Two-electron oxygen reduction reaction by high-loading molybdenum single-atom catalysts

Excerpt

Hydrogen peroxide (H₂O₂) is an important environmentally friendly chemical and potential energy carrier and has been widely used for a range of applications, such as paper and pulp bleaching, chemical synthesis and wastewater treatment [1, 2]. Nowadays, about 95% H₂O₂ is produced from the anthraquinone oxidation process, which unfortunately is energy intensive and far from economical [3]. The direct synthesis of H₂O₂ from H₂ and O₂ is a green method, but it has the potential hazard of explosion [4]. More recently, electrochemical production of H₂O₂ from the two-electron oxygen reduction reaction (2e-ORR) attracts quickly growing attention due to its mild operation condition and potential for decentralized production. To achieve so, suitable electrocatalysts with high activity and selectivity are needed. At present, the state-of-the-art candidates for 2e-ORR are noble metal alloys (PtHg or PdHg [5, 6], Au alloys [7]). However, their high costs greatly limit their practical applications. Single-atom catalysts (SACs) are an emerging class of materials. They are prepared by dispersing single transition metal atoms on high-surface-area supports. Their electronic structures can be regulated by tuning the metal atoms and surrounding coordinative environments for different purposes. For example, Choi et al. [8] reported that high-loading Pt SACs anchored on S-doped carbon could enable selective 2e-ORR instead of conventional 4e-ORR. Wang et al. shifted the reaction selectivity from 4e-ORR to 2e-ORR by changing the coordinative environment of Fe SACs from Fe–N–C to Fe–C–O [9]. …