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Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes
Electrochimica Acta 51 (2006) 5239–5245
Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes Raimundo R. Passos a,b , Valdecir A. Paganin a , Edson A. Ticianelli a,∗ a b
Instituto de Qu´ımica de S˜ao Carlos-USP, Departmento de Fisico Quimica, Av. Trabalhador Saocarlense, 400, C.P. 780, 13560-970 S˜ao Carlos, SP, Brazil Universidade Federal do Amazonas-UFAM, ICE/Departamento de Qu´ımica, Av. Gal. Rodrigo O.J. Ramos, 3000, Bloco H, 69077-000 Manaus, AM, Brazil Received 30 November 2005; received in revised form 23 January 2006; accepted 25 January 2006 Available online 28 February 2006
Abstract The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm−2 (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm−2 is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O2 pressure, which raises the fuel cell current densities to very high values (>4.0A cm−2 , for Nafion 112—50 m). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H2 /air PEMFCs, the positive effect of pressure is seen even for thick membranes. © 2006 Elsevier Ltd. All rights reserved. Keywords: PEM fuel cell; Catalyst layer; Nafion loading; Pt catalyst; Nafion membrane
1. Introduction Fuel cells are appearing as a good alternative to dirty and wasteful combustion engines for electric power generation. Among the several technologies, the proton exchange membrane fuel cell (PEMFC) has been one of the most studied systems, because of the several advantages for mobile and stationary applications [1,2], but strong efforts to develop more efficient and low cost PEMFC systems are still needed. There are many factors that may be considered to increase the efficiency, one of them being the optimization of the structure of the so called triple-phase boundary in the catalyst layer of the gas diffusion electrodes, where the reaction takes place. Therefore, many studies have been made aiming of optimizing the structure of the membrane and electrode assembly (MEA), including, for example, the catalyst and the Nafion contents in the active layer of the electrode [3–17].
∗
Corresponding author. Tel.: +55 16 3373 9945; fax: +55 16 3373 9952. E-mail address: [email protected] (E.A. Ticianelli).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.01.044
It has been observed that the performance of the PEMFC is significantly dependent on the Nafion content into the electrode. Several methods have been employed to incorporate this electrolyte into the electrode, and these include painting a Nafion solution in a previously prepared catalyst layer [3], preparing a Nafion/catalyst ink which is then applied, either directly or via a decal process, to the membrane [8–10], or painting the catalyst ink onto the diffusion layer and then hot press to the membrane [11–16]. Independent of the procedure, it has been found that an increase of the Nafion loading in the catalyst layer leads to two beneficial effects: one is to increase the electrochemical active area and the other is to improve the overall ionic conductivity. Nevertheless, there is an optimum Nafion loading, because too high Nafion contents may difficult gas diffusion in the polymer/catalyst agglomerate regions, resulting in mass transport limitations [17]. Despite the improvement of the performance related to the Nafion loading, the electrocatalysis of the oxygen reduction reaction (ORR) is still a critical problem for the PEMFC technology. Pt-based electrocatalysts are the most active materials for the ORR in the polymer electrolyte fuel cell. This is well
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recognized and still under intensive investigation, as demonstrated by recent reviews and articles [18–21]. Thus, efforts, either to reduce the Pt-loading or to maximize the Pt-utilization, are needed for building more efficient PEM fuel cell systems. In this work, the performance of a PEMFC with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated for the characterization of the effects of the Nafion content and the catalyst loading in the electrode, and also of the membrane thickness and gases pressures. Studies were conducted by cyclic voltammetry and steady state polarization measurements on a PEM single cell operated with H2 /O2 or H2 /air at 80–85 ◦ C.
and hydrogen to the counter/reference electrode. All experiments were obtained at a scan rate of 20 mV s−1 at room temperature. The polarization measurements were carried out in single cells (4.6 cm2 of active geometric area), with the reactant gases externally humidified using temperature controlled humidification bottles heated to a temperature 5 ◦ C higher than that of the cell for O2 or air and 15 ◦ C higher than that of the cell for hydrogen. Tests of the single cells were conducted in a specially designed test station [23], measuring the cell voltage as a function of the current density. 3. Results and discussion
2. Experimental The electrodes were prepared using platinum on carbon catalyst (20 wt.% Pt/C, E-TEK), a carbon powder (Vulcan XC-72, Cabot), a carbon cloth substrate (PWB-3, Stackpole), a Polytetrafluoretilene suspension (PTFE, TE-306A, DuPont) and a Nafion solution (5 wt.%, H+ form, Aldrich). To form the backing diffusion layer for all electrodes, a homogeneous water suspension of carbon and PTFE (85:15%, respectively) was filtered under vacuum onto both the faces of the carbon cloth substrate (3 mg cm−2 for each face). This composite structure was dried, then baked for 30 min at 280 ◦ C, and finally sintered at 330 ◦ C for 30 min. To prepare the catalyst layer, a suspension was formed from the desired amounts of the Pt/C catalyst and Nafion solution, having isopropanol as solvent. The material was dispersed in an ultrasonic bath for 30 min to form an ink, which was quantitatively deposited onto the membrane (cathode side) by painting the catalyst ink under vacuum at room temperature. This contrasts to the conventional procedure (anode side) in which the catalyst ink is deposited in one of the faces of the backing diffusion layer. After that, the sample was cured at 80 ◦ C for 1 h in an oven. The platinum loading was varied from 0.05 to 0.4 mg cm−2 , and the Nafion loading from 10 to 40 wt.% (dry weight of Nafion with respect to the total weight of the catalyst layer). In all cases the anode (0.4 mg Pt cm−2 , 1.1 mg Nafion cm−2 ) was manufactured by the conventional procedure [11]. The membrane and electrodes assemblies were prepared by hot pressing [3,11], a conventional anode and the backing diffusion layer for the cathode onto the Nafion membrane, at 125 ◦ C under a pressure of 5 MPa for 2 min. For all measurements, pre-treated [11] Nafion (DuPont) membranes with different thickness were used. Scanning electron microscopy (SEM, Zeiss-leica 440) was employed to analyze the MEA structural configuration. Cyclic voltammetry was employed to determine the electrochemical active areas of the electrodes [22], and the experiments were made using a Solartron model 1285 potentiostat/galvanostat. In these experiments, the cathode was used as the working electrode, while anode was employed as both the counter and a reversible hydrogen reference electrode (HRE). The cyclic voltammograms were obtained in the potential range of 0.075–1.2 V, after passing nitrogen to the working electrode
Fig. 1 shows a representative SEM image of a cross-section of a typical MEA prepared in this work. Different parts of the MEA are seen: the central area is the Nafion membrane (Nafion 115); A is the conventional electrode with 35 wt.% Nafion (C35) used in the anode; and B is the cathode in which catalyst layer was applied directly onto the Nafion membrane. Facing each catalyst layer is the carbon cloth-based diffusion layer. In case A it is observed that there is no uniformity in the catalyst layer, because of its strong penetration into the diffusion layer and/or the loss of contact with the membrane, as illustrated by the marked area in region A. This may lead to a decrease of the performance of the electrode in the cell, because the catalyst particles not contacting the membrane may not be active for the electrode reaction. On the contrary, in case B the catalyst layer is more uniform, allowing in principle higher utilization of the catalyst particles. 3.1. Effects of the Nafion loading Fig. 2 shows the hydrogen desorption region of the cyclic voltammograms for electrodes with different Nafion loadings. The electrochemical active areas of Pt were estimated from the charges for hydrogen desorption [3], assuming that
Fig. 1. Scanning electron micrograph of a MEA with a cathode (B) with 35 wt.% (1.1 mg cm−2 ) Nafion in the catalyst layer, 0.4 mg Pt cm−2 , and a conventional anode (A) with 35 wt.% Nafion, 0.4 mg Pt cm−2 .
