J. MATER. CHEM., 1991, 1(3), 441-445 Generation of Charge Carriers and an H/D Isotope Effect in Proton-conducting Doped Barium Cerate Ceramics Robert C. T. Slade* and Narendra Singh Department of Chemistrx University of Exeter, Stocker Road, Exeter EX4 4QD, UK Ionic conduction by H+ and D+ in doped perovskites BaCe,-,M,O,-, (M=Y, Gd; x=0.05, 0.10) at high temperatures in H,O-and D,O-moistened atmospheres is investigated. Conduction by hydrogen ions was confirmed by emf measurements on various gas cells using specimen ceramics as the solid electrolyte membrane separating moist and 'dry' nitrogen atmospheres. Ionic charge carrier densities in these materials are very low, as demonstrated by thermogravimetric and infrared measurements. Electrical conductivity (a) measurements were carried out in the temperature range T= 600-900 "C using complex impedance techniques.Plots of log aT versus 1/T showed Arrhenius behaviour in all cases. Attempt frequencies for charge-carrier migration correspond to deformation of the Ce/M-0-H bond angle, with H+ migrating between sites corresponding to attachment to neighbouring oxide ions. An isotope effect is seen in the activation barriers, €, (ca.33 kJ mol-' for H+ and ca. 43 kJ mol-' for D+). This results from tunnelling of charge carriers through a time-dependent barrier arising from coupling to lattice phonons. Keywords: Perovskite; Ionic conduction; Thermogra vimetric analysis; Infrared spectroscopy Doped barium cerate ceramics BaCe, -,M,03 (M =Y, Yb, Nd, La, Gd; x=O.O5, 0.10) exhibit protonic conduction (i.e.migration of H+ ions) in moist atmospheres at high tempera- tures (600-1000 OC).' These materials have a perovskite-type structure with Ce/M being octahedrally co-ordinated by oxygens. CI includes oxygen-ion vacancies. Other perovskite oxides have also been demonstrated to conduct ionically (when doped) via proton formation/incorporation, e.g. SrCe03,2-5 BaCe03,6 KTa03.'v8 The formation of con-ducting protons has also been studied in several other types of doped oxide in H2-containing atmospheres (e.g. Zr02,9.10 A1203," Y203,12 Th0213); these studies do not all involve a moist atmosphere. Protonic conductivities in other ceramic systems appear considerably lower than those associated with doped A"B"03 perovskites.l4 Protonic conduction in doped perovskites results from the oxygen vacancies which are introduced on substitution of lower valency cations M"' on some Ce" sites.The protons are said to arise from high-temperature equilibria between the condensed phase and a moist (H20-containing) atmos- phere:" viz. Vo" +to2400+2h' (1) H20+2he+2H.+302 (2) H2O+V0'*42H'+Oo (3) V;; denotes an oxygen vacancy, Oo an oxide ion at a normal lattice site, H' a proton (H') and h' a hole (electronic). Reaction (2) is the reaction of water with holes (localised on Ce", reaction giving Ce"') to generate H', which will co- ordinate to oxides to give hydroxyl ions (co-ordinated to octahedral metals, uiz. Ce/M). If this were a significant route for generation of H+ charge carriers, undoped BaCe03 would have a protonic conductivity comparable to the doped mater- ials; this is not the case.The generation of charge carriers in these systems can be described simply as water molecules reacting to fill vacant oxygen sites in the doped perovskites, thereby generating further hydroxyl ions as follows 00+Vo'. +H20 =2[OH]o (4) The protons then migrate/conduct by motion between sites, corresponding to attachment to adjacent oxygens. Studies of protonic conduction in solids have excited con- siderable interest internationally owing to the potential appli- cations of proton-conducting solid-electrolyte membranes in many novel electrochemical devices, e.g. advanced fuel cells, H2 sensors, steam electrolysers for H2 production and elec- trochromic optical displays.Proton-conducting materials at near ambient temperatures are commonly hydrated, e.g. solid acids and acid salts (acid phosphates, acid sulphates, hetero- polyacids) or layered double hydroxides. The hydrating water is easily lost when the temperature