Two Distinctive Hierarchical Products through the Hydrothermal

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Two Distinctive Hierarchical Products through the Hydrothermal Process for β‑Co(OH)2 Reacting with NaH2PO2 and Their Morphological Effect on Electrochemical Hydrogen Storage Dong Heon Lee, Myunggoo Kang, and Hyun Jung* Advanced Functional Nanohybrid Material Laboratory, Department of Chemistry, Dongguk University-Seoul Campus, 30 Pildong-ro 1-gil, Jung-gu, Seoul 04620, Republic of Korea S Supporting Information *

ABSTRACT: This paper reports a study of the reaction behavior of βCo(OH)2 with NaH2PO2 under hydrothermal conditions, depending on the concentration of NaOH (0−9.0 M). Uniform sized β-Co(OH)2 microplatelets, as the precursor, were prepared by the method of homogeneous precipitation using hydrolysis reaction with hexamethylenetetramine as the base. After the hydrothermal reaction, two distinctive products were obtained: cobalt phosphite [Co11(HPO3)8(OH)6] and hcp Co metal. The XRD analysis reveals that the Co11(HPO3)8(OH)6 appeared in the absence of NaOH. Then, Co11(HPO3)8(OH)6 and the hcp Co metal simultaneously appeared under 1.125 M NaOH. At 2.25−4.5 M NaOH, βCo(OH)2 and hcp Co metal appeared concurrently, and only pure hcp Co metal appeared under 9.0 M NaOH. The FE-SEM observations indicated that the obtained particles were dendritic-like Co11(HPO3)8(OH)6 and flower-like Co metal. We found that the solubility of β-Co(OH)2 and the role of the NaH2PO2 were strongly influenced by the concentration of NaOH during this reaction. To investigate the morphological effect of the two obtained products on the electrochemical hydrogen storage performance, materials with the same crystal structures yet with different morphologies were used for comparison. The evaluations of electrochemical performance proved that the two products showed better reversibility, and higher storage capacity and rate dischargeability than the comparative materials. Their relatively good performances can be attributed to their morphology, which resulted in increased surface area, reduced diffusion pathway, and the accommodation of volume change during cycling.

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INTRODUCTION Among the various materials investigated for energy storage applications, Co-based materials have recently been subjected to intensive study because they are low cost, have low toxicity, and are more naturally abundant than noble metal oxides such as RuO2 and IrO2.1 These Co-based materials have been demonstrated as having good electrochemical reversibility, high charge−discharge capacities, and good cycle stability; they are, therefore, generally used as electrode materials in energy storage devices such as nickel-metal hydride (Ni-MH) batteries, Li-ion batteries, and supercapacitors.1b,2 Among the various Co-based materials, cobalt phosphite [Co11(HPO3)8(OH)6] has attracted intense interest owing to its microporous structure and novel potential electrical, magnetic, catalytic, and optical properties.3 The basic structure of Co11(HPO3)8(OH)6 consists of octahedral cobalt(II) and tetrahedral phosphite (HPO32−) groups.3a These octahedral groups construct two types of open pore systems: a triangular pore with a diameter of 3 Å and a hexagonal pore with a diameter of 5 Å.3a Therefore, Co11(HPO3)8(OH)6 can act as an electrode material for energy storage because of its pore channels, which make efficient contact with electrolytes and allow rapid electrolyte transfer.3b,4 © XXXX American Chemical Society

Another Co-based material, Co metal, has attracted considerable attention because of its structure-dependent magnetic, electronic and catalytic electronic, catalytic, and magnetic properties, as well as not only because of its multiple various crystal structures (hcp and fcc).5 Recently, research has focused intensely on the use of Co metal as a Ni-MH battery electrode material for electrochemical storage of hydrogen, because it can adsorb a large amount of hydrogen in the electrode reaction, which is predicted to enhance electrochemical performances such as reversible discharge capacity, cycling ability, and rate capability.6 Generally, sodium hypophosphite (NaH2PO2) has been used as a mild reducing agent to prepare metals via the chemical reduction process.5a,b Depending on the reaction condition, the H2PO2− anion is not only used to reduce metal cations, but since disproportionation reactions can also take place (decomposition into HPO32− and P3− ions), it has been employed as the phosphite source for the generation of metal phosphite.3c,7 Xu et al. and Zhang et al. reported the successful synthesis of Co metal nanoplatelets by employing NaH2PO2 as Received: July 23, 2016

