- Email: [email protected]
Anode structure design for generating high stable power output for direct ethanol fuel cells
Fuel 113 (2013) 69–76
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Anode structure design for generating high stable power output for direct ethanol fuel cells Panuwat Ekdharmasuit a, Apichai Therdthianwong b,⇑, Supaporn Therdthianwong c a
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand Fuel Cells and Hydrogen Research and Engineering Center, Clean Energy System Group, PDTI, King Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand c Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand b
h i g h l i g h t s Three anode structures for direct ethanol fuel cells were designed. The presence of an anode MPL (both hydrophilic and hydrophobic) had a negative effect on DEFC performance. For DEFCs, the kinetic influence was more important than the fuel crossover effect. The MEA without an anode MPL exhibited the highest performance.
a r t i c l e
i n f o
Article history: Received 1 February 2013 Received in revised form 3 May 2013 Accepted 15 May 2013 Available online 4 June 2013 Keywords: Direct ethanol fuel cell (DEFC) Anode diffusion layer Microporous layer (MPL) Wettability Ethanol crossover
a b s t r a c t Three anode structures, one without a microporous layer (MPL), one with a hydrophilic MPL, and one with a hydrophobic MPL, were tested for direct ethanol fuel cells. The quick-scan polarization curve obtained from the membrane electrode assembly (MEA) without MPL exhibited a higher performance than those with MPL. From the anode potential measurement, electrochemical impedance spectroscopy (EIS) analysis and constant-current discharge measurement, it was proven that the existence of MPL at the anode diffusion layer could limit ethanol mass transport in the flow channel to the anode catalyst layer (CL), resulting in a low ethanol concentration in the anode CL, especially at higher current density operation. Moreover, the MEA with hydrophilic MPL showed lower cell performance than the one with hydrophobic MPL. The swelling of the Nafion ionomer at full hydration decreased pore size and porosity in the anode MPL, which then hindered the ethanol transport to the anode catalyst layer. Finally, it can be concluded that the negative effect from ethanol crossover seems to be less significant than the positive influence when keeping ethanol concentration high in the anode catalyst layer. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction During the last decade, direct alcohol fuel cells (DAFCs) have been widely studied because of their two main advantages. First, alcohols are usually in liquid form, which is convenient for storage and distribution. Second, liquid fuels usually contain high specific energy density of two of the most popular alcohols, methanol and ethanol, ethanol has more advantages, such as non-toxicity, availability from renewable sources and higher gravimetric energy density (6.09 and 8.00 kW h kg 1 for methanol and ethanol, respectively) [1]. Most research done on direct ethanol fuel cells (DEFCs) has emphasized on electro-catalyst development for the ethanol ⇑ Corresponding author. Tel.: +66 24709222x404; fax: +66 24709325. E-mail address: [email protected] (A. Therdthianwong). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.05.046
oxidation reaction (EOR). It is known that the best catalyst for ethanol electro-oxidation is PtSn-based catalysts [2] and its activity could be further improved by adding a third metal [3]. The stateof-the-art of low platinum and non-platinum electrocatalysts for both anode and cathode of DEFC were studied [4]. Membrane electrode assembly (MEA) was known to be the most important parts of proton electrolyte membrane based fuel cell e.g. proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), and DEFC. Many studies of MEA fabrication for PEMFC by replacing or processing different parts were reported [5]. However, one main issue of DEFCs’ problems is ethanol crossover through a Nafion membrane, causing mixed potential and catalyst poisoning at the cathode. A conventional Nafion-based membrane can be modified to decrease the ethanol crossover by adding some inorganic compounds [6]. Nevertheless, there is still one challenging technical problem that needs to be answered in
70
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
order to obtain high DEFC performance. This is the anode structure design involving mass transport issues. Basically, the simplest way to manage the mass transport problem in direct methanol fuel cells (DMFCs) using liquid fuel feed was to add an anode microporous layer (MPL) between the catalyst layer and the backing layer. The presence of an anode MPL in DMFC could reduce methanol crossover flux at high concentrations of methanol, and accelerate a two-phase transport due to the greater micro-pore distribution of MPL, resulting in the improvement of kinetic rate, as was found by Wu et al. [7] and Yang et al. [8]. However, some research has pointed out that the existence of an anode MPL increases the mass transfer resistance at the anode catalyst layer, leading to the reduction of the DMFC’s performance [9,10]. Normally, MPL is composed of carbon powder and a binder, such as PTFE or a Nafion ionomer. The polytetrafluoroethylene (PTFE) can be used to make the diffusion layer surface hydrophobic, whereas the Nafion ionomer makes the diffusion layer surface hydrophilic. The hydrophobic anode MPL was found to exhibit better cell performance in a DMFC because the PTFE created a greater region for free gas movement, reducing the liquid water accumulation and leading to a higher and more stable voltage, particularly during constant current discharge measurements [11]. On the other hand, some of the literature [12,13] states that DMFCs with hydrophilic anode MPLs performed better, because the Nafion ionomer provided a lower mass transfer resistance of liquid fuel from the flow channel to the catalyst layer. Although many previous studies on the anode structure of DMFCs have been published, but the benefits and the effects of the wettability of anode MPLs are still unclear. At present, the anode structure designed for DEFCs has been borrowed directly from DMFC technology. However, there are some differences in mass transport behavior between DEFCs and DMFCs. First is the difference of fuel crossover behavior. According to their molecular weights, a lower permeation of ethanol through the Nafion membrane with respect to methanol is expected. Song et al. [14] reported that ethanol had fewer negative effects of the crossover on the direct liquid fuel cell’s performance than did methanol. Second, the amounts of CO2 gas produced by the ethanol oxidation and the methanol oxidation reactions were different. The selectivity of CO2 in DEFCs has been reported to be less than approximately 15% (Faradaic yield) for both PtRu [15–19] and PtSn [19–22], while the CO2 formation in DMFCs using PtRu black as the anode catalyst has been found to be the main product. The higher production rate was in the range of 1.0E + 3 to 1.0E + 4 times than those of the other reaction products [23]. Hence, it is expected that the mass transport behavior in DEFC electrodes differed from that of DMFCs, and then the design of anode MPLs in DEFCs would not be the same. Unfortunately, few research works have studied the anode diffusion layers of DEFCs. Alzate et al. [24] investigated the effects of backing layer types and the presence of an MPL on the performance of a DEFC. They found that the performance of a cell consisting of carbon fiber paper with MPL was lower than that of a cell without MPL, particularly at high current density regions. There was no difference in the performances of the DEFCs with those two diffusion layers in low current density regions. It was concluded that due to the high PTFE content in the MPL as well as the very small pores and the lower porosity, the anode MPL had acted as a barrier to prevent the liquid fuel from flowing to the catalyst layer. However, only the polarization and power density curves were presented without other characterizations. Moreover, the effect of the wettability of the anode MPL on the DEFC’s performance was not explored in their work. Therefore, this work studied the design of an anode structure in a DEFC for suitable mass transport management. The presence of an anode MPL with a carbon fiber cloth, and the effect of anode MPL wettability on DEFC
performance were investigated by using various characterization techniques.
2. Experimental procedure 2.1. Materials A NafionÒ 115 membrane (DuPont) was pre-treated in three main steps to remove organic materials and to activate the membrane in a proton form. The membrane was cleaned in 3% H2O2 solution and then boiled in 0.5 M H2SO4 solution, followed by washing in de-ionized water. Each treatment process was conducted at 70 °C for 1 h. Pt-Sn (3:1 a/o) on Vulcan XC72 carbon (Johnson Matthey) was used as the anode catalyst. The cathode was the commercial gas diffusion electrode (E-LATÒ, E-TEK) containing 0.5 mg Pt/cm2 on Vulcan XC72 carbon. At the anode, three types of diffusion layers were used: (i) carbon cloth without a MPL (0 wt.% PTFE, Electrochem Inc.), (ii) carbon cloth with a hydrophilic MPL (20% Nafion), and (iii) carbon cloth with a hydrophobic MPL (20% PTFE), as detailed in Table 1.
2.2. MEA preparation The anode catalyst ink was composed of the catalyst NafionÒ ionomer solution (5 wt% in alcohol/water), and isopropyl alcohol. This mixture was ultrasonically agitated in an ultrasonic bath for 1 h to form a homogeneous ink. Since the catalyst-coated membrane (CCM) method provides higher power density than a catalyst-coated diffusion layer method [5], the anode catalyst layer in this work was formed by the CCM method, in which the catalyst ink was sprayed, by use of an ultrasonic auto-spray, onto a Nafion 115 membrane attached to a flat Pyrex glass at 50 °C. The 5 cm2 catalyst layer was dried for 1 h in a vacuum oven at 60 °C. The catalyst metal loading was 2 mg/cm2 for the anode, while the NafionÒ ionomer loading was 20 wt% of the dry catalyst layer. The anode MPL ink consisted of Vulcan XC72 carbon powder, binder (Nafion or PTFE) and isopropyl alcohol. This slurry was mixed in an ultrasonic bath for 1 h, and then was coated onto the carbon cloth via ultrasonic spraying to form an anode MPL. The MEAs were fabricated by hot-pressing the anode and the cathode onto each side of the treated NafionÒ membrane at 6.9 106 Pa at 150 °C for 90 s. The active area of the MEA made in this work was 5 cm2. Fig. 1 illustrates the configuration of different MEAs. The single cell was sandwiched between two graphite blocks with a parallel serpentine flow field, and was clamped on both sides by the current collector end-plates. A carbon cloth without an MPL, carbon cloth with a hydrophilic MPL, and a carbon cloth with a hydrophobic MPL were named MEA-1, MEA-2, and MEA-3, respectively.
