Nitrogen and Sulfur Co-Doped Carbon Nano-Onions for Efficient Electrochemical Conversion of Carbon Dioxide
Authors: Wanninayake Namal , Qianxiang Ai , Thomas Melonie , Kodithuwakku Udari Shyamika , Hoque Ariful , Guzman Marcelo I. , Guiton Beth , Risko Chad , Kim Doo Young 
Published: February 2020

Carbon dioxide(CO2) is the principal greenhouse gas contributing to global warming and climate change. Therefore, it is imperative to develop advanced methods and technologies to sequestrate or convert CO2 in the atmosphere. Recently, renewable energy driven electrochemical CO2 conversion has emerged as a promising route to convert CO2 into usable chemicals and fuels. Previous studies have shown that metal-free, hetero-atom-doped carbon-based electrodes are cost-effective and durable electrocatalyst for the reduction of CO2. However, studies related to co-heteroatom-doped carbon materials are marginal. These co-doped carbon catalysts lower the activation energy barriers of electrochemical CO2 reduction reaction(CO2RR) pathways by synergistically tuning the binding energies of CO2 molecules and the intermediates adsorbed on the active sites on the catalyst.
In this work, for the first time, both nitrogen(N) and sulfur(S) co-doped carbon nano-onions(CNOs) were investigated for CO2RR. CNOs are comprised of several fullerene-like carbon shells. The size of a CNO is typically below 10 nm. They have high electrical conductivity and large external surface area. Furthermore, the highly curved graphitic shell in CNOs shifts the electron density of graphene to the outer surface. This electron density enables the adsorption of reactants: thus, CNOs are ideal for catalytic applications.
In the present work, CNOs doped with N, S, as well as both N and S are systematically compared. The electrochemical tests were performed in an aqueous electrolyte by using a customized electrochemical cell. Products from the electrochemical reactions were characterized by gas chromatography and nuclear magnetic resonance spectroscopy. N doped CNOs(NCNOs) generated formic acid as the primary product(Maxium~50 % faradaic efficiency(FE) at -0.6 V vs. RHE). On the other hand, both N and S co-doped CNOs(NS-CNOs) produced carbon monoxide(CO) as a major product(Maximum~ 85% FE at -0.4V vs. RHE). The onset potential for the formation of CO was assessed by employing in-situ rotation ring disk electrode(RRDE) measurements. These measurements revealed that NS-CNOs could convert CO2 to CO at -0.20 V vs. RHE, close to its thermodynamic potential(-0.10 V vs. RHE). Finally, the stability of NS-CNOs were tested with an electrochemical setup equipped with a gas diffusion electrode(GDE). NS-CNOs on GDE maintained ~80 % FE at -0.4 V vs. RHE giving ~1.5 mA cm-2 current density for 8 hours.
X-ray photoelectron spectroscopic measurements were conducted for elemental and chemical state analysis. The morphology and microstructure of each doped catalyst, in particular, chemical structure related to heteroatom dopants, were revealed by high resolution scanning transmission electron microscopy. For Density functional theory(DFT) calculations, model structures of doped CNOs were built based on the findings of XPS and TEM characterizations. Then the DFT calculations were performed to understand the relationship between catalytic activity and the nature of the N and S environment on CNOs, and thereby viable electrochemical pathways of CO2RR were evaluated. Our results indicate that the electronic effect of N dopant and the geometric effect of S dopant in combination with the curvature of CNOs lead to a synergistic effect to catalyze CO2RR at low over-potential. These findings provide invaluable insights in developing efficient, selective and metal-free carbon-based catalysts for CO2RR.

Figure 1