Crystalline–amorphous Co@CoO core–shell heterostructures for efficient electro-oxidation of hydrazine

Abstract

Metal–metal oxide core–shell heterostructures have exhibited outstanding performances in many realms, especially in catalysis, due to enriched phase interfaces and strong metal/metal oxide interactions. Herein, Co@CoO core–shell heterostructures with a crystalline Co core and an amorphous CoO shell have been prepared using a simple solution reduction process and shown to be effective catalysts for the electro-oxidation of hydrazine in alkaline media. The synergistic effect between the crystalline metallic Co core and the amorphous CoO shell leads to a small onset potential of −1.10 V vs. Ag/AgCl. In addition, the Co@CoO electrode has been shown to exhibit good long-term catalytic stability.

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1. Introduction

Surface properties of a solid are contingent on the local structures near the surface regions that play a critical role in surface reactions,^1–3 such as electro-catalysis, photo-catalysis and surface energy storage. Tuning the structure of the existing materials can thus potentially enhance their inherent properties, and may even add new functional properties to them.^3–21 For example, it has been shown that deliberately introducing an amorphous surface layer on TiO₂ nanocrystals extends its optical absorption from ultraviolet to visible spectrum with enhanced photocatalytic activity⁶ and can also lead to strong microwave absorption.^7,8 Partially amorphized MnMoO₄ by hydrogenation treatment shows markedly higher catalytic activity towards the hydrogen evolution reaction and a 17-fold increase in specific capacitance as compared to MnMoO₄ crystals.⁹ Incorporation of a small number of heteroatoms (e.g. Fe, Co, and Ni) into the framework of metal sulfides can promote the catalytic activity for the hydrogen evolution reaction by reducing the kinetic energy barrier of H atom adsorption.^12,13 Electrochemical tuning enables olivine-type lithium transition-metal phosphates to have high catalytic activity towards water electrolysis due to the increase in active site density, valency of the transition metal centers, and covalent hybridization between the M-3d and O-2p states.¹⁴ Multi-shelled hollow structures have recently been attracting much attention owing to their unique architectures and highly tunable properties.^15–19 For instance, the capacitance of multi-shelled manganese oxide (Mn₂O₃) hollow microspheres is tunable by manipulating the shell number.¹⁸

Another strategy to promote surface reactions is to control the phase composition in the bulk or introduce phase interfaces near the surface regions where surface reactions occur.^22–38 For instance, compared to single-phase catalysts, both metallic and non-metallic alloys are believed to have higher intrinsic activity towards hydrogen evolution^22–24 and electro-oxidation of hydrazine.^25–27 Improved photocatalytic activity is also observed in biphasic TiO₂ nanocrystals owing to the enhanced charge separation driven by the difference in band gaps of different TiO₂ phases.^28,29 Recently, introducing phase interfaces near the surface regions of solid materials has been of great interest due to the unique physicochemical properties of the phase interface.^31–39 Among them, core–shell heterostructures have attracted more and more attention.^{20,33–38,40} For example, Ru@Pt core–shell nanoparticles show an unprecedentedly low light-off temperature (35°) for preferential oxidation of carbon monoxide in hydrogen.³³ Ni@NiO and Co@Co₃O₄ core–shell heterostructures present outstanding catalytic activity towards hydrogen evolution,^34–36 as the metal/oxide interface can promote water dissociation and hydrogen adsorption.³⁰ Similarly, Au@Co₃O₄ and NiFe@NiFeO_x core–shell nanocrystals demonstrate enhanced catalytic activity towards the oxygen evolution reaction owing to the synergistic effect provided by the core–shell structure.^37,38

We are interested in developing efficient catalysts for direct hydrazine fuel cells. Although metal–metal oxide core–shell structures have shown enhanced catalytic activity for water reduction (hydrogen evolution) and oxidation (oxygen evolution), to the best of our knowledge, such core–shell structures have not been explored as catalysts for the electro-oxidation of hydrazine. There has been a previous report showing that a Pd/WO₃ composite film, as compared to a Pd film, facilitates electro-oxidation of hydrazine,³⁹ pointing to the beneficial effect of metal/metal oxide interfaces on hydrazine electro-oxidation. We thus speculate that a metal–metal oxide core–shell nanostructure could further enhance the electro-oxidation of hydrazine owing to larger interface areas and stronger metal/oxide interactions. Among the relatively inexpensive first-row transition metals, metallic Co stands out because of its lower overpotential towards electro-oxidation of hydrazine than metallic Ni and Cu.^{25,27,41–43} Therefore, a cobalt–cobalt oxide core–shell structure is anticipated to offer high catalytic activity. Herein, we report the preparation of Co@CoO core–shell nanostructures with a crystalline Co core and an amorphous CoO shell, and show that the resulting Co@CoO core–shell structures are highly active catalysts towards electro-oxidation of hydrazine.

