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Dong Jiang, Ruibo Xu, Liang Bai, [Jonathan P. Hill](https://orcid.org/0000-0002-4229-5842), [Joel Henzie](https://orcid.org/0000-0002-9190-2645), Liyang Zhu, Wei Xia, Ran Bu, Yingji Zhao, Yunqing Kang, Takashi Hamada, [Renzhi Ma](https://orcid.org/0000-0001-7126-2006), Nagy Torad, Jie Wang, Toru Asahi, Xingtao Xu, [Yusuke Yamauchi](https://orcid.org/0000-0001-7854-927X)

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[Substrate Curvature‐Induced Regulation of Charge Distribution of Covalent Organic Frameworks Promotes Capacitive Deionization](https://mdr.nims.go.jp/datasets/fe4c00b5-c7b5-4461-92e2-cef182aae9ec)

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Substrate Curvature‐Induced Regulation of Charge Distribution of Covalent Organic Frameworks Promotes Capacitive DeionizationRESEARCH ARTICLEwww.afm-journal.deSubstrate Curvature-Induced Regulation of ChargeDistribution of Covalent Organic Frameworks PromotesCapacitive DeionizationDong Jiang, Ruibo Xu, Liang Bai, Jonathan P. Hill,* Joel Henzie, Liyang Zhu, Wei Xia,Ran Bu, Yingji Zhao, Yunqing Kang, Takashi Hamada, Renzhi Ma, Nagy Torad, Jie Wang,Toru Asahi, Xingtao Xu,* and Yusuke Yamauchi*Covalent organic frameworks (COFs) are promising high-performancecapacitive deionization (CDI) materials. Strategies to optimizeCDI performance of COFs focus largely on hybridization with conductivesubstrates, to improve their their intrinsically poor conductivity. A newstructure-function relationship between COFs and their substrates is proposedhere based on substrate-induced surface curvature. Graphene (zero-curvature)and carbon nanotubes (CNT, curved) are selected as COF growthsubstratesto assess the effect of curvature engineering effect on CDI performanceof TpPa-SO3H-COF. Ultrahigh ion (Na+) adsorption capacity (58.74 mg g−1) isachieved by CNT-COF hybrid (cf. compared to graphene-COF hybrid 34.20 mgg−1), demonstrating the significance of curvature engineering. Notably,the corresponding salt (NaCl) adsorption capacity of CNT-COF hybridreaches 149.25 mg g−1 in 1000 ppm at 1.2 V, representing state-of-the-artCDI performance, and the highest value among organic CDI electrodes.X-ray photoelectron spectroscopy and theoretical calculations subsequentlyreveal that substrate curvature can induce local strain, which regulates chargedistribution within the COF skeleton, causing a lower binding energy state forNa+ adsorption. Electrochemical quartz crystal microbalance measurementsrevealed faster Na+ adsorption kinetics of CNT-COF due to regulated chargedistribution within COF skeleton induced by substrate curvature. This workgives new insight into design of COF materials based on curvature engineering.D. Jiang, L. Zhu, Y. Zhao, Y. Kang, T. Hamada, Y. YamauchiDepartment of Materials Process EngineeringGraduate School of EngineeringNagoya UniversityFuro-cho, Chikusa-ku, Nagoya, Aichi 464-8603, JapanE-mail: y.yamauchi@uq.edu.auD. Jiang, L. Zhu, Y. Zhao, T. AsahiFaculty of Science and EngineeringWaseda University3-4-1 Okubo, Shinjuku, Tokyo 169-8555, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adfm.202407479© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly cited.DOI: 10.1002/adfm.2024074791. IntroductionThe growing demand for fresh water inagriculture, industry, and other sectors ne-cessitates the rapid development of tech-nologies that can be used to produce cleanwater from challenging sources includingseawater, wastewater, or brackish water.[1–5]Capacitive deionization (CDI) has emergedas a promising environmentally friendly,low-energy process to address the globalshortage of freshwater.[6,7] In contrast toother energy-intensive technologies such asreverse osmosis (RO), CDI removes ionsfrom saline water using a capacitor consist-ing of pairs of oppositely-charged electrodeswhere electrosorption[8] or redox reactionsoccur.[9,10] CDI is gaining traction as an al-ternative to established desalination tech-nologies. It is particularly effective for ap-plications at salt concentrations below 10 gL−1, such as in households or small busi-nesses where trained operators are unavail-able. CDI operates at low pressures andtemperatures, using voltages that are com-patible with standard consumer electricalR. Xu, L. Bai, X. XuMarine Science and Technology CollegeZhejiang Ocean UniversityZhoushan 316022, P. R. ChinaE-mail: xingtao.xu@zjou.edu.cnJ. P. Hill, J. Henzie, R. Ma, N. ToradResearch Center for Materials NanoarchitechtonicsNational Institute for Materials ScienceNamiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: jonathan.hill@nims.go.jpW. XiaSchool of Chemistry and Chemical EngineeringAnhui University of TechnologyMa’anshan 243032, P. R. ChinaAdv. Funct. Mater. 