is raised, with a subsequent drop in the conductivity of the electrolyte by several orders of magnitude (the hydrogen-bonded conduction pathway, and subsequently the entire structure, is destroyed). Oxide ceramics in which H+ conduction can be induced, on the other hand, offer structural and chemical stability at high temperatures. In the present study, the electrical properties of the doped ceramics BaCeo~g5Yo~050, -,have-,and BaCeo,90Gdo~lo03 been studied at high temperatures in atmospheres moistened with H20 and D20.Thermogravimetric analyses have detected changes in mass on reaction of powders in atmos- pheres and at temperatures characteristic of sintered device components, and the concentrations of charge carriers have been determined. An H/D isotope effect arising from the protonic nature of the conduction is reported. Experimental Doped perovskites BaCeo.95Yo.0503-, and -aBaCeo~goGdo~lo03 were prepared as described previously.' Y203 or Gd2O3, Ce02 (Aldrich, 99.9%) and BaC03 (BDH) were mixed in the stoichiometric ratios accord- ing to XBaCO +(1-x)Ce02+-M203 = 2 BaCe1-,M,03-a+C02 (M=Y, Gd) The powdered dry raw materials were mixed, ground and calcined in air in alumina crucibles at 1250 "C for 15 h.The products were characterised using X-ray powder diffraction (Philips diffractometer, Ni-filtered Cu-Ka radiation) to estab- lish the formation of single perovskite phases. Values of the cubic unit-cell constant a were 4.414(3) (BaCeo.95Yo.0503 -a) and 4.422(2) A (BaCeo~90Gdo~lo03-J. Thermogravimetric measurements on perovskite powders employed a Stanton Redcroft TG-750 thermobalance with flowing N2(g). Infrared spectra (KBr discs) were recorded using a Perkin- Elmer 88 1 instrument for (i) the dry 'raw' perovskite powders, (ii) powders treated in flowing moist N2(g) (passed through an H20 or D20 bubbler at ambient temperature) at 900 "C for 10 h.To form ceramic discs for the electrochemical studies the calcined powders were reground and pressed hydrostatically (10 tonnes) into pellets (16 mm diameter, ca. 1 mm thickness), with 1% by mass of camphor as volatile binder. Pellets were sintered in air at 1500 "C for 10 h. Porous platinum paste (Engelhard A4338) was applied (as a paint) to pellet faces as electrodes. Electrical conductivity measurements employed a.c. impedance techniques using a Hewlett-Packard HP 4192A LF impedance analyser (operating in the range 5 Hz-100 kHz) programmed via an IBM-compatible computer for data collec- tion and analysis (employing the EQUIVCRT modelling software of Boukamp). The purpose-built electrochemical cell assembly is shown in Fig.1. The whole assembly was mounted horizontally in an electric-tube furnace programmable down to 1 "C min-'. Two cell compartments were separated by the ceramic disc and individual controlled gas flows [either moist or 'dry' N2(g)] fed to each compartment. 'Moist' gas was produced by passage through a water (H20 or D20)bubbler at ambient temperature, whereas 'dry' gas was produced by passage through an activated molecular-sieve column (resulting in a very much lower partial pressure of water). A gold wire ring gasket provided the seal between the compart- ments, requiring the electrode/electrolyte assembly to be heated to 950 "C for ca. 1 h to obtain a gas-tight seal prior to electrochemical studies. Electrochemical potentials were measured with a Kiethley 6 14 electrometer.Electroly Disc Pt wire FURNACE ~ a Tube Au Ring Gasket Fig. 1 Schematic diagram of the electrochemical cell used in emf and conductivity measurements in the temperature range 600-900 "C(see text). Top, the gas flow paths; bottom, the electrode assembly J. MATER. CHEM., 1991, VOL. 1 Thermogravimetry Perovskite powders were heated (5 "C min-') to 900 "C and then soaked at that temperature for 10 h, all in moist flowing N,(g) (passed through an H20 bubbler at ambient tem-perature). The mass of each material initially decreased rapidly on heating to 900 "C. In an earlier study of doped strontium cerate, O2evolution on heating was detected by gas chroma- tography and was assigned