Cu 2 O Nanoparticles with Both {100} and {111} Facets for Enhancing the Selectivity and Activity of CO 2 Electroreduction to Ethylene

Abstract

Cu₂O nanoparticles (NPs) enclosed with different crystal facets, namely, c‐Cu₂O NPs with {100} facets, o‐Cu₂O NPs with {111} facets, and t‐Cu₂O NPs with both {111} and {100} facets, are prepared and their electrocatalytic properties for the reduction of CO₂ to C₂H₄ are evaluated. It is shown that the selectivity and activity of the C₂H₄ production depend strongly on the crystal facets exposed in Cu₂O NPs. The selectivities for the C₂H₄ production increases in the order, c‐Cu₂O < o‐Cu₂O < t‐Cu₂O, (with FE_C2H4 = 38%, 45%, and 59%, respectively). This study suggests that Cu₂O NPs are more likely responsible for the selectivity and activity for the C₂H₄ production than the metallic Cu NPs produced on the surface of Cu₂O NPs. This work provides a new route for enhancing the selectivity of the electrocatalytic CO₂ reduction by crystal facet engineering.

Carbon dioxide (CO₂) is highly responsible for the global warming and climate change, which makes it urgent to reduce the amount of CO₂ in atmosphere. In reducing the amount of CO₂, the electrocatalytic CO₂ reduction reaction (CO₂RR) is attractive because it can transform CO₂ into high‐valued feedstocks with high efficiency and can be combined easily with renewable energy sources such as solar or wind energy. Since the CO₂RR leads to various carbon products, it is necessary to improve the selectivity and activity for a single desired product. The production of C1 species such as carbon monoxide or formic acid has reached a very high selectivity (over 90%), but those of multicarbon species with higher commercial values have not.

Among the available electrocatalysts, copper is unique because it can produce various hydrocarbons and alcohols and because it can absorb CO intermediates well, facilitating the subsequent C–C coupling for C₂₊ production. Ethylene (C₂H₄), an important raw material, is one of the main C2 products over Cu electrodes. However, due to the simultaneous production of H₂ and other C1 species (i.e., CO, CH₄), the Faradaic efficiency for the production of C₂H₄ (FE_C2H4) on metallic Cu is usually low. Efforts to improve the FE_C2H4 of Cu‐based catalysts have focused on optimizing the sizes, morphologies, and exposed crystal facets of metallic Cu NPs. In producing C₂H₄ with high selectivity, Cu₂O NPs have recently been found to be more effective than metallic Cu NPs. For example, the FE_C2H4 of 36% was achieved from a Cu/Cu₂O catalyst prepared by electro‐redeposition method, and that of 57% from a nanodendritic Cu catalyst. Cu₂O NPs show a good performance probably because the low‐coordinate Cu⁺ ions present on the surface help the C–C coupling, thereby boosting the C₂H₄ production.

In controlling the activity and selectivity of electrocatalysts, it is important to understand how they are affected by the crystal facets. The crystal facets of metallic Cu NPs have a strong influence on the selectivity and activity of their catalytic reactions. For example, the Cu {111} facets lead preferentially to CH₄, while the Cu {100} and some high index planes to C2 products. Studies on Cu₂O NPs showed that those with different crystal facets exhibit different stabilities and different catalytic activities. For example, for the propylene oxidation under high temperature, Cu₂O NPs enclosed with the {111} facets are more catalytically active than those enclosed with the {100} or {110} facets. During a photocatalytic degradation of methyl orange on Cu₂O, electron transfer occurs from the {100} and {110} to {111} facets. During the electrochemical reduction over Cu₂O under negative potentials, metallic Cu NPs are formed on the surface of Cu₂O. It has not been unequivocal whether or not active catalysts during CO₂RR are the metallic Cu NPs produced on the surface of Cu₂O. The Cu NPs produced from Cu₂O NPs with different morphologies differ in size and aggregation, affecting their selectivity and activity for the C₂H₄ production. These observations prompt us to examine if the metallic Cu NPs derived from Cu₂O NPs possessing different crystal facets lead to different selectivities and different activities for the C₂H₄ production and consequently whether the Cu₂O or the metallic Cu NPs are responsible for the selectivity and activity of the CO₂RR.