2024, 34, 2407479 2407479 (1 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbHhttp://www.afm-journal.demailto:y.yamauchi@uq.edu.auhttps://doi.org/10.1002/adfm.202407479http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/mailto:xingtao.xu@zjou.edu.cnmailto:jonathan.hill@nims.go.jpwww.advancedsciencenews.com www.afm-journal.deequipment, enhancing its applicability and ease of use.[9] Thedevelopment of new electrode materials, which are the criti-cal components of CDI systems promoting adsorption perfor-mance, is a highly active research area. Carbon-based materi-als, including activated carbon, carbide-derived carbons, carbonnanotubes (CNTs), and graphene have all been successfully ap-plied for the fabrication of CDI electrodes.[11–14] However, con-ventional porous carbon-based electrodes often suffer from ionadsorption capacity bottleneck issues resulting from a limitationof salt adsorption capacity (SAC) based on electric double layers(EDLs).[15–17] To overcome this limitation, the introduction of newelectrode materials having higher SACs, but which are also ca-pable of immobilizing ions by several mechanisms rather thansolely through electrosorption, is highly desirable.Electrode materials based on covalent organic frameworks(COFs), a versatile class of crystalline porous 2D or 3D poly-meric organic frameworks, are promising candidates for thenext generation of sustainable CDI systems with enhanced SACsbased on their molecular design flexibility, which permits tun-ing of their physical and chemical properties.[18–20] In contrastto traditional polymers which lack ordered porosity, COFs pos-sess high porosity, uniform pore sizes, and high structuralmodularity, which are advantageous in establishing appropri-ate electrochemical performances and improved diffusion ofmolecules/ions to active sites.[21–24] For these reasons, COFs arewidely studied in the fields of batteries, supercapacitors, andother energy storage applications. More recently, some pioneer-ing works have demonstrated the use of COFs for CDI electrodes,achieving SACs of 29.34 mg g−1 for covalent triazine-basedframeworks[25] and 22.8 mg g−1 for 2,6-diaminoanthraquinone-1,3,5-triformylphloroglucinol (Tp)-COF.[26] At this stage, theSACs of COFs can only be comparable to carbons, which is farbelow the expected value.Generally, most COFs have limited intrinsic conductivitieseven if bulk aggregation induced by 𝜋–𝜋 stacking is overcome byexfoliating COFs; this is recognized as the main reason causingpoor CDI performance. Hybridization of COFs with conductivesubstrates/additives has addressed this issue due to improvedelectron and mass transfer characteristics.[27] Many of theseworks are aimed at the composite engineering of COFs and car-bon, and revealed that the carbon substrate has a strong influenceon the physical and chemical properties of the COFs.[28–31] For ex-ample, a composite of a polyimide COF and CNT showed a 71%utilization of redox-active sites for Li-ion batteries, a great im-provement over the pure polyimide COF where a 5% utilizationR. BuState Key Laboratory of Chemical EngineeringInstitute of Pharmaceutical EngineeringCollege of Chemical and Biological EngineeringZhejiang UniversityHangzhou 310027, P. R. ChinaJ. Wang, Y. YamauchiAustralian Institute for Bioengineering and Nanotechnology (AIBN)The University of QueenslandBrisbane, QLD 4072, AustraliaY. YamauchiDepartment of Chemical and Biomolecular EngineeringYonsei UniversityYonsei-ro, Seodaemungu, Seoul 03722, South Koreaof active sites was found under the same conditions.[29] In addi-tion to the high electrical conductivity of CNT, another importantparameter leading to this enhancement may be the surface curva-ture of CNT. As a widely studied approach in inorganic electrocat-alysts, surface curvature engineering of active sites has demon-strated a novel structure-function relationship that enables bet-ter electrochemical reactions. Unfortunately, however, the relatedcorrelation between the structural characteristics of carbon tem-plates and the resulting properties of the COF composites has re-ceived limited attention thus far. A detailed structure-function re-lationship affecting the various properties still remains obscure.In this work, we demonstrate that substrate curvature has asignificant role in affecting the properties of COF (Figure 1).Graphene and CNT, possessing well-defined and uniform sp2-hybridized carbon frameworks,[32] are selected as model sub-strates here for the exploration of structure-function relation-ships of COF-based materials. In contrast to graphene, theunique feature of the curved CNT substrate imparts curvature tothe in situ grown COF skeleton, which in turn affects the chem-ical properties of the resulting composite.