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A series of triple hierarchical micro-meso-macroporous N-doped carbon shells with hollow cores have been successfully prepared via etching N-doped hollow carbon spheres with CO2at high temperatures. The surface areas, total pore volumes and micropore percentages of the CO2-activated samples evidently increase with increasing activation temperature from 800 to 950 °C, while the N contents show a contrary trend from 7.6 to 3.8 at%. The pyridinic and graphitic nitrogen groups are dominant among various N-containing groups in the samples. The 950 °C-activated sample (CANHCS-950) has the largest surface area (2072 m2g−1), pore volume (1.96 cm3g−1), hierarchical micro-mesopore distributions (1.2, 2.6 and 6.2 nm), hollow macropore cores (~91 nm) and highest relative content of pyridinic and graphitic N groups. This triple micro-meso-macropore system could synergistically enhance the activity because macropores could store up the reactant, mesopores could reduce the transport resistance of the reactants to the active sites, and micropores could be in favor of the accumulation of ions. Therefore, the CANHCS-950 with optimized structure shows the optimal and comparable oxygen reduction reaction (ORR) activity but superior methanol tolerance and long-term durability to commercial Pt/C with a 4e−-dominant transfer pathway in alkaline media. These excellent properties in combination with good stability and recyclability make CANHCSs among the most promising metal-free ORR electrocatalysts reported so far in practical applications.

Hydrogen peroxide (H2O2) in situelectrosynthesis by O2reduction reaction is a promising alternative to the conventional Fenton treatment of refractory wastewater. However, O2mass transfer limitation, cathodic catalyst selectivity, and electron transfer in O2reduction remain major engineering obstacles. Here, we have proposed a systematic solution for efficient H2O2generation and its electro-Fenton (EF) application for refractory organic degradation based on the fabrication of a novel ZrO2/CMK-3/PTFE cathode, in which polytetrafluoroethylene (PTFE) acted as a hydrophobic modifier to strengthen the O2mass transfer, ZrO2was adopted as a hydrophilic modifier to enhance the electron transfer of O2reduction, and mesoporous carbon (CMK-3) was utilized as a catalyst substrate to provide catalytic active sites. Moreover, feasible mass transfer of O2from the hydrophobic to the hydrophilic layer was designed to increase the contact between O2and the reaction interface. The H2O2yield of the ZrO2/CMK-3/PTFE cathode was significantly improved by approximately 7.56 times compared to that of the conventional gas diffusion cathode under the same conditions. The H2O2generation rate and Faraday efficiency reached 125.98 mg·cm−2·h−1(normalized to 5674.04 mmol·g−1·h−1by catalyst loading) and 78.24% at −1.3 V versus standard hydrogen electrode (current density of −252 mA·cm−2), respectively. The high H2O2yield ensured that sufficient ·OH was produced for excellent EF performance, resulting in a degradation efficiency of over 96% for refractory organics. This study offers a novel engineering solution for the efficient treatment of refractory wastewater using EF technology based on in situhigh-yield H2O2electrosynthesis.