Lei Shi,Yu Yin,Hong Wu,Rajan Arjan Kalyan Hirani,Xinyuan Xu,Jinqiang Zhang,Nasir Rafique,Abdul Hannan Asif,Shu Zhang,Hongqi Sun,

1 School of Engineering,Edith Cowan University,Joondalup,WA 6027,Australia

2 College of Materials Science and Engineering,Nanjing Forestry University,Nanjing 210037,China

3 School of Environmental and Chemical Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,China

Keywords:Hollow structure Electrocatalytic N2 fixation NH3 synthesis Template-assisted strategy Ambient condition

ABSTRACT As a fascinating alternative to the energy-intensive Haber-Bosch process,the electrochemically-driven N2 reduction reaction (NRR) utilizing the N2 and H2O for the production of NH3 has received enormous attention.The development and preparation of promising electrocatalysts are requisite to realize an efficient N2 conversion for NH3 production.In this research,we propose a template-assisted strategy to construct the hollow electrocatalyst with controllable morphology.As a paradigm,the hollow Cr2O3 nanocatalyst with a uniform size (～170 nm),small cavity and ultrathin shell (～15 nm) is successfully fabricated with this strategy.This promising hollow structure is favourable to trap N2 into the cavity,provides abundant active sites to accelerate the three-phase interactions,and facilitates the reactant transfer across the shell.Attributed to these synergetic effects,the designed catalyst displays an outstanding behaviour in N2 fixation for NH3 production in ambient condition.In the neutral electrolyte of 0.1 mol·L-1 Na2SO4,an impressive electrocatalytic performance with the NH3 generation rate of 2.72 μg·h-1·cm-2 and a high FE of 5.31% is acquired respectively at -0.85 V with the hollow Cr2O3 catalyst.Inspired by this work,it is highly expected that this approach could be applied as a universal strategy and extended to fabricating other promising electrocatalysts for realizing highly efficient nitrogen reduction reaction (NRR).

1.Introduction

As one of the indispensable industrial feed-stocks,ammonia(NH3) is extensively used in the synthesis of various agricultural fertilizers and high-value added chemicals [1].Besides,it is regarded as an attractive carbon-free energy carrier containing a high H content of～17.8% (mass).In the industry,NH3generation is predominantly realized with the Haber-Bosch reaction,which is generally conducted with harsh operating conditions,e.g.,high pressures(20–40 MPa)and temperatures(400–600°C)[2].In addition to a high energy input,this process discharges enormous greenhouse gases due to the H2consumption stemming from the reforming of fossil sources.As a promising alternative,the electrochemically-driven nitrogen reduction reaction (NRR)involving the direct N2reduction and applying the H2O as the proton supplier has attracted extensive attention recently,on account of its mild ambient conditions,simple equipment,and environmentally benign process [3–5].

Nevertheless,the high bond energy (～941 kJ·mol-1),as well as its poor affinity and low water solubility,make the N2molecule extremely difficult to be cleaved and further react with activated H atom under ambient conditions [6].Design and preparation of promising catalysts with a desirable electronic and surface structure is considered as an efficient approach to address the issues[7–10].To this end,massive researches have been focused on the development of desirable electrocatalysts,and versatile strategies have been developed,such as the defect engineering,structural regulation,and interface tailoring [11–15].

In view of the structural manipulation,promising catalysts with various morphologies,such as the one-dimensional nanowire[16,17],two-dimensional nanosheet [18–20],and threedimensional (3D) hollow/porous structures [21,22] have been widely employed in NRR applications.Comparing with their counterparts,the 3D hollow structures display several unique advantages in accelerating N2reduction and improving the conversion efficiency.Up to now,several hollow micro-nanostructures of Bi2MoO6sphere [23],VO2microspheres [24],Au/Au-Ag2O nanocages [25,26],Ni–Fe@MoS2nanocubes [27],and CoP nanocages [28] have been demonstrated as attractive candidates in N2reduction.The hollow catalysts possess many unique properties,e.g.,high surface-to-volume ratio,enhanced electrical conductivity,and good stability[29,30].It is revealed that the hollow structures could enhance the reactant transfer and supply sufficient active sites on both surfaces [31].Particularly,due to the cage effect,the hollow cavity could facilitate the trapping of the reaction intermediates,which is beneficial to increase the contents of critical intermediate products involved in the rate–determining processes in N2reduction [25].

