Electrochimica Acta 50 (2005) 2837–2841

Anhydrous proton conductive membrane consisting of chitosan

Masanori Yamada, Itaru Honma∗

Nano-energy Materials Group, Energy Technology Research Institute, National Institute of Advanced Industrial Scienceand Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan

Received 9 June 2004; received in revised form 20 September 2004; accepted 1 November 2004

Abstract

We have investigated a low production cost anhydrous proton conductor consisting of a composite of chitosan, one of the world’s dis-carded materials, and methanediphosphonic acid (MP) having a high proton exchange capacity. This chitosan–200 wt.% MP compositematerial showed the high proton conductivity of 5× 10−3 S cm−1 at 150◦C under anhydrous conditions. Additionally, the proton con-ducting mechanism of the chitosan–MP composite material was due to proton transfer to the proton defect site without the assistance ofdiffusible vehicle molecules. The utilization of a biopolymer, such as chitosan, for PEMFC technologies is novel and challenging wherebiological products are usually considered as waste, non-hazardous, and environmentally benign. Especially, the low production cost oft ot only forP io-sensors,e©

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he biopolymer is an attractive feature. Anhydrous proton conducting biopolymer composite membranes may have potential nEMFCs operated under anhydrous conditions, but also for bio-electrochemical devices including an implantable battery, btc.2004 Elsevier Ltd. All rights reserved.

eywords:Polymer electrolyte membrane fuel cells (PEMFC); Proton conductivity; Biopolymer electrolyte; Chitosan; Acid–base composite mater

. Introduction

Chitin, a mucopolysaccharide composed ofN-acetyl-d-lucosamine, is present in the cell wall of fungi and in theutside skeleton of crustaceans and insects. Large amountsf chitin-enriched materials, such as crab and shrimp shells,ave been discarded as industrial waste around the world.hitosan, deacetylated chitin, is a highly specialized ba-ic biopolymer established as the main industrial derivativef chitin. Chitosan is a biodegradable, biocompatible, non-

oxic, and low-cost polymer, which shows many interestingroperties, such as wound healing, antibacterial activity, andinding in tissue[1]. Therefore, chitosan has been used forio- or medical-materials, such as a tissue engineering ma-

erial, surgical tape, and artificial skin[2–6]. However, ap-lications as an electrical or optical material have only beenarely reported. Applications of biopolymers in electrical de-

∗ Corresponding author. Tel.: +81 29 8615648; fax: +81 29 8615829.E-mail address:i.homma@aist.go.jp (I. Honma).

vices are not only interesting but also important for eronmental safety. Additionally, the production cost becolow by using a biopolymer instead of expensive engineepolymers.

Recently, proton conductors with a high proton cductivity under anhydrous and intermediate temperaconditions have attracted much attention as the electrfor a polymer electrolyte membrane fuel cell (PEMF[7–11]. The operation of the PEMFC at higher temperatimproves the carbon monoxide (CO) tolerance ofplatinum electrode and provides a higher energy efficiesimplified heat management, and co-generation[7–11].However, customary perfluorinated sulfonic acid mbranes, such as humidified Nafion®, are unstable at hightemperatures and proton conductivity abruptly decreaseto the evaporation of water from the electrolyte memband the destruction of the molecular structure by irreverreactions[12,13]. Additionally, the production cost of themembranes is extremely high, which makes it difficultthe industrialization of the PEMFC. Therefore, an anhyd

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2004.11.031

2838 M. Yamada, I. Honma / Electrochimica Acta 50 (2005) 2837–2841

proton conducting membrane with a low production cost,high proton conductivity, and high stability at intermediatetemperatures can offer multiple technological choices foradvanced membranes. In particular, acid–base compositematerials consisting of a low-cost artificial polymer havebeen reported as anhydrous electrolytes at intermediatetemperatures[10,11,14–24]. However, for the practicalutilization of a proton conducting membrane, the cost of thepolymer electrolyte has to be further reduced. Therefore,the utilization of biopolymer, one of discarded materials, toelectrolyte membrane has a high advantage.

In this study, we prepared an anhydrous proton conductingmembrane using a composite of chitosan, one of the basicbiopolymers with an amino group, and methanediphospho-nic acid (MP), which possesses a large proton exchangecapacity. This chitosan–MP composite material showedthe high proton conductivity of 5× 10−3 S cm−1 at 150◦Cunder anhydrous (water-free) conditions. Additionally, thethermal stability of this composite material was found toincrease with the mixing ratio of the MP molecule.

