Chemistry - A European Journal 10.1002/chem.202005032

COMMUNICATION Liquid-like phase of N, N-dimethylpyrrolidinium iodide impregnated into COFs endows fast lithium ion conduction in the solid state Zhenzhen Wu,[a,b] Qinchao Xu,[c] Juan Li,[a*] and Xian-Ming Zhang[a,d*]

[a] Zhenzhen Wu, Juan Li, and Prof. Dr. Xian-Ming Zhang Shanxi University Institute of Crystalline Materials Wucheng Rd., No. 92, Taiyuan 030006, China E-mail: [email protected] [b] Zhenzhen Wu Shanxi University Institute of Molecular Science [c] Qinchao Xu Chinese Academy of Science State key laboratory of Coal Conversion, Institute of Coal Chemistry 19 Kangle Street, Taiyuan 030001, China [d] Prof. Dr. Xian-Ming Zhang Shanxi Normal University School of Chemistry & Material Science 1 Gongyuan Street, Linfen 041004, P. R. China E-mail: [email protected]

Abstract: A novel kind of solid-state lithium electrolyte was fabricated by impregnating organic ionic plastic crystals (OIPCs) into the pores of covalent organic frameworks (COFs). The liquid-like phase of confined N,N-dimethylpyrrolidinium iodide (P1,1I) and the ordered nanochannels of COFs simultaneously stimulated the lithium ion conduction.

Organic ionic plastic crystals (OIPCs) are interesting solid state electrolyte materials for lithium batteries because of their special properties, including their thermal and electrochemical stability, fast ion transport and flexibility for chemical design. The key for Manuscript their good conductivity is their multiple reversible solid–solid phase transitions that often have larger entropy or enthalpy of Scheme 1. Schematic view of LiI@P1,1I impregnated into COFs and transition than those of the final melting transition. [1-11] The chemical structures of P1,1I and Tp-DB-COF. lithium ions can easily move via the vacancies or defects in the disordered domains. Typically, the doped lithium salt shares the same anion as OIPC and Li+ is generally considered to substitute for the plastic crystal cation within the OIPC lattice in Dry powder of P1,1I loaded COF, named after P1,1I@COFs was these systems. Doping small amount of lithium salt can lead to obtained by complete removal of methanol solvent under remarkable improvement in the ion conductivity of OIPC. vacuum. Different loading amounts were adopted to optimize the Detailed researches have proven that lithium-ion conduction was electrolyte components. The electrolytes P1,1I@COFs were the primary cause of this enhancement, which unmasked that named DMPI@COF. DMPI20@COF, DMPI40@COF, the majority of the ionic conductivity coming from Li+ ions. [12,13] DMPI60@COF, DMPI80@COF, and DMPI100@COF were In the past few years, covalent organic frameworks (COFs) have named according to the different proportions of P1,1I occupying been developed as an ideal platform to design fast ion the pore volume of Tp-DB-COF, respectively (Figure S1). conductors by the virtue of their ordered open nanochannels. [14- The powder X-ray diffraction (PXRD) patterns of the as- 21] It is found that introducing poly(ethylene oxide) (PEO) into the synthesized Tp-DB-COF and electrolyte DMPI@COFs are pores of COFs through physical or chemical methods can shown in Figure 1a. The peaks at 3.3° belong to (1 0 0) plane of Accepted induce PEO to change their phase transition behaviors Tp-DB-COF. The TEM image of Tp-DB-COF showed a large significantly why lithium ions can fasten their migration through area of regular crystal domains with oriented crystal lattice the liquid like phase. [18-21] Inspired by the phase-transition (Figure S2). After immobilization of P1,1I, the reflection peaks of behavior of PEO@COFs systems, we propose herein the Tp-DB-COF gradually disappeared and the intensity of the P1,1I loading of OIPCs based lithium electrolyte into the pore of COFs reflection peaks increased at the same time. The diffraction to fabricate a novel class of solid state lithium ion conductor. We pattern of DMPI20@COF and DMPI40@COF appeared to be fundamentally the same as that of the starting Tp-DB-COF, choose N,N-dimethylpyrrolidinium iodide (P1,1I), one of the OIPCs with two typical rotator phases here for its non-melting indicating that the framework structure of Tp-DB-COF does not characteristic and the unique static characteristic of iodide in decompose after loading the P1,1I salt. Apparent differences in [11,22,23] the relative intensities of the DMPI and Tp-DB-COF spectra P1,1I lattice. A robust COF with abundant carboxyl groups [24] indicates the presence of guest salts in the pore space of Tp- (Tp-DB-COF) was chosen to load P1,1I electrolytes. DB-COFs, similar to the reported works on ionic liquids included

