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Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery
Journal of Colloid and Interface Science xxx (xxxx) xxx
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Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery Mengchen Wu a, Bingkun Guo b, Anmin Nie c, Rui Liu a,⇑ a Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Department of Polymeric Materials, School of Materials Science and Engineering, Tongji University, Shanghai 201804, China b Materials Genome Institute, Shanghai University, Shanghai, China c Center for High Pressure Science, State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 3 September 2019 Revised 9 November 2019 Accepted 9 November 2019 Available online xxxx Keywords: ZIF Dopamine coating FeNi alloy ORR/OER Zn-air battery
a b s t r a c t As one type of bifunctional oxygen electrocatalyst for Zn-air battery, herein, FeNi alloy was successfully embedded into N-doped carbon with tailored architectures by integrating MOF precursor method and polymer coating/encapsulation strategy. The content of Fe in primary precursor has been proven to be able to obviously affect the morphology of the final catalyst. Benefiting from the mature active site (e.g. FeNi alloy) and the stable carbon matrix, a series of catalysts exhibited good performance towards ORR and OER. Of great significance, a particular ratio of Fe/Ni happened to be able to catalyze the growth of 1D bamboo-like carbon nanotubes, giving rise to a conductive network to diffuse ORR/OER-relevant species. Apparently, a low discharge-charge voltage gap (1.1 V) was acquired in a liquid Zn–air battery with 1.5FeNi@NCNT air cathode. Moreover, the solid-state Zn-air battery assembled on it also displayed a high open circuit voltage (1.38 V) and yielded a high power density of 81 mW cm2 at 0.83 V. This would leverage a choice to tailor carbon geometry of FeNi alloy-based active sites for ORR/OER and further serve for devices of practical significance. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Up against the ever-growing demand of energy, advanced energy conversion and storage systems are being installed world⇑ Corresponding author. E-mail address: [email protected] (R. Liu).
wide to mitigate the deficiency [1,2]. In the future-generation scenarios, Zinc-air battery stands as the most emblematical technology with large energy density, low cost and intrinsic safety [3,4]. However, as the keystone of rechargeable zinc-air battery, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are rather sluggish, impeding its massive implement. At the forefront of research effort is to develop highly effective, stable,
https://doi.org/10.1016/j.jcis.2019.11.033 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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M. Wu et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx
and inexpensive catalysts for these two reactions [5,6]. Prospectively, transition metal and their derivatives (e.g., alloy/oxides) have elucidated favorable catalytic activity for OER and ORR, which represent a group of appealing bi-functional catalysts [7–13]. Nevertheless, for the sake of practical operation, this promising paradigm fairly needs improvement in stability. One powerful knob is to fabricate active alloy-based carbon or heteroatom (N, P, or S) doped carbon matrix, which is substantially chemical stable [14,15,16]. Metal Organic Frameworks (MOFs), with the features of great variety and in-situ converting into alloy-carbon compound under heat treatment, have sprung up as ideal precursors for the scope of electrocatalysts [17,18,19]. For example, Dai et al. prepared FeNi/NiFe2O4/NC microboxes from carbonizing bimetal Fe/ Ni-MOF, which showed excellent OER performance [20]. The seeming simplicity of obtaining alloy whereas also comes great challenges. It entails the side-effect of aggregation and phase separation during the reduction for bimetallic phase under the heat treatment. Another bottleneck is the limited coordination of connectors (metal ions) and linkers (organic ligands), which restricts the choice of metal-ligand combo. To counteract the aforementioned shortages, a deposition of shell layers (e.g., coordination polymer, biomass) onto the core MOFs alongside of the envelope of external metal sources is a method of choice to obtain bimetallic carbon materials [21–23]. Telfer et al. prepared PtCo nanoparticles confined in hollow nitrogen-doped porous carbon capsules by carbonizing metaltannic acid coordination polymer coated ZIF-8 [24]. By virtue of the cornucopia of options for core and shell precursors, it gives rise to the possibility to construct various bimetallic carbon materials. Another blossoming interest is that some in-situ formed metals during high temperature can possibly catalyze carbon to form recognizable morphologies such as CNTs and graphene [25–28]. Previous observation suggested that the performance and stability of catalysts also depend sensitively on the morphology. Particularly, 1D nanotube structure outperforms others [29,30]. Mimicking catalysts bestowing the foregoing merits, herein, we constructed a series of N-doped carbon embedded with FeNi nanoparticles by integrating MOF precursor method and polymer coating/encapsulation strategy. Fe-doped ZIF-8 with different ratios of Fe dispersing throughout was used as a template. By virtue of an adherent dopamine-metal coating approach, MOF@PDA/ Ni2+ complex formed after mixing the template, dopamine monomers and nickel acetylacetonate in an alkaline solution [31]. The obtained core-shell complex was then carbonized into N-doped carbon embedded with FeNi nanoparticles via heat treatment. It was noteworthy that careful modulation of the content of Fe in the parent templates led to different phenomena, including 1) absence of obvious nanoparticles in dodecahedron, 2) bamboolike carbon nanotube embedded with alloy nanoparticles, and 3) aggregated nanoparticles supported on the carbon substrate. Encouragingly, 1D carbon nanotube attribute was propitious to offer pronounced catalytic activity for OER and ORR. Thus, it could drive both liquid and all-solid-state Zinc-air batteries to achieve much longer cycle life and higher efficiency compared with Pt/C and IrO2 catalysts. 2. Experimental 2.1. Sample preparation 2.1.1. Synthesis of 1Fe/Zn-ZIF, 1.5Fe/Zn-ZIF and 2Fe/Zn-ZIF During the synthesis step, by tuning the doping concentration of Fe, Fe-doped ZIFs with different Fe/Zn ratios were prepared. Typically for 1Fe/Zn-ZIF, 2-methylimidazole (1314 mg, 16 mmol) was dissolved in 15 mL methanol with stirring in flask I. Zinc nitrate hexahydrate (1190 mg, 4 mmol) and Iron(III) acetylacetonate
