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An advanced gas diffusion electrode for high performance phosphoric acid fuel cells
\ELSEVIER
Journal of Electroanalytic al Chemistry 413 ( 1996) 8 l-88
An advanced gas diffusion electrode for high performance phosphoric acid fuel cells Noriaki Hara a2b,Kazunori Tsurumi b, Masahiro Watanabe a3* a Laboratory
of Electrochemical b Tanaku Kikinzoku
Energy Cotwersion, Yamanashi University, Takeda 4-3, Kofu 400, Japan Kogyo Technical Center, Shin-machi 2-73, Hiratsuka 254, Japan
Received 25 March 1994; in revised form 25 May 1994 ’
Abstract An extremely thin fluorinated polyethylene (FPE) film was prepared on a carbon black (CB) surface and the properties were studied. It was found that polyethylene as the precursor can cover the whole surface of the CB particles with a thin film different from the conventional Teflon dispersion (PTFE), and it can be fluorinated easily to produce a Teflon-like hydrophobic material. The resulting FPE/CB material showed a distinctive hydrophobic property and a comparable thermal stability compared with PTFE under the operating conditions for phosphoric acid fuel cells (PAFCs). An advanced gas diffusion electrode for PAFCs is proposed in which a new design concept of the electrode structure is applied where the functions of electrolyte network and gas network are assigned completely to catalyzed CB and FPE/CB. The cathode optimized in the structure with the new FPE/CB exhibited a high performance for oxygen reduction compared with cathodes using conventional PTFE owing to the improved catalyst utilization and gas permeability, and showed a potential for long operational life owing to less performance degradation resulting from the prevention of flooding of the gas network with electrolyte during operation. Keywords: Gas diffusion electrode; Phosphoric
acid fuel cells
1. Introduction
In order to develop phosphoric acid fuel cells (PAFCs), it is essential to improve performance and lifetime. Field tests focused on commercial development have been demonstrated, but the operation time was at most about 15 000 h for cells using conventional electrode structures owing to the degradation of performance resulting from problems originating in the cells themselves or those induced by other difficulties. Conventionally, the gas diffusion electrode is composed of a mixture of fine particles of catalyzed carbon black (Pt/CB) and polytetrafluoroethylene (PTFE) dispersion, which form the electrolyte network and the gas network respectively. The balance of these networks is controlled by the PTFE content. PAFCs show a decay in performance with time. Restricted gas diffusion to catalyst clusters owing to flooding of the gas network with excess electrolyte is one of the major reasons for this
* Corresponding author. ’ Publication of this paper was delayed for reasons beyond the control of the publisher. 0022-0728/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03597-V
decay, particularly after operation for more than 10000 h. The gas diffusion property can be improved to some extent by increasing the content of PTFE, because of its high hydrophibicity, resulting in some improvement in lifetime. However, there is a concomitant decrease in performance because of the reduction of catalyst utilization in the gas diffusion electrode, i.e. the improvement of gas permeability and catalyst utilization are in opposition in the conventional electrode structure [l-3]. It is also believed that the carbon black (CB) surface in the gas network is not covered sufficiently with PTFE, because conventional PTFE particles are 10 times larger than CB particles, as shown schematically in Fig. l(A), and PTFE has a high viscosity at the sintering temperature in the electrode fabrication process, so that the hydrophobic property may not be maintained over a long period. In order to overcome these problems, Watanabe et al. [2] have proposed a new type of electrode, shown schematically in Fig. 2(B), which is based on the concept that the functions of the catalyst layer, or reaction layer, in the electrode are assigned completely to hydrophilic Pt/CB as the electrolyte network and CB highly wet-proofed with PTFE or analogous polymers as the gas-supplying network (GSP), and they
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diffusion electrode for such long-lived PAFCs, i.e. CB completely wet-proofed by fluorinated polyethylene (FPE/CB) and an electrode structure in which the functions of electrolyte network and gas network are assigned to Pt/CB and FPE/CB respectively. The aim of this paper is to demonstrate the advantageous features of FPE/CB and the electrode incorporating this material in more detail.
- 0.04~rn)
FPE(r
2. Experimental Denka Black (Denki Kagaku Kogyo) and HiZex (Mitsui Sekiyu Kagaku Kogyo) were used as the CB and polyethylene (PE) sources respectively for the preparation of FPE/CB. CB was mixed uniformly with PE, and the mixture was heat treated in the fluid condition at 170°C in N, atmosphere to obtain CB coated with a uniform thin PE film (PE/CB). Direct fluorination of PE/CB was carried out using specially programmed time-temperature and fluorine concentration sequences. The reaction conditions used were changed depending upon the thickness of PE film required. The degree of fluorination and the change in the fluorinated state were analyzed by gravimetry and X-ray photoelectron spectrometry (XPS) using a Shimazu ESCA-750s spectrometer with Mg Ka radiation. Details of the preparation and the effect of the conditions upon the properties of the FPE obtained will be published elsewhere. Several techniques were used to characterize CB samples coated with thin PE and FPE films. Their coating state was evaluated by Brunauer-Emmett-Teller (BET) surface area measurement using a Quantachrome Quantasorb or by scanning electron microscopy @EM) using a Jeol JSM6300F microscope. The degree of agglomeration of the film-coated CB was assessed using a particle sizer (Horiba Ltd. model LA-7001, and the porosity or pore-size distribution in CB powders or gas-diffusion electrodes was obtained using a mercury pore sizer (Micromeritics pore sizer 9310). The thermal stability of FPE and PTFE was evaluated by thermal gravimetry and differential thermal analysis (MacScience TG/DTA 2000).
(B)
(A)
Fig. I. Schematic structure of CB wet-proofed by (A) conventional PTFE particles and (B) fluorinated polyethylene (FPE) film. Symbols d and 1 denote particle diameter and film thickness respectively.
have demonstrated the possibility of designing an ideal electrode structure enabling 100% utilization of catalyst clusters without polarization losses caused by mass transport up to a high current density at the optimized structure. Wet-proofing reagents with relatively low melting viscosity, such as tetrafluoroethylene-hexafluoropropylene copolymer (FEP), combined with PTFE were also investigated by Watanabe et al. [4]; however, a progressive degradation of performance with time due to flooding of the gas network was still predicted as long as PTFE or similar polymers were used as the wet-proofing reagent because of the intrinsic properties of PTFE discussed above. Therefore it is essential to develop a new material for the gas network which can guarantee the wet-proofing property for a long period (e.g. 40000 h as required for commercial PAFCs) under the operating conditions. The fluorination of a bulky polyethylene surface in the presence of a small amount of oxygen has been found to provide hydrophilic surface properties [5], but there have been no studies of a hydrophobic film of fluorinated polyethylene (FPE) with properties similar to those of PTFE. We have published preliminary reports [6,7] of an ideal material and a new design concept for an advanced gas
(A)
(W
Electrolyte Fig. 2. Comparison
PTFE
Electrolyte
of the reaction layer structures of gas diffusion electrodes: (A) conventional concept using PTFE; (B, C) modem concepts completely electrolyte and gas networks by using CB wet-proofed by (B) PTFE and (C) FPE as the gas network. Pt/CB; catalyzed carbon black, GSP; gas-supplying powder.