A

DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the reducing agent.5a,b Also, Ni et al. reported the successful preparation of Co11(HPO3)8(OH)6 by employing NaH2PO2 as the phosphite source.3c Although the synthesis of Co11(HPO3)8(OH)6 and Co metal by using NaH2PO2 has been reported separately, to the best of our knowledge, no systematic study has been conducted of the reaction behavior, depending on the concentration of NaOH, which dramatically changes the role of NaH2PO2 during hydrothermal reaction. The aim of this study is to provide more systematic information for the generation of two different products, Co11(HPO3)8(OH)6 and Co metal, from β-Co(OH)2, depending on the role of NaH2PO2. We, therefore, carried out hydrothermal reaction with β-Co(OH)2 in the presence of NaH2PO2 and various concentrations of NaOH (0−9.0 M) to control the role of NaH2PO2 in the reaction. With a combination of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy-dispersive spectrometry (EDS) analyses, the phase, composition, morphology, and size transformation of the obtained samples were investigated. The results indicated that the concentration of NaOH strongly affects the solubility of β-Co(OH)2 and the reducing ability of NaH2PO2, which play a critical role in the determination of the product. Additionally, we investigated the electrochemical hydrogen storage properties of the obtained two distinctive products that were used as negative electrode materials in a Ni-MH battery system. To indicate the morphological effect on electrochemical hydrogen storage performance, materials with the same crystal structures yet with different morphologies were used for comparison.

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Oxford Instruments (AZtec ver. 2.1 EDS system) operating at an accelerating voltage of 15.0 kV. The pH−potential equilibrium diagrams of the P-H2O and Co-H2O systems at 25 °C are based on thermodynamic data from the Hydra/Medusa Chemical Equilibrium Database and Plotting Software with applied experimental concentration conditions. The pH−potential equilibrium diagrams show the thermodynamically stable area of H2O as two gray-colored dashed lines. Electrochemical Hydrogen Storage Measurements. Two Cobased materials were used as electrode materials for the electrochemical hydrogen storage performance. The first sample, Co11(HPO3)8(OH)6, was the obtained product in the absence of NaOH, and the second sample, Co metal, was the obtained product in 9 M of NaOH. The electrodes were prepared by mixing each Co-based material powder with polytetrafluoroethylene as a binder at the weight ratio of 8:2, and homogeneously molding the mixture in ethanol and coating on Ni foam with an exposed surface area of 1 × 1 cm2. After coating, the electrodes were dried at 40 °C for 4 h. To completely remove the solvent, the electrodes were dried at 80 °C for 12 h under vacuum and then pressed under a pressure of 3000 psi. Measurements were carried out in three-electrode systems with 1 M LiOH [Co11(HPO3)8(OH)6] and 6 M KOH (Co metal) aqueous electrolyte under ambient condition. The Co-based materials and Ni(OH)2/ NiOOH were selected as the working and counter electrodes, respectively. Ag/AgCl was selected as the reference electrode for the Co11(HPO3)8(OH)6 electrode, and Hg/HgO was selected as the reference electrode for the Co metal electrode, respectively. The negative electrode of Co11(HPO3)8(OH)6 was charged at a current density of 1.8 A/g for 0.5 h, and then discharged to −0.1 V at a current density of 0.6 A/g. For the Co metal, the electrode was charged at a current density of 0.5 A/g for 1 h, and then discharged to −0.1 V at a current density of 0.5 A/g. Moreover, to investigate the rate capability of the samples, the negative electrode was discharged to −0.1 V at a current density of 1.0−2.0 A/g. To indicate the morphological effect on electrochemical hydrogen storage performance, the same crystal structures with different morphologies were used for comparison. For comparison with the dendritic-like Co11(HPO3)8(OH)6 and the flower-like Co metal microsphere, the dumbbell-like Co11(HPO3)8(OH)6 was prepared by a previously reported method3c with some modification, and the spherical shaped Co metal with 2 μm particle size was purchased from Sigma-Aldrich Co. Detailed information about the characterizations of these compared materials is provided in the Supporting Information.

