Polymer electrolyte fuel cell

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

history

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

principle

Structure of a PEM fuel cell

Electrolyzer of a polymer electrolyte fuel cell

Chemical energy is converted into electrical energy using hydrogen (H ₂ ) and oxygen (O ₂ ) . The electrical efficiency is around 60 percent depending on the operating point. A solid polymer membrane , for example made of Nafion , is normally used as the electrolyte . The operating temperature is in the range from 60 to 120 ° C., temperatures between 60 and 85 ° C. being preferably selected for continuous operation. The membrane is coated on both sides with a catalytically active electrode , a mixture of carbon ( soot ) and a catalyst , often platinum or a mixture of platinum and ruthenium (PtRu electrodes), platinum and nickel (PtNi electrodes), or platinum and cobalt (PtCo electrodes). H ₂ molecules dissociate on the anode side and are oxidized to two protons each with the release of two electrons . These protons diffuse through the membrane. On the cathode side, oxygen is, by the electrons, which could perform in an external circuit electrical work previously reduced ; together with the protons transported through the electrolyte, water is produced. In order to be able to use the electrical work, the anode and cathode are connected to the electrical consumer.

Reaction equations

	equation
anode	${\displaystyle \mathrm {2\ H_{2}\to 4\ H^{+}+4\ e^{-}} }$ Oxidation / electron donation
cathode	${\displaystyle \mathrm {O_{2}+4\ H^{+}+4\ e^{-}\to 2\ H_{2}O} }$ Reduction / electron uptake
Overall response	${\displaystyle \mathrm {2\ H_{2}+O_{2}\to 2\ H_{2}O} }$ Redox reaction / cell reaction

The internal charge transport takes place by means of oxonium ions. The reaction requires water on the anode side, which it gives off again on the cathode side. Complex water management is necessary to cover the water requirement on the anode side. This is achieved, among other things, by back diffusion through the membrane and moistening of the educts .

Areas of application

The main areas of application are mobile applications without the use of waste heat, for example in fuel cell vehicles , submarines , spaceships or battery chargers for on the go. Small stationary systems with a waste heat level of around 60 to 80 ° C are also possible. In order to achieve a technically relevant electrical voltage, several cells (ten to several hundred) are connected in series to form a so-called stack . The temperature of the stack is regulated in its own additional cooling circuit.

It is also a heat-driven, stationary use, e.g. B. in residential buildings, possible at a useful heat level of 80 ° C, with approximately the same ratio of heat and electricity from bio-hydrogen or hydrogen, which is generated from natural gas using the Kværner process , are produced. This is a form of cogeneration where an overall efficiency of 90 percent is realistic.

CO tolerance

Since the reactions take place at relatively low temperatures (60 to 120 ° C), tolerance to carbon monoxide (CO) poses a problem. The CO concentration of the air supplied to the cathode and the hydrogen-rich gas mixture supplied to the anode side should be Platinum electrodes are well below 10  ppm and for platinum ruthenium electrodes are well below 30 ppm. Otherwise too many catalytically active centers on the membrane surface will be blocked by CO molecules. The oxygen molecules or hydrogen molecules can no longer be adsorbed and the reaction breaks down in a very short time. By flushing the fuel cell with pure inert gas or pure hydrogen, the CO can be removed from the membrane again. Even within the tolerance ranges, CO leads to accelerated, irreversible aging of the membrane; However, this effect can be neutralized by adding small amounts of air (≤ 1% by volume). In this case, operating times of more than 15,000 hours can be proven.

The aim of current research is therefore to also increase the CO tolerance of the membranes. Another approach is to develop high-temperature PEMFCs that work at up to 200 ° C. Due to the significantly higher temperature, the CO tolerance is up to 1%. Finding a suitable ionomer for this temperature range is currently still a problem . With Nafion, the electrical resistance increases too much and it loses its ability to conduct protons. For example, polyimides such as polybenzimidazole (PBI), which binds phosphoric acid as an electrolyte, can be used. If the water content in the fuel gas is too high, the discharge of phosphoric acid from the membrane can be problematic.

Sulfur content

Sulfur and sulfur compounds (here in particular hydrogen sulfide ) are strong catalyst poisons . This is caused by strong chemisorption on the catalytically active membrane surface. There is irreversible destruction. The concentration of these compounds in the gas flow must be in the lower double-digit ppb range in order to avoid such damage.

Advantages and disadvantages compared to other fuel cells

The advantages of a low temperature PEM (Nafion-based) are:

• Solid electrolyte, which means that no aggressive liquids can leak out.
• The cell has a high current density and
• has good dynamic behavior.
• Air can be used on the cathode side. No pure gas (oxygen) is required.
• The electrolyte is CO ₂ resistant

The disadvantages are:

• The cell type is very sensitive to contamination by CO, NH ₃ and sulfur compounds in the fuel gas.
• Water management is complex, as the membrane must be prevented from drying out. In addition, the water in the membrane must also be prevented from freezing.
• The system efficiency is rather low.

Web links

• Institute for Energy and Climate Research at Forschungszentrum Jülich - PEM Electrolysis , accessed on March 23, 2013
• www.pemfc.de
• Hydrogen-based storage power plant is to be built in Lusatia . heise.de , December 16, 2019 (a small test model)

Individual evidence

1. ^ Wissenschaft-Online-Lexika: Entry on polymer electrolyte membrane fuel cells in the Lexikon der Physik , accessed on January 5, 2009
2. ↑ Grubb, Willard Thomas. From: encyclopedia of earth , accessed August 7, 2012
3. ^ History. From: FuelCellToday , accessed August 7, 2012
4. ↑ ^{a b} Dominic A. Notter, Katerina Kouravelou, Theodoros Karachalios, Maria K. Daletou and Nara Tudela Haberlandad: Life cycle assessment of PEM FC applications: electric mobility and μ-CHP . In: Energy and Environmental Science 8, (2015), 1969–1985, doi : 10.1039 / C5EE01082A .
5. ↑ J. Scholta, J. Pawlik, N. Chmielewski, L. Jörissen: Longevity test results for reformate polymer electrolyte membrane fuel cell stacks . In: Journal of Power Sources 196, (2011), 5264-5271, doi : 10.1016 / j.jpowsour.2010.08.113 .
6. ↑ ^{a b} Peter Kurzweil: fuel cell technology . Basics, components, systems, applications. 2nd Edition. Springer Vieweg, Wiesbaden 2013, ISBN 978-3-658-00084-4 , p. 78; 176 , doi : 10.1007 / 978-3-658-00085-1 ( springer.com ): "Water balance problem"