2.3. Performance testing set-up and analysis 2.3.1. Microstructure and morphology characterization The contact angle measurements of different anode diffusion layer samples were measured by using a contact angle system (Kruss contact angle measuring system G10) at 25 °C. The measurement was made by placing a 10-lL de-ionized water droplet at the tip of a syringe close to the diffusion layer surface. The contact angle was measured at about 5 s after the droplet attached to the surface. Six different values from different random regions were averaged to obtain an accurate value. The BET surface areas and the pore volumes of the different diffusion layers were measured by a Gas Sorption Analyzer at 77 K.
71
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76 Table 1 Physical properties and wettability of the anode diffusion layers used for fabricating the MEAs. MEAs
Composition of anode diffusion layer
Pmax (mW/cm2)
Contact angle (°)
Diffusion layer thickness (lm)
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
MEA-1 MEA-2 MEA-3
Carbon cloth Carbon cloth with hydrophilic MPL Carbon cloth with hydrophobic MPL
30.3 21.0 24.7
130.6 ± 1.3 127.4 ± 2.6 133.2 ± 1.7
226.4 ± 3.0 393.3 ± 8.0 366.4 ± 2.0
84.4 141.4 133.6
0.07 0.36 0.33
3.3 10.3 9.9
2.3.2. Cell performance test Before testing the cell, the MEA was activated with ultra-high purity (99.995%) hydrogen and high purity (99.99%) oxygen reactant gases at the anode and the cathode sides respectively at 100 ml min 1 on each side. The cell activation was conducted at the cell and humidification temperatures of 75 °C and 15 psig at both the anode and the cathode for 4 h. After H2/O2 activation, an aqueous ethanol solution with the concentration of 1 M was fed by a digital micro-pump to the anode at a flow rate of 1 ml/ min. Unhumidified oxygen gas with a flow rate of 100 ml/min was supplied to the cathode compartment at atmospheric pressure. The cell activation by the ethanol solution remained for 60 min. These processes were performed to ensure a steady state cell operation. After that, cell polarization testing was carried out by a computer-controlled Fuel Cell Test station with an electronic load in a potentiodynamic polarization mode. For a measurement of voltage variation with time at constant current discharge, the cell current was controlled and the cell voltage was measured for 30 min. 2.3.3. Electrochemical measurements The anode polarization measurement of the ethanol oxidation reaction (EOR) was tested by using Potentiostat/Galvanostat apparatus (AutoLab). Ethanol solution was fed to the anode side at a flow rate of 1 mL/min, while the humidified hydrogen gas was supplied at a flow rate of 100 mL/min to the cathode side as a dynamic hydrogen electrode (DHE). The cell was conducted at 75 °C and performed in a potentiodynamic polarization mode with a scan rate of 2 mV/s. The anode impedance measured at constant current density was recorded by using Impedance/Gain-Phase Analyzer (AutoLab). The anode impedance spectra were measured by setting
MEA1 Anode
the anode fed with ethanol solution as working electrode (WE) and the cathode fed with hydrogen as a DHE. Impedance spectra were obtained at a frequency range between 5 kHz and 0.05 Hz with 10 points per decade, and the amplitude of the sinusoidal current signal did not exceed 10% of current discharged. 2.3.4. Ethanol crossover measurement by voltammetric method Potentiostat/Galvanostat apparatus (AutoLab) was used to measure the ethanol crossover and the methodology used in this study was the same as presented in a previous work [25]. The anode fed with an ethanol solution at a specific concentration acted as the counter electrode and also a dynamic hydrogen reference electrode (DHE). Pure nitrogen gas was fed to the cathode at 2 atm with a flow rate of 100 ml/min. The reaction occurring at the anode is the hydrogen evolution, while the reaction at the cathode is the oxidation of the ethanol that previously crossed from the anode to the cathode. When a dynamic potential versus DHE was applied, the permeated ethanol from the anode to the cathode was electrochemically converted in the cathode catalyst layer, and consequently, the hydrogen ions were pumped from the cathode to the anode. Then, the influence of the different MPLs on the ethanol crossover was measured by comparing the ethanol oxidation current at the cathode. The dynamic potential versus DHE was performed in the range of 0–1.2 V with a potential scanning rate of 2 mV/s. 3. Results and discussion 3.1. Physical characterization The BET surface area, mean pore diameter and pore volume of three different diffusion layers obtained from the N2 adsorption
MEA2 Cathode
Anode
Gas diffusion electrode (GDE)
MEA3 Cathode
Anode
Cathode
Gas diffusion electrode (GDE)
Gas diffusion electrode (GDE)
Membrane
Membrane
Membrane
Catalyst layer
Catalyst layer
Catalyst layer
Hydrophilic micro porous layer
Hydrophobic micro porous layer
Backing layer (carbon cloth)
Backing layer (carbon cloth) Fig. 1. Configuration of catalyst layer, diffusion layer, and membrane of three types of MEAs.
Backing layer (carbon cloth)
72
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
measurement are listed in Table 1. It should be noted that the BET surface area, pore volume, and mean pore diameter of MEA-1 should not be compared to MEA-2 and MEA-3, because the measured value represented the micropores in the fiber of the woven carbon cloth, and not the MPL. For MEA-2 and MEA-3, the BET surface area, pore volume, and mean pore diameter were about the same. Thereby, the microstructures in both of the MEAs with MPL slightly affected the cell performance as the microstructures did not cause much difference in mass transport resistance. In addition, the surface morphologies of these MPLs were examined by SEM. Their images are shown in Fig. 2, which confirms that there was a slight difference in the microstructures between the MEA-2 and MEA-3, corresponding to the BET results. The contact angle of each diffusion layer was measured by a contact angle meter. Their average values are presented in Table 1. Low contact angles show hydrophilic properties for the surface, while high contact angles give hydrophobic surfaces. The MEA-2, which has a Nafion ionomer in the anode MPL, showed the smallest contact angle while the MEA-3, containing PTFE in an anode MPL, gave the largest contact angle. The contact angle of MEA-1 was in between that of MEA-2 and MEA-3. However, the differences between the contact angles of MEA-2 and MEA-3 were small. It is believed that the use of ultra-sonic auto spraying to coat the MPL on the hot surface of the carbon cloth had uniformly
distributed either a Nafion ionomer or PTFE throughout the microporous layer, and had not allowed for phase separation of these binders to the top surface of the layer. 3.2. Cell performance test The quick-scan polarization curves of three MEAs are shown in Fig. 3. The results showed that in a low current density region (<20 mA/cm2), all three MEAs yielded similar cell performance. While the cell operated at low current density region, the required ethanol for generating the current was small. The impact of mass transport on cell performance was low, therefore all three anode structures displayed similar cell performance in low current density region. Nevertheless at higher current densities (>20 mA/ cm2), MEA-1 yielded the highest current densities at the same voltage. It was expected that the anode MPL in MEA-2 and MEA-3 would increase the mass-transfer resistance of the ethanol solution in the anode flow channel to the anode CL, resulting in an inadequate ethanol concentration in the anode CL. This finding was similar to a previous work reporting that the performance of MEA with anode MPL was inhibited, especially at high current density, by the hydrophobicity of the carbon fiber paper as a backing layer and the presence of the MPL [24]. The presence of the anode MPL prevents the flow of liquid to and from the CL due to the very small pores
Fig. 2. SEM images of anode diffusion media of carbon cloth without MPL (a) and (d), with hydrophilic MPL (b) and (e), and with hydrophobic MPL (c) and (f). Note (a–c) at 30 and (d–f) at 100.
73
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
35
0.9
30
Cell voltage (V)
0.7 25 0.6 0.5
20
0.4
15
0.3
10
0.2
MEA-1 MEA-2 MEA-3
0.1
5 0 100
0 0
20
40
60
Cell current density
Cell power density (mW/cm2)
0.8
[26]. Unlike DMFC, the swelling and expansion of the Nafion ionomer in the DEFC would affect only liquid phase transport, because only a small amount of gas was produced in the DEFC, especially using PtSn as the anode catalyst [27]. The MEA-3, having the hydrophobic MPL, yielded the anode limiting current density between that of MEA-1 and MEA-2, and it had lower cell performance than MEA-1. The mass transfer resistance was increased by adding PTFE in the anode MPL. However, a slight difference in the limiting current density between MEA-2 and MEA-3 was observed because the fact that the anode MPL, either hydrophobic or hydrophilic, would act as a barrier to obstruct the liquid mass transport resulting in the decrease of anode limiting current density.
