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Enhanced capacity and stability of K2FeO4 cathode with poly(3-hexylthiophene) coating for alkaline super-iron battery
Electrochimica Acta 213 (2016) 132–139
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced capacity and stability of K2FeO4 cathode with poly(3-hexylthiophene) coating for alkaline super-iron battery Suqin Wanga,b,* , Yaoyao Wanga , Shuiliang Chena , Haoqing Houa , Hongbo Lia,* a b
Department of Chemistry and Chemical engineering, Jiangxi Normal University, Ziyang Road 99, Nanchang 330029, China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C L E I N F O
Article history: Received 19 January 2016 Received in revised form 26 June 2016 Accepted 29 June 2016 Available online 30 June 2016 Keywords: Poly(3-hexylthiophene) K2FeO4 capacity stability super-iron battery
A B S T R A C T
Poly(3-hexylthiophene)-coated K2FeO4 (K2FeO4@P3HT) was prepared to enhance capacity and stability of K2FeO4. Scanning electron microscopy (SEM), Fourier transform infrared spectrum (FT-IR) and X-ray photoelectron spectra (XPS) were performed to characterize K2FeO4@P3HT. Discharge performance results showed that the Poly(3-hexylthiophene) (P3HT) coating layer enhanced the capacity of the K2FeO4 in 10 mol L1 KOH electrolyte. K2FeO4@P3HT-1% electrode showed a high discharge capacity of 351 mAh g1, about 13% increase comparing to the K2FeO4 electrode. Moreover, the stability of K2FeO4 electrode was obviously enhanced by P3HT coating, and the discharge capacity of the electrode which was stored in electrolyte for 6 h was improved to 314 mAh g1, increasing about 22.6% compared to that of 314 mAh g1. These desirable properties can be attributed to the in-situ formation of two-layer film on the surface of K2FeO4 crystal, which keep electrolyte from directly contacting with K2FeO4 and reduce the resistance of charge transfer. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that the discharge capacity of Zn/MnO2 alkaline battery is limited by the theoretical capacity of MnO2 cathode (308 mAh g1), compared to that of Zn anode (820 mAh g1). Litch et al. first introduced K2FeO4 as cathode material for alkaline battery to replace Zn/MnO2 alkaline primary battery [1]. K2FeO4 battery possesses high reduction potential, high specific capacity (406 mAh g1), environmentally benign and abundantly available starting materials[1–3]. Due to the high solid-state stability (<0.1% decomposition percent year) [4], K2FeO4 cathodes have been paid special attentions over the past two decades. However, the highest practical capacity of K2FeO4 reported have been about 180– 300 mAh g1, which are not higher than that of MnO2 cathode for Zn/MnO2 [5–8]. Therefore, it is necessary to explore new technique to enhance the practical capacity of K2FeO4. Meanwhile, the development of super-iron batteries are greatly restricted owing to their chemical instability in alkaline battery system [9,10]. To enhance the stability of ferrate(VI), many types of compounds
* Corresponding authors. E-mail addresses: [email protected] (S. Wang), [email protected] (H. Li). http://dx.doi.org/10.1016/j.electacta.2016.06.164 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
were as coatings. Walz et al. reported that BaFeO4 was coated with nanoparticulate thin films of TiO2 and SiO2, which was prepared by sol-gel techniques [9,11]. Yu et al. reported that K2FeO4 could be coated by Zirconia in an organic medium through the conversion of ZrCl4 to ZrO2 [12]. Zhang et al. reported that Y2O3 doped ZrO2 composites coating could improve the stability and charge transfer of K2FeO4 cathode in alkaline electrolytes [13,14]. Yang et al. reported that organic compounds (2,3-Naphthalocyanine, tetraphenylporphyrin, phthalocyanine) were used as coatings to enhance the stability of K2FeO4 [10,15,16]. The protective coating was the formation of a thick film, which was able to hinder the contact between electrolyte and K2FeO4 and reduce the decomposition of K2FeO4. Moreover, above-mentioned some compounds enhanced the charge transfer ability. However, these compounds showed comparatively low specific conductivity, and led to high internal resistance in the cathode. Therefore, after some time storing in electrolyte, the discharge capacity were still less than 200 mAh g1 [10,13–17]. According to our knowledge, there is no report to adopt conducting polymer to improve the stability and discharge capacity of K2FeO4 for alkaline battery cathode. Conducting polymers are promising materials for hybrid composites in electrochemical energy storage, due to electrically conductive
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characterization [18,19]. As one of conducting polymers, P3HT had been widely employed as electrode materials in supercapacitors and solar cells, etc., owing to the light weight, low-cost and flexible characteristics [20–22]. Moreover, P3HT is insoluble in aqueous media, thus suitable for use in alkaline primary battery with aqueous solution as electrolyte. In this paper, P3HT was firstly used as coating to enhance the capacity and stability of the K2FeO4 for alkaline super-iron battery. 2. Experimental 2.1. Preparation and characterization of K2FeO4 coated with P3HT K2FeO4 was prepared according to the previous report [5]. a certain amount of P3HT (regioregular, J&K Scientific LTD.) was dissolved in 30 ml chloroform to form a uniform solution; then added 2 g K2FeO4 crystal and mixed by mechanical stirring; Chloroform was removed by vacuum. Composite samples with P3HT content of 1%, 2% and 3% was prepared, and denoted as K2FeO4@P3HT-1%, K2FeO4@P3HT-2% and K2FeO4@P3HT-3%, respectively. The surface morphologies of the K2FeO4 and K2FeO4@P3HT samples were examined by SEM (TESCAN vega 3, Czech Republic). The FT-IR spectrum was conducted on using a Perkin Elmer 781 Fourier transform infrared spectrophotometer in a conventional KBr pellet. XPS were taken with a PHI Quantera Xray photoelectron spectrometer. 2.2. Electrode preparation K2FeO4 powder was mixed with acetylene black by weight ratio of 4/1, then n-hexane was added to form a slurry. The slurry was sandwiched into two pieces of nickel foam (2 cm 2 cm) to form the electrode and pressed to be thickness of 0.2 mm. The net weight of K2FeO4 on the electrode was controlled in the range of 0.06–0.10 g. For comparison, pure P3HT K2FeO4@P3HT electrodes were also prepared. The preparation procedure of P3HT and K2FeO4@P3HT electrodes were identical with the above-mentioned preparation of K2FeO4 electrode.
