WO2020251492A2

WO2020251492A2 - Application of hydrophobic polydimethylsiloxane (pdms) polymer at various amounts in pem fuel cell catalyst layer

Info

Publication number: WO2020251492A2
Application number: PCT/TR2020/050479
Authority: WO
Inventors: Ayşe BAYRAKÇEKEN YURTCAN; Hande UNGAN; Ayşenur ÖZTÜRK
Original assignee: Atatürk Üni̇versi̇tesi̇ Bi̇li̇msel Araştirma Projeleri̇ Bi̇ri̇mi̇
Priority date: 2019-06-13
Filing date: 2020-06-02
Publication date: 2020-12-17
Also published as: WO2020251492A3

Abstract

Invention relates to development of a catalyst layer solution (S) convenient for use in PEM fuel cells and mentioned solution (S) comprises Nafion solution (2) as binder and polydimetilsiloxane (PDMS) polymer (3) as a hydrophobic material. The invention also relates to a membrane electrode assembly (MEA) (9), which is formed by pressing anode (A) and cathode (B) electrodes having a catalyst layer (D) on their surfaces onto both sides of polymer electrolyte membrane (C), in where electrochemical half cell reactions occur, which is appropriate for use in PEM fuel cells and a method for preparation of it.

Description

APPLICATION OF HYDROPHOBIC POLYDIMETHYLSILOXANE (PDMS) POLYMER AT VARIOUS AMOUNTS IN PEM FUEL CELL CATALYST LAYER

Technical Field

Invention relates to a catalyst solution convenient for use in fuel cells.

Invention particularly relates to preparation of a catalyst layer solution comprising hydrophobic polydimethylsiloxane (PDMS) polymer at various amounts convenient for use in PEM fuel cells.

Current State of the Art

Meeting increasing energy demandof the world becomes much more difficult because of speed in growth of population as well as inadequacy of available sources. Also, damage to the ecosystem caused by use of fossil fuels, environmental issues such as air, water, soil pollution, vegetation and animal extinction has resulted in need for clean and sustainable energy sources.

The researches have indicated that hydrogen power is in the first place among the mentioned clean and sustainable energy sources. Fuel cells is the most popular application of hydrogen energy. Today fuel cells has found more application fields as power generation technology which are efficient, economical, silence and compatible with environment. Fuel cells are power sources converting chemical energy of fuel into usable energy in the forms of electric and heat without need for combustion and without use of any intermediate components. Particularly PEM (Proton Exchange Membrane) fuel cells are considerably convenient for portable and mobileapplications due to low working temperatures.

Providing water management in PEM fuel cells is considerably important in terms of cell performance. Because excesswater decreases cell performance. Various polymers are used in catalyst layer of cells in order to prevent it. Drying of polymer electrolyte membrane because of inadequate water amount and occurrence of flooding due to the excess water amount in cell are two cases that adversely affecting performance of PEM fuel cells and explained these two cases are opposite of each other. H⁺ ions transferred from anode through membrane are needed for completion of half cell reaction at cathode and maintaining this ionic conductivity is subject to humidifying of membrane in adequate level. On the other hand, gas transmission pathways should not be blocked because of excess water amount for good diffusion of reactant gases to catalyst layer and proceeding of reaction should not be interrupted. Establishing a good water balance between these two conflicting cases is considerably important in respect to cell performance. Maximum power amount that can be achieved from the system can be increased only by doing effective water management in PEM fuel cells. For that purpose, the top most frequently used polymers are fluorinated polymers such as polytetraflouroethylene (PTFE), fluorinated ethylene propylene (FEP), polyvinyl fluoride (PVDF). High performance values to be obtained from PEM fuel cell systems which will be one of the future energy technologies will increase the use of such systems for energy demand. However, the polymers that have used so far are inadequate in respect to mentioned performance increase and providing water management.

