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Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
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Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover A.M.I.N. Azam a, S.H. Lee b, M.S. Masdar a,b,c,*, A.M. Zainoodin a, S.K. Kamarudin a,b,c a
Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Chemical Engineering Program, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia c Research Centre for Sustainable Process Technology, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b
article info
abstract
Article history:
A parametric study was carried out to investigate the effect of fuel concentration (0.5 M
Received 15 June 2018
e3.0 M), operating temperature (ambient temperature to 85 C), flow rate of ethanol (0.5
Received in revised form
e5.0 mL min1) and air (100e600 mL min1) on the direct ethanol fuel cell (DEFC) perfor-
3 August 2018
mance. The operations were conducted in three operational modes, namely, passive, semi
Accepted 20 August 2018
passive, and active modes, and power generation were measured. Ethanol crossover was
Available online xxx
indicated by the carbon dioxide (CO2) concentration present at the cathode outlet and
Keywords:
with the increase of ethanol concentration, and ethanol and oxidant flow rate increased
DEFC
with temperature until DEFC reaches the optimum conditions, i.e., concentration and flow
Parametric
rate. Meanwhile, the DEFC performance significantly and proportionally increased with
Mass transport
operation temperature and reached values of up to 8.70 mW cm2 and 85 C at stable
Performance
conditions. Furthermore, fuel crossover, that is, ethanol flux, increased in proportion to the
Fuel crossover
ethanol concentration, i.e., 3.71 104 g m2 s1 and 8.79 104 g m2 s1 for 0.5 M and
measured by using a CO2 analyzer. Results indicated that DEFC performance increased
3.0 M ethanol concentration, respectively. At different modes of operation, the active DEFC system exhibited the highest performance, followed by the semi passive and passive DEFC system. These results indicated that optimizing ethanol, oxidant flow rate and temperature would enhance the mass transport in anodes and cathodes, and hence improve the electrochemical reactions and DEFC performance. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cell is one of the attractive alternative power sources [1,2]. Direct liquid fuel cells (DLFCs) produce electricity by transforming the combustion heat stored in liquid fuels through
electrochemical reactions. Methanol and ethanol fuels are among the common liquid fuel options for DLFC. For instance, ethanol is a better than methanol in DLFC application with respect to energy density (8.0 k W h kg1 vs. 6.1 k W h kg1, respectively) [3] and less toxic and thus safer in large-scale
* Corresponding author. Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia. E-mail address: [email protected] (M.S. Masdar). https://doi.org/10.1016/j.ijhydene.2018.08.121 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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applications. Furthermore, ethanol can be considered as renewable biofuel as nearly 95% ethanol is produced by the fermentation of sugar canes and corns [4]. Furthermore, ethanol has lower crossover rate in the membranes than methanol, which has smaller and more neutral molecules [5]. Direct ethanol fuel cells (DEFCs) have great potential for commercialization, but some challenges are encountered in the optimization of their performance, particularly the sluggish electro-oxidation of ethanol at anodes, water and heat management, and high cost of cell materials [3,6]. The electrooxidation of ethanol can occur through different pathways, most of which lead to the formation of partially oxidized products, such as acetaldehyde and acetic acid; this partial oxidation, which results because of nearly unbreakable CeC bonds, results in efficiency loss [7]. Meanwhile, although the ethanol crossover was lower in DEFC than in methanol in DMFC, the study on the fuel crossover is also important in DEFC. The permeation or crossover of ethanol from an anode to a cathode creates a mixed potential at the cathode and affects cell performance [8]. The fuel crossover is greatly influenced by the operating parameters of DEFC. For instance, Song et al. [9] mentioned that the crossover of ethanol is affected by temperature as the kinetics of the molecules is dependent on temperature and hence affects DEFC performance. Furthermore, parameter operations for DEFCs can affect mass transport and its performance. Effects of different operating parameters, including the cell operating temperature, ethanol concentrations and mass flow rates on cell performance in alkaline DEFC have been systematically studied in the previous works by Li et al. [10e12]. For instance, Pereira et al. [13] studied the effect of 1e3 M ethanol on cell performance and found that maximum power density is achieved by a cell at 2 M and started to drop beyond 2 M. Similar tendency was obtained by Li et al. [10,11]. They were suggested that too high ethanol concentration lowering the cell performance can be attributed to two reasons: i) more active sites in the anode catalyst layer are covered by ethanol and this counts against the adsorption of hydroxyl ions, thus lowering the ethanol oxidation reaction (EOR) kinetics, and ii) the concentrated ethanol that reduces the level of hydroxyl ionization creates a barrier for the transfer of hydroxyl ions, resulting in an increase in cell resistance [10]. For the operating temperature, Alzate et al. and Li et al. found that the performance of DEFC increases as temperature increases from 60 C to 90 C [14] and 23 C to 80 C [10]. Li et al. suggested that an increase in the cell operating temperature would enhance and increase; i) the electrochemical kinetics of both the anodic EOR and cathodic ORR, resulting in the reduction in the activation loss; ii) the conductivity of the hydroxyl ions, thus lowering the ohmic loss; iii) both the ethanol and oxygen transport, and thus decreasing the concentration loss [10]. Meanwhile, increase in fuel flow rate increases mass transport to the catalyst layer and thus possibly causes fuel crossover [15]. Alzate et al. [14] also investigated the effect of ethanol flow rates of 0.6e5.0 mL min1 on an active DEFC system, and their results showed that the increase of flow rates from 1.0 mL min1 to 5.0 mL min1 has no considerable effect on cell performance. Moreover, Li et al.
found that the effect of the fuel solution flow rate, i.e., 0.3e3.0 mL min1, is rather small in the low current density region, but the cell performance improves slightly with increasing solution flow rate in the high current density region. This latter feature is attributed to the enhanced masstransfer of both ethanol and OH ions from the flow-field to the catalyst layer [11]. The operational modes of DEFC contribute to mass transport behavior and its performance. In general, the DEFC system can be categorized into active, semi passive, and passive systems according to fuel delivery and handling concepts. An active system requires an external power supply that feed fuel and air or oxygen to the cell. This system has a high cost and suitable to large-capacity fuel cell systems. In a passive system, fuel is injected into cell reservoir before the operation. Mass transfer occurs by natural capillary forces, diffusion, and convection without the use of an additional power supply [13]. In a semi passive system, a pump is used for the feeding of air or oxygen to the cathode side only. For such operational modes, Li et al. have been studied the cell performance in alkaline DEFC for active mode [11], semi passive mode [10], and passive mode for the DEFC's stack [12]. For the direct formic acid fuel cell (DFAFC), Ong et al. [16] investigated the effects of operational modes on DFAFC performance. They found that the semi passive DFAFC with oxygen at cathode obtained the highest maximum current density followed by the semi passive DFAFC with air flow at cathode. However, for the active DFAFC system, the performance exhibited only a small improvement. They concluded that the change in flow rate at the cathode has a greater effect on cell performance than that at the anode. Previous research focused on electro-catalyst development and membrane modification for DEFC application. Some researcher examined the effect of operating parameters on cell performance were also conducted, although they did not consider ethanol crossover to the cathode. Therefore, in this work, the influences of operation parameters, such as concentration, temperature, reactant flow rate, and operation mode on DEFC performance and fuel crossover were investigated. The relationship between the operating parameters of DEFC and the fuel crossover were discussed on the basis of DEFC power generation.
Materials and methods Membrane electrode assembly and single cell DEFC Single cell DEFC was used in all parametric studies. A commercial membrane electrode assembly (MEA; Fuel Cell Store, US) with 2.5 cm2 active area was used and. This MEA consisted of a microporous layer, a catalyst layer PteRu on anode, and a Pt on cathode with loading of 4 mg cm2 on both sides. It used Nafion 117 types for its electrolyte membrane. A pretreatment process was conducted for MEA activation, and the steps were nearly similar to that described by Rejal et al. [17], who used hydrogen (H2) gas and air. The MEA was conditioned initially within the test fixture at 70 C under a H2/ air fuel cell operating mode for 1 h, and cell potential was
Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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maintained at 0.6 V. The H2 flow rate was set to 600 mL min1 at the anode and that of air was set to 1200 mL min1 at cathode. Both streams were humidified at 70 C prior to entering the cell. The current, voltage and power were monitored along 1 h MEA activation to ensure it reached at a stable performance.
