Article pubs.acs.org/JPCC
La2O3 Doped Carbonaceous Microspheres: A Novel Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions with Ultrahigh Mass Activity Xiaoxue Zhang, Qingqing Xiao, Yuxia Zhang, Xiong Jiang, Zhiyu Yang, Yifei Xue, Yi-Ming Yan,* and Kening Sun Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing, 100081, China S Supporting Information *
ABSTRACT: An efficient and robust bifunctional electrocatalyst for both ORR and OER is highly desired for the applications in renewable energy technologies. Here, we prepare the carbonaceous microspheres (CMSs) by a facile hydrothermal treatment of glucose precursor and then dope the CMSs with La2O3, resulting in a high performance bifunctional electrocatalyst of La2O3@ CMSs. In alkaline solution, the La2O3@CMSs catalyzes oxygen reduction reactions (ORR) with an onset potential of 0.80 V versus RHE and an overpotential only of 600 mV to achieve a current density of 1.3 mA cm−2. Meanwhile, oxygen evolution reaction (OER) at La2O3@CMSs electrode occurs at an onset potential of 1.60 V versus RHE and the overpotential is only 370 mV. Also, the as-prepared La2O3@CMSs exhibits high Faraday efficiency and long-term stability toward ORR and OER. Significantly, we demonstrate that La2O3@CMSs possesses surprisingly high mass activity, which is calculated to be 78.4 A g−1 for ORR and 831.5 A g−1 for OER, respectively. A potential window for ORR and OER at the modified electrode is estimated to be 0.80 V, implying a promising bifunctional electrocatalytical performance of La2O3@CMSs. The improvement of the bifunctional electrocatalytical activity may be due to the generation of active component of La−O and C−O at the surface and its synergistic interact with the La2O3@CMSs. This work not only provides a facile strategy for preparing highly efficient bifunctional electrocatalyst, but also offers an insight into the design of metal-oxides doped carbon materials for energy storage and conversion applications.
1. INTRODUCTION
catalysts are essentially desirable to achieve high efficiency of energy utilization. Pt, Ru, and their alloys are currently considered as the best ORR or OER catalysts.3,4,12 Unfortunately, the high cost and low abundance of noble-metals-based catalysts have largely limited their practical application. Toward this end, the activities of searching for bifunctional materials that are capable of catalyzing both ORR and OER are extremely important in practical applications, such as metal-air batteries and self-regenerative fuel cells.9,11
Oxygen electrochemistry has attracted significant attention due to continuous growing interests in green and sustainable energy conversion and storage technologies, such as fuel cells, metal-air batteries, water splitting, and solar fuel synthesis.1−16 In general, oxygen electrochemistry focuses on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, both reactions are complicated caused by the necessarily required multistep proton-coupled electron transfer, usually leading to high overpotential and low efficiency.6,7 For instance, ORR and OER govern the discharge and charge processes of metal-air batteries (such as Li-air, Zn-air, and Mg-air batteries), respectively, determining mainly the overall performance of the device. To expedite the sluggish kinetics, high efficient © XXXX American Chemical Society
Received: June 25, 2014 Revised: August 11, 2014
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synthesized sample was dried overnight in vacuum oven at 80 °C. The preparation for La2O3@CMSs was as follows: 0.6 g CMSs was dispersed in 30 mL of 0.02 M lanthanum nitrate solution and ultrasonicated for 30 min. The resulting suspension was magnetically stirred for 24 h and then aged for 1 h. After that, solid material was gathered through vacuum filtration and then washed with deionized water twice and dried in a vacuum oven at 50 °C for 12 h. Finally, the dried product was annealed in Ar at 900 °C for 1.5 h with a heating rate of 2 °C min−1 to produce the La2O3 crystal. La2O3@CMS samples with different lanthanum doping contents were realized by doping 0.02, 0.05, 0.1, and 0.2 M La(NO3)3 solution, and the resulting products were defined as La2O3@CMSs 9 wt %, La2O3@CMSs 11 wt %, La2O3@CMSs 16 wt %, and La2O3@ CMSs 22 wt %. Multiple-walled carbon nanotubes (CNTs, 20−30 nm in diameter) were treated in 4.0 M HNO3 for 10 h. The samples were prepared with the same procedure for La2O3@CMSs. The additive amount of CNTs 0.2 g and 10 mL 0.05, 0.1, 0.2, and 0.3 M La(NO3)3 solution were used to synthesize CNTs/La2O3 0.05, CNTs/La2O3 0.1, CNTs/La2O3 0.2, and CNTs/La2O3 0.3, respectively. Scanning electron microscope (SEM) images of the samples were taken using a QUANTA FEG 250 FEI field emission scanning electron microscope operated at 20 kV. Energy dispersive spectrometer (EDS) was