modification of solid state proton conductors

w.elsevier.com/locate/ssi

Solid State Ionics 176 (2

Modification of solid state proton conductors

A.B. Yaroslavtsev

Kurnakov Institute of General and Inorganic Chemistry RAS, Leninsky pr. 31, Moscow 119991, Russia

Abstract

A review of methods for ceramic proton conductor modification is presented. Point defects play the key role in the conductivity increase. The

increase in point defect concentration can be achieved in different ways. Heterogeneous and heterovalent homogeneous doping, synthesis of

nanomaterials and inorganic–organic composite materials can be considered as the most promising of these.

D 2005 Elsevier B.V. All rights reserved.

PACS: 74.62.Dh; 72.10.Fk; 73.30.+y; 73.40._c; 77.84.Lt

Keywords: Proton conductors; Conductivity; Defects; Modification; Nanomaterials; Heterovalent doping; Composite materials

1. Introduction

The demand for new power sources and environmental

monitoring determine the need for new materials possessing high

proton conductivity; this is the case for the production of fuel cells

or sensors. In recent years, only a few new materials with high

proton conductivity have been obtained [1]. This makes the

modification of known materials in order to improve their

conductive properties more relevant. The main goal of this article

is the review of basic methods for proton conductor modification.

It is well known that electrical conductivity is the product of

carrier mobility and their concentration. Both high concentra-

tion of proton defects and their mobility are needed to provide

good proton conductivity. The second factor is determined by

the nature of the mobile ion and the matrix of the compound in

which the ion transport occurs. Hence, it is rather difficult to

improve the ion mobility in the matrix of a material. The main

progress in the field of proton conductor modification is thus in

increasing carrier concentration. In the case of ceramic

electrolytes, proton transfer can only take place by ion defect

migration [2–4].

2. Defect formation

Different methods can be used for the defect formation in

solids. In this section, some of the most common methods will

be discussed.

0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2005.09.025

E-mail address: [email protected].

Equilibrium defect formation proceeds with temperature

increase. Thermal disordering of ions takes place according to

the thermodynamic laws and contributes to the conductivity

increase on heating. Because of the very small size of the proton,

Frenkel disordering is more preferable in proton conducting

materials. The carrier concentration is thus equivalent to the

concentration of proton vacancies or interstitials. In some cases,

non-equilibrium defects can be obtained by quenching.

Other methods for non-equilibrium defect formation include

material irradiation and mechanochemical treatment [5,6]. The

latter mechanism is the most important for a wide range of

mechanochemical reactions [7,8]. Usually, defects of this type

are characterized by low mobility and their formation does not

lead to high ionic conductivity. Nevertheless, non-equilibrium

defect generation methods are not promising because of the

rapid decrease in defect concentration due to high ionic mobility.

The most typical method for defect formation is doping of

ionic crystals with ions of different valence. Through charge

compensation, heterovalent substitution leads to the simulta-

neous formation of charged defects [2,9] (Section 3).

Synthesis of fine dispersed materials (nanoparticles) is one

of the most popular methods of material modification (Section

4). In this case, defect formation takes place at the particle

surface due to uncompensated chemical bonds [10–12]. Note

that surfaces usually adsorb different molecules; this region can

be considered as another phase with properties different from

the bulk.

The last method is the heterogeneous doping (Section 5). In

this case, defect formation takes place at the interface of two

005) 2935 – 2940

ww

Fig. 1. Schematic energy diagram for proton location in HZr2(PO4)3 (a)

H1�XZr2�XNbX(PO4)3 (b) and H1+XZr2�XScX(PO4)3 structures (c).

A.B. Yaroslavtsev / Solid State Ionics 176 (2005) 2935–29402936

solids. For example, contact of a solid electrolyte with a highly

dispersed oxide phase can result in the growth of defect

concentration near the interface and in a pronounced conduc-

tivity increase [13–17].

3. Heterovalent homogeneous doping

In general, heterovalent homogeneous doping is the most

widely used approach for the modification of solid state ionic

conductors, e.g., the doping of sodium chloride by calcium

ions and silver halogenides by cadmium ions [2,9,18]. The

higher charge of Cd2+ ion in the AgCl matrix (CdAg&) is

compensated by silver vacancy (VAgV) formation according to

the reaction:

CdCl2=AgClSCd&Ag þ VAgVþ 2ClCl: ð1Þ

In accordance with Kroeger and Wink [2], subscripts denote

the atom location in the lattice and superscripts ‘‘&’’ and ‘‘V’’indicate positive and negative charge of the species relative to

their lattice positions. Defect concentration should increase

almost proportionally to the heterovalent ion concentration.