80
(mA/cm2)
Fig. 3. Quick-scan DEFC polarization and power density curves of MEAs with different anode microporous layers (MPLs).
1 MEA-1 MEA-2 MEA-3 Model
and lower porosity. The difference between the cell performances of MEA-2 and MEA-3 was investigated later by the use of anode polarization and in situ EIS measurement. It was also observed that the open circuit voltage (OCV) of MEA-1 was slightly lower than that of the MEAs with GDL. This means that crossover of ethanol in the MEA-1 (without MPL) was slightly higher than that in the MEA-2 and the MEA-3.
-Z'' (ohm)
0.8 0.6 0.4 0.2 0
3.3. Cell characterization by electrochemical measurement
(a)
-0.2 0
0.2
0.4
0.6
0.8
1
1.2
Z' (ohm) 1
MEA-1 MEA-2 MEA-3 Model
0.8
-Z'' (ohm)
By using the cathode as a dynamic hydrogen electrode (DHE), the anode potentials of these three MEAs were measured, as illustrated in Fig. 4. The current density value at the high voltage region of approximately 0.5 V versus DHE, a so-called anode-limiting current density, was compared. The MEA-1 had the highest anode limiting current density (90 mA/cm2), which was approximately 25 and 30 mA/cm2 higher than those of the MEA-3 and the MEA-2, respectively. It was confirmed that the MEA without MPL (MEA1) had the lowest mass transfer resistance. The MEA-2, having the hydrophilic MPL, showed the lowest anode limiting current density. This behavior was also observed in DMFC as found in the work of Liu et al. [11]. It was believed that the mass transfer in hydrophilic MPL was inhibited by the swelling of the Nafion ionomer at full hydration, leading to a decrease of pore size and porosity in the anode MPL. Nafion absorbs water and other solvents and subsequently its dimension expands. The volume increase of Nafion in ethanol of approximately 181% was reported
0.6 0.4 0.2 0
(b)
-0.2 0
0.2
0.4
0.6
0.8
1
1.2
Z' (ohm) 1 MEA-1 MEA-2 MEA-3 Model
0.8 0.5 0.6 0.4
-Z'' (ohm)
Anode potential (V vs. DHE)
0.6
0.3
0.4 0.2
0.2 MEA-1 MEA-2 MEA-3
0.1
0
0
(c)
-0.2 0
20
40
60
80
100
Cell current density (mA/cm2) Fig. 4. Quick-scan anode polarization curves of MEAs with different anode diffusion layers.
0
0.2
0.4
0.6
0.8
1
1.2
Z' (ohm) Fig. 5. Anode half-cell Nyquist diagrams of MEAs with different anode diffusion layers at (a) 10, (b) 30 and (c) 50 mA/cm2.
74
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
R1
R2
R3
CPE 1 CPE2 Fig. 6. Equivalent circuit of the DEFCs anode (vs. DHE).
Another characterization technique performed for validating the mass transport loss was the EIS, which was carried out by controlling the current densities at 10, 30, and 50 mA cm 2. The corresponding Nyquist diagrams are shown in Fig. 5. To understand the effects of anode MPL, the obtained EIS data of the different MEAs were analyzed by fitting the anode spectra with an equivalent circuit model. The equivalent circuit model used in this work was modified from the conventional equivalent circuit models of DMFC [28], as shown in Fig. 6. The physical meanings of each element used in the model are described as follows: (1) R1 describes the ohmic resistance; (2) R2 and CPE1 are the contact resistance and the capacitive behavior between the membrane and the catalyst layer, respectively; (3) R3 and CPE2 are the charge transfer resistance and the capacitive behavior of the real anode with the roughness of the catalyst layer and non-uniform catalyst distribution, respectively. This equivalent circuit model matches the experimental data quite well (v2 0.03). The fitting parameters are summarized in Table 2, which shows that the simulated R1 value of MEA-1 tends to be slightly smaller than those of MEA-2 and MEA-3. It is believed that the diffusion layer without MPL offered good electron conduction due to the absence of an MPL binder, which was made of non-conductive material. From the results, in each MEA the R1 obtained from curve fitting at different current densities was averaged for the comparison. It was found that the lowest R1 of ca. 0.0373 X is obtained with the MEA-1, while R1 of MEA-2 and MEA-3 are 0.0385 X and 0.0381 X, respectively, slightly higher than that of MEA-1. However, the cell ohmic overpotential calculated by multiplying the R1 with the cell current showed that the % ohmic loss compared with the total loss did not exceed 2% in every MEA. The other losses such as kinetic and mass transport loss were the main losses of the fuel cell. Therefore, good electron conduction in the anode MPL of MEA-1 would not significantly enhance the cell performance. The main differences in EIS measurement are the R3 and the CPE2 caused by the kinetic reaction resistance of the anode electrode. By considering the simulated R3, it was found that the R3 of MEA-1was smaller than those of MEA-2 and MEA-3 at every current density. Obviously, at 50 mA/cm2, R3 of MEA-1 was approximately half of that of MEA-2 and MEA-3. Thus the lowest mass transfer resistance of MEA-1 was confirmed.
Comparing between MEA-2 and MEA-3, both MEAs have very close values of R3 especially at the current density of 50 mA/cm2 indicating the large kinetic resistance at the anode CL. The increase of kinetic resistance was caused by the low concentration of fuel in the anode CL, which has resulted from high mass transfer resistance from the anode diffusion layer to the anode catalyst layer. It is inferred that the existence of the MPL at the anode diffusion layers of both MEA-2 and MEA-3 limited the ethanol transport, but with different causes. The mass transfer resistance of MEA-3 was due to the exclusion of water by the hydrophobic PTFE, whereas that of MEA-2 was attributed from the swelling and expanding of the Nafion ionomer in hydrophilic MPL as discussed earlier. At low current densities (10 and 30 mA/cm2), the R3 of MEA-2 was slightly larger than that of MEA-3 (ca. 0.07–0.1 X) implying that the swelling of Nafion ionomer caused higher mass transfer resistance than the hydrophobic behavior. The evidence of the swelling of the Nafion ionomer could be also noticed from the increase of the CPE2-T, accounting for the roughness of the electrode’s surface. The CPE2-T of MEA-2 (0.4 F) was higher than that of MEA-1 and MEA-3 at around 0.2 F. In addition, some Nafion material in MPL possibly dissolved into the catalyst layer, and consequently, enlarged the ionomer network in the catalyst layer, leading to the increase of CPE2-T. Constant current-discharging experiments of all three MEAs at two current densities of 10 and 30 mA/cm2 were carried out to present the cell performance tendency under steady state condition compare to unsteady state condition and also to indicate how the accumulated species in the electrode affected mass transport. The cell voltage variation with time for 30 min is illustrated in Fig. 7. At low constant-current discharges (10 mA/cm2), all three MEA performances were quite the same. Nevertheless, at higher constant-current discharges of 30 mA/cm2, the cell voltages of both MEA-2 and MEA-3 declined much more than MEA-1 had. The cell voltage decay in the cell voltage variation was attributed to the enhancement of mass transport resistance at the anode catalyst layer, which was caused by the gradual accumulation of CO2 during constant current discharge. It was believed that there had been no liquid water accumulation in the cathode GDL and flow channels for all the MEAs, as there had been no obvious cell voltage fluctuations during constant current discharges as described in a previous work [11]. It should also be mentioned that the order of the cell performance observed in the constant current discharge experiment is quite consistent with the previous quick-scan polarization curve data at the corresponding current density. 3.4. Ethanol crossover measurement Fig. 8 shows the ethanol crossover current density versus the applied voltage of the MEAs using various types of gas diffusion layers. This crossover current density was ascribed to the permeated ethanol at the cathode [25]. Considering the peak of crossover current density in the present work, it was in the same range as
Table 2 Fitted parameters of the equivalent circuit model for the anode of the DEFCs operating at various current densities. Sample
I (mA/cm2)
R1 (X)
R2 (X)
CPE1-T (F)
CPE1-P
R3 (X)
CPE2-T (F)
CPE2-P
MEA-1
10 30 50
0.037 0.037 0.038
0.007 0.014 0.009
1.148 1.005 1.719
0.658 0.677 0.606
0.404 0.411 0.570
0.272 0.196 0.216
0.932 0.998 0.956
MEA-2
10 30 50
0.038 0.039 0.039
0.001 0.003 0.007
0.316 0.158 3.743
0.962 0.957 0.568
0.650 0.639 1.075
0.420 0.373 0.394
0.829 0.844 0.923
MEA-3
10 30 50
0.037 0.037 0.040
0.006 0.008 0.037
0.171 0.441 4.992
0.924 0.784 0.419
0.547 0.562 1.029
0.214 0.183 0.196
0.941 0.966 0.976
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
crossover was significantly reduced. However, the crossover effect seems to have less significant impact on the DEFC performance, because the MEA-1 without an anode MPL, which had the highest ethanol crossover, still displayed the highest performance.
