Fuel Cell

How does a fuel cell work?

Cell geometries

Types of fuel cells

Energy storage

Energy density

Strengths and weaknesses of fuel cells

Competitive technologies

How does a fuel cell work?

Hydrogen (H₂)and oxygen (O₂) react in a so called 'cold combustion' to water (H₂0) by providing energy.
The overall efficiency is app. 60%, while combustion engines operate at app. 35% (Carnot diagram). The energy that a fuel cell generates is available as electrical power and heat. Dependand of the actual load conditions of the split of electrical power and heat changes. The high efficiency, the direct generation of electrical power and the low to zero emissions make the fuel cell a superior technology.

Fuel cell structure (Source: www.ballard.com)

The fuel cell principle is the reversal of the water electrolysis, where water is split into hydrogen and oxygen by applying electrical power. The following reactions occur at the anode and cathode of a PEM fuel cell.

Fuel cell principle (Source: www.ecosoul.org)

Anode (hydrogen side)

In a first step, moleculare hydrogen H₂ is split in 2 hydrogen ions H⁺ and 2 electrons e^- under catalytic influence (e.g. Platin).

Reaction equation anode: 2 H₂ -> 4 H⁺ + 4 e^-

Electrolyte (membrane)

In a PEM fuel cell the electrolyte is conductive only for protons, therefore the positively charged H⁺ ions pass thru the membrane from the anode to the cathode. The negatively charged electrons have to travel thru the external load by providing electrical power.

Cathode (oxygen side)

The reaction on the cathode is much more complex. In principle a oxygen molecule O₂ from the surrounding air reacts with 2 H⁺ ions to H₂O₂ by consuming 2 electrons e^-. In a second step H₂O₂ reacts to 2 H₂0 (water) moelcules by consuming another 2 H⁺ ions and 2 electrons e^- .

Simplified cathode reaction equation: O₂ + 4 H⁺ + 4 e^- -> 2 H₂O

At the cathode is a lack of electrons while on the anode we have a surplus on electrons. If anode and cathode are electrically connected, a electrons pass from the anode to the cathode to balance this difference in electrical charge. This current flows as long as the reaction is taking place or in other words as long as hydrogen and oxygen is available to the anode/cathode of the fuel cell. A electrical load connected to this circuit makes it possible to utilize the so generated electrical power.

The theoretical open circuit of a fuel cell is 1,229V, in real applications it is possible to achieve app. 1,05V operating with pure oxygen and app. 0,95V operating with air. To achieve higher voltage levels, which is often desired to drive standard loads, several single fuel cells are connected together in series to a fuel cell stack.

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Cell geometries

While the basic structure of fuel cells always remains identical, the geometrie of a cell can be quite different. The basic geometries can be divided as such:

• Planar cell
• Tubular cell
• Coil cell

Planar single cell (Source: www.innovation-brennstoffzelle.de)

The most commonly used fuel cell geometrie in the planar cell. In a planar cell, anode, electrolyte (membrane) and cathode are put together in a sandwich structure.

Planar fuel cell stack (source: www.innovation-brennstoffzelle.de)

To achieve higher output voltages and higher power planar single cells are often combined to planar fuel cell stacks. In doing so multiple single cells are stacked togther and form a planar fuel cell stack. The planar fuel cell stack is electrically a serial connection of a certain number of single fuel cells. Therefore it is fairly easy to realize almost any output voltage requirement, since in a serial connection the single cell voltage just add up. For example, if you need a open circuit voltage of app. 17V (translates to a nominal voltage of app. 12V, since the MPP of a single cell is around 0.7V), 17 single cells need to be connected in series to form a stack with the desired output voltage. In theory planar stacks could consist of hundredes of single cells, in real applications there are limits caused by thermal management (heat removal), gas flow (hydrogen and oxygen) and water management (PEM fuel cells).

A second possibilty to combine multiple single planar cells is the so called stripe cell. Here the single cells are not piled together but rather combined in a single level. This geometrie is used when very flat geometries are needed (for example fuel cell to be integrated in laptop screen). sind.

With the tubular cell the elements anode, electrolyte and cathode form a tube. The inner side of the tube is normally the anode layer (hydrogen side) then covered by the electrolyte layer followed by the cathode layer (oxygen side).

Similar to the planar cells it is possible to connect multiple single tubular cells in series to form a array of tubular cells. Today tubular cells are utilized by the SOFC from Siemens Westinghouse.

Tubular cell (Source: www.h2-interpower.de)

The coil cell is another interesting geometrie for fuel cells. In principle the coil cell is a long planar cell, which is then wraped together like a coil. The coil geometrie promises low cost volume production, easy replacement of defect single cells in a series connection, but has also some technical problems like electrical connections to solve. The idea and development of the coil cell started at H2-Interpower in Schwabach near Nuremberg, Germany.

