La2O2CO3 Encapsulated La2O3 Nanoparticles Supported on Carbon

Nov 17, 2015 - M. Roca-Ayats , M.D. Roca-Moreno , M.V. Martínez-Huerta. International Journal of Hydrogen Energy 2016 41 (43), 19656-19663 ...
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La2O2CO3 Encapsulated La2O3 Nanoparticles Supported on Carbon as Superior Electrocatalysts for Oxygen Reduction Reaction Weiwei Gu, Jingjun Liu,* Mingan Hu, Feng Wang,* and Ye Song State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Constructing nanoscale hybrid materials with unique interfacial structures by using various metal oxides and carbon supports as building blocks are of great importance to develop highly active, economical hybrid catalysts for oxygen reduction reaction (ORR). In this work, La2O2CO3 encapsulated La2O3 nanoparticles on a carbon black (La2O2CO3@La2O3/C) were fabricated via chemical precipitation in an aqueous solution containing different concentrations of cetyltrimethyl ammonium bromide (CTAB), followed by calcination at 750 °C. At a given CTAB concentration 24.8 mmol/L, the obtained lanthanum compound nanoparticles reach the smallest particle size (7.1 nm) and are well-dispersed on the carbon surface. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results demonstrate the formation of La2O2CO3 located on the surface of La2O3 nanoparticles in the hybrid. The synthesized La2O2CO3@La2O3/C hybrid exhibits a significantly enhanced electrocatalytic activity in electrocatalysis experiments relative to pure La2O3, La2O2CO3, and carbon in an alkaline environment, by using the R(R)DE technique. Moreover, its long-term stability also outperforms that obtained by commercial Pt/C catalysts (E-TEK). The exact origin of the fast ORR kinetics is mainly ascribed to the La2O2CO3 layer sandwiched at the interface of carbon and La2O3, which contributes favorable surface-adsorbed hydroxide (OH−ad) substitution and promotes active oxygen adsorption at the interfaces. The unique covalent COC(O)OLaO bonds, formed at the interfaces between La2O2CO3 and carbon, can act as active sites for the improved ORR kinetics over this hybrid catalyst. Therefore, the fabrication of lanthanum compound-based hybrid material with an unique interfacial structure maybe open a new way to develop carbonsupported metal oxides as next-generation of ORR catalysts. KEYWORDS: La2O2CO3 encapsulated La2O3, carbon black, interfacial structure, chemical bonds, oxygen reduction reaction

1. INTRODUCTION During the past decades, oxygen reduction reaction (ORR) has always been considered as one of the most important electrochemical reactions because of its wide applications in advanced energy storage and conversion devices, such as fuel cells and metal-air batteries.1,2 However, because of the sluggish kinetics of ORR, there is great need for developing efficient electrocatalysts to promote the ORR process. Carbonsupported platinum (Pt/C) is currently thought as the most active electrocatalyst for the ORR process in both alkaline and acidic environments, which has been developed and utilized in various fuel cells and metal-air batteries, but the high cost and limited availability of Pt hinder its further application.3−5 Therefore, it is still a key challenge to develop an ORR catalyst © XXXX American Chemical Society

with low-cost, nonprecious metal, and high activity comparable to commercial Pt/C electrocatalyst.6 In recent years, construction of nanoscale hybrid catalysts using various metal oxides (MnxOy, NixOy, CoxOy, LaxOy, and etc.) and carbons including carbon black, carbon nanotube, and graphene as building blocks has become a research hotspot due to their improved electrocatalytic activity for the ORR.7−12 Such enhanced activity for carbon−metal oxide hybrids should be ascribed to the interfaces between metal oxide nanoparticles and carbon support, because coupling metal oxides and carbons Received: July 8, 2015 Accepted: November 17, 2015

A

DOI: 10.1021/acsami.5b06100 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ring-disk electrode techniques. We explored the possible origination of ORR electrocatalytic activity of the hybrid, which is assumed to stem from the formation of La2O2CO3 layer sandwiched at the interfaces between carbon and La2O3. By determining the unique chemical bonds formed at the interface, we can establish the interfacial structure-to-property relations of lanthanum compound-carbon hybrids, which is beneficial for the development of ORR as next-generation efficient and advanced non-Pt catalyst.