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Fig. 2. Cyclic voltammograms (20 mV s−1 ; 27 ± 2 ◦ C) of Nafion 117-coated electrodes prepared with 20 wt.% Pt/C and several Nafion contents. Table 1 Electrochemical surface area obtained from the cyclic voltammograms in Fig. 2, showing the influence of the Nafion loading Nafion loading (wt.%) Active Pt area (m2 g−1 )
10 30
15 32
20 49
25 49
30 56
35 58
40 59
210 C cm−2 is needed to produce a monolayer of adsorbed H on polycrystalline platinum. Table 1 shows these results expressed in terms of the active surface area per gram of platinum. From Fig. 2, it is seen that there is an increase of the hydrogen desorption currents when the Nafion content is increased. Consistently, in Table 1 it is observed that the platinum active surface area increases with the increase of Nafion content, but the effect is not so significant above 30 wt.%. Fig. 3 shows the cell performance for cathodes with different Nafion contents, while Fig. 4 presents the cell potential as a function of Nafion loading for several current densities. As can be seen, for low current densities, the fuel cell performance
Fig. 3. Cell potential vs. current density plots of Nafion 117-coated electrodes prepared with 20 wt.% Pt/C and several Nafion contents. Tcell = 80 ◦ C; TO2 = 85 ◦ C; TH2 = 95 ◦ C; PH2 /O2 = 0.1/0.1 MPa.
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Fig. 4. Cell potential as a function of Nafion loading at different current densities. The operation conditions are the same as in Fig. 3.
slightly increase, while for high current densities the performance decreases, with the increase of the Nafion content. Large amounts of electrolyte inside the catalyst layer may help to increase the electrode active area, but may make the access of the gas to the reactive sites difficult, leading to more significant mass transport overpotentials. To obtain more detailed information about the electrode kinetic parameters for the oxygen reduction in these electrodes, the polarization data were analyzed using a non-linear least squares method, using the equation [22,24,25]: E = Eo − b logi − Ri
(1)
where Eo = Er + b log io , Er is the reversible potential for the cell, b the Tafel slope and io is the exchange current density for the oxygen reduction reaction in the Pt/C catalyst. R represents the total contribution for the linear polarization components, which may include the charge transfer resistance of the hydrogen oxidation reaction, the ionic resistance of the electrolyte in the cell, the electronic resistance due to the insulation of the Pt/C catalyst particles and the linear diffusion terms associated to the diffusion limitations of the reactant gases [25]. Here, it is assumed that the resistance of the electrolyte is independent of the current density [26]. Since Eq. (1) does not include diffusion limitations other than linear contributions, and because a change in the Tafel slope is expected for the ORR at an electrode potential around 0.8 V [11], only the data above this potential were considered in the analyses. Table 2 shows the kinetic parameters Eo , b and R obtained by fitting Eq. (1) to the experimental data in Fig. 3. The inset in Fig. 3 shows iR corrected Tafel plots (E + iR) versus log i obtained using the R values in Table 2. As seen in Table 2, the Eo values increased with the increase of the Nafion load up to 30 wt.%, remaining constant after this. This shows that the Nafion content is important to increase the active area only up to this value, in agreement to the results in Table 1. On the other hand, significant variation of R only occurred when passing from 10 to 15 wt.% (a decrease) and from 35 to 40 wt.% (an increase). The first effect is surely related to an
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Table 2 Kinetic parameters obtained from the fitting of Eq. (1) to the experimental polarization results in Fig. 3, for the electrodes with different Nafion contents Nafion (wt.%)
Eo (V)
b (V dec−1 )
R ( cm2 )
10 15 20 25 30 35 40
0.943 0.948 0.950 0.953 0.957 0.958 0.956
0.069 0.066 0.067 0.065 0.064 0.061 0.062
0.26 0.21 0.22 0.22 0.21 0.23 0.36
increase of the electrolyte conductivity inside the catalyst layer of the electrode, while the second indicates that too high Nafion contents may block the gas penetration, leading to the inclusion of linear diffusion components to the R values. Although at high current densities the best fuel cell performance was found for 0.35 mg cm−2 (15 wt.%), the inset in Fig. 3 clearly shows that at low current densities the cell performance is better for the electrodes with higher Nafion contents. This shows that the beneficial effect of the increase of Nafion loading in raising the active area and reducing the ionic resistance of the catalyst layer is compensated by significant losses associated to oxygen mass transport limitations. This effect is confirmed by the larger decrease of the cell potential observed at high current densities, for Nafion loading above 15 wt.%. Results in Table 2 also show that the Tafel slope, b, is near to 65 mV dec−1 , in agreement to the value reported previously for the ORR in the PEMFC for cell potentials above 0.8 V [11]. This result is that expected for the reaction occurring at 80 ◦ C and at a Pt surface with a high adsorbed oxygen coverage [27–29], and indicates absence of structural effects of the Nafion load on the polarization behavior of the PEMFC cathode, which at low current densities behaves like a large area flat plate. 3.2. Effects of the catalyst loading Fig. 5 and Table 3 show the effects of the cathode platinum loading on the PEMFC performance and kinetic parameters for electrodes containing 15 wt.% of Nafion. At low current density (see inset in Fig. 5), as the Pt loading increases, the performance is enhanced. This indicates that there is an increase of the electrochemical active area when the Pt loading is increased, as confirmed from the values of Eo in Table 3. At moderate and high current densities the cathode activity increases with the increase of the Pt loading, reaching a maximum with 0.3/0.4 mg Pt cm−2 . Table 3 Kinetic parameters obtained from the fitting of Eq. (1) to the experimental polarization results in Fig. 5, for the electrodes with different Pt catalyst loading Pt loading (mg cm−2 )
Eo (V)
b (V dec−1 )
R ( cm2 )
0.05 0.1 0.2 0.3 0.4 0.8
0.911 0.924 0.935 0.944 0.948 0.968
0.070 0.067 0.068 0.070 0.066 0.064
