Elucidating the Role of Strain in Promoting C-C Coupling on Cu Surfaces

Abstract

The design of CO2 reduction catalysts with high selectivity toward energy-dense, multi-carbon products remains a challenge. In this study we demonstrate a substantial correlation between surface strain and selectivity in aqueous CO2 reduction on model Cu (001) surfaces. By tuning the thickness of epitaxially-grown Cu layers on Si (001) substrates, we vary the lattice mismatch-derived tensile strain that remains. In-plane X-ray diffractometry indicates an increasing in-plane strain, up to 0.48%, is achieved in Cu epitaxial layers as layer thickness decreases from 100 nm to 20 nm. A roughly order-of-magnitude enhancement in selectivity to multi-carbon products (C2H4, acetate, EtOH, and n-PrOH) as compared to single-carbon products (formate, CO, and CH4) in 0.1M KHCO3 electrolyte is seen with this increase in built-in strain, evidencing the tuning of the adsorption energy of key reaction intermediates. Moreover, the strained 20 nm epitaxially-grown Cu thin films exhibit 4.8x and 68.3x higher selectivity to multi-carbon products as compared with bulk single-crystal Cu(100) and polycrystalline Cu surfaces. The enhanced selectivity toward energy-dense products can be attributed to an increased adsorption energy of selectivity-determining intermediates such as *CO and *CHO on the strained surface and promotion of C-C coupling reaction. These findings indicate the opportunity for rigorous strain engineering to modify adsorption energy of key intermediates for enhanced selectivity in CO2 reduction. Figure. Characteristics and CO2 reduction performance of epitaxially grown Cu (001) thin film on Si (001) substrate. (a) Bulk in-plane tensile strain as a function film thickness. Selectivity ratio of (b) C2H4 to CH4 and (c) multi-carbon products to single-carbon products as a function of bulk in-plane tensile strain at -0.9V vs RHE in CO2-saturated 0.1 M KHCO3 electrolyte. Figure 1