can always give rise to unique interfacial structures that may be associated with superior catalytic activities.13 Although the effect of the interfacial features on electrochemical performances is debated, it is believed that ORR kinetics strongly depends on the unique electronic and chemical structures at the interfaces. According to the results available in literatures, the interfacial structures based on different interactions between metal oxides and carbons that dominate the origin of improved ORR electrocatalytic activity can be mainly divided into two categories, that is, chemical coupling and formation of new phase. One category involves with the formation of C−O−M covalent bonds across the interfaces between metal oxides and carbon supports through chemical coupling, which can effectively catalyze the ORR process.14 Dai4 reported nitrogen-doped graphene supported Co3O4 nanocrystals with Co− O−C bonds formed at the interfaces between Co3O4 and NrmGO, and found that the presence of the chemical linking among C, O, and Co atoms is responsible for the superior ORR activities of the hybrid material. Liu15 further proved that the high catalytic activities in ORR by carbon-supported Co3O4 nanoparticles are related to the strong electronic affinity of metal oxides through the formation of Co−O−C bonds in hybrid catalyst. This unique phenomenon about electronic affinity had been confirmed by Abrikosov16 and his co-workers. On the basis of their results about intra-atomic electronic redistribution caused by valence electron hybridization, the strong electronic affinity can led to unique electronic structure characteristics of hybrid catalysts, which facilitated the ORR catalytic process. The other important category refers to the formation of new phase at the interfaces between carbons and metal oxides. Recently, some researchers have realized that the superior performance of hybrid materials may also relate to the formation of new phase at the interface of hybrids, which can always lead to a great change of hybrid materials’ performance. For example, Song et al.17 reported a CuO nanosheet/graphene hybrid with a small quantity of Cu2O at the interfaces between CuO and graphene. Such small interfacial new phase results in obvious changes in electronic structure and electrochemical performance of the hybrid. Zhang et al.18 synthesized La2O3 on carbonaceous microspheres derived from glucose precursor as an efficient bifunctional electrocatalyst for ORR and OER, and the improved electrocatalytic activity of hybrid may be due to the active component of La−O and C−O formed at the interface in the hybrid. However, the nature of the synergistic effect at the interface on the ORR kinetics still remains unclear, since the exact interfacial structure of the hybrid materials has been obtained little concern. More importantly, the detailed ORR kinetics over these hybrids catalysts has been paid little attention, which hinders the development of efficient metal oxide-based catalysts for ORR. Therefore, it is very urgent to establish interfacial structure-to-property correlations of hybrids for ORR. In this work, we have fabricated La2O2CO3 encapsulated La2O3 nanoparticles on a carbon black (Vulcan XC-72, Cabot) as hybrid catalysts for ORR, by using a simple chemical precipitation method with the aid of CTAB, followed by calcination at 750 °C. The morphological and interfacial structures of the resulting hybrids were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The enhancement of electrocatalytic ORR performance for the hybrid in alkaline solutions was characterized by using rotating

2. EXPERIMENTAL SECTION 2.1. Preparation of Lanthanum Compound−Carbon Hybrid. During the synthesis process of hybrid, all reagents were analytical grade. Prior to the deposition of lanthanum compound nanoparticles, a carbon black (Vulcan XC-72, Cabot) was first subjected in the environment of HNO3 (14.6 M) at 120 °C for 10 h, and then filtering, washing the carbon black by deionized water, and drying in vacuum. After acid oxidation, there are functional groups like carboxyl formed on surfaces of the treated carbon so that lanthanum compound can easily deposit on it. Subsequently, carbon-supported lanthanum compound nanoparticles were synthesized with the following procedure: the acid-treated carbon was added into a mixture of 20 mL of LaCl3 (0.05 mol/L) and (NH4)2SO4 (0.2 wt %) at 85 °C to get a slurry. Another 20 mL of carbamide (0.3 mol/L) mixed with CTAB was added into the slurry at a fixed dropping rate of 1 mL/min. As a precipitating agent, the carbamide was first decomposed into CO2 and NH4OH, then the CO2 and NH4OH reacted with LaCl3 to form La(OH)CO3 precursor in an alkaline environment. Considering the impact of pH on the formation of La(OH)CO3 precursor in the reaction solution, it is needed to adjust pH of the reaction solution by using concentrated ammonia. As pH was fixed at 9−10, the lanthanum compound nanoparticles could form efficiently over the carbon surface. Therefore, the pH was fixed at 9−10 and the mixture was stirred for 2 h then tip-sonicated for 40 min. After reaction, the mixture was filtered, washed by deionized water and alcohol for several times, and evaporated at 80 °C for 2 h. Finally, the resulting mixture was heated in a tube furnace under argon atmosphere to 750 °C for 2 h with a heating rate of 5 °C/min. The theoretical loading of lanthanum compound is 50 wt % (relative to carbon), while the actual loading of lanthanum compound was determined by thermo gravimetric analysis (TGA) measurement. Pure La2O3 and La2O2CO3 nanoparticles were purchased from SINOPHARM Co. for comparison. To investigate the effect of CTAB concentration on the formation of lanthanum compound nanoparticles as a surfactant, we also synthesized a series of hybrid catalysts with different concentrations of CTAB at a fixed synthesized feeding of LaCl3 and carbon black. 2.2. Physical Characterizations. X-ray diffraction (XRD) data were carried out by Rigaku RINT 2200 V/PC with a scan rate of 5°/ min from 10° to 90°. The morphologies of the as-prepared catalysts were observed by using scanning electron microscopy (SEM) (FESEM, JEOL, JSM-6701F) and transmission electron microscopy (TEM) (JEOL TEM 2010 microscope). The electronic structures were performed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). Due to the insulation of lanthanum compound, such as La 2O 3 and La 2 O 2 CO 3 , which may cause electric charge accumulation effect and result in the shift of XPS spectra, all spectra were calibrated by the C 1s peak position at 285.0 eV. Besides, the fitting curve was based on a Shirley background during the analyzing process of XPS spectra, and the Lorentzian−Gaussian parameter was fixed at 20%. To investigate the phase structure of the hybrid, Ar+ bombardment accompanied by XPS spectra was carried out with increasing bombardment time. The Ar+ bombardment was applied with EX05 argon-ion gun with etching area 2 mm. The chamber pressure was kept at 1 × 10−6 Pa. The amount of lanthanum compound relative to carbon support was determined by thermogravimetric and differential thermogravimetric analysis (TG-DTA, Thermoplus TG8120, Rigaku). B

