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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Rare Earth- and Iridium-Decorated Silica Nanoparticle as a Single Catalyst for Carbon Dioxide Reduction and Water Oxidation: Buy One Get One Strategy Paramita Karfa,† Rashmi Madhuri,*,† and Prashant K. Sharma‡ †

Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826 004, India Functional Nanomaterials Research Laboratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826 004, India

‡

S Supporting Information *

ABSTRACT: Generation of alternate fuels with carbon content is the biggest challenge throbbing inside the scientist to meet the depletion of fossil fuel feedstocks. The present-day natural decrease in CO2 absorption and increase in CO2 emission leads to an increase in the demand for artificial CO2 sequestering agents to prevent global warming. Electrochemical reduction of CO2 is one of the leading research areas for renewable carbon-containing fuels, but it suffers from a high overpotential value, formation of several byproducts, low selectivity, and high catalyst cost. In this work, we synthesized a rare-earth element- and iridium-decorated mesoporous silica nanocomposite and examined its catalytic activity toward both CO2 electroreduction (CER) and the the oxygen evolution reaction (OER). The nanocomposite prepared from the europium precursor, SiO2@Eu2O3:Ir, proved to be the best electrocatalyst compared with the other five competitors (SiO2@Pr2O3:Ir > SiO2@Nd2O3:Ir > SiO2@Y2O3:Ir > SiO2@Dy2O3:Ir > SiO2@Yb2O3:Ir). The SiO2@Eu2O3:Ir nanocomposite can selectively reduce CO2 in formic acid (HCOOH) with a Faradaic efficiency of 92.7% at −0.7 V vs RHE. The nanocomposite was also tested for OER activity, and it was observed that the catalyst showed excellent OER performance with a low overpotential value (0.22 V), low onset potential (1.09 V), high current density (355 mA cm−2), and low Tafel slope (44 mV/dec). The same nanocomposite can be used as the cathode for CER and anode for the OER in a single electrochemical cell divided by a porous frit. The nanocomposite exhibits long-term stability with a constant current density of approximately −10 mA cm−2 during 12 h of electrolysis. The highly robust nature of the SiO2@Eu2O3:Ir nanocomposite was supported by its wide-range behavior in the two-electrode cell from −0.9 to 2.7 V vs RHE. KEYWORDS: Rare earth elements, CO2 electroreduction, Oxygen evolution reaction, Formic acid

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INTRODUCTION

The CO2 reduction reaction is extremely challenging, as CO2 is among the most chemically stable carbon-based molecules and is a quite inert molecule that can be activated only through kinetically constrained multielectron/multiproton processes.4 Meanwhile, the simultaneous hydrogen evolution reaction (HER) is almost inevitable during CO2 electroreduction (CER), which leads to a decrease in Faradaic efficiency (FE) and selectivity of the reaction. In addition, the electrocatalytic oxygen evolution reaction (OER), which is often coupled with CER processes at the anode, is a slow reaction that requires an overpotential in substantial excess to deliver an acceptable current density (e.g., 10 mA cm−2).5 Therefore, in order to achieve high selectivity for a given product and high production rates, an electrode material must be developed that is able to suppress the HER and promote CER, so that most of the

Carbon dioxide accumulation in the atmosphere is the main cause behind global warming resulting in climate change; however, it can be utilized and recycled as an abundant carbon feedstock via its reduction to useful chemicals or fuels.1 In recent years, conversion or recycling of CO2 into useful chemicals and fuels like formate/formic acid, carbon monoxide, or more highly reduced hydrocarbon products using electrochemical processes has gained a lot of interest of researchers and scientists.2,3 Electrochemical reduction of CO2 at a heterogeneous metal surface in aqueous solution is a promising technique because of its simplicity and control over product formation just by modifying the electroreduction conditions, such as the electrode and electrolyte. However, the popularity of electrochemical reduction of CO2 has yet to be achieved on appropriately large scales because of the unavailability of efficient, robust catalysts operating at low overpotential with high selectivity and current density. © XXXX American Chemical Society

Received: September 8, 2017 Revised: October 26, 2017

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DOI: 10.1021/acssuschemeng.7b03179 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

on their morphology, applied potential range, etc. In this work, we have designed nanocomposites of mesoporous silica nanoparticles modified with rare earth [praseodymium (Pr), neodymium (Nd), europium (Eu), dysprosium (Dy), ytterbium (Yb)] oxides in combination with iridium (Ir) for electrochemical reduction of CO2 to HCOOH in aqueous solution and the OER. In addition to the REEs, for comparative study we have also used yttrium (Y), which has properties almost similar to those of the f-block elements but belongs among the transition metals. Here the mesoporous silica nanoparticles are used to increase the surface area of the catalyst, which could possibly expose the analyte to the active sites of the catalyst, resulting in better or enhanced electrocatalytic activity. Here, incorporation of Ir, which possesses high catalytic activity, with a SiO2@Ln2O3 nanocomposite resolved the problem of high cost but increased the efficacy of the nanocomposite to higher levels. The prepared nanocomposites (SiO2@Ln2O3:Ir) were expected to achieve high electrochemical activity and performance durability with reduced Ir content. A comparative study of different Ln-modified nanocomposites in terms of the major product of CO2 reduction, overpotential, Faradaic efficiency, etc. It was found that the major product of CER on the SiO2@ Eu2O3:Ir nanocomposite is HCOOH with an incredibly low overpotential of 0.19 V. Besides, the total FE for the CO2 reduction product (HCOOH) reached 92.7% at a moderate potential of −0.70 V vs reversible hydrogen electrode (RHE). Furthermore, SiO2@Eu2O3:Ir exhibits long-term stability, as a constant current density of approximately −10 mA cm−2 was obtained during 12 h of electrolysis. The high selectivity for HCOOH in CO2 reduction may be ascribed to the combined effect of silica, Ir, and the REE, which can greatly diminish the HCOOH desorption energy and improve the catalytic selectivity for HCOOH. Additionally, we have also compared the electrochemical characteristics and stabilities of SiO2@ Ln2O3:Ir toward the OER, using bulk-metal catalysts of Ru, Ir, and Pt as benchmarks. The activity of the prepared catalysts for the OER was assessed by means of cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy measurements. In this work, we have fabricated a multifunctional catalyst that catalyzes two important reactions, reduction of carbon dioxide to formic acid (CER) and water oxidation in a one-compartment electrochemical cell.