For a proton conductor, HA (A� is electronegative matrix of

material), the situation is different because of the different radii

of the proton and the heterovalent ion. Usually, the new cation

(Mn+) cannot occupy the proton position, so it occupies some

other place and is formally not a direct obstacle to proton

migration:

MAn=HASMin& þ nVHVnAA ð2Þ

where the subscript ‘‘i’’ denotes an interstitial. On the other hand,

the positive charge of the new cation diminishes the possibility of

bond formation between hydrogen and neighboring electroneg-

ative atoms and prevents the proton transfer. Such substitution

usually results in a decrease in conductivity through the blocking

effect. The situation can be better in the case of two- or three-

dimensional channels. But, most probably, obstacle formation

prevents cooperative effects, which are of great importance for

proton transfer [19,20]. For example, the partial substitution of

hydrogen to copper ions in hydrated 12-tungstophosphoric acid

(three-dimensional channels) or to barium in hydrated tantalum

acid phosphate (two-dimensional channels) results in a conduc-

tivity decrease [21,22]. Proton defect concentration in the initial

compounds is high enough. Heterovalent doping of these

compounds results in a gradual decrease in proton concentration

and mobility. On the other hand, dehydration of HTa(PO4)2I2H2O

results in a sharp drop in conductivity and defect concentration.

Thus, the effect of additional defect formation becomes more

pronounced. Partial substitution of hydrogen for barium ions in

anhydrous tantalum acid phosphate results in a conductivity

increase at high temperatures [22]. A change in the interlayer

distance also takes place during this substitution [23], which

results in the change in proton mobility [20].

Heterovalent homogeneous substitution of polyvalent ions

in the acid salts can be much more useful. For example, it is

possible to substitute some zirconium ions for niobium in

acid zirconium phosphate with the NASICON structure

(HZr2(PO4)3IH2O). Due to charge compensation, proton

vacancies are formed in the structure of the obtained material

(H1�XZr2�XNbX(PO4)3IH2O) [24]:

NbZrðPO4Þ3=HZr2ðPO4Þ3¼VHVþ ZrxZr þ Nb&Zr þ 3PO4ðPO4Þ:

ð3Þ

In the same way, some of the zirconium ions can be

substituted for yttrium or scandium. In this case, additional

protons were introduced into interstitials:

H3Sc2ðPO4Þ3=HZr2ðPO4Þ3¼HxH þ 2H&

i þ 2ScZrVþ 3PO4ðPO4Þ:

ð4Þ

Electrostatic interaction of interstitials with the scandium ions

(negatively charged with respect to the lattice positions)

makes the proton energy in neighboring interstitials less than

that for the ordinary interstitials (Fig. 1). At high scandium

concentration, this interaction results in associate formation

according to the reaction:

H&i þ ScZrVSðH&

iScZrVÞassociate ð5Þ

In thesameway,protonvacanciesinH1�XZr2�XNbX(PO4)3IH2O

form associates with niobium ions (Fig. 1c). Because of this

interaction, the concentration of charge carriers and ionic

conductivity decreases for high heterovalent ion concentration.

Moreover, similar long-range interactions [18] may result in a

decrease in carrier mobility.

It is reasonable to note, that in a similar way, conductivity

can be improved only for low conducting materials with low

intrinsic defect concentration, e.g., HZr2(PO4)3IH2O. On the

other hand, in the high temperature range, the contribution of

extrinsic defects becomes negligible, so the conductivity of

HZr2(PO4)3 changes only slightly at high temperatures [24].

Heterovalent doping is widely used for the synthesis of high-

temperature proton conductors with the perovskite structure

[19,25–29], where oxygen vacancies are formed. Their inter-

action with water vapor results in proton interstitial formation:

V&&O þ H2O ¼ 2H&

i þ OxO ð6Þ

These protons formOH groups, but all oxygen atoms are bonded

to two polyvalent ions. As a result, O–H bonds are weak and

,

Fig. 3. Dependences of proton conductivity on particle size for Zr(HPO4)2IH2O

(a), FeH(SO4)2I4H2O(b) and InH(SO4)2I4H2O (c) [46].

