- Email: [email protected]
Response of a direct methanol fuel cell to fuel change
international journal of hydrogen energy 35 (2010) 11642–11648
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Response of a direct methanol fuel cell to fuel change T.J. Leo a,*, M.A. Raso b, E. Navarro c, E. Sa´nchez de la Blanca b, M. Villanueva c, B. Moreno d a
Dpto de Sistemas Ocea´nicos y Navales- ETSI Navales, Univ. Polite´cnica de Madrid, Avda Arco de la Victoria s/n, 28040 Madrid, Spain Dpto de Quı´mica Fı´sica I- Fac. CC. Quı´micas, Univ. Complutense de Madrid, Avda Complutense s/n, 28040 Madrid, Spain c Dpto de Motopropulsio´n y Termofluidodina´mica, ETSI Aerona´uticos, Univ. Polite´cnica de Madrid, Pza Cardenal Cisneros 3, 28040 Madrid, Spain d Instituto de Cera´mica y Vidrio, Consejo Superior de Investigaciones Cientı´ficas, C/Kelsen 5, Campus de la UAM, 28049 Cantoblanco, Madrid, Spain b
article info
abstract
Article history:
Methanol and ethanol have recently received much attention as liquid fuels particularly as
Received 18 December 2009
alternative ‘energy-vectors’ for the future. In this sense, to find a direct alcohol fuel cell that
Received in revised form
able to interchange the fuel without losing performances in an appreciable way would
8 February 2010
represent an evident advantage in the field of portable applications. In this work, the
Accepted 22 February 2010
response of a in-house direct methanol fuel cell (DMFC) to the change of fuel from
Available online 23 March 2010
methanol to ethanol and its behaviour at different ambient temperature values have been investigated. A corrosion study on materials suitable to fabricate the bipolar plates has
Keywords:
been carried out and either 316- or 2205-duplex stainless steels have proved to be adequate
DMFC
for using in direct alcohol fuel cells. Polarization curves have been measured at different
Performance
ambient temperature values, controlled by an experimental setup devised for this purpose.
Ethanol
Data have been fitted to a model taking into account the temperature effect. For both fuels,
Alcohol change
methanol and ethanol, a linear dependence of adjustable parameters with temperature is
Temperature effect
obtained. Fuel cell performance comparison in terms of open circuit voltage, kinetic and
Bipolar plate
resistance is established. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Within the last years there has been an increased interest in direct liquid fuel cells as potential power sources for portable devices, in this sense direct methanol (MeOH) fuel cells (DMFCs) have been identified as one of the most promising candidates to replace batteries in micropower applications. DMFCs have been attracting the majority of research and development efforts, because methanol shows both good electrochemical activity and a high energy density compared with other liquid fuel candidates. There are still two major unresolved drawbacks with the DMFCs: a relatively lower anode electrocatalytic activity and methanol crossover. Many
research efforts are devoted to find new membranes and anode catalysts [1–5]. In parallel with these research efforts the development of direct ethanol (EtOH) fuel cells (DEFCs) is drawing increasing attention due to the attractive advantages of EtOH as fuel: non-toxicity, natural availability, easy storage and higher power density among others. But similar problems to those appearing in DMFCs arise [6–11]. Nevertheless, MeOH and EtOH are considered valid as liquid reactants for anodes in fuel cells [12,13]. Therefore, to find a direct liquid fuel cell able to interchange the fuel, as MeOH and EtOH, without losing performances in an appreciable way would represent an evident advantage in the field of fuel cells. Some partial studies have been performed in this sense [14,15].
* Corresponding author. Tel.: þ34 913367147; fax: þ34 913363927. E-mail address: [email protected] (T.J. Leo). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.115
international journal of hydrogen energy 35 (2010) 11642–11648
2.
11643
Experimental
The single fuel cell consists of a Membrane Electrode Assembly (MEA) and two metallic bipolar plates including the reactants distribution channels. The cell design requirements revolved around two main topics: bipolar plates and MEA.
2.1.
Fig. 1 – Bipolar plates and MEA fabricated and used in this work.
The aim of this work is to investigate the response of a PEM direct liquid fuel cell to the change of fuel and its behaviour at different ambient temperature values. In order to achieve this objective, a DMFC single cell was assembled and an experimental setup allowing measurement and control of the ambient temperature has been designed and fabricated. After activation and normal operation with MeOH, the input fuel has been changed to EtOH. Performance comparison in terms of open circuit voltage, kinetic and internal resistance is established. Selection of the material used to fabricate the bipolar plates has been done after a corrosion study carried out with 6082-aluminium and three stainless steels (304SS, 316SS and 2205 duplex).
Bipolar plates
A study on materials suitable to fabricate the bipolar plates has been carried out in order to choose a cheap, durable one [16]. Three usual stainless steels (304SS, 316SS and 2205 duplex) and 6082-aluminium have been tested to check their durability in different media: Distilled water, MilliQ grade water and 1 M aqueous MeOH solution prepared in Distilled water and MilliQ, respectively. It is known that steel suffers corrosion when kept in water for a long period of time, but it is not clear if the presence of MeOH and the carbon cloth used in the electrodes has any influence on that phenomenon [17]. Two different carbon cloth types were used in the corrosion studies: B1 ASWP Standard Wet Proofing (without carbon coating) and E-TEK LT1400-W Low temperature ELAT GDL Microporous on Woven Web (with carbon coating).
2.2.
Electrochemical measurements
A Solartron 1250 frequency response analyser and a PAR 273A potentiostate–galvanostate have been used for electrochemical experiments. The open circuit voltage (OCV) of threeelectrode assemblies formed with one of the metals as working electrode, a saturated calomel electrode (SCE) as reference and
Fig. 2 – Schematic representation of the experimental setup.
