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Solid-Hydroxide Protonic Conductors: Superionic Conductivity,. Phase Transitions, Isotopic Effect, and Self-Organized Microheterogeneity. Yu. M. Baœkov and ...
ISSN 1063-7834, Physics of the Solid State, 2009, Vol. 51, No. 1, pp. 33–43. © Pleiades Publishing, Ltd., 2009. Original Russian Text © Yu.M. Baœkov, V.M. Egorov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 33–42.

SEMICONDUCTORS AND DIELECTRICS

Solid-Hydroxide Protonic Conductors: Superionic Conductivity, Phase Transitions, Isotopic Effect, and Self-Organized Microheterogeneity Yu. M. Baœkov and V. M. Egorov Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia e-mail: [email protected] Received April 8, 2008

Abstract—New members of the family of inorganic solid-state protonic conductors are synthesized on the basis of KOH, NaOH, and H2O. The equimolar eutectics KOH/NaOH and KOH/KOH · H2O and the monohydrate KOH · H2O are superprotonic conductors in the temperature ranges 360–458, 360–370, and 320–420 K, respectively, with the conductivities of above 1 mS/cm and activation energies below 0.4 eV. A characteristic feature of the eutectics is the presence of anomalies in the temperature dependences of the conductivity and heat capacity in the range 360 ± 1 K. The isotopic effect of the protonic conductivity for KOH · H2O and for the high-temperature form of KOH/NaOH is found to be 1.40 ± 0.15. The role played by the self-organized microheterogeneity of the KOH/NaOH and KOH/KOH · H2O solid eutectics in the substantial increase in the conductivity and in the change of its pattern as compared to the individual KOH and NaOH is discussed. PACS numbers: 64.70.kp, 66.10.Ed, 66.30.jp, 64.75.Yz, 65.40.-b, 64.70.Nd, 82.45.Un DOI: 10.1134/S106378340901003X

1. INTRODUCTION

to sidestep this intriguing question in the hope that the new members of the family of inorganic protonic conductors, which have until recently been unknown and are now studied by them, will reduce the number of “white spots” in the physics and ionics of hydrogencontaining solids. Starting from the 1970s, basic scientific interest in the protonic conduction in solid materials, both organic and inorganic, has been gradually complemented by exploration of a purely applied aspect of the problem which stemmed from the needs of hydrogen-based energetics, which focuses attention on solid materials with protonic conduction (protonics). The diversity of the well-known protonic conductors does not, however, warrant in itself successful progress on this way, because of the obvious need to match the electrolytes with electrodes, i.e., to bring in accord the conditions favoring a high protonic conductivity of an electrolyte with the high electrochemical activity of electrodes with respect to hydrogen-containing materials. The present trends in basic research suggest that to reach the above goals, one should raise the efficiency of proton transfer processes not only in the electrolyte but through the electrode–electrolyte heteroboundaries as well. An ideal solution to this problem would apparently be realization of a protonic heterojunction, i.e., of reversible proton exchange between the electrolyte and the electrode directly, i.e., without any intermediate reagents, for instance, of catalysts. It is this approach

1.1. Solid Protonic Conductors: Basic and Applied Aspects The term “protonic conduction” is used to identify the phenomenon of charge transport in an electric field by hydrogen cations, i.e., protons. Processes occurring at temperatures above 300 K are commonly described by various versions of the model treating transport in an electric field and diffusion of protons, which rest on the concept of the motion of the proton as a classical particle [1]. The model of thermally activated hopping over a grid of electrically negative ions, most frequently of oxygen ions, was proposed two centuries ago and called the Grotthuss-type mechanism [2]. This model is used presently also as a basis for treatment of other physicochemical processes as well, both in complex materials (such as polymers), and in biological objects (which can be exemplified by photosynthesis in green plants) [3]. Nonetheless, the specific features present in the structure and chemical composition require substantial modification of the concepts underlying the mechanism of proton transfer in a given material and/or a process, be it diffusion, electric transport or chemical reaction. A question may arise whether this “heterogeneity” of the concepts reflects the rich diversity of the Nature or a manifestation of the lack of basic knowledge concerning the processes involved in proton transfer? Being experimentalists, the present authors prefer 33