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2.3. Electrochemical performance The linear sweep voltammetry (LSV) and electrochemical impedance spectrum (EIS) tests were conducted by electrochemical working station (CHI660D). Zn foil was used both as counter electrode and reference electrode. The prepared K2FeO4 electrodes were served as working electrodes. The electrolyte was 10 mol L1 KOH. The scan direction was from 2 V to 0.8 V at 1 mV/s for LSV test. The frequency range for the EIS test was from 100 kHz to 0.01 Hz and the amplitude of the ac potential perturbation was 5 mV. The discharge behavior of K2FeO4 electrodes was tested in an open module battery using LAND CT2001A testing system. The battery was consisted of K2FeO4 cathode, PE separator and excess zinc foil anode. 10 mol L1 KOH aqueous solutions were as electrolyte. The K2FeO4 battery was discharged at rate of 0.1C to a cut-off voltage of 0.8 V after being stored in electrolyte for 0 h and 6 h. 3. Results and Discussion 3.1. Characterization of the K2FeO4 and K2FeO4@P3HT As shown in Fig. 1(a)–(d), the K2FeO4 particles are thin flake shaped polyhedral crystal with width of 25 um. After the coating process, the surface of the K2FeO4 crystal is covered by the P3HT overlayer (Fig. 1(e)–(h)). It is well known that K2FeO4 is a very strong oxidant [23], while P3HT is sensitive to oxidation [24], there would be redox reaction occurred between them. FT-IR and XPS analyses were conducted to characterize K2FeO4@P3HT. The FT-IR spectra of P3HT, K2FeO4 and K2FeO4@P3HT are shown in Fig. 2a. The signal at 776 cm1 could be attributed to the Fe-O symmetric stretching for K2FeO4. C-S stretching at 1376 cm1 and C H aromatics stretching at 3054 cm1, which are the main characterization peaks of thiophene [25], are observed for P3HT and K2FeO4@P3HT. And the peak at 726 cm1, which is the characteristic absorption of S atom on polythiophene ring [26], disappeared in the FT-IR for K2FeO4@P3HT. The oxidation of P3HT is proved by the band at 980 cm1, which belongs to S¼O
Fig. 1. SEM images for (a–d) K2FeO4 and (e–h) K2FeO4@P3HT.
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Fig. 2. (a) FT-IR spectra of K2FeO4, P3HT, and K2FeO4@P3HT; XPS spectra of (b) C1s, (c) S2p, and (d) Fe2p for K2FeO4@P3HT.
symmetric stretching vibration [27]. We could also find the redox reaction proofs from the XPS spectra. In one aspect, as shown in Fig. 2b, the C1 s peak at 288.5 eV can be attributed to the signal of carboxyl groups [24,28], which indicates that C in thiophene ring is oxidized. In another aspect, it can be seen in Fig. 2c, the fine S2p spectrum of K2FeO4@P3HT is deconvoluted into two peaks, 163.9 and 168.4 eV. The peak at 163.9 eV corresponds to non-oxidative sulfur S2p3/2 of the thiophene ring of P3HT [24,29,30], While the signal at 168.4 eV is assigned to highly oxidized species ( ) [28], indicating that S undergoes a chemical interaction [24]. The spectra of Fe2p3/2 and Fe2P1/2 at a binding energy of 710.16 and 724.89 eV, respectively, are assigned to Fe2O3 [31,32] (Fig. 2d). At the same time, the structure at 718.9 eV, which is a satellite peak of Fe(III) [31,33]. Above results demonstrated that the P3HT was partly oxidized by K2FeO4, which is in accordance with the results of FT-IR analysis.
the peak potential has a small negative shift compared to that of the K2FeO4, which is governed the decreasing concentration of K2FeO4 after coating according to the Nernst equation. FeO42 þ3H2O þ 3e ! FeOOH þ 5OH
3.2. Electrochemical performances of the K2FeO4 and K2FeO4@P3HT electrodes LSV tests are conducted to study the redox reaction on the K2FeO4 and K2FeO4@P3HTelectrodes. And the curves are shown in Fig. 3. The cathodic peaks of K2FeO4 at approximately 1.22 V (vs. Zn) presents the reduction reaction of K2FeO4 which proceeds with an overvoltage of 0.15 V [34]. There are two cathodic reduction peaks at 1.693 V (vs. Zn) and 1.192 V (vs. Zn) in P3HT LSV curve. For the K2FeO4@P3HT electrodes, the broad peak potential at 1.2 V (vs. Zn) is caused by the reduction of the K2FeO4 and P3HT due to the synergistic effect. And
Fig. 3. Linear sweep voltammograms of K2FeO4 and K2FeO4@P3HT.
