Solid State Ionics 40/41 (1990) 863-868 North-Holland

PROTON CONDUCTION IN SOME SOLID HYDRATES AND KDP-FERROELECTRIC FAMILY MATERIALS

Suresh CHANDRA and Ajay KUMAR Department of Physics, Banaras Hindu University, Varanasi 221005, India

Proton conduction in solid hydrates and KDP-ferroelectric family materials is briefly reviewed with emphasis on the materials studied in our laboratory. Typical experimental studies like coulometry, IR, transient ionic current for mobility and e versus l / T on APT-5H20, KDP and ADP materials have been specifically discussed. A mechanism for proton transport in KDP and ADP has been proposed, in which it is suggested that H...O...H bridge gets electrolysed on the application o fdc electric field resulting in H + and O H - ions as mobile ionic species.

I. Introduction

Solid proton conductors [ 1-7 ] are of great inter- est due to their potential uses in electrochemical de- vices and fuel cells. In general, the term "proton con- ductors" refers not only to H ÷ ion conducting materials but also materials showing complex ion motion like NH} , H 3 0 +, O H - , etc. Search of pro- ton conductors for application in different temper- ature ranges is being hotly pursued. Some examples are:

(a) High temperature proton conductors like SrCel_xMxO3 (M=Y, Yb, Zn, Nd) [8], oxides [9].

(b) Intermediate/low temperature proton con- ductors like tantalates, hydrates, clays, 13-aluminas, potassium dihydrogen phosphate (KDP), ammo- nium dihydrogen phosphate (ADP) etc. This paper deals with intermediate/low temperature proton conductors particularly hydrates and KDP- family materials. In fact, the best proton conductors available are mostly hydrates like molybdophos- phoric acid ( M PA ), tungstophosphoric acid (TPA), ammonium paratungstate pentahydrate (APT- 5H20), clays, etc.

2. Classification of hydrates

A list of proton conducting hydrates is given in [7 ]. Proton conducting hydrates can be classified [ 3 ] as:

0167-2738/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )

(a) Framework hydrates: Such hydrates are gener- ally hydrous oxides like Sb2Os'nH20 [10] having strongly bonded three-dimensional network of anion polyhedra inside which a close-packed H20 and H3 O + / O H - column occurs. (b) Layered hydrates: Water layers are included in their crystal structures, e.g., clays, HUP, etc. Clays like H-montmorillonite [ 11 ] have liquid-like net- work of interlayered water providing high proton conduction due to liquid-like motion of H ÷ ions (liquid-like transport mechanism). In HUP [ 12], the alternate layers of UOj- and PO7 ions have H20 and H3 O + layers providing proton conduction gov- erned by Grotthus mechanism. (c) Particle hydrates: The protons associated with the bound water are distributed over the particle sur- face. In this, the proton transport may be both by Grotthus mechanism (MPA, TPA [ 13] ) or liquid- like transport mechanism. ZrO2"nH20 and SnO2. nH20 [ 10] are examples of the latter type. (d) Swelling hydrates: They have intercalated water in multilayers. As the relative humidity increases, the number of intercalated water layers also increases causing the swelling of the compound. Example: V z O s - n H 2 0 [ 14] in which the conduction is due to both intrinsic surface proton transport and the elec- tron (hole) motion. ( e) Materials with structural protons: These are not specifically hydrates but contain proton inside the structure such that H...O...H type of bonding is pre-

864 S. Chandra, A. Kumar / Proton conduction in some solid hydrates

sented. KDP and ADP belong to this class o f ma- terials. According to Murphy [15], proton motion in KDP and ADP is due to L- and D-defects, while Harris and Vella [16] have also considered addi- tional A-defects (proton vacancies) for ADP. Sharon and Kalia [17] have suggested a synchronised ro- tation of H 2 PO2 units around three fold axes of PO4 tetrahedra. This may revert back to its initial posi- tion by bidirectional axial rotation. Our present ex- perimental results necessitate a "new look" for pro- ton conduction in these materials.

This paper reports some typical experimental studies on APT-5H20 , KDP and ADP.

3. Experimental

and found to be hydrogen only for KDP and ADP. For APT.5H20 , the cathode side gas was 70% H 2 and 30% NH3. Anode side gas (supposed to be ox- ygen or 02 + H20) could not be tested due to the lack o f the facility.

