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PVP-assisted synthesis of porous CoO prisms with enhanced electrocatalytic oxygen evolution properties
Accepted Manuscript
PVP-assisted synthesis of porous CoO prisms with enhanced electrocatalytic oxygen evolution properties
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Xueli Zhao , Wei Zhang , Rui Cao PII: DOI: Reference:
S2095-4956(17)30622-8 10.1016/j.jechem.2017.08.014 JECHEM 381
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
18 July 2017 18 August 2017 29 August 2017
Please cite this article as: Xueli Zhao , Wei Zhang , Rui Cao , PVP-assisted synthesis of porous CoO prisms with enhanced electrocatalytic oxygen evolution properties, Journal of Energy Chemistry (2017), doi: 10.1016/j.jechem.2017.08.014
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ACCEPTED MANUSCRIPT Highlights Synthesized a porous and hollow CoO tetragonal prism-like structure. Characterized this CoO material and its precursor Co3(OAc)5OH carefully. Examined this CoO material as a highly active catalyst for water oxidation. Demonstrated the important role of PVP in synthesis and catalysis. Revealed the porosity of CoO to be very valuable for electrocatalysis.
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PVP-assisted synthesis of porous CoO prisms with enhanced electrocatalytic oxygen evolution properties
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Xueli Zhaoa, Wei Zhanga,*, Rui Caoa,b,*
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an
710119, Shaanxi, China
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Department of Chemistry, Renmin University of China, Beijing 100872, China
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b
Corresponding author. Email: [email protected] (W. Zhang); [email protected] (R.
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Cao).
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Abstract Hollow metal oxide materials with nanometer-to-micrometer dimensions have attracted tremendous attention because of their potential applications in energy conversion and storage systems. Numerous efforts have been focused on developing
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versatile methods for the rational synthesis of various hollow structures to act as efficient water oxidation catalysts. In this work, a unique porous and hollow CoO tetragonal prism-like structure has been successfully synthesized via a facile and
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efficient co-precipitation method with polyvinylpyrrolidone (PVP K30) followed by a heating treatment of the resulted precipitates. The as-prepared porous and hollow CoO microprisms displayed a high activity and stability for water oxidation in 1.0 M KOH solution. To reach a current density of 10 mA/cm2, a low overpotential of 280 mV is
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required. The remarkable activity can be attributed to the synergistic effect between
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two different but well-distributed CoO crystalline phases, uniform particle size,
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ameliorative crystallinity, high surface area and the low mass transfer resistance
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benefitted from the unique porous structure.
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Keywords: Cobalt oxide; Electrocatalysis; Oxygen evolution; Porous structure; Water oxidation
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1. Introduction Energy and environmental problems related to the use of fossil fuels have been forcing people to find new energy carriers that are sustainable and environmentally benign [1–6]. Hydrogen is considered as an ideal energy carrier to replace fossil fuels
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[7–14], and the production of hydrogen through water splitting is an important component in artificial photosynthesis for energy conversion and storage [3,15,16]. As one of the half reactions of water splitting, water oxidation is thermodynamically
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uphill (1.23 V vs reversible hydrogen electrode, RHE) and is extremely slow in kinetics (i.e., it is a four-electron transfer pathway associated with O–H bond breaking and O–O bond formation) [1,17–21]. As a consequence, the efficiency of water oxidation has seriously hindered the progress of hydrogen-based new energy systems.
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Developing cheap and highly efficient water oxidation catalysts (WOCs) has thus
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attracted extensive research interests. Recent efforts lead to the identification of a
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variety of materials consisting of earth-abundant Mn [22–26], Fe [27–32], Co [33–42], Ni [43–47] and Cu [48–50] transition metal elements and also many mixed-metal
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materials [51–74] as active catalysts for water oxidation. However, substantial
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improvement of the activity and durability of WOCs is still required. Co-based materials, particularly Co oxides [34,35,39,40] and hydroxides [41],
have been shown to be active catalysts for water oxidation. Recently, we demonstrated that hierarchical Co(OH)F superstructure built by low-dimensional substructures is highly efficient by acting as a WOC [33]. Results from previous studies suggest that enhanced catalytic water oxidation activity can be realized by
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developing new materials with (1) intrinsically high catalytic features, (2) incorporated metal or non-metal elements to modulate the electronic structures of the catalytic active metal sites, (3) improved conductivities for rapid electron/charge transfer, (4) large surface-to-volume ratios to provide abundant active sites, and (5)
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appropriate porous structures for fast mass (i.e., substrate and product) transport. In these strategies, making high surface area structures is shown to be very valuable to significantly improve the electrocatalytic activity for water oxidation. Nevertheless,
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high surface area structures typically have quite poor crystallinity, which is adverse to the stability of catalyst materials [51]. As a consequence, it is desirable to fabricate porous WOC materials with abundant surface active sites and also sufficient crystallinity.
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Herein, we reported a facile and efficient solvothermal method as assisted by
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polyvinylpyrrolidone (PVP K30) to fabricate porous and hollow CoO tetragonal
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microprisms. In the presence of PVP, which has abundant N and/or C=O groups to bind metal ions, a uniform and discrete tetragonal prism-like structure can be formed
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under solvothermal conditions. Subsequent heating treatment gives novel porous and
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hollow CoO tetragonal microprisms. This CoO material is highly competent to function as an electrocatalyst for water oxidation by reaching a current density of 10 mA/cm2 at a low overpotential of 280 mV. Control experiments using its bulk material counterpart further show the outstanding catalytic activity and long-term stability of this CoO material for water oxidation. The remarkable activity can be attributed to the synergistic effect between two different but well-distributed CoO crystalline phases,
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uniform particle size, ameliorative crystallinity, high surface area and the low mass transfer resistance benefitted from the unique porous structure. This porous and hollow CoO structure is thus potentially valuable to be used in other electrocatalytic
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areas.