2. Experimental

2.1. Materials and preparation

dm akoP okyoK heirm sd –MPc –MPm df ft d byE

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The mixing ratio of the chitosan–MP composite materialwas widely varied from 0 to 500 wt.%. On the other hand,a pure chitosan membrane was prepared by the casting ofthe chitosan solution, which was dissolved in 5% aceticacid solution, onto the Teflon® plate. The solvents usedwere of analytical grade in all the experiments describedherein.

2.2. Characterization of chitosan–MP compositematerials

The thermal stability of the chitosan–MP composite mate-rials was determined by thermogravimetric–differential ther-mal analysis (TG–DTA) (TG–DTA 2000S, Mac SciencesCo., Ltd., Yokohama, Japan). The TG–DTA measurementwas done at the heating rate of 10◦C min−1 under a dry-nitrogen flow. The infrared (IR) spectra of the molecularstructure were characterized using an IR spectrophotometer(FTS-60, Bio-Rad Laboratories, Inc., PA) using the diamondattenuated total reflection (ATR) prism (Golden Gate Dia-mond ATR System, Specac Ltd., GA). The IR spectrum wasmeasured with the resolution of 4 cm−1.

2.3. Proton conductivity measurements of compositematerials

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Chitosan (MW = 1× 105, 80% deacetylated) anethanediphosphonic acid (MP) were obtained from Wure Chemical Industries Ltd., Osaka, Japan, and Tasei Kogyo Co., Ltd., Tokyo, Japan, respectively. Tolecular structures are shown inScheme 1. Chitosan waissolved in an aqueous MP solution. The chitosanomposite films were prepared as follows: a chitosanixed solution was cast onto the Teflon® plate and drie

or 24 h at room temperature or 70◦C. The mixing ratio ohe chitosan–MP composite material was determineq.(1):

ixing ratio of MP (wt.%) = 100× weight of MP

weight of chitosan(1)

cheme 1. Molecular structures of chitosan and methanediphosphonMP).

The proton conductivity of the chitosan–MP compoaterials was performed by the ac impedance me

n the frequency range from 1 Hz to 1 MHz usingI-1260 impedance analyzer (Solartron Co., HampsK) in a stainless steel vessel from room tempera

o 160◦C. The chitosan–MP composite material wandwiched between two gold-coated electrodes (diammm). The direction of the conductive measuremenerpendicular to the composite material. The condu

ties of the composite materials were determined froypical impedance response (Cole–Cole plots).easurements in this experiment were carried out udry-nitrogen flow. Additionally, the thickness of t

omposite material was measured after the impedeasurements.

. Results and discussion

.1. Preparation and characterization of chitosan–MPomposite materials

The chitosan–methanediphosphonic acid (MP) molution was cast onto a Teflon® plate and dried.Fig. 1hows a photograph of the chitosan–200 wt.% MP complm (thickness: approximately 100�m). The free-standinlm is transparent, flexible, and homogeneous. The fire stable enough for finger pulling.Fig. 2a and b show th

hermogravimetric (TG) and differential thermal analyDTA) of (1) pure chitosan, (2) chitosan–200 wt.% M

M. Yamada, I. Honma / Electrochimica Acta 50 (2005) 2837–2841 2839

Fig. 1. Photograph of chitosan–200 wt.% MP composite film with thicknessof ca. 100�m. A scale bar is 10 mm.

(3) chitosan–500 wt.% MP, and (4) pure MP materials.The pure chitosan material showed the TG weight loss ofseveral percent below 200◦C (line (1) in Fig. 2a). Thisweight loss is due to solvent evaporation from the film.The pure MP material indicated an endothermic peak bymelting at 179.0◦C (line (4) in Fig. 2). Surprisingly, thisendothermic peak disappeared by mixing with chitosan anda small exothermic behavior appeared (lines (2) and (3)in Fig. 2). These results suggested that the chitosan–MPcomposite material formed an acid–base complex in the

F witht n,( MPm

composite material through the electrostatic cross-linkingof the amino and phosphonic acid groups. In contrast, thechitosan–MP composite materials showed a TG weight lossbelow 170◦C. The composite material, presumably, formedthe random network structure by the mixing of chitosan andMP molecules. The network structure may contain residualwater due to the evaporation of water from the network.Therefore, the chitosan–MP composite material indicatesthe TG weight loss below 170◦C. On the other hand, thecomposite material indicated a high TG weight loss of30–40% at 250◦C. The DTA curve of the composite materialabove 200◦C appeared as a small exothermic behavior.Therefore, we think that this is due to the decomposition ofthe polymer backbone by the presence of a strong acid, suchas MP molecules, and the thermal energy activation.