(141 mg, 0.4 mmol) were dissolved in 30 mL methanol under ultrasound to form a clear solution in flask II. Then, the solution in flask II was subsequently added into flask I with vigorous stirring for 1 h at room temperature. The yellow solution was then transferred into an autoclave (100 mL) and heated at 120 °C for 4 h. The product was separated by centrifugation, washed with DMF and methanol in sequence and finally dried at 70 °C under vacuum. The synthesis of 1.5Fe/Zn-ZIF and 2Fe/Zn-ZIF was the same as the above, except the doping amount of Fe(acac)3 was 211.5 mg and 242 mg, respectively. 2.1.2. PDA-Ni coating In a typical experiment, 200 mg Fe/Zn-ZIFs were dispersed in Tris-HCl solution (pH = 8.5, 10 mM), labelled as solution I. Dopamine hydrochloride (200 mg) and nickel acetylacetonate (3 mg) were dissolved in a solution of ethanol (4 mL) and deionized water (6 mL), labelled as solution II. Solution II was then injected into solution I with stirring. The mixed solution was stirred continuously for 40 min at room temperature. PDA/Ni2+ complex coated ZIF was isolated by centrifugation and washed with water three times, and named as Fe/Zn-ZIF @PDA-Ni. 2.1.3. Synthesis of FeNi-based catalysts 1FeNi@NC, 1.5FeNi@NCNT and 2FeNi@NC were obtained by carbonizing the relative Fe/Zn-ZIF @PDA-Ni samples, respectively, at 900 °C with a heating rate of 5 °C min1 for 2 h under N2 atmosphere. 2.2. Structure characterization Scanning electron microscopy (SEM) images were obtained on Hitachi-S4800 scanning electron microscope (10 kV). X-ray diffraction (XRD) patterns were recorded by Bruker D8 Advance X-ray powder diffractometer. Nitrogen adsorption/desorption were tested on Micromeritics Tristar 3020 analyser at 77 K. BrunauerEmmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model were employed to calculate the specific surface area and pore size distribution. Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were collected on a JEM-2100. X-ray photoelectron spectroscopy (XPS) was performed on VG Thermo ESCALAB 250 (VG Scientific). 2.3. Electrochemical performance test ORR measurement was carried out on CHI760E in O2-saturated 0.1 M KOH. In the preparation of working electrodes, 10 mg of catalysts were dispersed in a mixture of 1.25 mL of ethanol and 30 mL of 5 wt% Nafion solution, then the mixture was put in ultrasound for 40 min. The loading of the prepared catalysts and Pt/C on working electrode was 0.36 and 0.10 mg cm2, respectively. The reference and counter electrode were Ag/AgCl electrode and platinum, and a rotating disk electrode (RDE) with a surface area of 0.196 cm2 was used as a working electrode. All the tested potentials were further calibrated to reversible hydrogen electrode (RHE) as follows: ERHE = EAg/AgCl + 0.059 pH + 0.1976. The transferred electron number (n) per oxygen molecule during ORR process was calculated by Koutecky–Levich equation:
1 1 1 1 1 ¼ þ ¼ þ J JK JL JK Bx1=2
ð1Þ
B ¼ 0:2nFDo 2=3 x1=2 v 1=6 C o
ð2Þ
OER performance of the catalysts was estimated from LSV in 1 M KOH at a scan rate of 2 mV s1 and the loading of catalyst was the same as that in ORR test.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
M. Wu et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx
Tafel slope was obtained according to the equation:
g ¼ b log ðjÞ þ a
ð3Þ
where g, j and b were the over potential, measured current density, and Tafel slope, respectively. 2.4. Fabrication and test of rechargeable Zn–air battery A liquid Zn–air battery was assembled by a carbon fiber paper (3.14 cm2) loaded with the catalyst, a zinc foil and electrolyte containing 6 M KOH and 0.2 M ZnCl2. The all solid-state Zn-air battery was fabricated by a polished zinc foil (1 3 cm) as anode and a piece of 1 1 cm carbon cloth coated with catalyst as cathode. A section of cellulose tape assisted to seal the battery. The solid electrolyte was prepared as follows: 1.0 g of polyvinyl alcohol (PVA) powder was added into 10 mL H2O at 90 °C under stirring. When the solution transformed into the transparent gel, 3 mL of 6 M KOH containing 0.2 M Zn(OAc)2 was added. After stirring for another 30 min, the gel was poured onto a glass and then frozen in a freezer at 20 °C. Before experiment, it should be melt at room temperature. Polarization curves were conducted on the CHI760E electrochemical workstation. LAND testing system was used to evaluate the performance of galvanostatic discharge. 3. Result and discussion 3.1. Structure and morphology analysis The preparation strategy was briefly depicted in Scheme 1. This started with the synthesis of Fe-doped ZIF-8 nanoparticles with different ratios of Fe/Zn, which were then mixed with dopamine monomer and nickel acetylacetonate in an alkaline solution. From TEM images (Fig. 1a, e and i), all the Fe/Zn-ZIF nanocrystals showed dodecahedral shape and the size increased along with the increasing amount of Fe. As a typical sample, 1.5Fe/Zn-ZIF retained the crystal structure of the pristine ZIF-8 from powder X-ray diffraction (XRD) patterns, well concurring with the former report (Fig. 2a) [32]. After a consistent PDA coating and metal encapsulation approach, a rough thin layer was observed on the surface with original morphology but less angularly distinct (Fig. 1b, f and j). Subsequently, these core-shell nanoparticles were carbonized under 900 °C under inert atmosphere, giving rise to 1FeNi@NC, 1.5FeNi@NCNT and 2FeNi@NC, respectively. By careful modulation of the Fe weight percent in the parent template, as nicely seen in TEM and SEM images, different morphologies were obtained, including: (1) dodecahedron with a shrinkage on surface where no obvious particles formed (Fig. 1c and d), (2) bamboo-like carbon nanotubes encapsulating some small nanoparticles (Fig. 1g and h), (3) carbon framework decorated with distinct particles (Fig. 1k and L).
Scheme 1. Illustration of the preparation process of the FeNi-based catalysts.
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The particular bamboo-like structure drove us to explore more information about this structural evolution. As seen in Fig.S1, 1.5Fe/Zn-ZIF was firstly selected as a reference sample and converted into dense dodecahedral particles (denoted as Fe@NC) after carbonization at 900 °C. [32] The comparison illustrated that PDA mediated bi-metal or alloy in 1.5Fe/Zn-ZIF@PDA-Ni would contribute to the evolution of bamboo-like carbon nanotube. [33] We then obtained a series of carbonized samples from 1.5Fe/ZnZIF@PDA-Ni as the temperature increased from 700 to 900 °C (Fig. S2). It was clear that, during the carbonization, there was a fusion and growth period before in-situ forming FeNi nanoparticles (Fig S2a and b). Then, the larger fusion at higher temperature led to the presence of fine FeNi species and at the same time, the nitrogen-rich ligands decomposed completely to afford essential sources of C and N (Fig. S2c). The as-formed FeNi alloy would catalyze the growth of carbon nanotubes while the appropriate content of N in the graphitic network would induce the curvature of the graphitic layer, accordingly promoting the evolution of bamboo-like structure [34,35]. A close comparison was further made in the series of FeNibased samples. 