5 Challenges and opportunities
In view of the various advantages of Ni-N-C electrocatalyst, such as low
cost, excellent performance, strong stability, high selectivity, etc.,
Ni-N-C
material has been focused by more and more researchers in the ECR field.
The rapid development of ECR technology shows more possibilities and
hopes. Although a considerable part of Ni-N-C electrocatalysts with
remarkable performance have been synthesized, the catalytic performance
has not reached the expectation in the practical application, because
there are still some difficulties and challenges to be overcome:
(i) Low hydrocarbon products. Hydrocarbon products have
higher energy storage capacity and chemical applications. Although
Ni-N-C materials possess the almost 100% CO selectivity, no low
hydrocarbon products are generated due to the active desorption of *CO
intermediate at the isolated active site limiting the further reduction
of CO2. The introduction of other metals with high
hydrocarbon selectivity (Cu, Mn, etc.) to develop the synergistic
bimetallic/polymetallic electrocatalysts, may be currently the most
effective way to improve the selectivity of hydrocarbon products.
(ii) Wide pH window. The environment of electrochemical
reaction has obvious influence on the activity output. At present, ECR
process is mostly performed in nearly neutral (bicarbonate) or alkaline
(KOH) solution. However, the dissolved CO2 is easily
converted into CO32-, which can reduce
the reaction efficiency and cause the cross contamination with counter
electrode. In contrast, acidic environment displays the obvious
advantage of speeding up the reaction process and requiring low
overpotential.[128,129] However, the low
solubility of CO2 in acidic environment and the easy
erosion of Ni SA/NPs reduce the electrode life. Most importantly, the
abundant proton in strong acid electrolyte facilitates the hydrogen
evolution process and is not conducive to the CO2reduction. Therefore, while it is necessary to improve the stability of
the electrocatalysts, the working environment of the catalysts should
also be adjusted to form a highly active and stable gas-liquid-solid
three-phase interface microenvironment.
(iii) Self-supporting materials. Most M-N-C materials as
powder form are attached to the electrode with the adhesive, which
increases the reaction resistance and reduces the electron transport.
It’s worth noting that the long-term testing will cause the power to
fall off, resulting in catalyst deactivation. The self-supporting
catalyst can effectively avoid the above problem due to the in-situ
generating active components. Furthermore, the self-supporting electrode
with abundant pores can accelerate the gas diffusion and the
high-efficiency, large-scale and rapid catalytic reaction.
(iv) Reasonable mechanism illumination. Accurately
controlling the reaction process is difficult due to the complex path of
CO2 reduction reaction. The reaction process was usually
tracked and via theoretical simulation and in-situ characterization
technique. However, the composition and structural changes of Ni-N-C
catalysts in detail (e.g., precise location and type of defects) cannot
be expressed precisely, while the establishment of theoretical
computational models is often based on idealized or incomplete
conditions. Therefore, the catalytic active center, the intermediate
species and the chemical evolution process should be determined by
various channels, which is essential for scientific and systematic
analysis and regulation of the intrinsic activity of catalysts.
(v) Industrialized application. The significance of
industrial development over the potential energy conversion is
self-evident. However, high overpotential, low current density, low
CO2 conversion and complex reaction units hinder the
practical ECR application. Therefore, this is inseparable from the
collaborative progress of basic research and engineering research, to
develop efficient and stable catalysts, as well as intelligent and
large-scale reaction devices. Of course, the mass-produced
electrochemical materials such as Ni-N-C, and the efficient separation
of electrochemical products are worthy of further consideration.