Porous Fe2O3 Nanorods on Hierarchical Porous Biomass Carbon as Advanced Anode for High-Energy-Density Asymmetric Supercapacitors

Introduction

Supercapacitors are efficient energy storage devices, and are clean and renewable with high efficiency, fast charge/discharge capability, and good cycling stability, which meet the increasing demands in various portable electronic devices (Zhai et al., 2011; Yu et al., 2015; Chen et al., 2020). Supercapacitors might be more practical when they obtain high-energy density at the same time to retain high specific power. It can increase the work voltage or the specific capacitance to enhance the energy density (Ike et al., 2015; Liu et al., 2016). Organic electrolytes or non-aqueous electrolytes with wider working voltages (3–4 V) suffer from lower ion conductivity and solvated ion size, which limits their future application (Meng et al., 2019; Zhang et al., 2019). Therefore, considerable efforts have to focus on fabrication of asymmetric supercapacitors (ASCs) based on the desirable negative and positive electrode with good electrochemical characteristics.

Various positive electrode materials have been used in ASC devices, such as carbon materials, transition metal oxides, and especially conducting polymers, with facile synthesis, low cost, and environmentally benign properties (Benzigar et al., 2018; Gao et al., 2019; Luo et al., 2019; Hong et al., 2020). In addition, multiscale porous carbon composites with conducting polymers have emerged as attractive electrodes in ASC devices owing to their high conductivity, abundant redox reactions, and mechanical stability (Yu M. H. et al., 2016; Meng et al., 2017; Jin et al., 2018). Recently, polyaniline nanorods directly grown on porous carbon derived from wheat flours demonstrated excellent supercapacitive performance (Wu et al., 2015; Yu P. P. et al., 2016; Yu et al., 2019), and thus may serve as a good cathode for ASCs.

Compared with the extraordinary advancement of anode nanomaterials that rely on high activity and large potential ranges, the lack of highly performing cathodes is still a bottleneck for making progress in advanced ASCs. Carbon materials play a role as negative electrodes, using the electrical double layer to accumulate energy, while the specific capacitance is trapped at an intermediate level of 100–200 F g⁻¹ (Zhu et al., 2011; Sahu et al., 2017; Wang et al., 2019). Recent studies have reported that pseudocapacitive anodes can deliver higher charge storage capacities than carbon materials, such as MoO₃ (Zhang et al., 2019), V₂O₅ (Guo et al., 2015), WO₃ (Yun T. G. et al., 2019), TiN Sun et al., 2020, Fe₃O₄ (Arun et al., 2019), FeOOH (Chen et al., 2016), and Fe₂O₃ (Yun X. et al., 2019; Le et al., 2020; Zhang et al., 2020). Among them, α-Fe₂O₃ is chosen as a potential candidate for ASCs because of its natural abundance, eco-friendly nature, excellent physicochemical stability, large theoretical capacitance (~3,625 F g⁻¹), and high hydrogen evolution potential in aqueous solution (Han et al., 2014; Nithya and Arul, 2016; Li et al., 2019). Its pseudocapacitive properties arise from the redox behaviors of the Fe³⁺/Fe²⁺ peaks (Han et al., 2018). However, agglomerating easily into large particles of α-Fe₂O₃ with low conductivity (10⁻¹⁴ S cm⁻¹) increases the electron transfer resistance during repeated ion charge/discharge cycles, showing a large effect on supercapacitance (Chen et al., 2014). These drawbacks will result in a reduction in the rate capacity and lifespan due to volume expansion, impeding the practical application as a negative electrode. By constructing integrated hybrid electrodes, α-Fe₂O₃ is directly deposited onto conductive skeletons of carbonaceous materials. Carbon nanotubes (Dong et al., 2018; Yue et al., 2018), porous carbon (Arun et al., 2019), and graphene (Xu et al., 2019) can overcome this vexing issue of low conductivity, which indeed demonstrates that integrated hybrid electrodes show better electrochemical performances than individual components because of the synergistic effect between the α-Fe₂O₃ nanostructure and the carbon configurations. Thus, far, the unsatisfactory dispersion and small surface area of α-Fe₂O₃ still causes a relatively low specific capacitance. Therefore, superior α-Fe₂O₃/carbon composites with excellent electrochemical performance are desired as negative electrodes, posing a major challenge for the preparation and design of ASCs with good electrochemical properties.

