Monolithic carbon aerogels for fuel cell electrodes

9 1998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors.

167

Monolithic c a r b o n aerogels for fuel cell e l e c t r o d e s G2VI. Pajonk ~), A. V e n k a t e s w a r a Rao ~2), N. Pinto ~), F.Ehrburger-Dolle ~3) and M.Bellido Gil ~3~ (~)LACE -UMR 5634 CNRS-Universit~ Claude Bernard Lyon 1, 43 boulevard du 11 Novembre 1918 - 69622 Villeurbanne Cedex, France (2) Airglass Laboratory-Department of Physics-Shivaji University - Kolhapur 416004 -India (8)ICSI-CNRS 15 rue Starcky - 68057 Mulhouse Cedex -France

ABSTRACT Electrical vehicles (Zero Emission Vehicles) represent a very elegant solution in order to decrease pollution in industrialised cities. To meet the pollution requirements, it is envisaged to replace gasoline engines by light fuell cells. They necessitate good electroconductive carbon monolithic electrodes. Monolithic organic resorcinol-formaldehyde aerogels (carbogels)can easily be made using the sol to gel method accompanied by the supercritical drying process using liquid CO~ (critical temperature = 31~ Thereafter, these copolymer monolithic carbogels are pyrolysed in an atmosphere of N2 at 10500C for 3 hours, in order to obtain monolithic carbon carbogels. The carbon carbogels are impregnated by H~PtCh to obtain a weight % of metal equal to 0.44. In this work, the unique solvent used was acetone in each step, - the sol to gel reactions were catalysed by perchloric acid, from the copolymer synthesis up to the final impregnation of the carbon monoliths. The advantages of this new preparation method are compared to the aqueous former one. INTRODUCTION Organic as well as inorganic, simple or composite aerogels based upon the well known sol-gel method,represent an interesting way for the preparation of highly divided catalytic materials, because the supercrtical drying process retain most of the very developed textural properties of the wet gel by avoiding the capillary stress gradients [1]. Around 1988, Pekala et al. [2,3] developed the first organic aerogel from the copolymerisation of formaldehyde (noted F) with resorcinol (noted R) in water with the help of sodium carbonate as catalyst. These searchers succeeded in making monolithic and more or less red transparent

168 organic carbogels which furtherly were pyrolysed at high temperature either in dinitrogen or carbon dioxide. Again black carbon monoliths were obtained after pyrolysis, which exhibited large porosities, good thermal and electrical conductivities [4-6]. Due to the chemistry used, it took no less than two weeks to to get a piece of carbon. In the catalytic literature one cannot find numerous papers describing the use of such carbon aerogels as supports, to the best of our knowledge only two articles relate catalytic results involving carbon aerogels (R-F) impregnated with Pt or Pd for the SCR of NO by NH~ on one hand [7], and another one dealing with a carbon aerogel support for Pt, originating from polyacrylonitrile and tested in the oxygen reduction reaction on the second hand [8]. In this paper, we describe a new method of synthesis of carbon aerogels derived of that of Pekala et al. [2-6], i.e a R-F series polymerised in acetone instead of water, and catalysed by an acid, perchloric acid, instead of a base like sodium carbonate.

EXPERIMENTAL Preparation of the Pt/C carbogels The following summarizes the multistep method used in this piece of work: resorcinol-formaldehyde sol gel in acetone catalysed by perchloric acid at 45~ -~ curing (aging) for 3 days at 45~ drying with supercritical carbon dioxide at 37~ -~ pyrolysis under dinitroge at 1050~ -* impregnation by chloroplatinic acid in acetone at room temperature -* supercritical drying at 37~ again -* calcination with dinitrogen at 450~ and reduction by dihydrogen at 450~ Synthesis of the R-F carbogels First, a solution of resorcinol in acetone (0.294 mold) is placed in a vessel and perchloric acid at 70 % in water (0.0294 mold) is added under stirring and finally formaldehyde at 36,5 % in water (0.588 mold) is added to the first mixture. At this stage it is important to respect the above order of mixing to avoid precipitation or flocculation phenomena. The vessel is placed at 45~ and the gel time is of the order of 75 min, then the gel is left to cure at the same temperature for 3 days before drying. If we note by C the concentration of the c a t a l y s t , this gel corresponds to a R/C value of 10, while the F/R ratio is 2.This last value corresponds to recommended value by Knop and Pilato [9]. The cured gel is placed in an autoclave already at the chosen supercritical temperature of 37~ ie about 5~ above the critical one, with an extra amount of acetone (500 ml), flushed with carbon dioxide, while the pressure attains 80 bar. Static and dynamic period of washing are run under the supercritical conditions for 6-7 hours and thereafter the R-F aerogel is recovered. It is a pinky and poorly

169 translucent solid. Figure 1 shows a schematic of the drying procedure used in this work.

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Figure 1. Supercritical drying mode flow charts "a) with alcohol (high temperature supercritical drying), b) with liquid COs (low temperature supercritical drying). Preparation of the carbon monolith. The copolymer carbogel is placed in an oven under a flow of nitrogen (3.6 1/hr) and gently raised to a temperature of 1050~ where it stays for 3 hours. Once cooled down to room temperature, the material is still under the form of a solid and its colour is black. Preparation of the Pt on carbon aerogel catalyst The carbon aerogel is placed in a vessel with a solution of hexachloroplatinic acid in acetone,in a concentration adjusted to give a final Pt weight content of 0.50. The impregnation operated under stirring lasted 36 hrs, then the solid is replaced in the autoclave to be dried in the same conditions than the copolymer.