DOI: 10.1021/acsami.5b06100 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces 2.3. Electrochemical Activity Characterizations. The ORR electrocatalytic activity for the hybrid materials was measured via a rotating ring-disk device (AFCBP1 type, PINE, USA) by using saturated calomel electrode (SCE) as a reference electrode and Pt wire as a counter electrode. All measurements were tested in 1 M NaOH solution, where the potential range is from 0.5 to −0.9 V. The working electrode was prepared by dispersing 10 mg of catalyst in a mixture of 2 mL of alcohol and 100 μL of Nafion and then pipetting 20 μL of suspension on the rotating ring-disk electrode (0.385 mg/cm2). In the cyclic voltammetry measurements, the scanning rate was 20 mV/s. For RRDE experiments, the potential of ring was fixed at 0.50 V (vs SCE) and scanning rate was 5 mV/s. The durability of the synthesized hybrid was tested respectively in a half-cell with three-electrode system, where the catalysts were first loaded onto the surface of a homemade gas diffusion electrode (GDE). Moreover, chronoamperometric curve for these catalysts was conducted by rotating disk electrode (RDE) by using CHI600E electrochemical workstation system in 1 M NaOH for 10000 s.

of the obtained XRD patterns has changed, implying that CTAB concentration has a significant impact on particle size of the lanthanum compound. Thus, the average particle sizes of these compounds were calculated by Scherrer’s formula with the data in Figure 1A and the resulting average particle sizes as a function of CTAB concentrations were shown in Figure 1B. As observed, the lanthanum compound particle sizes first decrease and then increase with the increasing CTAB concentration from 14.2 to 56.8 mmol/L. The particle size of lanthanum compound fabricated at 24.8 mmol/L reaches the minimum size among all the synthesized samples, indicating a close correlation between particle size and CTAB concentration. Moreover, CTAB concentration also exhibits a slight impact on the loading of the lanthanum compound relative to the carbon support, as shown in Figure 1C. For these hybrids, the actual loadings of lanthanum compound determined by TG/ DTA measurements slightly increase first then decline, accompanying the increase of CTAB concentration. As CTAB concentration reaches 24.8 mmol/L, there is a maximum actual loading (44.5 wt %), which is close to the theoretical loading (50 wt %). The impact of CTAB concentration on the lanthanum compound loading may be explained by both the amount of microemulsions formed by CTAB and the surfactant surface layer opening that controls two reactants (LaCl3 and carbamide) to mix and the precipitation reaction take place,19 considering that the precipitation reaction of La3+ ions with carbamide to generate the precipitate product (La(OH)CO3) belongs to a fast reaction. To further identify the dispersion of the nanoparticles on carbon, the SEM and high-resolution TEM images had been conducted for the hybrid fabricated at a given CTAB concentration of 24.8 mmol/L. As observed in Figure 1D, the synthesized lanthanum compound nanoparticles are homogeneously dispersed over the carbon surface without aggregated particles observed. As depicted by Figure 1D (inset), the observed particle size is about 7.5 nm, which is consistent with the result in XRD measurements (7.1 nm). Moreover, the lanthanum compound nanoparticles fabricated at the CTAB concentration 24.8 mmol/L show the smallest particle size and best dispersion over carbon support, compared with the samples fabricated at other CTAB concentrations shown in Figure S1. Therefore, it is concluded that the tunable particle sizes, loadings and dispersion for the synthesized lanthanum compound depend on the CTAB concentration that dominate the formation of oxide nanoparticles in this synthesizing system. However, the exact formation mechanism of the lanthanum compound nanoparticles on carbon with the aid of CTAB in the aqueous solution remains unclear. A possible formation mechanism of lanthanum compound nanoparticles on the carbon can be explained by the following steps shown in Figure 2, based on the space-confined deposition and growth of the compound capped by CTAB in this synthesis system.20 Initially, the carbon black as support was treated by concentrated nitric acid, which can generate functional groups carboxyl etc. on the carbon surface. These functional groups can act as anchoring sites for La3+ ions and carbamide that is precipitating agent for La3+ ions, as illustrated in Figure 2A. Subsequently, the anchored La3+ ions can react with carbamide to generate La(OH)CO3 by adjusting pH using NH3·H2O. The addition of surfactant CTAB can not only effect the nucleation of the lanthanum compound but also influence

