Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Abstract

•The CO2 reduction selectivity on Cu electrocatalysts was regulated by defect sites•The catalytic activity of Cu with abundant defects was illustrated by DFT calculations•The faradaic efficiency of ∼70% to C2+ alcohols was achieved in CO2 electroreduction Electrochemical reduction of carbon dioxide (CO2) is a promising approach to solve both renewable energy storage and carbon-neutral energy cycle. In order to improve the economic feasibility in applications, electrocatalytic CO2 reduction with high activity, selectivity, and stability toward multi-carbon products should be realized. When considering highly selective electrocatalysts for CO2 electroreduction, Cu catalysts have shown their potentials of producing multiple C2+ products in catalytic systems, while the selectivity among different C2+ products has yet to be optimized. Here, we demonstrated a rational strategy to synthesize the Cu catalyst under a CO-rich environment to induce the growth of defect-rich sites that are best for the adsorption of CO. During the electrochemical CO2 reduction process, these defect-rich sites enabled a high surface density of adsorbed ∗CO intermediates, allowing to tune the CO2 electroreduction pathways toward the formation of C2+ alcohols. Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation. Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation. Reducing the increasing atmospheric carbon dioxide (CO2) level is critical to tackling a variety of urgent environmental hazards, such as global warming. Powered by renewable solar or wind energy sources, electrocatalytic CO2 reduction reaction (CO2RR) is a promising approach to produce value-added chemicals/fuels and store renewable energy to fulfill the net-zero-emission goal.1Bushuyev O.S. De Luna P. Dinh C.T. Tao L. Saur G. van de Lagemaat J. Kelley S.O. Sargent E.H. What should we make with CO2 and how can we make it?.Joule. 2018; 2: 825-832Abstract Full Text Full Text PDF Scopus (518) Google Scholar C2+ alcohols are desirable CO2-synthesized products owing to their high energy densities and large global market capacities.2Birdja Y.Y. Pérez-Gallent E. Figueiredo M.C. Göttle A.J. Calle-Vallejo F. Koper M.T.M. Advances and challenges in understanding the electrocatalytic conversion of carbon dioxide to fuels.Nat. Energy. 2019; 4: 732-745Crossref Scopus (666) Google Scholar However, the building of alcohol-selective CO2RR electrocatalysts remains a challenge. Cu, one of the most effective catalysts toward C2+ products, typically favors the formation of hydrocarbons other than alcohols.3Jouny M. Lv J.J. Cheng T. Ko B.H. Zhu J.J. Goddard W.A. Jiao F. Formation of carbon–nitrogen bonds in carbon monoxide electrolysis.Nat. Chem. 2019; 11: 846-851Crossref PubMed Scopus (74) Google Scholar To date, strategies for building alcohol-selective Cu-based electrocatalysts, including geometric structure tuning,4Rahaman M. Dutta A. Zanetti A. Broekmann P. Electrochemical reduction of CO2 into multicarbon alcohols on activated Cu mesh catalysts: an identical location (IL) study.ACS Catal. 2017; 7: 7946-7956Crossref Scopus (98) Google Scholar,5Zhuang T.-T. Liang Z. Seifitokaldani A. Li Y. De Luna P. Burdyny T. Che F. Meng F. Min Y. Quintero-Bermudez R. et al.Steering post-C–C coupling selectivity enables high efficiency electroreduction of carbon dioxide to multi-carbon alcohols.Nat. 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Hung S.-F. et al.Cooperative CO2-to-ethanol conversion via enriched intermediates at molecule–metal catalyst interfaces.Nat. Catal. 2020; 3: 75-82Crossref Scopus (169) Google Scholar the corresponding partial current density remains below 10 mA·cm−2. Recent reports have demonstrated that increasing the local CO concentration and suppressing the deoxygenation of HOCCH∗ can favor the formation of alcohols.13Wang X. Wang Z. García de Arquer F.P. Dinh C.-T. Ozden A. Li Y.C. Nam D.