Measurement of DEFC performance We used and modified several parameter operations to investigate their effects on DEFC performance. For the ethanol concentration, the experiment was carried out in a semi passive mode, as shown in Fig. 1(a). A 0.5 M ethanol solution was fed to the anode reservoir of the cell, while the air was allowed to flow continuously at a flow rate of 200 mL min1 with an air mini pump. Different ethanol concentrations (0.5e3.0 M) were used at ambient temperature conditions. The investigation on the effect of operating temperature was carried out in a humidity chamber Model HCP108 Memmert (Memmert GmbH þ Co. KG, Germany). The cell was operated in a passive mode (as shown in the schematic diagram of Fig. 1(b)) in the range of 35 Ce85 C using 2.5 M ethanol concentration. Moreover, the relative humidity (RH) was fixed at RH 90% for all temperature conditions. For the active mode operation (shown in Fig. 1 (c)), ethanol fuel with 2.5 M in concentration was supplied at the anode by using a peristaltic pump, and the flow rate of air was fixed at 200 mL min1. The ethanol flow rate was manipulated within a range of 0.5e5.0 mL min1 at ambient temperatures. For the effect of oxidant flow rate, air was supplied to cathode at flow rate of 100e600 mL min1 by using 2.5 M ethanol in the reservoir. All these parameters were conducted at a constant voltage of 0.3 V, and the power generation during the 2 h operation was measured with Fuel Cell Monitor 3.0 (H-Tec Education GmbH, Germany).
a)
Measurement of ethanol crossover Ethanol crossover was identified by analyzing the CO2 produced at the cathode. As the ethanol at the anode side diffused through the membrane to the cathode, it was assumed to be oxidized and to have formed carbon dioxide on the basis of the report of James and Pickup [8]. During the two 2 h operation, the DEFC system was operated in either the semi passive or active mode. Air was allowed to flow at the cathode by using the air mini pump. The CO2 gas concentration was measured with a CO2 analyzer with Model 906 (Quantek Instruments Inc, US) in the form of percentage. The setup diagrams of the experiment are shown in Fig. 1(a) and (c). Fig. 1(d) shows the actual photo of the experiment setup. For the fuel crossover measurement, the evaporation of ethanol from the cathode was neglected, and all ethanol crossover to the cathode was assumed to be completely oxidized at the cathode catalyst and to have formed CO2 as show in Eq. (1). As the ethanol at the anode side diffused through the membrane to the cathode, and simultaneously reacts with oxygen at active site of catalyst cathode's catalyst layer. This assumption was referred to the previous study by Rejal et al. [17]. C2H5OH þ 3O2/2CO2 þ 3H2O.
(1)
It means, we assume that there are no intermediate reactions or partial oxidation reactions of ethanol involve as shown in Eq. (2)e(4) below [18] to produce intermediate products such as acetaldehyde (Eq. (2)), ethane-1,1-diol (Eq. (3)), or acetic acid (Eq. (4)). Even, if there are partial oxidation reactions of ethanol occur, the intermediate products are assumed to be extremely small and can be neglected. CH3CH2OH / CH3CHO þ 2Hþ þ 2e
(2)
b)
Monitor Data CollecƟon
Passive Ethanol SoluƟon
Anode Carbon Dioxide Analyzer
Air
Air Pump
Cathode
Passive Ethanol SoluƟon
Monitor (Data CollecƟon)
Anode
Fuel Cell Monitor
Monitor (Data CollecƟon)
Fuel Cell Monitor
Cathode
Air Breathing
Flow Meter
d)
c) Ethanol SoluƟon
Liquid Pump
Monitor (Data CollecƟon)
Monitor Data CollecƟon Anode Carbon Dioxide Analyzer
Air
Air Pump
Cathode
Flow Meter
Fuel Cell Monitor
Carbon dioxide analyzer Air mini pump
Electrochemical measurement unit CO2 outlet at anode
Single cell DEFC
1
Fig. 1 e Schematic diagram of experiment setup for the measurement on cell performance and ethanol flux; (a) semi-passive mode (b) passive mode (c) active mode, (d) actual photo for the semi-passive mode. Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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CH3CH2OH þ H2O / CH3CH(OH)2 þ 2Hþ þ 2e
(3)
CH3CH2OH þ H2O / CH3CHOOH þ 4Hþ þ 4e
(4)
Moreover, the amount of CO2 crossover from anode (product of ethanol oxidation at anode) to cathode through the MEA was assumed to be extremely small on the basis of the research of Han and Liu [19] and thus neglected. We greatly reduced the crossover of CO2 at the anode side was greatly reduced by the anode outlet design for the single cell of the DEFC to improve the exhaust of CO2 from the anode itself (Fig. 1(d)).
Results and discussion Effect of ethanol concentration The effect of ethanol concentration on DEFC performance was investigated with different ethanol concentrations (from 0.5 M to 3.0 M). The cell was operated in semi passive mode at an air flow rate of 200 mL min1. The voltage-current (VeI) and power-current (PeI) curves for the single-cell DEFC at different methanol concentrations is shows in Fig. 2. In Fig. 2, the opencircuit voltage (OCV) increases with increasing ethanol concentration and then decreases beyond the ethanol concentration of 2.0 M. The increase in OCV with ethanol concentration can be due to the increased rate of anode reactions. These phenomena are supported by Heysiattalab et al. [15], who stated that increment in ethanol concentration increases diffusion and ethanol concentration in the catalyst layer, thereby improving ethanol oxidation. Meanwhile, decrease in OCVs with increasing ethanol concentration
beyond 2.0 M may be due to the mixed potential caused by ethanol crossover. In general, the ethanol crossover may cause three technical problems which, i) ethanol may be oxidized to form the parasitic current, leading to a mixed potential on the cathode, so that the mixed-potential problem is insignificant in this fuel cell system; ii) effect on the water transport in the cell as ethanol will be directly oxidized to produce water at cathode, so that contributes the flooding phenomena at cathode [20,21]; iii) the ethanol crossover definitely results in a waste of fuel, decreasing the utilization efficiency [22]. We expected that the flux of ethanol crossover would increase with increasing ethanol concentration and then reduce the OCVs. Moreover, for the PeI curves in Fig. 2, the maximum power density increased with increasing ethanol concentration before it decreased at 3.0 M. The difference of the peak power density could be explained from three factors: i) thermodynamic voltage difference, ii) electrochemical kinetic loss, and iii) ohmic loss [23]. The maximum power density output for the single cell DEFC can reach 6.52 mW cm2 at 2.5 M ethanol concentration, whereas the limiting current was obtained at 46.7 mA cm2. At all ethanol concentrations, the optimum voltage at maximum power densities were obtained from 0.25 V to 0.30 V. Hence the 0.30 V was used as a constant voltage for 2 h single cell DEFC operation. Fig. 3 shows the profile of DEFC performance during 2 h operation at different ethanol concentrations and 0.30 V. At all concentrations, power density initially increases sharply and then slowly decreases before stabilizing with time. The high power densities were achieved at the first 1 min possibly because of the initial accumulation of ethanol at high concentration at anode surface before it decreased with time. These tendencies were similar to that observed in a previous
0.6
7
Semi Passive: 200 ml min-1 air Ambient temperature
0.4
Voltage [V]
6
V-I, 0.5M V-I, 1.0M V-I, 1.5M V-I, 2.0M V-I, 2.5M V-I, 3.0M
5
4
P-I, 0.5M P-I, 1.0M P-I, 1.5M P-I, 2.0M P-I, 2.5M P-I, 3.0M
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Power density [mW cm-2]
0.5
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0 0
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Current density [mA cm-2] Fig. 2 e VeI and PeI curves at different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature). Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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0.5M 2.0M
Power density [mW cm-2]
4.5
1.0M 2.5M
1.5M 3.0M
Semi passive: 200 mL min-1 air Ambient temperature 0.3 V
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
20
40
60
80
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120
Operation time [min]
Fig. 3 e Power density profile at different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature; constant voltage: 0.3 V).