We explored these questions by preparing Cu₂O NPs enclosed with different crystal facets, namely, cubic Cu₂O (c‐Cu₂O) NPs with {100} facets, octahedral Cu₂O (o‐Cu₂O) NPs with {111} facets, and truncated‐octahedral Cu₂O (t‐Cu₂O) NPs with both {111} and {100} facets, and then by evaluating the effect of the exposed crystal facets on the selectivity and activity for the C₂H₄ production. Our study shows that the selectivity and activity of the C₂H₄ production are strongly affected by the crystal facets exposed in Cu₂O NPs. We show that the selectivities of the Cu₂O NPs for the C₂H₄ production increases in the order, c‐Cu₂O < o‐Cu₂O < t‐Cu₂O, (with FE_C2H4 = 38%, 45%, and 59%, respectively). Our study suggests strongly that Cu₂O NPs are more likely responsible for the selectivity and activity for the C₂H₄ production than are the metallic Cu NPs produced on the surface of Cu₂O NPs.

The Cu₂O NPs were prepared by a wet chemical reduction method. A mixture of Cu₂O and carbon black was deposited on a glassy carbon electrode (GCE) to form a working electrode for the CO₂RR (for details, see the Supporting Information). The crystal structure of the samples are determined by X‐ray diffraction (XRD) (Figure S1, Supporting Information). The morphologies of the as‐prepared c‐Cu₂O, o‐Cu₂O, and t‐Cu₂O NPs were characterized by scanning electron microscopy (SEM). The three kinds of Cu₂O NPs have the sizes of 600–1000 nm, but their morphologies are different. c‐Cu₂O NPs exhibit cubic nanostructures (Figure 1a,b) enclosed with six {100} planes. The o‐Cu₂O NPs exhibit an octahedral morphology (Figure 1c,d) exposed with eight Cu₂O {111} planes. The t‐Cu₂O NPs exhibit a polyhedral morphology (Figure 1e,f) exposed with both {100} and {111} facets.

SEM images of a,b) c‐Cu2O NPs, c,d) o‐Cu2O NPs, and e,f) t‐Cu2O NPs.

We now compare the CO₂RR performances of the three Cu₂O electrodes by performing potentiostatic measurements in an H‐type electrochemical cell with CO₂ saturated aqueous 0.5 m KHCO₃ as electrolyte. The amounts of the gaseous and liquid products were determined by gas chromatography (GC) and nuclear magnetic resonance (NMR), respectively (Figures S2 and S3, Supporting Information). To examine the selectivity for the C₂H₄ production, we determine the Faradaic efficiency, FE_C2H4 = Q_C2H4/Q_tot, where Q_C2H4 is the amount of charge consumed to produce C₂H₄, and Q_tot the charge consumed to produce all products (for details see the Supporting Information). The FE_C2H4 values for the three Cu₂O electrodes are compared in Figure 2a. t‐Cu₂O NPs exhibit the highest selectivity at five selected potentials ranging from −0.9 to −1.3 V relative to reversible hydrogen electrode (RHE) (see Figure S4 in the Supporting Information for RHE calibration). The maximum FE_C2H4 reaches 59% at −1.1 V, which is comparable to the highest achieved in KHCO₃ so far by using plasma‐activated copper (60% at −1.1 V). The maximum FE_C2H4 for other Cu₂O NPs are lower, namely, 45% at −1.1 V for o‐Cu₂O NPs, and 40% at −1.2 V for c‐Cu₂O NPs. We evaluate the activities of the three Cu₂O NPs for the C₂H₄ production by calculating the currents consumed for the production, j_C2H4 = FE_C2H4 × j_total, where j_total is the total current used from the potentiostatic measurements. At any given potential, the j_C2H4 for t‐Cu₂O NPs is higher than those for o‐Cu₂O and c‐Cu₂O NPs (Figure 2b). Furthermore, the j_C2H4 for t‐Cu₂O and o‐Cu₂O NPs increases steadily with increasing the potential, but this is not the case for c‐Cu₂O NPs (Figure 2b). We now examine the stability of Cu₂O NPs by performing potentiostatic tests at a potential of −1.1 V for 2 h (Figure 2c). The current density of c‐Cu₂O, o‐Cu₂O, t‐Cu₂O NPs is maintained at about 11, 17, and 22 mA cm⁻², respectively with only ≈5% decrease in 2 h (For more details of the stability test, see Figure S5, Supporting Information). To probe the kinetics of the C₂H₄ production, we examine the Tafel plots for the three Cu₂O NPs (Figure 2d). The Tafel slope for t‐Cu₂O NPs (75 mV dec⁻¹) is lower than those of o‐Cu₂O (82 mV dec⁻¹) and c‐Cu₂O (97 mV dec⁻¹) NPs so that it has the lowest activation energy for the CO₂RR. In short, for the CO₂RR toward C₂H₄, t‐Cu₂O NPs exposed with {100} and {111} facets exhibit a better selectivity, activity than do o‐Cu₂O NPs with {111} facets and c‐Cu₂O NPs exposed with {100} facets.