0.32 0.30 0.28 0.19 0.21 0.23
Fig. 5. Cell potential vs. current density plots of Nafion 117-coated electrodes prepared with several Pt contents and 15 wt.% Nafion. Others conditions as in Fig. 3.
It is also seen from Table 3 that the cell resistance (R) decreases when the Pt loading increases up to 0.3 mg Pt cm−2 , after which the trend is reversed. These results denote three main effects of the increase of the Pt loading: (i) the expected increase of the active area caused by the increase of the catalyst amount; (ii) an initial improvement of the electronic conductivity due to the improvement of current collection caused by a more close contact of the Pt/C particles; (iii) the appearance of mass transport limitations for the higher catalyst loading caused by the increase of the electrode thickness, which favors flooding and increases the path length for the reactant gas diffusion. An important factor related to the membrane-coated electrode is the better characteristics of the catalyst-layer/membrane interface, even for low catalyst contents. Therefore, for the catalyst layer applied directly onto the membrane it is observed (Fig. 5) that a decrease of the usual Pt loading from 0.4 to 0.1 mg Pt cm−2 is possible, without having serious problems with the fuel cell performance. Finally, it must be mentioned that the fuel cell performance reported here is very good and had never been presented in the literature for the catalyst layer deposited directly onto the membrane, when using Nafion 117. 3.3. Effects of the membrane thickness and gas pressure Fig. 6 shows the fuel cell performance for different Nafion membrane thicknesses and in Table 4 are the corresponding kinetic parameters. As expected, a decrease of the membrane thickness favors the performance at all ranges of current densities, and this fact is related to a decrease of R (Table 4) caused by a lowering of the overall electrolyte resistance. However, it must be noted that the values of R are not directly proportional to the membrane thickness, with the cells with the thinner membranes showing too high values of R with respect to those for the thicker ones, or vice versa. In fact, the values of R in Table 4 (at 0.1 MPa) can be approximately represented
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Fig. 6. Cell potential vs. current density plots of Nafion 117-coated electrodes prepared with 0.4 mg Pt cm−2 and 15 wt.% Nafion. MEAs with different membrane thicknesses. Others conditions as in Fig. 3.
Fig. 7. Cell potential vs. current density plots for H2 /O2 —0.2/0.3 MPa. Nafioncoated electrodes prepared with 0.4 mg Pt cm−2 and 15 wt.% Nafion. MEAs with different membrane thicknesses. Tcell = 85 ◦ C; TO2 = 90 ◦ C; TH2 = 100 ◦ C.
by a linear formula of the type R = 0.053 + 9L, where L is the membrane thickness (cm) for R expressed in cm2 . This indicates that R contains a constant contribution, which probably includes the polarization resistance of the hydrogen electrode, contact resistances through the system, including those at the membrane/catalyst-layer/diffusion-layer interfaces, and other contributions due to discontinuities in the structure of the membrane between the surface and bulk regions, this being more important for thin membranes [30]. As expected, the Eo values remained essentially independent of the membrane thickness, except for Nafion 112. This deviation may be attributed to a larger gas crossover due to the small thickness of this membrane. The effects of increasing the gases pressure (H2 and pure O2 ) on the PEMFC performance and the kinetic parameters for different membrane thicknesses are shown in Fig. 7 and Table 4. For the thinner membranes (Nafion 112 and 1135) the effect of pressure is high, raising the current densities to very high values (>4.0 A cm−2 , for Nafion 112). This trend is not apparent for Nafion 115 and 117, for which the PEMFC performances seen in Fig. 7 are equivalent to those in Fig. 6, particularly at high current densities. On the other hand, in Table 4 it is seen that Eo increases with the increase of the gas pressure, as a result of the increase of the reversible potential of the fuel cell reactions
and the increase of the exchange current density of the oxygen reduction reaction due to the increased gases solubility [11]. The independence of the limiting current with the increase of pressure (Figs. 6 and 7) for the thicker membranes is a surprising result, and in principle must be related to an increased membrane resistance and/or to gas diffusion complication caused by the cathode flooding. To analyze this point, it may be recalled that the water is supplied to the membrane through the vapor that saturates the reactant gases and by the water formed in the cell reaction at the cathode side. At the same time, water is carried from anode to cathode together with the protons that carry the current through the membrane (electro-osmotic effect) [26] and this builds a higher water content on the cathode side compared to the anode. As a consequence, there is a back diffusion of water from the cathode to the anode, a fact that may reduce problems related to the membrane drying in the anode side and/or of the cathode flooding, but this effect is less effective for thick membranes [29]. Measurements of the membrane resistance at high current densities [30] have indicated the membrane drying at the anode face for Nafion 115 and 117 for single cells working with pure oxygen. Thus, in these cases the limiting current must be controlled by the proton transport in the membrane explaining the little effect of oxygen pressure for Nafion 117 and 115. The polarization results for H2 /air PEMFC (Nafion 117) single cells at several gases pressures with the membrane-coated
Table 4 Kinetic parameters obtained from the fitting of Eq. (1) to the experimental polarization results for MEAs with different membrane thicknesses and for the oxygen pressures of 0.1 and 0.3 MPa Membrane—thicknesses (m)
0.1/0.1 MPa Eo
N117—175 N115—125 N1135—90 N112—50
(V)
0.948 0.948 0.949 0.944
0.2/0.3 MPa b
(V dec−1 )
0.066 0.071 0.067 0.068
R
( cm2 )
0.21 0.18 0.15 0.10
Eo (V)
b (V dec−1 )
R ( cm2 )
1.005 0.994 1.001 0.999
0.068 0.072 0.068 0.078
0.19 0.14 0.14 0.09
Cathode catalyst layers with 15 wt.% Nafion, 0.4 mg Pt cm−2 . Tcell = 80 ◦ C; TO2 = 85 ◦ C; TH2 = 95 ◦ C; PH2 /O2 = 0.1/0.1 MPa and Tcell = 85 ◦ C; TO2 = 90 ◦ C; TH2 = 100 ◦ C; PH2 /O2 = 0.2/0.3 MPa.
