Achieving Efficient CO2 Electrochemical Reduction on Tunable In(OH

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Energy, Environmental, and Catalysis Applications

Achieving Efficient CO2 Electrochemical Reduction on Tunable In(OH)3 Coupled Cu2O Derived Hybrid Catalyst Tengfei Li, Hongmei Wei, Tianmo Liu, Gengfeng Zheng, Subiao Liu, and Jing-Li Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04580 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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ACS Applied Materials & Interfaces

Achieving Efficient CO2 Electrochemical Reduction on Tunable In(OH)3 Coupled Cu2O Derived Hybrid Catalyst Tengfei Li,a,b Hongmei Wei,b Tianmo Liu,b,* Gengfeng Zheng,c,* Subiao Liu,a Jing-Li Luoa,*

a

Department of Chemical and Materials Engineering, University of Alberta,

Edmonton, Alberta T6G 1H9, Canada b

College of Materials Science and Engineering, Chongqing University, Chongqing

400044, China c

Laboratory of Advanced Materials, Department of Chemistry and Shanghai Key

Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, China * Corresponding authors: Prof. Tianmo Liu (E-mail: [email protected]) Prof. Gengfeng Zheng (E-mail: [email protected]) Prof. Jing-Li Luo (E-mail: [email protected]) Abstract: Tunable In(OH)3 coupled Cu2O derived hybrid catalysts are facilely synthesized to boost the selectivity and efficiency of the electrochemical CO2 reduction reaction (CO2RR). The maximum Faradaic efficiency (FE) of 90.37% for CO production is achieved at −0.8 V vs. reversible hydrogen electrode (RHE). The mechanistic discussion suggests that the composition-dependent synergistic effect results in the enhanced selectivity for CO on the hybrid catalyst. By increasing the 1

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concentration of the electrolyte, a dramatically enhanced current density of 40.17 mA cm–2 was achieved at –1.0 V in 0.7

M

KHCO3. Furthermore, KHCO3 electrolyte with

high concentration promotes the selectivity of CO2RR over the low overpotential range. At a low overpotential of 290 mV, the increased FE for CO of 74.05% is obtained in 0.7

M

KHCO3 as compared to that of 57.04% in 0.1

M

KHCO3. Combining with the

synergistic effect of the catalyst and the concentration effect of the electrolyte, the hybrid catalyst achieves the high efficiency, high selectivity and high stability for CO2RR. Keywords: carbon dioxide; electrochemical reduction; hybrid catalyst; synergistic effect; concentration effect 1. Introduction The continuous increase of atmospheric carbon dioxide (CO2) is regarded as one of the main causes of the current global warming.1 The ever-increasing combustion of fossil fuels adversely contributes to the accumulation of CO2 in the atmosphere.2 Electrochemical CO2 reduction reaction (CO2RR) using renewable energies (e.g., wind, tidal and solar power) holds the promise to decrease CO2 emissions through converting CO2 to carbon-neutral energy-dense fuels without further releasing CO2 into our environment.3 For the large-scale industrial application to be feasible, the selective, efficient, stable and facile-synthesized catalysts are highly desirable. Various metal candidates have been extensively studied as the catalysts for CO2RR.4-8 Among them, Cu is a relatively inexpensive and earth-abundant candidate. It has attracted much attention due 2


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to its superior activity for reducing CO2 to multi-carbon products, such as ethylene (C2H4) and ethanol (C2H5OH).9-11 Except for hydrocarbons and oxygenates, carbon monoxide (CO) is another desirable product from CO2RR since it can be used as the precursor for the chemical synthesis in industries.12 However, much research work is needed in order to improve the selectivity of these products before proceeding to the next major step in this field. Benefitted from the synergistic effect, the foreign component modified Cu-based catalysts can get a higher selectivity than the pristine catalyst.13-17 Takanabe et al. reported a Cu-In bimetallic catalyst which exhibited the higher CO selectivity in CO2RR than Cu, indicating the synergistic effect between Cu and In.18 The highly selective Cu-In bimetallic catalyst was obtained by electrochemical reduction of CuInO2, demonstrating a facile strategy to synthesize bimetallic catalysts. However, not being able to regulate the ratio of the metal components is a major disadvantage of this strategy. Because the composition effect has been proved to play an important role in determining the catalytic activity and selectivity of multicomponent catalysts for CO2RR.19-21 Therefore, there is an urgent need for a facile, scalable and cost-effective strategy to synthesize the multicomponent hybrid catalysts with the tuneable composition.22 By the electrodeposition method, the dendritic Cu-In alloys with varying compositions were prepared which achieved the tunable selectivity between formate and syngas for the CO2RR.23 However, no synergistic effect favorable for CO evolution appeared in these electrodeposited dendritic Cu-In alloys. Therefore, the mechanism of 3



