Anode electrode with carbon buffer layer for improving methanol oxidation reaction in direct methanol fuel cell

Anode electrode with carbon buffer layer for improving methanol oxidation reaction in direct methanol fuel cell

Applied Surface Science 290 (2014) 246–251 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 290 (2014) 246–251

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Anode electrode with carbon buffer layer for improving methanol oxidation reaction in direct methanol fuel cell Yun Sik Kang a,b , Namgee Jung c , Kwang-Hyun Choi a,b , Myeong Jae Lee a,b , Minjeh Ahn a,b , Yong-Hun Cho d , Yung-Eun Sung a,b,∗ a

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, South Korea School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-742, South Korea c Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea d School of Advanced Materials Engineering, College of Engineering, Kookmin University, Seoul 136-702, South Korea b

a r t i c l e

i n f o

Article history: Received 24 July 2013 Received in revised form 14 November 2013 Accepted 14 November 2013 Available online 22 November 2013 Keywords: Direct methanol fuel cell (DMFC) Anode structure Carbon buffer layer Methanol crossover

a b s t r a c t An anode electrode with the carbon buffer layer is fabricated to increase the performance of direct methanol fuel cell (DMFC). The carbon buffer layer is located in the middle of the anode catalyst layers, consists of porous carbon and Nafion ionomer. Since the porous and relatively hydrophilic carbon buffer layer absorbs methanol, the flux of the methanol solution in the anode electrode can be controlled. And methanol crossover is decreased by the effect of the carbon buffer layer. Consequently, methanol can be oxidized more efficiently and the performance of DMFC increases. Therefore, the membrane electrode assembly (MEA) with the carbon buffer layer on the anode electrode exhibits higher open circuit voltage (OCV) and maximum power density compared to those of conventional MEA. Especially with 3.0 M methanol solution, the maximum power density is increased by ∼60%. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Direct methanol fuel cell (DMFC) is one of the most promising alternative energy devices because methanol as a fuel has high energy density compared to H2 gas and is used directly without reforming process. Due to this, DMFC can be used as a power source for mobile electrical devices. Therefore, DMFC has been attracted for portable devices. In order to commercialize DMFC, high concentration of methanol must be used for small volume of portable devices [1–4]. At such a high concentrated methanol solution, however, because of non-reacted methanol solution, methanol crossover occurs in the membrane electrode assembly (MEA) from the anode to the cathode. At the cathode catalyst layer, methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) occur simultaneously due to methanol crossover. This results in mixed potential in the cathode and decreases DMFC performance [5–8]. Therefore, to mitigate methanol crossover and increase DMFC performance, the MEA structures for decreasing methanol crossover have been developed [9–16]. New anode structures have been studied to reduce methanol crossover and to increase DMFC performance [17–20]. Liu et al. fabricated dual-layer anode

∗ Corresponding author. Tel.: +82 2 880 1889; fax: +82 2 888 1604. E-mail address: [email protected] (Y.-E. Sung). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.059

structure using pore-forming agent and the carbon nanotubes [17]. This morphology controls decreased the rate of methanol crossover and increased DMFC performance. Wan et al. fabricated novel MEA structures with methanol filter using various ratio of Pt-Ru catalyst [19]. This methanol filter suppressed the effect of methanol crossover and increased DMFC performance. These studies mostly focused on new anode structures using pore-forming agent and novel metal. However, the addition of these materials and the high loading of novel metal increase the manufacture cost of DMFC and complexify the MEA fabrication process. In this work, an anode electrode structure was modified by insertion of the carbon buffer layer. The price of carbon is cheaper than that of novel metal and pore-forming agent and there is no need for new fabrication process. 2. Experimental Catalyst slurry was prepared by mixing water, ionomer (5 wt% Nafion solution, Aldrich) and isopropyl alcohol (IPA) (Aldrich) with the catalyst. Samples of 75 wt% PtRu/C (Johnson Matthey, the ratio of Pt to Ru is 2:1.) and 60 wt% Pt/C (Johnson Matthey) were used for the anode and cathode catalyst slurries, respectively. The slurries were blended by ultrasonic treatment. In addition, the slurry for the carbon buffer layer was prepared by mixing porous carbon (Vulcan XC-72, Cabot), IPA and 5 wt% Nafion ionomer solution. The

