Solid State Ionics 260 (2014) 15–20

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Solid State Ionics

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Nonstoichiometry, thermal expansion and oxygen permeabilityof SmBaCo2 − xCuxO6 − δ

N.E. Volkova a, V.A. Kolotygin b,c, L.Ya. Gavrilova a, V.V. Kharton b,c, V.A. Cherepanov a,⁎a Department of Chemistry, Institute of Natural Sciences, Ural Federal University, Ekaterinburg, Russiab Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, Portugalc Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia

⁎ Corresponding author at: Department of Chemistry, InFederal University, Lenin av. 51, Ekaterinburg, 620000, Ru

http://dx.doi.org/10.1016/j.ssi.2014.03.0030167-2738/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 December 2013Received in revised form 18 February 2014Accepted 2 March 2014Available online xxxx

Keywords:Samarium–barium cobaltiteDilatometryOxygen deficiencyOxygen permeationElectronic conductivity

Phase relationship analysis in the pseudobinary SmBaCo2− xCuxO6− δ systemat 1000–1100 °С in air revealed theexistence of solid solutionswith the layered perovskite and 123-type structures, formedwithin the ranges of 0≤x≤1.2 and 1.5≤ x≤2.0, respectively. The substitution of Cu2+ for Co3+/4+was found to induce phase transitionfrom orthorhombic (space group Pmmm) into tetragonal (P4/mmm) polymorph at x N 0.1, and to increase oxy-gen deficiency studied by thermogravimetry and iodometric titration. The electrical conductivity and thermal ex-pansion of Cu-substituted cobaltites decrease on doping, while the steady-state oxygen permeability exhibits anopposite trend, in correlation with the oxygen content variations. The average thermal expansion coefficient ofSmBaCo1.4Cu0.6O6 − δ ceramics, where the level of ionic transport is comparable to that in most permeablecobaltite-based mixed conductors, is 16.9 × 10−6 K−1 at 25–1000 °С in air. At the same time, copper additionsincrease chemical reactivity of the materials with doped ceria electrolytes.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Among the variety of promising mixed-conducting materials thatcan be used for solid oxide fuel cell (SOFC) electrodes and oxygen sepa-ration membranes, a significant attention of researchers is centered onthe family of multicomponent solid solutions based on the layered pe-rovskites AA′Me2O6 − δ where A = Pr–Ho or Y, partially substitutedby alkaline earth metal cations A′, and Me corresponds to the 3d-transition metal cations, primarily of Mn, Fe, Co and Ni [1–8]. Their ad-vantages that are important for potential electrochemical applicationsinclude substantially high oxygen diffusivity, fast interfacial exchangekinetics and predominant electronic conductivity. In particular, numer-ous studies have been focused on cobalt-containing ABaCo2O6 − δ [2–9]which exhibits, in addition to attractive transport and electrochemicalproperties, lower thermal expansion coefficients (TECs) compared totheir disordered perovskite analogs (A,A′)CoO3 − δ. However, the TECsof the layered cobaltites are still essentially higher than those of com-mon solid oxide electrolytes, limiting thermomechanical compatibilityof these materials. In order to decrease contributions to the lattice ex-pansion associated with oxygen nonstoichiometry variations and elec-tronic transitions, cobalt is often replaced with other transition metalcations such as iron and copper [7,9–14]. In many cases, the lattertype of doping makes it also possible to improve sintering of ceramicmaterials and, thus, to reduce temperatures necessary for the electrode

stitute of Natural Sciences, Uralssia.

and membrane fabrication (e.g., [15,16]). Continuing our previous re-port [17] dealing with iron-doped SmBaCo2 − xFexO6 − δ, the presentwork was centered on the analysis of phase relationships in thepseudobinary SmBaCo2 − xCuxO6 − δ system and studies of the crystalstructure, defect chemistry, thermal expansion and mixed conductivityof Cu-substituted cobaltites.

