Hierarchical Ni–Fe layered double hydroxide/MnO₂ sphere architecture as an efficient noble metal-free electrocatalyst for ethanol electro-oxidation in alkaline solution

Abstract

Ni–Fe layered double hydroxide (LDH) nanosheets and hierarchical Ni–Fe LDH@MnO₂ spheres are synthesized by a facile and cost-effective approach for highly efficient ethanol electro-oxidation. The LDH@MnO₂ microspheres display excellent catalytic activity and robust durability for ethanol electro-oxidation, compared with the Ni–Fe LDH nanosheets. According to the analyses for nitrogen adsorption isotherms and electrochemical impedance spectra (EIS), it is inferred that the performance enhancement could be attributed to the MnO₂ increasing the concentration of OH_ads species on the Ni–Fe LDH surface. These OH_ads can react with C_1ad intermediate species to produce CO₂ or water soluble products, releasing the active sites on LDH for further electrochemical reactions. Therefore, it is expected that an effective noble-metal free catalyst for ethanol electro-oxidation could be obtained by tailoring structure and properties of LDHs and their composites.

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Introduction

Considerable efforts have been devoted to exploiting green, sustainable and efficient power sources owing to the increasing global demand for energy, coupled with the depletion of fossil fuels and the associated detrimental environmental impact.¹ Ethanol is a promising fuel for low temperature direct fuel cell reactions due to its high energy density, low toxicity, ease of storage and availability from different sources.^2–6 In direct ethanol fuel cells (DEFCs), noble metal based materials (Pt, Pd and their alloys) are the most effective catalysts due to their superior properties in the adsorption and dissociation of small organic molecules, but the high cost, limited availability as well as unsatisfactory cycle life restrict their practical applications.^2–13 In recent years, lots of explorations have been made to obtain advanced noble metal-based hetero-structures for improving the performance of DEFCs, such as graphene-supported Pt nanoparticles, free-standing Pd–Au bimetallic nanotubes and noble metal membranes.^14–22 Despite all this progress, these catalysts are also noble metal-based. How to develop effective noble metal-free anode catalysts for ethanol electro-oxidation remains a challenging goal.

Layered double hydroxides (LDHs) are a family of layered materials consisting of positively charged brucite-type octahedral layers where the charge-balancing anions and water molecules occupy the interlayer space.^14,15 They have been widely used in the fields of electrochemical sensors,¹⁶ super-capacitors¹⁷ and alkaline secondary batteries,¹⁸ owing to their novel structure and desirable properties. Recently, LDHs have been reported as promising noble metal-free electrode materials for the electrocatalytic oxidation of alcohol in alkaline medium.^14,15,19 However, a key challenge for LDHs in the application of anode catalysts is to enhance their electrochemical activity by tailoring their structures and properties, such as having a well-defined hierarchical architecture with high surface area and suitable pore-size distribution, in which all the electroactive species participate in the faradaic redox reaction and a fast mass transport and electron transfer are guaranteed, and finding a co-catalyst to influence intermediate adsorption–desorption.

Herein, we synthesized Ni–Fe LDH nanosheets and hierarchical Ni–Fe LDH@MnO₂ microspheres by a facile and cost-effective approach for highly efficient ethanol electro-oxidation. Furthermore, we hope to gain essential insight into the synergistic mechanism for electro-catalytic activity enhancement.

Experimental

Ni–Fe LDH synthesis

In a typical synthesis, co-precipitated Ni–Fe LDH was synthesized by simultaneous dropwise addition of 30 mL of 1.0 M Ni(NO₃)₂ and 30 mL of 0.5 M Fe(NO₃)₃ solutions to 100 mL of 1.5 M Na₂CO₃ solution while constantly stirring. The pH was adjusted to ∼10 by addition of NaOH solution. Then the vessel was transferred to a water bath at 65 °C for 6 hours. Finally the precipitate was filtered, washed with deionized water, and dried in air at 80 °C.