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catalyst-coated membranes (CCMs) as MEAs were prepared by direct spray coating method. The amount of Nafion ionomer in the catalyst layer was 30 wt% compared to that of the catalyst. Catalyst layer was fabricated by spraying slurries on Nafion 115 (DuPont) directly. Especially the anode electrode had a layered structure; inner catalyst layer, carbon buffer layer and outer catalyst layer. The carbon buffer layer, its loading is 0.5 mg cm−2 , was sandwiched between the anode catalyst layers, with a thickness ratio of 1:1. Inner and outer anode catalyst layer had the same total metal (PtRu) loading of 1.0 mg cm−2 . In addition, as a reference, the MEA without carbon buffer layer (conventional MEA) was fabricated. The total anode PtRu loading of two MEAs were identical, 2.0 mg cm−2 . To clearly express effect of methanol crossover, two MEAs with cathode Pt loading of 0.5 mg cm−2 were fabricated, MEA with the carbon buffer layer on the anode and conventional MEA. After spray coating of catalyst layer, the CCMs were sandwiched between the anode gas diffusion layer (GDL) (Toray TGPH-060) and cathode GDL (SGL 35 BC) without hot pressing process. The active area of the MEAs was 5.0 cm2 and the serpentine type flow channels were used at the anode and cathode, its total volume is 0.264 ml. Field emission scanning electron microscopy (FE-SEM) analysis was carried out using a JSM 6701F (JEOL Ltd.) to analyze the morphology of the anode electrode structure. The water drop contact angles were measured with a DSA 100 (KRÜSS) to examine the hydrophilicity of the anode catalyst layer and carbon buffer layer. The porosity of the anode catalyst layer and carbon buffer layer was measured with an AUTOPORE IV 9500 (V.106) (Micromeritics) by mercury intrusion porosimetry. For single cell performance test and electrochemical impedance spectroscopy measurement, 1.0 M or 3.0 M methanol solution and non-humidified air gas were supplied into the anode and cathode, respectively. Flow rates of methanol solution were 3 ml min−1 and 0.5 ml min−1 at 1.0 M and 3.0 M methanol solution, respectively.

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Flow rate of air was 200 ml min−1 . Electrochemical impedance spectroscopy (EIS) (IM6, Zahner) of the single cells was conducted at 0.45 V with an amplitude of 10 mV and in the frequency range from 0.1 Hz to 10 kHz. To analyze the quantity of permeated methanol, the limiting current method was used. The cathode was used as working electrode (WE) and the anode was used as reference electrode (RE) and counter electrode (CE). Methanol solution and humidified nitrogen gas were supplied to the anode and cathode, respectively. Meanwhile, a positive potential was applied to WE from 0 V to 0.75 V vs. RE with scan rate of 2 mV s−1 . All electrochemical measurement was committed at temperature 70 ◦ C. The constant current test at 0.15 A cm−2 was carried out to prove the effect of carbon buffer layer. During the experiment, 500 ml of 3.0 M methanol solution and dry oxygen gas was supplied to the anode and cathode, respectively at cell temperature 70 ◦ C. Once the methanol supply had been depleted and the electrical load was still on, the cell voltage decreased with time and the elapsed time was measured until the cell voltage reached zero. 3. Results and discussion Fig. 1 shows the schematic diagram of the MEA with carbon buffer layer on the anode. Cross-section images of the MEAs are shown in Fig. 2. As shown in Fig. 2(a), carbon buffer layer is placed in the middle of anode catalyst layer. As mentioned above, the thickness of the inner and outer catalyst layers was almost the same since the same amount of catalyst was loaded in Fig. 2(a) (9.4 ␮m (±0.2 ␮m) for inner catalyst layer and 9.2 ␮m (±0.2 ␮m) for outer catalyst layer). In addition, the sum of thickness of inner and outer catalyst layer is almost the same as the thickness of anode catalyst layer of conventional MEA (18.6 ␮m (±0.4 ␮m)). From this, it is proved that two MEAs have the same loading of the anode catalyst layer.

Fig. 1. The schematic diagrams of (a) the MEA with the carbon buffer layer and (b) the conventional MEA.

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Fig. 3. The polarization curves of the MEAs at methanol concentrations of (a) 1.0 M and (b) 3.0 M. Fig. 2. Cross-section images of (a) the MEA with the carbon buffer layer and (b) the conventional MEA.

