J. Cent. South Univ. (2021) 28: 2345-2359

DOI: https://doi.org/10.1007/s11771-021-4774-y

3D Se-doped NiCoP nanoarrays on carbon cloth for efficient alkaline hydrogen evolution

LIU Zi-xuan(刘紫轩), WANG Xiao-long(王晓龙), HU Ai-ping(胡爱平),TANG Qun-li(唐群力), XU Ya-li(徐亚利), CHEN Xiao-hua(陈小华)

College of Materials Science and Engineering, Hunan University, Changsha 410082, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: The exploration of stable and highly efficient alkaline hydrogen evolution reaction (HER) electrocatalysts is imperative for alkaline water splitting. Herein, Se-doped NiCoP with hierarchical nanoarray structures directly grown on carbon cloth (Se-NiCoP/CC) was prepared by hydrothermal reaction and phosphorization/selenization process. The experimental results reveal that Se doping could increase the electrochemical active sites and alter the electronic structure of NiCoP. The optimized Se-NiCoP/CC electrode exhibits outstanding HER activity in alkaline electrolyte, which only needs a low overpotential of 79 mV at the current density of 10 mA/cm². When serving as anode and cathode electrode simultaneously, the Se-NiCoP/CC electrodes achieve current density of 50 mA/cm² at a low voltage of only 1.62 V. This work provides a feasible way to rationally design high active HER electrocatalysts.

Key words: Se-NiCoP nanorods; multi-level array structure; hydrogen evolution reaction; alkaline solution; water splitting

Cite this article as: LIU Zi-xuan, WANG Xiao-long, HU Ai-ping, TANG Qun-li, XU Ya-li, CHEN Xiao-hua. 3D Se-doped NiCoP nanoarrays on carbon cloth for efficient alkaline hydrogen evolution [J]. Journal of Central South University, 2021, 28(8): 2345-2359. DOI: https://doi.org/10.1007/s11771-021-4774-y.

1 Introduction

Water electrolysis is acknowledged as a promising pathway to produce high pure hydrogen [1, 2]. However, electrolysis of water requires high energy consumption, so efficient catalysts are needed to speed up the reaction rate and improve the overall efficiency [3, 4]. Noble metals such as Pt have excellent catalytic activity, but their cost and resource restrict them for massive scale applications [5]. To date, considerable nonprecious metal-based HER electrocatalysts under acidic media have been developed as the substitutes for Pt-based catalysts [6, 7]. However, the harsh acidic environment and the lack of efficient catalysts for oxygen evolution reaction (OER) in acidic media make it urgent to explore efficient non-noble metal alkaline HER electrocatalysts [8-10].

Recently, bimetallic phosphides, such as Fe_xCo_1-xP, MoWP and CoMoP@C, have been demonstrated superior HER activity than their relative monometallic phosphide counterparts [11-14]. KIBSGAARD et al [15] found that Fe_0.5Co_0.5P has a more thermo-neutral Gibbs free energy of H adsorption (△G_H*) and exhibits higher activity than FeP or CoP. In addition, previous researches have pointed out that the composition greatly affects the hydrogen evolution performance [16-20]. For example, CAO et al [21] reported that the Ni_xCo_2-xP@NC NA/NF shows the best electrochemical performance when the ratio of Ni/Co is 1:1. Nevertheless, the hydrogen binding energy of NiCoP is far from the zero-energy level [16, 22]. Therefore, it is still a challenge to further improve the HER performance of NiCoP.

Heteroatom doping is considered one of the effective strategies to enhance the HER activity of transition metal phosphides (TMPs) [1, 23-26]. It could modulate the electronic structure and increase the active sites exposure so that the optimized hydrogen adsorption free energy and an enlarged electrochemical active surface area can be achieved [27]. For example, LIN et al [16] successfully developed S-doped NiCoP nanosheet arrays and it only requires 88 mV to obtain a current density of 10 mA/cm², which is substantially superior to NiCoP (151 mV). LIU et al [17] found that oxygen doping can optimize electrical conductivity and hydrogen adsorption Gibbs free-energy of NiCoP, which is good for HER activity. Inspired by the previous works, we consider that selenium doping into NiCoP is expected to enhance the HER performance based on the reasons as follows: 1) Due to the electronegativity of Se greater than P, similar as S and O, Se doping in NiCoP can tune the electronic state of NiCoP. In addition, its more metallic nature is beneficial to the improvement of electronic conductivity [19, 28]. 2) The larger atomic radius of selenium atom can lead to lattice defects after doping into the crystal structure of NiCoP and provides additional active sites for electrochemical reaction. For instance, HU and co-workers [29] reported that the incorporation of Se in (NiCo)S/OH nanosheets introduced structural defect and lattice distortion, which exhibited high HER performance with current density 10 mA/cm² at 103 mV potential.

