Alginic acid–imidazole composite material as anhydrous

proton conducting membrane

Masanori Yamada, Itaru Honma*

Nano-energy Materials Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),

Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan

Received 4 June 2004; received in revised form 16 September 2004; accepted 5 October 2004

Abstract

Recently, membranes with high anhydrous proton conducting have been attracted remarkable interest for the application to the polymer

electrolyte membrane fuel cell (PEFC). In this paper, we have investigated the anhydrous proton conductor consisting of alginic acid (AA),

one of the acidic biopolymers, and imidazole (Im). This AA–Im composite material showed the proton conductivity of 2!10K3 S cmK1 at

130 8C under anhydrous conditions. Additionally, these AA–Im composite materials have the highly mechanical property and thermal

stability. Furthermore, the biological products, such as biopolymer, are cheap, non-hazardous, and environmentally benign. The proton

conductive biopolymer composite material may have the potential for its superior ion conducting properties, in particular, under anhydrous

(water-free) or extremely low humidity conditions.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Proton conductivity; Polymer electrolyte membrane fuel cell; Biopolymer

1. Introduction

Alginic acid, one of the organized elements of marine

algae, is a natural polysaccharide containing of linear chains

of 1,4 0-linked b-D-mannuronic acid and a-L-guluronic acid.

Alginic acid is a biodegradable, biocompatible, non-toxic,

and low cost polymer, which shows many interesting

properties, such as wound healing, ion-exchange ability, and

absorption of metal ion [1–5]. Therefore, this natural

product has already found many applications in the food

and pharmaceutical industries, such as a tissue engineering

material, surgical tape, and artificial skin [1–3,6,7]. Many

investigations of alginic acid, so far, have been reported for

medical- [1–3,6,7] or environmental-materials [4,5], how-

ever applications for electrical or optical materials have not

been reported. Applications of alginic acid in electrical

devices are not only interesting as a material science but

also important for environmental safety and product cost

through a green chemistry approach.

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.10.017

* Corresponding author. Tel.: C81 29 8615648; fax: C81 29 8615829.

E-mail address: i.homma@aist.go.jp (I. Honma).

Polymer electrolyte membrane fuel cells (PEFC) are one

of the attractive energy conversion systems to be used in

many industrial applications including electric vehicles,

mobile telephone, and on-site power generations [8–14].

Recently, the operation of PEFC at intermediate tempera-

ture (100–200 8C) has been considered to provide many

advantages, such as improved carbon monoxide (CO)

tolerance of the platinum electrode, the higher energy

efficiency, simplified heat managements, and co-gener-

ations [8–14]. However, the customary perfluorinated

sulfonic acid membranes, such as Nafionw, is not so stable

at higher temperature (above 100 8C) and the proton

conductivity decreases abruptly due to a loss of water

from the membrane. Additionally, the production cost of

perfluorinated membranes is extremely high, which make it

difficult for an industrialization of the PEFC. Therefore, the

highly proton conducting membrane consisting of non-

expensive material and thermal stability at intermediate

temperature is necessary for the advanced PEFC technology

[15–18].

Recently, composite materials of strong acids and basic

polymers (molecules) have been considered to be polymer

Polymer 45 (2004) 8349–8354

www.elsevier.com/locate/polymer

Scheme 1. Molecular structure of alginic acid (AA) and imidazole (Im).

M. Yamada, I. Honma / Polymer 45 (2004) 8349–83548350

electrolytes of single proton conductors under intermediate

temperature [15–25] and anhydrous conditions [13–18,22–

25]. The proton transport under anhydrous (water-free)

conditions might be based on a non-vehicular mechanism

(Grotthuss mechanism), in which only protons are mobile

from site to site without an assistance of diffusible vehicle

molecules, such as water molecules [26]. This activation

energy of proton transport can primarily depend on the

distance between the hopping sites [26]. This type of the

mechanism has been reported for polybenzimidazole(PBI)-

strong acid [25,27,28], imidazole group immobilized

polymer-strong acid [29–33], sulfonated poly(ether–

ether–ketone) (S-PEEK)-heterocycle [34], polyacrylic

acid-heterocycle [35], mono-dodecylphosphate (MDP)–

benzimidazole (BnIm) composite material [15], and

2-undecylimidazole (UI)–MDP composite material [16].

