N-Doped Nanoporous Carbon Scaffolds As Catalysts for CO2 Reduction

Abstract

The need for clean energy solutions in order to significantly lower our carbon footprint is placing increasing attention on renewable energy, such as wind and solar. However, these technologies suffer from intermittency issues, thus making efficient energy storage and conversion a major requirement. One of the solutions to this problem is electrochemical CO2 reduction (CO2RR), using renewable energy to produce green fuels and chemicals. However, the reduction of CO2 is thermodynamically and kinetically unfavorable. To overcome this, the development of highly efficient and selective electrocatalysts is a key goal of CO2RR research.Carbon has been attracting interest as a promising candidate due to its high specific surface area, good conductivity, and low-cost. However, its catalytic performance is limited by the electroneutrality of the carbon atoms in the primarily graphitic lattice. In order to activate the CO2 molecules and enhance the adsorption of intermediates, a range of novel carbon catalysts has been developed, with nitrogen doping being the most common approach. The N atom has a similar size as C and thus the lattice mismatch after doping is minimal [1]. At the same time, the N atom also has a higher electronegativity than C which will break the electroneutrality of the carbon lattice and enhance the conductivity of the material [2-3]. Furthermore, N doping results in a range of N-based surface species, thus potentially allowing the tuning of the CO2RR products [4].In this work, a novel, self-supported, nanoporous carbon scaffold (NCS) was used as the carbon substrate. The NCS is a templated, binder-free mesoporous carbon material with tunable pore sizes, a high surface area, good conductivity, and scalability [5]. Here, N doping of the NCS was achieved by heat treatment in NH3. An in house flow cell that can be switched between flow-through and flow-by modes was developed, allowing the NCS to be used as a model material to understand the impact of fluid flow on mass transfer limitations in the CO2RR and on any local pH effects that may be present.The CO2RR performance was tested in the flow cell using a CO2-saturated KHCO3 solution. The bare NCS was confirmed to generated only H2, while the N-doped NCS gives roughly a 50:50 ratio of H2:CO at all potentials, with the onset potential being ca. -0.55 V vs RHE. The effect of the NCS pore size was also investigated, showing that an NCS membrane with a nominal 85 nm pore diameter produced roughly 45% CO, while the NCS-12 (12 nm pore size) material gave somewhat more CO (ca. 55-60%). At the same time, the NCS-12 gave lower current densities, despite its higher surface area, likely due to poorer pore accessibility. Furthermore, running the cell in the flow-through mode gave higher currents at all of the NCS-based catalysts, most likely due to removal of trapped gases. Current work is focused on determining the effect of the N content and the type of N-based functionalities attached to the NCS surface on the CO2RR performance.