A.B. Yaroslavtsev / Solid State Ionics 176 (2005) 2935–2940 2937

hydrogen atoms have high mobility, which is unusual for

hydrated oxides. Similar processes take place in phosphates of

rare-earth elements doped with M(II) [30,31].

4. Proton conducting nanomaterials

In recent years, finely dispersed compounds have became

popular objects of study because of their unusual properties

[32,33]. A new term–nanoionics–has appeared [34–37].

Size effects are well known in physics and chemistry.

When the particle size (L) becomes less than 10–100 nm,

material properties (mechanical, catalytic activity, phase

transition temperature, etc.) change dramatically. It is known

that conductivity and diffusion coefficients of some ions can

change by several orders of magnitude [34,38,39]. These

changes are determined by the high mobility of atoms along

the grain boundary [40]. High disordering of the grain

boundary regions in NiO was illustrated by Merkle using

high-resolution transmission electron microscopy [41]. This

can be explained by the formation of uncompensated bonds

on the particle surface, thereby changing the electrochemical

potential of the ions and defects in the thin (nanoscale) Debye

layer (d) on the particle surface [42] (Fig. 2a). The defect

concentration and diffusion coefficients thus increase near the

surface. For materials consisting of large particles, this

contribution is negligible. For very small particles (L�2d),the boundaries of the Debye layers can overlap and the defect

concentration increases in the bulk of the material (Fig. 2b–d)

[18].

It is reasonable to note that pillared or nanometric zirconium

phosphates and phosphonates are among the most popular

nano-ionic materials [39,43–46]. According to nuclear mag-

netic resonance data, protons on the surface of acid zirconium

phosphate are mobile at room temperature. Moreover, these

protons involve some of the bulk protons in the motion [47].

Proton conductivity of dispersed samples can be presented as:

r ¼ mrs þ ð1� mÞrb ¼ nðmls þ ð1� mÞlbÞ ð7Þ

where n is a total amount of mobile protons, rs, ls and rb, lb

are molar conductivity and mobility of the surface and bulk

protons and m is the fraction of surface protons.

Fig. 2. Schematic electrochemical potential (a) and defect concentration (b–d)

changes for particles with sizes L >2d (b), L >2d (c) and L <2d (d).

Hence, ion conductivity can be determined by the surface

for larger particles (L >2d). The typical dependence of proton

conductivity on particle size can be presented by two

straight-line log-plots to a first approximation (Fig. 3) [47].

For large particles, conductivity does not depend on the

dispersity and, for small particles, it increases as the particle

size decreases.

It is important to note that the interface can also create

additional resistance [14]. The use of finely dispersed materials

is thus not very promising for superionic materials with high

defect concentration and high conductivity.

5. Heterogeneous doping

Another approach for the modification of solid electrolytes

was suggested by Liang [48], who discovered that the

conductivity of low-temperature lithium iodide increased by

several orders of magnitude when contacted with highly

dispersed alumina. The theory of this phenomenon was

described in detail by Maier [13,49,50]. Addition of a dispersed

oxide phase to ionic crystals increases ion sorption at the

interface. Similar to finely dispersed materials, a change of

electrochemical potential for different ions and defects takes

place on the particle surface (Fig. 2a). As a result, a highly

defective Debye layer appears at the interface, and the

conductivity of the system increases sharply. Another impor-

tant reason for this phenomenon is the self-distribution and

‘‘melting’’ of ionic salts between the interface of the oxide

phases [16,17,51,52].

Heterogeneous doping with inert oxides (finely dispersed

alumina, silica and other oxides) has been widely used to

improve the conductivity of solid state proton conductors.

The most interesting results were obtained for systems

including cesium and rubidium sulfates by Ponomareva et

al. [53–57]. The addition of oxides increases the defect

concentration, which results in a decrease in activation energy

for conductivity and superionic phase transition temperature

[54,57]. Conductivity in these systems increases by several

order of magnitudes. Similar results were obtained for acid

iron sulfate and acid ammonium sulfate [58–60]. On the

other hand, attempts to increase the conductivity of zirconium


acid phosphate by silica or alumina addition were less

successful [61,62]. The main reason was perhaps the use of

finely dispersed material with an initially high defect

concentration. Mioc et al. report silica doped 12-tungstopho-

sphoric acid as a promising material for solid electrolytes

[63].