the synergistic effect between Cu and In remains to be studied. Herein, a series of In(OH)3 coupled Cu2O derived hybrid catalysts with different molar ratios of Cu to In have been successfully synthesized using a potentially scalable one-pot surfactant-free method in the aqueous solution, followed by the electrochemical reduction process. The composition-influences on the synergistic effect of the hybrid catalysts for CO2RR were systematically studied. The obtained hybrid catalyst with an optimal ratio of Cu to In achieves a maximum FE of 90.37% toward CO at an applied potential of –0.8 V vs. reversible hydrogen electrode (RHE; all potentials reported here are given on the RHE scale unless noted otherwise). In addition, the concentration effect of KHCO3 on the catalytic performance of the hybrid catalyst was systematically investigated. KHCO3 electrolyte with high concentration enables an increased current density of 40.17 mA cm–2 at –1.0 V. 2. Results and discussion The fabrication procedures of In(OH)3 coupled Cu2O hybrid material (ICC-HM) are illustrated in Figure S1, and the experimental details are summarized in the Supporting Information. Pure Cu2O nanocubes were synthesized by a surfactant-free method in aqueous solution based on a reported method.24 By controlling the introduced volumes (3, 6 and 7 mL) of In precursor solution, ICC-HM with varying In(OH)3 contents (ICC-HM-3, -6, and -7) were prepared, and the actual In/Cu ratios were determined by X-ray Fluorescence (XRF) analysis, as listed in Table S1. The crystal structures of ICC-HM-6 and pure Cu2O were investigated by the examination of X-ray diffraction (XRD) patterns. As shown in Figure S2a, pure Cu2O 4


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is indexed as cubic phase Cu2O with characteristic reflections at 29.6°, 36.4°, 42.3°, 61.4°, and 73.6° ascribed to (110), (111), (200), (220) and (311) planes of Cu2O (JCPDS no.78-2076). For ICC-HM-6, several additional peaks appear at 22.3°, 31.7° 45.4° and 51.2° corresponding to (200), (220), (400) and (420) peaks of In(OH)3 (JCPDS 851338). The morphologies were investigated by scanning electron microscopy (SEM). The as-prepared pure Cu2O has a nanocube structure (Figure S3a). ICC-HM-6 is a mixture of nanocubes and floccules (Figure S3b). The morphology and chemical composition of ICC-HM-6 were further analyzed by transmission electron microscopy (TEM), coupled with energy dispersive X-ray spectroscopy (EDS). Through analyzing the EDS elemental mapping (Figure 1a), it is found that Cu element is enriched in the nanocubes, while In element is mainly distributed in these floccules. It confirms that the nanocubes in ICC-HM-6 are Cu2O, whereas the floccules are In(OH)3. The crystal structures of Cu2O nanocubes of ICC-HM-6 were also characterized using high-resolution TEM (HRTEM), as shown in Figure 1b. The clear lattice fringes with distances of 0.213 and 0.302 nm corresponding to the reflections of the (200) and (110) planes of Cu2O (JCPDS No.78-2076) demonstrate a high crystalline nature of Cu2O nanocube. The obvious diffraction rings derived from In(OH)3 indicate its polycrystalline structure (Figure 1c). Prior to polarization at the stated potentials, all the electrodes were subjected to a cathodic current of –3.0 mA cm–2 in CO2-saturated KHCO3 electrolyte to reduce the In(OH)3 and Cu2O until the potential reached the steady-state values. The XRD patterns 5



