3D hierarchical CuO/Co₃O₄ core–shell nanowire array on copper foam for on-demand hydrogen generation from alkaline NaBH₄ solution

Abstract

In this work, we demonstrate the first use of a 3D hierarchical CuO/Co₃O₄ core–shell nanowire array in situ grown on copper foam as a robust and highly active catalyst toward the hydrolytic dehydrogenation of NaBH₄ for on-demand hydrogen generation in alkaline solutions. This monolithic catalyst exhibits a maximum hydrogen generation rate of 6162.55 mL min⁻¹ g_(CuO/Co3O4)⁻¹ under ambient conditions, which is higher than that of most reported Co-based catalysts, with an activation energy of 56.38 kJ mol⁻¹. It also shows good durability and reusability and can be used as an on/off switch for on-demand hydrogen generation.

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Presently, hydrogen is considered as a promising energy carrier with merits of high energy content, renewability and no emission of greenhouse gases.^1,2 However, one obstacle for hydrogen economy is the issue of hydrogen storage.^3,4 Hydrides are attractive hydrogen-storing materials for portable applications^5–7 and NaBH₄ has attracted considerable attention because of its good reducibility of metal precursors and organic compounds,^8–10 high hydrogen storage efficiency (10.8 wt%),¹¹ economical cost compared with other chemical hydrides,¹² stability of its solution at a high pH value,¹³ and potentially safe operation.¹⁴ NaBH₄ hydrolysis is a safe exothermic reaction that produces hydrogen according to eqn (1):¹⁵

NaBH₄ (aq) + 2H₂O → NaBO₂ (aq) + 4H₂ (1)

Nevertheless, it is usually kept under alkaline conditions to prevent its self-hydrolysis and an active catalyst must be utilized to activate and accelerate its hydrolysis for hydrogen generation. Noble metals like Pt,¹⁶ Rh,¹⁷ Ru,¹⁸ and Pd¹⁹ are the most active such catalysts, but the scarcity and high cost hinder their widespread uses.

In the past decades, Co based materials have been largely investigated as cost-effective catalysts for NaBH₄ hydrolysis with favourable activity,^20,21 such as Co nanoparticles,²² Au@Co core–shell nanoparticles,²³ Co₃O₄ powder,²⁴ CoO nanocrystals,²⁵ and LiCoO₂,²⁶ etc. However, the low surface area and difficult separation from fuel solutions limit the application of Co based powder materials in catalysing NaBH₄ hydrolysis for hydrogen generation. From the point of technology and operation, self-supported catalyst systems may conquer the above difficulties and have obvious advantages: free of aggregation, easy and rapid recovery from the reaction system for reusability and on-demand hydrogen generation.²⁷ Though several supported Co based catalysts have been utilized in the hydrolysis of NaBH₄, it is still a great challenge to design supported catalysts with larger surface area and more active sites.^1,28 Hierarchical structure is a promising method to solve the above problem, as the supported core–shell hierarchical structure grown directly on conductive substrate has fascinating synergetic properties and large surface area with some unique physical and chemical properties in catalyst systems.^29,30 Previous report has shown that copper foam can be oxided into nanostructured arrays and applied in catalyst systems.³¹ Further, Cu based catalysts have also been used as catalysts for NaBH₄ hydrolysis due to its cheap and environmental features.³² Thus, Cu–Co hierarchical nanostructure may have superior catalytic activity in NaBH₄ hydrolysis reaction due to the synergistic effect between Cu and Co and the large surface area of hierarchical nanostructure.³³ What's more, to the best of our knowledge, such Co–Cu hierarchical structure has not been applied in the hydrolysis of alkaline NaBH₄ solution.