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A pre-synthetic strategy to construct single ion conductive covalent organic frameworks† Cite this: Chem. Commun., 2020, 56, 2747 Juan Li, *a Fu-Qiang Zhang,b Falian Li,a Zhenzhen Wu,a Canliang Ma,ac Received 17th January 2020, Qinchao Xu,d Pengfei Wangd and Xian-Ming Zhang*abe Accepted 29th January 2020

DOI: 10.1039/d0cc00454e

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A pre-synthetic strategy was developed for the construction of As a novel class of porous crystalline solids, covalent organic single ion conductive covalent organic frameworks (COFs). A high frameworks (COFs) are excellent platforms for exploring single- Li+ conductivity of 1.6 103 Scm1 at 273 K was achieved, and ion conductors, which benefit from their chemical stability and single Na+ and K+ COFs were also obtained by using Na+ and K+ salts flexibility. Their ordered open channels would favor fast ion as monomers according to this synthetic method. migration in principle. Very recently, post-synthesized loading, modification and organometallic methods have been used to Crystalline porous materials, such as covalent organic frameworks construct single lithium MOFs, and good performance like (COFs), metal–organic frameworks (MOFs), zeolitic imidazolate 7.4 103 Scm1 has been achieved in those single-ion frameworks (ZIFs) and cages, have been reported as an advanced conductive MOFs and COFs,10–13,34–42 which nearly reached platform to design fast ion conductors over bulk ion conduction the practical application standard. However, we found that compounds.1–13 Their highly ordered nanochannels enable facile most of the crystalline porous single lithium conductors were electrolyte penetration and fast ion transportation through synthesized by using a post-synthetic method, but pre-synthetic them. In addition, the X-ray diffraction technique combined methods for single Li+ COFs have rarely been reported. with theoretical simulation methods can provide strong evidence In this study, we propose a pre-synthetic strategy to synthesize to establish their structure–performance relationships.2,9,14–24 single Li+ COFs by using lithium salt (lithium 2,5-diaminobenzene-

Their electronically insulating framework, well-defined pore sulfonate, PaSO3Li) as the monomer. A very small amount of acetic structure, and tunable surface polarities also offer them infinite acid was used as the catalyst to avoid the possible H+/Li+ exchange Published on 29 January 2020. Downloaded by Shanxi University 5/15/2020 12:26:40 PM. possibilities for their application as candidate ion conductors in problem during the synthesis process (Scheme 1). We first synthe- 2,18–23 solid state electrolytes. sized the lithium salt PaSO3Li through the acid–base reaction of Accumulation of anions at the anode is one of the primary 2,5-diaminobenzenesulfonic acid (PaSO3H) with lithium hydroxide culprits for decreasing lithium ion battery performance over (LiOH). The complete conversion to the lithium salt was verified 1 time. In single ion solid conductors, the anions are fixed to by the H-NMR spectrum. Then PaSO3Li and triformylphloro- the underlying matrix, which prevents the sedimentation of glucinol (Tp) were condensed to form a lithium salt-based COF

anions and eliminates the polarization effects present in dual (Tp-PaSO3Li-COF) through keto–enamine covalent bonds in a 25–28 ion electrolytes. Different matrixes such as polymers and solvothermal environment. The chemical structure of Tp-PaSO3Li- silicas have been adopted to construct single lithium ions.29–33 COF was characterized by solid-state 13CNMRspectrometry,FT-IR But the uncontrollability and great difficulties in synthesis spectroscopy, and powder X-ray diffraction (PXRD) analysis. In the limited the development of single Li+ conductors. 13C NMR spectrum in Fig. 1G, a sharp signal at d = 184.38 ppm (a) corresponds to the carbonyl carbon atom of the b-ketoenamine