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Fig. 8. Cell potential vs. current density plots for H2 /air of Nafion 117-coated electrodes with 15 wt.% Nafion, and conventional electrode (C-35, 35 wt.% Nafion). Tcell = 80 ◦ C; Tair = 85 ◦ C; TH2 = 95 ◦ C, PH2 /air = 0.1/0.1 MPa and Tcell = 85 ◦ C; Tair = 90 ◦ C; TH2 = 100 ◦ C for PH2 /air = 0.2/0.3 MPa and PH2 /air = 0.2/0.5 MPa.
electrode (15 wt.% Nafion) and a conventional (C-35) MEA are shown in Fig. 8. It is seen that the single cell performance is better for the membrane-coated electrode, independent of the gas pressure, and this can be attributed to a better oxygen reactant supply into the catalyst layer, as also observed with pure oxygen. The smaller apparent effect of membrane drying is a consequence of the smaller current densities attained with air in comparison to pure oxygen. 4. Conclusion Application of the catalyst layer directly onto the Nafion membrane favors an improvement of the catalystlayer/membrane interface structure, leading to an increase of the catalyst utilization compared to conventional electrodes. The increase of the Nafion content in these layers helps to improve the electrode active area and the electrolyte conductivity, but too high contents make the access of the gas to the reactive sites difficult, leading to more significant mass transport overpotentials. At high current densities the best fuel cell performance was found for 0.35 mg cm−2 (15 wt.%), which presented minimized mass transport limitations, while at low current densities the cell performance is better for the electrodes with higher Nafion contents due to the improved electrode active area.
The results denote three main effects of the increase of the Pt loading in the catalyst layer of the electrode: (i) the expected increase of the active area caused by the raise of the catalyst amount; (ii) an initial improvement of the electronic conductivity due to the improvement of current collection caused by a more close contact of the Pt/C particles; (iii) the appearance of mass transport limitations for the higher catalyst loading caused by the increase of the electrode thickness, which favors flooding and raises the length for the reactant gas diffusion. As a result, the cathode activity increases with the increase of the Pt loading, reaching a maximum with 0.3/0.4 mg Pt cm−2 , and decreasing after that. However, it is also observed that a decrease of the usual Pt loading from 0.4 to ca. 0.1 mg Pt cm−2 is possible, without introducing serious problems to the fuel cell performance. In fact, the fuel cell performance observed for such low Pt loading electrode is considerably higher than that obtained for the conventional electrode working with Nafion 117 under similar operational conditions [11]. As expected, a decrease of the membrane thickness favors the fuel performance at all ranges of current densities, and this fact is related to a decrease of R caused by a lowering of the overall electrolyte resistance. However, the results indicate that R contains a contribution not related to the membrane thickness, which probably represents all contact resistances through the system, including those related to the membrane surface and to the catalyst-layer/diffusion-layer interface. When pure oxygen is used in the cathode and for the thinner membranes (Nafion 112 and 1135) there is a positive effect of the increase of the O2 pressure, which raises the fuel cell current densities to very high values (>4.0 A cm−2 , for Nafion 112). This trend is not apparent for thicker membranes (Nafion 115 and 117), for which there is a negligible effect of pressure at high current densities. For H2 /air PEMFCs, the positive effect of pressure is seen even for thick membranes. On the other hand, the increase of the oxygen partial pressure leads to an increase of the reversible potential of the fuel cell reactions and of the exchange current density of the oxygen reduction reaction caused by an increase of the gases solubility. Acknowledgements The authors wish to thank the support of Coordenac¸a˜ o de Aperfeic¸oamento de Pessoal de N´ıvel Superior (CAPES/PQI), Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq), Brazil. The authors also thank Carlos A.S. Bento for the SEM experiments. References [1] A.J. Appleby, Sci. Am. 281 (1999) 58. [2] A.J. Charnah, J. Power Sources 86 (2000) 130. [3] E.A. Ticianelli, C.R. Derouin, A. Redondo, S. Srinivasan, J. Electrochem. Soc. 135 (1988) 2209. [4] S. Hirano, J. Kim, S. Srinivasan, Electrochim. Acta 42 (1997) 1587. [5] M.S. Wilson, J.A. Valerio, S. Gottesfeld, Electrochim. Acta 40 (1995) 355. [6] X. Cheng, B. Yi, M. Han, J. Zhang, Y. Qiao, J. Yu, J. Power Sources 79 (1999) 75.
