H2O2is a valuable chemical and has a diverse range of practical applications,including pulp and textile bleaching (Hage et al. 2006), chemical synthesis(Zhang et al. 2020), wastewater and ballast water treatment(Kim et al. 2020; Liu et al. 2018),exhaust air treatment(Yang et al. 2018),semiconductor cleaning (Seh et al. 2017), as well as medical and cosmetic uses (Melchionna et al. 2019). The global H2O2market demand exceeded 3.85 million tons in 2015 and is projected to surpass 6.0 million tons by 2024 due to its wide and promising applications (Kim et al. 2018).
At present, the industrial synthesis of H2O2predominantlyrelies on the highenergy-consuming and waste-producing anthraquinone process, which involves continuous hydrogenation and oxidation of anthraquinone molecules in the presence of expensive palladium catalysts (Xia et al. 2019). Anthraquinone process is suitable for large-scale production of H2O2, and the concentration of H2O2 is usually about 30%. Furthermore, the easy decomposition and instability of high-concentration H2O2lead togreat safety risks for transportation,handlingand storage (Li et al. 2020). As a simple, easily operable and eco-friendly process,electrochemical synthesis of H2O2 by ORRstands out as a potential alternative route to anthraquinone process, which could permit in situ generation to minimize transportation and storage costs (AlJaberi et al. 2019; Wang et al. 2021).
ORR has two pathways: the two-electron (2 e–) pathway where H2O2 is generated through partial reduction of O2(Eq.1 and 2), and the four-electron (4e–) pathway where oxygen (O2) is completely reduced to water instead (Eq.3 and 4). Thus, to produce H2O2effectively, it is necessary to promote the2 e– pathway andsuppressthe 4 e– pathway.
2 e– pathway:
4 e– pathway:
Various noble metals and their alloys have been suggested as prospective catalysts for H2O2 production (Jiang et al. 2018).For example, Pt-Hgwere highly active and selective for 2 e– ORR, on which the H2O2 selectivity was already 96% at a potential in the range 0.2−0.4 V versus RHE (Siahrostami et al. 2013).Unfortunately, the high cost and sophisticated fabrication process of noble metal-based electrocatalysts will be major bottlenecks in their industrial and household applications. Thus, metal-free carbon-based materials are regarded as promising substitutes due to their global abundance, low price and apparent ORR activity (Moreira et al. 2019; Pham-Truong et al. 2018). Currently, various carbonaceous materialshave been successfully developed as catalyticmaterials for selective H2O2synthesis, including graphite (Da Pozzo et al. 2005), graphite felt (Wang et al. 2015; Zhao et al. 2020), reticulated glassy carbon foam (Zhou et al. 2018), carbon felt (Pérez et al. 2018; Ma et al. 2019), and activated carbon fiber (Wang et al. 2005). In a recent study(Xia et al. 2015), polyacrylonitrile-based carbon fiber brush (PAN-CF) was employed to promote the 2 e- ORR. The H2O2 concentration reached 90.4 mg·L–1in 60 min, which may be attributed to catalytic selectivity of the nitrogen-doped structure in PAN-CF. Additionally, Zarei et al. (2009) found that theH2O2concentration obtained using carbon nanotube-PTFE electrode was almost 30 times as high as that of graphite felt electrode. The porous surface of the nanotube-PTFE electrode plays a key role in increasingoxygen transfer rates, thereby achieving enhanced mass transfer and high-efficiency H2O2production.
To further improve the catalytic activity and selectivity of carbon-basedcatalystsfor 2e- ORR, many modification methods have been utilizedto optimize their porous structures,surface functional groupsand surface area (Park et al. 2014; Thostenson et al. 2017; Shiraishi et al. 2015). For instance,Zhou et al. (2013) claimed that the modified graphite felt electrode displayed higher electrocatalytic activity for 2 e– ORR. The H2O2 concentration reached 247.2 mg·L–1, which was 2.6 times that of the unmodified electrode.Liu et al. (2015) found that the hierarchically porous carbon (HPC) maintained high H2O2 selectivity (80.9–95.0%) under acidic conditions, which was comparable to those of noble metal-based ORR electrocatalysts. They also demonstrated that the goodcatalytic activity and selectivity of HPC toward H2O2 were related to the high extent of sp3-C bonds and defects formed during hydrothermal treatment and carbonization process under H2.
Besides catalyst microstructures, electrode macro design parameters play critical roles in mass transfer and energy efficiency in different types of electrochemical systems(Oliaii et al. 2018). Nevertheless, little is known on the influence of cathode thickness and area on H2O2 electrosynthesis efficiency via ORR. In the present study, a series of cathodes were prepared by a simple coating method with stainless steel mesh as matrix, graphite powder as catalyst and polytetrafluoroethylene as binder, and the effects of electrode thickness, area and electrochemical process parameters on H2O2 yield and CE were systematically studied.