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.