Volkswagen High Temperature Fuel Cell

Press Release

Volkswagen Research has developed a type of high temperature fuel cell (HTFC) that is one of a kind in the world. It eliminates numerous disadvantages of the low temperature fuel cells (LTFC) previously known. A new high temperature mem­brane and electrodes specially adapted for this membrane will thus enable significantly more compact, cheaper and more efficient fuel cell systems for cars. The breakthrough in this type of propulsion has come a bit closer. However, the prognoses of many manufacturers for when the hydrogen fuel cell will be ready for series production and available on a grand scale have been repeatedly revised due to the imponderablities of research. This is why Volkswagen is sticking to the facts:

1999. VW Research begins the program for the development of a high temperature membrane.

2001. At the end of 2001 Volkswagen decided to carry out independ­ent development of the high temperature fuel cell – starting with basic research on the topic of membrane technology.

2003. Three years ago Volkswagen researchers achieved significant success in membrane development. But suitable electrodes were missing as the key to implementation.

2006. Today, in the Autumn of 2006, the electrode problem has for the most part been solved. The results are very promising: The high temperature fuel cells are currently being developed and tested in the Volkswagen Technology Center in Isenbüttel, specially constructed for the research of alternative drive systems and located at the gates of Volkswagen headquarters in Wolfsburg.

A peek into the future could look like this:

In about 2010. More higher performance high temperature fuel cell systems come about that are perfected step by step and will power the first research vehicles in 2010.

In about 2020. The first Volkswagens with a fuel cell drive that is affordable and suitable for everyday use – the decisive factors – could appear. Volkswagen sees no chance for the mass series pro­duction of low temperature fuel cells (LTFC) primarily being used by other car makers today due to the disadvantages related to their conceptual design.

Disadvantages of the LTFC. Low temperature fuel cells are oper­ated at a membrane temperature of approx. 80 degrees Celsius. If the temperature greatly exceeds this value fuel cell performance breaks down and irreparable damage is done to the fuel cell . This is why LT fuel cell vehicle prototypes – should they be able to pass driving test cycles similar to a combustion engine – place very high re­quirements on the cooling system, making it very expensive. In addi­tion, in an LT system the supply of hydrogen gas and air must be continuously humidified, because otherwise the production of energy will break down, permanently damaging the fuel cell and bringing the electric engine being powered to a stop. This humidification also takes space, weight and money.

Advantages of the Volkswagen HTFC. The high temperature mem­brane developed by Volkswagen can in combination with newly de­signed electrodes be “driven” at temperatures of up to 160 degrees at the same output of power. A medium operating temperature of 120° C is intended for vehicle operation. And this without additional hu­midification. A distinctly simpler cooling system and water man­agement is sufficient here, significantly reducing the need for space, weight and money!

FUEL CELLS IN GENERAL /

Chemical energy turns into electrical energy. To more precisely classify the advantages of the high temperature fuel cell will require a consideration of the general workings of fuel cells. The central element of each individual fuel cell – many of which are combined into a block (stack) – is a proton exchange membrane. It is located between the anode and cathode of each fuel cell. Hydrogen flows into the fuel cell on the anode side and the cathode is supplied with air. Many of these cells in combination generate enough energy to power a vehicle. Hydrogen and oxygen react inside each cell, pro­ducing water on the cathode side. Energy is released in this process. The fuel cells thus convert the chemical energy of an oxidation proc­ess, known as “cold combustion”, directly into electrical energy. The “exhaust” produced is nothing more than clean water vapor.

Electric, not combustion engine. The fuel cell is supplied via a hydrogen tank and an external air intake. The electrical energy – the power – generated by the fuel cell is delivered via a converter and a downstream static inverter to one or more electrical engines. Conse­quently, the car runs virtually without making a sound, but definitely emission free.

FUEL CELLS IN DETAIL

General process. Hydrogen is split into electrons and protons on the anode. The positively charged protons travel through the membrane to the other electrode, the cathode. The negatively charged electrons flow through an external circuit to the cathode. This current flow powers the electrical engine of the car. On the cathode the protons react with the inflowing oxygen and the electrons to form what is called product water, which for the most part escapes from the ex­haust pipe. Approximately sixty percent of the energy employed in the form of hydrogen is converted into electricity.

Disadvantages of the LT fuel cell: In the low temperature fuel cells primarily employed up to now the protons are carried from the anode to the cathode in the membrane via a short connection of the protons with the water in the membrane. To prevent the membrane from dry­ing out the reaction gasses hydrogen and air must therefore be hu­midified. This results in two decisive disadvantages: The membrane may not, as previously outlined, heat up to above 80 degrees Celsius. The result is a very small difference in temperature between the cooling medium and surrounding air. Continuous hill climbing and trailer operation are virtually impossible. Yet in order to attain suffi­cient cooling performance LT fuel cell systems – if being used for the everyday operation of a normal car – need a cooling surface ap­proximately three times as large as for a diesel engine (!). This is without taking into consideration situations such as hill climbing and the necessarily higher cooling performance associated with it.

Cooling problem in detail. Cars with conventional combustion en­gines produce more waste heat than vehicles with fuel cell powered electrical engines. Combustion engines can release this waste heat to the environment via the engine cooling system and exhaust fumes. The fuel cell does not have this option. As a consequence of the comparatively lower operating temperature, heat is almost exclu­sively released via the cooling system, but not via the exhaust sys­tem. The result is that at the same engine output more than twice the amount of heat has to be discharged via the car radiator. And this holds true despite the efficiency advantage of the fuel cell.

LT fuel cell gasses must be continuously humidified. There is another problem: The inflowing gasses hydrogen and air dry out the electrolyte – the water molecules stored in the membrane. And this also disrupts the flow of current. This is why a special unit humidi­fies the inflowing gasses. For these reasons, apart from the actual fuel cell other complex technology must be integrated in the vehicle. The result is that the overall system is heavier.

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.