Main group elements in electrochemical hydrogen evolution and carbon dioxide reduction

Abstract

Main-group elements are renowned for their versatile reactivities in organometallic chemistry, including CO₂ insertion and H₂ activation. However, electrocatalysts comprising a main-group element active site have not yet been widely developed for activating CO₂ or producing H₂. Recently, research has focused on main-group element-based electrocatalysts that are active in redox systems related to fuel-forming reactions. These studies have determined that the catalytic performances of heavier main-group element-based electrocatalysts are often similar to those of transition-metal-based electrocatalysts. Our group has recently reported the scope of including the main-group elements in the design of molecular catalysts and explored their applications in redox catalysis, such as the generation of H₂ upon coupling of two protons (H⁺) and two electrons (e⁻). This feature article summarizes our research efforts in developing molecular electrocatalysts comprising main-group elements at their active sites. Furthermore, we highlight their influence on the rate-determining step, thereby enhancing the reaction rate and product selectivity for multi-H⁺/multi-e⁻ transfer catalysis. Particularly, we focus on the performance of our recently reported molecular Sn- or Sb-centered macrocycles for electrocatalytic H₂ evolution reaction (HER) and on how their mechanisms resemble those of transition-metal-based electrocatalysts. Moreover, we discuss the CO₂ reduction reaction (CO₂RR), another promising fuel-forming reaction, and emphasize the recent progress in including the main-group elements in the CO₂RR. Although the main-group elements are found at the active sites of the molecular catalysts and are embedded in the electrode materials for studying the HER, molecular catalysts bearing main-group elements are not commonly used for CO₂RR. However, the main-group elements assist the CO₂RR by acting as co-catalysts. For example, alkali and alkaline earth metal ions (e.g., Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, and Ba²⁺) are known for their Lewis acidities, which influence the thermodynamic landscape of the CO₂RR and product selectivity. In contrast, the elements in groups 13, 14, and 15 are primarily used as dopants in the preparation of catalytic materials. Overall, this article identifies main-group element-based molecular electrocatalysts and materials for HER and CO₂RR.