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Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes Raimundo R. Passos a,b , Valdecir A. Paganin a , Edson A. Ticianelli a,∗ a b
Instituto de Qu´ımica de S˜ao Carlos-USP, Departmento de Fisico Quimica, Av. Trabalhador Saocarlense, 400, C.P. 780, 13560-970 S˜ao Carlos, SP, Brazil Universidade Federal do Amazonas-UFAM, ICE/Departamento de Qu´ımica, Av. Gal. Rodrigo O.J. Ramos, 3000, Bloco H, 69077-000 Manaus, AM, Brazil Received 30 November 2005; received in revised form 23 January 2006; accepted 25 January 2006 Available online 28 February 2006
Abstract The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm−2 (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm−2 is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O2 pressure, which raises the fuel cell current densities to very high values (>4.0A cm−2 , for Nafion 112—50 m). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H2 /air PEMFCs, the positive effect of pressure is seen even for thick membranes. © 2006 Elsevier Ltd. All rights reserved. Keywords: PEM fuel cell; Catalyst layer; Nafion loading; Pt catalyst; Nafion membrane
1. Introduction Fuel cells are appearing as a good alternative to dirty and wasteful combustion engines for electric power generation. Among the several technologies, the proton exchange membrane fuel cell (PEMFC) has been one of the most studied systems, because of the several advantages for mobile and stationary applications [1,2], but strong efforts to develop more efficient and low cost PEMFC systems are still needed. There are many factors that may be considered to increase the efficiency, one of them being the optimization of the structure of the so called triple-phase boundary in the catalyst layer of the gas diffusion electrodes, where the reaction takes place. Therefore, many studies have been made aiming of optimizing the structure of the membrane and electrode assembly (MEA), including, for example, the catalyst and the Nafion contents in the active layer of the electrode [3–17].
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Corresponding author. Tel.: +55 16 3373 9945; fax: +55 16 3373 9952. E-mail address: [email protected] (E.A. Ticianelli).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.01.044
It has been observed that the performance of the PEMFC is significantly dependent on the Nafion content into the electrode. Several methods have been employed to incorporate this electrolyte into the electrode, and these include painting a Nafion solution in a previously prepared catalyst layer [3], preparing a Nafion/catalyst ink which is then applied, either directly or via a decal process, to the membrane [8–10], or painting the catalyst ink onto the diffusion layer and then hot press to the membrane [11–16]. Independent of the procedure, it has been found that an increase of the Nafion loading in the catalyst layer leads to two beneficial effects: one is to increase the electrochemical active area and the other is to improve the overall ionic conductivity. Nevertheless, there is an optimum Nafion loading, because too high Nafion contents may difficult gas diffusion in the polymer/catalyst agglomerate regions, resulting in mass transport limitations [17]. Despite the improvement of the performance related to the Nafion loading, the electrocatalysis of the oxygen reduction reaction (ORR) is still a critical problem for the PEMFC technology. Pt-based electrocatalysts are the most active materials for the ORR in the polymer electrolyte fuel cell. This is well
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recognized and still under intensive investigation, as demonstrated by recent reviews and articles [18–21]. Thus, efforts, either to reduce the Pt-loading or to maximize the Pt-utilization, are needed for building more efficient PEM fuel cell systems. In this work, the performance of a PEMFC with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated for the characterization of the effects of the Nafion content and the catalyst loading in the electrode, and also of the membrane thickness and gases pressures. Studies were conducted by cyclic voltammetry and steady state polarization measurements on a PEM single cell operated with H2 /O2 or H2 /air at 80–85 ◦ C.
and hydrogen to the counter/reference electrode. All experiments were obtained at a scan rate of 20 mV s−1 at room temperature. The polarization measurements were carried out in single cells (4.6 cm2 of active geometric area), with the reactant gases externally humidified using temperature controlled humidification bottles heated to a temperature 5 ◦ C higher than that of the cell for O2 or air and 15 ◦ C higher than that of the cell for hydrogen. Tests of the single cells were conducted in a specially designed test station [23], measuring the cell voltage as a function of the current density. 3. Results and discussion
2. Experimental The electrodes were prepared using platinum on carbon catalyst (20 wt.% Pt/C, E-TEK), a carbon powder (Vulcan XC-72, Cabot), a carbon cloth substrate (PWB-3, Stackpole), a Polytetrafluoretilene suspension (PTFE, TE-306A, DuPont) and a Nafion solution (5 wt.%, H+ form, Aldrich). To form the backing diffusion layer for all electrodes, a homogeneous water suspension of carbon and PTFE (85:15%, respectively) was filtered under vacuum onto both the faces of the carbon cloth substrate (3 mg cm−2 for each face). This composite structure was dried, then baked for 30 min at 280 ◦ C, and finally sintered at 330 ◦ C for 30 min. To prepare the catalyst layer, a suspension was formed from the desired amounts of the Pt/C catalyst and Nafion solution, having isopropanol as solvent. The material was dispersed in an ultrasonic bath for 30 min to form an ink, which was quantitatively deposited onto the membrane (cathode side) by painting the catalyst ink under vacuum at room temperature. This contrasts to the conventional procedure (anode side) in which the catalyst ink is deposited in one of the faces of the backing diffusion layer. After that, the sample was cured at 80 ◦ C for 1 h in an oven. The platinum loading was varied from 0.05 to 0.4 mg cm−2 , and the Nafion loading from 10 to 40 wt.% (dry weight of Nafion with respect to the total weight of the catalyst layer). In all cases the anode (0.4 mg Pt cm−2 , 1.1 mg Nafion cm−2 ) was manufactured by the conventional procedure [11]. The membrane and electrodes assemblies were prepared by hot pressing [3,11], a conventional anode and the backing diffusion layer for the cathode onto the Nafion membrane, at 125 ◦ C under a pressure of 5 MPa for 2 min. For all measurements, pre-treated [11] Nafion (DuPont) membranes with different thickness were used. Scanning electron microscopy (SEM, Zeiss-leica 440) was employed to analyze the MEA structural configuration. Cyclic voltammetry was employed to determine the electrochemical active areas of the electrodes [22], and the experiments were made using a Solartron model 1285 potentiostat/galvanostat. In these experiments, the cathode was used as the working electrode, while anode was employed as both the counter and a reversible hydrogen reference electrode (HRE). The cyclic voltammograms were obtained in the potential range of 0.075–1.2 V, after passing nitrogen to the working electrode
Fig. 1 shows a representative SEM image of a cross-section of a typical MEA prepared in this work. Different parts of the MEA are seen: the central area is the Nafion membrane (Nafion 115); A is the conventional electrode with 35 wt.% Nafion (C35) used in the anode; and B is the cathode in which catalyst layer was applied directly onto the Nafion membrane. Facing each catalyst layer is the carbon cloth-based diffusion layer. In case A it is observed that there is no uniformity in the catalyst layer, because of its strong penetration into the diffusion layer and/or the loss of contact with the membrane, as illustrated by the marked area in region A. This may lead to a decrease of the performance of the electrode in the cell, because the catalyst particles not contacting the membrane may not be active for the electrode reaction. On the contrary, in case B the catalyst layer is more uniform, allowing in principle higher utilization of the catalyst particles. 3.1. Effects of the Nafion loading Fig. 2 shows the hydrogen desorption region of the cyclic voltammograms for electrodes with different Nafion loadings. The electrochemical active areas of Pt were estimated from the charges for hydrogen desorption [3], assuming that
Fig. 1. Scanning electron micrograph of a MEA with a cathode (B) with 35 wt.% (1.1 mg cm−2 ) Nafion in the catalyst layer, 0.4 mg Pt cm−2 , and a conventional anode (A) with 35 wt.% Nafion, 0.4 mg Pt cm−2 .
