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Insights into the Catalytic Activity of Barium Carbonate for Oxygen Reduction Reaction Xuecheng Cao, Tao Hong, Ruizhi Yang, Jinghua Tian, Chang-Rong Xia, Jin-Chao Dong, and Jianfeng Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08267 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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The Journal of Physical Chemistry

Insights into the Catalytic Activity of Barium Carbonate for Oxygen Reduction Reaction Xuecheng Cao†,∆, Tao Hongǁ, Ruizhi Yang†,∆,*, Jing-Hua Tian†,* Changrong Xiaǁ,*, Jin-Chao Dong┴ and Jian-Feng Li┴ †

College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Institute of Chemical Power Sources, Soochow University, Suzhou, Jiangsu 215006, China. ∆

ǁ

Institute of Chemical Power Sources, Soochow University, Suzhou, Jiangsu 215006, China.

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering& Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei, Anhui 230026, China.

┴

MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.

ACS Paragon Plus Environment

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ABSTRACT: Efficient electrocatalyst for oxygen reduction reaction (ORR) is crucial for the performance improvement of fuel cells and metal-air batteries. However, catalyst with high activity, easy fabrication process and low cost is still a daunting challenge. In this work, low cost BaCO3 nanorods have been demonstrated as efficient electrocatalysts toward the ORR in alkaline media for the first time. The activity of BaCO3 nanorods can be further enhanced by hybridizing with reduced graphene oxide (BaCO3/rGO). The mechanism of ORR on the surface of BaCO3 catalyst was investigated via in-situ electrochemical Raman spectroscopy (in-situ EC-Raman). Our findings suggest that the barium ions on the surface of catalyst play a key role in the adsorption of oxygen molecules and the formation/decomposition of intermediates. This work provides an important insight into the catalytic activity of BaCO3 for ORR, which can serve as a guide for the design of alkali-earth metal carbonate-based catalyst.


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Introduction Electrochemical reduction of oxygen gas is one of the most important reactions in fuel cells and metal-air batteries.1-3 Commonly, catalysts are used to accelerate the sluggish kinetics of oxygen reduction reaction (ORR) to improve the efficiency of fuel cells and metal-air batteries.2,4-7 Platinum and its alloy have been known to have the best performance,8-10 however, their high price and poor stability urge the search for alternative catalysts with comparable activity, higher durability and lower cost. Spinel based mixed valence transition metal (TM) oxides, such as Co3O4,11,12 MnCo2O4,13,14 CoFe2O415 and their hybrid with carbon,12,13,16 exhibit high ORR activity in alkaline electrolyte. The TM2+/TM3+redox couples in spinel facilitate the ORR and promote the reaction via a 4e- process.17 Perovskite based oxides also show promising catalytic performance for ORR, transition metal ions on B site (body center position in unit cell) of which have been reported to play an important role during the ORR process.18,19 The σ* orbital of transition metal and metal-oxygen covalency on the competition between O22-/OHdisplacement and OH- regeneration on the surface of perovskite have proven to have significant influences for the ORR.18 In recent years, carbon based materials doped with heteroatom show improved performance for electroreduction of oxygen.4,20,21 Heteroatom, such as N,22,23 P,24 B25,26 and S27,28, have been successfully doped into the carbon matrix and modify the charge density and spin density of carbon, thus providing substantial active sites for ORR. Although significant progress has been made in searching and synthesizing non-precious electrocatalysts for ORR, it is still an interesting and challenging issue to find an efficient electrocatalyst with low-cost and easy scale-up fabrication process to substitute commercial Pt/C catalyst. BaCO3 is a very cheap material, which is widely used in industry and manufacturing. Very recently, the application of BaCO3 nanoparticles as catalyst for high temperature oxygen