Two main methods are available in the known technique to provide effective water management in PEM fuel cell. The first and most common one is treatment of PEM fuel cell components with hydrophobic materials, and the second one is changing operating conditions of PEM fuel cell. In first method, various fuel cell components (gas diffusion layer, microporous layer, catalyst layer, polymer electrolyte membrane and bipolar layers etc.) are treated by various hydrophobic materials and made hydrophobic and thus it is aimed to facilitate the removal of excess water from the cell. In the currently known technique, the mostly studied hydrophobic polymer is polytetrafluoroethylene (PTFE) polymer, so far. The hydrophobic material in number two after PTFE polymer is fluorinated ethylene propylene (FEP) polymer which is also a kind of fluoropolymers. There are several examples concerning the use of these two hydrophobic polymers, mainly PTFE, in PEM fuel cells.

For instance, patent document numbered US5272017A is about the preparation of membrane and electrode assemblies for use in electrochemical cells having solid polymer electrolyte membranes. In this document, the each electrodes comprise a respective group of finely divided carbon particles, very finely divided catalytic particles supported on internal and external surfaces of these carbon particles and a proton conductive material interwined with these catalytic and carbon particles. Also, the mentioned polymer is polytetrafluoroethylene (PTFE) polymer.

Another study in the known technique is an article by Velayutham G. et. al. entitled with “Effect of PTFE Content in Gas Diffusion Media and Microlayer on the Performance of PEMFC Tested under Ambient Pressure” (D1 ). This article explains that when polymer electrolyte membrane fuel cell (PEMFC) is tested under the low temperature and medium pressure, polytetrafluoroethylene (PTFE) content in cell electrode plays an important role in cell performance. In this study, the cell performances are tested by using 7%, 15%, 23%, 26% and 30% weight percentages of PTFE polymer in PEM fuel cell catalyst layer and related graphic is given in Figure 1 a.

Another document in the known technique is the article by Mazu^'r P. et al. entitled with“Gas diffusion electrodes for high temperature PEM-type fuel cells: role of a polymer binder and method of the catalyst layer deposition” (D2). The subject of this study is optimization of a gas diffusion electrode (GDE) preparation procedure and chemical composition for use in PEM fuel cells at high temperatures. In here, phosphoric acid doped, a derivative of polybenzimidazole membrane was used as polymer electrolyte and effect of use of two different hydrophobic materials (PBI and PTFE) in catalyst layer on cell performance was studied. The result of the study can be seen in Figure 2. Graphic A of Figure 2 shows cell performance of PBI polymer and Graphic B shows cell performance of PTFE polymer in catalyst layer at different percentages by weight. In graphic for PTFE, triangle represents 5.1 % by weight, circle represents 10.1% by weight, down facing triangle represents 12% by weight and lastly cross represents 15.1% by weight.

In general, it is known that the best cell performances are obtained when PTFE and FEP polymers are added to PEM fuel cell component at high ratio such as 20% by weight. However, since FEP polymer is slightly dissoluble in almost all solvents, polymerization is effective only when a surfactant is used. This makes it difficult to use FEP polymer in the structure.

As a result, due to above described disadvantages and inadequacy of existing solutions, to make development in the related technique has been necessary.

Purpose of the Invention

The present invention relates to catalyst layer solution which meets the needs mentioned above, eliminates all disadvantages and provides some additional advantages.

Primary purpose of the invention is to develop a catalyst layer solution providing increase in performance of PEM fuel cell which is a clean and sustainable energy source by means of its polymer content. Another purpose of the invention is to develop a catalyst layer solution meeting energy need at a high efficiency by means of its low polymer content.

A further purpose of the invention is to develop a catalyst layer solution maximising cell performance and keeping membrane humidified by establishing a good and effective water balance by means of its polymer content.

A further purpose of the invention is to develop a catalyst layer solution eliminating blocking gas flow pathways because of excess water amount and providing occurrence of half cell reaction in electrode without interruption by means of its polymer content.

In order to achieve all purposes mentioned above and will be emerged from the detail explanations given below, present invention develops a catalyst layer solution appropriate for use in PEM fuel cells and this solution comprises Nation solution as binder and polydimethylsiloxane (PDMS) polymer as hydrophobic material. The invention also relates to a method for the preparation of a membrane electrode assembly (MEA), which is formed by pressing anode and cathode electrodes having a catalyst layer on their surfaces onto both sides of polymer electrolyte membrane, in where electrochemical half cell reactions occur, which is appropriate for the use in PEM fuel cells.

All characteristic features and advantages of the invention will be understood clearly with figures and detailed descriptions in their references given below; the assessments should be made taking into consideration the figures and explanations.

