High Facets on Nanowrinkled Cu via Chemical Vapor Deposition Graphene Growth for Efficient CO₂ Reduction into Ethanol

Abstract

Achieving high electrochemical conversion of carbon dioxide (CO 2 ) into valuable fuels and chemicals is one of the most promising directions to address environmental and energy challenges. Although several single-crystal based studies and simulation results have reported that rich in steps on Cu (100) surfaces are favorable to convert toward C 2 alcohol products, most studies are still stuck in low-index (100) facets or surface defectderived low density of step-sites. In the present work, we report the high production of ethanol by synthesizing a wrinkled Cu catalyst with high facets via a chemical vapor deposition (CVD) graphene growth process. Under our approach, we used graphene as a guiding material to produce wrinkled Cu film for use as an electrocatalyst. The graphene-grown Cu films are not only mass-producible but composed of a high density of step-sites with highfacet atomic arrangements, including the ( 200) and (310) facets, which are difficult to synthesize using existing methods. The wrinkled Cu film with a unique atomic arrangement showed high ethanol selectivity, achieving 40% faradaic efficiency (FE) at −0.9 V vs reversible hydrogen electrode (RHE), one of the largest selectivity values reported thus far for a Cu-based CO 2 conversion catalyst. The C 2 selectivity and productivity was 57% FE and −2.2 mA/cm 2 at −1.1 V vs RHE, respectively. Density functional theory (DFT) calculation results demonstrated that such a high ethanol productivity is mainly attributable to the (310) facet of the wrinkles, which feature a low C−C coupling barrier (0.5 eV) and a preferred reaction path toward ethanol among other products.

Structure‐Function Correlation and Dynamic Restructuring of Cu for Highly Efficient Electrochemical CO2 Conversion

“…For instance, ethanol is theoretically confirmed more thermodynamically favored on the Cu facet consisting of n(100) × m( 110), due to a lower energy barrier along the reaction way *COCHO + H + + e À !*CHOCHO. [40] To this end, Cu electrocatalysts having rich Cu(310) and ( 210) facets, such as Cu(OH) 2 /Cu [40] and wrinkled Cu film, [41,42] were rationally designed to achieve a higher ethanol production (Figure 5a,b). Notably, the synergistic interaction between Cu- (110) and Cu(100) played a key role.…”

Using pH dependence for understanding mechanisms in electrochemical CO reduction

“…Several studies have hypothesized the rate-limiting step in the production of C2+ products to be the first elementary step involving the dimerization of *CO to form the *OCCO species, which does not involve a proton transfer and is therefore pH-independent [19][20][21][22][23] . Alternative mechanisms based on the coupling of reaction intermediates in the later steps of the reaction have also been proposed [24][25][26][27] . When we consider the reaction kinetics (vide infra), we find that it is not possible to unequivocally exclude the involvement of a proton transfer in the rate-limiting step without pH-dependent activity studies performed in acidic conditions with sufficient concentration of proton donors other than H2O, e.g.…”

“…As outlined in the upper panel of Figure 1, under conditions where H2O is the proton donor, and with the high Tafel slopes that are generally observed in experiments 18,29,30 , there are three rate-limiting elementary steps that could satisfy the experimentally observed pH independence mentioned above (cf. Figure 1, C 2+ pathways): (1) the coupling of 2 *CO molecules to form the *OCCO dimer species [19][20][21][22][23] , (2) the protonation of the *OCCO dimer to *OCCOH 18 , and (3) the rate-limiting protonation of *CO to *COH/*CHO followed by the coupling with *CO in a later elementary step along the reaction pathway [24][25][26][27] . Similarly, the CO(2)R reaction mechanism towards methane (CH4) has been investigated in several theoretical 9,13,[31][32][33][34][35] and experimental studies 7,29,[36][37][38][39][40][41] .…”

Using pH dependence to understand mechanisms in electrochemical CO reduction

“…Several studies have hypothesized the rate-limiting step in the production of C2+ products to be the first elementary step involving the dimerization of *CO to form the *OCCO species, which does not involve a proton transfer 12,[20][21][22][23] . However, alternative mechanisms based on an initial proton-coupled electron transfer (PCET) to *CO followed by the coupling of reaction intermediates in the later steps of the reaction have also been proposed [24][25][26][27][28] . As we will show in detail, when we consider the reaction kinetics, we find that it is not possible to unequivocally exclude the involvement of a proton transfer in an initial rate-limiting step without activity studies with sufficient concentration of proton donors other than H2O, e.g.…”

“…hydronium (H3O + ) in acidic conditions or buffering anions 19,29 . Hence, four possible RLS satisfy the experimentally observed pH independence, as outlined in the upper panel of Figure 1: (1) the coupling of 2 *CO molecules to form the *OCCO dimer species 12,21-23 , (2) the protonation of the *OCCO dimer to *OCCOH 19 , and the rate-limiting protonation of *CO to (3) *COH or (4) *CHO followed by C-C coupling in a later elementary step along the reaction pathway [24][25][26][27][28] . This assignment of the possible RLS being an initial step in the reaction network is supported by the reported high Tafel slopes 19,30,31 , being the change in overpotential necessary for an order of magnitude change in current density, towards C2+ products in alkaline conditions.…”