carried out with a light element detector via the ZAF technique. Transmission electron microscopy (TEM) was conducted at 200 kV with a Philips Tecnai T30F field emission instrument equipped with a 2 K charge-coupled device (CCD) camera. X-ray Diffraction (XRD) patterns collected at room temperature were recorded on an automated Rigaku diffractometer (2500 D/MAX, Rigaku) using a Cu Kα (λ = 1.54056 Å) radiation. The measured patterns were evaluated qualitatively by comparison with entries from the ICDD-PDF-2 powder pattern database or with calculated patterns using literature structure data. X-ray photoelectron spectroscopy measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Kα X-ray source (E = 1486.6 eV) was operated at 15 kV and 15 mA. The hybrid mode was used as the lens mode. The base pressure during the experiment in the analysis chamber was 4 × 10−7 Pa. To account for charging effects, all spectra had been referenced to C (1s) at 284.4 eV. Thermal gravimetric analysis (TGA) curves in air were obtained in a TGA-DSC system. Analyses were carried out under air flow rate of 20 mL min−1 between room temperature and 800 °C at the ramping and cooling rate of 2 °C min−1. The ORR activity of the catalysts were measured on a rotating ringdisk electrode (RRDE; Model: AFMSRCE, PINE Research Instrumentation) by using Ag/AgCl (0.197 V) as reference electrode and Pt wire as counter electrode. The working electrode was scanned cathodically at a rate of 5 mV s−1 with varying rotating speed from 100 to 2500 rpm in 1.0 M KOH aqueous solution. Koutecky−Levich plots (J−1 vs ω−1/2) were analyzed at the same electrode potentials. The slopes of fitted linear lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky−Levich equation:
Transition metal oxides have been widely reported as bifunctional catalysts both in experimental synthesis and in theoretical calculations due to their promising electrocatalytical activity. For example, Dai and his co-workers14 have reported that Co3O4 nanocrystals on graphene showed high ORR and OER activities in alkaline solution. Meanwhile, Yang’s group15,17 found that LaMnO3 and LaNiO3 exhibited promising ORR and OER activities comparable to the performance of Pt/ C or IrO2. Moreover, Mn oxides,18 NiCo2O4,19 NiCo2S4@ graphene,20 MnCo2O4,21 and RuO2-Co3O422 have been prepared and extensively investigated as bifunctional catalysts for ORR and OER. Those reported catalysts were either utilized in metal-O2 battery cathodes or adopted as efficient catalysts for water splitting. However, as far as we are aware, most of the reported bifunctional catalysts were mainly limited to the 3d-M (Fe, Co, Ni, Mn) transition metal oxides and their mixed oxides.15,17,23 Moreover, the preparation of these electrocatalysts was mostly relying on the heavy content of metal in the materials, which definitely increases the cost of the catalyst and poses an environmental risk. In a view of practical application, the metal mass activity should be considered as an important parameter in addition to its electrochemical performance. Therefore, it is scientifically significant and practically meaningful to explore a novel bifunctional catalyst possessing high electrocatalytical activity,9 as well as exhibiting promising metal mass activity, in order to lower the usage of metallic materials. To the best of our knowledge, most researchers in the field of bifunctional catalysts have neglected this issue. In an attempt to develop a novel bifunctional catalyst with lower metal content while maintaining high performance, we report here the preparation of La2O3-doped carbonaceous microspheres (La2O3@CMSs) with a simple and facile synthetic strategy. The carbonaceous microspheres (CMSs) were deliberately synthesized by a modified hydrothermal treatment of glucose precursor.24−28 The presence of rich −OH groups at the surface of CMSs enables a readily absorption of lanthanum nitrate ions, therefore, generating a uniform and controllable La2O3 doping layer at CMSs surface after a simple sintering process. We discovered that La2O3-doped CMSs possess promising bifunctional electrocatalytical activities toward ORR and OER in alkaline solution, which is comparable to that of commercial Pt/C catalysts. Importantly, we highlight that the metal mass activity of the as-prepared catalysts surprisingly outperforms most of the reported bifunctional catalysts. Furthermore, we address that this reported synthetic strategy here is simple and reproducible, making it easy to understand the doping effects of La2O3. By adjusting the doping concentration of La2O3 on CMSs and comparing with La2O3 doped carbon nanotubes (CNTs), we illustrate the origination of prominent electrocatalytical activities of the novel catalyst and propose a reasonable mechanism for understanding the observed promising OER and ORR performance.