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reduction in solid oxide fuel cell (SOFC) was reported.29,30 However, the catalytic activity of BaCO3 toward ORR in alkaline electrolyte at room temperature, which is widely used in fuel cells and metal-air batteries, has been barely studied. It’s known that the oxygen reduction in SOFC occurs at the solid-gas interface at high temperature (for example 800˚C). This is totally different from the ORR in alkaline electrolyte, which involves the reaction at the solid-liquid-gas interface at room temperature. Moreover, the mechanistic origin of the catalytic activity of BaCO3 for ORR is still not understood. Herein, we studied the ORR activity of BaCO3 in alkaline electrolyte at room temperature and elucidated the mechanistic origin of the catalytic activity of BaCO3 by using in-situ electrochemical Raman spectroscopy (EC-Raman) for the first time. Our findings demonstrate that BaCO3 is an effective electrocatalyst for the ORR in alkaline electrolyte. The in-situ ECRaman results reveal that the barium ions on the surface of BaCO3 play a critical role for the adsorption of oxygen molecule and formation/decomposition of intermediate. The mechanism of ORR on BaCO3 has been proposed based on the in-situ EC-Raman results.

Experimental Synthesis of BaCO3 nanorods: The BaCO3 nanorods were synthesized by a facile precipitation method. In a typical synthesis, 0.2 g of BaCl2 was dissolved in a mixed solution that containing 10 mL of ethanol and 20 mL of deionized water. Meanwhile, 0.12 g of Na2CO3 was also dissolved in a mixed solution that containing 10 mL of ethanol and 20 mL of deionized water. Afterwards, the solution of Na2CO3 was slowly added to the solution of BaCl2under magnetic stirring for 20 min. The white precipitate was collected after washing several times with water and ethanol and dried at 80 ºC.


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Synthesis of BaCO3/rGO hybrid: Graphene oxide (GO) was prepared by a modified Hummers method.31 The as-prepared GO was dispersed in a mixed solution of ethanol and water with a volume ratio of Vethanol:Vwater = 1:2 to get the finial concentration of 0.16 mgmL-1. Then 0.1 g of BaCl2 and 0.04 g of NaHCO3 were added to 50 mL of GO suspension. After magnetic stirring for about 30 min, the mixture was transferred to an autoclave for hydrothermal reaction at 120 ˚C for 3 h and then cooled to room temperature. The product was collected by centrifugation and washed with ethanol and water. The resultant BaCO3/reduced GO hybrid is denoted as BaCO3/rGO. Physical Characterization: Powder X-ray diffraction (XRD) patterns of the samples were collected on a Bede D1 X-ray diffractometer (UK, Bede Scientific Ltd.) with Cu Kα radiation (λ = 0.15418 nm), which was operated at 40 kV and 45 mA. The morphology and microstructure of the samples were carried out on scanning electron microscopy (SEM, Hitachi S4700) operated at an accelerating voltage of 10 kV and on transmission electron microscopes (TEM, FEI TecnaiG20) operated at 200 kV. The element binding environment of the sample was studied with an X-ray photoelectron spectroscopy (XPS, VG ESCALAB MKII) equipped with a monochromatized Al Kα source (1486.6 eV). The measurement was carried out at a pass energy of 20 eV and an energy increment of 0.1 eV. The spectra were corrected for the background using the Shirley approach. Fourier transform infrared (FTIR) spectra of the samples were performed on a TENSOR 27 spectrometer (BRUKER OPTICS) in the 4000–400 cm−1 spectral range at a resolution of 4 cm−1. Pressed KBr pellets with a sample/KBr weight ratio of 1:200 were scanned. Electrochemical Measurements: The electrochemical measurements were performed by a rotating ring-disk electrode (RRDE) technique. The RRDE electrode consisted of a catalyst-