In this study, porous α-Fe₂O₃ nanowires were deposited onto conductive biomass hierarchical porous carbon (HPC) composites (Figure 1) by synthesizing via a hydrothermal process, and the nanowires acted as a Faradaic cathode with a pseudocapacitive anode (PANI/HPC) to assemble the ASC. HPC derived from wheat flour offers high conductivity and large surface area in both electrodes and thus improves the dispersion and stability of α-Fe₂O₃ and PANI nanowires, resulting in a good rate capacity. Porous α-Fe₂O₃ nanowires were well-arranged on the HPC surface directly, ensuring superior utilization and reduction in the ion diffusion length. The obtained α-Fe₂O₃/HPC//PANI/HPC ASC exhibited high energy/power density and electrochemical stability, indicating a wide prospect in future practical applications.

FIGURE 1

Figure 1. Schematic illustration of the fabrication of α-Fe₂O₃/hierarchical porous carbon (HPC) composites.

Experiment Section

Materials

All chemicals were used as achieved without treatment. Wheat flour was purchased from the Jingdong supermarket. KOH, Na₂SO₄, urea, Fe(NO₃)₃·9H₂O, aniline, and ammonium persulfate were purchased from Sinopharm Chemical Reagent Co., Ltd.

Synthesis of α-Fe₂O₃/HPC Composites

HPC was synthesized via a previously reported synthesis process (Yu P. P. et al., 2016). Briefly, the waste wheat flour (5 g), KOH (5 g), and urea (5 g) were mixed by stirring in 100 ml of distilled water (DI) before calcination in N₂ flux at 800°C for 1 h. The obtained HPC was washed with 5% HCl and DI water and then dried at 80°C.

Typically, the yellow solution in a stainless-steel autoclave (100 ml) consisted of 1.75 mM Fe(NO₃)₃·9H₂O, Na₂SO₄ (1.75 mM), and 50 g of deionized water. The mixture with uniformly dispersed HPC was heated in the autoclave to 120°C for 9 h. The obtained α-Fe₂O₃/HPC composites were added to DI water, collected by centrifugation, dried in vacuum at 60°C, and annealed under atmosphere at 300°C for 2 h. The α-Fe₂O₃ accounted for 35.4% of the weight of α-Fe₂O₃/HPC. The bare α-Fe₂O₃ was prepared via the same process without HPC.

Synthesis of PANI/HPC Composites

Aniline monomers (0.07 M) and HPC (150 mg) were sequentially added to 1 M H₂SO₄ under strong stirring; then, the uniform ammonium persulfate (0.07 M) 1 M H₂SO₄ solution was quickly loaded in the above solution with stirring for 10 min. The mixed solution was placed at −2°C for 14 h. Finally, the resulting PANI/HPC composites were washed and dried at 60°C. The loaded PANI was 26.2% in PANI/HPC.

Material Characterization

X-ray diffraction (XRD; Rifaku, Cu Kα radiation, 10–80°) and X-ray photoelectron spectroscopy (XPS; ESCA PHI 5000C) were used to analyze the crystalline and bonding energy of the samples. The morphology and structure were examined by scanning electron microscopy (SEM, Hitachi, SU8010) and transmission electron microscopy (TEM; JEOL JEM-2011). The Brunauer–Emmer–Teller (BET) specific surface area and pore size distribution of the composites were measured using a surface area analyzer (ASAP 2020, USA) and the Barrett–Joyner–Halenda (BJH) method.

