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 totest 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 quantifythe water content in the membrane,althoughit isclearly a crucial parameter regarding the performance and durability. Moreover, the time resolution is around 15 minutes,thusmuch 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 spatialresolutions 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.

Date: 2015-01-11; view: 853