Polyaniline@MOF fiber derived Fe–Co oxide-based high performance electrocatalyst

Abstract

Electrochemical energy conversion and storage are important and coupled with a number of electrocatalytic processes. Renewable hydrogen, as a promising energy carrier, is closely related to the oxygen evolution reaction (OER). However, the OER kinetics is slow due to the slow 4e⁻ transfer process. The low-cost transition metal-based catalysts provide broad prospects for the development of efficient and stable OER catalysts. Designing an efficient transition metal-based OER catalyst is beneficial to improve the overall efficiency of water decomposition. Here, we developed a new three-dimensional carbonized polyaniline fiber material loaded with Fe–Co oxide nanoparticle (denoted as 3D-CPF/FeCoO_x-Nanoparticles) material by doping Fe to activate the catalytic activity of cobalt-based catalysts, introducing new reaction pathways and using the synergistic enhancement between metal nanoparticles and carbonized polyaniline fibers. Benefiting from the synergistic enhancement of the conductive three-dimensional carbonized polyaniline fibers and the high catalytic activity of FeCoO_x nanoparticles, the 3D-CPF/FeCoO_x-Nanoparticles only need a potential of 1.63 V to obtain a current density of 10 mA cm⁻². Excitingly, the 3D-CPF/FeCoO_x-Nanoparticles have exhibited potential applications in supercapacitors.

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Introduction

The economic development of human society has made energy and environmental problems increasingly prominent. Water splitting can obtain clean and renewable energy, which is an effective way to solve the energy and environmental crisis. Water splitting includes the oxygen evolution reaction (OER, 4OH⁻ → 2H₂O + O₂ + 4e⁻ in base) and the hydrogen evolution reaction (HER, 2H₂O + 2e⁻ → H₂ + 2OH⁻ in base). The OER is more complex and challenging as it involves four sequential proton-coupled electron transfer steps as well as oxygen–oxygen bond formation, making the development of OER catalysts more challenging.^1,2 Pt-, Ru-, and Ir-based catalysts show excellent OER catalytic activity. However, it cannot be promoted on a global scale due to its low stability and abundance.³ The development of OER catalysts has become the key to solving energy problems.

It is demonstrated that transition metal-based catalysts such as Fe, Co, Ni, and Mo have OER catalytic activity,^4–15 in which Co-based OER catalysts have become the focus due to their excellent catalytic activity.^16–20 As a transition metal with higher abundance and lower price, cobalt become a popular metal in OER catalysts. Cobalt oxide,^21–24 monoatomic cobalt,^25–28 Co-based nanoparticles,^29–31etc. have shown broad prospects in the OER.

At present, quite a few researchers improve the conductivity and stability of the catalyst by loading cobalt on different carbon substrates or increase the number of active sites by increasing the distribution of cobalt. However, further improvement of the catalytic activity of Co-based catalysts is still a challenge. In response, improving the catalytic activity of Co-based catalysts by element doping has entered the vision of researchers. Different element doped Co-based catalysts by different methods have different effects on the OER performance. A large number of studies have shown that appropriate Fe doping can have a positive effect on the OER catalytic activity of Co-based catalysts. Moreover, Fe–Co oxide-based catalysts have become popular in recent research of OER catalysts and supercapacitors due to their low cost, high catalytic activity, and high specific capacitance.^32–38 Generally speaking, Fe has lower OER catalytic activity,³⁹ but the addition of Fe in the Co-based catalyst can activate the catalytic site of the Co-based catalyst and improve the reaction activity.^40–43 Considering the conductivity of Fe doped Co-based OER catalysts, the combination of an Fe–Co MOF and high conductivity carbon materials has become an ideal choice. Under the conditions of improving the catalytic activity of the Co-based catalyst, we have made a unique exploration on how to combine the Fe–Co MOF with three-dimensional structures and carbon materials.

