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Materials Today Energy 9 (2018) 11e18

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Materials Today Energy

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Eco-friendly cellulose based solid electrolyte with high performanceand enhanced low humidity performance by hybridizing withaluminum fumarate MOF

Jaeok Ko a, b, 1, Seong Ku Kim a, 1, Yeoheung Yoon a, c, Kyung Ho Cho d, Wooseok Song a,Tae-Ho Kim a, Sung Myung a, Sun Sook Lee a, Young Kyu Hwang d, Sang-Woo Kim b,Ki-Seok An a, *

a Advanced Materials Division, Korea Research Institute of Chemical Technology, Yuseong Post Office Box 107, Daejeon 305-600, Republic of Koreab School of Advanced Materials Science & Engineering, SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon440-746, Republic of Koreac Department of Materials Science & Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon305-701, Republic of Koread Green Carbon Catalysis Group, Korea Research Institute of Chemical Technology, Yuseong Post Office Box 107, Daejeon 305-600, Republic of Korea

a r t i c l e i n f o

Article history:Received 21 December 2017Received in revised form17 April 2018Accepted 23 April 2018

Keywords:Solid-state electrolytePolymer electrolyteAll-solid-state supercapacitorCelluloseMetal organic frameworks

* Corresponding author.E-mail address: ksan@krict.re.kr (K.-S. An).

1 These authors contributed equally.

https://doi.org/10.1016/j.mtener.2018.04.0072468-6069/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Abundant yet undeveloped, cellulose and various derivatives of cellulose are materials that we see andtouch every day. Here, we report a novel polymer proton conducting electrolyte synthesized by chemicalmodification of hydroxypropyl cellulose, which opens a new possibility of utilizing a renowned envi-ronmental friendly material for environmentally friendly applications. The modified cellulose byattaching sulfonic acid functional groups exhibits proton conductivity of 172 mS/cm at room temperatureunder 100% relative humidity with negligible electron or hole leakage through the membrane. By hy-bridizing the modified cellulose with aluminum fumarate metal-organic-framework (A520), dehydrationof the cellulose based electrolyte membrane is significantly moderated to enhance low humidity con-ductivity by more than an order of magnitude. The A520 hybridized sulfonated cellulose is utilized as theelectrolyte for all-solid-state supercapacitor, which demonstrated specific capacitance as high as135.14 F/g at 75% relative humidity with stable performance even at low humidity.

© 2018 Elsevier Ltd. All rights reserved.

1. Introduction

Development of a high performance solid electrolyte is essentialfor realization of number of devices in wide variety of applicationsranging from energy conversion/storage to electrochemical gassensors [1e6]. Especially for future energy devices such as fuel cellsand all-solid-state supercapacitors/batteries, solid state electrolytesare necessary component; hence, research on development of solidelectrolytes could never be emphasized enough. Furthermore, is-sues regarding chemical stability and safety inherit in liquid elec-trolytes [7e9] can be solved when they are replaced by solidelectrolytes. Among various classes of solid electrolytes, solid

polymer electrolytes can provide high conductivity even at roomtemperature, which makes them most promising for low temper-ature applications [10,11]. However, ionic conductivities ofcurrently available solid polymer electrolytes are still insufficient toreplace liquid electrolytes completely.

As a high performance solid polymer electrolyte, Nafion, arenowned perfluorinated polymer patented by DuPont, is consid-ered as one of the best with its high proton conductivity and highmechanical/chemical stability [10e12]. However, Nafions are arti-ficial polymers which contain large number of fluorine, which canbe environmentally detrimental if their wastes are not treatedproperly. For this very reason, we chemically modified cellulose as anovel polymer electrolyte.With its environmental friendliness, bio-compatibleness, and superb mechanical property, cellulose hassteadily gained interest of scientific communities in various fieldssuch asmedicine, pollutant capture, and electronics. Some of recentstudies have introduced electrolyte involving cellulose as the frame

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for ion conducting gel, salts, and polymers to utilize cellulose'ssuperb durability and reusability [13e16]. However, in this work,we directly modified this bio-polymer as a proton conducting solid-state electrolyte for the first time. Additionally, we addedaluminum fumarate metal organic framework (A520) to enhancethe electrolyte's low humidity performance. Moisture absorption ofA520 at low humidity is reported to exceed MIL-100 or MIL-101[17]; hence, its application as moisture retaining compound in ahybrid membrane is promising.

