PEFC, design

A bipolar conflguration similar to the PEFC design but for liquid electrolytes was developed by the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt, DLR) [24, 25]. 

In most PEFC designs, the reactant gases are actively supplied to the stack by feeding a given gas flow. The gas mass flow exceeds the quantity strictly needed for the electrochemical reaction, so that a sufficient concentration is present in the outlet section of the cell. Ideally, the gas flow should be distributed homogeneously to the flow channels of a cell and between the cells of a stack, but various processes can lead to inhomogeneous distributions as described below. 

The history of research on polymer electrolyte fuel cells spans about 50 years. PEFCs appeared in the focus of scientific interest toward the end of the 1980s. Generally, PEFC design is simple and all the needed components are available on the market. Take two gas-diffusion electrodes separated by a polymer electrolyte membrane and clamp this membrane-electrode assembly between two graphite plates with channels for hydrogen and air supply—the cell is ready. 

Figure 6.47 Planar stack design with open cathode design for natural breathing. Note the use of cell-to-cell interconnects instead of structural bipolar plates common to PEFC designs.

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

Detailed cost and design studies for both PEFC and SOFC systems at sizes ranging from 5kW to 1 MW were made that point to the fundamental differences between PEFC and SOFC technology that impact the system design and by implication the cost structure. These differences will be discussed in the following paragraphs. 

A. Turhan, K. Heller, J. S. Brenizer, and M. M. Mench. Passive control of liquid water storage and distribution in a PEFC through flow field design. Journal of Power Sources 180 (2008) 773-7 3. 

The previous discussion asserts that design, fabrication, and implementation of stable and inexpensive materials for membranes and catalyst layers are the most important technological challenges for PEFC developers. A profound insight based on theory and modeling of the pertinent materials will advise us how fuel cell components with optimal specifications can be made and how they can be integrated into operating cells. 

Two common threads will connect the various aspects considered in this chapter the pivotal yet double-edged role of water for the operation of PEFCs and the hierarchy of scales that has to be considered in theoretical modeling, physicochemical characterization, and materials design, as illustrated in Figure 6.2. ... 

Distributions of water and reactants are of high interest for PEFCs as the membrane conductivity is strongly dependent on water content. The information of water distribution is instrumental for designing innovative water management schemes in a PEFC. A few authors have studied overall water balance by collection of the fuel cell effluent and condensation of the gas-phase water vapor. However, determination of the in situ distribution of water vapor is desirable at various locations within the anode and cathode gas channel flow paths. Mench et al. pioneered the use of a gas chromatograph for water distribution measurements. The technique can be used to directly map water distribution in the anode and cathode of an operating fuel cell with a time resolution of approximately 2 min and a spatial resolution limited only by the proximity of sample extraction ports located in gas channels. 

For optimal design and operation of a SOFC, a fundamental and detailed understanding of transport and electrochemical kinetics is indispensable. Efforts are presently underway to understand the multiphysics and obtain the optimal design for SOFCs. For these purposes, a CFCD model, similar to those for PEFCs and DMFCs, becomes a valuable tool for design and operation of SOECs. 

One study specifically designed for PEFC was reported by Thompson et al.8 They used a direct current to measure the proton conductivity at low temperature. In conjunction with the DSC data, they found the dependency of crossover temperature (temperature where the activation energy changes) on water content and hysteresis between freezing and melting. 

The aforementioned numerical experiments, namely quasi-static drainage and steady-state flow simulations, are specifically designed to study the influence of microstructure and wetting characteristics on the underlying two-phase behavior and flooding dynamics in the PEFC CL and GDL. 

Two primary goals of atomistic modeling of PEFC are, first, to supplement the experiments performed in laboratory to study what has not been or cannot be experimentally studied and second, to conduct a computer design followed by virtual tests that the experiment in laboratory is difficult or impossible to be performed under current status of technology. This brings about a dilemma on the one hand, atomistic models can describe a phenomenon as microscopic as possible at the atomic level and on the other hand cannot describe the phenomenon in a system with a size as macroscopic as possible and in a timescale as long as possible. 

The early evidence on Ballard thinking about flexible graphite is presented by Wilkinson etal. (1996a, b). The latter patents are offspring of the 1994 370 Patent , well known in the PEFC business (Washington etal., 1994), and which sets out alternatives in flow plate design. See Figure 6.3. 

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