2. Experimental

Powders of SmBaCo2 − xCuxO6 − δ (0 ≤ x ≤ 2) were prepared by theglycine–nitrate technique using high-purity Sm2O3, BaCO3, CuO, metallicCo, nitric acid and glycine as starting materials. Final thermal treatmentwas performed at 1000–1100 °С in air for 120 h, employing several an-nealing steps (~20 h each) and intermediate grindings. The synthesizedsingle-phase powders were uniaxially pressed into disks and thensintered at 1200–1300 °С in air. The density of the sintered ceramicsamples was higher than 90% of the theoretical values, calculated fromthe X-ray diffraction (XRD) data. In order to achieve equilibrium withatmospheric oxygen at low temperatures, all the samples were cooleddown to room temperature at a rate of ~100 °C/h. XRD analysis was car-ried out employing a DRON-6 diffractometer (CuKα-radiation, anglerange was 2Θ = 20°–120°, step was 0.04°, 10 s/step) and an Equinox-3000 instrument (CuKα-radiation, equipped by a curved position-sensitive detector CPS-590; the angle interval was 2Θ= 10°–90°, acqui-sition time was 1–2 h, step was 0.012°) at room temperature. The unitcell parameters were calculated using the CelRef v.4.0 software, and thestructural parameters were refined by the full-profile Rietveld analysis

Fig. 1. Observed (circles), calculated (solid line) and difference (below) XRD profiles forthe final Rietveld refinement of SmBaCo2 − xCuxO6 − δ with x = 0.1 and 0.7. Insets showcharacteristic fragments of the XRD patterns, indicating the differences between ortho-rhombic (a) and tetragonal (b) phases.

Fig. 2.Unit cell parameters vs. copper concentration in SmBaCo2 − xCuxO6 − δ (x= 0–1.2)at room temperature.

16 N.E. Volkova et al. / Solid State Ionics 260 (2014) 15–20

with a Fullprof 2008 package [18,19]. Ceramic microstructure was ana-lyzed by scanning electron microscopy (SEM, Hitachi SU-70).

Thermogravimetric analysis (TGA)was carried out using a STA 409PCinstrument (Netzsch) within the temperature range 25–1100 °С. Inorder to assess oxygen nonstoichiometry variations in air, twomeasure-ment regimes were employed, namely static (10 h dwells at a constanttemperature, temperature steps of 25–50 °C) and dynamic (continuousheating/cooling rate of 1 °C/min). The absolute oxygen content was de-termined using a direct complete reduction by flowing hydrogen in theTGA instrument [17] and by iodometric titration. In the latter case, thepowdered samples (0.2 g) were dissolved in a 2 M aqueous solution ofhydrochloric acid in the presence of an excess amount of KI:

Co2−xCuxð ÞzMeþ þ 4zMe þ x−6ð ÞI−→ 2−xð ÞCo2þ þ xCuI↓þ 2zMe−3ð ÞI2ð1Þ

where zMe is the mean oxidation state of transition metal cations in theperovskite phase. The iodine formed in Reaction (1) was titrated by astandard Na2S2O3 solution with a preliminary determined concentra-tion; the equivalent point was registered using an automatic titratorAkvilon ATP-02.

Thermal expansion of the ceramic samples was measured using avertical alumina dilatometer Linseis L75V/1250 in air (temperaturerange of 25–1100 °С, heating/cooling rate of 5°/min). The total electrical

Table 1Atomic positions in orthorhombic SmBaCo1.9Cu0.1O6 − δ and tetragonal SmBaCo1.3Cu0.7O6 − δ, r

SmBaCo1.9Cu0.1O6 − δ (S.G. Pmmm)

Atom x y z Biso

Sm 0.5 0.241 (1) 0.5 0.36 (4)Ba 0.5 0.250 (1) 0 0.59 (4)Co(Cu)1 0 0.5 0.265 (2) 0.89 (4)Co(Cu)2 0 0 0.250 (2) 0.38 (4)O1 0 0 0 0.11 (5)O2 0 0.5 0 0.74 (4)O3 0 0.5 0.5 0.51 (5)O4 0 0 0.5 0.10 (4)O5 0.5 0 0.272 (3) 0.36 (5)O6 0.5 0.5 0.303 (3) 0.81 (5)O7 0 0.256 (2) 0.292 (1) 0.29 (4)a = 3.889 (1) Å; b = 7.822 (1) Å; c = 7.574 (1) Å; V = 230.42 (2) (Å)3; RBr = 6.41%;Rf = 9.58%; Rp = 11.7%

conductivity (σ) was studied by the 4-probe DC method at 25–1100 °Сunder atmospheric air, employing both cooling and heating regimes inorder to ensure equilibration. The experimental technique used to de-termine steady-state oxygen permeation fluxes (j) through denseSmBaCo2 − xCuxO6 − δ membranes was described elsewhere [15,16].Themeasurementswere performed at 800–950 °С in the range of oxygenpartial pressures at themembrane permeate side, p(O2)perm, from 0.01 to0.21 atm. In all cases, the oxygen partial pressure at the membrane feedside, p(O2)feed, was equal to atmospheric pressure.

