Electro-derived Cu-Cu2O nanocluster from LDH for stable and selective C2 hydrocarbons production from CO2 electrochemical reduction

Journal of Energy Chemistry 48 (2020) 169–180

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Electro-derived Cu-Cu2 O nanocluster from LDH for stable and selective C2 hydrocarbons production from CO2 electrochemical reduction Naveed Altaf, Shuyu Liang, Liang Huang, Qiang Wang∗ College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 2 November 2019 Revised 23 December 2019 Accepted 25 December 2019 Available online 28 December 2019 Keywords: Electrochemical Electro-derived strategy Layered double hydroxide Ethylene Cu2 O nanocomposites

a b s t r a c t Recently, CO2 conversion by electrochemical tool into value-added chemicals has been considered as one of the most promising strategies to offer sustainable development in energy and environment. In this contribution, we investigated electro-derived composites from Cu-based layered double hydroxide (LDH) for CO2 electrochemical reduction. The Cu-Cu2 O based nanocomposite (HPR-LDH) were derived by using electro-strategy from LDH having the stability up to 20 h and selectivity toward C2 H4 with faraday efficiency up to 36% by significantly suppressing CH4 and H2 with respect to bulk Cu foil. A highly negative reduction potential derived catalyst (HPR-LDH) maintained long-term stability for the selective production of ethylene over methane, and a small amount of Cu2 O was still observed on the catalyst surface after CO2 reduction reaction (CO2 RR). Moreover, such unique strategy for electro-derived composite from LDH having small nanoparticles stacked each other grown on layered structure, would provide new insight to improve durability of O–Cu combination catalysts for C–C coupling products during electrochemical CO2 conversion by suppressing HER. The XRD, SEM, ESR, and XPS analyses confirmed that the long-term ethylene selectivity of HPR-LDH is due to the presence of subsurface oxygen. The designed composite catalyst significantly enhances the stability and selectivity, and also decreases the over potential for CO2 electroreduction. We predict that the new designed LDH 2D-derived composites may attract new insight for transition metal and may open up a new direction for known structural properties of selective catalyst synthesis regarding effective CO2 reduction reaction. © 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

1. Introduction With the rapid advancement in industrialization and urbanization, many energy and environment issues have to be seriously concerned. For instance, extensive fossil fuels burning results in the significant increase of the concentration of greenhouse gases especially carbon dioxide (CO2 ), which could lead the world toward unmanageable environmental problems [1,2]. To meet this energy and environment crisis, converting CO2 into added-value compounds can not only solve the above-mentioned problems, but also be a pertinent way for future energy storage system (ESS) due to high demand and imbalanced supply of renewable energy sources [1,3,4]. Among different CO2 conversion technologies, electrochemical reduction process possesses many advantages, particularly it has the potential to produce value-added products with high energy density [5,6]. However, CO2 electro-reduction method also faces challenges, including poor stability and faradaic

∗

Corresponding author. E-mail address: [email protected] (Q. Wang).

efficiency, difficulty in controlling selectivity, and high competitive hydrogen evolution reaction (HER) from water splitting in the aqueous media [7]. It is believed that the structure, size, and morphology of catalysts have a diverse impact on CO2 conversion [8]. Recently, the electrochemical CO2 reduction reaction (CO2 RR) has mainly focused on the C1 chemicals production such as carbon monoxide (CO) or formate (HCOO− ) due to their potentially high demand and global usage [9,10]. Moreover, CO or formate can be synthesized via a two-electron (2e− ) reduction reaction, relatively compared with the reduction pathway of other products such as methane (CH4 ) and ethylene (C2 H4 ), which are 8e− and 12e− reduced products, respectively [11]. For instance, gold−iron core−shell nanoparticle catalyst was utilized with a CO faradic efficiency 97.6% [12] and black reduced porous SnO2 nanosheet catalyst was used for CO2 into formate with a faradic efficiency of 92.4% [13]. Meanwhile, the products toward more valuable chemicals have been proposed and one of the demanding candidates is C2 H4 due to its utilization in synthesis of many industrial chemicals. For this purpose, Cu is so far almost the only heterogeneous catalyst that can selectively produce C2 H4 during electro-

https://doi.org/10.1016/j.jechem.2019.12.013 2095-4956/© 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

