Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
IMPROVED CO2 REMOVAL CAPABILITY BY UTILIZING CNTS AS SECONDARY CATALYST SUPPORT FOR DMFC S. L. Chen 1 ,C. T. Lin 1 , M. C. Tsai 1 ,Y. S. Wu 1 , C. H. Wang 1 , T. K. Yeh 1 , C. H. Tsai 1 , F. G. Tseng 1, 2 1 Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan 2 Division of Mechanics, Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan Abstract: In this paper, the improvement of CO2 gas bubble removal efficiency by utilizing CNTs directly grown on carbon cloth (CC) as a secondary Pt support, designated as CNTs/CC, is proposed. The improvement of peak current density by high CO2 removal efficiency in half cell was also verified. An in­situ visualization system combined with electrochemical half cell test was employed to simultaneously examine the relationship between the methanol electroxidation current and CO2 gas bubble removal characteristics. The high speed bubble removal behavior and less bubble accumulation phenomena were observed on Pt coated CNTs/CC (Pt/CNTs/CC) electrode. Key words: DMFC, CNTs, CO2 gas bubble 1. INTRODUCTION Direct methanol fuel cell (DMFC) is a promising energy source for portable electrical applications due to its high efficiency, easy storage and transportation, simple system, as well as low cost. The performance of DMFC has been improved in recent years, but the poisoning of catalysts [1], fuel crossover [2], scale down and combination with MEMS [3], etc. are still issues of interest today. Typically the operation temperature of DMFC is under 100°C and methanol is fed in liquid phase. The anodic electro­catalytic reaction is described in Eq. (1): CH3OH + H2O à CO2 + 6H + + 6e ­ (1) In Eq. (1) methanol is in liquid phase and CO2 is in gas phase. CO2 gas is the byproduct of methanol oxidation and its accumulation in reaction regions has been a serious problem in direct methanol fuel cell (DMFC) when operated in high power density. If CO2 cannot be removed efficiently, not only the anode catalyst but the flow channel of liquid methanol will be blocked, leading to a mass transfer problem. To facilitate the CO2 exhaust, most of the previous works focused either on the flow pattern arrangement in flow channels or the backing layer design for rapid gas exhaustion. Scott et al. [4] studied gas removal capability on parallel channel and dot channel, and found that fuel can enter electrodes and carbon dioxide can be removed easier on dot channel. On the other hand, telflon coating on carbon cloth can also help on the separation of liquid flow from gas flow. Lu and Wang [5] also found out that smaller and more even bubbles can be formed on hydrophilic carbon cloth. Scott et al. also compared bubble removal ability on different carrier plates, and depicted 707s plate had the largest power density, because 707s had a large width of strand and the second smallest open area [6]. Bewer et al. [7] employed oxygen bubbles produced by hydrogen peroxide on platinum catalysis to imitate carbon dioxide bubbles on different channel designs, and found out the gas evolution rate can be set to same order of magnitude as in real direct methanol fuel cells. Yang et al. [8,9] further utilized CCD camera to observe the phenomenon of carbon dioxide removal on the anode of a fuel cell, and discussed the influence of current densities, flow velocities, operation temperatures to the efficiencies of fuel cells. In addition, In­situ visualization was also employed by Yang et al. and Liao et al. [10, 11] to find out the relationship between CO2 removal efficiency and cell performance [10, 11]. However, few papers discussed CO2 gas behavior close to the catalyst surface. In our group, R. X. Zhuang et al. reported different oxygen bubble removal ability for Pt film on substrates with various supports by H2O2 self catalyzing reaction [12]. S. K. Wang et al. has found the indirect electrochemical evidence of improved CO2 removal capability by utilizing CNTs grown on Si base as catalyst support in methanol half cell test [13]. Yet direct evidence is still not available. In this paper, the direct evidence of improved CO2 removal efficiency by utilizing CNTs/CC as Pt support is studied through direct bubble visualization and electrochemical signal measurement simultaneously for methanol oxidation, hopefully beneficial for the structure design of the anode surface. 2. EXPERIMENTAL SETUP 2.1 Electrodes fabrication
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Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
