Based on the literature, we selected Fe, Ni and Co atom for synthesizing the metal-doped carbon catalysts. These catalysts are known to produce carbon monoxide (CO) during electrochemical CO2 reduction. We presumed that dual-atomic Fe-Fe, Ni-Ni or Co-Co can facilitate the carbon-carbon coupling to form multi-carbon products such as ethanol or ethylene. Thus, we synthesized catalysts namely C2N900@Fe, C2N900@Ni, TAP 900@Fe and TAP 900@Ni. We observed that all of these catalysts mostly form single atomic sites with few dual-atomic sites as seen from HAADF-STEM images. Dual-atomic sites are very hard to stabilise and single-atomic sites are more thermodynamically favored. These materials exhibited excellent electrochemical CO2 reduction activity towards CO production and negligible amount of liquid products (formate, acetate, methanol, ethanol, propanol) were obtained.
Electrochemical facility was developed in the lab to screen the synthesized catalyst for the CO2 reduction. Due to COVID situation, the whole process took quite a long time from design to optimisation. Based on the literature, I have successfully designed an adaptable electrochemical cell for CO2 reduction testing. The surface area of the electrode was kept higher in comparison to the volume of the electrolyte used to detect even the minor liquid product formed. Benchmarking of the whole setup was carried out with the state-of-the-art Au/C (20 wt%) catalyst. GC was calibrated and optimised to analyse the gaseous products. Liquid 1H-NMR and High-performance liquid chromatography (HPLC) instrument was also optimised to analyse the liquid products formed during the CO2 reduction reaction.
Both TAP 900@Fe and TAP 900@Ni showed good CO2RR activity at lesser negative potential and approached a high CO selectivity (mean FECO ~ 95 %) at -0.55 V vs RHE. Bare TAP 900 showed very poor selectivity towards CO indicating the activity in TAP 900@M resulting predominantly from the M-Nx sites.
Despite the continuous effort towards improving the catalyst, we still have inconclusive idea about the actual improvement in catalyst activity through a proper comparison of the intrinsic activity. To understand this, we compared our synthesized catalyst with the best-reported catalysts for CO2 to CO production. We observed that all the best-reported catalysts seem to have similar TOF limitations and cannot exceed a particular TOF order range. Diving deeper we find a correlation between the observed limitations and the linear scaling relationship between the intermediates. Due to the existence of a scaling relationship between both the C-binding species (*CO and *COOH), tuning of only one intermediate is quite impossible. This is responsible for the persistent overpotential and limitation observed. Thus, it is quite difficult to achieve a high intrinsic activity/TOF and so all the best catalysts always overlapped each other.
During the course of MSCA fellowship, 3 articles were published in peer-reviewed journals, 2 manuscripts under communication and the results were presented in 2 conferences. Main manuscript related to this manuscript is still under communication and once the article is published, the results will be disseminated through youtube and twitter.
