Achieving Efficient CO2 Electrochemical Reduction on Tunable In(OH

May 31, 2019 - The continuous increase of atmospheric carbon dioxide (CO2) is regarded ... tidal, and solar power) holds promise to decrease CO2 emiss...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22346−22351

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Achieving Efficient CO2 Electrochemical Reduction on Tunable In(OH)3‑Coupled Cu2O‑Derived Hybrid Catalysts Tengfei Li,†,‡ Hongmei Wei,‡ Tianmo Liu,*,‡ Gengfeng Zheng,*,§ Subiao Liu,† and Jing-Li Luo*,† †

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China § 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 Downloaded via DALHOUSIE UNIV on August 5, 2019 at 02:51:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

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 versus reversible hydrogen electrode. The mechanistic discussion suggests that the composition-dependent synergistic effect results in the enhanced selectivity for CO on the hybrid catalyst. By increasing the 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, a 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 high efficiency, high selectivity, and high stability for CO2RR. KEYWORDS: carbon dioxide, electrochemical reduction, hybrid catalyst, synergistic effect, concentration effect selectivity than the pristine catalyst.13−17 Takanabe et al. reported a Cu−In bimetallic catalyst which exhibited 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 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 the synergistic effect between Cu and In remains to be studied.

1. INTRODUCTION The continuous increase of atmospheric carbon dioxide (CO2) is regarded as one of the main causes of 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 promise to decrease CO2 emissions through converting CO2 to carbonneutral 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 to its superior activity for reducing CO2 to multicarbon products, such as ethylene (C2H4) and ethanol (C2H5OH).9−11 Except for hydrocarbons and oxygenates, carbon monoxide (CO) is another desirable product from CO2RR because 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. Benefitting from the synergistic effect, the foreign component-modified Cu-based catalysts can achieve a higher © 2019 American Chemical Society

Received: March 14, 2019 Accepted: May 31, 2019 Published: May 31, 2019 22346

DOI: 10.1021/acsami.9b04580 ACS Appl. Mater. Interfaces 2019, 11, 22346−22351

Research Article

ACS Applied Materials & Interfaces

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

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

45.4°, and 51.2° corresponding to (200), (220), (400), and (420) peaks of In(OH)3 (JCPDS 85-1338). The morphologies were investigated by scanning electron microscopy. 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 the Cu element is enriched in the nanocubes, while the 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 ICCHM-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 the 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 a CO2-saturated KHCO3 electrolyte to reduce In(OH)3 and Cu2O until the potential reached the steadystate values. The XRD patterns 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. Because of 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

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 compositioninfluences 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 faradaic efficiency (FE) of 90.37% toward CO at an applied potential of −0.8 V versus 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. The 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 the 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 Xray fluorescence 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 is indexed as the 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°, 22347

DOI: 10.1021/acsami.9b04580 ACS Appl. Mater. Interfaces 2019, 11, 22346−22351

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etry. All the catalysts show 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 the 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 of only 3.26 mA cm−2 for OD-Cu. However, further increase of the In content in the hybrid catalyst does not continue to improve its ability to convert CO2 to CO. jCO 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 the In content of the hybrid catalysts increases 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, FEH2 of 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 (Figures 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 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 ratedetermining 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 electron relocation between Cu and In which alters its binging strength for reaction intermediates.

Figure 2a, the peaks at 932.97, 934.42, and 944.55 eV of the 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 correspond to the 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. Because of that the Cu 2p3/2 peaks related to Cu0 and Cu+ species are indistinguishable; it is difficult to discriminate the attribution of the 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). During the reduction process, a shift toward 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 of the peaks. It could be concluded that In(OH)3 was reduced to metallic In during the reduction process, which agreed well with previous reports.25,26 Furthermore, the core level binding energy of the In element 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 the fact that the binding energy of the Cu+ peak is very close to that of the Cu0 peak.27 In addition, the 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 gastight two-compartment cell to investigate the electrochemical CO2 reduction activity of ICC-HM-3, -6, and -7 derived hybrid catalysts (referred as HC-3, -6, and -7). Cu2O-derived catalyst (OD-Cu) was also studied for comparison. The CO 2 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 chronoamperom-

Figure 3. (a) Total current densities, (b) CO partial current densities, (c) FE 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. 22348

DOI: 10.1021/acsami.9b04580 ACS Appl. Mater. Interfaces 2019, 11, 22346−22351

Research Article

ACS Applied Materials & Interfaces The d-band 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 stronger electronegativity than In and a half-empty 4s band that can act as an electron acceptor.27,38 The positive chemical shift measured by XPS for In 3d of the Cu−In hybrid material further confirms 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 substrate−adsorbate interaction. The activation of CO 2 through the formation of a *COOH intermediate on Cu sites generally has an uphill energy barrier.39 The enhanced affinity of the Cu site to *COOH because of the upshifted dband 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 a higher O affinity than Cu.41 The additional O−In binding contributed by the Obinding 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. Because of 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. In addition, this 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 the Cu oxide is required for the synergistic effect of Cu−In to occur.25 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 h in CO2-saturated 0.1 M KHCO3. The FECO of HC-6 is almost uninfluenced stabilizing around 90% over the 12 h, demonstrating the good stability of selectivity toward CO2RR to CO (Figure 4a). During the 12 h of electrolysis, jtot increases by 33.67%. After the long-term electrocatalysis, a rise of the 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 may also be 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 CO 2 RR 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 the KHCO3 concentration increases from 0.1 to 0.7 M,

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.

indicating the remarkable influence of KHCO3 concentration on the catalytic activity. Similarly, jCO and jC2H4 also exhibit a KHCO3 concentration dependent enhancement (Figures 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(j CO ) versus log([KHCO3]) which is close to the value of 0.64 obtained by Li et al.44 The 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 and simultaneously acts as a proton donor.45,46 Furthermore, the selectivity of HC-6 can be altered by the effect of electrolyte concentration. By increasing the concentration of KHCO3, FECO at 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 re-plotted 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 a 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 the electrolyte concentration alters the selectivity of HC-6 over the low overpotential region where the data for the Tafel analysis are acquired. However, the 22349

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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 electrontransfer 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

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04580. Experimental details, fabrication procedures, XRD patterns of as-prepared Cu2O and ICC-HM-6 and freshly prepared and electrochemical reducted electrodes of ICC-HM-6, SEM images of Cu2O and ICC-HM-6, H2 partial current density, C2H4 partial current density, 1 H NMR spectrum, CO partial current density, C2H4 partial current density and FEs of C2H4, partial CO current density, FEs of CO (FECO) for HC-6, and In content of the as-prepared catalysts (PDF)

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REFERENCES

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3. CONCLUSIONS In summary, the novel In(OH)3-coupled Cu2O-derived hybrid catalysts were developed using a facile, scalable, and costeffective 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.L.). *E-mail: [email protected] (G.Z.). *E-mail: [email protected] (J.-L.L.). ORCID

Tengfei Li: 0000-0002-9826-4033 Gengfeng Zheng: 0000-0002-1803-6955 Subiao Liu: 0000-0001-6075-3301 Jing-Li Luo: 0000-0002-2465-7280 Notes

The authors declare no competing financial interest.

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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, 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. 22350

DOI: 10.1021/acsami.9b04580 ACS Appl. Mater. Interfaces 2019, 11, 22346−22351

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.9b04580 ACS Appl. Mater. Interfaces 2019, 11, 22346−22351