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that is embodied in one of the directions included in the Basic Research Program supported by the Presidium of the Russian Academy of Sciences (project P 03-02), more specifically, “Crystalline Ionic Materials and Related Heterostructures.” 1.2. Goal of the Study and Selection of Objects We chose for the family of the crystalline ionic materials to be studied within the above program protonic conductors based on alkali metal hydroxides. Solid hydroxides of alkali metals have been attracting interest of basic research people as fairly simple model objects for investigation of orientational phase transitions of the antiferroelectric type (see, e.g., [4]), of the chemical state of hydrogen in O–H bonds [5], and the of mechanism of protonic conduction [6–11]. Our goal consisted in a search for solid protonics based on mixtures of potassium and sodium hydroxides and water to operate in the range 360–460 K. While this temperature range is in itself most attractive for applications in electrochemical power engineering, it is the least studied because of the lack of suitable electrolytes capable of operating in heterostructures combined with metals of the type of Pd or Ti allowing hydrogenation. While being insufficient, this condition is necessary for formation of a protonic heterojunction at the interface [12–15]. A few attempts at proving the existence of a protonic heterojunction in high-temperature-perovskite-protonic–hydrogenated-metal heterostructures, in particular, in SrCe0.95Yb0.05O3 and V, Zr, Nb, and LaNi5 [16] or BaCe0.9Nd0.1O3, and Pd [17] were undertaken. A conductivity high enough to be acceptable for applications (0.1–1.0 mS/cm) could be reached in these protonics, however, only at temperatures above T = 600 K, where sustaining a stable hydrogen solution in metals at the necessary level (~0.1 H atom per metal atom) would require pressures substantially above the atmospheric level, a condition equally inappropriate both for basic research and main applications. A certain breakthrough in matching the temperatures of the metal–protonic heterostructure components has been reached in our recent studies which showed certain mixtures of the chemical compounds KOH, NaOH, and H2O to behave as solid protonics unknown at the time, namely, two eutectic compositions, KOH/NaOH (Tmelt = 448 K) and KOH/KOH · H2O (Tmelt = 372.5 K), as well as the monohydrate KOH · H2O (Tmelt = 419 K) [12–14]. These compositions correspond to characteristic points of the phase diagrams of the KOH–H2O [6, 18] and NaOH–KOH [19] systems. The search was prompted by earlier data (Baœkov et al.) suggesting a noticeable rate of hydrogen diffusion (DH > 10–8 cm2/s) in these materials in the solid state. The isotopic exchange method used in these studies revealed also that solid hydroxides are capable of activating molecular hydrogen themselves [20], a feature of considerable importance for a protonic heterojunc-