[33–35] As a result, theCNT-COF material exhibited a remarkable CDI performance in-cluding an ion (Na+) adsorption capacity (IAC (Na+)) of 58.74 mgg−1 and ion (Na+) adsorption rate (IAR (Na+)) of 1.96 mg g−1min−1 in 1000 ppm NaCl solution at 1.2 V, accompanied by a sub-stantial SAC of 149.25 mg g−1 for the whole cell. From a theoret-ical insight, such a performance improvement is ascribed to thefact that substrate curvature could induce local strain and regu-lation of charge distribution within the COF skeleton causing alower binding energy state for Na+ adsorption. Notably, this ul-trahigh Na+ adsorption capacity is significantly better than thoseof almost all the reported organic-based materials (For a compar-ison see Table S1, Supporting Information). This work demon-strates the significance of substrate curvature for the design ofsuperior CDI electrodes and also inspires further considerationof the structure-function relationship of COF-based materials.2. Results and Discussion2.1. Materials Design and CharacterizationTpPa-SO3H-COF was synthesized by the Schiff-base conden-sation reaction of Tp with 1,4-phenylenediamine-2-sulfonicacid (Pa-SO3H) to give a crystalline 2D COF in 91% yield(Figure S1 and see the experimental details in the Support-ing Information).[36] The condensation reaction was followed byan irreversible keto-enol tautomerization, which enhances thechemical stability and reduces the solubility of the COF.[37–39]Solvent plays a critical role in the crystallinity of the COFand the morphology of the corresponding CNT-COF core-shellstructure. After optimization, a mixed solvent system (1,4-dioxane/mesitylene = 1:4 (v/v)) containing 6 m acetic acid as acatalyst at 120 °C was found to yield the COF with suitable crys-tallinity or the CNT-COF composites having a well-formed core-shell structure. Different quantities of CNT were added to pro-duce TpPa-SO3H@CNT-X, where X = 10, 30, 50, and 70 wt.%of CNT. For this study, catalyst-free, multi-walled CNT were usedwith 50–60 nm outer diameter and a length range of 0.5–10 μm.In the presence of a carbon substrate, the COF tends to grow onthe surfaces of the CNT or graphene substrates induced by 𝜋–𝜋Adv. Funct. Mater. 2024, 34, 2407479 2407479 (2 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 1. Regulation of charge distribution of COF promotes CDI performance. a) Substrate curvature-induced engineering for the regulation of chargedistribution of COF, b) CDI process at the curvature COF-based electrode which presents the advantages of oxidation resistance, high electron conduc-tivity, and Na+ ion adsorption.interactions between the COF and the carbon substrates. Localinteractions between the curved CNT and the contacting COFlayer result in some distortion of the COF structure. Details of thestructure and morphologies of the different materials are shownin Figure 2a–h.The simulated structure of porous crystalline TpPa-SO3H-COFis shown in Figure 2a. In the structure, its hexagonal poresare stacked along the c-axis with some slight slippage from aneclipsed configuration. Fourier-transform infrared spectroscopy(FTIR) was used to confirm the successful condensation of TpPa-SO3H-COF and TpPa-SO3H@Carbon composites. As shown inFigure 2b, the absence of N-H stretching bands at 3429, 3403and 3337 cm−1 from the Pa-SO3H precursor in TpPa-SO3H-COFand any TpPa-SO3H@Carbon composites indicates the completeconsumption of Pa-SO3H in the polycondensation reactions. Thestrong formyl C═O vibration band observed for Tp (≈1650 cm−1)is almost absent in the TpPa-SO3H-COF, while the weak but ob-servable peak could be due to residual C═O bonds at the edgesof the structure. The peaks at 1282 and 1579 cm−1 in the spec-trum are due respectively to 𝛽-ketoenamine C─N moieties andC═C groups, suggesting that the COF exists in a 𝛽-ketoenamineform.