Porous Perovskite-Type Lanthanum Cobaltite as Electrocatalysts

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Porous Perovskite-Type Lanthanum Cobaltite as Electrocatalysts towards Oxygen Evolution Reaction Jaemin Kim, Xuxia Chen, Pei-Chieh Shih, and Hong Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02815 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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ACS Sustainable Chemistry & Engineering

Porous Perovskite-Type Lanthanum Cobaltite as Electrocatalysts towards Oxygen Evolution Reaction

Jaemin Kim, Xuxia Chen, Pei-Chieh Shih and Hong Yang*

Department of Chemical and Biomolecular Engineering, University of Illinois at UrbanaChampaign, 206 Roger Adams Laboratory, 600 South Mathews Avenue, Urbana, Illinois 61801, United States

* Correspondence to: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Porous lanthanum cobaltite (LaCoO3) were prepared by hydrothermal reaction and converted into hollow nanospheres through heat treatment. These hollow spheres were examined as electrocatalysts towards oxygen evolution reaction (OER) using the rotating disk electrode (RDE) technique in an alkaline solution. The obtained mass-specific OER activity was 7.51A/g for porous LaCoO3 particles and 12.58 A/g for hollow LaCoO3 nanospheres at 1.60 V. These values were more than 4-6 times higher than that of bulk LaCoO3 compound (1.87 A/g). The OER performance of these perovskite-type LaCoO3 compounds was characterized using the Tafel equation, which showed the hollow nanospheres had the fastest kinetics among the three morphologies. The amorphous surface of these porous structures could contribute to the enhanced OER performance. The electrocatalytic and structural analysis results show the porous nanostructures with amorphous surface layers are important to achieve high activity towards OER for water splitting.

KEYWORDS. Water splitting, OER, LaCoO3, porous perovskite, hydrothermal reaction


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INTRODUCTION

A large amount of effort has been placed on developing efficient electrocatalysts towards the generation of hydrogen through water splitting.1-13 However, the slow kinetics of oxygen evolution reaction (OER) has not been resolved yet and is an impediment for achieving a highly efficient process. The slow OER is mainly due to the sluggish proton-coupled electron transfer to the oxygen intermediates on catalyst surfaces.1,14-16 Although noble metal-based oxide catalysts such as IrO2 and RuO2 have shown excellent OER performance, their high cost imposes a limitation on large scale applications.17-19 So far water-splitting catalyst has not met the requirements for efficiency, stability, scalability and cost.17 Since the early reports on the OER catalytic activity of perovskite-type compounds,20,21 a variety of metal oxides have been synthesized and evaluated.15,22-25 Among them, earth-abundant transition metal oxides (MOx, M=Mn, Fe, Co and Ni) have been studied for possible high OER activity, and cobalt-based oxides have shown particularly high performance that was comparable to noble metal electrocatalysts.15,26,27 Subsequently, various types of cobalt oxides such as perovskite, double perovskite, layered double hydroxide (LDH) and cobalt phosphate (Co-Pi) have been studied experimentally and theoretically.15,22,28,29 These studies indicate that both local geometrical structure and amorphous surface are important for high OER performance. The octahedral coordination of Co, which facilitates the formation of Co-OH bond, appears to be the favored-electronic structure towards OER.26 The amorphous structure composed of distorted lattice sites that lower the activation energy and consequently improved the OER performance.30 Various synthetic techniques including electrodeposition, sol-gel synthesis, hydrothermal synthesis, and direct solid-state reaction have been used to prepare OER catalysts of metal oxides. Among them, hydrothermal synthesis is considered to be efficient for the preparation of metal oxides with multiple elements in the form of porous nanostructure with relatively uniform size and


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shape.31-33 Porous structure is highly desirable for the electrocatalytic reactions for its numerous catalytically active sites, favorable for the enhanced mass transport.34,35 We report herein the design, preparation and OER catalytic properties of perovskite-type LaCoO3 porous nanostructures and hollow spheres synthesized by the hydrothermal method. Their OER catalytic activity was evaluated in a 0.1-M KOH solution based on the rotating disk electrode (RDE) technique, while their structures were characterized with X-ray diffractometer (XRD), transmission electron spectroscopy (TEM) and scanning electron spectroscopy (SEM).