In this work, three-dimensional Se-doped NiCoP nanorod arrays on carbon cloth were constructed by hydrothermal and selenization/phosphorization process and their alkaline HER electrocatalytic performance were further investigated. The optimal Se-NiCoP/CC catalyst exhibits excellent HER activity in alkaline solution owing to several favorable advantages as follows: 1) the direct growth of the electrocatalysts on conductive substrate can not only increase the electronic conductivity of the catalyst, but also avoid the use of polymer binders, which is beneficial for increasing active sites exposure; 2) the open network and multi-level array structure facilitate the contact with electrolyte and the rapid release of gas; 3) Se doping can alter the electronic structure of NiCoP and increase active sites exposure. Thus, the reaction kinetics can be accelerated and the electrochemical active surface area can be enlarged. To further prove the competitive advantage of the Se-NiCoP/CC for practical applications, the performance for overall water splitting by applying Se-NiCoP-2/CC electrode as both cathode and anode was measured. The Se-NiCoP-2/CC||Se-NiCoP-2/CC pair only needs a low voltage of 1.62 V with 50 mA/cm² to drive the overall water splitting.

2 Experimental

2.1 Synthesis of NiCoP/CC

Carbon cloth was firstly immersed in nitric acid (35%) at 80 °C for 12 h [30], and then cleaned with acetone, deionized water and ethanol, respectively to increase its hydrophilicity. In a typical synthetic route, CoCl₂·6H₂O (1.5 mmol) and NiCl₂·6H₂O (1.5 mmol) were dissolved in 30 mL of deionized water under vigorous stirring. Then, NH₄F (8 mmol) and urea (15 mmol) were dispersed in the above solution to form a clear pink solution. The treated carbon cloth (1 cm×2 cm) was fixed on the autoclave wall through waterproof adhesive tape. The above solution was then poured into hydrothermal kettle and kept for 6 h at 120 °C. After natural cooling, the obtained NiCo-precursor was washed with DI water and dried overnight. To synthesize NiCoP/CC, the NiCo precursor was placed at the center of the tube furnace, and the NaH₂PO₂·H₂O (0.5 g) was placed on the upstream position. Then, the quartz tube was heated to 300 °C for 2 h under Ar atmosphere. After cooling naturally, the NiCoP/CC was obtained.

2.2 Synthesis of Se-NiCoP/CC

The NiCo precursor was placed at the center of the tube furnace, and the mixture of 0.5 g NaH₂PO₂·H₂O and 0.2 mmol selenium powder was placed in the upstream position. First, the Ar gas is pumped into the tube for 10 min to remove the air. Then, the device was heated to 300 °C (5 °C/min) for 2 h. After cooling naturally, the Se-NiCoP-2/CC was obtained. To optimize the Se doping, the amount of Se powder was adjusted to 0.1, 0.3 and 0.4 mmol, and the obtained samples were recorded as Se-NiCoP-x/CC (x=1, 3, 4), respectively.

2.3 Preparation of Pt/C(20 wt%) on carbon cloth

2 mg commercial Pt/C (20 wt%) was dissolved in a mixed solution (0.5 mL ethanol and 25 μL 5% Nafion) to form a uniform suspension. The suspension was then dropped onto carbon cloth (1 cm×1 cm) and dried at room temperature.

2.4 Materials characterization

D8 Advance X-ray diffractometer (Cu-Kα, λ=0.15418 nm), ESCALAB 250 Xi (Thermo Fisher Scientific, UK), S-4800 field emission microscope (Hitachi, japan) and JEM-2100F TEM (JEOL, Japan) were used.