On the other hand, when a small amount of MP molecules(≤100 wt.% MP) was mixed in the chitosan solution, thechitosan–MP composite material immediately precipitateddue to the intermolecular cross-linking by an electrostatic in-teraction and could not form the composite film. Therefore,we have mainly used the chitosan–MP (≥150 wt.%) compos-ite material in this study.

3.2. Molecular structure of chitosan–MP compositematerials

sitem lysisw oft san,( (d)c Thea g

F dif-f , (c)c ate-r

ig. 2. TG (a) and DTA (b) curves of chitosan–MP composite materialshe heating rate of 10◦C min−1 under dry nitrogen flow. (1) pure chitosa2) chitosan–200 wt.% MP, (3) chitosan–500 wt.% MP, and (4) pureaterials.

The molecular structures of the chitosan–MP compoaterial were characterized by an IR spectroscopic anaith the diamond ATR prism.Fig. 3shows the IR spectra

he chitosan–MP composite materials of (a) pure chitob) chitosan–50 wt.% MP, (c) chitosan–200 wt.% MP,hitosan–500 wt.% MP, and (e) pure MP materials.bsorption band at 1545 cm−1, attributed to the bendin

ig. 3. Infrared (IR) spectra of chitosan–MP composite materials witherent mixing ratio of MP: (a) pure chitosan, (b) chitosan–50 wt.% MPhitosan–200 wt.% MP, (d) chitosan–500 wt.% MP, and (e) pure MP mials.

2840 M. Yamada, I. Honma / Electrochimica Acta 50 (2005) 2837–2841

vibration of the amino group in chitosan[25], disappearedwhen MP molecules were mixed with the chitosan material.The absorption band at 1522 and 1616 cm−1 [25], thesymmetric and asymmetric bending vibrations of –NH3

+,respectively, appeared with the addition of MP molecules(seeFig. 3b–d). The disappearance of –NH2 and appearanceof –NH3

+ indicated the protonation of the amino group in thechitosan by the addition of the MP molecule. Additionally,the absorption band at 1641 cm−1, related to the COstretching peak of the chitin form, was hid by the appearanceof –NH3

+ band and the broad band of the phosphonic acidgroup. On the other hand, for the MP molecules composite,two new absorption bands appeared at 1082 and 990 cm−1,the stretching vibration band of –HPO3

− and –PO32−[14–18,26], respectively. These results suggest that the P–OHgroup of the MP molecule deprotonates due to the mixingwith chitosan and forms the P–O− group in the compositematerial. However, the definite decrease in the absorptionband of the P–OH groups at 1002 cm−1 was not observed bythe overlap of the chitosan peak. This free proton stronglyinteracts with the non-protonated –NH2 group of chitosan.Therefore, the complex of chitosan and MP produces theacid–base salt, such as (chitosan)–NH3

+. . .−O–P–(MPmolecule), in the chitosan–MP composite film.

3.3. Proton conductivity of chitosan–MP compositem

an–M aci z to1 ityw sames nfl itht thant thert rials.F e thee ore,t ydrousp re-s P at1 eri-a con-d ec thee con-d ithdM ,a thets hec aturea ,

Fig. 4. Typical impedance response (Cole–Cole plots) of chitosan–200 wt.%MP composite materials at 150◦C under dry nitrogen condition. Frequencyrange: (a) from 1 Hz to 1 MHz; (b) from 2 kHz to 1 MHz.

the chitosan–200 wt.% MP composite material indicated theanhydrous proton conductivity of 5× 10−3 S cm−1 at 150◦C.In contrast, the pure chitosan film and lightly mixed mate-rial (mixing ratio of≤100 wt.% MP), which could not formthe film, did not show any measurable proton conductivity(<10−8 S cm−1 at 150◦C). Additionally, the proton conduc-tivity of the pure MP material was 8× 10−5 S cm−1 at 150◦Cunder anhydrous conditions. These results suggest that the ad-dition of the MP molecule, one of the solid phosphonic acids,into the chitosan film is an effective technique to increase theanhydrous proton conductivity.