EXPERIMENTAL SECTION

Synthesis of the β-Co(OH)2 Precursor. β-Co(OH)2 platelets were prepared by a homogeneous precipitation method previously reported with some modification.8 In a typical procedure, 5 mmol of cobalt chloride hexahydrate (CoCl2·6H2O) was dissolved in 500 mL of a 9:1 solution of deionized (DI) water/ethanol (EtOH) under vigorous stirring. Then, 500 mL of hexamethylenetetramine (HMT) solution (60 mmol; 9:1 solution of DI water/EtOH) was added dropwise to the CoCl2 solution. The mixed solution was heated to 90 °C for 1 h under vigorous stirring. The resulting pink-colored product was cooled to room temperature (RT), and it was separated via centrifugation, washed with DI water and anhydrous EtOH several times, and finally air-dried at RT. Hydrothermal Reaction of β-Co(OH)2 in the Presence of NaH2PO2. The β-Co(OH)2 precursor (3 mmol) was dispersed in 20 mL of distilled water under stirring. Then, 20 mL of sodium hydroxide (NaOH) solution with different concentrations (0, 22.5, 45, 90, and 180 mmol) and 20 mL of sodium hypophosphite monohydrate (NaH2PO2·H2O) solution (24 mmol) were added dropwise to the dispersion under continuous stirring for 1 h at RT. The aqueous dispersion was transferred in a 100 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and kept in an oven at 110 °C for 24 h. The autoclave was then taken out and cooled to RT. The precipitate was centrifuged, washed with water and EtOH several times to eliminate any alkaline salt, and then dried at 60 °C in a vacuum oven for 12 h. Sample Characterizations. Structural analyses of the products were carried out by XRD using a Rigaku-Ultima IV equipped with Cu Kα radiation (λ = 1.5418 Å) with a graphite diffracted beam monochromator at a scan rate of 1°/min from 5° to 80°, an operating voltage of 40 kV, and a current of 30 mA. The morphology and size of the products were examined by FE-SEM (JEOL, JSM-7100F) at an accelerating voltage of 10.0 kV. For the FE-SEM measurements, the powder samples were attached to an Al mount with a carbon tape. In addition, the chemical composition of the samples was characterized by using an FE-SEM instrument coupled to EDS equipped with an

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RESULTS AND DISCUSSION Characterization of Prepared Samples. The structural change in the obtained samples according to the concentration of NaOH was investigated by XRD measurements. As shown in Figure 1a, the diffraction peaks of the precursor pink powder can be indexed to the hexagonal cell (space group P3̅m1) with lattice constants a = 0.3182 nm, and c = 0.4658 nm, matching well with that of standard brucite-like β-Co(OH)2 (JCPDS No. 74-1057). The sharp diffraction peaks suggest that the βCo(OH)2 precursor is highly crystalline, which can be attributed to the homogeneous precipitation employing HMT hydrolysis, resulting in a relatively slow nucleation.8 Figure 1b− f shows the XRD patterns of the products after the hydrothermal reaction as a function of NaOH concentrations of (b) 0, (c) 1.125, (d) 2.25, (e) 4.5, and (f) 9 M with a fixed NaH2PO2 concentration of 1.2 M. The diffraction peaks of the product in the absence of NaOH (Figure 1b) are indexed on a hexagonal unit cell (space group P63mc) with lattice constants of a = 12.817 Å and c = 4.976 Å, which are in good agreement with standard cobalt phosphite (Co11(HPO3)8(OH)6, JCPDS No. 81-1064) with only a small amount of Co metal impurity. At 1.125 M concentration of NaOH (Figure 1c), the diffraction peaks of the product showed mixed phases of Co11(HPO3)8B

DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX

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addition of NaOH to the reaction solution. At higher NaOH concentrations (2.25 and 4.5 M), the dendritic-like morphology disappeared (Figure 3A,B). Instead, the obtained products contained destroyed hexagonal platelets that coexist with flower-like microspheres. At the highest NaOH concentration of 9 M, the hexagonal platelets disappeared, as shown in Figure 3C, and only the flower-like microsphere morphology was obtained. The average diameter of the obtained flower-like microsphere was estimated to be ∼1.5 μm, and the individual reduced platelets in these microspheres had an average thickness of ∼7 nm. To determine the composition of the obtained products, elemental analyses were performed using EDS. As shown in Figure 2a, the EDS spectrum of the β-Co(OH)2 precursor shows that Co and O were the main elemental components. The analysis of elemental composition was based on the calculation of peak areas. The O/Co atomic ratio in the precursor was found to be 2.126, which is near the stoichiometric composition of β-Co(OH)2 within the error range. After the hydrothermal reaction, the EDS spectra of the obtained products without NaOH (Figure 2b) and with a NaOH concentration of 1.125 M (Figure 2c) revealed that Co, P, and O were main elemental components. The Co/P atomic ratios in the products were determined to be 1.223 and 1.588, which are near the stoichiometric composition of Co11(HPO3)8(OH)6 within the error range. At higher NaOH concentrations (2.25 and 4.5 M), the EDS spectra of the obtained products showed Co and O as the main elemental components (Figure 3a,b). The O/Co atomic ratios in parts of the destroyed hexagonal platelets were 1.933 (Figure 3(a-1)) and 1.859 (Figure 3(b-1)), respectively. The EDS spectra of the other part of the flower-like microsphere displayed mainly pure Co with only a small amount of oxygen (Figure 3(a-2) and (b2)). Figure 3c presents the EDS spectrum of the product at the highest NaOH concentration of 9 M, which indicates that the sample was essentially pure Co. The C peaks in all of the EDS spectra were attributed to the substrate. Through the whole scanning range of binding energies, no obvious peak belong to impurity was identified. These results further confirmed that the NaOH concentration has a strong effect on the reaction products. Detailed information on the products such as crystal structure, morphology, and elemental composition are summarized in Table 1. It is well-known that the solubility of materials essentially depends not only on the pressure, temperature, and pH of the solution but also on the physicochemical properties of the solute and solvent. Although the solubility of β-Co(OH)2 in water is very low (3.20 mg/L) at room temperature, we expected the solubility to increase along with increasing temperature and pressure under hydrothermal conditions. These conditions can improve the solubility of β-Co(OH)2, and dissolved Co2+ will react with NaH2PO2. Usually, H2PO2− ion not only is used as a reducing agent because of its low potential in an aqueous solution but also can decompose into HPO32− and PH3 through the disproportionation reaction.3c,7 The produced HPO32− ions rapidly combine with Co2+ ions in the presence of OH− ions originating from dissolved βCo(OH)2, and Co11(HPO3)8(OH)6 nuclei can finally be formed. The associated reactions can be represented as follows:

Figure 1. XRD patterns of (a) β-Co(OH)2 platelets and the products after hydrothermal reaction with different NaOH concentrations of (b) 0, (c) 1.125, (d) 2.25, (e) 4.5, and (f) 9 M.