In order to assess possible interaction of SmBaCo2 − xCuxO6 − δ withsolid oxide electrolytes, the corresponding mixtures with a 1:1 weightratio were annealed at various temperatures (900–1100 °С) for 24 hin air and then examined by XRD.

3. Results and discussion

XRD analysis showed that the solid solution formation ranges in thepseudobinary SmBaCo2 − xCuxO6 − δ system at atmospheric oxygenpressure correspond to 0 ≤ х ≤ 1.2 and 1.5 ≤ x ≤ 2.0. No secondaryphases are formed in these ranges; the endmembers of the two solid so-lution series were found to co-exist in the intermediate compositionalrange, х = 1.3–1.4. At x = 0.1 the crystal structure is orthorhombic(space group Pmmm), similar to undoped SmBaCo2O6 − δ [17]. Thisstructure can be described as ap × 2ap × 2ap where ар is the primitiveperovskite unit cell parameter. The Rietveld refinement results arepresented in Fig. 1a and Table 1.

efined by Rietveld analysis.

SmBaCo1.3Cu0.7O6 − δ (S.G. P4/mmm)

Atom x y z Biso

Sm 0.5 0.5 0 0.26 (4)Ba 0.5 0.5 0.5 0.10 (4)Co(Cu) 0 0 0.235 (1) 0.59 (4)O1 0 0 0 0.68 (4)O2 0 0 0.5 1.09 (4)O3 0 0.5 0.195 (2) 0.73 (4)

a = b = 3.900 (1) Å; c = 7.609 (1) Å; V = 115.72 (2) (Å)3; RBr = 4.99%;Rf = 4.73%; Rp = 7.65%

Fig. 3. Rietveld refinement profiles for Sm3Ba3Cu6O14 − δ.Fig. 4. Oxygen content variations in SmBaCo2 − xCuxO6 − δ at atmospheric oxygen pres-sure. The symbols correspond to the TGA regime with isothermal steps. Solid lines werecollected in the continuous cooling regime. Inset compares the oxygen deficiency levelsin SmBaCo2O6 − δ, SmBaCo1.4Cu0.6O6 − δ and SmBaCo1.4Fe0.6O6 − δ [17].

17N.E. Volkova et al. / Solid State Ionics 260 (2014) 15–20

For 0.2 ≤ x ≤ 1.2, the refinement showed formation of a tetragonalstructure related to the primitive perovskite lattice as ap × ap × 2ap,S.G. P4/mmm (Fig. 1b and Table 1). The unit cell parameter a remains es-sentially constant on doping (Fig. 2),while the c parameter and unit-cellvolume increase gradually with increasing copper concentration. Thistrend can be explained by the size factor (for octahedrally-coordinated transitionmetal cations the relevant radii are: rCo3þ=rCo4þ ¼0:75=0:67 ) and rCu2þ=rCu3þ ¼ 0:87=0:68 ) [20]).

For copper-rich materials (1.5 ≤ x ≤ 2.0), the crystal structure wasidentified as a derivative of the so-called 123-type (SmBa2Me3O6 − δ)where Ba2+ is partially substituted by Sm3+ cations. The correspondingchemical formula can bewritten as Sm1.5Ba1.5(Cu,Co)3O7 − δ or, alterna-tively, Sm3Ba3(CuxCo2 − x)3O14 − δ [21,22]. The structure is tetragonal,ap × ap × 3ap, with the space group P4/mmm. One example of theRietveld refinement plot is presented in Fig. 3; Table 2 lists the unitcell parameters and agreement factors.

SEM analysis coupled with energy-dispersive spectroscopy (EDS)did not reveal any significant compositional inhomogeneities in the ce-ramic materials. In all cases the average grain size varied in the range3–6 micrometers.

