Ammonia fuel cell

An ammonia fuel cell , more precisely and specifically the ammonia-oxygen fuel cell , converts ammonia and oxygen or air into water and nitrogen . Like all fuel cells , it generates electrical energy from the chemical energy of the substances used (through "cold combustion"). The theoretical voltage ( reversible cell voltage ) of a single ammonia-oxygen fuel cell is 1.17 V.

Advantages and disadvantages

What speaks in favor of ammonia as an energy carrier is that it is inexpensive and available in large quantities as a basic chemical produced on an industrial scale . The energy density of ammonia is 5.4 kWh / kg (19.4 MJ / kg, here and below - as usual - the oxygen required for combustion is not included). This is only about half as large as that of gasoline (12.1 kWh / kg) or diesel (11.8 kWh / kg), but corresponds to that of methanol (5.6 kWh / kg) and is thus far higher the energy density of batteries or accumulators. It is also higher than that of hydrogen in pressure tanks , and also higher than that of hydrogen in metal hydrides . Liquefied ammonia has an energy density comparable to that of gasoline . Apart from the hydrogen-oxygen fuel cell, almost all other fuel cells, e.g. B. the direct methanol fuel cell and the direct ethanol fuel cell , carbonaceous fuels . In contrast, the conversion of ammonia cannot produce carbon dioxide , and the products nitrogen and water are environmentally friendly. This makes ammonia an ideal source of energy, especially when it is obtained using power-to- ammonia processes with the help of renewable energy sources or with biological degradation processes. Ammonia fuel cells could make a contribution to a carbon-free and therefore climate-friendly energy economy. However, they are at the research and development stage and still a long way from being used. The serious disadvantage of the toxicity of ammonia also represents a hurdle when it comes to market introduction. However, the low odor threshold of ammonia contributes to the fact that accidents with personal injury when handling it are rather rare. Another problem with ammonia is its corrosive effect.

Demarcation

Ammonia can be split into hydrogen and nitrogen at sufficiently high temperatures (<700 ° C). The hydrogen obtained in this way can be converted in a suitable hydrogen-oxygen fuel cell. In contrast to this, the fuel cell discussed here, which can utilize ammonia without prior conversion to hydrogen, can also be referred to as a direct ammonia fuel cell.

Historical

The ammonia-oxygen fuel cell was developed in the 1960s by American industrial companies such as Allis-Chalmers (1961), Lockheed Missiles and Space Co. (1963) and General Electric (1967). The first generally accessible publication with data on an ammonia fuel cell appeared in 1968. It used concentrated potassium hydroxide as an electrolyte and thus belongs to the type of alkaline fuel cell (AFC). From 1980 solid oxide fuel cells (SOFC) were also operated with ammonia.

Reaction equations

The reaction equations apply to the reaction in alkaline solutions:

• Anode , negative pole: ${\displaystyle {\ce {2 NH3 + 6OH- -> N2 + 6H2O + 6e-}}}$
• Cathode , positive pole: ${\displaystyle {\ce {O2 + 2 H2O + 4 e- -> 4 OH-}}}$
• Overall reaction: ${\displaystyle {\ce {4 NH3 + 3 O2 -> 2 N2 + 6 H2O}}}$

In a cell with 54% KOH, a voltage of about 0.6 V could be obtained at a current density of 0.4 A / cm ² .

For the reaction in solid oxide fuel cells - the solid oxides conduct oxide ions - the reaction equations apply: ${\displaystyle {\ce {O^2-}}}$

• Anode, the negative pole: . The reaction takes place in sub-steps: ${\displaystyle {\ce {2 NH3 + 3 O^2- -> N2 + 3 H2O + 6e-}}}$
  
  • ${\displaystyle {\ce {2 NH3 + 5 O^2- -> 2 NO + 3 H2O + 10e-}}}$ (⋅3)
  • ${\displaystyle {\ce {2 NH3 + 3 NO -> 5/2 N2 + 3 H2O}}}$(⋅2)
    Since the intermediate product nitrogen monoxide NO occurs, it can also occur in the exhaust gas of the cell, from which it may have to be removed. It is also possible to optimize the cell with regard to a high yield of NO, which can then be further processed into nitric acid .
• Cathode, positive pole: ${\displaystyle {\ce {O2 + 4 e- -> 2 O^2-}}}$
• Overall reaction: ${\displaystyle {\ce {4 NH3 + 3 O2 -> 2 N2 + 6 H2O}}}$

Current research

Research projects started in 2017 aim, for example, to develop ammonia fuel cells that can be used in the transport sector or that are based on inexpensive solid oxide fuel cells and that can operate at relatively low temperatures.