separatingthe functions of
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The new structure of gas diffusion electrodes using FPE/CB is shown schematically in Fig. 2(C). In order to compensate for the lack of mechanical bonding strength of thin FPE film in the formation of the structure, PTFE was used as a binder to the electrolyte and gas networks composed of alloy catalyst supported on CB (denoted as Pt/CB) and FPE/CB respectively. The alloy catalyst was a Pt-containing ternary alloy (TEC77A20; alloy loading on CB was 20 wt.% by platinum base) supplied by Tanaka Kikinzoku Kogyo. The electrodes were prepared by the filter transfer method; the flock formed from a mixture of Pt/CB, FPE/CB and PTFE (DuPont PTFE-3OJ) suspended in water + isopropanol was filtered onto a Teflon membrane filter, transferred by cold-pressing onto a carbon paper which had previously been wet-proofed with 30 wt.% FEP and finally hot-pressed at 360°C for 3 s under 5 kg cm-*. FPE/CB (l/l) was used in a series of mixing ratios with Pt/CB to prepare the gas diffusion electrodes. The total CB weight per unit apparent electrode area (TCB) was fixed at 6.0 mg cm-’ for every electrode in order to keep the thickness of the reaction layer constant; therefore a different amount of alloy catalyst was loaded on each electrode. Gas diffusion electrodes prepared by adding F’PE/CB in six mixing ratios of FPE/TCB, i.e. 3/10,5/10,6/10,7/10, 8/10 and 9/10, were examined with various PTFE binder contents; the load of alloy catalyst on each electrode was 0.97 mg cm-*, 0.73 mg cm -*, 0.48 mg cm-‘, 0.37 mg cme2, 0.21 mg cmW2 and 0.048 mg cmp2 respectively in the Pt base and PTFE was added in PTFE/TCB ratios ranging from 5/100 to 70/100. Cathode performances of gas diffusion electrodes for pure oxygen and oxygen in air were measured under galvanostatic conditions in 100% phosphoric acid and ambient pressure at 190°C. Potentials were referred to a reversible hydrogen electrode (RHE) in the same electrolyte. Prior to the measurement, air and H, were supplied alternately to the gas chamber of the cell with the precaution of a N, blanket between gas switching. Switching the gas-supply accelerated the filling of the cathode catalyst layer with electrolyte. Cathode and anode performances were also measured during this process. After the cathode performance at 0.7 V/RHE had stabilized, its steady-state performance was measured fully. The ohmic
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resistances were corrected using a current-pulse generator (Nikko Keisoku NCPG-1010). The current density (j) on the polarization curve of each electrode loaded with X mg Pt cm-* was normalized to the current density j, of an electrode loaded with 0.54 mg Pt cme2 using the equation j, = 0.54 j/X. In order to evaluate the hydrophobic properties of the new FPE/CB by measuring the contact angle with hot phosphoric acid or the electrolyte occupation, porous sheets mimicking gas diffusion electrodes were prepared with FPE/CB in a similar manner to that above for catalyzed electrodes, but without the addition of Pt/CB. Powders or electrodes containing only CB and PTFE (PTFE/CB), with a PTFE CB ratio of 6/4 or 2/8, were also prepared in a similar manner as references for the FBE/CB powder or FPE/CB porous sheets. The hydrophobic properties of the gas diffusion electrodes prepared as described above were evaluated by measuring the contact angles to 105% phosphoric acid at 190°C using a contact angle meter (Erma Inc. Type G-II). Changes in the hydrophobic properties with time were evaluated by measuring the weight change with electrolyte occupation of the gas network in gas diffusion electrodes floating on a phosphoric acid pool at 190°C in air.
3. Results and discussion 3.1. Characterization
of PE/ C and FPE/
C samples
Scanning electron micrographs of CB, PE/CB and FPE/CB are shown in Figs. 3(A), 3(B) and 3(C) respectively. The primary CB particles of diameter ca. 50 nm can be seen in Fig. 3(A). It is found that the particles are covered with uniform thin PE film accompanying an increase in the particle size. No change in the covering state is observed after fluorination as shown in Figs. 3(B) and 3(C).
Table 1 gives the changes in the characteristics of CB powders, i.e. BET surface area, median diameter and porosity, during PE coating and the subsequent fluorination. All the data, except the median diameter, were normalized by the unit weight of CB. The BET surface area decreased by 64% compared with the original value after
Fig. 3. Scanning electron micrographs of (A) CB, (B) PE/CB and (Cl FPE/CB.
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PE coating, which suggests that the rough CB surface was well coated with a flat PE thin film. If it is assumed that the CB particles are separated from each other and maintain a mean particle diameter of 50 nm (although in practice they form aggregates) and that they can be fully coated with a PE film of uniform thickness (see Figs. 3(A) and 3(B)); the thickness is estimated to be ca. 7 nm. An increase in the median diameter after PE coating indicates agglomeration of CB aggregates by the binding action of PE, which results in an increase in the pore volume probably due to the increase in space between the agglomerates. The decrease in median diameter after fluorination of PE/CB probably occurred because the PE bonding between the agglomerates was partially destroyed during the formation of FPE, although no noticeable change in the BET surface area was observed. The pore volume decreased due to the increase in the film thickness on conversion of PE to FPE in the rigid structure of CB aggregates or agglomerates (see Figs. 3(B) and 3(C)). Comparison of the BET surface area of FPE/CB (27 m* (g CB)-’ ) with that of the PTFE/CB reference (59 m2 (g CB)- ’ ), clearly shows that the coating of CB with polymer film is much more complete with FPE/CB. Moreover, the porosity of FPE/CB is 10% higher than that of CB/PTFE, which must be preferable for gas diffusion when the former is to be used as the gas network material in gas diffusion electrodes. C 1s X-ray photoelectron spectra of compressed powders of PE/CB (4/9), FPE/CB (6/4) and PTFE/CB (6/4) are shown in Figs. 4(A), 4(B) and 4(C) respectively. The degree of fluorination for the FPE/CB on that of replacement of H atoms in PE with F atoms, was found to be ca. 100% by gravimetric determination. Although the C 1s peak on PE/CB is the same as that of CB, i.e. 285.0 eV, that of FPE/CB exhibits a large positive shift. The resulting peak at 292.4 eV coincides relatively well with that on PTFE/CB assigned to the CF,--CF, bond, suggesting the formation of a new Teflon-like material by direct fluorination of PE. A small number of H atoms remain as shown by the peak around 290 eV assigned to the CFHCF, bond. Since the C 1s spectrum on CB must be reduced by covering its surface with polymer films, the ratio of the peak area for CF,-CF, (292.5 eV) to that for CB (285.0 eV) reflects the increase in the surface coverage Table 1 Change in the typical characteristics coating and subsequent fluorination
CB PE/CB FPE/CB PTFE/CB
of carbon black during
BET (N,) surface area/ m* (g CB)- ’
Median diameter/ km
Pore volume/ cm3 (g CB)-
73 26 ( - 64%) 27(-63%) 59(-19%)
0.8 4.0 1.9 -
2.7 4.5 3.0 2.9
polyethylene Porosity/ %
’ 71 69 67 58
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A!!L (B)
CD)
Bonding
h loo energy / eV
0
290
Fig. 4. C IS X-ray photoelectron spectra of (A) PE/CB PTFE/CB (6/4), (C) FPE/CB (6/4) and (D) FPE/CB keeping in an oven at 205°C for 2000 h.