When discussing the variations of oxygen nonstoichiometry andtransport properties, one should mention that the substitution of cobalt

Table 3Oxygen content, mean oxidation states of the 3d metal cations in SmBaCo2 − xCuxO6 − δ, and e

Composition Oxygen content 2× (δx

TGA Redox titration

SmBaCo2O6 − δ 5.63 ± 0.01 –

SmBaCo1.6Cu0.4O6 − δ 5.47 ± 0.01 5.47 ± 0.05 0.32 ±SmBaCo1.4Cu0.6O6 − δ – 5.36 ± 0.05 0.54 ±SmBaCoCuO6 − δ – 5.15 ± 0.05 0.96 ±SmBaCo0.8Cu1.2O6 − δ 5.00 ± 0.01 – 1.26 ±

Table 2Unit cell parameters of Sm3Ba3(CuxCo2 − x)3O14 − δ, extracted from the Rietveld refinement re

Composition a, Å c, Å

Sm3Ba3Cu6O14 − δ 3.867 (1) 11.598 (1)Sm3Ba3Cu5.7Co0.3O14 − δ 3.873 (1) 11.566 (1)Sm3Ba3Cu5.4Co0.6O14 − δ 3.881 (1) 11.563 (1)Sm3Ba3Cu5.1Co0.9O14 − δ 3.883 (1) 11.548 (1)Sm3Ba3Cu4.8Co1.2O14 − δ 3.888 (1) 11.545 (1)Sm3Ba3Cu4.5Co1.5O14 − δ 3.892 (1) 11.534 (1)

by copper cations in SmBaCo2 − xCuxO6 − δ may induce at least twotypes of disordering processes caused by the fact that Cu is more elec-tronegative compared to Co. The dopant cations may thus act as anacceptor (Cu′Co), leading to the electron exchange:

Cu�Co þ Co�Co ¼ Cu0Co þ Co�Co ð2Þ

or to the formation of oxygen vacancies:

2Cu�Co þ O�

O ¼ 2Cu0Co þ V��O: ð3Þ

The values of the total oxygen content (6 − δ) determined by TGAand/or iodometric titration are listed in Table 3. Fig. 4 displays the tem-perature dependencies of the oxygen content under atmospheric air.Notice that the TGA data collected in the cooling and heating regimeswere found to completely coincide with each other within the limitsof experimental error, thus confirming reversibility of the oxygen up-take and desorption processes.

When copper is introduced into the cobalt sublattice of SmBaCo2− xCuxO6 − δ, the oxygen deficiency increases (Fig. 4). If assuming thatthe incorporation of acceptor-type Cu′Co is dominantly compensated by

stimated cobalt oxidation states at room temperature in air.

− δx = 0) Average oxidation state zMe Cobalt oxidation state zCo

3.13 3.130.12 2.97 3.210.12 2.86 3.230.12 2.65 3.300.04 2.50 3.25

sults.

V, (Å)3 RBr, % Rf, % Rp, %

173.45 (2) 1.14 1.25 8.23173.52 (2) 0.91 0.99 7.63174.17 (2) 1.13 1.31 7.93174.13 (2) 0.98 1.03 7.30174.57 (2) 1.18 1.68 8.07174.72 (2) 1.08 1.61 7.59

Fig. 5. Temperature dependence of the total conductivity of SmBaCo2 − xMexO6 − δ in air.The data on SmBaCo2O6 − δ and SmBaCo1.4Fe0.6O6 − δ [17] are shown for comparison.

Fig. 7. Temperature dependencies of the steady-state oxygen permeation fluxesthrough SmBaCo2 − xMexO6 − δ (Me = Cu, Fe) membranes under a fixed oxygenpartial pressure gradient. The data on La0.5Sr0.5CoO3 − δ [25], Ba0.5Sr0.5Co0.8Fe0.2O3 − δ [26]and SrCo0.4Fe0.3Cu0.3O3 − δ [16] are shown for comparison. Solid lines are a guide for the eye.