3. RESULTS AND DISCUSSION 3.1. Fabrication and Formation Mechanism for the Hybrid with the Aid of CTAB. To investigate the effect of CTAB concentration on deposition and growth of lanthanum compound nanoparticles on carbon, we initially fabricated a series of lanthanum compound-carbon hybrids in an aqueous solution by varying CTAB concentrations (14.2, 24.8, 42.6, and 56.8 mmol/L), respectively. Figure 1A shows the XRD patterns

Figure 1. (A) X-ray diffraction patterns for lanthanum compoundcarbon hybrids fabricated at different concentrations of CTAB (14.2 mmol/L, 24.8 mmol/L, 42.6 mmol/L and 56.8 mmol/L), respectively. Diffraction peaks marked with • and ⧫ correspond to hexagonal phase of La2O2CO3 and hexagonal phase of La2O3. (B) Nanoparticles size and (C) actual loadings of lanthanum compound in these above samples. (D) SEM and TEM (inset) images for the sample fabricated at 24.8 mmol/L.

for these above hybrids. For all the samples, the diffraction peak assigned at about 26° is ascribed to the (002) crystal plane of carbon. The peaks at approximately 26.1°, 29.1°, 39.5°, 46.0°, 52.1°, 55.4°, 55.9°, 60.3°, and 62.3° can be attributed to the hexagonal phase of La2O3 (JCPDS card 05-0602), while the other peaks at about 27.6°, 33.6°, 42.5°, 44.4°, and 75.6° correspond to the hexagonal phase of La2O3CO3 (JCPDS card 37-0804), indicating that the synthesized lanthanum compound is consisted of above two phases. Moreover, along with increasing CTAB concentration, the peak width at half height C

DOI: 10.1021/acsami.5b06100 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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3.2. Interfacial Structure of the Lanthanum Compound−Carbon Hybrid. To study the interfacial structure between La2O3CO3, La2O3, and carbon components in this hybrid, XPS measurements were performed for the synthesized hybrid (fabricated at 24.8 mmol/L of CTAB), La2O2CO3, and carbon support, respectively. The obtained results were shown in Figure 3. As shown in Figure 3A, the obtained La 3d5/2 spectrum for the hybrid is composed by a doublet with 835.5/ 839.2 eV,24 which is consistent with that of recorded for pure La2O2CO3 instead of La2O3 with a doublet at 834/837.6 eV.25 It strongly implies that the presence of La2O2CO3 phase in the hybrid, which is in agreement with the XRD results shown in Figure 1A. Since La2O3 is easily formed stable carbonate of lanthanum in ambient atmosphere, the observed La2O2CO3 should be located at the surface of La2O3 nanoparticles. To confirm this hypothesis, we performed Ar+ ion bombardment for the as-synthesized hybrid as a function of time, t = 0, 60, 120, 320, and 620 s, aiming to remove La2O2CO3 from the bulk La2O3 surface, as shown in Figure S2. After ion bombardment, for the La 3d5/2 and O 1s spectra of the hybrid, the corresponding component that is related to La2O3 is appeared, which provides a direct evidence of La2O2CO3 encapsulated La2O3 surface. The amount of La2O2CO3 component in the hybrid is about 11 wt %, relative to the total catalyst, determined by the TG-DTA measurements shown in Figure S3. More importantly, as shown in Figure 3A, the “shake-up” satellite with few higher binding energy (BE), separated from the main La 3d5/2 peak at about 835.5 eV that is related to surface La2O2CO3 component in this hybrid, is negatively shifted, compared to that recorded for La2O2CO3. It is believed that the binding energy of the “shake-up” satellite is remarkably dependent on local chemical environment of La3+ that is caused by hybridization of O2p and La4f orbitals.26 Thus, the obvious shift of the satellite peak of La 3d in the hybrid implies a strong chemical interaction between the La2O2CO3 component and carbon in this case, which leads to the shift of the satellite peak. To confirm this hypothesis, we fitted O 1s spectra for La2O2CO3@La2O3/C hybrid, pure La2O2CO3 and pure carbon support respectively, and the obtained results were shown in Figure 3B. In Figure 3B, the recorded O 1s spectrum for the hybrid can be fitted to three peaks at about 533.0, 530.1, and 531.4 eV, which should correspond to O in C−O, La−O, and CO32− bonds in La2O2CO3, respectively.24 However, the recorded peak at 531.4 eV for CO32− bonds is negatively

