Reversible fuel cell

Reversible fuel cells are special fuel cells whose energy-supplying work process is reversible. They are converter units that, like normal fuel cells , can provide electrical energy by converting fuels . However, the same converter unit can also be operated as an electrolyzer , so that electrical energy is converted into storable chemical energy . Reversible fuel cells have found isolated applications in space travel and in the military . Small reversible fuel cells are commercially available as training and demonstration objects. Otherwise they are still in the research and development stage, e.g. B. with regard to storage for the power grid .

On the concept of the reversible fuel cell

Reversible fuel cells in the narrower sense use the same electrode stack for the fuel cell process and for the electrolysis. Here the term is used in this narrower sense, in accordance with the usage in these sources. In the English-speaking world, however, the terms reversible fuel cell and regenerative fuel cell RFC are often used in a broader sense: they can also describe systems in which at least two modules, a fuel cell and a separate electrolyzer (and often a storage unit as a further component of the system), are combined into one system. For the systems in which both processes take place in a single cell stack , the term unitized reversible fuel cell URFC is then used - especially for cells with a polymer electrolyte membrane (PEM) .

Comparison of the directly reversible cell with cells with external reaction reversal

reversible fuel cell rBZ	Fuel cell with external reaction reversal
unitized regenerative fuel cells (URFC)	discrete regenerative fuel cells (DRFC)
chemical energy of fuels Fuel cell operation ("discharging") ↓ ↑ Storage mode ("Load") reversible fuel cell rBZ Fuel cell operation ("discharging") ↓ ↑ Storage mode ("Load") electrical power	chemical energy of fuels Fuel cell operation ("discharging") ↓ ↑ Storage mode ("Load") Fuel cell Electrolyzer Fuel cell operation ("discharging") ↓ ↑ Storage mode ("Load") electrical power

Most of the systems with reaction reversal tested to date did not have reversible fuel cells in the narrower sense described, but used a separate electrolyser, since the overall efficiency (back and forth reaction) is higher when using an electrolyser.

Example hydrogen fuel cell

Schematic structure of a PEM fuel cell

A hydrogen fuel cell consumes hydrogen (H ₂ ) and oxygen (O ₂ ) to produce electricity and water (H ₂ O); as a reversible fuel cell, it now has to produce hydrogen and oxygen again from water by electrolysis . The fuel cell is operated as an electrolyzer for this purpose.

Reversible process in a hydrogen fuel cell:

${\displaystyle \mathrm {2\ H_{2}\ +\ O_{2}\ \ \longrightarrow \ 2\ H_{2}O+\ \Delta H} }$

Reaction of hydrogen with oxygen to form water, releasing energy

${\displaystyle \mathrm {2\ H_{2}O\ {\xrightarrow {Elektrolyse}}2\ H_{2}+O_{2}} }$

Electrolysis of water to hydrogen and oxygen with the addition of energy

Voltage and efficiency

For the reaction between H ₂ , O ₂ and H ₂ O, the equilibrium or rest voltage is 1.23 V. Even with low current densities, the voltage of reversible H ₂ -O ₂ fuel cells drops below 1 V when discharging, while they are less than 1 V when charging is above 1.5 V. At high current densities, it drops below 0.8 V when discharging, while it rises to over 1.8 V when charging. Due to the considerable voltage differences (about one volt) between charging and discharging, the efficiency is correspondingly low: Typical efficiency of reversible PEM fuel cells is 40 to 50%.

Types of reversible fuel cells

Similar to normal fuel cells, hydrogen has so far been of the greatest importance as a fuel in reversible fuel cells, usually with (air) oxygen as the oxidizing agent. Since reversible fuel cells can be operated as closed systems, halogens are also investigated as oxidizing agents, e.g. B. with the combinations hydrogen- bromine H ₂ -Br ₂ or hydrogen- iodine H ₂ -I ₂ .

Most of the reversible fuel cells built so far are polymer electrolyte fuel cells (PEM). The overall efficiency of reversible fuel cells is in the range of 30 to 40% for alkaline electrolytes, 40 to 50% for PEM and 60 to 80% for solid oxide fuel cells (SOFC).

Practical use

Various small fuel cell demonstration devices with gas storage tanks are available for training purposes, some in combination with solar modules for electrolysis or with a model car.

One possible application is energy storage using the power-to-gas process. Reversible fuel cells are significantly smaller and less complex than the alternative with a separate electrolyzer. Both polymer electrolyte fuel cells (PEM, then also unitized regenerative fuel cell , or URFC for short ) and solid oxide fuel cells can be operated reversibly. The first systems were launched on the market in 2015.

The use of reversible fuel cells enables, compared to conventional technologies for fuel gas production by means of electrolysis, with good waste heat management, significantly higher electricity-to-electricity efficiencies of up to about 70% and lower costs.

In addition, a reversible fuel cell with a fuel storage device or by being connected to a distribution network can replace an accumulator , whereby a significantly lower and cheaper power-to-weight ratio can be achieved. At a maximum of 80%, the overall efficiency is less than that of competing battery systems, e.g. B. Lithium-ion batteries .

Historical

Considerations to recover the hydrogen fuel outside the fuel cell, e.g. B. by means of solar energy , were discussed as early as 1962. The first reversible fuel cell, which was also tested, was developed by General Electric in 1972–1973 ; In the 1990s, research on reversible fuel cells was carried out in particular at the Lawrence Livermore National Laboratory . Solar planes and emission-free vehicles were also named as possible fields of application .

