Proton exchange membrane fuel cell

The polymer electrolyte fuel cell ( engl. Polymer Electrolyte Fuel Cell, PEFC, and proton exchange membrane fuel cell, Eng. Proton Exchange Membrane Fuel Cell, PEMFC or solid polymer fuel cell, Eng. Solid Polymer Fuel Cell, SPFC ) is a low temperature fuel cell.

History

The PEMFC was developed in the early 1960s at General Electric. Willard Thomas Grubb developed in Schenectady ( New York), an ion exchange membrane on the basis of sulfonated polystyrene, on which Leonard Niedrach was three years later deposited platinum. In the English literature of this type of fuel cell is named in honor of the two GE scientists also Grubb Niedrach fuel cell. The mid-1960s came the polymer electrolyte fuel cell in the American space flight project Gemini for the first time used.

Principle

Using hydrogen (H2 ) and oxygen ( O2) is dry converted into electrical energy. The electrical efficiency is about 60 percent, depending on the operating point. As an electrolyte, in this case normally serves a solid polymer membrane, such as Nafion. The operating temperature is in the range of 60 to 120 ° C, preferably for continuous operating temperatures between 60 and 85 ° C are selected. The membrane is coated on both sides with a catalytically active electrode, a mixture of carbon (soot) and of a catalyst, often a mixture of platinum or platinum and ruthenium ( PtRu ) electrodes, platinum and nickel ( PtNi electrode ) or platinum and cobalt ( PtCo electrodes). H2 molecules dissociate on the anode side and are oxidized to two protons with the release of two electrons. These protons diffuse through the membrane. On the cathode side, oxygen is reduced by the electrons which have previously carry out electrical work in the external circuit; together with the transported through the electrolyte is water protons. In order to use the electrical work, the anode and cathode are connected to the electrical consumer.

Reaction equations

The internal charge transport takes place by means of oxonium ions. On the anode side, the reaction requires water, which it emits on the cathode side. To meet the demand for water on the anode side, a complex water management is required. This is realized inter alia by back diffusion through the membrane and humidification of the reactants.

Applications

Main applications are mobile applications without use of waste heat, such as in fuel cell vehicles, submarines, spaceships or Akkumulatorladegeräten are on the go to see. Even stationary small plants with a waste heat level by 60 to 80 ° C are possible. In order to achieve an industrially relevant electrical voltage, a plurality of cells ( tens to several hundred ) to a so -called stack (German: Stack ) are connected in series. The temperature control of the stack is carried out in a separate additional cooling circuit.

It is also a warm-led, stationary use, eg in houses, for a possible Nutzwärmeniveau of 80 ° C, wherein heat and electric current from bio-hydrogen, or hydrogen, which is produced by the Kvaerner process from natural gas, are produced in about the same ratio. This is a form of combined heat and power generation, in which an overall efficiency of more than 90 percent is realistic.

CO tolerance

Since the reactions at relatively low temperatures run ( 60-120 ° C), the tolerance for carbon monoxide ( CO), represents a problem, the CO concentration of the cathode - side supply air and the supplied to the anode side of hydrogen-rich gas mixture should be at platinum electrodes at 10 ppm and the platinum -ruthenium electrode are clearly well below 30 ppm. Otherwise, too many catalytically active centers of the membrane surface are blocked by CO molecules. The oxygen molecules and hydrogen molecules can not be adsorbed, and the reaction collapses in a short time. By purging the fuel cell using pure hydrogen or pure inert gas, the CO may be removed from the membrane. CO also leads within the tolerance ranges to an accelerated, irreversible aging of the membrane; However, this effect by an admixture low air quantities can ( ≤ 1 vol %) be repealed. In this case, operating times of more than 15,000 hours are detectable.

The aim of the current research is therefore to increase the CO tolerance of the membranes. Another approach is the development of high-temperature PEMFCs that operate at up to 200 ° C. Due to the significantly higher temperature, the CO tolerance of up to 1%. The problem is still a suitable ionomer for this temperature range to find. In Nafion, the electrical resistance increases too much and it loses to conduct protons his property. Used, for example, polyimides, such as polybenzimidazole (PBI), which binds phosphoric acid as electrolyte. At a too high water content in the combustion gas of the Phosphorsäureaustrag of the membrane can be problematic.

Sulfur

Sulfur and sulfur compounds ( hydrogen sulphide in particular ) are strong catalyst poisons. This is caused by a strong chemisorption on the catalytically active diaphragm surface. There is a non-reversible destruction. The concentration of these compounds in the gas stream must be in the low tens of ppb range to avoid such damage.

Advantages and disadvantages compared to other fuel cells

Advantages of a low-temperature PEM (Nafion - based systems):

  • Solid electrolyte, that is, it can not leak or aggressive liquids.
  • The cell has a high current density, and
  • Has a good dynamic behavior.
  • On the cathode side, air can be used. It is not a pure gas (oxygen ) is required.
  • The electrolyte is CO2 - resistant

Disadvantages are:

  • The cell type is very sensitive to contamination by CO, NH3, and sulfur compounds in the fuel gas.
  • Water management is very complex.
  • The system efficiency is rather low.
Schenectady, New York Inertgas Polybenzimidazole fiber Catalyst poisoning
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