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Preparation of porous electrodes and laminated electrode-membrane structures for polymer electrolyte fuel cells (PEFC)
__ .i-._ E!Fl
Ja!J ELSEWER
Solid State Ionics 77 ( 1995) 324-330
Preparation of porous electrodes and laminated electrodemembrane structures for polymer electrolyte fuel cells ( PEFC
)
K. Bolwin, E. Giilzow, D. Bevers, W. Schnumberger Institute for Technical Thermodynamics, German Aerospace Research Establishment (DLR). Pjaffenwaldring 38-40.070569 Stuttgart, Germany
AhStTXt Porous NaIion-PTFE-bonded gas-diffusion electrodes (GDEs), for use in (PEFC), were prepared by a cold rolling process. In this process, no solvents are required. The raw material for the production of these electrodes is a ternary mixture of Nation, PTFE and Platinum-loaded carbon blacks. Development goal of our recent activities is the optimisation of the raw material composition with respect to the structural and electrochemical properties of the electrode. Keywords: Porous electrodes; Gas-diffusion electrodes; Polymer electrolyte fuel cells (PEFC)
1. Introduction The energy conversion in fuel cells proceeds through charge transfer reactions at an interface between electrode and electrolyte. In fuel cells, charge transfer reactions take place at gas diffusion electrodes ( GDEs), since fuels and oxidant are gas and their solubility into electrolyte (i.e. the proton exchange membrane (PEM) in polymer electrolyte fuel cells) is very low. The development of fuel cell performance depends mainly on the enhancement of both GDE and PEM performance. With gas diffusion electrodes, only the three phase boundary regions behave as active sites. It is essential to clarify the reactions proceeding at the three phase boundary regions in order to improve the fuel cell electrode performance. Polymer electrolyte membrane fuel cells (PEFC) are attracting much attention as a power source of electric vehicles due to their high power densities capability. Although there has been much research and development done on PEFC recently, there are still
many problems to be solved. An important research and development approach is to increase the effrciency of proton conducting membranes and to reduce their costs drastically. This is also relevant for the production of electrodes and laminated membrane-electrode assemblies (MEAs) for low temperature fuel cells. Although these tasks are very important for the practical use of fuel cells, it will be mandatory to enhance the power density of fuel cells in order to decrease their costs. To meet this requirements, the enhancement of gas diffusion electrode performance must be pursued. In order to gain the understanding of kinetic phenomena of the hydrogen/oxygen reaction and the catalytic behavior of the electrode at the interface to the proton exchange membrane (PEM), electrochemical investigations as well as surface science analytics were performed.
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO167-2738(95)00256-g
K. Boiwin et al. /Solid State Ionics 77 (199s) 324-330
2. Preparation of key components for polymer membrane fuel cells
The technique to produce low temperature fuel cell electrodes by way of the reactive mixing of components and their rolling into an endless electrode belt and compound structures has been developed at the VARTA AG [ l-3 1. This very flexible and low cost production technique could also be suitable for electrodes and MEAs used in membrane fuel cells. Based on this knowledge compound components are developed including both electrodes and improved membranes. A considerable cost reduction for membrane fuel cells will be expected by this processoptimised production technique. For the development of low cost electrodes and for membrane electrode assemblies R + D-tasks have to be carried out: (i) process adaptation: catalyst preparation, rolling parameters of the electrode structures, membrane integration; (ii) qualification of the electrochemical and structural characteristics of rolled components; (iii) parameter studies to optimise the process technique of the single steps. Polymer electrolyte fuel cells differ from those types using liquid electrolytes mainly by the solid state character of the ion-conducting membrane. Thus, the electrolyte does not penetrate the electrode structure near the electrode-electrolyte interface and the formation of an extensive system of three-phase boundaries fails. However these three-phase boundaries are essential to support the energy conversion at the electrode, since on these sites the electrochemical reactions take place. An important task in electrode or MEA preparation for low temperature fuel cells with ion conducting solid state electrolytes is therefore the formation of three-phase boundaries in the electrode structure. Soaking of the electrode with a solution of the ion conducting polymer is established as a standard method to extend the three-phase boundaries. However the considered solvents are hazardous and to our opinion not suitable for an extended production of electrodes and electrode-membrane assemblies. However, since the fuel cell reaction needs threephase boundaries, electrolyte additives in the electrode material are essential. The penetration of the solid state ion conductors into the electrode structure