a linker. The absence of N–H stretching peaks (n = 3367 and Institute of Crystalline Materials, Shanxi University, Wucheng Rd, No. 92, 1 Taiyuan 030006, China. E-mail: [email protected], [email protected] 3293 cm ) due to the free diamine –NH2 group and of a peak 1 b Key Laboratory of Magnetic Molecules & Magnetic Information Materials due to the aldehyde group –CHO (n =1645cm )ofTpindicated (Ministry of Education), Shanxi Normal University, Linfen, 041004, China complete consumption of the reactants. The appearance of new c Institute of Molecular Science, Shanxi University, 030006, Taiyuan, China peaks at n = 1583 and 1234 cm1 could be ascribed to CQCand d Institute of Coal Chemistry, Chinese Academy of Sciences, 030001 Taiyuan, China C–N stretching bands (Fig. 1F). e Institute of Chemistry and Culture, School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, China Thermogravimetric analysis (TGA) revealed that Tp-PaSO3Li-COF † Electronic supplementary information (ESI) available. See DOI: 10.1039/ showed no decomposition up to 400 1C under a nitrogen atmo- d0cc00454e sphere (Fig. S1, ESI†). The permanent porosity of Tp-PaSO3Li-COF

This journal is © The Royal Society of Chemistry 2020 Chem. Commun., 2020, 56, 2747--2750 | 2747 NJC

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Layered-structure microporous poly(benzimidazole)-loaded imidazole for Cite this: New J. Chem., 2018, 42,1604 non-aqueous proton conduction†

Received 2nd November 2017, ab a a a a Accepted 27th December 2017 Juan Li, * Zhenzhen Wu, Hong Li, Heng Liang and Shuaishuai Li

DOI: 10.1039/c7nj04239f

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In this work, a microporous polybenzimidazole was designed and compared to nonporous one.2,7 However, due to dehydration, synthesised. By loading imidazole into its nanospace, we obtained phosphoric acid inside the pores can easily convert to pyropho- proton conductivity of 2.4 103 Scm1 at 130 8C under anhydrous sphate under high temperature, which impedes this adducts’ conditions, which proves that the imidazole loaded porous poly- conducting performance. benzimidazole is a superionic conductor. Heterogeneous hybridisation of proton-conductive molecules (or polymers) and solid supports has proven to be an effective Proton-exchange membrane fuel cells (PEMFCs) are a promising approach for realising anhydrousprotonconduction.Theuseof alternative technology for clean energy. For example, they have porous materials such as silicas,8–10 porous coordinative polymers/ been used in electric vehicles, portable electronic devices and metal organic frameworks (PCPs/MOFs),11–14 covalent organic residential power generators. A crucial component PEMFCs is frameworks (COFs)15 as the supporting matrix has been investi- proton-conducting solids: they play a key role in fast proton gated in detail. Their porosity enhances the mobility of adsorbed transfer. A state-of-the-art proton conductor is a perfluoro- organic aryl molecules such as histamine, benzimidazole, 12 sulphonic acid polymer, Nafion. Its proton conductivity is as triazole and imidazole. The composites His@[Al(OH)(ndc)]n 1 16 high as 0.1 S cm at high relative humidity. However, its proton and [Zn3(H2PO4)6(H2O)3](Hbim) have proton conductivities of conductivity sharply decreases at temperatures above 80 1C 1.7 103 Scm1 at 150 1C and 1.3 103 Scm1 at 120 1C, because of loss of continuity of hydrate proton pathways in the respectively, under absolutely anhydrous conditions. A mesoporous membrane.1,2 Membranes with high proton conductivities at COF (TPB-DMTP) loaded with imidazole has a considerably Published on 17 January 2018. Downloaded by Shanxi University 5/15/2020 12:26:46 PM. low humidity are required for practical applications, such as high conductivity of 4.37 103 Scm1.14 Nevertheless, operation at elevated temperatures.3 They can increase catalytic suffered from the chemical and hydrothermal instability, COFs activity, simplify water and heat management, and provide good and PCPs/MOFs have limited applications to proton conductors. tolerance of the Pt catalysts against CO poisoning. Developing Recently, a mesoporous tetrahedral polyimide loaded imidazole viable proton conducting solid with high thermal conductivity exhibited good proton conductivity at a wide temperature and high proton conductivity above 120 1Cisthereforeamajor range.17 Considering the recent progress in hybrid proton con- challenge. ductors, developing novel guest-loaded proton conductors with Presently, the most well-known high temperature PEM is high conductivity is of significance. based on the PBI (polybenzimidazole) materials.4–6 In the PBI In this work, we designed a new polybenzimidazole (Tp-DADMB) structure, benzimidazole units provide the base sites to load with two-dimensional microporous structure. In order to fabricate