study by Abdelkareem et al. [24]. It could be happen due to free-impurities, that is, products or intermediate products, such as CO2 and acetic acid, on the active site of the catalyst at the anode. Thus, electro-oxidation occurred in a good condition without any barrier between ethanol and anode surface. Then, the power densities slowly decreased with time as the ethanol fuel was consumed for the electro-oxidation reaction at the anode. At 2.5 M ethanol concentration (Fig. 3), the power density dropped more rapidly than those in the other ethanol concentrations. A previous study reported that active sites for electro-oxidation can be affected by acetic acid as an intermediate product from intermediate ethanol reactions and the anode catalyst Pt/Ru has a high tendency to produce acetic acid from ethanol oxidation [25]. The by-product of acetic acid is the factor that contributes to anode poisoning on a Pt-based catalyst. In Fig. 3, a high degradation rate was observed at a high power density, in which a high oxidation rate would have occurred and an increased amount of intermediate products could have been produced from the anode reactions. The effect of these by-products, especially acetic acid, may have resulted in catalyst poisoning in the anode's electrode. Therefore, the high degradation rate at 2.5 M ethanol concentration at a high power density was probably caused by the by-products attached on the active site of the catalyst and thus may have reduced cell performance. Furthermore, based on previous works by Li et. Al [11] and An et al. [26], a high ethanol concentration would blocks the mass transfer of hydroxyl ions, thus giving rise to an increase in the internal resistance. However, in this study, we do not measure the cell resistance, i.e., ohmic resistance. We assumed that the increment of ohmic resistance or variation with ethanol concentration was very small and could be neglected; since we only used diluted ethanol concentration from 0.5 M to 3.0 M. Our assumption was confirmed by Li et al., which they obtained a small increment of cell resistance for
operation using below 10 M ethanol concentration while a significant increase at above 10 M up to 17 M ethanol concentrations [11]. Moreover, Li et al. was clarified that for the case for the case of increasing ethanol concentration from 1.0 to 3.0 M, the decrease in activation and mass transport losses exceeds the increase in ohmic loss [10]. Ethanol crossover was investigated by measuring CO2 produced at the cathode outlet. Fig. 4 shows the percentage of CO2 profiles at the cathode outlet during the 2 h operation at different ethanol concentrations. In Fig. 4, the profiles of CO2 concentration fluctuated between 0.02% and 0.12%, and the magnitude of CO2 concentration varied according to ethanol concentration. The CO2 percentage in air recorded by the CO2 analyzer were approximately 0.04% ± 0.02% higher from its theoretical value, which is 0.04%. Moreover, the magnitude of CO2 concentration slightly increased with the increase of ethanol concentration at the anode (Fig. 4). Therefore, a low amount of ethanol fuel was permeated from anode to the cathode through the electrolyte membrane. The relationship among ethanol concentration, power density, and ethanol crossover at stable conditions (data at 90 min in Fig. 3) is illustrated in Fig. 5. For the ethanol flux, the average CO2 concentration at the cathode outlet was used for the calculation of the ethanol flux, and the power densities increased from 0.5 M to 2.5 M ethanol concentrations. Under stable operations (90 min), the highest power density (2.49 mW cm2) was obtained at the 2.5 M ethanol concentration. Further increase in ethanol concentration from 2.5 M to 3 M may decrease the power density of the DEFC. According to Alzate et al. [14], a DEFC cell will experiences difficulty in mass transfer at low ethanol concentration. With the increase of ethanol concentration from 0.5 M to 2.5 M, the mass transfer of ethanol molecules to the active sites of the catalyst can be improved, as shown in Fig. 5. The increase of ethanol concentration favored the production route of acetic acid, which yields four electrons per ethanol molecule in contrast to acetaldehyde production, which yields only two electrons
0.14
CO2 Concentration at cathode outlet [%]
5.0
0.5 M
1.0 M
1.5 M
2.0 M
2.5 M
3.0 M
Semi passive: 200 mL min-1 air Ambient temperature 0.3 V
0.12
0.10
0.08
0.06
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0.02 0
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80
100
120
Operation time [min]
Fig. 4 e CO2 concentration profile at the cathode outlet at different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature; constant voltage: 0.3 V).
Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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3.0
Power density
Effect of operating temperature
10 9
Semi passive: 200 mL min-1 air Ambient temperature 0.3 V
8 7
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6 1.5
5 4
1.0
3
Ethanol flux x 10-4 [g m-2 s-1]
2.5
Power density [mW cm-2]
Ethanol flux
2
0.5
1 0
0.0 0.0
0.5
1.0
1.5
2.0
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3.0
Ethanol concentration [M]
Fig. 5 e Relationship between DEFC performance and ethanol crossover at 90 min stable operation using different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature; constant voltage: 0.3 V).
[27]. This result suggested that the high-power density is the results of increased ethanol concentration. However, at high fuel concentrations (above the optimum level), the DEFC's performance was hindered by the fuel crossover and decreased in anode catalytic activity. These tendencies were similar to that observed in previous studies on DLFCs, such as DEFC [13], DMFC [27], and DFAFC [17]. In Fig. 5, the magnitude of ethanol crossover slightly increased with increasing of ethanol concentration at the anode. The magnitudes of fuel crossover were obtained at 3.71 104 and 8.79 3.71 104 g m2 s1 for 0.5 M and 3.0 M ethanol concentrations, respectively. However, these magnitudes were 30 times lower than the methanol crossover for DMFC and 10 times lower than formic acid crossover for DFAFC. The used of the thick electrolyte membrane of Nafion 117 instead of that of NRE 212 may be one of the reason for the low ethanol crossover obtained in this study. 10
Passive 2.5M ethanol 0.3 V
Power density [mW cm-2]
9 8 7 6 5 4 3 2
Fig. 6 shows the effect of operating temperature on DEFC performance at 2.5 M ethanol concentration and passive mode operation. The cell performance was studied within the range of ambient temperature ~28e85 C. The passive DEFC was operated at a constant voltage of 0.3 V inside a humidity chamber for the control of cell temperature and relative humidity. In Fig. 6, the power density significantly increased and was nearly directly proportional to the operating temperature. A low power density (1.65 mW cm2) was obtained for the operation at ambient condition possibly because of sluggish and low kinetic reactions at the anode. The increase of power density with increase the temperature could be due to; i) higher temperature lowers the activation loss of the redox reactions, ii) higher temperature decreases the cell resistance, and iii) higher temperature enhances the reactant transport [28]. Moreover, by increasing the operation temperature, the electrode kinetics and mass transfer properties, such as the ethanol diffusivity, can be accelerated [14,29], and the kinetics of ethanol oxidation at the anode and oxygen reduction at the cathode can be accelerated by using the Arrhenius equation, which shows the temperature dependence of the reaction rates, as shown in Eq. (5). The increment of temperature reduces the activation energy required for ethanol electrooxidation and oxygen reduction kinetic reactions and is related with molecular movement. Ea
k ¼ Ae RT :
(5)
High temperature improves the conductivity of membrane as the heat energy causes the Nafion polymer backbone to relax and expand and partly increases the transport rates of protons and electrons [9]. In other word, the increment of temperature will increase the ethanol crossover which is proton transport rates. Consequently, the increment of ethanol crossover could lead to a more negative effect on the OCV and DEFC performance (electron transport rates). The diffusivity of ethanol molecules is thereby enhanced owing to high kinetic energy, and this enhancement leads to the ethanol crossover to the cathode. Furthermore, increasing temperature creates a competitive effect between the reaction kinetic [30] and ethanol crossover. After reaching certain levels, these two effects balance each other out and therefore maintain the power density at a steady state. In this study, the highest power density was achieved at 8.70 mW cm2 for 85 C operation under stable operation. The power density was expected to increase at temperatures above 85 C, as a reported by Alzate et al. [14]. Therefore, the kinetic rate at the anode is more important and significant compared with ethanol crossover at temperature below 100 C.