EXPERIMENTAL SECTION Chemicals. Lanthanum (III) nitrate hexahydrate (La(NO3)36H2O, 99.9%) and cobalt (II) nitrate hexahydrate

(Co(NO3)26H2O,

98.0%)

were

purchased

from

Alfa

Aesar.

Glycine

(NH2CH2COOH, >99%), polyvinylpyrrolidone (PVP, MW 10,000, 55,000 and 1,300,000), Nafion 117 (5% aqueous solution), potassium hydroxide (KOH, 99.99%) were from Sigma-Aldrich. Ammonium hydroxide solution (NH4OH, 28.0-30.0%) and tetrahydrofuran (THF) were bought from Macron. Citric acid monohydrate (HOC(COOH)(CH2COOH)2H2O, >99%) and sodium hydroxide (NaOH, 99.4%) were obtained from Fisher Scientific. Vulcan carbon XC-72 was purchased from Cabot Corporation. Hydrogen (H2, 99.999%) and oxygen (O2, 99.999%) were supplied by S. J. Smith. All these solvents, gases and chemicals were used as received. Synthesis of LaCoO3 Porous Particles and Hollow Nanospheres by the Hydrothermal Route. 10 mL of 0.1-M La(NO3)36H2O solution and 10 mL of 0.1-M Co(NO3)26H2O solution were mixed with 20 mL of deionized water, followed by the addition of 900 mg of glycine and 300 mg of PVP (MW 10,000). The mixture was stirred for ~ 20 min to ensure all components were dissolved. This solution was then transferred into a 45-mL Teflon liner after adding 320 L of


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NH4OH solution (28.0-30.0%) to adjust the pH to ~8.9. Other types of PVP (MW: 55,000 and 1,300,000) and different amounts of NH4OH (250 L and 400 L) were used to optimize the procedures. The Teflon liner was placed in the autoclave, then transferred in a box furnace (Thermo Scientific) and heated at 180 °C for 12 h. The solid product was washed three times with DI water and centrifuged at 900 rpm for 10 min (Beckman Coulter, X-30), followed by further drying in a vacuum oven (VWR symphony, ~10 mmHg) at 65 C for 2 h. This powder was then transferred to a tube furnace (MTI, GSL-1500X), heated to 600 C at a rate of 5 C/min under air and maintained at this temperature for 2 h to make the porous LaCoO3 particles. Additional heat treatment was performed at 650 C for 2 h under air to prepare LaCoO3 hollow nanospheres. Synthesis of Bulk LaCoO3 Compound. A stoichiometric amount of La(NO3)36H2O (5 mmol), Co(NO3)26H2O (5 mmol) and citric acid monohydrate (10 mmol) were mixed with 50 mL of DI water. The mixed solution was stirred for ~30 min to dissolve all the chemicals. This solution was then heated to 80 C in an oil bath and kept at that temperature for ~5 h. After this reaction, water was removed by heating overnight until a brown gel was formed. Further removal of water from the gel was carried out in a vacuum oven (VWR symphony, ~10 mmHg) at 120 C for ~4 h. The temperature of the furnace (MTI, GSL-1500X) was then raised to 600 C at a heating rate of 5 C/min under air and maintained for 6 h, then to 900 C at a rate of 5 C/min and for 12 h. Electrochemical Study. Vulcan carbon XC-72 was used as the catalyst support. In a typical procedure, 1 mL of Nafion 117 aqueous solution (5%) was mixed with 0.1-M NaOH solution to adjust the pH value to 8 - 9. A mixture of 2 mg of carbon black and 10 mg of metal oxide was added in 2 mL of THF and 3 µL of the pre-processed Nafion solution, followed by sonication for 30 min to obtain a homogeneous suspension. 5 µL of this catalyst ink was drop-cast


