BESSY II: What drives ions through polymer membranes
Membrane. © HZB
Photoelectrolysers and electrolysis cells can produce green hydrogen or fossil-free carbon compounds – but they require ion-exchange membranes. An HZB team has now studied the transport of ions through the membrane in a hybrid liquid gas electrolyzer at the X-ray source BESSY II. Contrary to expectations, however, concentration differences hardly drive electric field ions. Diffusion is therefore the decisive process. This finding could help in the development of highly efficient and significantly more environmentally friendly membrane materials.
Ion exchange membranes are needed in (photo)electrolysers, fuel cells and batteries to separate ions and enable the desired processes. Polymeric membranes such as synthetically produced compounds like NAFION are particularly efficient, but they cannot be degraded. A ban on the use of these “eternal chemicals” is currently under discussion in the European Union. The development of suitable alternatives will be a major challenge. So, it is crucial to understand why NAFION and other established polymeric membranes work so well.
A team led by Dr. Marco Favaro of the HZB Institute for Solar Fuels has now investigated this using a special type of electrolysis cell. Here, the membrane sits on the outer wall and is in contact with both the liquid electrolyte and a gaseous external environment. It can act either as an anode or a cathode, depending on the polarity of the applied potential. This hybrid liquid-gas electrolyzer is considered particularly favorable for the electrochemical conversion of CO2. This is thanks to the higher CO2 concentrations that can be achieved in the gas phase. Thereby overcoming the poor solubility of CO2 in aqueous solutions.
For the study, Favaro and his team used commercially available ion-exchange membranes in contact with a model electrolyte like sodium chloride (NaCl) in water. Water vapor was fed to the gas phase, with the partial pressure of water close to its vapor pressure at room temperature. To analyze the migration of sodium and chloride ions through the membrane, they used in situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) at the SpAnTeX end-station at the KMC-1 beamline of BESSY II.
Ion exchange membranes are key component
“Indeed, we were expecting that the ion dynamics was determined, under applied potentials, by the electric fields generated between the anode and cathode of the electrolyzer, and that electromigration was therefore the main driver,” says Marco Favaro.
However, analysis of the data showed otherwise. Electromigration hardly plays a role. The ions simply diffuse across the membrane. The data could be perfectly simulated numerically with a diffusion model. “Our conclusion is that ions move through the polymer membranes in these types of electrolyzers due to hopping mediated by the ionized functional groups present in the membranes. In addition, since water diffuses as well through the polymer, the ions are “dragged” as well” explains Favaro.
These results are exciting for a number of reasons. These types of electrolyzers are a way to convert CO2 into valuable chemicals that can otherwise only be obtained from fossil fuels. Understanding how these devices work helps on the way to decarbonize the economy. On the other hand, the ion-exchange membranes that are a key component of these cells are themselves problematic. The European Union may soon ban the use of persistent chemicals. Understanding the relevant drivers of such transport processes will help to develop new membrane materials that are both efficient, durable, and environmentally friendly. Favaro now intends to take this project forward at HIPOLE. The new Helmholtz Institute in Jena, which will focus on polymer materials for new energy technologies.
- Susanne Nies appointed to EU advisory group on Green DealDr. Susanne Nies heads the Green Deal Ukraina project at HZB, which aims to support the development of a sustainable energy system in Ukraine. The energy expert has now also been appointed to the European Commission's scientific advisory group to comment on regulatory burdens in connection with the net-zero target (DG GROW).
- The future of corals – what X-rays can tell usThis summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO2 is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?
Prof. Dr. Tali Mass, a marine biologist from the University of Haifa, Israel, is an expert on stony corals. Together with Prof. Dr. Paul Zaslansky, X-ray imaging expert from Charité Berlin, she investigated at BESSY II the skeleton formation in baby corals, raised under different pH conditions. Antonia Rötger spoke online with the two experts about the results of their recent study and the future of coral reefs.
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.