Despite the observed superior properties,controllable synthesis and regulation of the hollow micro-nanostructures still remain as great challenges[32].For instance,hollow catalysts with desirable dimensions are extremely expected to supply sufficient inner space and plentiful active sites.Meanwhile,an ultrathin shell structure is indispensable for the reactant transfer and a smaller diffusion resistance.To meet these demands,various soft-and hardtemplating methods have been proven as efficient routes for preparing versatile hollow structures,realizing the tunable sizes and morphologies with the assistance of functional templates[33].However,the removal of the template is another concern with the template approaches.Generally,this procedure requires additional treatments and tedious operations,severely hindering its wide application and possibly destroying the micronanostructure of the obtained hollow materials.

To this end,applying the organic polymer of polystyrene (PS)beads as the template,a template-assisted strategy is proposed for the controllable preparation of hollow electrocatalyst in this work.On the one hand,the presence of PS beads induces the formation of homogeneous precursor nanospheres with a small size and uniform distribution.On the other hand,the hollow catalyst can be easily obtained with the subsequent calcination of precursor in air,while the PS template can be simultaneously removed without additional tedious procedures.Up to now,several Cr2O3based electrocatalysts,for example,multi-shelled hollow Cr2O3microspheres,Cr2O3nanofiber,and Cr2O3nanoparticles on graphene,have been reported for the NRR[34–36],exhibiting promising catalytic performance.Thus,taking the Cr2O3as a paradigm,the hollow Cr2O3nanocatalyst with controllable cavity and ultrathin shell was successfully synthesized with this reliable strategy.Comparing with the pristine Cr2O3microstructures (p-Cr2O3) with micro-scale diameters,the template-assisted Cr2O3(t-Cr2O3) displays a uniform size distribution with the diameter at～170 nm,and an ultrathin shell of～15 nm.Attributed to the attractive hollow structure and significantly enhanced active sites,the t-Cr2O3catalyst is demonstrated as a promising candidate in the efficient N2fixation.Evaluated in 0.1 mol·L-1Na2SO4electrolyte,an impressive electrocatalytic performance with the NH3production rate of 2.72 μg·h-1·cm-2and a high FE of 5.31% is acquired respectively at -0.85 V with the designed t-Cr2O3catalyst.

2.Experimental

2.1.Chemicals and materials

Styrene,sodium dodecyl sulfate,potassium persulfate,chromium chloride hexahydrate(CrCl3·6H2O),glucose,sodium salicylate (C7H5O3Na),sodium hypochlorite (NaClO),p-dimethylaminobenzaldehyde (p-C9H11NO),sodium hydroxide(NaOH),ethanol (C2H5OH),hydrochloric acid (HCl),sodium nitroferricyanide dihydrate (Na2[Fe(CN)5NO]·2H2O),hydrazine monohydrate (N2H4·H2O),and sodium sulfate (Na2SO4) were acquired from Sigma-Aldrich.Ultrapure water supplied with a Millipore system was used throughout the experiments.

2.2.Preparation of electrocatalysts

Firstly,the PS beads were prepared with an emulsion polymerization approach according to the literature [37,38].To synthesize the precursor solid of t-Cr2O3,3.38 g glucose and 0.2 g CrCl3·6H2O were dissolved in 30 ml water,following by adding the 3 ml PS template and stirring for 1 h at 25°C.The resultant precursor solution was subsequently transferred into the Teflon-lined autoclave,following by heating for 12 h at 180°C.The resultant product was thoroughly cleaned with ethanol and water,and finally dried at 55 °C for 12 h.To acquire the t-Cr2O3,the precursor solid product was calcinated at 500°C in the air atmosphere for 4 h.As a comparison,the precursor of p-Cr2O3was prepared in the absence of PS template under the same condition.

2.3.Fabrication of the catalyst modified working electrode

Firstly,5 mg as-prepared Cr2O3catalyst was measured and further dispersed in a mixture solution of 970 μl ethanol and 30 μl Nafion solution(5%(mass)).A homogeneous catalyst ink was subsequently obtained through sonicating the suspension solution for 0.5 h.Carbon paper(CP,1 cm×1 cm)was pre-cleaned through the sonication treatment in water and ethanol three times.Then,25 μl catalyst ink was carefully dropped onto the CP,and dried under the ambient condition.

