Water electrolysis

Under water electrolysis means the decomposition of water into hydrogen and oxygen by means of an electric current. The most important application of this electrolysis is the production of hydrogen, which up to now has only been used technically if cheap electrical energy is available, since until now the production of hydrogen from fossil fuels has been cheaper than the production of hydrogen by means of water electrolysis.

Due to the strong expansion of the use of renewable energies , it is assumed that water electrolysis as a component of power-to-gas plants will become very important for the production of synthesis gas in the medium to long term . With hydrogen as an energy storage device, the steady generation of electricity from renewable energies, especially wind power and photovoltaics , is promoted by the fact that surpluses of wind and solar power can be chemically stored. The hydrogen generated can be used for chemical processes or fed into the natural gas network directly or after subsequent methanation as methane . It is then available for various purposes such as B. as raw material for the chemical industry ( power-to-chemicals ), as drive energy for vehicles, ships and aircraft ( power-to-fuel ) or for reconversion in gas power plants or fuel cells .

The electrolysis of water is also important as a demonstration experiment; Hofmann's water decomposition apparatus is often used for this. Another application of water electrolysis is the enrichment of deuterium . Furthermore, water electrolysis is the most important side reaction of many technical electrolyses, e.g. B. the chlor-alkali electrolysis .

Reactions and their equations

Hofmann's decomposition apparatus : H ₂ and O ₂ behave largely like ideal gases . The measured gas volumes H ₂ to O _{2 have} the ratio 2: 1 and follow the stoichiometry of the electrolysis. The gas volumes are proportional to the electrical current that has flowed over the time of the measurement. The volumes are therefore proportional to the electrical charge .

The electrolysis of water consists of two partial reactions that take place at the cathode and anode electrodes . The overall reaction scheme for this redox reaction is:

${\displaystyle \mathrm {2\ H_{2}O(l)\ _{\overrightarrow {\rm {Elektrolyse}}}\ 2\ H_{2}(g)+O_{2}(g)} \qquad \Delta _{\text{R}}H^{\ominus }=\mathrm {+571,66\ kJ/mol} }$(at T = 298.15 K, p = 1.013 × 10 ⁵ Pa)

Electric current splits water into hydrogen and oxygen .

The electrodes are immersed in an electrolyte . The electrolyte is usually produced by adding an acid such as sulfuric acid or an alkali such as potassium hydroxide solution . Electrolysis is also successful when neutral salts such as sodium sulfate are used as the electrolyte. For example, hydrochloric acid or sodium chloride are unsuitable because chlorine is formed at the anode.

In acidic solution:

${\displaystyle \mathrm {Kathode{:}\ } }$	${\displaystyle \mathrm {4\ H_{3}O^{+}\ +\ 4\ e^{-}} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ 4\ H_{2}O} }$
${\displaystyle \mathrm {Anode{:}\ } }$	${\displaystyle \mathrm {6\ H_{2}O} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {\ O_{2}\ +\ 4\ H_{3}O^{+}\ +\ 4\ e^{-}} }$
${\displaystyle \mathrm {Gesamtreaktion{:}\ } }$	${\displaystyle \mathrm {2\ H_{2}O} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ O_{2}} }$

In alkaline solution:

${\displaystyle \mathrm {Kathode{:}\ } }$	${\displaystyle \mathrm {4\ H_{2}O\ +\ 4\ e^{-}} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ 4\ OH^{-}} }$
${\displaystyle \mathrm {Anode{:}\ } }$	${\displaystyle \mathrm {4\ OH^{-}} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {\ O_{2}\ +\ 2\ H_{2}O\ +\ 4\ e^{-}} }$
${\displaystyle \mathrm {Gesamtreaktion{:}\ } }$	${\displaystyle \mathrm {2\ H_{2}O} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ O_{2}} }$

In neutral sodium sulfate solution:

${\displaystyle \mathrm {Kathode{:}\ } }$	${\displaystyle \mathrm {4\ H_{2}O\ +\ 4\ e^{-}} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ 4\ OH^{-}} }$
${\displaystyle \mathrm {Anode{:}\ } }$	${\displaystyle \mathrm {6\ H_{2}O} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {O_{2}\ +\ 4\ H_{3}O^{+}\ +\ 4\ e^{-}} }$
${\displaystyle \mathrm {Gesamtreaktion{:}\ } }$	${\displaystyle \mathrm {\ 10\ H_{2}O} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ O_{2}\ +\ 4\ H_{3}O^{+}+\ 4\ OH^{-}} }$
${\displaystyle \mathrm {durch\ Neutralisation{:}\ } }$	${\displaystyle \mathrm {\ 10\ H_{2}O} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ O_{2}\ +\ 8\ H_{2}O} }$
${\displaystyle \mathrm {nach\ K{\ddot {u}}rzung{:}\ } }$	${\displaystyle \mathrm {\ 2\ H_{2}O} }$	${\displaystyle \mathrm {\longrightarrow } }$	${\displaystyle \mathrm {2\ H_{2}\ +\ O_{2}} }$

Between the half-cell to form concentration gradients in the electrolyte, since, depending on the conditions oxonium ions (H ₃ O ⁺ ) or hydroxide ions (OH ^- ) are formed or consumed. The oxonium ions migrate to the negatively charged cathode, the hydroxide ions to the positive anode. The ion mobility of H ₃ O ⁺ and OH ^- is comparatively high, since the ions do not migrate as a whole, but only protons (H ⁺ ) are shifted, see Grotthuss mechanism .

