Catalyst-coated membrane having a laminate structure

Abstract

A catalyst-coated membrane (CCM) for use in a water electrolyser, having a laminate structure comprising: a first layer comprising a first membrane component having a cathode catalyst layer disposed on a first face thereof; a second layer comprising a second membrane component having an anode catalyst layer disposed on a first face thereof; and an intermediate layer disposed between the first and second layers, comprising a third membrane component having a recombination catalyst layer disposed on a first face thereof is disclosed. The CCM is useful within a water electrolyser. The recombination catalyst layer reduces the risk associated with hydrogen crossover and allows thinner membranes with lower resistance to be used.

Claims

1. A catalyst-coated membrane for use in a water electrolyser, having a laminate structure comprising: a first layer comprising a first membrane component, the first membrane component having a cathode catalyst layer disposed on a first face thereof; a second layer comprising a second membrane component, the second membrane component having an anode catalyst layer disposed on a first face thereof; and an intermediate layer disposed between the first and second layers, comprising a third membrane component, wherein only a single face of the first membrane component has a catalyst layer thereon, and only a single face of the third membrane component has a catalyst layer thereon, the catalyst layer on the single face of the third membrane component is a recombination catalyst layer, and the recombination catalyst layer is located closer to the anode catalyst layer than to the cathode catalyst layer within the catalyst-coated membrane, and, wherein the first membrane component comprises an embedded reinforcing component and the third membrane component does not include an embedded reinforcing component.

2. The catalyst-coated membrane according to claim 1, wherein the first and second layers are each adjacent the intermediate layer.

3. The catalyst-coated membrane according to claim 1, wherein a second face of the first membrane component is adjacent the intermediate layer and a second face of the second membrane component is adjacent the intermediate layer, and the intermediate layer is oriented such that the recombination catalyst layer is disposed adjacent the second layer.

4. The catalyst-coated membrane according to claim 1, wherein the catalyst coated membrane has a total thickness of ≤120 μm.

5. The catalyst-coated membrane according to of claim 1, wherein the first, second and third membrane components are solid polymer electrolyte membrane components comprising an ionomer.

6. The catalyst-coated membrane according to claim 1, wherein one or more of the first, second and third membrane components further comprise a peroxide removal additive or a peroxy radical scavenger.

7. The catalyst-coated membrane according to claim 1, wherein the cathode catalyst comprises unsupported platinum black or platinum supported on carbon.

8. The catalyst-coated membrane according to claim 1, wherein the anode catalyst is selected from unsupported iridium oxide black or iridium on a support.

9. The catalyst-coated membrane according to any one of claim 1, wherein the recombination catalyst is selected from palladium on carbon, platinum on carbon, rhodium on carbon and platinum-group metal on silica.

10. The catalyst-coated membrane according to claim 1 consisting of the following layers, in order: (a) the cathode catalyst layer comprising a cathode catalyst; (b) the first membrane component; (c) the third membrane component; (d) the recombination catalyst layer comprising a recombination catalyst; (e) the second membrane component; and (f) the anode catalyst layer comprising an anode catalyst.

11. A method of making a catalyst-coated membrane according to claim 1 comprising: providing the first membrane component, the first membrane component having the cathode catalyst layer disposed on the first face of the first membrane component; providing the second membrane component, the second membrane component having the anode catalyst layer disposed on the first face of the second membrane component; providing the third membrane component, the third membrane component having the recombination catalyst layer disposed on the first face of the third membrane component; and laminating the first, second and third membrane components together to form a catalyst-coated membrane having a laminate structure, such that the third membrane component is disposed between the first and second membrane components within the laminate structure.

12. The method according to claim 11, wherein the first, second and third membrane components are laminated together in a single lamination step.

13. The method according to claim 11, wherein before lamination the membrane component are arranged such that a second face of the first membrane component faces a second face of the third membrane component and a second face of the second membrane faces the first face of the third membrane component.