(4/9), (6/4)
(B) after
of CB with films. The ratio is found to be 2.02 for FPE/CB, which is much larger than that for PTFE/CB (0.22). This difference clearly demonstrates that the coverage of CB with FPE is much greater than that with PTFE, and that the FPE obtained by the direct fluorination of PE can coat the CB surface much better than PTFE. This is supported by the following observation. A shoulder peak appears at around 295 eV on FPE/CB, which corresponds to the large peak at 295.8 eV on PTFE/CB. Since this peak disappears after the neutralization by an electron gun, it can be attributed to charged CF,-CF, detached from electron-conducting CB. Therefore, the small peak height obtained for FPE/CB clearly suggests that FPE coats CB with a thin film more uniformly than does PTFE, and that the PTFE particles in the conventional electrode do not exist in the film form on CB, but are present as much bulkier particles, as illustrated in Figs. l(A) and l(B). 3.2. Thermal stability of FPE/
CB
The thermal stability of FPE/CB (6/4) in air was examined by thermogravimetry and differential thermal analysis (TG-DTA) and compared with PTFE/CB (6/4). The data are shown in Fig. 5. Both samples decompose exothermically in two steps: the polymers undergo oxidative decomposition in the first step and CB is oxidized at a temperature above 500°C in the second step. It is found that FPE/CB starts to lose weight at 310°C which is about 100°C lower than observed for PTFE/CB. This is probably because the FPE has relatively lower molar mass than PTFE, because some of the C-C bonds may be broken by the direct fluorination reaction. The molar masses of FPE must be dispersed over a wide range, as indicated by the disappearance of a sharp melting point
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Table 2 XPS data for PTFE/CB
81-88
and FPE/CB CB or PE
%-loox
I
,
I
I
I
I
,
I
I
I
l
l
l
I
o-
300
(w -200,
%
1
.c i
85
CF, -CF,
Charged-up pe*
GSP PTFE/CB (6/41powder Bonding energy/eV 285.0 Peak area/% 64 Normalized ratio 1
292.1 14 0.22
295.8 22 0.35
FPE/CB f6/4)powder Bonding energy/eV Peak area/% Normalized ratio
285.0 32 1
292.5 65 2.02
294.5 4 0.11
FPE/ CB (6/4) powder Bonding energy/eV Peak area/% Normalized ratio
kept ut 2OPC for 2000 h 285.0 292.6 33 64 1 1.95
294.6 3 0.10
-!jcy$f
200
0
400
600
Temperature Fig. 5. Comparison PTFE/CB (6/4). rate of 10°C min-
Sheets with a series of FPE contents in FPE/CB, which mimicked gas diffusion electrodes, were prepared by adding 20 parts of PTFE as a binder to 80 parts of CB coated with FPE, and hot-pressing the mixture. The reference PTFE/CB sheet samples were composed of 20 and 60 parts of PTFE to CB uncoated with FPE. The results are shown in Table 3. All the data on the sheet samples show contact angles higher than the value of 110” reported for a flat Teflon sheet, probably because of the large real contact angle resulting from their microrough surface structure. The FPE/CB samples containing more than 10 wt.% FPE exhibit very high contact angles (greater than 161”) which is larger than the value of 159” for a PTFE/CB sample containing 60 wt.% PTFE. Fig. 6 shows the change of pore occupation by PA with time in the FPE/CB and PTFE/CB sheet samples discussed above. These data were obtained by gravimetry of these samples floated on a 105% PA pool at 190°C in air. The hydrophobic properties improve with increasing FPE content as shown by the lower occupation by PA. The samples containing more than 30 wt.% of FPE are almost completely hydrophobic, i.e. less than 10% occupation by PA after 150 h. However, more than 70% of the micropores of PTFE/CB samples were occupied by PA within a short time, i.e. 100 h even for the sample containing 60 wt.% PTFE. Therefore it is considered that FPE/CB containing more than 40 wt.% FPE is a suitable candidate for gas network material in gas diffusion electrodes. A long-
800
I “C
of the thermal stability of (A) FPE/CB (6/4) and (B) The temperature was increased to 800°C at a constant ’ in air.
like that of 340°C for PTFE/CB (Fig. 5(B)). Thus the new FPE/CB has a lower thermal stability than the conventional PTFE/CB. However, FPE/CB is still believed to be sufficiently stable around 200°C which is the conventional operating temperature of PAFCs. The long-range stability of FPE/CB was examined by keeping it in an oven at 205°C for 2000 h. The C 1s X-ray photoelectron spectrum is shown in Fig. 4(D) and the XPS data are given in Table 2. The difference between the data before and after the stability test is negligibly small. Consequently, we can conclude that FPE/CB is sufficiently stable for practical applications in gas diffusion electrodes. 3.3. Hydrophobic
properties
of FPE / CB
The hydrophobic properties of FPE/CB were examined by comparing the contact angle with 105% phosphoric acid (PA) at 190°C with that of conventional PTFE/CB. Table 3 Characteristics
layers wet-proofed
by fluorinated
Samples
of gas diffusion
A
B
C
D
E
F
G
FPE content in FPE/CB/% PTFE/CB ’ Total content of FPE and PTFE/% Contact angles *
0
0 6/4 60 159”
5
10
20
30
40
w3
2/8
2/g
2/8
w3
23 157”
2-I 161”
33 161”
40 164”
48 164”
2/8 20 158”
A, B, reference samples. ’ Ratio of added PTFE as a binder in the gas diffusion * Against hot phosphoric acid (19O”C, 105%, air).
layer.
polyethylene
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term test on changes in the hydrophobic properties of FPE/CB is required in the next stage of the investigation, but the results on the thermal stability and hydrophobic properties reported above suggest that the FPE/CB could be used instead of PTFE/CB for electrode structures in commercial PAFCs to ensure high performance and long life. 3.4. Performance of gas diffusion electrodes using FPE/ CB for 0, reduction The effects of the content of FPE/CB and PTFE used in gas diffusion electrodes as the gas network and binder respectively upon phenomena relating to the performance and lifetime of PAFC cathodes have been examined. The FPE or PTFE content is referred to the total CB (TCB) contained in FPE/CB and Pt/CB (TCB is fixed at 6 mg cmb2 of the apparent surface area of cathodes) in the following discussion. Fig. 7 shows the amount of electrolyte absorbed in gas diffusion electrodes as a function of content of the functional components in the electrodes. The amount of electrolyte was obtained by weighing each electrode before and after use. It is found that the amount of electrolyte absorbed at a constant PTFE content decreases with increasing FPE or FPE/CB content, and the amount of electrolyte is clearly reduced with increasing PTFE content, particularly in the region FPE/TCB > 7/10. PTFE probably works not only as a binder but also as a gas network in combination with FPE/CB, resulting in improvement of the gas-supplying property by suppressing
100 1
I
20
0
0 Time I h
Fig. 6. Change in pore occupation by 105% PA with time in Teflon-bonded gas diffusion layers wet-proofed by FPE at 190°C in air: w 5 wt.% FPE/CB; 0 10 wt.% FPE/CB; A 20 wt.% FPE/CB; IJ 30 wt.% FPE/CB; * 40 wt.% FPE/CB. In all these samples 20 parts of PTFE were added to CB as binder. A reference 1 (F’TFE/CB (2/8)), 0 reference 2 @TFE/CB (6/4)).