2. Experimental 2.1. General methods and materials
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All chemicals, including Co acetate tetrahydrate (99.999%, Alfa), KOH (98%, Sinopharm Chemicals), Nafion (5 wt%, DuPont), and PVP Κ30 were purchased from commercial suppliers and were used without further purification. Milli-Q water of 18 MΩ·cm was used in all experiments. The resulted crystal phase of the Co3(OAc)5(OH)
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precursor and porous and hollow CoO microprisms were characterized by powder
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X-ray diffraction (XRD) using a Rigaku D/Max2550VB+/PC, with Cu Kα radiation (λ
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= 1.5406 Å) at a voltage of 40 kV and a current of 100 mA. The surface morphology and microstructure of the samples were analyzed by field emission scanning electron
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microscopy (FESEM) using a Hitachi SU8000 and transmission electron microscopy
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(TEM) using a FEI Tecnai G2 F20 with an accelerating voltage of 200 kV. Thermo gravimetric analysis (TGA) was carried out with a temperature ramp of 5 °C/min under N2 flow at 100 mL/min over 25 °C to 800 °C in a TA Instruments SDT Q600. Brunauer-Emmett-Teller (BET) surface areas and pore size distributions were measured using a Micromeritics ASAP 2020 at liquid-nitrogen temperature. The X-ray photoelectron spectroscopy (XPS) analysis of the samples was carried out using
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a Kratos AXIS ULTRA XPS. Monochromatic Al Kα X-ray (hν = 1486.6 eV) was employed for analysis with photoelectron take-off angle of 90° with respect to surface plane. Correction of the binding energy was carried out using C 1s peak at 284.6 eV arising from the adventitious hydrocarbon. In addition, the XPS and Raman analysis
recorded on an ITO (5 × 5 mm2) electrode. 2.2. Synthesis of Co3(OAc)5(OH) precursor and CoO
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of the CoO samples after 12-h controlled potential electrolysis at 1.65 V was directly
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The Co3(OAc)5(OH) precursor was synthesized via a facile and efficient co-precipitation method in the presence of PVP. In a typical procedure, 1.50 g of PVP was dissolved in 50 mL of n-butyl alcohol at room temperature to form a transparent solution. Co(OAc)2·4H2O (0.249 g, 1 mmol) was then added to this solution. The
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obtained solution was transferred into a round bottom flask and refluxed at 85 °C for
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10 h. During this period, the solution gradually turns into cloudy with a light pink
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color. After refluxing at 85 °C for 10 h, the solution was cooled down to room temperature, and the precipitate that formed was separated by centrifugation and
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washed with n-butyl alcohol for at least 10 times to remove the attached PVP on the
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surface and then was dried at 60 °C for 2 h. In order to obtain the porous and hollow CoO structure, Co3(OAc)5(OH) was calcinated at 200 °C under Ar for 3 h with a slow heating rate of 2 °C/min, and then was cooled to ambient temperature under Ar. 2.3. Electrochemical measurements Typically, 4 mg of the catalyst powder was dispersed in a mixture of water and ethanol (v/v = 2/1, 1 mL) with 50 μL of Nafion solution under continuous
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ultrasonication for 0.5 h to give a homogenous ink. Then, 5 μL of the dispersive ink was casted to a polished glass carbon (GC) electrode and dried at ambient conditions. All electrochemical measurements were conducted with a CH Instruments (CHI 660E Electrochemical Analyzer) at 20 °C. Cyclic voltammetry (CV) was evaluated in a 1.0
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M KOH aqueous solution using a three-electrode cell with a 0.07 cm2 GC working electrode, a saturated Ag/AgCl reference electrode, and a platinum wire counter electrode. The GC electrode was polished with α-Al2O3 of decreasing sizes (1.0 μm to
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50 nm) and ultrasonically washed with deionized water and absolute ethanol before the loading of catalysts. The Pt electrode was routinely ultrasonicated in 3 M nitric acid to remove any deposited pollutants. Potentials were referenced to a reversible hydrogen electrode (RHE, all potentials are vs RHE in this work unless otherwise
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stated), according to the equation: ERHE = EAg/AgCl + (0.197 + 0.059 × pH). The
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overpotential (η) was calculated according to the following formula: η = ERHE − 1.23
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V. All CV measurements were conducted with iR compensation. The Tafel plots were derived from linear sweep voltammetry (LSV), which were tested at a scan rate of 5
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mV/s. The electrochemical impedance spectroscopy (EIS) was carried out in a
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frequency range from 0.1 Hz to 1 MHz with an AC amplitude of 5 mV. The stability was performed at a fixed current density of 10 mA/cm2 for 12 h in a cell with the indium tin oxide (ITO) working electrode (0.25 cm2). Current density was calculated based on the geometric area of electrodes.
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3. Results and discussion The fabrication of porous CoO structures contains two major steps. First, Co3(OAc)5OH particles with a uniform size were prepared by a co-precipitation method using Co acetate in the presence of PVP, which has strong coordination
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ability to metal ions through the N and/or C=O functional groups. The resulted Co3(OAc)5OH has a highly uniform and discrete tetragonal prism-like structure. The as-prepared Co3(OAc)5OH was then calcinated at 200 °C under Ar for 3 h with a
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ramping rate of 2 °C/min to afford the desired porous CoO structure (denoted hereafter as CoO-200).
The morphology and microstructure of as-prepared Co3(OAc)5(OH) and CoO were characterized by using SEM and TEM. As shown in Fig. 1(a-c), the
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Co3(OAc)5(OH) precursor has regular and uniform tetragonal prism-like morphology
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with a narrow size distribution and a broken surface providing possible active sites.