-H. Li J. Liu Y.-S. Wicks J. et al.Efficient electrically powered CO2-to-ethanol via suppression of deoxygenation.Nat. Energy. 2020; 5: 478-486Crossref Scopus (161) Google Scholar, 14Chen C. Yan X. Liu S. Wu Y. Wan Q. Sun X. Zhu Q. Liu H. Ma J. Zheng L. et al.Highly efficient electroreduction of CO2 to C2+ alcohols on heterogeneous dual active sites.Angew. Chem. Int. Ed. Engl. 2020; 59: 16459-16464Crossref PubMed Scopus (58) Google Scholar, 15Lum Y. Ager J.W. Sequential catalysis controls selectivity in electrochemical CO2 reduction on Cu.Energy Environ. Sci. 2018; 11: 2935-2944Crossref Google Scholar Combined with gas-diffusion electrodes, the alcohol faradaic efficiency has reached ∼50% regime at partial current densities above 100 mA·cm−2. However, to approach industrially relevant performance metrics for CO2-to-alcohol electroreduction, the selectivity toward alcohols should be further improved. Controlling the ∗CO surface coverage, which is typically accomplished via tandem catalyst design in CO2RR,13Wang X. Wang Z. García de Arquer F.P. Dinh C.-T. Ozden A. Li Y.C. Nam D.-H. Li J. Liu Y.-S. Wicks J. et al.Efficient electrically powered CO2-to-ethanol via suppression of deoxygenation.Nat. Energy. 2020; 5: 478-486Crossref Scopus (161) Google Scholar,15Lum Y. Ager J.W. Sequential catalysis controls selectivity in electrochemical CO2 reduction on Cu.Energy Environ. Sci. 2018; 11: 2935-2944Crossref Google Scholar is a strategy to enhance the alcohol selectivity. Here, we report an approach to enable high ∗CO coverage on pure Cu catalysts—the generation of abundant defective sites by synthesizing the catalyst under CO-rich conditions (Figures 1A and 1B). The adsorbed ∗CO species during catalyst processing promote the formation of a defect-site-rich Cu structure (designated as Cu-DS, Figure 1C). The Cu-DS catalyst converts CO2 into C2+ alcohols at faradaic efficiencies of ∼70% and high current densities over 100 mA·cm−2. In contrast, on a Cu control with flat surfaces formed without ∗CO adsorbates (designated as Cu-C), C2H4 dominates the product distribution (Figures 1D–1F). Furthermore, using this Cu-DS catalyst, we demonstrated a stable alcohol selectivity of ∼60% in a membrane electrode assembly (MEA) electrolyzer at a current density of ∼200 mA·cm−2, producing ∼500 mg C2+ alcohol per cm2 catalyst during a continuous 30-h operation. The Cu-DS catalyst was synthesized on carbon paper via a controlled electrochemical deposition method with a CO-bounded Cu(I) molecular complex as the precursor and a continuously delivered CO gas flow (Experimental Procedures). For comparison, the same electrochemical deposition was also applied with a CuCl precursor and argon (Ar) gas flow to grow the Cu-C sample. X-ray diffraction (XRD) patterns of both obtained Cu-DS and Cu-C samples showed three characteristic peaks of pure copper (JCPDS 04-0836), corresponding to the (111), (200), and (220) planes, respectively (Figure 2A). The chemical compositions and metal surface oxidation states of both samples were investigated by X-ray photoelectron spectroscopy (XPS, Figure 2B). The Cu 2p spectra presented two main peaks centered at 932.2 and 952.1 eV, respectively, suggesting that the Cu oxidation states for the as-synthesized Cu-DS and Cu-C were +1 or 0.16Kuang M. Wang Q. Han P. Zheng G. Cu, Co-embedded N-enriched mesoporous carbon for efficient oxygen reduction and hydrogen evolution reactions.Adv. Energy Mater. 