Effect of ethanol flow rate
1 0 20
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50
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70
80
Operation temperature [oC]
Fig. 6 e Power density at 90 min stable operation at different operating cell temperatures in a passive DEFC (ethanol concentration 2.5 M; constant voltage: 0.3 V).
90
We used the different flow rates of the ethanol solution (0.5e5.0 mL min1) to investigate their effects on DEFC performance. The test was carried out in the active DEFC system at a constant voltage of 0.3 V and ethanol concentration of 2.5 M. Fig. 7 shows the relationship between the power density and ethanol crossover at different ethanol flow rates. In Fig. 7,
Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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Ethanol flux
Power density [mW cm-2]
10 2.0 8 1.5 6
Active mode 200 mL min-1 air 2.5M ethanol Ambient temperature 0.3 V
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Ethanol flux x 10-4 [g m-2 s-1]
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-1
Ethanol flow rate [mL min ]
Fig. 7 e The relationship between DEFC's performance and ethanol crossover at 90 min stable operation using different ethanol flow rate in active mode DEFC (air flow rate: 200 mL min¡1; ethanol concentration: 2.5 M; ambient temperature; constant voltage: 0.3 V).
a significant increase in power density can be seen between the ethanol flow rate of 0.5 and 1.0 mL min1. The performance of the active DEFC slightly increased with increasing ethanol flow rate. This result confirmed that the effect of surface phenomenon is significant at low flow rates because of convention mechanism. Low ethanol flow rate can affect the removal of reaction products, such as CO2 and acetic acid, that block the active sites of reactions. Therefore, the utilization of available catalyst sites becomes low [14]. At increased anode flow rate, the mass transfer to the catalyst layer was improved, and thus DEFC cell performance increased. The highest power density of 2.59 mW cm2 was obtained at 4 mL min1 under a stable operation. In order to improve the mass transfer of ethanol, Li et al. have developed an innovative cell design with a metal foam-based all-in-one electrode that incorporates the flow field, backing layer, micro-porous layer, and catalyst layer into a whole. By using this cell, the liquid pump power can be saved when reducing the anode flow rate from 1.0 to 0.2 mL min1; with the cell performance of the all-in-one MEA still remains unchanged [31]. At 5 mL min1 ethanol flow rate, DEFC performance decreased significantly to about 1.18 mW cm2. The probable cause is the imbalance between stoichiometric reaction and high ethanol crossover. According to the overall electrochemical equation of DEFC, one molecule of ethanol needs to react with three molecules of oxygen so that ethanol achieves complete oxidation. However, further increase in ethanol flow rate resulted in an extremely high ethanol content in the cell reservoir, while oxygen molecules available for reactions with ethanol were insufficient. It contributes to the unbalance reaction within the DEFC system. A high fuel flow rate might also have a cooling effect to the fuel cell [32]. This would reduce the temperature on the MEA surface lower than the operating temperature and affect DEFC performance. For the mass transport of ethanol, CO2 concentration at the cathode outlet increased with increasing ethanol flow rate at anode, as shown in the increase of ethanol crossover
magnitude (as shown Fig. 7). The highest ethanol crossover was obtained at 12.53 104 g m2 s1 at 5 mL min1 operation and was higher compared with that observed when 3.0 M ethanol solution was used. Convection and diffusion mechanisms occurred at the active anode system. By contrast, only the diffusion mechanism occurred at the passive anode system, and hence increases the ethanol mass transfer rate and directly increases the magnitude of ethanol flux.
Effect of air flow rate Fig. 8 shows the effect of air flow rate on DEFC performance and the fuel crossover at the range of 100 and 600 mL min1. The power density slightly increased from 100 mL min1 to 200 mL min1. The highest power density (1.81 mW cm2) was achieved at 200 mL min1 under stable operation. A sufficient air convection was created by the mini air pump and hence improved air diffusion into the catalyst layer at a 200 mL min1 air flow rate. Moreover, the water removal rate was increased owing to the increase in air flow rate and then prevented water flooding at the cathode [17]. Nevertheless, higher air flow rate affects long-term DEFC operation. In Fig. 8, the power density slightly decreased at air flow rates greater than 200 mL min1. Abdelkareem and Nakagawa [33] and Rejal et al. [17], who studied DMFC and DFAFC, respectively, reported that a high air flow rate may results in electrolyte membrane dehydration because of the high forced convection at the cathode. Moreover, an extensive study on water transport in electrolyte was implemented by Li et al. They measured the water uptake and electro-osmotic drag coefficient and concluded that the water transport through the electrolyte membrane is caused by both the diffusion due to the concentration gradient and the electronosmotic drag due to the ion transport [20]. The proton transport from the anode to the cathode was slowed down and subsequently decreased the power density [34]. Therefore, the reason of low DEFC performance at a high air flow rate could
10
3.0 Power Density
Ethanol flux
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9 8 7
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Semi passive 2.5M ethanol Ambient temperature 0.3 V
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Ethanol flux x 10-4 [g m-2 s-1]
14 Power Density
Power density [mW cm-2]
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Fig. 8 e The relationship between DEFC's performance and ethanol crossover at 90 min stable operation using different air flow rate at cathode in semi-passive DEFC (ethanol concentration: 2.5 M; ambient temperature; constant voltage: 0.3 V).
Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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be explained by the same consideration, as the DEFC system was nearly similar to the DMFC and DFAFC system. Meanwhile, ethanol fluxes are nearly constant at different air flow rates (Fig. 8). By comparing the power density obtained at different air flow rate in Fig. 8, the fuel crossover increase at low current density and high air flow rates. However, the flux of ethanol obtained at a high oxidant flow rate was almost similar to that at low air flow rates. It was suggested at high flow rates, the driving force of mass transfer by forced convection at the cathode is larger than the free convection at the anode (molecular transport). According to Ong et al., the pressure gradient builds up between the anode and cathode, and slightly high pressure can be obtained at the cathode. Thus, the mass transfer of formic acid from anode to cathode can be minimized by using the pressure gradient, and the fuel crossover can become limited and nearly constant because of the pressure at the cathode [16], as shown in Fig. 8. Therefore, the oxidant flow rates at the cathode have a less significant impact on the DEFC performance and fuel crossover than the ethanol concentration and flow rate at the anode.
Comparison of DEFC performance with different operational modes Fig. 9 shows the power density profile for different operational modes of DEFC with air and oxygen at the cathode at 2.5 M ethanol at ambient room temperature. From Fig. 9, the active DEFC system with oxygen at the cathode exhibited the highest performance and had a maximum power density of up to 3.42 mW cm1 at a stable condition. The continuous feeding of ethanol at 4 mL min1 and oxygen at 200 mL min1 improved the mass transfer and diffusion rate of ethanol and air at the anode and cathode, thus enhancing electrochemical reactions. The active mode with oxygen at the cathode achieved higher power density than that with air flow. This difference
6
2.5M ethanol Ambient temperature 0.3 V
Power density [mW cm-2]
5
Passive Semi passive-air Semi passive-oxygen Active-air Active-oxygen
4
may be related to the composition of oxygen in air, which is only 21%. Oxygen content for the ORR at the cathode when pure oxygen was used was higher than that detected when air was directly used. This performance trend was similar to that observed in a semi passive system with oxygen and air at its cathode. The power density for semi passive operation with oxygen higher compared to that semi passive using air at almost 10%. A less desired performance was observed at passive mode DEFC. At a passive mode operation, water accumulation or flooding occurred may have occurred in the cathode without the air convection, that is, air supplied by the mini air pump. Thus, the pathway of oxygen into the cathode's active catalyst was blocked. Then, the mass transfer of oxygen to the catalyst layer and the kinetic of cathode reaction decreased without air or oxygen feeding [35]. Therefore, the power density obtained for the passive mode operation was lower than those obtained for the active and semi passive systems.