electrical energy supplied is consumed only in the CO2 reduction reaction with high selectivity, low FE, high current density, and slow CER kinetics. Electrochemical reduction of CO2 can proceed through two-, four-, six-, and/or eight-electron reduction pathways in gaseous, aqueous, and nonaqueous phases, resulting in a number of products or intermediates.6 Among the products formed upon CER, formic acid/formate is the most economically viable one, owing to its role as a hydrogen storage material,7 suitable energy carrier,8 liquid fuel in formic acid fuel cell applications,9 and a raw material for bacteria to generate higher alcohols as liquid fuels.10 Thus, the design and development of an efficient catalyst that can combat this electrochemically uphill multielectron reaction with high selectivity for formic acid production is a need of the day. Various electrode materials (including transition metals, metal oxides, metal complex, polymers, alloys or composite materials) have been reported in the literature for electrochemical reduction of CO2 to formate with low overpotential value and high selectivity.11,12 For example, Zhang et al. have prepared a zinc electrocatalyst that can reduce CO2 to formate with high selectivity and high faradic efficiency.13 Choi et al. have put forward a step toward preparation of tin−lead alloy based elecrocatalyst for the electroreduction of CO2 to formate.14 Kumar et al. have synthesized SnO2 porous nanowire as an efficient catalyst for electroreduction of CO2 to HCOOH.15 Kortlever et al. have prepared Pd−Pt nanoparticle and studied their electrocatalytic activity toward CO2 reduction to formic acid.16 Contrary to these, Zhang and his coworkers prepared metal free nitrogen-doped carbon nanotubes electrocatalyst for CO2 reduction to formate in aqueous medium.17 Wang et al. have synthesized novel tin gas diffusion electrode (GDE) for electrochemical reduction of CO2 to formic acid.18 The popularity of transition metal and their compounds in CER is probably because these metals have vacant orbitals and active d electrons, which are believed to be able to energetically facilitate the bonding between the metal and the CO2 for adduct formation and facilitate the desorption of the reduction products.19 However, besides the electronic and geometric properties of the metallic catalysis surface, the relation of size of the active centers and the elements forming them are also crucial components to block HER and promote CO2 reduction/O2 evolution. To-date IrO2 and RuO2 catalyst are considered to be the best anode materials for OER production in water oxidation.20 However, their alternatives are researched, due to their high cost,21 scarce availability in earth crust and their less applicability at large scale,22 which limits their role into industrial production. Therefore, development of efficient electrocatalysts having property similar to precious metal catalyst based on low-cost materials is necessary to decrease the present cost of electrolyzers. The present study was undertaken in order to investigate the electrochemical properties of a silica nanocomposite after modification with rare-earth elements (REEs). In the literature, incorporation of REEs with metal nanoparticles is very rare, and very few articles have been published on this topic. However, according to the Brewer−Engel theory,22 which has been comprehensively refined by Jaksic et al.,23 incorporation of REEs as intermetallic alloys or composites is expected to enhance the performance of the electrolyzer’s cathode toward the HER. In view of that, the electrocatalytic activity of the composite electrode will depend on the nature and composition of the modifying elements (i.e., REEs) as well as

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EXPERIMENTAL SECTION

Reagents and Materials. All of the chemicals were of analytical grade. Ammonium hydroxide, tetraethyl orthosilicate (TEOS), potassium hydrogen carbonate, potassium hexachloroiridate(IV), and potassium permanganate were purchased from Sigma-Aldrich, while other solvents such as ethanol and dimethylformamide (DMF) were purchased from Merck (India) and Spectrochem Pvt. Ltd. (India). Potassium hydroxide (flakes) was procured from TCI Chemicals (India). Europium(III) nitrate hexahydrate, neodymium(III) nitrate hexahydrate, praseodymium(III) nitrate hexahydrate, ytterbium(III) nitrate pentahydrate, yttrium(III) nitrate hexahydrate, and dysprosium(III) nitrate pentahydrate were purchased from Alfa Aesar. Synthesis of Mesoporous Silica Nanoparticles. To synthesize spherical mesoporous silica nanoparticles (MSNs), first 34.82 mL of water, 3.25 mL of NH4OH, and 30.0 mL of C2H5OH were mixed together and stabilized at 40 °C for half an hour. Then 6.2 mL of TEOS was added to the reaction mixture in a dropwise manner, and the reaction was conducted for 1 h. The resulting product was centrifuged at 550 rpm, washed with ethanol, and dried in a vacuum oven. Synthesis of SiO2@Ln2O3 (Ln = Y, Nd, Yb, Eu, Dy, Pr). For the preparation of SiO2@Ln2O3, 0.1 mmol of the rare earth nitrate and 0.3 B


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Figure 1. (A) XRD pattern, (B) FE-SEM image, (C) elemental maps, and (D) EDAX spectrum of mesoporous silica nanoparticles. (E−J) XRD patterns of various nanocomposites: (E) SiO2@Y2O3:Ir, (F) SiO2@Pr2O3:Ir, (G) SiO2@Nd2O3:Ir, (H) SiO2@Eu2O3:Ir, (I) SiO2@Dy2O3:Ir, and (J) SiO2@Yb2O3:Ir. mmol of citric acid were mixed in 5.0 mL of DMF, and the mixture was stirred for 30 min to make a homogeneous solution.24 To this, 5.0 mg of MSN was added through the incipient wetness technique, and the resulting product was dried in an oven for 24 h at 80 °C under vacuum and further calcined in air at 400 °C for 5 h. Synthesis of SiO2@Ln2O3:Ir. For the synthesis of SiO2@Ln2O3:Ir, 0.5 g of SiO2@Ln2O3 was mixed with 0.0414 g (0.08 mmol) of K2IrCl6 in 10 mL of DMF, and the reaction mixture was stirred overnight. The next day the reaction mixture was transferred into an autoclave and kept in an oven for 12 h at 200 °C. The resultant product was centrifuged at 10 000 rpm and dried in an oven at 60 °C. Prior to use, the prepared SiO2@Ln2O3:Ir was stored in a sealed container at room temperature. Characterization. The morphologies of the prepared mesoporous silica nanoparticles and nanocomposites were captured with a fieldemission scanning electron microscope (FE-SEM, Zeiss model Supra 55). Elemental imaging and energy-dispersive X-ray (EDAX) spectra were also obtained using the Zeiss model Supra 55 FE-SEM. Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å). The surface areas of the products were obtained by the Brunauer− Emmett−Teller (BET) method using nitrogen adsorption and desorption isotherms on a Quanta Chrome Nova 3200e analyzer. Xray photoelectron spectroscopy (XPS) was conducted with a Mg Kα

source, and the C 1s peak at 284.6 eV was used as an internal standard. UV−vis spectroscopy was performed on Shimadzu 1800 spectrophotometer. All of the experiments were performed at room temperature (25 ± 1 °C). Electrochemical Setup and Electrode Preparation. Electrochemical measurements were performed on a CH instrument (USA, model 660 C) in an electrochemical glass cell. Half-cell reactions (i.e., CER and the OER) were studied in a three-electrode cell configuration equipped with a nanomaterial-modified pencil graphite electrode (PGE) as the working electrode, a Pt counter electrode, and Ag/AgCl (in 3.0 M KCl electrolyte) as the reference electrode. The working electrode (i.e., PGE) was fabricated in our laboratory using graphite pencil leads purchased from Hi Par Camlin Ltd. (India) and micropipette tips procured from Tarsons Products Pvt. For modification of PGEs, 25.0 mg of nanomaterial and 10.0 μL of Nafion solution (5 wt %) were dispersed in 100 μL of water−ethanol solution with a volume ratio of 1:1. The mixture was ultrasonicated for 1 h to form a homogeneous ink. Then 20.0 μL of nanomaterial ink was drop-coated onto the cleaned surface of the PGE and dried at room temperature. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and controlled-potential bulk electrolysis (CPE) were carried out in aqueous (0.5 M) KHCO3 solution saturated with N2 (99.9%) or CO2 (99.9%). For saturation of the electrolyte, the corresponding gases C