325
can be achieved during the preparation process by adding powder of the ion conduction polymer (e.g. Nation) to the raw materials for the rolling process. The electrode consist mainly of an electronic conducting structure, that is loaded with a catalyst supporting the electrochemical reaction (e.g. platinum loaded carbon black - 100/6Pt on Vulcan XC72). On the other hand, PTFE is used in these electrodes not only as a binding agent but also to unfold the pore system due to its hydrophobic property. This pore system is necessary for the transportation of the gaseous reactants within the electrode. Thus, we need a ternary mixture of raw materials to consider the desired properties of the electrodes. For this ternary mixture of raw materials, the ion conducting polymer and PTFE must be prepared and characterised. Using the reactive mixing technique the raw material for the electrode preparation was formed by these powders and the carbon black. Using the ion conductor powder directly in the catalyst material, the reaction area within the electrodes increases, although no Nafton solvent is used in the preparation process. The ternary mixture should consist of about 20% PTFE to provide the electrode band with suffrcient stability and nearly 20% Nation powder to support the ionic conductivity and the formation of the three phase boundaries. Finally the mixture consist of nearly 60°h platinum loaded carbon black. The PTFE content that is necessary to stabilise the electrode depends on the particle size of the carbon black (d= 30 nm for Vulcan XC72). Thus the PTFE amount may vary by changing the catalyst powder. This ternary mixture is rolled in a calendar with two cylinders to produce a self carrying catalyst band. In a second rolling process, these catalyst band can be rolled onto the ion conducting membrane and the diffusion layers. The produced electrodes show a thickness of 250 pm. Thus the MEAs are of 670 pm thickness with conventional Nation 117 membranes. However the development goals are thicknesses below 100 nm for the electrode structures and 40-50 ).trnfor the advanced membranes which corresponds to a MEA thickness of less than 250 pm. As long as new ion conducting polymers are not available, Nation powder is used as protonic conductor in the ternary raw material. Since Nafion is available as granulated material only, it must be pulverised in a ball mill, which is cooled with liquid nitrogen.
326
K. Bolwin et al. /Solid State Ionics 77 (1995) 324-330
The powder resulting from this milling procedure must be classified and characterised with respect to the particle size distribution and the loading of the ball mill. In Fig. 1, the fractional part of particle with diameter less than 100 urn is shown versus the loading of the ball mill. As can be seen from this diagram, the loading should not exceed 500 mg. The particle size distribution N(d) of the Nafion powder was*determined by laser granulometric analysis. The result is shown in Fig. 2. Although the particle size distribution N(d) shows a distinct maximum around a diameter of d= 5 urn, the particles less than 10 urn diameter contribute 55OY6 to the overall particle surface only, as can be judged by the summation of the surface weighted particle size distribution J-86’N(6)d8 (see Fig. 2). However, the Nafion within the electrode have to serve both, the formation of three-phase boundaries and as protonic conductor. Thus, the small particle, which contribute 40% to the overall Nafion mass, form mainly the electrochemical active interface, the larger particle contribute to the protonic conductivity of the electrode structure. The active surface area of a certain amount of Nalion powder increases with a decreasing mean diameter in the particle size distribution. On the other
hand, small particle causes a decrease in the ionic conductivity of the electrode by increasing the number of contact resistance. Thus, the optimisation of the particle size distribution with regard to the electrochemical qualification of the electrode or membrane-electrode assembly is one of the important R + D task during the development of a new, low cost preparation method of electrodes and laminated electrode-membrane-structures.