14. A catalyst-coated membrane obtained by the method according to claim 11.

15. A water electrolyser comprising the catalyst-coated membrane according to claim 1.

16. A fuel cell comprising the catalyst-coated membrane according to claim 1.

17. A method of generating hydrogen gas comprising the steps of electrolysing water using the water electrolyser according to claim 15, and generating hydrogen gas.

Description

FIGURES

(1) FIG. 1 shows an exploded schematic view of a CCM according to one embodiment of the invention.

(2) FIG. 2 shows a schematic sectional view of a CCM according to one embodiment of the invention after lamination.

(3) FIG. 3 shows a schematic sectional view of a membrane component being coated with a catalyst layer while in position on a backing sheet.

(4) FIG. 4 shows a schematic discrete lamination process for producing a CCM according to one embodiment of the invention.

(5) FIG. 5 shows a schematic continuous lamination process (part of a “roll-to-roll” process) for producing a CCM according to one embodiment of the invention.

(6) FIG. 6 is a graph showing the performance of an electrolyser including state of the art CCMs and a CCM according to the invention.

(7) FIG. 7 is a plot of hydrogen crossover against current density for three different electrolysers, two including state of the art CCMs and one including a CCM according to one embodiment of the invention.

DETAILED DESCRIPTION

(8) FIG. 1 shows an exploded view of a CCM according to one embodiment of the invention. The CCM 1 consists of three individual catalyst-coated membrane components. A first membrane component 10 has a first face 10a and a second face 10b. A second membrane component 11 has a first face 11a and a second face 11b. A third membrane component 12 has a first face 12a and a second face 12b.

(9) A cathode catalyst layer 13 is located adjacent the first face 10a of the first membrane component 10. In practice, this layer is applied to the first face 10a of the first membrane component 10. The first face 10a of the first membrane component 10 faces outwards, so that the cathode catalyst layer 13 is located on an external surface of the CCM.

(10) An anode catalyst layer 14 is located adjacent the first face 11a of the second membrane component 11. In practice, this layer is applied to the first face 11a of the second membrane component 11. The first face 11a of the second membrane component 11 faces outwards, so that the anode catalyst layer 14 is located on an external surface of the CCM—the opposite surface to the cathode catalyst layer.

(11) A recombination catalyst layer 15 is located adjacent the first face 12a of the third membrane component 12. In practice, this layer is applied to the first face 12a of the third membrane component 12. The recombination catalyst layer 15 is also located adjacent the second face 11b of the second membrane component 11. The recombination catalyst layer is therefore sandwiched between the second and third membrane components 11 and 12, and lies in a position which is closer to the anode catalyst layer than it is to the cathode catalyst layer.

(12) FIG. 2 shows the same CCM as FIG. 1, assembled and in cross-section.

(13) Each of the first, second and third membrane components 10, 11 and 12 are coated with their respective catalyst layer while fixed to a backing sheet. FIG. 3 shows a process by which the first membrane component 10 may be coated with a cathode catalyst layer 13 while positioned on its backing sheet 13c. The membrane component is held in place using a heated vacuum bed (not shown) and the catalyst layer is deposited using an ultrasonic spray coater. In FIG. 3 the ultrasonic spray coater is represented by nozzle 20. The same process as shown in FIG. 3 may be used to coat membrane components 11 and 12 with their respective catalyst layers.

(14) FIG. 4 demonstrates the process by which the CCM of the invention may be prepared, showing the individual membrane components 10, 11 and 12 in position ready for lamination after being coated with the catalyst layers 13, 14 and 15 respectively. As shown in FIG. 4, the first membrane component 10 is coated with cathode catalyst 13, the second membrane component 11 is coated with anode catalyst 14 and the third membrane component 12 is coated with recombination catalyst 15.