guest molecule phosphoric acid (H3PO4) through acid–base anhydrous proton conductor, we introduced imidazole into its reaction. It was shown that the introduction of a defined nanochannels. Through this hybridisation method, we obtained

nanosized biphasic structure into the PBI/H3PO4 system could the composite Im@Tp-DADMB, which exhibited high proton lead to a considerable enhancement in the proton conductivity conductivity of 2.4 103 Scm1 at 130 1C under completely anhydrous conditions. This conductivity makes it suitable as a a Institute of Crystalline Materials, School of Chemistry & Chemical Engineering, PEM (proton exchange membrane) material. Shanxi University, Taiyuan 030006, China. E-mail: [email protected] Tp-DADMB was synthesized according to the typical solvo- b State Key Laboratory of Molecular Engineering of Polymers, Fudan University, thermal method,18 using 4,7-diamino-5,6-dimethyl-benzimidazole Shanghai, China † Electronic supplementary information (ESI) available: Experimental, PXRD, (DADMB) and triformylphloroglucinol (Tp) as the monomers, TGA, XPS, pore size distribution, characterization methods; pore size distribution and 6 M acetic acid as the catalyst. The schemes for synthesis is are provided. See DOI: 10.1039/c7nj04239f illustrated in Fig. 1. The imidazole-loaded Tp-DADMB composite

1604 | New J. Chem., 2018, 42, 1604--1607 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 RSC Advances

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Heterogeneous hybrid of propyl amino functionalized MCM-41 and 1H-1,2,4-triazole for Cite this: RSC Adv.,2017,7, 52321 high efficient intermediate temperature proton conductor†

Zhenzhen Wu, Juan Li * and Xian-Ming Zhang *

In this study, we introduced 1H-1,2,4-triazole into the ordered nanochannels of propyl amino functioned MCM-41 and then investigated the hybrids' proton conducting performance. A high proton conductivity of 8.34 103 Scm1 has been obtained at 120 C, with 5 mol% mole ratio of dangling propyl amino and fully loaded 1H-1,2,4-triazole in the silica. The activation energy is 0.55 eV and 1.303 eV in the Received 23rd September 2017 temperature range below and above 80 C respectively, indicating that the charge transfer mechanism Accepted 31st October 2017 of the material involved both vehicle and Grotthuss process. The above results showed that loading of 1- DOI: 10.1039/c7ra10535e H-1,2,4-triazole molecules into silica's ordered mesopores is a good strategy for excellent proton Creative Commons Attribution 3.0 Unported Licence. rsc.li/rsc-advances conductivity under intermediate temperature.

Introduction because rich nitrogen atoms are benecial to the effective formation of hydrogen networks inside the silicas' nano- Proton-conducting solids play a pivotal role in proton exchange channels and consequently high conductivity. Besides, with the membrane fuel cells (PEMFCs). Peruorosulfonic acid poly- discrete electronic density of triazole ring, 1-H-1,2,4-triazole mers, such as Naon, are the state-of-the-art in proton obtains higher electrochemical stability than other N- conductors, which can reach a proton conductivity of 0.1 S cm 1 heterocycles such as imidazole, suitable for fuel cell applica- This article is licensed under a in high relative humidity.1–3 However, when the operation tions and is able to effective proton conductivity under anhy- temperature is above 80 C, this conductivity always decreases drous conditions. In the past few decades, heterogeneous dramatically because of the water loss and nanochannel hybridization of proton source and porous matrix has proven to ff Open Access Article. Published on 13 November 2017. Downloaded 5/15/2020 12:26:46 PM. deformation. This limits PEMFCs' operating temperature to be one of the most e ective approaches to fabricate anhydrous 13–18 80 C. Novel electrolyte membranes with high proton conduc- proton conductors. With the connement effect and large tivity at temperatures above 120 C can minimize some of the pore volume of porous matrix, high conductivities have been key problems facing PEMFCs (i.e. CO poisoning, water and obtained for the porous coordination polymers (PCPs), covalent thermal management) hence reducing the overall cost of the organic frameworks (COFs), as well as other porous organic 19–26 fuel cells and increasing the total energy conversion efficiency. polymers. Kitagawa and co-workers reported that the imid- ¼ Therefore, developing an intermediate temperature proton azole loaded PCPs [Al(OH)(ndc)]n (ndc 1,4-naph- conductor independent of water is desperately in need.4–7 thalenedicarboxylate) exhibited a proton conductivity of 2.2 19 A classical approach to achieve anhydrous proton conduc- 10 5 Scm 1 at 120 C. In addition, the histamine loaded tion is to replace water as the proton carrier with amphoteric N- [Al(OH)(ndc)]n was also prepared and achieved an anhydrous 21 heterocycles, such as imidazole.8–12 In these amphoteric mole- proton conductivity of 1.7 10 3 Scm 1 at 150 C. Jiang et al. cules, the proton conduction is carried out by the formation and indicated that mesoporous COF [TPB-DMTP-COF] loaded with fracture of hydrogen bonds via Grotthuss-type mechanism, and imidazole molecules exhibited proton conductivity of 4.37 25 thus they can not only donate but also accept protons during 10 3 Scm 1 under anhydrous conditions. the proton transferring process. For better performance, 1-H- However, the poor hydrolysis resistance of the most PCPs 1,2,4-triazole, with three nitrogen atoms in ve membered ring and COFs limited the hybrid proton conductors' extensive structure, has been used as the non-water proton species applicability. From the point of view of atomic economy, it is sensible to utilize some other porous substrate with lower cost

