How to produce hydrogen - 2

The previous post touched upon the methodes of producing hydrogen (see How to produce hydrogen - 1). "How to produce hydrogen using renewable electricity" - one solution to this challenge is water electrolysis. Most of you supposedly learnt about water electrolysis in high school.

This post introduces briefly the water electrolysis, with a bit more details than what you had learnt in the past.

History of water electrolysis^[1-6]

The fact that water electrolysis is taught in high school indicates that its technology has been well understood and established. The chemical equation below describes the water splitting.
2H₂O --> 2H₂ + O₂ ...(1)

The first scientific report on water electrolysis dates back to 18th century. In 1789, Jan Rudolph Deiman（1752-1837）and Adriaan Paets van Troostwijk（1743-1808）reported about electrolysis of water.^[1] Two electrodes are immersed in an aqueous solution, and supplying eletric power between the electrodes yields hydrogen and oxygen.

The water splitting reaction driven by electric power can be desribed by the oxidation-reduction reactions (so called half-reaction):
Reduction reaction @ cathode: 2 H₂O + 2 e^- --> H₂ + 2 OH^- ...(2)
Oxidation reaction @ anode: 4 OH^- --> O₂ + 2 H₂O + 4 e^- ...(3)

The figure below summarizes the history of water electrolysis development, based on references.^[1-6]

* Sorry that the figure is in Japanese, but will be translated into English in future.
「水の電気分解」の発見 = Discovery of "water electrolysis", アルカリ水電解 = alkaline water electrolysis, 固体高分子形水電解 = polymer electrolyte membrane (PEM) water electrolysis, 固体高分子形水電解 = solid oxide water electrolysis

Alkaline water electrolysis
The water electrolysis process industrialized in 20th century is the alkaline water electrolysis. This type of electrolyzer employes aqueous solution at alkaline pH as electrolyte, in which the reactions equations (2) and (3) undergo, whereby hydrogen and oxygen evolve.

Polymer electrolyte membrane（PEM）water electrolysis
The middle of 20th century had witnessed the significant advancement of polymer chemistry, leading to the development of the ion-conductive polymer membrane. Among such membranes, some allow for quite rapid transport of the positively charged hydrogen ion (proton) H⁺. This discovery initiated the construction of another type of electrolyzer using such polymer electrolyte membrane (PEM). In this "new" type of electrolyzer, proton connects the oxidation and reduction reactions. Therefore, the half-reactions differ from the aforementioned alkaline one, and are described as follows:
Reduction reaction @ cathode: 2 H⁺ + 2 e^- --> H₂ ...(4)
Oxidation reaction @ anode: 2 H₂O --> O₂ + 4 H⁺ + 4 e^- ...(5)

Solide oxide water electrolysis
At the similar timing, solid oxide with high oxygen anion conductivity has been under development. The water electrolysis employing such an oxide as the electrolyte is called solide oxide water electrolysis. Distinct from the alkaline and PEM water electrolyzers, this solide oxide one typically requires operations temperatures higher than 500 C. It is simply because that high-temperature is needed to achieve high oxygen anion conductivity.

Overview of water electrolyzer

Among the electrolyzers, below an overviwe of alkaline and PEM electrolyzers are summarized in a figure.

* Sorry that the figure is in Japanese, but will be translated into English in future.
アルカリ水電解 = alkaline water electrolysis, 固体高分子形水電解 polymer electrolyte membrane water electrolysis, 陰極 = cathode, 隔膜 = diaphragm, 固体高分子膜 = polymer electrolyte membrane, 陽極 = anode

The water electrolyzer cell is basically composed of cathode, anode, and membrane/diaphragm separating them.

Alkaline water electrolyzer employes the diaphragm as a separator. The water molecule is reduced at the cathode (equation 2) into hydronge. The byproduct, hydroxide anion OH^-, is transported to the anode, where it is oxidized into oxygen.

In the PEM water electrolysis process, differnt ion participates in the cycle and flows in the different direction from the alkaline counterpart. The water moecule as the reactant is supplied to the anode, and oxidized into oxygen and proton. Because the PEM allows for the rapid transportation of proton, the generated proton is transported to the cathode, where it is reduced into hydrogen.

Coming posts will summarize more details on water electrolysis, such as the price of produced hydrogen by electrolysis.

References

1. R. de Levie, J. Electroanal. Chem. 1999, 476, 92.
2. K. Zeng, et al., Prog. Energy Combs. Sci. 2010, 36, 307.
3. W. Kreuter et al. Int. J. Hydrogen Energy 1998, 23, 661.
4. R. L. LeRoy, Int. J. Hydrogen Energy 1983, 8, 401.
5. P. W. T. Lu et al., J. Appl. Electrochem. 1979, 9, 269.
6. H. S. Spacil et al., J. Electrochem. Soc. 1969, 116, 1618.
7. Ø. Ulleberg, Int. J. Hydrogen Energy 2003, 28, 21.
8. M. Carmo et al., Int J. Hydrogen Energy 2013, 38, 4901.