(15) FIG. 4 shows a process for laminating the three coated membrane components 10, 11 and 12 to form a discrete CCM 1. After each membrane component 10, 11 and 12 has been coated with its catalyst layer, the backing sheets (not shown) are removed from each membrane component and the catalyst-coated membrane components are laid on top of one another in the correct order before being placed between two PTFE sheets 21, 22 and two titanium plates 23, 24 and pressed at temperature in a press pack. In the embodiment shown in FIG. 4, the central intermediate layer 12 is oriented such that the recombination catalyst layer 15 faces the second membrane component 11. This is because of the faster diffusion rate of H.sub.2 relative to O.sub.2 through the CCM. Orienting the membrane components in the manner shown means that, because the membrane components are of equal thickness, the recombination catalyst layer will be positioned closer to the anode catalyst layer in the final laminated CCM. As a result, the recombination catalyst layer will lie closer to the plane in which the levels of H.sub.2 and O.sub.2 are suitable for most effective recombination.

(16) Shen the catalyst-coated membrane components have been properly arranged they are laminated by heat pressing between the PTFE sheets 21, 22 and titanium plates 23, 24 at e.g. 170° C. and 800 psi (approx. 5500 kPa) for 2 minutes to consolidate the catalyst-coated membrane components into the CCM 1 shown in cross-section in FIG. 2.

(17) After the CCM has been prepared, suitable current collectors are positioned on each faces of the CCM to enable incorporation into a water electrolyser (PEMWE).

(18) FIG. 5 shows an alternative, continuous process for the production of a CCM 3 according to the invention. The product of the process is a long sheet of CCM 3 which can be stored as a roll (not shown). The first, second and third membrane components 30, 31 and 32 are also long sheets which can be stored as rolls (not shown) before processing into the CCM. The process shown in FIG. 5 is therefore the intermediate part of a roll-to-roll process.

(19) Membrane components 30, 31 and 32, coated with catalyst layers on each of their first face 30a, 31a and 32a respectively and backing sheets 30c, 31c and 32c respectively on the other face are shown in FIG. 5. Backing sheets 30c, 31c and 32c are removed from the membrane components 30, 31 and 32 by peeling the backing sheets 30c, 31c and 32c away from the membrane components 30, 31 and 32 and passing them over rollers 36a, 36b and 36c. The three membrane components 30, 31 and 32 coated with catalyst layers 30a, 31a and 32a respectively are then brought together as they approach rollers 37a and 37b until they are brought into contact. The second face 30b of first membrane component 30 contacts the second face 32b of third membrane component 32.

(20) Meanwhile, the second face 31b of second membrane component 31 contacts the first face 32a of third membrane component 32 (which is coated with recombination catalyst 35).

(21) After the membrane components are brought into contact, the laminate structure is subjected to heat and pressure to consolidate the components into a CCM sheet 3, with a cathode catalyst layer 33 on one surface and an anode catalyst layer 34 on the other surface, and an internal recombination catalyst layer 35 within. The sheet 3 may then be formed into a roll (not shown) for storage and transportation, and/or cut into individual CCMs of appropriate size as needed.

EXAMPLES

Example 1—CCM Preparation

(22) CCM 1 was prepared using three individual membrane components, Each membrane component had a nominal thickness of 17 μm, comprised a 900 EW Flemion™ ionomer from Asahi Glass Group with e-PTFE reinforcement and had a ceria hydrogen peroxide scavenger catalyst coated on one side. A cathode catalyst layer comprising Pt black in a dispersion of ionomer (aqueous Nafion 1100EW, 12 wt % w.r.t. Pt) was coated in an amount of 1 mgPt/cm.sup.2 onto one of the membrane components, on top of the scavenger catalyst. An anode catalyst layer comprising IrO.sub.2 black in a solution of ionomer (aqueous Nafion 1100EW from Chemours Corp, 12 wt % w.r.t. Ir) was coated in an amount of 2 mg Ir/cm.sup.2 onto another of the membrane components, on top of the scavenger layer. A recombination catalyst comprising Pd supported on carbon black in a solution of ionomer (Nafion 1100EW, 300 wt % w.r.t. carbon) was deposited onto the final membrane component in an amount of 0.04 mg Pd/cm.sup.2, on top of the scavenger catalyst. The catalyst layers were deposited using an ultrasonic spray coater (Sonotek ExactCoat with 120 kHz Impact coating head).