Ni–Fe LDH@MnO₂ sphere synthesis

To synthesize the composite electrocatalyst, 200 mg of MnO₂ spheres, prepared according to previous works,⁵ were dispersed into 100 mL deionized water and ultra-sonicated for 8 min. Then Ni–Fe LDH was synthesized and coated on the surface of MnO₂ sphere. The composite was harvested and denoted as LDH@MnO₂.

Material characterization

Powder X-ray diffraction (XRD) patterns of the as-prepared samples were collected on SmartLab XRD diffractometer using a Cu Kα source, with a scan step of 0.02° and a scan range between 5 and 90°. The morphology of the samples was investigated using a transmission electron microscopy (TEM JEOL JEM-2010 HR-TEM). The accelerating voltage was 200 kV. The specific surface area was measured using the Brunauer–Emmett–Teller (BET) method based on the nitrogen adsorption–desorption isotherm at 77 K on a Micrometritics ASAP2020 sorption analyzer.

Electrochemical characterization

Electrochemical measurements were performed on an electrochemical workstation (CHI 660D, CH Instruments Inc., Shanghai) using a traditional three-electrode mode. Pt plate was used as a counter electrode, Hg/HgO as a reference electrode, and glassy carbon (3 mm in diameter) coated with the as-prepared samples, as a working electrode. The working electrode was fabricated as follows: 4 mg of catalyst was dispersed in 2 mL of ethanol solution and sonicated for 5 min; 10 μL of the suspension was dripped onto the surface of a glassy carbon electrode and dried for 15 min; subsequently 2 μL of 0.5% Nafion solution (Sigma-Aldrich) was coated on the electrode surface and dried for another 5 min. Electrocatalytic oxidation of ethanol on the working electrode was measured in 1.0 M KOH + 1.0 M ethanol solution by cyclic voltammetry in the potential range from 0.2 to 0.7 V. Electrochemical impedance spectra (EIS) were measured at 0.5 V from 100 kHz to 0.01 Hz and the perturbing AC amplitude was 5 mV. Electrical current density was calculated by normalizing electrical current on the area of the 3 mm diameter glassy carbon electrode. The chronoamperometry (CA) was conducted at 0.5 V for 3600 s.

Results and discussion

The typical XRD pattern of the as-prepared Ni–Fe LDH is presented in Fig. 1(a), showing the characteristic diffraction peaks at 2θ of 11.5, 23.3 and 34.6° corresponding to the (003), (006) and (012) plane reflections of 2D hydrotalcite-like materials, evidencing the formation of LDH particles (JCPSD card no.: 40-0215). As shown in Fig. 1(b), all diffraction peaks can be unambiguously assigned to the single-phase γ-MnO₂ (JCPSD card no.: 14-0644). According to the XRD pattern shown in Fig. 1(c), it is found that the LDH@MnO₂ sphere is the phase mixture of γ-MnO₂ and Ni–Fe LDH. There is no new phase generated in the synthesis process.

Fig. 1 XRD patterns of Ni–Fe LDH (a), MnO₂ (b) and LDH@MnO₂ sphere (c).

The TEM image, as shown in Fig. 2(a), displays MnO₂ particles that are mainly spherical and the diameter of these microspheres is about 1–2 μm. Fig. 2(b) presents the morphology of Ni–Fe LDH and the nanosheet structure of this as-prepared sample is apparent. For LDH@MnO₂, as shown in Fig. 2(c), it holds the spherical morphology of the MnO₂ and is coated with Ni–Fe LDH nanosheets on its surface. Due to the adsorption of Ni²⁺ and Fe³⁺ on the MnO₂ spheres surface, Ni–Fe LDH nanosheets could cover the MnO₂ sphere uniformly when the precipitant was added. Furthermore, the heterogeneous interface between MnO₂ and the solution could facilitate the nucleation of Fe–Ni LDH. This may be another reason for MnO₂ to be covered uniformly with Fe–Ni LDH.

Fig. 2 TEM images of the as-prepared samples, (a) MnO₂ sphere; (b) Ni–Fe LDH and (c) LDH@MnO₂.

