Understanding ion exchange membranes
An ion exchange membrane (IEM) is a thin barrier that allows the selective passage of ions from one electrode to another of electrochemical devices, including but not limited to water electrolyzers and fuel cells.
IEMs are composed of a three-dimensional polymer matrix functionalized with charged (or ion exchange) groups. These fixed charge groups completely or partially repel similarly charged ions (co-ions) from the membrane and allow the movement of differently charged ions (counter-ions) through the membrane. In both water electrolyzers and fuel cells, efficient ion transport is important to achieve high performance, reduce overpotentials, and ensure the overall effectiveness of the electrochemical processes involved.
In this article, we’ll guide you through the key considerations when choosing the appropriate IEM for your water electrolysis and fuel cell applications.
Key considerations when choosing the right IEM
Fuel Cell and Water Electrolyzer Type
Depending on the type of the fixed charge groups found in their polymer backbones, IEMs can be classified into two: cation (CEM) and anion (AEM) exchange membrane. CEMs contain fixed negatively charged ions, which allows the movement of cations across the membrane. Conversely, AEMs have positively charged groups to selectively allow the transport of anions. On top of these two basic classifications, IEMs can also be proton exchange membranes (PEMs), bipolar, amphoteric, and mixed matrix membranes. In water electrolyzers and fuel cells, PEMs and AEMs are the most useful. PEM is a special type of CEMs that transports protons (H+ ions).
The first thing to consider when choosing IEMs is the type of fuel cell and water electrolyzer. Fuel cell and water electrolyzer operation involve the transport of ions; the type of ionic species that need to be transported dictates the appropriate type of IEM to be used. Polymer electrolyte membrane water electrolyzers (PEMWE) and fuel cells (PEMFC), which are operated under acidic conditions, require the transport of H+ ions. Therefore, a PEM is used for PEMFC and PEMWE. In contrast, in anion exchange membrane fuel cells (AEMFCs) and anion exchange membrane water electrolyzers (AEMWEs), which run in alkaline environments, OH– ions are transported. AEM is used in AEMFC and AEMWE.
Once the right type of IEM for your fuel cell and water electrolyzer stack has been determined, it is now time to dig into the specific properties that will yield your target efficiency and performance.
IEM properties and the balance between them
Generally, the properties of IEMs are determined by the properties of the polymer backbone and fixed charges that make up their structures. Particularly, the density, wettability (hydrophobicity or hydrophilicity), and morphology of the polymer matrix, as well as the type and concentration of the charged functional groups, affect the resulting properties of the IEMs. The mechanical, chemical, and thermal properties of IEMs are primarily influenced by the polymer backbone, whereas the electrochemical properties, conductivity, and permselectivity are determined by the concentration of the fixed charges. Learn more about the key properties of IEMs here.
A high performance IEM should exhibit high ionic conductivity, high ion exchange capacity, permselectivity close to unity, and excellent dimensional, chemical, mechanical, and thermal stabilities. However, hitting all these check boxes is nowhere near as possible. We can’t have it all and most of the time, we must find the perfect balance between these properties.
For example, high ionic conductivity improves cell efficiency. To achieve it, the IEM should have a high IEC. The downside to this is that adding more charged functional groups to the polymer matrix may increase the water uptake, which can potentially lead to the degradation of the IEM. For this case, performance–stability tradeoffs should be considered. Conversely, when we want a membrane that is highly dimensionally stable, membrane dehydration, which decreases ionic conductivity and consequently the cell performance, is a possibility. When choosing the IEM for your application, it is necessary to understand IEM properties and find the balance between them.
IONOMR Innovations offers advanced ion exchange membranes for next-generation water electrolyzers and fuel cells. AEMION+TM and PEMION+TM ion exchange membranes demonstrate a delicate balance between these key properties. AEMION+TM is a high-performance AEM that is stable in both highly alkaline and strongly acidic environments, enabling cost-effective hydrogen production and even carbon capture. PEMION+TM introduces a paradigm shift in PEM technology, replacing potentially harmful materials with more environmentally benign hydrocarbons. Both IEMs seek the balance between high ionic conductivity, IEC, and stability, recognizing the necessity of trade-offs in achieving optimal performance, whether for efficient hydrogen fuel cells or water electrolyzer applications.
The operating conditions, including temperature, pressure, and humidity levels, should also be considered when looking for the right IEM for your application. The operating conditions do not only affect the performance but also the stability of IEMs. Some IEMs perform optimally at high temperatures and pressures, while others are designed for lower-temperature applications. For wet operating conditions, a membrane with low water uptake may be suitable. In contrast, an IEM with a high water uptake is desired for dry operating conditions to avoid excessive membrane dehydration. Ensure that the chosen IEM can withstand your device’s environment.
Cost of the membrane material
Let’s say that we have found a perfectly optimized membrane with excellent performance and stability. Another factor to consider is the cost. Oftentimes, high performance and stability requirements require high material costs. As such, balancing performance requirements with budget constraints is crucial, as the cost of the IEM can affect the overall cost of the fuel cell and water system.
Special requirements (if there are any)
Lastly, when choosing IEMS, any special requirements unique to your fuel cell and water electrolyzer design should also be considered. Consider any special requirements unique to your application, such as radiation resistance for space applications or chemical resistance to specific contaminants in the environment.
Choosing the right ion exchange membrane is a pivotal point in the design of an efficient and durable water electrolyzer and fuel cell system. By carefully considering the factors mentioned above and conducting appropriate testing, you can make an informed decision that aligns with your specific application needs. Contact us, and our application engineers and in-house water electrolyzer and fuel cell experts will assist you in selecting the most suitable products tailored to your application. Together, let’s pave a way for the advancement of clean and sustainable energy solutions.