11644
international journal of hydrogen energy 35 (2010) 11642–11648
Table 1 – Experimental conditions of measurements. Parameter
Value
Alcohol (methanol/ethanol) concentration (mol/L) Alcohol flow rate (mL/min) Oxygen flow rate (mL/min) Oxygen pressure (bar) Temperature ( C)
1.0 3.0 250 1.0 25–75
platinum or a carbon cloth as counter-electrode, has been recorded versus time.
2.3.
Chemicals
MeOH was supplied by Panreac (99.9% purity), anhydrous EtOH by Scharlaw (99.9% purity). Distilled and MilliQ grade water have been used to prepare solutions.
2.4.
Electrodes
The electrodes, with an active area of 5 cm2, have been prepared following the procedure described by Jurado et al. [18]. The catalytic layer is aerographied onto ELAT GDL microporous layer on woven web. The total anode Pt loading was 1.75 mg/cm2 and was made using BASF 30% Pt:Ru(1:1)/ Vulcan XC-72. The total cathode Pt loading was 2.33 mg/cm2 and was prepared using BASF 40% Pt/Vulcan XC-72.
2.5.
Flow field design
The bipolar plates, which are used as current collectors and provide conduits for reactants flow, have been designed with parallel flow channels on one side. The other side of the planar metallic plates has been machined with the inlets and outlets of reactants and products (see Fig. 1).
2.6.
Membrane electrode assembly
Nafion 117 has been used as the proton conducting membrane in the MEA fabrication. Before the assembly, the membrane was protonated following this pre-treatment procedure: boiling 1h in 3 wt.% hydrogen peroxide at 80 C;
Fig. 3 – Open circuit voltage measurements of a 304stainless steel cathode against standard calomel electrode, using carbon cloth as counter-electrode.
rinsing in distilled water; boiling 1 h in distilled water at 80 C; boiling 1 h in 0.5 M sulphuric acid at 80 C; rinsing in distilled water and boiling 1 h in distilled water at 80 C. The MEA (Fig. 1) is prepared sandwiching the membrane between the two electrodes and keeping the ensemble under hot pressing at 4 MPa and 135 C for about 3 min. Homogeneity of the ensemble is checked by means of a procedure in which the electrical conductivity (s) of the MEA in different locations is compared, as described in [19]. In this case, logarithm of s did not vary more than 1% across the electrode surface. Electrochemical activation of the assembly was conducted at 70 C without the use of hydrogen following the procedure described by Reshetenko [20], optimized for this fuel cell and the equipment used. The cell was run with 1.5 M aqueous MeOH or EtOH solution with a flow rate of 6 mL/min over 2 h. After this, the polarization curve was registered and the process was repeated until no variation was observed. Additionally, before any tests, the cell was preconditioned following the procedure described by Eccarius [2], i.e., the cell was run under a constant load (0.150 mV) and operation conditions over 15 min.
2.7.
Experimental setup for polarization measurements
It is known that the Fuel Cell performance is affected in a significant way by the operation temperature. To register the polarization curves, i.e. voltage (V) versus current density ( j ), an experimental setup (a sketch is shown in Fig. 2) which allows modifying and controlling the temperature of the testing enclosure where the fuel cell is placed, has been devised and fabricated. For this purpose, a cross-flow fin-tube heat exchanger is placed inside. A fan is coupled to the heat exchanger to impel the air which is introduced into the testing enclosure from a bigger one containing it. Variation of the air mass flow is performed by changing the fan rotational speed. To modify the mass flow and the temperature of the liquid circulating through the heat exchanger, a Julabo F-33 ME refrigerated and heating circulator is used. The actual cell temperature is measured by T-type thermocouples connected to a Yokogawa MV100 data logger. A Jasco PU-2086 Plus HPLC pump is used to feed the fuel aqueous solution and Bronkhorst flow (model F-201) and pressure (model P-702) meter are used on the oxygen side. During the measurements, fuel and oxidant flow rates, oxidant pressure and fuel cell temperature were controlled. The experimental conditions used are shown in Table 1.
3.
Results and discussion
3.1.
A study for the choice of the material plates
To check the influence of water purity in the corrosion process, 304-stainless steel behaviour is tested when kept in four different media: Distilled water, MilliQ grade water, 1 M MeOH/ distilled water solution and 1 M MeOH in MilliQ grade water solution. Results are shown in Fig. 3. From the evolution of the OCV with time measurements it was concluded that the steel corrosion in 1 M MeOH/MilliQ water solution is the slowest one,
international journal of hydrogen energy 35 (2010) 11642–11648
11645
Fig. 4 – Assemblies to study corrosion of a) metal versus Pt and b) metal versus carbon cloth. 316SS samples are shown.
Fig. 5 – Metals after the corrosion study versus carbon cloth.
but the four steel samples reach the same degree of corrosion and approximately at the same OCV in the long term. Therefore, distilled water has been used in further experiments. The behaviour of the four metals in 1 M MeOH/MilliQ grade water solution has been studied by using a three-electrode configuration. Platinum, carbon cloth without carbon coating C(1) and carbon cloth with carbon coating C(2) have been used as counter-electrode in each case. Fig. 4 shows the arrangement of the working electrode. In Fig. 4a, a working electrode used against platinum can be seen whereas Fig. 4b shows the assembly of a working electrode with the carbon counter-electrodes.