Herein, we demonstrate the development of hierarchical CuO/Co₃O₄ core–shell nanowires array on copper foam (CuO/Co₃O₄ core–shell NA/CF) as a robust and cost-effective catalyst for on-demand hydrogen generation via the hydrolytic dehydrogenation of NaBH₄ in alkaline media, with an activation energy (E_a) of 56.38 kJ mol⁻¹. This 3D monolithic catalyst offers a maximum hydrogen generation rate (HGR) of 6162.55 mL min⁻¹ g_(CuO/Co3O4)⁻¹ at ambient conditions. It also exhibits good stability and reusability and behaves as an on/off switch for on-demand hydrogen generation. These attractive features promise its use as an earth-abundant catalyst material toward large-scale production of hydrogen fuels for applications.

CuO/Co₃O₄ core–shell NA/CF was fabricated by the following three steps in Fig. 1a (see ESI† for preparation details): (1) Cu(OH)₂ nanowires array was grown on commercially available CF by a wet-chemical route at room temperature; (2) CoCO₃(OH)₂·nH₂O core–shell NA/CF was subsequently grown on the Cu(OH)₂ backbones via chemical bath deposition (CBD) (Fig. S2†),³⁴ (3) the resulting Cu(OH)₂/CoCO₃(OH)₂·nH₂O core–shell NA/CF as precursor was converted to CuO/Co₃O₄ core–shell NA/CF by air annealing. Scanning electron microscopy (SEM) analysis suggests that macroporous copper foam (Fig. 1b) is completely covered with Cu(OH)₂ nanowires array after ammonium persulfate treatment, as shown in Fig. 1c and d. Fig. 1e and f show the SEM images of CuO/Co₃O₄ core–shell NA/CF, providing one piece of clear evidence to support the formation of hierarchical core–shell nanowires array on CF. Its energy-dispersive X-ray (EDX) spectrum confirms the existence of Cu, Co and O elements (Fig. S3†). The SEM and corresponding EDX elemental mapping images for such nanoarray demonstrate the uniform distribution of Cu, Co and O elements (Fig. 1g). The much stronger signals for Co and O also imply the shell nature for Co₃O₄ species.

Fig. 1 (a) A schematic diagram for illustrating the fabrication procedure of CuO/Co₃O₄ core–shell NA/CF. SEM images of (b) bare CF, (c and d) Cu(OH)₂ NA/CF and (e and f) CuO/Co₃O₄ core–shell NA/CF. (g) SEM and EDX elemental mapping images of Cu, Co, and O elements for CuO/Co₃O₄ core–shell NA/CF.

Fig. S4† shows the X-ray diffraction (XRD) pattern for CuO/Co₃O₄ core–shell NA/CF in the diffraction angle range of 30–80°. The three strong peaks at 43.30°, 50.43°, and 74.13° can be assigned to the (111), (200), and (220) planes of metallic Cu substrate (JCPDS no. 04-0836), respectively. Other peaks at 35.50°, 36.84°, 38.73°, 59.35° and 65.22° arise from CuO and Co₃O₄. The absence of additional diffraction peaks suggests the complete conversion of CoCO₃(OH)₂·nH₂O into CuO/Co₃O₄ after annealing. Fig. 2a shows the XPS survey spectrum, confirming the presence of only Cu, Co and O. The C 1s signals arise from exposure to atmosphere.³⁵ As shown in Fig. 2b, the CuO exhibits a Cu 2p_3/2 peak and Cu 2p_1/2 peak at binding energies (BEs) of 933.8 and 953.8 eV, respectively,^36,37 together with two high-intensity shake-up satellites (those located at 942.4, and 963.3 eV).³⁸ Fig. 2c shows the XPS spectra of Co. Two prominent peaks at BEs of 795.2 and 779.6 eV correspond to Co²⁺ 2p_1/2 and 2p_3/2, respectively.³⁹ Two small peaks at 803.5 and 788.9 eV can be attributed to 2p_3/2 and 2p_1/2 of Co³⁺, respectively, confirming the existence of Co₃O₄.^40,41 As shown in Fig. 2d, the O 1s core-level spectrum is broad, and two peaks are marked as I and II. Peak I, the lower energy of 529.7 eV, is in agreement with lattice O.^42,43 The high BE of 531.5 eV (peak II), is attributed to the low coordinated oxygen ions at the surface.^40,44

Fig. 2 XPS spectra of CuO/Co₃O₄ core–shell NA/CF. (a) XPS survey spectrum. XPS spectrum in the (b) Cu 2p, (c) Co 2p, and (d) O 1s regions.