3. Structural and morphological characterisationof electrodes and electrode-membraneassemblies The structural characterisation of electrodes and electrode-membrane assemblies is essential to relate the distribution of raw materials (i.e. Nalion, PTFE and catalyst-loaded carbon black), the pore size distribution and the morphology of the interface to the performance of the prepared components. These analytic methods are as more important, as the preparation techniques of PEFC components are under development. The characterisation will be carried out by means of ( ) scanning electron microscopy (SEM) to study the morphology of the ternary mixture of raw matel
deDendence on loading ( volume of milling reservoir : 10 ml )
0.5
1
1.5
2
2.5
3
3.5
loading [g]
Fig. 1. The fractional part of particle with diameter less than 100 pm is shown versus the loading of the ball mill.
K. Bolwin et al. /Solid State Ionics 77 (1995) 324-330 particle
321
size distribution
NW
15
m particle
25
30
35
40
45
50
diametar Cm]
Fig. 2. Particle size distribution of the Nafion powder determined by laser granulometric analysis after the milling process.
rials after different preparation steps as reactive mixing and rolling processes; ( l) energy dispersive X-ray emission analysis (EDX) to obtain the elemental distribution in the electrodes and the spatial distribution of raw materials; ( ) X-ray induced photoelectron spectroscopy (XPS) to obtain the surface sensitive elemental composition of both, the electrode and membrane interface, to investigate the chemical state of the surface and - in combination with Argon ion etching - to perform depth profiling of the components; ( ) X-ray diffraction (XRD) to investigate the morphology and crystalline structure of the raw materials; ( l) gas adsorption porosimetry (BET) to determine the pore size distribution and the specific surface area of the carbon black. To elucidate the research tasks and problems related to the development of preparation techniques of electrodes and laminated electrode structures examplaric results of these characterisation methods are discussed. Energy dispersive X-ray emission analysis and particularly scanning electron microscopy have been applied to investigate structural properties of gas diffusion electrodes [ 41. In Fig. 3, we show the SEM survey of a crosscut of an electrode prepared by the reactive l
l
mixing of catalyst and PTFE and rolling onto a porous diffusion layer. The granularity of the structure is clearly visible. The white grains can be identified as PTFE. However the porosity distribution of this electrode is not satisfactory. An essential question is the determination of the distribution of the Nation particles within these structures. EDX can be used to investigate the spatial elemental distribution within the prepared laminated structures. Since the ion conducting polymers differ from PTFE mainly in their polymeric structure but in their elemental composition by additional sulfonic groups ( S03H) only, the sulphur may be used to distinguish PTFE from the ion conductor during EDX measurements. Fig. 4 shows the X-ray emission spectra of PTFE and Nafion; the S Ku X-rays can distinctly be seen. Unfortunately, the S Ku X-ray signal coincide with X-rays from the Pt M series, excited by the platinum catalyst. Thus the investigation of the ion conductor distribution must be performed on samples prepared with P&catalyst free carbon. Whereas the EDX measurements are position sensitive but result in the elemental bulk composition of the sample only, from XP spectroscopy we obtain information of the chemical state of the surface atoms of the investigated sample. Thus we found different states of the C 1s level in PTFE and protonic conduc-
328
K. Bolwin et aI. /Solid State Ionics 77 (1995) 324-330
Fig. 3. SEM survey of a crosscut of an electrode prepared by the reactive mixing of catalyst and PTFE and rolling onto a porous diffusion layer.
A’i
15ooo~
BF
lOOO@
5ooo
C
K
2 3 energy in keV
1
2 3 energy in keV
Fig. 4. X-ray emission spectra of PTFE (A) and Nation (B ).
