Functionalized-carbon nanotube supported electrocatalysts and buckypaper-based biocathodes for glucose fuel cell applications

Electrochimica Acta 56 (2011) 7659–7665

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Functionalized-carbon nanotube supported electrocatalysts and buckypaper-based biocathodes for glucose fuel cell applications L. Hussein a,b,∗ , Y.J. Feng c,1 , N. Alonso-Vante c,1 , G. Urban a,b , M. Krüger a,b,∗∗ a b c

Freiburg Materials Research Centre (FMF), Laboratory for Nanosciences, University of Freiburg, Freiburg, Germany Institute for Microsystems Technology (IMTEK), Laboratory for Sensors, University of Freiburg, Freiburg, Germany Laboratory of Electrocatalysis, UMR-CNRS 6503, University of Poitiers, Poitiers, France

a r t i c l e

i n f o

Article history: Received 3 June 2011 Accepted 21 June 2011 Available online 30 June 2011 Keywords: Biofuel cell Oxygen reduction reaction (ORR) Glucose Functionalized carbon nanotubes (f-CNTs) Nanocatalysts

a b s t r a c t The preparation and testing for electrocatalytic activity of functionalized carbon nanotube (f-CNT) supported Pt and Au–Pt nanoparticles (NPs), and bilirubin oxidase (BOD), are reported. These materials were utilized as oxygen reduction reaction (ORR) cathode electrocatalysts in a phosphate buffer solution (0.2 M, pH 7.4) at 25 ◦ C, in the absence and presence of glucose. Carbon monoxide (CO) stripping voltammetry was applied to determine the electrochemically active surface area (ESA). The ORR performance of the Pt/f-CNTs catalyst was high (specific activity of 80.9 ␮A cmPt −2 at 0.8 V vs. RHE) with an open circuit potential within ca. 10 mV of that delivered by state-of-the-art carbon supported platinum catalyst and exhibited better glucose tolerance. The f-CNT support favors a higher electrocatalytic activity of BOD for the ORR than a commercially available carbon black (Vulcan XC-72R). These results demonstrate that f-CNTs are a promising electrocatalyst supporting substrate for biofuel cell applications. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Glucose is the most abundant monosaccharide. As a most important energy source (biofuel) for low-temperature, onecompartment biofuel cells, it has been the subject of many studies [1]. Particular attention has been paid to the development of electrocatalysts for mixed-reactant fuel cells, especially to the oxygen reduction reaction (ORR) process in relation to glucose tolerance of the catalysts. Large overpotential losses, due to mixed potentials resulting from mixed reactants and slow reaction kinetics, usually occur. One possibility for overcoming these difficulties is to conceive new efficient and selective electrocatalysts for oxygen reduction in the presence of glucose. Several ORR electrocatalysts based on Pt and oxophillic metals such as Pd, Co, Ni, Fe, Cu and Ag have been investigated [2]. The overall electrocatalytic processes at electrodes are rather complex and involve a number of adsorbed intermediates and by-products [3]. Electrocatalysts based on Pt or its alloys make

∗ Corresponding author at: Institute for Microsystems Technology (IMTEK), Laboratory for Sensors, University of Freiburg, Freiburg, Germany. Tel.: +49 7612 034781; fax: +49 7612 037262. ∗∗ Corresponding author at: Freiburg Materials Research Centre (FMF), Laboratory for Nanosciences, University of Freiburg, Freiburg, Germany. Tel: +49 7612 034755; fax: +49 7612 034701. E-mail addresses: [email protected] (L. Hussein), [email protected] (M. Krüger). 1 ISE member. 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.06.067