2. EXPERIMENTAL SECTION All reagents (purchased from Beijing Chemical Co., Ltd.) were analytical grade and used without further purification. Typically, the facile preparation of CMSs was as follows: 1.0 M glucose solution was prepared and stirred for 1 h. The solution was then hydrothermal treated at 190 °C for 5 h. After that, black products were obtained through centrifugation and washed with water and alcohol for several times. Finally, the as-
1/J = 1/JL + 1/JK = 1/(Bω1/2) + 1/JK
B = 0.62nFC0D0 2/3v−1/6 B
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Scheme 1. Schematic Illustration of the Synthetic Procedure Used to Obtain La2O3@CMSs
JK = nFkC0
Jmass = I /M
(1)
where I is the disk current we get directly from the RRDE equipment at the corresponding voltage for ORR and OER separately. M is the weight of the metal or metal oxide loading on surface of the electrode.
where J is the measured current density, JK and JL are the kinetic- and diffusion-limiting current densities, respectively, ω is the angular velocity, n is the transferred electron number, F is the Faraday constant, C0 is the bulk concentration of O2, v is the kinematic viscosity of the electrolyte, and k is the electrontransfer rate constant. Chronoamperometric responses (percentage of current retained versus operation time) of La2O3@ CMSs 16 wt % and Pt/C on glassy electrodes were measured at 0.60 V in O2-saturated 1.0 M KOH electrolytes, respectively. The peroxide yield (X, percentage of HO2− relative to total products) from the ring/disk currents (Ir/Id) can be calculated through the following equation: X = (200 × Ir /N )/(Id + Ir /N )
3. RESULTS AND DISCUSSION Scheme 1 shows the general process of synthesizing the La2O3@CMSs sample. It has been reported that a suitable hydrothermal treatment of glucose solution can generate sizecontrollable carbonaceous microspheres (CMSs) with welldefined surfaces, which possess plenty of functional groups such as −OH and −COOH. Earlier studies have indicated that such CMSs may offer a useful platform for coupling functional units toward different applications, including Fe, Co, Cu, Ni, and Zn.29 The key to this process is the use of carbonaceous particles with rich surface functional groups as scaffold for metal ions adsorption.31 We first carried out thermal gravimetric analysis (TGA) to calculate the weight ratio of the lanthanum species decorated on CMSs, as shown in Figure S1. It was found that the mass of the decorated lanthanum species gradually increased along with the concentration of initially used lanthanum solution, suggesting that lanthanum species were successfully introduced on the surface of CMSs in a controllable manner. We next investigated the morphology of the as-prepared La2O3@CMSs samples by using SEM and TEM. As shown in Figure 1a,c,e,g, La2O3@CMSs particles exhibits globular shape with a uniform size of approximately 5−6 μm. However, for a close comparison with undoped CMSs (as shown in Figure S2), La2O3@CMSs samples display much more rough surface, which should probably be due to the existence of small La2O3 particles produced during the sintering treatment. We next performed TEM to verify the existence of La2O3 particles at the surface of CMSs (Figure 1b,d,f,h). For the sample of La2O3@ CMSs 9 wt %, only sparse particles were observed, indicating a lower concentration of La2O3 doped at the surface. However, for the samples of La2O3@CMSs 11 wt % and La2O3@CMSs 16 wt %, more particles were clearly found along with the
(2)