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Each tetragonal prism has an average size of 1.0 μm in length, 500 nm in width, and 1.5 μm in height and has a pyramid-shaped apex at both ends. In contrast, the
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Co3(OAc)5(OH) particles synthesized in the absence of PVP are obviously
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agglomerate and non-uniform (Fig. S1). This difference highlights the important role of PVP during synthesis, which is likely due to the bulky and amorphous features of PVP and also its binding affinity to metal ions through numerous N and/or C=O functional groups [75–77]. In this context, PVP served as a surface stabilizer and nanoparticle dispersant, preventing the aggregation of Co3(OAc)5(OH) particles via the bulky hydrophobic carbon chains [78]. The TEM image clearly shows the solid
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and dense nature of the Co3(OAc)5(OH) prisms without discernible porosity (Fig. 1d), which also suggests the good distribution and uniform morphology. The chemical composition of Co3(OAc)5(OH) synthesized with or without PVP was analyzed by X-ray diffraction (XRD). As shown in Fig. 2(a), all the
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identified diffraction peaks can be unambiguously assigned to Co acetate hydroxide (JCPDS card no. 22-0582). Three large peaks at 7.42°, 8.50°, and 11.29° are in accordance with the Co3(OAC)5(OH) phase [79,80]. It is noteworthy that the XRD
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pattern of the precursor with addition of PVP does not display any additional peaks in addition to those strong diffractions from the Co3(OAc)5(OH) phase. This result suggests that the surfactant PVP can only regulate the morphology but not affect the composition of the resulted material. In addition, we also investigated the formation
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mechanism of the Co3(OAc)5(OH) precursor by a series of control experiments. For
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example, the SEM images of Co3(OAc)5(OH) synthesized with different amounts of
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PVP (Fig. S2a-c) and reaction temperatures (Fig. S3a-c) were depicted. As the amount of PVP varied, the Co3(OAc)5(OH) particles showed different densities and a
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variety of grain sizes. It is obvious that the amount of PVP used during the synthesis
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affects effectively the size and density of the Co3(OAc)5(OH) precursor. The effect of reaction temperatures on the formation of Co3(OAc)5(OH) was also studied (Fig. S3a-c). On the basis of these results, we obtained the optimal reaction conditions in the experiment section. The XRD patterns of Co3(OAc)5(OH) synthesized with different amounts of PVP are depicted in Fig. S4, which further confirms the aforementioned conclusion.
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Moreover, TGA and derivative thermogravimetric (DTG) analysis was performed to confirm the involvement of PVP, which was likely covered on the surface of the Co3(OAc)5(OH) microprism. It can be seen from Fig. 2(b), the Co3(OAc)5(OH) precursor starts to lose the weight at 200 °C, which may be due to the
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dehydration and gas release. Noticeably, there is a large mass loss (~56.57%) in the range of 200 to 400 °C, which is attributed to the loss of PVP. It is necessary to note that PVP alone decomposes around 400 °C in air, but it may start to decompose
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between 200 and 400 °C when it is covered on the surface of nanoparticles [81–83]. Upon elevated heating treatment, the Co3(OAc)5(OH) precursor eventually undergoes a phase transfer from Co acetate hydroxide to oxide.
The complete phase conversion via heating treatment of the Co3(OAc)5(OH)
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precursor at different temperatures was corroborated by XRD analysis. The results
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shown in Fig. 3(a) display typical XRD patterns of two CoO phases. Based on the
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standard PDF card (JCPDS no. 42-1300 and no. 48-1719), all the characteristic peaks of the obtained samples can be well assigned to two different CoO crystalline phases
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without any collateral peaks. When the Co3(OAc)5(OH) precursor was heated at
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200 °C for 3 h in Ar, the sample tended to form a dominant CoO (JCPDS no. 42-1300) phase with mixed weak signals from another CoO phase (JCPDS no. 48-1719). In contrast, increasing heating temperature resulted in a dominant CoO phase (JCPDS no. 48-1719) with five characteristic peaks, which were consistent with the (111), (200), (220), (311), and (222) facets. The corresponding SEM images of these samples are displayed in Fig. 4 and Fig. S5. The CoO samples maintained their initial tetragonal
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microprism morphology after the calcination treatment at relatively low temperatures, but started to lose the tetragonal prism-like structure at elevated temperatures above 350 °C. Importantly, the obtained CoO samples have a porous structure and improved crystallinity as compared with the Co3(OAc)5(OH) precursor. Importantly, the XRD
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pattern of CoO-150 (the sample obtained by heating at 150 °C) indicates that it has a dominant Co3(OAc)5(OH) phase (Fig. S6), which is consistent with the conclusion obtained by TGA and DTG analysis.
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These porous CoO structures were also analyzed by BET and TEM methods. For CoO-200, the N2 adsorption-desorption isotherm shows a porous structure with a high surface area of 50.78 m2/g (Fig. 3b). The average pore diameters are determined to be 21 nm. The adsorption-desorption isotherm plot also suggests the presence of
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meso-pores in the material. The inner architecture and detailed structure of
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as-synthesized porous prism-like samples are directly elucidated by TEM. It can be
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observed that both interior and surface of the samples are highly porous (Fig. 3c-e), consisting of numerous poly-crystalline primary particles with size of nanometers.
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This result is consistent with the BET result.
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To evaluate the electrocatalytic performance of the porous and hollow CoO
microprisms as OER electrocatalysts, the CoO samples were examined in a 1.0 M KOH solution using a typical three-electrode setup. All samples were drop-casted onto the GC electrode. The CV of CoO-200 displayed a large catalytic wave for water oxidation (Fig. 5a). A small overpotential of 280 mV is required for this CoO-200 sample to reach a current density of 10 mA/cm2. This sample is much more active
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than the Co3(OAc)5(OH) precursor. It was also noted that the catalytic performance of the Co3(OAc)5(OH) sample obtained with addition of PVP is improved as compared with that of the Co3(OAc)5(OH) sample obtained without addition of PVP. This difference is probably caused by the ability of PVP to effectively hinder the
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aggregation of catalyst particles. For comparison, Fig. 5(a) shows the CVs of the CoO-200 sample synthesized without PVP and the benchmark Ir/C (20 wt% of Ir). Our results show that the porous and hollow CoO catalyst is much more active for
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water oxidation than these controls. We also investigated the effect of different amounts of PVP during synthesis on electrocatalytic performance (Fig. S7). It is observed that the sample obtained with 1.5 g of PVP outperformed others for electrocatalytic water oxidation. As shown in Fig. 5(b), the CoO samples calcinated
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under higher temperatures (i.e., CoO-250, CoO-350 and CoO-400) showed
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significantly decreased activities. In contrast, CoO-150 showed slightly increased
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OER activity as compared with the Co3(OAc)5(OH) precursor. The enhanced electrocatalytic activity of CoO-200 can possibly be attributed to the following two
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factors: (1) the synergistic effect between two different CoO crystalline phases as
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found in XRD analysis; (2) the porous and hollow prism-like structure generated by the dehydration and gas release during the calcination. The Tafel plots of the CoO sample and the Co3(OAc)5(OH) precursor were
displayed in Fig. 5(c). The Tafel slope of the CoO sample (70 mV/dec) is smaller than the Tafel slope of the Co3(OAc)5(OH) precursor (95 mV/dec). Furthermore, the chronoamperometry and chronopotentiometry test using ITO working electrodes was
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conducted in a 1.0 M KOH solution to confirm the stability of the CoO sample for catalysis. As shown in Fig. S8, under an applied potential of 1.65 V, the CoO-200 sample can give a relatively stable current density of 10 mA/cm2, suggesting its durability for water oxidation. On the contrary, the Co3(OAc)5(OH) precursor can
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only deliver a current density of 4 mA/cm2. In chronopotentiometry test, the applied potential was stabilized at 1.60 V for the CoO-200 sample to deliver a constant current density of 10 mA/cm2 (Fig. 5d).