3/10 0
5/100
gj 40/100
5110
6110 7/10 8110 9/10 FPE/TCB •j 15/100 20/100 30/100
q
n
50/100
70/100 =PTFE/TCB
Fig. 7. Comparison of the amount of electrolyte absorbed in a series of gas diffusion electrodes containing FPE/CB and PTFE as gas network and binder respectively after the half-cell test at 190°C in 105% PA.
excess filling of the gas network with electrolyte. The gas-supplying property can be evaluated from the Tafel slope for oxygen reduction as described below. Many papers have been published concerning Tafel slope values for oxygen reduction in acid or alkaline electrolytes. In a simulation using the flooded agglomerate mode1 of a Teflon-bonded gas diffusion electrode, two different values of the Tafel slope, - 2.3RT/F (single Tafel slope) and - 2(2.3 RT)/F (double Tafel slope), were predicted in the absence and the presence of gas diffusion loss in the agglomerate particles [8,9]. These results have been used in discussions of floated electrodes [ 101 and gas diffusion electrodes [2] in hot phosphoric acid. It has also been shown that the Tafel slope decreases from - 110 to - 90 mV per decade as the specific surface area of platinum crystallites supported on CB increases from 10 to 80 m2 (g PtY’. Pt ill]. Figs. 8(A) and 8(B) show the changes in Tafel slope with FPE and PTFE contents for the reduction of oxygen and oxygen in air respectively. When the ratio of FPE to TCB is less than 6/10, the corresponding Tafel slopes become greater than 120 mV per decade regardless of the PTFE content because not only the charge transfer process but also the mass transport of dissolved 0, in electrolyte contribute to the increase in overpotential due to the low gas permeability in the gas network. However, the slope reaches a minimum for ratios of FPE to TCB in the range from 6/10 to 9/10 independent of PTFE content, i.e. ca. 100 mV and 90 mV per decade for air and oxygen
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t/996181-88
200 150 80 E 7
fi-i
100
2
50 I
-
0I
=
50 I
-
s \E;
-
1
60
!i E 20
G 100 I
-
150 I
-
200 I-
q q
0
3110
5/10
6/10 7110 FPE/TCB
5/100
•j 15000
40/100
n
8/10
9/10
20/100 gg 30/100
50/100 170/100
q q
=PTFE/TCB
Fig. 8. Change in Tafel slope of the various electrodes for the reduction of (A) oxygen and (B) oxygen in air with the electrode composition at l!WC in 105% PA.
respectively. This result corresponds to a kinetically controlled electrode process resulting from the condition that the gas supply process is improved by the addition of FPE/CB in cooperation with a PTFE binder where the PTFE-to-TCB ratio exceeds 20/ 100. When the ratio of FPE to TCB is greater than 9/10, the Tafel slope increases again despite the improved oxygen diffusion in the electrolyte network. It appears that protons included in the reaction scheme for oxygen reduction become a limiting reactant in the rate-determining step owing to the low conductivity of protons in the electrolyte network of such highly wet-proofed gas diffusion electrodes. Fig. 9 shows the change in mass activity with FPE and PIFE contents. The mass activity is defined as the current per unit platinum catalyst weight (A (g Pt)- ‘) at 0.9 V/RHE and is a measure of the change of utilization of catalyst clusters in gas diffusion electrodes. It can be seen that maximum mass activity is achieved for ratios of FPE to TCB in the range from 6/10 to 8/10, i.e. more than 80 A (g Pt))‘. This mass activity, or catalyst utilization, is more than 1.5 times greater than that obtained so far with electrodes of conventional structure. Reduction of the mass activity in the regions with FPE/TCB < 6/10 and FPE/TCB > 8/10 can be attributed to the drowning of catalyst clusters in the electrolyte network and starvation of them in the gas network respectively. These results are in good agreement with the discussion of Figs. 7 and 8.
5/100
fg 15/100
40/100
n
FPE/TCB 20/100
50/100
q
30/100
70/100 =PTFE/TCB
Fig. 9. Change in mass activity with FPE and P’I’FE contents for the reduction of (A) oxygen and (B) oxygen in air at 190°C in 105% PA.
As demonstrated previously [2], an ideal gas diffusion electrode structure can be achieved if the combination of electrolyte and gas network is optimized in an advanced 1000,
, 800 600 -
g
400 -
SJ 200 s \ 3 ‘$ z a
0 --I 200 400 600 -4 800 loo0
q q Fig.
PI 3/10 5/100
5/10
6/10 7/10 8/10 9/10 FPE/TCB •j 15/100 2O/lOO •j 30/100
40/100 H 50/100
q
70/100 =PTFE/TCB
10. Electrode potentials for the reduction of (A) oxygen at 300 mA and (B) oxygen in air at 200 mA cmm2 at 190°C in 105% PA on electrodes containing various contents of FPE and PTFE.
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electrode structure in which separate functions are assigned to the electrolyte or catalyst network and the gas network, such as those shown in Fig. 2(B). The functions can be diagnosedas described above from the electrolyte occupation, Tafel slope and massactivity. As a result of this diagnosis, we expect good performances at a high current density on the proposed new electrode structure using FPE/CB. Fig. 10 shows the electrode potentials at 300 mA cm-2 and 200 mA cme2 for the reduction of oxygen and oxygen in air respectively on electrodes containing various contents of FPE and PTFE. Very good performances of ca 0.8 V and 0.75 V are achieved at the optimized electrode composition, i.e. 6/10 < FPE/TCB < 8/10 and 30/ 100 < PTFE/TCB < 70/ 100, which are comparable with the best data for conventional electrodes, and a long electrode lifetime is expected owing to the superior wet-proofing properties of FPE/CB.
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improved catalyst utilization and gas permeability compared with cathodes using conventional PTFE. The results demonstrate a potential for replacing PTFE/CB with FPE/CB to achieve electrode structures with both high performance and long life for commercial PAFCs. Therefore further extensive research on this material and electrode is required, with particular emphasison stability over a long lifetime.
Acknowledgement The authors are grateful to Mr. Y. Kodera, a graduateof Yamanashi University, for his contributions to the experiments.
References 4. Summary We have proposeda new design concept and a new gas network material @PE/CB) for an advancedgas diffusion electrode for PAFCs in which completely separate functional materials were used for the gas network and the electrolyte network. It was shown that polyethylene could cover the whole surface of CB particles with a thin film, unlike the conventional Teflon dispersion,and could easily be fluorinated to produce a Teflon-like hydrophobic material. The resulting FPE/CB showed distinct hydrophobic properties and a thermal stability comparable with that of PTFE under PAFC operating conditions. Cathodeswith an optimum structure using the new FPE/CB material exhibited a good performance for oxygen reduction owing to the
[l] [Z] [3]
[4] [5] [6] [7] [8] [9] [lo] [I I]
M. Watanabe, M. Tomikawa and S. Motoo, J. Electroanal. Chem., 195 (1985) 81. M. Watanabe, K. Makita, H. Usami and S. Motoo, J. Electroanal. Chem., 197 (1986) 195. T. Mori, J. Imahashi, T. Kamo, K. Tamura and Y. Hishinuma. J. Electrochem. Sot., 133 (1986) 896. T. Mori, A. Honji and T. Hishinuma, J. Electrochem. Sot., 135 (1988) 1104. M. Watanabe and S. Motoo, J. Chem. Sot. Jpn., (1988) 1308. Y. Chong and N. Watanabe, Abstract 2DO8, Meeting of the Electrochemical Society of Japan. Yokohama, 1989. M. Watanabe, C. Shimura, K. Tsurumi and N. Hara. Chem. Lea., (1991) 1113. M. Watanabe, K. Tsurumi and N. Hara, US Patent 5, 137, 754. J. Giner and C. Hunter, J. Electrochem. Sot., 116 (1969) 1124. P. Stonehart and P.N. Ross, Electrochem. Acta, 21 (1976) 441. H.R. Kunz and G.A. Gruver, J. Electrochem. Sot., 122 (1975) 1279. L.J. Bregori, J. Electrochim. Acta, 23 (1978) 489.