Institute of Crystalline Materials, School of Chemistry & Chemical Engineering, and easier preparation method. Periodic mesoporous organo- Institute of Molecular Science, Shanxi University, Taiyuan 030006, PR China. silicas (PMOs), rst reported by Asefa et al. in 1999, directly E-mail: [email protected]; [email protected] forms hybrid organic–inorganic matrices by hydrolysis and † Electronic supplementary information (ESI) available. See DOI: condensation reactions with organosilane precursors.27–29 The 10.1039/c7ra10535e

This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7, 52321–52326 | 52321 PCCP

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Separating the redox couple for highly efficient solid-state dye-sensitized solar cells† Cite this: Phys. Chem. Chem. Phys., 2014, 16, 7334 Juan Li, Wei Zhang, Lu Zhang and Zhong-Sheng Wang*

To minimize the charge recombination between electrons and the electron acceptor in solid-state dye-sensitized solar cells, we propose a separated electron donor and acceptor of a redox couple in the Received 12th February 2014, Accepted 4th March 2014 photoanode and in the cathode, respectively. Owing to the absence of the acceptor in the photoanode initially, the charge recombination rate is retarded remarkably, resulting in an increase of the short- DOI: 10.1039/c4cp00628c circuit photocurrent by 42-fold, open-circuit photovoltage by 71 mV and power conversion efficiency by 42.5-fold. www.rsc.org/pccp

Introduction

Dye-sensitized solar cells (DSSCs) have been attracting considerable attention due to their potential low cost and high performance.1 Fig. 1 The chemical structure of DEII.

A typical DSSC comprises a dye-sensitized TiO2 film as the photoanode, a platinized conductive glass as the cathode and a redox couple based electrolyte sandwiched between the two deposited in the dye-loaded TiO2 layer for efficient dye regen- electrodes. The electron donor (e.g. IÀ) in the electrolyte fulfils eration preceded by electron injection from the excited dye to dye regeneration upon electron injection from the excited dye to the conduction band of TiO2, while the acceptor (mixture of À À À DEII and iodine at a molar ratio of 6 : 1, or I /I3 = 5 : 1, the the conduction band of TiO2 while the electron acceptor (e.g. I3 ) in the electrolyte accepts electrons at the cathode to complete the same below) was deposited on the cathode surface for efficient 2 À electron relay from the cathode to the inside of the cell for circuit. However, the presence of I3 in the photoanode layer À circuit completion. The cathode was placed on top of the

Published on 04 March 2014. Downloaded by Shanxi University 5/15/2020 12:26:57 PM. leads to charge recombination between electrons and I3 ,resulting in a drop of photovoltaic performance. photoanode to form a close contact between the donor and In fact, the photoanode layer only needs the electron donor for acceptor, which was then sealed into a solid device. As compared fast dye regeneration, while the electron acceptor should locate at to the redox mixture electrolyte based device A (Scheme 1), the surface of the counter electrode for efficient electron relay to the separating the electron donor and acceptor of a redox couple internal of the device. If the electron acceptor is moved from the in device B (Scheme 1) could significantly retard charge recom- À photoanode layer to the surface of the counter electrode, the charge bination because of the absence of I3 in the photoanode À recombination between electrons and I3 can be minimized due to the absence of triiodide in the photoanode initially and meanwhile fast electron transfer from the cathode to the internal of the device can also be maintained. Unfortunately, the separation of the donor and the acceptor cannot be realized with the liquid electrolyte,3 but it can be realized with the solid-state electrolytes for solid-state DSSCs (ssDSSCs).4–8 Herein, we prepared double-ester-substituted imidazolium iodide (DEII, Fig. 1) to act as the electron donor. The donor was

