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Electrochemical impedance diagnosis of micro porous layer in polymer electrolyte membrane fuel cell electrodes
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Electrochemical impedance diagnosis of micro porous layer in polymer electrolyte membrane fuel cell electrodes M. Maidhily, N. Rajalakshmi*, K.S. Dhathathreyan Centre for Fuel cell Technology, ARCI, IIT Madras Research Park, Phase 1, II floor, 6 Kanagam Road, Taramani, Chennai 600113, India
article info
abstract
Article history:
In the present study, Electrochemical Impedance Spectroscopy (EIS) is used to evaluate two
Received 27 April 2011
different types of gas diffusion electrodes for Polymer Electrolyte Membrane Fuel Cells
Received in revised form
(PEMFC). The over potential losses due to the components are determined without any
10 June 2011
reference electrode using symmetric gas supply of hydrogen or oxygen at various condi-
Accepted 15 June 2011
tions viz., open circuit potential (OCP), various load up to 0.7 A cm2, and various humidity
Available online 26 July 2011
conditions. Though it is very clear that the cathode impedance is a major contributor for voltage loss, it is observed that the two type of electrodes with different micro-porous
Keywords:
layers (DSGDL and SSGDL), show different behavior with respect to operating conditions
Fuel cells
like dry gas operation, humid condition etc., The DSGDL is favorable for operating the cell
Electrodes
at higher temperature and relative humidity while SSGDL is favorable under dry gas
Micro-porous layer
operation. However at higher current density and with humidity, Nernst diffusion plays
Impedance
a major contribution. The Nernst diffusion coefficient decreases with increasing current density for DSGDL and increasing for SSGDL, suggesting that the gas diffusion electrodes need to be engineered depending on the operating conditions and current density. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells are electrochemical reactors that convert the chemical energy of the fuel to electrical energy directly without any intermediate reactions. Polymer Electrolyte Membrane Fuel Cell (PEMFC) consists of an electrolyte with high ionic conductivity, two porous electrodes anode and cathode with electrocatalysts and gas distribution plates on either side [1]. Electrochemical impedance spectroscopy (EIS) is a powerful technique for characterizing the fuel cell electrodes at their interfaces [2e5]. Gas diffusion layer (GDL) is one of the most important components of PEMFC, which has a gas diffusion backing (GDB) and a Micro porous layer (MPL). The MPL placed between the catalyst layer and GDL, with proper pore structure and hydrophobicity, allows a better gas
transport and water removal from the catalyst layer [6]. The GDL structure and morphology is varied depending on the type of fuel/oxidant. Highly porous GDL with MPL is preferred for fuel from reformate gas and air as oxidant and GDL,while less porosity is enough if the fuel and oxidant are pure H2 and O2. Hence development of GDL becomes very important for the performance improvement in PEMFC. The role of bulk textile properties of four carbon clothes coated with the same MPL has been investigated in single fuel cell experiments under different operative conditions by steady-state polarization measurements combined with EIS [7]. Ramasamy et al. [8] investigated the interaction of micro and macro-porous layer on PEMFC and concluded that the diffusion media lose their hydrophobic nature during long term operation. Interestingly, the presence of an MPL mitigates the loss of
* Corresponding author. Tel.: þ91 44 66632707; fax: þ91 44 66632702. E-mail address: [email protected] (N. Rajalakshmi). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.084
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hydrophobicity, possibly due to the enhanced liquid resistance induced by the inclusion of MPL. They also observed that the effects of cathode MPL exhibit different behaviors when applied to different macro substrates, indicating that the existence of a complex interaction between the microand macro-layers of the porous diffusion electrodes. Nakajima et al. [9] studied the effect of addition of micro-porous layer to the gas diffusion layer of PEMFC on its performance and showed that the start-up performance of a PEMFC can be improved by suppressing the water accumulation at the electrode. Andreaus et al. [10] proposed that Oxygen Reduction Reaction (ORR) is the determining step for the fuel cell performance, and is also limited by the electrolyte resistance in series with the cathode impedance. Danzer et al. [11] has reported that the resistive effects are due to the charge transfer and ohmic voltage losses of electron and proton conduction. Parthasarathy et al. [12] performed ac impedance measurements for a platinum microelectrode and observed two major features in the complex impedance plot. At lower frequency they found a feature associated with the charge transfer resistance in parallel with interfacial double layer capacitance. The other feature was found at higher frequency, typically in the 10e100 kHz frequency domain, and was attributed to grain boundaries in the ionomeric membrane electrolyte. They showed a decrease in the charge transfer resistance with an increase in cathode over potential. Song et al. [13] studied polymer electrolyte fuel cell electrodes to find an optimal composition of the electrodes using ac impedance methods. They found a low frequency arc in the impedance spectrum at high over potentials, which are believed to reflect water related transport limitations. Freire and Gonza´lez [14] have studied the effect of temperature, membrane thickness and humidification conditions on the impedance response and concluded that the low frequency relaxation process is mainly due to the flooding of the cathode with liquid water and the consequential shortage of oxygen for the ORR due to its limited diffusivity in water. Gottesfeld and Zawodzinski [15] reported that the high over potential at the cathode is the most important source of loss in the PEMFC, as in all other low temperatures fuel cells. Wagner [16] characterized the membrane electrode assembly in polymer electrolyte fuel cells using impedance spectroscopy. He proved the possibility of separating the cell impedance into electrode impedances and electrolyte resistance with varying the operational conditions, current density, and various flow rates and by simulation of the measured EIS with an equivalent circuit. He calculated the VeI curves and voltage losses (over potential) in the fuel cell integrating over current densities of the individual impedance elements. Most of the studies concentrated on identifying the potential loss and they have been attributed to the ORR activity and electrolyte resistance. To our knowledge the contribution due to GDL with different types of MPL using EIS technique has not been reported. Since GDL play a major role for water and gas transport in PEMFC, the EIS analysis on GDL will help to understand the problems related to gas accessibility, water content, humidification etc., In this paper EIS has been used to
Catalyst layer
a Cathode Anode Membrane Water proofed carbon cloth+ MPL on double side
Catalyst layer
b
Anode
Cathode
Membrane Water proofed carbon cloth+ MPL on single side
Fig. 1 e a and b. Schematic diagram of the DSGDL and SSGDL.
resolve the various contributions to the polarization loss due to GDL with different types of MPL in a PEMFC electrode. Voltage losses from each component have been calculated by modeling the equivalent circuit and also reported the electrical transport properties of each cell component by symmetrical mode of gas supply.