where N is the current collection efficiency of RRDE, Id is the disk current, and Ir is the ring current. The OER activity measurements were carried out in a three-electrode configuration on the rotating disk electrode (RDE). All current versus potential curves were measured in a configuration with a sweep rate of 5 mV s−1 at 2000 rpm. Working electrodes were fabricated by depositing target materials on glassy carbon electrodes (PINE, 0.248 cm2 area). The surfaces of glass carbon (GC) electrodes were polished with Al2O3 suspension (5.0 and 0.25 μm, Allied High Tech Products, Inc.) before use. Sample powders (1.0 mg) were sonicated in ethanol (985 μL) and 5 wt % Nafion solution (15 μL) for more than half an hour. A total of 10 μL of drop of this solution was deposited into glassy carbon electrode and dried at room temperature, leading to a catalyst loading of 0.04 mg cm−2. The electrode was then dried at room temperature. The stability of the OER activity was tested by continuous cyclic voltammetry between 1.40 to 2.30 V at a sweep rate of 0.20 V s−1 and it was totally cycled 1500×. Potentials were calculated based on the formula (pH was equal to 14 in the whole paper): E RHE(V ) = EAg/AgCl + 0.197 + 0.0592 × pH
(4)
(3)
The mass activity is calculated in the following equation. C
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increase of the used lanthanum solution concentration. Particularly, a thin layer with a thickness of about 180 nm was formed at the surface of La2O3@CMSs 22 wt % sample. We noted that the TEM results exactly in agreement with the SEM images, from which an apparent increase of the roughness was observed along with the increase of La concentration. Meanwhile, we conducted Energy Dispersive Spectrometer (EDS) measurements to reveal the mapping of La elemental distribution at the sample surface (Figure S3b,d,e,h). It was found that the La element was evenly distributed at the surface, implying that La2O3 particles were well-dispersed at the surface. Apparently, the results demonstrate that La2O3 could be easily introduced to the surface of CMSs following the proposed strategy in this study. To clearly identify the La2O3 structure, the samples were examined by XRD, as shown in Figure 2a. For pure CMS samples, only a broad peak located at about 24° was observed, which could be assigned to carbon species. However, La2O3 with a rhombohedral structure (in P3̅m1 space group) were observed in the XRD patterns of La2O3@CMSs samples. Interestingly, as marked in the XRD patterns, small amount of La2O2CO3 also were observed in La2O3@CMSs samples. These two peaks can only match the peaks in 42.4° and 44.3° in La2O2CO3 [JCPDS No. 41−0672], while there are no other materials, which contain La, O, and C atoms, showing peaks at these positions. More importantly, as the dash line shows in Figure 2a, the peak at 44.3° is the third strongest peak in La2O2CO3. Besides, according to some previous work,30−33 we found that La2O2CO3 can exist at a high temperature. So we believe that the La2O2CO3 exists in our samples. Then, we conducted XPS to analyze the element composition of La2O3@
Figure 1. SEM images of (a) La2O3@CMSs 9 wt %, (c) La2O3@CMSs 11 wt %, (e) La2O3@CMSs 16 wt %, and (g) La2O3@CMSs 22 wt % with the same magnification; TEM images of (b) La2O3@CMSs 9 wt %, (d) La2O3@CMSs 11 wt %, (f) La2O3@CMSs 16 wt %, and (h) La2O3@CMSs 22 wt % with the same magnification.
Figure 2. (a) XRD patterns of CMSs, La2O3@CMSs 9 wt %, La2O3@CMSs 11 wt %, La2O3@CMSs 16 wt %, and La2O3@CMSs 22 wt % and the inset is XRD of the standard La2O3 (P3̅m1) crystal structure. The C (1s) XPS spectra (b) and O (1s) XPS spectra (c) peak in XPS spectra of La2O3@CMSs 9 wt %, La2O3@CMSs 11 wt %, La2O3@CMSs 16 wt %, and La2O3@CMSs 22 wt %. D
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Figure 3. (a) ORR polarization curves on La2O3@CMSs 9 wt %, La2O3@CMSs 11 wt %, La2O3@CMSs 16 wt %, La2O3@CMSs 22 wt %, CMSs, and La2O3 modified glassy carbon (GC) electrodes at 1600 rpm in O2-saturated 1.0 M KOH at a sweep rate of 5 mV s−1 (catalyst loading ∼ 0.04 mg cm−2 for all the samples). (b) Corresponding Koutecky−Levich plot (J−1 vs ω−1/2) at the different rotation rates indicated. (c) Chronoamperometric responses (percentage of current retained vs operation time) of La2O3@CMSs 16 wt % and Pt/C on glassy electrodes kept at 0.60 V vs RHE in O2saturated 1.0 M KOH electrolytes, respectively. La2O3@CMSs 16 wt % showed comparable superior stability to Pt/C in alkaline solutions and good ORR catalytic activity. (d) ORR mass activity for different materials.