tion. It is a detailed analysis of the conductivity of the above materials that is going to be undertaken in the present publication. 2. EXPERIMENTAL TECHNIQUE 2.1. Preparation of Hydroxide Electrolytes and Electrochemical Cells The starting materials for the ionic conductors were “potassium hydroxide, analytically pure” and “sodium hydroxide, analytically pure” (Chemapol) and doubly distilled water. Protium was replaced with deuterium in the hydroxides by isotopic exchange of anhydrous KOH with D2 at a temperature of 545 K and the anhydrous eutectic NaOH/KOH with D2 at 420 K by a special technique developed at the Ioffe Physicotechnical Institute, Russian Academy of Sciences [10, 20, 21]. The deuterated hydroxide samples contained deuterium at a level of 92.0–97.5 at. %. The KOD · D2O monohydrate was obtained by mixing deuterated hydroxide with heavy water. Our observations suggest that the solidification temperature of the deuterated samples was ~1.5–2.0 K higher than that of the protium-based ones. We refer to KOH · H2O in what follows as monohydrate, and the eutectics will be subsequently denoted by abbreviations, KOH/NaOH as EA (Eutectics Anhydrous), and KOH/KOH · H2O as EW (Eutectics watercontaining). When discussing hydrogen isotopic effects, these abbreviations will be complemented with symbols H or D, respectively. Special four-electrode electrochemical cells were constructed for measurements of the EA, EW, and monohydrate conductivities in a version similar to the one discussed in considerable detail in [10]. The comparatively low melting temperatures made it possible to prepare rod-shaped samples in a Teflon tube with an inner diameter of 6 and 40 mm long, which amounted to a cell geometry factor of ~0.1 cm. Palladium black and/or finely dispersed silver were used to prepare the current electrodes. The potential electrodes shaped as silver rings ~3 mm in diameter and the corresponding electrical leads were led through the walls of the Teflon tube before introduction of the melt. The resistance of the electrolyte samples prepared in this way ranged from 50 Ω to 10 kΩ; it was measured with dc current of up to 2 mA. To ensure proper reproducibility of the results obtained in studies of the behavior with temperature above the phase transition points (see below), the EA and EW samples had to be maintained within the corresponding temperature range. The matter is that as one crosses ~360 K, the texture of these samples underwent a distinct change. To obtain reproducible values of the resistance after the crossing of the phase transition point, one had sometimes to make annealing as long as 100 h. By contrast, the monohydrate behaves typically without any sharp changes in the conductivity within the 300–460-K region. The conductivity was measured with the samples placed in closed containers with facilPHYSICS OF THE SOLID STATE

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These measurements reduce actually to obtaining the I–V characteristic of an electrochemical cell (heterostructure) made up of current leads and the electrolyte under study. A comprehensive account of the techniques used to isolate the electrolyte contribution to the total cell conductivity, methods to separate the ionic from electronic components, to identify the type of majority carriers, etc. can be readily found in the literature (see, e.g., [22]). Determination of the electrical conductivity by the four-point probe method and establishing its ohmic character were effected by the quasistatic I–V curve approach, with the current varied in steps, with due account of the time required for the current and electrode potential to relax. In the dynamic regime, the I–V curve technique was employed to monitor the onset of steady state of the cell under variation in the temperature. The latter point was particularly important when measurements were performed in the region of phase transitions in the eutectics. Figure 1 shows a dynamic I–V curve obtained at the EA transition from the superprotonic phase (364.5 K) to the phase transition region (361 K); indeed, not only the conductivity changes here by a factor three but a nonlinearity appears. To investigate the stability of the state of the monohydrate above which the equilibrium vapor pressure changes in the range 300–420 K from 0.01 to 1 kPa, we used a heterostructure with two different Ti|KOH · H2O|C potential- and current-generating electrodes, which was completely isolated from the environment. It was capable of operating as a rechargeable battery [15]. The I–V curve of the heterostructure is determined in this case by the sum of the electrode polarization resistances, the electrolyte resistance and the known Ohmic load. Therefore, two silver wire rings, which served in this case as potential electrodes to estimate the resistance of the electrolyte itself, were immersed into the electrolyte of the battery, i.e., into the monohydrate. In this way we established that if the temperature in the region above 400 K was changed by three degrees, 10 min would be quite enough for equilibrium to obtain. Vol. 51

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2.2. Measurement of DC Conductivity

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Current, mA

ities provided to vary properly the gas pressure and composition. In view of the specifics involved in measurement of protonic conductivity, hydrogen was chosen as this gas after a few check experiments (see also [10]). Measurements of the electrical conductivity required a good enough knowledge of the electrolyte geometry factor and a particular care. Therefore, an experiment assumed a certain set of temperatures (four to five for each electrolyte) considered as reference points. To study the behavior with temperature over broader ranges of variation in the temperature and physicochemical conditions, which is needed for determination of the ion transference numbers and the isotopic effect of conductivity, specially modified electrochemical cells were designed to be discussed later on.

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0

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10 15 t, min

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Fig. 1. Cyclic I–V characteristic, 20 mV/s, from –0.1 to 2.0 V, 364.5 K (KOH/NaOH superprotonic phase (EA)). At t = 0, the temperature starts to decrease to the phase transition point (361 K): (1) region with a temperature decreasing in the eutectic; (2) oscillations ±0.3 K, the current decreases with the nearly linear I–V curve remaining unchanged; and (3) phase transition region, the current is stabilized, the I–V curve exhibits features indicated by the arrows.