[40,41] This result was further confirmed by the resonance(CP-MAS 13C NMR) measurements, showing the characteristicpeak at 184 ppm ascribed to the carbon atom of keto (─C═O)group (Figure S2, Supporting Information) Furthermore, X-rayphotoelectron spectroscopy (XPS) was conducted to study thebinding energy of different elements in TpPa-SO3H-COF, espe-cially for oxygen which is present at the main sodium adsorp-tion site in the framework. As shown in Figure S3 (SupportingInformation), the O 1s spectra for TpPa-SO3H-COF can be de-convoluted to two peaks centered at 530.24 and 531.87 eV, cor-responding respectively to C═O and S─O groups, respectively.Significantly, compared to TpPa-SO3H@Graphene, both peaks inTpPa-SO3H@CNT-50 show a shift to lower binding energy. Thisevidence indicates that the adsorption sites in the curved COFinduced by CNT could be more active for Na+ adsorption thanthose in TpPa-SO3H@Graphene. This will be discussed in moredetail below.Powder X-ray diffraction (PXRD) was used to confirm thestructure of the TpPa-SO3H-COF and its crystallinity in the TpPa-SO3H@Carbon composites. The experimental PXRD pattern ofTpPa-SO3H-COF closely matches the simulated PXRD pattern ofthe aligned A-A stacking model (Figure 2c). The characteristicdiffraction peaks at 2𝜃 = 4.6° and 26.2° are assigned to the (100)and (001) planes which represent the pore structure and layerstacking, respectively. The peak broadening and difference in thepeak intensity ratio between the experimental and simulated pat-terns may be due to the small particle size of the TpPa-SO3Hpowders and some deviation from a perfect crystalline structureand is consistent with previous reports.[42] The PXRD patternsof the TpPa-SO3H@Carbon composites show that the additionof the carbon substrate lowers the crystallinity of TpPa-SO3H-COF, with TpPa-SO3H@CNT-70 being the least crystalline ac-cording to PXRD data (Figure S4a, Supporting Information).The strong peaks ≈26.1° and 26.4° for TpPa-SO3H@CNT-50and TpPa-SO3H@Graphene represent the (002) plane for CNTand Graphene respectively (Figure S4b, Supporting Informa-tion). However, the PXRD patterns indicate that TpPa-SO3H-COF maintains the same structure in the COF@Carbon compos-ites. The morphologies of TpPa-SO3H-COF, TpPa-SO3H@CNT-X, and TpPa-SO3H@Graphene were observed by using ascanning electron microscope (SEM, Figure 2d–i; Figure S5,Adv. Funct. Mater. 2024, 34, 2407479 2407479 (3 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 2. Structural evolution characterization of TpPa-SO3H-COF, TpPa-SO3H@CNT-50 and TpPa-SO3H@Graphene a) the simulated structure ofTpPa-SO3H-COF. b) FTIR characterization of the building blocks for constructing COFs and the as-prepared TpPa-SO3H-COF, TpPa-SO3H@CNT-50,and TpPa-SO3H@Graphene.c) XRD patterns of the as-prepared TpPa-SO3H-COF. Red cycles: experimental PXRD pattern, blue line: computationalsimulation. d) SEM image of TpPaSO3H-COF. (e) HR-TEM image and corresponding FFT pattern of TpPa-SO3H-COF. f,g) SEM and TEM images ofTpPa-SO3H@Graphene. h,i) SEM and TEM images of TpPa-SO3H@CNT.Supporting Information). TpPa-SO3H-COF has a bulk solidrather featureless morphology while, in contrast, the TpPa-SO3H-COF appears to grow on the surfaces of the carbon substrateswhen CNT and graphene are present during synthesis leading tocorresponding plate-like and fibre-like morphologies. Tube-typecore-shell structures of TpPa-SO3H@CNT-X having the COFstructure grown around the external surface of the CNT couldbe observed (Figure 2h–I; Figure S6, Supporting Information).N2 adsorption-desorption isotherms were measured to in-vestigate the porosities of TpPa-SO3H-COF, TpPa-SO3H@CNT-50, and TpPa-SO3H@Graphene (Figure S7, Supporting Infor-mation). Compared to the pristine TpPa-SO3H (specific sur-face area = 220.62 m2 g−1), the specific surface area of TpPa-SO3H@CNT-50 decreases slightly to 195.66 m2 g−1. but in-creases to 317.67 m2 g−1 in TpPa-SO3H@Graphene. This differ-ence might be caused by the different properties of the CNT andgraphene substrates. The addition of CNT has a slight effect onthe specific surface area with values of 176.03, 196.13, 195.66,and 218.21 m2 g−1 found, respectively, for TpPa-SO3H@CNT-10, TpPa-SO3H@CNT-30, TpPa-SO3H@CNT-50 and TpPa-SO3-H@CNT-70.2.2. Sodium Storage Mechanism in the FrameworkIt is necessary to understand the sodium storage mechanism ofthe COF before studying their electrochemical properties andCDI performances. Hence, ex situ FTIR was performed to in-vestigate any structural evolution during charge/discharge pro-cesses involving sodium ions. The scheme shown in Figure 3aindicates that the Na+ insertion mechanism in pure TpPa-SO3H-COF involves the transportation of six electrons and one proton,thus offering seven Na+ charge/discharge active sites. As shownin Figure 3b, there are three different active sites for Na+ adsorp-tion in the COF structure. A new peak ≈1710 cm−1 after sodiumadsorption suggests the formation of -SO3Na, which is a resultof the cation exchange reaction during the capture processes.[43]The carbonyl O atoms of the 𝛽-keto units receive electrons andAdv. Funct. Mater. 