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Fig. 4. Nyquist plots of K2FeO4 electrode. Fig. 6. Nyquist diagrams of K2FeO4 electrode with fit results.
The Nernst equation for this reaction is shown as follows.
f ¼ fo þ
RT aðFeO42 Þ ln nF a5 ðOH Þ
Moreover, as increase of the ratio of P3HT, the reduction peak current of K2FeO4@P3HT electrode at 1.2 V (vs. Zn) decreases. The reduction peak current of K2FeO4@P3HT electrode is higher than that of K2FeO4. The reduction peak at 1.6 V (vs. Zn) for P3HT is more and more obvious with the increasing ratio of P3HT. The results show that P3HT enhances the charge transfer ability of K2FeO4 electrode and the charge transfer ability of K2FeO4@P3HT1% is the best. The results are further confirmed by the EIS. EIS was used to analyze the effect of P3HT on the electrochemical performance of the K2FeO4 electrode. Nyquist curve of K2FeO4 electrode was shown in Fig. 4, and the capacitive loop in the high frequency region is zoomed in on the top right corner of Fig. 4. The Nyquist curve comprises with two parallel RC loops in series. Rs is ohmic resistance which is related to electrolyte resistance. In the high frequency region, the capacitive loop is attributed to the contact capacitance (CPE1) in parallel with the contact resistance (Rc) among K2FeO4, acetylene black and nickel foam. In the low frequency region, the capacitive loop is ascribed to the chargetransfer resistance (Rt) in parallel with the double-layer capacitance (CPE2) that happen at the interface of the K2FeO4 electrode. The equivalent circuit for K2FeO4 electrode is obtained and shown in Fig. 5, which is similar with previous report [17]. Fig. 6 shows Nyquist diagrams of K2FeO4 electrode with fit result based on the equivalent circuit of Fig. 5 by Zview. Good fit are obtained and prove that the equivalent circuit for K2FeO4 electrode in Fig. 5 is
resonable. These fit parameters results of the elements in equivalent circuit have been obtained and tabulated in Table 1. Fig. 7 shows the geometric structure of K2FeO4 and K2FeO4@P3HT electrodes. Both K2FeO4 and K2FeO4@P3HT electrode, consist of nickel foam, acetylene black and K2FeO4. Comparing to the K2FeO4 electrode, the only difference of the K2FeO4@P3HT electrode is that a two-layer film (partly oxidized P3HT film, and Fe2O3 film) was coated on the surface of the K2FeO4 crystal. So the equivalent circuit for K2FeO4@P3HT electrode also comprises electrolyte resistance (Rs) and contact resistance (parallel Rc with CPE1) in series in high frequency region. According to the equivalent circuits for electrolyte/polymer coating/metal system [35–37] and one of our previous work which referred to the 2,3-Naphthalocyanine coated K2FeO4 [17], there should be three hierarchical parallel RC loops in the equivalent circuit for the K2FeO4@P3HT electrode. In the high frequency, the capacitive loop represents contact capacitance (CPE1) in parallel with the contact resistance (Rc) among K2FeO4, acetylene black and nickel foam. In the medium frequency region, two capacitive loops are attributed to the double layer capacitances (CPE3, CPE4) in parallel with the film resistances (Rf1, Rf2). Rf1 is partly oxidized P3HT coating film resistance. Rf2 is Fe2O3 coating film resistance. CPE3 and CPE4 are double layer capacitance corresponding to Rf1 and Rf2. In the low frequency region, the capacitive loop is ascribed to the charge-transfer resistance (Rt) in parallel with double layer capacitance (CPE2) that happen at the interface of the Table 1 Parameters of the elements in equivalent circuits for K2FeO4 and K2FeO4@P3HT cathodes obtained via Zview. Rs/V
Fig. 5. The equivalent circuit for K2FeO4 electrode.
Pure K2FeO4@P3HT1% K2FeO4@P3HT2% K2FeO4@P3HT3%
Rc/V
Rf1/
V
Rf2/
V
Rf1 + Rf2/
V
Rt/V
Rtotal/V
1.336 6.076 1.201 0.32437 2.095 3.536 5.631
25.86 33.272 16.27 23.426
1.259 0.40405 2.649 5.088 7.737
15.41
2.165
13.33 25.311
0.54697 2.975 6.294 9.269
24.810
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Fig. 7. The geometric structure of K2FeO4 and K2FeO4@P3HT electrode.
Fig. 8. Step by step zoomed Nyquist plots (a, b, c) and Bode plots (d, e) of K2FeO4 electrode.
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Fig. 9. The equivalent circuit for K2FeO4@P3HT electrode.