Previously, we reported the proton conducting membraneof the acid–base complex, such as MDP/BnIm[14], UI/MDP[15], or PVPA/heterocyclic molecule[18] composite mate-rials, under anhydrous and intermediate temperature condi-tions. In these cases, the Grotthuss-type diffusion mechanismwithout the assistance of diffusible vehicle molecules, suchas H3O+ or H5O2

+ ions, has been proposed, in which thetransport of the proton in the membrane can occur from pro-tonated molecules to a non-protonated molecule or from theneighboring proton to the proton defect site[27,28]. Namely,the proton conducting molecules in an acid–base complexmay act as proton donors and acceptors[27,28]. In our re-search, the proton conducting mechanism was postulated as

F dif-fMc

aterials under anhydrous condition

The proton conductivity measurements of the chitosP composite materials were demonstrated by the

mpedance method over the frequency range from 1 HMHz under a dry nitrogen flow. The proton conductivas obtained by four repeated measurements on theample from room temperature to 160◦C under nitrogeow conditions. The conductivity slightly decreased whe repetition and reached a steady state after morehree measurements. Additionally, the diffusible ions ohan the protons do not exist in the composite mateurthermore, these composite samples did not indicatlectronic conductivity under the dc condition. Theref

he measured impedance response indicates the anhroton conductivity.Fig. 4 shows the typical impedanceponse (Cole–Cole plots) of the chitosan–200 wt.% M50◦C. Typical Cole–Cole plots of the composite matls showed a feature similar to that of the high protonucting membrane, such as Nafion®. The resistances of thhitosan–MP composite material were obtained fromxtrapolation to the real axis. The anhydrous protonuctivities of the chitosan–MP composite materials wifferent MP mixing ratios of the (�) chitosan–150 wt.%P, (©) chitosan–200 wt.% MP, (×) chitosan–300 wt.% MPnd (�) chitosan–500 wt.% MP composite materials in

emperature range from room temperature to 160◦C arehown inFig. 5. The anhydrous proton conductivity of thitosan–MP composite material increased with tempernd reached a maximum conductivity at 150◦C. Especially

ig. 5. Proton conductivities of chitosan–MP composite materials witherent MP mixing ratio under anhydrous condition: (�) chitosan–150 wt.%P; (©) chitosan–200 wt.% MP; (×) chitosan–300 wt.% MP; (�)

hitosan–500 wt.% MP composite materials.

M. Yamada, I. Honma / Electrochimica Acta 50 (2005) 2837–2841 2841

Fig. 6. Proposed anhydrous proton conducting mechanism of chitosan–MPcomposite material. Arrows indicate the direction of proton transfer.

follows: the phosphate group in the MP molecule forms theproton defect site (–P–O−) by the electrostatic interactionwith the amino group in chitosan (see IR spectra ofFig. 3),and as a result, the neighboring proton in the MP moleculecan transfer to the proton defect site without the assistance ofdiffusible vehicle molecules.Fig. 6 shows the proposed an-hydrous proton conducting mechanism of the chitosan–MPcomposite material. The proton in these acid–base compositematerials can move to a neighboring molecule with a smallactivation energy, in particular, above 100◦C. In fact, thisactivation energy was approximately 0.5 eV. This value isalmost the same as other materials reported as anhydroussingle proton conductors, such as solid electrolytes[29,30],fullerene derivatives[31,32], or UI/MDP[15] composite ma-terials. Therefore, the chitosan–200 wt.% MP composite ma-terial showed a large proton conductivity of 5× 10−3 S cm−1

at 150◦C under anhydrous conditions. On the other hand,the distance between the amino groups in the pure chitosanmaterial was too long for the direct transfer of protons, andchitosan without the addition of MP did not show any mea-surable proton conductivity. In addition, for the lightly mixedmaterial (the mixing ratio of≤100 wt.% MP), the movableproton in the composite material disappeared by the strongelectrostatic interaction of the amino groups in the chitosanand phosphonic acid group in the MP molecules. As a result,no measurable anhydrous proton conductivity was obtained.

4

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low humidity conditions, but also in bio-electrochemicaldevices including an implantable battery, bio-sensors, etc.

Acknowledgement

This work was supported by the R&D program of PEFCby the New Energy and Industrial Technology DevelopmentOrganization (NEDO), Japan.

References

[1] M.N.V.R. Kumar, React. Funct. Polym. 46 (2000) 1.[2] M. Mochizuki, Y. Kadoya, Y. Wakabayashi, K. Kato, I. Okazaki, M.

