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Enhanced redox

aDepartment of Chemistry, Sungkyunkwan

sson@skku.edubKorea Basic Science Institute, Daejeon 3413cLaboratory of Nuclear Magnetic Resonance

Research Facilities (NCIRF), Seoul NationaldDepartment of Energy Engineering, Dank

E-mail: mhyang@dankook.ac.kr

† Electronic supplementary information (Eadditional cyclic voltammograms and TESee DOI: 10.1039/c8ta01379a

Cite this: J. Mater. Chem. A, 2018, 6,6233

Received 8th February 2018Accepted 12th March 2018

DOI: 10.1039/c8ta01379a

rsc.li/materials-a

This journal is © The Royal Society of C

activity of a hollow conjugatedmicroporous polymer through the generation ofcarbonyl groups by carbonylative Sonogashiracoupling†

Jaewon Choi,a Ju Hong Ko,a Chang Wan Kang,a Sang Moon Lee,b Hae Jin Kim,b

Yoon-Joo Ko,c MinHo Yang*d and Seung Uk Son *a

Redox active 1,4-bis(3-phenylpropynoyl)benzene moieties were

incorporated into a hollow conjugated microporous polymer via

carbonylative Sonogashira coupling of 1,3,5-triethynylbenzene with

1,4-diiodobenzene in the presence of carbonmonoxide. The resultant

redox active material showed enhanced energy storage performance

for pseudocapacitors.

Recently, conjugated microporous polymers (CMPs) have beenapplied as energy storage materials.1–5 Due to the porosities andhigh surface areas of these materials, electrolytes can diffuse intothe inner part of the materials to utilize the chemical sites. Thechemical stabilities and relatively low densities of CMP materialsare also benecial factors in use as energy storage materials.2

However, efforts to develop energy storage materials based onCMPs are in the early stages3–6 and the storage performancesreported until now are not sufficient. Thus, more exploration isrequired to enhance the electrochemical performance.

Supercapacitors are promising energy storage devices due totheir high-power properties.7 Organic polymers including CMPshave been applied as energy storage materials for super-capacitors.3,6 The capacitance of supercapacitors can be enhancedusing pseudocapacitive materials with additional redox behav-iors.7 In this regard, redox active moieties have been introducedinto CMP materials. For example, redox active building blockswere designed and used for the synthesis of CMPs.3a In addition,redox active moieties could be introduced into CMPs via post-synthetic modication.5a In these examples, although the

University, Suwon 16419, Korea. E-mail:

3, Korea

, The National Center for Inter-University

University, Seoul 08826, Korea

ook University, Cheonan 31116, Korea.

SI) available: Experimental procedures,M images of H-CMP and H-CMP-BPPB.

hemistry 2018

introduction of redox active moieties into CMPs enhanced theelectrochemical performance, it resulted in additional increasesof chemical components of materials, in addition to the basicskeletons of CMPs. Thus, more efficient synthetic approacheswhich can result in a minimal increase of chemical components,should be further explored for redox active CMPs.

Dicarbonyl compounds with conjugated connectors haveshown reversible redox behaviors and have been applied asorganic electrode materials for energy storage devices.8 Whenthe conjugated connectors consist of phenylene groups, redoxconversions between phenylene and quinone forms have beenwidely utilized as a principle of electrochemical energy storage.8

For example, according to our model studies, 1,4-bis(3-phenylpropynoyl)benzene (BPPB) showed a reversible two-electron redox behavior (see Fig. 1a and b and the ESI† fordetails).

Fig. 1 (a) A scheme of the reversible redox behavior of 1,4-bis(3-phenylpropynoyl)benzene (BPPB) and (b) cyclic voltammograms ofBPPB, 1,4-bis(2-phenylethynyl)benzene, and ferrocene (Fc).

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Fig. 3 SEM images of (a) H-CMP and (d) H-CMP-BPPB. TEM images of(b and c) H-CMP and (e and f) H-CMP-BPPB.