The fabrication processes of two catalytic substrates for methanol oxidation testing are shown in Fig. 1. Pt / CC Carbon cloth Pt chemical reduction (a) Reaction area definition Fig. 2: The SEM images of (a) Pt/CC (2,000X) and (b) Pt/CNTs/CC (20,000X) with chemically reduced Pt. Teflon Pt / CNTs / CC Carbon cloth 2.2 Electrochemical measurement Fig. 3 shows the schematic diagram of the electrochemical and in­situ optical system setup. The rectangular reaction chamber is 500 mL made by quartz. By utilizing this system, the dynamic CO2 gas bubble detached behavior and electrochemical I­t signal can be recorded coherently. The system was a standard three­electrode cell which was employed for electrochemical measurement. The sample with a definite area of 1cm 2 for CO2 gas bubbles visualization experiments, a platinum coated Titanium net, and a Saturated Calomel reference electrode (SCE) acted as working electrode, counter electrode and the reference electrode, respectively. The cyclic voltammetry (CV) from 1M CH3OH + 0.5M H2SO4 aqueous solutions (potential sweep range: ­0.2V~0.9V) were used to measure electro catalysis properties. All experiments were carried out at 25℃ with N 2 saturated and atmospheric pressure. All chemicals were reagent grade and DI water was used throughout. Ti film deposition Ni film deposition Teflon
CNTs growth (b) Pt chemical reduction Reaction area definition Fig. 1: The fabrication process of two different substrates for CO2 gas bubble removal experiment. Two kinds of substrates for catalyst support were employed for CO2 gas bubbles removal experiment, including carbon cloth (CC), CNTs on carbon cloth (CNTs/CC), as shown in Fig. 1. For CNTs/CC, 30nm Ti, and 10nm Ni were coated in order by E­beam deposition on CC substrates. The CNTs were grown by thermal chemical vapor deposition (CVD) with ethylene gas (C2H4) as carbon source at 800°C. After the growth of CNTs, Pt nanoparticles were deposited by chemical reduction as catalyst layer on CC, and CNTs/CC samples, respectively. The parameter is following：(1)Define the test sample area 2 cm 2 . (2) Immerse the sample in 2M H2SO4 aqueous solutions at 60℃ for 1 hour to hydrophilic the sample. (3) Immerse the sample in H2PtCl6 + Eg precursor solution in 60℃ for 24 hours. (4) Bake in 65℃, 5 minutes, for dry. Fig. 2(a) and (b) show the SEM images of the Pt nanoparticles reduced on CC and CNTs/CC substrates, respectively. The diameter of each support material is about 10μm for the fiber of carbon cloth, and 40­60nm for CNTs on carbon cloth. The Pt catalyst layers were coated on the CC, and CNTs/CC samples. Fig. 3: Schematics of the in­situ bubble visualization system (W: working electrode. R: reference electrode, SCE. C: counter electrode, Pt coated Ti net.) 3. RESULTS AND DISCUSSION 3.1 Electrocatalytic performance Fig. 4 shows the 5 th cyclic voltammogram (CV) results of the catalytical reaction by Pt on different carbon supports in 1M CH3OH+0.5M H2SO4 at room temperature 25°C with N2 saturated. The sweeping 422
Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
range is ­0.2V to 0.9V (SCE) and sweeping rate is 20mV/s. Two oxidation peaks in the cyclic voltammograms were observed and they were corresponded to the oxidation of methanol and the oxidation of carbon monoxide, with potentials ranges of 0.75V~0.85V and 0.65V~0.75V, respectively. Electrocatalytic properties of Pt film by cyclic voltammetry in 1M CH3OH + 0.5M H2SO4 aqueous solutions can be characterized by the peak current density and the peak potential from the oxidation of methanol. Higher peak current density (Ip) was found on Pt/CNTs/CC electrode, because of the combined benefits from larger reaction surface and better CO2 gas bubbles removal capability provided by CNTs. (a) (b) (c) Fig. 5: (a) The plot of peak current density, (b) charge transfer of hydrogen region Qh , and(c) Ip/Qh ratio in Pt/CC and Pt/CNTs/CC. Fig. 6 shows the reaction current with time (I­t) under 0.7 VSCE on the reaction electrode in methanol solution, and demonstrates a similar trend. Higher current and fluctuation frequency indicated that Pt/CNTs/CC substrate provided a more active surface. However, the more active surface also carried out a higher toxicity rate of Pt catalyst, leading to a more rapid reduction of current. Fig. 4: Cyclic voltammograms for Pt/CC and Pt/CNTs/CC at 20mVs ­1 in N2 saturated 1 M CH3OH + 0.5 M H2SO4 aqueous solutions. The CV curves were taken from the 5 th cycle at 25°C. (a) 3.2 Comparison of bubble detached ability Electrochemcial I­t curves were untilized to compare CO2 bubble detaching capability. Under diffusion control according to Cottrell equation (Eq. 2), the current I(t) is proportional to the electrode surface area A [14]. I(t) = Id(t) = (nFAD0 1/2 C0 * ) / (π 1/2 t 1/2 ) (2) Where n is Charge number, Id is diffusion limited current, F is Faraday constant, A is electrode surface area, D0 is diffusion coefficient, C0(x,0)=C0* Fig. 5 shows the Ip/Qh (Qh is the charge transfer quantity in hydrogen region of CV pattern in sulfuric acid) value in which the surface area effect can be normalized. Pt/CNTs/CC shows 34% Ip/Qh increment, indicating a more efficient reaction surface which might be attributed to higher CO2 removal capability. 423
