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Chia-Chun Wei, Po-Hung Lin, Chin-En Hsu, [Wen-Bin Jian](https://orcid.org/0000-0002-1898-9641), Yu-Liang Lin, Jiun-Tai Chen, Soumallya Banerjee, Chih-Wei Chu, Akhil Pradiprao Khedulkar, Ruey-An Doong, [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692)

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[Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous composite films](https://mdr.nims.go.jp/datasets/279f3da6-6293-4b28-b317-d2b2bbf912e8)

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Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous composite filmsllOPEN ACCESSArticleBoosting areal capacitance in WO3-basedsupercapacitor materials by stackingnanoporous composite filmsChia-Chun Wei, Po-Hung Lin,Chin-En Hsu, ..., Akhil PradipraoKhedulkar, Ruey-An Doong,Kazuhito Tsukagoshiwbjian@nycu.edu.twHighlightsGreen technologies aredemonstrated for the preparationof energy storage devicesLi-ion diffusion coefficient of1.12 3 10�7 cm2/s in WO3-basedsystemsAreal capacitance up to 496 mF/cm2 is demonstratedTradeoff between areal andgravimetric capacitances isexploredWei et al. develop electroexploding wire and spray-coating techniques to preparenanoporous WO3/MoO3 composite films. This work demonstrates bothsupercapacitive energy storage and methods to boost energy storage capabilitiesthrough the stacking of the nanoporous films.Wei et al., Cell Reports Physical Science 5,101836March 20, 2024 ª 2024 The Author(s).https://doi.org/10.1016/j.xcrp.2024.101836mailto:wbjian@nycu.edu.twhttps://doi.org/10.1016/j.xcrp.2024.101836llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836ArticleBoosting areal capacitance in WO3-basedsupercapacitor materials by stackingnanoporous composite filmsChia-Chun Wei,1 Po-Hung Lin,1 Chin-En Hsu,1 Wen-Bin Jian,1,7,* Yu-Liang Lin,2 Jiun-Tai Chen,2Soumallya Banerjee,3,4 Chih-Wei Chu,4 Akhil Pradiprao Khedulkar,1 Ruey-An Doong,5and Kazuhito Tsukagoshi1,6SUMMARYHigh-capacity energy storage devices are of interest in various ap-plications, but are not always easy to scale up, and they may experi-ence tradeoffs between areal and gravimetric capacitance. Here,nanoporous WO3/MoO3 films are prepared using electroexplodingwire and spray-coating techniques for the exploration of the trade-offs between areal and gravimetric capacitance. The nanoporousfilms are extended through nanoparticle stacking, which is accom-panied by increasing thickness. The diffusion coefficient measuredwith cyclic voltammetry increases exponentially with thickness andreaches 1.12 3 10�7 cm2/s for Li-ion intercalations. In galvanostaticcharge-discharge curves, the highest areal capacitance of 496 mF/cm2 is obtained at 0.5 mA/cm2 for an 18-mm-thick film, and the gravi-metric capacitance is 95.2 F/g at 0.13 A/g for a 1.6-mm-thick film. Thefilm thickness is adjusted to optimize either areal or gravimetric ca-pacitances, and high retention abilities imply the possibility of appli-cation in high-performance supercapacitive applications.1Department of Electrophysics, National YangMing Chiao Tung University, 1001 UniversityRoad, Hsinchu 300093, Taiwan2Department of Applied Chemistry, NationalYangMing Chiao Tung University, 1001 UniversityRoad, Hsinchu 300093, Taiwan3Department of Material Science andEngineering, National Yang Ming Chiao TungUniversity, 1001 University Road, Hsinchu 300093,Taiwan4Research Center for Applied Sciences,Academia Sinica, No. 128, Sec. 2, AcademiaRoad, Taipei 11529, Taiwan5Institute of Analytical and EnvironmentalSciences, National Tsing Hua University, Hsinchu300044, Taiwan6International Center for MaterialsNanoarchitectonics (WPI-MANA), NationalInstitute for Materials Science (NIMS), Tsukuba,Ibaraki 305-0044, Japan7Lead contact*Correspondence: wbjian@nycu.edu.twhttps://doi.org/10.1016/j.xcrp.2024.101836INTRODUCTIONThe huge consumption of fossil fuel energy has led to energy crises, CO2 emissions,and climate change. The building, industry, and transport sectors have dominatedamong all global energy consumption.1 In addition, the building sector hasconsumed approximately one-third of global energy and generated one-fourth ofthe CO2 emissions.1 To contain the CO2 emissions and prevent an energy crisis, sus-tainable and renewable energy resources with energy storage systems must bedeveloped.2 As for the transport sector and vehicles, they are under transition toelectrification, and, consequently, energy storage systems have played the mostimportant role.3 The continued development of energy storage systems will facili-tate the prevention of climate change and energy crises, and it has acceleratedthe applications of electric vehicles and portable electronics. One example is the po-tential to save energy using smart windows based on electrochromic (EC) mate-rials4–6 that can reduce energy consumption and CO2 emissions as well. Those ECmaterials were implemented for the development of supercapacitors7 as well asEC energy storage devices (ESDs).5The energy storage system manifested itself in the significant application of greenenergy and electric vehicles. There were several different types of energy storagesystems. The most well-established electrochemical energy storage systems wereCell Reports Physical Science 5, 101836, March 20, 2024 ª 2024 The Author(s).This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).1mailto:wbjian@nycu.edu.twhttps://doi.org/10.1016/j.xcrp.2024.101836http://creativecommons.org/licenses/by-nc-nd/4.0/llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836ArticlePb batteries, which still possessed the world’s largest market and underwent steadyinnovation in the design of their working electrodes with a high recycling rate.8 Inaddition, the technology of Li-ion batteries developed and evolved rapidly in thepast 3 decades due to specific features of high energy densities, low self-dischargeproperties, high open-circuit voltage, and long lifespan.9 The Li-ion batteries werebased on themechanisms of Li-ion intercalation and