Oxygen as oxidizing agent

Oxidation, reduction. We’ve heard this before, but it’s been a long time. What is it all about again? What is an oxidant? And what does all this have to do with fuel cell technology? A lot, because neither an EFOY Jupiter hydrogen fuel cell nor an EFOY direct methanol fuel cell can work without an oxidizer. To start with, let’s explain the difference: oxidation is a chemical reaction involving oxygen. Reduction is a deoxygenation reaction. Since the discussion below is primarily about the oxidant, the focus is, of course, on oxidation. Reactions with oxygen are typical for combustion processes, for example, to generate energy. They also occur in fuel cells. In their function as converters, fuel cells produce electricity with the help of chemical reaction energy. To do this, the fuel cell requires a combination of a continuously supplied fuel – hydrogen or methanol – and oxygen. Oxygen acts as an oxidant1 in the fuel cell. But more on this later.

The definition of an oxidizing agent is comparatively simple. An oxidizing agent is a chemical substance that is capable of oxidizing other substances, but is itself reduced during this process. Oxidizing agents can accept electrons, which are negatively charged elementary particles, while reducing agents release them.2 If the substances react together, a so-called redox reaction occurs. One reactant transfers electrons to the other. That’s it for the oxidant definition.3 In chemistry and especially in electrochemistry, this concept is of great importance. Typically, redox reactions take the form of combustion for conversion to kinetic energy. For example, in cars, trucks, ships and airplanes when gasoline, diesel and kerosene are burned in the engine.4 The redox reaction is even more impressive on New Year’s Eve. Rockets and sparklers also work thanks to oxidizing agents and reducing agents. So does the previously lit lighter or match.5

Particularly strong oxidizing agents

Basically, the fuel cell uses the same principle. Just like a battery, it also has electrodes: an anode and a cathode. Each of the electrodes is coated with a metal as a catalyst, for example nickel or platinum.6 These are very valuable metals, which SFC Energy recovers in the recycling process. 95 percent of an SFC Energy fuel cell is recyclable. The anode and cathode in the fuel cell are separated by an electrolyte. This gas-impermeable membrane ensures separate conduction of ions (positive charge) and electrons (negative charge). The continuously supplied fuel, for example hydrogen, flows around the anode. At the same time, oxygen reaches the cathode in its function as an oxidizing agent.7 Oxidation now takes place on the anode side. The hydrogen oxidizes by releasing electrons to form protons. The electrons subsequently flow from the anode to the cathode – in other words, current flows. The protons penetrate the electrolyte separating the anode and cathode and reach the cathode.8

In addition to the energy generated, the addition of oxygen as an oxidant in the fuel cell produces water and heat as a by-product. In this way, the fuel cell produces environmentally friendly electricity overall. The reaction heat generated during the process can even be used for heating. The fuel cell is therefore a true multi-talent. Just like the oxidant, which generates energy in combination with the reducing agent. However, oxygen is not the only oxidant. There are other and particularly strong oxidizing agents. After crypton difluoride, oxygen difluoride and fluorine are considered to be the strongest oxidizing agents of all.9 The latter is highly unstable, i.e. it reacts immediately. The conditions under which cryptone difluoride exists indicate this. It can only exist stably at temperatures of about -80°C. In order to produce it, it has to be stable. To be able to produce it, it must be much colder. The strongest oxidizing agent is formed in the production process under high electrical discharges from krypton and fluorine in the elemental state and exclusively at temperatures of about -196°C. The result is a colorless, colorless fluoride. The result is a colorless, crystalline substance.10

Oxidizing agents in everyday life

This all sounds very technical, and it’s hard to imagine oxidizers in everyday use. In addition to their use in fuel cells and combustion engines, there are far more mundane functions they perform. Hydrogen peroxide as an oxidizer is one such case. Presumably, numerous readers will already have come into contact with this pale blue, creamy substance. Hydrogen peroxide is a highly effective bleaching and disinfecting agent. Hairdressers rely on the power of hydrogen peroxide as a highly efficient bleaching cream. So anyone who has ever changed their own hair color will have used hydrogen peroxide. Dentists also use this powerful oxidizing agent. It whitens teeth and makes them sparkling white again after years of coffee and cigarettes. But be careful, hydrogen peroxide is highly corrosive. If it comes into contact with the skin, the area should be cleaned as soon as possible. It also does a great job as a disinfectant. Strongly diluted hydrogen peroxide helps in the mouth and throat area in the fight against bacteria. It also disinfects contact lenses and hands.11

In addition to oxygen, hydrogen peroxide and the strong oxidizing agents mentioned, such as cryptone difluoride, oxygen difluoride and fluorine, this group also includes numerous chlorites, nitrates and acids. It is often said that even gold and platinum can be used as oxidizing agents. This is true, but hardly practical. Precious metals such as these two representatives are only suitable as oxidizing agents to a limited extent. For one thing, they are very expensive, which makes their use too costly. For another, it is extremely difficult to bring them into solution at all. Therefore, the industry relies on other less expensive and easier-to-process oxidizing agents.12 This is not a problem; after all, there are enough of them. Oxidizer examples abound; too many to list them all here. They basically fall under the group of fire-promoting substances. Oxidizing agents can be clearly arranged in a table. For example, according to increasing oxidizing power. In this way, the so-called voltage series is obtained.13 However, this is not relevant for fuel cells from SFC Energy. EFOY fuel cells only need oxygen as an oxidant. And that is a good thing. After all, we are talking about nothing less than the production of clean and environmentally friendly energy.

1 Kurzweil, Peter: "Fuel Cell Technology – Fundamentals, Components, Systems, Applications"; Springer Fachmedien; Wiesbaden; 2003, 2013 – p. 3 ff.

2 University of Dusiburg/Essen: https://www.uni-due.de/~hc0014/S+WM/Definitions/Oxidants.htm

3 Spectrum of Science: https://www.spektrum.de/lexikon/biologie/redoxreaktionen/55953

4 Bavarian Radio: https://www.br.de/telekolleg/faecher/chemie/telekolleg-chemie-11-redoxreaktionen-102.html

5 Ibid.

6 Heise Medien: https://www.heise.de/hintergrund/Brennstoffzellen-ohne-Platin-275962.html

7 EnBW: https://www.enbw.com/energie-entdecken/mobilitaet/brennstoffzellenantrieb/

8 Töpler, Johannes; Lehmann, Jochen (eds.): "Hydrogen and Fuel Cell – Technologies and Market Perspectives"; Springer-Verlag Berlin, Heidelberg 2014.

9 ChemgaPedia/Wiley Information Services GmbH: http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/11/aac/vorlesung/kap_11/vlus/energiebetrachtungen.vlu/Page/vsc/de/ch/11/aac/vorlesung/kap_11/kap11_2/text_c.vscml.html

10 Latscha, Hans Peter; Kazmaier, Uli; Klein, Helmut Alfons: Organic Chemistry "Chemie-Basiswissen II"; Springer-Verlag GmbH Berlin, Heidelberg 2016.

11 University of Regensburg: https://www.uni-regensburg.de/chemie-pharmazie/anorganische-chemie-pfitzner/medien/data-demo/2010-2011/wasserstoffperoxid_mh.pdf

12 Spectrum of Science: https://www.spektrum.de/lexikon/chemie/gold/3827

13 https://www.chemie.de/lexikon/Elektrochemische_Spannungsreihe.html