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.