2.
Experimental
2.1.
Membrane electrode assembly (MEA) preparation
Gas diffusion electrodes are prepared using the teflonized gas diffusion backing (GDB) carbon cloth by the proprietary process developed at CFCT [17]. Two different types of MPL’s
Fig. 2 e Equivalent circuit of an electrochemical cell.
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0
a 0 .9
50 C 0
60 C
0 .8
0
70 C
0 .7
Voltage(V)
0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0 0
200
400
600
800
1000
1200
2
C u r r e n t d e n s i t y ( m A /c m )
b
1 .0
0
50 C
0 .9
0
60 C 0
70 C
0 .8 0 .7
Voltage(V)
0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0 0
100
200
300
400
500
600
700
800
900 1000
2
C u r r e n t d e n s it y ( m A /c m )
Fig. 3 e Polarization curves of the MEA’s with DSGDL (a) and SSGDL (b).
were prepared by coating carbon slurry consisting of Vulcan XC72, PTFE solution (66%), ammonium bicarbonate and iso Propyl Alcohol. The MPL slurry is coated on both sides of the backing layer viz., gas channel side and catalyst side until a loading of 3 mg cm2 and labelled as Double Side Gas Diffusion Layer (DSGDL). In the electrode labelled Single Side Coated (SSGDL), the slurry is coated only on one side, facing the catalyst side. The catalyst layers for the electrodes are prepared using Pt/C catalyst from M/s TKK, Japan by spreading the catalyst ink, consisting of 5% Nafion solution, uniformly in the electrode area of 5 cm2. Membrane Electrode Assemblies (MEA) are made by laminating a Nafion 212 membrane with two electrodes on both sides and hot pressed at 130 C at 1 ton. The schematic representation of the DSGDL and SSGDL are shown in Fig. 1a and b. The laminated MEA is inserted between two graphite blocks, which had a multichannel serpentine flow pattern.
The whole assembly is tightened with the help of end plates and bolts for proper compaction and leak prevention, by applying uniform torque to all the bolts. The fuel cell polarization experiments are conducted using an ARBIN test station, which was equipped with a gas humidifier, a mass flow controller, temperature controller, DC electronic load and data logging facility.
2.2.
Electrochemical characterization
2.2.1.
EIS at normal mode
The electrochemical characterization of the fuel cell is done using a Potentiostat/Galvanostat Auto lab, equipped with a frequency response analyzer (FRA) in the frequency ranging from 5 MHz to 100 kHz, in combination with an electronic load while drawing current using humidified hydrogen stream at 50 C.
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Fig. 4 e a and b SM Bode plots and Nyquist plots for SSGDL (B) and DSGDL (*).
2.2.2.
EIS at symmetrical mode
The anode and cathode transfer functions are determined independently using symmetric gas supply of hydrogen or air (oxygen) on both electrodes of the fuel cell at open circuit potential (OCP). Cathode and anode impedances are determined directly by two independent experiments and the measured cell impedance under load is split up into anode impedance, cathode impedance and electrolyte resistance by fitting to an equivalent modeling circuit [16,18]. The equivalent circuit for a simple electrochemical cell is used which includes a solution resistance, a double layer capacitor and a charge transfer or polarization resistance. The equivalent circuit for
the Randles cell is shown in Fig. 2. The double layer capacity is in parallel with the impedance due to the charge transfer reaction.
3.
Results and discussion
3.1.
Effect of MPL on cell performance
The polarization curves are obtained for the MEAs with DSGDL and SSGDL at various temperatures and are given in Fig. 3a and b respectively. From the figures it can be seen that the MEA with DSGDL showed comparatively better performance of
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a
b
Fig. 5 e a and b. Nyquist Plot for H2/air NM operation with DSGDL and SSGDL at RT and at humidification at various loads.
1.2 A cm2 at 0.1 V compared to SSGDL, which gave a higher current density of 0.9 A cm2 at the same voltage. However, the current density at activation and ohmic region is comparatively better for SSGDL. In the ohmic region (0.6V), MEA with SSGDL gave a current density of 0.587 A cm2 compared to DSGDL, where the current density is 0.546 A cm2.
interface namely charge transfer at intermediate frequency and the pseudo capacitance at low frequency. In the high frequency range, pseudo inductive impedance appears which arises due to all the metallic components of the complete single fuel cell assembly [18,19].
3.2.2. 3.2.
AC impedance characteristics
3.2.1.
EIS at symmetrical mode(SM)
EIS measurements are carried out under symmetric gas supply, by supplying with gas of (H2/H2) and (air/air) to the fuel cell electrodes at open circuit potential, in order to evaluate the anode and cathode impedance separately. The results are shown in Fig. 4a and b respectively for H2/H2 and air/air. As can be seen from the figures, the H2/H2 anode impedance is less than half of the air/air cathode impedance. In the case of DSGDL the anode impedance was found to be 0.127 U and for the cathode, it is 5.5U. Although the anode impedance in the case of SSGDL is same as 0.119U, the cathode impedance is 18U, which is three times higher than DSGDL. This high impedance is attributed to the two processes occurring at the
EIS at normal mode(NM): -
EIS measured on the PEMFC (H2/air) in the potential range from OCP to 450 mV by drawing current from the cell, for DSGDL and SSGDL is shown in Fig. 5a and b at RT and at humidification respectively .The Nyquist plot consists of three regions indicating the membrane contribution to the electrode at higher frequency, semicircle at medium frequencies,
Fig. 6 e Equivalent Circuit model for the electrode operated at RT and at humidification.