1FeNi@NC retained the dodecahedral structure with slight distortion whereas no obvious particles were observed. Owing to that the fusion of FeNi alloy particles became more easily with the increasing content of Fe, nanometer-scale particles then were formed in 1.5FeNi@NCNT. These existing nanoparticles further catalyzed the growth of CNTs alongside of the collapse of dodecahedral structure. Nevertheless, when it reached a much higher Fe content, the nanoparticles completely fused to bulk particles and then decorated onto the framework, as seen in 2FeNi@NC. High-resolution TEM (HRTEM) analyzed the characteristic of 1.5FeNi@NCNT. As shown in Fig. 3a and b, it exhibited bamboo-like structure and a lattice fringe of 2.07 Å, corresponding to (1 1 1) plane in cubic FeNi phase [36,37]. An obvious interface could be discerned between the rather tinny metal particles and the outer layers. The result validated that the growth of carbon nanotubes was assuredly catalyzed by the involved metal alloy. It could be expected according to previous researches that the interfacial effect would not only promote catalytic activity but also greatly enhance the stability of FeNi active sites. Energy-dispersive X-ray (EDX) elemental mapping showed the total mass of Ni and Fe in 1.5FeNi@NCNT was approximately 0.67% and 4.8% (Fig. 3c and Table S1). The existing FeNi nanoparticles were entrapped in the nanotube and the whole framework collapsed. Furthermore, XRD patterns of three FeNi-based materials showed two dominating peaks around 24° and 43° (Fig. 2b),[36,38] probably due to the comparatively more abundant carbon in all the samples. The results from X -ray photoelectron spectroscopy (XPS) (Fig. 4a) indicated the co-existence of C, N, O, Fe and Ni in the series of samples. On basis of binding energy, the N1s spectrum of all the samples could be fitted with three main peaks at 398.5, 499.4, and 401 eV (Fig. 4b, and Fig.S3a, 3b), attributing to pyridinic N, pyrrolic N and graphitic N. In Fe 2p spectrum, the peaks at 707.3 and 720.7 eV revealed the presence of Fe0, combined with some peaks around 709.8 and 723.2 eV coming from the surface oxidation of alloy particles (Fig. 4c and Fig.S3c, 3d). Besides, the spectrum of Ni 2p presented two peaks around 854.5 and 872.1 eV, ascribing to the subpeak of Ni0 (Fig. 4d and Fig.S3e, 3f) [37,38]. All of those aforementioned signified that there were most-likely FeNi alloy status in N-doped carbon martix. A Type-IV characteristic with an obvious hysteresis loop displayed in N2 sorption isotherms, suggesting the existence of mesopores in these catalysts (Fig. S4a). It was found here that adding Fe increased the pore volume and surface area in the system. To be specific, the surface area increased from 341.9 to 490.3 and 870 m2 g1 for 1FeNi@NC, 1.5FeNi@NCNT and 2FeNi@NC, respectively. The pore volume and pore distribution were also analyzed in Table S2 and Fig. S4b. It was probably
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 1. TEM images of (a) 1Zn/Fe-ZIF, (b) 1Zn/Fe-ZIF@PDA-Ni, (c) 1FeNi@NC, (e) 1.5Zn/Fe-ZIF, (f) 1.5Zn/Fe-ZIF@PDA-Ni, (g) 1.5FeNi@NCNT, (i) 2Zn/Fe-ZIF, (j) 2Zn/Fe-ZIF@PDANi and (k) 2FeNi@NC. SEM images of (d) 1FeNi@NC, (h) 1.5FeNi@NCNT and (l) 2FeNi@NC.
Fig. 2. XRD patterns of the as-prepared samples.
related to the fact that the carbon species were gradually consumed by metal particles, contributing to more voids and cavities. 3.2. Electrochemical performance of catalysts Having proved that our synthetic strategy could successfully yield appealing catalysts, we moved on to the study of their catalytic properties. Cyclic voltammetry (CV) tests were performed to preliminarily estimate the ORR activity with a rotating disk electrode in O2-saturated 0.1 M KOH [39]. As displayed in Fig. 5a, three FeNi-based catalysts held prominent oxygen reduction peaks around 0.85 V vs. reversible hydrogen electrode (RHE), which were more positive than Fe@NC. Within the linear sweep voltammetry (LSV) curves shown in Fig. 5b, especially, 1.5FeNi@NCNT delivered an ORR onset potential (E0) of 0.95 V and a half-wave potential (E1/2) of 0.86 V. These values were approaching to or even advancing those of commercial 20 wt% Pt/C catalyst (E0 = 0.96 V and E1/2 = 0.83 V). The corresponding Koutecky-Levich (K-L) plots (Fig.
S5) were used to calculate the electron transfer number (n). The parallel results with values close to theoretical value demonstrated that the catalytic process at the FeNi-based electrode entailed a four-electron ORR pathway. Having systematically probed ORR activity of all catalysts in terms of onset and half-wave potential, we made Fig. 5c and d to intuitively elucidate the variation and the origin of the improvement in catalytic activity. Additionally, the Tafel slopes were in the usual 50–80 mV dec1 range for FeNi-based catalysts (Fig. 6a). To shed insight on OER activity, the as-prepared materials were evaluated by LSV measurements in 1 M KOH at the scan rate of 2 mV s1. As shown in Fig. 6b, on balance, 1.5FeNi@NCNT exhibited the highest catalytic activity among the five samples. The overpotential required to achieve 10 mA cm2 for IrO2 was ca. 260 mV and significantly decreased to 230 mV on 1.5FeNi@NCNT. While for the other catalysts, the same catalytic current density was observed at 240 mV (1FeNi@NC), 260 mV (2FeNi@NC), and 290 mV (Fe@NC), respectively. Furthermore, Tafel slopes in
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 3. (a) HAADF-STEM, (b) High-magnification HRTEM images and (c) elemental mapping of 1.5FeNi@NCNT.
Fig. 6c reflected the activity trend. 1.5FeNi@NCNT showed the smallest value of 55 mV dec1 at low over-potentials, evidencing a faster kinetic process. Gleaning from the above results helped us in identifying the best excellent catalyst among the as-prepared ones. Most likely, the strong coupling between FeNi alloys and interwoven CNTs network should be helpful to enhance bifunctional catalytic activity and offer fast pathway for diffusing ORR/OER-relevant species, respectively. These variations simply originated from a change in
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Fe doping content. In the following, other key parameters in high ORR and OER performance were also examined. For the stability of ORR, chronoamperometry measurement at a constant potential was employed (Fig.S6a). After 12000 s of running, 1.5FeNi@NCNT remarkably exhibited 88% (70% for commercial Pt/C) retention of initial value. Moreover, the property of tolerance to methanol poison effect was also compared between 1.5FeNi@NCNT and Pt/C. Fig.S6b showed that current density changed slightly in 1.5FeNi@NCNT after injecting methanol into the electrolyte, indicating a superior tolerance than that of commercial Pt/C catalyst. Considering the low CO poisoning tolerance is one key factor to limit the catalytic activity for Pt/C, we thus tested the anti-COpoisoning property of 1.5FeNi@NCNT. In Fig.S7, when CO was introduced to O2-satuarated eletrolyte, the current density of 1.5FeNi@NCNT and Pt/C obviously decreased. However, 1.5FeNi@NCNT showed more retentation than Pt/C, thanking to porous structure to combine more O2. After another 500 s, O2 was replaced by CO again, 1.5FeNi@NCNT suffered lower CO poisoning than Pt/C compared to inital curve. The OER performance was inspected from the LSV curves of 1.5FeNi@NCNT before and after 2000 CV cycles, which showed that little had changed in the process (Fig. 6d). At the same time, performances of some relevant state-of-art catalysts towards ORR and OER have been compared together to highlight the attractive catalytic activity of 1.5FeNi@NCNT (Table S3). 3.3. Rechargeable Zn-air battery study Considering the bi-functional catalytic capability and stability of the 1.5FeNi@NCNT catalyst, a liquid Zn–air battery (ZAB) was further evaluated. The configuration consisted of a Zn plate (anode), a carbon paper coated with the catalyst (cathode), and 6.0 M KOH containing 0.2 M ZnCl2 solution (electrolyte), where oxygen was purged at the cathode side [40]. The open-circuit voltage of this ZAB based on 1.5FeNi@NCNT catalyst was found to be
Fig. 4. (a) XPS survey scan specta of 1.5FeNi@NCNT. XPS spectra of (b) N 1s, (c) Fe 2p, (d) Ni 2p in 1.5FeNi@NCNT.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 5. (a) CV responses of the as-prepared samples and Pt/C, (b) LSV of the as-prepared samples and Pt/C at 1600 rpm at a sweep rate of 10 mV s1 in 0.1 M KOH. (c) Onset potential (vs. RHE), and (d) half-wave potential of the as-prepared samples in O2-saturated alkaline medium.