4
329
K Bolwin et al. /Solid State Ionics 77 (1995) 324-330
tor membranes. These states are related to different chemical surroundings of the carbon atom e.g. C-C, C-CF2, SF-C, CF-CF2, CF2-CF2 and CF, corresponding to literature data [ 51. However a serious interpretation of the XP spectra is complicated by the decomposition of fluorinated hydrocarbons due to ionising radiation and requires the consideration of these effects. Fig. 5 shows the XP-spectrum of the C 1s level. The Cls binding energies shift by + 1.7 eV for the carbon (SF, state with respect to the (CF,), binding configuration of the carbon in PTFE or in the NaBon backbone. Binding energy shifts of -2.3 eV, -4.9 eV, -6.2 eV and -7.6 eV were found for the carbon in the CF-CF2, CF-C, C-CF2 and C-C binding configurations resp&ztivelc The binding energy for the (CF,), configuration was determined to E,=292.9 eV. Recently the chemical surface composition of the gas diffusion electrodes has been investigated XPS and electrochemical measurements depending on operation time to obtain basic data for long term operation effects and degradation mechanisms [ 61.
4. Conclusions The reactive mixing of catalyst powders and pulverised polymers in combination with rolling the raw material to a self carrying electrode belt is a promising method to produce low cost electrodes or laminated membrane electrode structures applied in polymer electrolyte fuel cells. However there are considerable requirements regarding both fundamental research and technical development to solve the problems that arise during the preparation of iaminated structures. We have demonstrated, that a number of commonly applied analytic methods may clarify questonaries connected with structural properties of the key components for PEFC. Other methods such as impedance spectroscopy and cyclic voltammetry will be used for the electrochemical characterisation. One of the key problems while preparing laminated structures is the formation of the three-phase boundaries within the electrode and a sufficient protonic conductivity from the reaction site to the ion conductor membrane. These properties depend on both, the particle size distribution of the ion conductor and its spatial distribution within the reaction and diffusion zones.
+ 295
290 bmding
energy
285
280
(eV)
Fig. 5. XP-spectrum of the Cls level of fluorinated hydrocarbons (e.g. PTFE and N&on). The different binding states are related to different chemical surroundings of the considered C-atom: e.g. C-C, (3-CF2, SF-C, SF-CF2, CF2-CF2 and SF,.
330
K. Bolwin et al. /Solid State Ionics 77 (1995) 324-330
Acknowledgements
The authors gratefully acknowledge financial support of the State of Baden-Wiirttemberg (Wirtschaftministerium ) . This work was performed as part of the Fuel Cell Program of Baden-Wiirttemberg.
References [ ] H. Sauer, Apparatus and method for a plastic-bound activated carbon layer for thin gas-diffusion electrodes (VARTA AG); Ger. Gffen. 2 941 774 (Cl. HOlM4/88); Apr. 30,1981, Appl. Ckt. 16,1979; 9 pp; CA 95: 118423e.
[ 21 M. Jung and X. Kroger, Activation of metal catalysts (VARTA AG); Ger. 1 156 768 (Cl. BOlj), Nov. 7, 1963, Appl. Dec. 17, 1960; 3 pp; CA Vol. 60 Col. 1158E. [ 31 A. Winsel, Porous gas electrodes (VARGA AG); Ger. Offen. DE 3 342 969 (Cl. C25B11/06), Jun. 5, 1985, Appl. Nov. 28,1983; 12 pp; CA 103: 448342. [4] E. Gtilzow and W. Schnumberger, Preparation and characterization of gas diffusion electrodes for alkaline fuel cells, Proc. Fuel Cell Seminar 1990; Phoenix ( 1990), p. 453. [ 51 D.T. Clark, D. Kilcast, W.J. Feast and W.K.R. Musgrave, J. Polym. Sci. Polym Chem. 11 (1973) 389. [6] E. Giilzow, B. Holzwarth, W. Schnumberger, M. Schulze, G. Steinhilber and N. Wagner, PTFE bonded gas diffusion electrodes for alkaline fuel cells, Proc. 9th World Hydrogen Energy Conf. (WHEC, Paris, 1992)-p. 1507.
