Next-Generation Electrochemical Energy Materials for Intermediate

Feb 10, 2017 - Next-Generation Electrochemical Energy Materials for Intermediate Temperature Molten Oxide Fuel Cells and Ion Transport Molten Oxide Me...
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Next-Generation Electrochemical Energy Materials for Intermediate Temperature Molten Oxide Fuel Cells and Ion Transport Molten Oxide Membranes Valery V. Belousov* Laboratory of Functional Ceramics, A.A. Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, 49 Leninskii Pr., 119334 Moscow, Russia CONSPECTUS: High temperature electrochemical devices such as solid oxide fuel cells (SOFCs) and oxygen separators based on ceramic materials are used for efficient energy conversion. These devices generally operate in the temperature range of 800−1000 °C. The high operating temperatures lead to accelerated degradation of the SOFC and oxygen separator materials. To solve this problem, the operating temperatures of these electrochemical devices must be lowered. However, lowering the temperature is accompanied by decreasing the ionic conductivity of fuel cell electrolyte and oxygen separator membrane. Therefore, there is a need to search for alternative electrolyte and membrane materials that have high ionic conductivity at lower temperatures. A great many opportunities exist for molten oxides as electrochemical energy materials. Because of their unique electrochemical properties, the molten oxide innovations can offer significant benefits for improving energy efficiency. In particular, the newly developed electrochemical molten oxide materials show high ionic conductivities at intermediate temperatures (600−800 °C) and could be used in molten oxide fuel cells (MOFCs) and molten oxide membranes (MOMs). The molten oxide materials containing both solid grains and liquid channels at the grain boundaries have advantages compared to the ceramic materials. For example, the molten oxide materials are ductile, which solves a problem of thermal incompatibility (difference in coefficient of thermal expansion, CTE). Besides, the outstanding oxygen selectivity of MOM materials allows us to separate ultrahigh purity oxygen from air. For their part, the MOFC electrolytes show the highest ionic conductivity at intermediate temperatures. To evaluate the potential of molten oxide materials for technological applications, the relationship between the microstructure of these materials and their transport and mechanical properties must be revealed. This Account summarizes the latest results on oxygen ion transport in potential MOM materials and MOFC electrolytes. In addition, we consider the rapid oxygen transport in a molten oxide scale formed on a metal surface during catastrophic oxidation and show that the same transport could be used beneficially in MOMs and MOFCs. A polymer model explaining the oxygen transport in molten oxides is also considered. Understanding the oxygen transport mechanisms in oxide melts is important for the development of new generation energy materials, which will contribute to more efficient operation of electrochemical devices at intermediate temperatures. Here we highlight the progress made in developing this understanding. We also show the latest advances made in search of alternative molten oxide materials having high mixed ion electronic and ionic conductivities for use in MOMs and MOFCs, respectively. Prospects for further research are presented. processes.8 Molten oxide electrolysis is considered as a promising route to oxygen generation for extraterrestrial exploration.9,10 There is a large amount of work on molten electrolytes and molten metal/molten oxide composite electrodes for battery and SOFC applications.11−14 Recently, the molten oxide fuel cell (MOFC) and molten oxide membrane (MOM) concepts have been proposed.15,16 MOFCs are a new type of intermediate temperature fuel cells, which combine the advantages of both solid oxide fuel cells (SOFCs, air at cathode side) and molten carbonate fuel cells (MCFCs, highly conductive

1. INTRODUCTION In the first half of the 19th century, Faraday1 discovered the molten oxide electrolytes. A review of early twentieth century successes with molten oxide electrolytes was published by Martin and Derge.2 This review describes ideas put forth as early as 1906 when Aiken was granted a patent in the United States for the production of iron by molten oxide electrolysis. The second half of the twentieth century saw important developments related to molten oxides in the Russian Federation, where scientists were interested in the ionic nature of oxide melts as well as their potential use for metal production.3,4 At the present time, molten oxides are widely used in metallurgy5,6 and crystal growth.7 They play important roles in geological © 2017 American Chemical Society