2017; 7: 1700193Crossref Scopus (381) Google Scholar The Cu valence states were further interrogated by operando X-ray absorption spectroscopy (XAS) under electrochemical CO2RR conditions. Unless specifically noted, all applied voltages in this work are referred to the reversible hydrogen electrode (RHE). At a constant applied potential of −1.08 V, the Cu K-edge spectra of the samples were recorded for 8 independent runs (i.e., 8 scans × 9 s each). All the X-ray absorption near edge structure (XANES, Figures 2C and 2D, left panels) spectra matched the reference Cu foil (black curve) well in both peak position and line shape but differed from the Cu2O reference (purple curve). No peak shifts were observed during the electrochemical CO2RR, confirming that the catalysts only contained Cu(0) species. Fittings of the extended X-ray absorption fine structure (EXAFS, Figures 2C and 2D, right panels, and Figure S1) spectra at the Cu K-edge of both Cu samples showed that the Cu local environments of the two catalysts were the same as that of the standard Cu foil, confirming that the catalysts during CO2RR were metallic Cu. The slight reduction in the amplitude was consistent with the larger aspect ratio in both Cu-DS and Cu-C catalysts than the bulk Cu foil. We then investigated the structural difference between Cu-DS and Cu-C using transmission electron microscopy (TEM) analysis. Both Cu-DS and Cu-C samples had similar dendritic structures (Figures 2E and 2F, left panels), in accord with their scanning electron microscopy (SEM) images (Figures S2 and S3). High-resolution TEM (HRTEM) images and the selected area electron diffraction (SAED) patterns (Figures 2E and 2F, right panels and insets) revealed the single-crystalline structure of each dendrite. 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To examine the stabilities of flat-Cu’(101) surface and stepped-Cu’(101) surface, we calculated the respective surface energies at different ∗CO coverages (Figure 3D). The surface energies of the stepped-Cu’(101) A and B surfaces decreased from 101 to 40 meV·Å−2 along with an increase in the ∗CO coverage. At the ∗CO coverage of 0.50 monolayer, the calculated surface energies of the stepped-Cu’(101) A and B surfaces were close to that of flat-Cu’(101) surface (36.91 meV·Å−2) and Cu(111) surface (34.25 meV·Å−2). Note that the reduced lateral interactions among ∗CO on defective surfaces would enable a higher density of ∗CO (∼0.58 monolayer). However, at the same ∗CO coverage, the flat-Cu’(101) surface becomes less stable. Hence, we concluded that defect-site-rich Cu surfaces are favorable at the high density of adsorbed ∗CO under a CO-rich growth environment, consistent with our characterization results. 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Soc. 2016; 138: 13802-13805Crossref PubMed Scopus (218) Google Scholar the theoretical limiting potentials (|UL|) of ∗CO formation in terms of the coordination numbers (CNs) of Cu atomic sites are summarized (Figure 3E). The limiting potential is the applied potential required to eliminate the energy barrier of the rate-limiting step on the free energy diagram of ∗CO formation (Figures 3F and 3G as examples). As an irregular surface containing sites with different CNs, the sites of stepped-Cu’(101) A, stepped-Cu’(101) B, and flat-Cu’(101) are labeled using green triangles, red triangles, and blue squares, respectively (Figure 3E). The regular Cu(100) and Cu(111) contain sites of CN = 8 and 9, respectively. These results indicate that most 6-CN or 7-CN sites (with |UL| ranging from 0.71 to 0.43 eV) have higher CO2-to-CO activities than the 8-CN and 9-CN sites (with |UL| ranging from 1.07 to 0.69 eV). The regular Cu(100) and Cu(111) surfaces have |UL| = 0.86 and 0.94 eV, respectively, which are inferior to the stepped-Cu’(101) surfaces that contain more sites with CN = 6 or 7. The atoms at the boundary of Cu(100)-like and Cu(111)-like surfaces do not exhibit high activity for CO2RR in our calculations (i.e., the sites of CN = 9 for Cu’(101), Figure 3E). A similar activity was also found for CO2 electroreduction to CO on stepped-Cu’(101) surfaces at high ∗CO coverage, compared with the low coverage scenario (Figure S11). Thus, we expected that stepped-Cu’(101) surfaces can lead to an increased local CO concentration, which increases the surface ∗CO coverage and promotes oxygenate production.15Lum Y. Ager J.W. Sequential