Tailoring N-Decorated MWCNTs as Efficient Metal-Free Catalysts for CO2 Selective Electroreduction to CO

The electrocatalytic conversion of CO2 into chemicals or energy vectors of added value is a highly challenging approach to cope with the steadily increasing demand for energy storage and valorization of renewable resources while limiting greenhouse gases (i.e. CO2) impact on global climate changes. To date, efforts in this area have been largely focused on the use of metal-containing catalysts and in particular precious and noble metals (i.e. Pd, Au, Ag, and Cu) that however suffer from poor sustainability both from an economical and an environmental viewpoint. The last few years have witnessed a rapid growth of interest towards carbon-based nanomaterials (CNMs), as such or in the form of light hetero-doped systems, to be employed as valuable alternative to metal-based catalysts in a variety of challenging transformations, including the CO2 reduction reaction (CO2RR). In particular, a series of N-doped systems have been widely exploited in this research area showing from good to excellent catalytic outcomes but offering only limited hints (due to the complex nature of their chemico-physical properties) to the comprehension of the real nature of the active sites engaged in the process. Recent findings from some of us have demonstrated how a fine tuning of the surface properties of CNMs can be conveniently achieved by chemical functionalization with tailored N-containing heterocycles. The chemical approach allows a precise control of N-dopants in terms of N-configuration and electronic charge distribution, offering at the same time a unique and powerful tool to the comprehension of the role of specific N-functionalities for the activation and conversion of small molecules. In this contribution we report the covalent chemical functionalization of MWCNTs with NH-aziridine species (MW@NAz) and their application as highly efficient and chemoselective metal-free electrocatalysts for CO2 reduction into CO (Figure 1). With a Faradaic efficiency (FE) close to 90% at -1.2 V (vs. Ag/AgCl/KClsat.) and a productivity as high as 48 NLCOh-1gN-1, MW@NAz ranks among the metal-free systems with the highest performance reported so far in the literature. Moreover, this approach to the tailored bottom-up synthesis of CO2RR electrocatalysts has provided a privileged view-point to the comprehension of the structure-reactivity relationship of this class of complex materials, shedding light on the supposed reaction mechanism. Electrocatalytic performance of N-decorated MWCNTs together with the postulated CO2RR mechanism will be discussed in detail.