Ja!J ELSEWER
Solid State Ionics 77 ( 1995) 324-330
Preparation of porous electrodes and laminated electrodemembrane structures for polymer electrolyte fuel cells ( PEFC
)
K. Bolwin, E. Giilzow, D. Bevers, W. Schnumberger Institute for Technical Thermodynamics, German Aerospace Research Establishment (DLR). Pjaffenwaldring 38-40.070569 Stuttgart, Germany
AhStTXt Porous NaIion-PTFE-bonded gas-diffusion electrodes (GDEs), for use in (PEFC), were prepared by a cold rolling process. In this process, no solvents are required. The raw material for the production of these electrodes is a ternary mixture of Nation, PTFE and Platinum-loaded carbon blacks. Development goal of our recent activities is the optimisation of the raw material composition with respect to the structural and electrochemical properties of the electrode. Keywords: Porous electrodes; Gas-diffusion electrodes; Polymer electrolyte fuel cells (PEFC)
1. Introduction The energy conversion in fuel cells proceeds through charge transfer reactions at an interface between electrode and electrolyte. In fuel cells, charge transfer reactions take place at gas diffusion electrodes ( GDEs), since fuels and oxidant are gas and their solubility into electrolyte (i.e. the proton exchange membrane (PEM) in polymer electrolyte fuel cells) is very low. The development of fuel cell performance depends mainly on the enhancement of both GDE and PEM performance. With gas diffusion electrodes, only the three phase boundary regions behave as active sites. It is essential to clarify the reactions proceeding at the three phase boundary regions in order to improve the fuel cell electrode performance. Polymer electrolyte membrane fuel cells (PEFC) are attracting much attention as a power source of electric vehicles due to their high power densities capability. Although there has been much research and development done on PEFC recently, there are still
many problems to be solved. An important research and development approach is to increase the effrciency of proton conducting membranes and to reduce their costs drastically. This is also relevant for the production of electrodes and laminated membrane-electrode assemblies (MEAs) for low temperature fuel cells. Although these tasks are very important for the practical use of fuel cells, it will be mandatory to enhance the power density of fuel cells in order to decrease their costs. To meet this requirements, the enhancement of gas diffusion electrode performance must be pursued. In order to gain the understanding of kinetic phenomena of the hydrogen/oxygen reaction and the catalytic behavior of the electrode at the interface to the proton exchange membrane (PEM), electrochemical investigations as well as surface science analytics were performed.
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO167-2738(95)00256-g
K. Boiwin et al. /Solid State Ionics 77 (199s) 324-330
2. Preparation of key components for polymer membrane fuel cells
The technique to produce low temperature fuel cell electrodes by way of the reactive mixing of components and their rolling into an endless electrode belt and compound structures has been developed at the VARTA AG [ l-3 1. This very flexible and low cost production technique could also be suitable for electrodes and MEAs used in membrane fuel cells. Based on this knowledge compound components are developed including both electrodes and improved membranes. A considerable cost reduction for membrane fuel cells will be expected by this processoptimised production technique. For the development of low cost electrodes and for membrane electrode assemblies R + D-tasks have to be carried out: (i) process adaptation: catalyst preparation, rolling parameters of the electrode structures, membrane integration; (ii) qualification of the electrochemical and structural characteristics of rolled components; (iii) parameter studies to optimise the process technique of the single steps. Polymer electrolyte fuel cells differ from those types using liquid electrolytes mainly by the solid state character of the ion-conducting membrane. Thus, the electrolyte does not penetrate the electrode structure near the electrode-electrolyte interface and the formation of an extensive system of three-phase boundaries fails. However these three-phase boundaries are essential to support the energy conversion at the electrode, since on these sites the electrochemical reactions take place. An important task in electrode or MEA preparation for low temperature fuel cells with ion conducting solid state electrolytes is therefore the formation of three-phase boundaries in the electrode structure. Soaking of the electrode with a solution of the ion conducting polymer is established as a standard method to extend the three-phase boundaries. However the considered solvents are hazardous and to our opinion not suitable for an extended production of electrodes and electrode-membrane assemblies. However, since the fuel cell reaction needs threephase boundaries, electrolyte additives in the electrode material are essential. The penetration of the solid state ion conductors into the electrode structure