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chemical CO2 reduction. Cu-based catalysts can also catalyze other multiple-electron transfer reactions beyond CO, formate, H2 , and CH4 [11,14]. During the investigation of Cu foil at an ideal potential, CH4 and significant amounts of H2 have been observed in products with ~40% faraday efficiency [15]. While Cu nanostructures have been recently demonstrated with higher C2 H4 selectivity. Many research groups have tried to enhance the active sites on Cu-based electrocatalysts for higher C2 H4 /CH4 production ratio [16,17], for instance, the use of copper/copper oxide mixtures [16,18]. In addition, some other scientific strategies have also been studied for higher C2 H4 production activity, such as using halide ions in the electrolyte, or inducing local pH deviation via control of surface morphology [19,20]. So far, copper based catalysts are considered as the most promising candidate for CO2 selective reduction into multicarbon hydrocarbons [11,21], including C2 H4 , ethanol (C2 H5 OH), and propanol (C3 H7 OH) [16,22,23]. It has been reported that the C–C coupling activities of Cu-based catalysts are highly dependent on the morphology, crystal structure, and the copper oxidation states [24,25]. The catalytic performance is also highly effected by many other factors, such as the role of bicarbonate anion on copper surface [26], under-coordinated active sites [27], subsurface residual oxygen [28], unconverted Cu+ copper states [29], local pH gradient effect [19,30], and cation effect [31]. To revamp the C2 H4 selectivity production, Cu nanostructures obtained from copper oxides by electro-reduction have been investigated, which can result in a low-coordinated Cu surface atom or residual oxygen, and thus raise strong CO active binding sites and enhance C–C coupling [29,32]. For example, ~40% C2 H4 faraday efficiency was achieved over an electrochemically synthesized Cu2 O/Cu electrode [27], and ~60% C2 H4 faraday efficiency by O2 plasma-treated Cu2 O/Cu and 70% C2 H4 faraday efficiency (highest) by branched copper oxide nanoparticles, selectivity production during CO2 RR [16,33]. Many other Cu-based catalysts were also investigated with a C2 H4 faradic efficiency in the ranges of 30%~40%, but still facing many experimental and electro-catalysis selectivity and stability challenges [16,24]. To date, there are various approaches towards C2+ chemicals production by CO2 electrochemical reduction but no specified solution has been proposed due to the complicated reaction pathway. Layered double hydroxides (LDHs) are new emerging composites and getting great allure due to the 2D nanostructure, having general formula [M2+ 1- x M3+ x (OH)2 ]y + (Az − )·nH2 O where M2+ is divalent cation (commonly Ca2+ , Mg2+ , Ni2+ , Zn2+ or Co2+ ), M3+ is trivalent cation (commonly Al3+ or Fe3+ ), and A is a chargebalancing anion (commonly CO3 2− , or NO3 − ) [34,35]. LDHs have been intensively investigated in many advanced applications such as anion conductive materials [36,37], capacitors [38], electrocatalysts [39], and photocatalysts [40]. Noble metals (Pt, Pd, and Au)loaded LDHs for CO2 photo-reduction have also been investigated, but their CO formation rates are limited [41]. Further, LDH loaded by metal catalysts such as CoFe alloy and its mixed oxides can be found effective as photo-catalysts for CO2 reduction into hydrocarbon compounds [42]. LDH derived composites are suitable for catalysts due to well-dispersed mixed metal oxides with large surface area, high adsorption capacity of CO2 , more reaction active sites and rich Lewis base sites [43]. With regards to the excellent electro-catalytic properties and high C2 H4 selectivity of copperbased catalysts for CO2 RR, and high specific surface area, excellent CO2 adsorption capacity, adjustable composition and marvelous catalytic properties of LDH derived materials, we are inspired to design a novel electro-derived Cu-Cu2 O nanocluster from LDH for stable and selective C2 H4 production from CO2 electrochemical reduction. In this work, novel Cu-Cu2 O composite catalyst prepared by electrochemical reduction treatment on Cu-containing LDHs was

investigated for CO2 electrochemical reduction for the first time. Among all obtained catalysts, the HPR-LDH has highly disperse Cu2 O nanoclusters grown on layer structure with more active sites for CO2 RR. The obtained catalyst exhibited good selectivity for hydrocarbon formation, in which CO and C2 H4 were the major products, with a faraday efficiency of 13.1% and 36.3%, respectively. HPR-LDH catalyst also showed good stability of C2 H4 faraday efficient production over 20 h at a potential of –1.1 V vs. reversible hydrogen electrode (RHE). The H2 production was remarkably suppressed and in the meantime, the CH4 production was significantly suppressed as compared to bulk Cu foil. We believed that this novel electro-derived composite from 2D LDHs can provide an alternative option for the design of advanced electro-catalysis for efficient CO2 electrochemical reduction in future. 2. Experimental 2.1. Catalyst preparations 2.1.1. Preparation of Cu based LDH The Cu5 Al-CO3 LDH was synthesized via a standard co-precipitation method. Therefore, an aqueous solution (100 mL) containing 0.125 mol Cu(NO3 )2 ·6H2 O and 0.025 mol Al(NO3 )3 ·9H2 O was added drop-wise into a vigorously stirred basic solution (100 mL) containing 0.05 mol Na2 CO3 . During the synthesis, the pH of the mixture solution was maintained at 10 by addition of a NaOH solution (4 M). The resulting slurry was stirred continuously for another 12 h. After aging, the precipitate was filtered, washed several times with deionized water until pH=7, then washed and stirred for 2 h with ethanol for better dispersion. 2.1.2. Electro-reduction of LDH and Cu2 O 50 mg of as-prepared Cu based LDH or commercially purchased Cu2 O (97% pure) was mixed with 40 μL Nafion and 1 mL ethanol and then sonicated for 1 h at room temperature. Meanwhile, the carbon cloth (CC) having dimension 1.5 × 1.5 cm as supporting conduction martial was pretreated by 5% H2 SO4 solution in water for 20 min, then washed with DI-water until the pH became neutral. The pretreated CC was then dried and weighted. After that, the as-prepared Cu based LDH or Cu2 O was loaded onto carbon paper by dropping method, followed by drying in oven for 20 min at 60 °C. After loading LDH or Cu2 O, the CC was weighted again. Electrochemical reduction of catalysts was performed by applying different potentials (vs. Ag/AgCl) at different times (–4 V vs. 1 h, –1 V vs. 4 h, and –1 V vs. 2 h) and the obtained three LDH derived composites were named as HPR-LDH, LPR-LDH-4 h, and LPR-LDH-2 h, respectively; and also three Cu2 O derived composites were named as HPR-Cu2 O, LPR- Cu2 O-4 h, and LPR- Cu2 O-2 h, respectively. 2.1.3. Electro-polishing of Cu foil To compare the catalytic activity of HPR-LDH with bulk Cu, a polycrystalline Cu foil (Alfa Aesar, 99.9999%, 0.1 mm) was used and electrochemically polished in phosphoric acid (Sigma Aldrich, ≥85 wt% in H2 O) at 4.0 V vs. Pt counter electrode for 300 s. The geometric area of the copper electrode was 0.50 cm2 . 2.2. Material characterizations The morphology and chemical composition of the as-prepared samples were examined by a scanning electron microscopy (SEM, HITACHI SU8010), a high-resolution transmission electron microscopy (HR-TEM, JEOL JEM 2100F) and an electron diffraction spectroscopy (EDS). The crystalline phases of samples were detected with X-ray diffractometer (XRD, Shimadzu XRD-70 0 0). The oxygen vacancy was measured by electro spin resonance (ESR)