We reasoned that the nanometerization of Co-based catalysts can fully expose the active sites of the catalytic reaction, and the introduction of Fe on the basis of cobalt may enhance the catalytic efficiency. However, nano-catalysts also need the support of the conductive matrix. In this study, we tried to use carbonized three-dimensional polyaniline fibers as the support matrix to explore the interaction between carbonized polyaniline fibers and nanoparticles, and through comparative experiments to explore the promotion mechanism of Fe doping, OER catalytic activity and supercapacitor performance of Co-based catalysts.

Experimental

Preparation of three-dimensional polyaniline fibers (3D-PF)

In this paper, 3D-PF are prepared by a rapid mixing reaction method. First, aniline (6.4 mmol) and (NH₄)₂S₂O₄ (1.6 mmol) were dissolved in 1 M HCl solution. Then the (NH₄)₂S₂O₄ solution was poured into the aniline solution under the conditions of high-speed magnetic stirring, until the solution turns dark green, the mixture solution of DMF : absolute ethanol : deionized water = 10 : 2 : 1 was centrifuged and washed multiple times to obtain 3D-PF.

Preparation of 3D-CPF/FeCoO_x-Nanoparticles

In the standard synthesis method of 3D-CPF/FeCoO_x-Nanoparticles, Co(NO₃)₂·6H₂O (68 mg), FeCl₂·4H₂O (90 mg) and H₄DOBDC (40 mg) were taken in a mixed solution of 30 mL DMF : absolute ethanol : deionized water = 10 : 2 : 1 and stirred to obtain a dark blue solution. Then 3D-PF was added, stirred well, poured into the reaction kettle and subjected to hydrothermal treatment at 120 °C for 24 hours. After the hydrothermal reaction, the solution was washed three times with absolute ethanol, centrifuged and dried at 60 °C for 12 hours; the resulting powder was placed in a tube furnace under nitrogen protection and pyrolyzed at 500 °C for 2 hours to obtain 3D-CPF/FeCoO_x-Nanoparticles.

Preparation of carbonized three-dimensional polyaniline fibers (3D-CPF)

We approached the fabrication of 3D-CPF by taking a small amount of 3D-PF, heated to 500 °C pyrolysis for 2 hours to obtain 3D-CPF.

Preparation of FeCoO_x

We approached the fabrication of FeCoO_x by taking 68 mg Co(NO₃)₂·6H₂O, 90 mg FeCl₂·4H₂O and 40 mg H₄DOBDC in a mixed solution of 30 mL mixture (DMF : absolute ethanol : deionized water = 10 : 2 : 1) and stirring to obtain a dark blue solution. Then the mixture was poured into the reaction kettle and subjected to hydrothermal reaction at 120 °C for 24 hours. After the hydrothermal reaction, it was washed three times with absolute ethanol, centrifuged and dried at 60 °C for 12 hours; the resulting powder was placed in a tube furnace under nitrogen protection and heated to 500 °C pyrolysis for 2 hours to obtain FeCoO_x.

Results and discussion

Scheme 1 clearly shows the synthesis process of 3D-CPF/FeCoO_x-Nanoparticles. In short, the 3D-PF is prepared by the rapid mixing reaction, and then a layer of Fe–Co oxide is wrapped on the 3D-PF by the hydrothermal method, and finally 3D-CPF/FeCoO_x-Nanoparticles are obtained through a pyrolysis process (see details in the Experimental section).

Scheme 1 Synthesis schematic of the 3D-CPF/FeCoO_x-Nanoparticles.