The evolution o f gases at both the electrodes on application o fdc electric field in KDP and ADP sug- gests the possible electrodissociation o f the H...O...H bridges generating H + and O H - / 0 2 - as mobile spe- cies, The following three possibilities have been considered:

I 11

2H20-~4H + + 2 0 2 - , 4H+ +4e --,2H2 , 202 - ~ 02 + 4e- ,

4 H 2 0 ~ 4 H + + 4 O H - , 4H + +4e ~ 2 H 2 , 4 O H - ~ 2H20 + 02 + 4e- ,

Hydrates are generally studied by the following ex- perimental techniques: (i) Transference numbers by coulometry or polarization method; (ii) Complex impedance plots and a versus 1 / T ; (iii) IR/ laser Raman for locating mobile ion sites; ( iv) N M R line- width and relaxation time for details o f proton mi- gration dynamics; (v) Neutron scattering for struc- tural studies; (vi) Transient ionic current for mobility; (vii) T G A / D T A for phase transition; (viii) EGA (electrogravimetric analysis) measuring loss of mass due to evolution of gases on application o f d c field recently proposed by Chandra and Kumar [18,19].

In the present paper, the experimental investiga- tions using coulometry, IR, transient ionic current and complex impedance techniques are reported on polycrystalline pellets of APT-5H20, KDP and ADP. These techniques are described in our earlier pub- lications [6,7,20,21 ].

4. Results and discussion

4.1. C o u l o m e t r y

On passing dc current through the pellets of KDP, ADP or APT. 5H20 gases evolved at both the cath- ode and the anode in each case. Typical plots of the evolved volume of gases for KDP and ADP are shown in fig. 1. Cathode side gas was gas chroma- tographically tested (Tracer Instruments Model-540)

III

4 H 2 0 ~ 4 H + + 4 O H - , 4H + + 4e- ~ 2He (at ca thode ) ,

2 H 2 0

4 O H - ' 02 + 4e (at anode) . back diff.

The mechanism III differs from II in the sense that for III it is taken that n 2 0 diffused back into the lat- tice after the charge transfer reaction and only 02 liberates at the anode, whereas in II both 02 and H20 liberate at the anode. I and III are similar as far as gas evolution is concerned and cannot be distin- guished. The transference numbers calculated for H ÷ and O H - / 0 2 - from the observed volumes of evolved gases considering all the three models are given in table 1. We expect (//)total tO be less than 1 even if the total transference is by ionic transport, since the actual gas volume measured would be less than the evolved due to their loss for various reasons (ad- sorption in mercury of coulometer, small leak, elec- trode reaction like H g + ~O2oHgO, etc. ). Therefore, it seems that in all these three cases of KDP and ADP H ÷ and O H - are the mobile species.

4.2. IR - spec t ra l s tud ies

(I) KDP: The IR-spectra of the original as well as sample after electrodissociation due to applied dc

S. Chandra, A. Kumar / Proton conduction in some sohd hydrates 865

0.07

0.06

0.05

T 0.0/.

£J

'~ 0.03 c

:~ 0.02 o

0.01

Cathode '~ . . e / .... • .... Anode .~ K DP (101.3~uAI J

- - m - - C a t h o d e - - ' - - A n o d e }ADP(68~uA)

I ' ~ , . " " ' + . . + . - - - -

, . . I I " ..-..,.

. ~ m , , 3"- _ . • . :_2." "~ " ~"

++'E_ - i+ ' . ' . . . . - - z ; - " + + . . - I ~

'+i '.~'~ L i t i i I I 1 1 0 15 30 45 60 75 90 105 120 135 150

Time in rain.

Fig. 1. The vo lume of evo lved gases in the cou lomete r as a funct ion of t ime for K D P and ADP pellets.

Table 1

Calcula ted t ransference number s of H +, NH + and O H - / 0 2 -

ions in KDP, A D P and A P T - 5 H 2 0 from coulometry .

Mater ia l s Model ln+ toll-to2- tNH~ (/+)tot

K D P I & l I I 0.64 0.49 1.13

II 0.64 0.16 0.80

ADP l & lII 0.54 0.48 1.02

II 0.54 0.16 0.70

A P T . 5 H 2 0 I & I I I 0.46 0.14 0.13 0.73

II 0.46 0.04 0.13 0.63

field are shown in fig. 2. The main features are: (a) Appearance of a new peak at 535 c m - ] after elec- trodissociation which has been assigned to symmet- ric P - O - P bond following the process illustrated in fig. 5 (discussed later); (b) disappearance of peak 1645 cm-1 after electrodissociation which has been attributed to the electrolysis of H...O...H bridge; (c) disappearance of the peak at 485 c m - 1. It is reported [ 22 ] that in K D P at higher temperatures H2PO4 ro- tates and acquires such a configuration that the in- tra-bond proton tunnelling between H 2 P O 4 units ceases and the corresponding peak (450 cm -] ) be-

comes feeble. In the present case, it appears that a similar situation is created on the application of dc field through a process o f H 2 P O 4 group rotation and subsequent electrodissociation (as shown in fig. 5) resulting in the disappearance of 485 c m - t peak.