the devices expensive and unsuitable for large scale applications. Replacing or reducing the amount of Pt, while maintaining the catalytic activity is an important goal for developing novel catalysts [4,5] and new supporting substrates [6]. In contrast to conventional direct fuel cells, the challenge to establish an implantable direct biofuel cell is to run the cell under physiological and mixedreactant conditions. The glucose fuel cell, as a mixed-reactant fuel cell, is considered to be an attractive power source for implantable medical devices [7]. Although this design is simpler than a conventional one, the electrocatalysts need to be highly active and resistant to the presence of other reactants (e.g. oxygen or glucose) under physiological conditions. The desired product of the oxygen reduction at the cathode is water rather than hydrogen peroxide since the later results from low active ORR catalysts, and accelerates the corrosion of the electrode material and polymer membrane [8]. With non-tolerant cathode catalysts, the glucose would depolarize the cathode due to electrooxidation at the electrode, resulting in direct electron transfer to oxygen. This results in an internal current flow and a reduced voltage compared to the thermodynamically expected potential of 1.23 V vs. RHE (Reversible Hydrogen Electrode) for the ORR [9]. Moreover, in order to minimize the additional ohmic drop, as well as mass transport limitation and manufacturing problems deriving from use of thick electrodes, catalysts for glucose fuel cells are usually based on non-supported active noble metals. It is well known that the occurrence of catalyst agglomeration limits their effectiveness and utilization in fuel cell systems [10]. In polymer electrolyte membrane fuel cells (PEMFCs), carbon black (Vulcan

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XC-72) is normally used as support material for Pt nanocatalyst to enhance their utilization. However, the degradation of carbon minimizes the long-term stability of the fuel cells [11]. Recently, carbon nanotubes (CNTs) have been explored as promising support material for fuel cell catalysts due to their high stability and unique mechanical properties [11,12]. The high amount of mesopores (2–50 nm in diameter) and the large specific surface area of CNTs can significantly increase the dispersion of metal nanoparticles and thus enhance their utilization and electrocatalytic activity. The mesoporous network of CNTs additionally facilitates the immobilization process and combined with their excellent electrical conductivity, enables an improved charge transport as well as easy accessibility of the reagent molecules to the catalytic sites. The consequence is that CNT supported nanocatalysts show a better catalytic performance than carbon black supported catalysts [12]. It is also expected that CNT supported electrocatalysts possess a better long-term stability compared to carbon black supported ones. Novel buckypaper (BP) fabricated from commercially available multi-walled carbon nanotubes (MWCNTs) is used in this work as electrode support for glucose fuel cell applications. BP is a selfsupporting mat of entangled assemblies (ropes and bundles) of MWCNTs which form membrane-type black films. It was originally developed to more easily handle carbon nanotubes [11]. BP is a highly porous, mechanically stable and electrically conductive material with various advantages over other carbon nanotube films [13–15]. Some physical properties of BP have been investigated for potential applications in supercapacitors [16], electromechanical actuators [17], artificial muscles [18], field emitters [19], transparent and conductive substrates [20], porous membranes [13,15,21], and thin protective layers for electromagnetic shielding [22]. Moreover, its use in fuel cell [23,24], biofuel cell electrodes [25,26] and sensor/actuator devices [27], has just started. Furthermore, it is reported that surface defects and functionalization under strongly oxidizing conditions can be used effectively as anchors for metal species [28]. Here we investigate the preparation and the electrochemical activity of functionalized multi-walled carbon nanotube (f-CNT) supported Pt and bimetallic Au–Pt nanoparticles (NPs) respectively as potential electrode materials for the cathodic oxygen reduction reaction (ORR) in a glucose fuel cell. A direct comparison is made with conventional carbon black (Vulcan XC-72R) as substrate since it is widely used as a catalyst support material. Additionally, CNT-BP has been utilized as electrode material. Both, the f-CNT and Vulcan XC-72R supports are used to immobilize the system consisting of bilirubin oxidase and electron-transfer mediator in order to test the electrocatalytic activity of the BP-based electrodes towards the ORR for biofuel cell applications in the presence of glucose at neutral medium (pH 7.4). 2. Materials and methods 2.1. Functionalization of carbon nanotubes Commercial MWCNTs (Baytubes C 150-HP, Bayer Material Science AG, Germany) were functionalised by harsh acid treatment (concentrated nitric acid, 65%) under reflux at 140 ◦ C for 3 h as previously reported [26]. The f-CNTs were collected by centrifugation, re-dispersed in de-ionized water, washed thoroughly over a nylon membrane filter with pore sizes of 0.45 ␮m (Whatman, UK) during constant vacuum filtration, until the filtrate was neutral. Afterwards the f-CNT-cake was dried in a vacuum oven at 50 ◦ C overnight.