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We have gured out that BPPB moieties can be introduced toCMP materials via carbonylative Sonogashira coupling9 ofphenyl alkynes and phenyl halides in the presence of carbonmonoxide. It is noteworthy that the change of chemicalcomponents of CMPs through introduction of carbonyl groupsis quite minimal, which can be benecial in consideration ofthe gravimetric capacitance of materials. Moreover, we engi-neered hollow CMPmaterials10 with BPPBmoieties via templatesynthesis. The thin shell thicknesses will be benecial in facilediffusion of electrolytes into CMP materials. In this work, wereport the preparation of redox active hollow CMP materials viacarbonylative Sonogashira coupling and their enhanced pseu-docapacitive performances as electrode materials ofsupercapacitors.

Fig. 2 shows synthetic schemes for a hollow CMP with BPPB(H-CMP-BPPB) and a hollow CMP without BPPB (H-CMP).Monodisperse silica spheres were prepared as hard templatesby the Stober method.11 As control materials, representativeCMP materials were synthesized on the silica spheres viaSonogashira coupling of 1 eq. 1,3,5-triethynylbenzene with 2 eq.1,4-diiodobenzene. Etching of silica templates resulted in H-CMP. Under the same synthesis conditions, CMP-BPPBs wereformed on silica spheres in the presence of carbon monoxide.Aer etching of the silica, H-CMP-BPPB was obtained.

The H-CMP and H-CMP-BPPB materials were investigated byscanning (SEM) and transmission electron microscopy (TEM)(Fig. 3). SEM and TEM images of H-CMP and H-CMP-BPPBrevealed the hollow structure of the materials. The averagediameters of H-CMP and H-CMP-BPPB were calculated to be213 � 8 and 218 � 11 nm, respectively. The average shellthicknesses of H-CMP and H-CMP-BPPB were measured as 19�1 and 23 � 3 nm, respectively.

Carbon and oxygen contents in the H-CMP and H-CMP-BPPBmaterials were analyzed by elemental analysis. H-CMP showed76.62 wt% of carbon and 1.37 wt% of oxygen. In comparison, H-CMP-BPPB showed 74.82 wt% of carbon and 12.11 wt% ofoxygen. The increased oxygen content in H-CMP-BPPB,compared with that of H-CMP, supports incorporation ofcarbonyl groups via carbonylative Sonogashira coupling. Basedon the increased oxygen content (10.74 wt%), the amount ofcarbonyl groups in H-CMP-BPPB was calculated to be�6.69 mmol g�1, which is a signicantly higher concentration,

Fig. 2 Synthetic schemes for H-CMP and H-CMP-BPPB.

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compared with the conventional amounts12 of active moietiesintroduced by pre-designed building blocks and post-syntheticmodication approaches.

Analysis of the N2 adsorption–desorption isotherm curves ofmaterials (77 K) based on Brunauer–Emmett–Teller (BET)theory revealed the surface areas of H-CMP and H-CMP-BPPB tobe 770 (micropore volume, Vmic: 0.20 cm3 g�1) and 622 m2 g�1

(Vmic: 0.18 cm3 g�1), respectively (Fig. 4a). Analysis of pore sizedistribution based on density functional theory (DFT) showedmajor micropores with sizes less than 2 nm (Fig. 4a). Theinfrared absorption (IR) spectroscopy of H-CMP showed vibra-tion peaks at 1580 and 831 cm�1 (Fig. 4b). These peaks wereattributed to the vibration of C]C and C–H bonds in thearomatic groups.12a The model compound, 1,4-bis(2-phenylethynyl)benzene also showed a similar IR absorptionspectrum to that of H-CMP. Another model compound,1,4-bis(3-phenylpropynoyl)benzene, for H-CMP-BPPB showedvery strong vibration peaks at 2197 and 1627 cm�1, corre-sponding to the enhanced vibration of internal alkyne andC]O, respectively. As shown in Fig. 4b, the IR absorptionspectrum of H-CMP-BPPB clearly showed strong vibration peaksat 2197 and 1645 cm�1, matching well with the spectrum of themodel compound, 1,4-bis(3-phenylpropynoyl)benzene.