Fig. 6. (a) The corresponding Tafel plots for ORR. (b) LSV polarization curves of the as-prepared samples. (c) Tafel plots from the obtained OER polarization curves. (d) Polarization curves of 1.5FeNi@NCNT before and after 2000 CV cycles.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 7. (a) Charge polarization curves and corresponding power density plots of liquid Zn-air battery. (b) Discharge curves of the battery with 1.5FeNi@NCNT at different current density. (c) The open-circuit voltage of all-solid-state Zn-air battery with 1.5FeNi@NCNT catalyst. (d) Schematic illustration of an all-solid-state Zn-air battery. (e) Discharge polarization curve and corresponding power density plots of all-solid-state Zn-air battery. (f) Charge and discharge cycling curve of flexible solid-state Zn-air battery under different bending conditions from 0° to 60° for every 2 h.
1.44 V (Fig.S8a). As shown in Fig. 7a, the ZAB with 1.5FeNi@NCNT showed a peak power density of 114 mW cm2, rivalling that of commercial Pt/C (90 mW cm2). When galvanostatically discharged at a current density of 5 mA cm2 for 12 h, the discharge voltage of 1.5FeNi@NCNT-based ZAB remained at 1.32 V (Fig. 7b). Fig.S8b showed that this rechargeable ZAB with 1.5FeNi@NCNT could continuously run more than 100 cycles at a constant discharging and charging current density of 10 mA cm2. It could been observed that the initial voltage gap was 1.1 V and negligible activity had lost after long operation, indicating a good rechargability. Fig.S9a presented a photograph of two connected primary liquid ZABs powering an LED light of 3 V. Alongside of the test of a home-made liquid ZAB, a flexible allsolid-state ZAB was also fabricated, as schematically illustrated in Fig. 7d [41]. An alkaline polyvinyl alcohol (PVA) gel was used as electrolyte, which was stretchable and bendable. 1.5FeNi@NCNT equipped ZAB showed an open-circuit voltage of 1.38 V (Fig. 7c). In Fig. 7e, it yielded a high power density of 81 mW cm2 at 0.83 V. In view of the flexible advantage, the all-solid-state ZAB with 1.5FeNi@NCNT catalyst was examined by folding in different bending angles of 0°, 30° and 60° for every 2 h. As shown in Fig. 7f, the initial voltage gap was only 0.81 V, and no obvious changes were observed after a cycle of bend. Fig.S9b displayed that two
all-solid-state ZABs in series can power a LED, which presumably offered new horizons for developing flexible all-solid-state ZABs.
4. Conclusion Generally, FeNi alloy has been encapsulated within N-doped carbon matrix, engineering from the combination of MOF precursor method and polymer encapsulation strategy. Principles from optimized morphology have been incorporated into the design of electrocatalysts, which depended on the content of Fe in the parent template. Significantly, the morphology of 1D bamboo-like tube with embedded FeNi nanoparticles effectively helped the transportion of ORR/OER species during the electrocatalytic process. This developed FeNi-based catalytic system ensured superior stability and high ORR/OER activity in alkaline condition, providing a new insight to explore advanced bi-functional catalysts. Furthermore, liquid and flexible all-solid-state ZABs with 1.5FeNi@NCNT as the catalyst gave a high open circuit potential of 1.44 V and 1.38 V, and both could entail a prolonged operation. Consequently, the stated method for the construction of distinct morphology and mature component would feature a light scope for practical applications.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Author contribution M. W. and R. L. designed experiments. M. W. conducted experiments and analyzed data. M. W., B. G., A. N. and R.L discussed and interpreted the results. M. W. and R.L. prepared the manuscript. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements We would like to acknowledge the Shanghai Municipal Natural Science Foundation (Grant No.17ZR1432200), the start-up funding from Tongji University and the Young Thousand Talented Program. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.11.033. References [1] J. Zhao, C. Li, R. Liu, Designed echinops-Like Ni@NiNC as efficient bifunctional oxygen electrocatalyst for zinc-air batteries, ChemElectroChem 6 (2) (2019) 342–348. [2] D.B. Wu, Y.B. Niu, C. Wang, H.J. Wu, Q. Li, B.H. Xu, H.L. Li, L.Y. Zhang, c-Fe2O3 nanoparticles stabilized by holey reduced graphene oxide as a composite anode for lithium-ion batteries, J. Coll. Interf. Sci. 552 (2019) 633–638. [3] X.T. Zhao, S.C. Abbas, M.X. Wu, Y.B. Wang, Robust and highly active FeNi@NCNT nanowire arrays as integrated air electrode for flexible solidstate rechargeable zn-air batteries, Adv. Mater. Interf. 5 (9) (2018) 1701448. [4] X. Zhu, T. Jin, C.C. Tian, C.B. Lu, X.M. Liu, X.D. Zhuang, H.L. Liu, S. Dai, In situ coupling strategy for the preparation of FeCo alloys and Co4N hybrid for highly efficient oxygen evolution, Adv. Mater. 29 (47) (2017) 1704091. [5] J. Zhao, N. Fu, R. Liu, Graphite-wrapped Fe core–shell nanoparticles anchored on graphene as pH-universal electrocatalyst for oxygen reduction reaction, ACS Appl. Mater. Inter. 10 (34) (2018) 28509–28516. [6] T.Y. Ma, J. Ran, S. Dai, M. Jaroniec, S.Z. Qiao, Phosphorus-doped graphitic carbon nitrides grown in situ on carbon-fiber paper: flexible and reversible oxygen electrodes, Angew. Chem., Int. Ed. 54 (15) (2015) 4646–4650. [7] H.H. Wu, M. Zeng, Z.Y. Li, X. Zhu, C.C. Tian, C. Xia, L. He, S. Dai, Coupling FeNi alloys and hollow nitrogen-enriched carbon frameworks leads to highperformance oxygen electrocatalysts for rechargeable zinc–air batteries, Sustain. Energ. Fuels 3 (1) (2019) 136–141. [8] Q.C. Wang, Y.P. Lei, Z.Y. Chen, N. Wu, Y.B. Wang, Y.D. Wang, Fe/Fe3C@ C nanoparticles encapsulated in N-doped graphene–CNTs framework as an efficient bifunctional oxygen electrocatalyst for robust rechargeable Zn–air batteries, J. Mater. Chem. A. 6 (2) (2018) 516–526. [9] S. Gupta, L. Qiao, S. Zhao, G. Wu, Highly active and stable graphene tubes decorated with FeCoNi alloy nanoparticles via a template-free graphitization for bifunctional oxygen reduction and evolution, Adv. Energy Mater. 6 (22) (2016) 1601198–2160210. [10] Y.Y. Zhang, D.B.Wu. Gong, Z. Li, Q. Li, L.W. Zheng, W.Y. Yuan, L.Y. Zhang, Facile fabrication of stable PdCu clusters uniformly decorated on graphene as an efficient electrocatalyst for formic acid oxidation, Int. J. Hydrogen Energ. 44 (2019) 2731–2740. [11] Y.Y. Gong, X.H. Liu, Y.Y. Gong, D.B. Wu, X.H. Xu, L. Bi, L.Y. Zhang, X.S. Zhao, Synthesis of defect-rich palladium-tin alloy nanochain networks for formic acid oxidation, J. Coll. Interf. Sci. 530 (2018) 189–195. [12] C.L. Li, M.C. Wu, R. Liu, High-performance bifunctional oxygen electrocatalysts for zinc-air batteries over mesoporous Fe/Co-NC nanofibers with embedding FeCo alloy nanoparticles, Appl. Catal. B: Environ. 