Water management issue

PEMFC is now a commercial product

As a result of intense research and development activities, the PEMFC is a large scale commercial product since few years. Several thousands of small power stationary plants units are installed in Japan since 4 years. Hyundai and Mercedes have prepared the introduction of fuel cell cars in the market within 2014-2015.

The CEA has developed fuel cell stacks and systems for several years, and successfully implemented them in prototypes of cars, boats or stationary power plants (Figure 2b). The next generation of CEA stack provides a power density as high as 2.5 kW/l and 2 kW/kg thanks to the use of optimized embossed stainless steel bipolar plates integrating the cooling circuit and the gas distribution of both anode and cathode in less than one millimeter in thickness. Durability tests are conducted during several thousands of hours. These specifications are at the best state-of-the-art level. However, these advertisements and good results must not hide the reality. PEMFC is a costly technology and cost reduction often goes at the expense of cost of performance and/or durability. The need of improvement is still crucial and still requires a deep understanding of the system components and behavior.

Current research shows that one of the main issue affecting power output, stability, and lifetime is the amount and distribution of water in the system, which are both strongly affected by the sorption and transport properties of the polymer electrolyte.The water management, i.e., the ability to maintain the dynamic balance of water in the membrane-electrode assembly (MEA) during operation in order to achieve proper membrane hydration without causing electrode flooding [2], is a critical issue affecting both performances and durability of low temperature FCs [3-8]. The great difficulty in developing an effective control of the water management is due to the complexity of the PEMFC operation, resulting from various, correlated, multiphysical, and multiscale phenomena. The overall amount of water within the MEA depends on a number of operating parameters as current density, hydration and flow of the inlet gases, temperature, pressure, etc. But, under given working conditions, the local water content is also related to the spatial heterogeneity of the cell, i.e., the design of the gas distribution channels and the layered structure of the different porous media. Reactants are progressively converted to products from the inlet to the outlet, so that water activity is expected to increase along the in-plane direction. But, at the same time, water is redistributed between anode and cathode through the membrane submitted to chemical and electrical gradients (Figure 3).

The consequence of the coupling of all phenomena is a fully 3D water repartition, inducing heterogeneity in FC performance (current density distribution) [9-13] and degradation [14,15].

Figure 3. Schematic representation of the 3D water repartition in a PEMFC single cell.

Thus, the need for a fundamental understanding of water transport in the PEMFC membrane-electrode assembly has motivated, over the past few years, the development of new operando diagnostic tools sensitive to the local water content in the components of the cell, namely in the channels of the gas distributors and in the gas diffusion layers, in the anode, the cathode and the membrane constituting the Membrane Electrode Assembly (MEA).

A variety of techniques (magnetic resonance imaging, confocal spectro-microscopy, small-angle neutron scattering,...) are available, with specific limitations: they are more or less intrusive and can be used in more or less representative conditions (cell geometry, membrane thickness, temperature, current density) [16].

Дата добавления: 2014-12-23