2.4.Electrocatalytic N2 reduction

Electrochemical tests were measured by the electrochemical workstation (CHI 760E,CH Instruments,Inc.).The three-electrode test system was applied here,containing the Cr2O3catalyst modified working electrode,the reference electrode of the Ag/AgCl electrode(E vs.Ag/AgCl),and the graphite rod as the counter electrode,respectively.With the equation:E (vs.RHE)=E (vs.Ag/AgCl) +0.197+0.059 × pH,all potentials were converted to E (vs.RHE).Through the normalization of the geometric area of the CP electrode,the current density was reported.

To avoid the potential oxidation of produced NH3on the counter electrode,the H-type cell was adopted in the work,and two compartment cells were separated by the Nafion 211 proton exchange membrane.Before the usage,the membrane was pretreated following the well-known procedure [39].Briefly,the membrane was sequentially treated in ultrapure water,H2O2(5%(mass)) and 0.5 mol·L-1H2SO4at 80 °C,and ultimately soaked in ultrapure water.Prior to the N2reduction reaction,the electrolyte of 0.1 mol·L-1Na2SO4was aerated with N2for 0.5 h.During the NRR experiment,the continuous bubbling of N2into the cell with the sparger was required.

2.5.Determination of reaction products

The formed NH3concentration was analysed by the indophenol blue method [40].Briefly,4 ml electrolyte was firstly collected from the cell after the reaction,in which 50 μl oxidizing solution(NaClO+NaOH),500 μl coloring solution (C7H5O3Na+NaOH),and 50 μl catalyst aqueous solution (Na2[Fe(CN)5NO]·2H2O) were added in sequence.To achieve the consistency and accuracy of the obtained data,the absorbance measurement of all samples was performed with the same incubation time.

For the analysis of potential N2H4content in the Na2SO4electrolyte,the developed method by Watt and Chrisp was adopted in this work.In detail,the coloring reagent was freshly prepared through dissolving 5.99 g p-C9H11NO with 300 ml C2H5OH and 30 ml HCl.Then,5 ml electrolyte was removed from the cell,and mixed with 5 ml coloring reagent for 20 min under ambient conditions.

2.6.Calculations of NH3 yield rate and faradaic efficiency (FE)

The yield rate of NH3is calculated with the Eq.(1):

The FE is estimated with the following Eq.(2):

where [NH3] is the detected NH3concentration,V is the volume of 0.1 mol·L-1Na2SO4electrolyte for the reaction,m is the loading mass of catalyst,t is the time for the reduction reaction,F is the Faraday constant(96485.3 C·mol-1),and Q is the consumed charge quantity.

3.Results and Discussion

The synthesis route of the t-Cr2O3is displayed in Fig.1(a).The PS beads are served as the cores for the growth of precursor nanospheres,which are subsequently calcinated in air to remove the PS template and finally generate the hollow t-Cr2O3.As depicted in the scanning electron microscopy (SEM) image,the PS beads with a uniform diameter of～380 nm are applied as the template in this work(Fig.S1(a),Supplementary Material).After the hydrothermal growth,the homogeneously dispersed precursor nanospheres with a diameter of～480 nm are observed (Fig.S1(b)),indicating that the precursor layer with an appropriate thickness of～50 nm is coated on the PS beads.As a comparison,the precursor microspheres with a larger size ranging from 2 to 5 μm are obtained in the absence of the PS template (Fig.S2(a)).It is revealed that the PS beads could induce the in-situ growth of the precursor on the surface,thus effectively regulating the size of the precursor nanospheres.