After the experiments, clear differences among the surfaces of these metals can be seen, Fig. 5. Both aluminium and 304-stainless steel reach a big OCV value in few hours, Fig. 6a, as an evidence of their high reactivity in that medium. On the other hand, 316 and duplex stainless steel samples reach an OCV near 0 V, as a proof of their surface passivation. Fig. 6b shows that duplex steel samples are somewhat nearer to 0 V than 316 steel ones. These behaviours remain unchanged for at least 3410 h. Therefore, either 316SS or 2205-duplex SS can be adequate. As duplex steel is more expensive than 316SS, bipolar plates have been machined using this last material.
Fig. 6 – Open circuit voltage measurements of a metal cathode against standard calomel electrode, using Pt, carbon cloth C(1) or carbon cloth C(2) as counter-electrode.(a) aluminium and 304-stainless steel cathodes; (b) duplex and 316-stainless steel cathodes.
11646
international journal of hydrogen energy 35 (2010) 11642–11648
Fig. 7 – Polarization (a) and power density (b) curves for the cell using 1 M MeOH as fuel.
Fig. 8 – Polarization (a) and power density (b) curves for the cell using 1 M EtOH as fuel.
3.2.
Polarization measurements
3.4.
Empirical model
Polarization curves have been registered at various ambient cell temperatures. Aqueous MeOH and EtOH solutions have been used as fuel, being pure oxygen the oxidant.
The experimental results shown in Figs. 7 and 8 have been used to fit the polarization curves to an empirical model and the effect of temperature on the model parameters has been derived. The model equation used is [21,22]:
3.3.
V ¼ E Alnj Ri j
Fuel-change study
The response of the fuel cell to the alcohol change has been studied in the temperature range from 25 to 75 C. Polarization and power curves for MeOH are shown in Fig. 7. EtOH measurements results can be found in Fig. 8. As expected, fuel cell response increases in both cases with temperature, although it is observed a decrease in the performance of the fuel cell when MeOH is changed by EtOH. For the later, the open circuit voltage diminishes about a 7% indicating a slower kinetic in the electrochemical reaction for EtOH. The current density values attained with MeOH are clearly higher than those attained with EtOH. The slopes of polarization curves diminish with increasing temperature in both cases. At current density values above 10 mA/cm2, the slopes of the V–j curves are higher when the fuel used is EtOH. Power curves show that the maximum of power density achieved with MeOH is much higher than that attained with EtOH at the same temperature.
(1)
where activation overvoltage at both electrodes has been taken into account [23]. Then E ¼ ENernst þ Alnb, A ¼ Aa þ Ac Aa =A and b ¼ j0a jA0cc =A with Aa ¼ RT=ðna aa FÞ, Ac ¼ RT=ðnc ac FÞ. The anodic and cathodic charge transfer coefficients are aa and ac , respectively; j0a and j0c stand for the anodic and cathodic exchange current density, respectively, and Ri is the cell resistance. A nonlinear fitting according to equation (1) is performed for each polarization curve. The three adjustable parameters
Table 2 – Linear dependence of fitting parameters with temperature. Parameter
E ðVÞ AðVÞ Ri ðU cm2 Þ
CH3OH
C2H5OH
0.18 þ 4.0 10 t ( C) 0.31–2.72 10L3t ( C) 8.1–0.071 t ( C) L3
0.11 þ 1.6 10L3t ( C) 0.35–1.0 10L3 t ( C) 15.8–0.165 t ( C)
international journal of hydrogen energy 35 (2010) 11642–11648
11647
Fig. 10 – Comparison between polarization curves of 1 M MeOH at 25 8C and 1 M EtOH at 74 8C.
Fig. 9 – Fitted parameters (a) E*, (b) A and (c) Ri as a function of temperature.
show linear temperature dependence with both fuels, MeOH and EtOH (Table 2). MeOH and EtOH parameter’s behaviour against temperature is compared in Fig. 9. At 25 C, E is higher when the fuel is MeOH (EMeOH ¼ 0:280 V, EEtOH ¼ 0:150 V), as occurs with the E values. The activation overpotential parameter A is higher for EtOH than for MeOH (AMeOH ¼ 0.242 V, AEtOH ¼ 0.325 V), as expected, because the electrochemical oxidation kinetics is slower for EtOH than for MeOH. Also, at this temperature the Ri value is higher for EtOH (Ri,MeOH ¼ 6.33 U cm2, Ri,EtOH ¼ 11.78 U cm2). As reported by Bagotski [12] the side-reactions products of the electrooxidation of EtOH, acetaldehyde or even acetic acid, will originate not only a drop in energy but a number of problems of removal and disposal of these products. Then, the ionic transport across the membrane when the fuel cell is operated with EtOH may be hindered by the presence of these species and the resistance term would be affected.
When temperature increases, the difference between the independent terms E* grows up (Fig. 9a), and a similar effect can be observed for the activation overpotential parameter A (Fig. 9b). Nevertheless, the difference between the ohmic loss parameters Ri diminishes in an appreciable way. At the higher working temperature (74 C) these parameter values for MeOH and EtOH get very close (Fig. 9c). As the working temperature increases the ethanol electrooxidation is favoured, thus the side products will decrease and will be also more easily eliminated. This fact agrees with the statement that when working at elevated temperatures EtOH fuel cells may yield electrical performance indicators comparable to those of MeOH fuel cells [12]. Thus, as at elevated working temperature values the resistance term becomes quite similar, an efficient fuel change from MeOH to EtOH would be achieved by reducing activation losses. Emphasizing that the studied cell has been prepared to use methanol as fuel, the great similarity observed between polarization curves for MeOH at 25 C and EtOH at 74 C in Fig. 10 shows that fuel change doesn’t seem to be an unattainable target.