Figure 2. Schematic diagram for the formation mechanism of lanthanum compound−carbon hybrid with the aid of CTAB. (A) HNO3 treatment of carbon support. (B) Formation of precursor La(OH)CO3 domains onto carbon surface. (C) Calcination of precursor La(OH)CO3 at 750 °C and formation of lanthanum compound nanoparticles.

its particle growth, coagulation and flocculation, as suggested by previous work.21 Moulik et al.22 proposed that there is a critical micelle concentration, CMC, of the surfactant that plays a key impact for controlling oxide nanoparticles sizes in the aqueous CTAB medium. In our case, as CTAB concentration is fixed at 24.8 mmol/L, the synthesized nanoparticles show the minimum particle size, compared to the other CTAB concentrations shown in Figure 1B. Since the critical micellar concentration of CTAB in aqueous solution is about 0.94 mM,23 it illustrates that CTAB indeed acted as a capping and stabling agent in controlling the size of the nanoparticles, which limit the growth of La(OH)CO3 domains, as shown in Figure 2B. Finally, the capped La(OH)CO3 domains were calcined at 750 °C under the protection of argon, eventually forming lanthanum compound nanoparticles on carbon. At this stage, the added (NH4)2SO4 can hinder the aggregation of the nanoparticles through decomposing into N2, SO2 and NH3 during calcination, as depicted in Figure 2C. On the basis of the above formation mechanism, the lanthanum compound nanoparticles, fabricated at 24.8 mmol/L of CTAB, can be rationalized to highly disperse on the carbon support, which has been verified by the results in Figure 1D.

Figure 3. XPS spectra for the synthesized samples. (A) La 3d5/2 spectra for the La2O2CO3@La2O3/C hybrid, La2O2O3, and La2O3, respectively, (B) O 1s spectra for the hybrid, La2O2CO3, and pure carbon, (C) C 1s spectra for the hybrid, La2O2CO3 and pure carbon, respectively. D

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were first conducted in the O2 and N2-saturated 1 mol/L NaOH solution at room temperature, respectively. The obtained results were shown in Figure 5A. For comparison,

shifted for the hybrid with respect to the pure La2O2CO3, accompanying that C−O bonds shifts negatively in its binding energy relative to that of pure carbon. It offers another proof of the chemical interaction between La2O2CO3 and carbon in this hybrid. To confirm this conclusion, we further fitted C 1s spectra for the hybrid, La2O2CO3, and pure carbon support respectively, and the obtained results were shown in Figure 3C. As observed, the recorded C 1s peaks of the hybrid can be fitted to three peaks, nonoxygenated C−C in aromatic rings (285.0 eV), C−O (286.3 eV), and CO32− (289.8 eV) groups,18 whereas the peak of CO32− is not observed in the pure carbon support. It reveals that the observed CO32− groups should originate from the La2O2CO3 phase formed rather than the carbon support. Moreover, in Figure 3C, the peak corresponding to C−O bonds in the hybrid shows an obviously positive shift relative to that of pure carbon support, further suggesting the chemical interaction between them. Together with the results from La 3d, O 1s, and C 1s spectra in Figure 3, the strong chemical interaction between La2O2CO3 layer sandwiched at the interface of carbon and La2O2CO3 in this hybrid, is present. On the basis of La2O2CO3 compound with so-called ordered lamellar structure, which is composed of carbonate and La2O22+ layers,27 there are three possible chemical linkage modes between the compound and carbon support in this hybrid. These modes can be defined as follows: (1) CC(O) OLaO bond, (2) COC(O)OLaO bond, and (3) COLaOC(O)O bond. Because of the presence of covalent CO bonds at the interfaces in the hybrid shown in Figure 3, mode 1 can be excluded because mode 1 has no such bond. Furthermore, as the CO2 is believed to be a strong electron withdrawing group, the presence of the group in mode 2 would lead to the shift of the bonding energy of the adjacent C and O atoms in the CO bond listed in mode 2. This deduction has been verified by the clearly positive and negative shifts of the bonding energy in C 1s and O 1s spectra for the synthesized hybrid, as shown in Figure 3. Therefore, we can deduce that the most probably chemically linked mode between La2O2CO3 and carbon is mode 2 rather than mode 3. The atomic structure scheme of the COC(O)OLaO bonds is shown in Figure 4. The formation of the unique chemical bonds in La2O2CO3@La2O3/C hybrid may be responsible for the improved electrocatalytic activity of the composite catalyst as catalyst for ORR. 3.3. Enhanced Electrocatalytic Activity of the Hybrid for ORR. To determine ORR elelctrocatalytic activity of the La2O2CO3@La2O3/C hybrid, cyclic voltammograms (CVs)