(b)
Fig.6: (a)Current to time (I­t) curve, (b)(I­ Imean)/Imean from 80s to 100s for Pt/CC and Pt/CNTs/CC at 0.7 V (SCE) in N2 saturated 1 M CH3OH + 0.5 M H2SO4 aqueous solutions. To verify the above hypothesis, direct visualization of bubble removal rates on different electrode surfaces was carried out. The in situ bubble removal characteristics are shown in Fig. 7 and the corresponding bubble size distribution is shown in Fig. 8. Higher CO2 bubble removal speed and less bubble accumulation behavior observed in the CCD for Pt/CNTs/CC can be evidenced to accompany with higher current and current vibration frequency obtained by the I­t signals (Fig. 6). This is because of the nanostructure of CNTs promotes CO2 bubble departure in smaller size while micro size cavities in CC tend to hold CO2 bubble longer from departure. The higher CO2 departure rate in Pt/CNTs/CC helps create more fresh catalytic sites thus promote higher electrochemical reaction/current and leads to a higher frequency current­fluctuation. Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
(a) developments of micro­direct methanol fuel cells (DMFC) Journal of Power Sources 163 743–754 [4] Scott K, Taama W M, Argyropoulos P 1998 Material aspects of the liquid feed direct methanol fuel cell Journal of Applied Electrochemistry 28 1389­1397 [5] Lu G Q, Wang C Y 2004 Electrochemical and flow characterization of a direct methanol fuel cell Journal of Power Sources 134 33–40 [6] Scott K, Argyropoulos P, Yiannopoulos P, Taama W M 2001 Electrochemical and gas evolution characteristics of direct methanol fuel cells with stainless steel mesh flow beds Journal of Applied Electrochemistry 31 823–832 [7] Bewer T, Beckmann T, Dohle H, Mergel J, Stolten D 2004 Novel method for investigation of two­phase flow in liquid feed direct methanol fuel cells using an aqueous H2O2 solution Journal of Power Sources 125 1–9 [8] Yang H, Zhao T S, Ye Q 2004 Addition of non­ reacting gases to the anode flow fieldof DMFCs leading to improved performance Electrochemistry Communications 6 1098–1103 [9] Yang H, Zhao T S, Ye Q 2005 In situ visualization study of CO2 gas bubble behavior in DMFC anode flow fields Journal of Power Sources 139 79–90 [10] Yang H, Zhao T S 2005 Effect of anode flow field design on the performance of liquid feed direct methanol fuel cells Electrochimica Acta 50 3243–3252 [11] Liao Q, Zhu X, Zheng X Y, Ding Y D 2007 Visualization study on the dynamics of CO2 bubbles in anode channels and performance of a DMFC Journal of Power Sources 171 644–651 [12] Zhuang R X, Tseng F G, Wu Y S, Wang S K 2005 The high speed departure of micro bubbles on carbon nanotubes The 22nd National Conference on Mechanical Engineering (Tauyoun, Taiwan, 25–26 November 2005) E7– 005. [13] Wang S K, Tseng F G, Yeh T K, Chieng C C Electrocatalytic properties improvement on carbon­nanotubes coated reaction surface for micro­DMFC Journal of Power Sources 167 413–419 [14] Wang J 2001 Analytical Electrochemistry (New York, WILEY Publishing) (b)
Fig. 7: The in­situ image captured at t=100s for (a) Pt/CC and (b) Pt/CNTs/CC with 0.7 V (SCE) applied in N2 saturated 1 M CH3OH + 0.5 M H2SO4 aqueous solutions. Fig. 8: The bubble size distribution on Pt/CNTs/CC and Pt/CC by applying 0.7V (SCE) in 1 M CH3OH + 0.5 M H2SO4 aqueous solutions. 4. CONCLUSION In the present study, it is concluded that electrodes with CNTs as catalyst support can directly limit the CO2 gas bubbles growth in electrochemical catalytic reaction and leads to the improvement of CO2 gas bubbles detached capability. This fast CO2 gas bubbles removal behavior is directly related to the high frequency current vibration. Also, the high bubble detached rate and smaller detached size observed in optical system is consistent with the high frequency of current vibration in electrochemical system. The results gives a evidence that electrode with nanostructure contributes to mass transfer for electro­ catalytic reaction composed of liquid reactants and gas products. REFERENCES [1] Hamnett A 1997 Mechanism and electrocatalysis in the direct methanol fuel cell Catalysis Today 38 445–457 [2] Heinzel A, Barragan V M 1999 A review of the state­of­the­art of the methanol crossover in direct methanol fuel cells Journal of Power Sources 84 70–74 [3] Kamarudin S K, Daud W R W, Ho S L, Hasran U A 2007 Overview on the challenges and ACKNOWLEDGEMENT We appreciate the support from National Science Council, Taiwan and the helps from CNT laboratory at NTHU for assistance in the thermal CVD. 424