N. Altaf, S. Liang and L. Huang et al. / Journal of Energy Chemistry 48 (2020) 169–180

Bruker E 500. The element oxidation state changes before and after the CO2 RR tests were measured by X-ray photoelectron spectroscopy (XPS) (Thermo-Fischer ESCALAB 250Xi). All spectra were calibrated to the C 1s peak at 284.6 eV. Immediately after the reaction, samples were taken out from the electrolyte, thoroughly rinsed with DI water, and then dried with N2 gas flow. To prevent further contamination, all the copper composites were vacuum packed and stored until X-ray measurement. 2.3. Electrochemical measurements The catalysts ink was first prepared separately by mixing 30 mg of the catalyst with 10 μL Nafion solution and 1 mL ethanol, sonicated for 1 h to get best dispersion. Then 30 μL catalyst ink was loaded onto the glassy carbon electrode (GCE 6 mm diameter) by drop method. The electrode was dried in an oven at 60 °C for 5 min and used as working electrode. The Ag/AgCl and Pt meshes were used as reference electrode and counter electrode, respectively. Electrochemically active surface area (EASA) of different LDH-derived catalysts was investigated in the same H-type electrochemical cell separated by Nafion proton exchange membrane, in which 0.1 M HClO4 was used as the electrolyte. Cyclic voltammetry (CV) was performed in a potential range (−0.15 to −0.31 V vs. Ag/AgCl) where faradaic processes are absent. Five different scan rates (100, 80, 60, 40 and 20 mV s−1 ) were carried out. The geometric current density was then plotted as a function of scan rate; the slope of this curve is equal to the double layer capacitance. Subsequently, for CO2 RR, electrolyte was prepared by dissolving reagent-grade potassium bicarbonate (KHCO3 , Sigma-Aldrich, 99.999% pure) in ultrapure water having 0.1 M KHCO3 . For each experiment, 0.1 M KHCO3 electrolyte was first saturated by CO2 for 40 min (pH=6.8), and both the catholyte and anolyte concentration in the H-type cell compartments were having constant volume (30 mL). The cyclic voltammetry (CV), linear sweep voltammetry (LSV), and i-t amperometric curve were measured by a potentiostat (CH Instruments, CHI600E). Constant electrolysis was done in range of –1.0 V ~ –1.3 V (vs. RHE) for 100 min for each potential respectively. For each potential, a new catalyst sample was used for electrochemical CO2 reduction. During the whole reaction, CO2 (30 mL min−1 ) was continuously fed into the working chamber, and the gas products were analyzed for every 10 min using a gas chromatography (GC, Agilent 7890A) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). High purity of N2 (99.999%) was used as the carrier gas for all GC compartments. GC for online gas analysis mainly detected gas products such as H2 , CO, C2 H4 , CH4 and negligible amounts of others compounds. All potentials were converted to potentials versus reversible hydrogen electrode (RHE) by using the following Eq. (1). Faradaic efficiencies of H2 , CO, CH4 and C2 H4 production were calculated following the Eq. (2). The total current density was measured by the potentiostat, and the partial current densities (iH2, CO, CH4, C2H4 ) were obtained from the areas of GC chromatogram peaks where VH2 or CO, CH4, C2H4n is the volume concentration of H2 , CO, CH4 , and C2 H4 , respectively, based on the calibration of GC. Q is flow rate of CO2 , F is Faradaic constant, P0 is pressure, T is room temperature and R is ideal gas constant, 8.314 J/mol/K.

E (V vs. RHE ) = E (V vs. Ag/AgCl ) + 0.209 V +0.0591 V × pH FEH2 =

or CO,CH4 ,C2 H4

VH2

(% ) =

or CO,CH4 ,C2 H4

itotal

iH2

or CO,CH4 ,C2 H4

×Q ×

itotal 2F P0 RTi

× 100

(1)

× 100 (2)