4.
Conclusions
The response of a DMFC to the change of fuel from methanol to ethanol has been investigated. Also, the behaviour of the fuel cell fed with methanol and with ethanol at different ambient temperature values has been studied. A single fuel cell and an experimental setup allowing the measurement and control of the ambient temperature have been designed and fabricated. After a corrosion study, 316SS and 2205 duplex stainless steels have proved to be appropriate materials for direct alcohol fuel cell bipolar plates. The polarization curves measurements show that the fuel cell performance decreases when methanol is changed by ethanol at every temperature studied. So, the fuel change from methanol to ethanol requires higher working temperatures, if the same performance is required.
11648
international journal of hydrogen energy 35 (2010) 11642–11648
Polarization curves measured at different temperatures have been fitted to a model which incorporates the activation overpotential and the ohmic losses. A linear dependence with temperature has been found for all the model parameters. Fitting results indicate that activation losses should be reduced to allow an efficient fuel change in a direct liquid fuel cell from methanol to ethanol. Further improvements on catalytic anode layer would lead to achieve this purpose. Those improvements would probably raise the ionic conductivity decreasing the amount of products of the ethanol electrooxidation side-reactions.
Acknowledgements This work has been financially supported by the Spanish Ministerio de Ciencia y Tecnologı´a in the frame of the Project Code N ENE2007-67584-C03-03ALT. The authors gratefully acknowledge Dr. E. Mora and Dr. F. Molleda for their help.
references
[1] Caracino P, Ballabio O, Colombo M, Sala E, Cavallo G. Polymeric fluorine-free electrolyte for application in DMFC. Int J Hydrogen Energy 2009;34:4653–60. [2] Eccarius S, Garcia BL, Hebling C, Weidner JW. Experimental validation of a methanol crossover model in DMFC applications. J Power Sources 2008;179:723–33. [3] Liu H, Song C, Zhang L, Zhang J, Wang H, Wilkinson DP. A review of anode catalysis in the direct methanol fuel cell. J Power Sources 2006;155:95–110. [4] Martin JJ, Qian W, Wang H, Neburchilov V, Zhang J, Wilkinson DP, et al. Design and testing of a passive planar three-cell DMFC. J Power Sources 2007;164:287–92. [5] Suo C, Liu X, Tang X, Zhang Y, Zhang B, Zhang P. A novel MEA architecture for improving the performance of a DMFC. Electrochem Commun 2008;10:1606–9. [6] Colmati F, Antolini E, Gonzalez ER. Effect of temperature on the mechanism of ethanol oxidation on carbon supported Pt, PtRu and Pt3Sn electrocatalysis. J Power Sources 2006;157:98–103. [7] 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.
[8] Lopes T, Antolini E, Gonzalez ER. Carbon supported Pt–Pd alloy as an ethanol tolerant oxygen reduction electrocatalyst for direct ethanol fuel cells. Int J Hydrogen Energy 2008;33: 5563–70. [9] 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. [10] Song SQ, Zhou WJ, Zhou ZH, Jiang LH, Sun GQ, Xin Q, et al. Direct ethanol fuel cells: the case of platinum based anodes. Int J Hydrogen Energy 2005;30:995–1001. [11] Wan CH, Chen CL. Mitigating ethanol crossover in DEFC: a composite anode with a thin layer of Pt50–Sn50 nanoparticles directly deposited into Nafion membrane surface. Int J Hydrogen Energy 2009;34:9515–22. [12] Bagotsky VS. Fuel cells, problems and solutions. Pennington: J.Wiley & Sons; 2009. p. 73–91. [13] Wang ZH, Li J, Dong X, Wang D, Chen T, Qiao H, et al. Ethanol oxidation on a nichrome-supported spherical platinum microparticle electrocatalyst prepared by electrodeposition. Int J Hydrogen Energy 2008;33:6143–9. [14] Cimenti M, Hill JM. Thermodynamic analysis of solid oxide fuel cells operated with methanol and ethanol under direct utilization, steam reforming, dry reforming or partial oxidation conditions. J Power Sources 2009;186:377–84. [15] Xu C, Su Y, Tan L, Liu Z, Zhang J, Chen S, et al. Electrodeposited PtCo and PtMn electrocatalysts for methanol and ethanol electrooxidation of direct alcohol fuel cells. Electrochim Acta 2009;54:6322–6. [16] Tawfik H, Hung Y, Mahajan D. Metal bipolar plates for PEM fuel cell – a review. J Power Sources 2007;163:755–67. [17] Li MC, Zeng CL, Luo SZ, Shen JN, Lin HC, Cao CN. Electrochemical corrosion characteristics of type 316 stainless steel in simulated anode environment for PEMFC. Electrochim Acta 2003;48:1735–41. [18] Jurado JR, Chinarro E, Colomer MT. Pat No. ES 2 209 657; 2005. [19] Chinarro E. Ph. D. thesis, Universidad Auto´noma de Madrid; 2003. [20] Reshetenko TV, Kim H, Lee H, Jang M, Kweon H. Performance of a direct methanol fuel cell (DMFC) at low temperature: cathode optimization. J Power Sources 2006;160:925–32. [21] Hirano S, Kim J, Srinivasan S. High performance proton exchange membrane fuel cells with sputter-deposited Pt layer electrodes. Electrochim Acta 1997;42:1587–93. [22] Ticianelli EA, Derouin CR, Redondo A, Srinivasan S. Methods to advance technology of proton exchange membrane fuel cells. J Electrochem Soc 1988;135:2209–14. [23] Larminie J, Dicks A. Fuel cell systems explained. 2nd ed. Chichester: J. Wiley & Sons; 2003.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Response of a direct methanol fuel cell to fuel change T.J. Leo a,*, M.A. Raso b, E. Navarro c, E. Sa´nchez de la Blanca b, M. Villanueva c, B. Moreno d a
Dpto de Sistemas Ocea´nicos y Navales- ETSI Navales, Univ. Polite´cnica de Madrid, Avda Arco de la Victoria s/n, 28040 Madrid, Spain Dpto de Quı´mica Fı´sica I- Fac. CC. Quı´micas, Univ. Complutense de Madrid, Avda Complutense s/n, 28040 Madrid, Spain c Dpto de Motopropulsio´n y Termofluidodina´mica, ETSI Aerona´uticos, Univ. Polite´cnica de Madrid, Pza Cardenal Cisneros 3, 28040 Madrid, Spain d Instituto de Cera´mica y Vidrio, Consejo Superior de Investigaciones Cientı´ficas, C/Kelsen 5, Campus de la UAM, 28049 Cantoblanco, Madrid, Spain b