Scheme 1 The device structures for device A and device B. A redox- Department of Chemistry, Lab of Advanced Materials, Collaborative Innovation couple mixture is used as the solid-state electrolyte in device A while the Center of Chemistry for Energy Materials, Fudan University, 2205 Songhu Road, electron donor and acceptor of the redox-couple are separated in device Shanghai 200438, P. R. China. E-mail: [email protected] B: the donor is in the photoanode layer but the acceptor (the mixture of † Electronic supplementary information (ESI) available. See DOI: 10.1039/ donor and acceptor) is on top of the photoanode in contact with the c4cp00628c cathode.

7334 | Phys. Chem. Chem. Phys., 2014, 16, 7334--7338 This journal is © the Owner Societies 2014 ChemComm

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Hydroxyethyl and ester co-functionalized imidazolium

Cite this: Chem. Commun., 2013, iodide for highly efficient solid-state dye-sensitized 49, 9446 solar cells† Received 2nd July 2013, Accepted 16th August 2013 Juan Li, Hong Wang, Gang Zhou and Zhong-Sheng Wang*

DOI: 10.1039/c3cc44940h

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Hydroxyethyl and ester co-functionalized imidazolium iodide has type of liquid material and the leakage during long-term operation is been designed and synthesized as an efficient solid state electrolyte unavoidable. Therefore, ssDSSCs with solid state imidazolium iodides for dye-sensitized solar cells. Single crystal X-ray analyses confirm having high conductivity are much desired for practical applications. the presence of ordered three dimensional ionic channels for iodide, Recently, we reported that replacement of the alkyl group with an which exhibits high conductivity when mixed with iodine and LiI ester group in alkyl-substituted imidazolium iodide could enhance and achieves a power conversion efficiency of 7.45%. ionic conductivity significantly and thus improve solar cell performance remarkably.10 On the basis of ester-alkyl-substituted Dye-sensitized solar cells (DSSCs) provide an economically credible imidazolium iodide, we designed and synthesized hydroxyethyl alternative to conventional inorganic photovoltaic devices.1 Owing to and ester co-functionalized imidazolium iodide (abbreviated as their high energy conversion efficiency and low production cost, they HEII, ESI,† Fig. 1a), a solid state ionic conductor (Fig. S1, ESI†), for have established themselves as the most promising low-cost alter- use in ssDSSCs. The oxygen and hydrogen in the hydroxyethyl native to Si-based solar cells for the conversion of solar energy to group are expected to participate in hydrogen bonding, which electricity.2,3 The working principle of these devices is based on ultra- is favorable for the ionic conductor to form a closely packed fast electron injection from a photo-excited dye into the conduction structure towards high conductivity. band of an oxide semiconductor and subsequent dye regeneration.4 The decomposition temperature of HEII was determined by The electrolyte containing a redox couple provides the internal thermogravimetric analysis (TGA). HEII starts to decompose at 1

Published on 16 August 2013. Downloaded by Shanxi University 5/15/2020 12:36:52 PM. electrical conductivity between working and counter electrodes 150 CasseenfromtheTGAcurve(Fig.S2a,ESI†).Fig.S2b(ESI†) and charge transfer occurs from the electrolyte to the oxidized dye displays differential scanning calorimetry (DSC) results for and from the counter electrode to the electrolyte. HEII. One sharp endothermic peak, where the ionic conductor Although the power conversion efficiency has been recorded undergoes phase transition from solid to liquid, was observed. for DSSCs with volatile organic liquid electrolytes,5 the stability is not good enough for long-term practical operation because of the leakage and evaporation of the liquid electrolyte caused by the technical difficulty in sealing. For this reason, considerable efforts have been devoted to the fabrication of solid-state DSSCs (ssDSSCs) by employing all-solid-state electrolytes due to their potential for providing long-term durability.6,7 Imidazolium iodides have been proven to be a promising material as ionic liquid electrolytes for high-performance DSSCs due to their wide electrochemical window, high electrical conductivity, negligible vapor pressure, and nontoxicity.8,9 However, they are still a