171

2.4. Liquid products identification After each test at a constant potential, 700 μL of catholyte was taken out from the electrochemical cell and mixed with 35 μL of an internal standard, composed of 10 mM dimethyl sulfoxide (DMSO, Sigma-Aldrich, 99.9%) and 50 mM phenol (SigmaAldrich, 99.0%~100.5%) dissolved in D2 O (Sigma-Aldrich, 99.9 atom%D). This mixture was transferred into a nuclear magnetic resonance (NMR) sample tube and analyzed by 1 H NMR spectroscopy with 700 MHz spectrometer (BRUKER AVANCE III HD). The peak of water was suppressed by pre-saturation sequence, and the ratio of peak area for formate to phenol, the ratio of peak areas for other products to DMSO were compared to the standard curves to quantify the concentration of each liquid product. 3. Results and discussion 3.1. Preparation of Cu-Cu2 O composite catalysts from LDH by electrochemical reduction method In this work, three Cu-based catalysts were prepared from Cu5 Al-CO3 LDH by either high potential reduction with short treatment time (–4 V vs. 1 h, named as HPR-LDH) or low potential reduction with long treatment time (–1 V vs. 4 h and – 1 V vs. 2 h, named as LPR-LDH-4 h and LPR-LDH-2 h, respectively). All samples were firstly characterized using XRD analysis, as shown in Fig. 1(a). For as-synthesized Cu5 Al-CO3 LDH, characteristic peaks at 2θ of 11.7°, 23.8°, 34.7°, 39.8°, and 47.9° were observed, which can be attributed to the reflections of (0 03), (0 06), (009), (015), and (018) planes of a standard LDHs unit cell (JCPDS #46–0099) [44], respectively. After electro-reduction treatment, Cu phases were observed in all electro-derived LPR-LDH-4 h, LPRLDH-2 h and HPR-LDH catalysts, with the characteristic peaks observed at 43.3°, 50.2°, and 74.1° (JCPDS #04–0836). These peaks match well with those of Cu-foil. However, in HPR-LDH, four additional peaks at 29.6°, 36.2°, 42.1° and 61.1° were also observed, which can be attributed to the reflections of (110), (111), (200) and (220) planes of Cu2 O nanocluster phase (JCPDS #78–2076). XRD analyses demonstrated that, with low potential vs. long time, Cu5 Al-CO3 LDH could be fully reduced to Cu phase; while with high potential vs. short time, Cu-Cu2 O composite catalyst could be obtained. The morphological changes of Cu5 Al-CO3 LDH after electroreduction treatment were also investigated by SEM analyses. The as-synthesized Cu5 Al-CO3 LDH showed flower morphology with ultrathin nanosheets act as petal (Fig. 1b). After low potential reduction, LPR-LDH-4 h and LPR-LDH-2 h still preserved its layered structure to certain extent (Fig. 1e and f). However, after high potential reduction, HPR-LDH completely changed to nanoparticles (Fig. 1c). The co-existence of both Cu and Cu2 O was confirmed by HRTEM analysis (Fig. 1d). The d-spacing values of 0.301 and 0.246 nm can be attributed to the (110) and (111) planes of Cu2 O, while the d-spacing of 0.285 nm matches well the (111) plane of Cu. The selected area electron diffraction (SAED) pattern also reflected the crystal structure of Cu2 O (the insert of Fig. 1d). The oxidation states of Cu in LPR-LDH-2 h, LPR-LDH-4 h, and HPR-LDH were further studied using XPS analysis. Fig. 1(g) shows the XPS spectra, from which no much difference in Cu 2px binding energy could be observed, because Cu+ and Cu0 states in Cu 2px peak are similar (only 0.1 eV difference) [45]. Therefore, further Cu LMM Auger spectra surface state analysis was performed (Fig. 1h) which reflected that Cu2 O (or Cu+ ) auger peak was at 569.9–570.0 eV in HPR-LDH. But for LPR-LDH-2 h and LPR-LDH4 h, only the Cu0 auger peak was observed at 567.7–567.9 eV

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Fig. 1. (a) XRD patterns of Cu based LDH, Cu foil and its derived composites, (b) SEM image of Cu5 Al-CO3 LDH, (c) SEM image of HPR-LDH, (d) HRTEM of HPR-LDH (insert figure is SAED pattern with 10 1/nm of scale bar), (e) SEM image of LPR-LDH-4 h, (f) SEM image of LPR-LDH-2 h, (g) Cu 2p XPS spectra of as-prepared HPR-LDH, LPR-LDH-4 h and LPR-LDH-2 h and (h) Cu LMM Auger spectra of as-prepared HPR-LDH, LPR-LDH-4 h and LPR-LDH-2 h.

[46,47]. In all, we demonstrated that the LPR-LDH-2 h and LPRLDH-4 h only have Cu crystals while HPR-LDH consists Cu-Cu2 O mix composites. In the following section, their electrochemical performance for CO2 conversion will be comparatively studied in detail.

3.2. Electrochemical performance for CO2 RR First, linear sweep voltammetry (LSV) curves of HPR-LDH, LPRLDH-2 h and LPR-LDH-4 h for CO2 RR were investigated at a sweeping rate of 50 mV/s in both N2 -saturated and CO2 saturated 0.1 M

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Fig. 2. Faradaic efficiency of gas products analyzed at the various applied potentials: (a) Cu-foil, (b) HPR-LDH, (c) LPR-LDH-4 h and (d) LPR-LDH-2 h.

KHCO3 electrolyte (Fig. S1a). In this comparison, of the CO2 saturated exhibited larger current density in the entire potential range as compared to N2 saturated electrolyte for all catalysts, which revealed the activation of CO2 during electrochemical reaction. Besides, HPR-LDH showed significantly the highest negative current density in CO2 environment among all catalysts, indicating HPRLDH is more efficient for CO2 RR as compared to the other two catalysts. Further, the CO2 RR gaseous products distribution of HPRLDH, LPR-LDH-2 h, LPR-LDH-4 h and flat Cu foil were comparatively studied at a potential region of –1~ –1.3 V vs RHE (Fig. 2). Regarding to selectivity, Cu-foil always preferred CH4 production in the whole applied potential regions (−1~−1.3 V vs. RHE), while both the CO and C2 H4 selectivities were quite low (<10%). This result suggested that the C−C coupling activity of bare Cu foil was not high regardless of the applied potentials. While for LPRLDH-2 h and LPR-LDH-4 h, the faradaic efficiency of CH4 was suppressed as compared to Cu foil, which was not higher than 19.4% at all applied potentials and the C2 H4 production was slightly improved from 10% to 20.4% at the best potential of −1.1 V vs. RHE. However, with HPR-LDH electro-catalyst, the CH4 production was further significantly suppressed, which was less than 4% at −1.25~−1.3 V vs. RHE, and became nearly 0% at −1.0~−1.2 V vs. RHE. In the meantime, the C2 H4 selectivity in HPR-LDH was significantly improved. Particularly at −1.1 V vs. RHE, the C2 H4 faradaic efficiency reached as high as 36.3%. The total gaseous products in