0.8 MEA-1
Cell voltage (V)
0.75
MEA-2
0.7
MEA-3
0.65
4. Conclusions
0.6 0.55 0.5
(a)
0.45 0.4 0
500
1000
1500
2000
Time (s) 0.8 MEA-1
0.75
Cell voltage (V)
75
MEA-2
0.7
MEA-3
0.65 0.6 0.55 0.5
(b)
0.45 0.4 0
500
1000
1500
2000
Time (s) Fig. 7. Cell voltage variation with time at constant-current discharge (a) 10 mA/cm2 and (b) 30 mA/cm2 for different MEAs with different anode diffusion layers.
Ethanol crossover current density (mA /cm2)
that in the work done by Song et al. [25]. At the same condition, the peak of crossover current density in our work was in the range of 60–75 mA/cm2, while it was around 80 mA/cm2 in their work. In Fig. 8, a peak crossover current density of MEA-1 without an anode MPL is at 75 mA/cm2, which is higher than that of the MEA-2 with hydrophilic MPL and the MEA-3 with hydrophobic MPL at 64 mA/cm2 and 58 mA/cm2, respectively. This means that the MEA without anode MPL resulted in higher ethanol crossover from the anode to the cathode. On the other hand, the MEA-2 with hydrophilic anode MPL and the MEA-3 with hydrophobic anode MPL was able to reduce the crossover current peak by 14.67% and 22.67%, respectively, compared to the MEA-1. This is because the anode MPL, either hydrophilic or hydrophobic, was able to act as a barrier diminishing the ethanol transfer from the anode flow channel to the anode catalyst layer. As a result, the concentration of ethanol in the anode catalyst layer was low and the ethanol
80 70 60 50 40 30 20 10 0 -10
MEA-1 MEA-2 MEA-3
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Voltage (V) Fig. 8. Ethanol crossover rate through the MEAs using different anode MPLs. (Anode: ethanol concentration = 1.0 M, flow rate = 1.0 ml/min, Cathode: Pure nitrogen pressure = 2.0 atm).
The effects of the anode MPLs on the performances of DEFCs were investigated. The quick-scan polarization curves of the three MEAs indicated that all three MEAs with an anode without MPL, an anode with a hydrophilic MPL, and an anode with a hydrophobic MPL, displayed similar cell performances at low current density regions, whereas MEA-1 without the anode MPL demonstrated the highest cell performance at high current densities. The quick-scan anode polarization curves and the anode half-cell Nyquist diagrams confirmed that the presence of the anode MPL had limited the supply of ethanol solution from the flow channel to the anode CL, resulting in an inadequate ethanol concentration in the reaction zone. The constant current discharge measurement also revealed that the cell voltage decay of the MEA with the anode MPL was higher than that of the one without the anode MPL, particularly at high constant current discharge. It was expected that the gradual accumulation of CO2 in the anode electrode had caused the cell voltage decay. The cell voltage of the MEA without the anode MPL was more stable in the long run, since CO2 had been more easily removed. In addition, the ethanol crossover measurement revealed that the MEA without the anode MPL had the highest ethanol crossover current of the three different MEAs. However, the kinetic influence seemed to be more important than the ethanol crossover effect for the DEFC, because the MEA-1 without the anode MPL had the best performance even though it had the highest ethanol crossover. Acknowledgements The authors would like to acknowledge the financial support from the Higher Education Research Promotion and Natural Research University (NRU) Project of Thailand, Office of the Higher Education Commission, the Joint Graduate School of Energy and Environment (JGSEE) of King Mongkut’s University of Technology Thonburi (KMUTT), and P.E. would also like to thank the Thailand Research Fund (TRF) for the Royal Golden Jubilee (RGJ) academic scholarship. References [1] Meyer M, Melke J, Gerteisen D. Modelling and simulation of a direct ethanol fuel cell considering multistep electrochemical reactions, transport processes and mixed potentials. Electrochim Acta 2011;56:4299–307. [2] Antolini E. Catalysts for direct ethanol fuel cells. J Power Sources 2007;170:1–12. [3] Tayal J, Rawat B, Basu S. Bi-metallic and tri-metallic PteSn/C, PteIr/C, PteIreSn/C catalysts for electro-oxidation of ethanol in direct ethanol fuel cell. Int J Hydrogen Energ 2011;36:14884–97. [4] Brouzgou A, Song SQ, Tsiakaras P. Low and non-platinum electrocatalysts for PEMFCs: Current status, challenges and prospects. Appl Catal B-Environ 2012;127:371–88. [5] Liu CY, Sung CC. A review of the performance and analysis of proton exchange membrane fuel cell membrane electrode assemblies. J Power Sources 2012;220:348–53. [6] Neburchilov V, Martin J, Wang H, Zhang J. A review of polymer electrolyte membranes for direct methanol fuel cells. J Power Sources 2007;169:231–8. [7] Wu QX, Zhao TS, Chen R, Yang WW. Effects of anode microporous layers made of carbon powder and nanotubes on water transport in direct methanol fuel cells. J Power Sources 2009;191:304–11. [8] Yang SH, Chen CY, Wang WJ. An impedance study for the anode micro porous layer in an operating direct methanol fuel cell. J Power Sources 2010;195:3536–45. [9] Casalegno A, Santoro C, Rinaldia F, Marchesia R. Low methanol crossover and high efficiency direct methanol fuel cell: The influence of diffusion layers. J Power Sources 2011;196:2669–75.
76
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
[10] Kang K, Lee G, Gwak G, Choi Y, Ju H. Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system. Int J Hydrogen Energ 2012;37:6285–91. [11] Liu F, Wang CY. Water and methanol crossover in direct methanol fuel CellEffect of anode diffusion media. Electrochim Acta 2008;53:5517–22. [12] Zhang J, Yin GP, Lai QZ, Wang ZB, Cai KD, Liu P. The influence of anode gas diffusion layer on the performance of low-temperature DMFC. J Power Sources 2007;168:453–8. [13] Lindermeir A, Rosenthal G, Kunz U, Hoffmann U. On the question of MEA preparation for DMFCs. J Power Sources 2004;129:180–7. [14] Song S, Zhou W, Liang Z, Cai R, Sun G, Xin Z, et al. The effect of methanol and ethanol cross-over on the performance of PtRu/C-based anode DAFCs. Appl Catal B-Environ 2005;55:65–72. [15] Li G, Pickup PG. Analysis of performance losses of direct ethanol fuel cells with the aid of a reference electrode. J Power Sources 2006;161:256–63. [16] Ghumman A, Vink C, Yepez O, Pickup PG. Continuous monitoring of CO2 yields from electrochemical oxidation of ethanol: Catalyst, current density and temperature effects. J Power Sources 2008;177:71–6. [17] Ghumman A, Li G, Bennett DV, Pickup PG. Online analysis of carbon dioxide from a direct ethanol fuel cell. J Power Sources 2009;194:286–90. [18] James DD, Bennett DV, Li G, Ghumman A, Helleur RJ, Pickup PG. Online analysis of products from a direct ethanol fuel cell. Electrochem Commun 2009;11:1877–80. [19] Wang Q, Sun GQ, Cao L, Jiang LH, Wang GX, Wang SL, et al. High performance direct ethanol fuel cell with double-layered anode catalyst layer. J Power Sources 2008;177:142–7. [20] Rousseau S, Coutanceau C, Lamy C, Leger JM. Direct ethanol fuel cell (DEFC): Electrical performances and reaction products distribution under operating
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
conditions with different platinum-based anodes. J Power Sources 2006;158:18–24. Zhu M, Sun G, Yan S, Li H, Xin Q. Preparation, structural characterization, and activity for ethanol oxidation of carbon-supported PtSnIn Catalyst. Energy Fuel 2009;23:403–7. Andreadis G, Stergiopoulos V, Song S, Tsiakaras P. Direct ethanol fuel cells: The effect of the cell discharge current on the products distribution. Appl Catal BEnviron 2010;100:157–64. Masdar MS, Tsujiguchi T, Nakagawa N. Mass spectroscopy for the anode gas layer in a semi-passive direct methanol fuel cell using porous carbon plate PartII. Releationship between the reaction products and the methanol and water vapor pressures. J Power Sources 2009;194:618–24. Alzate V, Fatih K, Wang H. Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance. J Power Sources 2011;196:10625–31. Song S, Wang G, Zhou W, Zhao X, Sun G, Xin Q, et al. The effect of the MEA preparation procedure on both ethanol crossover and DEFC performance. J Power Sources 2005;140:103–10. Affoune AM, Yamada A, Umeda M. Conductivity and surface morphology of Nafion membrane in water and alcohol environments. J Power Sources 2005;148:9–17. Jablonski A, Kulesza PJ, Lewera A. Oxygen permeation through Nafion 117 membrane and its impact on efficiency of polymer membrane ethanol fuel cell. J Power Sources 2011;196:4714–8. Yuan T, Zou Z, Chen M, Li Z, Xia B, Yang H. New anodic diffusive layer for passive micro-direct methanol fuel cell. J Power Sources 2009;192:423–8.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Anode structure design for generating high stable power output for direct ethanol fuel cells Panuwat Ekdharmasuit a, Apichai Therdthianwong b,⇑, Supaporn Therdthianwong c a
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand Fuel Cells and Hydrogen Research and Engineering Center, Clean Energy System Group, PDTI, King Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand c Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand b
h i g h l i g h t s Three anode structures for direct ethanol fuel cells were designed. The presence of an anode MPL (both hydrophilic and hydrophobic) had a negative effect on DEFC performance. For DEFCs, the kinetic influence was more important than the fuel crossover effect. The MEA without an anode MPL exhibited the highest performance.