The 3D-PF (Fig. 1a) synthesized by the rapid mixing reaction has a diameter of about 30 nm and a smooth surface with 3D branches, which provides favorable conditions for the next step of loading Fe–Co oxide on its surface. The Fe–Co oxide was loaded on the surface of the 3D-PF (Fig. 1b) in the form of fine particles by the hydrothermal method. The surface of the 3D-PF became rough and the diameter did not change. Then the 3D-CPF/FeCoO_x-Nanoparticles (Fig. 1c) is obtained by pyrolyzing the 3D-PF loaded with Fe–Co oxide, and the surface of the carbonized 3D-PF is loaded with nanoparticles with a diameter of less than 100 nm. The TEM image (Fig. 1d) shows that the diameter of about 10 nanometers is uniformly coated on the surface of the 3D-CPF/FeCoO_x-Nanoparticles, and the nanoscale transition metal particles provide more catalytically active sites. The lattice fringes in the HR-TEM (Fig. 1e) image of CoFe₂O₄ and C indicate that the nanoparticles contain CoFe₂O₄. In addition, the HADDF and EDS spectra (Fig. 1f) show the distribution of metal elements and oxygen elements. Fe, Co, and O are uniformly distributed on the 3D-CPF/FeCoO_x-Nanoparticles, and the distribution is the same, indicating that the nanoparticles on the surface of the 3D-CPF/FeCoO_x-Nanoparticles are Fe–Co oxide. This proves that with the help of the residual group on the surface of 3D-CPF, the Fe–Co MOF was successfully combined with carbon materials, and transition metal-based nanoparticles were generated on the surface of carbon materials.

Fig. 1 (a–c) SEM images of the 3D-PF, the 3D-PF coated with FeCoO_x and the 3D-CPF/FeCoO_x-Nanoparticles, respectively. (d) TEM image, (e) corresponding HR-TEM images, and (f) HADDF images and EDS elemental mapping images of the 3D-CPF/FeCoO_x-Nanoparticles.

The composition of the 3D-CPF/FeCoO_x-Nanoparticles is further explored by the XRD pattern (Fig. 2a). The (220), (311), (511), and (440) crystal planes of CoFe₂O₄ and the HR-TEM images prove that the nanoparticles contain CoFe₂O₄. The deconvoluted XPS spectra further verify the chemical composition of nanoparticles. The Co 2p spectrum (Fig. 2b) is deconvoluted, and 779.3 eV and 780.7 eV correspond to the 2p 3/2 orbitals of Co³⁺ and Co²⁺, respectively.⁴⁴ The combination of the EDS element distribution verified that there are other Co oxides besides CoFe₂O₄ on the nanoparticles of the 3D-CPF/FeCoO_x-Nanoparticles, but considering that there is no peak of Co oxide in XRD, lower crystallinity of Co oxide is indicated; the deconvoluted Fe 2p spectrum (Fig. 2c) shows that 709.9 eV and 724 eV belong to Fe³⁺ 2p 3/2 and 2p 1/2, respectively, which is consistent with the existence of CoFe₂O₄. The metal-oxo (M=O) complex formed after protonation of H₂O(OH⁻) and combining with metal is the key to forming O–O bonds and executing the OER.⁴⁵ The addition of Fe has different effects on Co^II/III and Co^III/IV redox transitions, decreasing the energetic barrier for disproportionation of di-μ-oxo bridged Co^III–Co^III motifs, thus activating the secondary reaction pathway and providing di-μ-oxo bridged Fe–Co sites.⁴⁶ A new reaction path enhanced the catalytic reaction activity and efficiency of the 3D-CPF/FeCoO_x-Nanoparticles. In the XPS spectrum (Fig. 2d) of C 1s, 83.6 eV corresponds to the C–C bond.⁴⁷ And the content ratio (Fe : Co = 62.07 : 37.93) of metal elements obtained by XPS also showed that there are other Co oxides in the nanoparticles besides CoFe₂O₄ (Fig. 2e).

Fig. 2 (a) XRD pattern, (b–e) high resolution XPS spectra of Co 2p, Fe 2p, and C 1s and full spectrum (inset shows the EDS elemental mapping images) of the 3D-CPF/FeCoO_x-Nanoparticles.

