Method for conditioning an electrolysis stack and electrolysis stack

Abstract

The present invention relates to a method (100) for treatment of an electrolysis stack (200). The method (100) comprises conditioning (101) the electrolysis stack (200) by flushing it with water for a predetermined time, wherein the water has a temperature higher than 75°C. Further, the invention relates to an electrolysis stack (200).

Description

[0001] Description

[0002] Title

[0003] Method for conditioning an electrolysis stack and electrolysis stack

[0004] The present invention relates to a method for conditioning an electrolysis stack and an electrolysis stack according to the attached claims.

[0005] State of the art

[0006] Electrolysis stacks are well known. Each electrolysis cell in an electrolysis stack has a layered construction, typically comprising an electrically insulating, but ion conducting composite polymer membrane with a respective cathode and anode, electrode layers provided on either side thereof, at which electrodes the actual electrochemical reaction takes place, such as H2O oxidation or oxygen evolution reaction at the anode and IT reduction or hydrogen evolution reaction at the cathode in case of electrolysis of water.

[0007] The anode and cathode catalyst layers comprise catalyst nano particles mixed with binder and support materials, typically an ionomer. This layered assembly of the membrane and the electrodes is often referred to as membrane electrode assembly (MEA) or catalyst coated membrane (CCM), which latter naming refers to the conventional MEA production process of coating the membrane with the electrode catalyst mixture dissolved in a volatile solvent.

[0008] An electrolysis cell further comprises a porous transport or gas diffusion layer (PTL / GDL) on either side of the CCM for carrying electric current to or away from the electrodes, while allowing electrolyte and / or the electrochemical reactants / products to be supplied to or carried away from a respective electrode side of the CCM. In some publications on electrochemical stacks, the MEA is defined to include the PTL / GDL. The PTL / GDL can be provided as a metal foam, sintered metal fibres / whiskers, woven or non-woven carbon fibres and the like. Effective porosity, average pore size, tortuosity, as well as electric conductance are all relevant characteristics of the PTL / GDL.

[0009] An electrolysis stack also comprises metal “bipolar” plates (BPP) shared between and mutually chemically separating adjacent cells for coupling electric current out-of, or into these cells, which plate can be corrugated such that it defines a fluid supply and distribution area and a fluid flow field for the transport of the electrolyte, the reactants / products and / or a cooling medium across the active area of the cell.

[0010] Alternatively to the CCM, the MEA could be designed as a PTE (Porous Transport Electrode) or CCS (Catalyst Coated Substrate) which is a solution claimed to enable lower precious metal catalyst loadings, such as Iridium, or Ruthenium, for example.

[0011] A stack comprising multiple cells is normally taken into operation by flushing it with water of low conductivity, to remove substances and adsorbed species to the different components after which it is brought to operating temperature.

[0012] Typically, this is a process that takes some time to allow the membrane to hydrate and build the water-soaked clusters to promote the proton conductance.

[0013] At the same time, the membrane swells in all directions, of which the swelling perpendicular to the in-plane direction allows for creating more contact positions between the membrane and the porous transport layers (PTL, GDL) on both sides.

[0014] After sufficient hydration, gradually electricity is applied in a step-wise procedure to allow the electrochemical reactions to take place and bring the stack into operation.

[0015] Disclosure of the invention

[0016] According to a first aspect, the present invention relates to a method for treatment of an electrolysis stack. The method disclosed herein comprises conditioning the electrolysis stack by flushing it with water for a predetermined time, wherein the water has a temperature higher than 75°C.

[0017] The method disclosed herein is based on the principle that during the process of conditioning, also called “activation” of the stack, which is carried out only once, in particular during or shortly after manufacturing of the stack, in particular between 100 and 500 hours of operation, in particular, before running the stack under regular operation conditions, the stack is flushed with water, in particular water with low conductivity, such as demineralized water, having a temperature higher than 75°C.

