[email protected]Co-FeLDHs as highly efficient electrocatalysts for oxygen evolution reaction

Catalysis Communications 133 (2020) 105826

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Short communication

PANI@Co-FeLDHs as highly efficient electrocatalysts for oxygen evolution reaction

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Xiulun Sun, Xinjie Liu, Rongmei Liu , Xueying Sun, Anran Li, Wen Li School of Biological and Chemical Engineering, Anhui Polytechnic University, Wuhu, Anhui 241000, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: PANI@Co-Fe LDHs Electrocatalyst OER Electronic synergy

We reported a new and facile synthesis route to prepare polyaniline (PANI) nanowire@Co-Fe LDHs nanosheets in two steps. The PANI@Co-Fe LDHs has excellent OER activity with a small overpotential of 261 mV at the current density of 10 mA·cm−2and a small Tafel slope of 67.85 mV·dec−1. The PANI@Co-Fe LDHs also has longterm performance and stability. The excellent electrocatalytic performance of the catalyst depends on the good electrical conductivity of PANI and the effect of the two-dimensional (2D) layered structure of Co-Fe LDHs. The work opens a new application for energy storage and conversion.

1. Introduction Environmental pollution and shortages of fossil fuels are driving people to find green-clean energy and design efficient energy storage and conversion devices [1–6]. Electrochemical water splitting is an important method to convert those energy sources into ideal chemical energy by producing hydrogen [7]. Electrochemical water splitting includes hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Because the large overpotential and the slow kinetic process of OER, it becomes the main factor restricting the electrocatalytic water splitting [8–10]. Therefore, it is a research hotspot to develop an OER catalyst with excellent performance [11]. Layered double hydroxides (LDHs) usually have a peculiar layered structure. This layered structure makes the LDHs have faster ion transport rate and higher specific surface area, which promotes its use in electrocatalytic water splitting. So far, different LDHs, such as Co-Ni [12–14], Co-Cr [15], Co-Fe [16], Ni-Fe [17,18] and Co-Mn [19], due to their excellent electrochemical performance have been reported as electrocatalysts for OER. Among them, Co-Fe LDHs have attracted great interests in the past few years due to its application in energy storage and conversion devices. Co-Fe LDHs is composed of a positively charged brucite-like matrix layer and a sandwich region. The octahedral vacancies between alternating pairs of OH planes of the brucite swatch are occupied by metal cations, while the charge-compensating anions and solution molecules fill in inter-layer regions [20–22]. Because Co and Fe ions can provide rich redox reactions during electrochemical reactions, researchers have paid more and more attention to the application of Co-

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Fe LDHs as an electrocatalyst in recent years. However, its electrochemical activity on OER is limited by the difference of its inherent conductivity. Meanwhile, due to agglomeration, it greatly hinders the transport of electrons and/or protons and reduces the number of available catalytic active sites. An effective method to solve this problem is to load it on conductive polymers. As a conductive polymer, polyaniline (PANI) has the characteristics of good electrical conductivity, high flexibility and low price. Simultaneously, after the addition of PANI, the dispersibility of the electrocatalysts is increased, thereby increasing the active site of the reaction. He et al. [23] prepared NiO/MnO2@PANI successfully. The electrocatalyst showed good electrochemical performance for OER in alkaline conditions. Zhao et al. [24] obtained the amorphous PANIFeCo/MWCNT nanohybrids via a in situ method with excellent OER activity in alkaline conditions. In this study, we have prepared a novel OER electrocatalyst by combining Co-Fe LDHs with PANI (denoted as PANI@Co-Fe LDHs). The electrochemical performances of as-prepared PANI@Co-Fe LDHs as OER electrocatalyst were tested scientifically. The PANI@Co-Fe LDHs has good OER performance with a very small overpotential of 261 mV at the current density of 10 mA·cm−2 and a low Tafel slope of 67.85 mV·dec−1, well than that of Co-Fe LDHs nanosheets electrocatalyst.

Corresponding author. E-mail address: [email protected] (R. Liu).

https://doi.org/10.1016/j.catcom.2019.105826 Received 29 May 2019; Received in revised form 2 September 2019; Accepted 18 September 2019 Available online 15 October 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.