deintercalation. However, therewas another type of energy storage system, the supercapacitor, that implementedthe mechanisms of ion adsorptions and surface redox reactions.2,7 When high-po-wer charging or discharging was required, supercapacitors could either replace orcomplement other types of batteries. Supercapacitors, also known as ultracapaci-tors or electrochemical capacitors, consisted of two kinds of charge storagemechanisms. The first kind of charge storage mechanisms was the electrochemicaldouble-layer capacitor (EDLC) that signified the adhesion of electrolyte ions onthe surface of charged electrodes to form the electric double layer with a nanoscaleseparation gap.10,11 This mechanism existed in carbon-decorated electrodes andrequired highly porous structures, and it exhibited a high-power density and remark-able cycling reversibility. The second kind of charge storage mechanism was thepseudocapacitor, which indicated the faradic reaction of oxidations and reductionsbetween the electrolytes and electrodes.2,7 Although the cycling reversibility andthe power density were not comparable to the EDLC, the pseudocapacitor revealedother benefits of high specific capacitances and energy densities. In the past 2 de-cades, syntheses of nanomaterials progressed rapidly12,13 and facilitated the ad-vances of supercapacitors.2 To integrate the advantages of the EDLC and the pseu-docapacitor, asymmetric supercapacitor cells were proposed to decorate either oneor both positive and negative electrodes.14Hydrous ruthenium oxides (RuO2) were discovered to be excellent pseudocapaci-tor materials that exhibited a high weight capacitance of up to 800 F/g,15 whereassuch noble metal oxide materials posed an intrinsic, noneconomic drawback and asevere problem in mass production. Abundant metal oxides such as Fe3O4 andMnO2 were proposed and used alternatively for supercapacitors. For example,the gravimetric capacitance of carbon-MnO2 was estimated to be �20 F/g.16 Inrecent years, WO3 manifested itself as a high potential material for EC applica-tions,4–7 and its hybrid composites had been demonstrated to be energy storagesmart windows.17 With the advances in nanotechnology, the bulk WO3 had beenconverted to nanomaterials, giving a high surface-to-volume ratio and enhancingthe supercapacitive property.18–21 Those nanostructured WO3s almost presenteda high specific capacitance of �400 F/g at 1 A/g,19 which was comparable tothat of RuO2 supercapacitors. In addition to pure, nanostructured WO3, therewere reports of hybridized composites and asymmetric supercapacitors basedon WO3 using several chemical or physical synthesis methods. For instance, anelectrochemical co-deposition method was used to prepare WO3-V2O5 compos-ites for supercapacitive applications that exhibited an areal capacitance (Careal)of �40 mF/cm2 at 0.5 mA/cm2.22 However, hydrothermal synthesis and electro-chemical polymerization were used to prepare honeycombed porous P5FIn/WO3nanocomposites for supercapacitors, revealing an area capacitance of �34 mF/cm2 at 0.1 mA/cm2.23 All of these studies pointed to the importance of preparingWO3-based porous materials. In addition, the areal capacitance normallyincreased with an increase in the thickness of metal-oxide materials, and the con-ductivity at outer surface and the probability of ion diffusing deeply into metal-ox-ide films brought about the poor performance of energy storage. In all of theseissues, the porosity of metal oxides was demonstrated to exhibit an essentialrole in enhancing energy storage.24,252 Cell Reports Physical Science 5, 101836, March 20, 2024llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836ArticleIn previous studies, we developed a scaling-up method to prepare nanoparticulatefilms of WO3 and MoO3 composites using the green technologies of electroexplod-ing wire and spray-coating techniques.5 We discovered that (WO3)87.5(MoO3)12.5composite films exposed optimum EC properties, such as high charge density,coloration efficiency, and transmittance variation. In the present work, we exploredeeply into energy storage capabilities for the same nanoparticulate (WO3)87.5(MoO3)12.5 composite films, presenting a potential application as supercapacitorelectrodes. The intrinsic properties of the nanoparticulate films, including the nano-porosity and stackability feature, are implemented for the development of highsupercapacitive performances that have rarely been explored. Currently, most ofthe high-capacitance supercapacitors based on WO3 are fabricated on carbon-based substrate such as carbon fabric and graphenes,26,27 which may lead to misin-terpretations of contributions from either intrinsic WO3- or carbon-based material.28The nanoporous (WO3)1-x(MoO3)x composite film manifests itself as an easy scaling-up structure on both sizes and areal capacities for supercapacitive applications. Thepresent study paves the way for developing high capacity on EC ESDs.RESULTS AND DISCUSSIONCharacterization of WO3 and MoO3 nanoparticles (NPs)WO3 and MoO3 NPs were synthesized using the electroexploding wire techniqueand mixed in solution. The transmission electron microscopy (TEM) images of thetwo species are displayed in Figure S1A. For a detailed inspection, a high-resolutionTEM image of WO3 is offered in Figure S1B, presenting an interplanar spacing of�0.38 nm for the (021) plane of monoclinic WO3.6 Accordingly, interplanar spacingsof �0.18 and �0.25 nm in Figure S1C are assigned to the (230) and (041) planes oforthorhombic MoO3. Further investigation of NPs, including size distributions, X-raydiffraction (XRD) analyses, and Raman spectroscopy analyses, were reportedpreviously.5Characterization of WO3/MoO3 composite filmsIn this study, we adopt nanoparticulate (WO3)87.5(MoO3)12.5 films for the furtherexploration of supercapacitive and energy storage capabilities. Figure 1A presentsa side view of a scanning electronmicroscopy (SEM) image for a (WO3)87.5(MoO3)12.5film prepared with 5 mL of mixed WO3 and MoO3 NP suspensions. The