A three-electrode device was used to detect the OER catalytic activity of the sample in 0.1 M KOH, and the 3D-CPF, FeCoO_x and the 3D-CPF/FeCoO_x-Nanoparticles were compared for OER performance (Fig. 3a). Among them, the potentials required for the 3D-CPF/FeCoO_x-Nanoparticles and FeCoO_x to reach a current density of 10 mA cm⁻² are 1.63 V and 1.75 V, respectively. In addition to a lower OER energy barrier and a new reaction path, the synergistic enhancement between FeCoO_x-Nanoparticles and 3D-CPF is an important reason why 3D-CPF/FeCoO_x-Nanoparticles have such a high current density at the same potential. An excellent OER catalyst should have appropriate oxygen adsorption free energy (ΔG(O*)) to tune the energetic barrier during adsorption and desorption. Especially in the process of transition metal-based nanoparticle catalysts catalyzing the OER, due to metal core's too stronger oxygen adsorption, the higher desorption barrier limits the OER catalytic efficiency.³⁰ The electrons of the metal core enter the graphitized carbon layer to increase the electron density of the graphitized carbon layer.^30,48 The electron densities of the FeCoO_x-Nanoparticles and 3D-CPF are reconciled with each other, which activates the OER catalytic activity of 3D-CPF and mediates the adsorption of oxygen by the FeCoO_x-Nanoparticles, thereby improving the catalytic activity of the FeCoO_x-Nanoparticles. In addition, 3D-CPF provides an efficient electron transmission channel for FeCoO_x-Nanoparticles to further improve 3D-CPF/FeCoO_x-Nanoparticles’ catalytic efficiency.

Fig. 3 (a) LSV curves of different samples in 0.1 M KOH. (b) Comparison chart of electrochemical surface area (ECSA) of different samples. (c) Comparison chart of the Tafel plot of different samples. (d) Stability plots of the 3D-CPF/FeCoO_x-Nanoparticles continuously catalyzing the OER in O₂-saturated 0.1 M KOH.

In order to further explore the synergistic enhancement between 3D-CPF and FeCoO_x-Nanoparticles, the capacitance property (Fig. S1, ESI†) was obtained by linear voltammetry and the electrochemically active surface area (ECSA). In the comparison of ECSA (Fig. 3b), larger ECSA of FeCoO_x (0.456 cm²) than that of the 3D-CPF/FeCoO_x-Nanoparticles (0.425 cm²) further confirmed that there is a synergistic enhancement between 3D-CPF and FeCoO_x-Nanoparticles, and 3D-CPF provides a high-speed electron transmission channel for FeCoO_x-Nanoparticles. The Tafel curve (Fig. 3c) showed the rate-determining step for the catalyst to catalyze the OER, in which the rate-determining step of the 3D-CPF/FeCoO_x-Nanoparticles (28 mV dec⁻¹) and FeCoO_x (47 mV dec⁻¹) is the desorption of oxygen, and the rate-determining step of the 3D-CPF (168 mV dec⁻¹) is OH⁻ adsorption. Fig. 3a shows the higher OER catalytic activity of the 3D-CPF FeCoO_x-Nanoparticles, which is probably related to new active reaction pathway induced by addition of Fe.⁴⁶ Considering the application of the 3D-CPF/FeCoO_x-Nanoparticles in daily life, a stability test (Fig. 3d) of 10 000 s was conducted on the 3D-CPF/FeCoO_x-Nanoparticles. After a long period of reaction, the 3D-CPF/FeCoO_x-Nanoparticles still maintain excellent catalytic activity, indicating the wide application of the 3D-CPF/FeCoO_x-Nanoparticles.

I = Cv^b (2)

i(V) = k₁ + k₂v^0.5 (3)

(1) The equation for calculating capacitance, where ‘I’ is the discharge current, ‘Δt’ is the discharge time, ‘m’ is the active mass of the nanocomposite, ‘ΔV’ is the voltage window, and the test voltage range is 0–0.5 V. (2) The equation for the capacitance effect is characterized by the volt-ampere response at various scan rates, where ‘I’ is the current, ‘v’ is the scan rate, and ‘C’ and ‘b’ are adjustable parameters. (3) The equation for calculating the contribution rate of capacitance, where ‘I’ is the current at a specific potential ‘V’, ‘v’ is the scan rate, ‘k₁’ and ‘k₂’ are potential functions, and ‘k₁’ and ‘k₂v^0.5’ represent the surface capacitance and diffusion control capacitance at a specific voltage, respectively.