catalysis controls selectivity in electrochemical CO2 reduction on Cu.Energy Environ. Sci. 2018; 11: 2935-2944Crossref Google Scholar Furthermore, our DFT calculations of both the ethylene and ethanol pathways (Figure S12 and Supplemental Experimental Procedures) also suggest that the high surface ∗CO density enhances the ethanol formation and suppresses the ethylene formation. The electrocatalytic CO2RR performance of the Cu-DS and Cu-C catalysts were first evaluated in a H-type electrochemical cell with a CO2-saturated 0.1 M KHCO3 electrolyte (Experimental Procedures). The major liquid products included ethanol (CH3CH2OH) and n-propanol (CH3CH2CH2OH), as confirmed by 1H-nuclear magnetic resonance (NMR, Figure S13). At −1.08 V, the Cu-DS catalyst presented peak FE values of ethanol and n-propanol as 53% and 18%, respectively (Figure 4A), outperforming the state-of-the-art alcohol-selective CO2RR electrocatalysts (Table S1).7Ren D. Deng Y. Handoko A.D. Chen C.S. Malkhandi S. Yeo B.S. 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Nam D.-H. Luo M. Wicks J. Chen B. Li J. Li F. de Arquer F.P.G. et al.Binding site diversity promotes CO2 electroreduction to ethanol.J. Am. Chem. Soc. 2019; 141: 8584-8591Crossref PubMed Scopus (181) Google Scholar, 49Ma S.C. Sadakiyo M. Luo R. Heima M. Yamauchi M. Kenis P.J.A. One-step electrosynthesis of ethylene and ethanol from CO2 in an alkaline electrolyzer.J. Power Sources. 2016; 301: 219-228Crossref Scopus (297) Google Scholar, 50Gabardo C.M. O’Brien C.P. Edwards J.P. McCallum C. Xu Y. Dinh C.-T. Li J. Sargent E.H. Sinton D. Continuous carbon dioxide electroreduction to concentrated multi-carbon products using a membrane electrode assembly.Joule. 2019; 3: 2777-2791Abstract Full Text Full Text PDF Scopus (154) Google Scholar A distinctively different selectivity was observed on Cu-C (Figure 4B), which produced C2H4 with a maximum FE of 60% at −1.23 V. In addition, the commercial Cu2O-derived catalyst and control catalysts synthesized using a NH3 coordination agent or CO2 gas flow did not show high selectivity toward C2+ alcohols (Figures S14 and S15). In comparison, the Cu-DS catalyst increased the alcohol-to-ethylene ratio from 0.128 to 10.3, corresponding to an over-80-fold increase in the selectivity toward alcohol products. The electrochemical active surface areas (ECSAs) of both Cu-DS and Cu-C catalysts, determined by their electrochemical double-layer capacitances (Cdl, Figure S16), were calculated as 1.08 (for Cu-DS) and 1.06 mF·cm−2 (for Cu-C). The similar ECSA values excluded the contribution of high surface areas to the enhanced alcohol selectivity on Cu-DS. As the Cu-DS catalyst did not show difference from Cu-C in the interaction with surrounding water molecules or surface protons, we suggested that the increased selectivity toward alcohols was attributed to the high ∗CO coverage, in agreement with our theoretical findings. The defect-site-rich Cu-DS catalyst synthesized under CO-rich conditions enabled a high ∗CO coverage during CO2 electroreduction, and facilitated to form alcohol products. To verify this hypothesis, we also conducted the CO electroreduction in flow cells using both Cu-DS and Cu-C catalysts (Figure S17). As expected, Cu-DS presented ∼55% faradaic efficiency toward ethanol and ∼15% faradaic efficiency toward n-propanol, comparable with those in CO2RR. In contrast, Cu-C was more alcohol selective under CORR compared with CO2RR. To demonstrate the potential capability for industrially relevant alcohol production, we used a gas-diffusion-electrode-based flow-cell system equipped with the Cu-DS catalyst to enhance the CO2 mass transfer. At −1.05 V applied potential, the overall current densities of Cu-DS and Cu-C reached ∼200 and 180 mA·cm–2, respectively (Figures 4C and 4D). The highest FE values for ethanol and n-propanol were 52% and 15% at −0.95 V, respectively. The alcohol partial current density was above 100 mA·cm–2 