2024, 34, 2407479 2407479 (4 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 3. Confirmation of the sodium adsorption sites in the TpPa-SO3H-COF structure. a) Na+ insertion mechanism of pure TpPa-SO3H-COF. b) FTIRcharacterization of TpPa-SO3H-COF before and after sodium ions adsorption. c,d) XPS spectra of the C 1s region of TpPa-SO3H-COF before and aftersodium ions adsorption. e) CV curves of TpPa-SO3H-COF, TpPa-SO3H@CNT-50, and TpPa-SO3H@Graphene. f) Capacity based on CV curves of CVcurves of TpPa-SO3H-COF, TpPa-SO3H@CNT-50, and TpPa-SO3H@Graphene.can coordinate sodium ions, as indicated by the new peak ≈1500cm−1 after Na+ adsorption, and which also further form 𝛼-C radi-cals stabilized by the enamine units and aromatic rings. The elec-trophilic 𝛼-C radicals subsequently accept additional electronsto form 𝛼-C anions, which can bind further sodium ions (1410cm−1).To investigate in more detail, X-ray photoelectron spectroscopy(XPS) spectra of TpPa-SO3H-COF were collected prior to and fol-lowing Na+ adsorption. As shown in Figure 3c,d, the C 1s spectraare deconvoluted into five peaks at 288.14, 284.73, 283.83, 283.03,and 283.71 eV for the as-prepared TpPa-SO3H-COF electrodes,assigned to C═O, C─S, C─C, C═C, and C─N bonding modes, re-spectively. After sodium insertion, the carbon-oxygen bond shiftsto 287.42 eV, implying a reduction of C═O to C─O, which is con-sistent with results obtained from FTIR spectra.[44] Also, an ad-ditional peak emerges at 289.63 eV, assigned to 𝛼-C – Na inter-action, which results from the insertion of sodium ions at the Catoms of 𝛽-ketoenamine groups.[45] These observations are con-sistent with previously reported observations occurring duringsodium ion charge/discharge at organic electrodes.[46,47]2.3. Electrochemical CharacterizationThe electrochemical properties of TpPa-SO3H-COF and the re-lated carbon hybrids were first investigated by cyclic voltamme-try (CV) in the −0.5 –0.5 V range. The three-electrode systemused comprises an aqueous 1 m NaCl solution as the electrolyte,a Pt electrode as the counter electrode, and a saturated calomelelectrode as the reference electrode. The CV curves in Figure 3econtain a pair of quasi-reversible redox peaks for the TpPa-SO3H-COF and TpPa-SO3H@Carbon hybrid structures. Thesepeaks correspond to redox processes of the COF structure in-volving the Na+ ions of the NaCl electrolyte as mentioned above.The adsorption sites in the COF structure confer excellent re-versible charge/discharge of sodium ions on the COF and TpPa-SO3H@Carbon hybrid structures, making them suitable for po-tential CDI applications. Additionally, the redox peak of TpPa-SO3H@CNT-50 shifts to zero potential compared with that ofTpPa-SO3H@Graphene and pristine TpPa-SO3H COF, indicat-ing easier activation of the adsorption sites in TpPa-SO3H@CNT-50. Furthermore, the integrated area of the CV curve of the TpPa-SO3H@CNT-50 hybrid is larger than those of pristine COF andTpPa-SO3H@Graphene, which suggests that the CNT-inducedcurved TpPa-SO3H COF will have improved capacitive perfor-mance and higher capacitance (Figure 3f). We further sum-marized the surface-controlled/diffusion-controlled processes incharge storage from the CV curves (Figure S8, Supporting Infor-mation). Obviously, as the scan rate increases, there is a corre-sponding increase in capacitive contributions for all of the sam-ples. Notably, the capacitive contribution of TpPa-SO3H@CNT-50 is higher than that of TpPa-SO3H@Graphene and TpPa-SO3H-COF, verifying that there is greater surface-controlled ca-pacitive behavior generated by the substrate curvature-inducedregulation of charge distribution of the framework. Galvano-static tests of TpPa-SO3H-COF, TpPa-SO3H@CNT, and TpPa-SO3H@Graphene at various rates were used to further study thecharge/discharge process for these electrodes. The GCD curvesare consistent with the CV curves as the TpPa-SO3H@CNT-50 electrode achieves the longest discharge time indicating theAdv. Funct. Mater. 2024, 34, 2407479 2407479 (5 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 4. CDI performance of TpPa-SO3H-COF, TpPa-SO3H@CNT-50 and TpPa-SO3H@Graphene. (a) Temporal variations in IAC (Na+) of the ma-terials, and (b) the corresponding Ragone plots for TpPa-SO3H-COF, TpPa-SO3H@CNT-50 and TpPa-SO3H@Graphene in 500 ppm (8.55 mM) NaClsolution. (c) Sodium chloride concentration dependency of CDI performance. (d) Cycling performance in 500 ppm (8.55 mM) NaCl solutions. (e) Com-parison of the desalination capacity of TpPa-SO3H@CNT-50 with other organic materials or carbon-based electrodes (The listed samples are listed inTable S1, Supporting Information).highest capacitance compared to the pure TpPa-SO3H-COF andTpPa-SO3H@Graphene electrodes (Figure S9, Supporting Infor-mation). Electrochemical impedance spectra (EIS) further revealthe merits of curvature engineering on the capacitive behavior.As shown in Figure S10 (Supporting Information), the fitted linefor Z’ versus square root of frequency (𝜔1/2) in the low-frequencyregion shows a steeper gradient for the curved TpPa-SO3H-COFinduced by CNT, indicating it has lower resistivity than both pureCOF and then COF@Graphene hybrid.