K2FeO4@P3HT electrode. Analyzing the Nyquist plots of K2FeO4@P3HT-2% step by step zoomed in Fig. 8(a–c), obviously, there are 2 capacitive loops: Z’ > 5 V, 5 V > Z’ > 3 V. So there are more than 2 capacitive loops at least. The Nyquist plot in 1.5 V > Z’ > 1.2 V is zoomed, there is an unconspicuous capacitive loop in 1.4 V > Z’ > 1.2 V. Then there are a capacitive loop in 3 V > Z’ > 1.4 V. i.e., there are 4 capacitive loops for K2FeO4@P3HT2%. On the other hand, phase angle Bode plot has been widely used to identify the number of time constant. Combining with Nyquist plots, the number of time constants could be concluded by the Bode plots phase angle peaks because each peak in the phase angle versus frequency curve corresponds to a time constant of a electrochemical process[38–40]. The Bode plot obviously exhibited 4 phase angle peaks indicating 4 time constants in Fig. 8 (d, e). The frequency range of time constants in the Bode plot is consistent with those of capacitive loops in the Nyquist plots. So there are 4 time constants in the equivalent circuit for K2FeO4@P3HT-2% electrode. The equivalent circuit for K2FeO4@P3HT is obtained and represented in Fig. 9. Fig. 10 presents Nyquist diagrams of K2FeO4@P3HT-1%, K2FeO4@P3HT-2%, K2FeO4@P3HT-3% with fit results based on the equivalent circuit of Fig. 9 by Zview. Good fit are obtained and prove that the equivalent circuit in Fig. 9 is resonable. These fit parameters results of the elements in equivalent circuit have been tabulated in Table 1. After coating, both the charge transfer resistance (Rct) and total resistance (Rtotal) decreases, because P3HT is conductive polymer and enhances the ability of electron transfer. The film resistance Rf1 and Rf2 increase with increasing the coating content, indicating that both the content of P3HT and its oxidized product and the content of Fe2O3 increased, respectively. The charge transfer resistance slightly decreases with increasing the amount of P3HT due to that P3HT could enhance the ability of electron transfer. The total resistance increases with increasing the amount of P3HT. This reason is that increasing the amount of P3HT could greatly increase the film resistance (Rf1 + Rf2). So the satisfying coating amount of P3HT is 1%. The discharge curves of K2FeO4 and K2FeO4@P3HT are shown in Fig. 11. Both the uncoated and coated K2FeO4 electrode exhibited nice discharge characteristics with similar discharge profiles. When tested immediately after assembling, the discharge capacity (310 mAh g1) of the K2FeO4 electrode is less than those of K2FeO4@P3HT electrodes which were 351 mAh g1 (for K2FeO4@P3HT-1%), 333 mAh g1 (for K2FeO4@P3HT-2%) and 315 mAh g1 (for K2FeO4@P3HT-3%), respectively. Increasing the ratio of P3HT coatings, the discharge capacity decreases. The discharge capacity of K2FeO4@P3HT-3% is almost equal with that of K2FeO4 electrodes. These results may be explained as follows. In the process of coating, because K2FeO4 is reduced to be Fe2O3 to be overlayer upon the K2FeO4 by P3HT, the mass of K2FeO4 which participate in discharge reaction decreases. So the higher ratio of P3HT, the lower electrode capacity obtained. Electrically insulating Fe2O3 is disadvantageous for K2FeO4 discharge. And during the discharge process, oxidative P3HT itself may have participated in the discharge reaction. The theoretical capacity for the P3HT discharge is calculated to be 315 mAh g1. The maximum capacity of the 1% P3HT is 3.15 mAh g1 (315 1% mAh g1), which only
Fig. 10. Nyquist diagrams of (a) K2FeO4@P3HT-1%, (b) K2FeO4@P3HT-2%, and (c) K2FeO4@P3HT-3% electrode with fit results.
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Technology Project of Jiangxi Province, China (No. 20121BBE50024, No. 20143ACB21015). We thank professor Jiulin Wang, associated professor Yanna Nuli at School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China for revising this paper. References
Fig. 11. Discharge curves for K2FeO4 and K2FeO4@P3HT after storing in 10 mol L1 KOH solution for 0 h and 6 h.
accounts for a relatively small portion of the 351 mAh g1 of total capacity for K2FeO4@P3HT-1% electrode. Although this ratio is small, the P3HT optimizes the K2FeO4 charge transfer. The P3HT coating is acting as a protecting and conducting layer which slows down the dissolution and decomposition rate of K2FeO4 and reduces the total resistance. So the discharge capacity of the K2FeO4 electrode is less than those of K2FeO4@P3HT electrode. Especially, the discharge capacity of K2FeO4@P3HT-1% is 351 mAh g1, which is much higher than those of K2FeO4 coated with other compounds reported [10,12–17]. The above results indicate that the partly oxidized P3HT coating could enhance the capacity of the K2FeO4 electrode. After storing the K2FeO4 and K2FeO4@P3HT electrodes in the 10 mol L1 KOH for 6 h, the change tendency is similar with that of being tested immediately after assembly. The discharge capacities are 256 mAh g1, 314 mAh g1, 305 mAh g1 and 268 mAh g1 for the electrodes of K2FeO4 and K2FeO4@P3HT-1%, K2FeO4@P3HT-2%, K2FeO4@P3HT-3%, respectively. The capacity ratio of the K2FeO4 is 82.6% of sample without storing in the electrolyte, which is less than those of K2FeO4@P3HT. The results indicate that the partly oxidized P3HT coating could improve the stability of the K2FeO4 electrode. 4. Conclusions Conductive polymer P3HT was successfully coated onto the K2FeO4. Two-layer films (partly oxidized P3HT film and Fe2O3 film) could be in-situ formed between the P3HT and K2FeO4 because that reduction-oxidation reaction occurs between P3HT and K2FeO4. The partly oxidized P3HT coating layer, in one aspect reduced the total inter-resistance and enhanced the ability of charge transfer of K2FeO4 electrode; in another aspect served as effective barriers to keep electrolyte from contacting with the K2FeO4 in the cathode, thus greatly improved the capacity and stability of the K2FeO4 for use as cathode in alkaline super-iron battery. Acknowledgements This research was supported by National Natural Science Foundation of China (No.21562015), General project of Jiangxi Provincial Education Department, China (No.GJJ150363), the Important Science and Technology Special Project of Jiangxi Province, China (No. 20114ABF05100) and the Science and