of electrodes of ICC-HM-6 before (fresh electrode) and after (used electrode) the reduction process are displayed in Figure S2b. After the reduction process, Cu2O has been completed reduced to metallic Cu within the detection limits of XRD measurement. Due to the low content of In, the peaks belonging to metallic In or In(OH)3 are absent from the fresh or used electrodes. Then, X-ray photoelectron spectroscopy (XPS) was conducted to test the surface valence states of the elements in the fresh and used electrodes of ICC-HM-6. As shown in Figure 2a, the peaks at 932.97, 934.42 and 944.55 eV of Cu 2p spectrum of the fresh electrode are corresponding to Cu0/+ (indistinguishable), Cu2+ species, and the satellite peak, respectively. The peak of Cu2+ is due to surface oxidation. However, the peaks corresponding to Cu2+ species and the satellite peaks disappear in the used electrodes, indicating the change of the surface valence states of Cu during the reduction process. Due to that Cu 2p3/2 peaks related to Cu0 and Cu+ species are indistinguishable, it is difficult to discriminate the attribution of Cu 2p3/2 peak in the used electrodes. According to the XRD data, metallic Cu is the main phase in the electrodes after electrochemical reduction. The binding energies of In 3d electrons are recorded at 445.95/453.51 and 445.22/452.78 eV for the fresh and used electrodes (Figure 2b). Upon the reduction process, a shift towards lower binding energies for In 3d peaks is observed. In addition, In 3d peaks broaden with respect to the used electrode for the fresh electrode, indicated by the full width at half maximum (FWHM) of the peaks. It could be concluded that In(OH)3 was reduced to metallic In during the reduction process, which agreed well with the previous reports.25-26 Furthermore, In core level 6


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binding energy is slightly blue-shifted for whether the fresh or used electrodes as compared with the reported values, indicating the electron relocation from In species to Cu species.26 No shift of the Cu 2p peak could be observed due to that the binding energy of the Cu+ peak is very close to that of the Cu0 peak.27 And Cu element accounts for the majority in the hybrid catalyst which may also rationalize the no shift of the Cu 2p peak. The CO2RR tests were carried out in a Nafion separated gas-tight twocompartment cell to investigate the electrochemical CO2 reduction activity of ICC-HM3, -6 and -7 derived hybrid catalysts (referred as HC-3, -6 and -7). Cu2O derived catalyst (OD-Cu) was also studied for comparison. The CO2 electrolysis experiments were performed in a CO2-saturated 0.1

M

KHCO3 electrolyte under ambient conditions.

Figure 3a shows the potential dependent total current densities (jtot) for OD-Cu, HC-3, -6 and -7, which were measured and averaged based on the steady-state currents from the chronoamperometry. All the catalysts show the similar jtot. The hydrogen evolution reaction (HER) is a competitive reaction for CO2RR. HC-3, -6 and -7 present lower H2 partial current densities (jH2) as compared with OD-Cu, suggesting that the hybrid catalysts could effectively inhibit HER (Figure S4). The hybrid catalysts demonstrate an efficient CO2-to-CO conversion. As shown in Figure 3b, CO partial current densities (jCO) of the hybrid catalysts significantly surpass that of OD-Cu over the more negative potential range. It reaches a maximum of 9.08 mA cm–2 at –1.0 V for HC-6 compared to that only 3.26 mA cm–2 for OD-Cu. However, the further increase of In content in the hybrid catalyst does not continue to improve its ability to convert CO2 to CO. jCO 7