Received: September 19, 2016 Published: February 10, 2017 273

DOI: 10.1021/acs.accounts.6b00473 Acc. Chem. Res. 2017, 50, 273−280

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Accounts of Chemical Research

oxidation process.27 When this structure is formed in oxide scale on copper at intermediate temperature, the catastrophic corrosion of copper occurs because of the accelerated oxygen transport along the liquid channels.28,29 However, the same rapid mass transport could be exploited beneficially in electrochemical devices such as MOFCs and MOMs. In addition to the technological applications, oxygen transport in molten oxide materials has high fundamental importance. Recently, significant strides have been made in the understanding of oxygen ion transport processes in molten oxide materials. Two major factors contributed to this. First, more accurate experimental techniques have been developed for measurement of transport properties of oxide melts. This measurement is frequently difficult due to high temperature and interaction between melt and container.30 A major problem in performing the measurements is the selection of a container material that is inert to the melt. A partly molten oxide method31 is developed to alleviate this problem. The oxygen permeation fluxes of several extremely reactive oxide melts have been measured by using this method under various oxygen electrochemical potential gradients and temperatures. The second factor is the development of model that has explained the oxygen mobility in some oxide melts.32 This Account highlights the progress made in understanding the mechanisms of oxygen ion transport in molten oxide materials. The further directions of research in this scientifically and technologically important area are presented.

molten electrolyte) and could be used for efficient electric power generation. MOMs are a new group of ion transport membranes (ITMs) for separation of ultrahigh purity oxygen from air.17 Interest in studying the ITM materials and processes is caused by their high potential for applications.18,19 SOFCs operate at high temperatures (800−1000 °C) and exhibit high efficiency of energy conversion compared to the traditional power generators. However, the high operating temperatures lead to accelerated degradation of the SOFC materials. Solution to this problem may be a lowering the operating temperatures.20−22 To achieve this goal, the ionic conductivity of SOFC ceramic electrolyte needs to be increased. Therefore, there is a need for finding new classes of highly conductive intermediate temperature electrolytes. Recently, alternative molten oxide electrolytes with highest oxygen ionic conductivity are developed for intermediate temperature MOFCs.15,23 Temperature dependences of the conductivities of MOFC electrolytes are shown in Figure 1 in comparison to the conventional SOFC and MCFC electrolytes.

2. CATASTROPHIC OXIDATION OF COPPER Copper experiences accelerated oxidation when its surface is coated by a low melting oxide (Bi2O3, MoO3, or V2O5) at elevated temperature in air.27 This is known as catastrophic oxidation of metals. Wetting of metal surface by molten oxide directly influences the metal degradation rate. Wetting of grain boundaries in an oxide scale formed on a metal surface during high temperature corrosion is also required for catastrophic oxidation. The formation of LGBS in oxide scale leads to the rapid transport of oxygen ions along the liquid channels. As a consequence, the metal oxidizes catastrophically. However, the same rapid mass transport could be exploited beneficially in electrochemical devices such as MOFCs and MOMs. Here we briefly consider the catastrophic oxidation of copper in a Cu− Bi2O3 system, which is studied most fully (detailed information is contained in the exhaustive reviews in this field28,29). Two stages, fast and superfast ones, were revealed in the catastrophic oxidation of Bi2O3-deposited copper.28 The fast stage is realized under a thin layer of the deposit and occurs by diffusion mechanism. The superfast stage is realized under a thick layer of the deposit and occurs by fluxing mechanism. Since we are only interested in the diffusion mechanism, we will consider the fast stage. It was established that the catastrophic oxidation of Bi2O3-coated copper occurs at 770 °C. Figure 2 shows the copper oxidation kinetics in parabolic coordinates (for different thicknesses of Bi2O3 deposit). Catastrophic oxidation obeys the parabolic rate law corresponding to a diffusion controlled process:

Figure 1. Temperature dependences of conductivities of innovative (MOFC) and traditional (SOFC and MCFC) electrolytes.