case of HPR-LDH at −1.1 V vs. RHE for C2 H4 production selective was calculated and up to 42.4% of the total gaseous products. The total products faradaic efficiency of HPR-LDH was closed to 100% (Fig. S1b). Comparatively, the HPR-LDH among all electro-derived catalysts along with Cu foil, suppressed CH4 selectively generated over C2 H4 which suggested that C−C bond formation even at low applied potentials may be occurred (−1 V vs. RHE). Further, the HPR-LDH catalyst at −1 V vs. RHE achieved up to 25% CO faradic efficiency, while the Cu foil, LPR-LDH-2 h and LPR-LDH-4 h only showed 10.5%, 13% and 12.9% of CO faradic efficiency, respectively. It can be implied that the HPR-LDH can also produce CO but C–C coupling became favorable toward more negative applied potential (Fig. 2b). Considering adsorbed CO as intermediate state of C2 H4 generation which can be demonstrated the potential dependent product change [29,30]. It was also found that all electro-catalysts have maximum C2 H4 production selectivity at −1.1 V vs. RHE. Inclusively, the HPR-LDH exhibited the highly enhanced C2 H4 production during CO2 RR with suppressed CH4 production and HER, as compared to other derived catalysts and the Cu foil (Fig. 2). Subsequently, the partial and total current densities of HPRLDH, LPR-LDH-4 h and LPR-LDH-2 h were analyzed (Fig. S2). The partial current densities of CH4 and C2 H4 depending on the applied potentials also reflected that the HPR-LDH has higher activity for C2 H4 production as compared to LPR-LDH-4 h and LPR-LDH-2 h composites. At −1.1 V vs. RHE, HPR-LDH achieved the highest C2 H4

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with nearly same H2 partial current density. It suggested that the electro-catalysts active sites might be contended between HER and CO2 to C2 H4 . Further, both the HPR-LDH and LPR-LDH-2 h were more active catalysts for selective C2 H4 production over CH4 than Cu-foil and LPR-LDH-4 h at the same applied potential region. This could be due to propose mechanism for C2 H4 generation pathway which is different from CH4 generation pathway [48]. For analyzing catalysts efficiency through the kinetics of charge transfer in the electro-derived composites, the electrochemical impedance spectroscopy (EIS) was performed in a three-electrode system. The semicircle radii are considered to be related to charge transfer and conductivity at the electrode and electrolyte interface. Fig. 3(f) demonstrated that HPR-LDH had the smallest semicircle radius with respect to other derived-catalysts. The efficient charge transfer at the electrode/electrolyte interface was investigated which can exalt the catalytic process efficiency. Therefore, the smaller semicircle of HPR-LDH illustrates its high charge transfers preferable efficiency with low resistance, which has leaded it to outstanding charge transferring performance contribution during CO2 RR. The charge transfers resistance (Rct ) of all electroderived catalysts was calculated as in Table S1. Besides, the EASAs of all catalysts were measured through cyclic voltammetry. The capacitance of HPR-LDH (376 μF) was measured to be higher than LPR-LDh-2 h and LPR-LDH-4 h (65.7 μF and 81.9 μF, respectively) (Fig. S3), which may support more active sites and lead to the higher current density with good performance. By comparing faradic efficiency and partial current density in all applied potential regions, it was found that the −1.1 V vs. RHE is the best potential for high C2 H4 and low H2 selectivity. Keeping this, all electro-catalysts are compared at the same applied potential with respect to 100 min CO2 RR. At −1.1 V vs. RHE in 100 min CO2 RR, HPR-LDH generated much higher C2 H4 selectivity (36.3%) than LPR-LDH-2 h (20.4%), LPR-LDH-4 h (18.8%) and Cu foil (10.6%), and lower H2 selectivity (33.1%) than LPR-LDH-2 h (36.7%), LPRLDH-4 h (40%) and Cu foil (53.6%). More importantly, the CH4 production was significantly suppressed closed to 0%, which is much lower than that over LPR-LDH-4 h (15.8%) and Cu foil (29.2%) (Fig. 3a). It has been devised that the HPR-LDH electro-catalysts generated better C2 H4 production stability than LPR-LDH-2 h, LPRLDH-4 h and Cu foil. Fig. 3(b) shows the stability of C2 H4 production of HPR-LDH, LPR-LDH-2 h, LPR-LDH-4 h, and Cu foil during 100 min CO2 RR. It could be seen that the C2 H4 faradaic efficiency for HPR-LDH slightly increased and finally reached up to 36.3%, while this value for LPR-LDH-2 h, LPR-LDH-4 h and Cu foil all decreased with operation time, from (20.4%~15.3%) (18.8%~14.3%) and (16.5%~6.2%), respectively. Therefore, it was cleared that HPR-LDH catalyst has more stable for C2 H4 production during this operation as compared to other catalysts. Meanwhile, the total current density comparison at −1.1 V vs. RHE for 100 min CO2 RR was measured and it was found that the Cu-foil total current density began to a more significant drop after 55 min than LPR-LDH-2 h, LPRLDH-4 h and HPR-LDH (Fig. 4a). Since the long-term stability is one of the key parameters to evaluate catalyst performance, all electro-catalysts were further compared at −1.1 V vs. RHE for longer CO2 RR. Fig. 3(c) shows that HPR-LDH has good stability up to 20 h in both total current density and C2 H4 faradaic efficiency at −1.1 V vs. RHE, and the faradaic efficiency of total gaseous products during long-term reaction are shown in Fig. 4(b). However, for LPR-LDH-4 h and LPR-LDH-2 h catalysts, the C2 H4 selectivity continuously decreased during 9 h and 3.5 h reactions, respectively (Fig. 3d and e); and their total gaseous products are in Fig. 4(c and d). The results showed that LPR-LDH-4 h and LPR-LDH-2 h were less stable for C2 H4 long term production as compared to HPR-LDH, which may be due to absence of Cu2 O nanocluster. Comparatively CH4 production selectivity at same potential, the HPR-LDH and LPR-LDH-2 h have neg-