2.5 Electrochemical measurements

The electrochemical performance of samples was measured on a CHI 660E electrochemical station in a three-electrode system. The Se-NiCoP-x/CC (1 cm×2 cm) was applied as the working electrode, and Hg/HgO electrode and graphite rod acted as reference electrode and counter electrode, respectively. In the electrochemical test, the area of the working electrode immersed in the electrolyte was 1 cm×1 cm. The potentials mentioned in this work were calibrated with reference to a reversible hydrogen electrode (RHE) by adding a value of (0.098+0.059pH) V. The linear sweep voltammogram (LSV) curves were conducted with a sweep rate of 5 mV/s. Cyclic voltammogram (CV) scans were performed in the above potential ranges with a sweeping rate of 100 mV/s to test the durability of samples. The long-term stability was measured through chronoamperometry under controlled potentials. Electrochemical impedance spectroscopy (EIS) test was conducted at the potentiostatic mode in the frequency range from 10⁵ to 0.1 Hz. All electrochemical measurements were IR compensated.

3 Results and discussion

The fabrication process for Se-NiCoP/CC nanorod arrays is depicted in Scheme 1. Firstly, NiCo-precursor arrays were synthesized on carbon cloth via a hydrothermal reaction. Then, the incorporation of Se and phosphidation treatment were simultaneously realized by employing Se- powder and NaH₂PO₂^.H₂O as the Se and P resources at 300 °C under Ar environment, respectively. In this process, the pink NiCo-precursor turns to black Se doped NiCoP sample. The SEM images of NiCoP/CC and Se-NiCoP-x/CC (x=1, 3, 4) are shown in Figures S1-S4 in Supporting information display similar morphologies, which demonstrates that the structure of NiCoP is basically consistent after Se doping.

To analyze the phase structures of obtained catalysts, the X-ray diffraction (XRD) patterns of NiCoP and Se-NiCoP-x (x=1, 2, 3, 4) were carried out (Figure 1(a)). The diffraction peaks of samples at 30.6°, 35.5°, 41.0°, 44.9°, 47.6° are in good coincidence with the (110), (200), (111), (201), (210) and (300) planes of the NiCoP (JCPDS card No. 71-2336), respectively [16, 22]. In addition, no peaks about selenide-based phase were detected. The (210) peaks of Se-NiCoP-x (x=1, 2, 3, 4) samples have a slightly negative shift compared to that of NiCoP, which is attributed to the lattice expansion caused by the substitution of P by Se with larger ratomic radius [31]. It suggests that Se atoms have been successfully incorporated into the crystal structure of NiCoP.

To further determine the roles of Se doping, the XPS measurements were carried out. The survey spectra reveal that the Se-NiCoP/CC sample contains Ni, Co, P, Se and O elements (Figure 1(b)). The O element may be caused by the oxidation of samples surface in contact with air [17]. From the high-resolution Co 2p spectra (Figure 1(c)), the peaks at 778.69 eV and 793.6 eV correspond to Co-P bonds, and the two peaks at 781.79 eV and 797.93 eV are related to Co—O bonds [32]. Other peaks at 786.47 eV and 803.25 eV can be ascribed to the satellite peaks [33]. In Figure 1(d), the peaks at 853.45 eV and 870.96 eV originate from Ni-P bonds, and the peaks at 856.51 eV and 874.2 eV can be ascribed to oxidized Ni species [34]. The peaks at 861.42 eV and 880.23 eV belong to the satellite peaks [16]. In the spectra of P 2p (Figure 1(e)), the peaks at 129.71 eV and 130.64 eV are assigned to phosphide [33], while the peak at 133.91 eV is related to metal oxidized phosphorus [35]. The peak at 137.5 eV corresponds to the Auger line of the Se element. In the spectrum of the Se 3d in Figure 1(f), the peaks at 54.4 eV and 55.0 eV correspond to Se 3d_5/2 and Se 3d_3/2, corresponding to metal-selenium bonds [31]. Notably, compared with pure NiCoP, the peaks of Ni 2p, Co 2p and P 2p in Se-NiCoP are all shifted to higher energy region, which suggest that Se doping has a strong influence on electronic interactions [19]. The increased binding energies of Ni 2p, Co 2p and P 2p in Se-NiCoP demonstrate the electrons transfer from Ni, Co and P to Se, owing to greater electronegativity of Se atom than P atom [17]. As a result, fewer electrons in Co and Ni atoms can interact with H. The binding interaction between Co, Ni atoms and H is weakened, which is beneficial for the HER process [33].