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Figure 2. (A) EDAX spectrum and (B) elemental mapping analysis of SiO2@Pr2O3:Ir. (C) EDAX spectrum and (D) elemental mapping analysis of SiO2@Y2O3:Ir. (E) EDAX spectrum and (F) elemental mapping analysis of SiO2@Nd2O3:Ir. (G) EDAX spectrum and (H) elemental mapping analysis of SiO2@Eu2O3:Ir. (I) EDAX spectrum of SiO2@Dy2O3:Ir. were bubbled through the setup for at least 30 min. The current density (j) was determined from the geometrical area of the electrode. CPE was conducted in a similar device adopted at different potentials, and the total current was monitored. The liquid product was sampled after that and monitored with UV−vis spectroscopy. The polarization curves and CV runs were recorded at a scan rate of 5.0 mV s−1 in 0.5 M KHCO3 aqueous solution saturated with 1 atm CO2 as the supporting electrolyte (pH 7.2). All potentials were recorded with reference to the Ag/AgCl electrode and are presented with respect to the RHE after conversion using the equation ERHE = EAg/AgCl + 0.059 × pH + E°Ag/AgCl, where EAg/AgCl is the experimental potential measured ° is the standard against the Ag/AgCl reference electrode and EAg/AgCl potential of Ag/AgCl at 25 °C. The catalytic water and CO2 electrolysis investigations were performed using a two-electrode glass cell in which the anode and the cathode (both of which were nanomaterial-modified PGEs) were separated by a membrane with very fine porosity. The membrane acts as a proton-transfer channel needed for CO2 reduction. Qualitative and Quantitative Detection of Formic Acid. Qualitative Detection by Baeyer’s Reagent Test. Herein, for the qualitative detection of formic acid produced during CER, 1% alkaline KMnO4 solution (i.e., Baeyer’s reagent) was added to the electrochemical cell, turning the cell solution purplish-pink, and successive

CPE runs were performed. Changes in color were observed and captured using a digital camera.25 Quantitative Detection by UV Spectroscopy. The product generated from the electrochemical setup at various potentials through CPE was analyzed by UV spectroscopy in the wavelength region of 190−240 nm to reveal the amount of HCOOH produced. The UV− vis spectra of known amounts of commercial HCOOH were taken at different concentrations. Deionized water was used as a reference, and the maximum absorbance peak at 207 nm from commercial HCOOH solution was matched with each resultant graph of samples. A calibration curve was plotted by taking the concentration and absorbance as the X and Y axes, respectively. From the slope and intercept of the calibration equation, we calculated the amount of HCOOH obtained through CPE from the measured absorbance. The sample collected from the electrochemical cell showed the peak at 207 nm, which was absent in the sample electrolyzed without CO2. Faradaic Efficiency Calculation. The Faradaic efficiency was calculated with respect to the theoretical amount of HCOOH produced from CO2 reduction. The amount of HCOOH produced was in turn calculated using the steady-state current obtained from the CPE, in which the current is given as the number of electrons passed per second. Under the assumption that CO2 reduction to formic acid is a two-electron process, this value was divided by 2 to give the amount of HCOOH formed per second. The theoretical amount of D


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Figure 3. (A) Elemental mapping analysis of SiO2@Dy2O3:Ir. (B) EDAX spectrum and (C) elemental mapping analysis of SiO2@Yb2O3:Ir. (D−I) FE-SEM images of SiO2@Ln2O3:Ir: (D) SiO2@Nd2O3:Ir, (E) SiO2@Y2O3:Ir, (F) SiO2@Yb2O3:Ir, (G) SiO2@Eu2O3:Ir, (H) SiO2@Pr2O3:Ir, and (I) SiO2@Dy2O3:Ir. HCOOH was obtained as moles of HCOOH s−1 = I/2F, where I is the current in amperes and F is the Faraday constant = 9.64853399 × 104 A s mol−1. Then the Faradaic efficiency is given by FE = (measured amount of HCOOH) × 100/(theoretical amount of HCOOH). H+ Adsorption Measurements. To study the proton adsorption capacity of the prepared nanocomposite (SiO2@Ln2O3:Ir), a dialysis method was used with a 5 mM HCl solution. The adsorption capacity experiments were conducted with 25 mg of catalyst and 10 mL of 5 mM HCl solution. The nanocomposite was dialyzed using a semipermeable membrane (molecular weight cutoff = 1000) in a 250 mL beaker containing 100 mL of 5 mM HCl as the dialysate. After stirring for different time intervals, 2 mL aliquots of dialysate were taken out, and the concentrations of the HCl solution were determined by titration of the aliquots against 5 mM NaOH solution.

from the JCPDS card no. 86-1561 for the presence of the (100) plane of Si. Then the incorporation of rare earth oxide and iridium to the MSNs was analyzed by recording their XRD patterns. Different rare earths used have different signature peaks in the XRD pattern. The XRD pattern of SiO2@Y2O3:Ir has peaks at 29.1°, 33.8°, 48.5°, and 56.3°, which could be assigned to the (222), (400), (440), and (541) planes of Y2O3 according to JCPDS card no. 79-1257 (Figure 1E). Besides the presence of silica and Ir peaks, the XRD pattern of SiO2@Pr2O3:Ir shows the characteristic peaks for Pr2O3 at 26.5°, 29.7°, 30.8°, and 53.4°, which could be indexed to the (100), (002), (101), and (103) planes, as confirmed from JCPDS card no. 47-1111 (Figure 1F). In the XRD pattern of SiO2@Nd2O3:Ir, the peaks for Nd2O3 were found at 27.9°, 32.8°, 47°, and 54.4° and assigned to the (222), (400), (440), and (622) planes, as confirmed from JCPDS card no. 21-0579 (Figure 1G). In the XRD pattern of SiO2@Eu2O3:Ir, the broad peak at 22° is attributed to the silica moiety, while other peaks at 28.4°, 32.9°, 47.3°, and 56.1° could be assigned to the (222), (400), (440), and (622) planes of Eu2O3 (Figure 1H). The presence of Ir was confirmed by the presence of some sharp peaks at 40.0° (111),

