Figure 6 (a) Schematic illustration of synthesis process of the Ni-N-CNS/CNT-X and Ni-N-CNS-Y catalyst. (b) TEM image Ni-N-CNS/CNT-20. Reproduced with permission.[91] Copyright 2021, American Chemical Society. (c) Schematic illustration of the preparation strategy for Ni-NC(HPU). (d) Low-magnification TEM image of Ni-NC(HPU). Reproduced with permission.[94] Copyright 2022, Wiley-VCH (e) Schematic illustration of the preparation process for CA/N-M aerogels. (f) FESEM image of CA/N-Ni aerogel (inset: digital photo). Reproduced with permission.[96] Copyright 2021, Wiley-VCH.
In recent years, carbon aerogel with the layered 3D porous network structure, which can provide highly exposed active sites, excellent electrolyte permeability and water wettability, as well as rich electronic interconnect channels, has also attracted wide attention.[95,96] This structure also provides a new opportunity to explore efficient carbon-based electrocatalysts for ECR. Hou et al. fabricated 3D porous aerogels with a large number of exposed M-N sites (CA/N-M, M = Ni, Fe, Co, Mn, Cu) through freeze-drying and carbonization processes (Figure 6e,6f).[96]The optimal CA/N-Ni aerogel can produce 98% FECO at −0.8 V and achieve the 91% FECO under 300 mA cm−2 in a flow cell. The highly exposed Ni-N sites from the multistage pore structure are proved to be true active centers for CO2 protonation and electron transfer. Furthermore, as the applied voltage was enhanced from −0.6 to −1.2 V, the stronger and stronger *COOH signal was captured by in-situ infrared spectroscopy, as well as CO2 was continuously consumed, indicating that *COOH formation is the rate-determining step in the ECR reaction.