Department of Chemistry, Lab of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, 2205 Songhu Road, Shanghai 200438, China. E-mail: [email protected]; Fax: +86-21-51630345; Tel: +86-21-51630345 † Electronic supplementary information (ESI) available: Experimental details, synthesis of HEII and its characterizations, MK2 dye structure, EIS, SEM images Fig. 1 (a) Chemical structure, (b) single crystal structure and packing diagram and long-term stability. CCDC 889350. For ESI and crystallographic data in CIF or viewed down the b axis for HEII. I (purple), O (red), N (blue), C (gray), H (white). other electronic format see DOI: 10.1039/c3cc44940h The labelled numbers indicate the distance (Å) of adjacent iodides.

9446 Chem. Commun., 2013, 49, 9446--9448 This journal is c The Royal Society of Chemistry 2013 NANO: Brief Reports and Reviews Vol. 9, No. 5 (2014) 1440006 (8 pages) © World Scienti¯c Publishing Company DOI: 10.1142/S1793292014400062

EFFECT OF SUBSTITUENTS IN THE IMIDAZOLIUM RING ON THE PERFORMANCE OF SOLID-STATE DYE-SENSITIZED SOLAR CELLS

JUAN LI, KAI LV, HONG SUN and ZHONG-SHENG WANG* Department of Chemistry, Lab of Advanced Materials Collaborative Innovation Center of Chemistry for Energy Materials Fudan University, 2205 Songhu Road Shanghai 200438, P. R. China *[email protected]

Received 14 January 2014 Accepted 13 March 2014 Published 17 April 2014

Three solid-state imidazolium iodides have been designed and synthesized for use as all-solid-state electrolytes in solid-state dye-sensitized solar cells (ssDSSCs). The e®ect of substituents in the imidazolium ring on the ionic conductivity and solar cell performance of ssDSSCs has been inves- tigated. As compared to the methyl-ethyl-substituted imidazolium iodide, replacement of one alkyl group (the methyl group) with an ester group increases the ionic conductivity and solar cell per- formance signi¯cantly, and further replacement of the other alkyl group (the ethyl group) with a hydroxyethyl group further increases the ionic conductivity and solar cell performance signi¯- cantly. A power conversion e±ciency of 7.45% has been achieved under the irradiation of simulated AM1.5G solar light (100 mW cm2) with the ssDSSC using the hydroxyethyl and ester co-func- by 54.152.109.166 on 09/10/15. For personal use only.

NANO 2014.09. Downloaded from www.worldscientific.com tionalized imidazolium iodide based solid-state electrolyte and a metal-free organic dye sensitizer.

Keywords: Solid state electrolyte; dye sensitized solar cells; ionic conductor.

1. Introduction working and counter electrodes and ful¯lls charge Dye-sensitized solar cells (DSSCs) provide an econ- transfer from the electrolyte to the oxidized dye and from the counter electrode to the electrolyte. omically credible alternative to conventional inor- ganic photovoltaic devices.1 Owing to their high Although the power conversion e±ciency has energy conversion e±ciency and low production cost, been recorded for DSSCs with volatile organic liquid 5 they have established themselves as the most prom- electrolytes, the stability is not good enough for ising low-cost alternative for the conversion of solar long-term practical operation because of the leakage energy to electricity.2,3 The working principle of and evaporation of the liquid electrolyte caused by these devices is based on ultra-fast electron injection the technical di±culty in sealing. For this reason, from a photo-excited dye into the conduction band of considerable e®orts have been devoted to the fab- an oxide semiconductor and subsequent dye regen- rication of solid-state DSSCs (ssDSSCs) by eration.4 The electrolyte containing a redox couple employing all-solid-state electrolytes for its poten- provides the internal electric conductivity between tial to provide long-term durability.6,7

1440006-1 RSC Advances

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Lithium-coordinating ionic conductor for solid- state dye-sensitized solar cells† Cite this: RSC Adv.,2015,5, 56967 Juan Li and Zhong-Sheng Wang*