article info
abstract
Article history:
Methanol and ethanol have recently received much attention as liquid fuels particularly as
Received 18 December 2009
alternative ‘energy-vectors’ for the future. In this sense, to find a direct alcohol fuel cell that
Received in revised form
able to interchange the fuel without losing performances in an appreciable way would
8 February 2010
represent an evident advantage in the field of portable applications. In this work, the
Accepted 22 February 2010
response of a in-house direct methanol fuel cell (DMFC) to the change of fuel from
Available online 23 March 2010
methanol to ethanol and its behaviour at different ambient temperature values have been investigated. A corrosion study on materials suitable to fabricate the bipolar plates has
Keywords:
been carried out and either 316- or 2205-duplex stainless steels have proved to be adequate
DMFC
for using in direct alcohol fuel cells. Polarization curves have been measured at different
Performance
ambient temperature values, controlled by an experimental setup devised for this purpose.
Ethanol
Data have been fitted to a model taking into account the temperature effect. For both fuels,
Alcohol change
methanol and ethanol, a linear dependence of adjustable parameters with temperature is
Temperature effect
obtained. Fuel cell performance comparison in terms of open circuit voltage, kinetic and
Bipolar plate
resistance is established. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Within the last years there has been an increased interest in direct liquid fuel cells as potential power sources for portable devices, in this sense direct methanol (MeOH) fuel cells (DMFCs) have been identified as one of the most promising candidates to replace batteries in micropower applications. DMFCs have been attracting the majority of research and development efforts, because methanol shows both good electrochemical activity and a high energy density compared with other liquid fuel candidates. There are still two major unresolved drawbacks with the DMFCs: a relatively lower anode electrocatalytic activity and methanol crossover. Many
research efforts are devoted to find new membranes and anode catalysts [1–5]. In parallel with these research efforts the development of direct ethanol (EtOH) fuel cells (DEFCs) is drawing increasing attention due to the attractive advantages of EtOH as fuel: non-toxicity, natural availability, easy storage and higher power density among others. But similar problems to those appearing in DMFCs arise [6–11]. Nevertheless, MeOH and EtOH are considered valid as liquid reactants for anodes in fuel cells [12,13]. Therefore, to find a direct liquid fuel cell able to interchange the fuel, as MeOH and EtOH, without losing performances in an appreciable way would represent an evident advantage in the field of fuel cells. Some partial studies have been performed in this sense [14,15].
* Corresponding author. Tel.: þ34 913367147; fax: þ34 913363927. E-mail address: [email protected] (T.J. Leo). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.115
international journal of hydrogen energy 35 (2010) 11642–11648
2.
11643
Experimental
The single fuel cell consists of a Membrane Electrode Assembly (MEA) and two metallic bipolar plates including the reactants distribution channels. The cell design requirements revolved around two main topics: bipolar plates and MEA.
2.1.
Fig. 1 – Bipolar plates and MEA fabricated and used in this work.
The aim of this work is to investigate the response of a PEM direct liquid fuel cell to the change of fuel and its behaviour at different ambient temperature values. In order to achieve this objective, a DMFC single cell was assembled and an experimental setup allowing measurement and control of the ambient temperature has been designed and fabricated. After activation and normal operation with MeOH, the input fuel has been changed to EtOH. Performance comparison in terms of open circuit voltage, kinetic and internal resistance is established. Selection of the material used to fabricate the bipolar plates has been done after a corrosion study carried out with 6082-aluminium and three stainless steels (304SS, 316SS and 2205 duplex).
Bipolar plates
A study on materials suitable to fabricate the bipolar plates has been carried out in order to choose a cheap, durable one [16]. Three usual stainless steels (304SS, 316SS and 2205 duplex) and 6082-aluminium have been tested to check their durability in different media: Distilled water, MilliQ grade water and 1 M aqueous MeOH solution prepared in Distilled water and MilliQ, respectively. It is known that steel suffers corrosion when kept in water for a long period of time, but it is not clear if the presence of MeOH and the carbon cloth used in the electrodes has any influence on that phenomenon [17]. Two different carbon cloth types were used in the corrosion studies: B1 ASWP Standard Wet Proofing (without carbon coating) and E-TEK LT1400-W Low temperature ELAT GDL Microporous on Woven Web (with carbon coating).
2.2.
Electrochemical measurements
A Solartron 1250 frequency response analyser and a PAR 273A potentiostate–galvanostate have been used for electrochemical experiments. The open circuit voltage (OCV) of threeelectrode assemblies formed with one of the metals as working electrode, a saturated calomel electrode (SCE) as reference and
Fig. 2 – Schematic representation of the experimental setup.