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In order to further investigate the surface chemical compositions and element valence states, the CoO sample was characterized by XPS before and after 12-h controlled potential electrolysis at 1.65 V. The XPS spectra of Co 2p and O 1s were depicted in Fig. 6(a) and 6(b), respectively. The Co 2p spectrum before electrolysis
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shows a Co 2p3/2 peak centered at 780.1 eV and a broad Co 2p3/2 satellite peak at
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786.5 eV, suggesting the existing of CoII ions. After electrolysis, the Co 2p spectrum
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was best fitted by two pairs of spin-orbit doublets that indicated the coexistence of divalent and trivalent cobalt ions. The Co 2p3/2 fitting peak at 779.5 eV and the weak
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but still distinguishable satellite peak at 790.1 eV both indicated the existence of CoIII.
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The O 1s spectrum can be resolved to two oxygen peaks. The fitting peak at 529.5 eV was attributed to the Co-O species, and the peak at 530.1 eV can be assigned to the chemisorbed oxygen species or the non-stoichiometric surface oxygen. The structural features were further confirmed by using Raman spectroscopy. The Raman spectrum of the CoO samples before and after electrolysis was depicted in Fig. 7(a) and 7(b), respectively. The peaks at 185, 459, 505, 599 and 658 cm−1 can be
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assigned to characteristic active vibrational modes F32g, Eg, F12g, F22g, and A1g of CoO, respectively (Fig. 7a). The peaks at 193, 479, 522, 619, and 68 cm−1 are assigned to F32g, Eg, F12g, F22g, and A1g modes of crystalline Co3O4, respectively (Fig. 7b). This result is consistent with the XPS result.
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The EIS data were measured in 1.0 M KOH solution to investigate the kinetics. The Nyquist plots of the CoO-200 sample and the Co3(OAc)5(OH) precursor were shown in Fig. 8(a). The semicircle of the CoO-200 sample is smaller than that of the
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Co3(OAc)5(OH) precursor, confirming that the porous electrocatalyst has a smaller charge transfer resistance during OER catalysis. The Nyquist plots of CoO samples obtained under different heating temperatures were provided in Fig. 8(b), showing that the CoO-200 catalyst has the smallest charge transfer resistance among these
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4. Conclusions
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samples.
In summary, we demonstrated that the porous CoO tetragonal prism-like
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hollow structure with mono-dispersed and uniform particle size can be successfully
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prepared by using PVP as a surface stabilizer and particle dispersant. The CoO-200 sample exhibited superior electrocatalytic activity for water oxidation by driving a current density of 10 mA/cm2 at a small overpotential of 280 mV in a 1.0 M KOH solution. A Tafel slope of 70 mV/dec indicated a fast water oxidation kinetics from this Co-based catalyst. The improved electrocatalytic performance of the porous CoO sample could be attributed to several factors, including the synergistic effect between
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two different but well-distributed CoO crystalline phases, uniform particle size, ameliorative crystallinity, high surface area and the low mass transfer resistance benefitted from the unique porous structure. This work shows the design of a new
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type of porous and hollow structure for potential applications in electrocatalysis.
Acknowledgments
We are grateful for the support from the Fundamental Research Funds for the
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Central Universities, the Starting Research Funds of Shaanxi Normal University, the National Natural Science Foundation of China (21101170, 21503126 and 21573139) and the “Thousand Talents Program” of China.
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Notes.
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The authors declare no competing financial interest.
Supporting Information.
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Supporting Information for this article is available on the website.
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Fig. 1. SEM (a-c) and TEM (d) images of prism-like Co3(OAc)5(OH).
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Fig. 2. (a) XRD patterns of the Co3(OAc)5(OH) precursor synthesized with (red) or
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without (black) PVP. (b) TGA and DTG analysis of the freshly prepared
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Co3(OAc)5(OH) sample.
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Fig. 3. (a) XRD patterns of porous CoO materials obtained by heating at different
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temperatures. (b) The isotherm plot and the corresponding pore distribution of the CoO-200 sample. (c-e) TEM images of Co3(OAc)5OH prisms after heating at
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different temperatures (c: room temperature; d: 200 °C; e: 350 °C).
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room temperature; b,e: 200 °C; c,f: 350 °C).
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Fig. 4. FESEM images of Co3(OAc)5(OH) after heating at different temperatures (a,d:
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Fig. 5. (a) CVs of CoO-200 synthesized with or without PVP, Co3(OAc)5(OH)
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precursor synthesized with or without PVP, and Ir/C catalysts. (b) CVs of Co3(OAc)5(OH) after heating at different temperatures. (c) Tafel plots of CoO-200 (blue) and Co3(OAc)5(OH) (red). (d) Chronopotentiometry of the Co3(OAc)5(OH)
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mA/cm2 current density.
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precursor synthesized with (blue) or without PVP (black), and CoO-200 (red) at 10
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Fig. 6. XPS spectra at the Co 2p (a) and O 1s (b) binding energy regions of CoO-200
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before and after water oxidation.
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Fig. 7. Raman spectra of CoO-200 before (a) and after (b) water oxidation.
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Fig. 8. (a) EIS Nyquist plots of the Co3(OAc)5(OH) precursor synthesized with (blue)
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or without (purple) PVP, and the CoO-200 sample (red). (b) EIS Nyquist plots of Co3(OAc)5(OH) after heating at different temperatures (blue: room temperature; green:
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150 °C; red: 200 °C; purple: 250 °C; gray: 350 °C; orange: 400 °C).
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Table of Contents
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PVP-assisted synthesis of porous CoO prisms is reported. The porous CoO microprisms, which are highly uniform and have discrete tetragonal prism-like
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structure, display enhanced activity for electrocatalytic water oxidation.
