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We develop ion conducting polymers for use as membranes or electrode binders.
Membranes are key components in fuel cells, water electrolysers, and batteries, and have a strong influence on the performance, efficiency and cost of electrochemical energy storage and conversion processes. This makes membranes essential for hydrogen technologies and energy storage.
Our activities range from polymer synthesis over membrane formation to the testing of membranes in the application.
1. Ion solvating membranes - Quaternary ammonium-free alternatives to AEMs (Anion Exchange Membranes)
State of the art are AEMs, which are functionalized with quaternary ammonium groups. The low alkaline stability of these groups impedes technologies like AEM water electrolysis.
Ion solvating membranes (ISM) do not have the known breaking points, and show high conductivity in alkaline solutions. However, they were initially investigated for use in highly alkaline systems.[1,2] We show that ISM can be designed to also show high conductivity in low alkaline solutions. Specifically, sulfonated para-PBI can be used in AEM water electrolysers, but needs to be chemically[3] or thermally[4] cured to prevent excessive membrane swelling and dissolution. While the membranes perform well, it is suspected that chemical crosslinkers might act as breaking point, and reduce the shelf-life of casting solutions. Thermal curing is a kinetically controlled process, which is difficult to control.
Our recent work:[5]
■ By using a partially sulfonated OPBI, curing can be omitted. The sulfonated repeat units provide high conductivity, while the non-sulfonated repeat units prevent excessive swelling.
■ Reinforcement with a PPS mesh from PVF GmbH prevents that sharp edges from the electrode can cut through the membrane.
■ Conductivity in 1 M KOH reaches 135 mS/cm at room temperature and 358 mS/cm at 80 C.
■ High alkaline stability: After 6 months in 2M KOH at 80 °C, conductivity did not decrease, and no significant change in molecular weight (e.g. no chain scission) was observed by GPC.
■ Good electrolysis performance: 4.8 A/cm2 at 2V, 80 C, 3M KOH
■ Hydrogen crossover (HTO) values are at safe levels and below those obtained with a commercial AEM.
■ Both K+ and OH- contribute to ion conductivity. The resulting movement of KOH from anode to cathode can be countered by mixing anolyte and catholyte.
■ Electrolyser performance with ISM strongly depends on anode pH
■ ISM promise high durability and performance, potentially can replace AEM, and can bridge the low alkaline region of AEMWE and the high alkaline region used in alkaline water electrolysis.
[1] Energy & Environmental Science, 15 (2022) 4362-4375. https://doi.org/10.1039/d2ee01922a
[2] Chem. Rev., 124 (2024) 6393-6443. https://doi.org/10.1021/acs.chemrev.3c00694
[3] Adv. Energy Mater. 13 (2023) 2302966. https://doi.org/10.1002/aenm.202302966
[4] Adv. Energy Mater. 15 (2025) 2500498. https://doi.org/10.1002/aenm.202500498
[5] Nature Energy (2025). https://doi.org/10.1038/s41560-025-01876-9
2. Flow Batteries - VRFB with a world-class energy efficiency of >90% at 80 mA/cm2 achieved
2. Flow Batteries - VRFB with a world-class energy efficiency of >90% at 80 mA/cm2 achieved
■ An overview over flow battery technology can be found here: www.dx.doi.org/10.1115/1.4037248
■ Polybenzimidazole membranes very effectively block transport of vanadium ions. As expected, the voltage efficiency increases, when the thickness is reduced from 35 to 15 µm. Most interestingly, the coulomb efficiency remains similar or even slightly increases as well. This seems to be related to the lower charging voltage for thin membranes, which should reduce side reactions and reduces the number of vanadium ions which enter the membrane by migration in the electric field.
http://pubs.acs.org/doi/abs/10.1021/acsami.7b10598
■ Polybenzimidazole (PBI) can be produced as a highly conductive gel and as a highly selective dense film. We combined these two forms of PBI into a membrane assembly (gel/film/gel), in which the gel PBI protects the thin PBI film. EE reached 90.5% at 100 mA cm-2.
https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202206284
■ The Figure below compares different membrane types developed by us.