244 (2019) 150–158. [13] S.Y. Zhao, B. Zhang, H. Su, J.J. Zhang, X. Wei, P.Y. Feng, Enhanced oxygen electroreduction over nitrogen-free carbon nanotube-supported CuFeO2 nanoparticles, J. Mater. Chem. A. 6 (10) (2018) 4331–4336. [14] Q. Wang, Y. Ji, Y. Lei, Y. Wang, Y. Wang, Y. Li, S. Wang, Pyridinic-N-dominated doped defective graphene as a superior oxygen electrocatalyst for ultrahighenergy-density Zn–air batteries, ACS Energy Lett. 3 (5) (2018) 1183–1191. [15] M. Wu, C. Li, J. Zhao, Y. Ling, R. Liu, Tannic acid-mediated synthesis of dualheteroatom-doped hollow carbon from a metal–organic framework for efficient oxygen reduction reaction, Dalton Trans. 47 (23) (2018) 7812–7818. [16] J. Zhao, C. Li, R. Liu, Enhanced oxygen reduction of multi-Fe3O4@carbon core– shell electrocatalysts through a nanoparticle/polymer co-assembly strategy, Nanoscale 10 (13) (2018) 5882–5887. [17] X.Z. Fang, L. Jiao, S.H. Yu, H.L. Jiang, Metal-organic framework-derived FeCo-Ndoped hollow porous carbon nanocubes for electrocatalysis in acidic and alkaline media, ChemSusChem. 10 (15) (2017) 3019–3024.
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Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery Mengchen Wu a, Bingkun Guo b, Anmin Nie c, Rui Liu a,⇑ a Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Department of Polymeric Materials, School of Materials Science and Engineering, Tongji University, Shanghai 201804, China b Materials Genome Institute, Shanghai University, Shanghai, China c Center for High Pressure Science, State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 3 September 2019 Revised 9 November 2019 Accepted 9 November 2019 Available online xxxx Keywords: ZIF Dopamine coating FeNi alloy ORR/OER Zn-air battery
a b s t r a c t As one type of bifunctional oxygen electrocatalyst for Zn-air battery, herein, FeNi alloy was successfully embedded into N-doped carbon with tailored architectures by integrating MOF precursor method and polymer coating/encapsulation strategy. The content of Fe in primary precursor has been proven to be able to obviously affect the morphology of the final catalyst. Benefiting from the mature active site (e.g. FeNi alloy) and the stable carbon matrix, a series of catalysts exhibited good performance towards ORR and OER. Of great significance, a particular ratio of Fe/Ni happened to be able to catalyze the growth of 1D bamboo-like carbon nanotubes, giving rise to a conductive network to diffuse ORR/OER-relevant species. Apparently, a low discharge-charge voltage gap (1.1 V) was acquired in a liquid Zn–air battery with 1.5FeNi@NCNT air cathode. Moreover, the solid-state Zn-air battery assembled on it also displayed a high open circuit voltage (1.38 V) and yielded a high power density of 81 mW cm2 at 0.83 V. This would leverage a choice to tailor carbon geometry of FeNi alloy-based active sites for ORR/OER and further serve for devices of practical significance. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Up against the ever-growing demand of energy, advanced energy conversion and storage systems are being installed world⇑ Corresponding author. E-mail address: [email protected] (R. Liu).
wide to mitigate the deficiency [1,2]. In the future-generation scenarios, Zinc-air battery stands as the most emblematical technology with large energy density, low cost and intrinsic safety [3,4]. However, as the keystone of rechargeable zinc-air battery, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are rather sluggish, impeding its massive implement. At the forefront of research effort is to develop highly effective, stable,
https://doi.org/10.1016/j.jcis.2019.11.033 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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and inexpensive catalysts for these two reactions [5,6]. Prospectively, transition metal and their derivatives (e.g., alloy/oxides) have elucidated favorable catalytic activity for OER and ORR, which represent a group of appealing bi-functional catalysts [7–13]. Nevertheless, for the sake of practical operation, this promising paradigm fairly needs improvement in stability. One powerful knob is to fabricate active alloy-based carbon or heteroatom (N, P, or S) doped carbon matrix, which is substantially chemical stable [14,15,16]. Metal Organic Frameworks (MOFs), with the features of great variety and in-situ converting into alloy-carbon compound under heat treatment, have sprung up as ideal precursors for the scope of electrocatalysts [17,18,19]. For example, Dai et al. prepared FeNi/NiFe2O4/NC microboxes from carbonizing bimetal Fe/ Ni-MOF, which showed excellent OER performance [20]. The seeming simplicity of obtaining alloy whereas also comes great challenges. It entails the side-effect of aggregation and phase separation during the reduction for bimetallic phase under the heat treatment. Another bottleneck is the limited coordination of connectors (metal ions) and linkers (organic ligands), which restricts the choice of metal-ligand combo. To counteract the aforementioned shortages, a deposition of shell layers (e.g., coordination polymer, biomass) onto the core MOFs alongside of the envelope of external metal sources is a method of choice to obtain bimetallic carbon materials [21–23]. Telfer et al. prepared PtCo nanoparticles confined in hollow nitrogen-doped porous carbon capsules by carbonizing metaltannic acid coordination polymer coated ZIF-8 [24]. By virtue of the cornucopia of options for core and shell precursors, it gives rise to the possibility to construct various bimetallic carbon materials. Another blossoming interest is that some in-situ formed metals during high temperature can possibly catalyze carbon to form recognizable morphologies such as CNTs and graphene [25–28]. Previous observation suggested that the performance and stability of catalysts also depend sensitively on the morphology. Particularly, 1D nanotube structure outperforms others [29,30]. Mimicking catalysts bestowing the foregoing merits, herein, we constructed a series of N-doped carbon embedded with FeNi nanoparticles by integrating MOF precursor method and polymer coating/encapsulation strategy. Fe-doped ZIF-8 with different ratios of Fe dispersing throughout was used as a template. By virtue of an adherent dopamine-metal coating approach, MOF@PDA/ Ni2+ complex formed after mixing the template, dopamine monomers and nickel acetylacetonate in an alkaline solution [31]. The obtained core-shell complex was then carbonized into N-doped carbon embedded with FeNi nanoparticles via heat treatment. It was noteworthy that careful modulation of the content of Fe in the parent templates led to different phenomena, including 1) absence of obvious nanoparticles in dodecahedron, 2) bamboolike carbon nanotube embedded with alloy nanoparticles, and 3) aggregated nanoparticles supported on the carbon substrate. Encouragingly, 1D carbon nanotube attribute was propitious to offer pronounced catalytic activity for OER and ORR. Thus, it could drive both liquid and all-solid-state Zinc-air batteries to achieve much longer cycle life and higher efficiency compared with Pt/C and IrO2 catalysts. 2. Experimental 2.1. Sample preparation 2.1.1. Synthesis of 1Fe/Zn-ZIF, 1.5Fe/Zn-ZIF and 2Fe/Zn-ZIF During the synthesis step, by tuning the doping concentration of Fe, Fe-doped ZIFs with different Fe/Zn ratios were prepared. Typically for 1Fe/Zn-ZIF, 2-methylimidazole (1314 mg, 16 mmol) was dissolved in 15 mL methanol with stirring in flask I. Zinc nitrate hexahydrate (1190 mg, 4 mmol) and Iron(III) acetylacetonate