A molten oxide material represents a composite consisting of ion-conducting liquid and electron-conducting solid oxide phases (mixed ionic electronic conductor, MIEC) or ionconducting liquid and insulating (or ion-conducting) solid oxide phases (electrolyte). The preparation of molten oxide materials occurs in two steps. In the first step, a ceramic composite is sintered. In the second step, so-called a liquidchannel grain-boundary structure (LGBS) is formed by the grain boundary (GB) wetting method.16 Making the ceramic composite includes pressing of the respective powder mixtures and subsequent sintering at a temperature below eutectic melting in air. Formation of the LGBS material includes heating the ceramic composite above the eutectic temperature, which leads to the appearance of the liquid existing in equilibrium with the solid. While being heated, the ceramic composite experiences GB wetting by a chemically compatible liquid at eutectic temperature resulting in LGBS material (or molten oxide material).16 The LGBS contains solid grains and intergranular liquid channels. Thanks to the intergranular liquid channels, the LGBS material demonstrates high ionic conductivity, ductility, and gas tightness. The ductility is necessary for the getting rid of the problem of thermal incompatibility. For the first time, a highly conductive LGBS was discovered in oxide scale formed on a copper surface in the catastrophic

⎛ m ⎞2 ⎜ ⎟ = k ″t ⎝S⎠

(1)

where k″ is the constant of parabolic rate, m is the weight gain, t is the time, and S is the surface area of copper. The parabolic rate constant for Bi2O3-coated copper oxidation is greater than 274

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in the volume fraction of liquid leads to the enhancement of ionic conductivity of the LGBS (Figure 4). Consequently, the

Figure 2. Oxidation kinetics of copper deposited with Bi2O3 layer of different thickness at 800 °C. Layer thickness ×105 m: (1) 0; (2) 3; (3) 6; (4) 9. 105k∥, kg2 m−4 s−1: (1) 0.012; (2) 1.6; (3) 4.9; (4) − 21.28

2 orders of magnitude higher than that of uncovered copper (pure copper). During the catastrophic oxidation, oxide scale is formed on the surface of copper. It includes CuO, Bi2CuO4, Bi2O3, and Cu2O.28 Analysis of the phase diagrams of Bi2O3−CuO33,34 revealed the coincidence of the eutectic temperature of Bi2O3−Bi2CuO4 (770 °C) with the threshold temperature of catastrophic oxidation of Bi2O3-coated copper. The GB wetting transition and LGBS formation in ceramic Bi2CuO4−Bi2O3 composites at 770 °C was experimentally confirmed (Figure 3).35

Figure 4. Liquid volume fraction dependence of oxygen ionic conductivity of Bi2CuO4−(5, 10, 15, or 20 wt % Bi2O3) LGBS at 800 °C.

LGBS oxide scale having a greater amount of liquid demonstrates higher ionic conductivity, which in turn significantly accelerates oxidation of copper (if electronic conductivity is sufficient). The electrochemical mechanism of catastrophic oxidation of copper is considered in ref 37. The parabolic rate constant is expressed as follows k″ =

2RTA2 σi 2 2 2

zVFc

ln

PO′ 2 PO″2

(2)

where T is the temperature, R is the universal gas constant, σi is the oxygen ionic conductivity of the scale, A is the atomic mass of oxygen, V is the molar volume of the scale, c is the oxygen concentration in the scale, z is the oxygen charge number, F is the Faraday constant, and P′O2 and P″O2 are the oxygen partial pressures at the interfaces of oxide scale−air and oxide scale− metal, respectively. The experimental values of the parabolic rate constant (Figure 2) and calculated ones from eq 2 have the same order of magnitude indicating that the developed model is consistent with experiment.

3. MOLTEN OXIDE MEMBRANES In this and the following sections, we will show how the LGBS concept can be beneficially used for the development of innovative electrochemical energy materials for MOFCs and MOMs. Recently, new oxygen transport MOMs have been developed.16 These MOMs are based on LGBS materials, which contain solid grains and intergranular liquid channels conducting electrons and oxygen ions, respectively. Besides, the intergranular liquid channels provide the membrane materials ductility and gas tightness. The ductility of MOMs solves a problem of thermal incompatibility (difference in coefficient of thermal expansion, CTE). The thermal incompatibility is one of the most important issues of the fragile ceramic membranes. Let us divide these materials into two groups depending on what type of oxygen ion-conducting melt,38 Bi2O3 or V2O5: (i) NiO−Bi2O3, In2O3−Bi2O3, ZnO−Bi2O3, Co3O4−Bi2O3 and CuO−Bi2O3; (ii) ZrO2−V2O5 and Bi2O3−V2O5.16,17,35 These membranes are made in two steps. In the first step, the ceramic composite is sintered. In the second step, the LGBS is formed.16 The result is MOM material comprising electronconducting solid grains (light phase in Figure 5) and ion- or