to the reversal of reaction (1) on heating in the absence of 02(g), thereby introducing more oxide vacancies and reducing the mean oxidation state of Ce. This explanation is unsatisfactory in that it implies a higher mobility for 02-ions than for the H+ involved in the subsequent equilibration (see below) which generates charge- carriers.An alternative explanation is that some oxygen is incorporated in dioxygen anions such as 0;-(barium per- oxide is a known compound) according to a reaction such as VO" +00 +302 =[02]20" (5) Self-diffusion of excess 0 atoms could then occur via a rapid bond-exchange (Grotthus-like) mechanism, which would not be a conduction mechanism (0;-+02-=02-+0;--transfer of neutral atoms). Generation of peroxide ions would not be accompanied by reduction of the cerium oxidation state.The amounts of oxygen evolved during heat- ing corresponded to a loss of 4.0 & 0.1 mol% of incorporated 0 atoms for BaCeo.90Gdo~lo02.95 and 3.0k0.1 mol% for BaCeo~95Yo~0502.975.No electronic-conductivity contribution was detected in the impedance studies described below. This is an important observation supporting the postulate of the incorporation of excess oxygen without reduction in the mean Ce oxidation state; the presence of an appreciable concen- tration of Ce"' would be expected to lead to significant electronic conductivity (by electron hopping) analogous to that in the defect fluorites Ce02-x. During soaking at 900 "C, the powders slowly increased in mass over a few hours to a stable final value, this arising from reaction (4) (reaction with H20).The observed mass changes corresponded to H contents for the final products at 900 "C of (i) 0.90f0.09 mol% for perovskite -aBaCeo~90Gdo~,o03 and (ii) 0.64 f0.09 mol% for BaCeo.95Yo.0503-a(the relatively large errors are due to the very small mass changes involved in this second stage, which introduces the charge carriers). The ionic charge-carrier con- centrations in these materials are thus very low. Analogous uptake by doped strontium cerate has been demonstrated by subsequent heating and gas chromatographic detection of evolved H20.I6 The thermogravimetric (TG) studies above were then repeated with replacement of H20 (in the bubbler) by D20.Initial mass losses (due to O2 loss) were very similar to the values above. Observed mass increases on high-temperature soaking were higher, for each material, for uptake of D20 than for uptake of H20. For instance, the observed mass change for perovskite BaCe0.,,Gdo. -a corresponded to a D content for the final product at 900°C of 1.43f0.09 mol%. The charge-carrier concentrations are thus higher in deuterated materials than in hydrogenated ones, indicating a shift to the right in equilibrium (4) on moving from H20 to D20 moistening of the flow gas (all other conditions being the same). Infrared Spectra Infrared spectra for the dry as-synthesised powders were very similar to those of the materials soaked at high temperature in moist flowing N2(g).Fig. 2 shows the spectra for as- J. MATER. CHEM., 1991, VOL. I I....!.,....I 4000 3200 2000 1600 1000 600 wavenumber/cm -' Fig. 2 Infrared spectra of as-synthesised BaCe,~,,Gd,~,,03 -a (top) and of the same material soaked for 10h in H,O-moistened flowing N,(g) and then for a further 5 h in D,O-moistened flowing N,(g) (bottom), both at 900 "C synthesised BaCeO~90GdO~1003 -aand that of the same material soaked for 10 h in H,O-moistened flow gas and then for a further 5 h in D,O-moistened flow gas, both at 900°C. Electrical Conductivity Conductivity measurements for ceramic discs BaCeo~90Gdo~lo03 -a were made in -a and BaCeo.95Yo.0503 flowing N2(g) in the temperature range 600-900 "C after pretreatment at T>600 "C in the same N2(g) (moistened with H20 or D20, as appropriate for studies of Hf or D+ conductivity) for 10 h.The values of the bulk electrolyte resistances were evaluated from complex impedance plots. Some typical impedance spectra are shown in Fig. 3 (for BaCeO~,,GdO~,,O,-a with D+ conduction). Spectra in the case of BaCeo~g5Yo~os0,-a were very similar in form. Such spectra are typical of ionic motion in solids with reversible electrodes. At T<700 "C a well defined depressed semicircle at higher frequencies is seen, with a linear impedance variation at lower frequencies. The impedance variations at low fre- quencies indicate a diffusion-related phenomenon, assigned in 15 10 . 5 ,/" Contrary to the assertion of Shin et a1." in their studies of 01'1.Y20,-doped SrZrO, and SrCe03, no information results as 30 40 soto the location of H in the conducting ceramic.This is not surprising in view of the very low H contents evident in the TG studies above. EMF Measurements Emfs of the cells using sintered discs of BaCeo~90Gdo.,o0,-a and BaCeo.95Yo.os03 -a were measured using the electro- chemical cell assembly (Fig. 1) at T2600 "C. Stable emfs were ru Iobserved on passage of moist (H,O) and 'dry' nitrogen gas through different compartments of the cell for a few hours. The negative electrode was that in the 'wet' compartment and the polarity of the cells reversed on interchanging the moist and 'dry' gas flows. This behaviour of the cells is explicable only in terms of the ceramic diaphragm acting as a protonic 16 20 24 28conductor.The emf arises from the different partial pressures of H20 in the two cell compartments and the following electrode reaction: H20(g)=2H+ (electrolyte) +302(g)+2e-(6) 4 At high temperature the equilibrium H20(g) =H2(g)+302(g) 3might be established and 02(g) could be available for gener- ation of possible 02-charge carriers by the reaction 2302(g)+2e-=O2-(electrolyte) (7) 1If this were the dominant electrode reaction and conduction was by 02-migration, the compartment with the higher partial pressure of water would have the higher partial pressure of oxygen and contain the positive electrode (contrary to experimental observation). Observed emfs decreased as the temperature increased, showing an electronic contribution to the conductivity to increase at higher temperatures. The observed emfs were for BaCeo~90Gdo~ro03-a: 65, 40, 21 mV at 600, 700,800 "C, and for BaCeo.95Yo.os03-a: 45,38, 18 mV at 600,700, 800 "C.I .0 1 I * 10 12 14 16 Z'/R Fig. 3 Typical complex-plane impedance plots as a function of tem- perature for BaCeo~,,Gd,-,,,03 -a in flowing N,(g) moistened with D,O (by passage through bubblers at ambient temperature). (a)600, (b) 700, (c) 800 "C 444 this case to diffusive processes in the electrode (Pt) or at the electrode/electrolyte interface. The depressed semicircular arc at higher frequencies arises from electrical charge transfer at the electrode/electrolyte interface, this giving rise to a charge- transfer resistance in parallel to the electrode double-layer capacitance at the interface.As anticipated, the value of the charge-transfer resistance gradually decreases with increasing temperature and finally vanishes at higher temperatures (T> 700 "C). No additional arcs corresponding to grain-boundary conductivity were observed. The bulk electrolyte resistance at a given temperature differed when measurements corresponded to H+ and D+ migration, i.e. in H20-and D20-moistened N2(g) flows. From the value of the bulk electrolyte resistance, R, the electrical conductivity, Q is evalu- ated using the expression IS=t/RA (8) where t is the thickness of the ceramic disc and A is the electrode cross-sectional area.Temperature dependences of the conductivities are shown in Fig. 4 for discs of BaCeo~90Gdo~1003-aand BaCeo~,,Yo~o,O,-a and for H+ and D+ conduction. Measure- ments refer to conductivities of the same discs in H20- and D20-moistened flowing gas and were fully reproducible on (i) temperature cycling and (ii) multiple cycles between H20 and D20 moistening of flow gas. Furthermore, very similar 1.2I I ' I0.2 0.8 0.9 1.0 1.1 1.2 103 KIT 1.0 0.8 0.6 0.4 0.2 I.0.0' * ' . ' 0.8 0.9 1.0 1.1 1.2 1.3 103 KIT Fig. 4 Temperature dependences of conductivities arising from H + and Df migration in ceramics BaCe,-,M,O,-, in moist (H20 or D20) flowing N2(g) (see text). Top, BaCeo~goGdo,,o03~Q; bottom, BaCeo,g5Yo~0503~~,.Solid lines correspond to the Arrhenius param- eters E, and A given in Table 1 H+ in the same host material (see Table 1).H+/D+ transfer J. MATER. CHEM., 1991, VOL. I data were obtained for different discs of the same materials. In all cases, the conductivity exhibits an Arrhenius-type T dependence explicable in terms of a thermally activated pro- tonic charge migration with aT= A exp (-E,/R'I) (9) The evaluated activation energies, E,, and pre-exponential factors, A, are given in Table 1. It is clear that an isotope effect is present in the ionic conduction occurring in both ceramic membranes investigated, with E, depending strongly on the migrating species (H+ or D'). Origins of Proton Mobility The H+ conductivity is related to the H+ self-diffusion coefficient D by the Nernst-Einstein equation Q =(Nq2/kBT)D (10) where N is the number density of charge carriers, q is the charge per carrier and kB is the Boltzmann constant. For a three-dimensional random walk, D is related to the jump frequency v by the equation D =A2v/6 (1 1) where 2 is the distance between neighbouring sites (taken as the O...Oseparation in this case).For an activated jump v =v, exp (-E,/R7) (12) Combining eqn. (8)-(lo), QT=(Nq2A2V,/6k~)exp (-E,/R'I) (13) Comparison of eqn. (9) and (13) gives an expression for the Arrhenius prefactor A, linking it to structure, migration pathway and jump-attempt frequency (also given in ref. 18) A =Nq2A2v,/6kB (14) The attempt frequency v, is thus given by V, =6AkB/Nq2i2 (15) For BaCeO~,,GdO~,,O, the experimental data give v, = 1.3 x lOI3and 1.8 x lOI3 Hz for H+ and D+ migration, respect- ively.This frequency is comparable to that for vibrations of metal-co-ordinated hydroxyl groups, and in particular to that for deformation of the Ce/M-0-H bond angle ('wagging', hydroxides being co-ordinated to the octahedrally co-ordi- nated metals in the structure). One might therefore anticipate that v, would be slightly lower (by a factor of ca. J2) for D than for H, a consequence of the higher reduced mass in the D case. However, both A and N are subject to experimental errors (A,in particular, being deduced by extrapolation) and the agreement between the deduced values is thus remarkable.It is evident in Table 1 that A is higher for D+ conduction than for H+; this peculiarity is a simple consequence of the higher carrier density, N, in the case of D', the attempt frequencies v, being similar. Activation energies for D+ migration are higher than for Table 1Arrhenius prefactors A and activation energies E, for H+/Df conduction in doped barium cerates BaCe, -xM,O, -a proton conduction deuteron conduction ceramic E,/kJ mol-' log (A/K S cm-') E,/kJ mol- log (A/K S cm-') ~ ~ BaCe0.90Gd0.1003 -01 35.6 0.9 2.63 k0.05 43.9 k0.7 2.96 f0.04 BaCe0.95Y0.0503 -a 30.6 k0.6 2.24 k0.04 43.2 1.7 2.78 f0.09 J. MATER. CHEM., 1991, VOL. 1 is believed to occur by tunnelling between sites corresponding to attachment to different oxygens.The question of the origin of E, therefore necessitates consideration of activated phenom- ena enabling tunnelling. Co-operative motions of the structure (phonons) will lead to modulation of O...O separations, with consequential modulation of the barrier to migration and hence of the frequency of tunnelling between sites (i.e.through the barrier). Therefore, we have combined variations of both the O...O separation (by phonons) and the Ce/M-0-H bond angle (the 'wagging'), the nature and potential associated with the latter differing in H and D cases. Phonon spectra are temperature dependent, as required. It is well known that the proton tunnels more readily than the deuteron (see for example ref.19), the tunnelling probability decreasing rapidly with mass. Combination of the above factors leads to a stronger temperature dependence for D+ of the frequency of achieving a critical O...O separation (to enable D+ to tunnel with a given probability), and hence to the observed relative activation energies. The discussion of the preceding two paragraphs omits consideration of the effects of charge-carrier introduction uia an equilibrium [reaction (4)]. A small