However, these anhydrous electrolytes membranes require

the artificial polymers or molecules. Therefore, these

membranes are high cost performance and not benign for

the environment. So, we have been investigated the

utilization of biopolymer, such as chitin or chitosan, as an

anhydrous proton conducting membrane for PEFC technol-

ogy [18].

In this study, we have studied anhydrous proton

conducting properties of membrane prepared from alginic

acids (AA), one of the acidic biopolymers, and imidazole

(Im) molecules, one of the basic heterocyclic molecules.

This AA–Im composite material showed the maximum

proton conductivity of 2!10K3 S cmK1 at 130 8C under an

anhydrous (water-free) condition. Additionally, these com-

posite materials indicated the high mechanical property at

intermediate temperature.

2. Experimental

2.1. Materials

Alginic acid (AA) and imidazole (Im) were obtained

from Wako Pure Chemical Industries Ltd., Osaka, Japan and

Tokyo Kasei Kogyo Co., Ltd., Tokyo, Japan, respectively.

Molecular structures are shown in Scheme 1. AA–Im

composite materials were prepared as follows: AA

(20 mg mlK1) was dissolved in Im solution. AA–Im

mixed solution (100 ml) was cast onto the glass or Teflonw

plates and dried for 24 h at room temperature. The molar

ratio (R) of AA–Im composite material was determined by

the Eq. (1).

RZ½Im�

½KCOOH group of AA�(1)

[Im] and [–COOH group of AA] of Eq. (1) are molar

concentration of Im and carboxyl group in AA, respectively.

The R value in AA–Im composite material was widely

changed from 0 to 5. Solvents were used an analytical grade

in all the experiments described.

2.2. Characterizations of composite materials

Dynamic mechanical data of pure AA and AA–Im

composite membranes were measured by dynamic visco-

elastometer (DMS6300, Seiko Instruments, Inc., Chiba,

Japan) within the temperature range of K50 to 150 8C with

a heating rate of 2 8C minK1. The frequency was 0.5, 1, 2, 5,

and 10 Hz. The thermal stability of these mixed materials

was analyzed by thermogravimetric-differential thermal

analysis (TG-DTA) (TG-DTA 2000S, Mac Sciences Co.,

Ltd., Yokohama, Japan). TG-DTA measurement was

demonstrated at a heating rate of 10 8C minK1 under

nitrogen flow. Infrared (IR) spectra were characterized by

the attenuated total reflection (ATR) method using IR

spectrophotometer (FTS-60, Bio-Rad Laboratories, Inc.,

PA). The IR spectrum was measured with a resolution of

4 cmK1.

2.3. Conductivity measurements

Proton conductivity of AA–Im composite materials was

measured by the a.c. impedance method in a frequency

range from 1 Hz to 1 MHz using an impedance analyzer SI-

1260 (Solartron Co., Hampshire, UK) in a stainless steel

vessel from RT to 180 8C. The AA–Im composite materials

(thickness: 50 mm) sandwiched between two gold-coated

electrodes (diameter: 5 mm). The direction of conductive

measurement is perpendicular to the material. All measure-

ments in this work were carried out under dry-nitrogen flow

[15–18].

Fig. 2. Dynamic mechanical behavior of (1) pure AA membrane and (2)

AA–Im (RZ2) composite membrane. Left and right axes show the storage

modulus (E0) and tan d of materials, respectively.

M. Yamada, I. Honma / Polymer 45 (2004) 8349–8354 8351

3. Results and discussion

Fig. 1 shows the photograph of AA–Im composite

membrane of (a) composite with RZ2 and (b) RZ5,

respectively. The thickness of membranes is ca. 50 mm. The

free-standing membrane is transparent, flexible, and homo-

geneous. Fig. 2 shows the dynamic mechanical analysis

(DMA) curves of (1) pure AA and (2) AA–Im (RZ2)

composite material, respectively. At low temperature, pure

AA shows a storage modulus (E 0) of approximately 1010 Pa.