Journal of Electroanalytic al Chemistry 413 ( 1996) 8 l-88
An advanced gas diffusion electrode for high performance phosphoric acid fuel cells Noriaki Hara a2b,Kazunori Tsurumi b, Masahiro Watanabe a3* a Laboratory
of Electrochemical b Tanaku Kikinzoku
Energy Cotwersion, Yamanashi University, Takeda 4-3, Kofu 400, Japan Kogyo Technical Center, Shin-machi 2-73, Hiratsuka 254, Japan
Received 25 March 1994; in revised form 25 May 1994 ’
Abstract An extremely thin fluorinated polyethylene (FPE) film was prepared on a carbon black (CB) surface and the properties were studied. It was found that polyethylene as the precursor can cover the whole surface of the CB particles with a thin film different from the conventional Teflon dispersion (PTFE), and it can be fluorinated easily to produce a Teflon-like hydrophobic material. The resulting FPE/CB material showed a distinctive hydrophobic property and a comparable thermal stability compared with PTFE under the operating conditions for phosphoric acid fuel cells (PAFCs). An advanced gas diffusion electrode for PAFCs is proposed in which a new design concept of the electrode structure is applied where the functions of electrolyte network and gas network are assigned completely to catalyzed CB and FPE/CB. The cathode optimized in the structure with the new FPE/CB exhibited a high performance for oxygen reduction compared with cathodes using conventional PTFE owing to the improved catalyst utilization and gas permeability, and showed a potential for long operational life owing to less performance degradation resulting from the prevention of flooding of the gas network with electrolyte during operation. Keywords: Gas diffusion electrode; Phosphoric
acid fuel cells
1. Introduction
In order to develop phosphoric acid fuel cells (PAFCs), it is essential to improve performance and lifetime. Field tests focused on commercial development have been demonstrated, but the operation time was at most about 15 000 h for cells using conventional electrode structures owing to the degradation of performance resulting from problems originating in the cells themselves or those induced by other difficulties. Conventionally, the gas diffusion electrode is composed of a mixture of fine particles of catalyzed carbon black (Pt/CB) and polytetrafluoroethylene (PTFE) dispersion, which form the electrolyte network and the gas network respectively. The balance of these networks is controlled by the PTFE content. PAFCs show a decay in performance with time. Restricted gas diffusion to catalyst clusters owing to flooding of the gas network with excess electrolyte is one of the major reasons for this
* Corresponding author. ’ Publication of this paper was delayed for reasons beyond the control of the publisher. 0022-0728/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03597-V
decay, particularly after operation for more than 10000 h. The gas diffusion property can be improved to some extent by increasing the content of PTFE, because of its high hydrophibicity, resulting in some improvement in lifetime. However, there is a concomitant decrease in performance because of the reduction of catalyst utilization in the gas diffusion electrode, i.e. the improvement of gas permeability and catalyst utilization are in opposition in the conventional electrode structure [l-3]. It is also believed that the carbon black (CB) surface in the gas network is not covered sufficiently with PTFE, because conventional PTFE particles are 10 times larger than CB particles, as shown schematically in Fig. l(A), and PTFE has a high viscosity at the sintering temperature in the electrode fabrication process, so that the hydrophobic property may not be maintained over a long period. In order to overcome these problems, Watanabe et al. [2] have proposed a new type of electrode, shown schematically in Fig. 2(B), which is based on the concept that the functions of the catalyst layer, or reaction layer, in the electrode are assigned completely to hydrophilic Pt/CB as the electrolyte network and CB highly wet-proofed with PTFE or analogous polymers as the gas-supplying network (GSP), and they
N. Ham
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diffusion electrode for such long-lived PAFCs, i.e. CB completely wet-proofed by fluorinated polyethylene (FPE/CB) and an electrode structure in which the functions of electrolyte network and gas network are assigned to Pt/CB and FPE/CB respectively. The aim of this paper is to demonstrate the advantageous features of FPE/CB and the electrode incorporating this material in more detail.
- 0.04~rn)
FPE(r
2. Experimental Denka Black (Denki Kagaku Kogyo) and HiZex (Mitsui Sekiyu Kagaku Kogyo) were used as the CB and polyethylene (PE) sources respectively for the preparation of FPE/CB. CB was mixed uniformly with PE, and the mixture was heat treated in the fluid condition at 170°C in N, atmosphere to obtain CB coated with a uniform thin PE film (PE/CB). Direct fluorination of PE/CB was carried out using specially programmed time-temperature and fluorine concentration sequences. The reaction conditions used were changed depending upon the thickness of PE film required. The degree of fluorination and the change in the fluorinated state were analyzed by gravimetry and X-ray photoelectron spectrometry (XPS) using a Shimazu ESCA-750s spectrometer with Mg Ka radiation. Details of the preparation and the effect of the conditions upon the properties of the FPE obtained will be published elsewhere. Several techniques were used to characterize CB samples coated with thin PE and FPE films. Their coating state was evaluated by Brunauer-Emmett-Teller (BET) surface area measurement using a Quantachrome Quantasorb or by scanning electron microscopy @EM) using a Jeol JSM6300F microscope. The degree of agglomeration of the film-coated CB was assessed using a particle sizer (Horiba Ltd. model LA-7001, and the porosity or pore-size distribution in CB powders or gas-diffusion electrodes was obtained using a mercury pore sizer (Micromeritics pore sizer 9310). The thermal stability of FPE and PTFE was evaluated by thermal gravimetry and differential thermal analysis (MacScience TG/DTA 2000).
(B)
(A)
Fig. I. Schematic structure of CB wet-proofed by (A) conventional PTFE particles and (B) fluorinated polyethylene (FPE) film. Symbols d and 1 denote particle diameter and film thickness respectively.
have demonstrated the possibility of designing an ideal electrode structure enabling 100% utilization of catalyst clusters without polarization losses caused by mass transport up to a high current density at the optimized structure. Wet-proofing reagents with relatively low melting viscosity, such as tetrafluoroethylene-hexafluoropropylene copolymer (FEP), combined with PTFE were also investigated by Watanabe et al. [4]; however, a progressive degradation of performance with time due to flooding of the gas network was still predicted as long as PTFE or similar polymers were used as the wet-proofing reagent because of the intrinsic properties of PTFE discussed above. Therefore it is essential to develop a new material for the gas network which can guarantee the wet-proofing property for a long period (e.g. 40000 h as required for commercial PAFCs) under the operating conditions. The fluorination of a bulky polyethylene surface in the presence of a small amount of oxygen has been found to provide hydrophilic surface properties [5], but there have been no studies of a hydrophobic film of fluorinated polyethylene (FPE) with properties similar to those of PTFE. We have published preliminary reports [6,7] of an ideal material and a new design concept for an advanced gas
(A)
(W
Electrolyte Fig. 2. Comparison
PTFE
Electrolyte
of the reaction layer structures of gas diffusion electrodes: (A) conventional concept using PTFE; (B, C) modem concepts completely electrolyte and gas networks by using CB wet-proofed by (B) PTFE and (C) FPE as the gas network. Pt/CB; catalyzed carbon black, GSP; gas-supplying powder.