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Table 1 e Parameteres obtained from the model fit for DSGDL at RT. Circuit simulation elements Physical interpretation Voltage (mV) 650 600 550 500 450
R1
C1
Anode activation kinetics
R2 Solution resistance related to membrane
R3
C2
Cathode activation kinetics
Ra (mU)
Ca (mF)
Rs (mU)
Rc(mU)
Cc (mF)
39.2 29.76 29.02 60.8 60.7
23.98 274.0 207.2 382 421
59.5 107.7 104.9 107.5 107.4
189.9 287.6 339 409 460
309.8 398 380 514 533
characteristic of the charge transfer processes associated with oxygen reduction at lower frequencies. From the impedance plot, it is clear that the high frequency intercept does not change much between OCP and at load. However, at the low frequency the diameter of the arc gradually increases from 0.466 to 0.626 U for DSGDL electrodes while for SSGDL, the impedance increases from 0.2956U to 0.640 U, with increasing current density. This is due to the increased driving force for the oxygen reduction reaction, which is limited by the mass transport due to increased water production at the cathode, leading to increased mass transfer resistance [20]. In both MEAs, there is no change in the membrane resistance at ambient and humidification condition with increasing current density. Under conditions of high ionic currents and or restricted transport in the membrane, the back diffusion of water influences the performance of the cell in terms of fluctuations in the cell voltage. However, the charge transfer resistance increases with increasing current density up to 0, 7 A cm2. Under humidification condition the impedance increases from 0.3 to 0.72 U for SSGDL and for DSGDL, the impedance increases from 0.45 U to above 0.7 U. From the Nyquist plots obtained shown in Fig. 5, it is clear that the MPL coated on both sides of the diffusion medium balanced the product water relatively better than the MPL coated only on single side of the diffusion medium. The two contributions, viz., the liquid water formed at the cathode, which affects the transport of oxygen, and hydration, that limit the transport in the membrane are seen in the Nyquist plot in the low frequency region [14]. The polarization resistance of the electrodes increases with increasing the load, due to the limited proton transport from anode to cathode at higher current densities through the electrolyte. When the
oxygen source is air, the diffusion of oxygen in the gas phase becomes a limiting factor as can be seen from the disturbances in the low frequency region due to the flooding in the cathode region [21]. In addition to the flooding, the other limitation that occur inside the gas diffusion electrodes due to mass transport arising from the accumulation of nitrogen blanket inside the gas diffusion electrodes, makes more difficult for the reactant to reach the catalyst site [22]. Hence mass transport resistance is very high at the cathode compared to anode and membrane, as they contribute towards ohmic resistance. Hence proper air and hydrogen stoichiometry and relative humidity is required to minimize the ohmic, charge and mass transfer resistance. The impedance plot also showed a zero frequency impedance inductive loops at 5 mHz, which is attributed to the side reactions and intermediates formed during the fuel cell operation, as suggested by Makharia et al. [23]. Roy et al [24]. also proposed that that the low frequency inductive loops observed in PEMFC are due to parasitic reactions in which the Pt catalyst reacts to form PtO and subsequently forms Ptþ ions. However, they could also be either due to non-stationary behavior, or the time required to make measurements at low frequencies. This non-stationary behavior could influence the shapes of the low-frequency features.
3.3.
Modeling of equivalent circuit
The most common method to analyze EIS spectra is equivalent circuit modeling. Equivalent circuit model of the electrode at RT and humidification is given in Fig. 6. At higher current density, an increase in the cell impedance is observed and is attributed to the additional over voltage, known as
Table 2 e Parameters obtained from the model fit for SSGDL at RT. Circuit simulation elements Physical interpretation Voltage (mV) 650 600 550 500 450
R1
C1
Anode activation kinetics
R2 Solution resistance related to membrane
R3
C2
Cathode activation kinetics
Ra (mU)
Ca (mF)
Rs (mU)
Rc(mU)
Cc (mF)
39.8 42.4 72.2 86.6 100
244.0 257.2 307.7 324 356
114.4 110.0 109.0 105.9 102.7
302 322 326 358 406
353 384 546 743 1010
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Table 3 e Parameteres obtained from the model fit for DSGDL at humidification. Circuit simulation elements
R1
C1
R2
C2
Cathode activation kinetics
R4
C3
Physical interpretation
Anode activation kinetics
Voltage (mV)
Ra (mU)
Ca (mF)
Rs (mU)
Rc (mU)
Cc (mF)
Rc (mU)
Cc (mF)
15.05 16.74 31.46 42.7 10.78
98.2 193.3 321 364 164.2
89.6 92.0 92.3 91.8 88.4
154.0 181.6 251.2 207.1 98.7
358 385 455 629 514
336 355 284.3 410 503
658 981 1834 1871 1125
R3
C2
R4
C3
650 600 550 500 450
Solution resistance related to membrane
R3
Nernst Diffusion Impedance related to additional process
Table 4 e Parameteres obtained from the model fit for SSGDL at humidification. Circuit simulation elements
R1
C1
R2
Physical interpretation
Anode activation kinetics
Voltage (mV)
Ra (mU)
Ca (mF)
Rs (mU)
Rc (mU)
Cc (mF)
Rc (mU)
Cc (mF)
18.21 17.76 20.27 13.92 9.79
144.2 152.6 188.6 175.7 42.0
102.7 104.7 105.6 106.1 103.4
172.7 162.8 141.0 136.3 133.7
213 205 323 504 468
236.5 223.6 360 382 482
400 388 384 759 820
650 600 550 500 450
Solution resistance related to membrane
diffusion over voltage “Nernst diffusion” [25]. Resistance and capacitance values are obtained by a linear least squares fit to the AC response data at various load currents. AC response data are then fitted to the circuit values calculated from the above load current fit. Model fit is reasonably accurate with a minimum number of parameters and without any nonideal circuit element. Numerical values extracted by fitting the data to the circuit elements correlated to physical or chemical properties.
