Electrochemical CO 2 reduction reaction (CO 2 RR) is an attractive strategy for sustainable production of chemicals and has mainly been implemented in alkaline or neutral electrolytes. However, part of input CO 2 is consumed by the formation of carbonate under these conditions. Herein, a space-confined strategy is proposed for CO 2 RR in acidic media, and Ni nanoparticles are encapsulated inside N-doped carbon nanocages as yolk−shell nanoreactors. By confining CO 2 RR in the cavities of nanoreactors, a Faradaic efficiency (FE) of 93.2% for CO is achieved at pH 7.2 and 84.3% FE for CO at pH 2.5. The inhibited proton diffusion within the Nernst layer of a nanoreactor is responsible for suppression of competing hydrogen evolution in acid. Moreover, CO 2 RR in an acidic flow electrolysis system offers enhanced current density and sustainable operation, in comparison with the conventional neutral pH system. This work shows that steering of mass transport via a unique structure is a viable avenue toward selective CO 2 conversion, and it provides a further understanding of the structure−performance relationship of electrocatalysts.

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Imine-nitrogen-doped carbon nanotubes (NCNTs) were prepared by pyrolysis using polyaniline as a major nitrogen source. They can electrocatalytically reduce CO 2 to CO with high selectivity and efficiency. The current density of CO presented a volcanic distribution in the applied potential between À 0.53 and À 0.93 V (vs RHE), reaching 13.8 mA • cm À 2 at À 0.83 V. The maximum Faraday efficiency reached 93 % at À 0.63 V, and the current density of CO was 6.8 mA • cm À 2 . Furthermore, the catalyst can directly reduce CO 2 in simulated flue gas (15 % CO 2, 77 % N 2 and 8 % O 2 ) and exhibited a high CO FE of 70 %. The high catalytic activity originates from rich content of iminenitrogen in NCNTs, serving as the active sites for catalysis.

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