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced capacity and stability of K2FeO4 cathode with poly(3-hexylthiophene) coating for alkaline super-iron battery Suqin Wanga,b,* , Yaoyao Wanga , Shuiliang Chena , Haoqing Houa , Hongbo Lia,* a b
Department of Chemistry and Chemical engineering, Jiangxi Normal University, Ziyang Road 99, Nanchang 330029, China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C L E I N F O
Article history: Received 19 January 2016 Received in revised form 26 June 2016 Accepted 29 June 2016 Available online 30 June 2016 Keywords: Poly(3-hexylthiophene) K2FeO4 capacity stability super-iron battery
A B S T R A C T
Poly(3-hexylthiophene)-coated K2FeO4 (K2FeO4@P3HT) was prepared to enhance capacity and stability of K2FeO4. Scanning electron microscopy (SEM), Fourier transform infrared spectrum (FT-IR) and X-ray photoelectron spectra (XPS) were performed to characterize K2FeO4@P3HT. Discharge performance results showed that the Poly(3-hexylthiophene) (P3HT) coating layer enhanced the capacity of the K2FeO4 in 10 mol L1 KOH electrolyte. K2FeO4@P3HT-1% electrode showed a high discharge capacity of 351 mAh g1, about 13% increase comparing to the K2FeO4 electrode. Moreover, the stability of K2FeO4 electrode was obviously enhanced by P3HT coating, and the discharge capacity of the electrode which was stored in electrolyte for 6 h was improved to 314 mAh g1, increasing about 22.6% compared to that of 314 mAh g1. These desirable properties can be attributed to the in-situ formation of two-layer film on the surface of K2FeO4 crystal, which keep electrolyte from directly contacting with K2FeO4 and reduce the resistance of charge transfer. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that the discharge capacity of Zn/MnO2 alkaline battery is limited by the theoretical capacity of MnO2 cathode (308 mAh g1), compared to that of Zn anode (820 mAh g1). Litch et al. first introduced K2FeO4 as cathode material for alkaline battery to replace Zn/MnO2 alkaline primary battery [1]. K2FeO4 battery possesses high reduction potential, high specific capacity (406 mAh g1), environmentally benign and abundantly available starting materials[1–3]. Due to the high solid-state stability (<0.1% decomposition percent year) [4], K2FeO4 cathodes have been paid special attentions over the past two decades. However, the highest practical capacity of K2FeO4 reported have been about 180– 300 mAh g1, which are not higher than that of MnO2 cathode for Zn/MnO2 [5–8]. Therefore, it is necessary to explore new technique to enhance the practical capacity of K2FeO4. Meanwhile, the development of super-iron batteries are greatly restricted owing to their chemical instability in alkaline battery system [9,10]. To enhance the stability of ferrate(VI), many types of compounds
* Corresponding authors. E-mail addresses: [email protected] (S. Wang), [email protected] (H. Li). http://dx.doi.org/10.1016/j.electacta.2016.06.164 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
were as coatings. Walz et al. reported that BaFeO4 was coated with nanoparticulate thin films of TiO2 and SiO2, which was prepared by sol-gel techniques [9,11]. Yu et al. reported that K2FeO4 could be coated by Zirconia in an organic medium through the conversion of ZrCl4 to ZrO2 [12]. Zhang et al. reported that Y2O3 doped ZrO2 composites coating could improve the stability and charge transfer of K2FeO4 cathode in alkaline electrolytes [13,14]. Yang et al. reported that organic compounds (2,3-Naphthalocyanine, tetraphenylporphyrin, phthalocyanine) were used as coatings to enhance the stability of K2FeO4 [10,15,16]. The protective coating was the formation of a thick film, which was able to hinder the contact between electrolyte and K2FeO4 and reduce the decomposition of K2FeO4. Moreover, above-mentioned some compounds enhanced the charge transfer ability. However, these compounds showed comparatively low specific conductivity, and led to high internal resistance in the cathode. Therefore, after some time storing in electrolyte, the discharge capacity were still less than 200 mAh g1 [10,13–17]. According to our knowledge, there is no report to adopt conducting polymer to improve the stability and discharge capacity of K2FeO4 for alkaline battery cathode. Conducting polymers are promising materials for hybrid composites in electrochemical energy storage, due to electrically conductive
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characterization [18,19]. As one of conducting polymers, P3HT had been widely employed as electrode materials in supercapacitors and solar cells, etc., owing to the light weight, low-cost and flexible characteristics [20–22]. Moreover, P3HT is insoluble in aqueous media, thus suitable for use in alkaline primary battery with aqueous solution as electrolyte. In this paper, P3HT was firstly used as coating to enhance the capacity and stability of the K2FeO4 for alkaline super-iron battery. 2. Experimental 2.1. Preparation and characterization of K2FeO4 coated with P3HT K2FeO4 was prepared according to the previous report [5]. a certain amount of P3HT (regioregular, J&K Scientific LTD.) was dissolved in 30 ml chloroform to form a uniform solution; then added 2 g K2FeO4 crystal and mixed by mechanical stirring; Chloroform was removed by vacuum. Composite samples with P3HT content of 1%, 2% and 3% was prepared, and denoted as K2FeO4@P3HT-1%, K2FeO4@P3HT-2% and K2FeO4@P3HT-3%, respectively. The surface morphologies of the K2FeO4 and K2FeO4@P3HT samples were examined by SEM (TESCAN vega 3, Czech Republic). The FT-IR spectrum was conducted on using a Perkin Elmer 781 Fourier transform infrared spectrophotometer in a conventional KBr pellet. XPS were taken with a PHI Quantera Xray photoelectron spectrometer. 2.2. Electrode preparation K2FeO4 powder was mixed with acetylene black by weight ratio of 4/1, then n-hexane was added to form a slurry. The slurry was sandwiched into two pieces of nickel foam (2 cm 2 cm) to form the electrode and pressed to be thickness of 0.2 mm. The net weight of K2FeO4 on the electrode was controlled in the range of 0.06–0.10 g. For comparison, pure P3HT K2FeO4@P3HT electrodes were also prepared. The preparation procedure of P3HT and K2FeO4@P3HT electrodes were identical with the above-mentioned preparation of K2FeO4 electrode.