Cathode activation kinetics
Nernst Diffusion Impedance related to additional process
Tables 1 and 2 shows the parameters evaluated from a fit of EIS for the equivalent circuit obtained for the DSGDL and SSGDL respectively at room temperature and Tables 3 and 4 shows for DSGDL and SSGDL at humidification condition respectively. From the parameter values obtained, it is clear that there is not much difference in the impedance related to membrane for both MEA’s and only a slight variation in electrolyte resistance is obtained with increasing current density. However under humid conditions, the impedance is lower for
Fig. 7 e Comparison of impedance plots for PEMFC with model fit for both Nyquist and bode plot.
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DSGDL compared to SSGDL electrode. However, the cathode impedance increases with increasing current density from 189.9 mU to 460 mU for DSGDL and 302 mUe406 mU for SSGDL electrodes. The effect of oxygen diffusion in the electrode is revealed in the charge transfer resistance, by the decreased low frequency arc as the over potential is increased, due to faster ORR kinetics. By performing fitting, the interfacial capacitances and ionic resistances of PEMFC membrane electrode assemblies (MEA’s) can be obtained at various current densities. We considered an additional parameter viz., Nernst diffusion to the equivalent circuit for the impedance simulation, under humidification condition and observed that this loss is higher compared to other three losses as shown in Tables 3 and 4. At humidification there is a change of 336e504 mU for DSGDL and 236e482 mU for SSGDL which is higher than that of the impedance obtained due to cathode for both the MEA’s.
3.4.
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Voltage loss calculated from the equivalent circuit (EC)
The impedance spectra measured at 650 mV for DSGDL at RT is shown in Fig. 7. A set of parameters were obtained to calculate the voltage loss due to various components of PEMFC from the product of the impedance parameters magnitude (Rct(A), Rct(M), Rct(C), Rct(N)) at various current densities. At room temperature the cathode loss is high compared to anode and electrolyte, while the loss due to “Nernst diffusion” is high at humidification. At lower current density region, the losses due to anode, membrane and cathode is comparatively lower for both the MEA’s. However, as the current density increasing, the loss due to three parameters rises immediately and the values are higher for both the MEA’s. As shown in Fig. 8a and b, it is clear that the diffusion component play a major role in the cathode impedance, as seen in the steep increase in voltage loss, due to increasing current density, causing an increase in the diffusion related issues mentioned
Fig. 8 e a and b .Voltage losses (E) at the PEMFC at RT and 50 C as a function of current density, calculated after simulation for DSGDL and SSGDL.
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as Nernst diffusion. From this result it is evident that the types of GDL play a major role in the performance in addition to the operation condition. As shown in Fig. 8, the plot obtained for voltage loss is in good agreement with the polarization curve obtained for both the MEA’s with different MPL’s.
4.
[9]
conclusion [10]
We have demonstrated an ac impedance measurements method for resolving voltage losses due to the individual resistance by comparing two different gas diffusion layers. The DSGDL is favorable for operating the cell at higher temperature and relative humidity while SSGDL is favorable under dry gas operation. The analysis of the experiment on “Symmetrical mode of gas supply” shows that, the cathode contribution for the voltage loss is very high compared to anode and electrolyte. However, the diffusion resistance plays a major role in the voltage loss, when the cells are operated at high current densities and high humidification. The parameters obtained from the model fit shows good agreement with the experimental analysis.
[11]
[12]
[13]
[14]
Acknowledgements
[15]
The authors would like to acknowledge Dr.G.Sundararajan, Director, and ARCI for his constant encouragement and support. The financial assistance from Department of Science and Technology, Govt of India is acknowledged herewith.
[16]
references
[17]
[18] [1] Vielstich W, Lamm A, Gasteiger H, editors. Handbook of fuel cells Fundamentals, Technology and Applications. England: John Wiley & Sons Ltd; 2005. p. 41. [2] MacDonald JR, Barsoukov E, editors. Impedance spectroscopy Theory, experiment and Applications. New Jersey: John Wiley & Sons Inc; 2005. p. 1e20. [3] Springer TE, Raistrick ID. Electrical impedance of a pore wall for the flooded-agglomerate model of porous gas-diffusion electrodes. Journal Electrochemistry Society 1989;136: 1594e603. [4] Asghari Saeed, Mokmeli Ali, Samavati Mahrokh. Study of PEM fuel cell performance by electrochemicalimpedance spectroscopy. International Journal of Hydrogen Energy 2010; 35:9283e90. [5] Wu Jinfeng, Zi Yuan Xiao, Wang Haijiang, Blanco Mauricio, Martin Jonathan J, Zhang Jiujun. Diagnostic tools in PEM fuel cell research:Part I Electrochemical techniques. International Journal of Hydrogen Energy 2008;33:1735e46. [6] Lim Chan, Wang CY. Effects of hydrophobic polymer content in GDL on power performance of a PEM fuel cell. Electrochimica Acta 2004;49:4149e56. [7] Omati Luca, Gallo Stampino Paola, Dotelli Giovanni, Brivio Davide, Grassini Paolo. Operative conditions effect on PEM-FC performance by in-situ and ex-situ analysis of gas diffusion media with different bulk textile structure. International Journal of Hydrogen Energy; 2011:1e10. [8] Ramasamy Ramaraja P, Kumbur Emin C, Mench Matthew M, Liu Wen, Moore Deborah, Murthy Mahesh. Investigation of