Conclusion A parametric study was carried out in DEFC to determine the effect of operating conditions on its performance and ethanol crossover. At all cases, the operating parameters and operational modes affected the DEFC performance and mass transport of fuel, that is, ethanol crossover, and DEFC performance increased with the increase of ethanol concentration. Meanwhile, ethanol and oxidant flow rates increased with temperature until DEFC reaches optimum conditions. DEFC performance significantly and proportionally increased with operation temperature, reaching up to 8.70 mW cm2 at 85 C at stable conditions. The flux increased in proportion to ethanol concentration and ethanol flow rate, although it remained constant at varying oxidant flow rates. These results indicated that the manipulation of both ethanol and oxidant flow rate to optimum levels may enhance mass diffusion in the anode and cathode and thus may improve the mass transport of fuel and DEFC performance.
Acknowledgments A part of this study was supported by Universiti Kebangsaan Malaysia (UKM) from grants GUP-2016-043 and DIP-2017-020.
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1
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Fig. 9 e Power density profile at different operation modes of DEFC: passive, semi passive with air and oxygen; oxidant flow rate: 400 mL min¡1, and active mode with air and oxygen at cathode; ethanol flow rate: 4 mL min¡1 and oxidant flow rate: 400 mL min¡1, (ethanol concentration: 2.5 M; constant voltage: 0.3 V; ambient temperature).
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Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover A.M.I.N. Azam a, S.H. Lee b, M.S. Masdar a,b,c,*, A.M. Zainoodin a, S.K. Kamarudin a,b,c a
Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Chemical Engineering Program, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia c Research Centre for Sustainable Process Technology, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b
article info
abstract
Article history:
A parametric study was carried out to investigate the effect of fuel concentration (0.5 M
Received 15 June 2018
e3.0 M), operating temperature (ambient temperature to 85 C), flow rate of ethanol (0.5
Received in revised form
e5.0 mL min1) and air (100e600 mL min1) on the direct ethanol fuel cell (DEFC) perfor-
3 August 2018
mance. The operations were conducted in three operational modes, namely, passive, semi
Accepted 20 August 2018
passive, and active modes, and power generation were measured. Ethanol crossover was
Available online xxx
indicated by the carbon dioxide (CO2) concentration present at the cathode outlet and
Keywords:
with the increase of ethanol concentration, and ethanol and oxidant flow rate increased
DEFC
with temperature until DEFC reaches the optimum conditions, i.e., concentration and flow
Parametric
rate. Meanwhile, the DEFC performance significantly and proportionally increased with
Mass transport
operation temperature and reached values of up to 8.70 mW cm2 and 85 C at stable
Performance
conditions. Furthermore, fuel crossover, that is, ethanol flux, increased in proportion to the
Fuel crossover
ethanol concentration, i.e., 3.71 104 g m2 s1 and 8.79 104 g m2 s1 for 0.5 M and
measured by using a CO2 analyzer. Results indicated that DEFC performance increased
3.0 M ethanol concentration, respectively. At different modes of operation, the active DEFC system exhibited the highest performance, followed by the semi passive and passive DEFC system. These results indicated that optimizing ethanol, oxidant flow rate and temperature would enhance the mass transport in anodes and cathodes, and hence improve the electrochemical reactions and DEFC performance. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cell is one of the attractive alternative power sources [1,2]. Direct liquid fuel cells (DLFCs) produce electricity by transforming the combustion heat stored in liquid fuels through
electrochemical reactions. Methanol and ethanol fuels are among the common liquid fuel options for DLFC. For instance, ethanol is a better than methanol in DLFC application with respect to energy density (8.0 k W h kg1 vs. 6.1 k W h kg1, respectively) [3] and less toxic and thus safer in large-scale
* Corresponding author. Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia. E-mail address: [email protected] (M.S. Masdar). https://doi.org/10.1016/j.ijhydene.2018.08.121 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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applications. Furthermore, ethanol can be considered as renewable biofuel as nearly 95% ethanol is produced by the fermentation of sugar canes and corns [4]. Furthermore, ethanol has lower crossover rate in the membranes than methanol, which has smaller and more neutral molecules [5]. Direct ethanol fuel cells (DEFCs) have great potential for commercialization, but some challenges are encountered in the optimization of their performance, particularly the sluggish electro-oxidation of ethanol at anodes, water and heat management, and high cost of cell materials [3,6]. The electrooxidation of ethanol can occur through different pathways, most of which lead to the formation of partially oxidized products, such as acetaldehyde and acetic acid; this partial oxidation, which results because of nearly unbreakable CeC bonds, results in efficiency loss [7]. Meanwhile, although the ethanol crossover was lower in DEFC than in methanol in DMFC, the study on the fuel crossover is also important in DEFC. The permeation or crossover of ethanol from an anode to a cathode creates a mixed potential at the cathode and affects cell performance [8]. The fuel crossover is greatly influenced by the operating parameters of DEFC. For instance, Song et al. [9] mentioned that the crossover of ethanol is affected by temperature as the kinetics of the molecules is dependent on temperature and hence affects DEFC performance. Furthermore, parameter operations for DEFCs can affect mass transport and its performance. Effects of different operating parameters, including the cell operating temperature, ethanol concentrations and mass flow rates on cell performance in alkaline DEFC have been systematically studied in the previous works by Li et al. [10e12]. For instance, Pereira et al. [13] studied the effect of 1e3 M ethanol on cell performance and found that maximum power density is achieved by a cell at 2 M and started to drop beyond 2 M. Similar tendency was obtained by Li et al. [10,11]. They were suggested that too high ethanol concentration lowering the cell performance can be attributed to two reasons: i) more active sites in the anode catalyst layer are covered by ethanol and this counts against the adsorption of hydroxyl ions, thus lowering the ethanol oxidation reaction (EOR) kinetics, and ii) the concentrated ethanol that reduces the level of hydroxyl ionization creates a barrier for the transfer of hydroxyl ions, resulting in an increase in cell resistance [10]. For the operating temperature, Alzate et al. and Li et al. found that the performance of DEFC increases as temperature increases from 60 C to 90 C [14] and 23 C to 80 C [10]. Li et al. suggested that an increase in the cell operating temperature would enhance and increase; i) the electrochemical kinetics of both the anodic EOR and cathodic ORR, resulting in the reduction in the activation loss; ii) the conductivity of the hydroxyl ions, thus lowering the ohmic loss; iii) both the ethanol and oxygen transport, and thus decreasing the concentration loss [10]. Meanwhile, increase in fuel flow rate increases mass transport to the catalyst layer and thus possibly causes fuel crossover [15]. Alzate et al. [14] also investigated the effect of ethanol flow rates of 0.6e5.0 mL min1 on an active DEFC system, and their results showed that the increase of flow rates from 1.0 mL min1 to 5.0 mL min1 has no considerable effect on cell performance. Moreover, Li et al.
found that the effect of the fuel solution flow rate, i.e., 0.3e3.0 mL min1, is rather small in the low current density region, but the cell performance improves slightly with increasing solution flow rate in the high current density region. This latter feature is attributed to the enhanced masstransfer of both ethanol and OH ions from the flow-field to the catalyst layer [11]. The operational modes of DEFC contribute to mass transport behavior and its performance. In general, the DEFC system can be categorized into active, semi passive, and passive systems according to fuel delivery and handling concepts. An active system requires an external power supply that feed fuel and air or oxygen to the cell. This system has a high cost and suitable to large-capacity fuel cell systems. In a passive system, fuel is injected into cell reservoir before the operation. Mass transfer occurs by natural capillary forces, diffusion, and convection without the use of an additional power supply [13]. In a semi passive system, a pump is used for the feeding of air or oxygen to the cathode side only. For such operational modes, Li et al. have been studied the cell performance in alkaline DEFC for active mode [11], semi passive mode [10], and passive mode for the DEFC's stack [12]. For the direct formic acid fuel cell (DFAFC), Ong et al. [16] investigated the effects of operational modes on DFAFC performance. They found that the semi passive DFAFC with oxygen at cathode obtained the highest maximum current density followed by the semi passive DFAFC with air flow at cathode. However, for the active DFAFC system, the performance exhibited only a small improvement. They concluded that the change in flow rate at the cathode has a greater effect on cell performance than that at the anode. Previous research focused on electro-catalyst development and membrane modification for DEFC application. Some researcher examined the effect of operating parameters on cell performance were also conducted, although they did not consider ethanol crossover to the cathode. Therefore, in this work, the influences of operation parameters, such as concentration, temperature, reactant flow rate, and operation mode on DEFC performance and fuel crossover were investigated. The relationship between the operating parameters of DEFC and the fuel crossover were discussed on the basis of DEFC power generation.