molarnumber of the 5-mL suspension corresponds to �5.2 3 10�6 mol (see the detailedcalculations in the supplemental experimental procedures). The thickness of theas-prepared composite film is estimated to be �500 nm, which is marked in Fig-ure 1A. The thicknesses of the other nanoparticulate (WO3)87.5(MoO3)12.5 films pre-pared with volumes of 10, 15, 20, and 25 mL are measured in SEM images shown inFigures S2A—S2D. The thickness as a function of molar number of (WO3)87.5(MoO3)12.5 composites is displayed in the inset to Figure 1A. With the thickness ofthe composite films determined, the sample names are designated accordingly aspresented in Table S1. The surface morphologies of (WO3)87.5(MoO3)12.5 compositefilms were examined using SEM. Figures S3A and S3C reveal the morphology of theas-prepared samples, whereas Figures S3B and S3D show the surface morphologyafter the electrochemical test, with Li ions intercalated. After performing electro-chemical measurements and putting the film in the colored state, additional clusters(see Figure S3B) and, sometimes, microrods (Figure S3D), will be observed at thesurface. These additional materials could be attributed to Li compounds when thecarbonate-based electrolyte was used.29,30 The elemental mappings of the side-view images of sample WMo-05 are provided in Figures S4A–S4C. It is observedthat the W and Mo elements are uniformly distributed in the composite films. Thereare no aggregations in the simple mixing of NP suspensions for the preparation ofCell Reports Physical Science 5, 101836, March 20, 2024 3Figure 1. Morphology and energy storage performance(A) Side-view SEM image of nanoparticulate WO3/MoO3 films of sample WMo-05, with a scale barof 1 mm. The thickness of �500 nm and the FTO substrate are marked on the SEM image. Thethicknesses are estimated to be 489 G 39, 1,000 G 46, 1,280 G 85, 1,670 G 120, and 2,080 G 160 nmfor samples WMo-05, WMo-10, WMo-13, WMo-16, and WMo-20, respectively. The inset presentsthe thickness in micrometers of the nanoparticulate films as a function of the molar number of WO3/MoO3 composites.(B) Top-view AFM image of sample WMo-05 in the 3-dimensional configuration. The root-mean-square roughness is �42.2 nm within an area of 5 3 5 mm2.(C) An EC ESD in its initial (top) and charged (colored) (bottom) states. The EC ESD is based onsample WMo-20, with an active area of 3.5 3 4.0 cm2 on 1 side of the FTO glass substrate. Thecharge storage system consists of 2 ESDs connected in series to offer sufficient voltage for drivingLEDs.(D) Discharging voltage as a function of time for the 2 ESDs connected with an LED. The dischargingperiod lasts longer than 8.5 min while the discharging voltage changes from �2.78 to �2.30 V.llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Article(WO3)87.5(MoO3)12.5 composite films. Moreover, Figure S5A offers Raman spectra ofthe (WO3)87.5(MoO3)12.5 composite films based on samples from WMo-05 to WMo-20. The spectra only display differences in intensity for different samples, whereasthe characteristic peaks remain in the same position. The Raman spectrum of sampleWMo-13 is magnified in Raman shifts ranging from 600 to 1,000 cm�1 and is pre-sented in Figure S5B. The spectrum is fitted with several Gaussian peaks and sepa-rated into bands at Raman shifts of 670, 713, 807, 816, and 896 cm�1. The Ramanbands at 713 and 807 cm�1 indicate the monoclinic structure of WO3 owing to thestretching vibrations of O–W–O bonds.5,31,32 The bands at 670 and 816 cm�1 revealthe orthorhombic MoO3, attributed to the stretching vibrations of the triply coordi-nated oxygen and the O=Mo and Mo–O–Mo bonds.5,33 In addition, the band at896 cm�1 denotes the monoclinic crystal phase of MoO3.5,33 In addition, XRDdata in Figure S6 reveal similar profiles for samples of different film thicknesses.X-ray photoelectron spectroscopy (XPS) spectra for the surface electronic structure,presented in Figure S7, also provide consistent results among samples of differentthicknesses. In Figure S8, the XPS spectra of W 4f, Mo 3d, and Li 1s core levels forsample WMo-20 are offered both before and after electrochemical testing. Accom-panied by negligible chemical shifting ofW 1s andMo 3d, an additional band of Li 1s4 Cell Reports Physical Science 5, 101836, March 20, 2024llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Articlewith a weak intensity can be observed in the spectrum after electrochemical testing.This result demonstrates the presence of the Li compound at the surface that is illus-trated in the SEM images in Figure S3.The nanoparticulate films reveal intrinsic nanoporosity due to the stacking ofdifferent sizes of NPs. Figure 1B exhibits a top-view atomic force microscopy(AFM) image of sampleWMo-05. Fruitful cavities of different sizes appear on the sur-face. In particular, the feature of nanoporosity benefits chemical or physical reac-tions on the surface and ion diffusions in cavities and channels. The mass of the(WO3)87.5(MoO3)12.5 composite films for sample WMo-05 is evaluated as being1.1 mg (see descriptions in the supplemental experimental procedures). For thosecompact films prepared by either thermal evaporation or sputtering, such films shallpossess a thickness of 296 nm, whereas the thickness of sample WMo-05 is�500 nm—much higher than that of compact films. With this preparation, the sur-face-to-volume ratio and the porous feature are enhanced; thus, the nanoparticulatefilms are suitable for physical and chemical reactions on the surface. The nanoparti-culate (WO3)87.5(MoO3)12.5 composite film based on sample WMo-20 is given as anegative electrode, and another empty fluorine tin oxide (FTO) glass substrate isused as a positive electrode for the assembly of a supercapacitive ESD. To demon-strate the function of the ESDs, two ESDs are connected in series to double thedriving voltage for lighting up a light-emitting diode (LED). Figure 1C illustratesthe initial state and the charged (colored) state of the supercapacitive ESDs. Thetwo