Proton Exchange Membrane Water Electrolyzers and Fuel Cells as More Efficient Technologies than Alkaline-based Systems
Alkaline water electrolyzers (AWE) and fuel cells (AFC) are the most mature hydrogen production and energy storage technologies that have been used extensively over the past decades. For example, AFCs were used in the Apollo space program. The AFCs used by NASA since the mid-1960s contain 31 cells connected in series that produce 27 to 31 V and deliver 1.5 kW of power. In AFCs and AWEs, a liquid caustic electrolyte solution, most commonly NaOH and KOH, is pumped into the system composed of electrodes separated by a porous diaphragm. While these designs are low cost owing to their compatibility for cheap catalysts, they suffer from large internal resistance between the diaphragm and liquid electrolyte. AWEs and AFCs operate under low current densities, which consequently requires bulky stack configurations at larger scales. Case in point, the AFCs that delivered 1.5 kW weighed a whopping 113 kg. This power density would have been only 10% of those of the current water electrolysis and fuel cells. Aside from these disadvantages, AWEs and AFCs require the use of ultra-pure feed materials to avoid the formation of alkaline carbonates, which are highly detrimental in the performance and stability of the device.
Proton Exchange Members as a Core Element of PEM Water Electrolyzer and Fuel Cells
To make the electrolyzer design more compact, solid ion conducting polymers are used instead of liquid electrolytes. Solid ion conducting polymers allows the crossing of ionic species between the electrodes (anode and cathode) during the electrochemical cell operation. These ion exchange membranes are composed of a polymer backbone infused with (or functionalized) with ionic charges to allow the selective permeation of the desired ions for the reactions.
Proton exchange membrane water electrolyzers (PEMWEs) and fuel cells (PEMFCs) use proton exchange membranes (PEMs). PEMs allow the conduction of protons (positively charged carriers) across the electrodes. On top of that, using PEMWEs reduces the possibility of fuel crossover (sometimes referred to as gas crossover for fuel cells) compared when liquid electrolytes are used. Fuel crossover causes degradation of the cell components and also
reduces the efficiency of the device. For example, if hydrogen crosses over in fuel cells, it might be used for competing reactions and not the hydrogen oxidation reaction, causing parasitic currents and mixed potentials on the cathode.
As discussed in the previous blog article, “Choosing the Right Ion Exchange Membrane for Your Water Electrolyzer and Fuel Cell Applications,” an ideal PEM possesses the following characteristics: (1) high proton conductivity, (2) high ion exchange capacity (IEC), (3) good thermal, mechanical, and chemical stability, (4) low reactant permeability, and (5) low cost. Of course, in reality, finding a PEM with all these 5 characteristics is difficult but it is critical to maximize the potential of PEMWEs and PEMFCs while ensuring long-term stability and maintaining economic viability.
Issues with the most commonly-used PEM in PEMWE and PEMFC
The most widely used PEMs in PEMWEs and PEMFCs are based on a perfluorosulfonic acid (PFSA) polymer with a polytetrafluoroethylene (PTFE) polymeric backbone. On one hand, the PTFE backbone confers the membrane with high chemical inertness, improving its chemical stability and reducing its reactant permeability.
On the other hand, the side chains consist of sulfonic acid (–SO3H) groups that give the membrane with proton conducting capabilities. This chemistry gives this PEM high proton conductivity and excellent electrochemical and mechanical stability. However, the application of these traditional PEMs is very much limited to temperatures up to 80 °C. At higher temperatures, membrane dehydration becomes an issue for these PFSA-based membranes. As these membranes are dehydrated, a lot of interconnected ionic transfer pathways are lost, translating to a loss in the conductivity and performance of the device. Aside from that, PFSA-based membranes are expensive, which drastically affects the large-scale commercialization of PEM-based technologies, including but not limited to water electrolyzers, fuel cells, and redox flow batteries.
Ban on the Use of Per- and Polyfluoroalkyl Substances (PFAS)
From an environmental standpoint, using perfluorosulfonic acid-based PEMs is a matter of concern. Perfluorosulfonic acid belongs to per- and polyfluoroalkyl substances (PFAS), which are known to persist in the environment even after prolonged periods of time. The C–F bond, which is among the strongest chemical bonds, in perfluorosulfonic acid polymers prevents the membrane from degrading completely. Due to their extreme persistence, PFAS can be easily bioaccumulated, potentially affecting human health and the environment. PFAS are known to cause cancer in humans. Some PFAS affect the human reproductive health, causing severe harm for the growth and development of fetuses and interfering with hormonal levels.
In February 2023, the European Chemicals Agency published a proposal to reduce the use of PFAS to make products and processes more safe for the human population and the environment. The proposal was led by five countries, namely Denmark, Germany, the Netherlands, Norway and Sweden. The proposal seeks to reduce the use of more than 12,000 PFAS. As PFAS are used in a wide range of industries, the idea of the proposal was not to completely eradicate the use of PFAS but to limit their usage to a minimum. A six-month consultation and scientific evaluation commenced last March 22, 2023. The risk assessment committee (RAC) will determine whether the proposed restriction indeed reduces the risk to human health and environment, whereas the socio-economic analysis committee (SEAC) will study the socio-economic impacts of the proposal. Finally, the opinions of the committees will be sent to the European Commission.
Learn more about the PFAS regulations here.
Effect of the PFAS-ban on Proton Exchange Membranes for Water Electrolyzers and Fuel Cells
Alternative materials for PEMs are sought after due to the impending ban of perfluorosulfonic acid PEMs. IONOMR’s PemionTM proton exchange membrane is based on sulfonated polyphenylene. Polyphenylene is chemically stable, that is, it is less likely to react with other chemicals. PFAS can undergo chemical reactions under certain conditions. Additionally, polyphenylene has a greater thermal stability than that of PFAS, extending the applicability of this PEM for high-temperature PEMFC and PEMWE applications. Polyphenylene is generally less expensive to produce than some specialized PFAS compounds, reducing the overall cost of building PEMWE and PEMFC stacks.
Pemion™, a sulfo-phenylated polyphenylene, exhibits an immense potential as an alternative to PFAS-based PEMS. Case in point, the performance of Pemion™-based fuel cells approaches that of the PFSA cells under different applications. In fully humidified H2/O2 environments, MEAs containing Pemion™ achieves a peak power density as high as 2.1 W cm−2, which is comparable to that of a MEA containing optimized short-chained PFSA.
Pemion™-based fuel cells outperform PFSA-based cells at elevated operating temperatures (80 °C to 110 °C) owing to the high-temperature resistance of polyphenylene. Moreover, PEMION™ exhibits an outstanding chemical stability, enduring stress tests with minimal degradation, marking a significant advancement in the development of next-generation MEA for PEMWEs and PEMFCs.
Aside from PFAS-free proton exchange membrane, IONOMR also offers a PFAS-free proton exchange binder, PEMION+™ – PP1-HNN8-00, that is put in catalyst ink formulations to maximize catalyst utilization in water electrolyzers and fuel cells.
For all your PEM water electrolyzer and fuel cell needs, look no further. CAPLINQ provides high-quality ion exchange membranes, including PFAS-free anion exchange membranes. Additionally, we supply essential components such as gas diffusion layers, porous transport layers, and flat gaskets. Contact us today to learn how we can support your projects.