The N₂ adsorption and desorption isotherms shown in Fig. 3(a) and (c) clearly indicate the presence of mesopores in the prepared samples, classified as type IV as defined by the International Union of Pure and Applied Chemistry (IUPAC). A hysteretic loop between the adsorption and desorption branches can be considered type H4, indicative of slit-like pores in the samples. The sample of Ni–Fe LDH shows a BET surface area of 208.33 m² g⁻¹, which is much larger than that of the LDH@MnO₂ composite (103.47 m² g⁻¹).The pore volumes of the as-prepared Ni–Fe LDH and LDH@MnO₂ microsphere are 0.883 and 0.208 cm³ g⁻¹, respectively. As shown in Fig. 3(b) and (d), the pore size distributions from the adsorption branch of the isotherms calculated using the Barrett–Joyner–Halenda (BJH) method reveal that the pore sizes of Ni–Fe LDH concentrate around 5–30 nm and for LDH@MnO₂, most of the pores have diameters under 20 nm.

Fig. 3 N₂ adsorption–desorption isotherms of (a) Ni–Fe LDH; (c) LDH@MnO₂ and pore size distribution of (b) Ni–Fe LDH; (d) LDH@MnO₂.

The catalytic activities of the as-prepared catalysts towards ethanol electro-oxidation were subsequently evaluated in an alkaline medium. The oxidation current was normalized to the electrode surface area; this allowed the current density to be directly used to compare the catalytic activity of different samples. The cyclic voltammograms of ethanol oxidation on the prepared catalysts are shown in Fig. 4. As shown in Fig. 4(a), the cyclic voltammograms of MnO₂ in alkaline solutions indicate that MnO₂ does not have catalytic activity for ethanol electro-oxidation. From Fig. 4(b) and (c), it is seen that the CVs of Ni–Fe LDH and LDH@MnO₂ recorded in 1.0 M KOH solution without ethanol consist of a pair of redox peaks, corresponding to the reversible redox of Fe²⁺/Fe³⁺ associated with OH⁻. In the presence of 1.0 M ethanol, both Ni–Fe LDH and LDH@MnO₂ display electrocatalytic behavior to ethanol oxidation. The onset potentials of the forward anodic peak for both Ni–Fe LDH and LDH@MnO₂ were 0.557 V vs. Hg/HgO. Furthermore, as shown in Fig. 4(d), the electrocatalytic behavior for ethanol oxidation is significantly enhanced for LDH@MnO₂ spheres in comparison with the Ni–Fe LDH nanosheets. Generally speaking, a larger specific surface area and pore volume can provide more electroactive sites as well as effective diffusion channels for electrolyte ions. In particular, abundant mesopores benefit the mass diffusion and electron transfer, which guarantees highly efficient electro-oxidation reactions. However, according to the analysis of the nitrogen adsorption isotherms (Fig. 3), though the LDH@MnO₂ spheres possess a smaller specific surface area and mesopore volume than the Ni–Fe LDH nanosheets, the electrocatalytic activity of the former is remarkably superior to that of the latter. Therefore, the performance improvement of LDH@MnO₂ cannot be attributed to the difference of surface and pore properties.

Fig. 4 (a–c) The cyclic voltammograms of MnO₂ (a), Ni–Fe LDH (b) and LDH@MnO₂ spheres (c); (d) cyclic voltammograms comparison of three catalysts; (e) EIS plots of Ni–Fe LDH and LDH@MnO₂ spheres at 0.5 V; (f) the equivalent electrical circuit; (g) potential cycling stability of Ni–Fe LDH and LDH@MnO₂ spheres and (h) CA curves of Ni–Fe LDH and LDH@MnO₂ spheres in 1.0 M KOH + 1.0 M ethanol solution at a potential of 0.5 V at 25 °C.