Figure 3. Scanning electron micrographs of polished cross sections of Bi2CuO4−20 wt % Bi2O3 composite (after cooling from 780 °C) confirm the GB wetting: (a) general view; (b) microstructure of intergranular channel.35

Therefore, the copper catastrophic oxidation is caused by the formation of LGBS in the oxide scale. Electrochemical properties of the Bi2CuO4−(5−20 wt % Bi2O3) LGBS were studied.36 The results showed that these LGBSs are mixed conductors where the electron transfer occurs in both solid Bi2CuO4 and intergranular liquid channels, but the oxygen ion transfer occurs only in intergranular liquid channels. An increase 275

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air is reduced to oxygen anions at the air−membrane interface (kinetics of surface exchange). In the second stage, the oxygen anions migrate along the intergranular liquid channel (chemical diffusion). In the third stage, the oxygen anions are oxidized to molecular oxygen at the membrane−oxygen interface (kinetics of surface exchange). Each stage can restrict the velocity of total oxygen permeation process. So it is necessary to measure the dependence of the oxygen flux on the membrane thickness to determine which one, chemical diffusion or the surface exchange reaction, is a limiting step. When the rate of oxygen permeation is limited by the diffusion process in the bulk membrane, the oxygen flux is inversely proportional to the thickness of the membrane (diffusion controlled kinetics; blue line in Figure 7). Such dependence of the oxygen flux is

mixed-conducting intergranular liquid channels (dark phase in Figure 5). Therefore, a gradient of electrochemical potential of

Figure 5. BSEM image of fracture face of BiVO4−10 wt % V2O5 MOM material after cooling from 640 °C (light and dark constituents are solid BiVO4 and liquid (molten V2O5 and BiVO4), respectively).31

oxygen stimulates the ambipolar conductivity of electrons and oxygen ions, which in turn provides the oxygen permeability of the membrane material (Figure 6). Both surface exchange reactions and chemical diffusion can limit the kinetics of oxygen

Figure 7. Experimental and calculated by eqs 3 and 5 thickness dependences of oxygen permeation fluxes of BiVO4−10 wt % V2O5 MOM material (lg(PO′ 2/PO″ 2) = 2.3, T = 650 °C).

observed for Co3O4−Bi2O3, ZrV2O7−V2O5, In2O3−Bi2O3, and NiO−Bi2O3 MOMs in the thickness and temperature ranges of 1−4 mm and 750−900 °C, respectively.16 In contrast, the oxygen flux dependence on the thickness of the BiVO4−V2O5 MOM differs from the linear at temperatures below 700 °C (black line in Figure 7). Apparently, in this case the oxygen transport is limited equally by chemical diffusion and surface exchange reactions (mixed controlled kinetics; red line in Figure 7). It was established that the characteristic thickness (hc in eq 5) for the BiVO4−V2O5 MOM is approximately equal to 0.7 mm.39 At a smaller thickness, the kinetics of oxygen permeation is limited by the surface exchange reactions. To compare the productivity of various membranes, we calculated the oxygen permeability (jO*2) by eq 6. The comparison of the calculated values of the oxygen permeability was made to materials having a high rate of surface exchange reactions.40

Figure 6. Schematic of oxygen ion and electron transfer through the MOM.

permeation through the MOM. The oxygen permeation flux (jO2) limited by chemical diffusion is usually described by Wagner’s equation39 jO = 2

PO′ RT σ ln 2 2 amb PO″2 16F h

(3)

where σamb is the ambipolar conductivity of electrons and oxygen ions and h is the thickness of membrane: σiσe σamb = σi + σe (4) where σi and σe are the ionic and electronic conductivities, respectively. When the permeation of oxygen through the MOM is limited by surface exchange reactions and chemical diffusion (mixed controlled kinetics), the flux can be expressed by eq 539 jO = 2