of CMSs should act as a reacting linkage for catching La3+, which was beneficial for the formation of La2O2CO3. In order to determine the electrocatalytical activity, we first investigated the ORR performance of the as-prepared samples (as shown in Figure 3a). The same amounts of the samples were modified on a RDE and subjected to LSVs measurements. Obviously, both La2O3 and CMSs only showed very poor ORR activity in the testing solution. However, for comparison, La2O3@CMSs samples showed surprisingly promising ORR activity with extremely higher limiting current density in 1.0 M KOH. We also noted that, among the four La2O3@CMSs samples, La2O3@CMSs 16 wt % exhibited the lowest overpotential, which was about 0.40 V, and highest limiting current density, which was about 1.3 mA cm−2 (0.63 V vs RHE). The enhanced ORR activity for La2O3@CMSs is probably associated with the binding energy shift of La−O and C−O in La2O3@CMSs samples, which will be discussed later. In addition, as verified by SEM and TEM results, the doped lanthanum species on the surface of CMSs was increased along with the increase of lanthanum solution concentration, providing more active sites and thus accelerating the ORR. However, for La2O3@CMSs 22 wt %, the CMSs were found densely wrapped with a lanthanum species layer, as observed in TEM image, therefore blocking the effective three-phase interface for ORR and leading to the decrease of current density. Electron transfer number (n) is an important kinetic parameter to evaluate the ORR efficiency of the electrocatalysts. Next, we recorded and compared the rotating disk polarization curves of La2O3@CMSs samples and Pt/C at various rotation
CMSs samples, as well as to further identify the existence of La2O2CO3. It can be seen that the peak at 289.3 eV in C (1s) spectra (Figure 2b) was observed for all La2O3@CMS samples, which was assigned to CO32− of La2O2CO3 at the surface of the samples. Further evidence was obtained from the O (1s) spectra, as seen in Figure 2c, which suggested the appearance of La−O bonds in La2O3@CMS samples. It is necessary to understand why the La2O2CO3 was formed at the surface because that the electrochemical activity of the samples was strongly depended on this component, which will be demonstrated in the following section. To clarify this, we prepared CNTs/La2O3 samples and compared the composition and surface components. As shown in Figure S4a, no La2O2CO3 component was observed in CNTs/La2O3 samples. Moreover, as shown in Figure S4b, we compared the C (1s) spectra of CMSs, La2O3@CMSs 16 wt %, CNTs and CNTs/ La2O3 0.3. Clearly, it demonstrated that a large amount of −COOH existed at the surface of CMSs, which were then substituted by CO32−, as shown in the C (1s) spectra of La2O3@CMSs. In comparison, no characteristic peaks for −COOH were found for pure CNT samples. Consequently, we did not observe any CO32− characters in the CNTs/La2O3 0.3 sample. Also, it could be confirmed by the O (1s) spectra of CNTs/La2O3 0.3, as shown in Figure S4c, which also implied that no CO32− was in existence at the surface of CNTs. Therefore, it seems that the presence of −COOH at the surface of CMSs is a prerequisite for the generation of the La2O2CO3 component. We deduce that the −COOH group at the surface E
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Figure 4. (a) Oxygen evolution current density of La2O3@CMSs 9 wt %, La2O3@CMSs 11 wt %, La2O3@CMSs 16 wt %, La2O3@CMSs 22 wt %, La2O3 and CMSs at 2000 rpm 1.0 M KOH at 5 mV s−1. (b) Tafel plots of OER currents in (a). (c) Percentage of oxygen evolution currents retained vs circle number of La2O3@CMSs 16 wt % and Pt/C on glassy electrodes kept at 2.0 V vs RHE in 1.0 M KOH electrolytes, respectively. (d) OER mass activity for different materials.