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2.3. Electrochemical performance The linear sweep voltammetry (LSV) and electrochemical impedance spectrum (EIS) tests were conducted by electrochemical working station (CHI660D). Zn foil was used both as counter electrode and reference electrode. The prepared K2FeO4 electrodes were served as working electrodes. The electrolyte was 10 mol L1 KOH. The scan direction was from 2 V to 0.8 V at 1 mV/s for LSV test. The frequency range for the EIS test was from 100 kHz to 0.01 Hz and the amplitude of the ac potential perturbation was 5 mV. The discharge behavior of K2FeO4 electrodes was tested in an open module battery using LAND CT2001A testing system. The battery was consisted of K2FeO4 cathode, PE separator and excess zinc foil anode. 10 mol L1 KOH aqueous solutions were as electrolyte. The K2FeO4 battery was discharged at rate of 0.1C to a cut-off voltage of 0.8 V after being stored in electrolyte for 0 h and 6 h. 3. Results and Discussion 3.1. Characterization of the K2FeO4 and K2FeO4@P3HT As shown in Fig. 1(a)–(d), the K2FeO4 particles are thin flake shaped polyhedral crystal with width of 25 um. After the coating process, the surface of the K2FeO4 crystal is covered by the P3HT overlayer (Fig. 1(e)–(h)). It is well known that K2FeO4 is a very strong oxidant [23], while P3HT is sensitive to oxidation [24], there would be redox reaction occurred between them. FT-IR and XPS analyses were conducted to characterize K2FeO4@P3HT. The FT-IR spectra of P3HT, K2FeO4 and K2FeO4@P3HT are shown in Fig. 2a. The signal at 776 cm1 could be attributed to the Fe-O symmetric stretching for K2FeO4. C-S stretching at 1376 cm1 and C H aromatics stretching at 3054 cm1, which are the main characterization peaks of thiophene [25], are observed for P3HT and K2FeO4@P3HT. And the peak at 726 cm1, which is the characteristic absorption of S atom on polythiophene ring [26], disappeared in the FT-IR for K2FeO4@P3HT. The oxidation of P3HT is proved by the band at 980 cm1, which belongs to S¼O
Fig. 1. SEM images for (a–d) K2FeO4 and (e–h) K2FeO4@P3HT.
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Fig. 2. (a) FT-IR spectra of K2FeO4, P3HT, and K2FeO4@P3HT; XPS spectra of (b) C1s, (c) S2p, and (d) Fe2p for K2FeO4@P3HT.
symmetric stretching vibration [27]. We could also find the redox reaction proofs from the XPS spectra. In one aspect, as shown in Fig. 2b, the C1 s peak at 288.5 eV can be attributed to the signal of carboxyl groups [24,28], which indicates that C in thiophene ring is oxidized. In another aspect, it can be seen in Fig. 2c, the fine S2p spectrum of K2FeO4@P3HT is deconvoluted into two peaks, 163.9 and 168.4 eV. The peak at 163.9 eV corresponds to non-oxidative sulfur S2p3/2 of the thiophene ring of P3HT [24,29,30], While the signal at 168.4 eV is assigned to highly oxidized species ( ) [28], indicating that S undergoes a chemical interaction [24]. The spectra of Fe2p3/2 and Fe2P1/2 at a binding energy of 710.16 and 724.89 eV, respectively, are assigned to Fe2O3 [31,32] (Fig. 2d). At the same time, the structure at 718.9 eV, which is a satellite peak of Fe(III) [31,33]. Above results demonstrated that the P3HT was partly oxidized by K2FeO4, which is in accordance with the results of FT-IR analysis.
the peak potential has a small negative shift compared to that of the K2FeO4, which is governed the decreasing concentration of K2FeO4 after coating according to the Nernst equation. FeO42 þ3H2O þ 3e ! FeOOH þ 5OH
3.2. Electrochemical performances of the K2FeO4 and K2FeO4@P3HT electrodes LSV tests are conducted to study the redox reaction on the K2FeO4 and K2FeO4@P3HTelectrodes. And the curves are shown in Fig. 3. The cathodic peaks of K2FeO4 at approximately 1.22 V (vs. Zn) presents the reduction reaction of K2FeO4 which proceeds with an overvoltage of 0.15 V [34]. There are two cathodic reduction peaks at 1.693 V (vs. Zn) and 1.192 V (vs. Zn) in P3HT LSV curve. For the K2FeO4@P3HT electrodes, the broad peak potential at 1.2 V (vs. Zn) is caused by the reduction of the K2FeO4 and P3HT due to the synergistic effect. And
Fig. 3. Linear sweep voltammograms of K2FeO4 and K2FeO4@P3HT.
