Volkswagen Research has developed a PEM (proton exchange membrane) fuel cell that operates at temperatures about 50% higher than the conventional low-temperature fuel cells prevalent in current hydrogen fuel-cell vehicle prototypes.
Low-temperature PEM fuel cells (LTFC) operate at a membrane temperature of approximately 80° C. If the temperature greatly exceeds this value, fuel-cell performance breaks down and the fuel cell can be irreparably damaged. Accordingly, vehicle prototypes with low temperature fuel cells have sophisticated and expensive cooling systems.
Furthermore, in LT systems the supply of hydrogen gas and air must be continuously humidified; humidification systems also add additional weight and cost.
In contrast, the high temperature fuel cell (HTFC) membrane developed by Volkswagen can, in combination with newly designed electrodes, operate at temperatures of up to 120° Celsius with no loss in performance and without humidification.
The HTFC uses phosphoric acid as the medium for the exchange of protons. The acid has good electrolytic properties similar to water, yet demonstrates a higher boiling point—the reason that a significantly simpler cooling system and water management is sufficient for the HTFC. The space required for the fuel cell system is also reduced by more than 30%.
The high temperature fuel cell independently developed by Volkswagen in seven years of research work will make the overall system in the car lighter, more compact, stable and cheaper. And those are the decisive criteria for putting fuel cells on the path towards mass series production.
We believe that the high temperature fuel cell is part of the future. In contrast, we no longer give much chance to low temperature fuel cells going into series production.—Prof. Jürgen Leohold, head of Volkswagen corporate research
A persistent obstacle in the development of the HTFC was the formation of product water via the reaction, which permeated the membrane and washed out the phosphoric acid. This, in turn, interrupted the flow of current.
Volkswagen’s breakthrough was to modify the electrodes to prevent the penetration of the product water. Volkswagen researchers used a special screen printing machine like the ones used in semiconductor production to coat several cloth elements made of carbon fiber with a new type of paste. The Volkswagen team found that the modified electrodes prevented the product water from penetrating the membrane.
Volkswagen Research expects to have its HTFC systems in research vehicles by 2010, and production versions ready for the public by about 2020.
Volkswagen has been involved in fuel-cell research for more than a decade. 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 and China).
The company’s resulting experience with low temperature fuel cells was instrumental in focusing its energy toward the development of high temperature fuel cell systems.