Figure 5. (a) Oxygen electrode activities within the ORR and OER potential window of La2O3@CMSs 9 wt %, La2O3@CMSs 11 wt %, La2O3@ CMSs 16 wt %, and La2O3@CMSs 22 wt % showed excellent catalytic activities for both ORR and OER. (b) Schematic of the reactions taken place on the electrodes.
electrocatalyst, as presented in Figure 3c. After 6000 s, a significant 70% loss of the initial current value was observed on Pt/C electrocatalyst. However, merely 17% loss of the initial value was occurred on La2O3@CMSs 16 wt % sample. The results strongly demonstrated that La2O3@CMSs 16 wt % has better stability than Pt/C electrocatalyst in alkaline electrolyte. Particularly, we note that the usage of metal in the electrocatalyst may raise possible environmental risk and improve the cost of the material. Thus, it is desirable to cut off the usage of the metal in electrocatalyst, while maintaining its high activity. To this end, we calculated the metal mass activity of La2O3@CMSs 16 wt % and compared it with some other commonly used metal-containing electrocatalysts,8,14,17 as shown in Figure 3d. We found that the metal mass activity of
rates, as shown in Figure S5a−e. To calculate the electron transfer number (n), Figure 3b shows the Koutecky−Levich plots of J−1 versus ω−1/2 of different electrocatalysts. By calculating the slop of Koutecky−Levich plots, the electron transfer number (n) of La2O3@CMSs 16 wt % was determined to be 3.7, suggesting a nearly 4e− ORR process. Specifically, we measured HO2− yield for La2O3@CMSs 16 wt %, as shown in Figure S5f, which was lowest at the whole potential range among all the examined samples. Again, the results indicate that La2O3@CMSs 16 wt % exhibits the best ORR performance among all the tested samples. Durability of the electrocatalysts is another important parameter in judging their practical applications. We next compared the durability of La2O3@CMSs 16 wt % and Pt/C F
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La2O3@CMSs 16 wt % is 78.4 A g−1, which is comparable to that of Pt/C and significantly higher than other cobalt-based electrocatalysts. Apart from the promising ORR performance, an ideal bifunctional catalyst should possess good OER performance. Thus, we further investigated the OER performance of La2O3@ CMSs in 1.0 M KOH at 2000 rpm (shown in Figure 4a). Again, among all the samples, La2O3@CMSs 16 wt % exhibited the best OER performance with a high current density of 33 mA cm−2 at 2.20 V and an onset potential of 370 mV. Moreover, Tafel curves of all La2O3@CMSs samples were recorded and shown in Figure 4b. As seen, the Tafel slopes ranged from 657 to 276 mV dec−1. It clearly suggested that La2O3@CMSs 16 wt %, which possessed the fastest OER kinetics. Moreover, La2O3@CMSs 16 wt % was modified on glassy carbon electrode to investigate the stability in a continuously testing condition. The results demonstrated that La2O3@CMSs 16 wt % can retain 98% of its initial current after 1500 cycles, while merely 82% of the initial value was maintained for Pt/C, revealing better durability of La2O3@CMSs 16 wt %. In addition, we calculated and compared the metal mass activity of different electrocatalysts,9,14,18 which is shown in Figure 4d. We noted that the metal mass activity of La2O3@CMSs 16 wt % is 831.5 A g−1, which is significantly higher the ever reported metal-containing OER electrocatalysts. Figure 5a shows the potential windows for ORR and OER at the La2O3@CMSs samples in alkaline solution. The potential windows for La2O3@CMSs 9 wt %, La2O3@CMSs 11 wt %, La2O3@CMSs 16 wt %, and La2O3@CMSs 22 wt % were 1.21, 1.19, 0.80, and 1.20 V, respectively. Among them, La2O3@ CMSs 16 wt % has smallest potential window (0.80 V), indicating a fast reaction kinetic and low overpotential. In other words, a high efficiency and low energy cost should be obtained by using La2O3@CMSs 16 wt % as a high performance bifunctional electrocatalyst. It is scientifically meaningful to discover the origination of the improved bifunctional activity for the La2O3@CMSs sample. We have assumed that formed C−O and La−O bonds at the surface should favors the OER and ORR process. To prove this, we carried out the control experiments by measuring the electrocatalytic activity for the ORR and OER at CNTs/La2O3 samples on the RRDE in the same conditions (shown in Figure S6). CNTs/La2O3 samples were fabricated using the very same way as the procedure of La2O3@CMSs samples. It was reported12 that CNTs can absorb