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onto the RDE and dried slowly to form a thin film working electrode. After the ink dried, 10 µL of the same Nafion-in-THF mixture was drop-cast on the working electrode and dried in air. A three-electrode cell was used to measure the performance. The cell is composed of a glassy carbon working electrode (surface area: 0.196 cm2), a counter electrode made of platinum wire (diameter: 0.5 mm) connected to a platinum foil (area: 1 cm2), and a hydrogen reference electrode (HydroFlex, Gaskatel). The reference electrode was calibrated in H2-saturated 0.1-M KOH solution before testing. All measurements were performed after purging with pure O2 in the KOH solution for at least 30 min. Cyclic voltammograms (CVs) were collected at a scan rate of 10 mV/s between 1.1 and 1.7 V. The RDE rotating speed was set at 1600 rpm, unless stated otherwise. The obtained OER current density was evaluated based on the disk area of RDE. The potential was corrected using the equation: Ecorrected = Eapplied – iR. The capacitance was corrected by taking the average of anodic and cathodic scans. Structural Characterization. The crystal structures were analyzed by XRD (Rigaku Miniflex 600) with Cu X-ray source. The measurement was performed in the scan range between 10º and 80º 2θ. TEM and HRTEM (JEOL 2100 Cryo) were carried out at an acceleration voltage of 200 kV. TEM specimen was prepared by dispersing a suspension in ethanol on a carbon-coated copper grid. SEM (Hitachi S4700) images were obtained at an acceleration voltage of 10 kV. SEM specimen was prepared by directly depositing catalyst powders on carbon tape on a SEM stub. UV-vis absorption was performed using Agilent Technologies, Cary 60 UV-vis spectrometer. BET surface area analysis was performed using a single point N2 isotherm adsorption measurement (Quantachrome Monosorb). The surface areas were measured to be 1.5 m2/g for the dense particles, 14.6 m2/g for the porous particles, and 11.4 m2/g for the hollow nanospheres, respectively.


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RESULTS AND DISCUSSION Synthesis of Porous LaCoO3 Nanostructures. The porous LaCoO3 particles and hollow nanospheres were synthesized in a mixture of glycine, PVP, and ammonia solution using the hydrothermal method, followed by thermal treatment at 600 and 650 °C, respectively. Figure 1 shows the schematic illustrations for the complexation and synthetic route for making the porous LaCoO3 nanostructures. The metal precursors are first dissolved into solvated ions and form a sixcoordinated complex, which is a preferred structure of cobalt and lanthanum ions in aqueous solutions (Figure 1a).36 Glycine could then react with ammonia and replace the coordinated water molecules because of the large formation constant of the metal-glycine complex.36 Experimentally, glycine-chelated metal complexes had a dark pink color in solution. UV-vis absorption analysis was performed for three different solutions to study the chelating mechanism between these metal ions and ligands (Figure S1). The absorption wavelength for the mixed solution containing both glycine and ammonia had the characteristic broad absorption range from 450 nm to 550 nm. The solution without ammonia showed similar absorption band width as that from the mixed solution, where the [M(H2O)6]n complex solution exhibited light pink color with decreased absorption intensity. On the other hand, the solution without glycine showed dark green color, which matched well with absorption band of [M(OH)2(H2O)4]n-2 complex in the range of 560-670 nm.36 After the hydrothermal reaction at 180 °C for 12 h, the faceted La-Co complexes were obtained from the mixed solution, as shown in Figure 1b. The porous LaCoO3 particles and hollow nanospheres could be obtained after further heat treatment at 600 °C and 650 °C, respectively, for 2 h in air. It is worthwhile to mention that glycine acts not only as the chelating agent but also as the pore-forming agent in this process.32,33