ligible amount of CH4 production selectivity while LPR-LDH-4 h has significant amount of CH4 production selectivity in long reaction. This may be due to different morphologies, their surface effect and the local pH difference which substantiated to root cause of different C2 H4 versus CH4 production preferences. Furthermore, several Cu-based electro-catalysts reported in recent studies were compared with our HPR-LDH catalyst, regarding C2 H4 faradaic efficiency, partial current density and stability. Table 1 indicates that although the C2 H4 selectivity for HPR-LDH is slightly lower than some other literature reports, it exhibited excellent long-term stability, with negligible amount of CH4 production during 20 h test. For catalysts surface investigation, the SEM images regarding morphological changes in all electro-derived composites were examined comparatively pre-post 100 min and long CO2 RR (Fig. S4). It is apparent that the morphologies of electro-catalysts obtained at different reduction potentials vs. different times are different. Moreover, electro-catalysts initial states are supreme for long-term activity understanding during the CO2 RR and their surface was reduced from oxidized copper into metallic copper during reaction [16,17]. Considering this significant surface activity factor, it was found that the as-prepared LPR-LDH-4 h and LPR-LDH-2 h morphologies were become deform after 100 min and long CO2 RR, while the HPR-LDH morphology was slightly changed in 100 min post and maximum changed in 20 h post CO2 RR which still has small amount of Cu2 O crystals at the catalyst surface. Further, the HPR-LDH catalyst performance and its morphological investigation in different applied conditions were conducted. First, the electrochemical reaction of HPR-LDH was performed in Ar gas flow instead of CO2 gas flow in same electrolyte (0.1 M KHCO3 ). After 100 min CO2 RR, we found only HER with no CO2 reduction products. The SEM analysis with compared to initial morphology and 100 min CO2 RR in 0.1 M KHCO3 showed that pre-morphology was nearly deformed in Ar flow but catalyst surface has still small Cu2 O crystals, while in CO2 flow, there was no big change observed in the morphology as compared to initial morphology (Figs. S4a and S5a). Such investigation revealed that CO2 RR is related to the interaction between the surface of the nanoparticle catalyst and the chemical species such as intermediates (i.e. ∗ CO or its coupled OCCO∗ intermediates). Moreover, products selectivity and the electrolyte effect with pH on HPR-LDH was explored. In this regard, the CO2 RR in the 0.1 M KCl (pH 3.9) was conducted. During 100 min CO2 RR, the nanocluster started deforming in 0.1 M KCl as compared to 0.1 M KHCO3 (Figs. S4a and S5b) and faradaic efficiency of C2 H4 decreased as well (Fig. 5a). This perceived that the electrolyte type related to pH can influence the crystal surface change and also effected the performance. It may indicate that the adsorbate species i.e. CO∗ or H∗ or the negative repulsive electrostatic force on the catalyst surface can influence the thermodynamic stability of the copper surface [49]. For further, the faradaic efficiency of C2 H4 for HPR-LDH was also compared with its precursor Cu5 Al-CO3 LDH and Cu2 O and its derived electro-catalysts at a fixed applied potential of −1.1 V vs. RHE in 0.1 M KHCO3 for 100 min CO2 RR. In case of Cu5 AlCO3 LDH, only CO and H2 production was found (Fig. 5b). In case of Cu2 O and its derived electro-catalysts HPR-Cu2 O, LPR-Cu2 O-2 h and LPR-Cu2 O-4 h, their XRD patterns (Fig. S6a) were first analyzed and found that after electrochemical treatment, the Cu2 O derived electro-catalysts have different peak patterns with respect to pure Cu2 O. Moreover, the XRD pattern of HPR-Cu2 O was found nearly similar to HPR-LDH. Further, performance was measured at −1.1 V vs. RHE in CO2 saturated 0.1 KHCO3 (Fig. 5c). The C2 H4 faradaic efficiencies of pure Cu2 O and its derived electro-catalysts, LPR-Cu2 O-2 h, LPR-Cu2 O-4 h and HPR-Cu2 O are 11%, 12%, 14% and 17%, respectively and their C2 H4 partial current densities are −0.42, −0.47, −0.56 and −0.74 mA cm−2 , respectively. As illustrated in Fig.

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175

Fig. 3. CO2 RR activity comparison of Cu-foil, LPR-LDH-2 h, LPR-LDH-4 h and HPR-LDH: (a) Faradaic efficiencies of H2 , CH4 , and C2 H4 at a fixed potential (−1.1 V vs. RHE) for Cu-foil, LPR-LDH-2 h, LPR-LDH-4 h, and HPR-LDH; (b) Stability of C2 H4 Faradaic efficiency for Cu-foil, LPR-LDH-2 h, LPR-LDH-4 h and HPR-LDH as a function of time (100 min); the total current density and the C2 H4 faradaic efficiency of: (c) HPR-LDH during 20 h operation, (d) LPR-LDH-4 h during 9 h operation, (e) LPR-LDH-2 h during 3.5 h operation and (f) electrochemical impedance spectra (EIS) measured in an aqueous solution of CO2 saturated 0.1 M KHCO3 aqueous solution, of HPR-LDH EIS curve.

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Fig. 4. Long-term bulk electrolysis showing gas product analysis of catalyats: (a) total current density of Cu-foil, LPR-LDH-2 h, LPR-LDH-4 h and HPR-LDH for 100 min CO2 RR, long term Faradaic efficiecny of: (b) (20 h) HPR-LDH, (c) (3.5 h) LPR-LDH-2 h, and (d) (9 h) LPR-LDH-4 h.

