Nitrogen-doped carbon materials (NCs) are extensively studied for the oxygen reduction reaction (ORR). However, the nature of active sites of pyridinic nitrogen (N_P) or graphitic nitrogen (N_G) on NCs is still under debate. Herein, we demonstrated that the ORR activity of NCs in alkaline media depended on the electronic synergism between N_P and N_G, rather than any single type of N. We measured the transferable electrons of NCs by absorption spectroscopy using 7′7′8′8-tetracyanoquinodimethane as an electron acceptor. The transferable electron amount of NCs is relevant to either N_P or N_G, leading to the [N_P] : [N_G] ratio as an electron transfer descriptor of NCs in a reverse volcano curve manner across nineteen NCs. The similar dependence of ORR activity on the [N_P] : [N_G] ratio of NCs was also discovered, demonstrating the synergistic effect of N_P and N_G. These results provide a new angle to understand the nature of ORR activity of NCs and optimize the ORR catalyst.

A novel AIE-active boron-difluoride complex (PTZ) was synthesized which exhibits multi-stimuli responsive characteristics. Its colours and emissions can be switched by mechanical grinding, organic solvent vapours and acid/base vapours. This complex can be utilized in data encryption and decryption based on the protonation–deprotonation effect.

The development of cheap and efficient electrodes for water splitting is an ongoing challenge. Herein, a nickel foam-based monolithic electrode is reported with high performance for both hydrogen and oxygen evolution reactions in neutral media. The karst landform-featured electrode consists of metallic Ni in the valley areas and Ni/α-Ni(OH)₂ heterostructures as towers on the surface. Remarkably, the electrode can switch between HER and OER reversibly and flexibly with long-term viability. Scanning electrochemical microscopy (SECM) analysis and computational studies proved the vital roles of the karst landform structure and the interface of the heterostructure in its efficient bifunctionality. Combined with a common photovoltaic cell, a water electrolysis cell with the bifunctional electrode can implement splitting of natural seawater (pH ∼ 8.0) with a high solar-to-hydrogen (STH) conversion efficiency of 16.5%. This cheap, flexible, robust, easily obtained and bifunctional electrode has potential applications for water splitting in the hydrogen economy.