a) FEC2H4 values for the c‐Cu2O, o‐Cu2O, and t‐Cu2O NPs as a function of the potential. b) jC2H4 values for the c‐Cu2O, o‐Cu2O, and t‐Cu2O NPs as a function of the potential. c) jC2H4 values for the c‐Cu2O, o‐Cu2O, and t‐Cu2O NPs at −1.1 V as a function of the reaction time. d) Tafel plots for the c‐Cu2O, o‐Cu2O, and t‐Cu2O NPs.

In our discussions so far, we have not examined the question whether the catalytic activities of the Cu₂O NPs are the intrinsic properties of these NPs or they originate from the metallic Cu NPs on the surface of the Cu₂O NPs produced during the CO₂RR. To explore this question, we carried out transmission electron microscopy (TEM) measurements for the Cu₂O NPs after the CO₂RR. As shown in Figure 3a–c, the morphologies of all the three Cu₂O nanoparticles can be well preserved after the stability test, with only some tiny nanoparticles on the surfaces, which could be probably ascribed to the Cu nanoparticles formed during electroreduction process. This finding is consistent with the stability tests discussed in Figure 2c. To further probe the change on composition and valence state of Cu₂O NPs after electrocatalytic reaction, Cu LMM Auger spectra were performed on the three Cu₂O samples before and after their use in the CO₂RR (Figure 3d–f). The peaks of Cu₂O and Cu are observed at 570.6 and 567.5 eV, respectively. The Auger spectra confirmed that before the stability test, all the three Cu₂O samples are mainly consisted of Cu⁺. While, a small part (≈5%) of Cu⁰ can be observed beside Cu⁺ after the stability test. According to these results, we conclude that the electrocatalytic activity of Cu₂O NPs is an intrinsic property of these oxides rather than the metallic Cu NPs produced on their surfaces.

TEM images and Cu LMM Auger spectra of a,d) c‐Cu2O, b,e) o‐Cu2O, and c,f) t‐Cu2O after CO2RR, respectively.

To probe a possible reaction mechanism for the CO₂RR, we examine how the amount of the gaseous products, C₂H₄, CH₄, CO, and H₂, arising from c‐Cu₂O, o‐Cu₂O, and t‐Cu₂O NPs, vary as a function of the potential as shown in Figure S6 (Supporting Information). The production of C₂H₄ and CO over all the three samples exhibit similar trends, where the FE_C2H4 initially increased sharply then decreased as the applied potential increased, while, the FE_CO decreased constantly with the increase of potential. This confirms that adsorbed CO species are intermediates for C₂H₄ production during CO₂RR. Comparing to the production of C₂H₄, the formation of CH₄ is much lower. That indicates C–C coupling to form C₂H₄ from adsorbed CO intermediates is preferable than does the formation of CH₄ on the surface of Cu₂O NPs. The production of hydrogen from the three Cu₂O NPs enclosed with different crystal facets are quite different. The FE_H2 for c‐Cu₂O is high (≈50%) at −0.9 V versus RHE, which sharply decreased with the increase of applied potential, then increased as the potential is more negative than −1.1 V versus RHE. For o‐Cu₂O, the FE_H2 is relatively lower than that from c‐Cu₂O as the potential is below (≈30%) −1.1 V versus RHE, then increased quickly as the potential further increased. This indicates the production of H₂ from {100} facets of Cu₂O could be more preferable than over Cu₂O {111} facets, which can explain why the FE_C2H4 over o‐Cu₂O is higher than that of c‐Cu₂O. However, for t‐Cu₂O, the FE_H2 was kept below 30% as the potential ranging from −0.9 to −1.3 V versus RHE. This indicates the production of H₂ can be effectively suppressed in t‐Cu₂O enclosed by both {100} and {111} facets, which lead to the highest FE_C2H4 among the three samples.