variation in charge carrier (H+) concentration with temperature has been noted in studies of analogous doped strontium cerates.16 It follows that E, values are likely to contain a small contribution from the equilibrium. Further consideration (e.g. by computational chemistry) of the origins and variations of E, are beyond the scope of this study.Shin et all7 studied H+ and D+ migration in Yz03-doped SrZrO, and SrCeO,. From conductivity ratios at given tem- peratures, they postulated an isotope effect, but dismissed the possibility of quantum effects (tunnelling). Their discussion and study did not, however, include consideration of vari- ations of activation energies E, with isotope exchange, nor the values of Arrhenius prefactors A. In particular, single- temperature ratios of conductivity have little meaning in the presence of E, variations. Scherban and Nowick" working on Yb,O,-doped SrCeO, at T1400"C (lower than in this study aimed at likely operating temperatures for devices) detected higher activation energies for D+ than for Hf migration, closely parallelling the results of this study.The observed isotope effects are, in themselves, primary evidence that the migrating species is H+, rather than OH-or even 0'-. It should be noted, however, that these systems are closely related to other defect perovskites which have been characterised as O2-ion conductors." Under appropriate conditions and at higher temperatures, the materials in this study would be expected to also conduct 02-ions (with E, >75 kJ mol- for that process2o), as postulated by Bonanos et a1." It has been the purpose of this study to work under conditions stimulating conduction by H only.+ Conclusion H conduction in M20,-doped barium cerates arises from + incorporation of water into the perovskite structure, H20 reacting with oxygen vacancies and an oxide ion to form two hydroxyl groups.Whereas others have discussed the low electronic conductivities of these materials in terms of low concentrations of Ce"' and small polaron motion,18 it is likely that excess oxygen is incorporated in these materials in dioxygen anions (without reduction of the mean Ce oxidation state). H+ migration is by hopping between sites correspond- ing to attachment (in hydroxyl groups) to neighbouring oxy- gens. The attempt frequencies measured correspond to deformation of the Ce/M-0-H angle, necessary for migration of H+ over the oxide network. An isotope effect is seen, activation energies E, being higher for D+ migration than for H +.This arises from interaction with lattice phonons, the jump involving tunnelling (with H tunnelling more + readily than D').A conductivity-limiting feature of these materials is the low charge carrier concentration. H -conductivities would be expected to rise at high steam pressures (e.g.in water electroly- sers) by driving reaction (4) further to the right, thereby increasing the charge-carrier density. Studies are in hand to investigate optimisation of the charge carrier density. We thank SERC and British Gas plc for co-funding this project and for permission to announce these results. N.S. thanks Gaya College (Magadh University, India) for study leave. References 1 R. C. T. Slade and N. 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Kofstad, Solid State Zonics, 1986, 20, 169. 13 D. A. Shores and R. A. Rapp, J. Electrochem Soc., 1972, 119, 300. 14 T. Norby, Solid State Zonics, 1990, 4/41, 857. 15 H. Uchida, H. Yoshikawa and H. Iwahara, Solid State Ionics, 1989,34, 103. 16 H. Uchida, H. Yoshikawa, T. Esaka, S. Ohtsu and H. Iwahara, Solid State Zonics, 1989, 36, 89. 17 S. Shin, H. H. Huang, M. Ishigame and H. Iwahara, Solid State Zonics, 1990, 4/41, 910. 18 T. Scherban and A. S. Nowick, Solid State Ionics, 1989, 35, 189. 19 P. W. Atkins, Physical Chemistry, Oxford University Press, Oxford, 3rd edn., 1986, p. 320. 20 X. Turillas, A. P. Sellars and B. C. H. Steele, Solid State Zonics, 1988,28-30,465. 21 N. Bonanos, B. Ellis, K. S. Knight and M. N. Mahmood, Solid State Zonics, 1989, 35, 179. Paper 0/05806H; Received 27th December, 1990