It is important to note that the optimal conductivity increase

can be achieved at relatively low oxide component concentra-

tion [54,58]. This is explained by the blocking effect of the

oxide particles. At low oxide concentrations, the conductivity

increases due to the appearance of highly conductive contacts.

A certain concentration of the oxide phase exists, which

provides a continuous contact between the dopant particles.

Above this critical concentration, the conductivity of the

system rapidly decreases due to the blocking effect of the

oxide phase. This percolation model for the conductivity of the

composite materials was described, for example, by Bunde et

al. and Uvarov et al. [64–66]. Another explanation for the

conductivity change in such systems was suggested by Maier

[67].

In order to minimize the contribution of the blocking effect,

it would seem very attractive to form a thin layer of highly

conductive phase on the surface of oxides or inorganic salt.

This layer can be obtained by chemical modification of the

particle surface. We have applied this approach through the

ion exchange Na+/H+ or Li+/H+ in acid zirconium phosphate

[68,69]. This takes place via the formation of layers of

new phase (MHZr(PO4)2InH2O, M1.5H0.5Zr(PO4)2InH2O or

M2Zr(PO4)2InH2O) on the surface of the particles. A sharp

conductivity increase was observed at the beginning of each

new stage (or step) of ion exchange, when the thickness of the

new phase was equal to the thickness of the Debye layer (Fig.

4) [68,69]. When the content of the new phase increases, the

highly conductive chains disappear and the conductivity of the

system decreases rapidly. This approach allows us to decrease

the dopant content and reduce its blocking effect.

Interesting results on ion conductivity enhancement in the

material formed by epitaxial CaF2 and BaF2 layers were

reported by Sata et al. [70]. Similar works in the field of proton

conductors may also be very promising.

Globular hydrates are usually formed of small particles with

a thin water layer sorbed on their surface. These can be

considered as good examples of conductor modification by a

Fig. 4. Dependence of conductivity on the degree of substitution (X) in

(H1�XNaX)2Zr(PO4)2InH2O.

thin layer of some new phase, e.g., hydrated oxides of

polyvalent elements and insoluble salts of 12-tungstopho-

sphoric acid with the heavy alkali metals [71–73].

6. Inorganic–organic composite materials

Inorganic–organic composite materials can be considered as

a new class of proton conductors and membrane materials

[74,75]. They exhibit specific physical and chemical properties

and can be divided into three large classes. The first is based on

inorganic component modification by organic fragments

containing functional acid groups. The best examples of this

class are zirconium phosphonates [43–46].

In the second class, mobile cations are generated by

inorganic component [76,77]. Such materials are formed, e.g.

in the reaction of a polymer matrix with mineral acids [78–80],

acid zirconium phosphate [81–83] or other acid compounds

[84]. Promising results were achieved in the case of hetero-

polyacid doping in a polymer matrix [85]. The polymer matrix

should be elastic and contain a number of polar groups to give

sufficient solubility of the inorganic component (polyethylene

oxide, polypropylene oxide, polysiloxane, etc.). Ionic conduc-

tivity of such hybrid materials decreases with a decrease in the

segmental mobility of polymer chains [77]. This enabled

Druger et al. to advance a percolation model of ion transport in

these systems [86]. The maximum conductivity of these

materials is attained for an optimum ratio of 0.2–0.4 between

the concentration of salt and monomer units of the polymer

[76,87,88].

The third type of material is an ion-exchange membrane

with functional groups and an inorganic dopant [89,90]. The

list of currently used inorganic dopants includes a number of

oxides of polyvalent elements, heteropolyacids and other

compounds [89–97]. The characteristic size of the dopant

species can vary from several nanometers to several hundreds

of nanometers [93]. In most cases, they form islands in the

membrane [98].

Mechanical strength and an increase in conductivity are

among the advantages cite for composite inorganic–organic

materials [74,89,99].

7. Conclusion

Modification of poor proton conductors is thus a very

promising way to enhance their conductivity and synthesize

new materials with a range of properties. Different methods

of modification can be used in different cases. All are based

on an increase in defect concentration, but it is almost

impossible to improve the conductivity of superionic con-

ductors with an initially high defect concentration. Further

investigations in the field of proton conducting materials

modification are necessary.

Acknowledgements

This work was supported by CRDF (grant #RE1-2528),

Norilskij Nickel and the Russian Government.

A.B. Yaroslavtsev / Solid State Ionics 176 (2005) 2935–2940 2939

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