18 N.E. Volkova et al. / Solid State Ionics 260 (2014) 15–20

the formation of oxygen vacancies (Eq. (3)), the concentrations of thesespecies should be interrelated as:

x ¼ 2� δx−δx¼0ð Þ: ð4Þ

Indeed, the values of 2 × (δx − δx = 0) listed in Table 3 are very closeto x, within the limits of experimental uncertainty. This gives unambig-uous evidence that the prevailing charge compensation mechanism inSmBaCo2 − xCuxO6 − δ is associatedwith the oxygen vacancy formation.As a result, the oxidation state of cobalt estimated assuming that allcopper ions are divalent (zCo) remains essentially unchanged, whereasthe average oxidation state of transition metal cations decreases withincreasing x (Table 3). Quite a similar behavior was recently observedfor the NdBaCo2 − xCuxO6 − δ system [23].

Fig. 5 compares the total electrical conductivity of undopedSmBaCo2O6 − δ, SmBaCo1.4Cu0.6O6 − δ and SmBaCo1.4Fe0.6O6 − δ [17] atatmospheric oxygen pressure. The shape of the conductivity vs. temper-ature dependencies is governed by several factors discussed elsewhere(see [17] and references therein). These primarily include a progressive

Fig. 6.Oxygen permeation fluxes through dense SmBaCo2 − xMexO6 − δ ceramics withoutsurface modification as a function of the oxygen partial pressure gradient at 850 °C. Themembrane thickness is 1.00 ± 0.02 mm.

decrease in the concentration of p-type electronic charge carriers onheating due to oxygen losses from the perovskite lattice (Fig. 4), transi-tion metal cation disproportionation and thermal activation visible inthe low-temperature range. Very similar reasons are responsible forthe conductivity decrease when copper cations are introduced inSmBaCo2 − xCuxO6 − δ. First of all, the significant increase in the oxygenvacancy concentration (Table 3) leads to breaking of the Co\O\Cobond network, thus decreasing holemobility. Second, the partial substi-tution of Co cations by Cu decreases the concentration of sites availablefor the p-type charge carriers. Third, despite the negligible effect ofdoping on the average oxidation state of Co cations, decreasing cobaltcontent results in a lower concentration of holes formed due to Co3+

disproportionation [17].The oxygen permeationmeasurements revealed a substantially high

level of ionic conduction in SmBaCo2 − xCuxO6 − δ perovskites. Exactidentification of the oxygen transport mechanisms and possible micro-structural effects [15,24] requires, however, further experimental stud-ies, which are now in progress. In this work, selected results are onlypresented (Figs. 6 and 7) in order to illustrate the relationships betweencomposition and oxygenpermeability. The data on the steady-state per-meation fluxes through dense SmBaCo2 − xMexO6 − δ (Me = Cu, Fe)membranes of various thicknesses, which will be summarized in a sep-arate publication, showed that oxygen transport is limited by both bulkionic conduction and surface exchange; the role of interfacial exchangekinetics increaseswith the oxygen partial pressure gradient. This type ofbehavior, well-known for other cobaltite-based perovskites with rela-tively high oxygen deficiency [15,25,26], makes it impossible to directlyextract ionic conductivity from the permeation fluxes as the ion-diffusion and surface-exchange coefficients are all dependent on the

Table 4Average TECs of SmBaCo2 − xCuxO6 − δ ceramics at atmospheric oxygen pressure.

Composition T, °C TEC × 106, К−1

SmBaCo2O6 − δ [17] 25–500 18.7500–1100 21.1

SmBaCo1.6Cu0.4O6 − δ 25–1000 18.1SmBaCo1.4Cu0.6O6 − δ 25–1000 16.9

Fig. 9. XRD patterns of the mixtures of SmBaCo2 − xCuxO6 − δ (x = 0.4, 1.0) andCe0.8Sm0.2O2 − δ solid electrolyte, fired at 900 °C and 1000 °C.

