Alkaline fuel cell

Schematic structure of an alkaline fuel cell

The alkaline fuel cell (engl. Alkaline Fuel Cell , AFC ) is a low-temperature fuel cell with a concentrated aqueous alkaline solution as electrolyte . Alkaline fuel cells were an important step in the development of fuel cell technology. You played an important role in space travel , especially in the Apollo program and the space shuttle . Today they are only of minor importance compared to the polymer electrolyte cell, and only a few companies - including AFC Energy based in the English county of Surrey - are developing them further.

By far the most important energy supplier (“ fuel ”) for the alkaline fuel cell is hydrogen . Alternatively, ammonia (see ammonia fuel cell ) or hydrazine ( hydrazine fuel cell ) can also be used as energy sources instead.

principle

An aqueous potassium hydroxide solution is usually used as the electrolyte . Since this lye is strongly alkaline - it has a high pH value - the cell is called an alkaline fuel cell. The hydrogen , which is mostly used as "fuel gas" , is fed to the anode and oxidized to H ^{+ there} . Together with OH ^- ions, which are formed at the cathode by the reduction of oxygen and reach the anode via the electrolyte, the reaction product water is formed at the anode, which must be constantly removed. The working temperature of the cell is 60–120 ° C (or 200–230 ° C for the Apollo cells).

The air supplied to the cathode should be free of CO ₂ , since CO ₂ after

${\displaystyle \mathrm {CO_{2}+2\,KOH\rightarrow K_{2}CO_{3}\downarrow +H_{2}O} }$

Potassium carbonate precipitates. The solid carbonate can e.g. B. clogging of porous electrodes lead to a decrease in the performance of the cell. According to one study, this does not apply to all cell types; in the case examined, the aging of the cells was independent of the CO ₂ content of the oxygen used.

Compared to the PEMFC , the AFC has a lower power density, but a slightly better efficiency due to the higher cell voltage. However, it does not currently achieve the high current densities of a PEMFC. The electrolyte also serves to regulate the temperature of the stack, so that no further cooling circuit is necessary.

Development of the cell and its areas of application

The invention of the alkaline fuel cell is attributed to an American and a French inventor on the basis of corresponding patents. The outstanding pioneer in the development of alkaline fuel cells - through which the development of fuel cells as a whole was promoted - was the English engineer Francis Thomas Bacon (1904–1992). He brought the fuel cell to maturity in England. Mostly American companies, e.g. B. Allis-Chalmers and Pratt & Whitney , then continued further development; besides, others worked, e.g. B. in Germany Eduard Justi , at the AFC. The readiness for use was initially demonstrated in demonstration projects such as the Allis Chalmers fuel cell tractor . After that, the AFC was used in manned space travel: the Apollo spacecraft used AFCs from Pratt & Whitney, which delivered a nominal power of 1.12 kW (maximum: 2.3 kW) at a voltage of 28 V and on nine flights to the moon participated. AFCs also used the Skylab , the LOK spaceship and the space shuttle . Siemens installed AFCs in submarines , but eventually turned to PEMFC . In 1998, a prototype of an alkaline fuel cell taxi drove in London, and the company also fitted a van with the AFC. This cell type was also used to drive the electric motor in the passenger boat Hydra (in service from 1999 to 2001). In 2002 these developments had to be stopped due to lack of money.

Reaction equations

	equation
anode	${\displaystyle \mathrm {2\ H_{2}+4\ OH^{-}\to 4\ H_{2}O+4\ e^{-}} }$ Oxidation / electron donation
cathode	${\displaystyle \mathrm {O_{2}+2\ H_{2}O+4\ e^{-}\to 4\ OH^{-}} }$ 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 hydroxide ions . On the cathode side, the reaction only needs pure oxygen and produces water on the anode side, which only needs pure hydrogen.

Advantages and disadvantages compared to other fuel cells

Advantages of an AFC are:

• robust system
• good dynamic behavior
• inexpensive catalysts ( nickel , silver )

Disadvantages are:

• possibly sensitive to pollution, especially by CO ₂ (possibly also not)
• short service life due to the corrosive electrolyte

Alkaline fuel cells have good potential for further development. Research deals in particular with different materials for membranes and electrodes. Electrodes with a gold catalyst have proven to be particularly effective here.

Individual evidence

1. ^ AFC Energy PLC - Producer of alkaline fuel cell systems. Retrieved June 18, 2019 (American English). 
2. ↑ ^{a b c} Peter Kurzweil: fuel cell technology . Basics, components, systems, applications. 2nd, revised and updated edition. Springer Vieweg, Wiesbaden 2013, ISBN 978-3-658-00085-1 , p. 65-66 , doi : 10.1007 / 978-3-658-00085-1 .
3. ↑ ^{a b} Erich Gülzow: Alkaline fuel cells: a critical view . In: Journal of Power Sources . tape 61 , no. 1-2 . Elsevier, July 1996, ISSN  0378-7753 , p. 99-104 , doi : 10.1016 / S0378-7753 (96) 02344-0 ( elsevier.com ).
4. ↑ Erich Gülzow, Jiri K. Nor, Peter K. Nor, Mathias Schulze: A renaissance for alkaline fuel cells . In: The Fuel Cell Review . tape 3 , no. 1 . Institute of Physics Publishing, February 2005, p. 19-25 .
5. ↑ Patent US0736016 : Process of Generating Electricity. Registered May 17, 1902 , published August 11, 1903 , inventor: James H. Reid (US). ‌
6. ↑ Patent US0736017 : Gas Battery. Registered December 20, 1902 , published August 11, 1903 , inventor: James H. Reid (US). ‌
7. ↑ Patent US0757637 : Gas Battery. Registered August 3, 1903 , published April 19, 1904 , inventor: James H. Reid (US). ‌
8. ↑ Patent FR350100 : Générateur électrique à gaz. Registered August 4, 1904 , published October 13, 1905 , inventor: Paul Gustave Leon Noel. ‌
9. ^ Fuel Cell, Apollo. March 11, 2016, accessed June 18, 2019 . 
10. ^ Collecting the History of Alkali Fuel Cells. In: Fuel Cell History Project. National Museum of American History, Smithsonian Institution, 2009, accessed June 20, 2019 (The Zero Emission Vehicle Company (ZEVCO) was renamed ZeTek Power in 2001). 
11. ↑ ^{a b} Alkaline Fuel Cell - AFC. In: Fuel Cells - Electrochemical Power> Types of fuel cells. Fuel Cell Norway ANS, 2006, accessed June 20, 2019 . 
12. ↑ Paola Quaino, Noelia B. Luque, Renat Nazmutdinov, Elizabeth Santos, Wolfgang Schmickler : Why is gold in an alkaline solution such a good catalyst for oxygen reduction ? In: Angewandte Chemie . tape 124 , no. 52 , 2012, p. 13171-13174 , doi : 10.1002 / anie.201205902 .