PO′ RT σamb ln 2 PO″2 16F (h + 2hc) 2

jO* = jO 2

2

h ln(PO′ 2 /PO″2)

(6)

The results presented in Table 1 confirm the high performance of the MOMs. Besides the high oxygen permeability, the MOMs show outstanding oxygen selectivity.17 A chromatogram of the gas permeating through the Co3O4−(36 wt % Bi2O3) MOM is presented in Figure 8 b. As we can see, the flux of nitrogen permeation (jN2) across the MOM is negligible (Figure 8 b). The detection limit of the gas chromatograph (10−8 g·mL−1) does not allow us to measure the nitrogen concentration, so we can estimate the oxygen selectivity as jO2/jN2 > 105. Apparently, the high selectivity of the MOM

(5)

where hc is the characteristic thickness of membrane at which the overall permeation control by surface exchange reactions is equal to the control by chemical diffusion. The oxygen permeation through the MOM occurs by three stages (Figure 6). In the first stage, the molecular oxygen from 276

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Accounts of Chemical Research Table 1. Oxygen Permeability (j*O2) of Ceramic (CER) and Molten (MOM) Membranes membrane

T, °C

Pr0.6Sr0.4Co0.5Fe0.5O3−δ (CER) Ba0.5Sr0.5Co0.8Fe0.2O3−δ (CER) La0.6Ca0.4Co0.8Fe0.2O3−δ (CER) In2O3−(55 wt % Bi2O3) (CER) In2O3−(48 wt %Bi2O3) (MOM) Co3O4−(36 wt % Bi2O3) (MOM)

900 900 900 800 850 850

jO2, mol·cm−2·s−1 2.3 1.1 1.2 5.3 8.7 1.9

× × × × × ×

10−7 10−6 10−7 10−8 10−8 10−7

h, mm

P′O2, atm

P″O2, atm

0.6 1.2 1.0 2.9 2.6 2.0

0.21 0.21 0.20 0.21 0.21 0.21

0.001 0.0065 0.0053 0.054 0.038 0.047

j*O2, mol·cm−1·s−1 2.6 3.8 3.3 1.1 1.3 2.5

× × × × × ×

10−9 10−8 10−9 10−8 10−8 10−8

refs 42 43 44 41 16 17

640 °C. Along with the low potential, this electrolyte consisting of TeO2 solid grains and oxygen ion-conducting intergranular liquid channels (molten TeO2 + Bi2Te4O11) shows an insufficiently high ionic conductivity. Since the solid grains of TeO2 do not conduct the oxygen ions, the liquid amount significantly limits ionic conductivity of the electrolyte. Lately, a highly conductive and ductile (Figure 9) δ-Bi2O3−(0.2 wt % B2O3)

Figure 8. Chromotograms of gas-permeating fluxes (a) of Co3O4− (36 wt % Bi2O3) ceramic composite (the chromatogram confirms that this ceramic material is not gastight) and (b) of Co3O4−(36 wt % Bi2O3) MOM (the chromatogram confirms that this MOM material is only oxygen-permeable).17

Figure 9. Photography of Bi2O3−(0.2 wt % B2O3) MOFC electrolyte: (a) initial sample and (b) after its deformation at 750 °C.

material is provided by the liquid phase in which the nitrogen is not dissolved. Thus, the high oxygen permeability, outstanding oxygen selectivity, and low manufacturing cost exhibit the promise of the Co3O4−(36 wt % Bi2O3) MOM material for technological applications.

