New method of electrode fabrication for phosphoric acid fuel cell

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Pergamon PII]

Int[ J[ Hydro`en Ener`y\ Vol[ 12\ No[ 00\ pp[ 0938Ð0942\ 0887 Þ 0887 International Association for Hydrogen Energy Elsevier Science Ltd All rights reserved[ Printed in Great Britain S9259Ð2088"87#99911Ð5 9259Ð2088:87 ,08[99¦9[99

NEW METHOD OF ELECTRODE FABRICATION FOR PHOSPHORIC ACID FUEL CELL RAK!HYUN SONG\ DONG!RYUL SHIN and CHANG!SOO KIM Korea Institute of Energy Research\ P[O[ Box 092\ Yousoung\ Taejon 294!232\ Korea

Abstract*The new combination method of electrode manufacturing for phosphoric acid fuel cell has been studied\ which takes advantage of the conventional coating and rolling methods[ In the combination method\ an electrocatalyst slurry is coated upon an electrode support\ dried at 114>C\ subjected to a rolling process\ and then to a sintering process at 249>C in an inert atmosphere[ The optimum coating and rolling processing conditions of the combination method were obtained when the thickness of the electrocatalyst layer was 59 mm\ and the rolling degree was 39 mm[ The single cell with an electrode of the combination method showed a good performance as compared with the conventional methods\ and their performance characteristics were discussed as related to the structure state of the electrocatalyst layer[ To decide the practicability of the combination method\ the performance characteristics of the single cells with a large electrode area of 1999 cm1 were examined[ Þ 0887 International Association for Hydrogen Energy

INTRODUCTION Among various types of fuel cells\ the phosphoric acid fuel cell "PAFC# represents the most advanced tech! nology in the research and development for practical use[ The performance of a PAFC is dependent on the electrochemical properties of the cell elements[ Of those\ electrodes are\ especially\ known to determine most of the performance of the fuel cell ð0\ 1Ł\ having a great in~uence upon the cell performance of producing elec! tricity[ A PAFC electrode consists of an electrode support and an electrocatalyst layer[ The electrode support is formed from porous carbon paper[ The electrocatalyst layer consists of platinum!dispersed carbon powders that are joined with polytetra~uoroethylene "PTFE# particles and is porous to gases\ as well[ While the electrode sup! port plays the role of supplying reactant gases such as hydrogen and oxygen to the electrocatalyst layer\ the electrochemical reactions occur in the electrocatalyst layer[ There are two well!known methods of manufacturing PAFC electrodes^ the coating method ð2\ 3Ł and the rol! ling method ð4Ł[ In the conventional coating method\ an

 To whom correspondence should be addressed[ Tel] 99 71 31 759 2467^ Fax] 99 71 31 759 2628^ E!mail] rhsongÝsun229[ kier[re[kr

electrode is obtained by coating a slurry of the elec! trocatalyst on the electrode support[ The process is com! pleted by drying and sintering the electrode[ This conventional coating method of fabricating electrodes has a major disadvantage in that an abundance of cracks occurs in the catalyst layer during the drying and sinter! ing[ An appropriate number of cracks in the catalyst layer may improve the performance of a fuel cell\ but a large number of cracks in the catalyst layer cause the phos! phoric acid electrolyte to ~ood into the catalyst\ greatly deteriorating the performance of the fuel cell[ It is pref! erable to prevent the occurrence of cracks in the catalyst layer as much as possible\ but it is extremely di.cult to avoid an abundance of cracks via the coating method[ Furthermore\ another signi_cant problem in the con! ventional coating method is that due to weak attractive forces between the carbon particles within the elec! trocatalyst layer\ the adhesion of the _lm to the support is very weak\ so the electrocatalyst layer is readily detached from the support[ In the conventional rolling method of manufacturing an electrode\ a modi_cation is _rst made to produce a gum!like catalyst slurry\ which is formed into a sheet! type catalyst layer by using a roller[ This sheet!type cata! lyst layer is attached to an electrode support by using a press[ As mentioned\ the gum!like catalyst layer is made in a sheet!like fashion\ but it is di.cult to make a wide catalyst layer that is uniform in thickness[ Also\ a press with a wide area is required to attach the wide catalyst

