Liquid water accumulated in the gas diffusion layers (GDL) can lead to reduced performance due to mass transport losses. Neutron dark-field imaging offers new possibilities to visualize water in the GDL, as this technique is selectively sensitive to microstructures. The dark-field image (DFI) is obtained simultaneously to the conventional transmission image (TI) when a neutron grating interferometer is placed in the beam.
The successful start-up of polymer electrolyte fuel cell stacks (PEFCs) under sub-zero conditions (cold-start) with a minimal input of auxiliary power is an important requirement for the broad market introduction of fuel cell cars. Typically, cold start failures occur when the water produced by the electrochemical reaction freezes and blocks the access of oxygen to the catalyst. However, water produced by the reaction in sub-zero conditions can remain in liquid (super-cooled) state . Methods that allow visualizing the location of freezing events during cold-starts help to understand which parameters influence the phase transitions.
The PEM (Proton Exchange Membrane) water electrolysers produce high purity hydrogen by electrochemical water splitting. The Major advantage of this technology is its dynamics, safety and simplicity of the whole pilot plant. The Anode where water is being split and cathode where hydrogen is evolved are isolated from each other by solid electrolyte what enables PEMWE (Proton Exchange Membrane Water Electrolysers) to electrochemically pressurize the gas up to 300bar.
The high penetration of neutrons through large thicknesses of metals (e.g. > 10 cm of aluminum) allows imaging the water distribution in single fuel cells of full size design, even between the thick compression bodies requested to ensure enough flatness over a large area. For the single cell to have thermal boundary conditions similar to those of a cell integrated in a stack, the compression bodies are heated and a specific temperature gradient is applied to mimic the temperature gradient in the cell itself.
In conventional through plane imaging the neutron beam is perpendicular to the membrane. To image the distribution between the different cell layers, high resolution in plane imaging can be used, where the neutron beam is parallel to the membrane. Due to the low thickness of the fuel cell layers (e.g. 200 µm for a GDL), high resolution is required. To optimize the trade-off between spatial and temporal resolution, we developed a set of anisotropic resolution enhancement methods.
Mass transport losses represent a significant limitation for the maximal reachable power density of polymer electrolyte fuel cells (PEFCs). The ability to measure them, in order to minimize them, is of high interest for the design of the structure of fuel cell components. Helox/oxygen voltage gain measurement is a known method to evaluate these losses.
During PEFC’s operation the reactant gas is fed to the cell through the gas diffusion layer (GDL) and through the same material the product water is removed. Water accumulation near the catalyst can block the gas access causing an increase of the mass transport losses. On the other hand, PEFC’s work with a solid electrolyte membrane that needs to be hydrated in order to be proton conductive. In order to obtain the maximum performance PEFC’s require a well-balanced water management.
PEFCs are penalized by their cost and complexity, due in part to needed cooling and humidification systems. Evaporative cooling concepts achieve simultaneous cooling and humidification by evaporating liquid water in the PEFC itself, thus allowing the removal of the costly external humidifiers and the use of much less water for cooling than a classical convective cooling system. However they have their own challenges, such as distributing the liquid water where it is most needed or avoiding flooding the cell.