Treating with the calcination in the air atmosphere,the Cr2O3catalysts with distinct morphologies are acquired with different precursors.For the t-Cr2O3,uniform nanospheres with a diameter of～170 nm are observed after the calcination,displaying hollow and transparent structures with ultrathin shells (Fig.1(b) and(c)).As further confirmed in the Fig.1(d),these hollow nanospheres consist of nanocrystals with an approximate diameter around 8–10 nm,and the shell of t-Cr2O3is estimated at～15 nm.Meanwhile,due to the ultrathin feature of as-prepared t-Cr2O3,partial fractures are observed on the catalyst surface.These merits would be beneficial to provide sufficient active sites and facilitate the interfacial diffusion of reactants for the NRR.It is also demonstrated that the PS template is successfully removed during the calcination process,following by the asymmetric constriction of the precursor.As revealed in the high-resolution TEM(HRTEM) image,the observed interplanar spacing of～0.363 nm within the lattice fringes is affiliated to the (012) plane of the Cr2O3phase (Fig.1(e)) [36].The obvious hollow nanostructure of t-Cr2O3is further verified in the energy-dispersive X-ray (EDX)mapping,and the homogeneously distributed Cr and O elements are confirmed on the framework of t-Cr2O3catalyst,as shown in Fig.1(f).As a contrast,the p-Cr2O3catalyst synthesized without the assistance of PS template possesses a distinct morphology.These microspheres exhibit multi-shelled hollow structures and large diameters around 1.5–2.0 μm (Fig.S2(b) and S3(a)).Accordingly,a shell at～45 nm much thicker than that of t-Cr2O3is observed(Fig.S3(b)).Similar to the t-Cr2O3,the constriction effect occurs within the calcination process for the synthesis of p-Cr2O3,leading to the size shrinkage when comparing with the precursor solid microspheres(Fig.S4(a)).Accompanying with the continuing calcination of the precursor and the removal of the carbon framework,the intermediate products of hollow microspheres with significantly reduced diameters are acquired (Fig.S4(b) and S4(c)).Finally,the p-Cr2O3catalyst with multi-shelled structures is fabricated after thorough calcination (Fig.S4(d)).

Fig.1.(a)Schematic route for the preparation of hollow of t-Cr2O3 nanocatalyst.(b)SEM image of t-Cr2O3.(c)and(d)TEM images of t-Cr2O3.(e)HRTEM image of t-Cr2O3.(f)Scanning TEM (STEM) image of t-Cr2O3,and the Cr and O element distribution in the EDX mapping.

The X-ray powder diffraction (XRD) patterns of designed p-Cr2O3and t-Cr2O3catalysts are illustrated in Fig.2(a).These typical diffraction peaks are ascribed to the Cr2O3(Joint Committee on Powder Diffraction Standards,JCPDS No.38-1479),and no obvious change except the variation in the peak intensity is observed.Moreover,the chemical states within the synthesized Cr2O3catalysts are further explored by the X-ray photoelectron spectroscopy(XPS).As revealed in Fig.2(b) and (c),the acquired O 1s and Cr 2p peaks confirm the co-existence of these elements.As for the Cr 2p section,these representative peaks located with binding energies at～576.5 and 586.1 eV belong to Cr 2p1/2and Cr 2p3/2,demonstrating the Cr(III) existence derived from the Cr2O3(Fig.2(b))[34,35].In the O 1s region(Fig.2(c)),a main peak with the binding energy at～530.2 eV is affiliated to surface lattice O associated with Cr(III).Based on these observations,the template introduction into the synthesis of Cr2O3induces few variations on the chemical properties of synthesized catalysts.Furthermore,N2adsorption–desorption isotherms of these catalysts are illustrated in Fig.2(d).As anticipated,a high Brunauer-Emmett-Teller (BET) surface area of t-Cr2O3is calculated as～175.5 m2·g-1,and the corresponding pore size displays a main distribution at～2.2 nm (Fig.2(e)).As a comparison,the p-Cr2O3possesses a smaller BET surface of～118.9 m2·g-1,with a wider distribution in the pore size.Obviously,the presence of PS template is indispensable to obtain hollow nanospheres with smaller sizes and ultrathin shells,eventually achieving significantly enhanced surface area.Finally,the electron transfer kinetics of the Cr2O3catalysts is explored with the electrochemical impendence spectra (Fig.S5).Displaying a smaller semicircle in the Nyquist plots,the t-Cr2O3possesses a lower electron transfer resistance compared with that of p-Cr2O3.All of these advantages of designed t-Cr2O3,such as smaller sizes with a hollow cavity,ultrathin shells and favourable chargetransfer kinetics,make it an attractive candidate for the efficient NRR.

Fig.2.(a)XRD patterns of p-Cr2O3 and t-Cr2O3 catalysts.(b)Cr 2p,and(c)O 1s XPS spectra of p-Cr2O3 and t-Cr2O3.(d)N2 adsorption and desorption curves of the p-Cr2O3 and t-Cr2O3.(e) The corresponding pore size distribution curves.