Materials and methods Membrane electrode assembly and single cell DEFC Single cell DEFC was used in all parametric studies. A commercial membrane electrode assembly (MEA; Fuel Cell Store, US) with 2.5 cm2 active area was used and. This MEA consisted of a microporous layer, a catalyst layer PteRu on anode, and a Pt on cathode with loading of 4 mg cm2 on both sides. It used Nafion 117 types for its electrolyte membrane. A pretreatment process was conducted for MEA activation, and the steps were nearly similar to that described by Rejal et al. [17], who used hydrogen (H2) gas and air. The MEA was conditioned initially within the test fixture at 70 C under a H2/ air fuel cell operating mode for 1 h, and cell potential was
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maintained at 0.6 V. The H2 flow rate was set to 600 mL min1 at the anode and that of air was set to 1200 mL min1 at cathode. Both streams were humidified at 70 C prior to entering the cell. The current, voltage and power were monitored along 1 h MEA activation to ensure it reached at a stable performance.
Measurement of DEFC performance We used and modified several parameter operations to investigate their effects on DEFC performance. For the ethanol concentration, the experiment was carried out in a semi passive mode, as shown in Fig. 1(a). A 0.5 M ethanol solution was fed to the anode reservoir of the cell, while the air was allowed to flow continuously at a flow rate of 200 mL min1 with an air mini pump. Different ethanol concentrations (0.5e3.0 M) were used at ambient temperature conditions. The investigation on the effect of operating temperature was carried out in a humidity chamber Model HCP108 Memmert (Memmert GmbH þ Co. KG, Germany). The cell was operated in a passive mode (as shown in the schematic diagram of Fig. 1(b)) in the range of 35 Ce85 C using 2.5 M ethanol concentration. Moreover, the relative humidity (RH) was fixed at RH 90% for all temperature conditions. For the active mode operation (shown in Fig. 1 (c)), ethanol fuel with 2.5 M in concentration was supplied at the anode by using a peristaltic pump, and the flow rate of air was fixed at 200 mL min1. The ethanol flow rate was manipulated within a range of 0.5e5.0 mL min1 at ambient temperatures. For the effect of oxidant flow rate, air was supplied to cathode at flow rate of 100e600 mL min1 by using 2.5 M ethanol in the reservoir. All these parameters were conducted at a constant voltage of 0.3 V, and the power generation during the 2 h operation was measured with Fuel Cell Monitor 3.0 (H-Tec Education GmbH, Germany).
a)
Measurement of ethanol crossover Ethanol crossover was identified by analyzing the CO2 produced at the cathode. As the ethanol at the anode side diffused through the membrane to the cathode, it was assumed to be oxidized and to have formed carbon dioxide on the basis of the report of James and Pickup [8]. During the two 2 h operation, the DEFC system was operated in either the semi passive or active mode. Air was allowed to flow at the cathode by using the air mini pump. The CO2 gas concentration was measured with a CO2 analyzer with Model 906 (Quantek Instruments Inc, US) in the form of percentage. The setup diagrams of the experiment are shown in Fig. 1(a) and (c). Fig. 1(d) shows the actual photo of the experiment setup. For the fuel crossover measurement, the evaporation of ethanol from the cathode was neglected, and all ethanol crossover to the cathode was assumed to be completely oxidized at the cathode catalyst and to have formed CO2 as show in Eq. (1). As the ethanol at the anode side diffused through the membrane to the cathode, and simultaneously reacts with oxygen at active site of catalyst cathode's catalyst layer. This assumption was referred to the previous study by Rejal et al. [17]. C2H5OH þ 3O2/2CO2 þ 3H2O.
(1)
It means, we assume that there are no intermediate reactions or partial oxidation reactions of ethanol involve as shown in Eq. (2)e(4) below [18] to produce intermediate products such as acetaldehyde (Eq. (2)), ethane-1,1-diol (Eq. (3)), or acetic acid (Eq. (4)). Even, if there are partial oxidation reactions of ethanol occur, the intermediate products are assumed to be extremely small and can be neglected. CH3CH2OH / CH3CHO þ 2Hþ þ 2e
(2)
b)
Monitor Data CollecƟon
Passive Ethanol SoluƟon
Anode Carbon Dioxide Analyzer
Air
Air Pump
Cathode
Passive Ethanol SoluƟon
Monitor (Data CollecƟon)
Anode
Fuel Cell Monitor
Monitor (Data CollecƟon)
Fuel Cell Monitor
Cathode
Air Breathing
Flow Meter
d)
c) Ethanol SoluƟon
Liquid Pump
Monitor (Data CollecƟon)
Monitor Data CollecƟon Anode Carbon Dioxide Analyzer
Air
Air Pump
Cathode
Flow Meter
Fuel Cell Monitor
Carbon dioxide analyzer Air mini pump
Electrochemical measurement unit CO2 outlet at anode
Single cell DEFC
1
Fig. 1 e Schematic diagram of experiment setup for the measurement on cell performance and ethanol flux; (a) semi-passive mode (b) passive mode (c) active mode, (d) actual photo for the semi-passive mode. Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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CH3CH2OH þ H2O / CH3CH(OH)2 þ 2Hþ þ 2e
(3)
CH3CH2OH þ H2O / CH3CHOOH þ 4Hþ þ 4e
(4)
Moreover, the amount of CO2 crossover from anode (product of ethanol oxidation at anode) to cathode through the MEA was assumed to be extremely small on the basis of the research of Han and Liu [19] and thus neglected. We greatly reduced the crossover of CO2 at the anode side was greatly reduced by the anode outlet design for the single cell of the DEFC to improve the exhaust of CO2 from the anode itself (Fig. 1(d)).
Results and discussion Effect of ethanol concentration The effect of ethanol concentration on DEFC performance was investigated with different ethanol concentrations (from 0.5 M to 3.0 M). The cell was operated in semi passive mode at an air flow rate of 200 mL min1. The voltage-current (VeI) and power-current (PeI) curves for the single-cell DEFC at different methanol concentrations is shows in Fig. 2. In Fig. 2, the opencircuit voltage (OCV) increases with increasing ethanol concentration and then decreases beyond the ethanol concentration of 2.0 M. The increase in OCV with ethanol concentration can be due to the increased rate of anode reactions. These phenomena are supported by Heysiattalab et al. [15], who stated that increment in ethanol concentration increases diffusion and ethanol concentration in the catalyst layer, thereby improving ethanol oxidation. Meanwhile, decrease in OCVs with increasing ethanol concentration
beyond 2.0 M may be due to the mixed potential caused by ethanol crossover. In general, the ethanol crossover may cause three technical problems which, i) ethanol may be oxidized to form the parasitic current, leading to a mixed potential on the cathode, so that the mixed-potential problem is insignificant in this fuel cell system; ii) effect on the water transport in the cell as ethanol will be directly oxidized to produce water at cathode, so that contributes the flooding phenomena at cathode [20,21]; iii) the ethanol crossover definitely results in a waste of fuel, decreasing the utilization efficiency [22]. We expected that the flux of ethanol crossover would increase with increasing ethanol concentration and then reduce the OCVs. Moreover, for the PeI curves in Fig. 2, the maximum power density increased with increasing ethanol concentration before it decreased at 3.0 M. The difference of the peak power density could be explained from three factors: i) thermodynamic voltage difference, ii) electrochemical kinetic loss, and iii) ohmic loss [23]. The maximum power density output for the single cell DEFC can reach 6.52 mW cm2 at 2.5 M ethanol concentration, whereas the limiting current was obtained at 46.7 mA cm2. At all ethanol concentrations, the optimum voltage at maximum power densities were obtained from 0.25 V to 0.30 V. Hence the 0.30 V was used as a constant voltage for 2 h single cell DEFC operation. Fig. 3 shows the profile of DEFC performance during 2 h operation at different ethanol concentrations and 0.30 V. At all concentrations, power density initially increases sharply and then slowly decreases before stabilizing with time. The high power densities were achieved at the first 1 min possibly because of the initial accumulation of ethanol at high concentration at anode surface before it decreased with time. These tendencies were similar to that observed in a previous
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Current density [mA cm-2] Fig. 2 e VeI and PeI curves at different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature). Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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Fig. 3 e Power density profile at different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature; constant voltage: 0.3 V).