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2. Experimental section 2.1. Chemicals All chemicals are analytical grade and require no further purification. Methyl Orange (≥85.0%), Ammonia solution (NH3, 25%~28%), Ethanol absolute (C2H6O2, ≥99.7%), Potassium hydroxide (KOH, ≥85.0%), Ammonium persulfate ((NH4)S2O8, APS, ≥98%), Aniline (AN, ≥99.5%), Cobalt nitrate (Co(NO3)2, ≥99%), Iron (II) chloride (FeCl2, ≥98.0%) and Nafion were purchased from Aladdin. The water used in this experiment is deionized water. 2.2. Materials synthesis 2.2.1. PANI nanowires 0.15 g of Methyl Orange was dissolved in 100 mL of water. To this solution, 1 mL of aniline (AN) was slowly added and stirred for 30 min. Then, 20 mL of 0.5 M cold treated ammonium persulfate (APS) was sluggish added, ice bath for 24 h. Finally, the prepared precipitate was filtered, washed with water and ethanol for several times, and dried at 60 °C. By calculation, the yield is 80%. 2.2.2. PANI@Co-Fe LDHs 0.1 g of PANI was dispersed in 20 mL of water. 0.582 g of Co(NO3)2 and 0.099 g of FeCl2 were added into this solution with stirring. After completely dissolved, 0.5 mL of ammonia was slowly added. Then, it was charged into a reaction vessel of 50 mL of polytetrafluoroethylene liner and heated at 120 °C for 6 h. After cooling to room temperature, the obtained precipitate was filtered, washed with water and ethanol for several times, and dried at 60 °C. By calculation, the yield is 100%. 2.2.3. Co-Fe LDHs 0.582 g of Co(NO3)2 and 0.099 g of FeCl2 were dissolved in 20 mL of water with stirring. Then, 0.5 mL of ammonia was slowly added. Next, it was charged into 50 mL of polytetrafluoroethylene liner and heated at same reaction condition. After cooling to room temperature, the obtained precipitate was filtered, washed with water and ethanol for several times, and dried at 60 °C. By calculation, the yield is 100%. 2.3. Materials characterization The morphology of the obtained samples was characterized by Hitachi S-4800 field emission scanning electron microscopy (SEM). TEM, HR-TEM was tested by FEI F30 on a Cu grid under a voltage of 200 kV. The synthesized nanomaterials were subjected to phase analysis using a BRUKER X-ray powder diffractometer (German D8 fous, Cu Kα, λ = 0.1541 nm). The XRD pattern was tested at range of 5° to 80° with a Cu target, and the test current and voltage decibels of 40 mA and 40 kV. X-ray photoelectron spectroscopy (XPS) of the samples was obtained on a Thermo Scientific Scientific ESCALAB 250Xi with Al Ka radiation. Fourier transform infrared (FTIR) spectroscopy was obtained on pure KBr Nicolet iS10 (Thermo Scientific). The conductivity of samples was tested on a ST2722 Powder Resistivity Tester.

Fig. 1. (a-b) SEM images and (c) TEM image of PANI@Co-Fe LDHs.

rate of 5 mV·s−1. Tafel slopes were calculation from the polarization curves at the current density from 1 to 10 mA·cm−2. Chronopotentiometric measurements are at the current density from 1 to 10 mA·cm−2. Electrochemical impedance spectroscopy (EIS) dates were obtained in the frequency range of 0.01–1000 kHz and bias potential of 1.5 V vs RHE.

2.4. Electrochemistry measurements 0.004 g of sample which was synthesized above was added to a mixed solution consisting of 1.5 mL of water and 0.5 mL of ethanol. After 1 h of ultrasound, 45 μL of Nafion was added. Then 15 μL of the mixed solution dripped on the glassy carbon electrode. The mixed solution was dried overnight at room temperature in air. The threeelectrode test system with the as-prepared samples as the working electrode, a saturated Ag/AgCl electrode as the reference electrode and a platinum wire electrode as the counter electrode was analyzed on a CHI660E electrochemical workstation. In addition, the electrolyte was a 1 M KOH solution.The polarization curves were measured at a scanning

3. Results and discussions Fig. 1a-b show the SEM images of PANI@Co-Fe LDHs, while Fig. S1 shows the scanning electron microscope (SEM) images of PANI. It can be seen from Fig. S1 that the prepared PANI has a uniform wirelike morphology, and with substantially the diameter is basically the same, about 150 nm. As shown in Fig. 1a-b, Co-Fe LDHs mainly exists in the form of sheet, and is uniformly dispersed on the surface of the PANI. 2