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coated glassy carbon (GC) disk (0.196 cm2 area) surrounded by a Pt ring (0.125 cm2area). A standard three-electrode electrochemical cell was used for electrochemical measurements at room temperature. A Pt wire was used as the counter electrode. The Pt counter electrode (Pine Research Instrumentation) was a coiled platinum wire (99.99% pure) mounted at the end of a chemically-resistant epoxy rod. A fritted glass tube was used to isolate the counter electrode from the main test solution, which minimizes the risk for Pt contamination on the working electrode. An Ag/AgCl (1 M Cl−, 0.20 V vs. NHE) reference electrode was used in a doublejunction reference chamber. All potential values in the text were converted to potential versus RHE according to ERHE=EAg/AgCl+E0 Ag/AgCl+0.059 pH. The electrolyte was a 0.1 M KOH solution prepared with ultrapure water (Millipore, 18.2 MΩ cm). For the preparation of catalysts ink, 5 mg of catalyst and 5 mg of acetylene black (AB) were mixed homogeneously using a mortar and a pestle. Then 95 µL of Nafion solution (5 wt.%) and 350 µL of ethanol was added to the above mixture followed by ultrasonic dispersion for at least 30 min. Accurate 7 µL of ink was dropped onto the GC disk by a pipette and dried at the room temperature. For AB electrode, only 5 mg AB was used for the preparation of catalyst ink. The catalyst mass loading was 0.4 mg cm-2. Cyclic voltammetry (CV) was carried out in N2- or O2-saturated 0.1 M KOH by sweeping the potential between 0.05and1.05 V (vs. RHE) at a scanning rate of 50 mVs-1. Linear sweep voltammograms (LSVs) were performed in O2-saturated 0.1 M KOH by sweeping the potential between 0.05and0.95 V (vs. RHE) at 10 mVs-1 with the electrode rotated from 400 to 2500 rpm. The ring potential was held at 1.5 V (vs. RHE), which is high enough to oxidize any HO2produced during the tests.14 The electron transfer number n was calculated using the following equation: n=

4 jD j D + ( jR / N )

(1)


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where jD and jR are the Faradaic currents at the disk and the ring, respectively; N = 0.22 is the collection efficiency. The diffusion-current-corrected Tafel plots of ORR on all samples have been calculated and compared, and the kinetic current was derived from the Equation (2): 1/J=1/JL+1/JD

(2)

in which J corresponds to the measured disk current density, JK and JD are the kinetic and diffusion-limiting current densities, respectively. In-situ electrochemical Raman Spectroscopy Measurements: In-situ electrochemical Raman Spectroscopy (EC-Raman) was used to investigate the species produced on the surface of catalysts during the oxygen reduction process. Raman spectra were obtained on a LabRam HR800 and XploRA confocal microprobe Raman system (HORIBA JobinYvon). A 50× magnification long working distance (8 mm) objective was used. The excitation wavelength was 633 nm from a He-Ne laser for the measurement in 0.1 M KOH electrolyte. Raman frequencies were calibrated using Si wafer and KOH solution spectra. Raman spectra were baseline corrected using the NGS LabSpec software. A custom-made spectro-electrochemical cell with a Pt wire counter electrode and an Ag/AgCl (1 M Cl−, 0.20 V vs. NHE) reference electrode was used for the measurements. All electrolytes were saturated with pure oxygen gas before EC-Raman measurements.

Results and discussion The X-ray diffraction (XRD) patterns of as-synthesized BaCO3 and BaCO3/rGO hybrid are shown in Figure 1a. It can be seen that both the samples have orthorhombic structure (Pmcn 62) with lattice parameters of a = 5.313 Å, b = 8.896 Å and c = 6.428 Å and no impurity phases are found. A typical scanning electron microscopy (SEM) image of BaCO3 clearly shows the


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formation of uniform nanorods with a diameter of about 200 nm (Figure 1b). As can be seen from the SEM image of the BaCO3/rGO hybrid (Figure 1c), the nanorods are supported with thin layers of graphene. The cross-linked graphene provides a conductive network for BaCO3 nanorods. The high-resolution transmission electron microscopy (HRTEM) image of an individual BaCO3/rGO hybrid shows clear lattice fringes with an interplanar distance of 0.322 and 0.279 nm, corresponding to (002) and (102) planes of BaCO3, respectively (Figure 1d).

Figure 1. (a) XRD patterns of BaCO3 nanorods and BaCO3/rGO hybrid. (b) SEM image of BaCO3 nanorods. (c) SEM image of BaCO3/rGO hybrid. (d) HRTEM image of BaCO3/rGO hybrid.