PVP-assisted synthesis of porous CoO prisms with enhanced electrocatalytic oxygen evolution properties
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Xueli Zhao , Wei Zhang , Rui Cao PII: DOI: Reference:
S2095-4956(17)30622-8 10.1016/j.jechem.2017.08.014 JECHEM 381
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
18 July 2017 18 August 2017 29 August 2017
Please cite this article as: Xueli Zhao , Wei Zhang , Rui Cao , PVP-assisted synthesis of porous CoO prisms with enhanced electrocatalytic oxygen evolution properties, Journal of Energy Chemistry (2017), doi: 10.1016/j.jechem.2017.08.014
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ACCEPTED MANUSCRIPT Highlights Synthesized a porous and hollow CoO tetragonal prism-like structure. Characterized this CoO material and its precursor Co3(OAc)5OH carefully. Examined this CoO material as a highly active catalyst for water oxidation. Demonstrated the important role of PVP in synthesis and catalysis. Revealed the porosity of CoO to be very valuable for electrocatalysis.
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PVP-assisted synthesis of porous CoO prisms with enhanced electrocatalytic oxygen evolution properties
a
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Xueli Zhaoa, Wei Zhanga,*, Rui Caoa,b,*
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an
710119, Shaanxi, China
*
Department of Chemistry, Renmin University of China, Beijing 100872, China
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b
Corresponding author. Email: [email protected] (W. Zhang); [email protected] (R.
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Cao).
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Abstract Hollow metal oxide materials with nanometer-to-micrometer dimensions have attracted tremendous attention because of their potential applications in energy conversion and storage systems. Numerous efforts have been focused on developing
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versatile methods for the rational synthesis of various hollow structures to act as efficient water oxidation catalysts. In this work, a unique porous and hollow CoO tetragonal prism-like structure has been successfully synthesized via a facile and
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efficient co-precipitation method with polyvinylpyrrolidone (PVP K30) followed by a heating treatment of the resulted precipitates. The as-prepared porous and hollow CoO microprisms displayed a high activity and stability for water oxidation in 1.0 M KOH solution. To reach a current density of 10 mA/cm2, a low overpotential of 280 mV is
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required. The remarkable activity can be attributed to the synergistic effect between
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two different but well-distributed CoO crystalline phases, uniform particle size,
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ameliorative crystallinity, high surface area and the low mass transfer resistance
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benefitted from the unique porous structure.
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Keywords: Cobalt oxide; Electrocatalysis; Oxygen evolution; Porous structure; Water oxidation
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1. Introduction Energy and environmental problems related to the use of fossil fuels have been forcing people to find new energy carriers that are sustainable and environmentally benign [1–6]. Hydrogen is considered as an ideal energy carrier to replace fossil fuels
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[7–14], and the production of hydrogen through water splitting is an important component in artificial photosynthesis for energy conversion and storage [3,15,16]. As one of the half reactions of water splitting, water oxidation is thermodynamically
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uphill (1.23 V vs reversible hydrogen electrode, RHE) and is extremely slow in kinetics (i.e., it is a four-electron transfer pathway associated with O–H bond breaking and O–O bond formation) [1,17–21]. As a consequence, the efficiency of water oxidation has seriously hindered the progress of hydrogen-based new energy systems.
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Developing cheap and highly efficient water oxidation catalysts (WOCs) has thus
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attracted extensive research interests. Recent efforts lead to the identification of a
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variety of materials consisting of earth-abundant Mn [22–26], Fe [27–32], Co [33–42], Ni [43–47] and Cu [48–50] transition metal elements and also many mixed-metal
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materials [51–74] as active catalysts for water oxidation. However, substantial
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improvement of the activity and durability of WOCs is still required. Co-based materials, particularly Co oxides [34,35,39,40] and hydroxides [41],
have been shown to be active catalysts for water oxidation. Recently, we demonstrated that hierarchical Co(OH)F superstructure built by low-dimensional substructures is highly efficient by acting as a WOC [33]. Results from previous studies suggest that enhanced catalytic water oxidation activity can be realized by
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developing new materials with (1) intrinsically high catalytic features, (2) incorporated metal or non-metal elements to modulate the electronic structures of the catalytic active metal sites, (3) improved conductivities for rapid electron/charge transfer, (4) large surface-to-volume ratios to provide abundant active sites, and (5)
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appropriate porous structures for fast mass (i.e., substrate and product) transport. In these strategies, making high surface area structures is shown to be very valuable to significantly improve the electrocatalytic activity for water oxidation. Nevertheless,
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high surface area structures typically have quite poor crystallinity, which is adverse to the stability of catalyst materials [51]. As a consequence, it is desirable to fabricate porous WOC materials with abundant surface active sites and also sufficient crystallinity.
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Herein, we reported a facile and efficient solvothermal method as assisted by
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polyvinylpyrrolidone (PVP K30) to fabricate porous and hollow CoO tetragonal
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microprisms. In the presence of PVP, which has abundant N and/or C=O groups to bind metal ions, a uniform and discrete tetragonal prism-like structure can be formed
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under solvothermal conditions. Subsequent heating treatment gives novel porous and
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hollow CoO tetragonal microprisms. This CoO material is highly competent to function as an electrocatalyst for water oxidation by reaching a current density of 10 mA/cm2 at a low overpotential of 280 mV. Control experiments using its bulk material counterpart further show the outstanding catalytic activity and long-term stability of this CoO material for water oxidation. The remarkable activity can be attributed to the synergistic effect between two different but well-distributed CoO crystalline phases,
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uniform particle size, ameliorative crystallinity, high surface area and the low mass transfer resistance benefitted from the unique porous structure. This porous and hollow CoO structure is thus potentially valuable to be used in other electrocatalytic
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areas.