11644
international journal of hydrogen energy 35 (2010) 11642–11648
Table 1 – Experimental conditions of measurements. Parameter
Value
Alcohol (methanol/ethanol) concentration (mol/L) Alcohol flow rate (mL/min) Oxygen flow rate (mL/min) Oxygen pressure (bar) Temperature ( C)
1.0 3.0 250 1.0 25–75
platinum or a carbon cloth as counter-electrode, has been recorded versus time.
2.3.
Chemicals
MeOH was supplied by Panreac (99.9% purity), anhydrous EtOH by Scharlaw (99.9% purity). Distilled and MilliQ grade water have been used to prepare solutions.
2.4.
Electrodes
The electrodes, with an active area of 5 cm2, have been prepared following the procedure described by Jurado et al. [18]. The catalytic layer is aerographied onto ELAT GDL microporous layer on woven web. The total anode Pt loading was 1.75 mg/cm2 and was made using BASF 30% Pt:Ru(1:1)/ Vulcan XC-72. The total cathode Pt loading was 2.33 mg/cm2 and was prepared using BASF 40% Pt/Vulcan XC-72.
2.5.
Flow field design
The bipolar plates, which are used as current collectors and provide conduits for reactants flow, have been designed with parallel flow channels on one side. The other side of the planar metallic plates has been machined with the inlets and outlets of reactants and products (see Fig. 1).
2.6.
Membrane electrode assembly
Nafion 117 has been used as the proton conducting membrane in the MEA fabrication. Before the assembly, the membrane was protonated following this pre-treatment procedure: boiling 1h in 3 wt.% hydrogen peroxide at 80 C;
Fig. 3 – Open circuit voltage measurements of a 304stainless steel cathode against standard calomel electrode, using carbon cloth as counter-electrode.
rinsing in distilled water; boiling 1 h in distilled water at 80 C; boiling 1 h in 0.5 M sulphuric acid at 80 C; rinsing in distilled water and boiling 1 h in distilled water at 80 C. The MEA (Fig. 1) is prepared sandwiching the membrane between the two electrodes and keeping the ensemble under hot pressing at 4 MPa and 135 C for about 3 min. Homogeneity of the ensemble is checked by means of a procedure in which the electrical conductivity (s) of the MEA in different locations is compared, as described in [19]. In this case, logarithm of s did not vary more than 1% across the electrode surface. Electrochemical activation of the assembly was conducted at 70 C without the use of hydrogen following the procedure described by Reshetenko [20], optimized for this fuel cell and the equipment used. The cell was run with 1.5 M aqueous MeOH or EtOH solution with a flow rate of 6 mL/min over 2 h. After this, the polarization curve was registered and the process was repeated until no variation was observed. Additionally, before any tests, the cell was preconditioned following the procedure described by Eccarius [2], i.e., the cell was run under a constant load (0.150 mV) and operation conditions over 15 min.
2.7.
Experimental setup for polarization measurements
It is known that the Fuel Cell performance is affected in a significant way by the operation temperature. To register the polarization curves, i.e. voltage (V) versus current density ( j ), an experimental setup (a sketch is shown in Fig. 2) which allows modifying and controlling the temperature of the testing enclosure where the fuel cell is placed, has been devised and fabricated. For this purpose, a cross-flow fin-tube heat exchanger is placed inside. A fan is coupled to the heat exchanger to impel the air which is introduced into the testing enclosure from a bigger one containing it. Variation of the air mass flow is performed by changing the fan rotational speed. To modify the mass flow and the temperature of the liquid circulating through the heat exchanger, a Julabo F-33 ME refrigerated and heating circulator is used. The actual cell temperature is measured by T-type thermocouples connected to a Yokogawa MV100 data logger. A Jasco PU-2086 Plus HPLC pump is used to feed the fuel aqueous solution and Bronkhorst flow (model F-201) and pressure (model P-702) meter are used on the oxygen side. During the measurements, fuel and oxidant flow rates, oxidant pressure and fuel cell temperature were controlled. The experimental conditions used are shown in Table 1.
3.
Results and discussion
3.1.
A study for the choice of the material plates
To check the influence of water purity in the corrosion process, 304-stainless steel behaviour is tested when kept in four different media: Distilled water, MilliQ grade water, 1 M MeOH/ distilled water solution and 1 M MeOH in MilliQ grade water solution. Results are shown in Fig. 3. From the evolution of the OCV with time measurements it was concluded that the steel corrosion in 1 M MeOH/MilliQ water solution is the slowest one,
international journal of hydrogen energy 35 (2010) 11642–11648
11645
Fig. 4 – Assemblies to study corrosion of a) metal versus Pt and b) metal versus carbon cloth. 316SS samples are shown.
Fig. 5 – Metals after the corrosion study versus carbon cloth.
but the four steel samples reach the same degree of corrosion and approximately at the same OCV in the long term. Therefore, distilled water has been used in further experiments. The behaviour of the four metals in 1 M MeOH/MilliQ grade water solution has been studied by using a three-electrode configuration. Platinum, carbon cloth without carbon coating C(1) and carbon cloth with carbon coating C(2) have been used as counter-electrode in each case. Fig. 4 shows the arrangement of the working electrode. In Fig. 4a, a working electrode used against platinum can be seen whereas Fig. 4b shows the assembly of a working electrode with the carbon counter-electrodes.