Here, we report a novel polymer electrolyte membrane based oncellulose with proton conductivity higher than that of Nafion at ahigh humidity condition. The proposed cellulose based solid elec-trolyte exhibited conductivity as high as 0.172 S/cm at room tem-perature. Additionally, we also synthesized sulfonated cellulose/A520 hybrid electrolytes to enhance the proton conductivity at lowhumidity condition, which resulted in enhancement of conductiv-ity more than an order of magnitude at relative humidity below40%.

2. Result and discussion

Chemical structure and synthetic scheme of the sulfonic acidfunctionalized cellulose are depicted in Fig. 1a. Details about theprocedure are included in Supporting Information. Fig. 1bed dis-plays the change in IR spectra as synthesis proceeds. First, epoxyfunctional groups are introduced to hydroxypropyl cellulosethrough nucleophilic substitutional reaction between chloride in

Fig. 1. Reaction scheme and IR spectra after each reaction step. a) Two step reaction schemobtained from b) hydroxypropyl cellulose, c) epi-cellulose after first reaction step, and d) s

epichlorohydrin and eOH of hydroxypropyl cellulose. The IRspectrum displayed in Fig. 1c is obtained from the product of thefirst step (epi-cellulose). IR peak corresponding to epoxide groupappears near 824 cm�1, clearly evincing successful attachment ofepoxide group to the cellulose backbone. Additionally, pH of thesolution during the first reaction gradually decreased over time,indicating that epoxides were attaching to hydroxypropyl cellulose.The IR peak corresponding to eOH group is apparent in Fig. 1c,because the spectrum was obtained in water.

The reaction between epi-cellulose and aminomethanesulfonicacid followed after the first step. For this step, renowned epoxidering-opening reaction was used to attach sulfonic acid group withaid of basic catalyst. The IR spectrum of the sulfonated cellulose isdisplayed in Fig. 1d. Significant differences in IR spectrum at wave-number between 900 and 1500 cm�1 (compared to Fig.1b) are clearevidence of successful attachment of sulfonic acid group. Especially,peaks at 1027 cm�1 and 1172 cm�1 can be attributed to symmetricand asymmetric stretchings of SO3

�, respectively, as frequently re-ported by articles on sulfonated membranes such as Nafion andsulfonated poly(ether ether ketone) [18e21]. Moreover, absence ofan IR peak at 1598 cm�1, which is typically assigned as primaryamine of aminomethanesulfonic acid (Fig. S1 in SupportingInformation), implies primary amines were converted to second-ary amines consequently due to proposed reaction [22].

For further processing, sulfonated cellulose can easily bedispersed in isopropyl alcohol, which is advantageous for advancedprocessing techniques such as printing. Processing of electrolytes in

e for functionalization of hydroxypropyl cellulose to sulfonated cellulose and IR resultsulfonated cellulose.

J. Ko et al. / Materials Today Energy 9 (2018) 11e18 13

various shapes and sizes are also possible by adjusting the castingframe (Fig. S2 in Supporting Information presents circular shapedelectrolyte with diameter of 9 cm). With a slow casting process,membrane with fine surface morphology can be produced asdemonstrated by SEM images in Fig. S3 in Supporting Information.Moreover, the electrolyte can be re-dispersed in isopropyl alcoholor IPA/water mixture for recycling purposes, which providesanother strong advantage over other polymer based electrolytes.