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Fig. 4. Nyquist plots of K2FeO4 electrode. Fig. 6. Nyquist diagrams of K2FeO4 electrode with fit results.
The Nernst equation for this reaction is shown as follows.
f ¼ fo þ
RT aðFeO42 Þ ln nF a5 ðOH Þ
Moreover, as increase of the ratio of P3HT, the reduction peak current of K2FeO4@P3HT electrode at 1.2 V (vs. Zn) decreases. The reduction peak current of K2FeO4@P3HT electrode is higher than that of K2FeO4. The reduction peak at 1.6 V (vs. Zn) for P3HT is more and more obvious with the increasing ratio of P3HT. The results show that P3HT enhances the charge transfer ability of K2FeO4 electrode and the charge transfer ability of K2FeO4@P3HT1% is the best. The results are further confirmed by the EIS. EIS was used to analyze the effect of P3HT on the electrochemical performance of the K2FeO4 electrode. Nyquist curve of K2FeO4 electrode was shown in Fig. 4, and the capacitive loop in the high frequency region is zoomed in on the top right corner of Fig. 4. The Nyquist curve comprises with two parallel RC loops in series. Rs is ohmic resistance which is related to electrolyte resistance. In the high frequency region, the capacitive loop is attributed to the contact capacitance (CPE1) in parallel with the contact resistance (Rc) among K2FeO4, acetylene black and nickel foam. In the low frequency region, the capacitive loop is ascribed to the chargetransfer resistance (Rt) in parallel with the double-layer capacitance (CPE2) that happen at the interface of the K2FeO4 electrode. The equivalent circuit for K2FeO4 electrode is obtained and shown in Fig. 5, which is similar with previous report [17]. Fig. 6 shows Nyquist diagrams of K2FeO4 electrode with fit result based on the equivalent circuit of Fig. 5 by Zview. Good fit are obtained and prove that the equivalent circuit for K2FeO4 electrode in Fig. 5 is
resonable. These fit parameters results of the elements in equivalent circuit have been obtained and tabulated in Table 1. Fig. 7 shows the geometric structure of K2FeO4 and K2FeO4@P3HT electrodes. Both K2FeO4 and K2FeO4@P3HT electrode, consist of nickel foam, acetylene black and K2FeO4. Comparing to the K2FeO4 electrode, the only difference of the K2FeO4@P3HT electrode is that a two-layer film (partly oxidized P3HT film, and Fe2O3 film) was coated on the surface of the K2FeO4 crystal. So the equivalent circuit for K2FeO4@P3HT electrode also comprises electrolyte resistance (Rs) and contact resistance (parallel Rc with CPE1) in series in high frequency region. According to the equivalent circuits for electrolyte/polymer coating/metal system [35–37] and one of our previous work which referred to the 2,3-Naphthalocyanine coated K2FeO4 [17], there should be three hierarchical parallel RC loops in the equivalent circuit for the K2FeO4@P3HT electrode. In the high frequency, the capacitive loop represents contact capacitance (CPE1) in parallel with the contact resistance (Rc) among K2FeO4, acetylene black and nickel foam. In the medium frequency region, two capacitive loops are attributed to the double layer capacitances (CPE3, CPE4) in parallel with the film resistances (Rf1, Rf2). Rf1 is partly oxidized P3HT coating film resistance. Rf2 is Fe2O3 coating film resistance. CPE3 and CPE4 are double layer capacitance corresponding to Rf1 and Rf2. In the low frequency region, the capacitive loop is ascribed to the charge-transfer resistance (Rt) in parallel with double layer capacitance (CPE2) that happen at the interface of the Table 1 Parameters of the elements in equivalent circuits for K2FeO4 and K2FeO4@P3HT cathodes obtained via Zview. Rs/V
Fig. 5. The equivalent circuit for K2FeO4 electrode.
Pure K2FeO4@P3HT1% K2FeO4@P3HT2% K2FeO4@P3HT3%
Rc/V
Rf1/
V
Rf2/
V
Rf1 + Rf2/
V
Rt/V
Rtotal/V
1.336 6.076 1.201 0.32437 2.095 3.536 5.631
25.86 33.272 16.27 23.426
1.259 0.40405 2.649 5.088 7.737
15.41
2.165
13.33 25.311
0.54697 2.975 6.294 9.269
24.810
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Fig. 7. The geometric structure of K2FeO4 and K2FeO4@P3HT electrode.
Fig. 8. Step by step zoomed Nyquist plots (a, b, c) and Bode plots (d, e) of K2FeO4 electrode.
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Fig. 9. The equivalent circuit for K2FeO4@P3HT electrode.