This mechanical property slightly decreased with the

increase of temperature and reached a constant value of

6!109 Pa at 150 8C. The mechanical property of AA–Im

composite materials with the mixing of Im molecules shows

the identical value, approx. 1010 Pa, with pure AA at K50 8C. However, the storage modulus decreases by the

melting of water content in composite membrane and

indicate the minimum peaks at 18.9 8C. Afterwards, the

mechanical property increases by the evaporation of water

from membrane. In fact, the tan d curve shows the

maximum and minimum peaks at 4.7 and 66.2 8C,

respectively. These results suggest that AA–Im composite

material has been maintained the highly mechanical

property of 108 Pa at the intermediate temperature region

(%150 8C). The strength is almost equivalent to the value of

Nafionw membrane which has been reported to have 107–

1010 Pa [36].

Fig. 3(a) and (b) show the thermogravimetric (TG) and

differential thermal analyses (DTA) of (1) pure AA, (2)

composite with RZ0.5, (3) RZ0.8, (4) RZ1, and (5) RZ2

(6) RZ5, respectively. The TG weight loss of all samples

below 160 8C is mainly due to evaporation of water from the

material (Fig. 3(a)). At the DTA analysis, the pure AA

materials indicate the endothermic peak by the thermal

decomposition at 161.3 8C (line (1) in Fig. 3(b)). In fact, the

pure AA showed the large TG weight loss above 180 8C

(line (1) in Fig. 3(a)). In the samples with small mixing

ratios (R%1), the exothermic peak, related to the thermal

decomposition, appeared at 170 8C (line (2)–(4) in Fig.

3(b)). Additionally, the sample with large mixing ratio

(RR2) showed the broad endothermic peak at 140 8C (line

(5) and (6) in Fig. 3(b)) and TG weight loss (line (5) and (6)

in Fig. 3(a)). These results suggested that AA–Im composite

Fig. 1. Photographs of AA–Im composite materials with thickness of ca.

50 mm. Mixing ratios of Im are (a) RZ2 and (b) RZ5, respectively.

material indicate the thermal decomposition above approx.

140 8C. In contrast, the TG weight loss of composite

material above 200 8C is due to the evaporation of the Im

molecule from the membrane and the thermal decompo-

sition. Additionally, AA–Im composite materials do not

produce the diffusible ion by the melting of samples.

The molecular structures of acid–base complex in the

AA–Im composite material were characterized by an IR

Fig. 3. TG (a) and DTA (b) curves of AA–Im composite materials with the

heating rate of 10 8C minK1 under dry nitrogen. (1), pure AA; (2),

composite with RZ0.5; (3), RZ0.8; (4), RZ1; (5), RZ2 and (6) RZ5. A

scale bar indicates 20 mV.

Scheme 2. Proposed molecular structure of AA–Im complex in composite

material.

M. Yamada, I. Honma / Polymer 45 (2004) 8349–83548352

spectroscopic analysis with the diamond ATR prism. Fig. 4

shows the IR spectra of the AA–Im composite materials of

(a) pure AA, (b) RZ0.2, (c) RZ0.5, (d) RZ0.8, (e) RZ1,

(f) RZ2, (g) RZ3, (h) RZ4, (i) RZ5, and (j) pure Im

material, respectively. The absorption band at 1730 cmK1

[37], attributed to the stretching vibration of CaO in –

COOH group, decreased when Im molecules were mixed in

the AA material. In addition, the symmetric and asymmetric

stretching vibration [37] of –COOK at 1600 and 1400 cmK1

increased with the mixing, respectively. These results

indicated that –COOH group of AA deprotonates by the

mixing of Im and form the –COOK group [35]. On the other

hand, the absorption band at ca. 3200 cmK1 in the AA–Im

composite material, the stretching vibration band of N–H

group in Im molecule [15–18,35,37], moderately increased

in comparison with the spectrum of pure Im. Additionally,

the absorption band [37] of CaN at 1445 cmK1 in AA–Im

composite material decreased ((g) and (h) in Fig. 4). These

results suggest that –COOH group deprotonates by the

doping of Im and form the –COOK group. This free proton

strongly interacts with the non-protonated –Na group of Im.