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Fig. 2. Cyclic voltammograms (20 mV s−1 ; 27 ± 2 ◦ C) of Nafion 117-coated electrodes prepared with 20 wt.% Pt/C and several Nafion contents. Table 1 Electrochemical surface area obtained from the cyclic voltammograms in Fig. 2, showing the influence of the Nafion loading Nafion loading (wt.%) Active Pt area (m2 g−1 )
10 30
15 32
20 49
25 49
30 56
35 58
40 59
210 C cm−2 is needed to produce a monolayer of adsorbed H on polycrystalline platinum. Table 1 shows these results expressed in terms of the active surface area per gram of platinum. From Fig. 2, it is seen that there is an increase of the hydrogen desorption currents when the Nafion content is increased. Consistently, in Table 1 it is observed that the platinum active surface area increases with the increase of Nafion content, but the effect is not so significant above 30 wt.%. Fig. 3 shows the cell performance for cathodes with different Nafion contents, while Fig. 4 presents the cell potential as a function of Nafion loading for several current densities. As can be seen, for low current densities, the fuel cell performance
Fig. 3. Cell potential vs. current density plots of Nafion 117-coated electrodes prepared with 20 wt.% Pt/C and several Nafion contents. Tcell = 80 ◦ C; TO2 = 85 ◦ C; TH2 = 95 ◦ C; PH2 /O2 = 0.1/0.1 MPa.
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Fig. 4. Cell potential as a function of Nafion loading at different current densities. The operation conditions are the same as in Fig. 3.
slightly increase, while for high current densities the performance decreases, with the increase of the Nafion content. Large amounts of electrolyte inside the catalyst layer may help to increase the electrode active area, but may make the access of the gas to the reactive sites difficult, leading to more significant mass transport overpotentials. To obtain more detailed information about the electrode kinetic parameters for the oxygen reduction in these electrodes, the polarization data were analyzed using a non-linear least squares method, using the equation [22,24,25]: E = Eo − b logi − Ri
(1)
where Eo = Er + b log io , Er is the reversible potential for the cell, b the Tafel slope and io is the exchange current density for the oxygen reduction reaction in the Pt/C catalyst. R represents the total contribution for the linear polarization components, which may include the charge transfer resistance of the hydrogen oxidation reaction, the ionic resistance of the electrolyte in the cell, the electronic resistance due to the insulation of the Pt/C catalyst particles and the linear diffusion terms associated to the diffusion limitations of the reactant gases [25]. Here, it is assumed that the resistance of the electrolyte is independent of the current density [26]. Since Eq. (1) does not include diffusion limitations other than linear contributions, and because a change in the Tafel slope is expected for the ORR at an electrode potential around 0.8 V [11], only the data above this potential were considered in the analyses. Table 2 shows the kinetic parameters Eo , b and R obtained by fitting Eq. (1) to the experimental data in Fig. 3. The inset in Fig. 3 shows iR corrected Tafel plots (E + iR) versus log i obtained using the R values in Table 2. As seen in Table 2, the Eo values increased with the increase of the Nafion load up to 30 wt.%, remaining constant after this. This shows that the Nafion content is important to increase the active area only up to this value, in agreement to the results in Table 1. On the other hand, significant variation of R only occurred when passing from 10 to 15 wt.% (a decrease) and from 35 to 40 wt.% (an increase). The first effect is surely related to an
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Table 2 Kinetic parameters obtained from the fitting of Eq. (1) to the experimental polarization results in Fig. 3, for the electrodes with different Nafion contents Nafion (wt.%)
Eo (V)
b (V dec−1 )
R ( cm2 )
10 15 20 25 30 35 40
0.943 0.948 0.950 0.953 0.957 0.958 0.956
0.069 0.066 0.067 0.065 0.064 0.061 0.062
0.26 0.21 0.22 0.22 0.21 0.23 0.36
increase of the electrolyte conductivity inside the catalyst layer of the electrode, while the second indicates that too high Nafion contents may block the gas penetration, leading to the inclusion of linear diffusion components to the R values. Although at high current densities the best fuel cell performance was found for 0.35 mg cm−2 (15 wt.%), the inset in Fig. 3 clearly shows that at low current densities the cell performance is better for the electrodes with higher Nafion contents. This shows that the beneficial effect of the increase of Nafion loading in raising the active area and reducing the ionic resistance of the catalyst layer is compensated by significant losses associated to oxygen mass transport limitations. This effect is confirmed by the larger decrease of the cell potential observed at high current densities, for Nafion loading above 15 wt.%. Results in Table 2 also show that the Tafel slope, b, is near to 65 mV dec−1 , in agreement to the value reported previously for the ORR in the PEMFC for cell potentials above 0.8 V [11]. This result is that expected for the reaction occurring at 80 ◦ C and at a Pt surface with a high adsorbed oxygen coverage [27–29], and indicates absence of structural effects of the Nafion load on the polarization behavior of the PEMFC cathode, which at low current densities behaves like a large area flat plate. 3.2. Effects of the catalyst loading Fig. 5 and Table 3 show the effects of the cathode platinum loading on the PEMFC performance and kinetic parameters for electrodes containing 15 wt.% of Nafion. At low current density (see inset in Fig. 5), as the Pt loading increases, the performance is enhanced. This indicates that there is an increase of the electrochemical active area when the Pt loading is increased, as confirmed from the values of Eo in Table 3. At moderate and high current densities the cathode activity increases with the increase of the Pt loading, reaching a maximum with 0.3/0.4 mg Pt cm−2 . Table 3 Kinetic parameters obtained from the fitting of Eq. (1) to the experimental polarization results in Fig. 5, for the electrodes with different Pt catalyst loading Pt loading (mg cm−2 )
Eo (V)
b (V dec−1 )
R ( cm2 )
0.05 0.1 0.2 0.3 0.4 0.8
0.911 0.924 0.935 0.944 0.948 0.968
0.070 0.067 0.068 0.070 0.066 0.064