Brief Description of the Drawings

Figure 1 a is graphic showing performance of cell described under document D1 wherein PTFE polymer is used

Figure 1 b is graph showing performance of cell described under the invention wherein PDMS polymer is used

Figure 2 is a graph showing effect of use of two different hydrophobic materials in catalyst layer at varying percentages on cell performance in document D2

Figure 3 is a image of membrane electrode assembly (MEA) of PEM fuel cell

Figure 4 is a image of measurements of contact angle on catalyst layer surfaces

Figure 5 is a flow chart showing part of invention in fuel cell

Figure 6a is an illustrative drawing of membrane electrode assembly (MEA)

Figure 6b is an illustrative drawing of single fuel cell Figure 6c is an illustrative drawing of fuel cell test station

Description of Invention References

1 Carbon Supported Platinum Catalyst

2 Binder (Nation ionomer)

3 PDMS polymer

4 2- propanol

5 Distilled Water

6 Gas Diffusion Layer

7 Fuel Cell Test Station

8 Bipolar Plate

9 Membrane Electrode Assembly

10 Current Collector Plates

1 1 Compressing Screws

12 Load Device

13Electronic Accessory

14 Flow meter

15 Pressure Gauge

16 Computer Control Panel

17 Gas Tube

18 Safety Button

19 Humidifying Tanks

A Anode

B- Cathode

C Polymer Electrolyte Membrane

D Catalyst Layer

E Gas Inlet

F Gas Outlet

G Gas Channels

S Catalyst Layer Solution

Detailed Description of the Invention

In this detailed description, the preferred embodiments of a catalyst layer solution (S) convenient for use in PEM fuel cells which is being subject of this invention have been disclosed solely for the purpose of better understanding of the subject and described in a manner not causing any restrictive effect. Present invention develops a catalyst layer solution (S) convenient for use in PEM fuel cells and this solution (S) comprises Nation solution (2) as binder and polydimethylsiloxane (PDMS) polymer (3) as hydrophobic material.

PDMS polymer (3) is a silicone type polymer with hydrophobic nature in which methyl (-CH3) groups are circulating around Si-O-Si main chain freely. It is different from other organic materials with its physical and chemical properties such as high and low temperature stability, good dielectric features, low interface energy, high surface activity, inertness, transparency. PDMS polymer (3) has also considerably hydrophobic property and thanks to this feature, it also provides hydrophobic property for the structure in where it is used even in low amounts.

By means of high hydrophobicity, PDMS polymer (3) provides super hydrophobic/hydrophobic features to surfaces which it is applied on and this can also be possible when PDMS polymer (3) is used in low amounts. In a preferred application of the invention, developed catalyst layer solution (S) comprises PDMS polymer (3) in weight percentages of 5%, 10% and 20% according to total solution (S) weight. Contact angle measurements were made on the catalyst layer (D) surfaces prepared in this way and results were shown in Figure 4. As it can be seen from the figure, following water dripping, spherical shape is formed on all surfaces. This case shows that catalyst layer (D) surface gets more hydrophobic when PDMS percentage increases. The increase in hydrophobicity of the surfaces parallel with increasing PDMS percentages was shown with results of contact angle measurements taken on catalyst layer (D) surfaces. It has been seen that higher cell performances were obtained in measurements made with cells containing PDMS polymer on catalyst layers according to fuel cell test results. Water management on PEM fuel cell gets easier when fuel cell components gain hydrophobic feature. Therefore, considerable performance increase was provided when water level in fuel cell was kept at optimum level.

In a preferred application of the invention, the developed catalyst layer solution (S) comprises carbon supported platinum catalyst (1 ). The catalyst (1 ) functioning as catalyst for electrochemical reactions occurring at PEM fuel cells is preferably Tanaka (67% Pt) catalyst commercially purchased, and loading amount of this catalyst (1 ) to anode (A) and cathode (B) electrode at PEM fuel cell is preferably 0,4 mg Pt/cm². The developed catalyst layer solution (S) also comprises Nation solution (2) as binder and this binder (2) makes contribution to electrical conductivity inside catalyst layer (D) and facilitates the adsorption of platinum nano particles to electrode surface. The developed catalyst layer solution (S) also comprises at least a solvent. The solvents are preferably 2-propanol (4) and distilled water (5).