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in the compound is divalent. [32,33]. Fig. S5b shows the XPS spectrum of Fe 2p with two Fe 2p peaks at 713.1 eV and 726.5 eV with satellite bands. This proves that the iron element is Fe (2) state [34]. In addition, Fig. S5c and Fig. S5d display that the quinoid imine peak shifts to lower binding energy and the C=NH+ peak shift to higher binding energy as compared to PANI can be owing to the existence of a certain interfacial interaction between PANI and Co-Fe LDHs [35]. Meanwhile, because we used Co(NO3)2 as cobalt source, an obvious peak from N 1 s at 406.9 eV (NO3−), which caused by the intercalating effect of NO3− on Co-Fe LDHs. A π-π* shake-up satellite peak appeared at 292 eV on the C 1 s are showed in Fig. S4c-d, representing aromatic or conjugated systems, which can improve the conductivity of material [36]. This also indicates that PANI@Co-Fe LDHs was synthesized successfully by this way. Fig. S6 is an FT-IR spectrum of different samples. It can be obtained from the spectrum that the positions of the two lines are basically the same. The infrared characteristic peak wavenumbesr of PANI are at 1541 cm−1, 1439 cm−1, 1309 cm−1, 1168 cm−1, and 788 cm−1. The absorption peaks at 1541 cm−1 and 1168 cm−1 are characteristic absorption peaks of the quinone-type structure. The absorption peak at 1439 cm−1 is the skeleton vibration absorption peak of the PANI benzene ring. At 1309 cm−1, it is the C-N stretching vibration absorption peak. Finally, at 788 cm−1, it is the C-H out-of-plane bending vibration peak of disubstituted benzene. From this it can be concluded that this sample is PANI. After loading Co-Fe LDHs, a typical absorption peak of carbonate ion appeared at 1384 cm−1, and a tensile vibration peak of CH was found at 2850 cm−1, and a peak of metal-O appeared at 669 cm−1 [37], which demonstrates the successful attachment of Co-Fe LDHs to the PANI surface. The OER performances of PANI@Co-Fe LDHs were studied in 1 M KOH aqueous solutions using a typical three-electrode device. Co-Fe LDHs nanosheets were used for comparison. Fig. S7 shows the SEM and TEM images, XRD pattern of the Co-Fe LDHs. As shown in the polarization curve of Fig. 4a, the OER onset potential of PANI@Co-Fe LDHs is 1.410 V, beyond which the anode current rises rapidly. On the other hand, Co-Fe LDHs nanosheets have a larger onset potential of 1.460 V and the current increase slowly, which show an inferior OER activity. In addition, the polarization curve of blank GCE is a straight line, which proved that the blank GCE substrate has no effect on OER. At a current density of 10 mA·cm−2, the potential are 1.491 V for PANI@Co-Fe LDHs and 1.512 V for Co-Fe LDHs, respectively. By calculation, the overpotential is 261 mV for PANI@Co-Fe LDHs and 282 mV for Co-Fe LDHs at this current density. The reason why the overpotential of PANI@CoFe LDHs less than Co-Fe LDHs is that PANI is an excellent conductive polymer, and the addition of PANI enhances the overall conductivity of

Because of the dispersibility, it is not easy to agglomerate together, which ensures the channels required for electron transport and increases its electrochemical activity. Transmission electron microscopy (TEM) was also used to study the structure and morphology of the samples. As shown in Fig. 1c and Fig. S2a-b, like the SEM images, PANI@Co-Fe LDHs consists of PANI nanowire and Co-Fe LDHs nanosheets attached to its surface. In addition, the lattice spacing shown in Fig. S2b is 0.48 nm, which corresponds to the (006) plane of Co-Fe LDHs. Fig. S2c shows the EDS mapping images of PANI@Co-Fe LDHs. The signals of C element was observed at the core, and the Co and Fe elements were clearly observed at the bone. In addition, as shown in Fig. S3, the signal of C and N elements are mainly concentrated in the middle and the signal of Co and Fe are mainly concentrated in the side. Those result confirm that Co-Fe LDHs was successfully attached to the surface of the PANI. Fig. 2 shows the XRD patterns of PANI and PANI@Co-Fe LDHs. The diffraction peaks at 2θ = 17.1°, 20.3° and 26.4° correspond to the (011), (020) and (200) planes of PANI, respectively. Among them, 17.1° is a consequence of the perpendicular periodicity, 20.3° is the spacing of the polymer chains alternately arranged, and 26.4o is the inter-planar spacing formed by the vertical and parallel folding of the polyaniline polymer chains [25–27]. The diffraction peaks at 2θ = 10.9°, 22.3°, 33.1°, 33.8° and 58.8°, correspond to (003), (006), (100), (102) and (110) respectively, which suggests a Co-Fe LDHs phase (JCPDS: 46–0605). In addition, there is a shift of the peaks [(003), (006)] in PANI@Co-Fe LDHs compared to pure Co-Fe LDHs, which indicated by Bragg's equation that the interlayer spacing increases and proved that an intercalating effect of Co-Fe LDHs on PANI nanowires [28,29]. This confirmes that the successful loading of Co-Fe LDHs on PANI and the PANI@Co-Fe LDHs were formed. The surface chemical states of elements in compounds were analyzed by X-ray photoelectron spectroscopy (XPS). Fig. S4a shows the XPS measurement spectrum obtained from the sample PANI, the peak of which is mainly the N 1 s, C 1 s, O 1 s region. The three peaks at 399.2 eV, 399.9 eV, and 400.8 eV in Fig. S4b corresponding to quinoid imine (=NH-), benzenoid amine (-NH-), and positively charged nitrogen (-NH+-) of PANI, respectively [30]. The three peaks from C 1 s at 284.6 eV (C=C or C-H), 285.4 eV (C-N or C=N), and 286.5 eV (C=NH+, protonated state) are showed in Fig. S4c [31]. After Co-Fe LDHs was loaded, the XPS spectrum of the sample added the peaks of Co 2p and Fe 2p, which is obtained in Fig. 3. Two peaks can be observed at 781.4 eV and 797.4 eV in the Co 2p spectrum of Fig. S5a, which correspond to Co 2p3/2 and Co 2p1/2, respectively. What's more, a satellite peak was obtained at 785.8 eV. This means that the valence of Co 3