Fig. 1. Scheme of the buckypaper fabrication process based on the vacuum filtration technique.

MWCNTs were dispersed in 200 mL of an aqueous solution containing 1 wt.% Triton X-100 (Sigma–Aldrich) under mechanical stirring for 30 min followed by ultrasound treatment for 3 h. The resulting suspension was centrifuged for 15 min using a Sigma 2–5 centrifuge at 2700 rpm to remove larger agglomerates of MWCNTs. The supernatant, containing a stable CNT-suspension, was then filtered through a nylon membrane filter (0.45 ␮m pore size) and compressed under vacuum by an oil-free diaphragm pump (KNF Neuberger, Germany). The obtained homogeneous black film was washed repeatedly with an excess of deionised water, followed by isopropyl alcohol and acetone. The prepared CNT-films were kept at room temperature for 30 min and then dried in a vacuum oven at 50 ◦ C overnight. 2.3. Preparation of electrocatalysts and BP-based biocathodes The f-CNT supported Pt NPs and Au70 Pt30 NPs (nominal Au:Pt molar ratio of 70:30) were prepared via a water in oil (W/O) microemulsion route with a metal mass loading of 40 wt.%, following the procedure reported by Habrioux et al. [29]. The BP-supported BOD catalysts were prepared according the scheme illustrated in Fig. 2. BP pieces (1.0 cm × 2.0 cm) were cut out of a 15 ␮m thick BP film. Bilirubin oxidase (BOD, Amano3 [EC 1.3.3.5], activity 2.44 U mg−1 ) from Myrothecium verrucaria was obtained from Amano Pharmaceutical Co. (Japan) and as electron-transfer mediator, 2,2 -azinobis(3-ethylbenzothiazoline6-sulfonate) diammonium salt (ABTS2− ) from Sigma–Aldrich was used. Typically, a BP biocathode based on f-CNTs was prepared using the biocatalyst inks consisting of 3 mg f-CNTs, 3 mg of BOD, 12 vol.% Nafion solution (Nafion® , 5 wt.%, Sigma–Aldrich) and 0.5 mM of ABTS2− , in 0.1 M phosphate buffer solution using Millipore-Q water (18.2 M cm). A good dispersion of f-CNTs has been ensured with the assistance of ultrasonic treatment for 15 min. Afterwards, the resulting ink was pipetted directly onto the BP to form BODABTS/f-CNT-BP with a total enzyme loading of 175 ␮g cm−2 . For direct comparison with f-CNTs, carbon black (Vulcan XC-72R, Cabot GmbH, Germany) was also used as BOD support to form BODABTS/C-BP catalysts, with the same enzyme loading. Afterwards the BP-based electrodes were cured and dried under nitrogen at room temperature for 2 h. Finally, the electrodes were tested directly after washing with a phosphate buffer solution (0.1 M, pH 7.4).

2.2. BP fabrication

2.4. Characterization of f-CNT supported catalysts and buckypaper

The BP fabrication was performed as reported in a previous work [26] and is depicted in Fig. 1. In short, 100 mg of as-received

Transmission electron microscopy (TEM) images of f-CNT supported and Vulcan XC-72R supported Pt and Au–Pt nanocatalysts

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Fig. 2. Schematic representation of the preparation of BP-based biocathodes. The BOD-ABTS system is immobilized either on (a) carbon black (Vulcan XC-72R) or (b) f-CNTs.