[0018] In particular, the water has a temperature greater than a temperature of the water under regular operating conditions of the stack.

[0019] During the flushing, membrane swelling takes place, resulting in membrane deformation, which improves contact between the membrane and the PTL.

[0020] According to an embodiment, conditioning comprises swelling of membranes in electrolysis cells of the stack, thereby maximizing contact positions between the membrane and porous transport layers on both sides of each cell. Especially on the anodic side this is crucial due to the usually slower OER (oxygen evolution reaction) being a limiting factor in the cell performance.

[0021] In experiments, it was surprisingly found that the number of contact points between CCM and PTL and the size of the impressions was maximized, i.e. significantly improved compared to a conditioning operation with water having a temperature lower than 75°C, when the temperature of the water was higher than 75 °C. This effect is based on a change in the properties of the membrane materials in a way that promotes significantly the contact between the porous PTL and the CCM.

[0022] Thus, flushing the stack with water having a temperature higher than 75°C enhances the contact between the CCM and PTL permanently, to improve the overall performance of the stack by reducing the interfacial contact resistance between CCM and PTL and by increasing the catalyst utilization.

[0023] The catalyst utilization is enhanced because more contact points are developed leading to a higher ratio of active so-called triple-phase-boundaries, which specifies contacts between the electron conducting PTL, the active catalyst and the proton conducting membrane.

[0024] The interfacial contact resistance is improved by an improved in-plane conduction of electrons through the improved number and size of the contact areas and an overall better distribution thereof.

[0025] According to another embodiment, the water has a temperature between 75°C and 100°C.

[0026] According to another embodiment, the water vapor is used with temperatures higher than 100°C.

[0027] Since use of water at higher temperatures promotes the change of the characteristics of the membrane and, therefore, the contact between the CCM and PTL is maximized permanently, also very high temperatures, such as 90°C or up to 120°C can be used, in particular for thicker membranes.

[0028] According to an embodiment, the predetermined time is greater than 1 minute, in particular greater than 1 hour.

[0029] The time for flushing may be chosen between 1 minute and several hours. In particular, the time may be chosen the shorter, the greater the temperature of the water and vice versa.

[0030] According to another embodiment, during conditioning, the stack is operated at, despite from the water temperature, regular operating conditions. In particular, a pressurized cathode, i.e. a cathode operated under pressurized hydrogen conditions, has been found to maximize the beneficial effects of the method disclosed herein.

[0031] Thus, according to another embodiment, after flushing the stack with water, the stack is operated under regular operating conditions with a water temperature lower than 75°C.

[0032] According to a second aspect the present invention relates to an electrolysis stack, wherein the electrolysis stack comprises a number of electrolysis cells, and wherein the electrolysis stack has been conditioned by the method disclosed herein.

[0033] The method disclosed is preferably used for conditioning the stack disclosed herein, which may include a stack with improved surface morphology of the PTL, such as a fiber type MPL, for example. Such an MPL enhances the conductivity by limiting macroscopic deformation while still allowing microscopic protrusion into the PTL pores and provides an interconnected, electrically conductive structure that allows a limited number of cracks in the catalyst layers and / or membrane due to lowered mechanical stress on these components to occur.

[0034] Further, the method disclosed herein enables the use of a stack with extreme low catalyst loadings. Such catalyst layers can be flexible enough to follow the shape of a deformed membrane and as such, form additional triple-phase-boundaries once the membrane deforms towards the inner fibres or particles of a PTL that otherwise would not be available as a reaction site.

[0035] According to an embodiment, each of the electrolysis cells comprise a porous transport electrode. When a catalyst is applied to the surface and within the porous structure of a PTL / GDL an electrochemical activity (HER, OER) is only achieved at locations with triple-phase-boundary. Thus, the protrusion of the membrane into the porous structure increases the utilization of catalyst that would otherwise not actively contribute to the reactions. According to another embodiment, the electrolysis cells comprise non-PFSA membranes. Non-PFSA membranes, such as Hydro-carbon based membranes, of which some have less water absorption capability and a different swelling behavior and are therefore less effective in creating a balanced contact to the PTL may be favourably treated by the method disclosed herein.

