WO1984002429A1

WO1984002429A1 - Chemo-electric cell with at least one gas electrode

Info

Publication number: WO1984002429A1
Application number: PCT/SE1983/000434
Authority: WO
Inventors: Olle Lindstroem
Original assignee: Lindstroem Ab Olle
Priority date: 1982-12-08
Filing date: 1983-12-07
Publication date: 1984-06-21
Also published as: JPS60500190A; SE8206994D0; SE8206994L; EP0157777A1

Abstract

With known gas electrodes for fuel cells, metal/air cells etc. the gaseous reactant is transported to the reaction sites in the electrode by diffusion. These electrodes are named gas diffusion electrodes. The invention is a further development of gas electrodes with integrated gas and electrolyte chambers whereby the diffusion process is replaced by a convection process. The new gas convection electrode is furnished with means for forced transport of gas from an upstream chamber (11) through the active part of the electrode (9) to a downstream chamber (12). One or both sides of the electrode is at the same time in contact with the electrolyte in bipolar or monopolar embodiments. A selective gas permeable membrane (36) on the upstream side of the electrodes is used in one embodiment in order to separate non-desirable components present in the reaction gas.

Description

Chemo-electric cell with at least one gas electrode.

Gas diffusion electrodes are used in fuel cells, metal/ air batteries, chloral ali cells with air electrodes, etc. Usually one side of the gas diffusion electrodes is in contact with electrolyte while the other side is in contact with the reaction gas, e.g. air. The gas diffu¬ sion electrode is thus facing a gas chamber, e.g. an air chamber, on one side and an electrolyte chamber on its other side. The Swedish patent 407721 describes, however, a chemo-electric cell where the gas- and electrolyte cham¬ bers have been integrated so that one or both sides of the gas diffusion electrode at the same time is in contact with the electrolyte and the reaction gas. The present invention is related to chemo-electric cells with electrodes of this kind where at least one side is in simultaneous contact with the reaction gas and the electrolyte.

Chemo-electric cells with gas diffusion electrodes accord¬ ing to the Swedish patent 407721 show great constructive simplicity. Weight and volume are reduced considerably by the integration of the gas- and electrolyte chambers. This reduction causes a somewhat higher resistance which, however, is compensated for by the reduction in weight and volume in relation to the area of the current flow.

The present invention is a surprising further development of the arrangement reported in the Swedish patent which in the first hand allows a considerably higher power density counted on the electrode area. The invention also improve the qualities of these electrodes in other respects.

Already the term gas diffusion electrode shows that the mass transfer by gas diffusion is an important phenomenon in these electrodes. Optimized gas diffusion electrodes, e.g. according to the Swedish patents 407721 or 360951 are so designed that the transport of the reactant to the reaction sitesin the inner parts of the gas electrode is rate-determining.

Design values for the amount of catalyst and inner surface area are taken somewhat in excess so that the mass transfer becomes the rate-determining factor. This design is most frequently based on empirical data though theoretical models can be of some help. The electrodes are made thin with a large specific inner surface.

Many other requirements have to be met at the same time e.g. mechanical strength, corrosion resistance, conduc¬ tivity, etc. A factor in common to the known gas diffusion electrodes is that the transport of the reactant to the reaction sites inside the electrode takes place through self-diffusion (to which is added internal convection because of the gas consumption through the reaction) .

Consequently the characteristic feature for the invention is that it comprises a chemo-electric cell with at least one gas electrode for electrochemical conversion of a gaseous reactant in primary and secondary iron/air and zink/air batteries, fuel cells for hydrogen/air and ethanol/air, electrochemical oxygen generators, cells for electrolysis of solutions of alkali metal halogenides etc. characterized in that gas spaces (11, 12) are disposed on each side of the gas electrode (6) whereby one gas space, the upstream space (T1) , is connected to means (13, 14) for supply of the reaction gas whereas the other gas space, the downstream space (12) is connected to means (15, 16) for discharge of the spent reaction gas which has been transported through the gas electrode.

Thanks to the invention gas diffusion is no longer the rate determining step which gives a considerable increase

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_____!_ ?_- of performance up to the limit which is governed by other properties e.g. electrical conductivity, feasible catalyst concentration, resistance in the electrolyte, etc. The invention will now be described by means of the drawing.