Electrochemical Measurements

A uniform paste was made by mixing HPC, α-Fe₂O₃/HPC, and PANI/HPC/carbon black/polytetrafluoroethylene at a ratio of 90/5/5, and was used to coat a 1.0-cm² piece of titanium mesh as the working electrode. A three-electrode cell consisting of a saturated calomel electrode (SCE) and Pt foil was used to test the above sample paste. The ASCs were assembled by a negative electrode of α-Fe₂O₃/HPC and a positive electrode of PANI/HPC, and the separator was a Celgard 3501. The loaded mass of α-Fe₂O₃/HPC and PANI/HPC composite was about 2.1 and 2.5 mg, as per the charge balance theory. Electrochemical studies of all electrodes were performed using a CHI electrochemical workstation (660D) in 1 M Li₂SO₄ electrolyte. The specific capacitance (C_sp) of HPC, α-Fe₂O₃/HPC, and PANI/HPC electrodes can be calculated depending on Equation (1):

$\begin{array}{ll}{C}_{sp}=\frac{It}{mV}& (1)\end{array}$

where Δt (s), I (A), ΔV (V), and m (g) stand for the discharge time, the current, the voltage difference, and the weight of the active materials.

The specific capacitance (C_asy) of α-Fe₂O₃/HPC//PANI/HPC ASC is obtained from Equation (2),

$\begin{array}{ll}{C}_{asy}=\frac{It}{MV}& (2)\end{array}$

where M is the total mass of the electrode materials.

Energy density (E, Wh kg⁻¹) and power density (P, W kg⁻¹) of α-Fe₂O₃/HPC//PANI/HPC ASC are according to the equations:

$\begin{array}{ll}E=\frac{C_{asy}{V}^2}{2\times 3.6}& (3)\end{array}$

$\begin{array}{ll}P=3600\times \frac{E}{t}& (4)\end{array}$

Results and Discussion

Figure 2a shows an interconnected hierarchical porous structure of HPC, similar to an open sponge with 100- to 200-nm thickness of carbon walls. The three-dimensional HPC served as a scaffold with high surface area and good conductivity for depositing α-Fe₂O₃ nanorods via a simple hydrothermal reaction. The α-Fe₂O₃ nanorods were directly and uniformly grown on the entire HPC surface (Figures 2b,c). The interior structure of α-Fe₂O₃/HPC was further tested via TEM (Figures 2d–f). The α-Fe₂O₃ nanorods, with diameters and lengths of ~10 and 50–100 nm, were vertically deposited on the scaffold of HPC to form a branched structure (Figure 2e), helping the fast transport of electrolyte ions. Moreover, the α-Fe₂O₃ nanorods have some pores (highlighted in the block diagram), resulting in a good specific surface to increase the number of accessible active sites for electrolyte ions. Meanwhile, high-resolution TEM (HRTEM) examination (Figure 2f) demonstrates the clear lattice fringes of the (110) plane of α-Fe₂O₃ with a spacing of 0.25 nm, indicating the high crystallinity of the α-Fe₂O₃ nanorods (Chen et al., 2015; Le et al., 2020).

FIGURE 2

Figure 2. (a) Scanning electron microscopy (SEM) image of HPC, SEM images (b,c), and transmission electron microscopy (TEM) images (d,e) of α-Fe₂O₃/HPC with the inset in (e) exhibiting the EDS spectrum. (f) High-resolution TEM (HRTEM) image of α-Fe₂O₃ from the red-marked region in (e).

Figure 3A shows XRD measurements to explore the structural and compositional properties of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC. The broad peaks of HPC at 2θ = 25° and 43.9° indicate the (002) and (101) planes of graphitized carbon (Yu P. P. et al., 2016; Liu et al., 2019). The other relevant characteristic diffraction peaks are very consistent with α-Fe₂O₃ (JCPDS 33-0664), which does not change in the α-Fe₂O₃/HPC composite. The detailed surface chemical composition of α-Fe₂O₃/HPC was evaluated by XPS measurements. Figure 3B demonstrates the coexistence of C (283.5 eV), O (527.6 eV), and Fe (Fe 2p peaks at 724.6 and 711 eV) (Yun X. et al., 2019; Racik et al., 2020). The high-resolution Fe 2p XPS spectrum exhibits typical Fe 2p_1/2 and 2p_3/2 peaks along with 719.5 eV of satellite peak corresponding to Fe³⁺ in α-Fe₂O₃ (Figure 3C) (Liang et al., 2018).

FIGURE 3

Figure 3. (A) X-ray diffraction (XRD) patterns of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC. (B) X-ray photoelectron spectroscopy (XPS) spectra of α-Fe₂O₃/HPC, (C) high-resolution Fe 2p XPS spectra. (D) The N₂ adsorption–desorption (inset: pore size distributions) of HPC and α-Fe₂O₃/HPC.