In an alternative application of the invention, there is preferably the 70:30 ratio by weight between carbon supported platinum catalyst (1 ) and binder (2). The ratio between 2-propanol (4) and distilled water (5) that can be selected as solvents, is preferably 2:1 by volume.

In the known technique, it is seen that PTFE polymer frequently preferred as hydrophobic material for fuel cell components, was behind of PDMS polymer (3) in respect to cell performance. For instance, present invention was compared to D1 document relating to the use of PTFE polymer and the details of which have been described, in terms of performance values. The graphics used for comparison were given in Figure 1 -a and Figure 1 b. According to the result, despite the fact that mass percentage is much, the use of PTFE polymer did not enable achievement of cell performances obtained by the use of PDMS polymer (3) in low amounts such as 5% and 10% by weight in present invention. Similar case is also seen in document D2 of the known technique. Polymer mass percentage/performance relationship under D2 document studying the effect of use of two different hydrophobic material, namely, PBI and PTFE in catalyst layer on cell performance, is far under present invention. As it can be seen from Figure 2, performance values obtained in the case of 12% PTFE loading by mass percentage are considerably inadequate. The obtained performance values are even under the performance data obtained for 5% PDMS which is the lowest mass percentage preferred under the invention. All those comparative studies show that use of PDMS polymer (3) as hydrophobic material to facilitate water management in PEM fuel cell catalyst layer is advantageous.

In addition, present invention develops a membrane electrode assembly (MEA) (9), which is formed by compression of anode (A) and cathode (B) electrodes having catalyst layer (D) on their surfaces onto both sides of polymer electrolyte membrane (C), in where electrochemical half cell reactions occur, which is convenient for use in PEM fuel cells; the mentioned catalyst layer (D) was prepared by a catalyst layer solution (S) comprising Nation solution (2) as binder and polydimethylsiloxane (PDMS) polymer (3) as hydrophobic material.

MEA(9) has an embodiment which is formed by compression of anode (A) and cathode (B) electrodes on both sides of polymer electrolyte membrane, having gas channels (G) upon, comprising a gas inlet (E) providing gas entrance into these channels (G) and a gas outlet (F) providing exit of gas from these channels (G). There is catalyst layer (D) on the surface of electrodes in MEA structure. Considered as heart of PEM fuel cell, the membrane electrode assembly (MEA) (9) was illustratively shown in Figure 3. Water management should be well done in order to execute electrochemical half cell reactions properly and achieve high cell performance. For that purpose, in the known technique, hydrophobic material is included in catalyst layer and mostly PTFE polymer is preferred for this. In our present invention, the use of PDMS polymer (3) giving better result than PTFE polymer was developed as an alternative and new hydrophobic material in PEM fuel cell catalyst layer. Thus fuel cells having better water management and resulting in higher performance by the use of lower amount of polymer were developed.

A method for the preparation of a membrane electrode assembly (MEA) (9), which is formed by pressing anode (A) and cathode (B) electrodes having a catalyst layer (D) on their surfaces onto both sides of polymer electrolyte membrane (C), in where electrochemical half cell reactions occur, which is appropriate for use in PEM fuel cells, was also disclosed in present invention and the mentioned method comprises of ;

- Preparation of a catalyst solution (S) containing PDMS polymer (3),

- Forming catalyst layer onto GDL by means of spraying the prepared catalyst solution (S) onto a gas diffusion layer (GDL) (6) which is providing transmission of reactant gases to catalyst layer by diffusion and functioning a physical support for catalyst layer (D),

- Compressing the anode (A) and cathode (B) electrodes, placing a polymer electrolyte membrane (preferably Nafion membrane) (C) between them, by hot pressing device. process steps. Illustrative drawing of MEA (9) was shown in Figure 6a.