K2FeO4@P3HT electrode. Analyzing the Nyquist plots of K2FeO4@P3HT-2% step by step zoomed in Fig. 8(a–c), obviously, there are 2 capacitive loops: Z’ > 5 V, 5 V > Z’ > 3 V. So there are more than 2 capacitive loops at least. The Nyquist plot in 1.5 V > Z’ > 1.2 V is zoomed, there is an unconspicuous capacitive loop in 1.4 V > Z’ > 1.2 V. Then there are a capacitive loop in 3 V > Z’ > 1.4 V. i.e., there are 4 capacitive loops for K2FeO4@P3HT2%. On the other hand, phase angle Bode plot has been widely used to identify the number of time constant. Combining with Nyquist plots, the number of time constants could be concluded by the Bode plots phase angle peaks because each peak in the phase angle versus frequency curve corresponds to a time constant of a electrochemical process[38–40]. The Bode plot obviously exhibited 4 phase angle peaks indicating 4 time constants in Fig. 8 (d, e). The frequency range of time constants in the Bode plot is consistent with those of capacitive loops in the Nyquist plots. So there are 4 time constants in the equivalent circuit for K2FeO4@P3HT-2% electrode. The equivalent circuit for K2FeO4@P3HT is obtained and represented in Fig. 9. Fig. 10 presents Nyquist diagrams of K2FeO4@P3HT-1%, K2FeO4@P3HT-2%, K2FeO4@P3HT-3% with fit results based on the equivalent circuit of Fig. 9 by Zview. Good fit are obtained and prove that the equivalent circuit in Fig. 9 is resonable. These fit parameters results of the elements in equivalent circuit have been tabulated in Table 1. After coating, both the charge transfer resistance (Rct) and total resistance (Rtotal) decreases, because P3HT is conductive polymer and enhances the ability of electron transfer. The film resistance Rf1 and Rf2 increase with increasing the coating content, indicating that both the content of P3HT and its oxidized product and the content of Fe2O3 increased, respectively. The charge transfer resistance slightly decreases with increasing the amount of P3HT due to that P3HT could enhance the ability of electron transfer. The total resistance increases with increasing the amount of P3HT. This reason is that increasing the amount of P3HT could greatly increase the film resistance (Rf1 + Rf2). So the satisfying coating amount of P3HT is 1%. The discharge curves of K2FeO4 and K2FeO4@P3HT are shown in Fig. 11. Both the uncoated and coated K2FeO4 electrode exhibited nice discharge characteristics with similar discharge profiles. When tested immediately after assembling, the discharge capacity (310 mAh g1) of the K2FeO4 electrode is less than those of K2FeO4@P3HT electrodes which were 351 mAh g1 (for K2FeO4@P3HT-1%), 333 mAh g1 (for K2FeO4@P3HT-2%) and 315 mAh g1 (for K2FeO4@P3HT-3%), respectively. Increasing the ratio of P3HT coatings, the discharge capacity decreases. The discharge capacity of K2FeO4@P3HT-3% is almost equal with that of K2FeO4 electrodes. These results may be explained as follows. In the process of coating, because K2FeO4 is reduced to be Fe2O3 to be overlayer upon the K2FeO4 by P3HT, the mass of K2FeO4 which participate in discharge reaction decreases. So the higher ratio of P3HT, the lower electrode capacity obtained. Electrically insulating Fe2O3 is disadvantageous for K2FeO4 discharge. And during the discharge process, oxidative P3HT itself may have participated in the discharge reaction. The theoretical capacity for the P3HT discharge is calculated to be 315 mAh g1. The maximum capacity of the 1% P3HT is 3.15 mAh g1 (315 1% mAh g1), which only
Fig. 10. Nyquist diagrams of (a) K2FeO4@P3HT-1%, (b) K2FeO4@P3HT-2%, and (c) K2FeO4@P3HT-3% electrode with fit results.
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Technology Project of Jiangxi Province, China (No. 20121BBE50024, No. 20143ACB21015). We thank professor Jiulin Wang, associated professor Yanna Nuli at School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, China for revising this paper. References
Fig. 11. Discharge curves for K2FeO4 and K2FeO4@P3HT after storing in 10 mol L1 KOH solution for 0 h and 6 h.
accounts for a relatively small portion of the 351 mAh g1 of total capacity for K2FeO4@P3HT-1% electrode. Although this ratio is small, the P3HT optimizes the K2FeO4 charge transfer. The P3HT coating is acting as a protecting and conducting layer which slows down the dissolution and decomposition rate of K2FeO4 and reduces the total resistance. So the discharge capacity of the K2FeO4 electrode is less than those of K2FeO4@P3HT electrode. Especially, the discharge capacity of K2FeO4@P3HT-1% is 351 mAh g1, which is much higher than those of K2FeO4 coated with other compounds reported [10,12–17]. The above results indicate that the partly oxidized P3HT coating could enhance the capacity of the K2FeO4 electrode. After storing the K2FeO4 and K2FeO4@P3HT electrodes in the 10 mol L1 KOH for 6 h, the change tendency is similar with that of being tested immediately after assembly. The discharge capacities are 256 mAh g1, 314 mAh g1, 305 mAh g1 and 268 mAh g1 for the electrodes of K2FeO4 and K2FeO4@P3HT-1%, K2FeO4@P3HT-2%, K2FeO4@P3HT-3%, respectively. The capacity ratio of the K2FeO4 is 82.6% of sample without storing in the electrolyte, which is less than those of K2FeO4@P3HT. The results indicate that the partly oxidized P3HT coating could improve the stability of the K2FeO4 electrode. 4. Conclusions Conductive polymer P3HT was successfully coated onto the K2FeO4. Two-layer films (partly oxidized P3HT film and Fe2O3 film) could be in-situ formed between the P3HT and K2FeO4 because that reduction-oxidation reaction occurs between P3HT and K2FeO4. The partly oxidized P3HT coating layer, in one aspect reduced the total inter-resistance and enhanced the ability of charge transfer of K2FeO4 electrode; in another aspect served as effective barriers to keep electrolyte from contacting with the K2FeO4 in the cathode, thus greatly improved the capacity and stability of the K2FeO4 for use as cathode in alkaline super-iron battery. Acknowledgements This research was supported by National Natural Science Foundation of China (No.21562015), General project of Jiangxi Provincial Education Department, China (No.GJJ150363), the Important Science and Technology Special Project of Jiangxi Province, China (No. 20114ABF05100) and the Science and
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