2. Experimental section

2.1. Material synthesis

Synthesis of cobalt hydroxide carbonate (Co(OH)_x(CO₃)_0.5(2−x)). In a typical synthesis, 1.0 mmol Co(NO₃)₂·6H₂O and 4.0 mmol urea were dissolved in a solvent mixture of 25 mL of deionized (DI) water and 10 mL of ethanol. A piece of nickel foam (∼12 cm²) was sonicated in 3 M HCl for 10 min to remove the possible surface oxide layer and then washed with DI water. The nickel foam was then transferred into the reaction solution and then a hydrothermal reaction was carried out in a Teflon lined stainless steel autoclave (50 mL) at 100 °C for 10 h. After being cooled to room temperature, the product was collected, washed with DI water, and dried at 100 °C for 10 h. The Co(OH)_x(CO₃)_0.5(2−x) powder on the bottom of the autoclave was also collected, washed with DI water, and dried at 100 °C for 10 h. The mass loading of Co(OH)_x(CO₃)_0.5(2−x) on the nickel foam was about 1.0 mg cm⁻².

Synthesis of cobalt@cobalt oxide (Co@CoO). Typically, 1.0 g of KOH and 0.5 g of NaBH₄ were dissolved in 35 mL of ethylene glycol. Then the solution was transferred to a 50 mL Teflon lined stainless steel autoclave with a piece of Ni foam covered with the as-prepared Co(OH)_x(CO₃)_0.5(2−x) in it. A solvothermal reduction reaction was performed at 180 °C for 10 h. After being cooled to room temperature, the product was collected, washed with DI water, and dried naturally.

2.2. Property characterization

Morphologies of the samples were examined using scanning and transmission electron microscopy (SEM and TEM). The SEM images were collected using a field emission scanning electron microscope (Supra55, Carl Zeiss). The TEM analyses were conducted on a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30 S-TWIN) equipped with a scanning transmission electron microscope. The electron accelerating voltage for the transmission electron microscope was 200 kV. Structural analysis was performed with the collected Co(OH)_x(CO₃)_0.5(2−x) powder on a wide-angle X-ray diffractometer (WAXD, D8 Advance, Bruker) using Cu Kα radiation (wavelength = 1.5418 Å). The elemental composition and chemical status of the electrodes were analyzed using X-ray photoelectron spectroscopy (XPS, EscaLab 250, Thermo Fisher Scientific) with monochromatic Al X-ray (hν = 1486.6 eV) as the excitation source.

2.3. Electrochemical characterization

Electrochemical measurements were carried out in a three-electrode system at room temperature. A Pt wire and an Ag/AgCl electrode were used as the counter and the reference electrode, respectively. Typically, 0.5 M N₂H₄ aqueous solution with 1.0 M KOH was used as the electrolyte. Linear sweep voltammetry was conducted at a scan rate of 5 mV s⁻¹ to evaluate the catalytic performance of the working electrode. A long-term stability test was carried out by cyclic voltammetry in the voltage range from −1.1 to −0.9 V vs. Ag/AgCl with repeated scans at a scan rate of 20 mV s⁻¹.

3. Results and discussion

The synthesis process of the Co@CoO electrode is schematically shown in Fig. 1. First, the Co(OH)_x(CO₃)_0.5(2−x) nanosheets were grown on Ni foam (Fig. S1, ESI†) through a hydrothermal reaction. Then the Co@CoO core–shell heterostructure was achieved by simply reducing the Co(OH)_x(CO₃)_0.5(2−x) nanosheets in the presence of KOH and NaBH₄ in ethylene glycol at 180 °C. Fig. 2A schematically shows the three-dimensional structure of the Co@CoO core–shell heterostructure on Ni foam with one particle showing the Co@CoO core–shell structure as a representative. Note that we simplified the structure in the schematic illustration. Fig. 2B and C display the SEM images of the Co@CoO heterostructures. Obviously, the nanosheet morphology of Co(OH)_x(CO₃)_0.5(2−x) evolved into sphere-like morphology of Co@CoO after the solvothermal reduction process. Magnified SEM (Fig. 2C) and TEM (Fig. 2D) images show that the sphere-like Co@CoO is relatively uniform in size. HRTEM images (Fig. 2E and F) reveal the crystalline core and amorphous shell structures. The core shows clear lattice fringes, while the shell presents amorphous feature (Fig. 2F). Additionally, the crystalline core is composed of crystals of different orientations, indicating the polycrystalline nature of the core. The d-spacing of 0.21 nm corresponds to the (100) plane of metallic cobalt (JCPDS 05-0727), indicating crystalline Co as the core. The amorphous layer has an average thickness of ∼4 nm.

Fig. 1 Schematic illustration of the synthesis process of the Co@CoO electrode.