at −1.05 V. The applied potentials for the optimal FE values were different for the flow cells and H-cells, which were attributed to the different electrolyte pH values in these two electrochemical systems. The detailed performance metrics of the Cu-DS and Cu-C catalysts were displayed in Tables S2–S5, respectively. In contrast, Cu-C retained its high C2H4 selectivity in flow cells. A peak C2H4 FE of 58% and a partial current density of 78.3 mA·cm−2 at −1.02 V were obtained. In flow cells, Cu-DS enhanced the alcohol-to-ethylene ratio by ∼54 folds compared with Cu-C. To demonstrate a stable and productive CO2-to-alcohol conversion, we operated a continuous 30-h test using a 5-cm2Birdja Y.Y. Pérez-Gallent E. Figueiredo M.C. Göttle A.J. Calle-Vallejo F. Koper M.T.M. Advances and challenges in understanding the electrocatalytic conversion of carbon dioxide to fuels.Nat. Energy. 2019; 4: 732-745Crossref Scopus (666) Google Scholar MEA electrolyzer equipped with Cu-DS at an applied full-cell voltage of −3.5 V (Figure 4E). A stable alcohol FE of ∼60% was recorded at a total current density of ∼200 mA·cm−2 during this 30-h continuous operation (Figures 4F and S18; Table S6), with the ethanol and total alcohol production of ∼400 and 500 mg per cm2 catalyst, respectively (Figure S19). The full-cell energy efficiency of producing C2+ alcohols was ∼20%, indicating a ∼1.2× increase compared with the best prior reports.12Li F. Li Y.C. Wang Z. Li J. Nam D.-H. Lum Y. Luo M. Wang X. Ozden A. Hung S.-F. et al.Cooperative CO2-to-ethanol conversion via enriched intermediates at molecule–metal catalyst interfaces.Nat. Catal. 2020; 3: 75-82Crossref Scopus (169) Google Scholar,13Wang X. Wang Z. García de Arquer F.P. Dinh C.-T. Ozden A. Li Y.C. Nam D.-H. Li J. Liu Y.-S. Wicks J. et al.Efficient electrically powered CO2-to-ethanol via suppression of deoxygenation.Nat. Energy. 2020; 5: 478-486Crossref Scopus (161) Google Scholar After CO2RR, both Cu-DS and Cu-C were examined by TEM (Figure S20), indicating that the surface features of both samples were retained. In addition, the electron holographic studies showed that the post-CO2RR Cu-DS catalyst (Figures S20 and S21) preserved a similar high density of charge fluctuation as the pre-CO2RR one (Figure 2I), which was still much higher than the post-CO2RR Cu-C sample under similar electrochemical conditions. These results suggested that the defect-site-rich structure of Cu-DS was stable during CO2RR, in agreement with our DFT findings that ∗CO can stabilize Cu defects. In summary, we have demonstrated an electrochemical deposition method under a CO-rich environment to prepare Cu catalysts with abundant defective sites for selective CO2 electroreduction to C2+ alcohols. This high alcohol selectivity is suggested as a CO-rich condition promotes the formation of defective Cu surfaces via stabilizing the surface energy, and the defect-site-rich surfaces greatly enhance the CO2-to-alcohol reduction pathways. Using operando spectroscopies and theoretical calculations, we have revealed that the defect-site-rich surfaces enhance local CO production and, thereby, increase the surface ∗CO coverage and CO2-to-alcohol selectivity. As a result, the defect-site-rich Cu-DS electrocatalyst achieves ∼70% faradaic efficiency toward C2+ alcohols in an H-cell system. In flow electrolyzers, an alcohol partial current density of more than 100 mA cm−2 is obtained with 67% corresponding faradaic efficiency. Furthermore, using the defect-site-rich Cu-DS catalyst in a MEA electrolyzer, a stable FE of ∼60% has been achieved for a 30-h continuous electrochemical operation. With continuous efforts contributed to developing more alcohol-selective CO2 reduction electrocatalysts and carbonate-formation-free systems with high carbon efficiency, more exciting opportunities will be revealed in renewable fuel electroproduction with high selectivity, productivity, and CO2 single-pass utilization.