Hierarchical porous Co₃O₄@Co_xFe_3−xO₄ film as an advanced electrocatalyst for oxygen evolution reaction

Abstract

Fabricating a delicate structure for water oxidation is critical for developing highly efficient electrocatalysts, which hold significant promise for energy conversion devices. Herein, we show an effective approach for constructing a Co₃O₄@Co_xFe_3−xO₄ hierarchical nanostructure with obviously improved electrocatalytic activity relative to Co_xFe_3−xO₄ nanoplate and Co₃O₄ nanowire films. The enhancement is attributed to the formation of secondary Co_xFe_3−xO₄ nanoplates outside of the Co₃O₄ nanowires, leading to a fantastic porous architecture that results in a more active electrocatalyst.

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Hierarchical nanostructured inorganic materials have attracted world-wide interest due to their unique physical and chemical properties.^1–5 The growth of the hierarchical nanostructures in a vertical fashion to form geometrically confined nano-array architectures has been identified as one of the most efficient routes to greatly boost their performances in various fields including energy storage,^6–8 sensors⁹ and photoelectrochemical applications.^10,11 Recently, the potential application of hierarchical nanostructures in electrochemical catalysis has been investigated, and superior performances^12–15 have been demonstrated, especially for the oxygen evolution reaction (OER), in which oxygen is generated by water oxidation. This strategy will bring the following several advantages. First, the oriented hierarchical nano-array architecture offers a large contact surface area and high porosity, thus facilitating ion diffusion.¹⁶ Second, the direct growth of active catalysts on conductive substrates ensures good electric conductivity and mechanical adhesion, which accelerates electron transport and stabilises the surface morphology and structure.¹⁷ Finally, the nanostructured architecture has been demonstrated to significantly reduce the size of released as-formed oxygen gas bubbles, thus offering a stable and fast current increase.¹⁸

In addition to structural design, catalyst screening is equally crucial to achieve an excellent overall performance. Since four electrons are required to generate one molecular oxygen, the kinetics of this reaction are relatively sluggish, and huge overpotentials are required to drive the reaction.^19,20 An effective electrocatalyst can obviously reduce the overpotential, thereby improving the energy efficiency. To date, the most active OER catalysts are RuO₂ and IrO₂;^21,22 however, these two metal oxides are scarce and high-cost, restricting their large-scale usage. Alternatively, compounds based on earth-abundant metals (Co, Ni, Fe, Mn) with comparable activities for OER have been discovered.^23–28 In particular, Co₃O₄ is an active, stable and inexpensive catalyst in alkaline conditions, and Co has been identified as the active centre for OER.^29–31 Moreover, the introduction of other transition metals into the Co matrix was demonstrated to result in superior OER activity for either of the parent metal oxides due to the creation of new active sites with lower activation energies.³² Therefore, we believe that mixed transition metal oxides are worth exploring due to their great potential in delivering outstanding OER activities.

Herein, we employed a three-step synthetic protocol to fabricate a hierarchical porous Co₃O₄@Co_xFe_3−xO₄ thin film with Co₃O₄ nanowires as the core and mixed Co and Fe oxide nanoplates as the shell. It was found that the mixed oxides are derived from Co₂(OH)₂CO₃ and CoFe layer double hydroxide precursors (CoFe–LDH). The electrochemical results suggested that the hierarchical porous Co₃O₄@Co_xFe_3−xO₄ was a novel electrocatalytic material with high OER activity and stability in basic solutions; its activity and stability were much better than those of Co_xFe_3−xO₄ nanoplate and Co₃O₄ nanowire films. The intrinsically high activity and unique structure of Co₃O₄@Co_xFe_3−xO₄ were believed to be responsible for the high electrocatalytic performance. This study not only provided a new and efficient OER catalyst, but also created a new opportunity for constructing advanced structures for next generation of water splitting and metal–air battery devices.

The synthetic process for hierarchical porous Co_xFe_3−xO₄ involved three steps, as shown in Fig. 1. We chose nickel foam as the substrate because of its high porosity and zigzag skeleton, which endowed the film with a high surface area. The Co₂(OH)₂CO₃ nanowires were first grown on the nickel foam by a hydrothermal method. Subsequently, the CoFe layered double hydroxide nanoplates were constructed in situ along the nanowire skeleton using a similar hydrothermal process. Finally, the hierarchical porous Co₃O₄@Co_xFe_3−xO₄ was formed by a calcination process. The morphological evolution was examined by scanning electron microscopy (SEM), which showed that Co₂(OH)₂CO₃ nanowires grew uniformly on the substrate with smooth surfaces (Fig. 2A and B). After the second hydrothermal process, it was observed that a large amount of nanoplates had grown outside the nanowires (Fig. S1†). Each nanoplate was hexagonal in shape and approximately 400 nm in length, and the calcination process did not destroy the porous architecture (Fig. 2C–F). The energy dispersive spectroscopy (EDS) results (Fig. S2†) showed that the metal elements were uniformly distributed on the hierarchical porous structure with molar ratios of ∼1 : 1 for Co and Fe (Table S1†).

