Why is an activation procedure or break-in necessary for a membrane electrode assembly (MEA)? A good reason for performing an activation procedure or break-in is to properly humidify the membrane portion of the MEA that was dried out during the hot press stage of the membrane electrode assembly (MEA) production. MEAs will not work well when they are not fully humidified (see article: Why is Humidity / Moisture Control Important in a Fuel Cell?).
How do I Humidify a Membrane Electrode Assembly (MEA)?
There are different MEA products manufactured in the market and each one would require a specific humidification process. The most common MEA configurations are the 3-layer, 5-layer and 7-layer ones.
In a 3-layer MEA, anode and cathode catalysts are usually deposited on the membrane surface and hot pressed in order to improve the bonding of these layers, and there are no gas diffusion layers. Such a 3-layer MEA (which is also known as catalyst coated membrane or CCM) can be submerged into a deionized water bath outside of the electrochemical cell for its humidification and then assemble in the cell while it is wet. Dry assembly method can also be used for the 3-layer MEAs where the MEA is assembled in the electrochemical cell in a fully dry form and then humidified while it is constrained in the cell.
With a 5-layer MEA, a different protocol will be needed compared to the 3-layer MEA products. 5-layer MEAs are usually manufactured via hot pressing of the electrodes on the desired membrane surface. For this 5-layer MEA configuration, initially the anode and cathode catalysts are deposited on the desired gas diffusion layers with microporous layer, and then post coated with more ionomer in order to improve the bonding to the membrane surface. After the post coating process, the finalized gas diffusion electrodes are hot pressed on the membrane component. Since there is significant difference in the expansion of the membrane and GDE components, 5-layer MEAs cannot be soaked in a bath of deionized water (outside of the electrochemical cell or stack) as this will result in complete delamination of the electrodes from the membrane surface. It is recommended that the 5-layer MEA is assembled into the electrochemical cell or stack first and then clamped down in order to make sure there is sufficient mechanical load on the 5-layer MEA. After the assembly of the 5-layer MEA in the cell, anode and cathode reactant gases are brought into the electrochemical cell or stack in the humidified form (i.e., the dry gases are transferred from a pressurized tank such as K-type bottle with the help of a mass flow controller into a bubble humidifier or some other humidification device) and then it is transferred through heated transfer lines to the electrochemical cell. Since the 5-layer MEA will be under a mechanical load, the humidified anode and cathode reactants will help the membrane component to slowly pick up moisture from the respective gases as it is being activated or broken in and this usually prevents the delamination of the electrodes from the membrane surface. If the electrochemical cell is generating by-product water such as the case in a cathode side of the PEM fuel cells, there will be additional humidification of the membrane as a result of this process as well.
While there are no standardized MEA activation or break-in protocols that can be used by the researchers and end-users, there are very good resources and published documentation in the public domain and it is recommended that some of these resources to be reviewed prior to purchasing and testing the membrane electrode assemblies. Master's thesis and PhD dissertations are some of the most beneficial resources where the authors of those resources will usually provide detailed steps compared to the short experimental sections that can be found in articles.
After installing the 5-layer MEA in the electrochemical cell (also known as single cell) or stack, one can initiate the break-in process in which it is operated in an air-starved mode periodically (and this is a process that is used predominantly at the single cell level to quickly hydrate the 5-layer MEA, not recommended for short stack or full scale stack operations). Operate normally (with a suitable load applied) and then turn off the gas flow on the Air/Oxygen side until the power drops significantly and begins to settle out (around a few minutes depending on the volume present in the transfer lines and single cell) and make sure the cell voltage does not drop below 0.1 V. Allowing the cell voltage to drop below 0.0 V (versus NHE) will potentially result in cell voltage reversal phenomenon and may damage the MEA components. Then reintroduce the air/oxygen flow until the power increases and stabilizes again. Repeat this as necessary until there are no further performance gains.
What this air starvation protocol does is allow for the MEA to produce water instantaneously as it consumes the residual oxygen in the system when the gas flow is removed and force the membrane to pick up more moisture either from cathode side or shuttling it from anode towards the cathode side as a result of concentration gradient in the headspace section. In addition to this, since the microporous layer of the commercial gas diffusion layers are significantly more hydrophobic compared to the rest of the GDL, the by-product water generated as a result of the air-starvation will have more residence time at the cathode catalyst and membrane interface and be captured by the hygroscopic proton exchange membrane.
7-layer MEA products are 5-layer MEA products that have sub-gasketing or sub-framing element to the perimeter of the membrane and can use the same activation or break-in protocol as the 5-layer MEAs. 7-layer MEAs will also require dry assembly of the MEA into the electrochemical cell first and then initiate the activation process. Submerging a 7-layer MEA in a deionized water bath outside of the electrochemical cell will result in complete delamination of the electrodes from the memrbane surface.
Short stacks and full scale stacks should not use air starvation method for MEA activation since there is a high likelihood of pushing the MEAs into cell voltage reversal condition.
Membrane Electrode Assembly Activation Procedure
The start-up procedure for a new fuel cell membrane electrode assembly MEA may vary somewhat from application to application. What is important in any research or production environment is to be consistent with the break-in procedure that you use. How the MEA is initially broken in can have long-lasting effects on the ultimate performance of the MEA. Published procedures vary in specifics, but almost all follow a similar sequence:
- Initial Start-Up
- Load Cycling
- Final Performance
The US Fuel Cell Council (USFCC) published a standard for single cell testing that includes specific break-in procedures(beginning on page 15):
- Fuel: Hydrogen, 1.2 Stoich, 100% RH
- Oxidant: Air, 2.0 Stoich, 100% RH
- Temperature (C): 80
- Pressures (psig): 25
Initial Startup: As required to reach 80C
- Cycle Step 1 (Perform Once): Hold 0.6V for 60 mins
- Cycling Step 2 (Perform 9 times): Hold 0.7V for 20 mins, than hold 0.5V for 20 mins
- Constant Current Operation:Hold at 200 mA/cm2 for 720 mins (12 hrs)
Verify break-in status by repeating the polarization curve sequence three times, or as necessary, to ensure that the cell is broken-in. Remain at each sequence step for 20 minutes. The cell is considered broken in when less than a 5 mV deviation from the previous polarization curve is recorded at 800 mA/cm². A waiting period of 10 minutes should be observed between polarization curves. During this period, return the gas flow rates to the equivalent of 10 stoic at 200 mA/cm² and set the current to 800 mA/cm².
For additional questions or inquiries regarding the activation/break-in procedures of membrane electrode assemblies (MEA) contact us, your membrane electrode assembly experts.
Also, do not forget to check out our standard membrane electrode assemblies here. If you would like a quotation for a custom membrane electrode assembly please contact one of our fuel cell specialists.
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