were recorded on a Zeiss LEO 912 Omega microscope with an accelerating voltage of 120 kV. Nitrogen adsorption/desorption isotherms were conducted at −196 ◦ C on an automatic analyzer (Sorptomatic 1990, Porotec, GmbH). Prior to the experiment, the samples were degassed by heating at 250 ◦ C for 5 h under high vacuum. The texture properties were analyzed using advanced data processing software (ADP version 5.1, Thermo Electron Corporation). The specific surface areas (given in m2 g−1 ) of carbon black and f-CNT powder samples were evaluated from the linear part of the BET adsorption plot of N2 , according to the standard Brunauer–Emmett–Teller (BET) method, at a relative pressure range of 0.05–0.3 p/p0 [30]. The mean diameter of mesopores (2–50 nm), the mesoporous surface area (given in m2 g−1 ) and the volume (given cm3 g−1 ) were deduced from the desorption branch of the isotherm relative pressure in the range of 0.4–0.999 p/p0 using a model developed by Barrett, Joyner and Halenda (BJH) [30]. The X-ray diffraction (XRD) measurements were carried out on a powder diffractometer (Siemens D-5000) with a Cu K␣ radiation ˚ at 40 kV and 30 mA using 2 scanning from 30◦ to 90◦ ( = 1.5406 A) with a step size of 0.015◦ per 1.5 s to ensue obtaining fine crystalline structure of metal nanoparticles. The electrical conductivity measurements of BPs were carried out on a four-point probe instrument (QuadPro resistivity system, Lucas Signatone) and the thicknesses of BPs were measured using a micrometer gauge (resolution 1 ␮m, Mitutoyo). All potentiodynamic measurements were performed in a conventional thermostated three-electrode electrochemical cell at 25 ◦ C, using a potentiostat/galvanostat apparatus (Autolab PGSTAT30). The working electrode was a 3.0 mm diameter (0.07 cm2 ) ˚ glassy carbon disk, successively polished with Al2 O3 powder (5 A) using emery paper prior to the deposition of the catalyst ink. The catalyst ink was prepared by dispersing 10 mg of the catalyst in a 200 ␮L Nafion solution and 1200 ␮L Millipore-Q water (18.2 M cm) in an ultrasonic bath for 2 h. The catalyst ink (1.8 ␮L in the case of 40 wt.% CNT supported Pt NPs or Au–Pt NPs catalysts, and 3.6 ␮L in case of 20 wt.% Pt/C (E-TEK) catalyst), was deposited onto the glassy carbon disk to obtain a metal mass loading of 72.7 ␮g cm−2 and then dried under nitrogen at room temperature for 2 h. A KCl saturated calomel electrode (SCE) and a glassy carbon plate (1 cm2 ) were used as reference and counter electrodes, respectively. The reference electrode was separated from the electrochemical cell by a Luggin capillary. All the potentials are quoted vs. RHE by directly measuring the potential of the

SCE against a commercial RHE (Hydroflex, Gaskatel GmbH, Kassel, Germany). All the linear-sweep voltammetries were conducted at a rotating speed of 2500 rpm using a potential scan rate of 5 mV s−1 in O2 -saturated phosphate buffer solution (0.2 M, pH 7.4) in the absence and the presence of glucose (0.2 M). The obtained currentpotential curves were recorded at a potential scan rate of 50 or 20 mV s−1 , after performing 20 or 5 potential cycles in the absence and the presence of glucose respectively, in a nitrogen-purged electrolyte which cleaned and activated the electrode surface. For the testing of the BP-based biocathodes, the electrodes were mounted in a home-made single-cell test fixture, as described in literature [26]. 2.5. Potentiodynamic CO stripping The electrochemically active surface areas (ESA) of the electrocatalysts were obtained using the electrochemical carbon monoxide (CO)-stripping of an adsorbed monolayer [31]. Initially, N2 gas was purged into a phosphate buffer solution (0.2 M, pH 7.4) for 30 min. Thereafter, 20 potential cycles were performed at 50 mV s−1 between 0.1 and 1.2 V vs. RHE. The solution was then bubbled with CO (99.9% purity) for 5 min. It was then subjected to a bias of 0.1 V vs. RHE for further 3 min. Finally, the dissolved CO was removed by bubbling N2 gas again into the solution for 30 min. The potential was cycled at 5 mV s−1 between 0.1 and 1.2 V vs. RHE for two complete oxidation and reduction cycles. The ESA was determined by calculating the charge corresponding to the CO oxidation peak. By assuming a nearly monolayer of adsorbed CO, a faradaic charge of 420 ␮C cm−2 was used for the calculation [31]. 3. Results and discussion 3.1. Structure and morphology of nanocatalysts and buckypaper Fig. 3 shows transmission electron microscopy (TEM) images of (A) f-CNT supported (40 wt.%) Pt-NPs, (B) Vulcan XC-72R supported 20 wt.% Pt-NPs (E-TEK), and (C) f-CNT supported (40 wt.%) Au–Pt NPs. In general, the nanoparticles were well-dispersed on the f-CNT as for Vulcan XC-72R, with an average particles size of 5.01 nm, 4.60 nm and 5.09 nm respectively and measured by TEM image analysis using iTEM Desktop software (Olympus Soft Imaging Solutions GmbH, Münster, Germany).