Fig.3.(a) Chronoamperometry test curves in N2 reduction with the t-Cr2O3 catalyst at various applied potentials.(b) UV–vis absorption spectra of the various electrolytes after the N2 reduction reaction.(c) NH3 yield rate,and (d) the related FEs of t-Cr2O3 catalyst for the NRR.(e) NH3 production rates and corresponding FEs of p-Cr2O3 and t-Cr2O3 catalysts conducted at -0.85 V.(f) NH3 yield rates in different control and blank tests.

Subsequently,two kinds of Cr2O3catalysts are evaluated in the NRR tests.During the electrocatalytic reaction,the N2bubbling was maintained in the electrolyte of 0.1 mol·L-1Na2SO4.Compared with these corrosive media (e.g.,HCl and KOH),the use of mild electrolytes attracts great attention owing to the environmentally friendly operation conditions,and low possibility to the material corrosion [7,35,41].The chronoamperometry test curves of t-Cr2O3catalyst at different electrocatalytic potentials are depicted in Fig.3(a).With decreasing the potentials from -0.55 V to-1.05 V,gradually accelerated interfacial reaction rates with increased current densities are obtained.For each test,a subtle change in current density is observed,indicating that a stable reaction interface is maintained within the duration of 2 h.To analyse the reaction products,the widely used indophenol method,and the Watt and Chrisp approach are employed respectively to determine the concentrations of NH3and possible N2H4.As shown in Fig.S6 and S7,these fitting curves exhibit acceptable linear relations between the NH3or N2H4concentration versus the corresponding absorbance value.Fig.3(b) displays the typical ultraviolet–visible(UV–vis) absorption spectra of various electrolytes collected after the reduction reaction.Comparing with the baseline of the blank sample,the enhanced absorption peaks confirm the successful synthesis of NH3within the applied potential ranges.Accordingly,the NH3yield rates and related FEs obtained at different potentials are estimated,as displayed in Fig.3(c) and (d).At an appropriate potential of -0.85 V,an impressive generation rate of 2.72 μg·h-1·cm-2and a high FE of 5.31% are acquired for the NH3synthesis.Applying with a higher potential,for instance,-0.55 V,the driving force is not sufficient to effectively trigger the N2activation and reduction on the catalyst surface.While introducing a more negative potential,i.e.,-1.05 V,the competitive H2evolution reaction becomes predominant on the most active centres of the catalyst,resulting in the significantly reduced FE.The performance comparison of t-Cr2O3catalyst and other reported electrocatalysts is shown in Table S1.Amongst various electrocatalysts,the t-Cr2O3catalyst possesses an acceptable activity comparing with some metal catalysts[21,25,42–44],metal-sulfides[45,46],metal oxides[47,48] and metal-free catalysts [49–52].As for the N2H4,no detectable N2H4is obtained after the catalytic reactions within all applied potentials (Fig.S8),suggesting an excellent selectivity in the N2reduction process for NH3synthesis.To explore the effect of shell thickness on the catalytic performance,t-Cr2O3(T) with a thicker shell (in some region up to～30 nm,Fig.S9) was prepared with higher CrCl3·6H2O and glucose concentrations.The catalytic activity of as-prepared t-Cr2O3(T) catalyst was tested,and a decreased NH3yield rate of 2.36 μg·h-1·cm-2and FE of～5.06%were acquired (Fig.S10).Obviously,a thin shell of hollow t-Cr2O3catalyst is preferable to achieve an excellent performance,while an increased shell thickness is not only unfavourable for the reactant transfer,but also hinders the electron migration.