( I I ) ADP: The results for ADP are similar to KDP as shown in fig. 2 with only minor difference with respect to the peak positions. The respective peak positions referred in (a) , (b) and (c) above (for K D P ) are the following for ADP: 525 cm -1, 1650 c m - 1 and 460 c m - 1.

( I I I ) APT. 5H20: The results are discussed in de- tail elsewhere [23]. Two new peaks appear at 1240 c m - 1 and 3000 c m - 1 in the NH3 bending and N - H stretching regions respectively as a result of partial discharge of H + from NH + ions under dc bias.

4. 3. Transient ionic current measurement for mobility

In these experiments, the sample is first polarised and then the polarity is reversed. The current versus time is monitored. The number of peaks give the number of mobile species while the time taken for peaking is related to the mobility [ 21 ]. The TIC plots

866 S. Chandra, A. Kumar / Proton conduction in some solid hydrates

LLI

Z < m p,,

0 Li3

133 <

"~F"~/~'-~ A D p SAMPLE

KDP 1645 E LECTROLYSED SAMPLE

535 I I I I I I I I I I I I I

4000 3500 3000 2500 2000 1800 1600 ~00 1200 1000 800 600 400 200 WAVE NUMBER (cm -1)

Fig. 2. IR absorption spectra of the original and electrolysed samples of KDP and ADP.

TI H20

KDP

1 I 1 I I 20 Zo 6'0 8'0 160 ' ' i,.0 160 180 220 210 2;0 120 2OO

Time in sec.

Fig. 3. Transient ionic current studies on APT- 5H20, KDP and ADP.

S. Chandra, A. Kumar / Proton conduction in some solid hydrates 867

i i I

10-4 i

• '~,l~, .! I 165 • ~ \ ". (O-bulk)

'~ Ik,, i-. ' / p K D P ei "L \

l ~ i. Atmosphere

166 'C~ ' ~ TE '~ ~. ,w) lOP "'. .

I.) ~ Xl~ W. ,,.

~-E %.1 '~, ' . , .~, ', ""~-~" )',(10 KHz)

b 10_7 - k ~ ADP k, ~ Vac u u m

" e ,

10-8 ~ ' ,

(10KHz) I i KDP

1o-91 , i t t

2-0 2.4 2.8 3.2 3.6

103/T (K -1 )

Fig. 4. Temperature dependence of electrical conductivity of KDP and ADP in air and vacuum.

are given in fig. 3. The results can be summar i sed as: K D P - two peaks; possible species: H + and O H -

#H+ = 3 . 6 X 10 -5 , #oH- = 9 . 3 X 10-6 cmZ/V s ,

A D P - two peaks; possible species: H + and O H -

/tH+= 1.34X 10 - 4 , #OH- = 3.30X 10 -5 cm2/V s ,

A P T ' 5 H 2 0 - three peaks; possible species: H +, NH + and O H - / O z -

# H + = 2 . 2 7 X 10 -4 , J 2 N H ~ " = 1.24X 10 -4 ,

#oH-/o2- = 9.10 X 10 - s cm2/V s .

(a) o \

/ HO

0 \ /OH P

r ] / X /LOH H0, 0 P

\ o

o\ / (b)

/P \o

(c) 0

0 j 0 / ~ p

HO" 0

Rear rangement

o OH

A f t e r Elect rolysis

OH J P

0 4- + H +OH-

Fig. 5. Schematic illustration of the mechanism of proton trans- port in KDP and ADP.

The peaks in fig. 3 cannot be assigned unambigu- ously for H +, NH~- o r O H - / O 2 - . However, the rel- at ive ionic size have been taken as a guide to identify /tH+, #NHg or #oH-/o2 . The presence o f H + was conf i rmed by gas chromatography and NH + by chemical test.