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a

activity than Co-Fe LDHs. The high performance is related to the high electrical conductivity of PANI. Because the electronic properties of PANI@Co-Fe LDHs also affect its OER activity, the electrical properties were determined by electrochemical impedance characterization. Fig.S10a is the impedance map of PANI@Co-Fe LDHs and Co-Fe LDHs, with the addition of PANI, the diameter of the semicircle becomes smaller and the value intersecting with the real axis is reduced, which suggested that the electrocatalyst improves OER performance from two ways. On the one hand, the electrocatalyst can improve charge transfer efficiency, on the other hand, the ohmic resistance is reduced. What's more, the conductivity of Co-Fe LDHs and PANI@Co-Fe LDHs also tested by using Microscopic Four-Point Probe Technique. The conductivity of Co-Fe LDHs and PANI@Co-Fe LDHs are 0.001 S·cm−1 and 0.002 S·cm−1, which proved that the conductivity is also improved. These resaults also show that PANI@Co-Fe LDHs have excellent electrochemical activity for OER. The long-term performance and stability is another significant criterion to evaluate an electrocatalyst. The stability and durability of asprepared catalysts were evaluated by chronopotentiometry response at the current density of 10 mA·cm−2. As shown in Fig.S10b, at the beginning, the potential of the PANI@Co-Fe LDHs catalyst is 1.491 V, after over 11 h, the potential is 1.51 V. Under the same conditions, the Co-Fe LDHs have a more instability during the course of the measurement. The activation of PANI not only improves the electrical conductivity of the material, but also avoids material accumulation and increases the active site of the reactions. These measurements confirm the good electrochemical stability of PANI@Co-Fe LDHs under OER conditions.

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In summary, PANI@Co-Fe LDHs were facile synthesized through two-step method. The PANI@Co-Fe LDHs have excellent OER activity with a very small overpotential of 261 mV at the current density of 10 mA·cm−2and a small Tafel slope of 67.85 mV·dec−1.The sample also has long-term performance and stability. These results provide an easy way to prepare high performance OER electrocatalysts.

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Acknowledgements

Fig. 4. (a) LSV curves of PANI@Co-Fe LDHs, Co-Fe LDHs and blank GCE and (b) Tafel slops of Co-Fe LDHs and PANI@Co-Fe LDHs.

The work was supported by the National Natural Science Foundation of China (No. 21301002), the Natural Science Research Project of Anhui University (No. KJ2018A0107).

the material. At the same time, because Co-Fe LDHs is uniformly dispersed on the surface of PANI, it not only provides a channel for the transfer of charge, but also greatly increases the active area of Co-Fe LDHs nanosheets. The PANI@Co-Fe LDHs also exhibits a linear increase in TOF with overpotential, affording higher TOFs than Co-Fe LDHs nanosheets (Fig. S8a). The Tafel slopes were then measured to investigate the electrode kinetics. As shown in Fig. 4b, the corresponding Tafel slope values were 67.85 and 98.67 mV·dec−1, meaning that the PANI@Co-Fe LDHs has faster electron transport speeds and was the most efficient OER electrocatalyst. The mass activity of PANI@Co-Fe LDHs is 58.4 A·g−1 at a current density of 10 mA·cm−2, which is less than 65.3 A·g−1 of Co-Fe LDHs (see Fig. S8b and Table S1). Compared with other similar OER electrocatalysts in Table S2, the PANI@Co-Fe LDHs also have excellent electrochemistry performance. Fig. S9a-b are the CV curves of the two samples at different scan rates. Fig.S9c-d are the peak current difference diagrams at different scan rates, the double layer capacitance (Cdl) of the materials can be obtained from the slope. By fitting, The Cdl values of PANI@Co-Fe LDHs and Co-Fe LDHs are 67 mF·cm−2, 97 mF·cm−2, respectively. What's more, the active area of PANI@Co-Fe LDHs is 0.135 cm2, less than 0.1932 cm2 of Co-Fe LDHs, which also indicates why the activity quality of PANI@Co-Fe LDHs is less than Co-Fe LDHs. The above results indicate that PANI@Co-Fe LDHs have higher OER

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