Figure 1 shows completely schematically the principle, exemplified by an iron/air battery with a gas convection electrode for air in the monopolar embodiment. Figure 2 explains the design of the air electrode in Figure 1. Figure 3 shows a bipolar electrode embodiment. Figure 4 shows a bipolar electrode with a corrugated shape. Fi¬ gure 5 shows a gas convection electrode covered with a selective gas permeable coating. Figures 6-10 show in the same principal way as Figure 2 how the invention can be used with different known chemoelectric cells with gas electrodes. Figure 11 shows voltage current curves.

The purpose with Figure 1 is only to demonstrate the prin¬ ciple. The measures in the figure as well as in the other figures are not correct. Furthermore the figures show only the most important components. The porous iron electrodes (1) , which could be manufactured according to the Swedish patent 360952, are mounted in plastic frames (2) , so as to provide intermediate electrolyte chambers (3) which contain a so-called third electrode (4) supported by a plastic grid (5) . The new gas con¬ vection electrode (6) has several things in common with prior art gas diffusion electrodes according to the Swedish patent 407721 with integrated air and electrolyte chambers. The parts which carry electrolyte comprise ribs (7) of a porous material which has been sintered onto the air electrode (6) . This feature is more clear on Figure 2, which shows a part of the air electrode seen from above. Figure 2 demonstrates the design of the air electrode (6) in more detail. A nickel wire mesh (8) , serving as a support and electrical conductor, carries a layer (9) of electrocatalytically active material, e.g. Raney silver bonded with polytetrafluorethylene. The wire mesh is not coated in the area of contact (10) between the sepa¬ rator rib (7) and the wire mesh (8) so as to provide a good bond between the separator rib and.the electrode and a good electrolyte contact.

Figure 1 and Figure 2 show two gas chambers on each side of the electrode, i.e. the gas chambers (11) and (12) respectively. A continuous electrolyte contact is deve¬ loped between the electrolyte chamber (3),. the porous iron electrode (1), the separator rib (7) and the naked part of the air electrode (10) and the active part (9) of the air electrode _L which contains a thin electrolyte film whereas it is at the same time permeable for gas.

The characteristic feature of the invention is that gas is brought to flow from the gas chamber (11) through the exposed part of the electrode (9) to the gas chamber (12)

Gas, in this case air, is fed by means of channel system, which is only indicated in the figure by means of the bi-channel (13), to the gas chamber (11). This chamber is in the following called the upstream chamber, since it is located upstream in the gas flow. The bi-channel

(13) is connected to an outer connection (14) by means of a main channel, which is not shown. The connection

(14) is also only indicated in Figure 1.

The gas is leaving the second gas chamber (12) , which in the following is called the down-stream chamber by means of the bi-channel (15), which is in connection with an

OMPI outer connection (16), only indicated in the Figure.

Gas is thus delivered to the system by means of the connection (14) and is then distributed to the upstream chambers (11) by means of the bi-channels (13). The gas is then flowing through the electrode to the downstream chambers (12) and leaves by the bi-channel(15) and the connection (16).

Figure 1 shows also a bi-channel (17), which is disposed in the lower part of the downstream chamber. This bi- channel is connected to the outer connection (18) with a valve (19). This conduit can be used among other things for draining of the downstream chamber. The upstream- chamber (11) is also furnished with an upper bi-channel (20) which is connected to an outer conduit (21) con- taining the valve (22). This conduit can be used for venting the upstream chamber, e.g. in ,a special embodi¬ ment where a gas selective coating is disposed on the surface of the electrode facing the upstream chamber.

Electrolyte is carried to the electrolyte chamber (3) by the bi-channel (23) , which is connected to an outer connection (24)_; part of the not shown external electrolyte system. This external electrolyte system contains a surge tank, heat exchangers, pumps, concentration indicators etc. This system is using circulating electrolyte as described in the U.S. patent 3,801,376 where the electro¬ lyte is leaving the upper part of the electrolyte chamber by means of an overflow to a surrounding container.

The current collectors (25) to the iron anodes are connec¬ ted to a main collector (26) . The air cathodes are in the same way furnished with current collectors (27) connected to a main collector (28) . The third electrodes are also connected with the current collectors (29) joined to the main collector (30) . It is evident from this description that it should be no difficulty for the artisan using the spirit of this invention to convert an iron/air battery according to the earlier status of this technology, e.g. represented by the Swedish patent 407721, from the mode of gas diffusion operation to the mode of gas convection opera¬ tion. This could be done by means of channels and gas separators to form an upstream side and a downstream side of the gas electrode for convective transport of gas through the electrode.