separatingthe functions of
N. Haru
et al./Journal
ofElecrroanalyrica1
The new structure of gas diffusion electrodes using FPE/CB is shown schematically in Fig. 2(C). In order to compensate for the lack of mechanical bonding strength of thin FPE film in the formation of the structure, PTFE was used as a binder to the electrolyte and gas networks composed of alloy catalyst supported on CB (denoted as Pt/CB) and FPE/CB respectively. The alloy catalyst was a Pt-containing ternary alloy (TEC77A20; alloy loading on CB was 20 wt.% by platinum base) supplied by Tanaka Kikinzoku Kogyo. The electrodes were prepared by the filter transfer method; the flock formed from a mixture of Pt/CB, FPE/CB and PTFE (DuPont PTFE-3OJ) suspended in water + isopropanol was filtered onto a Teflon membrane filter, transferred by cold-pressing onto a carbon paper which had previously been wet-proofed with 30 wt.% FEP and finally hot-pressed at 360°C for 3 s under 5 kg cm-*. FPE/CB (l/l) was used in a series of mixing ratios with Pt/CB to prepare the gas diffusion electrodes. The total CB weight per unit apparent electrode area (TCB) was fixed at 6.0 mg cm-’ for every electrode in order to keep the thickness of the reaction layer constant; therefore a different amount of alloy catalyst was loaded on each electrode. Gas diffusion electrodes prepared by adding F’PE/CB in six mixing ratios of FPE/TCB, i.e. 3/10,5/10,6/10,7/10, 8/10 and 9/10, were examined with various PTFE binder contents; the load of alloy catalyst on each electrode was 0.97 mg cm-*, 0.73 mg cm -*, 0.48 mg cm-‘, 0.37 mg cme2, 0.21 mg cmW2 and 0.048 mg cmp2 respectively in the Pt base and PTFE was added in PTFE/TCB ratios ranging from 5/100 to 70/100. Cathode performances of gas diffusion electrodes for pure oxygen and oxygen in air were measured under galvanostatic conditions in 100% phosphoric acid and ambient pressure at 190°C. Potentials were referred to a reversible hydrogen electrode (RHE) in the same electrolyte. Prior to the measurement, air and H, were supplied alternately to the gas chamber of the cell with the precaution of a N, blanket between gas switching. Switching the gas-supply accelerated the filling of the cathode catalyst layer with electrolyte. Cathode and anode performances were also measured during this process. After the cathode performance at 0.7 V/RHE had stabilized, its steady-state performance was measured fully. The ohmic
Chemiswy
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resistances were corrected using a current-pulse generator (Nikko Keisoku NCPG-1010). The current density (j) on the polarization curve of each electrode loaded with X mg Pt cm-* was normalized to the current density j, of an electrode loaded with 0.54 mg Pt cme2 using the equation j, = 0.54 j/X. In order to evaluate the hydrophobic properties of the new FPE/CB by measuring the contact angle with hot phosphoric acid or the electrolyte occupation, porous sheets mimicking gas diffusion electrodes were prepared with FPE/CB in a similar manner to that above for catalyzed electrodes, but without the addition of Pt/CB. Powders or electrodes containing only CB and PTFE (PTFE/CB), with a PTFE CB ratio of 6/4 or 2/8, were also prepared in a similar manner as references for the FBE/CB powder or FPE/CB porous sheets. The hydrophobic properties of the gas diffusion electrodes prepared as described above were evaluated by measuring the contact angles to 105% phosphoric acid at 190°C using a contact angle meter (Erma Inc. Type G-II). Changes in the hydrophobic properties with time were evaluated by measuring the weight change with electrolyte occupation of the gas network in gas diffusion electrodes floating on a phosphoric acid pool at 190°C in air.
3. Results and discussion 3.1. Characterization
of PE/ C and FPE/
C samples
Scanning electron micrographs of CB, PE/CB and FPE/CB are shown in Figs. 3(A), 3(B) and 3(C) respectively. The primary CB particles of diameter ca. 50 nm can be seen in Fig. 3(A). It is found that the particles are covered with uniform thin PE film accompanying an increase in the particle size. No change in the covering state is observed after fluorination as shown in Figs. 3(B) and 3(C).
Table 1 gives the changes in the characteristics of CB powders, i.e. BET surface area, median diameter and porosity, during PE coating and the subsequent fluorination. All the data, except the median diameter, were normalized by the unit weight of CB. The BET surface area decreased by 64% compared with the original value after
Fig. 3. Scanning electron micrographs of (A) CB, (B) PE/CB and (Cl FPE/CB.
84
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oj’Electroanalytical
PE coating, which suggests that the rough CB surface was well coated with a flat PE thin film. If it is assumed that the CB particles are separated from each other and maintain a mean particle diameter of 50 nm (although in practice they form aggregates) and that they can be fully coated with a PE film of uniform thickness (see Figs. 3(A) and 3(B)); the thickness is estimated to be ca. 7 nm. An increase in the median diameter after PE coating indicates agglomeration of CB aggregates by the binding action of PE, which results in an increase in the pore volume probably due to the increase in space between the agglomerates. The decrease in median diameter after fluorination of PE/CB probably occurred because the PE bonding between the agglomerates was partially destroyed during the formation of FPE, although no noticeable change in the BET surface area was observed. The pore volume decreased due to the increase in the film thickness on conversion of PE to FPE in the rigid structure of CB aggregates or agglomerates (see Figs. 3(B) and 3(C)). Comparison of the BET surface area of FPE/CB (27 m* (g CB)-’ ) with that of the PTFE/CB reference (59 m2 (g CB)- ’ ), clearly shows that the coating of CB with polymer film is much more complete with FPE/CB. Moreover, the porosity of FPE/CB is 10% higher than that of CB/PTFE, which must be preferable for gas diffusion when the former is to be used as the gas network material in gas diffusion electrodes. C 1s X-ray photoelectron spectra of compressed powders of PE/CB (4/9), FPE/CB (6/4) and PTFE/CB (6/4) are shown in Figs. 4(A), 4(B) and 4(C) respectively. The degree of fluorination for the FPE/CB on that of replacement of H atoms in PE with F atoms, was found to be ca. 100% by gravimetric determination. Although the C 1s peak on PE/CB is the same as that of CB, i.e. 285.0 eV, that of FPE/CB exhibits a large positive shift. The resulting peak at 292.4 eV coincides relatively well with that on PTFE/CB assigned to the CF,--CF, bond, suggesting the formation of a new Teflon-like material by direct fluorination of PE. A small number of H atoms remain as shown by the peak around 290 eV assigned to the CFHCF, bond. Since the C 1s spectrum on CB must be reduced by covering its surface with polymer films, the ratio of the peak area for CF,-CF, (292.5 eV) to that for CB (285.0 eV) reflects the increase in the surface coverage Table 1 Change in the typical characteristics coating and subsequent fluorination
CB PE/CB FPE/CB PTFE/CB
of carbon black during
BET (N,) surface area/ m* (g CB)- ’
Median diameter/ km
Pore volume/ cm3 (g CB)-
73 26 ( - 64%) 27(-63%) 59(-19%)
0.8 4.0 1.9 -
2.7 4.5 3.0 2.9
polyethylene Porosity/ %
’ 71 69 67 58
Chemistry
413
(1996)
81-88
A!!L (B)
CD)
Bonding
h loo energy / eV
0
290
Fig. 4. C IS X-ray photoelectron spectra of (A) PE/CB PTFE/CB (6/4), (C) FPE/CB (6/4) and (D) FPE/CB keeping in an oven at 205°C for 2000 h.