Yamada, T. Sato, N. Sakairi, N. Nishi, M. Nomizu, FASEB J. 17(2003) 875.

[3] K. Kato, A. Utani, N. Suzuki, M. Mochizuki, M. Yamada, N.Nishi, H. Matsuura, H. Shinkai, M. Nomizu, Biochemistry 41 (2002)10747.

[4] J.K.F. Suh, H.W.T. Matthew, Biomaterials 21 (2000) 2589.[5] T. Chandy, C.P. Sharma, Biomater. Artif. Cells Artif. Organs 18

(1990) 1.[6] I.V. Yannas, J.F. Burke, D.P. Orgill, E.M. Skrabut, Science 215

(1982) 174.[7] K.D. Kreuer, Chem. Phys. Chem. 3 (2002) 771.[8] J.A. Kerres, J. Membrane Sci. 185 (2001) 3.[9] K.D. Kreuer, J. Membrane Sci. 185 (2001) 29.

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[ hus-

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. Conclusion

We have investigated the anhydrous proton conduonsisting of chitosan, one of the discarded biopolymnd methanediphosphonic acid possessing a high pxchange capacity. This chitosan–200 wt.% MP cosite material showed the high proton conductivity× 10−3 S cm−1 at 150◦C under anhydrous conditions. Ttilization of a biopolymer, such as chitosan, for PEM

echnologies is novel and challenging for biological prodhat are cheap, usually considered as waste, non-hazand environmentally benign. Especially, the fuel cell withroton conducting biopolymer electrolyte can be expecteduce the cost. The biopolymer composite material mayhe potential not only for its superior ion conducting propies, in particular, under anhydrous (water-free) or extrem

,

10] M. Rikukawa, K. Sanui, Prog. Polym. Sci. 25 (2000) 1463.11] M.F.H. Schuster, W.H. Meyer, M. Schuster, K.D. Kreuer, Ch

Mater. 16 (2004) 329.12] Q. Li, R. He, J.O. Jensen, N.J. Bjerrum, Chem. Mater. 15 (2

4896.13] Y. Sone, P. Ekdunge, D. Simonsson, J. Electrochem. Soc. 143 (

1254.14] M. Yamada, I. Honma, Electrochim. Acta 48 (2003) 2411.15] M. Yamada, I. Honma, J. Phys. Chem. B 108 (2004) 5522.16] M. Yamada, I. Honma, Chem. Phys. Chem. 5 (2004) 724.17] M. Yamada, I. Honma, Angew. Chem. Int. Ed. 43 (2004) 368818] M. Yamada, I., Honma, submitted.19] H. Pu, W.H. Meyer, G. Wegner, Macromol. Chem. Phys. 202 (2

1478.20] A. Schechter, R.F. Savinell, Solid State Ionics 147 (2002) 18121] A. Bozkurt, W.H. Meyer, Solid State Ionics 138 (2001) 259.22] M. Schuster, W.H. Meyer, G. Wegner, H.G. Herz, M. Ise, M. Sc

ter, K.D. Kreuer, J. Maier, Solid State Ionics 145 (2001) 85.23] H.G. Herz, K.D. Kreuer, J. Maier, G. Scharfenberger, M.F.H. Sc

ter, W.H. Meyer, Electrochim. Acta 48 (2003) 2165.24] F. Sevil, A. Bozkurt, J. Phys. Chem. Solid 65 (2004) 1659.25] R.M. Silverstein, F.X. Webster, Spectrometric Identification of

ganic Compounds, Wiley, New York, 1998.26] K.D. Berlin, G.M. Blackburn, J.S. Cohen, D.E.C. Corbridge, D

Hellwege, Topics in Phosphorus Chemistry, Wiley, New York, 127] K.D. Kreuer, Chem. Mater. 8 (1996) 610.28] T. Dippel, K.D. Kreuer, J.C. Lassegues, D. Rodriguez, Solid Sta

Ionics 61 (1993) 41.29] Y.M. Li, M. Hibino, M. Miyayama, T. Kudo, Solid State Ionics 1

(2000) 271.30] P. Colomban, A. Novak, J. Mol. Struct. 177 (1988) 277.31] K. Hinokuma, M. Ata, Chem. Phys. Lett. 341 (2001) 442.32] Y.M. Li, K. Hinokuma, Solid State Ionics 150 (2002) 309.