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macro- and micro-porous layer interactionin polymer electrolyte fuel cells. International Journal of Hydrogen Energy 2008;33:3351e67. Nakajima Hironori, Konomi Toshiaki, Kitahara Tatsumi. Direct water balance analysis on a polymer electrolyte fuel cell (PEFC): effects of hydrophobic treatment and microporous layer additionto the gas diffusion layer of a PEFC on its performance during a simulated start-up operation. Journal of Power Sources 2007;171:457e63. Andreaus Mc, Evoy AJ, Scherer GG. Analysis of performance losses in polymer electrolyte fuel cells at high current densities by impedance spectroscopy. Electrochimica Acta 2002;47:2223e9. Danzer MA, Hofer EP. Electrochemical parameter identification - An efficient method for fuel cell impedance characterization. Journal of Power Sources 2008;183:55e61. Parthasarathy D, Srinivasan A, Appleby S, Martin AJ. CR. Pressure Dependence of the oxygen reduction reaction at the platinum Microelectrode/Nafion interface: electrode Kinetics and mass transport. Journal of Electrochemistry Society 1992;139:2856e62. Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method. Journal of Power Sources 2001;94:78e84. Freire TJP, Gonzalez ER. Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells. Journal of Electro Analytical Chemistry 2001;503:57e68. Gottesfeld S, Zawodzinski TA. Polymer electrolyte fuel cells. In: Alkire RC, Gerischer H, Kolb DM, Tobias W, editors. Advances in electrochemical Science and Engineering. , Weinheim: Wiley-VCH; 1997. p. 195e301. Wagner N. Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy. Journal of Applied Electrochemistry 2002;32:859e63. Rajalakshmi N, Dhathathreyan KS. Catalyst layer in PEMFC electrodes -Fabrication, characterisation and analysis. Chemical Engineering Journal 2007;129:31e40. Wagner N, Schnurnberger W, Muller BM, Lang M. Electrochemical impedance spectra of solid-oxide fuel cells and polymer membrane fuel cells. Electrochimica Acta 1998; 43:3785e93. alez L, Rubio MA, Agila W, Villanueva E, Hombrados AG, Gonz Guinea D, et al. Symmetrical electrode mode for PEMFC characterization using impedance spectroscopy. Journal of Power Sources 2005;151:25e31. Yan X, Houb M, Sun L, Liang D, Shen Q, Xu Hongfei, et al. AC impedance characteristics of a 2 kW PEM fuel cell stack under different operating conditions and load changes. International Journal of Hydrogen Energy 2007; 32:4358e64. Paganin VA, Oliveira CLF, Ticianelli EA, Springer TE, Gonzalez ER. Modelistic interpretation of the impedance response of a polymer electrolyte fuel cell. Electrochimica Acta 1998;43:3761e6. Zhu WH, Payne RU, Cahela DR, Tatarchuk BJ, Center for Microfibrous Materials Manufacturing Department of chemical engineering 230 Ross Hall, Auburn University, AL 36849. Makharia R, Mathias MF, Baker DR. Measurement of catalyst layer electrolyte resistance in PEFCs using electrochemical impedance spectroscopy. Journal of Electrochemistry Society 2005;52(5). A9A9770e7. Roy SK, Orazem ME, Tribollet B. Interpretation of lowfrequency inductive loops in PEM fuel Cells. Journal of Electrochemistry Society 2007;154:B1378e88. Gabrielli C. Technical Report 24. solar instruments; 1999.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical impedance diagnosis of micro porous layer in polymer electrolyte membrane fuel cell electrodes M. Maidhily, N. Rajalakshmi*, K.S. Dhathathreyan Centre for Fuel cell Technology, ARCI, IIT Madras Research Park, Phase 1, II floor, 6 Kanagam Road, Taramani, Chennai 600113, India
article info
abstract
Article history:
In the present study, Electrochemical Impedance Spectroscopy (EIS) is used to evaluate two
Received 27 April 2011
different types of gas diffusion electrodes for Polymer Electrolyte Membrane Fuel Cells
Received in revised form
(PEMFC). The over potential losses due to the components are determined without any
10 June 2011
reference electrode using symmetric gas supply of hydrogen or oxygen at various condi-
Accepted 15 June 2011
tions viz., open circuit potential (OCP), various load up to 0.7 A cm2, and various humidity
Available online 26 July 2011
conditions. Though it is very clear that the cathode impedance is a major contributor for voltage loss, it is observed that the two type of electrodes with different micro-porous
Keywords:
layers (DSGDL and SSGDL), show different behavior with respect to operating conditions
Fuel cells
like dry gas operation, humid condition etc., The DSGDL is favorable for operating the cell
Electrodes
at higher temperature and relative humidity while SSGDL is favorable under dry gas
Micro-porous layer
operation. However at higher current density and with humidity, Nernst diffusion plays
Impedance
a major contribution. The Nernst diffusion coefficient decreases with increasing current density for DSGDL and increasing for SSGDL, suggesting that the gas diffusion electrodes need to be engineered depending on the operating conditions and current density. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells are electrochemical reactors that convert the chemical energy of the fuel to electrical energy directly without any intermediate reactions. Polymer Electrolyte Membrane Fuel Cell (PEMFC) consists of an electrolyte with high ionic conductivity, two porous electrodes anode and cathode with electrocatalysts and gas distribution plates on either side [1]. Electrochemical impedance spectroscopy (EIS) is a powerful technique for characterizing the fuel cell electrodes at their interfaces [2e5]. Gas diffusion layer (GDL) is one of the most important components of PEMFC, which has a gas diffusion backing (GDB) and a Micro porous layer (MPL). The MPL placed between the catalyst layer and GDL, with proper pore structure and hydrophobicity, allows a better gas
transport and water removal from the catalyst layer [6]. The GDL structure and morphology is varied depending on the type of fuel/oxidant. Highly porous GDL with MPL is preferred for fuel from reformate gas and air as oxidant and GDL,while less porosity is enough if the fuel and oxidant are pure H2 and O2. Hence development of GDL becomes very important for the performance improvement in PEMFC. The role of bulk textile properties of four carbon clothes coated with the same MPL has been investigated in single fuel cell experiments under different operative conditions by steady-state polarization measurements combined with EIS [7]. Ramasamy et al. [8] investigated the interaction of micro and macro-porous layer on PEMFC and concluded that the diffusion media lose their hydrophobic nature during long term operation. Interestingly, the presence of an MPL mitigates the loss of
* Corresponding author. Tel.: þ91 44 66632707; fax: þ91 44 66632702. E-mail address: [email protected] (N. Rajalakshmi). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.084
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hydrophobicity, possibly due to the enhanced liquid resistance induced by the inclusion of MPL. They also observed that the effects of cathode MPL exhibit different behaviors when applied to different macro substrates, indicating that the existence of a complex interaction between the microand macro-layers of the porous diffusion electrodes. Nakajima et al. [9] studied the effect of addition of micro-porous layer to the gas diffusion layer of PEMFC on its performance and showed that the start-up performance of a PEMFC can be improved by suppressing the water accumulation at the electrode. Andreaus et al. [10] proposed that Oxygen Reduction Reaction (ORR) is the determining step for the fuel cell performance, and is also limited by the electrolyte resistance in series with the cathode impedance. Danzer et al. [11] has reported that the resistive effects are due to the charge transfer and ohmic voltage losses of electron and proton conduction. Parthasarathy et al. [12] performed ac impedance measurements for a platinum microelectrode and observed two major features in the complex impedance plot. At lower frequency they found a feature associated with the charge transfer resistance in parallel with interfacial double layer capacitance. The other feature was found at higher frequency, typically in the 10e100 kHz frequency domain, and was attributed to grain boundaries in the ionomeric membrane electrolyte. They showed a decrease in the charge transfer resistance with an increase in cathode over potential. Song et al. [13] studied polymer electrolyte fuel cell electrodes to find an optimal composition of the electrodes using ac impedance methods. They found a low frequency arc in the impedance spectrum at high over potentials, which are believed to reflect water related transport limitations. Freire and Gonza´lez [14] have studied the effect of temperature, membrane thickness and humidification conditions on the impedance response and concluded that the low frequency relaxation process is mainly due to the flooding of the cathode with liquid water and the consequential shortage of oxygen for the ORR due to its limited diffusivity in water. Gottesfeld and Zawodzinski [15] reported that the high over potential at the cathode is the most important source of loss in the PEMFC, as in all other low temperatures fuel cells. Wagner [16] characterized the membrane electrode assembly in polymer electrolyte fuel cells using impedance spectroscopy. He proved the possibility of separating the cell impedance into electrode impedances and electrolyte resistance with varying the operational conditions, current density, and various flow rates and by simulation of the measured EIS with an equivalent circuit. He calculated the VeI curves and voltage losses (over potential) in the fuel cell integrating over current densities of the individual impedance elements. Most of the studies concentrated on identifying the potential loss and they have been attributed to the ORR activity and electrolyte resistance. To our knowledge the contribution due to GDL with different types of MPL using EIS technique has not been reported. Since GDL play a major role for water and gas transport in PEMFC, the EIS analysis on GDL will help to understand the problems related to gas accessibility, water content, humidification etc., In this paper EIS has been used to
Catalyst layer
a Cathode Anode Membrane Water proofed carbon cloth+ MPL on double side
Catalyst layer
b
Anode
Cathode
Membrane Water proofed carbon cloth+ MPL on single side
Fig. 1 e a and b. Schematic diagram of the DSGDL and SSGDL.
resolve the various contributions to the polarization loss due to GDL with different types of MPL in a PEMFC electrode. Voltage losses from each component have been calculated by modeling the equivalent circuit and also reported the electrical transport properties of each cell component by symmetrical mode of gas supply.
2.
Experimental
2.1.
Membrane electrode assembly (MEA) preparation
Gas diffusion electrodes are prepared using the teflonized gas diffusion backing (GDB) carbon cloth by the proprietary process developed at CFCT [17]. Two different types of MPL’s
Fig. 2 e Equivalent circuit of an electrochemical cell.
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0
a 0 .9
50 C 0
60 C
0 .8
0
70 C
0 .7
Voltage(V)
0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0 0
200
400
600
800
1000
1200
2
C u r r e n t d e n s i t y ( m A /c m )
b
1 .0
0
50 C
0 .9
0
60 C 0
70 C
0 .8 0 .7
Voltage(V)
0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0 0
100
200
300
400
500
600
700
800
900 1000
2
C u r r e n t d e n s it y ( m A /c m )
Fig. 3 e Polarization curves of the MEA’s with DSGDL (a) and SSGDL (b).
were prepared by coating carbon slurry consisting of Vulcan XC72, PTFE solution (66%), ammonium bicarbonate and iso Propyl Alcohol. The MPL slurry is coated on both sides of the backing layer viz., gas channel side and catalyst side until a loading of 3 mg cm2 and labelled as Double Side Gas Diffusion Layer (DSGDL). In the electrode labelled Single Side Coated (SSGDL), the slurry is coated only on one side, facing the catalyst side. The catalyst layers for the electrodes are prepared using Pt/C catalyst from M/s TKK, Japan by spreading the catalyst ink, consisting of 5% Nafion solution, uniformly in the electrode area of 5 cm2. Membrane Electrode Assemblies (MEA) are made by laminating a Nafion 212 membrane with two electrodes on both sides and hot pressed at 130 C at 1 ton. The schematic representation of the DSGDL and SSGDL are shown in Fig. 1a and b. The laminated MEA is inserted between two graphite blocks, which had a multichannel serpentine flow pattern.
The whole assembly is tightened with the help of end plates and bolts for proper compaction and leak prevention, by applying uniform torque to all the bolts. The fuel cell polarization experiments are conducted using an ARBIN test station, which was equipped with a gas humidifier, a mass flow controller, temperature controller, DC electronic load and data logging facility.
2.2.
Electrochemical characterization
2.2.1.
EIS at normal mode
The electrochemical characterization of the fuel cell is done using a Potentiostat/Galvanostat Auto lab, equipped with a frequency response analyzer (FRA) in the frequency ranging from 5 MHz to 100 kHz, in combination with an electronic load while drawing current using humidified hydrogen stream at 50 C.
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Fig. 4 e a and b SM Bode plots and Nyquist plots for SSGDL (B) and DSGDL (*).
2.2.2.
EIS at symmetrical mode
The anode and cathode transfer functions are determined independently using symmetric gas supply of hydrogen or air (oxygen) on both electrodes of the fuel cell at open circuit potential (OCP). Cathode and anode impedances are determined directly by two independent experiments and the measured cell impedance under load is split up into anode impedance, cathode impedance and electrolyte resistance by fitting to an equivalent modeling circuit [16,18]. The equivalent circuit for a simple electrochemical cell is used which includes a solution resistance, a double layer capacitor and a charge transfer or polarization resistance. The equivalent circuit for
the Randles cell is shown in Fig. 2. The double layer capacity is in parallel with the impedance due to the charge transfer reaction.
3.
Results and discussion
3.1.
Effect of MPL on cell performance
The polarization curves are obtained for the MEAs with DSGDL and SSGDL at various temperatures and are given in Fig. 3a and b respectively. From the figures it can be seen that the MEA with DSGDL showed comparatively better performance of
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a
b
Fig. 5 e a and b. Nyquist Plot for H2/air NM operation with DSGDL and SSGDL at RT and at humidification at various loads.
1.2 A cm2 at 0.1 V compared to SSGDL, which gave a higher current density of 0.9 A cm2 at the same voltage. However, the current density at activation and ohmic region is comparatively better for SSGDL. In the ohmic region (0.6V), MEA with SSGDL gave a current density of 0.587 A cm2 compared to DSGDL, where the current density is 0.546 A cm2.
interface namely charge transfer at intermediate frequency and the pseudo capacitance at low frequency. In the high frequency range, pseudo inductive impedance appears which arises due to all the metallic components of the complete single fuel cell assembly [18,19].
3.2.2. 3.2.
AC impedance characteristics
3.2.1.
EIS at symmetrical mode(SM)
EIS measurements are carried out under symmetric gas supply, by supplying with gas of (H2/H2) and (air/air) to the fuel cell electrodes at open circuit potential, in order to evaluate the anode and cathode impedance separately. The results are shown in Fig. 4a and b respectively for H2/H2 and air/air. As can be seen from the figures, the H2/H2 anode impedance is less than half of the air/air cathode impedance. In the case of DSGDL the anode impedance was found to be 0.127 U and for the cathode, it is 5.5U. Although the anode impedance in the case of SSGDL is same as 0.119U, the cathode impedance is 18U, which is three times higher than DSGDL. This high impedance is attributed to the two processes occurring at the
EIS at normal mode(NM): -
EIS measured on the PEMFC (H2/air) in the potential range from OCP to 450 mV by drawing current from the cell, for DSGDL and SSGDL is shown in Fig. 5a and b at RT and at humidification respectively .The Nyquist plot consists of three regions indicating the membrane contribution to the electrode at higher frequency, semicircle at medium frequencies,
Fig. 6 e Equivalent Circuit model for the electrode operated at RT and at humidification.
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Table 1 e Parameteres obtained from the model fit for DSGDL at RT. Circuit simulation elements Physical interpretation Voltage (mV) 650 600 550 500 450
R1
C1
Anode activation kinetics
R2 Solution resistance related to membrane
R3
C2
Cathode activation kinetics
Ra (mU)
Ca (mF)
Rs (mU)
Rc(mU)
Cc (mF)
39.2 29.76 29.02 60.8 60.7
23.98 274.0 207.2 382 421
59.5 107.7 104.9 107.5 107.4
189.9 287.6 339 409 460
309.8 398 380 514 533
characteristic of the charge transfer processes associated with oxygen reduction at lower frequencies. From the impedance plot, it is clear that the high frequency intercept does not change much between OCP and at load. However, at the low frequency the diameter of the arc gradually increases from 0.466 to 0.626 U for DSGDL electrodes while for SSGDL, the impedance increases from 0.2956U to 0.640 U, with increasing current density. This is due to the increased driving force for the oxygen reduction reaction, which is limited by the mass transport due to increased water production at the cathode, leading to increased mass transfer resistance [20]. In both MEAs, there is no change in the membrane resistance at ambient and humidification condition with increasing current density. Under conditions of high ionic currents and or restricted transport in the membrane, the back diffusion of water influences the performance of the cell in terms of fluctuations in the cell voltage. However, the charge transfer resistance increases with increasing current density up to 0, 7 A cm2. Under humidification condition the impedance increases from 0.3 to 0.72 U for SSGDL and for DSGDL, the impedance increases from 0.45 U to above 0.7 U. From the Nyquist plots obtained shown in Fig. 5, it is clear that the MPL coated on both sides of the diffusion medium balanced the product water relatively better than the MPL coated only on single side of the diffusion medium. The two contributions, viz., the liquid water formed at the cathode, which affects the transport of oxygen, and hydration, that limit the transport in the membrane are seen in the Nyquist plot in the low frequency region [14]. The polarization resistance of the electrodes increases with increasing the load, due to the limited proton transport from anode to cathode at higher current densities through the electrolyte. When the
oxygen source is air, the diffusion of oxygen in the gas phase becomes a limiting factor as can be seen from the disturbances in the low frequency region due to the flooding in the cathode region [21]. In addition to the flooding, the other limitation that occur inside the gas diffusion electrodes due to mass transport arising from the accumulation of nitrogen blanket inside the gas diffusion electrodes, makes more difficult for the reactant to reach the catalyst site [22]. Hence mass transport resistance is very high at the cathode compared to anode and membrane, as they contribute towards ohmic resistance. Hence proper air and hydrogen stoichiometry and relative humidity is required to minimize the ohmic, charge and mass transfer resistance. The impedance plot also showed a zero frequency impedance inductive loops at 5 mHz, which is attributed to the side reactions and intermediates formed during the fuel cell operation, as suggested by Makharia et al. [23]. Roy et al [24]. also proposed that that the low frequency inductive loops observed in PEMFC are due to parasitic reactions in which the Pt catalyst reacts to form PtO and subsequently forms Ptþ ions. However, they could also be either due to non-stationary behavior, or the time required to make measurements at low frequencies. This non-stationary behavior could influence the shapes of the low-frequency features.
3.3.
Modeling of equivalent circuit
The most common method to analyze EIS spectra is equivalent circuit modeling. Equivalent circuit model of the electrode at RT and humidification is given in Fig. 6. At higher current density, an increase in the cell impedance is observed and is attributed to the additional over voltage, known as
Table 2 e Parameters obtained from the model fit for SSGDL at RT. Circuit simulation elements Physical interpretation Voltage (mV) 650 600 550 500 450
R1
C1
Anode activation kinetics
R2 Solution resistance related to membrane
R3
C2
Cathode activation kinetics
Ra (mU)
Ca (mF)
Rs (mU)
Rc(mU)
Cc (mF)
39.8 42.4 72.2 86.6 100
244.0 257.2 307.7 324 356
114.4 110.0 109.0 105.9 102.7
302 322 326 358 406
353 384 546 743 1010
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Table 3 e Parameteres obtained from the model fit for DSGDL at humidification. Circuit simulation elements
R1
C1
R2
C2
Cathode activation kinetics
R4
C3
Physical interpretation
Anode activation kinetics
Voltage (mV)
Ra (mU)
Ca (mF)
Rs (mU)
Rc (mU)
Cc (mF)
Rc (mU)
Cc (mF)
15.05 16.74 31.46 42.7 10.78
98.2 193.3 321 364 164.2
89.6 92.0 92.3 91.8 88.4
154.0 181.6 251.2 207.1 98.7
358 385 455 629 514
336 355 284.3 410 503
658 981 1834 1871 1125
R3
C2
R4
C3
650 600 550 500 450
Solution resistance related to membrane
R3
Nernst Diffusion Impedance related to additional process
Table 4 e Parameteres obtained from the model fit for SSGDL at humidification. Circuit simulation elements
R1
C1
R2
Physical interpretation
Anode activation kinetics
Voltage (mV)
Ra (mU)
Ca (mF)
Rs (mU)
Rc (mU)
Cc (mF)
Rc (mU)
Cc (mF)
18.21 17.76 20.27 13.92 9.79
144.2 152.6 188.6 175.7 42.0
102.7 104.7 105.6 106.1 103.4
172.7 162.8 141.0 136.3 133.7
213 205 323 504 468
236.5 223.6 360 382 482
400 388 384 759 820
650 600 550 500 450
Solution resistance related to membrane
diffusion over voltage “Nernst diffusion” [25]. Resistance and capacitance values are obtained by a linear least squares fit to the AC response data at various load currents. AC response data are then fitted to the circuit values calculated from the above load current fit. Model fit is reasonably accurate with a minimum number of parameters and without any nonideal circuit element. Numerical values extracted by fitting the data to the circuit elements correlated to physical or chemical properties.
Cathode activation kinetics
Nernst Diffusion Impedance related to additional process
Tables 1 and 2 shows the parameters evaluated from a fit of EIS for the equivalent circuit obtained for the DSGDL and SSGDL respectively at room temperature and Tables 3 and 4 shows for DSGDL and SSGDL at humidification condition respectively. From the parameter values obtained, it is clear that there is not much difference in the impedance related to membrane for both MEA’s and only a slight variation in electrolyte resistance is obtained with increasing current density. However under humid conditions, the impedance is lower for
Fig. 7 e Comparison of impedance plots for PEMFC with model fit for both Nyquist and bode plot.
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DSGDL compared to SSGDL electrode. However, the cathode impedance increases with increasing current density from 189.9 mU to 460 mU for DSGDL and 302 mUe406 mU for SSGDL electrodes. The effect of oxygen diffusion in the electrode is revealed in the charge transfer resistance, by the decreased low frequency arc as the over potential is increased, due to faster ORR kinetics. By performing fitting, the interfacial capacitances and ionic resistances of PEMFC membrane electrode assemblies (MEA’s) can be obtained at various current densities. We considered an additional parameter viz., Nernst diffusion to the equivalent circuit for the impedance simulation, under humidification condition and observed that this loss is higher compared to other three losses as shown in Tables 3 and 4. At humidification there is a change of 336e504 mU for DSGDL and 236e482 mU for SSGDL which is higher than that of the impedance obtained due to cathode for both the MEA’s.
3.4.
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Voltage loss calculated from the equivalent circuit (EC)
The impedance spectra measured at 650 mV for DSGDL at RT is shown in Fig. 7. A set of parameters were obtained to calculate the voltage loss due to various components of PEMFC from the product of the impedance parameters magnitude (Rct(A), Rct(M), Rct(C), Rct(N)) at various current densities. At room temperature the cathode loss is high compared to anode and electrolyte, while the loss due to “Nernst diffusion” is high at humidification. At lower current density region, the losses due to anode, membrane and cathode is comparatively lower for both the MEA’s. However, as the current density increasing, the loss due to three parameters rises immediately and the values are higher for both the MEA’s. As shown in Fig. 8a and b, it is clear that the diffusion component play a major role in the cathode impedance, as seen in the steep increase in voltage loss, due to increasing current density, causing an increase in the diffusion related issues mentioned
Fig. 8 e a and b .Voltage losses (E) at the PEMFC at RT and 50 C as a function of current density, calculated after simulation for DSGDL and SSGDL.
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as Nernst diffusion. From this result it is evident that the types of GDL play a major role in the performance in addition to the operation condition. As shown in Fig. 8, the plot obtained for voltage loss is in good agreement with the polarization curve obtained for both the MEA’s with different MPL’s.
4.
[9]
conclusion [10]
We have demonstrated an ac impedance measurements method for resolving voltage losses due to the individual resistance by comparing two different gas diffusion layers. The DSGDL is favorable for operating the cell at higher temperature and relative humidity while SSGDL is favorable under dry gas operation. The analysis of the experiment on “Symmetrical mode of gas supply” shows that, the cathode contribution for the voltage loss is very high compared to anode and electrolyte. However, the diffusion resistance plays a major role in the voltage loss, when the cells are operated at high current densities and high humidification. The parameters obtained from the model fit shows good agreement with the experimental analysis.
[11]
[12]
[13]
[14]
Acknowledgements
[15]
The authors would like to acknowledge Dr.G.Sundararajan, Director, and ARCI for his constant encouragement and support. The financial assistance from Department of Science and Technology, Govt of India is acknowledged herewith.
[16]
references
[17]
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