Individual evidence

1. ↑ ^{a b c} Angelika Heinzel, Ansgar Rau, Marc Valerius, Ursula Wittstadt: Energy storage with a reversible electrolysis / fuel cell system. In: Services and Results - Annual Report 2001. Fraunhofer Institute for Solar Energy Systems ISE, 2002, accessed on April 23, 2019 . 
2. ↑ ^{a b} Tom Smolinka: Reversible fuel cell systems. In: Annual Report 2005 - Achievements and Results. Fraunhofer Institute for Solar Energy Systems ISE, 2006, accessed on April 23, 2019 . 
3. ↑ Angela Schmid: Reversible fuel cells: Electricity storage with hydrogen. In: Edison. Handelsblatt GmbH, January 8, 2019, accessed on April 22, 2019 . 
4. ↑ Mohamed Gabbasa, Kamaruzzaman Sopian, Ahmad Fudholi, Nilofar Asim: A review of unitized regenerative fuel cell stack: Material, design and research achievements . In: International Journal of Hydrogen Energy . tape 39 , no. 31 , October 2014, p. 17765–17778 , doi : 10.1016 / j.ijhydene.2014.08.121 ( elsevier.com ).
5. ↑ ^{a b c} Biddyut Paul, John Andrews: PEM unitised reversible / regenerative hydrogen fuel cell systems: State of the art and technical challenges . In: Renewable and Sustainable Energy Reviews . tape 79 , November 2017, p. 585-599 , doi : 10.1016 / j.rser.2017.05.112 ( elsevier.com ).
6. ^ ^{A b c d} Fred Mitlitsky, Blake Myers, Andrew H. Weisberg: Regenerative Fuel Cell Systems . In: American Chemical Society ACS (Ed.): Energy & Fuels . tape 12 , no. 1 , January 1998, ISSN  0887-0624 , p. 56-71 , doi : 10.1021 / ef970151w ( acs.org ).
7. ^ ^{A b c} Yifei Wang, Dennis YC Leung, Jin Xuan, Huizhi Wang: A review on unitized regenerative fuel cell technologies, part-A: Unitized regenerative proton exchange membrane fuel cells . In: Renewable and Sustainable Energy Reviews . tape 65 , November 2016, p. 961–977 , doi : 10.1016 / j.rser.2016.07.046 ( hw.ac.uk [PDF]).
8. ↑ Regis P. Dowd, Venkata Yarlagadda, Dhrubajit Konwar, Guangyu Lin, Guoming Weng: A Study of Alkaline-Based H ₂ -Br ₂ and H ₂ -I ₂ Reversible Fuel Cells . In: Journal of The Electrochemical Society . tape 163 , no. 14 , 2016, ISSN  0013-4651 , p. F1471-F1479 , doi : 10.1149 / 2.0011614jes .
9. ↑ Reversible Fuel Cell With Storage. In: Fuel Cell Store> Education> Demonstration> Dr. FuelCell Model Car Kit - Demo. Fuel Cell Store, accessed May 5, 2019 . 
10. ^ Fuel Cell 10. In: Fuel Cell Cars. Fuel Cell Store, accessed May 5, 2019 . 
11. ↑ Winlab.de Shop: Complete model car with reversible fuel cell. In: School> Demonstration Devices & Materials> Fuel Cell Technology. Windaus-Labortechnik GmbH & Co KG, accessed on May 5, 2019 . 
12. ↑ Solar / hydrogen power generation kit. In: data sheets. Conrad.de, accessed on May 5, 2019 . 
13. ↑ Data sheet model car with reversible fuel cell. ( Memento of March 13, 2017 in the Internet Archive ) (PDF; 340 kB)
14. ↑ ^{a b} S. H. Jensen, C. Graves, M. Mogensen, C. Wendel, R. Braun: Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO ₂ and CH ₄ . In: Energy & Environmental Science . tape 8 , no. 8 , 2015, ISSN  1754-5692 , p. 2471-2479 , doi : 10.1039 / C5EE01485A ( rsc.org ).
15. ↑ Alberto Varone, Michele Ferrari, Power to liquid and power to gas: An option for the German Energiewende . In: Renewable and Sustainable Energy Reviews 45, (2015), pp. 207–218, here p. 209, doi: 10.1016 / j.rser.2015.01.049 .
16. ^ D. Noel Buckley, Colm O'Dwyer, Nathan Quill, Robert P. Lynch: Electrochemical Energy Storage . In: Issues in Environmental Science and Technology . Royal Society of Chemistry, Cambridge 2018, ISBN 978-1-78801-399-4 , pp. 115–149 , doi : 10.1039 / 9781788015530-00115 ( rsc.org [accessed May 5, 2019]).
17. ↑ John J. Rowlette: Investigation of new solar regenerative fuel cell system. In: Technical Report, Electro-Optical Systems, Inc., Pasadena, Calif. US Department of Energy, Office of Scientific and Technical Information OSTI, January 10, 1962, accessed May 2, 2019 . 
18. ↑ PJ Chludzinski, IF Gdansk, AP Fickett, DW Craft: Regenerative fuel cell development for satellite secondary power. Final technical report, Apr 1972-Apr 1973. [H / sub 2 // O / sub 2 / fuel cells] . Ed .: US Department of Energy, Office of Scientific and Technical Information OSTI. AD-762923; AFAPL-TR-73-34. General Electric Co., Lynn, Mass. (USA), June 1, 1973 ( osti.gov [accessed May 5, 2019]).
19. ↑ Fred Mitlitsky, Blake Myers, Andrew H. Weisberg: Lightweight pressure vessels and unitized regenerative fuel cells . CONF-961107 - Absts., 460339, December 31, 1996, doi : 10.2172 / 460339 ( osti.gov [accessed May 5, 2019]).