After the experiments, clear differences among the surfaces of these metals can be seen, Fig. 5. Both aluminium and 304-stainless steel reach a big OCV value in few hours, Fig. 6a, as an evidence of their high reactivity in that medium. On the other hand, 316 and duplex stainless steel samples reach an OCV near 0 V, as a proof of their surface passivation. Fig. 6b shows that duplex steel samples are somewhat nearer to 0 V than 316 steel ones. These behaviours remain unchanged for at least 3410 h. Therefore, either 316SS or 2205-duplex SS can be adequate. As duplex steel is more expensive than 316SS, bipolar plates have been machined using this last material.
Fig. 6 – Open circuit voltage measurements of a metal cathode against standard calomel electrode, using Pt, carbon cloth C(1) or carbon cloth C(2) as counter-electrode.(a) aluminium and 304-stainless steel cathodes; (b) duplex and 316-stainless steel cathodes.
11646
international journal of hydrogen energy 35 (2010) 11642–11648
Fig. 7 – Polarization (a) and power density (b) curves for the cell using 1 M MeOH as fuel.
Fig. 8 – Polarization (a) and power density (b) curves for the cell using 1 M EtOH as fuel.
3.2.
Polarization measurements
3.4.
Empirical model
Polarization curves have been registered at various ambient cell temperatures. Aqueous MeOH and EtOH solutions have been used as fuel, being pure oxygen the oxidant.
The experimental results shown in Figs. 7 and 8 have been used to fit the polarization curves to an empirical model and the effect of temperature on the model parameters has been derived. The model equation used is [21,22]:
3.3.
V ¼ E Alnj Ri j
Fuel-change study
The response of the fuel cell to the alcohol change has been studied in the temperature range from 25 to 75 C. Polarization and power curves for MeOH are shown in Fig. 7. EtOH measurements results can be found in Fig. 8. As expected, fuel cell response increases in both cases with temperature, although it is observed a decrease in the performance of the fuel cell when MeOH is changed by EtOH. For the later, the open circuit voltage diminishes about a 7% indicating a slower kinetic in the electrochemical reaction for EtOH. The current density values attained with MeOH are clearly higher than those attained with EtOH. The slopes of polarization curves diminish with increasing temperature in both cases. At current density values above 10 mA/cm2, the slopes of the V–j curves are higher when the fuel used is EtOH. Power curves show that the maximum of power density achieved with MeOH is much higher than that attained with EtOH at the same temperature.
(1)
where activation overvoltage at both electrodes has been taken into account [23]. Then E ¼ ENernst þ Alnb, A ¼ Aa þ Ac Aa =A and b ¼ j0a jA0cc =A with Aa ¼ RT=ðna aa FÞ, Ac ¼ RT=ðnc ac FÞ. The anodic and cathodic charge transfer coefficients are aa and ac , respectively; j0a and j0c stand for the anodic and cathodic exchange current density, respectively, and Ri is the cell resistance. A nonlinear fitting according to equation (1) is performed for each polarization curve. The three adjustable parameters
Table 2 – Linear dependence of fitting parameters with temperature. Parameter
E ðVÞ AðVÞ Ri ðU cm2 Þ
CH3OH
C2H5OH
0.18 þ 4.0 10 t ( C) 0.31–2.72 10L3t ( C) 8.1–0.071 t ( C) L3
0.11 þ 1.6 10L3t ( C) 0.35–1.0 10L3 t ( C) 15.8–0.165 t ( C)
international journal of hydrogen energy 35 (2010) 11642–11648
11647
Fig. 10 – Comparison between polarization curves of 1 M MeOH at 25 8C and 1 M EtOH at 74 8C.
Fig. 9 – Fitted parameters (a) E*, (b) A and (c) Ri as a function of temperature.
show linear temperature dependence with both fuels, MeOH and EtOH (Table 2). MeOH and EtOH parameter’s behaviour against temperature is compared in Fig. 9. At 25 C, E is higher when the fuel is MeOH (EMeOH ¼ 0:280 V, EEtOH ¼ 0:150 V), as occurs with the E values. The activation overpotential parameter A is higher for EtOH than for MeOH (AMeOH ¼ 0.242 V, AEtOH ¼ 0.325 V), as expected, because the electrochemical oxidation kinetics is slower for EtOH than for MeOH. Also, at this temperature the Ri value is higher for EtOH (Ri,MeOH ¼ 6.33 U cm2, Ri,EtOH ¼ 11.78 U cm2). As reported by Bagotski [12] the side-reactions products of the electrooxidation of EtOH, acetaldehyde or even acetic acid, will originate not only a drop in energy but a number of problems of removal and disposal of these products. Then, the ionic transport across the membrane when the fuel cell is operated with EtOH may be hindered by the presence of these species and the resistance term would be affected.
When temperature increases, the difference between the independent terms E* grows up (Fig. 9a), and a similar effect can be observed for the activation overpotential parameter A (Fig. 9b). Nevertheless, the difference between the ohmic loss parameters Ri diminishes in an appreciable way. At the higher working temperature (74 C) these parameter values for MeOH and EtOH get very close (Fig. 9c). As the working temperature increases the ethanol electrooxidation is favoured, thus the side products will decrease and will be also more easily eliminated. This fact agrees with the statement that when working at elevated temperatures EtOH fuel cells may yield electrical performance indicators comparable to those of MeOH fuel cells [12]. Thus, as at elevated working temperature values the resistance term becomes quite similar, an efficient fuel change from MeOH to EtOH would be achieved by reducing activation losses. Emphasizing that the studied cell has been prepared to use methanol as fuel, the great similarity observed between polarization curves for MeOH at 25 C and EtOH at 74 C in Fig. 10 shows that fuel change doesn’t seem to be an unattainable target.
4.