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The N2 -physisorption measurement showed that f-CNTs have BET surface areas (ABET ) of ca. 279 m2 g−1 , median pore sizes of 54.50 nm and average pore sizes of 30.40 nm (calculated using the equation: 4VBJH /ABET ) compared to ca. 232 m2 g−1 , 35.39 nm and 9.53 nm respectively, for Vulcan XC-72R. Moreover, significant increases in the BJH-mesopore area (ABJH ) of 240.4 m2 g−1 and BJHmesopore volume (VBJH ) of 2.2 cm3 g−1 have been found for f-CNTs, in comparison to 75.77 m2 g−1 and 0.552 cm3 g−1 respectively, for Vulcan XC-72R [26]. Additionally, a four-point probe measurement of the electrical conductivity indicated that the BP based on as-received CNTs (Baytubes) was highly conducting with conductivities of ca. 25 S cm−1 , in contrast to carbon black with a reported conductivity value of ca. 5 S cm−1 [32]. 3.2. Determination of the electrochemically active surface area

Fig. 3. Transmission electron microscopy (TEM) images of (a) f-CNTs supported Pt NPs (40 wt.%), (b) carbon black Vulcan XC-72R supported Pt NPs (E-TEK, 20 wt.%) and (c) f-CNTs supported Au–Pt NPs (40 wt.%).