of HC-7 were lower than that of HC-6 at all the potentials. It suggests that the hybrid catalyst of HC-6 possess the optimal composition ratio of Cu to In to obtain the best catalytic ability to convert CO2 to CO. The potential dependent selectivity of gaseous products for OD-Cu, HC-3, -6 and -7 are compared and displayed in Figure 3c. OD-Cu shows low FEs for CO (FECO< 37.29%), while primarily generating H2 (FEH2> 39.81%) over the entire potential range. As In content of the hybrid catalysts increase from HC-3 to HC-6, FEH2 decreases obviously. Inversely, the remarkably enhanced FECO is achieved by the hybrid catalysts of HC-3 and -6 as compared with OD-Cu, indicating that In plays a crucial role in enhancing the selectivity for CO. In particular, the highest FECO of 90.37% is achieved on HC-6 at −0.8 V. In light of previous research on CO2RR, the maximum FECO achieved by the oxide-derived Cu catalysts is about 60% and In(OH)3-derived catalysts generally favor CO2RR to formate.28-33 The enhanced selectivity for CO of HC-3 and -6 as compared with OD-Cu is attributed to the synergistic effect between Cu and In. However, HC-7 which is coupled with more In content compared with HC-6 fails to achieve a further enhanced selectivity for CO. On the contrary, FEH2of HC-7 increases as relative to that of HC-6 over the low overpotential range. It is due to that the high In content weakens the synergistic effect of the hybrid catalysts. Furthermore, the catalytic conversion activity of OD-Cu to hydrocarbon (e.g., C2H4) gradually decreases with the addition of In (Figure 3c and S5). It is concluded that the composition effect between Cu and In coordinatively drives the CO2RR activity. To better understand the origin of enhanced catalytic selectivity of the hybrid 8


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catalysts as compared with OD-Cu, a Tafel plot analysis was explored to obtain better insights into the kinetics for the CO2RR. It was proposed by Hori that a two-electron transfer is involved in the CO2RR to CO.34 The first proton-coupled electron-transfer step to form the *COOH intermediate is generally affirmed as the rate-determining step with a Tafel slope value of around 118 mV dec–1.35 As shown in Figure 3d, Tafel slopes of OD-Cu, HC-3, -6 and -7 for CO production are 137.89, 129.92, 147.86 and 111.42 mV dec–1, respectively, close to the theoretical value of 118 mV dec–1, implying that all the catalysts share the same RDS. The observed enhanced selectivity for CO of tunable In(OH)3 coupled Cu2O derived hybrid catalyst is attributed to the synergistic effect of Cu and In. To interpret the synergistic reaction mechanism, the electronic structure of the hybrid catalyst surface must be taken into consideration. It could be modified by the electron relocation between Cu and In which alters its binging strength for reaction intermediates. The dband center is one of most widely-used reactivity descriptors for the chemisorption models on transition-metal surfaces and their alloys.36 Generally, a metal site with a higher d-band center exhibits a stronger affinity to adsorbates.37 The combination of Cu with In causes the charge transfer from In to Cu because Cu has a stronger electronegativity than In and a half-empty 4s band that can act as an electron acceptor.27, 38

The positively chemical shift measured by XPS for In 3d of Cu-In hybrid material

further confirm the electron transfer from In to Cu. Such charge transfer would result in an up-shift of the d-band center of surface Cu sites which strengthens the substrateadsorbate interaction. The activation of CO2 through the formation of a *COOH 9



intermediate on Cu sites generally has an uphill energy barrier.39 The enhanced affinity of the Cu site to *COOH due to the upshifted d-band center significantly decreases the energy barrier for the formation of *COOH. In addition to the desirable electronic structure, the favorable adsorption geometry is also indispensable.14, 40 In is identified as a metal with higher O affinity than Cu.41 The additional O-In binding contributed by the O-binding site of In adds up to the stability of *COOH on the Cu sites. In this work, the first proton-coupled electron-transfer step to form the *COOH intermediate is demonstrated as the rate-determining step based on the Tafel analysis. Thus, the improved *COOH stability results in a lowered energy barrier for CO2 reduction to CO on the Cu-In hybrid catalyst. Furthermore, the local pH effect would also play a role in the electrocatalytic performances of the oxide-derived catalysts. Due to the nanostructured surface, the local pH effect would have a more pronounced influence on the oxide-derived catalysts than the planar catalyst (e.g. metal foil). The reaction rate of the competing HER is suppressed in alkaline electrolytes. Hence, the increased local pH near the oxidederived catalyst surface could improve the selectivity of the CO2RR over the HER. For this work, the local pH effect associated with the oxide-derived catalysts plays an indispensable role in determining the improved performance of the Cu-In hybrid catalyst along with the electronic and geometric structure effect. And that may be why the synergistic effect is absent from the microfabricated Cu-In system prepared on the Cu substrate and the electrodeposited Cu-In alloy.23, 25 Hence, the presence of Cu oxide is required for the synergistic effect of Cu-In to occur.25 10