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layer to the support[ It is important to provide uniform stress during pressing[ If the stress is not uniform\ the catalyst layer is already in part\ detached[ In practice\ the manufacture of a press capable of applying a uniform stress over a wide area is expensive and technically di.! cult[ The purpose of this study is to overcome the problems encountered with the conventional coating and rolling methods and to provide a new method for producing electrodes\ the combination method\ which utilizes advantages of both the coating and rolling processes[ The e}ects of processing conditions in the combination method were examined\ and the performance charac! teristics of a single cell\ with an electrode of small area were evaluated and compared with those of conventional methods[ Also\ a single cell with a large electrode area\ made by the combination method\ was tested with a view to practical application[ EXPERIMENTAL Figure 0 is a schematic of the combination method for manufacturing an electrode of a fuel cell[ An elec! trocatalyst slurry and an electrode support are prepared separately[ A sheet of carbon paper with a dimension of

Fig[ 0[ Electrode manufacturing process for PAFC by com! bination method[

499 mm×599 mm was waterproofed by immersing it in an aqueous waterproo_ng solution for 29 s and dried in air for a day[ Then\ the carbon paper was subjected to sintering at 269>C for 19 min\ to give a waterproofed electrode support[ The waterproo_ng of the carbon paper prevents phosphoric acid electrolyte or water from in_l! trating the carbon paper and closing its pores[ As seen in Fig[ 0\ the electrocatalyst slurry is obtained through two mixing steps[ In the _rst step\ platinum! dispersed carbon powder with a platinum amount of 19 wt) was mechanically stirred for 29 min in a solvent to make a homogeneous mixture[ In a second step\ a PTFE emulsion was added to this mixture\ stirred by ultra! sonication[ Subsequently\ another mechanical stirring was executed for 29 min after adding a bridge!builder and a peptization agent\ to produce an electrocatalyst slurry[ In the present study\ the PTFE is added at an amount of 34 wt) of the resulting electrocatalyst layer[ Using a coating apparatus\ the electrocatalyst slurry is coated upon the electrode support[ An electrode support is _xed on a ~at die\ on which a suitable amount of electrocatalyst slurry is deposited by running a coating blade parallel to the ~at die[ The thickness of the elec! trocatalyst layer can be controlled by adjusting the height of the coating blade[ The electrode is dried for a day in air and then for 29 min in an inert atmosphere at 114>C[ This drying temperature completely removes the solvent while the inert atmosphere prevents the platinum in the catalyst layer from being oxidized[ The rolling process is carried out by passing the dried electrode between the two rolls of a rolling apparatus\ with the aim of modifying the PTFE in the electrocatalyst layer to prevent cracks in the catalyst layer as well as increasing the adhesion between the electrocatalyst layer and the electrode support[ It is preferable to maintain the proper rolling degree\ that is\ the di}erence in the thickness of the electrode before and after the rolling[ The e}ect of the rolling degree on the electrode structure was examined[ In order to prevent the electrode from sticking to the surfaces of the rolls\ the opposite sides of the electrode are preferably covered with thin protective _lms before the rolling process[ Nevertheless\ under some rolling conditions\ the electrocatalyst layer was detached from the electrode support\ and this is described in detail later[ Finally\ the rolled electrode is subjected to sintering for 29 min at 249>C in an inert atmosphere\ to produce a PAFC electrode[ The single cells of PAFC were fabricated with the electrodes and tested for the capacity of the electrode[ The large electrode was cut in the proper areas for the cell test[ Single cells with e}ective electrode areas of 09 and 1999 cm1 were constructed in such a way that an electrolyte layer was placed between two electrodes in which electrocatalyst layers faced each other[ The elec! trolyte layer was prepared by impregnating a porous SiC sheet 9[0 mm thick with high!concentrated 094 wt) phosphoric acid[ A single cell was tested at 029 and 079>C while hydrogen gas was made to ~ow into the anode side of the cell\ and oxygen gas into the cathode side[