series-connected ESDs are charged for �3 min, with �8 V applied on one nega-tive electrode and one positive electrode grounded. The charged ESDs are thenconnected to an LED. Figure 1D exposes the lighting up of an LED and the variationof the potential as a function of time across the two ESDs. The period of the LEDlighting up is longer than 8.5 min, with a stable change of discharging voltagesfrom �2.45 to �2.30 V. The period is longer than that of the �2 min measured inour previous studies.5 The energy storage capability of the ESDs is promoted simplydue to the increment of the nanoparticulate film thickness. The energy storage capa-bility is further investigated with regard to the thickness of the nanoparticulate(WO3)87.5(MoO3)12.5 composite films.Cyclic voltammetry (CV) of WO3/MoO3 composite filmsEnergy storage capabilities of the nanoparticulate (WO3)87.5(MoO3)12.5 compositefilms are inspected by either CV or galvanostatic charge-discharge (GCD) curves.Figure 2A exhibits the CV curves of samples WMo-05 to WMo-20 at a scan rate of20 mV/s, with a scan direction indicated. For samplesWMo-16 andWMo-20, a widerpotential window from �2.0 to 2.0 V is used because the redox peaks shift to highervoltages. All CV curves reveal reversible redox reactions in loops, implying an inser-tion and extraction of Li+ ions. Meanwhile, the color of the nanoparticulate filmchanges to dark blue from the original transparent at negative voltages when Li+ions are intercalated into (WO3)87.5(MoO3)12.5 composites. In the deintercalatedprocess at positive voltages, the films return to their transparent (bleached) color.The CV loops are enlarged with an increase in film thickness that infers many moreLi+ ions stored in the supercapacitive materials.In the oxidation and reduction processes, the highest currents in the loop giveanodic (deintercalation) and cathodic (intercalation) peak currents for the evaluationof diffusion coefficients (D) of Li+ ions in the nanoparticulate films. The diffusion co-efficient at 25�C is evaluated through the Randles-Sevcik equation: jp = 2:693105n3=2D1=2Cv1=2, where n= 1 is the number of electrons transferred in the redoxCell Reports Physical Science 5, 101836, March 20, 2024 5Figure 2. CV measurement(A) CVs of samples WMo-05, WMo-10, WMo-13, WMo-16, andWMo-20 at a voltage scanning rate of20 mV/s. The voltage scanning direction is indicated on the graph.(B) Diffusion coefficients and areal capacitances of nanoparticulate (WO3)87.5(MoO3)12.5 films as afunction of film thickness. The red circles and the black squares at top represent diffusioncoefficients obtained in reduction (intercalation) and oxidation (deintercalation) processes,respectively. The black squares, the red circles, and the blue triangles at bottom give the arealcapacitance evaluated at voltage scanning rates of 5, 10, and 20 mV/s, respectively.llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Articlereaction, C is the concentration of electrolyte, and v is the scan rate of the sweepingpotential.The diffusion coefficients for intercalation and deintercalation are 2.96 3 10�11 and5.51 3 10�10 cm2/s for sample WMo-05 that are much smaller than the values of3.39 3 10�10 and 1.18 3 10�10 cm2/s obtained previously,5 owing mainly to thedecrement of the scan rate from 150 to 20 mV/s. In the present study, the diffusioncoefficients for intercalation and deintercalation significantly increase up to 1.72 310�9 and 7.78 3 10�10 cm2/s for sample WMo-20 at the slow scan rate of 20 mV/s.The diffusion coefficients in the logarithmic scale as a function of the nanoparticulate(WO3)87.5(MoO3)12.5 film thickness are exposed in the upper panel of Figure 2B. Thediffusion coefficient increases exponentially with an increase in the nanoparticulatefilm thickness. The nature of Li+ ion diffusion in the electrolyte does not change,whereas the (WO3)87.5(MoO3)12.5 films for the embodiment of ion diffusion and redoxreactions are extended from two-dimensional (thin film) to three-dimensional (thickfilm) structures. Such an extension leads to the outcome of exponentially raised diffu-sion coefficients. As revealed in Figure 2B, the diffusion coefficient of intercalation is�2–3 times higher than that of deintercalation for thicker films, whereas it is�6 timeshigher for sampleWMo-05. For the thicker nanoporous film, the surface area expandsexponentially, which accelerates ion intercalation and deintercalation near the sur-face without diffusing deeply into the crystalline bulk. Thus, the difference in diffusioncoefficients between the intercalation and deintercalation processes becomessmaller. In addition, we adopt another method to estimate the diffusion coefficient.Figures S9A–S9D exhibit the CV curves of samplesWMo-05, WMo-13, WMo-20, andWMo-180 at scan rates (v) from 2 to 20 mV/s. The linear least-squares fitting to thedata of the peak current density (jp) and v1=2 give slopes that are used to estimatediffusion coefficients at 25�C.34–36 Figure S9E shows the jp as a function of the squareroot of the voltage scan rate. The inset to Figure S9F points to the same feature of anexponential increase in the diffusion coefficient as a function of the film thickness. Forthe film that is�18 mm in thickness in Figure S9F, thediffusion coefficient seems not toincrease so rapidly, which is in line with methanol diffusion in poly(vinyl acetate)6 Cell Reports Physical Science 5, 101836, March 20, 2024llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Articlefilms.37 In addition, the diffusion coefficients of intercalation and deintercalation forthe thickest film reach up to 1.123 10�7 and 3.193 10�8 cm2/s, which are almost thehighest values ever recorded.The CV curves are also used for the evaluation of areal capacitances according to theequation Careal = ðR JdtÞ=ð2v$DVÞ, where J is the current density, v is the scan rate,and DV is the voltage range of the potential window. TheCareal as a function of nano-particulate film thickness is offered in the lower panel of Figure 2B. The Careal at