5(c), HPR-LDH showed the maximum selectivity toward C2 H4 as compared to pure Cu2 O and its derived composite. Furthermore, the performance of HPR-LDH that reduced at −4 V for 60 min was also compared with other Cu5 Al-CO3 LDH derived catalysts that obtained at a same potential for different intervals (−4 V vs. 10, 30, 90 min) at −1.1 V vs. RHE in 0.1 M KHCO3 for 100 min CO2 RR, as shown in Fig. 5(d). The XRD patterns and morphology are shown in Figs. S6(b) and S7, respectively. In this regard, the HPR-LDH for C2 H4 production selectivity as compared to other different intervals derived composites is the highest with minimum amount of hydrogen production. 3.3. XRD, ESR and XPS analysis of all electro-derived LDH based composites For surface analysis, we performed detailed XRD characterization analysis of pre-post CO2 RR at fixed potential −1.1 V vs. RHE for all electro-derived LDH catalysts (HPR-LDH, LPR-LDH-4 h and LPR-LDH-2 h) (Fig. 6). The HPR-LDH has mainly a crystalline Cu2 O and Cu peaks patterns after electrochemical treatment applied on Cu5 Al-CO3 LDH, but LPR-LDH-4 h and LPR-LDH-2 h electrocatalysts have no Cu2 O crystallinity and have only Cu peak patterns. Withal, the HPR-LDH composite had intensity of Cu2 O patterns at 2θ = 29.6°, 36.2°, 42.08° and 61.1° exhibited mixed states with Cu2 O–Cu on surface. For the HPR-LDH 100 min CO2 RR post, a

significant small amount of Cu2 O crystal signal was noticeably observed, and the copper peaks became sharp after 20 h bulk CO2 RR (Fig. 6a). This was also consistent with the HPR-LDH SEM results of 100 min and 20 h post CO2 RR (Figs. S4b and S4c). However, LPRLDH-4 h and LPR-LDH-2 h were lack of Cu2 O peak and were also passed through 100 min and long CO2 RR post, and was found that their copper peaks became sharp as compared to initial pattern without any further appearance of new peak (Fig. 6b and c). Further, CO2 RR pre-post effect was analyzed on the three composites surface through XPS for C2 H4 production selectivity. The catalysts surfaces with Cu+ and Cu0 states were almost alike in Cu 2p XPS pre-post CO2 RR due to very low binding energies difference (0.1 eV), similar to that in Fig. 1(g). In this regard, Cu LMM auger peak surface state analysis for pre-post CO2 RR was performed which clearly exhibited this binding energies effect (Fig. S8). The ex-situ all XPS surface analysis measurements were conducted for all electro-composites. The surface state of catalysts may be destroyed due to the ex-situ analysis condition because the copper and its states are highly active. Therefore, careful sample storage and transfer are required. Herein, the HPR-LDH XPS was analyzed in detail with respect to pre-post CO2 RR, and Cu LMM analysis disclosed that both Cu+ and Cu0 states were present after 100 min CO2 RR (Fig. 7). This revealed that after 100 min reaction, HPR-LDH catalyst still has the mixed states of Cu2 O and metallic copper.

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Table 1. C2 H4 selectivity and stability comparison of reported Cu based CO2 RR electro-catalysts. Catalyst

Electrolyte

HPR-LDH (This work) LPR-LDH-2 h (This work) LPH-LDH-4 h (This work) Cu foil (This work) Anodized copper [55] Cu2 O NP/C [56] CuO nanoparticles [33] Cu+ ions in Cu4.16 CeOx [57] CuZn alloy [58] Cu-mesocrystal [59] Plasma-Cu [16]

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

M M M M M M M M M M M

KHCO3 KHCO3 KHCO3 KHCO3 KHCO3 KHCO3 KHCO3 KHCO3 KHCO3 KHCO3 KHCO3

Prism-Cu [60] Amino-Cu [61] Electrodeposition of Mesoporous Cu2 O [20] Cu2 O film [62]

0.1 M KHCO3 0.1 M KHCO3 0.5 M NaHCO3 0.1 M KHCO3

Substrate

Experimental condition

Faradaic efficiency (%) C2 H 4

CH4

C2 H4 partial current density (mA/cm2 )

Stability (h)

glassy carbon glassy carbon glassy carbon – cu foil glassy carbon plate glassy carbon glassy carbon glassy carbon polished copper discs electro-polished polycrystalline cu foils – – cu foam on cu wafer

−1.1 V vs. RHE −1.1 V vs. RHE −1.1 V vs. RHE −1.1 V vs. RHE −1.08 V vs. RHE −1.1 V vs. RHE −1.0 V vs. RHE −1.1 V vs. RHE −1.1 V vs. RHE −0.99 V vs. RHE −0.9 V vs. RHE

36.3 20.4 18.8 10.6 38.1 57.3 70 47.6 33.3 27.2 60

– – 15.8 29.2 1.3 1.9 – 14.4 – 1.47 5

~4.2 1.30 1.31 3.45 7.3 ~11 13.5 – 2.01 ~6.8 ~6.6

20 3.5 9 0.83 40 10 12 6 15 6 5

−1.15 V vs. RHE −1.9 V vs. Ag/AgCl −0.8 V vs. RHE

30 13 20

– 24 –

~12 ~1.4 ~1

1.5 12 –

–

−0.99 V vs. RHE

37.5

~12.9

1

Fig. 5. Comparison of 100 min CO2 RR: (a) HPR-LDH in 0.1 M KCl (pH=~3.9) and 0.1 M KHCO3 (pH=~6.8) electrolytes at −1.1 V vs. RHE, (b) HPR-LDH faradic efficiency vs. Cu5 AlCO3 -LDH in 0.1 M KHCO3 (pH=~6.8) electrolytes at −1.1 V vs. RHE, (c) HPR-LDH faradic efficiency vs. Cu2 O and its derived composites in 0.1 M KHCO3 and (d) HPR-LDH faradic efficiency vs. other Cu5 AlCO3 -LDH derived catalysts (fix applied potential vs. different time intervals) in 0.1 M KHCO3 .