metal ions easily due to the large amount of active −OH groups on the surface. But the current densities for both ORR and OER decreased when La2O3 was grown on the surface of the CNTs, as shown in Figure S6. To discover the reason for the big difference between the La2O3@CMSs and CNTs/ La2O3 samples, XRD and XPS were processed. As shown in Figure S4a, there are three important messages we can get from them. First, in all four CNTs/La2O3 samples, we cannot find the trace of the new products La2O2CO3 existing in the material (Figure S4a). And then, comparing the surface of CMSs with the surface of CNTs, there is a large amount of −OH and −COOH in CMSs while there is only a group of −OH (Figure S4b). What is the most important, the binding energy of C−O in La2O3@CMSs 16 wt % shifts from 285.4 to 285.9 eV, which means the binding energy increases 0.5 eV, while there is no movement for binding energy in CNTs/La2O3 sample (Figure S4b). In O (1s) spectra, C−O binding energy in La2O3@CMSs 16 wt % is about 0.6 eV larger than in CNTs/La2O3 0.3 (Figure S4c). And La−O binding
energy in La2O3@CMSs 16 wt % is 531.7 eV, while it is 532.0 eV in CNTs/La2O3 0.3, which means the binding energy shifts 0.3 eV to high energy zone34−39 (Figure S4c). Based on all the XRD and XPS analyses, doping La on CMSs forms La2O2CO3 in the assistance of rich −COOH group on the surface of the CMSs. Besides, owing to the existence of the new product, binding energy of C−O and La−O shift higher in La2O3@ CMSs 16 wt %, resulting in the enhanced catalytic performance for ORR and OER. Figure 5b shows the carton picture illustrating the binding effect of La2O3 on the surface of CMSs. As shown in Figures 3a and 4a, the La2O3 and CMSs alone hardly generate ORR or OER, respectively. But when La atoms doped at the surface of the CMSs, the catalytic activity for oxygen reactions is largely enhanced. The reason for the significant change is discussed as followed. With the formation of La2O3 on the surface of the CMSs, partial La atoms connect to the CMSs, convinced by La2O2CO3 (Figure 2a). Owing to the doping effect on the CMSs, the binding energy of C−O and La−O are shifted higher. Therefore, the area around La2O3 where there are both La−O and C−O bonds proposes to be the catalytic active zone, where ORR and OER happened. And during the reaction process, O2 reacted to H2O2 and H2O because of 3.7 e− transferred during ORR process and H2O is converted to O2 during the OER process. Therefore, it is reasonable to assume that the formed unique bonds of La−O and C−O contribute to improving the electrocatalytical activities of the resulted hybrid material. More efforts of computational calculations are undertaking to get further insight into the exact reaction pathway.
4. CONCLUSION In summary, we have prepared La2O3@CMSs as a novel and efficient bifunctional electrocatalyst for ORR and OER. Although La2O3 or CMS alone has poor electrcatalytic activity toward ORR and OER, the as-prepared La2O3@ CMSs samples exhibits unexpected, surprisingly high ORR and OER activities in alkaline solutions. Specifically, the performance of the La2O3@CMSs 16 wt % is comparable to fresh commercial Pt/ C (5 wt %) electrocatalyst but far exceeding Pt/C in stability and durability. More importantly, we found that metal mass activity of this novel electrocatalyst is prominently higher than that of some other commonly used bifunctional electrocatalysts. Moreover, we explored the possible origination of the electrocatalytical activity of the La2O3@CMSs, which is assumed to stem from the formation of C−O and La−O bonds and their shift to higher binding energy, as a part of La2O2CO3 compounds easily produced at the surface of CMSs.
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ASSOCIATED CONTENT
* Supporting Information S
Thermal gravimetric analysis (TGA) of the samples; SEM image of CMSs; EDS results of the samples; The C (1s) and O (1s) XPS spectrum of samples; XRD of CNTs/La2O3 samples; LSVs of La2O3@CMSs and Pt/C samples; ORR and OER comparisons of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +86-10-68918696. Tel.: +86-10-68918696. G
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Ministry of Science and Technology (2012DFR40240), National Natural Science Foundation of China (Grant Nos. 21175012), and the Chinese Ministry of Education (Project of New Century Excellent Talents in University) is gratefully acknowledged.
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