(141 mg, 0.4 mmol) were dissolved in 30 mL methanol under ultrasound to form a clear solution in flask II. Then, the solution in flask II was subsequently added into flask I with vigorous stirring for 1 h at room temperature. The yellow solution was then transferred into an autoclave (100 mL) and heated at 120 °C for 4 h. The product was separated by centrifugation, washed with DMF and methanol in sequence and finally dried at 70 °C under vacuum. The synthesis of 1.5Fe/Zn-ZIF and 2Fe/Zn-ZIF was the same as the above, except the doping amount of Fe(acac)3 was 211.5 mg and 242 mg, respectively. 2.1.2. PDA-Ni coating In a typical experiment, 200 mg Fe/Zn-ZIFs were dispersed in Tris-HCl solution (pH = 8.5, 10 mM), labelled as solution I. Dopamine hydrochloride (200 mg) and nickel acetylacetonate (3 mg) were dissolved in a solution of ethanol (4 mL) and deionized water (6 mL), labelled as solution II. Solution II was then injected into solution I with stirring. The mixed solution was stirred continuously for 40 min at room temperature. PDA/Ni2+ complex coated ZIF was isolated by centrifugation and washed with water three times, and named as Fe/Zn-ZIF @PDA-Ni. 2.1.3. Synthesis of FeNi-based catalysts 1FeNi@NC, 1.5FeNi@NCNT and 2FeNi@NC were obtained by carbonizing the relative Fe/Zn-ZIF @PDA-Ni samples, respectively, at 900 °C with a heating rate of 5 °C min1 for 2 h under N2 atmosphere. 2.2. Structure characterization Scanning electron microscopy (SEM) images were obtained on Hitachi-S4800 scanning electron microscope (10 kV). X-ray diffraction (XRD) patterns were recorded by Bruker D8 Advance X-ray powder diffractometer. Nitrogen adsorption/desorption were tested on Micromeritics Tristar 3020 analyser at 77 K. BrunauerEmmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model were employed to calculate the specific surface area and pore size distribution. Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were collected on a JEM-2100. X-ray photoelectron spectroscopy (XPS) was performed on VG Thermo ESCALAB 250 (VG Scientific). 2.3. Electrochemical performance test ORR measurement was carried out on CHI760E in O2-saturated 0.1 M KOH. In the preparation of working electrodes, 10 mg of catalysts were dispersed in a mixture of 1.25 mL of ethanol and 30 mL of 5 wt% Nafion solution, then the mixture was put in ultrasound for 40 min. The loading of the prepared catalysts and Pt/C on working electrode was 0.36 and 0.10 mg cm2, respectively. The reference and counter electrode were Ag/AgCl electrode and platinum, and a rotating disk electrode (RDE) with a surface area of 0.196 cm2 was used as a working electrode. All the tested potentials were further calibrated to reversible hydrogen electrode (RHE) as follows: ERHE = EAg/AgCl + 0.059 pH + 0.1976. The transferred electron number (n) per oxygen molecule during ORR process was calculated by Koutecky–Levich equation:
1 1 1 1 1 ¼ þ ¼ þ J JK JL JK Bx1=2
ð1Þ
B ¼ 0:2nFDo 2=3 x1=2 v 1=6 C o
ð2Þ
OER performance of the catalysts was estimated from LSV in 1 M KOH at a scan rate of 2 mV s1 and the loading of catalyst was the same as that in ORR test.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Tafel slope was obtained according to the equation:
g ¼ b log ðjÞ þ a
ð3Þ
where g, j and b were the over potential, measured current density, and Tafel slope, respectively. 2.4. Fabrication and test of rechargeable Zn–air battery A liquid Zn–air battery was assembled by a carbon fiber paper (3.14 cm2) loaded with the catalyst, a zinc foil and electrolyte containing 6 M KOH and 0.2 M ZnCl2. The all solid-state Zn-air battery was fabricated by a polished zinc foil (1 3 cm) as anode and a piece of 1 1 cm carbon cloth coated with catalyst as cathode. A section of cellulose tape assisted to seal the battery. The solid electrolyte was prepared as follows: 1.0 g of polyvinyl alcohol (PVA) powder was added into 10 mL H2O at 90 °C under stirring. When the solution transformed into the transparent gel, 3 mL of 6 M KOH containing 0.2 M Zn(OAc)2 was added. After stirring for another 30 min, the gel was poured onto a glass and then frozen in a freezer at 20 °C. Before experiment, it should be melt at room temperature. Polarization curves were conducted on the CHI760E electrochemical workstation. LAND testing system was used to evaluate the performance of galvanostatic discharge. 3. Result and discussion 3.1. Structure and morphology analysis The preparation strategy was briefly depicted in Scheme 1. This started with the synthesis of Fe-doped ZIF-8 nanoparticles with different ratios of Fe/Zn, which were then mixed with dopamine monomer and nickel acetylacetonate in an alkaline solution. From TEM images (Fig. 1a, e and i), all the Fe/Zn-ZIF nanocrystals showed dodecahedral shape and the size increased along with the increasing amount of Fe. As a typical sample, 1.5Fe/Zn-ZIF retained the crystal structure of the pristine ZIF-8 from powder X-ray diffraction (XRD) patterns, well concurring with the former report (Fig. 2a) [32]. After a consistent PDA coating and metal encapsulation approach, a rough thin layer was observed on the surface with original morphology but less angularly distinct (Fig. 1b, f and j). Subsequently, these core-shell nanoparticles were carbonized under 900 °C under inert atmosphere, giving rise to 1FeNi@NC, 1.5FeNi@NCNT and 2FeNi@NC, respectively. By careful modulation of the Fe weight percent in the parent template, as nicely seen in TEM and SEM images, different morphologies were obtained, including: (1) dodecahedron with a shrinkage on surface where no obvious particles formed (Fig. 1c and d), (2) bamboo-like carbon nanotubes encapsulating some small nanoparticles (Fig. 1g and h), (3) carbon framework decorated with distinct particles (Fig. 1k and L).