(OH)6 and Co metal with the hcp phase (JCPDS No. 5-727) between 40° and 50°. At higher concentrations (2.25 and 4.5 M) of NaOH, the diffraction peaks of the products revealed that the β-Co(OH)2 phase appeared with the hcp Co metal phase (Figure 1d,e). With 9 M NaOH, the diffraction peaks of the product showed only the hcp Co metal phase (Figure 1f). There is no obvious impurity peak of CoO or Co3O4 found that indicates a good purity of the obtained product. The relative intensity of the the hcp Co metal phase increased as a function of the NaOH concentration, along with the dominant (100), (002), and (101) diffraction peaks due to the anisotropic nature of the obtained product.9 From the XRD measurements, we concluded that the NaOH concentration can strongly influence the reaction products. Specifically, the Co11(HPO3)8(OH)6 phase was dominant in the absence of NaOH, and the pure hcp phase Co metal was obtained with 9 M NaOH. The changes in the size and shape of the obtained products were investigated by FE-SEM measurements. The typical FESEM images shown in Figure 2A indicate that the β-Co(OH)2 precursor has a hexagonal platelet shape. The widths of the βCo(OH)2 were estimated to be in the range of 7−8 μm, and the thickness of these platelets was measured to be ∼50 nm. The aspect ratio of the platelets was ∼150, which is much larger than the previously reported value.8 These results demonstrate that the obtained β-Co(OH)2 platelets have a uniform hexagonal shape, and high aspect ratio. As shown in Figure 2B, the obtained products without NaOH have a dendritic-like morphology with a stem and orderly branches distributed on both directions of the stem with apparent self-similarity, in the form of approximately 7−8 μm in length for the dendrite stem and 3−4 μm in length for the feathers in the branches. As the concentration of NaOH was increased to 1.125 M (Figure 2C), the dendritic-like morphology became imperfect with decreased sizes. The lengths of the dendrite stem reached approximately 3−4 μm, and those of the feather in the branches were estimated to reach 1−2 μm. Such behavior suggests that the dendritic architecture could be limited in growth with the C

3H 2PO2− + OH− → 2HPO32 − + PH3 + H 2O

(1)

Co(OH)2 → Co2 + + 2OH−

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Figure 2. FE-SEM images and EDS spectra of (A, a) β-Co(OH)2 platelets and the products after hydrothermal reaction with different NaOH concentrations of (B, b) 0 and (C, c) 1.125 M.

11Co2 + + 8HPO32 − + 6OH− → Co11(HPO3)8 (OH)6

The redox reaction occurs if the whole value of electrochemical potential (ΔE) is positive. Therefore, we identified the pH−potential equilibrium diagrams of the P-H2O (Figure 4a) and Co-H2O systems (Figure 4b) derived from literature data and the Hydra/Medusa Chemical Equilibrium Database by applying experimental concentration conditions.5b,10 According to the pH−potential equilibrium diagrams (Figure 4), when the NaOH concentration was raised from 0 to 9 M (pH 7−14), the potential value of eq 4 decreased from −0.94 to −1.58 V (Figure 4a), and that of eq 5 decreased from −0.29 to −0.73 V (Figure 4b). Thus, the ΔE value of this redox reaction increased from 0.65 to 0.85 V. This implies that the redox reaction can occur spontaneously with a strong driving force, and the reduction ability of the H2PO2− is intense enough to reduce β-Co(OH)2 to Co metal when the NaOH concentration is reached to 9 M. The two possible reaction pathways for β-Co(OH)2 in the presence of NaH2PO2 depending on NaOH concentration are presented in Scheme 1. First, in the absence of NaOH, the H2PO2− ion was decomposed into HPO32− and PH3 through the disproportionation reaction. The β-Co(OH)2 then reacted with HPO32− to form dendritic-like Co11(HPO3)8(OH)6. Second, under 9 M NaOH, the H2PO2− ion was used as a reducing agent due to its low potential in conditions with

(3)

In the absence or with a small amount (1.125 M) of NaOH, the dismutation reaction of H2PO2− ion dominated. Subsequently, an impurity amount of the hcp phase Co metal was generated by the reduction reaction of the remaining βCo(OH)2. On the other hand, as the concentration of NaOH increased, the reduction capacity of the H2PO2− ion became stronger than the dismutation tendency, and the solubility of βCo(OH)2 decreased proportionately due to the common ion effect (OH− ion). Therefore, Co11(HPO3)8(OH)6 was not generated above the NaOH concentration of 2.25 M. Instead, the obtained products with NaOH concentrations of 2.25 and 4.5 M were found to be β-Co(OH)2 and the hcp Co metal phase. Furthermore, when the NaOH concentration was increased to 9 M, β-Co(OH)2 was completely reduced to pure hcp Co metal. The redox reaction between β-Co(OH)2 and H2PO2− is as follows: H 2PO2− + 3OH− → HPO32 − + 2H 2O + 2e−

(4)

Co(OH)2 + 2e− → Co + 2OH−

(5) D

DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. FE-SEM images and EDS spectra of the products after hydrothermal reaction with different NaOH concentrations of (A, a) 2.25, (B, b) 4.5, and (C, c) 9 M.