Moreover,the catalytic performance of p-Cr2O3catalyst is also explored under the same condition,as shown in Fig.3(e).In view of a lower NH3yield rate (1.87 μg·h-1·cm-2) and FE (3.86%)achieved at -0.85 V,the p-Cr2O3catalyst displays an inferior performance to that of t-Cr2O3.Based on these results,the templateassisted strategy is demonstrated as an effective approach to design highly-effective catalysts,achieving an extremely enhanced conversion efficiency in NRR.The superior performance of t-Cr2O3catalyst could be mainly ascribed to the following factors.(i) The hollow structure is favourable to trap N2into the cavity and promote the N2reduction undergoing the high-frequency collisions;(ii) the nanostructures with a smaller size distribution are expected to provide sufficient active sites on both the inner and outer surface,and accelerate the three-phase interactions between the N2,the electrolyte,and the t-Cr2O3catalyst;and(iii)the ultrathin shell of t-Cr2O3catalyst could facilitate the reactant transfer across the shell,and simultaneously reduce the diffusion resistance on the interface.As well known,the ubiquity of NH3sources in surroundings,such as the air atmosphere,aqueous solutions,and solid surfaces,would introduce non-negligible measurement errors[53].Therefore,to further verify that detected NH3is generated through electrocatalytic NRR instead of exogenous NH3pollutant,a series of control and blank operations are performed.As depicted in Fig.3(f)and Fig.S11,negligible NH3contents are obtained both with the open circuit potential and in the Ar-saturated electrolyte.These results verify that the designed t-Cr2O3catalyst possesses reliable activity for the generation of NH3through the electrocatalytic NRR.

Moreover,the potential activity of bare CP toward N2reduction is also explored.As shown in Fig.S12,an ultralow NH3amount is detected,manifesting its poor catalytic activity in NRR.The structural stability is a critical parameter of the designed catalyst,and an excellent stability is crucial for practical applications.To this end,a recycling test is performed with the t-Cr2O3catalyst.As displayed in Fig.4(a),subtle variations are observed within five successive evaluations,confirming the unexceptionable stability and activity of the catalyst.Meanwhile,no obvious variation in the current density is acquired with t-Cr2O3catalyst during a long-term test of 24 h (Fig.4(b)),also implying a good structural stability.Furthermore,the XPS and TEM were conducted to explore the valence states and structures of the t-Cr2O3after the NRR test.As anticipated,the XPS spectra of t-Cr2O3exhibit subtle changes after the NRR test,indicating that the catalyst is still Cr2O3in nature(Fig.S13),while the t-Cr2O3retains the initial hollow nanophere morphology (Fig.S14).All these results demonstrate that asprepared t-Cr2O3catalyst possesses an excellent electrochemical and structural stability for NRR.

Fig.4.(a)Recycling evaluation of t-Cr2O3 catalyst for five tests at-0.85 V.(b)Recorded current density conducted with the t-Cr2O3 catalyst for the long-term test of 24 h.(c)Schematic distal NRR pathway on the Cr2O3 surface.

To explore the underlying NRR mechanism,density functional theory (DFT) calculation is considered as an efficient tool [54,55].As for Cr2O3catalysts,the potential-determining step is confirmed as *N2→*NNH in the first hydrogenation process with the DFT simulations [34,36].In view of no N2H4is monitored after the NRR procedure,a probable mechanism is also proposed in this work.According to these previous investigations,the two neighboured Cr sites are estimated to play a pivotal role in inert N2chemisorption,forming an associative route for subsequent N2hydrogenation.The subsequent hydrogenation steps would prefer to follow the distal mechanism,while the partially alternative route is also energetically possible.As for the distal mechanism(Fig.4(c)),the proton prefers to interact with the terminal N atom.Once the first NH3is generated and disassociated from the catalyst surface,the N atom adjacent to the catalyst surface would participate in the following hydrogenation steps to generate the second NH3.

4.Conclusions

In this work,a template-assisted approach is proposed for the controllable synthesis of hollow Cr2O3electrocatalyst.The PS template can not only effectively regulate the dimension and thickness of the precursor,but also provide a feasible removal during the calcination process.The prepared hollow Cr2O3nanocatalyst displays a small size,controllable cavity and ultrathin shell,which endow a feasible electrocatalyst for ambient N2reduction for NH3synthesis.Accordingly,an impressive performance with the NH3generation rate of 2.72 μg·h-1·cm-2and a high FE of 5.31% is acquired at-0.85 V with the designed catalyst.Meanwhile,the designed Cr2O3nanocatalyst possesses a remarkably excellent reusability and long-term stability.Based on this discovery,it is highly anticipated to develop various kinds of hollow nanocatalysts with controllable morphologies through the addition of PS beads with different sizes.Moreover,this versatile strategy may inspire the design of other metal-oxide based NRR electrocatalysts with attractive hollow nanostructures.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author (Sun) would like to express his thanks for the support from Vice-Chancellor’s Professorial Research Fellowship.The work was partially supported by Australian Research Council Discovery Projects (DP170104264 and DP190103548).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.11.016.