thescan rate of 5 mV/s is �6.46 mF/cm2 for sample WMo-05 and it is increased �5.6times up to 36.2 mF/cm2 for sample WMo-20. The Careal0s for sample WMo-20 are36.2, 27.6, and 19.6 mF/cm2 at scan rates of 5, 10, and 20 mV/s. The Careal decreaseswith an increasing scan rate for thicker films, whereas it does not change considerablyfor thin films such as in sampleWMo-05. The higherCareal at a lower scan rate impliessufficient time for Li+ ion diffusion in nanoparticulate (WO3)87.5(MoO3)12.5 films.To analyze the charge storage mechanism, the data of current densities and voltagescan rates are fitted according to the equation jp = anb,38–40 where a is a constant, n isthe voltage scan rate, jp is the redox peak current density, and b is the exponent. Theexponent b shall theoretically be in the range of 0.5–1. Figure S10 presents the dataof logðjpÞ as a function of logðvÞ for samples WMo-05, WMo-13, WMo-20, andWMo-180, and the b values are the slopes of those linear fittings. The exponent b in ionintercalation does not vary with increasing thickness, whereas the b in ion deinterca-lation decreases from 0.97 to 0.51. It implies a transition of the mechanism from theEDLC to the pseudocapacitor during the discharging process. Electrochemicalimpedance spectroscopies of samples WMo-05, WMo-20, and WMo-180 are pro-vided in Figure S11. The data do not show noticeable semicircles in the high-fre-quency region that indicate a capacitive behavior. Moreover, the straight-linefeature in the low-frequency region suggests a low ion diffusion resistance, makingthem promising for supercapacitor applications. The intercept on the real imped-ance axis (Z0) represents the equivalent series resistance (Rs) of 87.9, 86.1, and88.1 U $ cm2 for samples WMo-05, WMo-20, and WMo-180, respectively.GCD of WO3/MoO3 composite filmsThe measurement of GCD curves provides another method to assess the energystorage capability of the nanoparticulate films. It is measured at a constant currentsupplied to the nanoparticulate film for both charging and discharging processes,and the voltage is recorded as a function of time. In this study, GCD curves at severaldifferent current densities are collected for the estimation of the specific capaci-tance. Figure 3A unveils GCD curves of samples WMo-05–WMo-20 at a current den-sity of 0.1 mA/cm2 and a GCD curve of sample WMo-180 at a current density of 0.5mA/cm2. For the thickest film of sample WMo-180, the GCD cycle takes a muchlonger time at a low current density; thus, a higher current density of 0.5 mA/cm2is used. When the thickness of the nanoparticulate (WO3)87.5(MoO3)12.5 film in-creases, the charging and discharging time increases. For all of the samples, thecharging time is very close to the discharging time, which implies a relatively highreversibility in electrochemical reactions.41 The discharging curve of sample WMo-180 deviates away from the ideal triangular shape that indicates the redox processand manifests itself as pseudocapacitor- and battery-type materials. The arealcapacitance is extracted using the equation Careal = ðJDtÞ=DV , where J is the dis-charging current density, Dt is the discharging time, and DV is the voltage rangeof the potential window. From the GCD curves, the Careal0s are estimated to be15.45 and 87.27 mF/cm2 for samples WMo-05 and WMo-20. Those areal capaci-tances are �2.4 times higher than the values of 6.46 and 36.2 mF/cm2 obtainedCell Reports Physical Science 5, 101836, March 20, 2024 7Figure 3. GCD measurement and comparison of areal capacitance(A) GCD curves of samples WMo-05, WMo-10, WMo-13, WMo-16, and WMo-20 at a constantcurrent density of 0.1 mA/cm2 and of sample WMo-180 at 0.5 mA/cm2.(B) Areal capacitances of nanoparticulate (WO3)87.5(MoO3)12.5 films as a function of film thickness atcurrent densities of 0.1 (black squares), 0.2 (red circles), and 0.3 mA/cm2 (blue triangles). Theorange symbols point to the areal capacitances of the film, with thicknesses of �9.0, 13, and 18 mm,measured at current densities of 0.5, 1.0, and 1.5 mA/cm2. The inset shows a magnified area of thedashed-line rectangle.(C) Areal capacitances at various current densities of our samples and other groups’ materials,including (i) WO3-V2O5 composites,22 (ii) porous P5Fin/WO3 nanocomposites,23 (iii) Gd-dopedWO3 nanofilms,45 (iv) Mo-doped a-WO3 thin films,46 (v) Mo-doped WO3 thin films,47 (vi) WO3-ZnOcomposite thin films,44 and (vii) WO3/PEDOT:PSS films.48 The inset gives the configuration of theareal capacitance measurements.llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Articlefrom the CV curves at the scan rate of 5 mV/s. In particular, the areal capacitance ofsample WMo-180 gives an outstanding benchmarked value of 496 mF/cm2Figure 3B shows the areal capacitance as a function of the thickness of the nanopar-ticulate (WO3)87.5(MoO3)12.5 film. High current densities are used in GCD measure-ments for samples WMo-90, WMo-130, and WMo-180. The areal capacitances areestimated at the 10th loop of the GCD curve. With current densities increasedfrom 0.1 to 0.3 mA/cm2, the areal capacitance of the thinnest film (sample WMo-05) does not vary apparently, whereas that of sample WMo-20 shows a significantvariation, giving a lower areal capacitance at a higher current density. For sampleWMo-20, the Careal of 87.3 mF/cm2 at 0.1 mA/cm2 drops down to 62.4 and 58.2mF/cm2 at current densities of 0.2 and 0.3 mA/cm2, respectively. The same trendis observed for thicker films of samples WMo-90, WMo-130, and WMo-180 that8 Cell Reports Physical Science 5, 101836, March 20, 2024llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Articlethe areal capacitance of the thickest film (WMo-180) reveals an abrupt decrease, withcurrent densities increased from 0.5 to 1.5 mA/cm2. Alternatively, the areal capaci-tance does increasemonotonically with the increasing film thickness, which indicatesthe capability of the stacking features for nanoparticulatematerials. It is worth notingthat theCareals of sampleWMo-180 are 496, 291, and 