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Characterization of Porous LaCoO3 Nanostructures. The different nanostructures of LaCoO3 compounds were studied using SEM, TEM, and XRD. Figure 2 shows the representative SEM images of the faceted crystals of La-Co complex formed at 180 °C under the hydrothermal condition. The synthesized crystal of La-Co complex had a morphology reflecting the R-3c symmetry of a rhombohedral unit cell structure. These crystals were uniform in size, with an edge length between 250 and 350 nm, and had flat crystal surfaces. Truncation was found at certain corners on most, if not all, of the La-Co complex crystals. The formation of truncated morphology could be due to the change in molar ratio between the metal precursors and organic molecules.37 PVP was known to stabilize colloidal particles in solution.33 Thus we further examined the formation of La-Co complex crystals using PVP with different molecular weights, and at varying amounts of ammonia (Figure S2). As the molecular weight of PVP increased, the size of the crystals decreased, because the growth of crystals was confined by the long chains of PVP (Figure S2a and b). The shape and uniformity of the crystals were greatly affected by the amount of ammonia used. Different types and sizes of truncated and overgrown crystals were observed when 250 L of ammonia was added to the synthetic solution (pH ~8.7) (Figure S2c), while the particles with rounded-corners or even spheroids were found when 400 L of ammonia was used to adjust the pH to ~9.1 (Figure S2d). These results indicate the size of La-Co complex particle was mainly controlled by the type and amount of PVP used and their shapes were determined primarily by the amount of ammonia used. Finally, PXRD study shows the formed La-Co complex precursors are made of mixed metal-organic compounds (Figure S3). Porous LaCoO3 particles were obtained after heating at 600 °C for 2 h (Figure 3). The SEM study shows that after the treatment, the size of the faceted La-Co crystals decreased slightly to an edge length of ~200 nm, but the morphology did not change significantly (Figure 3a). The surface


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became rough, and pores were formed, which may result from the removal of organic molecules, such as glycine and PVP (Figure 3b). TEM study indicates that the pores were produced through interconnected small nanoparticles (Figure 3c). Their linkage after the sintering led to the formation of distributed nanopores over the entire particle structure. High-resolution TEM (HRTEM) image reveals the nanoparticles are made of highly crystalline materials covered by amorphous surface layers of 2-3 nm thick (Figure 3d). The amorphous layers were likely due to the relatively lower synthetic temperature. The morphology of these porous LaCoO3 particles changed dramatically after further heat treatment at 650 °C for 2 h. They became hollow nanosphere, though the porous structure maintained (Figure 4). SEM images show the spherical morphology and interconnected porous structures (Figure 4a and b). The overall diameter of the hollow nanospheres was in the order of ~200 nm, which was in the similar size with that of porous LaCoO3 particles. This morphology change can be driven by the minimization of interfacial surface energy during the crystallite growth of LaCoO3 with the thermal energy.38 TEM study further indicates interconnectivity of the primary nanocrystals and the formed pores (Figure 4c). Necking was observed among the hollow spheres, which was a structural feature appearing in the inorganic ceramic-type products after sintering. The crystal structure of nanosphere was like that of porous particles, in which amorphous surface layers of 2-3 nm thick were on top of highly crystalline structures (Figure 4d). Dense perovskitetype LaCoO3 powders were also made as a control, using the sol-gel synthesis method (Figure S4). The particle product showed no control over the uniformity of size, shape, and morphology. The surface of the powders did not present any porosity. The crystal lattices matched well with the typical LaCoO3 crystal structure throughout without the amorphous surface layers.


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Figure 5 shows PXRD of the nanostructured powders and the illustration of a unit cell of LaCoO3 perovskite. The PXRD patterns for the porous particles, hollow nanospheres, and dense powder reference could all be indexed to the perovskite-type structure of LaCoO3 (Figure 5a). No diffraction peaks from the likely impurities such as LaOx, and CoOx were detectable. The broadening of the diffraction peaks indicates the primary crystals of these porous structures were in the nanometer-sized range. Further analysis shows these samples are the rhombohedral phase of LaCoO3 perovskite (PDF # 025-1060), which has the unit cell with octahedral CoO6 sub-units connected with each other through corner-shared oxygen ions (Figure 5b). OER Kinetic Study. Rotating disk electrode (RDE) technique was used to study the OER catalytic activity of these LaCoO3 perovskites with different morphologies. All tests were carried out in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV/s in a potential range between 1.1 and 1.7 V. Figure 6a shows CV curves for LaCoO3 perovskites in the form of dense particle, porous particle, and hollow nanosphere, respectively. The carbon support showed negligible current density under the same evaluation condition, suggesting the contribution of current from carbon support can be ignored. Among these three nanostructures, LaCoO3 hollow nanospheres showed the best OER performance. The onset potentials were observed to be ~1.50 V for both porous particles and hollow nanospheres, while dense particle form had an onset potential of ~1.55 V, which is in good agreement with those of LaCoO3 oxides reported previously (~1.55 V - 1.57 V in 0.1-M KOH solution).22,39 It is noteworthy that the amorphous surface structure may greatly affect the onset potential because it is favored in the absorption of oxygen species and fast dissociation of intermediates.26,27,30,40,41 Thus, the porous particles and hollow nanospheres with an amorphous surface layer showed the enhanced OER performance over the dense particles. Turnover frequency (TOF) was calculated to evaluate further the OER performance (inset in