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RESULT AND DISCUSSION Characterization of the Prepared Nanocomposite. Surface Group and Compositional Characterization of the Nanocomposite. The XRD pattern of the mesoporous silica nanoparticles (MSNs) was recorded, and it shows a single and broad peak in the range of 15° to 30° (Figure 1A). The broad nature of the peak could be attributed to the diffraction of its amorphous structure. The formation of MSNs was confirmed E


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Figure 4. Deconvoluted XPS spectra of (A) O 1s, (B) Eu 3d, (C) Ir 4f, and (D) Si 2p. (E) Cyclic voltametric and (F) LSV runs of all nanocomposites (SiO2@Ln2O3:Ir) in 0.5 M KHCO3 at 5.0 mV s−1 for the OER. (G) Overpotential calculation graph from LSVs of all of the nanocomposites at a current density of 10 mA cm−2. (H) Tafel slope and (I) EIS spectra of the nanocomposites.

that Ir is well-incorporated into the prepared nanocomposites. No trace levels of other impurities could be seen within the detection limit of the EDAX spectra. After the careful elemental characterization of the nanocomposites, XPS spectra of SiO2@Eu2O3:Ir were also recorded to examine the oxidation states of the elements present in the nanocomposite. The positions of various photoemission peaks are marked in the survey spectrum for elements present in the nanocomposite. Further detailed scans were performed for O 1s, Si 2p, Eu 3d, and Ir 4f. The deconvoluted spectrum of O 1s (Figure 4A) shows the presence of two peaks at 529.3 and 531.0 eV. The main intense peak centered at 529.3 eV is attributed to Eu−O in Eu2O3.26 This shows that oxygen ions in the nanocomposite are bonded to the europium(III) ions. The peak at 531.0 eV confirms the presence of O 1s and represents the O 2− form of oxygen in the nanocomposite. The incorporation of Eu in the form of Eu2O3 was further confirmed by the Eu 3d XPS spectrum (Figure 4B), where the peaks at 1135 and 1127 eV indicate the presence of Eu 3d5/2 and Eu 3d3/2, respectively.27 The XPS spectrum of the Ir 4f region (Figure 4C) consists of the 4f7/2 and 4f5/2 peaks at 61.31 and 63.35 eV, respectively, which can be assigned to

46.5° (200), and 67.9°(220), which represents the zero oxidation state of Ir in the nanocomposite (JCPDS card no. 88-2342). The formation of the nanocomposite of Dy2O3 was confirmed from the peaks at 28.8°, 33.5°, 48.2°, and 57.2°, which are assigned to the (222), (400), (440), and (622) planes (JCPDS card no. 780-388) (Figure 1I). The formation of the SiO2@Yb2O3:Ir nanocomposite was confirmed by the peaks at 29.5°, 34.4°, 49.3°, and 58.9° corresponding to the (222), (400), (440), and (622) planes from JCPDS card no. 781-690 (Figure 1J). To further investigate the chemical composition, elemental maps and EDAX spectra of all of the nanocomposites were recorded, and the results are shown in Figures 2 and 3. The EDAX spectrum and elemental maps for MSNs (displayed in Figure 1C,D) clearly confirmed the presence of Si and O in MSNs. The absence of other elements indicates that the prepared nanoparticles have a high purity level. Similarly, the EDAX spectra of the nanocomposites showed the presence of Y, Pr, Nd, Eu, Dy, and Yb in their respective SiO2@Ln2O3:Ir in addition to Si, O, and Ir. The EDAX spectra and corresponding elemental maps confirmed the successful synthesis of the nanocomposites. In addition, the results gave solid evidence F


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ACS Sustainable Chemistry & Engineering metallic Ir(0).28 The Si 2p spectrum of the silicon particles contains a single peak at 103.1 eV (Figure 4D), corresponding to the Si−O (silicon oxide) bond.29 The XPS results confirmed the presence of europium(III) oxide, Ir, and SiO2 in the prepared nanocomposite (SiO2@Eu2O3:Ir), which is in complete agreement with the XRD results. Morphological Characterization of the Nanocomposites. From the FE-SEM analysis, the morphological structures of the prepared MSNs and nanocomposites were analyzed. The MSNs show a good arrangement of spherical-shaped nanoparticles having diameters of around 40−50 nm (Figure 1B). The nanoparticles are not perfectly dispersed, and slight agglomerations at some places can also be observed. Figure 3D−I shows FE-SEM images of the SiO2@Ln2O3:Ir nanocomposites. It can be clearly observed that all of the nanocomposites have a spherical shape, while their sizes, dispersions, and morphologies are different from each other. Closer observation reveals that the SiO2@Eu2O3:Ir nanocomposite shows very beautiful, distinguishable, and welldispersed spheres/balls (Figure 3G). These balls have very uniform shape and size with a very smooth surface. On the other hand, the other nanocomposites possesses nonuniform surfaces with roughness and agglomeration. For details about the surface area and mesoporous arrangement of the prepared nanocomposites, BET surface area analysis was performed. A typical hysteresis loop with a relative pressure ranging from 0.0 to 1.0 was obtained for each nanocomposite material, and their surface areas and pore size distributions are summarized in Table S1. The corresponding nitrogen adsorption−desorption isotherms of the prepared nanocomposites are shown in Figure S1A. The specific surface areas and pore volumes of the prepared nanocomposites are found in the given order: SiO2@Eu2O3:Ir > SiO2@Pr2O3:Ir > SiO2@Nd2O3:Ir > SiO2@Y2O3:Ir > SiO2@Dy2O3:Ir > SiO2@ Yb2O3:Ir. The Eu-based nanocomposite shows a higher surface area in comparison with the others. Furthermore, their electroactive surface area (after coating on electrode) was also calculated and portrayed in Table S1. The electrochemical activity and electroactive surface area were examined using 5.0 mM Fe[(CN)6]4− in 1.0 M KCl via the CV technique (Figure S1B). As shown in the figure and the corresponding table, modification of the bare PGE with nanocomposites enhances the charge transfer kinetics by increasing the electroactive surface area, and therefore, more current is obtained at the nanohybrid-modified PGE in comparison with the bare PGE. The electroactive surface area was estimated according to the Randles−Sevcik equation:30