(4/9), (6/4)
(B) after
of CB with films. The ratio is found to be 2.02 for FPE/CB, which is much larger than that for PTFE/CB (0.22). This difference clearly demonstrates that the coverage of CB with FPE is much greater than that with PTFE, and that the FPE obtained by the direct fluorination of PE can coat the CB surface much better than PTFE. This is supported by the following observation. A shoulder peak appears at around 295 eV on FPE/CB, which corresponds to the large peak at 295.8 eV on PTFE/CB. Since this peak disappears after the neutralization by an electron gun, it can be attributed to charged CF,-CF, detached from electron-conducting CB. Therefore, the small peak height obtained for FPE/CB clearly suggests that FPE coats CB with a thin film more uniformly than does PTFE, and that the PTFE particles in the conventional electrode do not exist in the film form on CB, but are present as much bulkier particles, as illustrated in Figs. l(A) and l(B). 3.2. Thermal stability of FPE/
CB
The thermal stability of FPE/CB (6/4) in air was examined by thermogravimetry and differential thermal analysis (TG-DTA) and compared with PTFE/CB (6/4). The data are shown in Fig. 5. Both samples decompose exothermically in two steps: the polymers undergo oxidative decomposition in the first step and CB is oxidized at a temperature above 500°C in the second step. It is found that FPE/CB starts to lose weight at 310°C which is about 100°C lower than observed for PTFE/CB. This is probably because the FPE has relatively lower molar mass than PTFE, because some of the C-C bonds may be broken by the direct fluorination reaction. The molar masses of FPE must be dispersed over a wide range, as indicated by the disappearance of a sharp melting point
N. Hara
of Electroanalytical
et ul./Journal
Chemistry
413 (1996)
Table 2 XPS data for PTFE/CB
81-88
and FPE/CB CB or PE
%-loox
I
,
I
I
I
I
,
I
I
I
l
l
l
I
o-
300
(w -200,
%
1
.c i
85
CF, -CF,
Charged-up pe*
GSP PTFE/CB (6/41powder Bonding energy/eV 285.0 Peak area/% 64 Normalized ratio 1
292.1 14 0.22
295.8 22 0.35
FPE/CB f6/4)powder Bonding energy/eV Peak area/% Normalized ratio
285.0 32 1
292.5 65 2.02
294.5 4 0.11
FPE/ CB (6/4) powder Bonding energy/eV Peak area/% Normalized ratio
kept ut 2OPC for 2000 h 285.0 292.6 33 64 1 1.95
294.6 3 0.10
-!jcy$f
200
0
400
600
Temperature Fig. 5. Comparison PTFE/CB (6/4). rate of 10°C min-
Sheets with a series of FPE contents in FPE/CB, which mimicked gas diffusion electrodes, were prepared by adding 20 parts of PTFE as a binder to 80 parts of CB coated with FPE, and hot-pressing the mixture. The reference PTFE/CB sheet samples were composed of 20 and 60 parts of PTFE to CB uncoated with FPE. The results are shown in Table 3. All the data on the sheet samples show contact angles higher than the value of 110” reported for a flat Teflon sheet, probably because of the large real contact angle resulting from their microrough surface structure. The FPE/CB samples containing more than 10 wt.% FPE exhibit very high contact angles (greater than 161”) which is larger than the value of 159” for a PTFE/CB sample containing 60 wt.% PTFE. Fig. 6 shows the change of pore occupation by PA with time in the FPE/CB and PTFE/CB sheet samples discussed above. These data were obtained by gravimetry of these samples floated on a 105% PA pool at 190°C in air. The hydrophobic properties improve with increasing FPE content as shown by the lower occupation by PA. The samples containing more than 30 wt.% of FPE are almost completely hydrophobic, i.e. less than 10% occupation by PA after 150 h. However, more than 70% of the micropores of PTFE/CB samples were occupied by PA within a short time, i.e. 100 h even for the sample containing 60 wt.% PTFE. Therefore it is considered that FPE/CB containing more than 40 wt.% FPE is a suitable candidate for gas network material in gas diffusion electrodes. A long-
800
I “C
of the thermal stability of (A) FPE/CB (6/4) and (B) The temperature was increased to 800°C at a constant ’ in air.
like that of 340°C for PTFE/CB (Fig. 5(B)). Thus the new FPE/CB has a lower thermal stability than the conventional PTFE/CB. However, FPE/CB is still believed to be sufficiently stable around 200°C which is the conventional operating temperature of PAFCs. The long-range stability of FPE/CB was examined by keeping it in an oven at 205°C for 2000 h. The C 1s X-ray photoelectron spectrum is shown in Fig. 4(D) and the XPS data are given in Table 2. The difference between the data before and after the stability test is negligibly small. Consequently, we can conclude that FPE/CB is sufficiently stable for practical applications in gas diffusion electrodes. 3.3. Hydrophobic
properties
of FPE / CB
The hydrophobic properties of FPE/CB were examined by comparing the contact angle with 105% phosphoric acid (PA) at 190°C with that of conventional PTFE/CB. Table 3 Characteristics
layers wet-proofed
by fluorinated
Samples
of gas diffusion
A
B
C
D
E
F
G
FPE content in FPE/CB/% PTFE/CB ’ Total content of FPE and PTFE/% Contact angles *
0
0 6/4 60 159”
5
10
20
30
40
w3
2/8
2/g
2/8
w3
23 157”
2-I 161”
33 161”
40 164”
48 164”
2/8 20 158”
A, B, reference samples. ’ Ratio of added PTFE as a binder in the gas diffusion * Against hot phosphoric acid (19O”C, 105%, air).
layer.
polyethylene
86
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term test on changes in the hydrophobic properties of FPE/CB is required in the next stage of the investigation, but the results on the thermal stability and hydrophobic properties reported above suggest that the FPE/CB could be used instead of PTFE/CB for electrode structures in commercial PAFCs to ensure high performance and long life. 3.4. Performance of gas diffusion electrodes using FPE/ CB for 0, reduction The effects of the content of FPE/CB and PTFE used in gas diffusion electrodes as the gas network and binder respectively upon phenomena relating to the performance and lifetime of PAFC cathodes have been examined. The FPE or PTFE content is referred to the total CB (TCB) contained in FPE/CB and Pt/CB (TCB is fixed at 6 mg cmb2 of the apparent surface area of cathodes) in the following discussion. Fig. 7 shows the amount of electrolyte absorbed in gas diffusion electrodes as a function of content of the functional components in the electrodes. The amount of electrolyte was obtained by weighing each electrode before and after use. It is found that the amount of electrolyte absorbed at a constant PTFE content decreases with increasing FPE or FPE/CB content, and the amount of electrolyte is clearly reduced with increasing PTFE content, particularly in the region FPE/TCB > 7/10. PTFE probably works not only as a binder but also as a gas network in combination with FPE/CB, resulting in improvement of the gas-supplying property by suppressing
100 1
I
20
0
0 Time I h
Fig. 6. Change in pore occupation by 105% PA with time in Teflon-bonded gas diffusion layers wet-proofed by FPE at 190°C in air: w 5 wt.% FPE/CB; 0 10 wt.% FPE/CB; A 20 wt.% FPE/CB; IJ 30 wt.% FPE/CB; * 40 wt.% FPE/CB. In all these samples 20 parts of PTFE were added to CB as binder. A reference 1 (F’TFE/CB (2/8)), 0 reference 2 @TFE/CB (6/4)).