Scheme 1. Illustration of the preparation process of the FeNi-based catalysts.
3
The particular bamboo-like structure drove us to explore more information about this structural evolution. As seen in Fig.S1, 1.5Fe/Zn-ZIF was firstly selected as a reference sample and converted into dense dodecahedral particles (denoted as Fe@NC) after carbonization at 900 °C. [32] The comparison illustrated that PDA mediated bi-metal or alloy in 1.5Fe/Zn-ZIF@PDA-Ni would contribute to the evolution of bamboo-like carbon nanotube. [33] We then obtained a series of carbonized samples from 1.5Fe/ZnZIF@PDA-Ni as the temperature increased from 700 to 900 °C (Fig. S2). It was clear that, during the carbonization, there was a fusion and growth period before in-situ forming FeNi nanoparticles (Fig S2a and b). Then, the larger fusion at higher temperature led to the presence of fine FeNi species and at the same time, the nitrogen-rich ligands decomposed completely to afford essential sources of C and N (Fig. S2c). The as-formed FeNi alloy would catalyze the growth of carbon nanotubes while the appropriate content of N in the graphitic network would induce the curvature of the graphitic layer, accordingly promoting the evolution of bamboo-like structure [34,35]. A close comparison was further made in the series of FeNibased samples. 1FeNi@NC retained the dodecahedral structure with slight distortion whereas no obvious particles were observed. Owing to that the fusion of FeNi alloy particles became more easily with the increasing content of Fe, nanometer-scale particles then were formed in 1.5FeNi@NCNT. These existing nanoparticles further catalyzed the growth of CNTs alongside of the collapse of dodecahedral structure. Nevertheless, when it reached a much higher Fe content, the nanoparticles completely fused to bulk particles and then decorated onto the framework, as seen in 2FeNi@NC. High-resolution TEM (HRTEM) analyzed the characteristic of 1.5FeNi@NCNT. As shown in Fig. 3a and b, it exhibited bamboo-like structure and a lattice fringe of 2.07 Å, corresponding to (1 1 1) plane in cubic FeNi phase [36,37]. An obvious interface could be discerned between the rather tinny metal particles and the outer layers. The result validated that the growth of carbon nanotubes was assuredly catalyzed by the involved metal alloy. It could be expected according to previous researches that the interfacial effect would not only promote catalytic activity but also greatly enhance the stability of FeNi active sites. Energy-dispersive X-ray (EDX) elemental mapping showed the total mass of Ni and Fe in 1.5FeNi@NCNT was approximately 0.67% and 4.8% (Fig. 3c and Table S1). The existing FeNi nanoparticles were entrapped in the nanotube and the whole framework collapsed. Furthermore, XRD patterns of three FeNi-based materials showed two dominating peaks around 24° and 43° (Fig. 2b),[36,38] probably due to the comparatively more abundant carbon in all the samples. The results from X -ray photoelectron spectroscopy (XPS) (Fig. 4a) indicated the co-existence of C, N, O, Fe and Ni in the series of samples. On basis of binding energy, the N1s spectrum of all the samples could be fitted with three main peaks at 398.5, 499.4, and 401 eV (Fig. 4b, and Fig.S3a, 3b), attributing to pyridinic N, pyrrolic N and graphitic N. In Fe 2p spectrum, the peaks at 707.3 and 720.7 eV revealed the presence of Fe0, combined with some peaks around 709.8 and 723.2 eV coming from the surface oxidation of alloy particles (Fig. 4c and Fig.S3c, 3d). Besides, the spectrum of Ni 2p presented two peaks around 854.5 and 872.1 eV, ascribing to the subpeak of Ni0 (Fig. 4d and Fig.S3e, 3f) [37,38]. All of those aforementioned signified that there were most-likely FeNi alloy status in N-doped carbon martix. A Type-IV characteristic with an obvious hysteresis loop displayed in N2 sorption isotherms, suggesting the existence of mesopores in these catalysts (Fig. S4a). It was found here that adding Fe increased the pore volume and surface area in the system. To be specific, the surface area increased from 341.9 to 490.3 and 870 m2 g1 for 1FeNi@NC, 1.5FeNi@NCNT and 2FeNi@NC, respectively. The pore volume and pore distribution were also analyzed in Table S2 and Fig. S4b. It was probably
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 1. TEM images of (a) 1Zn/Fe-ZIF, (b) 1Zn/Fe-ZIF@PDA-Ni, (c) 1FeNi@NC, (e) 1.5Zn/Fe-ZIF, (f) 1.5Zn/Fe-ZIF@PDA-Ni, (g) 1.5FeNi@NCNT, (i) 2Zn/Fe-ZIF, (j) 2Zn/Fe-ZIF@PDANi and (k) 2FeNi@NC. SEM images of (d) 1FeNi@NC, (h) 1.5FeNi@NCNT and (l) 2FeNi@NC.
Fig. 2. XRD patterns of the as-prepared samples.
related to the fact that the carbon species were gradually consumed by metal particles, contributing to more voids and cavities. 3.2. Electrochemical performance of catalysts Having proved that our synthetic strategy could successfully yield appealing catalysts, we moved on to the study of their catalytic properties. Cyclic voltammetry (CV) tests were performed to preliminarily estimate the ORR activity with a rotating disk electrode in O2-saturated 0.1 M KOH [39]. As displayed in Fig. 5a, three FeNi-based catalysts held prominent oxygen reduction peaks around 0.85 V vs. reversible hydrogen electrode (RHE), which were more positive than Fe@NC. Within the linear sweep voltammetry (LSV) curves shown in Fig. 5b, especially, 1.5FeNi@NCNT delivered an ORR onset potential (E0) of 0.95 V and a half-wave potential (E1/2) of 0.86 V. These values were approaching to or even advancing those of commercial 20 wt% Pt/C catalyst (E0 = 0.96 V and E1/2 = 0.83 V). The corresponding Koutecky-Levich (K-L) plots (Fig.
S5) were used to calculate the electron transfer number (n). The parallel results with values close to theoretical value demonstrated that the catalytic process at the FeNi-based electrode entailed a four-electron ORR pathway. Having systematically probed ORR activity of all catalysts in terms of onset and half-wave potential, we made Fig. 5c and d to intuitively elucidate the variation and the origin of the improvement in catalytic activity. Additionally, the Tafel slopes were in the usual 50–80 mV dec1 range for FeNi-based catalysts (Fig. 6a). To shed insight on OER activity, the as-prepared materials were evaluated by LSV measurements in 1 M KOH at the scan rate of 2 mV s1. As shown in Fig. 6b, on balance, 1.5FeNi@NCNT exhibited the highest catalytic activity among the five samples. The overpotential required to achieve 10 mA cm2 for IrO2 was ca. 260 mV and significantly decreased to 230 mV on 1.5FeNi@NCNT. While for the other catalysts, the same catalytic current density was observed at 240 mV (1FeNi@NC), 260 mV (2FeNi@NC), and 290 mV (Fe@NC), respectively. Furthermore, Tafel slopes in
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 3. (a) HAADF-STEM, (b) High-magnification HRTEM images and (c) elemental mapping of 1.5FeNi@NCNT.