Conclusions
The response of a DMFC to the change of fuel from methanol to ethanol has been investigated. Also, the behaviour of the fuel cell fed with methanol and with ethanol at different ambient temperature values has been studied. A single fuel cell and an experimental setup allowing the measurement and control of the ambient temperature have been designed and fabricated. After a corrosion study, 316SS and 2205 duplex stainless steels have proved to be appropriate materials for direct alcohol fuel cell bipolar plates. The polarization curves measurements show that the fuel cell performance decreases when methanol is changed by ethanol at every temperature studied. So, the fuel change from methanol to ethanol requires higher working temperatures, if the same performance is required.
11648
international journal of hydrogen energy 35 (2010) 11642–11648
Polarization curves measured at different temperatures have been fitted to a model which incorporates the activation overpotential and the ohmic losses. A linear dependence with temperature has been found for all the model parameters. Fitting results indicate that activation losses should be reduced to allow an efficient fuel change in a direct liquid fuel cell from methanol to ethanol. Further improvements on catalytic anode layer would lead to achieve this purpose. Those improvements would probably raise the ionic conductivity decreasing the amount of products of the ethanol electrooxidation side-reactions.
Acknowledgements This work has been financially supported by the Spanish Ministerio de Ciencia y Tecnologı´a in the frame of the Project Code N ENE2007-67584-C03-03ALT. The authors gratefully acknowledge Dr. E. Mora and Dr. F. Molleda for their help.
references
[1] Caracino P, Ballabio O, Colombo M, Sala E, Cavallo G. Polymeric fluorine-free electrolyte for application in DMFC. Int J Hydrogen Energy 2009;34:4653–60. [2] Eccarius S, Garcia BL, Hebling C, Weidner JW. Experimental validation of a methanol crossover model in DMFC applications. J Power Sources 2008;179:723–33. [3] Liu H, Song C, Zhang L, Zhang J, Wang H, Wilkinson DP. A review of anode catalysis in the direct methanol fuel cell. J Power Sources 2006;155:95–110. [4] Martin JJ, Qian W, Wang H, Neburchilov V, Zhang J, Wilkinson DP, et al. Design and testing of a passive planar three-cell DMFC. J Power Sources 2007;164:287–92. [5] Suo C, Liu X, Tang X, Zhang Y, Zhang B, Zhang P. A novel MEA architecture for improving the performance of a DMFC. Electrochem Commun 2008;10:1606–9. [6] Colmati F, Antolini E, Gonzalez ER. Effect of temperature on the mechanism of ethanol oxidation on carbon supported Pt, PtRu and Pt3Sn electrocatalysis. J Power Sources 2006;157:98–103. [7] 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.
[8] Lopes T, Antolini E, Gonzalez ER. Carbon supported Pt–Pd alloy as an ethanol tolerant oxygen reduction electrocatalyst for direct ethanol fuel cells. Int J Hydrogen Energy 2008;33: 5563–70. [9] 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. [10] Song SQ, Zhou WJ, Zhou ZH, Jiang LH, Sun GQ, Xin Q, et al. Direct ethanol fuel cells: the case of platinum based anodes. Int J Hydrogen Energy 2005;30:995–1001. [11] Wan CH, Chen CL. Mitigating ethanol crossover in DEFC: a composite anode with a thin layer of Pt50–Sn50 nanoparticles directly deposited into Nafion membrane surface. Int J Hydrogen Energy 2009;34:9515–22. [12] Bagotsky VS. Fuel cells, problems and solutions. Pennington: J.Wiley & Sons; 2009. p. 73–91. [13] Wang ZH, Li J, Dong X, Wang D, Chen T, Qiao H, et al. Ethanol oxidation on a nichrome-supported spherical platinum microparticle electrocatalyst prepared by electrodeposition. Int J Hydrogen Energy 2008;33:6143–9. [14] Cimenti M, Hill JM. Thermodynamic analysis of solid oxide fuel cells operated with methanol and ethanol under direct utilization, steam reforming, dry reforming or partial oxidation conditions. J Power Sources 2009;186:377–84. [15] Xu C, Su Y, Tan L, Liu Z, Zhang J, Chen S, et al. Electrodeposited PtCo and PtMn electrocatalysts for methanol and ethanol electrooxidation of direct alcohol fuel cells. Electrochim Acta 2009;54:6322–6. [16] Tawfik H, Hung Y, Mahajan D. Metal bipolar plates for PEM fuel cell – a review. J Power Sources 2007;163:755–67. [17] Li MC, Zeng CL, Luo SZ, Shen JN, Lin HC, Cao CN. Electrochemical corrosion characteristics of type 316 stainless steel in simulated anode environment for PEMFC. Electrochim Acta 2003;48:1735–41. [18] Jurado JR, Chinarro E, Colomer MT. Pat No. ES 2 209 657; 2005. [19] Chinarro E. Ph. D. thesis, Universidad Auto´noma de Madrid; 2003. [20] Reshetenko TV, Kim H, Lee H, Jang M, Kweon H. Performance of a direct methanol fuel cell (DMFC) at low temperature: cathode optimization. J Power Sources 2006;160:925–32. [21] Hirano S, Kim J, Srinivasan S. High performance proton exchange membrane fuel cells with sputter-deposited Pt layer electrodes. Electrochim Acta 1997;42:1587–93. [22] Ticianelli EA, Derouin CR, Redondo A, Srinivasan S. Methods to advance technology of proton exchange membrane fuel cells. J Electrochem Soc 1988;135:2209–14. [23] Larminie J, Dicks A. Fuel cell systems explained. 2nd ed. Chichester: J. Wiley & Sons; 2003.