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Tuning nanocavities of Au@Cu2O yolk–shell nanoparticles for highly selective electroreduction of CO2 to ethanol at low potential

The electrosynthesis of high-value ethanol from carbon dioxide and carbon monoxide addresses the need for the large-scale storage of renewable electricity and reduction of carbon emissions. However, the electrosynthesis of ethanol by the CO2 reduction reaction (CO2RR) has suffered from low selectivity and energy efficiency. Here, we report a catalyst composed of Au nanoparticles in Cu2O nanocavities (Au@Cu2O) that is very active for CO2 reduction to ethanol through the confinement of the CO intermediate. The architecture shows tandem catalysis mechanisms in which CO2 reduction on Au yolks produces CO filling Cu nanocavities, where a sufficiently high CO concentration due to the confinement effect promotes ethanol formation and then results in an ethanol faradaic efficiency of 52.3% at −0.30 V versus the reversible hydrogen electrode (vs. RHE) via regulating the hollow size of the Cu2O nanocavities. Such a strategy provides a new way of fabricating various tandem catalysts with high selectivity and efficiency for the CO2RR.


Introduction
The electrocatalytic reduction of CO 2 to valuable carbon-based fuels and chemicals offers a route to reduce CO 2 emissions and facilitate the long-term storage of renewable electricity. [1][2][3][4][5][6] In particular, C 2 and C 2+ products from the CO 2 reduction reaction (CO 2 RR) have attracted considerable attention due to their relatively high energy density, added-value and widespread use as feedstocks in polymer synthesis, etc. 7-9 Excellent electrocatalysts have been developed to boost the activity and selectivity of the CO 2 RR towards C 2 and C 2+ productions. [10][11][12][13][14] Among currently available materials, Cu-based catalysts have been reported as the most promising electrodes for producing C 2 and C 2+ compounds under aqueous conditions in CO 2 RR, and have been extensively studied. 8,12,[15][16][17][18][19][20][21] However, there still present some scientic challenges, such as poor selectivity, low faradaic efficiency and durability, which need to be managed primarily in further work.
To date, several avenues have been employed to improve the selectivity of Cu-based catalysts for CO 2 RR to specic products, including altering size, structure, composition, surface state, and so on. [17][18][19][20] Hori et al. declared that the product selectivity of CO 2 RR shied greatly with the crystal orientation, where Cu(100) yielded mainly C 2 H 4 and Cu(111) beneted CH 4 production. 22 Sargent's group constructed a core-shell vacancy engineering catalyst (Cu 2 S-Cu-V) to steer products beyond alkenes and toward ethanol at certain potentials. 23 More recently, theoretical and experimental investigations demonstrate that the activity and selectivity for C 2 and C 2+ of Cu-based catalysts can be greatly advanced by bimetallic strategy or constraining the local CO concentration at the catalyst-electrolyte interface. 24,25 Au/Cu bimetallic electrocatalyst was obtained with improved activity and selectivity for electrochemical transformation of CO 2 to alcohols over hydrocarbons at low overpotentials, and a tandem catalysis mechanism has been proposed where Au nanoparticles (Au NPs) reduce CO 2 to CO near the copper surface, driving a high CO coverage. 25 When supplied directly with CO instead of CO 2 as a feedstock, oxidederived nanocrystalline copper electrodes produce ethanol with up to 30% faradaic efficiency at modest overpotentials under alkaline conditions. 26 In addition, the connement effect is known to alter thermodynamic and transport properties of uids. [27][28][29][30] Prior studies of porous catalysts exploited connement effect to implement a selectivity shi by extending the retention of C 1 or C 2 species, and then boost C 2 or C 3 production, 16,24 which lack in-depth exploration. As such, developing a new strategy of combining bimetallic strategy and connement effect is highly imperative for boosting the selectivity to ethanol in CO 2 electroreduction and its practical application.
Herein, we further apply the connement effect with a bimetallic yolk-shell structure of Au nanoparticle in Cu 2 O nanocavity (Au@Cu 2 O), whereas Au shows high catalytic activity for converting CO 2 to CO at low potentials, 31,32 and Cu is able to catalyze the second step of the tandem reaction, the reduction of CO to C 2 products at low potentials. [33][34][35] The Au@Cu 2 O catalyst has higher selectivity for ethanol compared with hydrocarbons at lower overpotentials. We propose that the selectivity shiing from C 1 to C 2 originates from a tandem catalysis mechanism, where Au NPs reduce CO 2 to CO in the Cu nanocavities, producing a high CO concentration, thus boosts the ethanol production due to the connement of CO intermediate. Meanwhile, we adjust the CO concentration in the nanocavities by regulating the hollow size of the Cu 2 O nanocavities, and then achieve an ethanol faradaic efficiency of 52.3% at À0.30 V vs. RHE.