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Figure 6a), in which the amount of generated oxygen molecules is divided by the total number of cobalt ions under the condition of 100% Faradaic efficiency.42 The TOF values were 1.2×10-3 s-1 for the dense particles, 4.8×10-3 s-1 for the porous particles and 8.0×10-3 s-1 for the hollow nanospheres at the overpotential of 370 mV. This result indicated the porous LaCoO3 nanostructures had higher OER activities than the dense particles. The trend was the same at the overpotential of 415 mV. The obtained TOF values were 4.6×10-3 s-1 for the dense phase, 1.7×102 -1

s for the porous particles, and 2.6×10-2 s-1 for the hollow nanospheres. Figure 6b shows the Tafel plots of these three catalysts for the mass-specific OER activity.

The Tafel slopes were 76 mV/dec for the dense particles, 74 mV/dec for the porous particles, and 72 mV/dec for the hollow nanospheres. These values agree well with those of the LaCoO3 perovskite structure previously reported (~70 mV/dec - 75 mV/dec in 0.1-M KOH solution).21,43 The OER stability of the catalysts was examined by performing 50 CV cycles in a potential range between 1.1 V and 1.7 V (Figure S5). The current density of the catalysts decrseased slightly upon the consecutive cycles, likely resulting from the minor structural degradation due to the high overpotential and high pH conditions. Tafel slopes of the catalysts remained unchanged after 50 cycles, though the potential change at the fixed current density was larger for the dense particles of LaCoO3 than the two porous nanostructures. The OER mass current density at 1.60 V was 7.51 A/g for the porous particles, and 12.58 A/g for the hollow nanospheres, while the dense phase had a value of 1.87 A/g (Figure 6c). Figure 6c further shows the reaction rate at zero overpotential, that is, exchange mass current density ( ) of these catalysts, which are calculated using the Tafel equation,

/

, where η is the

overpotential, α represents the transfer coefficient, F is the Faraday constant (96,485 A·s/mol), R is the universal gas constant (8.324 J/mol·K), and T is the temperature in Kelvin. The exchange


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current density was 26.9 μA/g for the dense particle form of LaCoO3, 85.0 μA/g for the porous particles and 192.2 μA/g for the hollow nanospheres, indicating the OER kinetics near the equilibrium was enhanced in the order of dense particle < porous particle < hollow nanosphere. The capacity-corrected OER mass current densities of these catalysts were further fitted to the Tafel equation as shown in Figure 6d. The experimental data fitted very well with the simulated curves (long dashed lines), that were calculated using the Tafel equation. The corresponding transfer coefficients were determined to be 0.74, 0.77 and 0.79 for the dense particles, porous particles, and hollow nanospheres, respectively. The obtained transfer coefficients were close to the value previously determined for the LaCoO3 perovskite (0.84).21 Additional analysis of OER kinetics after 50 CV cycles was shown in Figure S6. Mechanism of Porous LaCoO3 Nanostructure-catalyzed OER. The OER on oxide surface typically follows the four-electron transfer pathways with multiple steps as shown in Figure 7a.22,26 In this process, the cobalt ions have the oxidation states of 3+ and 4+, and act as the sites for the redox reaction in the OER process. First-principle calculation suggests that the ratedetermining step (RDS) can be evaluated using the Gibbs free energy change in each step, where either the formation of O-O bond in OOH adsorbate (Step 2) or the deprotonation from OOH adsorbate to form peroxide ions (Step 3) could be the RDS.26,44 This model explains the thermochemistry of OER well for metal oxide catalysts; however, it cannot reflect sufficiently the OER kinetics when surface layers are amorphous and have many crystal defects. The amorphous surface could accommodate different degrees of defect sites, inducing distorted lattices, which might result in the reduction of OER activation energy in energetically favored intermediate steps.30,40,41 Thus, the defects were proposed to influence the OER activity by acting as the sites to activate water molecule or adsorb hydroxide ion in the initial OER step (Step 1).3,45 As a result,