We examined the catalytic activity of the monolithic CuO/Co₃O₄ core–shell NA/CF toward NaBH₄ hydrolysis by putting them in alkaline NaBH₄ solution. Co₃O₄ NA/CF and CuO NA/CF were also tested for comparison. As observed, slow hydrogen release occurs for CuO NA/CF and Co₃O₄ NA/CF (Fig. S5a and b†) while a large amount hydrogen gas appears vigorously on CuO/Co₃O₄ core–shell NA/CF (Fig. S5c and Movie S1†). Fig. S5d† compares the catalytic activities for the NaBH₄ hydrolysis on CuO NA/CF, Co₃O₄ NA/CF and CuO/Co₃O₄ core–shell NA/CF. Obviously, CuO/Co₃O₄ core–shell NA/CF shows the highest catalytic activity as compared to the same amount of others. This excellent performance may be attributed to the following factors: (1) this nanostructured branched metal oxide has large reactive sites and facilitates the diffusion of materials; (2) CuO plays the role of substrate and synergistic effect to the hydrolysis reaction of NaBH₄.^45,46

The catalytic activity of CuO/Co₃O₄ core–shell NA/CF was further investigated by hydrolyzing alkaline NaBH₄ solutions under different conditions. Fig. 3a presents the effect of NaBH₄ concentration on hydrogen generation. The HGR maintains almost constant with increasing NaBH₄ concentration from 0.2 wt% to 2.0 wt%, suggesting that the catalytic hydrolysis reaction over CuO/Co₃O₄ core–shell NA/CF catalyst follows zeroth order kinetics in this concentration range.⁴⁷ The reaction rate equation based on zero-order kinetics can be described as follow:⁴⁸

−4 × d[NaBH₄]/dt = d[H₂]/dt = r_H2 (2)

where r_H2 is the hydrogen release rate (mL min⁻¹).

Fig. 3 Effect of concentrations of NaBH₄ and NaOH as well as catalyst loading on the HGR: (a) and (b) [NaOH] = 1 wt%, CuO/Co₃O₄ loading = 0.9 mg. (c) [NaBH₄] = 1 wt%, CuO/Co₃O₄ loading = 0.9 mg. (d) [NaOH] = 1 wt%, [NaBH₄] = 1 wt%. All hydrolysis experiments were done at 298 K and ambient pressure.

It has been well understood that the reaction order with respect to NaBH₄ concentration changes with NaBH₄ concentration.^49,50 From Fig. 3b, it can be shown that the HGR increases with further rising NaBH₄ concentration, but this enhancement is limited to NaBH₄ concentration of 8 wt%. The HGR decreases when NaBH₄ concentration is further increased to 10 wt%. Similar results were also reported by other researchers.^51,52 It can be explained as follows: (1) increased NaBH₄ concentration could increase the viscosity of the fuel solution; (2) the NaBO₂, as a hydrolysate, has lower solubility than NaBH₄ and thus could decrease mass transfer rate.^53,54 Fig. 3c shows the effect of different NaOH concentrations on the release rate of hydrogen, suggesting an increase in NaOH concentration causes decreased HGR. It can be explained as follows. NaOH is an inhibitor to slow down the self-hydrolysis of NaBH₄ in solution and thus more NaOH gives less H₂ via NaBH₄ decomposition. Increased NaOH concentration also increases the solution viscosity and reduces the available free H₂O to react with NaBH₄.¹⁵ The hydrogen release rate may be expressed using eqn (3):^48,55

r_H2 = k/(1 + k₁[NaOH]^0.3) (3)

where k is rate constant and k₁ the coefficient for the effect of NaOH concentration. It was also found that the catalyst loading has great influence on hydrogen generation performance. As shown in Fig. 3d, an increase of catalyst loading leads to a higher r_H2. Fig. S6† depicts the correlation of catalyst loading on r_H2. The value of r_H2 was calculated from the slope of the linear part of each plot shown in Fig. 3d. The logarithmic plot of the calculated initial reaction rate vs. m_CuO/Co3O4 has a slope of 1.03 ≈ 1.0 indicating that the hydrolysis is first order with respect to the catalyst loading.⁵⁶

