CHEM-E4185 - Electrochemical Kinetics, 25.02.2019-29.05.2019

The term “electrolyzer” refers to an electrochemical device in which decomposition of water into its molecular constituents, hydrogen and oxygen, takes place. The reactions are essentially the same as those occurring in a hydrogen fuel cell, but proceed in a reversed, non-spontaneous direction. Consequently, in an electrolyzer, electrical energy is converted into chemical bond energy. In this section, we introduce the two most common elecrolyzer systems, namely alkaline and polymer electrolyte electrolyzers.

Aqueous KOH electrolyte (see Figure 9.7)-based alkaline electrolyzer technology is more mature than acidic polymer electrolyte membrane electrolyzers (PEME). In the former system, a diaphragm is used to separate the anode and cathode compartments to prevent mixing of the generated H₂ and O₂ gases. The alkaline environment enables use of affordable electrode materials, and nickel-based (hydro)oxides are often used to catalyze the electrochemical reactions which proceed according to reactions:

Anode:          2 OH⁻  \(\ce{ -> }\)  ½ O₂ + H₂O + 2e⁻                                                                   E^o = 0.00 vs. SHE

Cathode:       2H₂O + 2e⁻ \(\ce{ -> }\)  H₂ + OH⁻                                                                          E^o = 0.00 vs. SHE

Overall:          H₂O \(\ce{ -> }\)  H₂ + ½ O₂

PEME consists of similar components to PEFC: an acidic membrane electrolyte and noble metal electrodes. The benefits of a PEME over an alkaline electrolyzer are a higher current density and a shorter response time. PEMEs can also operate at higher pressures, and gas crossover is lower, producing high quality pure hydrogen. However, expensive noble metal-based catalysts, Pt for the cathode and Ir for the anode, are required and, hence, investment costs are high. Both the catalyst materials are scarce and alternatives are being sought. As stated above, reactions are similar to those in a PEFC but proceed in a reverse direction. 

Figure 9.7. Operation principle of an alkaline electrolyzer for generating O₂ and H₂ from water.

Electrolyzers and other electrochemical devices enabling the conversion of electrical energy to chemical energy are attractive choices for renewable energy storage. In particular, the number of solar and wind power stations is drastically increasing, generating an excess of electricity during peak production hours. In order to maximize their benefit, this surplus needs to be converted or stored. Hydrogen has several uses: In addition to the above-mentioned fuel application, hydrogen is consumed in several chemical industry processes, and it can be mixed with natural or city gas or stored in containers for subsequent use.