Unambiguously, proton conductivity is an important perfor-mance parameter to be considered for solid electrolytes. For accu-rate measurement of conductivity of the electrolyte, impedancespectroscopy was employed [23,24]. Fig. 2a consists of a Nyquistplot of the sulfonated cellulose electrolyte obtained using imped-ance spectroscopy. The Nyquist plots obtained by impedancemeasurements at various temperatures and humidities areanalyzed using the equivalent circuit proposed in Fig. 2b. The circuitconsists of three serial elements, where Rc represents the contactresistance, Rm and Cm represent resistance and capacitance fromthe electrolyte, respectively, while Re and Ce are for electric doublelayers at electrolyte-electrode interfaces. Consequently, when thesample measurement was performed using four-probe ac-imped-ance, the low frequency semi-circle (related to Re and Ce) disap-pears as demonstrated in Fig. S4 in Supporting Information.

Using the Rm values extracted from the Nyquist plots, conduc-tivity (s) of the electrolyte was evaluated. The conductivity depen-dence to relative humidity (RH) at room temperature is plotted inFig. 2c. The room temperature conductivity of the electrolyte under100%RH is as high as 0.172 S/cm, which is even higher than Nafion[12,25,26]. Even when compared to various other polymer electro-lytes (Fig. S5 in Supporting Information), the conductivity of sulfo-nated cellulose is outstanding [12,27e31]. Even when comparedwith a proton conducting nanocellulose paper (cellulose sulfonatedwith a shorter branching chain on high crystalline nanocellulose)synthesized by Bayer et al., our sulfonated cellulose outperforms itwith conductivity that is more than 30 times higher [29]. Strongdependence on relative humidity is a clear indication that themeasured conductivity is mainly due to motion of protons throughhydrated channels formed inside the electrolyte. The temperaturedependent conductivity at 100%RH is presented in Fig. 3d. As

Fig. 2. Conduction property and thermal stability of the sulfonated cellulose electrolyte. a)impedance analysis. Proton conductivity of sulfonated cellulose membrane with respect tohumidity. e) Result of dc-polarization experiment. f) TGA/DTA curves for evaluation of ther

expected, the conductivity increased with increasing temperaturewhen dehydration is insignificant, and the activation energy (Ea) ofconduction was 0.118 eV [25,26]. It is often regarded that whenactivation energy is <0.3 eV, transport occurs by protons jumpingfrom one sulfonate group to another as reported frequently forsulfonated polymer electrolytes (e.g. the reported activation energyis ~0.145 eV for Nafion 117 and ~0.155 eV for Nafion 112 and 120)[12,25,32]. The Bode plot in Fig. S5 in Supporting Information alsoleads to similar conclusion with n of Almond-West expression(Equation (3) in Supporting Information)near0.55at high frequencyregion, indicating planar transport is occurring in the electrolyte(proton hopping on surface of water channels) [6,33,34].

DC-polarization test was also performed to obtain the trans-ference number of the electrolyte [6,35]. For polarization, relativelyinactive gold electrodes were utilized and only low DC potentials(100 mVe500 mV) were applied. The difference between the initialcurrent and the steady state current after complete polarization ismore than 38 times, which indicates transference number higherthan 0.974 is observed from this sample.

Thermal stability of the material was evaluated using thermalgravimetric analysis (TGA) as presented in Fig. 2f. Due to highmoisture absorption property of the electrolyte, the weight loss isobserved at low temperature, even though the sample was driedprior to TGA. The gradual weight loss due to dehydration continuesuntil 220 �C, at which an abrupt weight loss is featured due to de-functionalization of the electrolyte followed closely by endo-thermic decomposition of the cellulose backbone near 285 �C. FromTGA, it is apparent that the membrane is thermally stable up toabout 220 �C, which is well above the operating temperatures ofpolymer based fuel cells and electrochemical sensors [1e5].

Even though sulfonated cellulose exhibited high proton con-ductivity under high humidity condition, its performance at lowhumidity condition still needs improvement. Since even the state ofart polymer electrolytedNafiondhas been suffering from dehy-dration, materials such as zeolite or silica were added to Nafionmembrane to enhance its performance at low humidity [36e38].Recently, reports have revealed outstanding moisture absorptionpropertyof variousMOFs (metal organic frameworks). Among them,aluminum fumarate MOF (A520) is known to absorb moisture even

Nyquist plot obtained from sulfonated cellulose and b) proposed equivalent circuit forc) relative humidity under constant temperature and d) temperature under constantmal stability of the membrane.