a r t i c l e
i n f o
Article history: Received 1 February 2013 Received in revised form 3 May 2013 Accepted 15 May 2013 Available online 4 June 2013 Keywords: Direct ethanol fuel cell (DEFC) Anode diffusion layer Microporous layer (MPL) Wettability Ethanol crossover
a b s t r a c t Three anode structures, one without a microporous layer (MPL), one with a hydrophilic MPL, and one with a hydrophobic MPL, were tested for direct ethanol fuel cells. The quick-scan polarization curve obtained from the membrane electrode assembly (MEA) without MPL exhibited a higher performance than those with MPL. From the anode potential measurement, electrochemical impedance spectroscopy (EIS) analysis and constant-current discharge measurement, it was proven that the existence of MPL at the anode diffusion layer could limit ethanol mass transport in the flow channel to the anode catalyst layer (CL), resulting in a low ethanol concentration in the anode CL, especially at higher current density operation. Moreover, the MEA with hydrophilic MPL showed lower cell performance than the one with hydrophobic MPL. The swelling of the Nafion ionomer at full hydration decreased pore size and porosity in the anode MPL, which then hindered the ethanol transport to the anode catalyst layer. Finally, it can be concluded that the negative effect from ethanol crossover seems to be less significant than the positive influence when keeping ethanol concentration high in the anode catalyst layer. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction During the last decade, direct alcohol fuel cells (DAFCs) have been widely studied because of their two main advantages. First, alcohols are usually in liquid form, which is convenient for storage and distribution. Second, liquid fuels usually contain high specific energy density of two of the most popular alcohols, methanol and ethanol, ethanol has more advantages, such as non-toxicity, availability from renewable sources and higher gravimetric energy density (6.09 and 8.00 kW h kg 1 for methanol and ethanol, respectively) [1]. Most research done on direct ethanol fuel cells (DEFCs) has emphasized on electro-catalyst development for the ethanol ⇑ Corresponding author. Tel.: +66 24709222x404; fax: +66 24709325. E-mail address: [email protected] (A. Therdthianwong). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.05.046
oxidation reaction (EOR). It is known that the best catalyst for ethanol electro-oxidation is PtSn-based catalysts [2] and its activity could be further improved by adding a third metal [3]. The stateof-the-art of low platinum and non-platinum electrocatalysts for both anode and cathode of DEFC were studied [4]. Membrane electrode assembly (MEA) was known to be the most important parts of proton electrolyte membrane based fuel cell e.g. proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), and DEFC. Many studies of MEA fabrication for PEMFC by replacing or processing different parts were reported [5]. However, one main issue of DEFCs’ problems is ethanol crossover through a Nafion membrane, causing mixed potential and catalyst poisoning at the cathode. A conventional Nafion-based membrane can be modified to decrease the ethanol crossover by adding some inorganic compounds [6]. Nevertheless, there is still one challenging technical problem that needs to be answered in
70
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
order to obtain high DEFC performance. This is the anode structure design involving mass transport issues. Basically, the simplest way to manage the mass transport problem in direct methanol fuel cells (DMFCs) using liquid fuel feed was to add an anode microporous layer (MPL) between the catalyst layer and the backing layer. The presence of an anode MPL in DMFC could reduce methanol crossover flux at high concentrations of methanol, and accelerate a two-phase transport due to the greater micro-pore distribution of MPL, resulting in the improvement of kinetic rate, as was found by Wu et al. [7] and Yang et al. [8]. However, some research has pointed out that the existence of an anode MPL increases the mass transfer resistance at the anode catalyst layer, leading to the reduction of the DMFC’s performance [9,10]. Normally, MPL is composed of carbon powder and a binder, such as PTFE or a Nafion ionomer. The polytetrafluoroethylene (PTFE) can be used to make the diffusion layer surface hydrophobic, whereas the Nafion ionomer makes the diffusion layer surface hydrophilic. The hydrophobic anode MPL was found to exhibit better cell performance in a DMFC because the PTFE created a greater region for free gas movement, reducing the liquid water accumulation and leading to a higher and more stable voltage, particularly during constant current discharge measurements [11]. On the other hand, some of the literature [12,13] states that DMFCs with hydrophilic anode MPLs performed better, because the Nafion ionomer provided a lower mass transfer resistance of liquid fuel from the flow channel to the catalyst layer. Although many previous studies on the anode structure of DMFCs have been published, but the benefits and the effects of the wettability of anode MPLs are still unclear. At present, the anode structure designed for DEFCs has been borrowed directly from DMFC technology. However, there are some differences in mass transport behavior between DEFCs and DMFCs. First is the difference of fuel crossover behavior. According to their molecular weights, a lower permeation of ethanol through the Nafion membrane with respect to methanol is expected. Song et al. [14] reported that ethanol had fewer negative effects of the crossover on the direct liquid fuel cell’s performance than did methanol. Second, the amounts of CO2 gas produced by the ethanol oxidation and the methanol oxidation reactions were different. The selectivity of CO2 in DEFCs has been reported to be less than approximately 15% (Faradaic yield) for both PtRu [15–19] and PtSn [19–22], while the CO2 formation in DMFCs using PtRu black as the anode catalyst has been found to be the main product. The higher production rate was in the range of 1.0E + 3 to 1.0E + 4 times than those of the other reaction products [23]. Hence, it is expected that the mass transport behavior in DEFC electrodes differed from that of DMFCs, and then the design of anode MPLs in DEFCs would not be the same. Unfortunately, few research works have studied the anode diffusion layers of DEFCs. Alzate et al. [24] investigated the effects of backing layer types and the presence of an MPL on the performance of a DEFC. They found that the performance of a cell consisting of carbon fiber paper with MPL was lower than that of a cell without MPL, particularly at high current density regions. There was no difference in the performances of the DEFCs with those two diffusion layers in low current density regions. It was concluded that due to the high PTFE content in the MPL as well as the very small pores and the lower porosity, the anode MPL had acted as a barrier to prevent the liquid fuel from flowing to the catalyst layer. However, only the polarization and power density curves were presented without other characterizations. Moreover, the effect of the wettability of the anode MPL on the DEFC’s performance was not explored in their work. Therefore, this work studied the design of an anode structure in a DEFC for suitable mass transport management. The presence of an anode MPL with a carbon fiber cloth, and the effect of anode MPL wettability on DEFC
performance were investigated by using various characterization techniques.
2. Experimental procedure 2.1. Materials A NafionÒ 115 membrane (DuPont) was pre-treated in three main steps to remove organic materials and to activate the membrane in a proton form. The membrane was cleaned in 3% H2O2 solution and then boiled in 0.5 M H2SO4 solution, followed by washing in de-ionized water. Each treatment process was conducted at 70 °C for 1 h. Pt-Sn (3:1 a/o) on Vulcan XC72 carbon (Johnson Matthey) was used as the anode catalyst. The cathode was the commercial gas diffusion electrode (E-LATÒ, E-TEK) containing 0.5 mg Pt/cm2 on Vulcan XC72 carbon. At the anode, three types of diffusion layers were used: (i) carbon cloth without a MPL (0 wt.% PTFE, Electrochem Inc.), (ii) carbon cloth with a hydrophilic MPL (20% Nafion), and (iii) carbon cloth with a hydrophobic MPL (20% PTFE), as detailed in Table 1.
2.2. MEA preparation The anode catalyst ink was composed of the catalyst NafionÒ ionomer solution (5 wt% in alcohol/water), and isopropyl alcohol. This mixture was ultrasonically agitated in an ultrasonic bath for 1 h to form a homogeneous ink. Since the catalyst-coated membrane (CCM) method provides higher power density than a catalyst-coated diffusion layer method [5], the anode catalyst layer in this work was formed by the CCM method, in which the catalyst ink was sprayed, by use of an ultrasonic auto-spray, onto a Nafion 115 membrane attached to a flat Pyrex glass at 50 °C. The 5 cm2 catalyst layer was dried for 1 h in a vacuum oven at 60 °C. The catalyst metal loading was 2 mg/cm2 for the anode, while the NafionÒ ionomer loading was 20 wt% of the dry catalyst layer. The anode MPL ink consisted of Vulcan XC72 carbon powder, binder (Nafion or PTFE) and isopropyl alcohol. This slurry was mixed in an ultrasonic bath for 1 h, and then was coated onto the carbon cloth via ultrasonic spraying to form an anode MPL. The MEAs were fabricated by hot-pressing the anode and the cathode onto each side of the treated NafionÒ membrane at 6.9 106 Pa at 150 °C for 90 s. The active area of the MEA made in this work was 5 cm2. Fig. 1 illustrates the configuration of different MEAs. The single cell was sandwiched between two graphite blocks with a parallel serpentine flow field, and was clamped on both sides by the current collector end-plates. A carbon cloth without an MPL, carbon cloth with a hydrophilic MPL, and a carbon cloth with a hydrophobic MPL were named MEA-1, MEA-2, and MEA-3, respectively.