ELECTRODE FABRICATION FOR PHOSPHORIC ACID FUEL CELL

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RESULTS AND DISCUSSION Effects of processin` conditions on the electrode structure in the combination method Figure 1 shows the surface morphology of the electrode with a thickness of 59 mm\ manufactured by the com! bination method[ Cracks were observed in the elec! trocatalyst layer[ Crack creation in the combination method occurs mainly during the drying of the coated electrocatalyst layer due to the di}erent shrinkage rates of electrocatalyst and electrode support\ which depends on the thickness of the electrocatalyst layer[ Table 0 shows the e}ect of thickness of the electrocatalyst layer on the amount of cracks[ The amount of cracks increased with thickness[ The presence of cracks in the elec! trocatalyst layer is of importance because the cracks may induce electrolyte ~ooding and thus decrease the elec! trode performance ð5Ł[ Figure 2 shows the e}ect of thickness of the elec! trocatalyst layer on the performance of a single cell at 079>C[ A single cell with an electrocatalyst layer 59 mm thick displayed a good performance[ Generally\ the cell performance increases with increasing thickness of elec! trocatalyst layer due to the increased platinum amount per unit electrode area ð6Ł[ However\ in the case of 89 mm\

Fig[ 1[ Surface morphology of the electrode fabricated by com! bination method[

Table 0[ E}ect of thickness of the electrocatalyst layer on an amount of cracks after sintered at 249>C for 29 min in the inert gas atmosphere Thickness of the electrocatalyst layer "mm# Amount of the crack in the electrocatalyst layer

29

59

89

Small

Medium

Large

Fig[ 2[ E}ect of thickness of electrocatalyst layer on the per! formance of single cell with an electrode area of 09 cm1 manu! factured by combination method[

even though the thickness of the electrocatalyst layer is increased\ thereby also increasing the platinum loading amount\ the cell performance decreases[ This is believed to be related to the presence of a large amount of cracks as shown in Table 0\ which would greatly induce the electrolyte ~ooding through the cracks[ Thus\ we con! clude that the optimum thickness of the electrocatalyst layer in the combination method is about 59 mm[ In addition\ the optimum condition in the rolling pro! cess of the combination method is important because the electrocatalyst layer may be detached from the electrode support during this step[ Table 1 shows the e}ect of rolling condition on the electrode structure[ As the rolling degree decreases\ the amount of the detached elec! trocatalyst layer from the electrode support was reduced[ When the rolling degree was below 39 mm\ the elec! trocatalyst layer was not detached[ Moreover\ in case of large rolling degree\ the surface of electrocatalyst layer was rough due to the detachment of electrocatalyst layer\ but in case of small rolling degree such as 29 mm\ the electrocatalyst layer was partially not rolled[ Therefore\ it is considered that when the space between the two rolls is too big or too small\ the rolling state of the elec!

Table 1[ E}ect of the pressing condition on an amount of the detached electrocatalyst layer from the electrode support and on a surface structure of the electrocatalyst layer in the rolling process of the combination method  Rolling Amount of the detached degree "mm# electrocatalyst layer 59 49 39 29

Large Small Not detached Not detached

Surface of the electrocatalyst layer Rough Partially rough Smooth Not rolled and partially rough

 Rolling degree is de_ned as the di}erence in the thickness of the electrocatalyst layer before and after the rolling[

