New and Revised Aspects of the Electrochemical Synthesis of Hydrogen Peroxide: From Model Electrocatalytic Systems to Scalable Materials

Hydrogen peroxide is a crucial commodity with a wide range of applications in a variety of industrial processes, including disinfection and water treatment. With the neologism green H2O2, we can classify hydrogen peroxide produced by oxygen reduction or water oxidation making use of electricity obtained from renewable sources without emitting carbon dioxide into the atmosphere. The production of environmentally friendly H2O2 through oxygen reduction, and even more so through water oxidation, is currently impeded by the slow advancement of efficient electrocatalysts, along with the lack of suitable reactors. Nonetheless, the realization of producing green H2O2 is within reach. In this regard, this review paper aims to evaluate and resume not only the existing literature considering the synthetic procedures and performances of different electrocatalysts but also to put in order and clarify aspects relating to the mechanism of oxygen reduction and the role of active sites in the various functional materials proposed (both as model system or scalable one), while also highlighting good practices for the correct electrochemical screening of electrocatalysts and thus for obtaining reliable performance parameters. Hence, a comprehensive evaluation of catalysts, including those based on noble metals, noble-metal-free, and metal-free catalysts, is presented and critically discussed. This evaluation encompasses not only activity and selectivity but also considers a sustainability perspective. A thorough assessment of methods and techniques for the detection and quantification of H2O2 is conducted, with particular attention to understanding various applications: transitioning from qualitative to quantitative analysis involves employing methods to unravel the mechanism of H2O2 synthesis and utilizing techniques capable of identifying and quantifying H2O2 in various applications including environmental and biomedical ones. The emphasis is on identifying potential cross-contamination between fields that, although distinct in their aspects, may involve H2O2. Furthermore, an effort is made to emphasize technological advancements and outline expectations for the development of H2O2 electrolyzers, taking into consideration various factors, including the rate of conversion, accumulation, and operating voltage.