The electrocatalytic performance of BaCO3 and BaCO3/rGO hybrid was first evaluated with a cyclic voltammetry (CV) in N2- and O2-saturated 0.1M KOH electrolyte (Figure 2a). In the pure BaCO3 case, BaCO3 was mixed with acetylene black (AB) due to the poor electronic conductivity of BaCO3. Accordingly, the performance of free AB was also included for


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comparison. The ORR peak potential of BaCO3+AB is 0.63 V (vs. RHE), which is about 110 mV more positive than that of the pristine AB (0.52 V). This suggests that BaCO3 demonstrates electrocatalytic activity for ORR and AB mostly serves as conductive additive. The BaCO3/rGO hybrid exhibits a further enhanced catalytic activity for ORR with a more positive peak potential (0.65 V), which is 20 mV higher than that of BaCO3+AB. The ORR activity of these samples was further studied by using RRDE technique (Figure 2b).

(b)

(a) BaCO3+AB

BaCO3/rGO -2

2 mAcm 0.0

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0.4 0.6 0.8 1.0 Potential (V vs. RHE)

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65 mVdec 73 mVdec

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1.0

-1

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0.7 80 mVdec 0.6

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-1

AB BaCO3+AB BaCO3/rGO 0.1

1 -2 Current Density (mAcm )

Figure 2. (a) CV curves of AB, BaCO3+AB and BaCO3/rGO hybrid in N2-(dash lines) and O2 (solid lines)saturated 0.1M KOH aqueous solution at a scan rate of 50 mVs-1. (b) Linear sweeping voltammograms (LSVs) on disk electrode for AB, BaCO3+AB and BaCO3/rGO hybrid in O2-saturated 0.1M KOH with a scan rate of 10 mVs-1 at 1600 rpm. (c) Calculated electron number n for catalysts. (d) Tafel plots of catalysts towards ORR.

At a rotating rate of 1600 rpm, the potential at a measured current density of 1.5 mA cm-2 increases from 0.56 V on AB to 0.68 V on BaCO3+AB and further to 0.72 V on BaCO3/rGO,


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which indicates clearly the high activity of BaCO3+AB and BaCO3/rGO. Note that the ORR activities of these samples were also measured with glassy carbon (GC, diameter: 5 mm, length: 80 mm) counter electrode. As can be seen in Figure S1, the ORR activities of the samples measured with GC counter electrode and Pt counter electrode are consistent with each other. The number of transferred electron, n, is calculated from the disk and ring currents (Figure S2-S4) by using equation (1) (Figure 2c). The electron transfer number of ORR on BaCO3+AB is 2.6 – 3.3 in the potential range of 0 – 0.67 V, indicating a mixed two and four electron pathway during ORR. While the electron transfer number of ORR on BaCO3/rGO increases to 3.2–3.7 in the same potential range. This indicates the critical role of graphene network in the kinetics enhancement of ORR on BaCO3/rGO. The diffusion-current-corrected Tafel plots of specific ORR activities of the catalysts are shown in Figure 2d. In the Tafel plots, the kinetic currents were derived from mass transport correction by using equation (2). The Tafel slopes of AB, BaCO3+AB and BaCO3/rGO are 80, 73 and 65 mV dec-1, respectively. The smaller Tafel slope of BaCO3+AB and BaCO3/rGO reveal the intrinsic high catalytic activity of BaCO3 for ORR. The Tafel slope of BaCO3/rGO is even lower than the value of commercial Pt/C (20 wt.%) reported elsewhere,32 which indicates its excellent intrinsic catalytic activity. To get insight into the catalytic activity of BaCO3/rGO, the X-ray photoelectron spectroscopy (XPS) were performed on BaCO3 and BaCO3/rGO hybrid. As shown in Figure 3a, a positive shift of 0.76 and 0.87 eV exist in Ba 3d5/2 and Ba 3d3/2 (Figure 3a) for BaCO3/rGO, respectively, as compared with BaCO3, which indicates the existence of chemical coupling between BaCO3 and rGO.33 The spectra of C 1s in BaCO3 can be fitted to three peaks (Figure 3b), which can be assigned to C=C-C (284.6 eV), C-O (286.2 eV) and lattice C-O binding of CO32- in BaCO3