Fig. 1 Schematic illustration of the fabrication process for hierarchical porous Co₃O₄@Co_xFe_3−xO₄ thin film grown on nickel foam.

Fig. 2 (A and B) SEM images of Co₃O₄ nanowire film with different magnifications; (C–F) SEM images of Co₃O₄@Co_xFe_3−xO₄ hierarchical nanostructure with different magnifications.

The morphological evolution induced by the hydrothermal process usually brought about a phase transformation, which was revealed by the X-ray diffraction (XRD) patterns of all the precursors and final products. The red line in Fig. 3A demonstrated that the nanowires were mainly composed of the Co₂(OH)₂CO₃ phase (JCPDF: 48-0083), which was then converted into the Co₃O₄ phase (JCPDF: 42-1467) by the calcination process (black line, the peaks marked ‘#’ denote the Ni substrate). Fig. 3B revealed that after the second growth step, a series of CoFe–LDH reflections (red line) was observed in addition to the Co₂(OH)₂CO₃ phase, demonstrating a successful transformation from an orthorhombic structure to a layered hexagonal structure. The final product (black line) showed a spinel structure, which can be attributed to the CoFe₂O₄ reflection (JCPDF: 03-0864), while the slightly shifted positions might result from variation in the ratio between Co and Fe.^33–35 The X-ray photoelectron spectroscopy (XPS; Fig. S3†) results further confirmed the existence of both elements in the porous film with oxidation states of +2 and +3 for Co and Fe, respectively.

Fig. 3 XRD patterns of as-prepared samples: (A) the Co(OH)₂CO₃ (red) and the obtained Co₃O₄ after calcination (black); and (B) the CoFe–LDH (red) and the obtained hierarchical porous Co₃O₄@Co_xFe_3−xO₄ (black).

In order to gain more structural information, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were employed (Fig. 4). The TEM image clearly showed that the hexagonal nanoplates grew outside the Co₃O₄ nanowire backbone (Fig. 4A), consistent with the SEM images. The HRTEM results (Fig. 4B) further confirmed a core–shell structure, and the observed lattice spacings matched well with a spinel-type structure.

Fig. 4 Hierarchical porous Co₃O₄@Co_xFe_3−xO₄ thin film: (A) a typical TEM image; and (B) a typical HRTEM image (inset is FFT).

The formation mechanism of CoFe–LDH outside of the Co₂(OH)₂CO₃ nanowires can be interpreted by the dissolution and co-precipitation of Co²⁺ and Fe³⁺ ions. Since the pH of the precursor solution containing FeCl₃ and HCl was below 2, the Co was not stable, and it might have been dissolved.^32,36 Prolonging the reaction time would result in the substitution of Co²⁺ for Fe³⁺, and the gradual hydrolysis of Fe³⁺ and Co²⁺ would result in formation of LDH associated with the existence of CO₃²⁻ in the Co₂(OH)₂CO₃ nanowires as the intercalation anion.

Such a hierarchical porous film would facilitate the electrochemical reactions on the electrode surface due to the enlarged surface area and abundant active sites. Here, we evaluated the OER activity of the hierarchical Co₃O₄@Co_xFe_3−xO₄ film using a typical three-electrode setup in O₂-saturated 1 M KOH electrolyte. The Co_xFe_3−xO₄ nanoplate film (SEM, EDS and XRD results are shown in Fig. S4†) and Co₃O₄ nanowire film were also tested as controls. The polarisation curves demonstrated that the hierarchical Co₃O₄@Co_xFe_3−xO₄ film exhibited an onset potential (Fig. 5A, defined as the starting point of the linear range of the Tafel slope) of ∼1.54 V vs. RHE and an overpotential of ∼390 mV at ∼150 mA cm⁻² (η₁₅₀). On the contrary, the Co_xFe_3−xO₄ nanoplate film showed a similar onset potential (∼1.54 V), but the increase in OER current was much slower, leading to a higher overpotential (∼437 mV) at ∼150 mA cm⁻². For the Co₃O₄ nanowire film, the onset potential was only ∼1.59 V with an η₁₅₀ value of ∼510 mV, demonstrating that the secondarily formed Co_xFe_3−xO₄ was a more active electrocatalyst for OER. To gain more insight into the OER activity, Tafel plots derived from polarisation curves were constructed (Fig. 5B). The resulting Tafel slope of the hierarchical Co₃O₄@Co_xFe_3−xO₄ film was ∼53 mV dec⁻¹, which was similar to that of the Co_xFe_3−xO₄ nanoplate film, but much smaller than that of the Co₃O₄ nanowire film (∼96 mV dec⁻¹). This indicated that the formation of Co_xFe_3−xO₄ was beneficial for accelerating the OER kinetics.^37,38

Fig. 5 (A) Polarisation curves of the hierarchical Co₃O₄@Co_xFe_3−xO₄ film (black line), Co_xFe_3−xO₄ nanoplate film (blue line) and Co₃O₄ nanowire film (red line); (B) Tafel plots of the Co₃O₄@Co_xFe_3−xO₄ film (black line), Co_xFe_3−xO₄ (blue line), and Co₃O₄ (red line); (C–E) electrochemical surface area (ESA) measurements of the hierarchical Co₃O₄@Co_xFe_3−xO₄ film, Co_xFe_3−xO₄ nanoplate film and Co₃O₄ nanowire film, respectively; and (F) stability of the hierarchical Co₃O₄@Co_xFe_3−xO₄ film at high current density at an applied potential of 1.61 V after IR correction.

