In comparison with the developing nano-carbon catalysts, some small organic molecules are also emerging as catalysts with typical features, however, their working mechanism is still unclear. Here, we synthesized a series of viologen-based heterogeneous catalysts with the same molecular skeleton but different substituent groups through anion exchange engineering. These viologen-based molecules were used as a model catalyst to investigate the underlying structure–function relationship for small molecules-based H₂O₂ electrosynthesis. Differing from the commonly reported carbon-based electrocatalysts, viologens can produce H₂O₂ in a synergistic manner, which means that viologens can not only directly catalyze oxygen reduction but also serve as a redox mediator. We found that the ring current and H₂O₂ selectivity of viologens deliver an increasing trend with the increase of the alkyl chain length of alkyl-substituted viologens and further increase when using benzyl as the substituent group. As a result, a benzyl-substituted viologen (BV) delivers the best electrocatalytic performance among the samples, including the highest H₂O₂ selectivity of 96.9% at 0.6 V and the largest ring current density of about 13.6 mA·mmol⁻¹. Furthermore, density functional theory (DFT) calculations disclose that the carbon atoms bonded with positively charged N are the active sites and the small highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap of BV is beneficial to the synergistic mechanism for H₂O₂ production. This work sheds new insight into the efficient H₂O₂ production in a synergistic manner for small molecules-based electrocatalysts.

Rational design and tailoring of the structural features of Co–N–C catalysts are urgently required to construct highly efficient bifunctional non-noble metal electrocatalysts for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, we report a series of carbon-based catalysts with varied structural features, specifically the graphitic degree of carbon, porosity, and the configuration of active sites, and their effects on bifunctional oxygen electrocatalytic reactions. Through the synergistic tuning of these structural factors, the well-tailored Co–N–C catalyst exhibits a high bifunctional electrocatalytic activity, as revealed by a half-wave potential of 0.84 V for ORR and a low overpotential of 420 mV at 10 mA·cm⁻² for OER. More impressively, the Zn-air battery using the optimum catalyst delivers excellent performance including a peak power density of 125.2 mW·cm⁻² and a specific capacity of 790.8 mAh·g_Zn⁻¹, as well as stable cycling durability, outperforming the noble metals-based catalysts. The first-principles calculations reveal that the interlayer interaction between the pyridinic N-doped graphene and the confined Co nanoparticles increases the electronic states of the active C atoms near the Fermi level, thus enhancing the adsorption of the HOO* intermediate and generating superior catalytic activity for bifunctional oxygen electrocatalysis. By comprehensively studying the structural factors of catalysts, the bifunctional catalytic behaviors, the use in a practical Zn-air device, and theoretical simulations, this work may also give inspirations to the design, use, and understanding of other kinds of catalysts.

Dual-doping of carbon, especially the combination of nitrogen and a secondary heteroatom, has been demonstrated efficient to optimize the oxygen reduction reaction (ORR) performance. However, the optimum dual-doping is still not clear due to the lack of strong experimental proofs, which rely on a reliable method to prepare carbon materials that can rule out the interference factors and then emphasize only the doping effects. In this work, an inside-out doping method is reported to prepare carbon submicrotubes (CSTs) as a material to study the principles of designing dual-doping catalysts for ORR. The interference factors including the metal impurities and doping gradient in the bulk phase are excluded, and the doping effects including the structural and chemical variation of carbon are studied. P-doping exhibited a higher pore-forming ability to perforate carbon and a lower doping content, but a higher ORR catalytic activity as compared with S- and B-doped N-CSTs, demonstrating the N, P co-doping is more efficient in making carbon-based catalysts for ORR. First-principle calculations reveal that the edge C situated around the oxidized P site nearby a graphitic N atom is the active site that shows the lowest ORR overpotential comparable to Pt-based catalysts. This study suggests that the catalytic activity of dual-heteroatoms-doped carbons not only depends on the intrinsic chemical bonding between heteroatoms and carbon, but also is affected by the structural variation generated by introducing different atoms, which can be extended to the study of other kinds of functionalization of carbon and potential reactions besides ORR.