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such surface features contribute to the enhanced OER performance for the two porous LaCoO3 nanostructures.26,41 Another important factor for the high OER activity of LaCoO3 nanostructured catalysts should be related to the atomic structures, which results in LaCoO3 outperforming many other oxides towards OER.22 Based on the molecular orbital theory analysis, d-electron configuration of the catalytic center determines the OER activity of perovskite structures, where a single electron in eg orbital is critical and used as a good descriptor for OER activity.22,46 The d-electron configuration of Co3+ in LaCoO3 perovskite has an intermediate spin state. Thus, a single electron occupies eg orbital, and the rest electrons remain in degenerated t2g orbitals as shown in Figure 7b. LaCoO3 has the favored binding energy towards the oxygen species, neither too strong (eg < 1) nor too weak (eg > 1) in bonding, resulting in enhanced OER.46

CONCLUSION Both LaCoO3 porous particles and hollow nanospheres have been made using a hydrothermal method. They had amorphous surface structures, which favored the OER performance, showing both a low onset potential of ~50 mV and high current density, which is more than six times than those of the dense LaCoO3 particles. Our findings point to the new possibility in which a porous nanostructure with an amorphous surface can be important to achieve high catalytic activity towards OER for water splitting application.


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Figure 1. (a) Reaction for the complex formation between metal cation and glycine chelate, and (b) illustration of the synthesis route for making the porous LaCoO3 particles and hollow nanospheres, respectively.


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Figure 2. Representative (a) low and (b) high magnification SEM images of La-Co complex crystal precursors synthesized at 180 °C through the hydrothermal synthesis.


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Figure 3. (a, b) SEM and (c, d) TEM images of LaCoO3 porous particles showing (a) large population of the faceted crystals formed, (b-c) porous structures and (d) an amorphous layer over the crystalline perovskite structure. Inset in d) shows the low-resolution TEM image of the crystal indicating that the amorphous layer surrounds the compound. The scale bar is 10 nm.


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Figure 4. (a, b) SEM and (c, d) TEM images of LaCoO3 hollow spheres showing (a) large population of the spheres, (b-c) porous structures and (d) an amorphous layer over the crystalline perovskite structure. Inset in d) shows the low-resolution TEM image of the crystal indicating that the amorphous surface layer. The scale bar is 10 nm.


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Figure 5. (a) X-ray diffraction patterns of LaCoO3 for dense particles, porous particles, and hollow nanospheres, respectively, and (b) schematic illustration of rhombohedral structured LaCoO3 perovskite. Color codes are La (green), Co (blue) and O (red).


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Figure 6. (a) CV curves for OER and the TOF for O2 gas evolution at various overpotential (inset), (b) Tafel plot of mass activity, (c) bar chart of mass current activity at 1.60 V and the exchange mass current density, and (d) fitted curves to Tafel equation (long dashed lines) of capacitycorrected OER mass current density with calculated transfer coefficients, α, (inset) for dense particles, porous particles and hollow nanospheres of LaCoO3, respectively. All experiments were performed at least three times. Error bars represent standard deviation from each measurement.


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Figure 7. (a) Proposed mechanism of LaCoO3 towards OER in basic conditions and (b) electron configurations of Co3+ in the LaCoO3 perovskite showing the intermediate spin state with an occupied single electron on eg orbital.


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Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. UV-vis absorption spectra of precursor solutions, SEM images and XRD patterns of LaCo complex faceted crystals, SEM and TEM images of dense particles of LaCoO3, CV and Tafel plot of LaCoO3 catalysts, Tafel equation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by a start-up fund from the University of Illinois. The EM characterizations were carried out in the Frederick Seitz Materials Research Laboratory Central Facilities. The X-ray diffraction was carried out at the George L. Clark X-Ray Facility and 3M Materials Laboratory, School of Chemical Science at UIUC.


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Table of Contents

Synopsis: Enhanced oxygen evolution kinetics for the porous, nanostructured electrocatalysts of perovskite-type lanthanum cobaltite.


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Porous Perovskite-Type Lanthanum Cobaltite as Electrocatalysts

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