Table 1. Crystal Structure, Morphology, and Elemental Composition of Products elemental composition (atomic %)c reaction conditions

crystal structurea

morphologyb

O

P

Co

NaOH 0 M NaOH 1.125 M NaOH 2.25 M

Co11(HPO3)8(OH)6 Co11(HPO3)8(OH)6 and hcp phase Co metal β-Co(OH)2 and hcp phase Co metal β-Co(OH)2 and hcp phase Co metal hcp phase Co metal

dendritic-like shape dendritic-like shape hexagonal platelet and flower-like microsphere hexagonal platelet and flower-like microsphere flower-like microsphere

67.92 60.92 65.90 4.75 65.02 4.69 4.13

14.43 15.10

17.65 23.98 34.10 95.25 34.98 95.32 95.87

NaOH 4.5 M NaOH 9 M a

Crystal structure obtained from XRD measurement. bThe morphology was obtained from FE-SEM measurement. cElemental composition was calculated via peak area of the EDS spectrum.

sufficient OH− ions. Therefore, β-Co(OH)2 reacted with H2PO2− to form flower-like Co metal. This result provides more detailed information on the role of NaH2PO2 and the generation of the two distinctive products of Co11(HPO3)8(OH)6 and Co metal from β-Co(OH)2. Also, this implies that the reaction pathways could be utilized to prepare other types of transition metals or metal phosphites, such as Mn, Fe, or Ni. Electrochemical Hydrogen Storage Properties of Two Co-Based Products. In a Ni-MH secondary battery system, the electrochemical hydrogen storage process under alkaline electrolyte solution can be explained as follows.2d,11 On the negative electrode:

M (hydrogen storage material) + H 2O + e− ⇌ MH + OH−

(6)

On the positive electrode: Ni(OH)2 + OH− ⇌ NiOOH + H 2O + e−

(7)

The overall reaction: M + Ni(OH)2 ⇌ MH + NiOOH

(8)

Here, the H in the zero oxidation state was formed by the electrochemical H2O reduction in the aqueous solution of alkaline electrolyte when the potential of the electrode becomes E

DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX

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Then, the electron transfer reaction takes place at the interface, which can be represented in the aqueous solution of alkaline electrolyte by − M + H 2O(b) + e− ⇌ M−H(ad) + OH(s)

(10)

The reduced zero oxidation state H is chemically adsorbed on the surface of M, and then the adsorbed H can be diffused into the bulk of the M host: M−H(ad) ⇌ M−H(abs)

(11)

−

The OH ion is then transferred into the bulk of the aqueous solution of alkaline electrolyte. − − OH(s) ⇌ OH(b)