Membrane type 1: PBI pre-swollen in 10 M sulfuric acid (Chem. Eng. J. 435 (2022) 134902)
Membrane type 2: Stack of 3 membranes: soft protective gel PBI, selective dense PBI, soft protective gel PBI (Small 18 (2022) 2206284)
Membrane type 3: sulfonated polystyrene coated with selective PBI layer (Adv. Energy Mater. 2024)
Membrane type 4: sulfonated para-PBI (Adv. Energy Mater. 2024)
3. Water electrolysis in low alkaline solutions using Anion exchange membranes
■ We developed a simple method for testing the true-hydroxide conductivity in water, which can be easily adopted by most labs. This solves the issue of CO2 absorption from air, which results in erroneously low conductivity values.
https://www.mdpi.com/2077-0375/12/10/989
■ Industrial scale alkaline water electrolysers can have an active area of up to 3 m2. Looking ahead, large scale AEM water electrolysers may reach similar dimensions, and low dimensional changes during cell assembly and start of electrolyser operation will become essential. We suggest to pre-swell the membranes with a high-boiling solvent like ethylene glycol to minimize dimensional changes.
https://www.sciencedirect.com/science/article/pii/S0376738822010894
■ Reinforcing the membranes may be necessary to guarantee high mechanical strength and low swelling. The different swelling of porous supports and ion conducting matrices results in delamination and increased gas crossover. We approach this by engineering a strong interphase and covalently connecting a PBI nanofiber mat (support) with the AEM matrix.
https://www.sciencedirect.com/science/article/pii/S0376738821007766
4. HT PEMFC - 2000 hours stable operation at 800 mA/cm2 achieved
Performance of our membrane in the HT PEM Fuel Cell
HT PEMFC membranes are usually based on phosphoric acid (PA) doped polybenzimidazole. We develop new crosslinking methods (avoding potentially unstable C-N linkages), but are also interested in other chemistries, and develop membranes with pyridine or tetrazole groups in the side chain. Because of the low basicity of the tetrazole group, it cannot be easily doped with PA and is not attractive for HT PEMFC. However, we found a way to increase the basicity of tetrazole groups. In detail, we found that 5-(2,6-dioxyphenyl) tetrazole is forced into a coplanar arrangement, allowing resonance stabilization of the positive charge in protonated tetrazolium ions.
Tetrazole: http://pubs.rsc.org/is/content/articlehtml/2015/ta/c5ta01936b
Pyridine: http://www.sciencedirect.com/science/article/pii/S0013468616326342
Crosslinking via aromatic sulfone bonds:
http://www.sciencedirect.com/science/article/pii/S0376738817326273
http://pubs.rsc.org/-/content/articlehtml/2017/ta/c6ta07653j
Recently, we achieved a stable performance at very challenging 800 mA/cm2 for over 2000 hours.
5. Membranes for Actuators - exceptionally fast response and large mechanical power density
Ionic polymer actuators are materials which move when a potential is applied. The resulting force can be used to propel underwater vehicles or to move endoscopic surgery tools. By blending a sulfonated polymer with a sulfonated phthalocyanine, we prepared membranes which showed the fastest response to the applied voltage and the largest mechanical power density reported so far. This work was in collaboration with the group of Dr. Chongmin Koo.
6. Make membranes resist the mechanical stresses they experience in the applications
Continuous swelling and shrinking of the membranes due to changes in temperature and/or humidity lead to cracks in the electrode and membrane, membrane-electrode delamination, and formation of cracks along the edges of the active area.
We developed two technologies which help to stabilize the membranes:
a) Membranes with a shape memory effect (SME)
By drying membranes in a fixed geometry (e.g. clamped on a board, or held in shape by rolls), the membranes can only shrink in the thickness. When humidified again, the membranes mainly swell in the thickness, regaining their original shape. In the fuel cell the membranes are clamped between the bipolar plates, and the SME is re-established in every drying cycle. Therefore, the membranes always keep a smooth surface, no wrinkles and folds are observed.
http://onlinelibrary.wiley.com/doi/10.1002/macp.201500063/full
http://pubs.rsc.org/is/content/articlehtml/2014/ta/c4ta01467g
b) Selective strengthening of the most sensitive membrane areas
Gas inlet/outlet areas and the edge of the active area experience most mechanical stress. With a metal stamp these areas can be selectively heated to temperatures at which the membranes start to crosslink and/or lose ion conducting groups. The process is fast (just a few minutes), and leaves the active area intact.
By using this process, high IEC membranes (which easily fail in the fuel cell due to excessive swelling) can be used without failure, while untreated membranes brake along the edge of the active area.
http://www.sciencedirect.com/science/article/pii/S0376738813009733
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