In Fig. 3(a) and (b), the performances of the MEAs with catalyst loading of 0.5 mg cm−2 on the cathodes are shown. Since these MEAs have low catalyst loading on the cathodes, the performances were influenced easily by the concentration of the methanol solution because the cathode with low Pt loading was unable to offset the effect of mixed potential caused by methanol crossover. From this, the effect of methanol crossover could be clearly shown [21]. At 1.0 M and 3.0 M methanol solution, the open circuit voltages (OCV) of the MEA with the carbon buffer layer were 0.769 V and 0.722 V, respectively, which were higher than those of conventional MEA (0.739 V at 1.0 M and 0.653 V at 3.0 M methanol solution). In addition, the maximum power densities of the MEA with the carbon buffer layer were 130 mW cm−2 at 1.0 M and 105 mW cm−2 at 3.0 M methanol solution, which were larger than those of the conventional MEA (101 mW cm−2 at 1.0 M and 63 mW cm−2 at 3.0 M methanol solution). It is believed that higher OCV and performance of DMFC result from decrease of methanol crossover [22]. Based on this, the effect of methanol crossover may have been decreased by the carbon buffer layer. From Fig. 4, it is proved that methanol crossover was suppressed by the carbon buffer layer. In Fig. 4(a) and (b), the limiting current densities from the oxidation of permeated methanol in the cathodes are shown at 1.0 M and 3.0 M methanol solution. The limiting current density of MEA with the carbon buffer layer was smaller than that of conventional MEA. This result coincides with the increase in OCV of the performances of the MEAs in Fig. 3. It may be possible that the carbon buffer layer blocked physically

and slowed the flux of methanol, which resulted in the decrease of methanol crossover [23]. It may be able to be comprehended that the carbon buffer layer on the anode has a very high resistance to methanol transport. Therefore, when the electrical load increases, the concentration loss by interfered methanol transport would be occurred and performance of DMFC would be decreased. However this explanation is not suitable. Another explanation is proper that the carbon buffer layer on the anode acts as a reservoir of methanol, not the resistance to methanol transport. By the porosimetry result (shown in Table 1) and water drop contact angle measurement data (shown in Fig. 5), it is proved that the carbon buffer layer is a little more porous and relatively hydrophilic than anode catalyst layer (As contact angle becomes smaller, the surface becomes more hydrophilic). Because of these properties, the carbon buffer layer can absorb methanol and act as a methanol reservoir in the anode. From this, the flux of methanol solution was controlled and a high concentration of methanol in the outer catalyst layer was achieved. Reversely, in the inner catalyst layer, a lower concentration of methanol was maintained compared to the concentration of methanol in

Table 1 Porosimetry data of carbon buffer layer and anode catalyst layer at 18 ± 1 ◦ C and 40 ± 1 % RH. Porosity (%) Carbon buffer layer Anode catalyst layer

75.9 73.8

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Fig. 5. Measurement of contact angle of (a) the carbon buffer layer and (b) the anode catalyst layer.

Fig. 4. Comparison of current density of MOR by permeated methanol at (a) 1.0 M and (b) 3.0 M.

the outer catalyst layer. Commonly, it is believed that the activity of the catalyst layer changes with distance from the Nafion membrane since ionic conductivity of the catalyst layer decreases with distance from the membrane [24]. That is, the outer catalyst layer had lower activity than that of the inner catalyst layer. However, because of the carbon buffer layer, a higher concentration of methanol in the outer catalyst layer was maintained and it compensated the lower activity. Contrary to this, the catalyst

Fig. 6. The diagrams of expected methanol concentration in the anode during DMFC operation.

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Fig. 7. Measurement of the cell voltage and elapsed time during the constant current experiment; (a) total region and (b) after depletion of methanol supply.

activity of inner catalyst layer was higher than that of the outer catalyst layer. Therefore, most of the methanol in the inner catalyst layer can be easily oxidized. The diagram of expected methanol concentration in the anode during DMFC operation is shown in Fig. 6. At OCV condition, the methanol concentration in the inner catalyst layer is maintained lower than that in the outer catalyst layer because of the effect of carbon buffer as described above. As the electrical load (produced current density) increases, methanol in the catalyst layer is oxidized. However, methanol in the carbon buffer layer wasn’t oxidized and the concentration gradient is established. This non-reacted methanol comes from the carbon buffer layer to the inner and outer catalyst layers. (Arrows in Fig. 6 indicate the flux of methanol solution in the anode) During this time, the difference in methanol concentration between the inner and outer catalyst layer was maintained. Therefore, more methanol in anode electrode could be oxidized with decrease of methanol crossover. That is, because of the carbon buffer layer, the concentration gradient of methanol on anode electrode was established and the condition that more methanol can be oxidized efficiently was achieved in the inner and outer catalyst layer. Consequently, the carbon buffer layer increased the retention time of methanol and redistributed the methanol solution in the anode. Therefore, the MEA with carbon buffer layer showed enhanced performance of the anode electrode without any concentration loss caused by this layer. To prove that the carbon buffer layer can absorb methanol, the constant current test was conducted. In Fig. 7, the cell voltage and elapsed time were recorded. As expected, MEA with the