2. Experimental 2.1. General methods and materials
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All chemicals, including Co acetate tetrahydrate (99.999%, Alfa), KOH (98%, Sinopharm Chemicals), Nafion (5 wt%, DuPont), and PVP Κ30 were purchased from commercial suppliers and were used without further purification. Milli-Q water of 18 MΩ·cm was used in all experiments. The resulted crystal phase of the Co3(OAc)5(OH)
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precursor and porous and hollow CoO microprisms were characterized by powder
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X-ray diffraction (XRD) using a Rigaku D/Max2550VB+/PC, with Cu Kα radiation (λ
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= 1.5406 Å) at a voltage of 40 kV and a current of 100 mA. The surface morphology and microstructure of the samples were analyzed by field emission scanning electron
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microscopy (FESEM) using a Hitachi SU8000 and transmission electron microscopy
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(TEM) using a FEI Tecnai G2 F20 with an accelerating voltage of 200 kV. Thermo gravimetric analysis (TGA) was carried out with a temperature ramp of 5 °C/min under N2 flow at 100 mL/min over 25 °C to 800 °C in a TA Instruments SDT Q600. Brunauer-Emmett-Teller (BET) surface areas and pore size distributions were measured using a Micromeritics ASAP 2020 at liquid-nitrogen temperature. The X-ray photoelectron spectroscopy (XPS) analysis of the samples was carried out using
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a Kratos AXIS ULTRA XPS. Monochromatic Al Kα X-ray (hν = 1486.6 eV) was employed for analysis with photoelectron take-off angle of 90° with respect to surface plane. Correction of the binding energy was carried out using C 1s peak at 284.6 eV arising from the adventitious hydrocarbon. In addition, the XPS and Raman analysis
recorded on an ITO (5 × 5 mm2) electrode. 2.2. Synthesis of Co3(OAc)5(OH) precursor and CoO
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of the CoO samples after 12-h controlled potential electrolysis at 1.65 V was directly
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The Co3(OAc)5(OH) precursor was synthesized via a facile and efficient co-precipitation method in the presence of PVP. In a typical procedure, 1.50 g of PVP was dissolved in 50 mL of n-butyl alcohol at room temperature to form a transparent solution. Co(OAc)2·4H2O (0.249 g, 1 mmol) was then added to this solution. The
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obtained solution was transferred into a round bottom flask and refluxed at 85 °C for
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10 h. During this period, the solution gradually turns into cloudy with a light pink
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color. After refluxing at 85 °C for 10 h, the solution was cooled down to room temperature, and the precipitate that formed was separated by centrifugation and
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washed with n-butyl alcohol for at least 10 times to remove the attached PVP on the
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surface and then was dried at 60 °C for 2 h. In order to obtain the porous and hollow CoO structure, Co3(OAc)5(OH) was calcinated at 200 °C under Ar for 3 h with a slow heating rate of 2 °C/min, and then was cooled to ambient temperature under Ar. 2.3. Electrochemical measurements Typically, 4 mg of the catalyst powder was dispersed in a mixture of water and ethanol (v/v = 2/1, 1 mL) with 50 μL of Nafion solution under continuous
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ultrasonication for 0.5 h to give a homogenous ink. Then, 5 μL of the dispersive ink was casted to a polished glass carbon (GC) electrode and dried at ambient conditions. All electrochemical measurements were conducted with a CH Instruments (CHI 660E Electrochemical Analyzer) at 20 °C. Cyclic voltammetry (CV) was evaluated in a 1.0
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M KOH aqueous solution using a three-electrode cell with a 0.07 cm2 GC working electrode, a saturated Ag/AgCl reference electrode, and a platinum wire counter electrode. The GC electrode was polished with α-Al2O3 of decreasing sizes (1.0 μm to
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50 nm) and ultrasonically washed with deionized water and absolute ethanol before the loading of catalysts. The Pt electrode was routinely ultrasonicated in 3 M nitric acid to remove any deposited pollutants. Potentials were referenced to a reversible hydrogen electrode (RHE, all potentials are vs RHE in this work unless otherwise
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stated), according to the equation: ERHE = EAg/AgCl + (0.197 + 0.059 × pH). The
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overpotential (η) was calculated according to the following formula: η = ERHE − 1.23
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V. All CV measurements were conducted with iR compensation. The Tafel plots were derived from linear sweep voltammetry (LSV), which were tested at a scan rate of 5
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mV/s. The electrochemical impedance spectroscopy (EIS) was carried out in a
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frequency range from 0.1 Hz to 1 MHz with an AC amplitude of 5 mV. The stability was performed at a fixed current density of 10 mA/cm2 for 12 h in a cell with the indium tin oxide (ITO) working electrode (0.25 cm2). Current density was calculated based on the geometric area of electrodes.
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3. Results and discussion The fabrication of porous CoO structures contains two major steps. First, Co3(OAc)5OH particles with a uniform size were prepared by a co-precipitation method using Co acetate in the presence of PVP, which has strong coordination
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ability to metal ions through the N and/or C=O functional groups. The resulted Co3(OAc)5OH has a highly uniform and discrete tetragonal prism-like structure. The as-prepared Co3(OAc)5OH was then calcinated at 200 °C under Ar for 3 h with a
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ramping rate of 2 °C/min to afford the desired porous CoO structure (denoted hereafter as CoO-200).
The morphology and microstructure of as-prepared Co3(OAc)5(OH) and CoO were characterized by using SEM and TEM. As shown in Fig. 1(a-c), the
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Co3(OAc)5(OH) precursor has regular and uniform tetragonal prism-like morphology
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with a narrow size distribution and a broken surface providing possible active sites.
