Several reaction mechanisms having the metal centers as active sites have been proposed in the past. (3) However, the design of optimal catalysts first calls for a better understanding of the electrochemical reaction mechanisms, particularly the one related to the anodic OER and for a rationalization of the reasons behind the often observed changes in the catalyst’s electrochemical activity (either as positive or negative trend) during operation. From the point of view of speeding up the widespread market penetration of water electrolyzers, the development of a highly active, stable, and inexpensive electrocatalyst is highly demanded. Therefore, still open questions and great challenges orbit around the OER. (3) Neither the reaction mechanism nor the ideal catalyst in terms of activity and stability has been revealed so far. Even though the electrochemical splitting of water has been known since the 19th century when Paets van Troostwijk/Deiman and Nicholson/Carlisle made one of the most exciting scientific discoveries ever made, unveiling that electricity can decompose water into hydrogen and oxygen (see Figure 1), the oxygen evolution reaction (OER), that is, the anodic reaction of water electrolyzers, is still an enigma.
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