prepared Eu-based nanocomposite showed better electrochemical activity. It is also expected that the high surface area of the nanocomposite may expose the catalytic sites, facilitating the mass and charge transfer of electrochemical reactions, which may lead to their better performance as catalysts for CER and the OER. Electrochemical Performance of Nanocomposites toward the OER. Oxygen evolution or oxidation is a kinetically sluggish reaction because of its multistep transfer of four electrons with high activation energy and large overpotential requirements.32 However, it plays a major role in several electrochemical devices, such as rechargeable metal− air batteries, water electrolyzers, electrosynthesis reactors, etc. Therefore, the OER has a number of applications in several fields of energy conversion and storage. Here the OER activities of the prepared electrocatalysts were determined in a threeelectrode system. The electrochemical cell was purged for 30 min with oxygen prior to the reaction. Before the electrochemical analysis was started, different experimental parameters such as scan rate, catalyst loading amount, and electrolyte concentration were optimized to obtain the highest current density (Figure S2). By optimization, it was found that catalysts showed the lowest onset potential and highest current density when the scan rate was 5.0 mV s−1 , the electrolyte concentration was 0.5 M KHCO3, and the loading mass was 10.0 mg. At first, CV runs were taken at a scan rate of 5 mV s−1 over the potential range of +0.5 V to +2.8 V for all of the nanocomposites under the same analytical conditions (Figure 4E). All of the nanocomposites showed similar behavior, that is, an anodic sweep toward higher potential with an increase in the current density, in which the maximum current density was observed for SiO2@Eu2O3:Ir compared with the other moieties. Similarly, their polarization curves were also recorded and are shown in Figure 4F. An almost similar trend in current density was recoded for LSV runs also. Additionally, the corresponding onset potentials for the nanocomposites were also calculated: SiO2@Eu2O3:Ir (1.09 V) < SiO2@Pr2O3:Ir (1.25 V) < SiO2@ Nd2O3:Ir (1.37 V) < SiO2@Y2O3:Ir (1.45 V) < SiO2@Dy2O3:Ir (1.53 V) < SiO2@Yb2O3:Ir (1.57 V). From the LSV graph, it can be seen that SiO2@Eu2O3:Ir has the lowest onset potential compared with the other rare earth composites. According to the literature, lower onset potential represents a higher capability of the electrocatalyst for the OER.33 The reason for the exceptionally lower onset value of 1.09 V for the OER was very nicely explained in the recent repot by Chen et al.34 According to them, the potentials for the OER and HER decrease with increasing pH of an electrolyte solution, and if we are able to combine a high HER potential in an acidic electrolyte solution with the low OER potential in a basic electrolyte solution, the overall driving voltage should be lower than the theoretical one (1.23 V).34 Herein also, at higher pH (i.e., 7.2) the standard OER potential has decreased to 0.817 V, compared with 1.23 V at pH 0. The proposed catalysts showed the value of 1.09 V for the onset potential for the OER, which is close to the value of 0.817 V and lower than those of the other reported catalysts. To determine the general trend, an overpotential comparison of the OER catalysts was performed at 10 mA cm−2 current density, which is considered to be related to hydrogen fuel synthesis and estimated as a value of 10% efficiency of a solarto-fuel device.33 Herein, the most active catalyst, i.e., SiO2@ Eu2O3:Ir, achieved a current density of 10 mA cm−2 at an

Ip = 2.69 × 105n3/2AD1/2v1/2C

where Ip, A, n, D, v, and C are the peak current (in μA), electroactive surface area, number of electrons involved in the reaction, diffusion coefficient of the analyte, scan rate, and concentration of the analyte, respectively. In general, the redox reaction of Fe(CN)63−/4− involves one-electron transfer (n = 1), and their diffusion coefficient (D) can be taken as 0.76 × 10−5 cm2 s−1.31 From the above equation, the electroactive surface area of SiO2@Eu2O3:Ir was calculated and found to be ∼10 times higher than that of the bare electrode. In addition, the electroactive surface areas of the nanocomposites followed a similar trend as obtained by BET analysis. It may be concluded that as a result of the high surface area and pore volume, the G


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Figure 5. (A) LSVs for SiO2@Eu2O3:Ir in N2- and CO2-saturated 0.5 M KHCO3 electrolyte at 5.0 mV s−1. (B) LSVs for all of the nanocomposites (SiO2@Ln2O3:Ir) in 0.5 M KHCO3 at a scan rate of 5.0 mV s−1. (C) Variations of the electrolysis current density with the electrolysis time during the electrochemical reduction of CO2 on the SiO2@Eu2O3:Ir electrode at electrolysis potentials of −0.3 to −0.9 V. (D) Variations of the Faradaic efficiency for producing formic acid and the current density with the electrolysis potential during the electrochemical reduction of CO2 on the SiO2@ Eu2O3:Ir electrode. (E) Tafel slopes for CER. (F) Faradaic efficiencies for formation of HCOOH and byproduct H2 for SiO2@Eu2O3:Ir at various potentials. (G, H) Faradaic efficiencies for formation of HCOOH and byproduct H2 for all of the other nanocomposites at various potentials.

nanocomposite a good electrocatalyst. In addition, the large surface area, homogeneous spherical morphology, and large number of porous sites allow the easy access of O2 to the active sites as well as the accessibility of the electrolyte, making the material worthy for OER activity. Electrochemical impedance spectroscopy (EIS) experiments were performed for the entire nanocomposite using a threeelectrode system to work out the charge transfer resistance during OER measurements (Figure 4I). The Nyquist plots of these composites consist of a semicircle region that represents charge transfer resistance (Rct) at high frequency, which delivers the resistance of the electrochemical reactions occurring on the electrode. The frequency of the AC voltage was swept over the range from 100 kHz to 1 Hz, and the impedance data were fitted to the semicircle to calculate the Rct values. The lower charge transfer resistance of SiO2@Eu2O3:Ir (i.e., 80 Ω) indicates that highly efficient electron transport for electrochemical reactions can occur at this nanocomposite-

overpotential (η) value of 220 mV. The respective overpotential values for the other nanocomposites are as follows (Figure 4G): SiO2@Eu2O3:Ir (220 mV) < SiO2@Pr2O3:Ir (290 mV) < SiO2@Nd2O3:Ir (310 mV) < SiO2@Y2O3:Ir (340 mV) < SiO2@Dy2O3:Ir (400 mV) < SiO2@Yb2O3:Ir (420 V). The Tafel slope is also an important parameter that provides a valuable approach toward the kinetics of the OER process by underlining the influence of the potential or overpotential on the steady-state current density. Herein the Tafel slopes were also calculated from the LSV plot, and it was observed that the η versus log(j) plot was linear for all of the nanocomposites, which indicates that they may have followed similar reaction mechanism pathway. In general, a lower Tafel slope means O2 is more easily adsorbed on the electrocatalytic surface, and here the lowest Tafel slope was observed for SiO2@Eu2O3:Ir (i.e., 44 mV/dec). The Tafel slopes for the other nanocomposites are shown in Figure 4H. It can be concluded from the results that the lower the Tafel slope is, the faster is the charge transfer from the electrolyte to the electrode, which makes the H


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Figure 6. CPE runs in a two-electrode system setup after addition of Baeyer’s reagent at different potentials: (A) −0.3 V, (B) −0.4 V, (C) −0.5 V, (D) −0.6 V, (E) −0.7 V, and (F) −0.8 V. (G) Dependence of the adsorption of H+ over the nanocomposites on the adsorption time. (H) Stability of SiO2@Eu2O3:Ir for CO2 reduction at a potential of −0.70 V vs RHE for 12 h. (I) Long-term CPE using SiO2@Eu2O3:Ir electrodes as both the anode and cathode between 1.0 and 2.5 V vs RHE.

