Utility of neutrons and background of the proposers
Neutrons are a very relevant and non-intrusive probe to quantify the water content in the various components of a fuel cell, especially during operation:
The metallic materials constituting the fuel cell are transparent to neutrons
The neutrons are extremely sensitive to protons due to the exceptional incoherent cross section of hydrogen atoms with respect to any other atomic species. Therefore, the neutrons probe quite exclusively the properties of protons contained in the ionic phase of the membrane (i.e. charge carriers and hydration protons) and/or in the liquid water located in the components of the MEA and in the gas distribution channels.
Our group has developed since several years a method based on Small angle neutron scattering (SANS). It is the only method that allows to quantify simultaneously the local water content in the membrane (via the direct relation between microstructure and water content of perfluorosulfonated ionomers) and the amount of liquid water outside this component (producing an incoherent proton scattering signal) [16]. Accordingly, the operando technique has provided, up to now, invaluable insights into the water distribution in the working FC [17-26]. A PEMFC transparent to neutrons can be built using standard MEA materials and without significant modification of the state-of-art design (Figure 4a). SANS measurements have thus been carried out under representative conditions of gas stoichiometry, temperature (from -10 to 80 °C), current density (up to 800 mA.cm-2), pressure (up to 3 bars) and with relative humidity (RH) of fed gases varying from 0% to 100% [20,22,23,25] (Figure 4b).
The local water content in an operating fuel cell has been probed every 30 seconds in front of the rib and the channel of the bipolar plate. These results are currently under publication [26].
N117:
175 µm
Figure 4. (a) 25 cm² single cell developed at CEA for SANS experiments on D22 beamline at ILL (Picture of the single cell at -10°C inserted). (b) SANS spectra recorded in different areas of an operating PEMFC (30 seconds, sub-millimeter resolution) and used to determine water repartition at 80°C_1.6 bar H2/Air 60/40%RH_St. 1.2/2 counter-flow.
Novelty and goals of the subject Difference in cell design
The previous experiments using SANS were conducted in 25 cm² single cells which are conventionally used worldwide to characterize the performance and the durability of the components of the MEA at the laboratory scale. They allow to test the fuel behavior cell at low cost and on reasonable timescales, in conditions as representative as possible of the real setup, yet slightly different, especially regarding the cell geometry. Basically, the size of the cell, and consequently of the MEA, in a real stack is always larger than those characterized previously by SANS.
The integration of MEA into a real size fuel cell leads to additional effects compared to conventional small single cells :
It is not possible to reproduce exactly the design of the flow field channel of a stack in a small size single cell, although it is well-known that if significantly affects the water repartition.
Compared to a single cell, the mechanical stress applied onto the MEA and the local temperature are inherently more heterogeneous in a real stack, which affect both the operation and degradation processes.
Therefore, in the current state of the developments, unveiling the influence of the operating parameters on the water repartition in full-scale experiments is required to improve the design of the stacks.
The repartition of liquid water in the channel of a PEMFC stack has been probed by other groups using neutron imaging. Most of these experiments have been conducting at Paul Scherrer Institute and at NIST on the beamline BT2 neutron imaging facility (NIF) which has been financed by General Motors (GM) [16,27,28]. Nevertheless, only the presence of liquid water can be probed with neutron imaging. In other words, it is not possible to quantify the water content in the membrane, although it is clearly a crucial parameter regarding the performance and durability. Moreover, the time resolution is around 15 minutes, thus much longer compared to SANS experiments. This can lead to some misinterpretation of the water distribution. It is indeed not possible to differentiate water which is accumulating in the probed area and droplets of water which are flowing in front of the beam.
We demonstrated in the last SANS experiments in small single cell that our method allows to reach the time and spatial resolutions relevant to study the behavior of real size PEMFC. So, from now on, efforts can be implemented to perform these studies.
In this post-doc, we plan to perform SANS experiments on a large size single cell with the design and constraints of a prototype stack presenting the best state-of-the-art performance and durability. For the first time, it will be possible to quantify the water content in the membrane and outside the membrane in a real full-scale PEMFC.
Current density distribution
The distribution of the current density will be measured onto the full scale single cell (same design than for SANS experiments) using a commercial device from S++. This kind of measurements is commonly carried out at CEA. The knowledge of the distribution of both current density and water on a common real size cell design is of prime importance for a deep understanding of fuel cell operation and is one of the most original results expected at the end of the post-doc.
Complete 3D characterization of water distribution
We also propose to design a specific single cell in order to perform, for the first time, on the same cell SANS experiments, high resolution and low resolution neutron imaging in order to have a complete view of the three dimensional repartition of water, that-is-to-say water amount within the membrane (through-plane) and within the Anode and Cathode Gas Diffusion Layers (in-plane).
Summary of the goals
This post-doc has two main goals: i/ quantify the effect of operating parameters and ageing on the current density and water distribution in a full-scale single PEMFC fully representative of those integrated in cars, boats or stationary power plants have three main goals (Figure 5). ii/ Design a specific cell to conduct SANS, low and high resolution neutron imaging and perform characterizations in representative operating conditions.
The project will provide, for the first time, crucial information on the water management of real-size PEMFC which are required for the improvement of stack prototypes for transportation and stationary applications.
Figure 5. Picture of full-scale single PEMFC