Preparation of Au NPs
The Au NPs with a diameter of about 22 AE 2 nm were synthesized and used as the core materials for the Au@Cu 2 O yolkshell particle fabrication. 36,37 Au NPs were prepared by a standard citrate reduction procedure. Typically, 150 mL of 0.025 M HAuCl 4 solution was added into a three-necked round bottom ask and heated to boiling under continuously magnetic stir with condensing and reuxing conditions. Then 7.5 mL of 0.02 M sodium citrate solution was added. Aer reaction for 30 min, heating resource was removed and the solution was allowed to cooled down naturally to room temperature. Finally, the Au NPs were collected from the solution by centrifugation (10 000 rpm), washed with deionized water several times and redispersed in 10 mL of deionized water.

Preparation of Au@Cu 2 O yolk-shell NPs with different hollow sizes
Au@Cu 2 O yolk-shell NPs was prepared by hydrazine hydration reduction method. 36,38 Typically, 1 g of PVP powders was added into 10 mL of 0.01 M Cu(NO 3 ) 2 solution under constant stirring until the PVP powders dissolved completely. Then a certain amount of as-obtained Au NPs solution was added, followed by immediate introduction of trace hydrazine hydrate. Aer reaction, the Au@Cu 2 O yolk-shell NPs were synthesized and centrifuged (10 000 rpm), washed three times with water and isopropyl alcohol, and then dried in an oven at 60 C for later use. Au@Cu 2 O NPs with different hollow sizes can be controlled through adjusting the amount of reducing agent and the reaction time (Fig. 1a).

Preparation of Cu 2 O NPs
Cu 2 O NPs was prepared in parallel by the same method as Au@Cu 2 O yolk-shell NPs except for no addition of Au NPs and PVP powders.

Preparation of working electrode
To prepare the catalyst inks, 3 mg of catalyst powder, 10 mL of Naon solution (5%) and 1200 mL of isopropyl alcohol were mixed and treated under sonication for 30 min. Then, 600 mL of the asprepared ink was pipetted onto the two sides of a carbon cloth with area of 1 Â 1.5 cm 2 giving a catalyst loading of 1 mg cm À2 .
Characterizations X-ray diffraction (XRD) pattern was carried out on a Rigaku D/ Max-2500 diffractometer equipped with a Cu Ka1 radiation (l ¼ 1.54Å). Scanning electron microscopic images (SEM) were collected on a JEOL scanning electron microscope (S-4800, Japan). Transmission electron microscopic images (TEM) were obtained by a JEM-2100F microscope (JEOL, Japan) equipped with an EDS detector (Oxford Instrument, UK). X-ray photoelectron spectroscopy (XPS) was performed on an ESCALab220i-XL electron spectrometer (VG Scientic, UK) with a monochromatic Al Ka source. The gas products for CO 2 reduction were measured on a gas chromatography (GC, Agilent Technologies 7890B). The liquid products were analyzed with a Bruker AVANCE 600 using dimethyl sulphoxide (DMSO) as an internal standard.

Electrochemical performance test
Electrochemical measurements were performed on a CHI 660E electrochemical workstation in a typical H-type electrolysis cell under ambient pressure and room temperature using platinum mesh (1 Â 1 cm 2 ) and Ag/AgCl electrode (saturated KCl) as counter and reference electrodes, respectively. The cathode and anode compartments were separated by a proton-exchange membrane (Naon 117). The 0.1 M KHCO 3 aqueous solution was used as electrolyte directly without any purication. The electrode potentials were converted to the reversible hydrogen electrode reference (RHE) scale using the following Nernst equation: Before test, the catalyst was measured at À0.2 V vs. RHE for 30 min in Ar-saturated 0.1 M KHCO 3 solution to ensure all Cu 2 O-shells had been reduced to Cu-shells. Then, the catalyst was immediately transfer into CO 2 -saturated 0.1 M KHCO 3 solution to perform linear scanning voltammetry (LSV) test from 0.2 to À1.1 V vs. RHE until the performance of the electrode became stable. Finally, CO 2 RR experiment at a constant applied voltage spanned over 3600 seconds. The electrochemically surface area (ECSA) was determined by measuring the double layer capacitance (C dl ), which was derived from the CV curves at various scan rates.