Figure 5. (A) Cyclic voltammograms for La2O2CO3@La2O3/C hybrid, La2O3, and carbon support in O2-saturated (solid line) or N2-saturated (dash line) 1 M NaOH. (B) Polarization curves for these above samples and a commercial Pt/C (20 wt % of Pt, relative to carbon, E-TEK) in an O2-saturated 1 M NaOH solution at a rotating rate of 1600 rpm. (C) Rotating ring-disk electrode voltammograms of the hybrid at different rotating rates. (D) Transferred electron number (n) calculated derived from panel C.

the CV curves for pure La2O3 and carbon support were also shown in this figure. By comparing the CVs curves for O2 and N2 saturated electrolyte, the hybrid clearly displays only one oxygen reduction peak under O2-saturated condition in the polarization potential region from −0.2 to −0.6 V, revealing that the ORR process involves with a single-step oxygen reduction process (4-electron pathway for the reduction of O2 to OH−). Moreover, the observed strong oxygen reduction peak is located at around −0.41 V and its peak current is very high, which further implies the fast kinetics of ORR on this hybrid with respect to the other two samples. However, both La2O3 and carbon material exhibit very poor ORR catalytic activity shown in Figure 5A, because the resultant CV profiles for them just show a typical characteristic of pseudo- and purecapacitive behaviors, respectively. Thus, it can be concluded that the ORR process catalyzed by the synthesized hybrid is remarkably improved, compared with that of La2O3 and carbon alone. Furthermore, the electrocatalytic activities of these samples were further evaluated by rotating disk electrode (RDE) measurements at a fixed rotating rate of 1600 rpm. The ORR polarization plots for these samples and a commercial Pt/C catalyst were shown in Figure 5B. The measured half-wave potentials for oxygen reduction on these catalysts follow the order: Pt/C > the hybrid > carbon > La2O3, with the half-wave potential values of −0.17, −0.22, −0.30, and −0.41 V versus SCE electrode, respectively. Apparently, the as-synthesized hybrid presents a significantly enhanced electrocatalytic activity, compared with La2O3 and carbon material. Although the halfwave potential for the hybrid is negatively shifted by 50 mV relative to that obtained by the commercial Pt/C, the electrocatalytic activity of the hybrid, estimated from the onset potential of the ORR in Figure 5B, is comparable with

Figure 4. Atomic structure diagram of the covalent bonds formed at interfaces at La2O2CO3 and carbon components in the hybrid. E

DOI: 10.1021/acsami.5b06100 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Except for the superior electroactivity toward ORR, the longterm durability is another key electrochemical property of the synthesized hybrid for practical application. Therefore, chronopotentiometry method was employed to investigate the durability of the hybrid catalyst and it was compared with that of commercial Pt/C catalyst here. The corresponding results were shown in Figure 6A. As observed, the measured

that of the commercial Pt/C catalyst (E-TEK). However, the diffusion limiting plateau of the hybrid is well above that of Pt/ C, and it can be explained by the relatively poor dispersion of the hybird catalyst on glassy carbon electrode relative to the commercial Pt/C. To confirm that the improved ORR activity of the hybrid is mainly originated from the chemical interaction between the La2O2CO3 and carbon components shown in Figure 3, it is useful to collect the RDE data of La2O3 or La2O3CO3 mechanically mixed with carbon for comparison. As shown in Figure S4, the La2O2CO3@La2O3/C hybrid shows much better ORR activity than that obtained by La2O3 or La2O2CO3 mechanically mixed with carbon. It indeed indicates that the enhanced electrocatalytic activity for ORR is mainly attributed to the chemical interaction between the La2O2CO3 and carbon components at the interfaces in the hybrid. To further identify the ORR pathway over the hybrid catalyst, rotating ring-disk measurements were conducted in O2-saturated 1 M NaOH solution, as shown in Figure 5C. As observed, the measured disk currents are much larger than the ring currents at different rotating rates, revealing that the ORR catalytic process may undergo a four-electron pathway for this hybrid material. To confirm this conclusion, the electron transfer number (n) during the ORR was calculated from the data in Figure 5C, based on an emprical formula available in the ref 28. The obtained results were shown in Figure 5D. Expectedly, the calculated number of transferred electron during ORR on the La2O2CO3@La2O3/C hybrid catalyst is ca.4 over the entire range of scanning potentials, and it is evident that the ORR process mainly involves a near four-electron pathway. However, for the carbon−metal oxide hybrids, the contribution of individual carbon and metal oxide to ORR electrocatalysis should not be neglected. Recently, some investigators29−31 proposed that carbon or metal oxide in their hybrids can influence both the electrical conductivity of electrode layer and the ORR mechanism itself. It is believed that carbon plays an active role in catalyzing 2-electron reduction of oxygen to HO2− while the metal oxides (such as, transition or perovskite-type oxides) are primary catalysts for the hydrogen peroxide decomposition to produce OH− and O2 that improve the ORR activity in an apparent 4-electron process (indirect 4-electron pathway). In our case, for the hybrid with respect to corresponding physical mixtures shown in Figure S4, the superior ORR activity is associated with the formation of the COC(O)OLaO bonds at the interfaces between lanthanum compound and carbon in Figure 4. To confirm the ORR over the covalent hybrid catalyst via a direct 4-electron pathway, RRDE measurements had been performed to monitor the production of hydrogen peroxide on different samples, such as the hybrid, La2O3 or La2O3CO3 mechanically mixed with carbon, respectively. The obtained results were shown in Figure S5. As observed, the recorded production of hydrogen peroxide on the hybrid catalyst is much lower than that obtained by La2O3 or La2O3CO3 mechanically mixed with carbon. It offers a valid proof that the ORR catalyzed by the hybrid catalyst undergoes via a nearly direct four-electron pathway, that is, a faster ORR kinetics, where the active oxygen could be directly reduced to OH−. The detail 4electron mechanism of the ORR catalyzed by this hybrid will be discussed in following section 3.4. The higher production of hydrogen peroxide on the above two physical mixtures may be due to the low hydrogen peroxide disproportionation chemically catalyzed by La2O3 or La2O2CO3.