It is also possible - without too complicated measures - to use the invention for bipolar electrodes, which is shown in Figure 3. This figure, related to the de¬ tailed drawing in Figure 2,shows the separating wall or collector (31) of the bipolar electrode, which sepa¬ rating wall is electrically conducting but impermeable for electrolyte. One side of this separating wall is in contact with in this case the iron electrode (1) , whereas the other side is in contact with the active layer (5) of the gas electrode by means of contact ele¬ ments (32) which connect electrically the air electrode with the separating wall. Gas is flowing from the upstream chambers (11) to the downstream chambers (12).

Figure 4 shows an embodiment, where the air electrode has an undulated shape. The nodes on one side are in contact with the separating wall in the points (33) whereas the nodes on the other side are in contact with the separator (34) in the points (35) . The gas is flowing from the upstream chambers (11) to the downstream chambers (12) .

The new gas convection electrode lends itself also to an unexpected process improvement which has a great im¬ portance. One problem with iron/air batteries with

O alkaline electrolyte is that the electrolyte is picking up carbon dioxide from the air. The electrolyte has to be exchanged after about fifty cycles or so in case the system is not equipped with a special scrubber. Figure 5 shows how this problem can be eliminated or at least reduced considerably in importance. The surface of the electrode which is facing the upstream chamber is here coated with a thin film of a selectively permeable po¬ lymer (36) . The other surfaces in the upstream chamber are sealed with a tight polymer film (37) . The selec¬ tively permeable film or coating permits transport of oxygen molecules to the active part of the air electrode and from there to the downstream chamber (12) . Most of the carbon dioxide stays in the upstream chamber (11) and is carried away from there by means of the bi-channel (20) connected to the conduit (21) with the valve (22). The gas permeable film could be made of silicone rubber. It could also consist of poly-carbonate, poly-urethane, poly-acetate, various vinyl polymers, cellulose-acetate, crosslinked poly-glutamine etc. The thickness of the film is frequently below 1 μm. It is advisable . to use a backing of e.g. porous polytetrafluorethylene or poly-sulphon (38) so as to support the film and prevent formation of small holes in the film.

The pressure differences which have to be maintained between the upstream side and the downstream side are in this case fairly small and the transport capacity through the film corresponds to the consumption at prac-

2 tical current densities around 100 mA/cm . Thanks to the selective transport of oxygen through the electrode the oxygen concentration is also increased in the active part of the electrode, which improves the electrode performance further.

This technique can also be used with the same advantages with hydrogen containing non-desirable components like nitrogen, smaller quantities of oxygen, carbon dioxide, by means of suitable polymers. A suitable polymer has to be chosen from case to case by means of laboratory tests.

It is not difficult for the artisan to select a suit¬ able film material and film thickness relying on the know how and state of art with gas permeable polymer membranes now under development so as to accomplish separation of the different gas mixtures within the cell itself. Thus , the new gas convection electrode has paved the way for a surprising spin-off development of great importance for the technology of fuel cells and metal air batteries.

Figures 6-12 show in the same principal way as Figure 2 how the invention can be used with different chemo- electric cells with gas electrodes. Figure 6 thus shows the principal design of a bipolar electrode for a cell with phosphoric acid as electrolyte. This design con¬ stitutes an application of the invention on. the so- called ABA electrode developed by Engelhard Industries. Figure 6 shows only the hydrogen side of the bipolar electrode. The air side is designed in the same way but the air flow is carried transversely to the flow on the hydrogen side. The impermeable separator wall of the bipolar electrode (31) is here a densified graphite paper. The separating wall is disposed in contact with an element of a carbonized felt structure (39) which is furnished with fine channels (40) so as to facilitate the in-transport of gas (40) . This element (39) corre- sponds to the so-called A-element of the ABA electrode whereas the separating wall (31) corresponds to the B element. With my invention the element (39) is serving as an upstream chamber (11). The felt structure (39) is nermeable for gas and contains at the same time a reserve of electrolyte.