In order to normalize the current densities of our electrocatalysts to their Pt content, the ESAs for the all Pt-based electrocatalysts were investigated. The charge associated with the adsorption of a monolayer of hydrogen atoms to the surfaces of Pt atoms is 210 ␮C cm−2 [33]. Green and Kucernak [34] found that hydrogen adsorption and stripping is not suitable for the assessment of the active surface area of many alloy catalysts, since the charges associated with H and other oxidation processes overlap (i.e. occurrence of a flat shoulder peak). The hydrogen adsorption process is more complex either due to the absorption of H into the metal lattice or the redox behavior of surface-active groups, which mask the platinum hydrogen adsorption–desorption characteristics. Thus, the electro-oxidation of an adsorbed monolayer of CO is used as the preferred method for the in situ measurement of the surface area of electrocatalysts [31]. It is generally assumed that CO is bonded linearly to a single metal surface atom, and therefore a 1:1 ratio between the number of adsorbed CO molecules and metal surface atoms exists. Therefore, the active surface areas for all catalysts were obtained by CO stripping experiments. It is known that the shape of the CO-stripping peak depends on the surface nature of the catalysts. In the case of Pt, the oxidation wave of the adsorbed CO monolayer centers at 0.75 V vs. RHE. ESAs for Au–Pt/f-CNT, Pt/C (E-TEK) and Pt/f-CNT catalysts were determined (Figs. S1, S2 and S3 in Supplementary data). The resulting ESA values based on Pt loadings of the above mentioned catalysts are: 48.60, 71.50 and 10.56 m2 g−1 , respectively. The important difference of these values between Pt/C (E-TEK) and Pt/f-CNT can be due to differences in average particle size and/or due to agglomeration of nanoparticles [35] (see Fig. 3a and b). The oxidation of adsorbed carbon monoxide usually takes place at more negative potentials (E ca. −0.1 V) on Pt/f-CNTs than on Pt/C (E-TEK) (Figs. S2 and S3 in Supplementary data). This phenomenon can be attributed to a substrate effect, which has been observed on Pt deposited onto oxide sites [36]. The reason for this effect is not clear. However, on the Au–Pt bimetallic nanoparticales, the broad CO oxidation wave (from 0.68 to 1.2 V vs. RHE) reveals the complex interplay of the Au and Pt atoms on these alloy nanoparticles (Fig. S1 in Supplementary data). Moreover, the crystalline structure of these nanoalloys was characterized by XRD measurement (Fig. S4 in Supplementary data). From the XRD pattern, one deduces that the peaks at 2-theta (38.7◦ , 44.8◦ , 65.4◦ , 78.4◦ ), can be attributed to Au–Pt nanoalloy structure rather than the mixture of individual nanoparticles [29,37,39]. 3.3. Surface electrochemistry of electrocatalysts Fig. 4a shows cyclic voltammetry curves of commercially available 20 wt.% Pt/C (E-TEK), and of home-made 40 wt.% Pt/f-CNT and 40 wt.% Au70 Pt30 /f-CNT catalysts after electrochemical stabi-

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Fig. 4. Cyclic voltammetry curves (CVs) of (a) 20 wt.% Pt/C (E-TEK), 40 wt.% Pt/f-CNTs and 40 wt.% Au70 Pt30 /f-CNTs catalysts at the 20th potential cycle in a N2 -purged phosphate buffer solution (0.2 M, pH 7.4), in the absence of 0.2 M glucose, (b) the CVs at the 5th potential cycle in the presence of 0.2 M glucose were performed. A sweep rate of 5 mV s−1 was applied at 25 ◦ C.

lization. These curves were usually achieved, after 20 potential cycles at a sweep rate of 20 mV s−1 in N2 -purged phosphate buffer solution (0.2 M, pH 7.4). Characteristic peaks for the hydrogen adsorption/desorption emerge between 0.1 and 0.35 V vs. RHE. As expected, these waves are strongly attenuated in case of the Au–Pt alloy. The double layer region in the present electrolyte medium is not as well defined as in KOH medium, which was observed by Hsueh et al. [38]. For all investigated catalysts, the reduction peak of the Pt-oxide species is centered at ca. 0.7 V vs. RHE. It can be seen that on Au–Pt/f-CNTs this process is also affected by the presence of gold and the peak is shifted to 0.6 V vs. RHE. The curves have been recorded at 25 ◦ C after 5 initial potential cycles at a potential scan rate of 20 mV s−1 . Fig. 4 b shows the faradaic oxidation currents for glucose (0.2 M) oxidation for the various catalysts. For the Pt/C (E-TEK) catalyst the oxidation wave on the positive-going potential is limited between the onset potential value (ca. 0.25 V) and 1.0 V vs. RHE, due to the formation of Pt-oxide species. This oxidation process is recovered again in the negative-going scan. In contrast to this, glucose oxidation at the Au–Pt catalyst occurs at 0.15 V indicating that either at Au sites oxygenated species formed at low potentials, or a reduced overpotential occurs for the glucose oxidation process (as expected). The oxidation potential is sustained up to 1.25 V vs. RHE due to the fact

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Fig. 5. ORR curves at the 10th potential cycle of (a) 20 wt.% Pt/C (E-TEK), 40 wt.% Pt/f-CNTs and 40 wt.% Au70 Pt30 /f-CNTs in O2 -saturated phosphate buffer solution (0.2 M, pH 7.4) in the absence (solid curves) and presence (dotted curves) of 0.2 M glucose, (b) the corresponding Tafel plots of the same ORR curves. A sweep rate of 5 mV s−1 at a rotating speed of 2500 rpm, was applied at 25 ◦ C.