In light of above descriptions, an application of the invention is executed as follows: First of all, materials to form solution (S) for PEM fuel cell catalyst layer are provided. Preferably, carbon supported catalyst (1 ) containing platinum at 67% by mass and preferably, commercial Nafion solution (2) containing Nafion at 15% by mass are weighed in needed quantities and put into bottle. Preferred loading amount of carbon supported platinum catalyst (1 ) in each electrode, namely anode (A) and cathode (B), is 0.4 mg Pt/cm². As the active area in fuel cell is 4.41 cm², the required amount of carbon supported platinum catalyst (1 ) is calculated on the basis of two values. Illustrative drawing of single fuel cell comprising bipolar plate (8), Membrane Electrode Assembly (9), compressing screws (10), current collector plates (1 1 ) and load device (12) was shown in Figure 6b. Despite of the presence a ratio between carbon supported platinum catalyst (1 ) and Nafion solution (2) as preferably 70:30 by weight, since PDMS polymer (3) is also added, the mass percentage of 30% Nafion solution (2) is shared between Nafion (2) and PDMS polymer (3) based on loading amount of PDMS polymer (3). Consequently, PDMS polymer (3) of varying mass percentages of 5%, 10% and 20% is added into bottle containing carbon supported platinum catalyst (1 ) and Nation solution (2). Finally, preferably 2-propanol (4) and distilled water (5) are added into bottle as solvents preferably 2:1 ratio by volume to form final catalyst layer solution (S). Prepared catalyst layer solution (S) is mixed for preferably 3-4 minutes by the help of a mixer (preferably homogenizer) and homogenous solution is obtained.

After having homogenous solution, preferably an air compressor spray gun is used to transfer it onto a gas diffusion layer (6) (GDL 34 BC) purchased commercially. At this stage, prepared homogenous solution is put into reservoir of gun and is sprayed onto gas diffusion layer (6) to form catalyst layer (D). Gas diffusion layer (6) is weighed when it is empty before spraying and also is weighed during spraying at certain time intervals and it is checked for if desired weight for catalyst layer (D) is achieved or not. When the desired weight is reached, spraying process is stopped. When calculating the weight of catalyst layer (D), weight of carbon supported platinum catalyst (1 ), dry Nation (2) and PDMS polymer (3) are taken into account. Since the spraying operation is conducted at preferably 60Ό on a heater vacuum plate, 2-propanol (4) and distilled water (5) used as solvents are assumed to evaporated.

Catalyst layer (D) is prepared separately on gas diffusion layer (6) for anode (A) and cathode (B) electrodes. Cathode electrode (B) catalyst layer (D) preparation procedure is as described at previous paragraph. Procedure for the preparation of catalyst layer (D) for anode electrode (A) is the same except PDMS polymer (3). Preference of PDMS polymer (3) at cathode electrode (B) is due to the fact that water is released as a result of oxygen reduction reaction occurring at this electrode and elimination of damage caused by such excess water by hydrophobic PDMS polymer (3). The electrodes in which the catalyst layers (D) are constructed must be brought together by means of placing commercial Nafion membrane (C), which is polymer electrolyte membrane, between them in order to form membrane electrode assembly (MEA) (9). Nafion membrane (C) is a perfluorosulfonic acid based polymeric membrane used as electrolyte in fuel cell. While it is allowing passing of only H⁺ ions from anode (A) to cathode side, it does not allow passing of electrons. Electrons are transferred to cathode side by external circuit. Transfer of H⁺ ions continue if the adequate humidification of Nafion membrane is provided during the supply of hydrogen gas. Anode (A) and cathode (B) and Nafion membrane (C) are kept at hot-pressing device preferably at 130Ό and at preferably 400 psi press ure preferably for 3-4 minutes. At the end of hot-pressing, a triple structure comprising anode-electrode(A) - Nafion membrane (C) - cathode electrode (B) is obtained as membrane electrode assembly (MEA) (9). Prepared MEA structures are now used in fuel cell. Performance measurements of the fuel cells are performed at Henatech trademark computer controlled fuel cell test station (7) which has maximum 600 W power. Hydrogen is supplied to anode side and oxygen gas is supplied to cathode side at 1 :1 stoichiometric ratio as reactant gases. By setting the temperature of working cell and humidification temperatures of anode (A)/cathode (B) to 70Ό, relative humidity of reactant gases is ensured to be 100%. After the cell to be tested is placed at fuel cell testing station (7), firstly, sweeping with nitrogen gas for one hour is performed and then cell temperature and tank temperatures are provided to reach to desired temperatures. Illustrative drawing of fuel cell test station comprising bipolar plate (8), membrane electrode assembly (9), current collector plate (10), compressing screws (1 1 ), load cell (12), electronic accessory (13), flow meter (14), pressure gauge (15), computer control panel (16), gas tube (17), safety button (18), humidification tanks (19) was given in Figure 6c. Then reactant gases are supplied into the system and after 30 minutes, the load device connected to single fuel cell is opened and the values of current supplied by cell corresponding to adjustable potential values are measured and recorded. This measurement process is performed by reducing potential values at 0.05 V intervals starting from open circuit voltage and repeated at each 30 minutes. Cell is left in operating state at 0.6 V at intervals where measurement is not made and thus bringing the cell more active is provided. Potential/current values at each half an hour are recorded along day and cell performance curve is obtained. Measurement process is continued until current values start to become stable. The method process steps described above were shown in flow diagram given in Figure 5.