Analysis of CO 2 reduction products
We measure the reduction performance of the catalyst by calculating the faradaic efficiency (FE) of the CO 2 reduction product. The FE of products can be calculated using the following formula: 39 i: the specic reduction product, CO, HCOO À , C 2 H 5 OH or H 2 ; Q total : the total charge of CO 2 RR, C; Q i : charge used for the reduction of certain product, C; N i : number of moles for certain product, mol; n: number of electrons transferred for the CO 2 -to-CO, HCOO À and C 2 H 5 OH conversion or water-to-H 2 reduction, which is 2, 2, 12, 2 for CO, HCOO À , C 2 H 5 OH and H 2 , respectively; F: faradaic constant, which is 96 485 C mol À1 .

Results and discussion
The Au NPs with uniform size and morphology were obtained according to previous literature (Fig. S1 †). 36,37 Subsequently, the Au nanoparticle was encapsulated within a porous Cu 2 O shell with an average size of 140 nm and the hollow size was precisely controlled by regulating the reaction time and the amount of reducing agents, as depicted in Fig. 1a. with a characteristic interplanar angel of 90 on the shell (Fig. 2a). 40 Dark-eld scanning transmission electron microscopy (DF-STEM) image (Fig. 2b) exhibits the loose and porous structure of the shell material, which will promise efficient mass transport for potential electrochemical applications. Meanwhile, the corresponding energy dispersive X-ray spectroscopic (EDS) elemental mapping images (Fig. 2c-f) clearly display that Au distributes homogeneous on the core part while Cu and O are distributed mainly across the nanoparticle shell section, conrming the yolk-shell structure of Au@Cu 2 O. The composition and crystalline structure of these samples were investigated by X-ray diffraction (XRD) technique. All of the recorded diffraction peaks in the typical XRD patterns (Fig. 3a) can be well indexed to cubic Au (JCPDS no. 89-3697) and cubic Cu 2 O (JCPDS no. 78-2076), corresponding to the HRTEM results. Further clues can be seen in X-ray photoelectron spectroscopic spectra (XPS) (Fig. 3b and c). By comparing the Cu 2p and Cu-LMM XPS peaks of Cu 2 O and Au@Cu 2 O-MC, it could be clearly seen that the Cu atom in Au@Cu 2 O-MC is Cu + , which is benecial for CO 2 RR according to previous reports. 15,16,26,41 In addition, since XPS is a surface analysis technique with investigation depth of 2-5 nm and the Au-core is coated by Cu 2 O, the signal of Au element is not detected. 42 Based on these results, it can be concluded that Au-core encapsulates in porous Cu 2 Oshell particle, which constitute the tandem catalyst.
Consequently, it is suggested that the active Cu sites of the catalyst during CO 2 RR are derived from Cu 2 O.
The CO 2 RR performances were evaluated over the asobtained samples supported on the carbon cloth in 0.1 M KHCO 3 solution saturated with CO 2 using H-cell set-up. The products of CO 2 RR were analyzed and quantied by online gas chromatography (GC) for the gas products and 1 H nuclear magnetic resonance spectroscopy ( 1 H NMR) for the liquidphase products. We can determine the electrochemical activity by linear sweep voltammetry (LSV) towards CO 2 RR, initially. Compared with the current density under Ar-saturated 0.1 M KHCO 3 , it has a signicant increase under CO 2 -saturated electrolyte, indicating that Au@Cu 2 O-MC has superior CO 2 RR performance (Fig. S4a †). From Fig. S4b, † it can be seen the difference of current density between these samples, and Au@Cu 2 O-MC has the maximum current density at the same potential, showing the excellent CO 2 RR performance. For porous Cu 2 O NPs, at low applied cathodic potential of À0.3 and À0.35 V vs. RHE, the only detectable product is ethanol and the faradaic efficiency (FE C 2 H 5 OH ) reaches 16% at À0.3 V vs. RHE (Fig. 4a), demonstrating the Cu 2 O NPs has intrinsic C-C coupling potential. At more cathodic potentials, a substantial difference in the product distribution is found with the products of C 2 and signicant increases in the CO, HCOO À production rates. Aer introducing Au NPs in the cavities (Fig. 4b), the selectivity