243mF/cm2 at 0.5, 1.0, and 1.5mA/cm2, respectively. The effective diffusion length of Li+ ions in thick films islonger, but the diffusion requires a longer time under the high current density. Alonger diffusion time is required for a thick nanoporous film with a high areal current;thus, the Careal descends with increasing current densities.42,43 Here, the Careal de-creases with an increase in the current density, and the rate capability is used forsuch an evaluation. For example, the rate capabilities of samples WMo-05 andWMo-20 are �75.4% and �66.7%, respectively, with a current density increasedfrom 0.1 to 0.3 mA/cm2. For sample WMo-180, it significantly decreases to�48.9%, with current densities tripled from 0.5 to 1.5 mA/cm2. Similar trendshave been reported previously. For example, the rate capability is �61.9% forP5FIn/WO3 nanocomposites23 when the current density increases from 0.03 to 0.1mA/cm2. It is �77% for WO3-ZnO composites,44 with current densities varyingfrom 0.3 to 1.1 mA/cm2, and it is �77% for WO3/graphene/polyaniline films, withcurrent densities changed from 0.05 to 0.15 mA/cm2The outstanding Careal of 87.3 and 496 mF/cm2 for sample WMo-20 and WMo-180are benchmarked against recent works such as 38.75 mF/cm2 for WO3-V2O5 com-posites,22 34.1 mF/cm2 for porous P5FIn/WO3 nanocomposites,23 62.43 mF/cm2for Gd-doped WO3 nanofilms,45 39.2 mF/cm2 for Mo-doped a-WO3 thin films,46117.1 mF/cm2 for Mo-dopedWO3 thin films,47 15.24mF/cm2 for WO3-ZnO compos-ite thin films,44 and 5.3 mF/cm2 for WO3/poly(3,4-ethylenedioxythiophene) polysty-rene sulfonate (PEDOT:PSS) films.48 The comparison is summarized in Figure 3C,with the current density indicated. It is noted that sample WMo-20 exhibits a compa-rably highCareal, whereas the film thickness can be piled up to further aggrandize andraise the Careal. Sample WMo-180 shows the highest Careal, which is more than fivetimes higher than those reported previously, and it is measured in the same three-electrode system, consisting of a Pt counterelectrode, an LiClO4 electrolyte, andan FTO (or InSn) current collector.Gravimetric capacitance of WO3/MoO3 composite filmsThe gravimetric capacitance in F/g units is calculated to explore more about the en-ergy storage mechanism in the nanoporous films. We calculate the loading mass forall of our samples as described in detail in the supplemental experimental proced-ures. For example, the mass loadings for samples WMo-20 and WMo-180 areevaluated to be �0.95 and 9.5 mg/cm2. The gravimetric capacitance of samplesWMo-20 and WMo-180 at 0.1 and 0.05 A/g are calculated to be �91.9 and 52.2F/g, respectively, which are higher than the 24 F/g for WO3 coated on glassy carbonsubstrates49 and lower than 436F/g at 1A/g forWO3nanofibers.19 Figure 4Adisplaysgravimetric capacitances of nanoparticulate (WO3)87.5(MoO3)12.5 composite filmswith a thickness of less than 2.0 mm at lower current densities of 0.1, 0.2, and 0.3mA/cm2. At the current density of 0.1 mA/cm2, gravimetric capacitances of all ofthe samples vary between �81.3 and �91.9 F/g. In addition, the gravimetric capac-itance increases to a maximum and decreases with increasing film thickness. Thedecrease in gravimetric capacitance comes from the stacking nature of the incrementof the third dimension of the film thickness. Similar behaviors have been reported inthe literature.50 Li et al. report that the gravimetric capacitance decreases when thethickness of their films increases. The feature could be attributed to a gradual loss ofsurface conductivity in thicker films.50 Figure 4B presents gravimetric capacitancesCell Reports Physical Science 5, 101836, March 20, 2024 9Figure 4. Comparison of gravimetric capacitance(A) Gravimetric capacitance of (WO3)87.5(MoO3)12.5 composite films with a thickness of less than2.0 mm at discharging current densities of 0.1, 0.2, and 0.3 mA/cm2.(B) Gravimetric capacitance of (WO3)87.5(MoO3)12.5 composite films with a thickness greater than9.0 mm under discharging current densities of 0.5, 1.0, and 1.5 mA/cm2. (C) Gravimetric capacitanceof all of the samples as a function of gravimetric current density in logarithmic scale. The blackdashed lines are the least-squares fitting lines to data in the low- and high-current density groups.llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Articlefor thicker composite films at a current density higher than 0.5 mA/cm2. The gravi-metric capacitance increases with an increase in film thickness that indicates the po-tential to raise the gravimetric capacitance further with NP stacking. However, thegravimetric capacitance significantly decreases to�52.2 F/g due to a high areal cur-rent density, indicating that the ions in the electrolyte must take a longer time todiffuse in the third dimension (the thickness) of the nanoporous composite films.The gravimetric capacitances of all of the samples with respect to gravimetric currentdensity are shown in Figure 4C. The data are categorized into two groups, with onemeasured at areal current densities smaller than 0.5mA/cm2 and the other measuredat a higher areal current density. In the group of low areal current densities, the high-est gravimetric capacitance of 95.2 F/g is obtained at 0.13 A/g for sample WMo-16.However, the gravimetric capacitance presents an incremental tendency withincreasing film thickness at a decreasinggravimetric current density. The results high-light the tradeoff between the areal and the gravimetric capacitances when the nano-porous composite film expands in the direction of the film thickness. It was argued inthe reference papers of Wang et al.54 that the weight of the substrate as well as the10 Cell Reports Physical Science 5, 101836, March 20, 2024llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836Articleelectrode should be included in the estimation of gravimetric power density and en-ergy density. That may underscore the importance of the scalable technique overpursuing a high gravimetric capacitance.51 Furthermore, the measurement of themass of