Apparent diffusion of an oxonium ion by shifting protons.

Apparent diffusion of a hydroxide ion by shifting protons.

Technical water electrolysis

This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

The energy efficiency of the electrolysis of water is between a little over 60% and 85%, depending on the detailed process used.

Ambiguities or ranges of such information may arise. a. from the difference between the calorific value and calorific value of hydrogen, which is around 18%. Since the electrolyte concentration and the temperature of an electrolyte solution have a major influence on the cell resistance and thus on the energy costs, a 25 to 30 percent potassium hydroxide solution is used for this, the temperature is around 70–90 ° C. The current density is around 0.15-0.5 A / cm², the voltage around 1.90 V. In practice, for example, an electrical energy of 4.3 is required to produce 1 m³ of hydrogen (at normal pressure ) -4.9 kWh required. The overvoltage can be reduced by approx. 80 mV using electrocatalysts (for cathodes e.g. Ni-Co-Zn, Ni-Mo, for anodes: nickel-lanthanum perovskite, nickel-cobalt spinel) .

There is also the option of separating distilled water through electrolysis. A proton- charged Nafion membrane is used in SPE hydrogen electrolysis . The thin perforated electrodes are located on the surface layer ( English zero gap " gap -free cell geometry") of the membrane. As electrode material, for. B. ruthenium oxide hydrates (anode) or platinum (cathode) can be used. SPE electrolysis appears to be gaining acceptance in the market for small electrolysers.

Research is also being carried out into high-temperature steam electrolysis (at 800 to 1000 ° C) on solid electrolytes . Yttrium-stabilized zirconium dioxide (YSZ) is usually used as the solid electrolyte . Alternatively, Sc or Ca-doped ZrO ₂ , Gd or Sm-doped CeO ₂ or electrolytes with a perovskite structure (e.g. based on LaGaO ₃ ; doped with Sr and / or Mg) can be used. Due to the increased operating temperature, the required voltage in the thermo-neutral operating point can be reduced to 1.30 V, the current density was 0.4 A / cm².

The electrical efficiency is particularly important when using hydrogen as a seasonal energy store, the so-called power-to-gas process. Electrolysis hydrogen (or, if necessary, methane after a subsequent methanation ) is used to compensate for the fluctuating generation of some regenerative energy sources and thus to achieve a stable power supply. The reconversion of electricity can take place in different ways; u. a. to gas power plants , cogeneration plants or fuel cells are used. Since power-to-gas results in high energy losses due to the very low efficiency of the energy chain electricity => hydrogen / methane => electricity, a future energy system should be designed in such a way that there is as little long-term storage requirement as possible, for which this technology is required .

Side reactions

1. Formation of ozone

   On smooth platinum anodes, in cooled (the ozone decomposition is then less), strongly acidic solutions with high current densities, ozone is formed. The formation of ozone is lower in an alkaline environment; it is even completely absent on nickel anodes.

   If alternating current is superimposed on the direct current, ozone is formed even at lower current densities.

2. Formation of hydrogen peroxide

   Hydrogen peroxide can mainly form on the cathode.

See also

• Electrolyzer
• List of hydrogen technologies

literature

• Michael Sterner , Ingo Stadler (ed.): Energy storage. Need, technologies, integration. 2nd edition, Berlin Heidelberg 2017, ISBN 978-3-662-48893-5 .

Web links

Commons : Water Electrolysis  - Collection of Pictures, Videos and Audio Files

Wiktionary: water electrolysis  - explanations of meanings, word origins, synonyms, translations

Individual evidence

1. ↑ See Volker Quaschning , Regenerative Energiesysteme. Technology - calculation - simulation . 9th updated edition. Munich 2015, p. 54f.
2. ↑ ^{a b} Peter Kurzweil, Paul Scheipers: Chemistry: Basics, Structural Knowledge, Applications and Experiments , 8th edition, Vieweg + Teubner, Wiesbaden, 2010, p. 198. Restricted preview in Google book search
3. ^ Karl-Heinz Lautenschläger, Wolfgang Weber: Taschenbuch der Chemie , 22nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2018, p. 366.
4. ↑ S. Milanzi, C. Spiller, B. Grosse, L. Hermann, J. Kochems, J. Müller-Kirchenbauer: Technical status and flexibility of the power-to-gas process. (PDF; 1.16 MB) Technische Universität Berlin , August 29, 2018, accessed on November 6, 2019 (Fig. 4-2 on page 7/18). 
5. ^ Carl Jochen Winter , Joachim Nitsch : Hydrogen as an energy carrier: technology and systems. Springer Verlag, 1986.
6. ↑ Singhal, Subhash C .; Kendall, Kevin (Eds.): High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications . 2003.
7. ↑ Gerd Sandstede: Possibilities for hydrogen generation with reduced carbon dioxide emissions for future energy systems. In: Chem.-Ing.-Tech. Volume 63, No. 6, 1993, pp. 586-589, doi: 10.1002 / cite.330630608 .
8. ↑ Günther Brauner: Energy systems: regenerative and decentralized. Strategies for the energy transition . Wiesbaden 2016, p. 89.
9. ^ Fritz Foerster: Electrochemistry of aqueous solutions. Publisher by Johann Ambosius Barth, 1915, p. 206ff.