Figure 3D shows the N₂ adsorption/desorption isothermal analysis of HPC and α-Fe₂O₃/HPC. The inset image shows the corresponding pore size distribution. The HPC and α-Fe₂O₃/HPC both exhibit IV type curves along with an H3 type hysteresis loop, which indicates the existing micropores and mesopores. Compared to HPC (977 m² g⁻¹), α-Fe₂O₃/HPC has a lower surface area of 700 m² g⁻¹, but is still superior to the reported carbon/α-Fe₂O₃ composites. The pore size distributions of α-Fe₂O₃/HPC were analyzed from the desorption isotherm, and a sharp peak was in the 1.0- to 4.0-nm range (inset of Figure 3D). This result is also confirmed by SEM and TEM images showing an interconnected multiple size porous structure to contribute to the large value of surface area, which ensures a good accessible electrolyte ion and facilitates ion transport.

The electrochemical properties of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC electrodes were investigated through cyclic voltammetry (CV) curves, galvanostatic charge/discharge (GCD) plots, and electrochemical impedance spectroscopy (EIS) plots using 1 M Li₂SO₄ electrolyte in a three-electrode configuration. Figure 4A shows the compared CV curves of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC recorded at 50 mV s⁻¹. A Nearly symmetrical rectangular shape demonstrates an ideal double-layer capacitor characteristic of the HPC electrode. Remarkably, the α-Fe₂O₃/HPC electrode shows a larger CV curve area than HPC and bare α-Fe₂O₃ nanorods, demonstrating a large enhancement in capacitive behavior owing to the synergistic effect on the double-layer capacitor characteristics of the HPC electrode and the pseudocapacitor of the α-Fe₂O₃ nanorods. There are no obvious anodic/cathodic peaks in the α-Fe₂O₃ and α-Fe₂O₃/HPC CV curves because of the pseudo-constant rate of charging/discharging throughout the entire voltammetric cycle, accompanied by a fast Faradic reaction between alkaline cations (Na⁺). This phenomenon results from the enhanced electrical conductivity and hierarchical porous structure resulting in fast ion diffusion under charge/discharge cycles. At 200 mV s⁻¹, α-Fe₂O₃/HPC maintains a shape similar to that of the CV curves, demonstrating good capacitive performance (Figure 4B). Nearly symmetrical GCD profiles at the current density range of 1–10 A g⁻¹ indicate the high coulombic efficiency (Supplementary Figure 1), which is consistent with CV curves.

FIGURE 4

Figure 4. (A) Cyclic voltammetry (CV) curves of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC at 50 mV s⁻¹. (B) CV curves of α-Fe₂O₃/HPC at different scan rates. (C) The galvanostatic charge/discharge (GCD) curves at a current of 1 A g⁻¹, (D) gravimetric specific capacitance vs. current densities, (E) the Nyquist plots, and (F) cycling stability at 1 A g⁻¹ of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC.

