Volkswagen High Temperature Fuel Cell

Press Release

HIGH TEMPERATURE FUEL CELL IN DETAIL

Advantages of the HT fuel cell. It’s precisely these problems that the fuel cell with high temperature membrane developed by Volks­wagen doesn’t have. This is because the protons are carried via other liquid electrolytes such as phosphoric acid. The acid has good elec­trolytic properties similar to water, yet demonstrates a higher boiling point.

The advantage is that no addition humidification is necessary. In this way, it is possible to increase the maximum operating temperature up to 130 degrees Celsius with no loss in performance. The high temperature fuel cell developed by Volkswagen makes a significant contribution towards making the system lighter, smaller an cheaper.

How the HT fuel cell is made. Simply stated, high temperature membranes are produced by dipping a film into a bath of phosphoric acid. The phosphoric acid permeates the film within a matter of min­utes. The membrane is then integrated into a fuel cell stack. At first a sheet of pressed carbon fiber is first placed on the pre-configured cells. Air will later flow through the grooves in this sheet. Attaching a seal follows as the next step. Then comes a cloth of carbon fiber which is doped with a catalytically active platinum paste, the gas diffusion electrode. It is simultaneously used as a gas distribution layer and cathode. The membrane doped with phosphoric acid is spread over the cathode. Following this is another seal and a cloth­shaped counter electrode, this time used as an anode. Hydrogen flows through the grooves of the last sheet. The back of the sheet is cooled with water. Finally, every cell is pressed together under pres­sure and joined together into a fuel cell stack. Conventional elec­trodes don’t work. But there has been one problem up to now: Simi­lar to the low temperature membrane, product water formed on the cathode. The water permeated the membrane and washed out the phosphoric acid. This in turn interrupted the flow of current. At this point all attempts up to now to make a high temperature fuel based on familiar materials useable have failed. Intensive Volkswagen

basic research came to the result that in addition to a new membrane special modifications of the electrodes are necessary which are able to prevent product water from penetrating the membranes.

New electrodes delivered the breakthrough. The solution: On a special screen printing machine like the ones used in the field of semiconductor technology the researchers at the Volkswagen Tech­nology Center in Isenbüttel coated cloth elements made of carbon fiber with a new type of paste. The newly created electrodes then underwent extensive testing in fuel cell stacks. The clear result: The product water can no longer leach out. HT technology is thus ready for use. The new cells now work in a considerably wider temperature window than fuel cells known up to now. The membrane is also less sensitive to air impurities due to the higher temperatures. Using the high temperature fuel cell developed by Volkswagen approximately one third of overall system components can be dispensed with. This makes HT systems lighter, cheaper and suitable for vehicles. And this make the future of fuel cells exciting.

VOLKSWAGEN TECHNOLOGY CENTER IN ISENBÜTTEL

Fuel cell milestones. Volkswagen has for decades been involved in the area of fuel cell research. The milestones include the Capri Pro­ject (1996-2000, hybrid drive in the Golf Variant with 20 kW fuel cell), the Bora HyMotion (2000, fuel cell hybrid car with 30 kW fuel cell continuous power rating), the PSI Bora in cooperation with the Paul Scherer Institute (2001, driving tests over the 2,005 meter high Simplon Pass with 40 kW fuel cell) and the Touran HyMotion (since 2004, integration of a fuel cell with 65 kW continuous power rating with no restrictions on available space, including field tests in Cali­fornia, China and continuously in Berlin as part of the Clean Energy Partnership).

Technology Center opened in 2001. In order to ideally bundle the forces for this research project oriented towards the far future, in the previous decade the company decided to establish its own technol­ogy center located near its headquarters in Wolfsburg. The choice was made for Isenbüttel located about 15 kilometers away. The Volkswagen Technology Center for fuel cell and electric vehicle technology was established there in 2001 over an area of 38,000 square meters. Investments amounted to 20 million euros alone for testing and measuring facilities.

Ideal infrastructure. The floor space for test stands and vehicle construction is 6,800 square meters and offers sufficient space for every imaginable research and development infrastructure. The of­fice building of the technology center comprises 2,800 square me­ters, offering room for 100 engineers. A test stand park specially geared towards the needs of the high temperature fuel cell supports the testing of all stack (fuel cell stack) and system components. This includes test stands for small, individual laboratory fuel cell and vehicle fuel cell stacks with output of up to 100 kW as well as fuel cell components and complete fuel cell systems. For the development of the high temperature fuel cell there are laboratories and produc­tion lines for individual cells and stacks with all necessary facilities,

from screen printing machine to seal injection robot. Research using special measuring technology such as impedance spectroscopy, cyclovoltametry, current density distribution measurement and gas chromatography are part of the standard program. For research into electric vehicle components an electric drive test stand and a battery test stand were integrated. Fuel cell vehicles can be assembled on lifting platforms and put into operation. Initial tests in the driving cycle and consumption measurements of the fuel cell vehicle can be carried out on a roller dynamometer.

Own hydrogen production and hydrogen filling station: The in­frastructure of the technology center includes a hydrogen filling station, allowing vehicles to be filled with liquid hydrogen at -253 degrees Celsius or compressed hydrogen at 350 bar with the option for upgrading up to 700 bar. The hydrogen for the pressurized filling station is produced regeneratively using solar energy from a 50 square meter photovoltaic field installed on site. The foundations are thus laid under ideal conditions to assist Volkswagen’s high tem­perature fuel cell on its way towards success.