325
can be achieved during the preparation process by adding powder of the ion conduction polymer (e.g. Nation) to the raw materials for the rolling process. The electrode consist mainly of an electronic conducting structure, that is loaded with a catalyst supporting the electrochemical reaction (e.g. platinum loaded carbon black - 100/6Pt on Vulcan XC72). On the other hand, PTFE is used in these electrodes not only as a binding agent but also to unfold the pore system due to its hydrophobic property. This pore system is necessary for the transportation of the gaseous reactants within the electrode. Thus, we need a ternary mixture of raw materials to consider the desired properties of the electrodes. For this ternary mixture of raw materials, the ion conducting polymer and PTFE must be prepared and characterised. Using the reactive mixing technique the raw material for the electrode preparation was formed by these powders and the carbon black. Using the ion conductor powder directly in the catalyst material, the reaction area within the electrodes increases, although no Nafton solvent is used in the preparation process. The ternary mixture should consist of about 20% PTFE to provide the electrode band with suffrcient stability and nearly 20% Nation powder to support the ionic conductivity and the formation of the three phase boundaries. Finally the mixture consist of nearly 60°h platinum loaded carbon black. The PTFE content that is necessary to stabilise the electrode depends on the particle size of the carbon black (d= 30 nm for Vulcan XC72). Thus the PTFE amount may vary by changing the catalyst powder. This ternary mixture is rolled in a calendar with two cylinders to produce a self carrying catalyst band. In a second rolling process, these catalyst band can be rolled onto the ion conducting membrane and the diffusion layers. The produced electrodes show a thickness of 250 pm. Thus the MEAs are of 670 pm thickness with conventional Nation 117 membranes. However the development goals are thicknesses below 100 nm for the electrode structures and 40-50 ).trnfor the advanced membranes which corresponds to a MEA thickness of less than 250 pm. As long as new ion conducting polymers are not available, Nation powder is used as protonic conductor in the ternary raw material. Since Nafion is available as granulated material only, it must be pulverised in a ball mill, which is cooled with liquid nitrogen.
326
K. Bolwin et al. /Solid State Ionics 77 (1995) 324-330
The powder resulting from this milling procedure must be classified and characterised with respect to the particle size distribution and the loading of the ball mill. In Fig. 1, the fractional part of particle with diameter less than 100 urn is shown versus the loading of the ball mill. As can be seen from this diagram, the loading should not exceed 500 mg. The particle size distribution N(d) of the Nafion powder was*determined by laser granulometric analysis. The result is shown in Fig. 2. Although the particle size distribution N(d) shows a distinct maximum around a diameter of d= 5 urn, the particles less than 10 urn diameter contribute 55OY6 to the overall particle surface only, as can be judged by the summation of the surface weighted particle size distribution J-86’N(6)d8 (see Fig. 2). However, the Nafion within the electrode have to serve both, the formation of three-phase boundaries and as protonic conductor. Thus, the small particle, which contribute 40% to the overall Nafion mass, form mainly the electrochemical active interface, the larger particle contribute to the protonic conductivity of the electrode structure. The active surface area of a certain amount of Nalion powder increases with a decreasing mean diameter in the particle size distribution. On the other
hand, small particle causes a decrease in the ionic conductivity of the electrode by increasing the number of contact resistance. Thus, the optimisation of the particle size distribution with regard to the electrochemical qualification of the electrode or membrane-electrode assembly is one of the important R + D task during the development of a new, low cost preparation method of electrodes and laminated electrode-membrane-structures.