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(289.0 eV). When BaCO3 is combined with rGO, a positive shift of 0.7 eV is shown for lattice CO binding. The O 1s peak is fitted to lattice Ba-O binding (532.4 eV) and C-O binding (530.9 eV) in BaCO3 (Figure 3c). Similarly, a positive shift of 0.8 eV can be observed for lattice C-O binding in BaCO3/rGO as compared with that in BaCO3. The positive shifts of C-O binding in both C 1s and O 1s further confirm the coupling between BaCO3 and rGO in BaCO3/rGO hybrid.

Intensity (a.u.)

(a)

B a 3d

+0.87 eV

+0.76 eV B aC O 3 /rGO

B aC O 3 805

800

Intensity (a.u.)

795

790

785

780

775

B inding Energy (eV)

(b)

C 1s

BaCO 3 /rG O

BaCO 3 294

292

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286

284

282

B inding Energy (eV )

(c)

C-O

O 1s

lattice Ba-O

Intensity (a.u.)

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BaCO 3 /rG O C-O

lattice Ba-O BaCO 3 536

534

532

530

528

Binding Energy (eV )

Figure 3. XPS spectra for Ba 3d (a), C 1s (b) and O 1s (c) in BaCO3 and BaCO3/rGO hybrid.


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The results reveal the important role of both Ba2+ and CO32- in the coupling between BaCO3 and rGO. The covalent binding between BaCO3 and rGO provides a pathway for a fast charge transfer through the interface and thus promotes the electrocatalytic activity of BaCO3/rGO toward ORR. This can also be verified by the higher activity of BaCO3/rGO (Figure S4) than that of BaCO3+rGO (Figure S5) and rGO (Figure S6). Besides the high electrocatalytic activity, the BaCO3+AB and BaCO3/rGO exhibit excellent stability as measured by chronoamperometic measurements (Figure S7). After 80,000 s continuous operation, the relative ORR currents of BaCO3+AB and BaCO3/rGO maintain 90% and 80%, respectively; while the commercial Pt/C demonstrates a large current decrease of about 60%. The large activity loss of Pt/C catalyst mainly results from the agglomeration of platinum nanoparticles and the corrosion of carbon.8-10 In electrochemical impedance spectroscopy (EIS) measurements (Figure S8), the BaCO3/rGO catalyst shows a smaller semicircle than rGO, BaCO3+AB and BaCO3+rGO. This reveals a much smaller charge transfer resistance for ORR on BaCO3/rGO. The improved charge transfer at the interface is facilitated by the covalent binding between BaCO3 and rGO. This could be the major reason for the enhanced ORR activity of BaCO3/rGO hybrid as compared to BaCO3+AB catalyst. To understand the ORR mechanism on the surface of BaCO3, in-situ electrochemical Raman spectroscopy (EC-Raman) measurements were carried out to monitor the ORR process by holding the working electrode at a series of different potentials. Firstly, we studied the ORR process on the pristine acetylene black (AB) electrode with EC-Raman. Figure 4a and Figure 4b show the in-situ EC-Raman data collected on AB electrode at various potentials during cathodic and anodic scanning, respectively. In the negative ORR potential range, the peak observed at the 1150 cm-1 is assigned to the O-O stretching mode of superoxide, ν(O2-). This clearly indicates


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(a)

(b)

-1

-1

1150cm

(c)

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1510cm

@0.55 V anodic scanning

-1

1150cm

0.25 V

adsorbed O2

Intensity (a.u.)

