Metal-Free Heterogeneous Nanoporous Carbon for CO2 electrochemical Reduction to CO and CH4 in Aqueous Solution
Authors: Li Wanlu , Herkt Barbara , Seredych Mykola , Bandosz Teresa J 
Published: February 2020

Recently, the electrochemical conversion of CO2 has gained a significant attention. It provides a potential solution for minimizing the CO2 concentration level in the environment and also offers an alternative way to convert it to renewable fuels. Various metals (Ag, Au, Cu, etc.) and metal complexes are used as heterogeneous catalysts for CO2 reduction. However, the limitations in their application are in the fact that many polycrystalline metals show a low product selectivity and high overpotential. Also, the rising cost of noble metals and a low stability of the catalysts are the main hindrance towards their practical applications. Therefore it is important to develop a new class of catalysts, which are less expensive than metals and show a sufficient performance in CO2reduction.Metal-free N-doped carbon nanofibers and N-doped carbon nanotubes (NCNTs) have been addressed in the literature as a catalysts for the reduction of CO2 to CO. Nanoporous carbons, which exhibit well-developed porosity combined with specific surface chemistry might be potential candidates for CO2 reduction catalysts. The previous research on nanoporous heteroatom doped (S- and N-) polymer-derived carbon indicated that the catalytic performance is governed by the positively charged sites in the carbon matrix. Pyridinic nitrogen was found as crucial since it brings positively charged sites to the carbon atoms, which stabilizes the CO2
·
¯
and COOH* intermediates for CO formation. The surface basicity and high volume of ultramicropores improved the CO2reduction process.

In this research, various treatments were used to incorporate nitrogen groups to commercial wood-based activated carbon (BAX-1500, MeadWestvaco). The obtained carbons were studied for CO2 electrochemical reduction in KHCO3 electrolyte (0.1 M) saturated with CO2 (ultrapure). Testing was performed in an airtight three-electrode and two compartments cell. The electrocatalytic behavior of carbons were evaluated by Faradaic efficiency (FE). A chronoamperometry (CA) was run under a constant potential for 6h between -0.4 V and -1.2 V vs. RHE. During the CA run, the gas phase from the headspace of the cathodic compartment of the electrochemical cell was analyzed periodically (1h) using gas chromatography (GC) (model SRI 8610C, column Carboxen1000). The carbon treated with melamine and heat treated at 950 oC under N2 (BAX-M-950) showed the best electrochemical performance. The FE of 24.1% for CO and 0.9% for CH4 at -0.79 V vs. RHE were measured. The XPS results showed that nitrogen is mainly in pyridinic (36.3%), pyridonic (48.0%) and pyridine-N-oxides (15.7%) configurations. The obtained carbon has large surface area (1500 m2/g) with micro/mesoporous structure (Vmic/Vt~ 0.5).

For BAX-M-950, the CO formation (Figure 1a) started at -0.49 V vs. RHE with a CO FE of approximately 6.3%. The maximum FE reached 24.1% at the reducing potential of -0.79 V vs. RHE for 4h. This voltage corresponds to an overpotential of 0.68 V as the CO2/CO dynamic equilibrium potential is -0.11 V vs. RHE. The FE decreased when reducing potential is more negative than -0.79V vs. RHE. Similar behavior has also been reported in the literature. The best Faradaic efficiency for CH4 formation is 0.71% and it is observed at -0.89 V vs. RHE.

Stability of a catalyst is another main focus in this research. The results indicated that FE with respect to CO increased from 16.5% (at 0.5h) to 24.1% reaching it maximum at 4h (Figure 1c). That increase in FE with time could be either due to the increase in mass transport of CO2 to the carbon surface or due to the favorable change in surface chemistry during reduction. Thus the electrode was reduced for 4h performing chronoamperometry (CA) at -0.79V vs. RHE in N2 saturated electrolyte (It is referred to as CA pretreatment). When CO2was introduced to electrolyte the carbon showed the best FE for the CO formation reaching 39.9% and FE only increased 2.5% during first 3h reduction. The electrode after the CA pretreatment was studied by XPS. The results showed that the content of pyridinic N (~ 40 %) increased after 4h CA pretreatment compared to the initial material. This increase in the performance is linked to an increased contribution of this species on the surface.

Figure: a) Faradaic efficiency for the CO formation of BAX-M-950 at different potentials; b) Faradaic efficiency for the CH4 formation of BAX-M-950 at different potentials; c) Carbon monoxide FE Comparison with and without CA pretreatment at -0.79V vs. RHE; d) Comparison of the FE after CA pretreatment for all carbon tested. (5 mg·cm-2 catalyst loading)

Figure 1