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Each tetragonal prism has an average size of 1.0 μm in length, 500 nm in width, and 1.5 μm in height and has a pyramid-shaped apex at both ends. In contrast, the
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Co3(OAc)5(OH) particles synthesized in the absence of PVP are obviously
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agglomerate and non-uniform (Fig. S1). This difference highlights the important role of PVP during synthesis, which is likely due to the bulky and amorphous features of PVP and also its binding affinity to metal ions through numerous N and/or C=O functional groups [75–77]. In this context, PVP served as a surface stabilizer and nanoparticle dispersant, preventing the aggregation of Co3(OAc)5(OH) particles via the bulky hydrophobic carbon chains [78]. The TEM image clearly shows the solid
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and dense nature of the Co3(OAc)5(OH) prisms without discernible porosity (Fig. 1d), which also suggests the good distribution and uniform morphology. The chemical composition of Co3(OAc)5(OH) synthesized with or without PVP was analyzed by X-ray diffraction (XRD). As shown in Fig. 2(a), all the
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identified diffraction peaks can be unambiguously assigned to Co acetate hydroxide (JCPDS card no. 22-0582). Three large peaks at 7.42°, 8.50°, and 11.29° are in accordance with the Co3(OAC)5(OH) phase [79,80]. It is noteworthy that the XRD
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pattern of the precursor with addition of PVP does not display any additional peaks in addition to those strong diffractions from the Co3(OAc)5(OH) phase. This result suggests that the surfactant PVP can only regulate the morphology but not affect the composition of the resulted material. In addition, we also investigated the formation
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mechanism of the Co3(OAc)5(OH) precursor by a series of control experiments. For
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example, the SEM images of Co3(OAc)5(OH) synthesized with different amounts of
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PVP (Fig. S2a-c) and reaction temperatures (Fig. S3a-c) were depicted. As the amount of PVP varied, the Co3(OAc)5(OH) particles showed different densities and a
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variety of grain sizes. It is obvious that the amount of PVP used during the synthesis
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affects effectively the size and density of the Co3(OAc)5(OH) precursor. The effect of reaction temperatures on the formation of Co3(OAc)5(OH) was also studied (Fig. S3a-c). On the basis of these results, we obtained the optimal reaction conditions in the experiment section. The XRD patterns of Co3(OAc)5(OH) synthesized with different amounts of PVP are depicted in Fig. S4, which further confirms the aforementioned conclusion.
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Moreover, TGA and derivative thermogravimetric (DTG) analysis was performed to confirm the involvement of PVP, which was likely covered on the surface of the Co3(OAc)5(OH) microprism. It can be seen from Fig. 2(b), the Co3(OAc)5(OH) precursor starts to lose the weight at 200 °C, which may be due to the
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dehydration and gas release. Noticeably, there is a large mass loss (~56.57%) in the range of 200 to 400 °C, which is attributed to the loss of PVP. It is necessary to note that PVP alone decomposes around 400 °C in air, but it may start to decompose
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between 200 and 400 °C when it is covered on the surface of nanoparticles [81–83]. Upon elevated heating treatment, the Co3(OAc)5(OH) precursor eventually undergoes a phase transfer from Co acetate hydroxide to oxide.
The complete phase conversion via heating treatment of the Co3(OAc)5(OH)
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precursor at different temperatures was corroborated by XRD analysis. The results
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shown in Fig. 3(a) display typical XRD patterns of two CoO phases. Based on the
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standard PDF card (JCPDS no. 42-1300 and no. 48-1719), all the characteristic peaks of the obtained samples can be well assigned to two different CoO crystalline phases
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without any collateral peaks. When the Co3(OAc)5(OH) precursor was heated at
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200 °C for 3 h in Ar, the sample tended to form a dominant CoO (JCPDS no. 42-1300) phase with mixed weak signals from another CoO phase (JCPDS no. 48-1719). In contrast, increasing heating temperature resulted in a dominant CoO phase (JCPDS no. 48-1719) with five characteristic peaks, which were consistent with the (111), (200), (220), (311), and (222) facets. The corresponding SEM images of these samples are displayed in Fig. 4 and Fig. S5. The CoO samples maintained their initial tetragonal
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microprism morphology after the calcination treatment at relatively low temperatures, but started to lose the tetragonal prism-like structure at elevated temperatures above 350 °C. Importantly, the obtained CoO samples have a porous structure and improved crystallinity as compared with the Co3(OAc)5(OH) precursor. Importantly, the XRD
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pattern of CoO-150 (the sample obtained by heating at 150 °C) indicates that it has a dominant Co3(OAc)5(OH) phase (Fig. S6), which is consistent with the conclusion obtained by TGA and DTG analysis.
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These porous CoO structures were also analyzed by BET and TEM methods. For CoO-200, the N2 adsorption-desorption isotherm shows a porous structure with a high surface area of 50.78 m2/g (Fig. 3b). The average pore diameters are determined to be 21 nm. The adsorption-desorption isotherm plot also suggests the presence of
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meso-pores in the material. The inner architecture and detailed structure of
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as-synthesized porous prism-like samples are directly elucidated by TEM. It can be
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observed that both interior and surface of the samples are highly porous (Fig. 3c-e), consisting of numerous poly-crystalline primary particles with size of nanometers.
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This result is consistent with the BET result.
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To evaluate the electrocatalytic performance of the porous and hollow CoO
microprisms as OER electrocatalysts, the CoO samples were examined in a 1.0 M KOH solution using a typical three-electrode setup. All samples were drop-casted onto the GC electrode. The CV of CoO-200 displayed a large catalytic wave for water oxidation (Fig. 5a). A small overpotential of 280 mV is required for this CoO-200 sample to reach a current density of 10 mA/cm2. This sample is much more active
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than the Co3(OAc)5(OH) precursor. It was also noted that the catalytic performance of the Co3(OAc)5(OH) sample obtained with addition of PVP is improved as compared with that of the Co3(OAc)5(OH) sample obtained without addition of PVP. This difference is probably caused by the ability of PVP to effectively hinder the
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aggregation of catalyst particles. For comparison, Fig. 5(a) shows the CVs of the CoO-200 sample synthesized without PVP and the benchmark Ir/C (20 wt% of Ir). Our results show that the porous and hollow CoO catalyst is much more active for
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water oxidation than these controls. We also investigated the effect of different amounts of PVP during synthesis on electrocatalytic performance (Fig. S7). It is observed that the sample obtained with 1.5 g of PVP outperformed others for electrocatalytic water oxidation. As shown in Fig. 5(b), the CoO samples calcinated
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under higher temperatures (i.e., CoO-250, CoO-350 and CoO-400) showed
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significantly decreased activities. In contrast, CoO-150 showed slightly increased
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OER activity as compared with the Co3(OAc)5(OH) precursor. The enhanced electrocatalytic activity of CoO-200 can possibly be attributed to the following two
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factors: (1) the synergistic effect between two different CoO crystalline phases as
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found in XRD analysis; (2) the porous and hollow prism-like structure generated by the dehydration and gas release during the calcination. The Tafel plots of the CoO sample and the Co3(OAc)5(OH) precursor were
displayed in Fig. 5(c). The Tafel slope of the CoO sample (70 mV/dec) is smaller than the Tafel slope of the Co3(OAc)5(OH) precursor (95 mV/dec). Furthermore, the chronoamperometry and chronopotentiometry test using ITO working electrodes was
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conducted in a 1.0 M KOH solution to confirm the stability of the CoO sample for catalysis. As shown in Fig. S8, under an applied potential of 1.65 V, the CoO-200 sample can give a relatively stable current density of 10 mA/cm2, suggesting its durability for water oxidation. On the contrary, the Co3(OAc)5(OH) precursor can
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only deliver a current density of 4 mA/cm2. In chronopotentiometry test, the applied potential was stabilized at 1.60 V for the CoO-200 sample to deliver a constant current density of 10 mA/cm2 (Fig. 5d).
