Highly Selective Atomically Dispersed Copper Electrocatalyst for CO2 Reduction to Ethanol

Abstract

Introduction Ethanol has been extensively blended in gasoline as fuel to improve the combustion efficiency. Electrochemical CO2 reduction reaction (CO2RR) to ethanol and other value-added chemicals offers a promising “carbon-neutral” or even “carbon-negative” strategy1 when combined with renewable electricity, making the approach not only environmentally sound but also economically attractive. A key challenge for CO2RR is the lack of electrocatalyst that is capable of converting CO2 to a single product with high current efficiency, i.e. Faradaic efficiency (FE), low overpotentials and high durability2. Here, we report the preparation of commercial carbon-supported Cu SA catalysts by an amalgamated Cu–Li method. The catalysts demonstrated highly selective CO2-to-ethanol conversion with FE reaching ~91% at -0.7 V versus RHE and an onset potential as low as -0.4 V with FE of ~15%. The FE of the catalysts maintained at roughly 90% without degradation for 16 h. The CO2-to-ethanol FE is favored by high initial dispersion of single Cu atoms. Operando synchrotron X-ray absorption spectroscopy revealed that such dispersion played a critical role in a reversible transformation between Cu SA and Cu n (n= 3 and 4) as the active sites during the electrocatalytic reaction. Materials and Methods Preparing single atom catalyst starts with melting metal in liquid lithium in an Ar-filled glovebox. Lithium was heated to 200 °C, then the target metal (Cu in this case) was simultaneously added and kept for 4 h. The Li-Cu solid solutions were removed from the glovebox. The prepared amalgamated Cu-Li was oxidized to LiOH and mixed with carbon support materials before the LiOH was leached out with copious amounts of DI water, leaving the metal single atoms imbedded in the carbon support. Results and Discussion High-angle annular dark-field and aberration-corrected scanning transmission electron microscopy (HAADF-STEM), and X-ray absorption spectrum (XAS) are used to characterize the resulting samples. Fig. 1a shows a HAADF-STEM image of the atomically disperse Cu in sample Cu/C-0. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) are used to characterize the coordination structures and the electronic structure of the Cu atoms. Based on these XAS techniques, the Cu atoms are identified as being coordinated with four oxygens (Figure 1b). Figure 1c shows the FEs and CO2RR product distributions as the function of applied potentials from -0.4 V to -1.2 V over Cu/C-0.4. We detected the active potential for ethanol formation as low as -0.4 V with a decent FE of ~ 15%. The peak FE for ethanol is ~91% at a low potential of only -0.6 V and -0.7 V. To our knowledge, this value represents the highest FE for direct electrocatalytic CO2-to-ethanol conversion among Cu-based catalysts. The Cu/C-0.4 catalyst over a 16-hour span at -0.7 V showed an excellent stability in both current density and FE of CO2-to-ethanol over Cu/C-0.4 (Fig. 1d). Fig. 1e shows the FEs of the ethanol formation at different potentials obtained from individual catalyst studied. For Cu/C-0.1, Cu/C-0.4 and Cu/C-0.8, the FE for ethanol formation followed nearly identical profile with the active potential starting at as low as -0.4 V and reaching peak value > 90% at -0.7 V. Once the Cu loading reached to 1.6 wt.% or higher, however, the ethanol FE profile underwent a drastic reduction. Operando XAS study under the reaction conditions revealed a crucial dynamic catalytic mechanism, a reversible transformation between atomically dispersed Cu to Cun (n=3 or 4) clusters (Fig. 1f).Fig 1 (a) Representative HAADF-STEM images of Cu/C-0.4 showing the presence of isolated Cu species; (b) Fourier transform of k2-weighted R-space χ EXAFS data of the catalysts plus Cu(AcAc)2 as reference; (c) FE and the product distribution at different polarization potentials. The data were averaged over three repeated measurements with the standard deviations marked by black error bar for ethanol and red error bar for the total products; (d) FE and current density as the function of time during chronoamperometric electrolysis at -0.6 V; (e) FE of CO2-to-ethanol at different potentials over catalysts of different Cu loadings. (f) The hypothesised reaction mechanism suggested by the operando measurements. Significance Understanding the role of metal single atoms on the CO2 electrochemical reduction reaction is fundamentally important in designing new CO2RR catalyst for the large-scale production of value-added liquid fuels and chemicals. Reference 1 Xu, H. P. et al Highly selective electrocatalytic CO2 reduction to ethanol by metallic clusters dynamically formed from atomically-dispersed copper, Nature Energy, 2020, 5, 623-632.2 Zhou, Y. S. et al Dopant-induced electron localization drives CO2 reduction to C2 hydrocarbons. Nature Chemistry, 2018, 10, 974–980. Figure 1