3/10 0
5/100
gj 40/100
5110
6110 7/10 8110 9/10 FPE/TCB •j 15/100 20/100 30/100
q
n
50/100
70/100 =PTFE/TCB
Fig. 7. Comparison of the amount of electrolyte absorbed in a series of gas diffusion electrodes containing FPE/CB and PTFE as gas network and binder respectively after the half-cell test at 190°C in 105% PA.
excess filling of the gas network with electrolyte. The gas-supplying property can be evaluated from the Tafel slope for oxygen reduction as described below. Many papers have been published concerning Tafel slope values for oxygen reduction in acid or alkaline electrolytes. In a simulation using the flooded agglomerate mode1 of a Teflon-bonded gas diffusion electrode, two different values of the Tafel slope, - 2.3RT/F (single Tafel slope) and - 2(2.3 RT)/F (double Tafel slope), were predicted in the absence and the presence of gas diffusion loss in the agglomerate particles [8,9]. These results have been used in discussions of floated electrodes [ 101 and gas diffusion electrodes [2] in hot phosphoric acid. It has also been shown that the Tafel slope decreases from - 110 to - 90 mV per decade as the specific surface area of platinum crystallites supported on CB increases from 10 to 80 m2 (g PtY’. Pt ill]. Figs. 8(A) and 8(B) show the changes in Tafel slope with FPE and PTFE contents for the reduction of oxygen and oxygen in air respectively. When the ratio of FPE to TCB is less than 6/10, the corresponding Tafel slopes become greater than 120 mV per decade regardless of the PTFE content because not only the charge transfer process but also the mass transport of dissolved 0, in electrolyte contribute to the increase in overpotential due to the low gas permeability in the gas network. However, the slope reaches a minimum for ratios of FPE to TCB in the range from 6/10 to 9/10 independent of PTFE content, i.e. ca. 100 mV and 90 mV per decade for air and oxygen
N. Hum
rt ~l./Jnurmi
ofEiectroanulytica1
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413
87
t/996181-88
200 150 80 E 7
fi-i
100
2
50 I
-
0I
=
50 I
-
s \E;
-
1
60
!i E 20
G 100 I
-
150 I
-
200 I-
q q
0
3110
5/10
6/10 7110 FPE/TCB
5/100
•j 15000
40/100
n
8/10
9/10
20/100 gg 30/100
50/100 170/100
q q
=PTFE/TCB
Fig. 8. Change in Tafel slope of the various electrodes for the reduction of (A) oxygen and (B) oxygen in air with the electrode composition at l!WC in 105% PA.
respectively. This result corresponds to a kinetically controlled electrode process resulting from the condition that the gas supply process is improved by the addition of FPE/CB in cooperation with a PTFE binder where the PTFE-to-TCB ratio exceeds 20/ 100. When the ratio of FPE to TCB is greater than 9/10, the Tafel slope increases again despite the improved oxygen diffusion in the electrolyte network. It appears that protons included in the reaction scheme for oxygen reduction become a limiting reactant in the rate-determining step owing to the low conductivity of protons in the electrolyte network of such highly wet-proofed gas diffusion electrodes. Fig. 9 shows the change in mass activity with FPE and PIFE contents. The mass activity is defined as the current per unit platinum catalyst weight (A (g Pt)- ‘) at 0.9 V/RHE and is a measure of the change of utilization of catalyst clusters in gas diffusion electrodes. It can be seen that maximum mass activity is achieved for ratios of FPE to TCB in the range from 6/10 to 8/10, i.e. more than 80 A (g Pt))‘. This mass activity, or catalyst utilization, is more than 1.5 times greater than that obtained so far with electrodes of conventional structure. Reduction of the mass activity in the regions with FPE/TCB < 6/10 and FPE/TCB > 8/10 can be attributed to the drowning of catalyst clusters in the electrolyte network and starvation of them in the gas network respectively. These results are in good agreement with the discussion of Figs. 7 and 8.
5/100
fg 15/100
40/100
n
FPE/TCB 20/100
50/100
q
30/100
70/100 =PTFE/TCB
Fig. 9. Change in mass activity with FPE and P’I’FE contents for the reduction of (A) oxygen and (B) oxygen in air at 190°C in 105% PA.
As demonstrated previously [2], an ideal gas diffusion electrode structure can be achieved if the combination of electrolyte and gas network is optimized in an advanced 1000,
, 800 600 -
g
400 -
SJ 200 s \ 3 ‘$ z a
0 --I 200 400 600 -4 800 loo0
q q Fig.
PI 3/10 5/100
5/10
6/10 7/10 8/10 9/10 FPE/TCB •j 15/100 2O/lOO •j 30/100
40/100 H 50/100
q
70/100 =PTFE/TCB
10. Electrode potentials for the reduction of (A) oxygen at 300 mA and (B) oxygen in air at 200 mA cmm2 at 190°C in 105% PA on electrodes containing various contents of FPE and PTFE.
cm-’
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electrode structure in which separate functions are assigned to the electrolyte or catalyst network and the gas network, such as those shown in Fig. 2(B). The functions can be diagnosedas described above from the electrolyte occupation, Tafel slope and massactivity. As a result of this diagnosis, we expect good performances at a high current density on the proposed new electrode structure using FPE/CB. Fig. 10 shows the electrode potentials at 300 mA cm-2 and 200 mA cme2 for the reduction of oxygen and oxygen in air respectively on electrodes containing various contents of FPE and PTFE. Very good performances of ca 0.8 V and 0.75 V are achieved at the optimized electrode composition, i.e. 6/10 < FPE/TCB < 8/10 and 30/ 100 < PTFE/TCB < 70/ 100, which are comparable with the best data for conventional electrodes, and a long electrode lifetime is expected owing to the superior wet-proofing properties of FPE/CB.
Chemistry
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(19%)
81-88
improved catalyst utilization and gas permeability compared with cathodes using conventional PTFE. The results demonstrate a potential for replacing PTFE/CB with FPE/CB to achieve electrode structures with both high performance and long life for commercial PAFCs. Therefore further extensive research on this material and electrode is required, with particular emphasison stability over a long lifetime.
Acknowledgement The authors are grateful to Mr. Y. Kodera, a graduateof Yamanashi University, for his contributions to the experiments.
References 4. Summary We have proposeda new design concept and a new gas network material @PE/CB) for an advancedgas diffusion electrode for PAFCs in which completely separate functional materials were used for the gas network and the electrolyte network. It was shown that polyethylene could cover the whole surface of CB particles with a thin film, unlike the conventional Teflon dispersion,and could easily be fluorinated to produce a Teflon-like hydrophobic material. The resulting FPE/CB showed distinct hydrophobic properties and a thermal stability comparable with that of PTFE under PAFC operating conditions. Cathodeswith an optimum structure using the new FPE/CB material exhibited a good performance for oxygen reduction owing to the
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