0.1 V

1510cm

300

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600 900 1200 -1 Raman Shift (cm )

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2-

CO3

1800

adsorbed O2 -

O2

Ba-O -

Ba-(O2 )

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@0.55 V anodic scanning adsorbed O2 -

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2-

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600 900 1200 -1 Raman Shift (cm )

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Figure 4. In-situ EC-Raman spectra of AB electrode from 1 V to 0.1 V at an interval of 0.15 V during a cathodic scan (a), from 0.25 V to 1 V at an interval of 0.15 V during an anodic scan (b). (c) One spectra of AB electrode at 0.55 V during the anodic scan. In-situ EC-Raman spectra of BaCO3+AB electrode from 1 V to 0.1 V at an interval of 0.15 V during a cathodic scan (d), from 0.25 V to 1 V at an interval of 0.15 V during an anodic scan (e). (f) One spectra of BaCO3+AB electrode at 0.55 V during the anodic scan. In-situ EC-Raman spectra of BaCO3/rGO electrode from 1 V to 0.1 V at an interval of 0.15 V during a cathodic scan (g), from 0.25 V to 1 V at an interval of 0.15 V during an anodic scan (h). (i) One spectra of BaCO3/rGO electrode at 0.55 V during the anodic scan. The specific potential for each curve is 1, 0.85, 0.7, 0.55, 0.4, 0.25, 0.1 V during cathodic scanning and 0.25, 0.4, 0.55, 0.7, 0.85, 1 V during anodic scanning.


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that superoxide is one of the main intermediates during electroreduction of oxygen on the acetylene black surface in basic media, which is similar to that observed on Au electrode.34,37 The presence of superoxide also suggests that the first step in ORR on pristine acetylene black is the single electron transfer from O2 to O2-. The peak observed at 1510 cm-1 may originate from two different modes. One is O-O stretch mode of oxygen gas molecule adsorbed on surface.34 The other possible mode is attributed to the strong interaction between O2- and carbon surface. The graphitic ring stretching may be distorted by the surface coupling with O2-–like component.35 The peaks at 1150 cm-1 and 1510 cm-1 on AB electrode can be seen more clearly in a magnified spectrum at a specific potential of 0.55 V (Figure 4c). Note that the two strong peaks at 1329 cm1

and 1589 cm-1 are the well-known D-band and G-band of carbon, respectively. For BaCO3+AB electrode (Figure 4d and Figure 4e) and BaCO3/rGO hybrid electrode

(Figure 4g and Figure 4h), the peak at 1058 cm-1 is due to the asymmetric stretching vibration of CO32-, while the Ba-O lattice mode is at about 150 cm-1.38 During the oxygen reduction process, a broad hump develops at about 550 cm-1 in the potential between 0.55 and 0.1 V, which comes from the formation of Ba-(O2-) on the surface of BaCO3.34 This intermediate comes from the first one electron reduction of Ba-(O2) species and will be further reduced to OH- or HO2-. The formation of Ba-(O2-) indicates that the barium ions on the surface act as active sites for the adsorption of oxygen. The magnified spectrum at 0.55 V during the anodic scanning (Figure 4f and Figure 4i) shows O2- species (1150 cm-1) and O-O stretching mode of surface adsorbed oxygen (also may come from the distortion of graphitic ring stretching mode) (1510 cm-1), and a broad peak at about 760 cm-1, which may come from the Ba-OH bending mode.34,36 Figure 5a and Figure 5b show the normalized in-situ EC-Raman intensities of the Ba-O lattice mode and the asymmetric stretching vibration of CO32- for BaCO3/rGO electrode at different


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potentials. The CV of oxygen reduction on BaCO3/rGO is also included. As can be seen, when oxygen reduction starts at about 0.75 V (vs. RHE), the peak intensities of both Ba-O lattice mode(Fig.5a) and CO32- vibration mode (Fig.5b) increase. This is due to the formation of Ba-(O2) on surface during ORR can affect the Ba-O lattice mode and CO32- vibration mode. The results confirm the crucial role of barium ions on the catalyst surface for the adsorption of oxygen molecule

and

the

formation/decomposition

0.0

0.0

-1.0 -1.5 -2.0 0.2


1.0

-2

-2

-2.5 0.0

of

intermediates.

Relative intensity (a.u.)