Fig. 3. Structural, morphological, and thermal properties of composite electrolytes. a) Structure of aluminum fumarate MOF (A520). SEM images of A520-SC membranes with A520content of b) 5 wt%, c) 10 wt%, d) 20 wt%, and e) interparticle distance between A520 particles measured from SEM images for each A520-SC sample. f) XRD pattern for compositemembranes compared with XRD of A520 and g) TGA/DTA curves for analysis of thermal stability.

J. Ko et al. / Materials Today Energy 9 (2018) 11e1814

at relative humidity lower than 25%, thus, is very fitting to serve asmoisture absorber [39,40]. For this reason, A520 was added to sul-fonated cellulose to improve performance at low humidity.

Fig. 3a contains the structural description of A520. The A520lattice is expected to absorb water molecules by confining theminside the organic chains, and also attain chain of extra water mol-ecules through adhesion at hydroxyl groups surrounding the lattice[39,40]. Fig. 3bed present the SEM images obtained from 5 wt%,10 wt%, and 20 wt% A520 added sulfonated cellulose (5 wt% A520-SC, 10 wt% A520-SC, and 20 wt% A520-SC), respectively. As theweight percentage of the added MOF increases, higher density ofA520 particles was observed. The particles were distributedthroughout the sample uniformly enough to enhance formation/retainment of water channels. The observed average distance be-tween the center of two closest particles were 3.29±1.61, 1.81±0.54,and 1.25±0.67 mm for 5 wt%, 10 wt%, and 20 wt% A520-SC, respec-tively (Fig. 3e).

XRD (Fig. 3f) and IR (Fig. S6) were performed on the samples.The XRD pattern at the bottom of Fig. 3f is diffraction pattern ob-tained from A520. For clear comparison of XRD patterns, the loca-tions of aluminum fumarate peaks are marked by dotted lines. Allcomposites samples, from 5 wt% to 20 wt%, contain the peaksobserved from A520. Furthermore, the intensity of the peaksidentified as A520 increases accordingly as the MOF content

increases from 5 wt% to 20 wt%. The IR results in Fig. S6 includespectra obtained from sulfonated cellulose and A520-SC mem-branes. A small peak appeared at 1574.5 cm�1 in addition to typicalspectrum for sulfonated cellulose, of which intensity dependeddirectly on concentration of A520. The peak may be attributed tostretching of C]C, which exists in the organic chain of A520. Toconfirm thermal stability, TGA and DTA were obtained, which aredisplayed in Fig. 3g. The TGA results of all composite membranesare very similar to that of sulfonated cellulose, exhibiting goodthermal stability at least up to 220 �C. The TGA thermograms of allcomposite membranes exhibited another weight loss near 285 �Cfrom decomposition of cellulose backbone followed by gradualweight loss due to detachment of organic chains in A520 [39,41].

Fig. 4a and b include the relative humidity dependent protonconductivity of 5 wt%, 10 wt%, and 20 wt% A520-SC electrolytescompared with sulfonated cellulose. Fig. 4a clearly demonstratesthe effect ofMOF addition to the electrolyte, where the conductivityreduction from dehydration is clearly suppressed due to addition ofA520. While the sulfonated cellulose without MOF additiondemonstrated conductivity reduction of more than two orders ofmagnitude as humidity decreases from 100% to 35%, 20 wt% A520-SC showed less than an order of magnitude decrease in conduc-tivity. The conductivity at different humidity is plotted against MOFconcentrations in Fig. 4b, from which more than an order of

Fig. 4. Transport properties of the A520-SC composite electrolytes. Proton conductivities measured from A520-SC a) in function of relative humidity for different composition andb) their dependence on MOF concentration at different relative humidity. c) Arrhenius plot which shows temperature dependent proton conductivity, and d) activation energydependence on added MOF concentration. e) Result of dc-polarization measurement and f) transference number of samples with different A520 concentration.