[48] This means that TpPa-SO3H@CNT-50 has a larger ion diffusion rate again suggestingit is a promising candidate for capacitive deionization.2.4. CDI PerformanceAs a proof-of-concept, asymmetric CDI devices containing ei-ther of the active materials TpPa-SO3H@CNT-50 or TpPa-SO3H@Graphene as the cathode for Na+ capture, with AC asthe anode for Cl− capture, were constructed. For comparison,CDI cells using CNT or Graphene as one electrode were alsooperated under similar conditions., The asymmetric CDI con-figuration more easily achieved improved desalination perfor-mance than the symmetric CDI configuration due to a highersafe operating voltage and greater flexibility in the choice of elec-trode materials for higher Na+ or Cl− selectivity. These featuresalso make it more suitable for the in-depth study of the cationcharge/discharge process in the cathode. The desalination per-formance of all devices was conducted at an initial NaCl con-centration of 500 ppm at 1.2 V, which is an acceptable operatingvoltage for asymmetric CDI cells. The corresponding conductiv-ity and current profiles are shown in Figure S11 (Supporting In-formation). During the CDI process, no bubbles were observed,indicating that no side reactions such as chlorine generation orhydrogen evolution were occurring. The influence of the thick-ness of the COF on its CDI performance was investigated ini-tially. As shown in Figure S12 (Supporting Information), TpPa-SO3H@CNT-50 shows the highest Na+ capacity with an IAC of44.79 mg g−1 within 30 min. Further decreasing the thicknessof COF by adding more CNT should not increase the overall ca-pacity. However, it is worth noting that the materials with thethinnest COF components display higher adsorption rates withinthe first few minutes. This could be due to better electrochemicalaccessibility of active sites for the materials having thinner COFouter layers on the CNT.[37] After optimizing the effects of thick-ness on the CDI performance, the effects of curvature engineer-ing on performance were investigated. Figure 4 shows the desali-nation performances of TpPa-SO3H-COF, TpPa-SO3H@CNT-50 and TpPa-SO3H@Graphene. TpPa-SO3H@CNT-50 exhibits ahigh IAC (Na+) of 44.79 mg g−1, which is much greater than thosefound for TpPa-SO3H@Graphene (28.12 mg g−1) and pure TpPa-SO3H-COF (20.04 mg g−1) (Figure 4a). For the whole CDI cell,the TpPa-SO3H@CNT-50 also shows a high SAC of 113.8 mg g−1,far exceeding those of TpPa-SO3H-COF (50.92 mg g−1) and TpPa-SO3H@Graphene (71.45 mg g−1) (Figure S13, Supporting Infor-mation). As shown in Figure 4b, the corresponding CDI Ragoneplots of TpPa-SO3H@CNT-50 shift toward the upper right regionsuggesting higher IAC (Na+) and faster IAR (Na+) than thoseof pristine TpPa-SO3H-COF and TpPa-SO3H@Graphene, high-lighting the enhancement of CDI performance in curvature en-gineered COF with a carbon substrate. In contrast, the carbonAdv. Funct. Mater. 2024, 34, 2407479 2407479 (6 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.desubstrate displays limited CDI performance, with SAC value of8.1 mg g−1 for Graphene and 6.2 mg g−1 for CNT (Figure S14,Supporting Information).It is important to achieve excellent CDI performance over awide range of concentrations because real-world feedwaters forpractical CDI applications are usually brines of varying ionicstrengths. Therefore, the CDI performance of TpPa-SO3H-COF,TpPa-SO3H@CNT-50, and TpPa-SO3H@Graphene were furtherinvestigated using NaCl solutions having concentrations in therange from 50 to 1000 ppm, corresponding to a NaCl salinitywindow of 0.86 to 17.11 mM. As expected, all three samples ex-hibited increasing IAC values with increasing NaCl concentra-tion. In particular, TpPa-SO3H@CNT-50 exhibits an ultrahighIAC (Na+) of 58.74 mg g−1 and IAR (Na+) of 1.96 mg g−1 min−1in 1000 ppm NaCl solution, together with a very high SAC (NaCl)of 149.25 mg g−1 for the whole cell (Figure 4c; Figure S15, Sup-porting Information). This performance is significantly higherthan those of TaPa-SO3H-COF (IAC (Na+): 24.8 mg g−1; SAC(NaCl): 63.01 mg g−1) and TpPa-SO3H@Graphene (IAC (Na+):34.20 mg g−1; SAC (NaCl): 86.87 mg g−1). We further conductedthe CDI test for