Figure 4C shows the GCD plots of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC electrodes collected at 1 A g⁻¹. Longer discharge time of the α-Fe₂O₃/HPC electrode indicates a higher capacitance than the HPC and α-Fe₂O₃ electrodes, convincingly revealing enhanced conductivity from HPC for the disposition of active α-Fe₂O₃ nanorods, which is consistent with the above CV curve analysis. This phenomenon demonstrates that the skeleton of HPC can improve the capacitance performance of α-Fe₂O₃, reduce the internal resistance between the interfacial and HPC, and offer more active sites for pseudocapacitors. Figure 4D shows the calculated C_sp values of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC electrodes under different current densities. Compared to the values of bare Fe₂O₃ nanorods (370 A g⁻¹) and HPC (323 A g⁻¹) electrodes at 1 A g⁻¹, the maximum C_sp of the α-Fe₂O₃/HPC hybrid electrode is 706 F g⁻¹, indicating a larger capacitive property. This result is superior to other recently reported iron oxide-based electrodes at the same current density, such as Fe₂O₃/graphene (226 F g⁻¹) (Wang et al., 2013), Fe₂O₃/CNT (204 F g⁻¹) (Yue et al., 2018), Fe₂O₃/N-doped CNT (264 F g⁻¹) (Gnana Sundara Raj et al., 2020), α-Fe₂O₃/C (280 F g⁻¹) (Dong et al., 2018), and Fe₂O₃/hemp straw (256 F g⁻¹) (Jiang et al., 2020). At 10 A g⁻¹, the C_sp of α-Fe₂O₃/HPC is 410 F g⁻¹, showing a decrease as the current density increases. This value retains 58% of the initial capacitance, exhibiting a high-rate capability of α-Fe₂O₃/HPC, which is comparable to other reported electrodes such as Fe₂O₃/graphene (40% at 10 A g⁻¹) (Wang et al., 2013), Fe₂O₃ nanospheres/diatomite (42% at 10 A g⁻¹) (Jiang et al., 2018), and SiC@ Fe₂O₃ (51% at 12 A g⁻¹) (Zhao et al., 2018). EIS analyses of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC electrodes were conducted. The charge transfer resistance of α-Fe₂O₃/HPC (0.91 Ω) was much smaller than that of bare α-Fe₂O₃ (1.63 Ω) and higher than that of pure HPC (0.52 Ω), indicating the enhanced conductivity of α-Fe₂O₃/HPC after adding the high conductive HPC as the scaffold (Figure 4E). The α-Fe₂O₃/HPC electrode has a more ideal straight line with small Warburg resistance, leading to fast transfer of electrolyte ions into hybrid electrode. Figure 4F shows the corresponding cycling stability of HPC, α-Fe₂O₃, and α-Fe₂O₃/HPC, evaluated at 1 A g⁻¹. Notably, the cycling stability of the α-Fe₂O₃/HPC electrode is 95.8% of its original capacitance over 5,000 cycles, which is higher than 68% of the bare α-Fe₂O₃ electrode.

To further evaluate the possibility of α-Fe₂O₃/HPC electrode materials in practical applications of energy storage, the ASC device was built with α-Fe₂O₃/HPC and PANI/HPC as the cathode and anode, respectively, to achieve a high voltage range in 1 M Li₂SO₄ electrolyte. PANI nanorod arrays were aligned on interconnected porous surfaces of HPC through in situ polymerization (Supplementary Figures 2A,B), which shows a C_sp of 506 F g⁻¹ at 10 mV s⁻¹ measured in 1 M Li₂SO₄ electrolyte (Supplementary Figure 2C). The C_sp of PANI/HPC dropped to 343 F g⁻¹, presenting capacitance retention of 67.8% at 200 mV s⁻¹ (Supplementary Figure 2D). The optimized mass ratio of the α-Fe₂O₃/HPC (380 F g⁻¹)/PANI/HPC (441 F g⁻¹) electrode was ~0.85, according to the corresponding C_sp value at 50 mV s⁻¹ to maintain charge balance (Figure 5A). Based on the separate potential window, the α-Fe₂O₃/HPC//PANI/HPC ASC can still exhibit an ideal capacitive characteristic at 2.0-V work voltage.

FIGURE 5

Figure 5. (A) CV curves of α-Fe₂O₃/HPC and pseudocapacitive anode (PANI)/HPC at 50 mV s⁻¹. Electrochemical properties of α-Fe₂O₃/HPC//PANI/HPC asymmetric supercapacitors (ASC). (B) CV curves at various scan rates, (C) GCD curves, and (D) capacitance curves against current densities, (E) cycling stability at 1 A g⁻¹, and (F) Ragone plots.