study by Abdelkareem et al. [24]. It could be happen due to free-impurities, that is, products or intermediate products, such as CO2 and acetic acid, on the active site of the catalyst at the anode. Thus, electro-oxidation occurred in a good condition without any barrier between ethanol and anode surface. Then, the power densities slowly decreased with time as the ethanol fuel was consumed for the electro-oxidation reaction at the anode. At 2.5 M ethanol concentration (Fig. 3), the power density dropped more rapidly than those in the other ethanol concentrations. A previous study reported that active sites for electro-oxidation can be affected by acetic acid as an intermediate product from intermediate ethanol reactions and the anode catalyst Pt/Ru has a high tendency to produce acetic acid from ethanol oxidation [25]. The by-product of acetic acid is the factor that contributes to anode poisoning on a Pt-based catalyst. In Fig. 3, a high degradation rate was observed at a high power density, in which a high oxidation rate would have occurred and an increased amount of intermediate products could have been produced from the anode reactions. The effect of these by-products, especially acetic acid, may have resulted in catalyst poisoning in the anode's electrode. Therefore, the high degradation rate at 2.5 M ethanol concentration at a high power density was probably caused by the by-products attached on the active site of the catalyst and thus may have reduced cell performance. Furthermore, based on previous works by Li et. Al [11] and An et al. [26], a high ethanol concentration would blocks the mass transfer of hydroxyl ions, thus giving rise to an increase in the internal resistance. However, in this study, we do not measure the cell resistance, i.e., ohmic resistance. We assumed that the increment of ohmic resistance or variation with ethanol concentration was very small and could be neglected; since we only used diluted ethanol concentration from 0.5 M to 3.0 M. Our assumption was confirmed by Li et al., which they obtained a small increment of cell resistance for
operation using below 10 M ethanol concentration while a significant increase at above 10 M up to 17 M ethanol concentrations [11]. Moreover, Li et al. was clarified that for the case for the case of increasing ethanol concentration from 1.0 to 3.0 M, the decrease in activation and mass transport losses exceeds the increase in ohmic loss [10]. Ethanol crossover was investigated by measuring CO2 produced at the cathode outlet. Fig. 4 shows the percentage of CO2 profiles at the cathode outlet during the 2 h operation at different ethanol concentrations. In Fig. 4, the profiles of CO2 concentration fluctuated between 0.02% and 0.12%, and the magnitude of CO2 concentration varied according to ethanol concentration. The CO2 percentage in air recorded by the CO2 analyzer were approximately 0.04% ± 0.02% higher from its theoretical value, which is 0.04%. Moreover, the magnitude of CO2 concentration slightly increased with the increase of ethanol concentration at the anode (Fig. 4). Therefore, a low amount of ethanol fuel was permeated from anode to the cathode through the electrolyte membrane. The relationship among ethanol concentration, power density, and ethanol crossover at stable conditions (data at 90 min in Fig. 3) is illustrated in Fig. 5. For the ethanol flux, the average CO2 concentration at the cathode outlet was used for the calculation of the ethanol flux, and the power densities increased from 0.5 M to 2.5 M ethanol concentrations. Under stable operations (90 min), the highest power density (2.49 mW cm2) was obtained at the 2.5 M ethanol concentration. Further increase in ethanol concentration from 2.5 M to 3 M may decrease the power density of the DEFC. According to Alzate et al. [14], a DEFC cell will experiences difficulty in mass transfer at low ethanol concentration. With the increase of ethanol concentration from 0.5 M to 2.5 M, the mass transfer of ethanol molecules to the active sites of the catalyst can be improved, as shown in Fig. 5. The increase of ethanol concentration favored the production route of acetic acid, which yields four electrons per ethanol molecule in contrast to acetaldehyde production, which yields only two electrons
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Fig. 4 e CO2 concentration profile at the cathode outlet at different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature; constant voltage: 0.3 V).
Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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Fig. 5 e Relationship between DEFC performance and ethanol crossover at 90 min stable operation using different ethanol concentrations in semi-passive DEFC (air flow rate: 200 mL min¡1; ambient temperature; constant voltage: 0.3 V).
[27]. This result suggested that the high-power density is the results of increased ethanol concentration. However, at high fuel concentrations (above the optimum level), the DEFC's performance was hindered by the fuel crossover and decreased in anode catalytic activity. These tendencies were similar to that observed in previous studies on DLFCs, such as DEFC [13], DMFC [27], and DFAFC [17]. In Fig. 5, the magnitude of ethanol crossover slightly increased with increasing of ethanol concentration at the anode. The magnitudes of fuel crossover were obtained at 3.71 104 and 8.79 3.71 104 g m2 s1 for 0.5 M and 3.0 M ethanol concentrations, respectively. However, these magnitudes were 30 times lower than the methanol crossover for DMFC and 10 times lower than formic acid crossover for DFAFC. The used of the thick electrolyte membrane of Nafion 117 instead of that of NRE 212 may be one of the reason for the low ethanol crossover obtained in this study. 10
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Fig. 6 shows the effect of operating temperature on DEFC performance at 2.5 M ethanol concentration and passive mode operation. The cell performance was studied within the range of ambient temperature ~28e85 C. The passive DEFC was operated at a constant voltage of 0.3 V inside a humidity chamber for the control of cell temperature and relative humidity. In Fig. 6, the power density significantly increased and was nearly directly proportional to the operating temperature. A low power density (1.65 mW cm2) was obtained for the operation at ambient condition possibly because of sluggish and low kinetic reactions at the anode. The increase of power density with increase the temperature could be due to; i) higher temperature lowers the activation loss of the redox reactions, ii) higher temperature decreases the cell resistance, and iii) higher temperature enhances the reactant transport [28]. Moreover, by increasing the operation temperature, the electrode kinetics and mass transfer properties, such as the ethanol diffusivity, can be accelerated [14,29], and the kinetics of ethanol oxidation at the anode and oxygen reduction at the cathode can be accelerated by using the Arrhenius equation, which shows the temperature dependence of the reaction rates, as shown in Eq. (5). The increment of temperature reduces the activation energy required for ethanol electrooxidation and oxygen reduction kinetic reactions and is related with molecular movement. Ea
k ¼ Ae RT :
(5)
High temperature improves the conductivity of membrane as the heat energy causes the Nafion polymer backbone to relax and expand and partly increases the transport rates of protons and electrons [9]. In other word, the increment of temperature will increase the ethanol crossover which is proton transport rates. Consequently, the increment of ethanol crossover could lead to a more negative effect on the OCV and DEFC performance (electron transport rates). The diffusivity of ethanol molecules is thereby enhanced owing to high kinetic energy, and this enhancement leads to the ethanol crossover to the cathode. Furthermore, increasing temperature creates a competitive effect between the reaction kinetic [30] and ethanol crossover. After reaching certain levels, these two effects balance each other out and therefore maintain the power density at a steady state. In this study, the highest power density was achieved at 8.70 mW cm2 for 85 C operation under stable operation. The power density was expected to increase at temperatures above 85 C, as a reported by Alzate et al. [14]. Therefore, the kinetic rate at the anode is more important and significant compared with ethanol crossover at temperature below 100 C.