Catalyst layer solution developed under the invention provides establishment of a better and more effective water balance and thus generating power demand at higher efficiency from PEM fuel cell which are clean and sustainable power sources.

Claims

1. Invention is a development of a catalyst layer solution (S) convenient for use in PEM fuel cells characterized in comprising; Nafion solution (2) as binder and polydimetilsiloxane (PDMS polymer (3) as a hydrophobic material.

2. The solution (S) according to claim 1 characterised in comprising PDMS polymer (3) in 5% ratio based on total solution (S) weight.

3. The solution (S) according to claim 1 characterised in comprising PDMS polymer (3) in

10% ratio based on total solution (S) weight.

4. The solution (S) according to claim 1 characterised in comprising PDMS polymer (3) in 20% ratio based on total solution (S) weight.

5. The solution (S) according to claim 1 characterized in comprising a carbon supported platinum catalyst (1 ).

6. The solution (S) according to claim 5 characterized by; mentioned catalyst (1 ) comprising Tanaka catalyst containing 67% Pt by weight.

7. The solution according to claim 5 characterized by; catalyst (1 ) loading amount on each of PEM fuel cell electrodes is 0.4 mg Pt/cm².

8. The solution (S) according to claim 1 characterized in comprising at least one solvent.

9. The solution (S) according to claim 8 characterized by; mentioned solvents are 2- propanol (4) and distilled water (5).

10. The solution (S) according to claim 1 and claim 5 characterized by; there is ratio of 70:30 by mass between the mentioned carbon supported platinum catalyst (1 ) and the mentioned binder (2).

11. The solution (S) according to claim 9 characterized by; there is ratio of 2:1 by volume between the 2-propanol (4) and distilled water (5).

12. The invention relates to a membrane electrode assembly (MEA) (9), which is formed by compression of anode (A) and cathode (B) electrodes having catalyst layer (D) on their surfaces onto both sides of polymer electrolyte membrane (C), in where electrochemical reactions occur, which is convenient for use in PEM fuel cells and characterized by; mentioned catalyst layer (D) being prepared by a catalyst layer solution (S) comprising Nation solution (2) as binder and polydimethylsiloxane (PDMS) polymer (3) as hydrophobic material.

13. A method for preparation of membrane electrode assembly (MEA) (9) according to claim

12 characterized in comprising process steps of:

- preparation of a catalyst solution (S) containing PDMS polymer (3),

- forming catalyst (D) layer onto GDL by means of spraying the prepared catalyst solution (S) onto a gas diffusion layer (GDL) (6) which is providing transmission of reactant gases to catalyst layer by diffusion and functioning a physical support for catalyst layer (D),

- obtaining MEA (9) through compressing anode (A) and cathode (B) electrodes placing a polymer electrolyte membrane (C) between them, by hot-pressing device.

14. The method according to claim 13 characterized in polymer electrolyte membrane (C) is Nation membrane.

15. The method according to claim 13 characterized in spraying process is being performed by an air compressor with spray gun.

16. The method according to claim 13 characterized in spraying process is being performed on a heater vacuum plate at temperature of 60TT

17. The method according to claim 13 characterized in polymer electrolyte membrane (C) is being a perfluorosulfonic acid based polymeric membrane.

18. The method according to claim 13 characterized in anode (A), cathode (B) and polymer electrolyte membrane (C) is being waited in hot-pressing device at 130Ό and pressure of 400 psi for 3-4 minutes.