EIS was further applied to analyze the electrocatalytic performance of Ni–Fe LDH and LDH@MnO₂. Fig. 4(e) represents the EIS plots of Ni–Fe LDH and LDH@MnO₂ at 0.5 V in 1.0 M KOH + 1.0 M ethanol solution and the data were analyzed by an equivalent circuit shown in Fig. 4(f). In the equivalent circuit, R_s is the sum of resistance of electrolyte, electrode material and the contact resistance at the interface of the active material/current collector; Q is a constant angle element, which represents the double layer capacitance; R_t is the charge transfer resistance; R_c is the resistance of intermediate ad-layer and L is the inductance induced by the intermediate. The values of R_s, Q, R_t, R_c and L were calculated from the CNLS fitting of the experimental impedance spectra and their resulting values are listed in Table 1.

Table 1 Fitting results of EIS

	Rs (Ω)	Q (Y s^−n)	n	Rt (Ω)	Rc (Ω)	L (H)
Ni–Fe LDH	6.149	2.88 × 10^−5	0.7833	2157	1747	1.463 × 10^4
LDH@MnO2	6.682	2.523 × 10^−5	0.7883	2053	1154	9610

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According to the fitting results in Table 1, it can be seen that the resistances for LDH@MnO₂, no matter whether R_t or R_c, are smaller than those for Ni–Fe LDH. In other words, the former has a smaller charge transfer resistance and intermediate ad-layer resistance. Moreover, it is found from the EIS results that mass diffusion is not a key factor for the electro-oxidation process and thus both samples may provide effective diffusion channels despite the differences of surface area and pore volume. Additionally, it is well known that the adsorption of OH_ads species onto MnO₂ is more favorable.⁶ Therefore, it is inferred that MnO₂ could increase the concentration of OH_ads species on Ni–Fe LDH surface, and these OH_ads can react with C_1ad intermediate species to produce CO₂ or water soluble products, releasing the active sites on LDH for further electrochemical reactions.^5,13–15,23 This results in a better electrocatalytic performance in the ethanol electro-oxidation process than Ni–Fe LDH. Further investigations of the mechanism are ongoing at our lab.

To evaluate the electrocatalytic activity and stability of Ni–Fe LDH and LDH@MnO₂ composite under continuous operating conditions, CV cycling tests were carried out in a 1.0 M KOH + 1.0 M ethanol solution, as shown in Fig. 4(g). After 100 potential cycles, 90.8% of the initial catalytic activity was still maintained for the LDH@MnO₂ spheres. This is highly superior to the Ni–Fe LDH nanosheets (60.2%), indicating the greatly improved stability of the LDH@MnO₂ spheres. In addition, the cycling stability of these two samples was also studied by testing CA curves. Fig. 4(h) shows the current density curves versus time recorded at 0.5 V for 3600 s. It was found that the oxidation current density on the LDH@MnO₂ spheres is much higher than that on Ni–Fe LDH nanosheets over the whole time range, further demonstrating a significantly enhanced electrocatalytic activity. Moreover, this also indicates that the LDH@MnO₂ catalyst possesses good long-term durability for ethanol electro-oxidation in alkaline media.

Conclusions

In this work, Ni–Fe LDH nanosheets and hierarchical Ni–Fe LDH@MnO₂ spheres were synthesized by a facile and cost-effective approach for highly efficient ethanol electro-oxidation. According to the CV curves, it is shown that MnO₂ has no catalytic activity for ethanol electro-oxidation, and the LDH@MnO₂ microspheres display excellent catalytic activity and robust durability for ethanol electro-oxidation, compared with the Ni–Fe LDH nanosheets. This can be ascribed to MnO₂ increasing the concentration of OH_ads species on the Ni–Fe LDH surface, these OH_ads can react with C_1ad intermediate species to produce CO₂ or water soluble products, releasing the active sites on LDH for further electrochemical reactions. It is expected that an effective noble-metal free catalyst for ethanol electro-oxidation could be obtained by tailoring the structure and properties of LDHs and their composites.

Acknowledgements

The authors are grateful for the financial support by One Hundred Talent Program of Chinese Academy of Sciences, Chinese National Programs for High Technology Research and Development (2014AA06A513), as well as by the NSFC (51302264) of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15718h

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