−0.21 V, which is approximately −0.37 V higher than the potential required for the HER. To validate the effects of different lanthanides on the electrocatalytic performance of the nanocomposites (i.e., SiO2@Ln2O3:Ir) for CER, their LSV runs were recorded under similar conditions. As shown in Figure 5B, in the comparative LSV graph, the electrocatalytic CO2 reduction by SiO2@Eu2O3:Ir has a more positive onset potential (−0.21 V) with a very low overpotential (190 mV) and high current density (−54 mA/cm2) compared with the other nanocomposites. The onset potentials for other nanocomposites were found to be SiO2@Pr2O3:Ir (−0.31 V), SiO2@Nd2O3:Ir (−0.41 V), SiO2@Y2O3:Ir (−0.53 V), SiO2@Dy2O3:Ir (−0.64 V), and SiO2@Yb2O3:Ir (−0.75 V). For CO2 reduction, a more positive onset potential and higher current density were achieved by SiO2@Eu2O3:Ir having the smallest spherical structure, which may improve the specific surface area and number of active sites of the nanocomposite. As CER and the HER work simultaneously, it is very difficult to trace which electrochemical reaction is going on, as we cannot give a conclusive statement from the current density and

modified PGE, which results in enhancement of its performance toward the OER as well as CER. Electrochemical Performance of the Nanocomposites toward CO2 Reduction. The electrocatalytic performance of the nanocomposites toward reduction of CO2 was evaluated in a 0.5 M aqueous solution of KHCO3 (pH 7.2). The loading amount, scan rate, and electrolyte concentration were optimized prior to the electrochemical setup (Figure S3). First, the performances of the nanocomposites were compared using the CV technique (Figure S4). For SiO2@Eu2O3:Ir, the increase in current density started from about −0.21 V, while for others the change was observed at lower potential values. Afterward, the polarization curve of the SiO2@Eu2O3:Ir electrode was recorded in both the presence and absence of CO2, and the results are displayed in Figure 5A. As shown in the figure, the LSV curve recorded with SiO2@Eu2O3:Ir in N2saturated 0.5 M KHCO3 shows an onset potential of −0.57 V, which can be ascribed to the HER; this is a high overpotential for the HER. However, when the electrolyte was purged with CO2, the reduction reaction was initiated at a potential of I


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where n is the total number of electron transferred to convert to CO2 to formic acid and k is the rate constant for converting CO2 to HCOOH (including the value of K).38 Converting the values according to the equations, finally the Tafel slope can be written as

overpotential values only. Thus, it is important to monitor the reaction under controlled potential electrolysis at different potentials. Here we performed the electrolyses at different potentials in the range of −0.3 to −0.9 V (Figure 5C). The effects of the different electrolysis potentials on the electrolysis current density and the Faradaic efficiency for producing formic acid during the electrochemical reduction of CO2 were investigated. It was observed that the electrolysis current density increases as the electrolysis potential decreases from −0.3 to −0.9 V (Figure 5C). The Faradaic efficiency for producing formic acid increases as the electrolysis potential decreases from −0.3 to −0.7 V and reaches a maximum of 92.7% (Figure 5D). As the electrolysis potential decreases further, the Faradaic efficiency begins to decrease. The obtained liquid product of electrolysis was qualitatively detected via Baeyer’s test, and its quantification was done through UV-vis spectroscopy (Figure S5). For Baeyer’s test, 20.0 μL of potassium permanganate solution was added to one of the compartments of the electrochemical cell at different potentials, and the change in the solution color was captured using a camera. As shown in Figure 6A−F, the purplish-pink color of the solution fades to brown with increasing concentration of HCOOH formed during the reaction: 2KMnO4 + 4HCOOH + 2H+ → 2MnO2 + 4CO2 + 4H2O + H2 + 2K+.35 It was observed that minimum HCOOH production started to be formed at an applied electrode potential of −0.30 V (maximum is H2) and the FE for HCOOH production gradually increased as more negative potentials were applied, reaching 60% at −0.60 V. There was a significant rise in FE when the applied potential was switched from −0.60 (FE = 76%) to −0.70 V (FE = 92.7%) (Figure 5F). As the more negative potentials were applied, the FE decreased because of mass transport limitations.36 The Faradaic efficiencies of different silica rare earth nanocomposites toward H2 and HCOOH formation at different potentials were also evaluated and are displayed in Figure 5G,H. It was observed that nanocomposites other than SiO2@Eu2O3:Ir did not show specific behavior toward CO2 reduction to HCOOH, and the observed FEs for HCOOH production were very low compared with that for SiO2@Eu2O3:Ir. However, similar to SiO2@Eu2O3:Ir, at more negative potentials the FEs for formic acid were enhanced for the other nanocomposites also and increased to the following values: SiO2@Pr2O3:Ir (82%), SiO 2 @Nd 2 O 3 :Ir (70%), SiO 2 @Y 2 O 3 :Ir (69%), SiO 2 @ Dy2O3:Ir (45%), and SiO2@Yb2O3:Ir (10%). Additionally, Tafel analyses were also performed to elucidate the fast kinetics associated with the reduction of CO2 to formic acid, and the Tafel plots are shown in Figure 5E for different nanocomposites. The Tafel slope value for SiO2@Eu2O3:Ir was also calculated and found to be 119 mV/dec, which is very close to the reported value of 118 mV/dec for rate-limiting single-electron transfer at the electrode.37 Mechanism and Kinetics of CER. A relationship between the HCOOH current (iHCOOH) and overpotential (η) can be expressed as follows: iHCOOH ∝ PCO2 exp(αFη/RT), where F is the Faraday constant, R is the gas constant, T is the absolute temperature, PCO2 is the partial pressure of CO2, and α is the transfer coefficient.38 From Henry’s law, the concentration of the dissolved CO2 in the electrolyte can be expressed as CCO2 = KPCO2, where K is a constant. Thus, the equation becomes