To gain insight into the reason why t‐Cu₂O has a better performance for the C₂H₄ production than does c‐Cu₂O and o‐Cu₂O NPs, we carry out DFT calculations to examine the adsorption capabilities of the reaction intermediate CO as well as the product C₂H₄ on the Cu₂O {100} facets, {111} facets and the joint interface between {100} and {111} facets. As shown in Figure 4, CO is more strongly adsorbed on the Cu₂O {100} facets and the joint interface between {100} and {111} facets than on the Cu₂O {111} facets. This would subsequently facilitate the C–C coupling to produce C₂₊ products during CO₂RR. On the other hand, C₂H₄ can be adsorbed more weakly on the joint interface between Cu₂O {100} and {111} facets and Cu₂O {111} facets than on the Cu₂O {100} facets. That means c‐Cu₂O enclosed by {100} facets could facilitate the C–C coupling to produce C₂₊ products, but the formed C₂₊ products (e.g., C₂H₄) can hardly escape from the surface of Cu₂O {100} facet because of its stronger adsorption ability of C₂H₄. For o‐Cu₂O enclosed by {111} facets, although the adsorption of CO intermediates is lower than that on Cu₂O {100} facets, once C₂H₄ was formed, it can be easily desorbed from the surface of Cu₂O {111} facets because of its weaker adsorption ability. Meanwhile, for t‐Cu₂O enclosed by both {100} and {111} facets, the CO intermediates can not only be strongly adsorbed on the joint interface between {100} and {111} facets to promote the C–C coupling, but also the as‐formed C₂H₄ can be easily desorbed from the joint interface to promote the C₂H₄ production.

Adsorption energies of a) CO and b) C2H4 on the {100} surfaces, {111} surfaces and the interface of {100} and {111} surfaces of Cu2O. The corresponding adsorption configurations are also shown.

In addition, the reason why t‐Cu₂O provides a much better catalytic performance than does c‐Cu₂O and o‐Cu₂O may be related to the fact that the Fermi level of Cu₂O is lower on the {111} than on the {100} facets. This could subsequently facilitate the charge transfer between Cu₂O {111} and {100} facets, and further promote the multielectron involved kinetics for ethylene production in Cu₂O nanoparticles enclosed by both {111} and {100} facets (Figure 5a–c).

Formation of C2H4 on the a) {100} facets of c‐Cu2O NPs, b) {111} facets of o‐Cu2O NPs, and c) {100} and {111} facets of t‐Cu2O NPs.

In summary, the t‐Cu₂O NPs enclosed with both {100} and {111} facets exhibit the FE_C2H4 and j_C2H4 values of 59% and 23.1 mA cm⁻², respectively, for the CO₂RR at −1.1 V in 0.5 m KHCO₃. These are better than those of the o‐Cu₂O NPs with {111} facets (45%, 16.4 mA cm⁻²) and c‐Cu₂O NPs with {100} facets (38%, 10.6 mA cm⁻²). Our study suggests that the electrocatalytic activity of Cu₂O NPs is an intrinsic property of these oxides rather than the metallic Cu NPs produced on their surfaces during the CO₂RR. The enhanced performance of t‐Cu₂O NPs can be attributed to the synergistic effect of {100} and {111} facets, which can not only facilitate the C–C coupling and C₂H₄ desorption, but also be able to promote the multielectron involved kinetics for ethylene production. Our work may provide a new route for enhancing the selectivity of the electrocatalytic CO₂ reduction by crystal facet engineering.