Scheme 1 Synthesis process of Se doped NiCoP nanorod arrays on carbon cloth

Figure 1 (a) XRD patterns of pure NiCoP and Se-doped NiCoP; (b) XPS survey spectra; (c) Co 2p, (d) Ni 2p and (e) P 2p of NiCoP/CC and Se-NiCoP-2/CC; (f) Se 3d of Se-NiCoP-2/CC

The SEM images of Se-NiCoP-2/CC are shown in Figures 2(a) and (b). It displays an open network and interlaced nanosheets architecture. The carbon cloth substrate is fully covered by Se-NiCoP nanosheets arrays. These 400-500 nm thick nanosheets arrays are composed of nanorods, which grow vertically and interconnect with each other on carbon cloth. Such highly open networks and multi-level array structure not only facilitate the contact between the electrode and electrolyte, but also improve the transmission rates of electroactive component and the diffusion rate of bubbles. Its TEM images (Figure 2(c)) show that the nanorods with a diameter of 50-60 nm and a length of 600- 700 nm. Moreover, there are many pores with different sizes distribution on the surface of nanorods. The high-resolution TEM (HRTEM) image (Figure 2(d)) shows the lattice fringes of 0.22 nm, attributing to the (111) plane of NiCoP [21]. Notably, in selected white-dashed areas of Se-NiCoP-2/CC, a large number of lattice mismatch were observed. It demonstrates that the incorporation of Se in NiCoP nanorods can induce lattice distortion, which may provide additional active sites for electrochemical reaction. The corresponding elemental mapping of Se-NiCoP-2/CC reveals the uniform distribution of Ni, Co, P and Se elements, which further confirms the successful incorporation of Se into the sample (Figure 2(e)). The EDS spectra of Se-NiCoP-x/CC (x=1, 2, 3, 4) show that the Se contents in the samples are 2.81 at%, 5.91 at%, 9.78 at% and 11.70 at%, respectively (Figure S5).

The electrocatalytic HER performance of all samples was measured in 1.0 mol/L KOH solution. From the polarization curves in Figure 3(a), the commercial Pt/C electrocatalyst shows the best HER activity and the pure carbon cloth has almost no activity for the HER. The overpotential of Se-NiCoP-1/CC, Se-NiCoP-2/CC, Se-NiCoP-3/CC and Se-NiCoP-4/CC at 10 mA/cm² is 87, 79, 84 and 90 mV, respectively, which is less than that of NiCoP/CC (99 mV), indicating that Se doping can effectively improve the HER activity of NiCoP/CC. Obviously, among Se-NiCoP/CC electrodes, the Se-NiCoP-2/CC shows the best HER electrocatalytic activity. It also displays a superior performance than the reported HER catalysts in Table S1 (Supporting information). As shown in Table S2, at a large current density of 100 mA/cm², the overpotential of Se-NiCoP-2/CC is 158 mV, which is also lower than those of NiCoP/CC (178 mV), Se-NiCoP-1/CC (168 mV), Se-NiCoP-3/CC (160 mV), Se-NiCoP-4/CC (174 mV), respectively, which further suggests the outstanding HER activity of Se-NiCoP-2/CC electrode. The reaction kinetics in the HER process were analyzed by Tafel plots (Figure 3(b)). The Tafel slope of Se-NiCoP-2/CC is 81.6 mV/dec, indicating that its hydrogen evolution process may be followed by the Volmer-Heyrovsky mechanism, and the electrochemical desorption is rate determined step [28, 36, 37]. In comparison, the Tafel slope of NiCoP/CC is 118.8 mV/dec, significantly higher than that of Se-NiCoP-1/CC (105 mV/dec),Se-NiCoP-2/CC (81.6 mV/dec), Se-NiCoP-3/CC (86 mV/dec) and Se-NiCoP-4/CC (108 mV/dec). The small Tafel slope of Se-NiCoP-2/CC is more beneficial for practical applications because it can significantly increase HER rate and reduce overpotential [22, 38].