The CV profiles of the α-Fe₂O₃/HPC//PANI/HPC ASC deviate only slightly from the quasi-rectangular shape as the scan rate increases (10–200 mV s⁻¹), which indicates good reversibility (Figure 5B). A couple of redox peaks in each CV curve is attributed to faradaic reactions of the electrolyte ions insertion/extraction. Nearly symmetric GCD curves were observed at 1–10 A g⁻¹ (Figure 5C), indicating an excellent coulombic efficiency and superior capacitance of the α-Fe₂O₃/HPC//PANI/HPC ASC. The maximum C_asy of α-Fe₂O₃/HPC//PANI/HPC ASC is 212 F g⁻¹ at 1 A g⁻¹ (Figure 5D), superior to these published α-Fe₂O₃/C//MnO₂ (Dong et al., 2018), Fe₂O₃/N-CNT//CuCo₂O₄ (Gnana Sundara Raj et al., 2020), and CF (carbon fiber)-rGO/Fe₂O₃//CF-MnOx (Serrapede et al., 2019), comparable to that of α-Fe₂O₃@C//CNTs-COOH (Xu et al., 2019) (Supplementary Table 1). Figure 5E shows the C_asy vs. cycle number plot of the α-Fe₂O₃/HPC//PANI/HPC ASC, conducted at 1 A g⁻¹, which is only 5.8% decay of the original capacity after 5,000 cycles. Supplementary Figure 3 displays the Nyquist plot of the α-Fe₂O₃/HPC//PANI/HPC ASC at the first cycle and after the 5,000th cycle. The corresponding charge transfer resistances are 8.1 and 14 Ω, respectively. These small resistance values indicate that the as-assembled ASC has good conductivity. Moreover, Figure 5F shows the Ragone plot of the α-Fe₂O₃/HPC//PANI/HPC ASC suggesting a relationship of energy density (E) vs. power density (P). The as-fabricated α-Fe₂O₃/HPC//PANI/HPC ASC has a gravimetric E value of 117 Wh kg⁻¹ at 1.0 kW kg⁻¹. At the highest P-value of 9.3 kW kg⁻¹, the E value is maintained at 102 Wh kg⁻¹. The E and P values were compared with previously published devices, as listed in Supplementary Table 1, such as Fe₂O₃@PPy//MnO₂ (Liang et al., 2018), porous Fe₂O₃/N-CNT//CuCo₂O₄ (Gnana Sundara Raj et al., 2020), α-Fe₂O₃/G//CoNi-layer double hydroxide/CNT (Chen et al., 2015), α-Fe₂O₃//ZnCo₂O₄@MnO₂ (Ma et al., 2015), Fe₂O₃@PPy//MnO₂ thin film (Le et al., 2020), and Fe₂O₃//MnO₂ thin film (Gund et al., 2015).

Based on these results, the α-Fe₂O₃/HPC//PANI/HPC ASC exhibits superior electrochemical behaviors arising from the synergistic interaction of the components. The HPC, as a robust scaffold, performs an important function to accommodate the volume variation of α-Fe₂O₃ or PANI and impedes the erosion and deformation of the as-prepared electrode. The high conductivity of HPC can enhance the integrated conductivity of α-Fe₂O₃/HPC and provide rapid electron transmission channels, resulting in a small charge transfer resistance. Furthermore, the well-ordered α-Fe₂O₃ or PANI nanorod arrays were decorated in the porous surfaces of HPC to enhance the pseudocapacitance and cycle stability resulting from effective avoidance of swelling/shrinking after long-term cycling. Therefore, the α-Fe₂O₃/HPC//PANI/HPC ASC presents exceptional capacitor properties and is therefore an attractive candidate in commercial energy storage devices.

Conclusions

In summary, a facile hydrothermal route was used to fabricate the α-Fe₂O₃/HPC electrode. Compared to α-Fe₂O₃ (370 F g⁻¹), the enhanced C_sp value of the α-Fe₂O₃/HPC anode is 706 F g⁻¹. Furthermore, assembled α-Fe₂O₃/HPC//PANI/HPC ASC device delivers a maximum E value of 117 Wh kg⁻¹ at 1.0 kW kg⁻¹ and retains 102 Wh kg⁻¹ at 9.3 kW kg⁻¹, accompanied with excellent capacity retention by 5.8% loss in original capacitance. This study offers a plausible way to assemble α-Fe₂O₃ ASCs with excellent electrochemical properties as next-generation energy storage devices.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding author/s.

Author Contributions

PY wrote the paper and designed the study. WD prepared the experiments. YJ polished the manuscript. All authors contributed to the article and approved the submitted version.

Funding

This work was funded by the National Natural Science Foundation of China (Grant No. 51802124) and Jiangsu Province (BK 20180626), and the Postdoctoral Science Foundation of China (Grant No. 2019M651693).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2020.611852/full#supplementary-material

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