Basic Curiosity 3. Water oxidation: What and Why?

Water oxidation reaction

When water is oxidized it could mean that either oxygen is produced, or electrons are removed. Water oxidation is becoming more and more essential reaction in the environmental and energy chemistry, because using water as abundant energy source, the products can be coupled to further hydrogen evolution reaction, CO2 reduction or NH3 production to allow storage of renewable energy.

Hydrogen (H2) is predicted to be a promising energy storage medium or carrier with high energy density of 140 MJ∙Kg-1, which far exceeds those of gasoline and coal. However, nearly 95% of H2 is still produced from hydrocarbons, such as fossil fuels or biomass which is severely restricted by its high air emission and high cost. Thus, water electrolysis is expected to be a greener method which requires only water and electricity. In addition, electricity can be supplied by electricity surplus from wind, hydro power, or photovoltaics [1].

Thyssenkrupp have been established the industrially scaled water electrolysis system to produce green hydrogen. What’s interesting is that they rely their production cycles only on renewable energies. They have used renewable electricity produced by wind and solar and green hydrogen to produce, for example, fertilizers, fuels and chemicals.

Watch a full video at https://www.youtube.com/watch?v=ftAmgrCC4z0 .

To improve water oxidation efficiency and lower the process cost, it is important to use efficient, low-cost catalyst. We need Oxygen Evolution Reaction (OER) catalyst at anode, where the oxidation of water takes place when applied with positive potential. On the other side, we need Hydrogen Evolution Reaction (HER) catalyst at cathode, where reduction of protons takes place when applied with negative potential. In this report, LDH (Layered-Double-Hydroxide) was used for water electrolysis in alkaline solution.

Research have been active to prove the in-depth mechanism of water oxidation in various catalysts. Real movements of atoms and molecules can be different in respect to the difference in composition and morphology of the catalyst, temperature and amount of usage of the system, and electrolyte solution. For example, in order to probe the mechanism of the removal of 4 protons and 4 electrons, the transfer of electrons and protons can be happened in a concerted way (at the same time) or in a sequential way. In sum, whether the Proton-Coupled Electron Transfer (PCET) happen or not can be determined by finding isotopic thermodynamic and kinetic effect.

Reference

[1] Z. Cai, X. Bu, P. Wang, J. C. Ho, J. Yang and X. Wang, J. Mater. Chem. A, 2019, DOI: 10.1039/C8TA11273H