Fig. 2 (A) Schematic illustration of Co@CoO on Ni foam with one particle showing the core–shell structure as a representative. SEM (B and C), TEM (D), and HRTEM (E and F) images of Co@CoO. The inset in (D) schematically shows the Co@CoO core–shell heterostructures.

XRD and XPS characterization were conducted to analyze the phase composition and surface chemical states, respectively, of Co@CoO. Fig. 3A shows the XRD pattern of Co@CoO. Only signals from metallic Co were observed. This agrees well with the HRTEM characterization. It is worth noting that the crystalline metallic Co core is composed of two different phases: a cubic phase (JCPDS 15-0806) and a hexagonal phase (JCPDS 05-0727). The average grain size was calculated to be ∼9 nm using Scherrer's equation: d = (kλ)/(β cos θ), where k is the shape factor with a typical value of 0.9, λ is the X-ray wavelength, β is the full width at half maximum, θ is the Bragg angle and d is the grain size. The XPS survey spectrum suggested the presence of Co and O (Fig. S2a, ESI†). Ni was not detected (Fig. S2a and b, ESI†), indicating the absence of Ni in Co@CoO. Fig. 3B displays the Co 2p core-level XPS spectrum, which was calibrated with the C 1s peak (284.6 eV). Two typical primary peaks at 796.8 eV (Co 2p_1/2) and 780.8 eV (Co p_3/2) were observed, corresponding to the signals from the Co²⁺ ions in CoO.^44–46 This confirms CoO as the amorphous surface layer on the crystalline Co core.

Fig. 3 (A) XRD pattern and (B) Co 2p XPS spectrum of Co@CoO. # and * indicate two different metallic Co phases, i.e., a cubic phase (JCPDS 15-0806) and a hexagonal phase (JCPDS 05-0727), respectively.

The catalytic activity of the crystalline–amorphous Co@CoO core–shell heterostructure towards electro-oxidation of hydrazine was evaluated in a typical three-electrode system with 1.0 M KOH + 0.5 M N₂H₄ as the electrolyte. Fig. 4 shows the polarization curves of the Co@CoO electrode. For comparison, the polarization curve of bare Ni foam is included. The bare Ni foam showed a small oxidation current density of 2.6 mA cm⁻² at −0.90 V vs. Ag/AgCl, while the Co@CoO electrode delivered a remarkably high current density of 67.6 mA cm⁻² at −0.90 V vs. Ag/AgCl. In addition, the Co@CoO electrode presented a very low onset potential of −1.10 V vs. Ag/AgCl, which is among the lowest reported in the literature for electro-oxidation of hydrazine (Table 1). These results confirm the high catalytic activity of the Co@CoO core–shell structure towards hydrazine oxidation. It has been reported that copper hydroxide/oxide layers in situ formed on the Cu nanocubes play an important role in enhancing the catalytic activity of the catalyst towards electro-oxidation of hydrazine.⁴⁷ The high catalytic activity of the Co@CoO catalyst is thus attributed to the synergistic effect between the crystalline Co core and the amorphous CoO shell. That is, the interface regions offer the highest activity. The metallic Co core likely acts as the electron reservoir for fast transfer of electrons from hydrazine to the catalyst electrode while the Co²⁺ sites on the amorphous CoO shell promote the adsorption of OH⁻ due to the strong electrostatic affinity to the positively charged Co²⁺ species and more unfilled d orbitals in Co²⁺ species than metallic Co.^34,48 It has been proposed that the adsorption of OH⁻ is the first step in the electro-oxidation of hydrazine⁴⁹ and that the initial one-electron transfer between N₂H₄ and OH⁻ (N₂H_4ads + OH⁻ → N₂H₃ + H₂O + e⁻) is the rate-determining step.^47,50 Therefore, the fast charge transfer and the enhanced adsorption of OH⁻ collectively accelerate the hydrazine oxidation reaction on the surface of the Co@CoO catalyst.

Fig. 4 Polarization curves of the Co@CoO electrode in 1.0 M KOH with 0.5 M N₂H₄.