[0036] The less ductile nature of non-PFSA membranes can be overcome by the method disclosed herein using a flushing of these membranes with water having a temperature greater than 75°C, thereby making use of changed properties, such as a modulus of elasticity of the base material.

[0037] It is to be understood that the features mentioned above, and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. In particular, the features of the different aspects, embodiments, and / or methods of the invention can be combined with each other unless noted to the contrary.

[0038] The foregoing and other features and advantages of the invention will become further apparent from the following detailed description read in conjunction with the accompanying drawings. In the drawings, like reference numerals refer to like elements.

[0039] Figure 1 shows schematically an embodiment of the method disclosed herein,

[0040] Figure 2 shows schematically an embodiment of the electrolysis stack disclosed herein, and

[0041] Figure 3 shows a potential profile of a temperature of water used for conditioning in the method according to Figure 1.

[0042] In Fig. 1 , a method 100 for treatment of an electrolysis stack is shown. The method 100 comprises a conditioning step, in which the stack is conditioned during manufacturing, before being operated for the first time under normal operating conditions, by flushing it with water for a predetermined time, wherein the water has a temperature greater than 75°C.

[0043] In Fig. 2, an electrolysis stack 200 is shown. The electrolysis stack 200 comprises a plurality of electrolysis cells 201 , which have been treated by the method 100. Thus, the stack 200 shows a maximized overall performance, since the electrolysis cells show a minimal interfacial contact resistance between CCM and PTL and a maximized catalyst utilization.

[0044] In Fig. 3, a diagram 300 is shown, which shows time on its x-axis and temperature of water used for flushing the stack on its y-axis.

[0045] In an activation or conditioning period 301 , the temperature is elevated over 75 °C. After the conditioning period 301 , the temperature sinks to an ambient temperature for an installation period 303 in which the flushing of the stack is paused.

[0046] After the installation period 303, an operation period 305 starts, in which the temperature of the water is elevated to an operating temperature lower than 75 °C, as indicated by the delta 307.

Claims

Claims1 . Method (100) for treatment of an electrolysis stack (200), the method (100) comprising:Conditioning (101) the electrolysis stack (200) by flushing it with water for a predetermined time, wherein the water has a temperature higher than 75°C.

2. Method (100) according to claim 1 , characterized in that, conditioning (101) comprises swelling of membranes in electrolysis cells (201) of the stack (200), thereby maximizing contact positions between the membrane and porous transport layers on both sides of each cell (201).

3. Method (100) according to claim 1 or 2, characterized in that, the water has a temperature between 75°C and 100°C.

4. Method (100) according to claim 1 or 2, characterized in that, the water is water vapor and has a temperature higher than 100°C.

5. Method (100) according to any of the preceding claims, characterized in that, the predetermined time is greater than 1 minute.

6. Method (100) according to any of the preceding claims, characterized in that, the predetermined time is greater than 1 hour.

7. Method (100) according to any of the preceding claims, characterized in that,during conditioning (101), the stack (200) is operated at, despite from the water temperature, regular operating conditions.

8. Method (100) according to any of the preceding claims, characterized in that, after flushing the stack with water, the stack (200) is operated under regular operating conditions with a water temperature lower than 75°C.

9. Electrolysis stack (200), wherein the electrolysis stack (200) comprises a number of electrolysis cells (201), and wherein the electrolysis stack (200) has been conditioned by a method (100) according to one of claim 1 to 8.

10. Electrolysis stack (200) according to claim 9, characterized in that, each of the electrolysis cells (201) comprises a porous transport electrode.11 . Electrolysis stack (200) according to claim 9 or 10, characterized in that, the electrolysis cells (201) comprise non-PFSA membranes.

Images