19N.E. Volkova et al. / Solid State Ionics 260 (2014) 15–20

oxygen vacancy concentration. In such cases, the partial ionic conduc-tivity can only be calculated by modeling employing additional dataon equilibrium oxygen nonstoichiometry in the entire range of condi-tions used for the oxygen permeation tests [25]. Nonetheless, takinginto account the correlation between oxygen exchange and diffusioncoefficients [27], the relationships between oxygen fluxes and cationcomposition can still be used for the assessment of ionic conductivityvariations in a solid solution series [28]. One should also mention thatin this work, the permeation measurements were performed in a rela-tively narrow range of oxygen partial pressure gradients when the lim-iting role of bulk ionic transport is dominant. Consequently, comparisonof the oxygen permeability of SmBaCo2O6 − δ, SmBaCo1.4Cu0.6O6 − δ andSmBaCo1.4Fe0.6O6 − δ (Fig. 6) displays a clear correlation with the oxy-gen non-stoichiometry level in these phases (inset in Fig. 4), indicatingthat the ionic transport is governed by the oxygen vacancy concentra-tion. The apparent activation energies varying in the range of40–60 kJ/mol are consistent with the vacancy diffusion mechanism.The oxygen permeability of undoped SmBaCo2O6 − δ is lower thanthat of La0.5Sr0.5CoO3 − δ (Fig. 7), probably due to large lattice strainscaused by the A and A′ cation radius mismatch in the former case [29].SmBaCo1.4Cu0.6O6 − δ exhibits, however, substantially faster ionic trans-port, almost comparable to that in Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF)under similar external conditions. Note that BSCF is among the mostpermeable ceramic materials known up to now [30].

Another positive impact of copper doping is related to lowering ofthe average TECs (Table 4). Furthermore, the nonlinearity of the dilato-metric curves, observed for undoped SmBaCo2O6 − δ due to the ortho-rhombic → tetragonal phase transition on heating [17], disappears inthe case of SmBaCo1.4Cu0.6O6 − δ where the orthorhombic structure isstable within the entire temperature range important for practical ap-plications (Fig. 8). Although the TECs of SmBaCo2 − xCuxO6 − δ (x =0.4–0.6) are yet higher than those of zirconia- and ceria-based solidelectrolytes, the results show that thermomechanical compatibility ofthese materials can be further improved via cobalt-site doping.

Finally, testing of chemical compatibility of SmBaCo2 − xCuxO6 − δ

and Ce0.8Sm0.2O2 − δ solid electrolyte, performed in the temperaturerange 900–1100 °С in air, indicated no interaction between the mate-rials when the copper content is moderate (x ≤ 0.4). Increasing xleads to a higher reactivity. For example, in the case of SmBaCoCuO6 − δ,the reaction becomes significant at 1000 °С. Selected examples of theXRD patterns are presented in Fig. 9. Taking into account that processingof SOFC electrodes and catalytic layers deposited onto mixed-conductivemembranes requires thermal treatments at 950–1100 °С, this tendencymakes it necessary to avoid the use of copper-richmaterials in the elec-trochemical devices. The compositional optimization of SmBaCo2O6 − δ

and its analogs can be based, therefore, on co-doping with Cu2+ and

Fig. 8. Dilatometric curves of SmBaCo2 − xCuxO6 − δ ceramics in air.

Ni2+/3+ or on a modest increase of acceptor-type cation concentrationin the A-sublattice.

4. Conclusions

According to the results of XRD analysis, solid solutions with the lay-ered perovskite and 123-type structures are formed in the pseudobinarySmBaCo2 − xCuxO6 − δ system within the ranges of 0 ≤ x ≤1.2 and1.5 ≤ x ≤2.0, respectively. As for Fe-substituted SmBaCo2O6 − δ

[17], increasing dopant content leads to the transition of orthorhombic(S.G. Pmmm) into tetragonal polymorph (P4/mmm) at x N 0.1. The in-corporation of Cu2+ cations in SmBaCo2 − xCuxO6 − δ (0≤ x ≤ 1.2) de-creases oxygen content, thermal expansion and total conductivity. Theoxygen permeation fluxes were found to correlate with the oxygenvacancy concentration and, thus, to increase on doping. As a result,the level of oxygen ionic transport in SmBaCo1.4Cu0.6O6 − δ ceramicsbecomes quite close to that inmost permeable cobaltite-basedmate-rials, such as Ba0.5Sr0.5Co0.8Fe0.2O3 − δ. However, copper additionspromote also a chemical interaction with solid oxide electrolyte ma-terials, such as Ce0.8Sm0.2O2 − δ. The significant reactivity observedfor SmBaCoCuO6 − δ even at 1000 °С, makes it necessary to limitCu2+ concentration in the materials for practical electrochemicalapplications.

Acknowledgments

This work was financially supported by the Russian Foundation forBasic Research (project nos. 13-03-00958_a and 13-03-12409), theMinistry of Education and Science of the Russian Federation (project14.B25.31.0018), and the FCT, Portugal.

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