(23) The three catalyst-coated membrane components were then arranged with the membrane component having the recombination catalyst layer in the middle, sandwiched between the other two membrane components with the anode and cathode catalyst layers facing outwards. The central membrane component was oriented such that the recombination catalyst layer faced the membrane component which carried the anode catalyst layer (as in FIG. 2).

(24) These three layers were then laminated to form CCM 1.

Comparative Example 1—CCM Preparation

(25) CCM A was prepared using a Nafion 117 (RTM) membrane (perfluorinated ion-exchange membrane of thickness 177.8 μm; equivalent weight (EW)=1100), A cathode catalyst comprising Pt black in a solution of ionomer (aqueous Nafion 1100EW, 12 wt % w.r.t. Pt) was coated in an amount of 1 mgPt/cm.sup.2 on one side of the Nafion 117 membrane and an anode catalyst comprising IrO.sub.2 black in a solution of ionomer (aqueous Nafion 1100EW, 12 wt % w.r.t. Ir) was coated in an amount of 2 mg Ir/cm.sup.2 on the other side, to provide CCM A. The catalysts were deposited using an ultrasonic spray coater (Sonotek ExactCoat with 120 kHz Impact coating head).

Comparative Example 2—CCM Preparation

(26) CCM B was prepared using three individual membrane components identical to those used for CCM 1, with e-PTFE reinforcement and a peroxide scavenger layer. The three membrane components were laminated to form a laminated membrane before being coated on one side with a cathode catalyst (as used in Membrane 1) and on the other side with an anode catalyst (as used in Membrane 1) to give CCM B.

Example 2—Electrolyser CCM Performance

(27) Each CCM was tested at 60° C. using an electrolyser test station and a QCF25 cell fixture from Baltic Fuel Cells with parallel flow fields machined out of carbon on the cathodic side and titanium anodic side. The CCMs were assembled between a carbon base gas diffusion layer on the cathodic side (SGL 10BB) and a gold coated porous titanium sinter (Mott) on the anodic side. The assembled MEA was then heated in place using the reactant water passing on the cathodic side which in turn was heated via a tube-in-tube heat exchanger and water bath. The polarisation curves were recorded from 2 A cm.sup.−2 down to 0.1 A cm.sup.−2 holding at each point for 170 s. The hydrogen in oxygen was measured after cooling the anode exhaust gas with a heat exchanger using a four port thermal conductivity detector (GE-XMTC) with pure oxygen as the reference gas and calibrated against a 10% H.sub.2 in N.sub.2 reference gas and corrected for the different thermal conductivities of O.sub.2 and N.sub.2. The crossover data was logged continuously and the final points at each current density reported.

(28) As shown in FIG. 6, improved performance (lower applied voltages at a given current) is seen for both CCM B and CCM 1 due to the reduced thickness of both membranes relative to CCM A. The improved performance can be attributed to the reduced resistance of the CCM from the lowering of the gradient of the polarisation curves for CCM 1 and CCM B compared to CCM A.

(29) However, as shown in FIG. 7, the hydrogen crossover observed for CCM B was unacceptably high for the beginning of life. Typically, a limit of 2% hydrogen in oxygen is used to ensure that a flammable mixture does not exist in the system and such a high initial level will lead to a reduced operating lifetime of the system due to membrane thinning increasing crossover with time. The lowest level of hydrogen crossover was observed for CCM 1. Although the hydrogen crossover for CCM A was not as high as for CCM B, it was still higher than CCM 1.

(30) CCM 1 was the only membrane tested which demonstrated both good performance and acceptably low levels of hydrogen crossover