Oxygen reduction reaction (ORR) is an important cathode reaction in fuel cells and metal–air batteries. Composites of transition-metal sulfides (TMSs) and nitrogen-doped carbon (NC) are promising alternative ORR catalysts because of their high catalytic activity. However, the agglomeration of TMS particles limits practical applications. Here, a confinement catalyst composed of Co9S8@NC with a flower-like morphology was derived from metanilic intercalated Co­(OH)2 through interlayer-confined carbonation accompanied with host-layer sulfidation. The surface of the Co9S8 particles is covered with a few layers of nitrogen-doped graphene, which can prevent the Co9S8 particles from agglomeration and also produce catalytic activity affected by internal Co9S8. Thus, the Co9S8@NC material achieves excellent ORR performance with a half-wave potential of 0.861 VRHE. In addition, an oxide layer on the surface of Co9S8@NC is fabricated shortly after the ORR starts. Further tests and density functional theory calculations indicated that this cobalt oxide layer can increase the electrochemically active area of Co9S8@NC as well as reduce the ORR energy barrier, thereby providing more catalytic active sites and enhancing the intrinsic catalytic activity, thus achieving a self-activation effect during the electrochemical reaction process.

The key to the design of electrochemical nitrogen reduction (NRR) catalysts is that the reaction sites can not only activate the NN bond but also have high catalytic selectivity. Vacancy engineering is an effective way to modulate active sites, and cation vacancies are considered to have enormous potential in tuning catalytic selectivity. However, research on NRR activity is still at an early stage due to the difficulty in preparation and precise regulation. Here, we provided an adjusted method of cation vacancy through topotactic transformation, which combines solvothermal reduction with etching via lattice confinement effect to accomplish precursor reduction and vacancy construction while maintaining consistent material morphologies. Based on the topotactic transformation, NiAl-LDH precursor was reduced to Ni metal nanoflower, while Al is simultaneously etched by alkali, thus the precise tunability of the cation vacancy can be achieved by adjusting the Al content in the LDH. The Ni nanoflower achieved excellent stability and high ammonia yield by adjusting the vacancy concentration. In addition, the insight into the selectivity and intrinsic activity of cation vacancies on NRR process has been revealed. For the reaction selectivity, the cation vacancy is beneficial to activate NN but not conducive to the HER process. For the intrinsic NRR activity, the generation of cation vacancies can also significantly reduce the energy barrier of NRR process and accelerate the reaction kinetics.

Cu(ii) ions are one of the most common forms of copper present in water and can cause bioaccumulation and toxicity in the human body; therefore, sensitive and selective detection methods are required.

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