Experimental Section

Preparation of the c‐Cu₂O, o‐Cu₂O, and t‐Cu₂O Particles

Cu₂O particles were synthesized by wet chemical reduction method according to previous reports. In a typical synthesis, polyvinylpyrrolidone (PVP, MW 24 000) (0 g for c‐Cu₂O, 4 g for t‐Cu₂O, and 6 g for o‐Cu₂O) was added into 100 mL CuCl₂ · 2H₂O aqueous solution. Then, 10.0 mL NaOH aqueous solution (2.0 m) was added dropwise into the above solution. After stirring for 30 min, 10.0 mL ascorbic acid solution (0.60 m) was added dropwise into the dark brown solution. The mixture was aged for 3 h and the solution gradually transferred into turbid red. All of the procedure was carried out under constant stirring and heated in a water bath at 55 °C. The resulting precipitate was collected by centrifugation and decanting, followed by washing with distilled water 3 times and absolute ethanol 3 times and finally dried under vacuum at 60 °C for 6 h.

Preparation of the Electrodes

Typically, an ink of Cu₂O/C particles was prepared by adding 2 mg catalyst (c‐Cu₂O, o‐Cu₂O, or t‐Cu₂O particles) and 8 mg Carbon Black into the ink‐base of 800 µL of isopropanol, 100 µL of H₂O, and 100 µL of 5% nafion solution and then ultrasonicating the solution for 3 h. We deposited 10 µL of the sample inks on the GCE (diameter, 5 mm) to form the sample electrodes.

Electrochemical Experiments

The linear sweeping voltammetry (LSV) measurements were carried out with an Ag/AgCl reference electrode (with saturated KCl as the filling solution), a platinum electrode as the counter electrode and the as‐prepared samples as the working electrode. The product analysis was carried out in a two‐compartment electrochemical cell with an anion exchange membrane separating the working and counter electrodes. The potentiostatic measurements were performed using a three‐electrode system to determine the value of the consumed coulomb, and the amounts of the gases produced were measured by the GC and GCMS instruments. The electrolyte was potassium bicarbonate saturated with CO₂ by bubbling high‐purity CO₂ gas, before each experiment, at a flow rate of 50 mL min⁻¹ for 1 h to remove all oxygen from the electrolyte. The working electrode was tested 20 times before the plot is recorded at a scan rate of 50 mV s⁻¹. All potentials were transformed to the reversible hydrogen electrode reference by using the calibrated relationship, E_RHE = E_Ag/AgCl + 0.657 V.

Characterization

Crystal structures of the as‐obtained products were characterized by XRD measurements with a Bruker AXS D8 diffractometer using Cu Kα radiation. Fourier transform infrared (FTIR) spectra were obtained on a Bruker ALPHA‐T spectrometer using KBr pellets. Raman spectra were recorded on a microscopic confocal Raman spectrometer (Horiba JobinYvon, LabRAM HR) with an excitation of 613 nm laser light. Morphologies and microstructures of the products were characterized by scanning electron microscopy (Hitachi S‐4800) equipped with an Energy Dispersive Spectrometer (EDS) and transmission electron microscopy using a Philips Tecnai 20U‐Twin microscope at an acceleration voltage of 200 kV. (JEOL JEM‐2100F). X‐ray photoelectron spectroscopy (XPS) measurement was performed using a Thermo Fisher Scientific Escalab 250 spectrometer with monochromatized Al Kα excitation, and C1s (284.6 eV) was used to calibrate the peak positions of various elements. All electrochemical experiments were carried out using the electrochemical workstation CHI660E. The gas products from the compartment were examined with a gas chromatograph equipped with a TDX‐01 column with a flame ionization detector (FID) and a H₂‐detection GC (ShiweipxGC‐7806) with a thermal conductivity detector (TCD). Gas chromatograph‐mass spectrometer (GCMS) was used to determine the concentration of liquid products with a Max capillary column.

The faradaic efficiency (FE) was calculated by the following equation [Image Omitted. See PDF]

Where α is the number of the electrons transferred for CO, F is the Faraday constant, Q is the charge, I is the current, t is the running time, and n is the total amount of CO (in moles).

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (21972078 and 21333006), and the Shandong Provincial Natural Science Foundation (ZR2019MEM004). Z.Y.W. acknowledges the support from Shandong University multidisciplinary research and innovation team of young scholars (2020QNQT11), Qilu Young Scholar Program of Shandong University, Young Scholars Program of Shandong University (2015WLJH35) and the Fundamental Research Funds of Shandong University (2018JC039). B.B.H acknowledges the support from the Taishan Scholar Foundation of Shandong Province.

Conflict of Interest

The authors declare no conflict of interest.