Fig. 6c reflected the activity trend. 1.5FeNi@NCNT showed the smallest value of 55 mV dec1 at low over-potentials, evidencing a faster kinetic process. Gleaning from the above results helped us in identifying the best excellent catalyst among the as-prepared ones. Most likely, the strong coupling between FeNi alloys and interwoven CNTs network should be helpful to enhance bifunctional catalytic activity and offer fast pathway for diffusing ORR/OER-relevant species, respectively. These variations simply originated from a change in
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Fe doping content. In the following, other key parameters in high ORR and OER performance were also examined. For the stability of ORR, chronoamperometry measurement at a constant potential was employed (Fig.S6a). After 12000 s of running, 1.5FeNi@NCNT remarkably exhibited 88% (70% for commercial Pt/C) retention of initial value. Moreover, the property of tolerance to methanol poison effect was also compared between 1.5FeNi@NCNT and Pt/C. Fig.S6b showed that current density changed slightly in 1.5FeNi@NCNT after injecting methanol into the electrolyte, indicating a superior tolerance than that of commercial Pt/C catalyst. Considering the low CO poisoning tolerance is one key factor to limit the catalytic activity for Pt/C, we thus tested the anti-COpoisoning property of 1.5FeNi@NCNT. In Fig.S7, when CO was introduced to O2-satuarated eletrolyte, the current density of 1.5FeNi@NCNT and Pt/C obviously decreased. However, 1.5FeNi@NCNT showed more retentation than Pt/C, thanking to porous structure to combine more O2. After another 500 s, O2 was replaced by CO again, 1.5FeNi@NCNT suffered lower CO poisoning than Pt/C compared to inital curve. The OER performance was inspected from the LSV curves of 1.5FeNi@NCNT before and after 2000 CV cycles, which showed that little had changed in the process (Fig. 6d). At the same time, performances of some relevant state-of-art catalysts towards ORR and OER have been compared together to highlight the attractive catalytic activity of 1.5FeNi@NCNT (Table S3). 3.3. Rechargeable Zn-air battery study Considering the bi-functional catalytic capability and stability of the 1.5FeNi@NCNT catalyst, a liquid Zn–air battery (ZAB) was further evaluated. The configuration consisted of a Zn plate (anode), a carbon paper coated with the catalyst (cathode), and 6.0 M KOH containing 0.2 M ZnCl2 solution (electrolyte), where oxygen was purged at the cathode side [40]. The open-circuit voltage of this ZAB based on 1.5FeNi@NCNT catalyst was found to be
Fig. 4. (a) XPS survey scan specta of 1.5FeNi@NCNT. XPS spectra of (b) N 1s, (c) Fe 2p, (d) Ni 2p in 1.5FeNi@NCNT.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 5. (a) CV responses of the as-prepared samples and Pt/C, (b) LSV of the as-prepared samples and Pt/C at 1600 rpm at a sweep rate of 10 mV s1 in 0.1 M KOH. (c) Onset potential (vs. RHE), and (d) half-wave potential of the as-prepared samples in O2-saturated alkaline medium.
Fig. 6. (a) The corresponding Tafel plots for ORR. (b) LSV polarization curves of the as-prepared samples. (c) Tafel plots from the obtained OER polarization curves. (d) Polarization curves of 1.5FeNi@NCNT before and after 2000 CV cycles.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Fig. 7. (a) Charge polarization curves and corresponding power density plots of liquid Zn-air battery. (b) Discharge curves of the battery with 1.5FeNi@NCNT at different current density. (c) The open-circuit voltage of all-solid-state Zn-air battery with 1.5FeNi@NCNT catalyst. (d) Schematic illustration of an all-solid-state Zn-air battery. (e) Discharge polarization curve and corresponding power density plots of all-solid-state Zn-air battery. (f) Charge and discharge cycling curve of flexible solid-state Zn-air battery under different bending conditions from 0° to 60° for every 2 h.
1.44 V (Fig.S8a). As shown in Fig. 7a, the ZAB with 1.5FeNi@NCNT showed a peak power density of 114 mW cm2, rivalling that of commercial Pt/C (90 mW cm2). When galvanostatically discharged at a current density of 5 mA cm2 for 12 h, the discharge voltage of 1.5FeNi@NCNT-based ZAB remained at 1.32 V (Fig. 7b). Fig.S8b showed that this rechargeable ZAB with 1.5FeNi@NCNT could continuously run more than 100 cycles at a constant discharging and charging current density of 10 mA cm2. It could been observed that the initial voltage gap was 1.1 V and negligible activity had lost after long operation, indicating a good rechargability. Fig.S9a presented a photograph of two connected primary liquid ZABs powering an LED light of 3 V. Alongside of the test of a home-made liquid ZAB, a flexible allsolid-state ZAB was also fabricated, as schematically illustrated in Fig. 7d [41]. An alkaline polyvinyl alcohol (PVA) gel was used as electrolyte, which was stretchable and bendable. 1.5FeNi@NCNT equipped ZAB showed an open-circuit voltage of 1.38 V (Fig. 7c). In Fig. 7e, it yielded a high power density of 81 mW cm2 at 0.83 V. In view of the flexible advantage, the all-solid-state ZAB with 1.5FeNi@NCNT catalyst was examined by folding in different bending angles of 0°, 30° and 60° for every 2 h. As shown in Fig. 7f, the initial voltage gap was only 0.81 V, and no obvious changes were observed after a cycle of bend. Fig.S9b displayed that two
all-solid-state ZABs in series can power a LED, which presumably offered new horizons for developing flexible all-solid-state ZABs.
4. Conclusion Generally, FeNi alloy has been encapsulated within N-doped carbon matrix, engineering from the combination of MOF precursor method and polymer encapsulation strategy. Principles from optimized morphology have been incorporated into the design of electrocatalysts, which depended on the content of Fe in the parent template. Significantly, the morphology of 1D bamboo-like tube with embedded FeNi nanoparticles effectively helped the transportion of ORR/OER species during the electrocatalytic process. This developed FeNi-based catalytic system ensured superior stability and high ORR/OER activity in alkaline condition, providing a new insight to explore advanced bi-functional catalysts. Furthermore, liquid and flexible all-solid-state ZABs with 1.5FeNi@NCNT as the catalyst gave a high open circuit potential of 1.44 V and 1.38 V, and both could entail a prolonged operation. Consequently, the stated method for the construction of distinct morphology and mature component would feature a light scope for practical applications.
Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033
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Please cite this article as: M. Wu, B. Guo, A. Nie et al., Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.033