2.3. Performance testing set-up and analysis 2.3.1. Microstructure and morphology characterization The contact angle measurements of different anode diffusion layer samples were measured by using a contact angle system (Kruss contact angle measuring system G10) at 25 °C. The measurement was made by placing a 10-lL de-ionized water droplet at the tip of a syringe close to the diffusion layer surface. The contact angle was measured at about 5 s after the droplet attached to the surface. Six different values from different random regions were averaged to obtain an accurate value. The BET surface areas and the pore volumes of the different diffusion layers were measured by a Gas Sorption Analyzer at 77 K.
71
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76 Table 1 Physical properties and wettability of the anode diffusion layers used for fabricating the MEAs. MEAs
Composition of anode diffusion layer
Pmax (mW/cm2)
Contact angle (°)
Diffusion layer thickness (lm)
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
MEA-1 MEA-2 MEA-3
Carbon cloth Carbon cloth with hydrophilic MPL Carbon cloth with hydrophobic MPL
30.3 21.0 24.7
130.6 ± 1.3 127.4 ± 2.6 133.2 ± 1.7
226.4 ± 3.0 393.3 ± 8.0 366.4 ± 2.0
84.4 141.4 133.6
0.07 0.36 0.33
3.3 10.3 9.9
2.3.2. Cell performance test Before testing the cell, the MEA was activated with ultra-high purity (99.995%) hydrogen and high purity (99.99%) oxygen reactant gases at the anode and the cathode sides respectively at 100 ml min 1 on each side. The cell activation was conducted at the cell and humidification temperatures of 75 °C and 15 psig at both the anode and the cathode for 4 h. After H2/O2 activation, an aqueous ethanol solution with the concentration of 1 M was fed by a digital micro-pump to the anode at a flow rate of 1 ml/ min. Unhumidified oxygen gas with a flow rate of 100 ml/min was supplied to the cathode compartment at atmospheric pressure. The cell activation by the ethanol solution remained for 60 min. These processes were performed to ensure a steady state cell operation. After that, cell polarization testing was carried out by a computer-controlled Fuel Cell Test station with an electronic load in a potentiodynamic polarization mode. For a measurement of voltage variation with time at constant current discharge, the cell current was controlled and the cell voltage was measured for 30 min. 2.3.3. Electrochemical measurements The anode polarization measurement of the ethanol oxidation reaction (EOR) was tested by using Potentiostat/Galvanostat apparatus (AutoLab). Ethanol solution was fed to the anode side at a flow rate of 1 mL/min, while the humidified hydrogen gas was supplied at a flow rate of 100 mL/min to the cathode side as a dynamic hydrogen electrode (DHE). The cell was conducted at 75 °C and performed in a potentiodynamic polarization mode with a scan rate of 2 mV/s. The anode impedance measured at constant current density was recorded by using Impedance/Gain-Phase Analyzer (AutoLab). The anode impedance spectra were measured by setting
MEA1 Anode
the anode fed with ethanol solution as working electrode (WE) and the cathode fed with hydrogen as a DHE. Impedance spectra were obtained at a frequency range between 5 kHz and 0.05 Hz with 10 points per decade, and the amplitude of the sinusoidal current signal did not exceed 10% of current discharged. 2.3.4. Ethanol crossover measurement by voltammetric method Potentiostat/Galvanostat apparatus (AutoLab) was used to measure the ethanol crossover and the methodology used in this study was the same as presented in a previous work [25]. The anode fed with an ethanol solution at a specific concentration acted as the counter electrode and also a dynamic hydrogen reference electrode (DHE). Pure nitrogen gas was fed to the cathode at 2 atm with a flow rate of 100 ml/min. The reaction occurring at the anode is the hydrogen evolution, while the reaction at the cathode is the oxidation of the ethanol that previously crossed from the anode to the cathode. When a dynamic potential versus DHE was applied, the permeated ethanol from the anode to the cathode was electrochemically converted in the cathode catalyst layer, and consequently, the hydrogen ions were pumped from the cathode to the anode. Then, the influence of the different MPLs on the ethanol crossover was measured by comparing the ethanol oxidation current at the cathode. The dynamic potential versus DHE was performed in the range of 0–1.2 V with a potential scanning rate of 2 mV/s. 3. Results and discussion 3.1. Physical characterization The BET surface area, mean pore diameter and pore volume of three different diffusion layers obtained from the N2 adsorption
MEA2 Cathode
Anode
Gas diffusion electrode (GDE)
MEA3 Cathode
Anode
Cathode
Gas diffusion electrode (GDE)
Gas diffusion electrode (GDE)
Membrane
Membrane
Membrane
Catalyst layer
Catalyst layer
Catalyst layer
Hydrophilic micro porous layer
Hydrophobic micro porous layer
Backing layer (carbon cloth)
Backing layer (carbon cloth) Fig. 1. Configuration of catalyst layer, diffusion layer, and membrane of three types of MEAs.
Backing layer (carbon cloth)
72
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
measurement are listed in Table 1. It should be noted that the BET surface area, pore volume, and mean pore diameter of MEA-1 should not be compared to MEA-2 and MEA-3, because the measured value represented the micropores in the fiber of the woven carbon cloth, and not the MPL. For MEA-2 and MEA-3, the BET surface area, pore volume, and mean pore diameter were about the same. Thereby, the microstructures in both of the MEAs with MPL slightly affected the cell performance as the microstructures did not cause much difference in mass transport resistance. In addition, the surface morphologies of these MPLs were examined by SEM. Their images are shown in Fig. 2, which confirms that there was a slight difference in the microstructures between the MEA-2 and MEA-3, corresponding to the BET results. The contact angle of each diffusion layer was measured by a contact angle meter. Their average values are presented in Table 1. Low contact angles show hydrophilic properties for the surface, while high contact angles give hydrophobic surfaces. The MEA-2, which has a Nafion ionomer in the anode MPL, showed the smallest contact angle while the MEA-3, containing PTFE in an anode MPL, gave the largest contact angle. The contact angle of MEA-1 was in between that of MEA-2 and MEA-3. However, the differences between the contact angles of MEA-2 and MEA-3 were small. It is believed that the use of ultra-sonic auto spraying to coat the MPL on the hot surface of the carbon cloth had uniformly
distributed either a Nafion ionomer or PTFE throughout the microporous layer, and had not allowed for phase separation of these binders to the top surface of the layer. 3.2. Cell performance test The quick-scan polarization curves of three MEAs are shown in Fig. 3. The results showed that in a low current density region (<20 mA/cm2), all three MEAs yielded similar cell performance. While the cell operated at low current density region, the required ethanol for generating the current was small. The impact of mass transport on cell performance was low, therefore all three anode structures displayed similar cell performance in low current density region. Nevertheless at higher current densities (>20 mA/ cm2), MEA-1 yielded the highest current densities at the same voltage. It was expected that the anode MPL in MEA-2 and MEA-3 would increase the mass-transfer resistance of the ethanol solution in the anode flow channel to the anode CL, resulting in an inadequate ethanol concentration in the anode CL. This finding was similar to a previous work reporting that the performance of MEA with anode MPL was inhibited, especially at high current density, by the hydrophobicity of the carbon fiber paper as a backing layer and the presence of the MPL [24]. The presence of the anode MPL prevents the flow of liquid to and from the CL due to the very small pores
Fig. 2. SEM images of anode diffusion media of carbon cloth without MPL (a) and (d), with hydrophilic MPL (b) and (e), and with hydrophobic MPL (c) and (f). Note (a–c) at 30 and (d–f) at 100.
73
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
35
0.9
30
Cell voltage (V)
0.7 25 0.6 0.5
20
0.4
15
0.3
10
0.2
MEA-1 MEA-2 MEA-3
0.1
5 0 100
0 0
20
40
60
Cell current density
Cell power density (mW/cm2)
0.8
[26]. Unlike DMFC, the swelling and expansion of the Nafion ionomer in the DEFC would affect only liquid phase transport, because only a small amount of gas was produced in the DEFC, especially using PtSn as the anode catalyst [27]. The MEA-3, having the hydrophobic MPL, yielded the anode limiting current density between that of MEA-1 and MEA-2, and it had lower cell performance than MEA-1. The mass transfer resistance was increased by adding PTFE in the anode MPL. However, a slight difference in the limiting current density between MEA-2 and MEA-3 was observed because the fact that the anode MPL, either hydrophobic or hydrophilic, would act as a barrier to obstruct the liquid mass transport resulting in the decrease of anode limiting current density.