Figure 2 (a, b) SEM images; (c) TEM images; (d) HRTEM images; (e) EDS elemental mapping of Se-NiCoP-2

Figure 3 (a) HER polarization curve; (b) Tafel slope plots; (c) C_dl values of obtained samples; (d) Electrochemical impedance spectroscopy; (e) Polarization curves of Se-NiCoP-2/CC electrode initially and after CV cycles; (f) Chronoamperometry J–t curve

The electrochemical active surface area (ECSA) of catalyst is often used to evaluate intrinsic catalytic activity [21, 39]. In order to estimate the ECSA of as-prepared catalysts, the double-layer capacitance (C_dl) of each catalyst was obtained by cyclic voltammetry (CV) test (Figure S6). The C_dl of Se-NiCoP-2/CC (63.7 mF/cm²) is larger than that of NiCoP/CC (31.6 mF/cm²) and other Se-NiCoP sample, suggesting that the Se-NiCoP-2/CC has a highest electrochemical active surface area and the most exposed active sites (Figure 3(c)). Since the Se-NiCoP-x/CC (x=1, 2, 3, 4) and NiCoP/CC samples have similar morphologies, the amount of doped Se plays an important role in the incensement of ECSA.

To further obtain insight into the HER reaction kinetics, the interfacial charge transfer resistance of different catalysts in HER process was investigated by the Nyquist plots (Figure 3(d)). As the selenium content increases, the interfacial charge transfer resistance of the doped electrode decreases and then increases compared to the undoped electrode. This may be caused by the following reasons: On the one hand, doped selenium atoms with more metallic properties in NiCoP can induce lattice distortion to improve the electronic conductivity [28, 29]; On the other hand, excessive selenium doping will weaken the delocalization ability of electrons in Co and Ni atoms and increase the resistance [38]. The Se-NiCoP-2/CC exhibits a smaller diameter of semicycle, indicating that it has the lowest interfacial charge transfer resistance.

The electrochemical stability of Se-NiCoP-2/CC was evaluated by continuous CV test and chronopotentiometry test. As shown in Figure 3(e), there is no obvious deviation of current density after 1000 cycle sweeps even in the high overpotential region. Under the constant voltage (Figure 3(f)), the current density of Se-NiCoP-2/CC electrode shows no obvious decrease after the continuous hydrogen release for 20 h, which indicates the good durability of Se-NiCoP-2/CC electrode in long-term HER process. The SEM image (Figure S7(a)) and XRD pattern (Figure S7(b)) of Se-NiCoP-2/CC electrode after durability test show that the nanosheets arrays structure and phase of Se-NiCoP-2 were well retained, demonstrating the excellent stability of Se-NiCoP-2/CC electrode. The XPS spectra (Figure S8) of Se-NiCoP-2/CC electrode after durability test show that the peak positions of Co 2p, Ni 2p, P 2p and Se 3d change little, which further demonstrates that Se-NiCoP-2/CC electrode has good chemical stability. Based on the analysis above, the better HER activity of Se-NiCoP-2/CC could be attributed to the fact that Se atoms effectively modulate the electronic structure and introduce abundant defects, leading to the accelerated reaction kinetics and the increased electroactive site, which is in accordance with the reports of literature [29].

The OER performance of all samples was also measured in alkaline solution, as shown in Figure 4. The overpotential for Se-NiCoP-2/CC is 303 mV at 50 mA/cm², which is much lower than that of NiCoP/CC (335 mV), Se-NiCoP-1/CC (310 mV), Se-NiCoP-3/CC (306 mV), Se-NiCoP-4/CC(324 mV) (Figure 4(a)). The Tafel slope of the Se-NiCoP-2/CC catalyst (107.97 mV/dec) is also smaller than those for NiCoP (165.43 mV/dec), Se-NiCoP-1/CC (118.36 mV/dec), Se-NiCoP-3/CC (112.21 mV/dec), Se-NiCoP-4/CC (146.22 mV/dec) (Figure 4(b)). These results suggest that Se doping can also improve the OER activity of NiCoP.

Finally, using Se-NiCoP-2/CC as cathode and anode, respectively, the overall water splitting performance of Se-NiCoP-2/CC in 1 mol/L KOH alkaline electrolytic cell was further studied. The Se-NiCoP-2/CC||Se-NiCoP-2/CC only needs a low voltage of 1.62 V at 50 mA/cm² (Figure 4(c)). The catalytic activity of Se-NiCoP-2/CC is superior to the reported electrocatalysts in Table S2 (Supporting information). In addition, it can be clearly observed that obvious hydrogen and oxygen bubbles are generated on the electrodes of the cathode and anode. Furthermore, the Se-NiCoP-2/CC showed stable durability in the long-term electrochemical process (Figure 4(d)). The voltage only increased 20 mV after working for 40 h at 10 mA/cm².