3. Structural and morphological characterisationof electrodes and electrode-membraneassemblies The structural characterisation of electrodes and electrode-membrane assemblies is essential to relate the distribution of raw materials (i.e. Nalion, PTFE and catalyst-loaded carbon black), the pore size distribution and the morphology of the interface to the performance of the prepared components. These analytic methods are as more important, as the preparation techniques of PEFC components are under development. The characterisation will be carried out by means of ( ) scanning electron microscopy (SEM) to study the morphology of the ternary mixture of raw matel
deDendence on loading ( volume of milling reservoir : 10 ml )
0.5
1
1.5
2
2.5
3
3.5
loading [g]
Fig. 1. The fractional part of particle with diameter less than 100 pm is shown versus the loading of the ball mill.
K. Bolwin et al. /Solid State Ionics 77 (1995) 324-330 particle
321
size distribution
NW
15
m particle
25
30
35
40
45
50
diametar Cm]
Fig. 2. Particle size distribution of the Nafion powder determined by laser granulometric analysis after the milling process.
rials after different preparation steps as reactive mixing and rolling processes; ( l) energy dispersive X-ray emission analysis (EDX) to obtain the elemental distribution in the electrodes and the spatial distribution of raw materials; ( ) X-ray induced photoelectron spectroscopy (XPS) to obtain the surface sensitive elemental composition of both, the electrode and membrane interface, to investigate the chemical state of the surface and - in combination with Argon ion etching - to perform depth profiling of the components; ( ) X-ray diffraction (XRD) to investigate the morphology and crystalline structure of the raw materials; ( l) gas adsorption porosimetry (BET) to determine the pore size distribution and the specific surface area of the carbon black. To elucidate the research tasks and problems related to the development of preparation techniques of electrodes and laminated electrode structures examplaric results of these characterisation methods are discussed. Energy dispersive X-ray emission analysis and particularly scanning electron microscopy have been applied to investigate structural properties of gas diffusion electrodes [ 41. In Fig. 3, we show the SEM survey of a crosscut of an electrode prepared by the reactive l
l
mixing of catalyst and PTFE and rolling onto a porous diffusion layer. The granularity of the structure is clearly visible. The white grains can be identified as PTFE. However the porosity distribution of this electrode is not satisfactory. An essential question is the determination of the distribution of the Nation particles within these structures. EDX can be used to investigate the spatial elemental distribution within the prepared laminated structures. Since the ion conducting polymers differ from PTFE mainly in their polymeric structure but in their elemental composition by additional sulfonic groups ( S03H) only, the sulphur may be used to distinguish PTFE from the ion conductor during EDX measurements. Fig. 4 shows the X-ray emission spectra of PTFE and Nafion; the S Ku X-rays can distinctly be seen. Unfortunately, the S Ku X-ray signal coincide with X-rays from the Pt M series, excited by the platinum catalyst. Thus the investigation of the ion conductor distribution must be performed on samples prepared with P&catalyst free carbon. Whereas the EDX measurements are position sensitive but result in the elemental bulk composition of the sample only, from XP spectroscopy we obtain information of the chemical state of the surface atoms of the investigated sample. Thus we found different states of the C 1s level in PTFE and protonic conduc-
328
K. Bolwin et aI. /Solid State Ionics 77 (1995) 324-330
Fig. 3. SEM survey of a crosscut of an electrode prepared by the reactive mixing of catalyst and PTFE and rolling onto a porous diffusion layer.
A’i
15ooo~
BF
lOOO@
5ooo
C
K
2 3 energy in keV
1
2 3 energy in keV
Fig. 4. X-ray emission spectra of PTFE (A) and Nation (B ).