A new solid-state ionic conductor is synthesized by linking an ether group to the nitrogen-atom of 1,2- dimethylimidazole with an iodide counter anion, and the single crystal structure is determined using X- ray crystallographic analysis. Replacement of the butyl group in 1-butyl-2,3-dimethylimidazolium iodide with an ether group induces a significant improvement in conductivity. When the solid ionic conductor is mixed with LiI alone, conductivity enhancement is more remarkable for the ether-containing ionic conductor due to the lithium coordination to the ether oxygen, which is able to avoid the aggregation of lithium cations with iodides and hence improves the transport properties of Li+. Owing to the p-stacking of imidazolium rings for the ether-containing ionic conductor, the increment of ionic conductivity is also

more significant upon further doping with I2. The ether-containing ionic conductor mixed with LiI alone Received 23rd May 2015 as the solid electrolyte can even make the solid-state dye-sensitized solar cells work. Further doping Accepted 23rd June 2015 with iodine achieves power conversion efficiency of 7.1%, which is much higher than that (5.3%) for the DOI: 10.1039/c5ra09688j alkyl analogue due to the positive shift of the conduction band edge of titanium dioxide and suppression www.rsc.org/advances of charge recombination caused by the ether group.

Introduction organic solvent based electrolyte for high-performance organic liquid electrolyte based DSSCs. However, the solid 1,2,3-tri-

Dye-sensitized solar cells (DSSCs) employing nanoporous TiO2 alkylimidazolium iodide mixed with iodine and LiI without electrodes1 have been attracting considerable attention in adding organic solvent cannot make the solid-state DSSCs industrial and academic elds because of their potential low- (ssDSSCs) work efficiently due to the pretty low ionic conduc- cost and high efficiency.2–5 Although high performance can be tivity.9 To improve the ionic conductivity of the solid IL, achieved with volatile organic liquid electrolytes, solid-state different functional groups have been introduced to the imi- Published on 24 June 2015. Downloaded by Shanxi University 5/15/2020 12:42:57 PM. DSSC (ssDSSC) devices mediated with solid-state electrolytes dazolium ring,6,9 which results in signicant improvement of have high application potential due to stability and safety photovoltaic performance of ssDSSCs. concerns.6–10 In this study, we synthesized a new ionic conductor, 1,2- Ionic liquids (ILs) have attracted growing interest for use in dimethyl-3-methoxyethylimidazolium iodide (DOII, Fig. 1a), DSSCs due to their wonderful properties such as negligible based on 1-butyl-2,3-dimethylimidazolium iodide15 (DBII, vapor pressure, excellent thermal stability, good ionic conduc- Fig. 1b) by replacing one CH2 unit in the butyl group with an tivity and wide electrochemical window.11,12 1-Methyl-3- oxygen atom. The aim of introducing an ether group is to induce propylimidazolium iodide is oen used to construct solvent- p–p stacking of imidazolium rings through the H/O hydrogen free DSSCs.13 To make the DSSC device more stable, a series bonds, which is favourable for efficient charge exchange and of solid imidazolium iodides with various functional groups can thus increase ionic conductivity. Furthermore, lithium have been developed for use in ssDSSCs.6,9 coordination to the ether oxygen can avoid strong aggregates of The acidic hydrogen at C-2 position in imidazolium ring lithium cations with iodides, thus improving their transport usually causes the IL unstable.14 Replacement of the H atom at properties, which may inuence the conduction band level of C-2 position can improve the stability and meanwhile enhance titania and the interfacial charge recombination. As compared the melting point signicantly.14 The solid 1,2-dimethyl-3- to DBII, the introduction of an ether group to the imidazolium propylimidazolium iodide is an oen used additive in the improved the ionic conductivity signicantly for the pure ionic conductors and their mixtures with LiI and with LiI and I2 as well. When DOII mixed with appropriate amount of LiI and I Department of Chemistry, iChEM (Collaborative Innovation Centre of Chemistry for 2 Energy Materials), Lab of Advanced Materials, Fudan University, 2205 Songhu was employed as the solid electrolyte, the ssDSSC with a metal- 16 Road, Shanghai 200438, P. R. China. E-mail: [email protected] free organic dye (FNE29, Fig. S1†) achieved a power conversion † Electronic supplementary information (ESI) available. CCDC 948215. For ESI efficiency of 7.1% under AM1.5G full-sun illumination, which and crystallographic data in CIF or other electronic format see DOI: was much higher than that (5.3%) with the DBII/LiI/I2 solid- 10.1039/c5ra09688j

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