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trocatalyst layer is bad[ In this work\ the optimum rolling condition was obtained when the rolling degree was 39 mm[ Comparison of sin`le cell performances of the electrodes fabricated by several methods Figure 3 shows the performances of single cells with an electrode area of 09 cm1 manufactured by the con! ventional and combination methods at 079>C[ The single cell by the combination method indicated a good per! formance as 9[54 V\ 159 mA:cm1[ This is related to the di}erence of structure state in the electrocatalyst layer depending on the manufacturing process[ The surface structures of the electrocatalyst layers were examined using optical and scanning electron microscopes[ In case of the conventional rolling method\ the electrocatalyst layer has a smooth surface and no cracks[ Also the Te~on particles\ included in the electrocatalyst as a binder\ for! med a _bre shape[ Thus the density of the electrocatalyst layer in the rolling method is relatively high[ On the other hand\ the electrode made by the con! ventional coating method had a weak adhesion between electrocatalyst layer and electrode support\ and the den! sity of its electrocatalyst layer was low in comparison with the rolled electrode[ Also\ the conventionally coated electrocatalyst layer had many cracks\ which is because the Te~on particles in the electrocatalyst layer did not form the _bre shape[ In case of the electrode manu! factured by combination method\ since the rolling was executed after coating the electrocatalyst layer\ many cracks did not exist in the electrocatalyst layer but cracks exist to some degree[ The density of this electrocatalyst layer was higher than the case of conventional coating method[ If the density of the electrocatalyst layer is low and cracks in the electrocatalyst layer exists in large quan! tities\ electrolyte ~ooding occurs easily\ which reduces the reaction area of the electrode and increases the resist! ance to gas di}usion[ Thus\ the performance of a single cell produced by the coating method is considered to be low[ On the other hand\ if cracks do not exist at all and

Fig[ 4[ E}ect of operation temperature on the performance of single cell with an electrode area of 1999 cm1 manufactured by combination method[

the density of the electrocatalyst layer is high\ as in the case of the rolling method\ resistance to electrolyte ~ood! ing becomes large\ but formation of the reaction area in the electrocatalyst layer is not easy[ Therefore\ it seems reasonable to suppose that\ since the combination method gave the proper quantity of crack and density in the electrocatalyst layer\ the single cell based on the combination method exhibited a good performance[ Performance characteristics of a sin`le cell with a lar`e electrode area of 1999 cm1 manufactured by combination method To examine the possibility of practical use\ a single cell with an electrode area of 1999 cm1 was tested at two temperatures and the results are presented in Fig[ 4[ It produced a performance of 299 A\ 9[5 V at 079>C[ In this result\ it should be noted that the increase of the oper! ation temperature increased the cell performance\ but decreased the open circuit voltage "OCV# greatly[ The OCV depends mainly on temperature and gas cross! over under the conditions of a constant electrode and electrolyte[ Temperature e}ects were calculated by a theoretical thermodynamic equation ð7Ł and the drop in thermodynamic OCV was evaluated to be 03 mV between 029 and 079>C[ Since the decreased amount of OCV in Fig[ 4 is 004 mV\ the contribution from gas cross!over is 090 mV[ To con_rm the existence of gas cross!over\ the hydrogen content in the oxygen electrode side was exam! ined by using GC[ We detected 6) of hydrogen at 079>C[ Thus it is concluded that the decrease in OCV is mainly caused by gas cross!over\ which is attributed to the decreased gas bubble pressure of the phosphoric acid electrolyte due to its low viscosity at high temperature[

CONCLUSIONS Fig[ 3[ Performance characteristics of the single cells with an electrode area of 09 cm1 manufactured by several methods[

A new combination method of the electrode fab! rication for PAFC\ which takes advantage of the con! ventional coating and rolling processes has been

ELECTRODE FABRICATION FOR PHOSPHORIC ACID FUEL CELL

developed for making electrodes with a large area[ When the thickness of the electrocatalyst layer is 59 mm and the rolling degree*the di}erence in the electrode thickness before and after rolling*is 39 mm\ a large electrode was fabricated e.ciently[ The performance characteristics of single cells\ with an electrode area of 09 cm1\ manufactured by the con! ventional and combination methods were examined[ A single cell by the combination method showed a good performance of 9[54 V\ 159 mA:cm1[ This is considered to be caused by the proper structural formation of the electrocatalyst layer such as the number of cracks and its density[ To judge the possibility of practical use\ a single cell\ with a large electrode area of 1999 cm1\ manufactured by combination method was tested[ A single cell at 079>C showed a performance of 299 A\ 9[5 V[ The increased operation temperature decreased the open circuit voltage of the single cell\ which is mainly caused by the gas cross! over e}ect rather than by the thermodynamic factor\ probably as a result of the decreased gas bubble pressure of the electrolyte matrix[

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