4.4. Electrical conductivity

The tempera ture dependence of electrical conduc- t ivi ty of K D P and A D P has been shown in fig. 4. The true bulk conduct iv i ty was evaluated from the com- plex admit tance plots. The initial decrease in the bulk conduct iv i ty o f K D P and A D P with the tempera ture rise is an art ifact and is due to the loss o f surface ad- sorbed water. When the measurements are done in vacuum, cr versus I / T plot looks like a normal Ar- rhenius plot. The onset of a dip in K D P after ~ 423 K may be a t t r ibu ted to the beginning of some struc-

868 S. Chandra, A. Kumar / Proton conduction in some solid hydrates

tural change w h i c h is also seen in D T A [22 ] .

On the bas is o f ou r e x p e r i m e n t a l results , the sug-

ges ted m e c h a n i s m for K D P a n d A D P is: A th ree - fo ld

ro t a t i on o f H2PO~- uni t s abou t any o f the axes o f

PO4 t e t r ahed ra results in a s i tua t ion like O. . .H. . .H. . .O

( the D - d e f e c t ) as s h o w n in fig. 5a. It r ea r ranges i t se l f

as H. . .O . . .H . . .O (fig. 5b) , a highly uns t ab l e s i tua t ion

wh ich gets e l e c t r o l y s e d / d i s s o c i a t e d on the appl ica-

t ion o f dc e lectr ic f ield gene ra t i ng H ÷ and O H as

mob i l e ionic spec ies and a P - O - P b o n d resul ts as

s h o w n in fig. 5c.

References

[ 1 ] S. Chandra, Superionic solids - principle and application (North-Holland, Amsterdam, 1981 ).

[2]F.W. Poulsen, Riso Report M-2244 (Riso National Laboratory, Riso, Denmark, 1980).

[ 3] J.B. Goodenough, in: Solid state protonic conductors I1, eds. J.B. Goodenough, J. Jensen and M. Kleitz (Odense Univ. Press, Odense, Denmark, 1983)p. 123.

[4] S. Chandra, Mater. Sci. For. 1 (1984) 153. [ 5 ] S. Chandra, N. Singh and S.A. Hashmi, Proc. Ind. Nat. Sci.

Acad., 52 (1986) 338. [6]S. Chandra, in: Solid state ionic devices, eds. B.V.R.

Chowdari and S. Radhakrishna (World Scientific, Singapore, 1988) p. 265.

[7 ] S. Chandra, in: Superionic solids and solid electrolytes - Recent trends, eds. A.L. Laskar and S. Chandra (Academic Press, New York, 1989) p. 185.

[ 8 ] H. Iwahara, T. Esaka, H. Uchida and N. Maeda, Solid State Ionics 3/4 (1981) 359.

[9] T. Norby, Solid State lonics 40/41 (1990) 000, this volumc. [ 10] W.A. England, M.G. Cross, A. Hamnett, P.J. Wiseman and

J.B. Goodenough, Solid State lonics 1 (1980) 231. [ 11 ] S.H. Sheffield and A.T. Howe, Mat. Res. Bull. 14 (1979)

929. [12] A.T. Howe, S.H. Sheffield, P.E. Childs and M.G. Shilton,

Thin Solid Films 67 (1980) 365. [ 13 ] O. Nakamura, T. Kodama, I, Ogino and Y. Miyake, Chem.

Letters (1979) 17. [ 14] P. Barboux, N. Baffier, R. Morineau andJ. Livage, in: Solid

state protonic conductors III, eds. J.B. Goodenough, J. Jensen and A. Potier (Odense Univ. Press, Odense, Denmark, 1985) p. 173.

[ 15] E.J. Murphy, J. Appl. Phys. 35 (1964) 2609. [ 16 ]L.B. Harris and G.J. Vella, J. Chem. Phys. 58 ( 1973 ) 4550. [ 17 ] M. Sharon and A.K. Kalia, J. Solid State Chem. 21 ( 1977 )

171. [ 18 ] S. Chandra and A. Kumar, Mat. Res. Bull. 24 ( 1989 ) 417. [ 19 ] S. Chandra and A. Kumar, Solid State lonics 40/41 (1990)

000, this volume. [20] S. Chandra, N. Singh and B. Singh, Solid State Commun.

57 (1986) 519. [21 ] S. Chandra, S.K. Tolpadi and S.A. Hashmi, Solid State lonics

28-30 (1988) 651. [22] R. Blinc, V. Dimic, D. Kolar, G. Lahajnar, J. Stepisnik, S.

Zumer, N. Vene and D. Hadzi, J. Chem. Phys. 49 (1968) 4996.

[23]S. Chandra, S.K. Tolpadi and S.A. Hashmi, J. Phys.: Condens Matt. 1 (1988) 9101.