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In order to further investigate the surface chemical compositions and element valence states, the CoO sample was characterized by XPS before and after 12-h controlled potential electrolysis at 1.65 V. The XPS spectra of Co 2p and O 1s were depicted in Fig. 6(a) and 6(b), respectively. The Co 2p spectrum before electrolysis
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shows a Co 2p3/2 peak centered at 780.1 eV and a broad Co 2p3/2 satellite peak at
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786.5 eV, suggesting the existing of CoII ions. After electrolysis, the Co 2p spectrum
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was best fitted by two pairs of spin-orbit doublets that indicated the coexistence of divalent and trivalent cobalt ions. The Co 2p3/2 fitting peak at 779.5 eV and the weak
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but still distinguishable satellite peak at 790.1 eV both indicated the existence of CoIII.
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The O 1s spectrum can be resolved to two oxygen peaks. The fitting peak at 529.5 eV was attributed to the Co-O species, and the peak at 530.1 eV can be assigned to the chemisorbed oxygen species or the non-stoichiometric surface oxygen. The structural features were further confirmed by using Raman spectroscopy. The Raman spectrum of the CoO samples before and after electrolysis was depicted in Fig. 7(a) and 7(b), respectively. The peaks at 185, 459, 505, 599 and 658 cm−1 can be
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assigned to characteristic active vibrational modes F32g, Eg, F12g, F22g, and A1g of CoO, respectively (Fig. 7a). The peaks at 193, 479, 522, 619, and 68 cm−1 are assigned to F32g, Eg, F12g, F22g, and A1g modes of crystalline Co3O4, respectively (Fig. 7b). This result is consistent with the XPS result.
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The EIS data were measured in 1.0 M KOH solution to investigate the kinetics. The Nyquist plots of the CoO-200 sample and the Co3(OAc)5(OH) precursor were shown in Fig. 8(a). The semicircle of the CoO-200 sample is smaller than that of the
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Co3(OAc)5(OH) precursor, confirming that the porous electrocatalyst has a smaller charge transfer resistance during OER catalysis. The Nyquist plots of CoO samples obtained under different heating temperatures were provided in Fig. 8(b), showing that the CoO-200 catalyst has the smallest charge transfer resistance among these
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4. Conclusions
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samples.
In summary, we demonstrated that the porous CoO tetragonal prism-like
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hollow structure with mono-dispersed and uniform particle size can be successfully
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prepared by using PVP as a surface stabilizer and particle dispersant. The CoO-200 sample exhibited superior electrocatalytic activity for water oxidation by driving a current density of 10 mA/cm2 at a small overpotential of 280 mV in a 1.0 M KOH solution. A Tafel slope of 70 mV/dec indicated a fast water oxidation kinetics from this Co-based catalyst. The improved electrocatalytic performance of the porous CoO sample could be attributed to several factors, including the synergistic effect between
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two different but well-distributed CoO crystalline phases, uniform particle size, ameliorative crystallinity, high surface area and the low mass transfer resistance benefitted from the unique porous structure. This work shows the design of a new
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type of porous and hollow structure for potential applications in electrocatalysis.
Acknowledgments
We are grateful for the support from the Fundamental Research Funds for the
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Central Universities, the Starting Research Funds of Shaanxi Normal University, the National Natural Science Foundation of China (21101170, 21503126 and 21573139) and the “Thousand Talents Program” of China.
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Notes.
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The authors declare no competing financial interest.
Supporting Information.
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Supporting Information for this article is available on the website.
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Fig. 1. SEM (a-c) and TEM (d) images of prism-like Co3(OAc)5(OH).
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Fig. 2. (a) XRD patterns of the Co3(OAc)5(OH) precursor synthesized with (red) or
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without (black) PVP. (b) TGA and DTG analysis of the freshly prepared
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Co3(OAc)5(OH) sample.
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Fig. 3. (a) XRD patterns of porous CoO materials obtained by heating at different
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temperatures. (b) The isotherm plot and the corresponding pore distribution of the CoO-200 sample. (c-e) TEM images of Co3(OAc)5OH prisms after heating at
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different temperatures (c: room temperature; d: 200 °C; e: 350 °C).
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room temperature; b,e: 200 °C; c,f: 350 °C).
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Fig. 4. FESEM images of Co3(OAc)5(OH) after heating at different temperatures (a,d:
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Fig. 5. (a) CVs of CoO-200 synthesized with or without PVP, Co3(OAc)5(OH)
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precursor synthesized with or without PVP, and Ir/C catalysts. (b) CVs of Co3(OAc)5(OH) after heating at different temperatures. (c) Tafel plots of CoO-200 (blue) and Co3(OAc)5(OH) (red). (d) Chronopotentiometry of the Co3(OAc)5(OH)
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mA/cm2 current density.
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precursor synthesized with (blue) or without PVP (black), and CoO-200 (red) at 10
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Fig. 6. XPS spectra at the Co 2p (a) and O 1s (b) binding energy regions of CoO-200
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before and after water oxidation.
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Fig. 7. Raman spectra of CoO-200 before (a) and after (b) water oxidation.
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Fig. 8. (a) EIS Nyquist plots of the Co3(OAc)5(OH) precursor synthesized with (blue)
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or without (purple) PVP, and the CoO-200 sample (red). (b) EIS Nyquist plots of Co3(OAc)5(OH) after heating at different temperatures (blue: room temperature; green:
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150 °C; red: 200 °C; purple: 250 °C; gray: 350 °C; orange: 400 °C).
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Table of Contents
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PVP-assisted synthesis of porous CoO prisms is reported. The porous CoO microprisms, which are highly uniform and have discrete tetragonal prism-like
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structure, display enhanced activity for electrocatalytic water oxidation.