Table 1 Comparison of onset potentials for various materials in electro-oxidation of hydrazine

Sample	Electrolyte	v (mV s^−1)	Con. of N2H4 (M)	Onset potential^a (V)	Ref.
a All the potentials are relative to Ag/AgCl.
Ni60Co40 alloy	1.0 M KOH	20	0.1	−1.17	25
Co	1.0 M KOH	20	0.1	−1.06	27
Ni nanoflowers on Ni foam	3.0 M KOH	1	0.5	−1.05	41
Cu Nanowires on Cu foil	3.0 M NaOH	25	1.0	−0.82	42
Cu film on Cu foil	3.0 M NaOH	5	1.0	−0.76	43
Nano-on-microsized Cu	1.0 M KOH	10	0.5	−1.10	This work
	2.0 M KOH	10	0.5	−1.13	This work

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The effect of KOH concentration on the catalytic performance was studied at a fixed N₂H₄ concentration of 0.5 M. Fig. 5A displays the polarization curves of the Co@CoO electrode at different KOH concentrations. The catalytic performance improved significantly with increasing KOH concentration. For example, the onset potential for hydrazine oxidation on the Co@CoO electrode was −1.07 V vs. Ag/AgCl in 0.5 M KOH; a negative shift of the onset potential was observed when the KOH concentration increased to 1.0 M; the onset potential shifted to a more negative value of −1.13 V vs. Ag/AgCl as the KOH concentration further increased to 2.0 M. These findings suggest that the hydrazine electro-oxidation reaction is highly dependent on the concentration of OH⁻, which agrees well with the previous assertion that OH⁻ plays a critical role in breaking hydrazine into nitrogen and water during the electro-oxidation of hydrazine in alkaline media.^47,49,50 The effect of N₂H₄ concentration on the catalytic performance was also evaluated at a fixed KOH concentration of 2.0 M. Fig. 5B shows the polarization curves of the Co@CoO electrode at varying N₂H₄ concentrations. Up to N₂H₄ concentration of 0.5 M, the current density at 0.95 V vs. Ag/AgCl increased considerably. At N₂H₄ concentrations in the range of 0.5–0.75 M, the increase in current density at 0.95 V vs. Ag/AgCl became less sensitive to increasing N₂H₄ concentration. There was negligible increase in current density at 0.95 V vs. Ag/AgCl when the N₂H₄ concentration increased from 0.75 to 1.0 M.

Fig. 5 (A) Polarization curves of the Co@CoO electrode in 0.5 M N₂H₄ with different KOH concentrations. (B) Polarization curves of the Co@CoO electrode in 2.0 M KOH with varying N₂H₄ concentrations.

The stability of a catalyst is a key parameter for its practical applications. The catalyst stability of the Co@CoO catalyst towards hydrazine electro-oxidation was studied by cyclic voltammetry to measure the current density at −0.9 V vs. Ag/AgCl as a function of the number of cycles. Fig. 6A shows the 1st and the 100th cyclic voltammograms of the Co@CoO electrode. The current density at −0.9 V vs. Ag/AgCl showed a loss of 11% after 100 cycles. This relatively large drop in current density is partly due to the limited mass transport of the electrolyte, as evidenced by comparing the polarization curves before and after 100 cycles (Fig. 6B), where a smaller decrease of 6.2% in current density at −0.9 V vs. Ag/AgCl was observed. These results confirm that the Co@CoO catalyst has good stability. To reveal the origin of the catalyst decay, SEM, HRTEM and XPS studies were conducted on the cycled Co@CoO electrode. SEM detected no obvious morphological change of the cycled Co@CoO (Fig. S3, ESI†) relative to the pristine one (Fig. 2C). The HRTEM image (Fig. S4, ESI†) showed that the average thickness of the amorphous CoO layer on the Co core decreased sharply to around 1 nm after 100 cycles, while the d-spacing of 0.21 nm which corresponds to the (100) plane of metallic cobalt (JCPDS 05-0727) confirmed the well-maintained crystalline Co core. XPS analysis showed that the surface chemical states of cobalt were maintained (Fig. S5, ESI†), confirming the presence of the surface CoO layers on the cycled Co@CoO. It is thus clear that the catalyst decay is caused by the deterioration of surface amorphous CoO layers. On the other hand, this confirms the important role of surface amorphous CoO layers in enhancing the catalytic activity of nanostructured metallic Co catalysts towards electro-oxidation of hydrazine.

Fig. 6 Catalyst stability test using cyclic voltammetry in 1.0 M KOH + 0.5 M N₂H₄. (A) The 1st (black) and the 100th (red) cyclic voltammograms of the Co@CoO electrode during 100 cycles. (B) Polarization curves of the Co@CoO electrode collected before (black) and after (red) 100 cycles.

Conclusions

In summary, well-defined crystalline–amorphous Co@CoO core–shell heterostructures have been readily synthesized by solvothermal reduction of cobalt hydroxide carbonate. The synergistic effect between the crystalline metallic Co core and the amorphous CoO shell endows Co@CoO with high catalytic performance towards electro-oxidation of hydrazine in alkaline media, showing a small onset potential of −1.10 V vs. Ag/AgCl and good long-term stability. This study offers insight into the rational design of advanced catalysts for direct hydrazine fuel cells.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Z. P. acknowledges the support from the National Science Foundation (DMR1308577). X. Y. thanks the University of Missouri-Kansas City, School of Graduate Studies, for supporting this research.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00401j

‡ These authors contributed equally.

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