shis clearly and CO, HCOO À , C 2 H 5 OH products are generated at À0.3 and À0.35 V vs. RHE. The This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 19192-19198 | 19195 production of CO and HCOOH on the Au@Cu 2 O-SC as well as its absence on the pure porous Cu 2 O NPs, together with the promoted FE C 2 H 5 OH (36%) at low potentials, prove the effect of the suggested tandem reaction mechanism within the nano-conned space with both catalytic sites located in cavities. The Au-core is active for electroreduction of CO 2 to CO, yet the Cu-shell is able to reduce the retention of CO in the cavities to improve ethanol production (Fig. 4e). When the hollow size of the catalyst was increased to 35 nm, the largest FE C 2 H 5 OH was improved to 52.3% at À0.3 V vs. RHE (Fig. 4c). The reduction of the FE C2H5OH at higher cathodic potentials from À0.45 to À0.6 V vs. RHE indicates that the previously described tandem mechanism is almost inactive in this potential window for Au@Cu 2 O-MC due to the overwhelmingly competitive HER from the exposed metal species (Fig. S5 †). Further increasing the hollow size to 48 nm, the largest FE C 2 H 5 OH is 38% at À0.35 V vs. RHE (Fig. 4d), which is slightly less than that of Au@Cu 2 O-MC. However, the active potential window of tandem reaction mechanism for Au@Cu 2 O-LC is broader than that of Au@Cu 2 O-MC, and all of the FE C 2 H 5 OH at whole potentials from À0.27 to  À0.45 V vs. RHE is higher than that of Au@Cu 2 O-SC, as well as a reduction of the detected CO at À0.27 and À0.3 V vs. RHE for Au@Cu 2 O-MC and Au@Cu 2 O-LC, suggesting the reaction mechanism of CO 2 RR can be monitored via regulating the concentration of CO intermediate by steering the hollow size of the Cu 2 O cavity. 43 Therefore, advisable hollow size for tandem reaction drove by spatial connement effect is very important. Moreover, the ECSA of all the samples have been determined from the CV curves at different scan rates from 10 to 50 mV s À1 (Fig. S6 †) to estimate the effect from morphology. As shown in Fig. S7, † the Au@Cu 2 O-SC shows a C dl value of 2.71 mF cm À2 , similar to 2.08 and 2.23 mF cm À2 of Au@Cu 2 O-MC and Au@Cu 2 O-LC, further verify the reaction mechanism we have proposed previously.
The durability of each catalyst was further assessed by chronoamperometry (CA) measurement since it is another important performance parameter for estimating an electrocatalyst. Fig. 4f exhibits the consecutive over 13 h CA curves at a constant potential of À0.3 V vs. RHE for Au@Cu 2 O-MC, and it outputs a steady current density for ethanol production with a nearly unchanged FE C 2 H 5 OH . The FE C 2 H 5 OH was retained >50% during the entire period, suggesting the excellent long-term durability of the catalyst, comparable with or outperforming most of other stateof-the-art Cu-based CO 2 RR catalysts (Table S1 †). In addition, the catalysts aer CO 2 RR tests were further characterized. TEM image (Fig. S8a †) and XRD pattern (Fig. S8b †) reveal that Au@Cu 2 O was in situ electroreduction to Au@Cu, hinting that the actual active sites are from Au-core and Cu-shell, which is in agreement with previously reported results. 16,25,26 Conclusions In summary, we have developed a bimetallic catalyst of Au@Cu 2 O yolk-shell with improved activity and selectivity for the electrochemical reduction of CO 2 to ethanol at lower potential. It was demonstrated that the spatial connement of different active sites with a tandem catalysis mechanism leads to the selectivity shi from C 1 to C 2 . The Au-core can reduce CO 2 to CO in the copper nanocavity, producing a high CO concentration, thus the Cu-shell transforms CO to ethanol production. In addition, we conne the CO concentration in the nanocavity by optimizing the hollow size of the Cu 2 O nanocavity, and then perform an ethanol faradaic efficiency of 52.3% at À0.30 V vs. RHE. These results suggest that the present strategy may shed light on the design and preparation of highly active tandem catalysts for other electrochemical reactions.

Conflicts of interest
There are no conicts to declare.