2.3. Determination of the Carrier Type and Proton Transference Numbers The determination of the carrier type in the hightemperature phase of EA and the monohydrate was performed on samples enclosed in Teflon tubes. The ends of the tubes were connected with two different containers in which one could vary independently the pressure of hydrogen. The contribution of protons to the total conductivity was defined by the transference number tH using the relation linking the experimental with theoretical values of the emf of such a cell through Nernst’s expression which is written here for protons tH = 2∆U/[ηt ln(p1/p2)], where ∆U is the potential difference across the cell, p1 and p2 are partial hydrogen pressures at the electrodes; and ηt = kBT is the thermal potential, with kB being the Boltzmann constant (see, e.g., [23]). Ref. [23] explains also how to separate out the contribution of other ions and electrons to the total conductivity. 2.4. Impedance Spectroscopy In the particular case considered in the present paper, impedance measurements were of not more than auxiliary importance which served to check the data obtained by the four-probe method. Major attention was focused here on the possible contribution of heteroboundaries to the cell resistance to be measured.

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ties were studied in nitrogen environment on a DSC-2 Perkin-Elmer calorimeter, with the heating and cooling rates varied in the range of 10–1.25 K/min. The temperature scale was calibrated against the melting points of ice (273.1 K) and indium (429.7 K), and the heat flow scale, against the white sapphire heat capacity.

1 7

10–2

3. RESULTS 3.1. DC Conductivity

6 σ, S/cm

1 3 10–4 4 5

2

10–6

10–8 1.6

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2.4 2.8 103/T, K–1

3.2

Fig. 2. General pattern of the conductivity–temperature curves: (1) KOH · H2O (solid), (2) KOH/KOH · H2O, (3) KOH/NaOH, (4) KOH [8], (5) NaOH [8, 10], (6) CsHSO4 [24–28], and (7) KOH · H2O (melt).

In principle, impedance measurements permit one to find the frequency response of ionic conductivity; it was not, however, established within the limits of precision of our equipment and design limitations of our electrochemical cells (heterostructures) provided for such measurements throughout the temperature range covered (250–450 K). 2.5. Differential Scanning Calorimetry (DSC) Both eutectics, EA and EW, revealed in experiments a sharp change in the behavior of the conductivity with temperature, which occurred practically at the same temperature of ~360 K, a feature which for individual compounds is usually associated with phase transitions. In this connection we carried out DSC measurements of thermal properties of all the three electrolytes within the range from 300 K up to their melting points. We used as starting material the polycrystalline ingots prepared by slow cooling of the corresponding melts down to room temperature in silver crucibles in nitrogen environment. From these ingots, samples ~20 mg in mass were cut off for calorimetric measurements in a dry atmosphere. The vials were of silver. Thermal proper-

Figure 2 plots in the log σ–T –1 coordinates the results of our conductivity measurements performed within the 300–420-K region for three compositions under study, namely, the KOH · H2O monohydrate and two equimolar eutectic mixtures, EA and EW. Also presented are the data on NaOH and KOH obtained earlier [8–10], as well as measurements for CsHSO4 taken from [24–28]. While the monohydrate (curve 1), a chemical compound, features a smooth variation in the conductivity within a factor ~50 (it ranges from 0.3 to 16 mS/cm), for the eutectics (curves 2 and 3) the temperature range of interest breaks into two regions, above and below 360 K. Above this temperature, the conductivity of the eutectics (from 15 to 80 mS/cm) is higher than that of the monohydrate, and the activation energy does not exceed 0.28 eV. The behavior with temperature of the conductivity of the solid monohydrate corresponds to an activation energy of ~0.4 eV throughout the temperature range studied (curve 1 joins curve 7 identified with the melt of the monohydrate). Thus, the electrolytes studied by us can be classed among the family of superionic conductors whose distinctive features may be conventionally specified as conductivities above 1 mS/cm and activation energies