Can cheaper and more efficient fuel cells be developed by dispersing ceramic nanoparticles in the polymeric membrane? Several studies report on improved performances using so-called fillers, but MoZEES PhD student Xinwei Sun has found few credible rationalisations of why they work. Find out more in her recently published review article.

The development of fuel cells as energy converting devices has received increasing interest over the past decades. A part of the research in MoZEES contributes towards production of fuel cells with lower cost and higher efficiency in order to meet the 2025 US DOE targets in the transport sector. The Group for Electrochemistry at University of Oslo contributes to the development of proton conducting membranes based on composites of polymers and ceramic nanoparticles for polymer electrolyte membrane fuel cells (PEMFCs) and electrolysers (PEMEs).

Xinwei Sun (Photo: Ann Kristin Støversen)

The pursuit of higher operating temperatures

The heart of a PEM electrochemical cell consists of a solid polymer electrolyte sandwiched between two electrodes. The electrodes are further assembled using porous gas transport layers and bipolar plates acting as current collectors and connecting cells in series. When the PEMFC is in operation (Figure 1), humidified H2 is supplied to the anode, being oxidized into protons and electrons. The hydrated protons, namely H3O+ ions are transported through the electrolyte to the cathode, while the electrons must migrate there through an external circuit with the load. At the cathode, the recombination of protons and electrons occurs via reduction of O2 to form water as the only product.

Figure 1 Schematic of a PEMFC [1]
Conventional low temperature PEMFCs (LT-PEMFCs) that typically operate below 80 °C suffer from slow reaction kinetics, water flooding of the electrodes, complex heat management, gas crossover, and poisoning of the catalysts.

Most of the above-mentioned issues can be overcome by operating the PEMFCs at higher temperatures. The high temperature PEMFC, or HT-PEMFC, is very much like the traditional PEMFC, but the proton-conducting electrolyte, traditionally Nafion®, which would dehydrate above 100 °C, is modified or replaced with a high-temperature tolerant polymer or composite. Higher temperatures facilitate the electrode reaction kinetics significantly, and in turn improve the fuel cell and electrolyser performance as a whole. Moreover, the waste heat released is of higher temperature and hence quality and may find broader utilization, such as producing hot water and ambient heating. Higher temperatures result in the reduction of water flooding at the electrodes and simplify the water management, as water is in vapour phase. The purity of fuel and catalyst poisoning are becoming less critical as well. The CO tolerance is increased by about three orders of magnitude if the PEMFC is operated at 160 °C.

Are ceramic fillers the answer?

The performance of the PEMFC is partly characterized by the protonic conductivity and water content of the electrolyte, which are dependent on the temperature and relative humidity (RH). Many studies attempt to improve the protonic conductivity, water content and stability of the polymer membranes at elevated temperature and lower RH through the dispersing a secondary ceramic phase (filler) so as to make a polymer-ceramic composite.

One example of such fillers is silica, SiO2. Silica is not new to us, we find silica gel packets in all kinds of products we buy (Figure 2) – water is adsorbed on the surfaces of its millions of tiny pores and keeps the relative humidity of the surrounding air low, in order for the products to be preserved longer. For instance, silica packets in leather products limit moisture that would otherwise increase the growth of mould.

Figure 2 Silica Gel packets as desiccant [2]
Inspired by this, people have tried to prepare composite membranes by introducing nanostructured particles of silica into the polymer structure to improve the water uptake and retention. In addition to silica, the same principle is exploited with other ceramics of various kinds (metal-organic frameworks (MOFs), metal phosphates, solid acids, clays, carbon structures etc.) that are believed to strongly absorb water. Such materials have been introduced to both the traditional polymer electrolytes like Nafion®, as well as HT polymers such as polybenzimidazole (PBI) and polyether ether ketone (PEEK). Ceramic dispersions may increase the hardness and temporarily the thermal stability of the membrane, but they on the other hand also increase the brittleness of the membrane. Are they really of help in terms of water retention, hence protonic conductivity, at high temperatures? For this and other reasons, we did an extensive literature review.

Indeed, we have seen in some cases large improvements in protonic conductivity, power density, or stability operated at higher temperatures and lower RH. However, full PEMFC tests at the US DOE target temperature of 120 °C are missing from the majority of these works, and they are of paramount importance in order to assess the compatibility, stability and lifetime of such composite membranes. We have seen few credible rationalisations of why fillers work. Some refer to the use of proton conductive particles, all of which are recently understood to exhibit only protonic conduction instead in adsorbed water or acids. This encourages us to study PEM composites onwards with emphasis on well-characterised microstructures, as well as a well-founded assignment of protonic conduction appropriately to bulk solid polymer and ceramic phases and liquid phases, adsorbed water layers, and interfaces.

Interested in knowing more? Check out our latest critical review article regarding composite membranes as proton-conductive electrolyte for its application in PEMFCs and PEMEs with OPEN access: https://www.mdpi.com/2077-0375/9/7/83. It’s completely free!

Xinwei is supervised by Prof. Truls Norby and Dr. Athanasios Eleftherios Chatzitakis.

References:

  1. https://en.wikipedia.org/wiki/Proton-exchange_membrane_fuel_cell. Date: 01-12-2019
  2.  https://science.howstuffworks.com/innovation/science-questions/question206.htm. Date: 09-12-2019