Fig. 1: Fuel cell designed for X-ray scattering experiments. In the center, the 6 recesses specially made for the beam. Typical size of the cell is 10 cm.
A fuel cell generates electricity, heat and water. Water distribution strongly impacts the performance of the battery: the how and why deserves attention! Researchers at Inac and Liten had already put an operating fuel cell in a neutron beam. Now they have placed the system in an X-ray beam at ESRF. The spatial resolution improves from the centimeter to the millimeter, the temporal resolution from 10 minutes to15 seconds. X-rays thus provide observations of the water at the heart of the battery with very fine accuracy and in transient regime.
Fig. 2: Small-angle X-ray scattering spectra. When the current density increases, the peak grows and shifts to smaller angles, indicating the swelling of the membrane, much more marked under the rib than in the channel.
Water is a reaction product of the proton exchange membrane fuel cell (PEMFC): it is formed at the cathode. It is also the vector of proton transport: without water, the electrolyte membrane does not conduct. Water is however inhomogeneously distributed (see inset). This uneven distribution causes serious performance and degradation problems. Knowledge of local operating conditions by non-intrusive measurement methods is essential for understanding the degradation mechanisms and for optimizing PEMFC operation.
For several years, Liten and Inac have worked together to determine the water concentration profiles in the PEMFC membranes. The analysis of the early work done by neutron scattering revealed non-uniform concentration profiles in the thickness of the membrane. The origin of these profiles is the competition between the electro-osmosis process, i.e. the process that drives a flow of water molecules from the anode to the cathode by the protons, and the backscattering process, i.e.the diffusion of water from the cathode to the anode under the effect of a concentration gradient.
Fig. 3: Microfluidic simulations predicting the amount of water in the membrane: it is higher under the rib than in the channel. On both sides of the membrane are the electrodes and gas diffusion layers.
More accurate and faster
In-operando characterization of a fuel cell has recently been adapted to small angle X-ray scattering (Fig. 1). Why X-rays? To significantly improve the spatial and temporal resolution. Spatially, the area of analysis is reduced from 1 cm² with neutrons to 1 mm² with X-rays. Therefore, a much finer local analysis is possible. For instance, it is possible to compare the amount of water under a rib and in front of a channel of the monopolar plate. As for the temporal aspect, with X-rays, measurementsevery 15 s are possible. This is a very important asset for the study of transient phenomena: when you switch on the battery for example, or when studying the kinetics of hydration. The experiment performed at ESRF is the only non-invasive technique delivering this kind of information with such a resolution for an operating fuel cell.
Two important results have already been obtained:
• At the cathode, the amount of water in the membrane increases between the inlet and the outlet, although water is generated continuously and everywhere in the membrane;
• The amount of water is always higher in front of a rib than in front of a channel (Fig. 2)
These results are in good agreement with fluid modeling of water transfer through the various parts of the fuel cell (Fig. 3). Further analysis of the results will specify the local water concentration profiles in the membrane. Later on, these studies will be carried out for thin membranes and operating temperatures below 0 ° C and above 100 ° C.
Water and gases: where do they go?
The anode and the cathode of a PEMFC are both composed of a monopolar plate in which a gas circulates. It is distributed in a serpentine channel, while the ribs press the stack consisting of a gas diffusion layer (porous carbon cloth), an electrode where the chemical reaction takes place (carbon and platinum catalyst) and finally the proton exchange membrane, impermeable to gases.
The heterogeneity of the water concentration in a cell occurs as well between the two electrodes - the cathode is always wetter than the anode - as between gas inlet and outlet. The water is extracted by gravity and is recycled in particular for humidifying gases.
Maj : 17/02/2014 (924)