## 1 Introduction  
Hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), a valuable environmentally friendly and clean oxidant that emits only water as a byproduct, has received increasing demands in the areas of chemical synthesis, pulp bleaching, pharmaceutical industry, wastewater treatment and so on [1]. Currently, most H\({}_{2}\)O\({}_{2}\) in the world is produced indirectly via the anthraquinone auto-oxidation process (AOP) with Pd catalysts, which has high H\({}_{2}\)O\({}_{2}\) yield and efficiency but still suffers from a complicated multistep reaction process, large consumption of organic solvents, high energy demands and the generation of organic byproduct waste [2]. Furthermore, the transport, storage and handling of concentrated H\({}_{2}\)O\({}_{2}\) cause an extra safety risk, making the in situ H\({}_{2}\)O\({}_{2}\) synthesis highly desirable. Thus, an efficient and inexpensive in situ green H\({}_{2}\)O\({}_{2}\) synthesis route is urgently needed to overcome the disadvantages of AOP. Nowadays, the attractive alternatives are H\({}_{2}\)O\({}_{2}\) direct synthesis from H\({}_{2}\) and O\({}_{2}\)[3] and H\({}_{2}\)O\({}_{2}\) electroproduction through an oxygen reduction reaction (ORR) [4]. However, the contact of H\({}_{2}\) and O\({}_{2}\) during the H\({}_{2}\)O\({}_{2}\) direct synthesis process has the potential risk of explosion over a wide composition range (5-95 vol%) and hinders the practical application [4]. Hence, the H\({}_{2}\)O\({}_{2}\) electrosynthesis through the 2e\({}^{-}\) ORR becomes the most promising approach, since it can simultaneously achieve efficient and safe H\({}_{2}\)O\({}_{2}\) production under mild conditions. Furthermore, the H\({}_{2}\)O\({}_{2}\) produced in acid electrolyte with a specific pH and concentration could be directly used in many industry applications for water treatment and pulp bleaching, which leads to more favorable H\({}_{2}\)O\({}_{2}\) electroproduction. Generally, the ORR involves a multielectron transfer process in which O\({}_{2}\) can be reduced to produce H\({}_{2}\)O via the four-electron (4e\({}^{-}\)) pathway (Equations (1) and (2)) or to form H\({}_{2}\)O\({}_{2}\) via the two-electron (2e\({}^{-}\)) pathway (Equations (3) and (4)) [5]. The final product of the 4e\({}^{-}\) pathway ORR is H\({}_{2}\)O, which is undesirable for H\({}_{2}\)O\({}_{2}\) production. Moreover, the sluggish kinetics and high overpotential for the 2e\({}^{-}\) ORR severely impede the energy efficiency of H\({}_{2}\)O\({}_{2}\) electrosynthesis. Therefore, it is necessary to develop a catalyst with high selectivity and activity toward the 2e\({}^{-}\) ORR [6]. \[O_{2}+2H_{2}O+4e^{-}\to 4 OH^{-}(alkaline),\ \ E^{{}^{\circ}}=0.4\ V \tag{1}\]  
\[O_{2}+4H^{+}+4e^{-}\to 4H_{2}O(acid),\ \ E^{{}^{\circ}}=1.23\ V \tag{2}\]  
\[O_{2}+H_{2}O+2e^{-}\to OH^{-}+HO_{2}^{-}(alkaline),\ \ E^{{}^{\circ}}=0.06\ V \tag{3}\]  
\[O_{2}+2H^{+}+2e^{-}\to H_{2}O_{2}(acid),\ \ E^{{}^{\circ}}=0.7\ V \tag{4}\]  
Many scientists and researchers have made efforts to find the proper catalyst. To date, the most efficient catalysts for the 2e\({}^{-}\) ORR are noble-metal-based materials (e.g., Pd-Au [7], Pt-Hg [8] and Pd-Hg [9]), which require a small overpotential and possess high H\({}_{2}\)O\({}_{2}\) selectivity (up to 98%). However, the scarcity of noble metals significantly raises the costs and limits the large-scale application. Carbon-based materials, because of their high earth abundance, electronic conductivity and chemical stability, have shown great prospect as candidates for H\({}_{2}\)O\({}_{2}\) electrosynthesis. Moreover, the activity and selectivity of carbon-based materials can be improved by adjusting the electronic and geometric structures, such as introducing nanomaterials (e.g., graphene [10], carbon nanotube [11] and carbon nitride [12]) and doping heteroatoms (e.g., O [13], N [14], S [15], etc.). However, such modifications usually suffer from complicated procedures or harsh pH condition, and thus, impede the intensive application to produce H\({}_{2}\)O\({}_{2}\).