PEFC Systems

A schematic diagram of a methanol-fueled PEFC system is shown in Fig. 27-65. A methanol reformer (to convert CH3OH to H9 and CO9... 

Polymer electrolyte fuel cells (PEFC) deliver high power density, which offers low weight, cost, and volume. The immobilized electrolyte membrane simplifies sealing in the production process, reduces corrosion, and provides for longer cell and stack life. PEFCs operate at low temperature, allowing for faster startups and immediate response to changes in the demand for power. The PEFC system is seen as the system of choice for vehicular power applications, but is also being developed for smaller scale stationary power. For more detailed technical information, there are excellent overviews of the PEFC (1,2). 

When used in a PEFC system, the reformate must pass through a preferential CO catalytic oxidizer, even after being shifted in a shift reactor. Typically, the PEFC can tolerate a CO level of only 50 ppm. Work is being performed to increase the CO tolerance level in PEFC. At least two competing reactions can occur in the preferential catalytic oxidizer ... 

Advances in Fuel Processing for PEFC Systems for Transport," R. Dams, P. Hayter, and S. Moore, Wellman CJB Ltd., Pg. 238, Fuel Cell Seminar Abstracts, Courtesy Associates, Inc., November 1998. 

As mentioned, the primary motivation for the PEFC development was the anticipated applicability in transportation. However, the economics of stationary use are more forgiving, and commercialization of the technology will likely begin as grid-independent power supphes. Figure 24-51 shows a 5-kW PEFC system operating on natural gas. 

FIG. 24-51 Natural gas-fueled 5-kW PEFC system. (Courtesy of Plug Power)... 

FIG. 24-52 System diagram for reformer-based PEFC system. 

PAFC systems are commercially available from UTC Power as 200-kW stationary power sources operating on natural gas. The stack cross section is 1 m (10.8 ft ). It is about 2.5 m (8.2 ft) tall and rated for a 40,000-h life. It is cooled with water/steam in a closed loop with secondary heat exchangers. Fuel processing is similar to that in a PEFC system, but a preferential oxidizer is not needed. These systems are intended for on-site power and heat generation for hospitals, hotels, and small businesses. 

The most important electrolyte property is ionic conductivity. For the PEFC system, water and proton transport in the polymer electrolyte occurs concurrently. Springer et al. correlated the proton conductivity (in S/cm) in the polymer membrane with its water content as follows... 

Two other important electrolyte properties for the PEFC system are the water diffusion coefficient and electro-osmotic drag coefficient. These two param-... 

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

In the PEFC system, the mean pore radii of catalyst layers are of the order of 0.1 pm. The Knudsen diffusion coefficients at 80 °C for O2 and H2O through the catalyst layer are thus estimated to be 0.32 and 0.43 cm /s, respectively. These values are comparable to the respective ordinary diffusion coefficients, indicating that Knudsen diffusion further restricts the rates of oxygen and water transport through the cathode catalyst layer in PEFCs and should be taken into account. 

R D on PEFC System. METI and NEDO will continue R D on fuel cells to meet performance targets, durability, and cost reductions toward future commercialisation, primarily focusing on PEFCs, R D for relevant system and production technologies, and components such as membrane, electrode, MEA and separators. The budget allocations for 2001-2003 total 14.7 billion the 2003 allocation is 5.11 billion. 

Establishment of codes and standards (Millennium Project). In order to promote the establishment of codes and standards, the project focuses on R D on testing methodologies and data acquisition for PEFC vehicles and stationary PEFC systems. The goal is to promote international standardisation, fuel standard studies, establish performance and safety testing methods for public use, and reviewing of regulations. The 2003 allocation is 3.87 billion. 

For successful operation a selective CO oxidation catalyst in a reformer-PEFC system must be operated at ca. 353-373 Kin a complex feed consisting of CO, 02, H2, C02, H20 and N2, and be capable of reducing CO concentrations from about 1% to below 50 ppm - this is equivalent to a CO conversion of at least 99.5% [4, 54, 60], In addition, this conversion must be achieved with the addition of equimolar 02 (twice the stoichiometric amount) and the competitive oxidation of H2 must be minimized. This is expressed as selectivity, which is defined as the percentage of the oxygen fed consumed in the oxidation of CO for commercial operation a selectivity of 50% is acceptable, since at this selectivity minimal H2 is oxidized to water. 

The PEFC system itself will have to face the consequences of irreversibility due to lack of circulators. Crucially, the system must have a hydrogen mine . Perhaps Ballard will need to join with Methanex and Air Products to solve this major development and capital investment problem. There are no signs of that happening. 

Recupero, V., Pino, L., Vita, A., Cipiti, F., Cordaro, M., and Lagana, M. Development of a LPG fuel processor for PEFC systems Laboratory scale evaluation of autothermal reforming and preferential oxidation subunits. International Journal of Hydrogen Energy, 2005, 30 (9), 963. 

A number of different methods exist for the production of catalyst layers [97-102]. They use variations in composition (contents of carbon, Pt, PFSI, PTFE), particle sizes and pds of highly porous carbon, material properties (e.g., the equivalent weight of the PFSI) as well as production techniques (sintering, hot pressing, application of the catalyst layer to the membrane or to the gas-diffusion layer, GDL) in order to improve the performance. The major goal of electrode development is the reduction of Pt and PFSI contents, which account for substantial contributions to the overall costs of a PEFC system. Remarkable progress in this direction has been achieved during the last decade [99, 100], At least on a laboratory scale, the reduction of the Pt content from 4.0 to 0.1 mg cm-2 has been successfully demonstrated. 

Sasaki H, Soga T, Yatake T, Kano A (2002) Development of 30 kW class PEFC system with pure hydrogen fuel. FCDIC Fuel Cell Symp Proc 9 117-121... 

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