that glucose oxidation is still carried out on the Au-oxide/Au–OH surface species which is in synergy with the Pt-oxide/Pt–OH species [29] and can be attributed to the alloying and “bifunctional” or “ligand” effects [39]. Moreover the multiple anodic peaks can be attributed to the oxidation of glucose and resulting intermediates, as observed during the positive-going scans on the Au–Pt/f-CNTs catalyst for the potential region between 0.15 and 1.25 V vs. RHE. The activity of all catalysts is well visualized when it takes into consideration the current with respect to the ESA of Pt content, see Fig. 4b. Summing up, nanocatalysts (Pt/f-CNT and Pt/C (E-TEK)) are less active towards glucose electro-oxidation as compared to Au–Pt/f-CNTs catalyst.

3.4. Electrochemical oxygen reduction reaction – ORR To test the degree of selectivity (tolerance) towards ORR and/or the glucose oxidation reaction, carbon supported Pt and Au–Pt nanocatalysts were measured using the rotating disk electrode (RDE) technique. Fig. 5a shows the results for Pt/C (E-TEK), Pt/f-CNT, and Au–Pt/f-CNT catalysts in O2 -saturated phosphate buffer solution (0.2 M, pH 7.4) in the absence (solid curves) and the presence (dotted curves) of 0.2 M glucose at 25 ◦ C.

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Table 1 Specific activity of the ORR electrocatalysts (total metal NPs loading 72.7 ␮g cm−2 ) in a phosphate buffer solution (0.2 M, pH 7.4), containing 0.2 M glucose at 25 ◦ C using rotation speed of 2500 rpm. Nanocatalyst

Onset potential (V vs. RHE)

ESA (m2 gPt −1 )

Specific activity at 0.8 V vs. RHE (␮A cmPt −2 )

Pt/f-CNTs Pt/C (E-TEK) Au70 Pt30 /f-CNTs

0.89 0.90 0.54

10.56 71.50 48.60

80.9 14.6 NA

In the absence of glucose, both carbon supported Pt catalysts (either on Vulcan XC-72R or f-CNTs) show similar kinetics, as judged by the shape of the Tafel plots (see Fig. 5b). A similar trend is also observed for Au–Pt/f-CNT in this activation region. The differences between the Au–Pt bimetallic and Pt monometallic based catalysts appear at higher overpotential. Indeed, the ORR leading to water (4-electron charge transfer) is less pronounced on the Au–Pt nanoalloy, since the current in the diffusion region is ca. 88% of the diffusion current attained by Pt catalysts. This decrease can be attributed to the hydrogen peroxide formation. Furthermore, it clearly appears that in the activation region, the substrate and the nature of the catalyst influence the overpotential. The overpotential is decreasing according to following order: Au–Pt/f-CNT > Pt/f-CNT > Pt/C (E-TEK). On the other hand, in the presence of 0.2 M glucose, both Pt monometallic based catalysts display a high tolerance towards glucose, since only a depolarization of ca. −74 mV or −94 mV for Pt/f-CNT and Pt/C (E-TEK) can be observed, respectively (Fig. 5b). However, a limiting current density on Pt catalysts is more sustained on f-CNTs than on Vulcan XC-72R. Furthermore, as expected, on Au–Pt/f-CNT the depolarization is evident (ca. −400 mV). The lower limiting current density on Pt/C (E-TEK) can be attributed to a lower accessibility of oxygen in the presence of glucose. Furthermore, the results summarized in Table 1 clearly demonstrate that the Pt/f-CNTs electrocatalyst has a high electrocatalytic activity (80.9 ␮A cmPt −2 at 0.8 V vs. RHE) for the ORR, exhibits glucose-tolerance, and is stable during repetitive potential cycles as compared to the Pt/C (E-TEK) electrocatalyst in a phosphate buffer solution (0.2 M, pH 7.4) containing 0.2 M glucose. This can be ascribed to the good dispersion and high utilization of Pt-NPs on f-CNTs due to effective mass transport assisted by the presence of large numbers of mesoporous in f-CNTs, which enhance the catalytic activity and stability for ORR in comparisons to Vulcan XC-72R which contains mainly micropores [26]. 3.5. ORR activity of f-CNT supported BOD To investigate the substrate effect of f-CNT, two BOD-based biocathodes (geometric area 2.0 cm2 ) were prepared using f-CNT and Vulcan XC-72R as the supporting substrate, and named BOD/f-CNTBP and BOD/C-BP, respectively. Fig. 6 shows the electrocatalytic activity of BOD/C-BP and BOD/f-CNT-BP towards ORR under steady-state conditions in O2 -saturated phosphate buffer solution (0.2 M, pH 7.4), containing 10 mM glucose. Here, we observe a plateau-like current density at 0.6 V vs. RHE: −0.272 mA cm−2 for BOD-ABTS/C-BP and −0.70 mA cm−2 for BOD-ABTS/f-CNT-BP. The current density of the later one is about 2.5 times higher. It is worth to mention here that the reported limiting current density of −0.3 mA cm−2 (at 0.95 V vs. RHE), using RDE at 100 rpm, in air-saturated solution, was obtained by Habrioux et al. [40]. This suggests that the f-CNT substrate enhances the reaction rate on BOD/f-CNT-BP. To the best of our knowledge, this is the first time that such an effective oxygen electro-reduction promotion of f-CNT supported BOD towards ORR is reported in the presence of glucose in direct comparison to a BOD/C-BP cathode. This enhanced per-