The surface areas of three samples (Co₃O₄ nanowire film, hierarchical Co₃O₄@Co_xFe_3−xO₄ film and Co_xFe_3−xO₄ nanoplate film) were also tested by both the BET method and electrochemical testing (Fig. 5C–E and S5†). The BET results revealed that the hierarchical Co₃O₄@Co_xFe_3−xO₄ film showed the highest surface area (0.689 m² cm⁻²), much higher than those of the other two samples (0.466 m² cm⁻² for the Co_xFe_3−xO₄ nanoplate film and 0.389 m² cm⁻² for the Co₃O₄ nanowire film). The electrochemical results were in good accordance with the BET results. A high double layer capacitance (∼171.76 mF cm⁻²) was observed for the hierarchical Co₃O₄@Co_xFe_3−xO₄ film, corresponding to a roughness factor of ∼2862.³⁹ On the contrary, the Co₃O₄ nanowire film and Co_xFe_3−xO₄ nanoplate film showed significantly inferior electrochemical surface areas (∼82.79 mF cm⁻² and ∼90.16 mF cm⁻² for Co_xFe_3−xO₄ nanoplate and Co₃O₄ nanowire films, respectively), further demonstrating the effectiveness of hierarchical construction in improving the porosities of the materials.

The long-term stability of a catalytic electrode is another critical issue to consider for commercial applications. In this case, a high and stable current density (>200 mA cm⁻²) with negligible degradation for 10 h was observed for the hierarchical Co₃O₄@Co_xFe_3−xO₄ film (Fig. 5F), indicating a prominent stability at a constant applied potential. The combination of the above features (small overpotential and high durability) validated the hierarchical Co₃O₄@Co_xFe_3−xO₄ film as an advanced and promising OER electrode.

We attributed the significantly enhanced OER performance to the secondarily formed Co_xFe_3−xO₄ nanoplates, which act as a more active electrocatalyst for OER and result in the formation of a highly porous hierarchical structure. The hierarchical Co₃O₄@Co_xFe_3−xO₄ film showed a high intrinsic OER activity (a low onset potential of ∼1.54 V and a small Tafel slope of ∼53 mV dec⁻¹), similar to that of the Co_xFe_3−xO₄ nanoplate film, but much higher than that of the pure Co₃O₄ nanowire film. This indicated that the introduction of Fe into the Co matrix would greatly enhance the intrinsic OER activity. Co is generally considered as the active centre for OER because of its suitable binding energy (not too strong and not too weak) to oxygen intermediates. Moreover, a recent study reported that an Fe-doped Co₃O₄ replica showed a much enhanced OER activity compared to pure Co₃O₄, which was attributed to the formation of the spinel phase of CoFe₂O₄. The different electronic structure of the material might affect the conductivity and charge transferability, thereby influencing the OER activity.^32,40 Therefore, it is reasonable that Co_xFe_3−xO₄ derived from CoFe–LDH is a potential electro-catalyst for water oxidation.

More importantly, the unique hierarchical architecture should also play an essential role in the enhanced OER performance. Compared with the nanowire film, the hierarchical structure can offer an even higher surface area and porosity, while the conductivity is well preserved.^15,41 The hierarchical porosity could accelerate the diffusion of OH⁻ ions, accelerating the kinetics. Therefore, the improved OER performance of the hierarchical Co₃O₄@Co_xFe_3−xO₄ film could be easily understood as a combination of inducing a more active electro-catalyst and constructing a hierarchical architecture at the electrode surface.

In summary, we have developed a facile and cost-effective method to fabricate a hierarchical Co₃O₄@Co_xFe_3−xO₄ film with mixed Co and Fe oxide as the shell and Co₃O₄ as the core on Ni foam. This hierarchical film offers a large surface area, high porosity and good conductivity, which are all beneficial in promoting surface electrochemical reactions. As an example, the hierarchical Co₃O₄@Co_xFe_3−xO₄ film exhibited a significantly enhanced OER performance relative to the Co_xFe_3−xO₄ nanoplate film and Co₃O₄ nanowire film. The highly active Co_xFe_3−xO₄ nanoplates and the 3D porous structure are responsible for the high OER performance. This work provides a propitious route for designing low-cost, high-performance and scalable OER electrodes, which are necessary components for water splitting and metal–air batteries aimed at achieving maximum market penetration of intermittent renewable energy.

Acknowledgements

This work was supported by the Nature Science Foundation of Hebei Province (E2013407124) and the Education Department of Hebei Province (ZH2012039, Z2010258).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, XPS survey, mapping data results. See DOI: 10.1039/c4ra13122c

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