Necari

operation in an automotive environment. Included are frequent start-up/shut-down, rapid load changes, and vibration/accelerations resulting from both normal operation and impact.

Elaboration of the technology was conducted in four main technical areas; cell development, membrane electrode assembly (MEA), stack engineering, and manufacturing process development.

Cell Development

Work started with investigation and screening of many design concepts, bipolar plate designs, and flow field techniques to increase the specific power output, and reduce the volume and weight of the cell. For example, a new flow field pattern was devised to make optimal use of the envelope area, resulting in an increase of area utilization from about 36% to greater than 80%. Area utilization is defined as the ratio of electro-chemically active area to cell-envelope area. The remaining area is used for fluid headers, seals, and tie-rods. The increase was made possible by a novel cell arrangement. The number of tie-rods was reduced from sixteen to four, the tie-rods were located inside the gas headers, and the flow fields were routed around the headers.

Membrane Electrode Assembly

The MEA development focused on designs to consistently provide good performance, under all operating conditions, using the various fuel gas mixtures for the required lifetime. Many different MEA designs were developed using combinations of a variety of the basic elements (membranes, catalysts, and gas diffusion electrodes).

Designs were initially proven on a I-cell basis, using beginning-of-Iife performance as the first screen, and Mk 5 V-I curves as the performance baseline. Promising candidates were run for 1000 hours, to ensure adequate performance stability, and mechanical integrity over the period. Preferred designs were operated in ten-cell stacks to prove operation in the serial configuration, and to give statistical significance to the data.

Stack Design

Using successful designs of cells and MEAs, stack designs were produced. Included are details of the MEA, the bipolar plates, seals, stack endplates, compression hardware, and tooling to enable repeatable manufacture and dependable stack assembly. Success is determined by the ability to produce stacks of approximately 150 cells that give full performance projected from 10-cell stacks.

Manufacturing Process Development

Manufacturing process development was conducted concurrently with the evolution of the design. Early in the program, a study was conducted to review the manufacturing capabilities for the Mk 5 design, and project the suitability to the requirements for the emerging Mk 7 concepts. Considerations included repeatability, reproducibility, tooling requirements, capacities, capabilities, material logistics, and quality plan. A second study culminated in a model for production volumes and cost reduction in various phases. These included laboratory scale, pilot scale, pre-commercial, and finally full commercial production, representing increasing volumes and decreasing costs over the next 15 years.

As concepts evolved to optimize the Mk 7, designs were reviewed regarding manufacturability. Processes for manufacture at the increasing rates were conceptualized. Vendors were sought out and qualified. Processes suitable for the laboratory scale production required during the course of this project were developed in detail. Suitable tooling was specified and purchased when possible, modified if necessary, or designed and built otherwise. Prototype parts were produced and checked to determine capability and cycle times of the processes, the tooling, and the operators.

Verification of Mk 7 Technology

Many cells, short stacks, and full sized stacks were built and tested during the program. The Mk 7 design has been tested for over 1 million cell-hours. Individual cells and 10-cell stacks have been operated continuously for more than 2500 hours. To date,-production has included 17 full sized stacks, dozens of short stacks ranging from 10 to 50 cells, and hundreds of single cells totaling thousands of flow field plates and MEAs.

In addition to electrical performance and durability, many other aspects of the design have been verified. These include long-term compatibility of materials, corrosion resistance, resistance to vibration and impact, exposure to temperature extremes (including freezing), electrical isolation, influence of cycling loads/fluid pressures.

A photo of the Mk 7 side-by-side with the Mk 5 predecessor is shown in Figure 1, and a summary of specifications of the two fuel cell stacks is contained in Table 1.

Figure 1: Mk 5 and Mk 7 Fuel Cell Stacks

Specifications

Mk5

Mk 7

Length (mm)

0 0

Post a comment