OV-T -

The gas convection electrode (6) comprises in this case a porous carbon paper, which is catalyzed by means of a noble metal electro-catalyst. The downstream chamber (12) contains a felt structure (40) furnished with chan- nels (41). The felt structure (40) is a non-conductor and is made of e.g. fibres of PTFE which can withstand the chemical environment in question. The other side of the element (40) is in direct contact with the electrolyte element (41), which comprises a porous layer of silicon carbide bonded by PTFE which is completely filled up with phosphoric acid. The downstream chamber (12) is thus situated between the gas convection electrode (6) and the electrolyte element (41). The fuel cell is of course furnished with bi-channels which are disposed so that hydrogen will flow from the upstream chamber (^'11) through the gas convection electrode (6) to the downstream cham¬ ber (12) . A similar arrangement is disposed on the air- side of the bipolar electrode. ^'

Figure 7 shows one side of a bipolar electrode for a fuel Cell with molten carbonate electrolyte according to the invention. Here the so-called electrolyte tile (42) is furnished with ribs (43) , which are in contact with the gas convection electrode (6) , which in this case is a cathode of porous nickel oxide. The separating wall (31) is in this case an undulated stainless steelplate which is furnished with ribs (44) which are disposed in electronic contact with the gas convection electrode. An upstream chamber (11) is formed between one- side of the gas convection electrode (6) and a downstream cham- ber (12) on the other side. Other bipolar fuel cells with solid electrolyte, e.g. ion conducting oxide elec¬ trolyte for high temperature cells or so-called SPE cells with an electrolyte consisting of an ion exchange membrane like Nafion U are designed in the same way as in Figure 7. Practical measures are described e.g. in US patent 4,175,165. Figure 8 shows a so-called oxygen generator for electro¬ chemical concentration of the oxygen of the air to pure oxygen. The oxygen generator contains an air cathode and an oxygen anode. The oxygen is developed anodic- ally on one side of the separator wall (31) , which is in contact with the ribbed separator (46) . The oxygen gas is leaving through the electrolyte filled channels (47) . The ribbed separator (46) is in its turn in con¬ tact with another ribbed separator (48) , the ribs of which (49) are located in corresponding pockets in the undulated gas convection electrode (6) , which are point welded at the separator wall. The upstream chamber (11) and the downstream chamber (12) are formed on each side of the gas convection electrode (6) . The separator ele- ment (46) and (48) have a micro-porous structure so as to prevent leakage of air in the upstream chambers and in the downstream chambers through the separator into the anolyte spaces (47) .

Figure 9 shows a detail of a chlor alkali cell with air cathodes, which are designed as gas convection electrodes. The chlorine gas is developed at the dimensionally stable anode (50). The anolyte space (51) is separated from the catholyte space (52) by the membrane (53) which could - be e.g. Nafion , The catholyte space (52) contains the upstream chamber (11) and the downstream chamber (12) separated by the air electrode (6) . The upstream cham¬ ber (11) contains elements (49) of the same kind as shown in Figure 6. The air electrode (6) is in electronic con¬ tact with the separator wall (31) by means of contact elements (32) .

It has to be pointed out that it is not necessary to use so-called third electrodes shown in Figure 1 for charging, when gas convection electrodes are used in metal/air batteries. The gas convection electrode can be coated with a material with a low oxygen over-voltage, e.g. nickel, on the upstream side for oxygen development during charge. The gas convection electrode can also be designed as a two layer electrode (compare Swedish patent 360952) , whereby the layer for oxygen develop¬ ment is facing the upstream side. It may also be possible to furnish the downstream side with a similar layer (three layer electrode) . The layers for oxygen de¬ velopment have to be porous so that gas can penetrate from the upstream side to the downstream side. We are thus in this case not concerned with barrier layers of the type used in conventional two-layer gas diffusion electrodes. The outer layer for oxygen development should have larger pores than the layer for oxygen re- duction.

The gas convection electrode according to the invention could also be used with other rechargeable metal/air batteries than iron/air, e.g. zinc/air batteries. The absence of free electrolyte in the space between the electrodes reduces the tendency to dendrite formation to¬ wards the air electrode during charge. Zinc electrodes stabilized with different polymers therefore show a con¬ siderable increase of cycle life, when they are used together with gas convection electrodes according to the invention.

