Proton Conducting Ceramics for Use in Intermediate

Journal of New Materials for Materials for Electrochemical Systems 5, 25-30 (2002) c J. New. Mat. Electrochem. Systems

Proton Conducting Ceramics for Use in Intermediate Temperature Proton Conducting Fuel Cells

D. Browning, M. Weston, J.B. Lakeman1 , P. Jones1 , M. Cherry2 , J.T.S. Irvine3 , D.J.D. Corcoran3 QinetiQ, Haslar Marine Technology Park, Haslar Road, Gosport, Hampshire, PO12 2AG, UK 1 DSTL, Portsdown West, Fareham, PO17 6AD, UK 2 Cambridge Discovery Chemistry, Merrifield Centre, Rosemary Lane, Cambridge, CB1 3LQ, UK 3 School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK ( Received June 5, 2001 ; received in revised form November 27, 2001 ) Abstract: A new proton conducting perovskite material Sr3 CaZr0.9 Ta1.1 O8.55 (SCZT) was synthesised and the mode of conduction elucidated through modelling. The changes in protonic conductivity under fuel cell conditions were examined using AC impedance techniques and a test fuel cell was constructed. The material proved less conductive than the standard BaCe0.95 Y0.05 O2.975 material but was more stable under reformate conditions. The maximum power output generated by the SCZT85 cell was 0.2mW/cm2 at 600◦ C although this was for an electrolyte considerably thicker (1.5 mm) than would be used in an actual cell. Key words : Intermediate proton conducting, ceramics, fuel cell, materials, construction, SOFC

ceramic materials based on the perovskite structure have been developed, which may offer a way to overcome the associated drawbacks of the above-mentioned systems.

1. INTRODUCTION For many reasons, both economic and environmental, fuel cells have become increasingly popular and therefore been the focus of much recent research and development. Two main systems have reached commercialisation-the PEMFC (Polymer Electrolyte Membrane Fuel Cell) and SOFC (Solid Oxide Fuel Cell), however these systems are not without their inherent problems. Low temperature fuel cells, such as the PEMFC, require a hydrogen-rich fuel feed, which is low in potential poisons such as carbon monoxide (< 20 ppm) and sulphur compounds (ppb levels) found in typical fuel reformate. Alternatively, high temperature fuel cell systems e.g. SOFC, are CO tolerant and could potentially use methane as a fuel instead of hydrogen. However, significantly higher temperatures (~1000◦ C) are necessary for the electrolytes to be sufficiently conductive to sustain operation at reasonable power levels, and thus these systems suffer from materials-related problems. A new series of proton-conducting

The intermediate temperature proton conducting fuel cell (ITPCFC) operates by proton conduction across a ceramic electrolyte and should have an operating temperature of between 200-600 ◦ C. This lower temperature of operation will allow easier cell construction, enabling cheaper materials to be used, and will reduce the problem of thermal stress. The incorporation and conduction of protons in perovskite-type oxides was first recognised by Iwahara, et al [1] in samples of SrCeO3 over twenty years ago. While in dry, hydrogen free conditions these materials demonstrated p-type (hole) conductivity. However, following introduction of water vapour or hydrogen at high temperature, the electronic conductivity was seen to fall and the onset of protonic conductivity was observed. That the conduction is indeed protonic has been independently verified by electrochemical and analytical techniques [2], although some contri-

∗ To whom correspondence should be addressed: Fax: +44 (0)2392 212025; e-mail: [email protected]

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D. Browning et al./ J. New Mat. Electrochem. Systems 5, 25-30 (2002)

bution from oxide conduction has been noticed at elevated temperatures (>700◦ C) [3]. A pure perovskite material is in itself not a good proton conductor; to exhibit proton conduction the presence of oxygen vacancies is required. These vacancies can be created through doping the perovskite with an aliovalent metal ion as demonstrated in the example below of a gadolinia doped barium cerate.

Our work is currently concerned with the development of more stable electrolyte materials for application in fuel cells. The work reported here deals with the characterisation of the new triple perovskite Sr3 Ca1+x+y Zr1−x Ta1−y O9−δ (SCZT) where δ = (2x+3y)/2. An alteration of the ratios of the B-site ions leads to a wide range of formulations containing differing concentrations of oxygen vacancies, as can be seen on table 1.

Table 1: Formulations and oxide deficiencies of SCZT

x 2BaO + Gd2 O3 → 2BaxBa + 2Gd0Ce + 5OO + VO··

Formulation When such a material is heated in an atmosphere containing sufficient quantities of water these vacancies are readily occupied by ‘transient’ hydroxyl ions generated through the reaction below.

Sr3 CaZr0.75 Ta1.25 O8.625 Sr3 CaZr0.9 Ta1.1 O8.55 Sr3 CaZrTaO8.5 (Parent Phase) Sr3 Ca1.3 Zr0.85 Ta0.85 O8.125 Sr3 Ca1.5 Zr0.75 Ta0.75 O7.875

Identifying Code SCZT86 SCZT85 SCZT SCZT81 SCZT78

Oxide deficiency 0.375 0.45 0.5 0.875 1.125

H2 O(g) + VO·· → 2OHO 1.1 Since these early results for doped SrCeO3 , many other proton conducting ceramics based of the form AB1−x Mx O3 have been synthesised and their physical properties fully characterised. Initial studies showed that barium cerate based ceramics exhibited the greatest conductivity (10−2 to 10−3 Scm−1 ) but were thermodynamically unstable towards the reaction with carbon dioxide or water [4]. Subsequent replacement of ceria by zirconia led to improved stability but resulted in a significant reduction in conductivity (10−3 to 10−4 Scm−1 ). Recent work by Nowick and co-workers [5,6] has focused on another class of perovskite oxides that offer a wider range of compositions and non-stoichiometry without the need for doping. These complex perovskites have the formula 0 00 A3 B 1+x B 2−x O9−δ , where A is divalent, and B’ and B” are divalent and pentavalent ions respectively. When stoichiometric (i.e. x=0) these materials contain few oxygen vacancies and thus are poor proton conductors. If however the stoichiometry is shifted (i.e. x6=0) the resultant charge imbalance can be compensated by the formation of oxygen vacancies, indicated here by the value of δ which equals 3x/2. Results from these materials have proved promising with the reported protonic conductivity for non-stoichiometric Ba3 CaNb2 O9 bettering that of Nd doped BaCeO3 [6]. Work on similar perovskites has also illustrated the expected strong correlation between the number of oxygen vacancies present and the conductivity. This has resulted in the examination of a number of highly oxygen deficient perovskites such as Ba2 YSnO5.5 and Ba2 In2 O5 that can take up 0.5 and 1 protons per ABO3 unit. While these results are certainly impressive when compared with the value of 0.18 protons typically obtained for doped perovskites, problems with stability in CO2 and H2 containing atmospheres limits their effectiveness.

Modelling

The conduction of protons through solid oxide electrolytes has been the subject of some discussion [7] and it is generally believed to occur through a ‘hopping’ mechanism. In this the conduction occurs by the proton associating with an oxygen atom, forming a transient hydroxyl species that is ‘transported’ through the material by the migration of the hydrogen ion from one oxygen atom to a neighbouring oxygen atom. Since the exact details remain unclear, dynamic quantum mechanical modelling was used to better understand the mechanism of proton conduction in perovskite oxides. This was achieved by expanding upon Cherry’s earlier study of proton migration in perovskites [8] to produce a more robust dynamic 3D model. The results produced indicated that the major constituent of the proton activation energy was associated with a relaxation of the oxide lattice making the transfer of the proton more energetically favourable. This would explain why the relatively short distance between the oxide ions and the flexibility of the corner sharing octahedral oxide lattice in certain perovskites is conducive to good proton conduction. 2.

EXPERIMENTAL

The SCZT perovskite was prepared by solid-state synthesis. This was achieved by mixing SrCO3 , CaCO3 , ZrO2 and Ta2 O5 (all 99.5%+ purity from Aldrich Chemicals) together in the appropriate stoichiometric ratios determined by the formula i.e.

Sr3 Ca1+x+y Zr1−x T a1−y O9−δ = 3SrCO3 + (1 + x + y) CaCO3 + (1 − x) ZrO2 +

(1 − y) T a2 O5 2

Proton Conducting Ceramics for Use in Intermediate Temperature . / J. New Mat. Electrochem. Systems 5, 25-30 (2002)

The mixture was ground in a ball mill for around 20 minutes and then dried in an oven set at 170◦ C to drive off any residual solvent. The resultant powder was placed in a furnace, heated to 1000◦ C and left for 24 hours to allow complete decarbonation of the precursors. The powder was then reground and uniaxially pressed (300 kPa for 1 minute) into pellets that were calcined at 1200◦ C for 12 hours. These pellets were broken up and ground for a further 20 minutes before being pressed in a cold isostatic press at 300MPa for two minutes. These new pellets were sintered at 1400◦ C over a period of two days with an intermediate regrinding and pressing step after the first day. This resulted in pellets of thicknesses of approximately 1.5 mm. Thinner samples were obtained by carefully smoothing down these pellets using a selection of silicon carbide papers.

sponse from the grain boundary may interfere (~10−10 Fcm−1 ), especially when it is significantly larger. Figure 1 shows the test rig developed to measure the conductivity of the samples. The sample used is a dense pellet (~ 1.5 mm thick) which is painted with thick platinum electrodes, and clamped between the two electrical connections. The atmosphere humidity could be partially controlled by passing gas through a series of water bubblers or a drying column to achieve moist or dry gas conditions respectively.

This process was used to synthesis the perovskites listed in table 1 as well as a yttira doped barium cerate standard (BaCe0.95 Y0.05 O2.975 - BCY). An XRD (X-Ray Diffraction) analysis of each of the perovskite materials was undertaken. In each case the plots obtained indicated the presence of a single perovskite phase. Of the SCZT samples, SCZT85 was chosen for initial study, since pellets with higher concentration of vacancies tended to be difficult to press effectively. It was further observed that pellets of SCZT81/71 tended to crack if left in moist conditions, suggesting that the uptake of water was causing a significant change in the lattice. 2.1

Figure 1: AC Impedance Test Rig

Thermogravimetric Analysis (TGA)

Thermogravimetric analysis using a Rheotherm TG 1000M was carried out on a series of SCZT85 samples that had undergone pre-treatment to alter the water content. These treatments were either vacuum degassing at elevated temperature to remove the presence of protons in the sample or, treating the sample in a water-filled, Teflon bomb at 220◦ C for 24 hours to completely fill the oxygen vacancies. Intermediate levels of hydration were achieved by exposing the sample to a flow of wet hydrogen for a predetermined time. A colour change associated with the degree of hydration was noticed as samples changed from a pale grey to a light blue as water content increased. The TGA results showed that a dried sample would take up protons at temperatures above 300◦ C, while at temperatures above 500◦ C most protons would be lost. 2.2

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AC Impedance

The protonic conductivity is commonly determined by impedance spectroscopy that allows the bulk resistances to be determined from the complex impedance as a function of frequency. In such AC impedance spectra, or Cole-Cole plots, the bulk response appears as a high frequency arc that corresponds to a specific capacitance in the range 10−13 – 10−12 Fcm−1 . While this arc is easy to identify in single crystals it can be difficult to effectively resolve this arc in polycrystalline materials as the re-

2.3

Fuel Cell Studies

A number of designs were proposed for the fuel cell test rig from which the one shown on figure 2 was chosen. This design can be thought of as a series of concentric tubes held in place by steel tube fittings / graphite ferrules. The test pellet had platinum electrodes painted on either side and was firmly held in place by the opposing tubes and a gas tight seal made by the application of high temperature cement. Gases were fed to the electrodes through the central tubes and exhaust through the slightly larger ones. Similarly, the electrical connections were made by threading platinum wire through the gas inlets and connecting the other end to the relevant tube fitting. It was at these points that the external connections to a Solartron Electrochemical Interface (1286) were made. Its particular merits were ease of initial construction, since the quartz tubes and connectors were readily available, and safety since the outer nitrogen blanket should minimise the possibility of all but catastrophic crossover i.e. cracking of the pellet itself. For safety reasons the main gases used initially were a 10% mix of hydrogen in nitrogen, and compressed air. However in later experiments, once the safety of the rig had been demonstrated, the use of a gas mixer and pure hydrogen and oxygen offered a range of compositions in 10% increments

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The slope of such a graph is equal to –E /k , where E is the activation energy and k is the Boltzmann Constant. Arrhenius plots for BCY and SCZT85 are shown on figure 4. The activation energy for the BCY is 0.44 eV. However, for SCZT85 the calculation is less clear since two separate zones exist, caused by the loss of water from the sample, hence reduced proton concentration, at elevated temperatures. If the lower ‘wet’ value alone is considered, assuming that improved humidification could minimise the water loss, an activation energy of 0.4 eV is obtained. This assumption is practical, as in a working fuel cell water is being continuously produced and this should help improve the conductivity.

Figure 2: Fuel Cell Test Rig

3.

RESULTS AND DISCUSSION

The conductivity of the perovskite samples with increasing temperature was recorded at 100◦ C intervals in a wet hydrogen (~10% in nitrogen) atmosphere. The results for BCY and SCZT85 are combined and shown on figure 3, also shown for further comparison are similar results for yttria-stabilised zirconia (YSZ), the standard SOFC electrolyte. Figure 4: Arrhenius plots for BCY and SCZT85 To further elucidate the effects of humidity a hydrated sample was heated to 440◦ C in a moist atmosphere containing 5% hydrogen and left over night. The following day the sample was then exposed to dry gas of an identical composition and the changes in conductivity recorded. Some of the impedance plots generated have been combined to produce figure 5, which clearly indicates the decrease in conductivity as the water content of the perovskite falls.

Figure 3: Conductivity of BCY, SCZT85 and YSZ with temperature in a wet H2 atmosphere The conductivity of the SCZT85 sample can be seen to rise steadily until it reaches a plateau above 300◦ C. This can be explained by considering the loss of water illustrated by the TGA studies. In all cases the conductivity of the BCY is considerably higher than that of the SCZT85. However both are more conductive than YSZ over a large part of the temperature range of interest (200 – 500◦ C). The activation energies for proton conduction can be determined from an appropriate Arrhenius plot, in this case log(σT) vs T−1 .

Table 2 summarises the conductivity results obtained from the SCZT85 and BCY as well as some results for other perovskite materials, from previous work at St. Andrews University, included for comparison. It can be seen that, as predicted, the conductivity increases with increasing oxygen deficiency. A number of power curves produced from a BCY fuel cell operating at different temperatures were combined and are shown on figure 6. Figure 7 illustrates the relationship between pellet thickness and power output. The results show that as the thickness of the electrolyte decreases there is a trend towards an increase in power output, this relationship does not appear to be linear which precludes any easy predictions of power outputs for thinner layers. The maximum power output obtained was ~8mW/cm2 for a 0.6mm thick pellet running on 10% H2 and air at 600◦ C.

Proton Conducting Ceramics for Use in Intermediate Temperature . / J. New Mat. Electrochem. Systems 5, 25-30 (2002)

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Figure 6: Power output vs. temperature for Pt | BCY | Pt fuel cell running on 10% H2 /Air Figure 5: Impedance plots for SCZT85 showing decrease in conductivity as the water content of the perovskite falls.

Table 2: Conductivity of SCZT85 and other perovskite Compound BaCe0.95 Y0.05 O2.975 Sr3 CaZr0.9 Ta1.1 O8.55 Sr3 Ca1.1 Nb1.9 O8.85 (δ =0.15) Sr3 Ca1.18 Ta1.82 O8.73 (δ =0.27) Sr3 Ca(Zr0.9 Ta1.1 )O8.55 (δ =0.45)

Temperature (o C) 300 300 800

Conductivity (Scm−1 ) 4.38 x 10−4 7.62 x 10−5 8.34 x 10−6

800

6.35 x 10−4

800

2.52 x 10−3

The results for the SCZT85 electrolyte operating at a similar range of temperatures are shown on figure 8. It can seen that the maximum power output for the SCZT85 (0.2mW/cm2 at 600◦ C) is much less than for the comparative BCY system. However, issues such as purity and long term stability have yet to be addressed. To test the materials tolerance to operation on reformed hydrocarbon fuels, a gas mixture simulating a typical reformate was used instead of hydrogen as fuel. A synthetic reformate was used which simulated a worse case scenario reformer output i.e. before gas clean-up (43 % H2 , 25 % CO2 , 1 % CO, 31 % CH4 ). This composition contained methane as would be expected directly after the pre-processor. Two fuel cells were constructed; one using the standard yttria doped barium cerate (BCY) electrolyte material and the other with the new electrolyte, strontium calcium zirconium tanta-

Figure 7: Comparison of Power Output vs. Pellet Thickness for BCY fuel cell at 600o C

late (SCZT85). The cells were operated on a fuel containing a concentration of hydrogen equivalent to that of the reformate (~40%) and voltage/current studies were undertaken. Following this the gas supply was switched and the synthetic reformate added. The cell was left for 30 minutes before voltage/current studies were again undertaken. To test for any permanent effects the gas was switched back to the 40% H2 and given 30 minutes to settle. A final set of voltage/current readings were then obtained. The results for the BCY can be seen on Figure 9. It can be seen that a maximum power of 0.16mW/cm2 is obtained before exposure to the reformate. Under reformate conditions the power falls to roughly half its original value. On returning to the clean hydrogen the power does not return to its original level and achieves only 50% improvement.

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Figure 8: Power output vs. temperature for Pt | SCZT85 | Pt fuel cell running on 10% H2 /Air

Figure 10: Pt | SCZT85 | Pt fuel cell running on synthetic reformate at 500◦ C

and SOFC/MCFC systems. However, there obviously exists a need for continued improvements in material conductivity and rigorous testing of these materials, especially those with higher degrees of oxygen deficiency, once certain materials problems have been addressed. REFERENCES [1] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics, 3/4, 359 (1981). [2] R.A. De Souza, J.A. Kilner, B.C.H. Steele, Solid State Ionics, 77, 180 (1995). Figure 9: Pt | BCY | Pt fuel cell running on synthetic reformate at 500◦ C

[3] T. Norby, Solid State Ionics, 125, 1 (1999).

The results obtained for the SCZT85 are shown on Figure 10. The curves for hydrogen and reformate operation are very similar with small differences that can be easily explained by the slight differences in fuel concentrations and flow rates.

[5] K.C. Liang, Y. Du, A.S. Nowick, Solid State Ionics, 69, 117 (1984).

4.

[7] E. Matsushita, T. Sasaki, Solid State Ionics, 125, 31 (1999).

CONCLUSION

The work reported here shows that the Sr3 Ca1+x+y Zr1−x Ta1−y O9−δ family of perovskites offers some benefits over more established proton conducting ceramics. While the proton conduction for SCZT85 is less than that of BaCe0.95 Y0.05 O2.975 the material does not exhibit the same degree of chemical instability under typical fuel cell conditions, at least in the short term. The low power densities reported here may be improved upon by utilising thin film technologies, or even by simple spraying. The results obtained at low temperatures highlight the possibility of developing an intermediate temperature fuel cell capable of bridging the gap between PEM

[4] S.V. Bhide, A.V. Virkar, J. Electrochem. Soc., 146, 4386 (1999).

[6] A.S. Nowick, Y. Du, K.C. Liang, Solid State Ionics, 125, 303 (1999).

[8] M. Cherry, M.S. Islam, J.D. Gale, C.R.A. Catlow, Solid State Ionics, 77, 207 (1995).