Fig. 8. The EIS of the MEAs at methanol concentrations of (a) 1.0 M and (b) 3.0 M.

carbon buffer layer exhibited higher cell voltage and a longer retention time than those of conventional MEA after depletion of the methanol supply. Methanol in the catalyst layer was entirely oxidized during the test. However, methanol in the carbon buffer layer wasn’t oxidized. Therefore, higher concentration of methanol in the carbon buffer layer was established compared to that in the anode catalyst layer. Due to this concentration difference, non-reacted methanol flowed from the carbon buffer layer to the anode catalyst layer. Because of this, MEA with the carbon buffer layer maintained its cell voltage for a longer time than the conventional MEA. In addition, when the methanol supply was depleted, the cell voltage started to increase at 5979 s (arrow in Fig. 7(a)). The increase of cell voltage after depletion of methanol, as shown in Fig. 7(b) resulted from the decrease of methanol crossover. Since methanol wasn’t supplied from the outside, the flux of methanol solution from the anode to cathode was reduced and the effect of methanol crossover decreased. In this situation, since MEA with the carbon buffer layer had more methanol solution in the anode, more methanol could be oxidized in the MEA. From this, a higher slope is exhibited compared to conventional MEA in Fig. 7(b). To find out the effect of carbon buffer layer more exactly on DMFC performance, electrochemical impedance spectroscopy (EIS) was measured and is shown in Fig. 8(a) and (b). The MEA with the carbon buffer layer exhibited smaller charge transfer resistance than the conventional MEA without any changes in ohmic resistance. This means that the electrochemical reactions of the MEA

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with the carbon buffer layer were faster than those of the conventional MEA. There are two semicircles in the EIS results, but only the second one (at lower frequencies) changes. From previous study about EIS of DMFC [25,26], it means that second semicircle is associated to the methanol oxidation reaction (MOR). The smaller second semicircles of EIS data means that the rate of electrochemical reaction has been increased, especially MOR in the anode and cathode, and the effect of methanol crossover in the cathode decreased by the carbon buffer layer. That is, due to the existence of carbon buffer layer, more methanol could be oxidized efficiently in the anode, and methanol crossover could be controlled. As a result, the effect of mixed potential by methanol crossover in the cathode could be minimized and the rate of cathode reaction slowed by permeated methanol could be improved. 4. Conclusions An anode electrode with the carbon buffer layer was fabricated to increase the performance of DMFC. The carbon buffer layer absorbed methanol, therefore the flux of the methanol solution was slowed. Because of this, a high concentration of methanol in the outer catalyst layer and a low concentration of methanol in the inner catalyst layer were achieved, respectively. Therefore, methanol in the anode was oxidized more efficiently and methanol crossover was decreased especially at a high concentration of methanol. Due to this effect, the mixed potential in the cathode was decreased and OCV and maximum power density of the MEA was increased. Acknowledgments We acknowledge financial support by the Institute for Basic Science (IBS) and by the Joint Research Project funded by the Korea Research Council of Fundamental Science & Technology (KRCF) as part of the “Development and mechanism study of high performance and durable components for high-temperature PEMFCs”. YHC acknowledges a financial support by Priority Research Centers Program through NRF funded by the MEST (2012-0006680). References [1] C.M. Lai, J.C. Lin, K.L. Hsueh, C.P. Hwang, K.C. Tsay, L.D. Tsai, Y.M. Peng, On the electrochemical impedance spectroscopy of direct methanol fuel cell, Int. J. Hydrogen Energy 32 (2007) 4381–4388. [2] K.-H. Choi, K.-S. Lee, T.-Y. Jeon, H.Y. Park, N. Jung, Y. Chung, Y.-E. Sung, High alloying degree of carbon supported Pt–Ru alloy nanoparticles applying anhydrous ethanol as a solvent, J. Electrochem. Sci. Tech. 1 (2010) 19–24. [3] F. Liu, G. Lu, C.-Y. Wang, Low crossover of methanol and water through thin membranes in direct methanol fuel cells, J. Electrochem. Soc. 153 (2006) A543–A553.

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