0.32 0.30 0.28 0.19 0.21 0.23
Fig. 5. Cell potential vs. current density plots of Nafion 117-coated electrodes prepared with several Pt contents and 15 wt.% Nafion. Others conditions as in Fig. 3.
It is also seen from Table 3 that the cell resistance (R) decreases when the Pt loading increases up to 0.3 mg Pt cm−2 , after which the trend is reversed. These results denote three main effects of the increase of the Pt loading: (i) the expected increase of the active area caused by the increase of the catalyst amount; (ii) an initial improvement of the electronic conductivity due to the improvement of current collection caused by a more close contact of the Pt/C particles; (iii) the appearance of mass transport limitations for the higher catalyst loading caused by the increase of the electrode thickness, which favors flooding and increases the path length for the reactant gas diffusion. An important factor related to the membrane-coated electrode is the better characteristics of the catalyst-layer/membrane interface, even for low catalyst contents. Therefore, for the catalyst layer applied directly onto the membrane it is observed (Fig. 5) that a decrease of the usual Pt loading from 0.4 to 0.1 mg Pt cm−2 is possible, without having serious problems with the fuel cell performance. Finally, it must be mentioned that the fuel cell performance reported here is very good and had never been presented in the literature for the catalyst layer deposited directly onto the membrane, when using Nafion 117. 3.3. Effects of the membrane thickness and gas pressure Fig. 6 shows the fuel cell performance for different Nafion membrane thicknesses and in Table 4 are the corresponding kinetic parameters. As expected, a decrease of the membrane thickness favors the performance at all ranges of current densities, and this fact is related to a decrease of R (Table 4) caused by a lowering of the overall electrolyte resistance. However, it must be noted that the values of R are not directly proportional to the membrane thickness, with the cells with the thinner membranes showing too high values of R with respect to those for the thicker ones, or vice versa. In fact, the values of R in Table 4 (at 0.1 MPa) can be approximately represented
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Fig. 6. Cell potential vs. current density plots of Nafion 117-coated electrodes prepared with 0.4 mg Pt cm−2 and 15 wt.% Nafion. MEAs with different membrane thicknesses. Others conditions as in Fig. 3.
Fig. 7. Cell potential vs. current density plots for H2 /O2 —0.2/0.3 MPa. Nafioncoated electrodes prepared with 0.4 mg Pt cm−2 and 15 wt.% Nafion. MEAs with different membrane thicknesses. Tcell = 85 ◦ C; TO2 = 90 ◦ C; TH2 = 100 ◦ C.
by a linear formula of the type R = 0.053 + 9L, where L is the membrane thickness (cm) for R expressed in cm2 . This indicates that R contains a constant contribution, which probably includes the polarization resistance of the hydrogen electrode, contact resistances through the system, including those at the membrane/catalyst-layer/diffusion-layer interfaces, and other contributions due to discontinuities in the structure of the membrane between the surface and bulk regions, this being more important for thin membranes [30]. As expected, the Eo values remained essentially independent of the membrane thickness, except for Nafion 112. This deviation may be attributed to a larger gas crossover due to the small thickness of this membrane. The effects of increasing the gases pressure (H2 and pure O2 ) on the PEMFC performance and the kinetic parameters for different membrane thicknesses are shown in Fig. 7 and Table 4. For the thinner membranes (Nafion 112 and 1135) the effect of pressure is high, raising the current densities to very high values (>4.0 A cm−2 , for Nafion 112). This trend is not apparent for Nafion 115 and 117, for which the PEMFC performances seen in Fig. 7 are equivalent to those in Fig. 6, particularly at high current densities. On the other hand, in Table 4 it is seen that Eo increases with the increase of the gas pressure, as a result of the increase of the reversible potential of the fuel cell reactions
and the increase of the exchange current density of the oxygen reduction reaction due to the increased gases solubility [11]. The independence of the limiting current with the increase of pressure (Figs. 6 and 7) for the thicker membranes is a surprising result, and in principle must be related to an increased membrane resistance and/or to gas diffusion complication caused by the cathode flooding. To analyze this point, it may be recalled that the water is supplied to the membrane through the vapor that saturates the reactant gases and by the water formed in the cell reaction at the cathode side. At the same time, water is carried from anode to cathode together with the protons that carry the current through the membrane (electro-osmotic effect) [26] and this builds a higher water content on the cathode side compared to the anode. As a consequence, there is a back diffusion of water from the cathode to the anode, a fact that may reduce problems related to the membrane drying in the anode side and/or of the cathode flooding, but this effect is less effective for thick membranes [29]. Measurements of the membrane resistance at high current densities [30] have indicated the membrane drying at the anode face for Nafion 115 and 117 for single cells working with pure oxygen. Thus, in these cases the limiting current must be controlled by the proton transport in the membrane explaining the little effect of oxygen pressure for Nafion 117 and 115. The polarization results for H2 /air PEMFC (Nafion 117) single cells at several gases pressures with the membrane-coated
Table 4 Kinetic parameters obtained from the fitting of Eq. (1) to the experimental polarization results for MEAs with different membrane thicknesses and for the oxygen pressures of 0.1 and 0.3 MPa Membrane—thicknesses (m)
0.1/0.1 MPa Eo
N117—175 N115—125 N1135—90 N112—50
(V)
0.948 0.948 0.949 0.944
0.2/0.3 MPa b
(V dec−1 )
0.066 0.071 0.067 0.068
R
( cm2 )
0.21 0.18 0.15 0.10
Eo (V)
b (V dec−1 )
R ( cm2 )
1.005 0.994 1.001 0.999
0.068 0.072 0.068 0.078
0.19 0.14 0.14 0.09
Cathode catalyst layers with 15 wt.% Nafion, 0.4 mg Pt cm−2 . Tcell = 80 ◦ C; TO2 = 85 ◦ C; TH2 = 95 ◦ C; PH2 /O2 = 0.1/0.1 MPa and Tcell = 85 ◦ C; TO2 = 90 ◦ C; TH2 = 100 ◦ C; PH2 /O2 = 0.2/0.3 MPa.
