Engineering Cu surfaces for the electrocatalytic conversion of CO ₂ : Controlling selectivity toward oxygenates and hydrocarbons

Abstract

In this study we control the surface structure of Cu thin-film catalysts to probe the relationship between active sites and catalytic activity for the electroreduction of CO 2 to fuels and chemicals. Here, we report physical vapor deposition of Cu thin films on large-format (∼6 cm 2 ) single-crystal substrates, and confirm epitaxial growth in the <100>, <111>, and <751> orientations using X-ray pole figures. To understand the relationship between the bulk and surface structures, in situ electrochemical scanning tunneling microscopy was conducted on Cu(100), (111), and (751) thin films. The studies revealed that Cu(100) and (111) have surface adlattices that are identical to the bulk structure, and that Cu(751) has a heterogeneous kinked surface with (110) terraces that is closely related to the bulk structure. Electrochemical CO 2 reduction testing showed that whereas both Cu(100) and (751) thin films are more active and selective for C-C coupling than Cu(111), Cu(751) is the most selective for >2e− oxygenate formation at low overpotentials. Our results demonstrate that epitaxy can be used to grow singlecrystal analogous materials as large-format electrodes that provide insights on controlling electrocatalytic activity and selectivity for this reaction.carbon dioxide reduction | epitaxy | electrocatalysis | copper T he electrochemical reduction of CO 2 (CO 2 R) is a process that could couple to renewable energy from wind and solar to directly produce fuels and chemicals in a sustainable manner. However, developing catalysts is a major challenge for this reaction, and significant advances are needed to overcome the issues of poor energy efficiency and product selectivity. One reason for these issues is that there are a limited number of catalysts that can effectively convert CO 2 to products that require more than two electrons (>2e − products), e.g., methane, methanol, ethylene, etc. (1, 2). Therefore, developing catalysts that are effective for CO 2 R to >2e − products would greatly improve prospects for utilization, and such an endeavor requires a deeper understanding of the relevant surface chemistry.Out of the polycrystalline metals, Cu is the only one that has shown a propensity for CO 2 R to >2e − products at considerable rates and selectivity (2, 3). To date, its uniqueness is reflected by how nearly all work on catalysts with improved activity and selectivity for >2e − products is based on Cu (4-6). However, polycrystalline Cu is not particularly selective toward any one >2e − reduction product (7). Thus, it is critical to understand what active site motifs lead to this unique selectivity for further reduced products and to apply this knowledge to develop new materials with this electrocatalytic behavior.Single-crystal studies on Cu have shown that CO 2 R activity and selectivity are extremely sensitive to surface structure. In particular, facet sensitivities for C-C coupling are the most widely studied, with experimental reports concluding that Cu(100) terraces and any orientation of step sites are more...

Effects of Anion Identity and Concentration on Electrochemical Reduction of CO2

“…Cu thin films with specific crystal orientations were prepared by rf sputtering of Cu onto silicon (Si) single crystal substrates. This approach is based on previous studies demonstrating the epitaxial relationship between Cu and Si substrates of different orientations ,,. Cu(100)‐oriented thin films were used for the present study because it has been shown that this surface exhibits a high selectivity to the desired C 2+ hydrocarbons and oxygenates (e.g., C 2 H 4 and CH 3 CH 2 OH) and has been shown to be electrochemically stable .…”

Promoter Effects of Alkali Metal Cations on the Electrochemical Reduction of Carbon Dioxide

“…Epitaxially grown Cu thin films were used as electrocatalysts in order to have well-defined catalytic surfaces. 27 These films were prepared by radio-frequency sputtering of Cu onto silicon (Si) single crystal substrates, taking advantage of the epitaxial relationship between Cu and Si substrates of different orientations. [27][28] Most of our studies were conducted on the Cu(100) surface because it has been…”

Standards and Protocols for Data Acquisition and Reporting for Studies of the Electrochemical Reduction of Carbon Dioxide

“…The Cu(100) surface exhibits an activity for generating C 2+ products roughly an order of magnitude higher than that for Cu(111), as reported elsewhere. 13,16 This facet dependence can cause polycrystalline Cu foils obtained from different vendors or even different batches from the same vendor to exhibit large differences in electrocatalytic activity and selectivity that arise due to variations in surface faceting. In contrast to Cu, Ag predominately produces H 2 and CO, with CO Faradaic efficiencies exceeding 90% at an applied potential of -1 V vs RHE.…”