80
(mA/cm2)
Fig. 3. Quick-scan DEFC polarization and power density curves of MEAs with different anode microporous layers (MPLs).
1 MEA-1 MEA-2 MEA-3 Model
and lower porosity. The difference between the cell performances of MEA-2 and MEA-3 was investigated later by the use of anode polarization and in situ EIS measurement. It was also observed that the open circuit voltage (OCV) of MEA-1 was slightly lower than that of the MEAs with GDL. This means that crossover of ethanol in the MEA-1 (without MPL) was slightly higher than that in the MEA-2 and the MEA-3.
-Z'' (ohm)
0.8 0.6 0.4 0.2 0
3.3. Cell characterization by electrochemical measurement
(a)
-0.2 0
0.2
0.4
0.6
0.8
1
1.2
Z' (ohm) 1
MEA-1 MEA-2 MEA-3 Model
0.8
-Z'' (ohm)
By using the cathode as a dynamic hydrogen electrode (DHE), the anode potentials of these three MEAs were measured, as illustrated in Fig. 4. The current density value at the high voltage region of approximately 0.5 V versus DHE, a so-called anode-limiting current density, was compared. The MEA-1 had the highest anode limiting current density (90 mA/cm2), which was approximately 25 and 30 mA/cm2 higher than those of the MEA-3 and the MEA-2, respectively. It was confirmed that the MEA without MPL (MEA1) had the lowest mass transfer resistance. The MEA-2, having the hydrophilic MPL, showed the lowest anode limiting current density. This behavior was also observed in DMFC as found in the work of Liu et al. [11]. It was believed that the mass transfer in hydrophilic MPL was inhibited by the swelling of the Nafion ionomer at full hydration, leading to a decrease of pore size and porosity in the anode MPL. Nafion absorbs water and other solvents and subsequently its dimension expands. The volume increase of Nafion in ethanol of approximately 181% was reported
0.6 0.4 0.2 0
(b)
-0.2 0
0.2
0.4
0.6
0.8
1
1.2
Z' (ohm) 1 MEA-1 MEA-2 MEA-3 Model
0.8 0.5 0.6 0.4
-Z'' (ohm)
Anode potential (V vs. DHE)
0.6
0.3
0.4 0.2
0.2 MEA-1 MEA-2 MEA-3
0.1
0
0
(c)
-0.2 0
20
40
60
80
100
Cell current density (mA/cm2) Fig. 4. Quick-scan anode polarization curves of MEAs with different anode diffusion layers.
0
0.2
0.4
0.6
0.8
1
1.2
Z' (ohm) Fig. 5. Anode half-cell Nyquist diagrams of MEAs with different anode diffusion layers at (a) 10, (b) 30 and (c) 50 mA/cm2.
74
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
R1
R2
R3
CPE 1 CPE2 Fig. 6. Equivalent circuit of the DEFCs anode (vs. DHE).
Another characterization technique performed for validating the mass transport loss was the EIS, which was carried out by controlling the current densities at 10, 30, and 50 mA cm 2. The corresponding Nyquist diagrams are shown in Fig. 5. To understand the effects of anode MPL, the obtained EIS data of the different MEAs were analyzed by fitting the anode spectra with an equivalent circuit model. The equivalent circuit model used in this work was modified from the conventional equivalent circuit models of DMFC [28], as shown in Fig. 6. The physical meanings of each element used in the model are described as follows: (1) R1 describes the ohmic resistance; (2) R2 and CPE1 are the contact resistance and the capacitive behavior between the membrane and the catalyst layer, respectively; (3) R3 and CPE2 are the charge transfer resistance and the capacitive behavior of the real anode with the roughness of the catalyst layer and non-uniform catalyst distribution, respectively. This equivalent circuit model matches the experimental data quite well (v2 0.03). The fitting parameters are summarized in Table 2, which shows that the simulated R1 value of MEA-1 tends to be slightly smaller than those of MEA-2 and MEA-3. It is believed that the diffusion layer without MPL offered good electron conduction due to the absence of an MPL binder, which was made of non-conductive material. From the results, in each MEA the R1 obtained from curve fitting at different current densities was averaged for the comparison. It was found that the lowest R1 of ca. 0.0373 X is obtained with the MEA-1, while R1 of MEA-2 and MEA-3 are 0.0385 X and 0.0381 X, respectively, slightly higher than that of MEA-1. However, the cell ohmic overpotential calculated by multiplying the R1 with the cell current showed that the % ohmic loss compared with the total loss did not exceed 2% in every MEA. The other losses such as kinetic and mass transport loss were the main losses of the fuel cell. Therefore, good electron conduction in the anode MPL of MEA-1 would not significantly enhance the cell performance. The main differences in EIS measurement are the R3 and the CPE2 caused by the kinetic reaction resistance of the anode electrode. By considering the simulated R3, it was found that the R3 of MEA-1was smaller than those of MEA-2 and MEA-3 at every current density. Obviously, at 50 mA/cm2, R3 of MEA-1 was approximately half of that of MEA-2 and MEA-3. Thus the lowest mass transfer resistance of MEA-1 was confirmed.
Comparing between MEA-2 and MEA-3, both MEAs have very close values of R3 especially at the current density of 50 mA/cm2 indicating the large kinetic resistance at the anode CL. The increase of kinetic resistance was caused by the low concentration of fuel in the anode CL, which has resulted from high mass transfer resistance from the anode diffusion layer to the anode catalyst layer. It is inferred that the existence of the MPL at the anode diffusion layers of both MEA-2 and MEA-3 limited the ethanol transport, but with different causes. The mass transfer resistance of MEA-3 was due to the exclusion of water by the hydrophobic PTFE, whereas that of MEA-2 was attributed from the swelling and expanding of the Nafion ionomer in hydrophilic MPL as discussed earlier. At low current densities (10 and 30 mA/cm2), the R3 of MEA-2 was slightly larger than that of MEA-3 (ca. 0.07–0.1 X) implying that the swelling of Nafion ionomer caused higher mass transfer resistance than the hydrophobic behavior. The evidence of the swelling of the Nafion ionomer could be also noticed from the increase of the CPE2-T, accounting for the roughness of the electrode’s surface. The CPE2-T of MEA-2 (0.4 F) was higher than that of MEA-1 and MEA-3 at around 0.2 F. In addition, some Nafion material in MPL possibly dissolved into the catalyst layer, and consequently, enlarged the ionomer network in the catalyst layer, leading to the increase of CPE2-T. Constant current-discharging experiments of all three MEAs at two current densities of 10 and 30 mA/cm2 were carried out to present the cell performance tendency under steady state condition compare to unsteady state condition and also to indicate how the accumulated species in the electrode affected mass transport. The cell voltage variation with time for 30 min is illustrated in Fig. 7. At low constant-current discharges (10 mA/cm2), all three MEA performances were quite the same. Nevertheless, at higher constant-current discharges of 30 mA/cm2, the cell voltages of both MEA-2 and MEA-3 declined much more than MEA-1 had. The cell voltage decay in the cell voltage variation was attributed to the enhancement of mass transport resistance at the anode catalyst layer, which was caused by the gradual accumulation of CO2 during constant current discharge. It was believed that there had been no liquid water accumulation in the cathode GDL and flow channels for all the MEAs, as there had been no obvious cell voltage fluctuations during constant current discharges as described in a previous work [11]. It should also be mentioned that the order of the cell performance observed in the constant current discharge experiment is quite consistent with the previous quick-scan polarization curve data at the corresponding current density. 3.4. Ethanol crossover measurement Fig. 8 shows the ethanol crossover current density versus the applied voltage of the MEAs using various types of gas diffusion layers. This crossover current density was ascribed to the permeated ethanol at the cathode [25]. Considering the peak of crossover current density in the present work, it was in the same range as
Table 2 Fitted parameters of the equivalent circuit model for the anode of the DEFCs operating at various current densities. Sample
I (mA/cm2)
R1 (X)
R2 (X)
CPE1-T (F)
CPE1-P
R3 (X)
CPE2-T (F)
CPE2-P
MEA-1
10 30 50
0.037 0.037 0.038
0.007 0.014 0.009
1.148 1.005 1.719
0.658 0.677 0.606
0.404 0.411 0.570
0.272 0.196 0.216
0.932 0.998 0.956
MEA-2
10 30 50
0.038 0.039 0.039
0.001 0.003 0.007
0.316 0.158 3.743
0.962 0.957 0.568
0.650 0.639 1.075
0.420 0.373 0.394
0.829 0.844 0.923
MEA-3
10 30 50
0.037 0.037 0.040
0.006 0.008 0.037
0.171 0.441 4.992
0.924 0.784 0.419
0.547 0.562 1.029
0.214 0.183 0.196
0.941 0.966 0.976
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
crossover was significantly reduced. However, the crossover effect seems to have less significant impact on the DEFC performance, because the MEA-1 without an anode MPL, which had the highest ethanol crossover, still displayed the highest performance.