Therefore, the composite of AA and Im produce the acid–

base salt, i.e. imidazolium organic salts (shown in Scheme

2) [35].

The proton conductivity measurements of the AA–Im

composite materials were demonstrated by the a.c. impe-

dance method over the frequency range from 1 Hz to 1 MHz

under dry nitrogen flow. In contrast, these composite

samples did not indicate the electronic conductivity at the

DC condition. Additionally, the diffusible ions other than

proton have not existed in the composite samples. There-

fore, the measured impedance response indicates the

Fig. 4. IR spectra of AA–Im composite materials with different mixing ratio

of Im. (a), pure AA; (b), composite with RZ0.2; (c), RZ0.5; (d), RZ0.8;

(e), RZ1; (f), RZ2; (g), RZ3; (h), RZ4; (i), RZ5; and (j), pure Im

materials. A scale bar indicates the transmittance of 50%.

anhydrous proton conductivity [15–18]. Fig. 5 shows the

typical impedance response (Cole–Cole plots) of the AA–

Im (RZ2) composite material at 130 8C. A typical Cole–

Cole plots of composite materials showed a feature similar

to that of the highly proton conducting membrane, such as

Nafionw, the organic–inorganic hybrid membrane mixed

with heteropolyacids [19–21], MDP-BnIm [15], UI–MDP

[16], and uracil (U)–MDP [17] composite materials. The

resistances of AA–Im composite material were obtained

from the extrapolation to the real axis. The anhydrous

proton conductivities of the AA–Im composite materials

with different Im mixing ratios of the (6) RZ0.8, (,) RZ1, (B) RZ2, ($) RZ3, and (7) RZ5 in the temperatures

range from RT to 160 8C are shown in Fig. 6. The anhydrous

proton conductivity of the AA–Im composite material

increased with the temperature. Although the composite

material of the large mixing ratios (RR2) showed the high

anhydrous proton conductivity of 10K3 S cmK1, the con-

ductivity decreased above approximately 140 8C. This

decrease is due to the thermal decomposition (see the TG-

DTA analyses). In particular, RZ5 composite material

Fig. 5. Typical impedance response (Cole–Cole plots) of AA–Im (RZ2)

composite materials at 130 8C under dry nitrogen condition. Frequency

range (a) from 1 Hz to 1 MHz and (b) from 2 kHz to 1 MHz.

Fig. 6. Proton conductivities of AA–Im composite materials with different

Im mixing ratio under anhydrous condition. Mixing ratios of Im are (6),

RZ0.8; (,), RZ1; (B), RZ2; ($), RZ3; and (7), RZ5, respectively.

M. Yamada, I. Honma / Polymer 45 (2004) 8349–8354 8353

showed the large decrease of conductivity by the thermal

decomposition and the evaporation of excessive imidazole

molecules. Additionally, the composite materials of low

mixing ratios (R!2) also decreased by the thermal

decomposition above 160 8C. So we compared the anhy-

drous proton conductivity at 130 8C where no thermal

decomposition occurs. Fig. 7 shows the proton conductivity

at 130 8C of the composite material as a function of Im

mixing ratios. The conductivity increased with the mixing

ratio and finally reached a maximum conductivity of

approximately 2!10K3 S cmK1 at the Im mixing ratio of

RZ2. In contrast, the pure AA membrane did not show any

measurable proton conductivity (!10K8 S cmK1 at

130 8C). Additionally, the anhydrous proton conductivity

of pure imidazole molecule could not be measured by the

melting and evaporation at intermediate temperature; i.e. in

our experiment, since the proton conductive measurements

have been performed by the sandwich of two electrode, the

conductivity of material at the liquid state could not

determine. These results suggest that biopolymer-hetero-

cycle composite material becomes proton conductive by

forming acid–base ionic pairs.