170

RESULTS Textural Properties For the copolymer carbogel, the BET surface area, measured with N2 at 77K was found equal to 390 m2/g including a microporous surface,calculated with the ,,t,, method of 90 m2//g. After pyrolysis under dinitrogen, the BET surface area quite doubled, it was now of 745 m2/g, and it developed a microporous surface of about the half i.e 371 m~/g. Thermoporometry measurements made with water, clearly showed t h a t both types of carbogels, the organic and the carbon ones, were not mesoporous at all. They were micro and macro porous only. Figures 2 and 3 show the pore size distributions and the cumulated pore volumes for the non pyrolysed copolymer and the carbon respectively.It is clear that, as demonstrated in figure 3, the pyrolysis treatment resulted in a shift of the macropore volumes towards larger ones with respect to the organic aerogel, without any change of the macropore radii ,centered around 50 nm. Nevertheless, both samples essentially keep the same micro and macro porous textures. The pyrolysis treatment developed a doublefold increase of the macropore volume (V. . . . = 0.121 against 0.2 cma/g respectively). This conclusion can be extended to the contributions of the micropores to the surface areas where a fourfold increase of the microporous surface is recorded (90 against 371 m2/g respectively ).

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150

171 Due to the interaction of the support with dioxygen, it was not possible to determine the dispersion of the metal with the well known H2-O2 titration method. An attempt to chemisorb dihydrogen was performed and it gave a dispersion of 23 %, figure 4, in very good agreement with the chemisorption of CO at low pco (not shown here), taking into account the weight percentage in platinum given by the chemical analysis i.e 0.44. Its BET surface area was of 531 m~/g including a microporous surface of 230 m2/g as shown by its corresponding t-plot in figure 5.The complete N2 isotherm clearly indicates that the solid is essentially micro-and macro porous (figure 6). The electrical resistance measurements of the copolymer carbogel show that it is an electrical insulator while those of the carbon and Pt-C carbogels give evdence that they were good electricity conductors with electrical conductivities equal to 3.10 -2 and 9.10 .8 S/cm for the carbon and Pt-C carbogels respectively.

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The new conditions used in this work to synthesize the organic copolymer RF give materials which are different from those obtained by Pekala et al [2-6] because of one major change in the nature of the catalyst, acidic here, which lead to a reaction mechanism beginning by a protonation of a formaldehyde moleclule giving a hydroxymethylene carbonium ion which further adds on the resorcinol molecule.The global reaction is an electrophilic substitution between the two reactants [9] .This R-F copolymer exhibits a particular porous texture showing the presence of only micro-and macro-pores which is retained by the pyrolysed carbon material. The solvent preferred in this synthesis, acetone instead of water, avoid s the tedious and time consuming solvent exchange step necessary in the case of the former work [2-6]. Moreover, the sol to gel and cure steps are performed at 45~ instead of 85~ and lasted 3 to 4 days only, the drying step needed only one day more in order to obtain the carbon carbogel. The impregnation of Pt on a carbon aerogel led to a rather poor metal dispersion, the catalyst did not differ in its electrical propery from its pure carbon aerogel support.

174 Contrary to Pekala's carbon aerogels [10-12] which are quite non macroporous, those depicted in this paper contain a substantial amount of macropores. REFERENCES

1. 2. 3. 4. 5.

G.M.Pajonk, Rev. Chimie Appliqu~e., 24 (1989) 13. R.W. Pekala, J. Mater. Sci., 24 (1989) 3221. R.W. Pekala, Rev. Chimie Appliqu~e, 24 (1989) 33. R.W. Pekala, C.T. Alviso and J.D. LeMay, J. Non Cryst, Sol., 125 (1990) 67. R.W. Pekala, C.T. Alviso, F.M. Kong and S.S. Hulsey, J. Non Cryst. Sol., 145 (1992) 90. 6. R.W. Pekala, C.T. Alviso, J.K. Nielsen, T.D. Tran, G.A.M Reynolds and M.S Dresselhaus, Mat. Res. Soc. Symp. Proc. No. 393 (1995) 413. 7. J.L. Zhao, R.J. Willey and R.W. Pekala, Mater. Res. Full Symp. (1990) Boston. 8. S. Ye, A.K. Vijh and Le.H. Dao, J. Non Cryst. Sol., submitted. 9. A. Knop and L. A. Pilato, Phenolic Resins, Springer Verlag. (1985). 10. H. Tamon, H. Ishizaka, M. Mikami and lY[ Okazaki, Carbon., 35 (1997) 791. 11. Y. Hanzawa, IL Kaneko, R.W. Pekala and M.S. Dresselhaus, Langmuir, 12 (1996) 6167. 12. R.W. Pekala, S.T. Mayer J.L. Kaschmitter and F.M. Kong, Sol-Gel Processing and Applications, Y.A Attia (ed), Plenum Press (1994) 369.