0.8 MEA-1
Cell voltage (V)
0.75
MEA-2
0.7
MEA-3
0.65
4. Conclusions
0.6 0.55 0.5
(a)
0.45 0.4 0
500
1000
1500
2000
Time (s) 0.8 MEA-1
0.75
Cell voltage (V)
75
MEA-2
0.7
MEA-3
0.65 0.6 0.55 0.5
(b)
0.45 0.4 0
500
1000
1500
2000
Time (s) Fig. 7. Cell voltage variation with time at constant-current discharge (a) 10 mA/cm2 and (b) 30 mA/cm2 for different MEAs with different anode diffusion layers.
Ethanol crossover current density (mA /cm2)
that in the work done by Song et al. [25]. At the same condition, the peak of crossover current density in our work was in the range of 60–75 mA/cm2, while it was around 80 mA/cm2 in their work. In Fig. 8, a peak crossover current density of MEA-1 without an anode MPL is at 75 mA/cm2, which is higher than that of the MEA-2 with hydrophilic MPL and the MEA-3 with hydrophobic MPL at 64 mA/cm2 and 58 mA/cm2, respectively. This means that the MEA without anode MPL resulted in higher ethanol crossover from the anode to the cathode. On the other hand, the MEA-2 with hydrophilic anode MPL and the MEA-3 with hydrophobic anode MPL was able to reduce the crossover current peak by 14.67% and 22.67%, respectively, compared to the MEA-1. This is because the anode MPL, either hydrophilic or hydrophobic, was able to act as a barrier diminishing the ethanol transfer from the anode flow channel to the anode catalyst layer. As a result, the concentration of ethanol in the anode catalyst layer was low and the ethanol
80 70 60 50 40 30 20 10 0 -10
MEA-1 MEA-2 MEA-3
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Voltage (V) Fig. 8. Ethanol crossover rate through the MEAs using different anode MPLs. (Anode: ethanol concentration = 1.0 M, flow rate = 1.0 ml/min, Cathode: Pure nitrogen pressure = 2.0 atm).
The effects of the anode MPLs on the performances of DEFCs were investigated. The quick-scan polarization curves of the three MEAs indicated that all three MEAs with an anode without MPL, an anode with a hydrophilic MPL, and an anode with a hydrophobic MPL, displayed similar cell performances at low current density regions, whereas MEA-1 without the anode MPL demonstrated the highest cell performance at high current densities. The quick-scan anode polarization curves and the anode half-cell Nyquist diagrams confirmed that the presence of the anode MPL had limited the supply of ethanol solution from the flow channel to the anode CL, resulting in an inadequate ethanol concentration in the reaction zone. The constant current discharge measurement also revealed that the cell voltage decay of the MEA with the anode MPL was higher than that of the one without the anode MPL, particularly at high constant current discharge. It was expected that the gradual accumulation of CO2 in the anode electrode had caused the cell voltage decay. The cell voltage of the MEA without the anode MPL was more stable in the long run, since CO2 had been more easily removed. In addition, the ethanol crossover measurement revealed that the MEA without the anode MPL had the highest ethanol crossover current of the three different MEAs. However, the kinetic influence seemed to be more important than the ethanol crossover effect for the DEFC, because the MEA-1 without the anode MPL had the best performance even though it had the highest ethanol crossover. Acknowledgements The authors would like to acknowledge the financial support from the Higher Education Research Promotion and Natural Research University (NRU) Project of Thailand, Office of the Higher Education Commission, the Joint Graduate School of Energy and Environment (JGSEE) of King Mongkut’s University of Technology Thonburi (KMUTT), and P.E. would also like to thank the Thailand Research Fund (TRF) for the Royal Golden Jubilee (RGJ) academic scholarship. References [1] Meyer M, Melke J, Gerteisen D. Modelling and simulation of a direct ethanol fuel cell considering multistep electrochemical reactions, transport processes and mixed potentials. Electrochim Acta 2011;56:4299–307. [2] Antolini E. Catalysts for direct ethanol fuel cells. J Power Sources 2007;170:1–12. [3] Tayal J, Rawat B, Basu S. Bi-metallic and tri-metallic PteSn/C, PteIr/C, PteIreSn/C catalysts for electro-oxidation of ethanol in direct ethanol fuel cell. Int J Hydrogen Energ 2011;36:14884–97. [4] Brouzgou A, Song SQ, Tsiakaras P. Low and non-platinum electrocatalysts for PEMFCs: Current status, challenges and prospects. Appl Catal B-Environ 2012;127:371–88. [5] Liu CY, Sung CC. A review of the performance and analysis of proton exchange membrane fuel cell membrane electrode assemblies. J Power Sources 2012;220:348–53. [6] Neburchilov V, Martin J, Wang H, Zhang J. A review of polymer electrolyte membranes for direct methanol fuel cells. J Power Sources 2007;169:231–8. [7] Wu QX, Zhao TS, Chen R, Yang WW. Effects of anode microporous layers made of carbon powder and nanotubes on water transport in direct methanol fuel cells. J Power Sources 2009;191:304–11. [8] Yang SH, Chen CY, Wang WJ. An impedance study for the anode micro porous layer in an operating direct methanol fuel cell. J Power Sources 2010;195:3536–45. [9] Casalegno A, Santoro C, Rinaldia F, Marchesia R. Low methanol crossover and high efficiency direct methanol fuel cell: The influence of diffusion layers. J Power Sources 2011;196:2669–75.
76
P. Ekdharmasuit et al. / Fuel 113 (2013) 69–76
[10] Kang K, Lee G, Gwak G, Choi Y, Ju H. Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system. Int J Hydrogen Energ 2012;37:6285–91. [11] Liu F, Wang CY. Water and methanol crossover in direct methanol fuel CellEffect of anode diffusion media. Electrochim Acta 2008;53:5517–22. [12] Zhang J, Yin GP, Lai QZ, Wang ZB, Cai KD, Liu P. The influence of anode gas diffusion layer on the performance of low-temperature DMFC. J Power Sources 2007;168:453–8. [13] Lindermeir A, Rosenthal G, Kunz U, Hoffmann U. On the question of MEA preparation for DMFCs. J Power Sources 2004;129:180–7. [14] Song S, Zhou W, Liang Z, Cai R, Sun G, Xin Z, et al. The effect of methanol and ethanol cross-over on the performance of PtRu/C-based anode DAFCs. Appl Catal B-Environ 2005;55:65–72. [15] Li G, Pickup PG. Analysis of performance losses of direct ethanol fuel cells with the aid of a reference electrode. J Power Sources 2006;161:256–63. [16] Ghumman A, Vink C, Yepez O, Pickup PG. Continuous monitoring of CO2 yields from electrochemical oxidation of ethanol: Catalyst, current density and temperature effects. J Power Sources 2008;177:71–6. [17] Ghumman A, Li G, Bennett DV, Pickup PG. Online analysis of carbon dioxide from a direct ethanol fuel cell. J Power Sources 2009;194:286–90. [18] James DD, Bennett DV, Li G, Ghumman A, Helleur RJ, Pickup PG. Online analysis of products from a direct ethanol fuel cell. Electrochem Commun 2009;11:1877–80. [19] Wang Q, Sun GQ, Cao L, Jiang LH, Wang GX, Wang SL, et al. High performance direct ethanol fuel cell with double-layered anode catalyst layer. J Power Sources 2008;177:142–7. [20] Rousseau S, Coutanceau C, Lamy C, Leger JM. Direct ethanol fuel cell (DEFC): Electrical performances and reaction products distribution under operating
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
conditions with different platinum-based anodes. J Power Sources 2006;158:18–24. Zhu M, Sun G, Yan S, Li H, Xin Q. Preparation, structural characterization, and activity for ethanol oxidation of carbon-supported PtSnIn Catalyst. Energy Fuel 2009;23:403–7. Andreadis G, Stergiopoulos V, Song S, Tsiakaras P. Direct ethanol fuel cells: The effect of the cell discharge current on the products distribution. Appl Catal BEnviron 2010;100:157–64. Masdar MS, Tsujiguchi T, Nakagawa N. Mass spectroscopy for the anode gas layer in a semi-passive direct methanol fuel cell using porous carbon plate PartII. Releationship between the reaction products and the methanol and water vapor pressures. J Power Sources 2009;194:618–24. Alzate V, Fatih K, Wang H. Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance. J Power Sources 2011;196:10625–31. Song S, Wang G, Zhou W, Zhao X, Sun G, Xin Q, et al. The effect of the MEA preparation procedure on both ethanol crossover and DEFC performance. J Power Sources 2005;140:103–10. Affoune AM, Yamada A, Umeda M. Conductivity and surface morphology of Nafion membrane in water and alcohol environments. J Power Sources 2005;148:9–17. Jablonski A, Kulesza PJ, Lewera A. Oxygen permeation through Nafion 117 membrane and its impact on efficiency of polymer membrane ethanol fuel cell. J Power Sources 2011;196:4714–8. Yuan T, Zou Z, Chen M, Li Z, Xia B, Yang H. New anodic diffusive layer for passive micro-direct methanol fuel cell. J Power Sources 2009;192:423–8.