were 0.50  0.01 V and 0.35  0.01 V in the light and dark state, respectively, and the maximum power output density (Pmax) under 10 mW cm 2 light illumination was enhanced by a factor of 22 compared to the value without irradiation, reaching 23.65 mW cm 2 (RSD = 1.8 %, n = 5) (Figure 3 A). Furthermore, the photoresponse when switching from dark to light was fast, and no obvious power output loss was observed after 8 cycles (Figure 3 B), indicating excellent stability during continuous discharge. Because of the high stability of pTTh, the BFC retained 94.3 % of its original power output

Table 1. Comparison of different microfabricated biofuel cells and photoelectrochemical fuel cells. Fuel cell type

Anode

Cathode

Eoc [V]

Pmax [mW cm 2]

Ref.

microfluidic biofuel cells

glucose oxidase/SWCNT glucose oxidase/SWCNT glucose oxidase/Fc-C6-LPEI glucose dehydrogenase/CNTs-IL glucose dehydrogenase/poly MB/SWNHs anatase TiO2/ITO

laccase/SWCNT CotA laccase/SWCNT MWCNTs/laccase/TBAB-Nafion bilirubin oxidase/CNTs-IL f-TiO2-Pt NPs Pt wire bilirubin oxidase/CNTs-IL

glucose dehydrogenase/porphyrin/SnO2

Hg/Hg2SO4

1.57 1.65 64 13.5 10.74 4.81 (350 nm) 1.32 (380 nm) 36 (H2O) 47 (glucose) 19

[1]

TiO2 nanotube

0.43 0.44 0.54 0.56 0.43 0.57 0.48 0.93 1.00 0.75

glucose dehydrogenase/TCPP/NTDMF glucose oxidase/TTF-OMC

Pt black pTTh

0.74 0.53

33.94 23.65

paper-based biofuel cell hybrid biofuel cell photoelectrochemical fuel cell (UV light)

photoelectrochemical biofuel cell (visible light) microfabricated visible lightenhanced biofuel cell

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

[2] [3] [4] [5] [6] [7] [8] this work

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CHEMSUSCHEM COMMUNICATIONS Experimental Section Screen-printed carbon electrodes (SPCEs) were prepared by printing conductive carbon ink onto a 0.8 cm  2.0 cm PMMA plate through a patterned screen, and then drying in a vacuum oven at 60 8C for 1 h. The electrode diameter was fixed as 1 mm by masking a piece of Scotch tape. In situ polymerization of TTh on the screen-printed carbon electrode proceeded in acetonitrile solution containing 50 mm TTh and 0.1 m LiClO4. Potentiostatic deposition was performed at 1.0 V and the thickness of pTTh was controlled through the amount of electrodeposited charge. OMC with defined pore size was synthesized according to a previous report.[27, 30] The TTF-OMC composite was prepared as follows: 3 mg of OMC (pore diameter 3.9 nm) and 10 mg of TTF were dispersed in 5 mL ethanol in water (2:1 v/v) solution. The suspension was sonicated for 30 min followed by magnetic stirring for 12 h at room temperature. The resulting suspension was subsequently centrifuged to remove free TTF and washed with fresh water/ethanol mixed solvent more than 3 times, and then vacuum-dried at 70 8C for 12 h. The obtained sample was denoted as TTF-OMC composite. Then, 0.5 mg of TTF-OMC composite was ultrasonically dispersed in 1 mL aqueous Nafion (0.1 wt %) solution, and then 5 mL of the resulting colloidal dispersion was spread onto the screen-printed carbon electrode followed by drying at room temperature. This sample is denoted as TTF-OMC electrode. To prepare the bioanode, GOD (2 mg mL 1 in 10 mm pH 7.0 PBS) was mixed with Nafion (0.1 wt %) and 8 mL of the mixture was coated onto the TTF-OMC electrode, and this assembly was dried at 4 8C overnight (noted as GOD/TTF-OMC electrode). Electrochemical measurements were conducted with an electrochemical workstation CHI 832B (Shanghai Chenhua Instrument Corporation, PR China). Except for the BFC assembly, a conventional three-electrode cell was used, with a Ag/AgCl electrode (saturated KCl) as reference electrode and platinum wire as counter electrode. SEM images were obtained with an XL30 ESEM field-emission scanning electron microscope at an accelerating voltage of 15 kV. XRD profiles were detected by using a D8 ADVANCE instrument (Germany) with CuK (l = 1.5406 ) radiation. UV/Vis absorption measurements were performed on a Cary 50 UV-Vis spectrometer (Varian). A home-made portable light source (the UV component was filtered) was used in the assays involving light; the photon flux applied to the electrode surface was 10 mW cm 2. All of the experiments were carried out at room temperature.

Acknowledgements This work was supported by National Natural Science Foundation of China (21075116 and 21375123) and the 973 Project (2010CB933603 and 2011CB911002). Keywords: carbons · electrochemistry · energy conversion · fuel cells · mesoporous materials [1] M. J. Moehlenbrock, S. D. Minteer, Chem. Soc. Rev. 2008, 37, 1188 – 1196. [2] R. A. Bullen, T. C. Arnot, J. B. Lakeman, F. C. Walsh, Biosens. Bioelectron. 2006, 21, 2015 – 2045.

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COMMUNICATIONS L. L. Zhang, Z. K. Xu, B. H. Lou, L. Han, X. W. Zhang, S. J. Dong* && – && Visible-Light-Enhanced Electrocatalysis and Bioelectrocatalysis Coupled in a Miniature Glucose/Air Biofuel Cell

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

BFCs forever: A miniature glucose/air biofuel cell (BFC) converts both chemical and light energy into electricity. The power output of the BFC exhibits an enhancement by a factor of ca. 22 upon visible-light illumination, with excellent stability and a fast photoresponse. Fabricating a BFC in this manner provides an energy conversion model that offers high efficiency at low cost, paving an avenue for practical solar energy conversion on a large scale.

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