Next, we determined the activation energy of proton

Fig. 7. Change of the proton conductivities at 130 8C as a function of a

mixing ratio of Im. Proton conductive measurements were demonstrated

under dry-nitrogen condition.

conduction for AA–Im composite material. Fig. 8 shows the

Arrhenius plot of AA–Im composite material in the

temperature range from 80 to 140 8C. Solid lines in Fig. 8

are the results of the least-squares fitting. The activation

energies (Ea) estimated from the slop are 1.2–1.6 and 0.2–

0.4 eV at the small (R%1) and large (RR2) mixing ratios,

respectively. The activation energy of 0.2–0.4 eV at RR2

are one order higher than that of fully swelled Nafionw

membrane, and almost the same as other materials, such as

solid electrolytes [38–40], fullerene derivatives [41–43],

MDP-BnIm [15], UI–MDP [16], and U–MDP [17] compo-

site materials for anhydrous proton conductors. Therefore,

the transport of the proton in AA–Im (RR2) composite

material is suggested to occur from protonated Im molecules

to non-protonated neighboring Im molecules with some

activation energy. Namely, at the high mixing ratio, the

proton conducting pathway has been constructed in the

composite material and the distance between the hopping

sites has been shorten. As a result, AA–Im (RR2)

composite material indicated the high anhydrous proton

conductivity of 2!10K3 S cmK1 at 130 8C. On the other

hand, the activation energy, such as 1.2–1.6 eV, at R%1 is

extremely larger than that of other reported materials. This

high activation energy is due to the long distance between

the hopping sites. Additionally, the proton hopping distance

of pure AA or small mixed material (R%0.5) was too long

to hop to neighboring site, as a result, these materials could

not show any measurable proton conductivity (!10K8 S

cmK1).

Previously, we have reported the biomolecular compo-

site material, such as chitin phosphate-heterocyclic mol-

ecules composite materials, as an anhydrous proton

conducting membrane [18]. In these cases, the composite

materials showed the high proton conductivity of R10K3 S

cmK1 at 150 8C under anhydrous condition. Additionally,

these materials had a high thermal stability. However,

AA–Im composite materials did not indicate the satisfactory

Fig. 8. Arrhenius plots of the conductivity of AA–Im composite materials.

Solid lines are results of the least-squares fitting. Activation energies of the

proton transport under anhydrous condition were estimated from the slope.

Mixing ratios of Im in the composites are (6), RZ0.8; (,), RZ1; (B),

RZ2; ($), RZ3; and (7), RZ5, respectively.

M. Yamada, I. Honma / Polymer 45 (2004) 8349–83548354

conductivity and thermal stability in comparison with the

reported materials [15–18]. One of reasons might be the

effect of pKa value (pKaZ3.1) of –COOH group in AA [44].

In previous reports [15–18], we had used the phosphonic

acid group, which is stronger than carboxylic acid. The

phosphonic acid group and basic group forms a strong acid–

base complex in composite membrane, as a result, the free

proton from phosphonic acid strongly interacts with non-

protonated –Na [15–18]. However, a weak acid, such as –

COOH group in AA, cannot form the strong acid–base in

composite membrane and not provide enough mobile-

protons to Im molecules, so that AA–Im composite showed

the lower anhydrous proton conductivity than the phospho-

nic acid composite materials.

4. Conclusion

We have investigated the anhydrous proton conductor

consisting of a biological polymer of AA and Im. This AA–

Im (RZ2) composite material showed the proton conduc-

tivity of 2!10K3 S cmK1 at 130 8C under anhydrous

conditions. Additionally, these AA–Im composite materials

have the highly mechanical property and thermal stability.

Furthermore, the biological products, such as biopolymer,

are non-hazardous and environmentally benign. The proton

conductive biopolymer composite material may have the

potential for its superior ion conducting properties, in

particular, under anhydrous (water-free) or extremely low

humidity conditions but also in bio-electrochemical devices

including an implantable battery, bio-sensors, and others.

Acknowledgements

This work was supported by R&D program of PEFC by

New Energy and Industrial Technology Development

Organization (NEDO), Japan.

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