Fig. 6. ORR curves at the second potential cycle of (a) BOD-ABTS/C-BP and (b) BODABTS/f-CNT-BP, in O2 -saturated phosphate buffer solution (0.2 M, pH 7.4) in the presence of 10 mM glucose without any rotating of the electrode. A sweep rate of 3 mV s−1 was applied.

formance may result from a better electrical conductivity of the support, lower impurities, a larger number of mesopores and a higher total BJH-mesopore surface area of ca. 240 m2 g−1 for f-CNT which is three-fold larger than for Vulcan XC-72R (ca. 76 m2 g−1 ). Even if the later possesses a similar value range for the overall specific surface area as f-CNT, not all of the pores seem to be effective for mass transport and accessible for enzyme molecules because the majority of its pores are micropores (less than 2 nm in diameter) [26]. 4. Conclusions We have investigated the substrate effect of functionalized multi-walled carbon nanotube (f-CNT) as supporting substrate for Pt and Au–Pt NPs and bilirubin oxidase (BOD) catalyst on the electrocatalytic activity for the ORR in a phosphate buffer solution (0.2 M, pH 7.4), in the absence and presence of glucose at 25 ◦ C. In comparison with carbon black (Vulcan XC-72R), the f-CNT substrate shows an effective promotion for BOD towards ORR and it favors the selective enhancement for the ORR in the presence of glucose. The new Pt/f-CNTs catalyst is more active than the commercially available Pt/C (E-TEK) catalyst. This can be rationalized based on different factors: The lower charge-transfer resistance at the carbon/electrolyte interface, the larger pore sizes, higher dispersion of Pt NPs and the better affinity of the carbon support to water might be responsible for the higher catalytic activity. Further investigation of the device performance with respect to biocompatibility of BP-based electrodes for various fuel cells and sensor applications are currently in progress. Acknowledgements We are very grateful to the German Science Foundation (DFG) within the graduate school Micro Energy Harvesting (GRK 1322) for the financial support. Thanks are due to Prof. B. Kokoh, Dr. Gregory B. Stevens, Dr. Andreas Schreiber, Dr. Ralf Thomann, Dr. Ralf Sorgenfrei, and Dr. Nagham Mehaibes for their support and fruitful discussions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2011.06.067.

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