working materials usually produces significant errors that lead to overstate-ments of performance.52 As a consequence, the areal and gravimetric capacitancesare essential in the applications of different directions, and, in some situations, theconcept of areal capacitance plays a larger role in performance characterization.53The energy storage capability of the (WO3)87.5(MoO3)12.5 composite films shalldepend largely on the electrolyte used in the GCD measurements. To investigate itsvariation, we also performed GCD measurements for sample WMo-20 under the elec-trolyte of 1 M H2SO4. Figure S12A presents the GCD curves at a high constant currentdensity of 1.0 mA/cm2 within the potential window between�0.4 and 0.5 V. The GCDcurves have a moderately symmetric feature between the charging and dischargingprocesses that points to a high electrochemical reversibility. The Careal0s of the 1stand 10th cycles are estimated to be 151.7 and 121.9 mF/cm2. Comparing this withthat in the electrolyte of 1 M LiClO4 + propylene carbonate (PC), the applied currentdensity is 10 times higher and the potential window is approximately one-half smallerin the GCD measurements in 1 M H2SO4. Figure S12B provides a comprehensivecomparison of the Careal of the nanoparticulate (WO3)87.5(MoO3)12.5 composite films,obtained in electrolytes of 1 M H2SO4 and 1 M LiClO4 + PC at different currentdensities. It is noted that for the electrolyte of 1 M LiClO4 + PC, the Careal is expectedto be lower than 87.3 mF/cm2. Nevertheless, it increases to 114.1 mF/cm2 when theelectrolyte of 1 M H2SO4 is applied to the GCD measurements. These results signifythe important roles of electrolytes for the estimation of the specific capacitance.Retention ability and performance of symmetric devicesThe retention ability of the energy storage capability is further explored through theGCD measurements on samples WMo-20 and WMo-90 at current densities of 1.8and 2.8 mA/cm2, respectively. Figures 5A and 5C exhibit several cycles of chargingand discharging GCD curves for samples WMo-20 and WMo-90 within a potentialwindow in the range between 1.2 and �0.8 V. For sample WMo-20, the chargingand discharging time continuously decreases, as indicated by the orange arrow,whereas the charging and discharging time of sample WMo-90 increases in the first160 cycles and starts to decrease thereafter until the end.According to the GCD curves, the retention of areal capacitance and theCoulombic efficiency of samples WMo-20 and WMo-90 for 5,000 cycles are evalu-ated and given in Figures 5B and 5D. The insets to Figures 5B and 5D offer thecorresponding areal capacitance with respect to cycle number. The capacitanceretention of sample WMo-20 exhibits a stable decay to 60% of its initial value.The areal capacitance at 1.8 mA/cm2 in the 1st and the 5,000th cycles are 27.8and 15.9 mF/cm2, respectively. For sample WMo-90, the areal capacitance at2.8 mA/cm2 is �59.3 mF/cm2 in the beginning and it decreases to 46.6 mF/cm2in the 5,000th cycle. The capacitance retention of sample WMo-90 surpasses100% within �1,000 cycles (see the red dashed line frame in Figure 5D), and it rea-ches the highest value of �139% in the 160th cycle. This phenomenon is in linewith that reported in the literature.54 It could be attributed to more and moremixed regions of partially active and inactive regions in thicker films. These regionsare activated after certain cycles of charging and discharging GCD measurements,and the retention ability could surpass its initial state, especially for thicker nano-porous films. The Coulombic efficiency of both samples changes a little bit and re-mains at �99.6% and 99.7% of their original value throughout the measurement.Cell Reports Physical Science 5, 101836, March 20, 2024 11Figure 5. Capacitance retention test(A) Several loops of GCD curves of sample WMo-20 in the voltage range from 1.2 to �0.8 V at acurrent density of 1.8 mA/cm2. The orange arrow provides a guide to the variation according to theGCD loop number.(B) Retention rate of capacitance (black squares) and Coulombic efficiency (blue dots) of sampleWMo-20 as a function of cycle number, measured at a current density of 1.8 mA/cm2.(C) Several loops of GCD curves of sample WMo-90 at voltages ranging from 1.2 to �0.8 V at acurrent density of 2.8 mA/cm2. The orange arrow shows the evolution trend of the GCD curve withan increment of cycle number.(D) Capacitance retention (black squares) and Coulombic efficiency (blue dots) of sample WMo-90as a function of cycle number, measured at a current density of 2.8 mA/cm2. The red dashed-lineframe points to the region where the capacitance retention surpasses 100%. The insets toFigures 5B and 5D offer areal capacitances as a function of cycle number.llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836ArticleThe results imply a good retention ability of energy storage for nanoparticulate(WO3)87.5(MoO3)12.5 composite films. The GCD curves and areal capacitancesabove are measured in a three-electrode potentiostat. To check the areal capaci-tances of ECDs, we prepared a symmetric supercapacitive ESD (see Figure S13A)based on sample WMo-20. The GCD measurements at a current density of 0.1 mA/cm2 are depicted in Figure S13B. The GCD measurements are carried out in thepotential window ranging from �1.5 to 1.5 V. The areal capacitance of the sym-metric supercapacitive ECD is �18.1 mF/cm2, which is approximately one-fifth ofthe areal capacitance of the film measured in the potentiostat.Overall, the nanoporous (WO3)1-x(MoO3)x composite filmpresents promise for scalabil-ity toward both areal and gravimetric supercapacitive applications. This study is oneentry in the increasing efforts to design and implement high-capacity EC and ESDs.EXPERIMENTAL PROCEDURESResource availabilityLead contactFurther information and requests for resources and reagents should be directed to andwill be fulfilled by the corresponding author, Wen-Bin Jian (wbjian@nycu.edu.tw).12 Cell Reports Physical Science 5, 101836, March 20, 2024mailto:wbjian@nycu.edu.twllOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836ArticleMaterials availabilityThis study did not generate new unique materials.Data and code availabilityAll of the datasets used in this study are available within the article and are describedin the supplemental information or are available from the corresponding authorupon reasonable request. There was no code generated in this study.MoO3 and WO3 NPs suspension productionMoO3 andWO3 NPs were synthesized in deionized water by the electroexploding wiretechnique at a direct current voltage of 36 V using an NP generator (Metal Xano, Nano-vie, Taiwan). The concentrationof eitherMoO3orWO3NP suspensionwas estimated tobe �43 10�3 M and it was simply mixed to prepare (WO3)87.5(MoO3)12.5 suspensions.