To obtain the E_a for the hydrolysis of NaBH₄ catalyzed by CuO/Co₃O₄ core–shell NA/CF, the hydrolysis reactions were performed at different temperatures (Fig. 4a). It is quite clear that the higher reaction temperature leads to larger HGR. The E_a was determined as 56.38 kJ mol⁻¹ according to Arrhenius plot of ln(HGR) vs. 1/T for the catalyst (Fig. 4b).⁵⁷ A HGR of 6162.55 mL min⁻¹ g_(CuO/Co3O4)⁻¹ was achieved under 298 K and ambient pressure. This value compares favourably to the behaviours of most reported Co-based catalysts (Table S1†). Such superior catalytic performances can be rationally attributed to the following two reasons: (1) the nanoarray format leads to more exposed active sites for catalysis;³¹ (2) the 3D macroporous configuration of CF and the open spaces between the nanowires allow for enhanced diffusion of the fuel and hydrogen gas generated.⁵⁸

Fig. 4 (a) Hydrogen generation kinetics curves for alkaline NaBH₄ solutions (2 mL) catalyzed by CuO/Co₃O₄ NW/CF using the 1 wt% NaOH and 1 wt% NaBH₄ solution at different solution temperatures. (b) The corresponding Arrhenius plot. (c) Recycling of CuO/Co₃O₄ NW/CF in aqueous solution (2 mL) containing 1.0 wt% NaBH₄ and 1.0 wt% NaOH at 298 K and ambient pressure. (d) On-demand hydrogen generation in the fuel solution using CuO/Co₃O₄ NW/CF as an on/off switch (CuO/Co₃O₄ loading = 0.9 mg, [NaOH] = 1 wt%, [NaBH₄] = 1 wt%, T = 298 K).

High recycle stability is really crucial for every practical catalyst. Therefore, hydrolysis experiments were performed to reuse the catalyst several times at room temperature for durability test. The catalytic activity remains unchanged after 10 cycles (Fig. 4c). Compared to powdery catalyst systems, the monolithic CuO/Co₃O₄ core–shell NA/CF can be easily separated from the fuel solution, promising its use as an efficient on/off switch for on-demand hydrogen generation.^28,59 Fig. 4d shows the temporal control of hydrogen generation by repeated activation of the catalyst system. In the absence of CuO/Co₃O₄ NA/CF (OFF), hydrolysis was terminated without hydrogen release. Hydrogen release occurs again when immersing CuO/Co₃O₄ core–shell NA/CF into the fuel solution (ON). XPS analysis for CuO/Co₃O₄ core–shell NA/CF suggests the valence state of Co does not change obviously after reaction (Fig. S7a†). But, in contrast, two distinct peaks at BEs of 932.4 and 952.5 eV can be attributed to Cu(I),^60,61 which demonstrate some CuO was reduced to Cu₂O during reaction (Fig. S7b†).

In summary, hierarchical CuO/Co₃O₄ core–shell nanowires array on copper foam has been proven as an efficient 3D monolithic catalyst with an E_a of 56.38 kJ mol⁻¹ for on-demand generation ration from the hydrolysis of alkaline NaBH₄ solution. It shows a high HGR of 6162.55 mL min⁻¹ g_(CuO/Co3O4)⁻¹ with high recyclability and durability. Our study not only provides us an attractive low-cost catalyst material toward efficient NaBH₄ hydrolysis for mass production of hydrogen fuels, but would open new opportunities in designing and utilizing monolithically integrated nanoarrays as an on/off switch for on-demand hydrogen generation from borohydrides as fuel feed to cells in portable devices.⁶²

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c6ra17340c

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