4 Conclusions

In summary, we successfully synthesized Se-NiCoP/CC hierarchical nanoarray structures by hydrothermal method and selenization/ phosphorization process with Se powder and NaH₂PO₂^.H₂O as the Se and P resources, respectively. Benefiting from the multi-level array structure, the excellent electron transport rate and abundant accessible active sites, the Se-NiCoP/CC electrode manifests outstanding HER catalytic performance in alkaline solutions. Additionally, when applied for overall water splitting, the Se-NiCoP/CC electrode also displays a superior performance and stability in alkaline solution. Consequently, the optimal Se-doped NiCoP electrode shows an excellent potential for practical applications for overall water splitting. This work may contribute to our deeper understanding of the effect of anion doping and can be applied to the reasonable design of other electrocatalysts.

Figure 4 (a) OER polarization curve; (b) Tafel slope plots; (c) Polarization curves of overall water splitting;(d) Chronoamperometry curve

Contributors

LIU Zi-xuan: Investigation, methodology, writing-original draft. WANG Xiao-long, TANG Qun-li, and XU Ya-li:Investigation, validation. HU Ai-ping and CHEN Xiao-hua: Supervision, conceptualization, writing-reviewing & editing, funding acquisition.

Conflict of interest

The authors declare no competing financial interest.

Supporting information

Figure S1 SEM images of NiCoP/CC with different resolutions

Figure S2 SEM images of Se-NiCoP-1/CC with different resolutions

Figure S3 SEM images of Se-NiCoP-3/CC with different resolutions

Figure S4 SEM images of Se-NiCoP-4/CC with different resolutions

Figure S5 EDS of (a) Se-NiCoP-1/CC, (b) Se-NiCoP-2/CC, (c) Se-NiCoP-3/CC and (d) Se-NiCoP-4/CC

Table S1 Comparison with previous reports in alkaline media

Table S2 Overpotential of different catalysts at current densities of 10 mA/cm² and 100 mA/cm²

Figure S6 CVs curves of (a) NiCoP/CC, (b) Se-NiCoP-1/CC, (c) Se-NiCoP-2/CC, (d) Se-NiCoP-3/CC, (e) Se-NiCoP-4/CC and (f) pure carbon cloth (CC)

Figure S7 (a) SEM image and (b) XRD pattern of Se-NiCoP-2/CC after durability test

Figure S8 High resolution of (a) Co 2p, (b) Ni 2p, (c) P 2p and Se 2p (d) XPS spectra of Se-NiCoP-2/CC after durability test

Table S2 Comparison of cell voltages of different electrodes for overall water splitting

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(Edited by YANG Hua)

中文导读

碳布负载三维Se掺杂NiCoP纳米阵列作为碱性条件下的析氢催化剂

摘要：探索稳定高效的碱性析氢(HER)电催化剂对碱性条件下的电解水具有重要意义。通过水热反应和磷化/硒化工艺在碳布上原位生长了具有分级纳米阵列结构的Se-NiCoP/CC析氢催化剂。实验结果表明，Se掺杂可以增加NiCoP的电化学活性位点，调节NiCoP的电子结构。优化后的Se-NiCoP/CC电极在碱性电解质中表现出优异的析氢催化活性，在10 mA/cm²的电流密度下的过电位仅为79 mV。当将Se-NiCoP/CC电极同时作为碱性水电解槽的正极和负极时，在仅1.62 V的低电压下实现了50 mA/cm²的电流密度。本工作为合理设计高活性HER电催化剂提供了一种可行的方法。

关键词：Se-NiCoP 纳米棒；多级阵列结构；析氢反应；碱性条件；全水解

Foundation item: Projects(51772086, 51872087, 51971089) supported by the National Natural Science Foundation of China; Project(2018TP1037-202102) supported by Open Fund of Hunan Provincial Key Laboratory of Advanced Materials for New Energy Storage and Conversion, China; Project supported by Student National SIT Innovation Program, China; Project(2020CB1007) supported by Hunan Joint International Laboratory of Advanced Materials and Technology for Clean Energy, China

Received date: 2021-02-20; Accepted date: 2021-06-15

Corresponding author: HU Ai-ping, PhD, Professor; E-mail: hudaaipinghu@126.com; CHEN Xiao-hua, PhD, Professor; E-mail: xiaohuachen@hnu.edu.cn; ORCID: https://orcid.org/0000-0003-1054-1487