If a third electrode is used in a zinc/air battery den¬ drite formation against the third electrode could be reduced considerably, if the third electrode is by mechanical means brought to move up and down slowly duri charge, Figure 10. It may also in this case be possible to use a zinc electrode of the solution type which during charge is precipitated on a perforated thin iron plate (55) the backside of which is in contact with the separa¬ tor ribs of. the gas convection electrode. The separating

OM?ϊ elements in the electrolyte space could in this case consist of plates of highly porous sintered polymer

(56) with abrasive elements which are bonded to the third electrode. In this way the zinc electrode will be brushed during charge by means of mechanical means

(57) to produce a smooth and compact zinc coating. The gas development from the third electrode is facilitated by means of channels (58) disposed in the separator elements (56) adjacent to the third electrode.

In this example the upstream chambers have for the sake of simplicity been disposed on one side of the electrode in its entirety and the downstream chambers on the other side of the electrode in its entirety. It is, however, also possible to dispose a sequence of downstream and upstream chambers on each side of the electrode which will produce a gas movement through the electrode back and forth several times _^from the inlet to the outlet.

The above applications are well described in the litera¬ ture in this area and it should be no difficulty for the artisan to modify these known designs so as to furnish them with gas convection electrodes according to the in¬ vention.

The technical effect of the invention is easy to demon¬ strate. It is apparently possible to run the battery according to Figure 1 in the conventional way by adding

, air to both the upstream and the downstream sides and to discharge air at the same rate from the upstream and the downstream chambers. In this case there will be no con- vective gas transport through the electrode, only diffu- sion from both the_. upstream and downstream sides. Figure 13 shows a voltage current curve for such a battery operating in the mode of gas diffusion. The air electrodes were designed according to Figure 2. The air flows were adjusted so that about 50% of the available oxygen was reacting at the electrode. In a comparative experiment the valve (22) was closed almost completely so that about 95% of the incoming air had to pass through the air electrode whereas only 5% was vented through the channel (20) and •its connection (21). This change im¬ proved cell performance considerably as is evident from Figure 11.

In an additional comparative experiment a film of sili- con rubber which was 0,3 ym thick was applied to the electrode according to Figure 5. In this case the air flow was increased to about 6 times the stochiometric flow whereby 1/5 of the gas flow was discharged via the downstream side. The electrode performance was increased further, curve c in Figure 13. The carbon dioxide up¬ take of the electrolyte was reduced to less than half compared to the other cases.

There is no difficulty to optimize the new gas electrode for all of the applications, which have been discussed above. The convection electrode will in general be somewhat thicker with coarser pores and a smaller speci¬ fic surface compared to the diffusion electrode whereas the total surface, however, will be somewhat larger. The electrolyte content of the electrode is also somewhat higher. These features provide not only for better per¬ formance but also for better life properties and endur¬ ance towards abnormal operation situations - compared to corresponding conventional gas diffusion electrodes.

Claims

Patent claims

1. Chemo-electric cell with at least one gas electrode for electrochemical conversion of a gaseous reactant in primary and secondary iron/air and zinc/air batte¬ ries, fuel cells for hydrogen/air and methanol/air, electrochemical oxygen generators, cells for electro¬ lysis of solutions of alkali metal halogenides etc. c h a r a c t e r i z e d in that gas spaces (11 , 12) are disposed on each side of the gas electrode (6) whereby one gas space, the upstream space (11), is connected to means (13, 14) for supply of the re¬ action gas whereas the other gas space, the down¬ stream space (12) is connected to means (15, 16) for discharge of the spent reaction gas after its passage^through the gas electrode.

2. Chemo-electric cell according to claim T c h a r a c- t e r i z e d in that the gas convection electrode

(6) is a monopolar electrode.

3. Chemo-electric cell according to claim 1 c h a r¬ a c t e r i z e d in that the gas convection elec¬ trode (6) is a bipolar electrode.

4. Chemo-electric cell according to any of the claims 1,2 or 3 c h a r a c t e r i z e d in that the gas convection electrode (6) is in contact with an electrolyte in the liquid state.

5. Chemo-electric cell according to any of the claims 1,2 or 3 c h a r a c t e r i z e d in that the gas convection electrode (6) is in contact with a solid electrolyte. 6. Chemo-electric cell according to any of the preced¬ ing claims c h a r a c t e r i z e d in that the surface of the gas convection electrode facing the upstream space (11) is coated with a selective permeable film (36) of a polymer.