The reverse reaction occurs during the discharging reaction.2d In this system, we evaluated the electrochemical hydrogen storage performances of the two samples obtained in the absence and presence of 9 M NaOH. Through the above characterizations, we determined that the obtained sample in the absence of NaOH was the main product of Co11(HPO3)8(OH)6 with a dendritic-like morphology, while that obtained in 9 M NaOH was the main product of Co metal with a flowerlike microsphere morphology. To identify the morphological effect on electrochemical hydrogen storage properties, the obtained dendritic-like Co11(HPO3)8(OH)6 was compared with the dumbbell-like form, and the obtained flower-like Co metal microsphere was compared with the spherical form. First, to determine their capacities for electrochemical hydrogen storage, the discharge capacities of the two Co11(HPO3)8(OH)6 samples were investigated. The discharge curves for two Co11(HPO3)8(OH)6 electrodes at the discharge current density of 0.6 A/g are represented in Figure 5a. As shown in the figure, the plateaus of the discharge curves of the two Co11(HPO3)8(OH)6 samples were observed to be around −0.80 V, which is consistent with those observed in the previous literature.1a,2b,4,12 These plateaus in the discharge curves could be attributed to the desorption of hydrogen on the Co11(HPO3)8(OH)6 electrodes.1a,2b,4,12 Also, the discharge capacities of dendritic- and dumbbell-like Co11(HPO3)8(OH)6 were about 145 and 100 mA h/g, respectively. This suggested that the higher capacity of the dendritic-like Co11(HPO3)8(OH)6 can be attributed to the increased surface area in its morphology.13 To evaluate the cycle performance, 30 charge− discharge cycles with two Co11(HPO3)8(OH)6 electrodes were conducted at the charge and discharge current densities of 1.8 and 0.6 A/g, respectively, as shown in Figure 5b. The discharge capacities of these two Co11(HPO3)8(OH)6 gradually increased and then became saturated. The dendritic-like and dumbbelllike Co11(HPO3)8(OH)6 showed maximum storage capacities of about 150 and 103 mA h/g, respectively. Also, their capacities remained at over 90% even after 30 continuous charge−discharge measurements. The maximum storage capacity of the dendritic-like Co11(HPO3)8(OH)6 is moderately higher than the dumbbell-like form. In addition, the cycle stability of dendritic-like Co11(HPO3)8(OH)6 is comparatively better than that of the dumbbell-like form, indicating that the dendritic-morphology can effectively accommodate volume changes during the charge−discharge procedure, thus enhancing the cycle stability.13 To investigate the kinetic performances of the two Co11(HPO3)8(OH)6 electrodes, the rate capability and cycling reversibility were evaluated at various discharge current densities in the range of 1.0−2.0 A/g. As shown in

Figure 4. pH−potential equilibrium diagram of the (a) P-H2O system and (b) Co-H2O system at 25 °C.

Scheme 1. Schematic Representation of the Two Possible Reaction Pathways Depending on NaOH Concentration

more negative than the equilibrium redox potential.11 The H2O molecule is the hydrogen source resulting in OH− ion as the conjugated-base product after electron transfer according to the following reaction:2d,11 H 2O + e− ⇌ H + OH−

(12)

(9) F

DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Discharge curves and (b) cycle stability test for Co11(HPO3)8(OH)6 electrodes at charge and discharge current densities of 1.8 and 0.6 A/g, respectively. (c) Discharge capacities and (d) rate capability of Co11(HPO3)8(OH)6 electrodes under various discharge current densities.

flower-like and spherical shaped Co metal were about 352, and 268 mA h/g, respectively. In order to evaluate the cycle stability, 50 charge−discharge cycles for the two Co metal electrodes were conducted at the same charge−discharge current densities, as shown in Figure 6b. As shown in Figure 6b, the flower-like and spherical shaped Co metal show the maximum storage capacities of 361 and 297 mA h/g, and their capacities were 345 mA h/g (95%) and 250 mA h/g (84%) after 50 continuous charge−discharge cycles, respectively. The maximum storage capacity of the flower-like Co metal is higher than that of the spherical shaped Co metal even though they have a similar particle size. The hydrogen storage capacity is known to be dependent on the specific surface areas.14 Therefore, the higher capacity of the flower-like Co metal can be attributed to the increased surface area in its morphology as expected.14 Also, the cycle stability of the flower-like Co metal is better than that of the spherical shaped form, indicating that the flower-like morphology can efficiently buffer the volume changes during the charge−discharge process, thus enhancing the cycle stability.14 To examine the kinetic properties of the two Co metal electrodes, the rate capability and cycling reversibility were evaluated at various discharge current densities in the range of 1.0−2.0 A/g. As shown in Figure 6c, the discharge capacities of