Coil cell (Source: www.h2-interpower.de)

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Types of fuel cells

Even so all fuel cells share the basic function principles there are many different fuel cell types available. This is similar to the batterie technology where a lot of different batterie types do exist. In the fuel cell technology the electrolyte is used to determine which type of fuel cell type it is. The following table provides an overview on the different fuel cell types:

The various types of fuel cells are suited for different applications. The PEMFC offers low operating temperature and quick turn on times but require very clean hydrogen fuel since CO content does damage the noble catalysts. These features make the PEMFC a good fit for portable and mobile applications, while high power stationary systems are more suited for SOFC or MCFC.

Fuel cell types like SOFC and MCFC are operated at much higher temperatures and therefore do not need noble metal catalysts. For this reason it is possible to use hydrogen rich fuel which may contain CO_x und NO_x, which normally would damage the noble metal catalysts. Therefore these fuel cells can be fueled with natural gas or require only little external reforming. While the fuel cells operating at higher temperatures are less sensitive to CO_x und NO_x, the turn on time and power on and off cycles are much off a problem. Each power on and off cycle may degrade and reduce the performance of such a cell, therefore they are best suited for a uninterrupted operation.

The following table lists the key manufacturers by fuel cell type:

Energy storage

Today the storage of hydrogen is the biggest challenge for the broadrange introduction of fuel cell technology. While we have a working infrastructure for liquid fuels such as gasoline, oil or diesel the broad availability of hydrogen will be a very demanding task. For pure hydrogen the following storage concepts are available:

• Metal hydrid storage
• Compressed hydrogen (CGH₂)
• Liquid hydrogen (LH₂)

Metal hydride have the ability to bind hydrogen molecules in their atomic structure under certain conditions. These conditions includes certain pressure level and temperature conditions. When filling up a metal hydride storage tank thermal energy generated and the tank needs ro be cooled to a certain temperature level. In reversal thermal energy is needed to release hydrogen from the metal hydride storage tank. In a lot of cases the hydrogen release rate at room temperature is sufficient, while in other cases external heat sources (for example the fuel cell itself) are needed. The process of charging and discharging a metal hydride storage tank is a reversible process. Metal hydride storage tanks can store up to 500x of the actual tank volume at pressure levels only slightly above atmospheric pressure.

Storage of compressed hydrogen in steel tanks is a long known technology. Advanced technologies allow pressure level up to 70MPa (700 bar) and can store up to 700x of the tank volume. Currently research is done on new materials for the tanks to increase pressure levels and reduce weight.

Liquid hydrogen LH₂ is a the most compact way to store hydrogen. The energy to volume ratio of LH₂ is app. 1/4 to that of gasoline. In addition LH₂ allows to fill up the tank quickly. On the other side the storage tank for LH₂ is quite complex, because it needs excellent thermal insulation (boiling point of hydrogen is at - 252,77 °C). The generation of LH₂ requires extra energy to convert the hydrogen gas into the liquid stage.

Methanol is a pretty good alternative to hydrogen and can be used for DMFC's as well as other fuel cell types when a reformer is used. The use of methanol does reduce the storage problem significantly. The energy to volume ratio of methanol is 4,4kWh/l and is app. 1/2 that of gasoline. Methanol has a boiling point of about 65 °C and can be stored in tanks easily.

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Energy density

For a better understanding of the the competitive technologies such as fuel cell, batteries (incl. metal-air-cells) and combustion engines the specific energy density of the utilized fuels are very important. The following table provides key figures about energy to volume and energy to weight ratios of various fuels.

When looking at these figures it becomes obvious, that beside the efficiency of the energy conversion technology, the fuel is a key factor to determine, which technology is suited for a targeted application.

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Strength and weaknesses of fuel cells

Pro

• Exhaust is pure water(H₂O)
• No CO_x or NO_x emissions (when pure hydrogen is used)
• High efficiency
• No to little noise

Pro

• High overall efficiency >60%
• High efficiency at partial load
• Direct generation of electricity
• No moving parts, reliability
• Mobile, portable and stationäre applications possible (from mW to MW)

Con

• Price
• Hydrogen infrastructure suffers

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Competitive technologies

Pro

• Proven technology
• Compact size
• Fuel infrastructure available

Con

• CO_x and NO_x emissions
• Overall efficiency of max 35%
• Fossile fuels are limited

Pro

• Proven technology
• No noise

Con

• Limited lifetime
• Use heavy metals, critical to reycyle
• Limited in capacity and power

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