LGBS electrolyte in which both δ-Bi2O3 solid grains and intergranular liquid channels (molten Bi2O3 + B2O3) conduct the oxygen ions has been developed.23 The temperature dependence of conductivity of the electrolyte is presented in Figure 1. An inflection on the conductivity curve at 620 °C is associated with the eutectic melting and LGBS formation.49 The α → δ-Bi2O3 polymorphic transformation at 735 °C is accompanied by increase in ionic conductivity of oxygen.50,51 The highest ionic conductivity of δ-Bi2O3 is related to a large amount of oxygen vacancies and high O2− anion mobility. The large amount of oxygen vacancies (25% of the lattice sites) is a result of obtaining the fluorite structure with a Bi3+ cation. The high anion mobility is related to the high polarizability of the Bi3+ cation with its lone pair of electrons. The intergranular liquid channels comprising predominantly molten Bi2O3 also conduct the oxygen ions.38 The oxygen ion transport number of δ-Bi2O3−(0.2 wt % B2O3) LGBS electrolyte is 0.96 ± 0.03 at 750 °C.23 Despite the fact that this electrolyte exhibits sufficient ionic conductivity and oxygen ion transport number, its application is complicated because molten Bi2O3 is reduced to its metallic state at a low potential, approximately 0.45 V in air. Therefore, at the present time there is a need for stable molten oxides at a potential equal to or above 1 V in air. Presumably, B2O3 can claim this role because it melts at 450 °C and is reduced at a potential approximately 1.8 V at 500 °C in air. In the sequel, to assess the potential of molten oxide electrolytes based on B2O3 for technological applications, it is necessary to understand their ionic conductivity as well as behavior in a reducing atmosphere. Overall, the development of the MOFC concept may result in a new generation of fuel cells operating at intermediate temperatures.

4. MOLTEN OXIDE FUEL CELLS SOFCs convert chemical energy into electric power with high performance and little pollutant emissions. However, before SOFCs will take a considerable proportion of the energy market, it is necessary to solve a major problem. This problem lies in the need for development of new SOFC electrolytes having high ionic conductivities at intermediate temperatures. Here we consider the applicability of molten oxide materials as alternative MOFC electrolytes. Fuel cells are divided into high and low temperature.45 SOFCs are high temperature fuel cells (800−1000 °C). The high operating temperatures lead to accelerated degradation of SOFC materials.46 To create durable intermediate temperature SOFCs, considerable efforts of researchers have been aimed at development of different ionic conductors in the temperature range of 500−800 °C. Recently, ceria doped with a rare earth element and binary or ternary eutectic carbonate melt composite electrolytes have been developed.24,47,48 However, these electrolytes have both advantages (high ionic conductivity at intermediate temperatures) and disadvantages (mixture of oxygen and carbon dioxide is required as a cathode gas). To get rid of this disadvantage, a new concept of the molten oxide fuel cell (MOFC) has been proposed.15 In this MOFC, air is used as a cathode gas. The air/chromel/TeO2−(16 wt % Bi2Te4O11) LGBS/chromel/CO cell power was investigated. While air was used as a cathode gas and carbon monoxide was used as a fuel, the cell showed low open circuit voltage (OCV) of 0.36 V at 277

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5. MODELING A general topic among the new MOM and MOFC materials, considered in this Account, is the oxygen transfer mechanisms in oxide melts. This section focuses on a polymer model for transport of oxygen ions in oxide melts.32 We briefly review the main points of this model. More detailed atomic scale modeling for oxide melts can be found in the comprehensive reviews.52,53 The polymer model adapts the theory of Wagner for oxide melts and gives the interesting interpretation of mobility of oxygen ions in melts. The schematic structure of oxide melt is shown in Figure 10. The model is valid under the following assumptions: (i) a polymer structure is typical for melt; (ii) the polymer chain average length is a function of temperature; (iii) the disconnection and connection of chains take place; this process occurs stochastically; (iv) a chain disconnection leads to formation of two new chains; an oxygen pseudovacancy is also formed at the end of one new chain; (v) the chains connection leads to the formation of one free ion of oxygen; (vi) this free ion of oxygen takes up the nearest vacancy; (vii) the most mobile carriers of charge in the molten oxide material are electrons and oxygen ions; (viii) the ambipolar conductivity of electrons and oxygen ions provides the oxygen permeability through the molten oxide material; (ix) the migration of electrons and oxygen ions occurs so that the overall current through the molten oxide material is zero. According to the model, the flux of the oxygen permeation through the molten oxide material is expressed by eq 7, jO = 2