1=

αF (∂η /∂ log iHCOOH) 2.3RT

In this equation, with the value of the Tafel slope = 119 mV/ dec for SiO2@Eu2O3:Ir, the transfer coefficient α was calculated to be 0.49. This value of α (i.e., less than 0.5) suggests that the rate-determining step for the CO2 reduction to formic acid is the initial electron transfer to CO2 on the nanocompositemodified electrode. It is a general concept that in the KHCO3 electrolyte medium, the product of CER may be potassium formate or formic acid. It is true that in a basic medium the possibility of formate formation is greater than that of HCOOH. However, in this work, 0.5 M KHCO3 was used, which has a pH of 7.2, close to a neutral medium. According to the literature, the formation of HCOOH should be always more favorable in comparison with the formation of other reduction products in the process of electrocatalytic reduction of CO2.39,40 A number of literature reports also support the statement and showed the successful synthesis of HCOOH as a major product of CO2 reduction in neutral media. For example, Kumar et al.15 showed that an electrochemically reduced SnO2 porous nanowire catalyst with a high density of grain boundaries exhibits an efficient conversion of CO2 into HCOOH, higher than analogous catalysts in a 0.1 M KHCO3 medium. Yadav and Purkait41 synthesized Co3O4 and used it as an electrocatalyst for H2O oxidation to generate protons for CO2 reduction to form HCOOH. It was found that at low applied voltages, bicarbonate-based electrolytes show higher Faradaic efficiencies than carbonate-based electrolyte solutions and that HCOOH was the only product formed at all applied voltages in different electrolyte solutions. They proposed an elementary mechanism toward CO2 reduction also and showed that initially CO2 radical anion is formed by accepting an electron from the cathode, which is protonated to form a neutral radical after interaction with a water molecule. The neutral radical so formed further gained an electron to form formate, which accepts a proton to form HCOOH. On the basis of the calculations and earlier reported literature, it was concluded that the main mechanism behind the reaction of formic acid formation is the adsorption of dissolved CO2 molecules on active facets of the rare-earthbased nanocomposite-modified PGE:41

CO2 (s) → CO2 (ads) CO2 (ads) + e− → CO2•−(ads)

CO2•−(ads) + H+ → HCOO•(ads) HCOO•(ads) + e− → HCOO−(aq)

HCOO−(aq) + H+ → HCOOH

The overall reaction for the formation of HCOOH is CO2(s) + 2H+ + 2e−→ HCOOH. In the rate-determining step, one electron is transferred to the adsorbed CO2 to generate the surface-adsorbed CO2•−. After that, a proton transfer to the CO2•− radical occurs, followed by the second electron transfer

iHCOOH ∝ nFkCCO2 exp(αFη /RT ) J


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Table 1. Comparative Study of SiO2@Ln2O3:Ir and Earlier-Reported Electroctalysts for CO2 Reduction to Formic Acida electrolyte

product

potential (V)

Jp (mA cm−2)

Faradaic efficiency (%)

M KHCO3 M KH2PO4 M KHCO3 MKHCO3 M KHCO3 M KHCO3

HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH CH3OH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH

−0.80 V −0.4 V vs RHE −1.8 V vs Ag/AgCl −2 V −1.5 V −0.7 V vs RHE

3−4 ∼5 13.46 ± 3.2

−1.9 V vs RHE −1.8 V vs Ag/AgCl −1.2 V vs Ag/AgCl − − −1.4 V vs SCE −0.18 V −0.40 V 1.11 to −1.40 V −0.7 V

∼5 10.56 ± 3.12 − − − − − 10 − 9.18

HCOOH = 78 HCOOH = 88 HCOOH = 72.99 HCOOH = 60 HCOOH = 78.54 HCOOH = 68 CH3OH= 11 HCOOH = 70 HCOOH = 71 HCOOH = 36 HCOOH = 90 HCOOH = 90 ± 10 HCOOH = 66 HCOOH = 49 HCOOH = 10 HCOOH = 66.0 HCOOH = 92.7

entry

electrode

1 2 3 4 5 6

SnO2 nanowires C@Pd−Pt NPs Sn (GDE) Pb2O/Gr plate Zn and Co3O4 Cu-CDots nanocorals

0.1 0.1 0.5 0.5 0.1 0.5

Phy@Gr electrode BDD electrode SGDE Cu complex Co with amine B-graphene AgPd-edged AuNPs hollow Cu fiber Sn/SnOx film SiO2@Ln2O3:Ir

PBS 0.075 M RbOH 0.5 M KHCO3 0.1 M n-Bu4NBF4 DMF−water 0.1 M KHCO3 0.1 M LiClO4 0.3 M KHCO3 KHCO3 0.5 MKHCO3

7 8 9 10 11 12 13 14 15 16

− ∼4.2

ref 15 16 18 39 41 42 43 44 45 46 47 48 49 50 51 this work

a

Definitions: Jp = partial current density; C@Pd−Pt NPs = carbon-supported Pd−Pt nanoparticles; Phy@Gr electrode = in protoporphyrin immobilized on a pyrolytic graphite electrode; PBS = phosphate buffer solution; BDD = boron-doped diamond electrode; SGDE = Sn-loaded gas diffusion electrode; Cu complex = [Cu(cyclam)](ClO4)2 complex; Co with amine = molecular cobalt complexes with pendant amines; B-graphene = boron-doped graphene; AuNPs = gold nanoprisms.

catalyst performance after different numbers of cycles and storage for a few months. After CV runs of the catalyst for OER measurements were taken, almost no changes in the onset potential and current density (355, 349, 335, 323, and 319 mA cm−2) were observed after the first, 50th, 100th, 500th, and 1500th cycle (Figure S6). It was also observed that following the same path for CO2 reduction, the LSV plot recorded after the 1000th cycle also did not exhibit any significant change in the current density (Figure S6). The storage stability of the prepared electrocatalyst (SiO2@ Eu2O3:Ir) is an equally important parameter if we are concerned about the cost of the material. LSV runs of the catalyst for the OER at an interval of 1 month showed no considerable changes in onset potential and current density (Figure S7). After 3 months of storage, only a 1% change in the current density with similar onset potential and overpotential was observed. LSV runs for CER were also recorded with variation in storage duration, and a similar response was observed (Figure S7). To study the OER durability, a chronoamperometry run was performed, and only a 3% change in the relative current was observed after 1 day. After 3 days, the change in the current density was approximately 6% (Figure S8A). We tested the long-term durability of the SiO2@Eu2O3:Ir catalyst at a constant applied electrode potential of −0.7 V. The current density remained steady at around −10 mA cm−2 throughout the 12 h test period with FEs of 90−93% (Figure 6H). In addition to the durability and storage stability study, regeneration of the nanocomposite was also performed. For this, the used catalyst was placed in a Petri dish and kept in a hot-air oven for 10 min at 100 °C. The catalyst was removed, and the current density was measured after each regeneration step. As shown in Figure S8B, the catalyst maintained a constant current density after 20 regeneration cycles, which suggest its good reusability during the process. To explore the change in morphology after these electrochemical studies, we recorded the FE-SEM image of SiO2@