Figure 5c, the discharge capacities of the dendritic-like Co11(HPO3)8(OH)6 are 128, 123, 115, 110, 102, and 97 mA h/g at the high rates of 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 A/g, respectively, corresponding to the capacity utilization of 85, 82, 77, 74, 68, and 65%, respectively, in comparison to the maximum discharge capacity (150 mA h/g) of this sample at 0.6 A/g (Figure 5d). However, the discharge capacities of the dumbbell-like Co11(HPO3)8(OH)6 are 68, 64, 59, 51, 45, and 43 mA h/g at the same rates (Figure 5c), corresponding to the capacity utilization of 66, 62, 57, 50, 44, and 42%, respectively, in comparison to the maximum discharge capacity (103 mA h/ g) of this sample at 0.6 A/g (Figure 5d). Since the influence of high current density on the cycle stability of the dendritic-like Co11(HPO3)8(OH)6 is relatively small, the electrode for this sample showed a better kinetic performance.13 Next, to determine the electrochemical hydrogen storage performances of the two Co metal electrodes, the discharge capacities were investigated. Figure 6a represents the discharge curves of the Co metal electrode at the same charge−discharge current density of 0.5 A/g. As shown, the average discharge voltage plateaus were at −0.8 V, which is consistent with those observed in a previous report.1a,12 The plateau in the discharge curve should be attributed to the desorption of hydrogen on the Co metal electrode.1a,12 Also, the discharge capacities of the G

DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) Discharge curves and (b) cycle stability test of Co metal electrodes at the same charge−discharge current density of 0.5 A/g. (c) Discharge capacities and (d) rate capability of Co metal electrodes under various discharge current densities.

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the flower-like Co metal are 350, 343, 336, 327, 320, and 313 mA h/g at the rates of 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 A/g, corresponding to the capacity utilization of 97, 95, 93, 91, 89, and 87%, respectively, in comparison to the maximum discharge capacity (360 mA h/g) of this sample at 0.5 A/g (Figure 6d). Moreover, the discharge capacities of the spherical shaped Co metal are 260, 249, 232, 221, 208, and 199 mA h/g at the same rates (Figure 6c), corresponding to the capacity utilization of 88, 84, 78, 74, 70, and 67%, respectively, in comparison to the maximum discharge capacity (296 mA h/g) of the sample at 0.5 A/g (Figure 6d). The reversible discharge capacity decreased to a certain extent with increasing current density, which is consistent with previous reports.15 At a high rate of 2.0 A/g, the discharge capacity of the flower-like Co metal was retained (87%); however, that of the spherical shaped form (67%) was relatively suppressed. These results demonstrate that better reversibility, higher hydrogen storage capacity, and higher rate dischargeability were achieved in this study due to the morphological effect of these Co-based materials.

CONCLUSION In this work, a study of the hydrothermal reaction of βCo(OH)2 with NaH2PO2 under various concentrations of NaOH (0−9.0 M) was carried out. In the absence of NaOH, the Co11(HPO3)8(OH)6 was dominant. Under 1.125 M NaOH, Co11(HPO3)8(OH)6 and hcp Co metal simultaneously appeared. When the NaOH concentration was increased to 2.25 and 4.5 M, β-Co(OH)2 and hcp Co metal concurrently appeared, respectively, and the hcp Co metal phase gradually increased. At 9 M NaOH, only pure hcp Co metal appeared. Our research demonstrated that the NaOH concentration strongly affects the solubility of β-Co(OH)2 and the reducing ability of NaH2PO2, both of which play a critical role in the determination of the product. In addition, the electrochemical properties of the two obtained products were investigated for electrochemical hydrogen storage. The electrochemical performances of the obtained dendritic-like Co11(HPO3)8(OH)6 and flower-like Co metal electrodes were compared with the dumbbell-like Co11(HPO3)8(OH)6 and the spherical shaped Co metal, respectively, in order to examine the morphology effect. The electrochemical performances of the two products exhibited better reversibility, higher hydrogen storage capacity, and higher rate dischargeability than those of their comparH

DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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isons, indicating that their unique morphology can enhance the specific surface area for hydrogen storage, and effectively buffer volume changes during the charge−discharge procedure, thereby improving the electrochemical hydrogen storage properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01731. Structural, morphological, and elemental analysis data for the compared materials (dumbbell-like Co11(HPO3)8(OH)6 and the spherical shaped Co metal) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hyun Jung: 0000-0002-4879-9208 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF2016R1D1A1B01009640).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01731 Inorg. Chem. XXXX, XXX, XXX−XXX