PO′ (1 − τi)Di ηρ ln 2 480Mh PO″2

Figure 10. Structure of (a) solid and (b) molten V2O5 and (c) schematic of oxygen ion transport in oxide melt based on V2O5: blue circles, mobile oxygen; red circles, terminal oxygen; green circles, bridging oxygen; open circles, pseudovacancy of oxygen.32

(7)

where M is the melt molar mass, ρ is the melt density, η is the melt volume fraction, τi is the transference number of oxygen ions, and Di is the diffusion coefficient of oxygen. Under a gradient of electrochemical potential of oxygen, the transference of oxygen through the molten oxide material occurs as follows. Atmospheric oxygen is chemisorbed on the air−molten oxide material interface (the first step in Figure 10 c) and then takes up the nearest vacancy (the second step in Figure 10 c). The chains connection generates a free oxygen ion (the third step in Figure 10 c), which takes up the nearest vacancy, etc. The free oxygen thus reaches the opposite side of the molten oxide material (the fourth step in Figure 10 c). At the same time, there is a transfer of electrons. The conductivity of electrons in the molten oxide materials was considered in refs 4 and 16. In conclusion, this model describes the oxygen transport in a specific class of oxide melts, though the approach is interesting and can provide the basis for the development of a general model.

temperatures.20−22 Here we demonstrated that the LGBS based on low-melting-point oxides are appropriate materials. In particular, the LGBS membrane materials show outstanding oxygen selectivity and sufficient oxygen permeability and the LGBS electrolytes exhibit highest oxygen ionic conductivity at intermediate temperatures. The Account was focused on the latest results of oxygen ion transport in potential LGBS materials for MOMs and MOFCs. An oxygen transport model and the kinetics of catastrophic oxidation, stimulated by the formation of highly conductive LGBS oxide scale on metal surface, were considered. The rapid oxygen ion transfer in the LGBS oxide scale resulted in accelerated oxidation of metal. We also showed that the same rapid mass transport can be exploited beneficially in electrochemical devices such as MOMs and MOFCs. However, at the present time there is not sufficient understanding how mass transport and thermodynamic factors combine to control the channel location and width in LGBS. This understanding is important to be able to predict which combinations of solid and liquid oxides will form LGBS and what the resulting microstructure will be. Similarly, to evaluate the potential of LGBS as mixed-conducting MOM materials and ion-conducting MOFC electrolytes, the relationship between microstructure and transport and mechanical

6. CONCLUSIONS AND OUTLOOK In this Account, we showed the ample opportunities for molten oxides as innovative electrochemical energy materials. The necessity for new electrochemical energy materials is dictated by the fact that conventional electrochemical devices (SOFCs, oxygen separators, etc.) operate efficiently at lower 278

DOI: 10.1021/acs.accounts.6b00473 Acc. Chem. Res. 2017, 50, 273−280

Article

Accounts of Chemical Research

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properties of LGBS materials must be revealed. Thus, the broad aim of this Account is to take the first steps in developing this understanding. The LGBS materials contain a large amount of solid/liquid interfaces, which can significantly affect oxygen transport. It is established that highly conductive space charge regions formed near the interfaces.54−56 The mass and charge transfer along the interfaces can stimulate the appearance of an interface tension gradient, which in turn initiates the Marangoni flow and eases the oxygen transport.57 The proposed liquid-channel grainboundary nanostructure (LGBN) concept58 opens up possibilities for the design of intermediate temperature nanomaterials with enhanced ionic conductivity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +7-499-135-2060. Fax: +7-499-135-4513. ORCID

Valery V. Belousov: 0000-0002-0832-4945 Notes

The author declares no competing financial interest. Biography Valery V. Belousov is currently principal scientist at A.A. Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences (Moscow). He received M.Sc. degree from Chelyabinsk State University, Ph.D. from Ural State University (Yekaterinburg), and D.Sc. from NUST MISiS (Moscow) in Physical Chemistry and Materials Science. His primary research interest is electrical and mass transport processes in oxide materials.

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ACKNOWLEDGMENTS I thank the Russian Science Foundation for funding this project, Grant No. 16-19-10608.

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REFERENCES

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DOI: 10.1021/acs.accounts.6b00473 Acc. Chem. Res. 2017, 50, 273−280