Solid state 13C nuclear magnetic resonance (NMR) spec-troscopy of H-CMP showed 13C peaks of internal alkyne at95–85 ppm (Fig. 4c). In the aromatic region, three major peakswere observed at 136, 131, and 123 ppm. 1,4-bis(2-phenylethynyl)benzene showed similar 13C NMR spectra withinternal alkyne peaks at 91 and 89 ppm and aromatic peaks at132, 128, and 123 ppm. The rst signicant point in the solidstate 13C NMR spectrum of H-CMP-BPPP is the appearance ofa 13C peak of carbonyl groups at 175 ppm. The second signi-cant point is the increased intensity of a downeld aromaticpeak at 145–134 ppm. These observations resulted from theintroduction of carbonyl groups into materials. In the case ofthe model compound, the 13C NMR spectrum of 1,4-bis(3-phenylpropynoyl)benzene showed a 13C peak of carbonylgroups at 177 ppm. A downeld shied aromatic peak wasobserved at 141 ppm, matching well with the trend in the 13C

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Fig. 4 (a) N2 adsorption–desorption isotherm curves of H-CMP andH-CMP-BPPB which were obtained at 77 K and their pore size distri-bution diagrams (by the DFT method), (b) IR absorption and (c) 13CNMR spectra of model compounds (1,4-bis(2-phenylethynyl)benzeneand 1,4-bis(3-phenylpropynoyl)benzene), H-CMP, and H-CMP-BPPB.(d) TGA curves and (e) PXRD patterns of H-CMP and H-CMP-BPPB.

Fig. 5 Electrochemical performance of symmetric coin cell typepseudocapacitors (electrolyte: 1 M H2SO4) of H-CMP and H-CMP-BPPB. (a, b) Cyclic voltammograms, (b) charge–discharge profiles, (c)rate performance, and (d) Nyquist plots of H-CMP-BPPB and H-CMP.

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NMR spectrum of H-CMP-BPPB. The IR and 13C NMR studiesindicate that the carbonyl groups were successfully incorpo-rated into CMP materials through carbonylative Sonogashiracoupling.

According to thermogravimetric analysis (TGA), H-CMP-BPPB was stable up to 303 �C (Fig. 4d). Powder X-ray diffrac-tion (PXRD) patterns of H-CMP and H-CMP-BPPB indicated theamorphous feature of the materials, which are conventionalproperties of CMP materials in the literature1 (Fig. 4e).

It has been a well-established strategy that the incorporationof additional redox species into the electrode materials ofsupercapacitors can enhance storage capacitance throughpseudocapacitive behaviors. Considering the redox moieties ofBPPB in H-CMP-BPPB, we studied the electrochemical perfor-mance of energy storage materials for supercapacitors,

This journal is © The Royal Society of Chemistry 2018

compared to H-CMP. Symmetric coin cell type pseudocapacitorswere fabricated using H-CMP and H-CMP-BPPB as electrodematerials. Fig. 5 and 6 summarize the results.

Cyclic voltammograms of H-CMP-BPPB showed enhancedpseudocapacitive behaviors with redox curves, compared tothose of H-CMP (Fig. 5a and S1 in the ESI†). Fig. 5b clearlyshows the redox assisted charge–discharge proles of H-CMP-BPPB and the resultant enhanced capacitances, compared toH-CMP. The current density dependent rate performance wasstudied and is shown in Fig. 5c. H-CMP-BPPB showed capaci-tances of 220, 193, 161, 133, 120, and 108 F g�1 at currentdensities of 0.5, 1, 2, 6, 10, and 20 A g�1, respectively. Incomparison, H-CMP showed capacitances of 130, 113, 95, 77,63, and 53 F g�1 at current densities of 0.5, 1, 2, 6, 10, and20 A g�1, respectively. The Nyquist plots obtained throughelectrochemical impedance spectroscopy (EIS) of H-CMP-BPPBand H-CMP showed charge transfer resistances (Rct) of 5.9and 12.3 U, respectively. The enhanced capacitances andconductivities of H-CMP-BPPB, compared with those of H-CMP,can be attributable to the redox behaviors of BPPB moieties inthe materials. In 2017, Li et al. reported redox active triaza-truxene based CMP materials with a capacitance of 183 F g�1 @