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Fig. 6. XRD analysis of HPR-LDH, LPR-LDH-4 h and LPR-LDH-2 h: (a) HPR-LDH per-post 100 min and 20 h of CO2 RR, (b) LPR-LDH-4 h pre-post 100 min and 9 h of CO2 RR, (c) LPR-LDH-2 h pre-post100 min and 3.5 h of CO2 RR and (d) ESR spectra of HPR-LDH, LPR-LDH-2 h and LPR-LDH-4 h composites measured at T = 298 K.

In addition to Cu XPS analysis, the detailed XPS O 1 s peak study was investigated to explore the surface oxygen role in all electro-catalysts pre-post CO2 RR (Fig. S9). Whereas, the HPR-LDH catalyst has Cu2 O phase with more surface oxygen than LPR-LDH4 h and LPR-LDH-2 h composites. It was confirmed by the higher ratio of O to Cu in HPR-LDH (the value was determined to be 9.0) than that in LPR-LDH-4 h and LPR-LDH-2 h (4.2 and 4.1, respectively). Further, the atomic concentration percentage of post CO2 RR of all electro-catalysts was also calculated (Table S2). This investigation disclosed that the HPR-LDH surfaces have relatively higher oxygen abundant than the LPR-LDH-4 h and LPR-LDH-2 h even after 100 min post CO2 RR. Secondly, it was also perceived that the oxygen peak shifting from as prepared 530.6 eV to 100 min post CO2 RR 531.7 eV in mix state HPR-LDH may be due to oxygen vacancy generation occurrence, which may lead toward C2 H4 longer stability selectivity production. However, this attribute was not found in LPR-LDH-2 h and LPR-LDH-4 h, which means that the more abundant oxygen (Cu+ ) state in HPR-LDH may lead to better C2 H4 long term stability and selectivity. Notably, the peak that closed to 532 eV may also due to other oxygen components such as OH, H2 O or carbonate species deposited on the surface of electro-catalysts [50,51].

To further investigate this, the presence of surface oxygen vacancies in as-prepared HPR-LDH, LPR-LDH-2 h and LPR-LDH-4 h catalysts was measured at room temperature by ESR analysis (Fig. 6d). The three catalysts all exhibited the signal centered at approximately 3514 G (Gauss), but from the peak height (H) decreases and the resonance signal intensity weakens in the sequence of HPR-LDH, LPR-LDH-4 h, and LPR-LDH-2 h, it demonstrates that HPR-LDH possesses the highest oxygen vacancies concentration [52]. Such result suggested that in HPR-LDH, the presence of high oxygen vacancy might enhance its electrochemical efficiency in terms of electron transport. After detailed investigations, it was finally revealed that the surface oxidation state might affect the C2 H4 selectivity. By observing detailed HPR-LDH O 1s spectra analysis and ratio (O/Cu) (Table S2) before and after CO2 RR (Fig. 7c), it is also perceived that the HPR-LDH electro-catalyst surfaces still have mixed state of Cu-Cu2 O even after the 100 min and 20 h CO2 RR as compared to the LPR-LDH-4 h and LPR-LDH-2 h catalysts. Such observation related to high oxygen content in terms of the mixed state on the active catalysts may lead to stable and selective C2 H4 production, while other two derived electro-catalysts (LPR-LDH-4 h and LPR-LDH-2 h) are unable to generate long term stable or high

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Fig. 7. XPS analysis of HPR-LDH: (a) Cu 2p XPS spectra, (b) Cu LMM Auger spectra of as-prepared and after 100 min of CO2 RR of HPR-LDH electro-catalyst and (c) HPR-LDH XPS O 1s spectra pre- and post-CO2 RR (as-prepared Cu2 O, 530.6 eV, 100 min post 530.9 eV, 20 h post 531.7 eV; H2 O: 533.5 eV, and Cu(OH)2 531.5 eV).

selectivity toward C2 H4 production due to less subsurface oxygen. Such mix state in HPR-LDH may be selective species for C2 H4 production or the structural changes outcome. Furthermore, past investigations related to in-situ spectroscopy revealed that the mix copper states reduces to metallic Cu0 under CO2 RR by applying the negative biased treatment may be a contribution from the surface oxygen species [16,17]. Moreover, it is also concluded that subsurface oxygen role in C2 H4 generation has been suggested, although no detailed investigation has been done related to stable subsurface oxygen species in the copper matrix [53,54]. Our work proposes that a high potential treatment applied to Cu-based LDH can affect the surface properties of the oxygen containing on the copper surface and thus contributes to the C2 H4 production with expanded durability. Finally, it presumed that this HPR-LDH composite from Cu based LDH is an efficient and environmental friendly CO2 reduction electro-catalyst, which may contribute a new insight for 2D metals for CO2 electrochemical reduction into prolific chemicals. 4. Conclusions Here, in this work, our electro-derived HPR-LDH catalyst achieved a C2 H4 faradaic efficiency up to 36.3% with 20 h stability during CO2 RR. We also found that the high potential electrochemical reduction of Cu based LDH gave rise Cu2 O that is the main

cause of the highly C2 H4 selectivity and stability. While comparing HPR-LDH with LPR-LDH-2 h and LPR-LDH-4 h derivatives, the HPRLDH has more subsurface oxides than other two derived composites, which affected the choice of product selectivity of C2 H4 over CH4 and it can also be the main cause for C–C coupling for longterm C2 H4 generation. Moreover, the ex-situ surface analysis XRD, SEM, ESR and XPS of electro-catalysts proved that high C2 H4 selectivity during short and long-term reaction was due to high oxygen content and generation of surface oxygen vacancy on the surface of HPR-LDH catalyst, which is in the form of mix oxides Cu-Cu2 O. We hopeful that this design of electro-engineering for LDH based electro-derived composites will be applied for further stable and selective electro-catalysts development for CO2 reduction to valueadded products especially C2 H4 . Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We gratefully acknowledge the Fundamental Research Funds for the Central Universities (2019YC17), the National Natural Science

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