J. Ko et al. / Materials Today Energy 9 (2018) 11e18 15

magnitude enhancement in proton conductivity at 40%RH can beobserved as the A520 content increased from 0wt% to 20 wt%. Suchenhancement in low humidity proton conductivity is especiallyadvantageous when utilizing the electrolyte in applications thatcannot afford controlled environment (such as electrochemicalsensors).

The temperature dependent conductivity of the electrolytes isplotted in Fig. 4c, from which the activation energy was obtainedand compared with sulfonated cellulose in Fig. 4d. Gradually yetclearly, the measured activation energy increases with increasingA520 concentration. On the other hand, the proton conductivity at100% humidity decreases with increasing MOF content from172 mS/cm for sulfonated cellulose to 154 mS/cm for 20 wt% A520-SC (Fig. 4b). Such interesting trends can be explained by thechanges in association energy with introduction of MOF into theelectrolyte. It is well-established among researchers that activationenergy of ion conduction is directly related to association enthalpyof ions to hopping site especially at low temperature [42e44]. Thestrong absorption/adsorption properties of A520 causes extra as-sociation force to mobile ions, consequently, the conductivity at

high humidity is reduced slightly by the additional associationforce when A520 is added.

Although addition of MOF is clearly advantageous whenenhancing the proton conductivity at low humidity, it is importantto confirm that high transference number is still maintained evenafter MOF addition. The polarization measurement was performedon the composite electrolytes (Fig. 4e) from which resultingtransferences for A520-SC membranes were calculated. The trans-ference numbers (Fig. 4f) were 0.988, 0.990, and 0.989 for 5 wt%,10 wt%, and 20wt% A520-SC, respectively, indicating that negligibleamount of electron or hole conducts through the electrolytemembrane. Additionally, the sulfonated cellulose and A520-SCelectrolytes all exhibited excellent stability in high humidity(RH ¼ 90±4%) at least up to a month. Furthermore, the electrolytemembrane remained unchanged even after storing in ambient formore than 3 months. By altering the amount of sulfonation, themechanical property (such as flexibility) of the membrane can bemodified as well.

As a method to verify the applicability of the membrane as solidelectrolyte, all-solid-state supercapacitors were fabricated. All-

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solid-state supercapacitors provide few compelling advantage overliquid-electrolyte supercapacitor such as high chemical stability,safety, flexibility, and excellent processability to micro-sized de-vices [45e47]. For the solid state electrolytes, hydroxypropyl cel-lulose, sulfonated cellulose, and 20 wt% A520-SC were used andperformance was compared. A schematic diagram to describe thestructure of the solid-state-supercapacitor is provided in Fig. 5a.The detailed device fabrication process is included in SupportingInformation.

Fig. 5b displays the charge and discharge curves of the super-capacitors under 75% relative humidity, demonstrating typicalcharging-discharging performance. Fig. S8a and S8b in SupportingInformation are the cyclic voltammetry results obtained fromsupercapacitor made with sulfonated cellulose and 20 wt% A520-SC

Fig. 5. Schematic and performances of all-solid-state supercapacitors. a) Schematic diagramcurves obtained from the supercapacitor. Cyclic voltammetry results obtained from supercavarious humidity. e) Comparison of humidity dependent specific capacitance between the sthe supercapacitor with proposed equivalent circuit for impedance fitting.