TpPa-SO3H-COF, TpPa-SO3H@CNT-50, andTpPa-SO3H@Graphene at various voltages. With an increase inthe operating voltage, the sodium adsorption capacity of TpPa-SO3H-COF, TpPa-SO3H@CNT-50, and TpPa-SO3H@Grapheneall increase, with the TpPa-SO3H@CNT-50 hybrid having consis-tently the highest capacity among these electrodes (Figure S16,Supporting Information). Long-term cycling stability is also animportant criterion for estimating practical performance. Here,the long-term cycling property of the optimized hybrid, TpPa-SO3H@CNT-50, was further evaluated at 1.2 V. For compar-ison, pure TpPa-SO3H-COF and TpPa-SO3H@Graphene werealso studied under similar conditions. As shown in Figure 4d,all samples possess excellent cycling stability without obviousdeterioration of performance even after 200 cycles. In addi-tion, SEM images of the samples obtained after cycling (FigureS17, Supporting Information) indicate that all samples main-tain their nanostructure even after long-term cycling. PXRD wasused to further confirm that TpPa-SO3H, TpPa-SO3H@CNT-50, and TpPa-SO3H@Graphene retained their crystallinity afterthe long-cycling operation. (Figure S18, Supporting Information)Notably, this ultrahigh Na+ adsorption capacity is significantlybetter than those of almost all the reported organic-based ma-terials (Figure 4e).2.5. Discussion of Curvature Effects on CDI PerformanceTo gain a deeper insight into the origin of the enhancementof desalination properties of the curved TpPa-SO3H@CNT-50,both experimental analysis and theoretical calculations were con-ducted for the sodium insertion process in TpPa-SO3H@CNT-50 and TpPa-SO3H@Graphene. Ex situ XPS was used to studyany variations in the states of the elements within the mate-rials prior to and following sodium insertion taking particu-lar note of changes in O atoms, the main adsorption sites forNa+. As mentioned previously, the bonding energy of C═O andS─O in TpPa-SO3H@CNT-50 shows a shift to a lower energystate (C═O: 531.87 eV, S─O: 530.23 eV) compared to those ofTpPa-SO3H@Graphene (C═O: 532.50 eV, S250O: 530.65 eV)(Figure 5a), indicating the greater activity of the O sites forsodium ion adsorption in the curved COF grown on CNT. Thischange is consistent with the CV analysis where the redox peakof TpPa-SO3H@CNT-50 shifts to zero potential compared to thatof TpPa-SO3H@Graphene, indicating an easier activation of theadsorption sites in TpPa-SO3H@CNT-50 (Figure S10a, Support-ing Information). Following the insertion of sodium ions, C═Oand S─O in TpPa-SO3H@CNT-50 maintain a lower binding en-ergy state than those in TpPa-SO3H@Graphene. Notably, twonew peaks emerge representing S─O─Na and C─O─Na, indicat-ing the sodium insertion process involving these groups in theframework. Due to the different adsorption activity induced bythe local strain effect of CNT and graphene, the binding energyof the new S-O-Na and C-O-Na peaks for TpPa-SO3H@CNT-50(S-O-Na: 532.79 eV, C─O─Na: 535.22 eV) also present at lower en-ergy than the corresponding groups in TpPa-SO3H@Graphene(S-O-Na: 533.11 eV, C─O─Na: 535.8 eV) (Figure 5b). This sug-gests a higher Na+ charge/discharge activity for S-O-Na and C-O-Na in the former case (TpPa-SO3H@CNT-50). This has beenconfirmed by considering the EIS data analysis mentioned previ-ously in this article (Figure S10f, Supporting Information). Vari-ations in the density-of-states at the Fermi level occurring in thepores of TpPa-SO3H growing on the different carbon templates,and the structural stability, were further investigated by usingVASP AIMD simulations. We note that the density-of-state atthe Fermi level is predominantly attributed to the p orbitals ofcarbon (C) and oxygen (O) atoms contained in the frameworks.As shown in Figure 5a,b, O and C play a central role in estab-lishing connections with Na+ during the sodium insertion pro-cess. This highlights the significance of the specific interactionsof O and 𝛼-C of 𝛽-ketoenamine with sodium ions. The overalldensity-of-states of curved TpPa-SO3H on CNT is obviously lowerthan that of flat TpPa-SO3H layered on graphene. This furtherconfirms that curved COFs offer increased Na+ adsorption ac-tivity over flat COF, highlighting the importance of local strainand curvature effects on CDI performance. The theoretical cal-culations are thus consistent with the experimental ex situ XPSanalysis.For further discussing the desalination mechanism of the ac-tive materials, the mass transport during the Na+ adsorption pro-cess was discussed via the in situ electrochemical quartz crystalmicrobalance (EQCM) measurement which has been applied toquantify mass changes during the electrochemical energy stor-age field. The active material to be interrogated was coated ontoan Au-coated quartz crystal sensor and used as the working elec-trode in a three-electrode cell. An increase in electrode masscauses a decrease in the crystal’s oscillation frequency accordingto the Sauerbrey equation.