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To further investigate the stability of the hybrid catalysts, the long-term performance of HC-6 was evaluated at a constant potential load of –0.8 V for 12 hours in CO2-saturated 0.1 M KHCO3. The FECO of HC-6 is almost uninfluenced stabilizing around 90% over the 12 hours, demonstrating the good stability of selectivity towards CO2RR to CO (Figure 4a). During the 12 hours of electrolysis, jtot increases by 33.67%. After the long-term electrocatalysis, a rise of pH value of the electrolyte is detected which may be caused by the accumulation of liquid products detected by 1H NMR spectroscopy (Figure S6). The increased pH of the electrolyte maybe also an indicator of the enhanced concentration of HCO3–. It is assumed that the enhanced activity is attributed to the change in the electrolyte composition during the long-term electrocatalysis. The effect of KHCO3 concentration on the catalytic performance of CO2RR on HC-6 was systematically investigated. jtot of HC-6 in KHCO3 solution with different concentrations are shown in Figure 4b. At –1.0 V, jtot dramatically enhances from 10.88 to 40.17 mA cm–2 when KHCO3 concentration increases from 0.1 to 0.7 M, indicating the remarkable influence of KHCO3 concentration on the catalytic activity. Similarly, jCO and jC2H4 also exhibit a KHCO3 concentration dependent enhancement (Figure S7 and S8a). It is proposed that HCO3– enhances CO2RR rates by increasing the effective reducible CO2 concentration in solution through rapid equilibrium exchange between dissolved CO2 and HCO3–.42-43 The reaction order study with respect to HCO3– is conducted. As shown in Figure S9, a slope of 0.65 is obtained for the plot of log(jCO) vs. log([KHCO3]) which is close to the value of 0.64 obtained by Li et al.44 The 11



deviation from the expected first-order dependence may stem from the bifunctional role of HCO3– in CO2RR which increases the concentration of CO2 surrounding the catalyst surface, simultaneously, acts as a proton donor.45-46 Furthermore, the selectivity of HC-6 can be altered by the effect of electrolyte concentration. With increasing the concentration of KHCO3, FECO at the low overpotentials could be further boosted. As shown in Figure 4c, at a low overpotential of 290 mV, the increased FECO of 74.05% in 0.7 M KHCO3 is obtained as compared to that of 57.04% in 0.1 M KHCO3. The enhancement of FEs for other products (e.g., C2H4) in more concentrated KHCO3 results in the decrease of FECO at more negative potentials (Figure S8b). Increasing the concentration of KHCO3 aqueous solution would result in an enhancement of pH. The pH values of CO2-saturated 0.1, 0.3, 0.5, and 0.7 M KHCO3 are 6.8, 7.2, 7.5, and 7.6, respectively. To investigate whether there is a pH effect leading to the increased FE of CO over the low overpotentials, Figure 4c is replotted using the standard hydrogen electrode (SHE) scale (Figure S10). On the SHE scale, HC-6 shows almost the same value of FECO over the low overpotentials in CO2-saturated KHCO3 solution with different concentrations. It is concluded that the observed enhancement of FECO over the less negative potential range on the RHE scale is reasonably attributed to the pH effect. The reaction rate of the competing HER is suppressed in alkaline electrolytes. Hence, CO2RR is promoted in highly concentrated KHCO3 electrolyte, resulting in the increased FECO. The effect of electrolyte concentration alters the selectivity of HC-6 over the low overpotential region where the data for the Tafel analysis are acquired. However, the 12