See also

• Power-to-ammonia

Individual evidence

1. ^ Neil V. Rees, Richard G. Compton: Carbon-free energy: a review of ammonia- and hydrazine-based electrochemical fuel cells . In: The Royal Society of Chemistry (Ed.): Energy & Environmental Science . tape 4 , no. 4 , 2011, ISSN  1754-5692 , p. 1255 , doi : 10.1039 / c0ee00809e ( rsc.org ).
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. 6; 74; 126; 228; 230 , doi : 10.1007 / 978-3-658-00085-1 .
3. ↑ ^{a b c d} Ahmed Afif, Nikdalila Radenahmad, Quentin Cheok, Shahriar Shams, Jung H. Kim: Ammonia-fed fuel cells: a comprehensive review . In: Renewable and Sustainable Energy Reviews . tape 60 , July 2016, p. 822-835 , doi : 10.1016 / j.rser.2016.01.120 ( elsevier.com ).
4. ↑ ^{a b c d e} Rong Lan, Shanwen Tao: Ammonia as a Suitable Fuel for Fuel Cells . In: Frontiers Research Foundation (Ed.): Frontiers in Energy Research . tape 2 , August 28, 2014, ISSN  2296-598X , 35, doi : 10.3389 / fenrg.2014.00035 ( frontiersin.org ).
5. ↑ ^{a b} Hellmuth Nordwig: Alternative energy generation - researchers investigate ammonia fuel cells. In: Current Research. Deutschlandradio, November 22, 2013, accessed on June 17, 2019 .  
6. ^ ^{A b c} Hermann Matschiner: Use of ammonia to generate energy. In: Archive. Fördergesellschaft Erneuerbare Energien eV fee-ev.de, December 10, 2007, accessed on June 17, 2019 .  
7. ↑ Marion O'sullivan: Hydrogen breakthrough Could be a game-changer for the future of car fuels. In: Chemistry, Materials Science. Science X Network, phys.org, June 24, 2014, accessed June 17, 2019 (American English).  
8. ↑ Trevor Brown: GenCell launches commercial alkaline fuel cell using cracked ammonia fuel. In: Ammonia Energy. Ammonia Energy, July 6, 2018, accessed June 17, 2019 .  
9. ↑ ^{a b} Herbie Schmidt: New hope for fuel cells through hydrogen from ammonia | NZZ . Neue Zürcher Zeitung, August 8, 2018, ISSN  0376-6829 ( nzz.ch [accessed June 17, 2019]).
10. ↑ Patrick Karl Ewald Preuster: Development of a reactor for the dehydrogenation of chemical hydrogen carriers as part of a decentralized, stationary energy store . Dissertation at the Friedrich-Alexander University Erlangen-Nuremberg. Erlangen-Nuremberg March 7, 2017, state of the art, p. 8–9 ( kobv.de [PDF; 17.8 MB ; accessed on April 1, 2019]).
11. ↑ RA Wynveen: The Preliminary Appraisal of the Ammonia Fuel Cell System . In: American Chemical Society, Division of Petroleum Chemistry (Ed.): ACS Division Proceedings . Meeting 140: September 3-8, 1961, Chicago, IL, Symposium on Fuel Cells, September 1961, pp. B49-B59 ( acs.org [PDF]).
12. ^ ^{A b c d e} Elton James Cairns, EL Simons, Arthur D. Tevebaugh: Ammonia – Oxygen Fuel Cell . In: Nature . tape 217 , no. 5130 , February 1968, ISSN  0028-0836 , p. 780-781 , doi : 10.1038 / 217780a0 ( nature.com ).
13. ^ ^{A b} Roger D. Farr, Constantinos G. Vayenas: Ammonia High Temperature Solid Electrolyte Fuel Cell . In: The Electrochemical Society ECS (Ed.): Journal of The Electrochemical Society . tape 127 , no. 7 , 1980, pp. 1478 , doi : 10.1149 / 1.2129934 ( ecsdl.org ).
14. ↑ Jason C. Ganley: Ammonia Fuel Cell Systems. 2005 Annual NH3 Fuel Conference • Ammonia - The Key to a Hydrogen Economy October 13-14, 2005 • Argonne National Laboratory. NH3 Fuel Association, October 2005, accessed June 18, 2019 .  
15. ↑ Constantinos G. Vayenas, Roger D. Farr: Cogeneration of Electric Energy and Nitric Oxide . In: Science . tape 208 , no. 4444 , May 9, 1980, ISSN  0036-8075 , p. 593-594 , doi : 10.1126 / science.208.4444.593 ( sciencemag.org ).
16. ↑ Direct Ammonia Fuel Cells for Transport Applications. In: Transportation Fuels. Advanced Research Projects Agency - Energy arpa-E, US Department of Energy, December 15, 2016, accessed June 18, 2019 .  
17. ^ Cost-effective, intermediate-temperature fuel cell for carbon-free power generation. In: Transportation Fuels. Advanced Research Projects Agency - Energy arpa-E, US Department of Energy, December 15, 2016, accessed June 18, 2019 .