(WO3)1-x(MoO3)x composite film depositionThe mixed NP suspensions were used to deposit nanoporous, nanoparticulate filmson the FTO glass by a spray coater (Xano Spray Coating, Nanovie). The FTO glasses,having a thickness of 2.2 mm and a sheet resistance of 7 U=sq, were purchased fromRui Long Optoelectronics (Taiwan). During the spray-coating deposition, the FTOglass substrates were heated at 300�C on a hot plate and the spray nozzle waskept at a distance of 9 cm away from the substrate. The details about preparationsand characterizations of NPs and (WO3)1-x(MoO3)x composite films were describedin our previous paper.5WO3/MoO3 supercapacitor ESDThe fabrication of supercapacitive ESDs with (WO3)87.5(MoO3)12.5 composite films onthe negative electrode was described as follows. (WO3)87.5(MoO3)12.5 suspensionswere spray coated on an FTO glass substrate with an area of 4.0 3 3.5 cm2 thatwas used as a negative electrode for the supercapacitive ESDs. The coated FTOglasssubstrate was affixed on its four edges with frame spacers using double-sided acrylictape (Kinsten Industrial, Taiwan) with a thickness of 1 mm. Another empty FTO glasssubstrate was attached on the acrylic tape on the positive electrode to form a super-capacitive ESD. The edges of the ESD were sealed with a UV sealant (Padico, Japan)while leaving a small hole for the injectionof the electrolyte. Finally, a 1-MLiClO4+PCelectrolyte5 was injected into the ESD using a syringe mounted with a sharp needle.Theworking area of the ESDwas�4.03 3.5 cm2. The same procedure was applied tofabricate symmetric ESDs in which (WO3)87.5(MoO3)12.5 composite films were coatedon the two opposite FTO glass substrates of the symmetric ESD.CharacterizationMorphologies and lattice structures of WO3 and MoO3 NPs were inspected usingTEM (JEOL ARM200F, Japan). The thicknesses of the nanoparticulate (WO3)87.5(MoO3)12.5 films on the FTO glass substrates were measured using an SEM (HitachiSU-8010, Japan), and the surfaces of the films were inspected using an AFM(SPA300HV, Seiko Instruments, Japan). The Raman spectra of the nanoparticulatefilms on the FTO substrates were taken with a 532-nm laser source by a Raman spec-troscope (MRI, ProTrustTech, Taiwan). Crystalline structures were examined usingXRD (D8 Advance, Bruker AXS, Germany) with a Cu Ka radiation source (l =1.5406 Å). Surface electronic states of nanoparticulate films were collected usingXPS (PHI 5000 VersaProbe, UlVAC-PHI Instruments, Japan). CV and GCD measure-ments were performed in a three-electrode system using a potentiostat (SP-50 Po-tentiostat, BioLogic, France). Electrochemical impedance spectroscopy data werecollected using an electrochemical station (Autolab PGSTAT302N, Metrohm AG,Cell Reports Physical Science 5, 101836, March 20, 2024 13llOPEN ACCESSPlease cite this article in press as: Wei et al., Boosting areal capacitance in WO3-based supercapacitor materials by stacking nanoporous com-posite films, Cell Reports Physical Science (2024), https://doi.org/10.1016/j.xcrp.2024.101836ArticleSwitzerland). Pt meshes were used for the counterelectrode, and the reference elec-trode was the standard hydrogen electrode. The 1 M LiClO4 + PC electrolyte wasused in the potentiostat. The voltage scanning rates of 2, 5, 10, and 20 mV/s wereadopted in the CV measurements, and the current densities of 0.1, 0.2, 0.3, 0.5,1.0, and 1.5 mA/cm2 were applied in the GCD measurements.SUPPLEMENTAL INFORMATIONSupplemental information can be found online at https://doi.org/10.1016/j.xcrp.2024.101836.ACKNOWLEDGMENTSThis work was supported by the National Science and Technology Council, Taiwan,under grant nos. NSTC-111-2124-M-A49-008 and NSTC-111-2112-M-A49-038. Thiswork was also financially supported by the Center for the Semiconductor Technol-ogy Research from the Featured Areas Research Center Program within the frame-work of the Higher Education Sprout Project by the Ministry of Education, Taiwan,and supported in part by the National Science and Technology Council, Taiwan, un-der grant no. NSTC 111-2634-F-A49-008.AUTHOR CONTRIBUTIONSConceptualization, C.-C.W. and W.-B.J. Methodology, C.-C.W. and W.-B.J. Investi-gation, C.-C.W., P.-H.L., C.-E.H., Y.-L.L., S.B., and A.P.K. Formal analysis, C.-C.W.Validation, W.-B.J. Writing – original draft, C.-C.W. Writing – review & editing,W.-B.J. Resources, J.-T.C., C.-W.C., R.-A.D., and K.T. Project administration,W.-B.J. Supervision, K.T. Funding acquisition, W.-B.J. The manuscript was writtenthrough the contributions of all of the authors. All of the authors have given approvalto the final version of the manuscript.DECLARATION OF INTERESTSThe authors declare no competing interests.Received: October 20, 2023Revised: December 21, 2023Accepted: January 25, 2024Published: February 26, 2024REFERENCES1. González-Torres, M., Pérez-Lombard, L.,Coronel, J.F., Maestre, I.R., and Yan, D. (2022).A review on buildings energy information:Trends, end-uses, fuels and drivers. EnergyRep. 8, 626–637. https://doi.org/10.1016/j.egyr.2021.11.280.2. Simon, P., and Gogotsi, Y. (2008). Materials forelectrochemical capacitors. Nat. Mater. 7,845–854. https://doi.org/10.1038/nmat2297.3. Khaligh, A., and Li, Z. (2010). 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