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slopes of Tafel plots for CO production obtained for HC-6 in 0.1, 0.3, 0.5, and 0.7 M KHCO3 are close to the value of 118 mV dec–1, indicating that the first proton-coupled electron-transfer step to form the *COOH intermediate is still the RDS (Figure 4d). It demonstrates that the effect of electrolyte concentration does not change the mechanisms of the synergistic effect of the hybrid catalyst for CO formation.47 3. Conclusions In summary, the novel In(OH)3 coupled Cu2O derived hybrid catalysts were developed using a facile, scalable and cost-effective strategy. The selectivity of the hybrid catalysts can be easily tuned by adjusting the ratios of Cu2O to In(OH)3. The HC-6 catalyst with an optimized ratio reaches a FECO of 90.37% at −0.8 V, attributing to the synergistic effect between Cu and In. This study suggests a facile yet general strategy that makes it feasible to develop multicomponent hybrid catalysts for efficient CO2RR. Furthermore, we demonstrated that the activity and selectivity of the Cu-In hybrid catalysts in CO2 electroreduction can be promoted via the effect of electrolyte concentration. Supporting Information Experimental details, Figures S1−S10 and Table S1. Acknowledgments The authors thank the following funding agencies for supporting this work: the Natural Sciences and Engineering Research Council of Canada (RGPIN-2016-05494), the National Key Research and Development Program of China (2017YFA0206901, 2018YFA0209401), the National Natural Science Foundation of China (21473038, 13



21773036), the Science and Technology Commission of Shanghai Municipality (17JC1400100), and the Shanghai Pujiang Program (18PJ1401300). T.L. gratefully acknowledges the financial support from the China Scholarship Council (CSC) for his research at the University of Alberta. References (1) Karl, T. R.; Trenberth, K. E. Modern Global Climate Change. Science 2003, 302, 1719–1723. (2) Hoffert, M. I.; Caldeira, K.; Benford, G.; Criswell, D. R.; Green, C.; Herzog, H.; Jain, A. K.; Kheshgi, H. S.; Lackner, K. S.; Lewis, J. S.; Lightfoot, H. D.; Manheimer, W.; Mankins, J. C.; Mauel, M. E.; Perkins, L. J.; Schlesinger, M. E.; Volk, T.; Wigley, T. M. Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet. Science 2002, 298, 981–987. (3) Li, Q.; Fu, J.; Zhu, W.; Chen, Z.; Shen, B.; Wu, L.; Xi, Z.; Wang, T.; Lu, G.; Zhu, J. J.; Sun, S. Tuning Sn-Catalysis for Electrochemical Reduction of CO2 to CO via the Core/Shell Cu/SnO2 Structure. J. Am. Chem. Soc. 2017, 139, 4290–4293. (4) Liu, S.; Wang, X.-Z.; Tao, H.; Li, T.; Liu, Q.; Xu, Z.; Fu, X.-Z.; Luo, J.-L. Ultrathin 5-fold Twinned Sub-25 nm Silver Nanowires Enable Highly Selective Electroreduction of CO2 to CO. Nano Energy 2018, 45, 456–462. (5) Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P.; Safaei, T. S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.; Kelley, S. O.; Sargent, E. H. Enhanced Electrocatalytic CO2 Reduction via Field-Induced Reagent Concentration. Nature 2016, 14


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Figure 1. (a) Bright-field TEM image and corresponding EDS elemental maps of ICCHM-6, and HRTEM images and the corresponding diffractograms of (b) Cu2O and (c) In(OH)3 of ICC-HM-6.

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Figure 2. (a) Cu 2p and (b) In 3d XPS spectrum and the corresponding fitting results of the fresh and used electrodes of ICC-HM-6.

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Figure 3. (a) Total current densities, (b) CO partial current densities, (c) Faradaic efficiency for gas products as a function of potential, and (d) Tafel plots of CO partial current density for OD-Cu, HC-3, HC-6 and HC-7 in CO2-saturated 0.1 M KHCO3. The current density is normalized to the geometric area.

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Figure 4. (a) Long-term stability test of HC-6 at –0.8 V for 12 h in CO2-saturated 0.1 M KHCO3. (b) Total current density, (c) FE for CO, and (d) Tafel plots of CO partial current density on HC-6 in CO2-saturated KHCO3 solution with different concentrations. 27



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Achieving Efficient CO2 Electrochemical Reduction on Tunable In(OH

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