electrolyte membranes at various scan rates ranging from 2 mV/s to20 mV/s at relative humidity of about 75%. The specific capacity wascalculated using the results above which equaled to 115.89 F/g(sulfonated cellulose) and 135.14 F/g (20 wt% A520-SC) per mass ofactive materials in electrodes. On the other hand, significant dif-ference in specific capacitance between sulfonated cellulose and20 wt% A520-SC was observed when they were measured in wet(100%RH) and dry (~41%RH) environments. The humidity depen-dent cyclic voltammetry results obtained at scan rate of 2 mV/s areincluded in Fig. 5c and d, and the resulting specific capacitances aresummarized in Fig. 5e. While the performance of 20 wt% A520-SCdevice was almost unchanged, a notably reduced performancewas observed from sulfonated cellulose capacitor probably due tosubstantial dehydration of the electrolyte. Dehydration significantly

of all-solid-state supercapacitor based on cellulose electrolyte and b) charge-dischargepacitors fabricated using c) sulfonated cellulose and d) 20 wt% A520-SC electrolytes atulfonated cellulose and 20 wt% A520-SC supercapacitors. f) Nyquist plot obtained from

J. Ko et al. / Materials Today Energy 9 (2018) 11e18 17

reduces number of mobile charges for constructing electric doublelayers at the electrode-electrolyte interface, thus, specific capaci-tance was reduced about three folds. On the other hand, dehydra-tion of 20 wt% A520-SC membrane was not significant enough toaffect the amount of mobile charges, thus, negligible change inspecific capacitance was observed. On the contrary, the capacitorbased on hydroxypropyl cellulose (Fig. S9) exhibited poor perfor-mance as expected from ordinary capacitor from simply dielectricmaterial.

Fig. 5e presents the Nyquist plots obtained from impedancespectroscopy measurement on sulfonated cellulose supercapacitorwith an equivalent circuit for impedance fitting. Almost identicalspectrum was observed from 20 wt% A520-SC supercapacitor aswell. In the equivalent circuit model, Rc represents the contactresistance, Rm and Cm are resistive and capacitive elements relatedto the electrolyte respectively, Wi is the Warburg element associ-ated with diffusion of protons at the interface [48,49], and Re and Ceare resistance and capacitance from the electric double layerformed at the electrolyte-electrode interface [49,50]. Lm is includedin the equivalent circuit to account for the inductance caused by themeasurement set-up. The impedance results clearly indicate thatsupercapacitor with very small cell resistance of about 0.8 U wasproduced with negligible resistance associated to contact(Rc < 0.8 U) or diffusion (WR ~ 1 U) at the interface, suggestingexcellent compatibility between activated carbon electrodes andthe sulfonated cellulose based electrolytes. The all-solid-statesupercapacitor maintained more than 90% of its capacitance evenafter 10,000 cycles of repeated charging and discharging asdemonstrated by Fig. S10 and Fig. S11 in Supporting Information.

3. Conclusion

An environmentally friendly electrolyte was synthesized bychemical functionalization of a renowned eco-friendly and bio-compatible material, hydroxypropyl cellulose. The sulfonated cel-lulose electrolyte exhibited high proton conductivity reaching0.172 S/cm at room temperature under relative humidity of 100%.The electrolyte was stable enough to maintain its conductivity for amonth under high humidity condition despite its hydrophilic na-ture, and remained intact when stored in air for more than threemonths. However, significant decrease in proton conductivity wasobserved at low humidity condition (RH < 40%), which wasimproved by more than an order of magnitude through addition ofA520. Moreover, the performances of the proto-type all-solid-statesupercapacitors based on these electrolytes were satisfactorydespite its simple design. Specific capacitance of the all-solid-statesupercapacitors fabricated using sulfonated cellulose and A520-SCelectrolytes reached 135 F/g, with negligible dependence to hu-midity down to ~40%RH when 20 wt% A520-SC electrolyte wasused. The supercapacitors exhibited superb stability at least up to10,000 charge-discharge cycles. This work may open up anotherpossibility for cellulose: an eco-friendly, naturally abundant, andbio-degradable super-material.

Acknowledgement

This work was supported by the Korea Research Fellowship(KRF) Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science and ICT (NRF-2016H1D3A1938211) as well as Multi-Ministry Collaborative R&DProgram through the National Research Foundation of Korea (NRF)funded by KNPA, MSIT, MOTIE, ME, NFA (NRF-2017M3D9A1073858). We thank Dr. Jong-San Chang for his fruitful discussion andcomments.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttps://doi.org/10.1016/j.mtener.2018.04.007.

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