Considering that the Fe–Co MOF derivatives have supercapacitor performance, we conducted a cyclic voltage test (Fig. S2, ESI†) of the 3D-CPF/FeCoO_x-Nanoparticles, the 3D-CPF and the FeCoO_x in 2 M KOH at different sweep rates (2–60 mV s⁻¹). We found that the 3D-CPF/FeCoO_x-Nanoparticles, the 3D-CPF and the FeCoO_x may have the potential of a supercapacitor, and the pseudo-capacitance properties of the 3D-CPF/FeCoO_x-Nanoparticles have been confirmed by the constant current charge–discharge curve (Fig. 4a). The rate characteristic curve (Fig. 4b) is calculated using eqn (1), and we found that the specific capacitance of the 3D-CPF/FeCoO_x-Nanoparticles (2136 F g⁻¹) at a current density of 1 A g⁻¹ is significantly higher than that of the 3D-CPF (510 F g⁻¹) and the FeCoO_x (550 F g⁻¹), showing that the synergistic enhancement effect of the 3D-CPF and the FeCoO_x enhances the capacitive properties of the 3D-CPF/FeCoO_x-Nanoparticles. The b value (Fig. 4c) of Fig. S3a (ESI†) was calculated through eqn (2). In addition to the diffusion-controlled redox intercalation at 0.35 V, the 3D-CPF/FeCoO_x-Nanoparticles exhibit non-diffusion-controlled surface redox reaction contribution capacitance at different potentials. By combining Fig. S3b (ESI†) with eqn (3), the capacitance contribution is maintained at 89% (Fig. 4d), indicating that the capacitive charge storage contribution is dominant.⁴⁹ The increase of the scan rate has a lower effect on the capacitance contribution, and the insertion contribution decreases with the increase of the scan rate because the redox embedding process is diffusion controlled, and the time for the ions to diffuse into the main lattice will increase with the scan rate and lower. Therefore, the redox pseudocapacitive behavior will dominate charge storage at a high rate. The electrical conductivity and ion migration rate of the 3D-CPF/FeCoO_x-Nanoparticles were further studied by electrochemical impedance spectroscopy (Fig. S4a, ESI†). The arc radius of the 3D-CPF/FeCoO_x-Nanoparticles in the high-frequency region is smaller than that of the FeCoO_x, indicating that the 3D-CPF/FeCoO_x-Nanoparticles overcome the disadvantage of poor conductivity of the FeCoO_x. The greater linear slope of the low-frequency part indicates the smaller internal mass transfer resistance. The lower mass transfer resistance of the 3D-CPF/FeCoO_x-Nanoparticles demonstrates the ideal capacitance property. The stability test (Fig. S4b, ESI†) shows that the stability of the 3D-CPF/FeCoO_x-Nanoparticles has great potential for improvement.

Fig. 4 (a) Chrono-potential curve at different current densities of the 3D-CPF/FeCoO_x-Nanoparticles. (b) Magnification characteristic curve of different samples. (c) The b value of the 3D-CPF/FeCoO_x-Nanoparticles at different potentials. (d) Percentage capacitance contribution of the 3D-CPF/FeCoO_x-Nanoparticles at different scan speeds.

Conclusions

In summary, the introduction of Fe activates the OER catalytic activity of the Co-based catalyst, and further enhances the catalytic efficiency through synergistic enhancement, and the 3D-CPF/FeCoO_x-Nanoparticles have the potential to become a supercapacitor. In OER catalysis, the 3D-CPF/FeCoO_x-Nanoparticles require 1.63 V to achieve a current density of 10 mA cm⁻², demonstrating the excellent effect of a synergistic enhancement effect. In the property test of supercapacitors, the specific capacitance of the 3D-CPF/FeCoO_x-Nanoparticles reached 2136 F g⁻¹ at a current density of 1 A g⁻¹. This research provides a new idea for the synthesis of carbonized polyaniline fiber as an efficient iron-doped Co-based catalyst.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation (Grant No. 21605057 and No. 21705056), the Natural Science Foundation of Shandong Province (No. ZR2016BQ07), the Open Founds of State Key Laboratory of Electroanaytical Chemistry (SKLEAC201907), and Study Abroad Fund.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj05423b

‡ Qiqi Sha and Jianrong Wang contributed equally to this work.

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