Catalyst Morphology Engineering: Towards a Better Understanding of the Effects of Surface Structure and Mass Transport in Copper Electrodes for the Electrochemical CO2 Reduction Reaction

The modern global energy economy and chemical industry are both heavily dependent on fossil resources, but with rising concerns over the depletion of these resources and their negative impacts on the environment, there has been a strong push for the development of sustainable, renewable alternatives. One potential solution is the electrochemical reduction of carbon dioxide (CO₂) to fuels and chemicals. However, the CO₂ reduction reaction (CO₂RR) is notoriously difficult to catalyze. To date, only copper has shown a propensity to produce high value products such as ethylene and ethanol, but copper still requires large overpotentials (excess energy input) to reduce CO₂ and generates a variety of products requiring separation.^1,2

Recent literature reports have shown that modifying catalyst morphology can be a path towards improved efficiency and selectivity for the CO₂ reduction reaction.^3,4 However, the origin of these improvements is not well understood since nanostructured morphologies can alter both the crystal surface structure and the local mass transport. While single crystal studies have clearly shown that the CO₂RR activity and selectivity are facet–dependent¹, the influence of transport phenomena has only been characterized to a limited degree.⁵ It is thought though that in mass transport–limited systems, accumulation of interfacial OH^- generated during the CO₂RR can lead to a higher local pH, a phenomenon which has been attributed to increased selectivity towards products such as ethylene and ethanol.^4,6 Due to the random, disordered nature of nanostructured catalysts that have been previously reported, it has been difficult to decouple these two potential impacts of catalyst morphology. In this work, catalyst morphology engineering is used in an attempt to isolate and explore the effects of mass transport/pH on the CO₂reduction reaction. There are two main methods being used: nanostructuring the catalyst itself and depositing an inert, porous layer onto a planar catalyst. The first method will leverage the unique processability of silicon to create highly ordered vertical nanowire arrays with tunable control over the nanowire length, diameter, and pitch, which will then be coated with copper, allowing us to examine the impacts of the nanowire dimensions and surface area. The second method will utilize mesoporous oxides and membranes to create a diffusion barrier layer on top of copper, perturbing the local mass transport to the catalyst surface.

References:

[1] Y. Hori, Modern Aspects of Electrochemistry. 2008, 89

[2] K. Kuhl et al., Energy Environ. Sci. 2012, 5, 7050

[3] C.W. Li et al., J. Am. Chem. Soc. 2012, 134, 7231

[4] D. Ren et al., ACS Catalysis. 2015, 5, 2814

[5] M. Singh et al., Phys. Chem. Chem. Phys. 2015, 17, 18924

[6] R. Kas et al., ChemElectroChem. 2015, 2, 354