[49] Figure 6 shows the EQCM responsefor the third overtone of the TpPa-SO3H-COF and correspond-ing hybrid materials-coated Au-sensor immersed in 500 ppmNaCl aqueous solution during chronoamperometry scan. TheEQCM response tracks well with the charge process. Comparedto pure TpPa-SO3H-COF and TpPa-SO3H@Graphene, TpPa-SO3H@CNT displays the largest shift in both integrated charge(Q) and frequency (f3), indicating the largest theoretical Na+ ad-sorption capacity and actual total mass increase. The EQCM fur-ther confirms that the curved COF shows a great advantage forthe Na+ adsorption in terms of dynamics, agreeing with ex situXPS analysis and theoretical calculations results.Adv. Funct. Mater. 2024, 34, 2407479 2407479 (7 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 5. Illustration of the importance of local strain and curve effects on Na+ insertion properties. XPS spectra of the O 1s region of a)TpPa-SO3H@CNT-50 and b) TpPa-SO3H@Graphene before and after sodium ions adsorption. Demonstration of density of state of c,e) TpPa-SO3H@Graphene and d,f) TpPa-SO3H@CNT-50 before and after sodium ions adsorption.Figure 6. In situ EQCM analysis of TpPa-SO3H-COF, TpPa-SO3H@CNT-50, and TpPa-SO3H@Graphene during chronoamperometry scan at the poten-tial of 1.0 V versus Ag/AgCl in 500 ppm NaCl aqueous solution. a) Integrated charge and b) f3 responses.Adv. Funct. Mater. 2024, 34, 2407479 2407479 (8 of 10) © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.de3. ConclusionIn summary, a curved COF layer formed by controlled growthon CNT template has been designed and constructed for de-liberate enhancement of the desalination performance and tostudy the effect of COF curvature on CDI performance. Adetailed structure-function relationship underlying the diverseNa+ charge/discharge properties is demonstrated. CNT-inducedcurved COF shows easier activation for the insertion of sodiumions compared to flat COF grown on graphene. This importantfeature is confirmed by applying ex situ X-ray photoelectron spec-troscopy analysis. The overall density-of-states of curved TpPa-SO3H on CNT is lower than that of flat TpPa-SO3H layered ongraphene, further confirming that curved COF has better Na+adsorption activity than flat COF, in turn highlighting the impor-tance of local strain and curvature effects on CDI performance.Local strain induces lower binding energy states of the adsorp-tion sites in the COF structures, thus facilitating the sodium in-sertion process. Thus, curved COF materials show remarkableCDI performances with large ion adsorption capacity for Na+(IAC (Na+)) of 58.74 mg g−1 and high Na+ adsorption rate (IAR(Na+)) of 1.96 mg g−1 min−1 in 1000 ppm NaCl solution at 1.2 V,as well as a very high salt (NaCl) adsorption capacity (SAC (NaCl))of 149.25 mg g−1 for the whole cell. This ultrahigh Na+ adsorp-tion capacity is significantly greater than that of the control flatCOF materials that include a COF@Graphene hybrid and pris-tine COF. This discovery will open new avenues to design en-hanced activity COFs or other conventional 2D materials for ca-pacitive deionization applications.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsD.J. and R.X. contributed equally to this work. The authors acknowledgefinancial support from the JST-ERATO Yamauchi Materials Space-Tectonics Project (JPMJER2003), the ARC Australian Laureate Fellowship(FL230100095), and the UQ-Yonsei International Joint Research Project.This work used the Queensland node of the NCRIS-enabled AustralianNational Fabrication Facility (ANFF). Y.K. thanks the support from JSPSPostdoctoral Fellowships for Research in Japan. 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Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 45, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202407479 by National Institute For, Wiley Online Library on [13/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.de Substrate Curvature-Induced Regulation of Charge Distribution of Covalent Organic Frameworks Promotes Capacitive Deionization 1. Introduction 2. Results and Discussion 2.1. Materials Design and Characterization 2.2. Sodium Storage Mechanism in the Framework 2.3. Electrochemical Characterization 2.4. CDI Performance 2.5. Discussion of Curvature Effects on CDI Performance 3. Conclusion Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords