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Ion transport in alkaline and earth alkaline hydrogen fluorides
Solid State Ionics 154 – 155 (2002) 487 – 495 www.elsevier.com/locate/ssi
Ion transport in alkaline and earth alkaline hydrogen fluorides C. Kro¨ger a, H. Niggemeier a, H.-D. Wiemho¨fer a,*, O. Glumov b, I. Murin b a
Institute of Inorganic and Analytical Chemistry and SFB 458, University of Mu¨nster, Wilhelm-Klemm-Str. 8, 48149 Mu¨nster, Germany b Department of Solid State Chemistry, State University, Universitetsky Pr. 2, Staryi Petergof, RUS-198504 Saint Petersburg, Russia Accepted 11 March 2002
Abstract The impedance of the hydrogen fluorides KHF2, NH4HF2 and BaHF3 was investigated as a function of temperature and in the presence of HF and H2O. A comparison of results on different electrode materials (graphite, silver, palladium, SnjSnF2, and SnASnF2ALaF3) was used to elucidate the contribution of fluoride ions and protons to the net charge transport. All three investigated materials exhibited a considerable and nearly reversible increase of conductivity in the presence of gases that act as proton donors. In the case of BaHF3, the results indicate both the mobility of fluoride ions (EA = 0.54 eV) as well as a net mobility of protons. The latter is explained by proton exchange due to internal acid – base reactions of the protonated anions. At ambient temperature, KHF2 only becomes conducting when exposed to HF or H2O. NH4HF2, on the other hand, shows a higher conductivity than BaHF3 already in dry atmosphere increasing less with humidity than the other two materials. This indicates an intrinsic proton mobility in NH4HF2. D 2002 Elsevier Science B.V. All rights reserved. PACS: 66.30h Keywords: Hydrogen fluorides; Fluoride ion conductivity; Proton conductivity
1. Introduction Fluoride ion conductors are the most interesting class of anion conductors besides oxygen ion conducting materials [1]. There exists a considerable number of ionic metal fluorides with fluoride ion conduction. Just like oxides, the anion lattice of fluorides is able to dissolve different species such as O2 , OH and probably S2 [2,3]. Accordingly, interaction of fluorides is known to occur with gaseous H2O and H2S as well as with O2 and S2 at elevated temperatures. In the
*
Corresponding author. Tel.: +49-251-83-33115; fax: +49-25183-33169. E-mail address: [email protected] (H.-D. Wiemho¨fer).
past, there have been efforts to apply CaF2 and LaF3 as solid electrolytes in gas sensors [2,3]. There is also increasing evidence for proton transport in fluoride materials as well as in composites containing ion conducting fluorides. This led to some interest to test ion conducting fluorides for the use in low temperature fuel cells and gas sensors [4]. In order to investigate the effects of coexisting protons in a fluoride lattice, we focus here on the transport properties of hydrogen fluorides which contain HF 2 anions. Hydrogen bonds and acid – base properties of these and other protonated species should play a central role for proton transport in fluorides. We expect an analogy to oxides and salts with oxo-anions where internal acid – base equilibria with protons lead to proton transport along the anion lattice [5]. Ion
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transport in metal hydrogen fluorides MFn(HF)m has been investigated much less as compared to the binary metal fluorides. Simple hydrogen fluorides such as KHF2 and NH4HF2 only contain the linear HF 2 unit with a strong and symmetric hydrogen bond, but no single fluoride ions [6,7], whereas hydrogen fluorides of two and three valent cations also contain single F ions besides the HF 2 anions. Of course, the ionic point defects and the ion transport mechanisms will be affected by the presence of HF 2 and its ability to dissociate into HF and F . Both fluoride ions of the linear HF 2 anion are virtually equivalent and the F –F distance is rather short [8]. One may consider reorientation, bending and dissociation of the HF 2 units as preconditions for ion transport. A transport of neutral HF molecules is very probable, too. Accordingly, the possible types of defects contributing to ion mobility are more complicated than in simple fluorides. Conductivity data for KHF2 were first reported by Davis and Westrum [8] who found a strong increase in conductivity at the transition from the tetragonal a- to the cubic h-phase. Indications for proton transport were obtained by Pollock and Sharan [9] from electrolysis experiments with KHF2. The quantity of the electrolytically generated hydrogen-gas was only explainable by a predominating contribution of protons to the charge transport. Impedance data for KHF2 were obtained on pressed polycrystalline pellets and on single crystals by Bruinink and Broers [10,11]. They found higher conductivities and a higher temperature dependence for the polycrystalline pellets as compared to single crystals which was attributed to a surface excess conductivity of the grains. From EMF and electrolysis experiments, the authors deduced that the proton is the predominant charge carrier (tH near 1) in the low temperature phase ( = a-phase below 196 jC). In the high temperature h-phase, the transference numbers of the fluoride and potassium ions were derived from a quantitative analysis of the reaction products at the electrodes to be tF = 0.75, tK = 0.25 whereas the protons gave no contribution (tH = 0). In contradiction to that, however, refined NMR-experiments by Kurukawa and Kiriyama [12] gave a coupled motion of fluorine and hydrogen which strongly suggested the transport of the HF 2 ions as a whole in the h-phase. One should note that the latter results can also be explained by the
transport of neutral HF in similarity to the Grotthus mechanism of OH motion in water. Recently, Zhu et al. [4] carried out experiments on fuel cells with some alkaline and earth alkaline fluorides containing composites as solid electrolytes. Based on the results, they suggested defect models to explain the observed proton and oxygen ion conductivity for pure and composite fluoride materials at elevated temperatures. To sum up, there is clear evidence for a net hydrogen transport for fluoride systems as well as for hydrogen fluorides such as KHF2. On the other hand, up to now, ion transport was not investigated in earth alkaline hydrogen fluorides to our knowledge. This is surprising, because BaHF3 for instance exhibits a considerable thermal stability and may thus represent an interesting model system for a lattice with coexistence of F and HF 2 ions. We therefore studied the conduction in BaHF3 and made some additional experiments with KHF2 and NH4HF2.
2. Experimental 2.1. Sample preparation Samples of KHF2 and NH4HF2 were mainly obtained by recrystallisation of hydrogen fluorides (Fluka, p.a.) from aqueous HF. BaHF3 was prepared by reaction of the carbonate or the binary fluoride (Aldrich) with HF (48 –51%, ACROS) according to BaCO3ðsÞ þ 3HFðaqÞ ! BaHF3ðsÞ # þH2 OðlÞ þ CO2ðgÞ z BaF2ðsÞ þ HFðaqÞ ! BaHF3ðsÞ or by treatment of the carbonate with pyridiniumpolyhydrogenfluoride (PPHF, Aldrich) as described by Singh and Padma [13]. A second series of samples of KHF2 and NH4HF2 were also prepared according to this second approach from the respective carbonates or fluorides. The precipitate was washed with water and dried under a flow of nitrogen. Phase purity was controlled by X-ray powder diffraction. Only after long time storage, small lines of the respective binary fluorides were detectable. Pellets of 6 mm diameter and 0.5 – 4 mm thickness were pressed at about 600
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MPa giving between 84% and 99% of the theoretical crystallographic densities. 2.2. Electrical measurements Several electrode materials were applied to the sample pellets, in particular thermally evaporated Pd films (99.95% Goodfellow), silver paste, graphite spray (Graphit 33, Kontakt-Chemie), and SnjSnF2. The latter was premelted before use. The sample pellets with the as-prepared electrodes were placed between two planar platinum foils in a shielded PTFEvessel enclosed in an electric furnace. Impedance spectra were recorded using a frequency response analyzer and a dielectric interface (SI 1260/DI 1296, Solartron/Schlumberger) in the frequency range 0.01 Hz – 1 MHz as a function of temperature and of partial pressures of H2O (achieved by saturating dry nitrogen over sulfuric acid at different concentrations) or HF (provided by controlled decomposition of NaHF2 or electrolysis in a cell with a solid LaF3 membrane). The impedance data were evaluated in terms of suitable equivalent circuits using ‘‘Zview’’ software (Scribner Associates).
3. Results and discussion 3.1. BaHF3 Because of the existence of isolated F ions besides HF 2 in BaHF3, we expected a certain fluoride ion-mobility in similarity to that in binary BaF2. According to the structural data of Ref. [14], the crystal structure of BaHF3 seems to have room for ion transport via interstitial sites which might give rise to a Frenkel type disorder. The interatomic distance ˚ (Ba2 + is between Ba2 + and F in BaF2 is 2.663 A surrounded by 8 F ions) [15] which is shorter than the shortest Ba – F distances in BaHF3 (the shortest ˚ ). The Ba2 + cations Ba – F distances are 2.69 –2.73 A in BaHF3 are surrounded by nine fluorine atoms (belonging to F and HF 2 anion sites) forming a tricapped trigonal prism. The structure also offers space for dissociation and rotational reorientation of the [F– H – F] units and for formation of hydrogen bonds between HF (from dissociation) and F ions on normal or interstitial lattice sites. HF molecules
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from dissociation of HF 2 may diffuse via interstitial sites or suitable vacancies in the anion lattice. In this way, a coupled motion of F anions and of HF molecules in opposite directions may give an efficient proton transport without breaking the HF bond. This bears some similarity to the discussion of Grotthus and vehicle mechanisms in proton conducting systems with H3O + /H2O as the basic acid –base couple. Impurities such as univalent alkali ions or oxygen ions are expected to lead to an increased anion vacancy concentration by charge compensation, trivalent cations on the other hand should lead to cation vacancies or more probably to fluoride interstitial. HF 2 units can dissociate and exchange fluoride ions with neighbouring vacancies on F sites or with interstitial sites. A first series of our experiments concerned the comparison of impedance of cells with the fluoride ion conducting SnSnF2 contacts and C or AgjC as blocking contacts in order to distinguish between fluoride and net proton transport. A comparison of Figs. 1 and 2 shows that, in dry N2, SnjSnF2 electrodes yielded the lowest impedance (taken vs. zero frequency). The impedance vs. the lowest frequencies corresponds to a net F transport. Therefore, the result give clear evidence for F mobility under an inert dry gas atmosphere, i.e. under a low thermodynamic activity of HF. The graphite contacts (C) in Fig. 2 were highly porous. In order to achieve nonporous contacts, we applied silver paste electrodes on top of the graphite contacts (in order to prevent formation of AgF which might exchange F and thus would be nonblocking). The results with mere graphite contact (Fig. 2) show intermediate impedance values between those for SnjSnF2 (Fig. 1) and AgjC (Fig. 2), i.e. a factor of 10 higher as compared to SnjSnF2. Sealing the carbon electrodes with silver increases the impedance by another factor of 10 which is demonstrated by the two measurements on the same sample in Fig. 2. At temperatures in the range 100 –130 jC (not shown here), the difference of the impedance results between the graphite contacts with and without Ag becomes much smaller, e.g. at 100 jC the impedance with AgjC contacts is no more than a factor of 2 higher than that measured with C contacts. One should note that although these results verify a net F transport, they do not exclude a mobility of protons or HF
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Fig. 1. Temperature-dependent impedance spectra of BaHF3 as measured with symmetric Sn/SnF2 contacts in dry nitrogen atmosphere.
molecules. Rather the large impedance decrease for porous graphite compared to gas tight contacts suggests an additional electrode reaction with formation
Fig. 2. Impedance spectra of BaHF3 in dry nitrogen as measured with contacts consisting of porous carbon (C) and of carbon covered with silver-paste (AgjC). The coverage with Ag paste leads to a complete ion blocking feature with high impedance.
of gaseous hydrogen. A very probable cell reaction with graphite is: xHF + C ! x/2H2 + CFx which is blocked for a gas tight electrode. This reaction implies a compensation of the consumed HF and thus a transport of HF and F . Fig. 3 shows the DC conductivity of undoped BaHF3 as measured with fluoride ion conducting SnjSnF2 electrodes in dry N2. The values were calculated from the limiting value of the impedance for low frequencies where the imaginary part became zero. The activation energy from the temperature dependence in Fig. 3 (0.54 eV) is of the same order of magnitude as in BaF2 at low temperatures (0.58 eV according to Ref. [16]). The conductivity of 10 9 S/ cm at ambient temperature is much higher than that of undoped BaF2 ( < 10 12 S/cm at room temperature [17]). An additional BaHF3 sample was doped with GdF3 which is known to produce interstitial fluoride-ions in BaF2. Fig. 4 compares the impedance of GdF3 doped and undoped material. The results show a clear increase of the DC conductivity indicating that indeed Frenkel disorder seems to occur in the anion lattice with mobile F interstitial. The results given in the previous figures were for sample pellets measured in dry nitrogen. Further experiments concerned the influence of gaseous HF and H2O on the conductivity of BaHF3. According to thermodynamics, varying the partial pressure of HF
Fig. 3. Logarithm of the conductivity of BaHF3 vs. inverse temperature (from extrapolation of the data in Fig. 1 to low frequencies).
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Fig. 4. Impedance spectrum of undoped BaHF3 in comparison to a sample doped with 1 mol% GdF3.
should induce a variation of the nonstoichiometry x in the lattice with regard to the general formula BaF2(HF)1 x. Fig. 5 shows the change of the impedance spectra with time at 20 jC after switching to an atmosphere of streaming N2 with 10 4 mol/l HF. After 2 h, the low frequency impedance was decreased by a factor of 3 to 4. It is evident that dissolution of additional HF increases the conductivity of F ions. One cannot exclude, however, that the primary impedance change after exposing to HF containing atmosphere mainly results from a surface excess conductivity of the crystal grains as long as the HF concentration is not yet equilibrated throughout the bulk. Nevertheless, HF clearly enhances the ion transport. An explanation for the increased conductivity should be based on the assumption of a HF excess in the lattice. We assume that interstitial HF may stabilise F interstitial by the equilibrium HF + F WHF 2 . Fluoride ion jumps could then be favored by diffusion of HF at times forming HF 2 and acting as a F carrier. Thus, the couple HF 2 /HF increases the F conductivity. One should note, however, that a second explanation may be considered for the conductivity enhancement. Interstitial HF can transfer protons to the HF 2 ions on normal lattice sites giving rise to the formation
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of [H –F. . .H –F] dimers. In this case, the acid –base couple (HF)2/HF 2 should lead to an additional net proton transport. We consider this as probable in a similar manner, because HF also acts on SnjSnF2 and may loosen the condition of a mere F transport through the interface. One should note that the dissociation HF 2 WHF + F is the analogue of a Frenkel defect formation, if the remaining F (on the HF 2 lattice site) is understood as a HF vacancy and the free HF molecule as a HF interstitial. Therefore, there must be a certain nonzero equilibrium concentration of these defect species in every crystal. It is difficult in the present stage to decide between detailed models for the influence of HF on the conduction process. However, we do not consider a proton transport by direct proton exchange between HF 2 units, because this would imply the complete two-step dissociation of the strongly hydrogen bonded HF 2. Additional impedance measurements were carried out on BaHF3 pellets with evaporated Pd contacts in the presence of H2O in N2. Fig. 6a shows the impedance spectra as a function of the relative humidity at ambient temperature. The presence of gaseous water affects the conductivity in the same way as gaseous HF. The conductivity as derived from the impedance vs. zero frequency becomes up to two orders of magnitude higher than the mere F conductivity in dry N2 (cf. Fig. 3). The following hydrolysis reaction (using Kroeger-Vink symbols) may
Fig. 5. Influence of HF on the impedance spectra of BaHF3 at 20 jC: (1) in dry N2, (2) directly after exposing to N2 + 10 4 mol/l HF, (3) after 65 min in N2 + 10 4 mol/l HF, (4) after 95 min in N2 + 10 4 mol/l HF.
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lead to irreversible conductivity changes. But in general, the observed conductivity changes were virtually reversible. Therefore, the main effect is attributed to an in-diffusion of HF. Pd electrodes are not reversible for F ions. Accordingly, the conductivity increase is more probable due to a net proton transport by the (HF)2/HF 2 couple. Fig. 6b shows the dependence of the conductivity on the water partial pressure. At high humidities, the frequency-dependent impedance could only be fitted after including a Warburg impedance. This indicates the onset of an ambipolar transport at the electrodes caused by proton exchange. Of course, the question arises as to which degree the water interaction at the grain surfaces causes a surface excess concentration of mobile defects. We consider this as probable, because the surface hydrolysis consumes negative fluoride or hydrogen fluoride ions and leads to enrichment of hydroxide ions at the surface as well as an increased surface concentration of dissolved HF. Therefore, one may also expect surface and space charge effects from the hydrolysis reaction. 3.2. KHF2 and NH4HF2
Fig. 6. Influence of humidity on the impedance of the cell PdABaHF3APd and the conductivity of BaHF3: (a) frequency dependence of the impedance as a function of humidity in nitrogen (relative percentage of water saturation pressure), (b) semilogarithmic plot of the conductivity (from the impedance in the limit of small frequencies and long times of exposition to humidity) vs. the water partial pressure.
explain the similarity between the influence of H2O and HF: H2 Oad þ FF WðHFÞi þ ðOHÞF H2 Oad þ FHFFHF W2ðHFÞi þ ðOHÞFHF According to these hydrolysis reactions, HF is formed at the grain surfaces and can diffuse into the bulk of the grains. Of course, a diffusion of OH into the bulk may occur, too, and has to be expected at least for long times. One can expect that this would
Extensive impedance measurements as a function of time after exposing to humidity were carried out with KHF2 and NH4HF2. Both materials react very sensitively on the presence of humidity. In general, the impedance decreases by one to two orders of magnitude. For moderate treatment with not too high water partial pressure (below 80% relative humidity), any conductivity changes were found to be reversible. A large volume expansion accompanied the water interaction for high relative humidities proving that an uptake of water and possibly of the HF from hydrolysis occurs. For more extensive treatments with high water partial pressures over several days, the pellets became enlarged and porous. This was most clearly seen for NH4HF2. On the other hand, NH4HF2 samples were not stable when stored under completely dry atmosphere. Apparently, the crystal structure needs stabilisation by dissolved excess HF or H2O. Therefore, measurements on NH4HF2 were not easily reproducible. Fig. 7 shows Cole – Cole plots for the cell Pd AKHF2APd in a typical experiment where the impe-
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Fig. 8. Time dependence of the conductivity of KHF2 during exposition to nitrogen with constant humidity. The values at t > 75 103 s correspond to the impedance spectra of Fig. 7.
Fig. 7. Influence of humidity on the conductivity of KHF2. Note in this figure that the results from the measurement in dry atmosphere describe the beginning of a very huge semicircle (the points are seen close to the imaginary axis). This means that, in the absence of HF and H2O, the conductivity practically vanishes.
The suggested transport mechanism is supported by the characteristics of the KHF2 structure. The tetragonal structure can be viewed as the packing of alternating K + and HF 2 layers [6]. Within one anion layer of the low temperature phase, the HF 2 ions are lined up in parallel. Partial rotational disorder is possible (the high temperature phase shows statistical
dance was monitored for several hours after an initial stepwise change from dry to humid N2. The impedance decreased rapidly in the beginning followed by a much slower, but significant decrease for longer time (e.g. during more than 1 day for a sample with 1.5 mm thickness and 6 mm diameter). Fig. 8 shows a semilogarithmic plot of the change of the conductivity with time (minus the extrapolated conductivity r(l) for t ! l) vs. the time elapsed after switching to a humid atmosphere. Assuming bulk diffusion and an exponential time dependence proportional to exp[ Dt/L2] with L as effective diffusion length, the slope gives a diffusion coefficient D of the order of 10 7 cm2 s 1 which seems reasonable. In accord with this, the impedance in the low frequency range shows a Warburg-type frequency dependence with a huge semicircle. This gives additional evidence that the sample exhibits a net proton transport via an ambipolar diffusion (of HF and HF 2 ). This is in agreement with NMR results (as mentioned in Introduction) which pointed out a coupled transport of hydrogen and fluorine [12].
Fig. 9. Influence of humidity on the impedance of NH4HF2. As compared to the other two investigated materials, there is a much lower humidity dependence of the conductivity in this case.
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C. Kro¨ger et al. / Solid State Ionics 154 – 155 (2002) 487–495 conductivity for low activities HF 2 ) shows a low F of HF and H2O, KHF2 has no significant ionic conductivity under HF free or dry conditions (cf. Fig. 7). This has to be expected as no free F ions are involved in the crystal structure of KHF2 . NH4HF2, on the other hand, shows a medium impedance also under dry atmosphere. The latter is a strong hint that NH4HF2 shows an intrinsic proton conductivity under dry conditions due to the presence of the ammonium ions. It is probable that an acid – base equilibrium of the following type exists leading to a proton mobility
Fig. 10. Short-term time dependence of conductivity of NH4HF2 exposed to humidity: the conductivity changes are much faster than measured for BaHF3 and KHF2.
NHþ 4 þ HF2 WNH3 þ H2 F2 WNH3 þ 2HF:
4. Conclusion
orientations of the FHF anions). Transport of HF, F and HF 2 seems feasible within the anion layers. A view along the c-axis in one of the HF 2 layers of the KHF2 structure reveals a second possible transport mechanism where a fluoride ion or a complete HF can jump from one tetragonal K + -cage to the next neighbouring cage (the shortest distance between the F of ˚ one HF 2 to the centre of the next HF2 is 2.871 A [6]) probably passing an interstitial site. In principle, the structure should allow the occupation of interstitial sites by excess HF which then forms a hydrogen bond to one neighbouring HF 2 . This is also supported by the existence of the compound KH2F3 containing the hydrogen bonded (but nonlinear) H2F 3 unit. Corresponding results for NH4HF2 are shown in Fig. 9. The impedance of this material becomes very low for increasing humidity. The conductivity under these conditions becomes similar to that of the LaF3 contact material (used here as F conducting solid electrolyte). Apparently, the proton exchange is considerably enhanced. The semilogarithmic plot of the conductivity vs. time in Fig. 10 shows a linear slope from which we estimated a diffusion coefficient of the order of 10 5 cm2 s 1, distinctly higher than for KHF2. 3.3. Comparison between KHF2, NH4HF2 and BaHF3 It became evident that there are differences between the three analyzed materials: whereas BaHF3 in accord with its structure (existence of both F and
The conductivity of metal hydrogen fluorides is strongly increased by the presence of HF and H2O. Although in the case of earth alkaline hydrogen fluorides there is also a F mobility, the results made evident a contribution from protonated species that can cause a net proton transport by dissociation and proton transfer. The observations are explained in accord with the chemical properties of the hydrogen fluorides, namely the high tendency to form hydrogen bonded aggregates of HF and F . Thus, the metal hydrogen fluorides represent an interesting class of compounds, which are extremely sensitive to proton transport in the presence of proton donor gases. Acknowledgements The authors would like to thank the DFG for financial support of the work within the SFB 458 and for financing the stay of O. Glumov in Muenster. References [1] T. Kudo, Survey of types of solid electrolytes, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, 1996, p. 195. [2] P. Fabry, E. Siebert, Survey of types of solid electrolytes, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, 1996, p. 329.
C. Kro¨ger et al. / Solid State Ionics 154 – 155 (2002) 487–495 [3] S. Kumata, N. Miura, N. Yamazoe, T. Seiyama, Chem. Lett. (1984) 981. [4] B. Zhu, I. Albinsson, B.-E. Mellander, Solid State Ionics 135 (2000) 503. [5] K.D. Kreuer, Chem. Mater. 8 (1996) 610. [6] J.A. Ibers, J. Chem. Phys. 40 (1964) 402. [7] T.R.R. McDonald, Acta Crystallogr. 13 (1960) 113. [8] M.L. Davis, E.F. Westrum, J. Phys. Chem. 65 (1961) 338. [9] J.M. Pollock, M. Sharan, J. Chem. Phys. 47 (10) (1967) 4064. [10] J. Bruinink, G.H.J. Broers, J. Phys. Chem. Solids 33 (1972) 1713.
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[11] J. Bruinink, J. Electroanal. Chem. Interfacial Electrochem. 51 (1974) 141. [12] Y. Kurukawa, H. Kiriyama, Bull. Chem. Soc. Jpn. 51 (1978) 3438. [13] R.N. Singh, D.K. Padma, J. Fluorine Chem. 67 (1994) 211. [14] W. Massa, E. Herdtweck, Acta Crystallogr. C39 (1983) 509. [15] J.M. Leger, J. Haines, A. Atouf, O. Schulto, S. Hull, Phys. Rev., B 52 (1995) 13247. [16] V.P. Zhukov, V.M. Zainullina, Phys. Solid State 40 (1998) 1827. [17] A.V. Chadwick, Solid State Ionics 8 (1983) 209.
Ion transport in alkaline and earth alkaline hydrogen fluorides C. Kro¨ger a, H. Niggemeier a, H.-D. Wiemho¨fer a,*, O. Glumov b, I. Murin b a
Institute of Inorganic and Analytical Chemistry and SFB 458, University of Mu¨nster, Wilhelm-Klemm-Str. 8, 48149 Mu¨nster, Germany b Department of Solid State Chemistry, State University, Universitetsky Pr. 2, Staryi Petergof, RUS-198504 Saint Petersburg, Russia Accepted 11 March 2002
Abstract The impedance of the hydrogen fluorides KHF2, NH4HF2 and BaHF3 was investigated as a function of temperature and in the presence of HF and H2O. A comparison of results on different electrode materials (graphite, silver, palladium, SnjSnF2, and SnASnF2ALaF3) was used to elucidate the contribution of fluoride ions and protons to the net charge transport. All three investigated materials exhibited a considerable and nearly reversible increase of conductivity in the presence of gases that act as proton donors. In the case of BaHF3, the results indicate both the mobility of fluoride ions (EA = 0.54 eV) as well as a net mobility of protons. The latter is explained by proton exchange due to internal acid – base reactions of the protonated anions. At ambient temperature, KHF2 only becomes conducting when exposed to HF or H2O. NH4HF2, on the other hand, shows a higher conductivity than BaHF3 already in dry atmosphere increasing less with humidity than the other two materials. This indicates an intrinsic proton mobility in NH4HF2. D 2002 Elsevier Science B.V. All rights reserved. PACS: 66.30h Keywords: Hydrogen fluorides; Fluoride ion conductivity; Proton conductivity
1. Introduction Fluoride ion conductors are the most interesting class of anion conductors besides oxygen ion conducting materials [1]. There exists a considerable number of ionic metal fluorides with fluoride ion conduction. Just like oxides, the anion lattice of fluorides is able to dissolve different species such as O2 , OH and probably S2 [2,3]. Accordingly, interaction of fluorides is known to occur with gaseous H2O and H2S as well as with O2 and S2 at elevated temperatures. In the
*
Corresponding author. Tel.: +49-251-83-33115; fax: +49-25183-33169. E-mail address: [email protected] (H.-D. Wiemho¨fer).
past, there have been efforts to apply CaF2 and LaF3 as solid electrolytes in gas sensors [2,3]. There is also increasing evidence for proton transport in fluoride materials as well as in composites containing ion conducting fluorides. This led to some interest to test ion conducting fluorides for the use in low temperature fuel cells and gas sensors [4]. In order to investigate the effects of coexisting protons in a fluoride lattice, we focus here on the transport properties of hydrogen fluorides which contain HF 2 anions. Hydrogen bonds and acid – base properties of these and other protonated species should play a central role for proton transport in fluorides. We expect an analogy to oxides and salts with oxo-anions where internal acid – base equilibria with protons lead to proton transport along the anion lattice [5]. Ion
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C. Kro¨ger et al. / Solid State Ionics 154 – 155 (2002) 487–495
transport in metal hydrogen fluorides MFn(HF)m has been investigated much less as compared to the binary metal fluorides. Simple hydrogen fluorides such as KHF2 and NH4HF2 only contain the linear HF 2 unit with a strong and symmetric hydrogen bond, but no single fluoride ions [6,7], whereas hydrogen fluorides of two and three valent cations also contain single F ions besides the HF 2 anions. Of course, the ionic point defects and the ion transport mechanisms will be affected by the presence of HF 2 and its ability to dissociate into HF and F . Both fluoride ions of the linear HF 2 anion are virtually equivalent and the F –F distance is rather short [8]. One may consider reorientation, bending and dissociation of the HF 2 units as preconditions for ion transport. A transport of neutral HF molecules is very probable, too. Accordingly, the possible types of defects contributing to ion mobility are more complicated than in simple fluorides. Conductivity data for KHF2 were first reported by Davis and Westrum [8] who found a strong increase in conductivity at the transition from the tetragonal a- to the cubic h-phase. Indications for proton transport were obtained by Pollock and Sharan [9] from electrolysis experiments with KHF2. The quantity of the electrolytically generated hydrogen-gas was only explainable by a predominating contribution of protons to the charge transport. Impedance data for KHF2 were obtained on pressed polycrystalline pellets and on single crystals by Bruinink and Broers [10,11]. They found higher conductivities and a higher temperature dependence for the polycrystalline pellets as compared to single crystals which was attributed to a surface excess conductivity of the grains. From EMF and electrolysis experiments, the authors deduced that the proton is the predominant charge carrier (tH near 1) in the low temperature phase ( = a-phase below 196 jC). In the high temperature h-phase, the transference numbers of the fluoride and potassium ions were derived from a quantitative analysis of the reaction products at the electrodes to be tF = 0.75, tK = 0.25 whereas the protons gave no contribution (tH = 0). In contradiction to that, however, refined NMR-experiments by Kurukawa and Kiriyama [12] gave a coupled motion of fluorine and hydrogen which strongly suggested the transport of the HF 2 ions as a whole in the h-phase. One should note that the latter results can also be explained by the
transport of neutral HF in similarity to the Grotthus mechanism of OH motion in water. Recently, Zhu et al. [4] carried out experiments on fuel cells with some alkaline and earth alkaline fluorides containing composites as solid electrolytes. Based on the results, they suggested defect models to explain the observed proton and oxygen ion conductivity for pure and composite fluoride materials at elevated temperatures. To sum up, there is clear evidence for a net hydrogen transport for fluoride systems as well as for hydrogen fluorides such as KHF2. On the other hand, up to now, ion transport was not investigated in earth alkaline hydrogen fluorides to our knowledge. This is surprising, because BaHF3 for instance exhibits a considerable thermal stability and may thus represent an interesting model system for a lattice with coexistence of F and HF 2 ions. We therefore studied the conduction in BaHF3 and made some additional experiments with KHF2 and NH4HF2.
2. Experimental 2.1. Sample preparation Samples of KHF2 and NH4HF2 were mainly obtained by recrystallisation of hydrogen fluorides (Fluka, p.a.) from aqueous HF. BaHF3 was prepared by reaction of the carbonate or the binary fluoride (Aldrich) with HF (48 –51%, ACROS) according to BaCO3ðsÞ þ 3HFðaqÞ ! BaHF3ðsÞ # þH2 OðlÞ þ CO2ðgÞ z BaF2ðsÞ þ HFðaqÞ ! BaHF3ðsÞ or by treatment of the carbonate with pyridiniumpolyhydrogenfluoride (PPHF, Aldrich) as described by Singh and Padma [13]. A second series of samples of KHF2 and NH4HF2 were also prepared according to this second approach from the respective carbonates or fluorides. The precipitate was washed with water and dried under a flow of nitrogen. Phase purity was controlled by X-ray powder diffraction. Only after long time storage, small lines of the respective binary fluorides were detectable. Pellets of 6 mm diameter and 0.5 – 4 mm thickness were pressed at about 600
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MPa giving between 84% and 99% of the theoretical crystallographic densities. 2.2. Electrical measurements Several electrode materials were applied to the sample pellets, in particular thermally evaporated Pd films (99.95% Goodfellow), silver paste, graphite spray (Graphit 33, Kontakt-Chemie), and SnjSnF2. The latter was premelted before use. The sample pellets with the as-prepared electrodes were placed between two planar platinum foils in a shielded PTFEvessel enclosed in an electric furnace. Impedance spectra were recorded using a frequency response analyzer and a dielectric interface (SI 1260/DI 1296, Solartron/Schlumberger) in the frequency range 0.01 Hz – 1 MHz as a function of temperature and of partial pressures of H2O (achieved by saturating dry nitrogen over sulfuric acid at different concentrations) or HF (provided by controlled decomposition of NaHF2 or electrolysis in a cell with a solid LaF3 membrane). The impedance data were evaluated in terms of suitable equivalent circuits using ‘‘Zview’’ software (Scribner Associates).
3. Results and discussion 3.1. BaHF3 Because of the existence of isolated F ions besides HF 2 in BaHF3, we expected a certain fluoride ion-mobility in similarity to that in binary BaF2. According to the structural data of Ref. [14], the crystal structure of BaHF3 seems to have room for ion transport via interstitial sites which might give rise to a Frenkel type disorder. The interatomic distance ˚ (Ba2 + is between Ba2 + and F in BaF2 is 2.663 A surrounded by 8 F ions) [15] which is shorter than the shortest Ba – F distances in BaHF3 (the shortest ˚ ). The Ba2 + cations Ba – F distances are 2.69 –2.73 A in BaHF3 are surrounded by nine fluorine atoms (belonging to F and HF 2 anion sites) forming a tricapped trigonal prism. The structure also offers space for dissociation and rotational reorientation of the [F– H – F] units and for formation of hydrogen bonds between HF (from dissociation) and F ions on normal or interstitial lattice sites. HF molecules
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from dissociation of HF 2 may diffuse via interstitial sites or suitable vacancies in the anion lattice. In this way, a coupled motion of F anions and of HF molecules in opposite directions may give an efficient proton transport without breaking the HF bond. This bears some similarity to the discussion of Grotthus and vehicle mechanisms in proton conducting systems with H3O + /H2O as the basic acid –base couple. Impurities such as univalent alkali ions or oxygen ions are expected to lead to an increased anion vacancy concentration by charge compensation, trivalent cations on the other hand should lead to cation vacancies or more probably to fluoride interstitial. HF 2 units can dissociate and exchange fluoride ions with neighbouring vacancies on F sites or with interstitial sites. A first series of our experiments concerned the comparison of impedance of cells with the fluoride ion conducting SnSnF2 contacts and C or AgjC as blocking contacts in order to distinguish between fluoride and net proton transport. A comparison of Figs. 1 and 2 shows that, in dry N2, SnjSnF2 electrodes yielded the lowest impedance (taken vs. zero frequency). The impedance vs. the lowest frequencies corresponds to a net F transport. Therefore, the result give clear evidence for F mobility under an inert dry gas atmosphere, i.e. under a low thermodynamic activity of HF. The graphite contacts (C) in Fig. 2 were highly porous. In order to achieve nonporous contacts, we applied silver paste electrodes on top of the graphite contacts (in order to prevent formation of AgF which might exchange F and thus would be nonblocking). The results with mere graphite contact (Fig. 2) show intermediate impedance values between those for SnjSnF2 (Fig. 1) and AgjC (Fig. 2), i.e. a factor of 10 higher as compared to SnjSnF2. Sealing the carbon electrodes with silver increases the impedance by another factor of 10 which is demonstrated by the two measurements on the same sample in Fig. 2. At temperatures in the range 100 –130 jC (not shown here), the difference of the impedance results between the graphite contacts with and without Ag becomes much smaller, e.g. at 100 jC the impedance with AgjC contacts is no more than a factor of 2 higher than that measured with C contacts. One should note that although these results verify a net F transport, they do not exclude a mobility of protons or HF
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Fig. 1. Temperature-dependent impedance spectra of BaHF3 as measured with symmetric Sn/SnF2 contacts in dry nitrogen atmosphere.
molecules. Rather the large impedance decrease for porous graphite compared to gas tight contacts suggests an additional electrode reaction with formation
Fig. 2. Impedance spectra of BaHF3 in dry nitrogen as measured with contacts consisting of porous carbon (C) and of carbon covered with silver-paste (AgjC). The coverage with Ag paste leads to a complete ion blocking feature with high impedance.
of gaseous hydrogen. A very probable cell reaction with graphite is: xHF + C ! x/2H2 + CFx which is blocked for a gas tight electrode. This reaction implies a compensation of the consumed HF and thus a transport of HF and F . Fig. 3 shows the DC conductivity of undoped BaHF3 as measured with fluoride ion conducting SnjSnF2 electrodes in dry N2. The values were calculated from the limiting value of the impedance for low frequencies where the imaginary part became zero. The activation energy from the temperature dependence in Fig. 3 (0.54 eV) is of the same order of magnitude as in BaF2 at low temperatures (0.58 eV according to Ref. [16]). The conductivity of 10 9 S/ cm at ambient temperature is much higher than that of undoped BaF2 ( < 10 12 S/cm at room temperature [17]). An additional BaHF3 sample was doped with GdF3 which is known to produce interstitial fluoride-ions in BaF2. Fig. 4 compares the impedance of GdF3 doped and undoped material. The results show a clear increase of the DC conductivity indicating that indeed Frenkel disorder seems to occur in the anion lattice with mobile F interstitial. The results given in the previous figures were for sample pellets measured in dry nitrogen. Further experiments concerned the influence of gaseous HF and H2O on the conductivity of BaHF3. According to thermodynamics, varying the partial pressure of HF
Fig. 3. Logarithm of the conductivity of BaHF3 vs. inverse temperature (from extrapolation of the data in Fig. 1 to low frequencies).
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Fig. 4. Impedance spectrum of undoped BaHF3 in comparison to a sample doped with 1 mol% GdF3.
should induce a variation of the nonstoichiometry x in the lattice with regard to the general formula BaF2(HF)1 x. Fig. 5 shows the change of the impedance spectra with time at 20 jC after switching to an atmosphere of streaming N2 with 10 4 mol/l HF. After 2 h, the low frequency impedance was decreased by a factor of 3 to 4. It is evident that dissolution of additional HF increases the conductivity of F ions. One cannot exclude, however, that the primary impedance change after exposing to HF containing atmosphere mainly results from a surface excess conductivity of the crystal grains as long as the HF concentration is not yet equilibrated throughout the bulk. Nevertheless, HF clearly enhances the ion transport. An explanation for the increased conductivity should be based on the assumption of a HF excess in the lattice. We assume that interstitial HF may stabilise F interstitial by the equilibrium HF + F WHF 2 . Fluoride ion jumps could then be favored by diffusion of HF at times forming HF 2 and acting as a F carrier. Thus, the couple HF 2 /HF increases the F conductivity. One should note, however, that a second explanation may be considered for the conductivity enhancement. Interstitial HF can transfer protons to the HF 2 ions on normal lattice sites giving rise to the formation
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of [H –F. . .H –F] dimers. In this case, the acid –base couple (HF)2/HF 2 should lead to an additional net proton transport. We consider this as probable in a similar manner, because HF also acts on SnjSnF2 and may loosen the condition of a mere F transport through the interface. One should note that the dissociation HF 2 WHF + F is the analogue of a Frenkel defect formation, if the remaining F (on the HF 2 lattice site) is understood as a HF vacancy and the free HF molecule as a HF interstitial. Therefore, there must be a certain nonzero equilibrium concentration of these defect species in every crystal. It is difficult in the present stage to decide between detailed models for the influence of HF on the conduction process. However, we do not consider a proton transport by direct proton exchange between HF 2 units, because this would imply the complete two-step dissociation of the strongly hydrogen bonded HF 2. Additional impedance measurements were carried out on BaHF3 pellets with evaporated Pd contacts in the presence of H2O in N2. Fig. 6a shows the impedance spectra as a function of the relative humidity at ambient temperature. The presence of gaseous water affects the conductivity in the same way as gaseous HF. The conductivity as derived from the impedance vs. zero frequency becomes up to two orders of magnitude higher than the mere F conductivity in dry N2 (cf. Fig. 3). The following hydrolysis reaction (using Kroeger-Vink symbols) may
Fig. 5. Influence of HF on the impedance spectra of BaHF3 at 20 jC: (1) in dry N2, (2) directly after exposing to N2 + 10 4 mol/l HF, (3) after 65 min in N2 + 10 4 mol/l HF, (4) after 95 min in N2 + 10 4 mol/l HF.
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lead to irreversible conductivity changes. But in general, the observed conductivity changes were virtually reversible. Therefore, the main effect is attributed to an in-diffusion of HF. Pd electrodes are not reversible for F ions. Accordingly, the conductivity increase is more probable due to a net proton transport by the (HF)2/HF 2 couple. Fig. 6b shows the dependence of the conductivity on the water partial pressure. At high humidities, the frequency-dependent impedance could only be fitted after including a Warburg impedance. This indicates the onset of an ambipolar transport at the electrodes caused by proton exchange. Of course, the question arises as to which degree the water interaction at the grain surfaces causes a surface excess concentration of mobile defects. We consider this as probable, because the surface hydrolysis consumes negative fluoride or hydrogen fluoride ions and leads to enrichment of hydroxide ions at the surface as well as an increased surface concentration of dissolved HF. Therefore, one may also expect surface and space charge effects from the hydrolysis reaction. 3.2. KHF2 and NH4HF2
Fig. 6. Influence of humidity on the impedance of the cell PdABaHF3APd and the conductivity of BaHF3: (a) frequency dependence of the impedance as a function of humidity in nitrogen (relative percentage of water saturation pressure), (b) semilogarithmic plot of the conductivity (from the impedance in the limit of small frequencies and long times of exposition to humidity) vs. the water partial pressure.