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We used the different flow rates of the ethanol solution (0.5e5.0 mL min1) to investigate their effects on DEFC performance. The test was carried out in the active DEFC system at a constant voltage of 0.3 V and ethanol concentration of 2.5 M. Fig. 7 shows the relationship between the power density and ethanol crossover at different ethanol flow rates. In Fig. 7,
Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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Fig. 7 e The relationship between DEFC's performance and ethanol crossover at 90 min stable operation using different ethanol flow rate in active mode DEFC (air flow rate: 200 mL min¡1; ethanol concentration: 2.5 M; ambient temperature; constant voltage: 0.3 V).
a significant increase in power density can be seen between the ethanol flow rate of 0.5 and 1.0 mL min1. The performance of the active DEFC slightly increased with increasing ethanol flow rate. This result confirmed that the effect of surface phenomenon is significant at low flow rates because of convention mechanism. Low ethanol flow rate can affect the removal of reaction products, such as CO2 and acetic acid, that block the active sites of reactions. Therefore, the utilization of available catalyst sites becomes low [14]. At increased anode flow rate, the mass transfer to the catalyst layer was improved, and thus DEFC cell performance increased. The highest power density of 2.59 mW cm2 was obtained at 4 mL min1 under a stable operation. In order to improve the mass transfer of ethanol, Li et al. have developed an innovative cell design with a metal foam-based all-in-one electrode that incorporates the flow field, backing layer, micro-porous layer, and catalyst layer into a whole. By using this cell, the liquid pump power can be saved when reducing the anode flow rate from 1.0 to 0.2 mL min1; with the cell performance of the all-in-one MEA still remains unchanged [31]. At 5 mL min1 ethanol flow rate, DEFC performance decreased significantly to about 1.18 mW cm2. The probable cause is the imbalance between stoichiometric reaction and high ethanol crossover. According to the overall electrochemical equation of DEFC, one molecule of ethanol needs to react with three molecules of oxygen so that ethanol achieves complete oxidation. However, further increase in ethanol flow rate resulted in an extremely high ethanol content in the cell reservoir, while oxygen molecules available for reactions with ethanol were insufficient. It contributes to the unbalance reaction within the DEFC system. A high fuel flow rate might also have a cooling effect to the fuel cell [32]. This would reduce the temperature on the MEA surface lower than the operating temperature and affect DEFC performance. For the mass transport of ethanol, CO2 concentration at the cathode outlet increased with increasing ethanol flow rate at anode, as shown in the increase of ethanol crossover
magnitude (as shown Fig. 7). The highest ethanol crossover was obtained at 12.53 104 g m2 s1 at 5 mL min1 operation and was higher compared with that observed when 3.0 M ethanol solution was used. Convection and diffusion mechanisms occurred at the active anode system. By contrast, only the diffusion mechanism occurred at the passive anode system, and hence increases the ethanol mass transfer rate and directly increases the magnitude of ethanol flux.
Effect of air flow rate Fig. 8 shows the effect of air flow rate on DEFC performance and the fuel crossover at the range of 100 and 600 mL min1. The power density slightly increased from 100 mL min1 to 200 mL min1. The highest power density (1.81 mW cm2) was achieved at 200 mL min1 under stable operation. A sufficient air convection was created by the mini air pump and hence improved air diffusion into the catalyst layer at a 200 mL min1 air flow rate. Moreover, the water removal rate was increased owing to the increase in air flow rate and then prevented water flooding at the cathode [17]. Nevertheless, higher air flow rate affects long-term DEFC operation. In Fig. 8, the power density slightly decreased at air flow rates greater than 200 mL min1. Abdelkareem and Nakagawa [33] and Rejal et al. [17], who studied DMFC and DFAFC, respectively, reported that a high air flow rate may results in electrolyte membrane dehydration because of the high forced convection at the cathode. Moreover, an extensive study on water transport in electrolyte was implemented by Li et al. They measured the water uptake and electro-osmotic drag coefficient and concluded that the water transport through the electrolyte membrane is caused by both the diffusion due to the concentration gradient and the electronosmotic drag due to the ion transport [20]. The proton transport from the anode to the cathode was slowed down and subsequently decreased the power density [34]. Therefore, the reason of low DEFC performance at a high air flow rate could
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Fig. 8 e The relationship between DEFC's performance and ethanol crossover at 90 min stable operation using different air flow rate at cathode in semi-passive DEFC (ethanol concentration: 2.5 M; ambient temperature; constant voltage: 0.3 V).
Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121
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be explained by the same consideration, as the DEFC system was nearly similar to the DMFC and DFAFC system. Meanwhile, ethanol fluxes are nearly constant at different air flow rates (Fig. 8). By comparing the power density obtained at different air flow rate in Fig. 8, the fuel crossover increase at low current density and high air flow rates. However, the flux of ethanol obtained at a high oxidant flow rate was almost similar to that at low air flow rates. It was suggested at high flow rates, the driving force of mass transfer by forced convection at the cathode is larger than the free convection at the anode (molecular transport). According to Ong et al., the pressure gradient builds up between the anode and cathode, and slightly high pressure can be obtained at the cathode. Thus, the mass transfer of formic acid from anode to cathode can be minimized by using the pressure gradient, and the fuel crossover can become limited and nearly constant because of the pressure at the cathode [16], as shown in Fig. 8. Therefore, the oxidant flow rates at the cathode have a less significant impact on the DEFC performance and fuel crossover than the ethanol concentration and flow rate at the anode.
Comparison of DEFC performance with different operational modes Fig. 9 shows the power density profile for different operational modes of DEFC with air and oxygen at the cathode at 2.5 M ethanol at ambient room temperature. From Fig. 9, the active DEFC system with oxygen at the cathode exhibited the highest performance and had a maximum power density of up to 3.42 mW cm1 at a stable condition. The continuous feeding of ethanol at 4 mL min1 and oxygen at 200 mL min1 improved the mass transfer and diffusion rate of ethanol and air at the anode and cathode, thus enhancing electrochemical reactions. The active mode with oxygen at the cathode achieved higher power density than that with air flow. This difference
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may be related to the composition of oxygen in air, which is only 21%. Oxygen content for the ORR at the cathode when pure oxygen was used was higher than that detected when air was directly used. This performance trend was similar to that observed in a semi passive system with oxygen and air at its cathode. The power density for semi passive operation with oxygen higher compared to that semi passive using air at almost 10%. A less desired performance was observed at passive mode DEFC. At a passive mode operation, water accumulation or flooding occurred may have occurred in the cathode without the air convection, that is, air supplied by the mini air pump. Thus, the pathway of oxygen into the cathode's active catalyst was blocked. Then, the mass transfer of oxygen to the catalyst layer and the kinetic of cathode reaction decreased without air or oxygen feeding [35]. Therefore, the power density obtained for the passive mode operation was lower than those obtained for the active and semi passive systems.
Conclusion A parametric study was carried out in DEFC to determine the effect of operating conditions on its performance and ethanol crossover. At all cases, the operating parameters and operational modes affected the DEFC performance and mass transport of fuel, that is, ethanol crossover, and DEFC performance increased with the increase of ethanol concentration. Meanwhile, ethanol and oxidant flow rates increased with temperature until DEFC reaches optimum conditions. DEFC performance significantly and proportionally increased with operation temperature, reaching up to 8.70 mW cm2 at 85 C at stable conditions. The flux increased in proportion to ethanol concentration and ethanol flow rate, although it remained constant at varying oxidant flow rates. These results indicated that the manipulation of both ethanol and oxidant flow rate to optimum levels may enhance mass diffusion in the anode and cathode and thus may improve the mass transport of fuel and DEFC performance.
Acknowledgments A part of this study was supported by Universiti Kebangsaan Malaysia (UKM) from grants GUP-2016-043 and DIP-2017-020.
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Fig. 9 e Power density profile at different operation modes of DEFC: passive, semi passive with air and oxygen; oxidant flow rate: 400 mL min¡1, and active mode with air and oxygen at cathode; ethanol flow rate: 4 mL min¡1 and oxidant flow rate: 400 mL min¡1, (ethanol concentration: 2.5 M; constant voltage: 0.3 V; ambient temperature).
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Please cite this article in press as: Azam AMIN, et al., Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.121