4
329
K Bolwin et al. /Solid State Ionics 77 (1995) 324-330
tor membranes. These states are related to different chemical surroundings of the carbon atom e.g. C-C, C-CF2, SF-C, CF-CF2, CF2-CF2 and CF, corresponding to literature data [ 51. However a serious interpretation of the XP spectra is complicated by the decomposition of fluorinated hydrocarbons due to ionising radiation and requires the consideration of these effects. Fig. 5 shows the XP-spectrum of the C 1s level. The Cls binding energies shift by + 1.7 eV for the carbon (SF, state with respect to the (CF,), binding configuration of the carbon in PTFE or in the NaBon backbone. Binding energy shifts of -2.3 eV, -4.9 eV, -6.2 eV and -7.6 eV were found for the carbon in the CF-CF2, CF-C, C-CF2 and C-C binding configurations resp&ztivelc The binding energy for the (CF,), configuration was determined to E,=292.9 eV. Recently the chemical surface composition of the gas diffusion electrodes has been investigated XPS and electrochemical measurements depending on operation time to obtain basic data for long term operation effects and degradation mechanisms [ 61.
4. Conclusions The reactive mixing of catalyst powders and pulverised polymers in combination with rolling the raw material to a self carrying electrode belt is a promising method to produce low cost electrodes or laminated membrane electrode structures applied in polymer electrolyte fuel cells. However there are considerable requirements regarding both fundamental research and technical development to solve the problems that arise during the preparation of iaminated structures. We have demonstrated, that a number of commonly applied analytic methods may clarify questonaries connected with structural properties of the key components for PEFC. Other methods such as impedance spectroscopy and cyclic voltammetry will be used for the electrochemical characterisation. One of the key problems while preparing laminated structures is the formation of the three-phase boundaries within the electrode and a sufficient protonic conductivity from the reaction site to the ion conductor membrane. These properties depend on both, the particle size distribution of the ion conductor and its spatial distribution within the reaction and diffusion zones.
+ 295
290 bmding
energy
285
280
(eV)
Fig. 5. XP-spectrum of the Cls level of fluorinated hydrocarbons (e.g. PTFE and N&on). The different binding states are related to different chemical surroundings of the considered C-atom: e.g. C-C, (3-CF2, SF-C, SF-CF2, CF2-CF2 and SF,.
330
K. Bolwin et al. /Solid State Ionics 77 (1995) 324-330
Acknowledgements
The authors gratefully acknowledge financial support of the State of Baden-Wiirttemberg (Wirtschaftministerium ) . This work was performed as part of the Fuel Cell Program of Baden-Wiirttemberg.
References [ ] H. Sauer, Apparatus and method for a plastic-bound activated carbon layer for thin gas-diffusion electrodes (VARTA AG); Ger. Gffen. 2 941 774 (Cl. HOlM4/88); Apr. 30,1981, Appl. Ckt. 16,1979; 9 pp; CA 95: 118423e.
[ 21 M. Jung and X. Kroger, Activation of metal catalysts (VARTA AG); Ger. 1 156 768 (Cl. BOlj), Nov. 7, 1963, Appl. Dec. 17, 1960; 3 pp; CA Vol. 60 Col. 1158E. [ 31 A. Winsel, Porous gas electrodes (VARGA AG); Ger. Offen. DE 3 342 969 (Cl. C25B11/06), Jun. 5, 1985, Appl. Nov. 28,1983; 12 pp; CA 103: 448342. [4] E. Gtilzow and W. Schnumberger, Preparation and characterization of gas diffusion electrodes for alkaline fuel cells, Proc. Fuel Cell Seminar 1990; Phoenix ( 1990), p. 453. [ 51 D.T. Clark, D. Kilcast, W.J. Feast and W.K.R. Musgrave, J. Polym. Sci. Polym Chem. 11 (1973) 389. [6] E. Giilzow, B. Holzwarth, W. Schnumberger, M. Schulze, G. Steinhilber and N. Wagner, PTFE bonded gas diffusion electrodes for alkaline fuel cells, Proc. 9th World Hydrogen Energy Conf. (WHEC, Paris, 1992)-p. 1507.