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Fig. 6 (a) Cycling performance of symmetric coin cell type pseudo-capacitors (inset) of H-CMP-BPPB (current densities: 1 and 6 A g�1). (b)Nyquist plots and (c) TEM images of H-CMP-BPPB before and after10 000 cycles (refer to Fig. S2 in the ESI† for more TEM images).

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1 A g�1.3a Moreover, carbonized CMP materials showingcapacitances of �75 and 164 F g�1 @ 1 A g�1 were reported in2015 and 2016, respectively.4b,c In addition, redox active covalentorganic frameworks (COFs) with capacitances of 40–167 F g�1 @0.1 A g�1 were reported in 2013 and 2015.6 Considering theseresults, although the electrochemical performance (193 F g�1 @1 A g�1) of H-CMP-BPPB is not superior to the best peformancesreported so far, it is superior or comparable to those of recentCMP related materials in the literature.3,4,6

The cycling stability of the coin cell type pseudocapacitors ofH-CMP-BPPB was studied (Fig. 6a). Aer 10 000 cycles, thepseudocapacitor cells maintained capacitances of 170 (90% ofthe rst cycle capacitance, 189 F g�1) and 115 F g�1 (85% of therst cycle capacitance, 135 F g�1) at current densities of 1 and6 A g�1, respectively. The Nyquist plots of H-CMP-BPPB beforeand aer 10 000 cycles showed a very small change of Rct from5.8 U to 6.3 U, indicating the stability of cells (Fig. 6b). TEManalysis on the cycled H-CMP-BPPB showed complete retentionof the original hollow structure. Interestingly, a signicantincrease (�2 times) of shell thickness from 23 � 3 nm to 39 �8 nm was observed (Fig. 6c and S2 in the ESI†), indicating theefficient participation of materials in electrochemical events.We speculate that the hollow structure of H-CMP-BPPB could bebenecial to the utilization of redox species in the materials.13

In conclusion, this work shows that carbonylative Sonoga-shira coupling can be an efficient synthetic strategy for intro-duction of redox active species into CMPs. The BPPB moietiesgenerated in CMPs can be utilized in electrochemical energystorage materials to show enhanced capacitances through theirpseudocapacitive behaviors. We believe that the syntheticstrategy in this work can be applied to more various redox activeCMPs through screening of the building blocks.

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Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was supported by the Basic Science ResearchProgram (2016R1E1A1A01941074) through the NationalResearch Foundation of Korea (NRF) funded by the Ministry ofScience, ICT and Future Planning and the grants CAP-15-02-KBSI (R&D Convergence Program) of the National ResearchCouncil of Science & Technology (NST) of Korea.

Notes and references

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12 In our previous work, the contents of chemical sites wereanalyzed in the range of 0.86–1.76 mmol g�1 (a) N. Park,D. Kang, M. C. Ahn, S. Kang, S. M. Lee, T. K. Ahn,J. Y. Jaung, H. W. Shin and S. U. Son, RSC Adv., 2015, 5,47270–47574; (b) M. H. Kim, T. Song, U. R. Seo, J. E. Park,K. Cho, S. M. Lee, H. J. Kim, Y. J. Ko, Y. K. Chung andS. U. Son, J. Mater. Chem. A, 2017, 5, 23612–23619.

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