-0.5

Current density (mAcm )

(b) 0.5

Current density (mAcm )

(a) 0.5 Relative intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.5 -1.0 -1.5 -2.0 -2.5 0.0

0.2


1.0

Figure 5. Normalized in-situ EC-Raman intensities of the Ba-O lattice mode (a) and the asymmetric stretching vibrations of CO32- (b) for BaCO3/rGO at different potentials. The CV of oxygen reduction on BaCO3/rGO is included.

Based on the electrochemical results and in-situ EC-Raman measurements, two reaction mechanisms for ORR on BaCO3 catalyst are proposed, as shown in Figure 6a and Figure 6b, which are two and four electron transfer processes respectively. During the two electron transfer process (Figure 6a), oxygen gas is firstly adsorbed on the barium ions of catalysts to from Ba(O2) species (step 1) and transform to Ba-(O2-) (step 2) by the reduction via one electron. Then water molecule is introduced and reduced to OH- and HO2- species (step3 and step4). During the four electron transfer process (Figure 6b), oxygen gas may bridge-like adsorbed on two adjacent barium ions and O=O bond is broken to form Ba-(O-) species (step 1 and step 2). Then Ba-(O-)


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can adsorb another H2O molecular and get reduced to form OH- (step 3 and step 4). Finally, the bare barium ions are regenerated for the next adsorption of oxygen molecule. The electrochemical results suggest that the ORR on BaCO3 contains both two and four electron transfer processes, so the above proposed mechanisms can both occur during the electroreduction of oxygen. (a) e-

O--Ba--O

ΔE

ΔE

O2

(1)

(2)

total reaction : O2 + H2O + 2e- = HO2- + OH-

O--Ba--O

O--Ba--O

H2O (4)

HO2-

ΔE

(3)

· e-

ΔE

O--Ba--O OHO=O

(b)

Ba

ΔE O

O2

2e-

Ba O

O

(1)

ΔE (2)

OBa O

2OH-

Ba O

O

total reaction : O2 + 2H2O + 4e- = 4OH-

· H O

ΔE

Ba O

(4)

2e-

Ba O

O

2H2O

(3)

ΔE

O

Ba O

H·

O-

Ba O

O

2OH-

Figure 6. Proposed reaction mechanism of ORR on the surface of BaCO3. (a) Two electron transfer process; (b) Four electron transfer process.

Conclusions


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In summary, the potential application of BaCO3 as a non-precious catalyst for ORR in alkaline electrolyte has been demonstrated. The activity of BaCO3 can be further enhanced by hybridizing with rGO due to the improved charge transfer at the interface, which is facilitated by covalent coupling between the BaCO3 and rGO. In-situ EC-Raman results reveal the crucial role of surface barium ions for the adsorption of oxygen molecule and the formation/decomposition of intermediates during ORR process. Insights into the catalytic activity of BaCO3 toward ORR are provided. This work may open the way to efficient non-precious electrocatalysts for ORR based on low-cost alkali-earth metal carbonate.

ASSOCIATED CONTENT Supporting Information available: Disk and ring current densities of ORR on AB, BaCO3+AB, BaCO3/rGO, rGO and BaCO3+rGO. I-t chronoamperometic responses for the ORR on BaCO3+AB, rGO, BaCO3+rGO, BaCO3/rGO and commercial Pt/C (20 wt.%). EIS of BaCO3+AB, rGO, BaCO3+rGO, BaCO3/rGO.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (R. Yang); [email protected] (J.-H. Tian); [email protected] (C. Xia). Tel: +86 512 65221519. Notes


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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by National Natural Science Foundation of China (Nos. 51272167, 51572181, 21206101 and 21303114), Natural Science Foundation of Jiangsu Province, China (BK20151226).


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Table of Contents Graphic:

2-

(Ba-O2 ) (CO3 -1) (Ba-O) 550cm-1 1058cm -1 150cm

AB

0.1 V

BaCO3+AB

BaCO3/rGO -2

2 mAcm 0.2 0.4 0.6 0.8 1.0 Potential (V vs. Ag/AgCl)

1.2 300

1V 600 900 1200 1500 1800 -1 Raman Shift (cm )


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Insights into the Catalytic Activity of Barium ... - ACS Publications

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