explain the similarity between the influence of H2O and HF: H2 Oad þ FF WðHFÞi þ ðOHÞF H2 Oad þ FHFFHF W2ðHFÞi þ ðOHÞFHF According to these hydrolysis reactions, HF is formed at the grain surfaces and can diffuse into the bulk of the grains. Of course, a diffusion of OH into the bulk may occur, too, and has to be expected at least for long times. One can expect that this would
Extensive impedance measurements as a function of time after exposing to humidity were carried out with KHF2 and NH4HF2. Both materials react very sensitively on the presence of humidity. In general, the impedance decreases by one to two orders of magnitude. For moderate treatment with not too high water partial pressure (below 80% relative humidity), any conductivity changes were found to be reversible. A large volume expansion accompanied the water interaction for high relative humidities proving that an uptake of water and possibly of the HF from hydrolysis occurs. For more extensive treatments with high water partial pressures over several days, the pellets became enlarged and porous. This was most clearly seen for NH4HF2. On the other hand, NH4HF2 samples were not stable when stored under completely dry atmosphere. Apparently, the crystal structure needs stabilisation by dissolved excess HF or H2O. Therefore, measurements on NH4HF2 were not easily reproducible. Fig. 7 shows Cole – Cole plots for the cell Pd AKHF2APd in a typical experiment where the impe-
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Fig. 8. Time dependence of the conductivity of KHF2 during exposition to nitrogen with constant humidity. The values at t > 75 103 s correspond to the impedance spectra of Fig. 7.
Fig. 7. Influence of humidity on the conductivity of KHF2. Note in this figure that the results from the measurement in dry atmosphere describe the beginning of a very huge semicircle (the points are seen close to the imaginary axis). This means that, in the absence of HF and H2O, the conductivity practically vanishes.
The suggested transport mechanism is supported by the characteristics of the KHF2 structure. The tetragonal structure can be viewed as the packing of alternating K + and HF 2 layers [6]. Within one anion layer of the low temperature phase, the HF 2 ions are lined up in parallel. Partial rotational disorder is possible (the high temperature phase shows statistical
dance was monitored for several hours after an initial stepwise change from dry to humid N2. The impedance decreased rapidly in the beginning followed by a much slower, but significant decrease for longer time (e.g. during more than 1 day for a sample with 1.5 mm thickness and 6 mm diameter). Fig. 8 shows a semilogarithmic plot of the change of the conductivity with time (minus the extrapolated conductivity r(l) for t ! l) vs. the time elapsed after switching to a humid atmosphere. Assuming bulk diffusion and an exponential time dependence proportional to exp[ Dt/L2] with L as effective diffusion length, the slope gives a diffusion coefficient D of the order of 10 7 cm2 s 1 which seems reasonable. In accord with this, the impedance in the low frequency range shows a Warburg-type frequency dependence with a huge semicircle. This gives additional evidence that the sample exhibits a net proton transport via an ambipolar diffusion (of HF and HF 2 ). This is in agreement with NMR results (as mentioned in Introduction) which pointed out a coupled transport of hydrogen and fluorine [12].
Fig. 9. Influence of humidity on the impedance of NH4HF2. As compared to the other two investigated materials, there is a much lower humidity dependence of the conductivity in this case.
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C. Kro¨ger et al. / Solid State Ionics 154 – 155 (2002) 487–495 conductivity for low activities HF 2 ) shows a low F of HF and H2O, KHF2 has no significant ionic conductivity under HF free or dry conditions (cf. Fig. 7). This has to be expected as no free F ions are involved in the crystal structure of KHF2 . NH4HF2, on the other hand, shows a medium impedance also under dry atmosphere. The latter is a strong hint that NH4HF2 shows an intrinsic proton conductivity under dry conditions due to the presence of the ammonium ions. It is probable that an acid – base equilibrium of the following type exists leading to a proton mobility
Fig. 10. Short-term time dependence of conductivity of NH4HF2 exposed to humidity: the conductivity changes are much faster than measured for BaHF3 and KHF2.
NHþ 4 þ HF2 WNH3 þ H2 F2 WNH3 þ 2HF:
4. Conclusion
orientations of the FHF anions). Transport of HF, F and HF 2 seems feasible within the anion layers. A view along the c-axis in one of the HF 2 layers of the KHF2 structure reveals a second possible transport mechanism where a fluoride ion or a complete HF can jump from one tetragonal K + -cage to the next neighbouring cage (the shortest distance between the F of ˚ one HF 2 to the centre of the next HF2 is 2.871 A [6]) probably passing an interstitial site. In principle, the structure should allow the occupation of interstitial sites by excess HF which then forms a hydrogen bond to one neighbouring HF 2 . This is also supported by the existence of the compound KH2F3 containing the hydrogen bonded (but nonlinear) H2F 3 unit. Corresponding results for NH4HF2 are shown in Fig. 9. The impedance of this material becomes very low for increasing humidity. The conductivity under these conditions becomes similar to that of the LaF3 contact material (used here as F conducting solid electrolyte). Apparently, the proton exchange is considerably enhanced. The semilogarithmic plot of the conductivity vs. time in Fig. 10 shows a linear slope from which we estimated a diffusion coefficient of the order of 10 5 cm2 s 1, distinctly higher than for KHF2. 3.3. Comparison between KHF2, NH4HF2 and BaHF3 It became evident that there are differences between the three analyzed materials: whereas BaHF3 in accord with its structure (existence of both F and
The conductivity of metal hydrogen fluorides is strongly increased by the presence of HF and H2O. Although in the case of earth alkaline hydrogen fluorides there is also a F mobility, the results made evident a contribution from protonated species that can cause a net proton transport by dissociation and proton transfer. The observations are explained in accord with the chemical properties of the hydrogen fluorides, namely the high tendency to form hydrogen bonded aggregates of HF and F . Thus, the metal hydrogen fluorides represent an interesting class of compounds, which are extremely sensitive to proton transport in the presence of proton donor gases. Acknowledgements The authors would like to thank the DFG for financial support of the work within the SFB 458 and for financing the stay of O. Glumov in Muenster. References [1] T. Kudo, Survey of types of solid electrolytes, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, 1996, p. 195. [2] P. Fabry, E. Siebert, Survey of types of solid electrolytes, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, 1996, p. 329.
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