Figure 6. (A) Chronopotentiometry curves of the hybrid and commercial Pt/C (E-Tek) during the ORR with a sweep rate of 50 mV/s at a current density of 3000 A/m2. (B) Hydrogen peroxide production on these catalysts during the ORR.

oxygen reduction potential for the hybrid is slightly negative with respect to that of the commercial Pt/C, but the catalyst has a more stable ORR activity, because no any decay in ORR polarization potential has been observed in continuous operation of 10 h. In contrast, the reduction potential during ORR over the Pt/C catalysts shifts toward the negative direction under the same operation condition, revealing a slightly decay of ORR activity. These outcomes illustrate that the hybrid has superior long-term durability to the Pt/C catalyst. To explain the reason for the superior durability of the hybrid, we monitored the production of hydrogen peroxide on the hybrid and commercial Pt/C in 1 M NaOH solution during ORR, by using a ferricyanide detection method14 and the results were shown in Figure 6B. As observed, the hydrogen peroxide production on the hybrid catalyst (0.037 mmol/L) is slightly higher than that of the Pt/C catalysts (0.026 mmol/L) at the beginning of testing. However, with increasing the operation time, the hydrogen peroxide yield on the hybrid continuously decreases, while it increases for Pt/C catalyst. To verify this outcome, we used RRDE to study the production of hydrogen peroxide on the hybrid and Pt/C catalyst, respectively. The obtained results were shown in Figure S6. The recorded production of hydrogen peroxide is comparable with that obtained by Pt/C catalyst. It underlines a close correlation between hydrogen peroxide production and stability for these catalysts. More importantly, the durability of the above two mixtures were carried out for comparison with the hybrid material. The obtained results were shown in Figure S7. As observed, the durability of the hybrid is much better than the above two mechanically mixtures. Thus, It is concluded that the enhanced durability for the hybrid catalyst should be attributed to the chemical interaction between La2O2CO3 and carbon support in this hybrid, since the covalent coupling of metal oxide nanoparticles to carbon can not only remarkably reduce the production of hydrogen peroxide but also anchor nanoparticles on the host carbon surfaces through the strong chemical interaction between them.14 3.4. Origin of Enhanced Activity for ORR on the Hybrid. In this work, we deeply explored the origin of enhanced ORR activity of the hybrid catalyst. On the basis of the unique interfacial structure of the hybrid catalyst, where  F

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2), which contributes to trigger the ORR process. Such electron transfer process has been confirmed by UV−vis and Raman measurements for the synthesized hybrid and pure carbon support respectively, as shown in Figure S8. The possible electron transfer pathway may be as following: covalent electron transfers from the carbon atoms on the carbon host surface to O(CO)O species in La2O2CO3 through the bridging COC(O)OLaO at the interface in this hybrid material. Moreover, considering the negligible HO2− production on this hybrid catalyst has been detected in Figure 6, it can be deduced that step 4 (fourelectron transfer pathway) is faster than step 5 (two-electron pathway). As a result, step 4 is no longer a rate-determined step during the ORR over the catalyst, because step 5 is always regarded as a relative fast step with respect to other steps during the ORR. In our case, the accelerated step 4 can be explained by easily removing of coordinated (lattice) oxygen (O−) in La2O2CO3, and it leads to the fast step 4 during ORR. Thus, the hybrid catalyzing the oxygen reduction process undergoes following the whole cycle and each cycle can consume 4 electron and produce 4 OH− rather than 2 HO2− produced via the half cycle (two-electron pathway), as shown in Figure 7A. On the basis of the proposed mechanism of the ORR above, therefore, the overall process of the ORR catalyzed by the hybrid catalyst is shown in Figure 7B. The most of dissolved O2 is reduced to OH− at the surfaces of La2O2CO3 layer sandwiched at the interface of carbon and La2O3. The nature of the enhanced electrocatalytic activity for the hybrid should be attributed to the oxidation of carbon and the reduction of the lanthanum compound at the interface. Therefore, through achieving synergy between inexpensive lanthanum carbonate and carbon, it may be open a new way to developing new and efficient ORR catalysts.35,36