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Fig. 8. Cell potential vs. current density plots for H2 /air of Nafion 117-coated electrodes with 15 wt.% Nafion, and conventional electrode (C-35, 35 wt.% Nafion). Tcell = 80 ◦ C; Tair = 85 ◦ C; TH2 = 95 ◦ C, PH2 /air = 0.1/0.1 MPa and Tcell = 85 ◦ C; Tair = 90 ◦ C; TH2 = 100 ◦ C for PH2 /air = 0.2/0.3 MPa and PH2 /air = 0.2/0.5 MPa.
electrode (15 wt.% Nafion) and a conventional (C-35) MEA are shown in Fig. 8. It is seen that the single cell performance is better for the membrane-coated electrode, independent of the gas pressure, and this can be attributed to a better oxygen reactant supply into the catalyst layer, as also observed with pure oxygen. The smaller apparent effect of membrane drying is a consequence of the smaller current densities attained with air in comparison to pure oxygen. 4. Conclusion Application of the catalyst layer directly onto the Nafion membrane favors an improvement of the catalystlayer/membrane interface structure, leading to an increase of the catalyst utilization compared to conventional electrodes. The increase of the Nafion content in these layers helps to improve the electrode active area and the electrolyte conductivity, but too high contents make the access of the gas to the reactive sites difficult, leading to more significant mass transport overpotentials. At high current densities the best fuel cell performance was found for 0.35 mg cm−2 (15 wt.%), which presented minimized mass transport limitations, while at low current densities the cell performance is better for the electrodes with higher Nafion contents due to the improved electrode active area.
The results denote three main effects of the increase of the Pt loading in the catalyst layer of the electrode: (i) the expected increase of the active area caused by the raise of the catalyst amount; (ii) an initial improvement of the electronic conductivity due to the improvement of current collection caused by a more close contact of the Pt/C particles; (iii) the appearance of mass transport limitations for the higher catalyst loading caused by the increase of the electrode thickness, which favors flooding and raises the length for the reactant gas diffusion. As a result, the cathode activity increases with the increase of the Pt loading, reaching a maximum with 0.3/0.4 mg Pt cm−2 , and decreasing after that. However, it is also observed that a decrease of the usual Pt loading from 0.4 to ca. 0.1 mg Pt cm−2 is possible, without introducing serious problems to the fuel cell performance. In fact, the fuel cell performance observed for such low Pt loading electrode is considerably higher than that obtained for the conventional electrode working with Nafion 117 under similar operational conditions [11]. As expected, a decrease of the membrane thickness favors the fuel performance at all ranges of current densities, and this fact is related to a decrease of R caused by a lowering of the overall electrolyte resistance. However, the results indicate that R contains a contribution not related to the membrane thickness, which probably represents all contact resistances through the system, including those related to the membrane surface and to the catalyst-layer/diffusion-layer interface. When pure oxygen is used in the cathode and for the thinner membranes (Nafion 112 and 1135) there is a positive effect of the increase of the O2 pressure, which raises the fuel cell current densities to very high values (>4.0 A cm−2 , for Nafion 112). This trend is not apparent for thicker membranes (Nafion 115 and 117), for which there is a negligible effect of pressure at high current densities. For H2 /air PEMFCs, the positive effect of pressure is seen even for thick membranes. On the other hand, the increase of the oxygen partial pressure leads to an increase of the reversible potential of the fuel cell reactions and of the exchange current density of the oxygen reduction reaction caused by an increase of the gases solubility. Acknowledgements The authors wish to thank the support of Coordenac¸a˜ o de Aperfeic¸oamento de Pessoal de N´ıvel Superior (CAPES/PQI), Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq), Brazil. The authors also thank Carlos A.S. Bento for the SEM experiments. References [1] A.J. Appleby, Sci. Am. 281 (1999) 58. [2] A.J. Charnah, J. Power Sources 86 (2000) 130. [3] E.A. Ticianelli, C.R. Derouin, A. Redondo, S. Srinivasan, J. Electrochem. Soc. 135 (1988) 2209. [4] S. Hirano, J. Kim, S. Srinivasan, Electrochim. Acta 42 (1997) 1587. [5] M.S. Wilson, J.A. Valerio, S. Gottesfeld, Electrochim. Acta 40 (1995) 355. [6] X. Cheng, B. Yi, M. Han, J. Zhang, Y. Qiao, J. Yu, J. Power Sources 79 (1999) 75.
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