from the electrode to form adsorbed formate ion, which receives one proton from solution, resulting in formic acid. Reason behind the Better Performance of SiO2@ Eu2O3:Ir toward the OER and CER. From all of the electrochemical studies, it is very clear that among the five different types of REE-decorated silica-Ir-based electrocatalysts, SiO2@Eu2O3:Ir is better in terms of current density, onset potential, and overpotential values. The better performance compared with the others could be explained on the basis of their high surface area, high electroactive area, and more homogeneous or uniform morphology. However, for the improved CER performance with the OER, the good adsorption capability of the electrocatalyst for H+ is also very important. Here we studied the H+ adsorption behavior of the prepared catalysts also, which may be a reason behind the enhanced performance of any electrocatalyst toward CER. The dependence of the adsorption of H+ by the mesoporous silica nanocomposites on the adsorption time is shown in Figure 6G. The amount of adsorbed H+ in SiO2@Eu2O3:Ir (based on the quantity of HCl) was calculated and found to be around 11.59 mg/mg, which is around 7 times higher than for SiO2@ Yb2O3:Ir (which showed the lowest performance among the others). The high adsorption capacity of SiO2@Eu2O3:Ir toward H+ could also be a valid reason for the improved efficiency of the proposed catalyst. It can be concluded from the experiments that at a potential of −0.7 V the possibilities for the formation of other byproducts like H2 are already reduced, but for the selective formation of HCOOH, a good feedstock of H+ is needed. The problem can be easily sorted out by the good H+ adsorption capacity of the proposed SiO2@ Eu2O3:Ir nanocomposite, which promotes CO2 reduction to HCOOH.42 Stability of the Proposed Nanocomposite. After all of the quantitative and qualitative electrochemical studies, the robustness and stability of the catalyst against the supporting medium over an extended period of time was also studied. For the stability study, the most active catalyst, SiO2@Eu2O3:Ir, was taken as the reference. We checked the stability in terms of the K


Research Article

ACS Sustainable Chemistry & Engineering Eu2O3:Ir that was stored for 3 months under normal roomtemperature conditions. No significant change in the morphology was observed, which suggests that the proposed catalyst is highly stable under long-term storage conditions also (Figure S9). Real Application. The ability of SiO2@Eu2O3:Ir to act as an electrocatalyst for both water oxidation and CO2 reduction provides a basis for performing both reactions concurrently in a two-compartment electrolysis cell. In a normal electrochemical cell, a porous membrane is used to separate the produced gases (Figure S10). In this experiment, both the anode and the cathode were SiO 2 @Eu 2 O 3 :Ir-modified PGEs, and the supporting electrolyte was 0.5 M NaHCO3. The corresponding overall CV graph is shown in Figure S11. In the electrolysis experiments using a two-electrode system (illustrated in Video S1), the electrode compartments were separated by a membrane that allowed H+ transfer between the compartments to balance the proton content and charge, avoiding slow proton transfer diffusion. Therefore, CO2 was reduced to HCOOH using a proton generated after water oxidation. During the CV measurements, the potential applied to the anode and cathode was monitored, which measured the opencircuit potential using a reference electrode. The overvoltage for the cathode and anode relative to the tentative standard voltage of the CO2 reduction to HCOOH, together with the Ohmic loss, was determined by measuring the rest potentials for both the cathode and anode with the application of a reference electrode (Ag/AgCl). The overpotentials were 220 mV for O2 evolution and 190 mV for CO2 reduction to HCOOH. CO2 splitting occurred at an applied cell potential of ∼1.66 V [= 1.09 V − (−0.57 V)] with an overvoltage of ∼0.76 V (= 1.66 V − 0.90 V). The current densities in both the anodic and cathodic waves were within a 95% match to those obtained in the study of the half-reactions. Figure 6I shows the current−time profiles obtained by controlled potential electrolysis between 1.0 and 2.5 V. In the cathodic compartment, a high FE for HCOOH of ∼90% was maintained. During the long-term CPE, the oxidation and reduction processes were sustained with no decrease in the catalytic current densities in the range of 1−8 mA cm−2. Table 1 summarizes the Faradaic efficiencies, applied voltages, and current densities that have been reported for CER to formic acid with some of the recent electrocatalysts. As indicated in Table 1, the proposed catalyst shows high catalytic activity and maximum FE at relatively high current density value.

overpotential values for the OER, which makes it a simple, inexpensive, robust, and efficient catalyst.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03179. BET surface areas, pore volumes, electroactive surface areas, and nitrogen adsorption−desorption isotherms of the nanocomposites; electrocatalytic activity study for all of the catalysts; optimization of loading mass, scan rate, and supporting electrolyte for the OER and CER; CV run for CO2 reduction; UV−vis absorption spectra of standard formic acid solutions; multiple CV runs for the OER; multiple LSV runs for CER; storage stability study toward the OER and CER; chronoamperometric study for the OER; reusability of SiO2@Eu2O3:Ir; FE-SEM image of SiO2@Eu2O3:Ir after long-term storage; singlecompartment electrochemical cell setup for both the OER and CER; overall CV graph for the OER and CER in a single electrolysis cell (PDF) Electrolysis experiments using the two-electrode system (AVI)

■

AUTHOR INFORMATION

Corresponding Author

*R. Madhuri: E-mail: [email protected]. Tel: +91 9471191640. ORCID

Rashmi Madhuri: 0000-0003-3600-2924 Prashant K. Sharma: 0000-0001-5283-0901 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are thankful to DST, BRNS, and ISM for sponsoring the research projects of R.M. (SERB/F/2798/201617, SB/FT/CS-155/2012, FRS/43/2013-2014/AC, and 34/ 14/21/2014-BRNS) and P.K.S. (SERB/F/2798/2016-17, SR/ FTP/PS-157/2011, FRS/34/2012-2013/APH, and 34/14/21/ 2014-BRNS). Authors are also thankful to Dr. D. Kumar (Indian Institute of Science, Banglore) for his kind assistance and help in materials characterization.

■

■

CONCLUSION In this work, we prepared five nanocomposites using different rare earth elements, SiO2@Ln2O3:Ir, among which SiO2@ Eu2O3:Ir was found to better with higher activity toward both the OER and CER in 0.5 M KHCO3. The use of mesoporous silica nanoparticles enhances the porosity, which provides active sites for electrochemical oxidation and reduction reactions, and the incorporation of Ir enhances the electrocatalytic activity of the nanocomposite. The prepared nanocomposite can act as both the anode and the cathode, so a single electrochemical cell with a porous frit separator was built, where the OER and CER were performed simultaneously. The prepared nanocomposite shows high Faradaic efficiency with high product selectivity and high stability for CER. Similarly, the catalyst shows a very low onset potential with considerable Tafel slope and low

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