COC(O)OLaO bonds formed at the interfaces between La2O2CO3 and carbon components shown in Figure 4, the improved activity of this catalyst may be substantially associated with the formation of the chemical bonds, which should act as active sites for the ORR. On account of the overall process of the ORR in an alkaline electrolyte shown in Figure 7A, a possible kinetic mechanism of the ORR via four-electron or two-electron transfer pathways over the hybrid catalyst can be explained by the following five steps:

4. CONCLUSION We synthesized La2O2CO3 encapsulated La2O3 nanoparticles on carbon via a chemical precipitation in aqueous solution containing different concentrations of CTAB, followed by calcination at 750 °C. The particle size, loading and dispersion of the obtained lanthanum compound nanoparticles on carbon strongly depend on the CTAB concentrations, and the minimum particle size (7.1 nm) and maximum actual loading (44.5 wt %) of the nanoparticles are achieved at the CTAB concentration of 24.8 mmol/L. Compared with pure La2O3, La2O2CO3, and carbon support, the synthesized hybrid catalyst exhibits a remarkably improved ORR electrocatalytic activity, which are comparable to commercial Pt/C (20 wt %). Moreover, the hybrid catalyst also shows a better long-term durability than Pt/C catalysts. The enhanced ORR electrocatalytic performance is substantially associated with La2O2CO3 layers sandwiched at the interface between carbon and La2O3, where the La2O2CO3 compound chemically connects the carbon support via COC(O)OLaO bonds. The ORR catalyzed by the hybrid catalyst undergoes a direct four-electron pathway, and the process of oxygen reduction are followed four steps: (1) surface-adsorbed hydroxide (OH−ad) substitution, (2) surface peroxide (OOH−) formation, (3) surface oxide (O−) formation, and (4) surface-adsorbed hydroxide regeneration. The presence of the unique covalent COC(O)OLaO bonds in the hybrid catalyst would result in favorable surface-adsorbed hydroxide (OH−ad) desorption and promoted active oxygen adsorption that occur at step 1 during the ORR, which remarkably leads to

Figure 7. (A) The proposed ORR mechanism on the hybrid material in alkaline solution. The ORR proceeds via four steps: (1) surface hydroxide displacement, (2) surface peroxide formation, (3) surface oxide formation, (4) surface hydroxide regeneration, and (5) generation of HO2−. (B) Schematic illustrating the overall ORR process catalyzed by the La2O2CO3@La2O3/C hybrid.

Although the detailed kinetic process of ORR is still debated, it is believed that competition between the O2−/OH−ad displacement (step 1) or OH−ad regeneration (step 4) on the surface of metal oxides are key rate-determined steps of ORR in alkaline solution.32−34 In our case, since the formation of chemical coupling between La2O2CO3 and carbon components has been proposed in Figure 4, the electron transfer from carbon to La2O2CO3 through the formed covalent CO C(O)OLaO bonds would occur, and it makes the relatively negatively charged surfaces for the carbon atoms in the OCOLaO bonds. As a result, since the adsorbed OO− species have been regarded as strong electron acceptors, these negatively charged carbon atoms would establish favorable sites for the OO− surface adsorption and facilitate the desorption of surface-adsorbed hydroxide (OH−ad) (step G

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enhanced ORR performance. The exact understanding of ORR mechanism catalyzed by the hybrid catalysts is beneficial for the development lanthanum compound-carbon hybrid as nextgeneration ORR electrocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06100. SEM and TEM images for the lanthanum compoundcarbon hybrids synthesized with different CTAB concentrations, XPS spectra of La 3d5/2 and O 1s for lanthanum compound-carbon hybrid at different ion bombardment time, TG-DTA curves for the La2O2CO3@La2O3/C hybrid, electrocatalytic activity of La2O2CO3@La2O3/C hybrid, La2O3 or La2O2CO3 physically mixed carbon for ORR, hydrogen peroxide production on La2O2CO3@La2O3/C hybrid and commercial Pt/C catalyst (20 wt %), chronoamperometric measurements for La2O2CO3@La2O3/C hybrid, La2O3 or La2O2CO3 physical mixed with carbon in 1 M NaOH, UV−vis and Raman spectra for La2O2CO3@La2O3/C hybrid and carbon support. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 10 64411301. Fax: +86 10 64411301. *E-mail: [email protected]. Tel: +86 10 64411301. Fax: +86 10 64411301. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Funds of China (Grant Nos. 51272018, 51125007). REFERENCES

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DOI: 10.1021/acsami.5b06100 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX