Novel Pr-Doped BaLaInO₄ Ceramic Material with Layered Structure for Proton-Conducting Electrochemical Devices

Abstract

One of the urgent tasks of applied materials science is the creation of novel high-effective materials with target properties. In the area of energy systems, there is a problem in the conversion of chemical energy to electricity without mechanical work. Hydrogen energy provides a way using electrochemical devices such as protonic ceramic fuel cells. Novel advanced proton-conducting materials with the top characteristics of target properties are strictly needed. Layered perovskites are a novel and promising class of protonic conductors. In this work, the layered perovskite BaLa_0.9Pr_0.1InO₄ was obtained and investigated as a protonic conductor for the first time. The possibility for water intercalation and proton transport is proved. It was shown that isovalent doping Pr³⁺ → La³⁺ leads to an increase in the crystal lattice size, proton concentration and proton mobility. The proton conductivity value for doped BaLa_0.9Pr_0.1InO₄ composition is 18 times greater than for undoped BaLaInO₄ composition. Layered perovskites based on BaLaInO₄ are promising materials for application in proton-conducting electrochemical devices.

1. Introduction

Applied materials science plays an important role in the development of various areas of human life. Critical areas such as medicine, energy and mechanical engineering cannot develop without the creation of new materials with targeted properties. Ceramic materials are required for very different needs such as medical applications (endoprosthetics) [1,2,3,4,5,6] and components for various electrochemical devices [7,8,9,10,11,12], as examples. The high priority of sustainable development necessitates the development of advanced energy technologies, one of them being hydrogen energy [13,14,15,16]. The switchover to renewable energy is, actually, not only for increasing energetically efficiency, but, also, due to the pursuit in decreasing climate changes and the lowering of cases of respiratory diseases [17,18,19,20,21,22,23]. Hydrogen energy is considered as an actual, sustainable and promising approach for energy generation [24,25,26,27,28,29,30,31,32,33,34,35]. This industry includes aspects such as the production, transportation and use of hydrogen as an energy source. The conversion of the chemical energy of hydrogen oxidation to electrical energy can be implemented using electrochemical devices such as protonic ceramic fuel cells [36,37,38,39,40,41]. This requires new high-effective materials such as electrodes, electrolytes and interconnectors with good compatibility between each other. The traditional and most investigated proton-conducting materials are barium cerate zirconates BaCeO₃-BaZrO₃, which are characterized by perovskite structure [42,43]. However, they are characterized by a relatively low concentration of protons and low chemical resistance to carbon dioxide. Consequently, novel advanced proton-conducting materials with top characteristics of target properties are strictly needed.

Layered perovskite structure is related to classical perovskite structure. The main difference is the separation of the perovskite framework by the layers with different structures. Layered perovskites with the general formula AA’_nB_nO_3n+1 belong to members of layered perovskite family such as compositions with the Ruddlesden–Popper structure. Perovskite slabs A’BO₃ are divided by the rock salt layers AO in this structural type, and the existence of compositions with a different “n” is possible. Layered structures provide a concentration of protons an order of magnitude higher than that of classic perovskites. The realization of proton transport was proved for monolayer perovskites such as BaNdInO₄ [44,45,46,47,48], BaNdScO₄ [49], SrLaInO₄ [50,51,52,53,54], BaLaInO₄ [55,56,57,58,59,60] and compositions based on them in the last few years [61]. Such compositions can be potentially used as electrolytic materials in the proton-conducting fuel cells. Various types of doping, including cationic [62] and oxyanionic [63], were investigated. However, the substitutions were implemented by the ions with a stable oxidation state. Meanwhile, the introduction of ions capable of changing their valence can provide control over the contributions of the electronic and ionic components of conductivity. In the future, this can ensure the creation of electrode and electrolyte materials with the same crystal structure and similar chemical composition, which should provide excellent compatibility. In this work, a Pr-doped ceramic material based on BaLaInO₄ was obtained and investigated for the first time. The possibility of protonic transport was revealed.

2. Experimental Procedure

The phase BaLa_0.9Pr_0.1InO₄ was synthesized via the nitrate–citrate route according to [52]. The starting reagents Ba(NO₃)₂, In(NO₃)₃∙6H₂O, Pr(NO₃)₃∙6H₂O and La(NO₃)₃∙9H₂O were used. The X-ray diffraction analysis (XRD) was implemented at the Cu K_α diffractometer Bruker Advance D8. The full profile Le Bail regiments were implemented via the FullProf Suite software. The method of scanning electron microscopy (SEM) of powder and ceramic samples was realized using microscope VEGA3 TESCAN coupled with an energy-dispersive X-ray spectroscopy system (EDS).

The thermogravimetry (TG) measurements were implemented by the Netzsch Analyser STA 409 PC. The samples were initially hydrated using the method of cooling from 1000 to 150 °C (0.5 °C/min) at wet Ar flow. During TG-measurements, the samples were heated from 40 to 1000 °C with the speed 10 °C/min at dry Ar flow.

The resistance of ceramic samples was collected via the impedance spectrometer Elins Z-1000P. The ceramic pellets with a 10 mm diameter and 2 mm thickness were pressed for the investigations. Pt-electrodes were applied on the surfaces of the samples. The temperature range 200–1000 °C was covered, and the speed of cooling was 1°/min. The dry atmosphere was obtained by the circulating of air or Ar through phosphorus pentoxide (pH₂O = 3.5 × 10⁻⁵ atm). The wet atmosphere was obtained by the passing of air or Ar through a saturated solution of potassium bromide (pH₂O = 2 × 10⁻² atm).

3. Results and Discussions

The phase characterization of composition BaLa_0.9Pr_0.1InO₄ was made using XRD analysis. Figure 1a represents the Le Bail refinement of the X-ray obtained data. The composition is single phase and it has orthorhombic symmetry (Pbca space group). The image of the crystal structure of the monolayer perovskite is presented in the inset of Figure 1a. The introduction of Pr³⁺ ions into the La³⁺ sublattice (isovalent doping) leads to the expansion of the crystal lattice (

Table 1. The geometric characteristics of the crystal lattice for the compositions BaLa_0.9Pr_0.1InO₄ and BaLaInO₄.

Composition	a, Å	b, Å	c, Å	V, (Å3)
BaLa0.9Pr0.1InO4	12.968 (1)	5.911 (9)	5.917 (9)	453.17 (7)
BaLaInO4 [56]	12.932 (3)	5.906 (0)	5.894 (2)	450.19 (5)

). An increase in the size of the unit cell is observed during doping despite the close ionic radii of trivalent metals Pr³⁺ and La³⁺ ($r_{{\mathrm{La}}^{3+}}$ = 1.216 Å, $r_{{\mathrm{Pr}}^{3+}}$ = 1.179 Å [64]). The presence of ions with different electronegativity (${\mathsf{\chi }}_{\mathrm{La}}$ = 1.10, ${\mathsf{\chi }}_{\mathrm{Pr}}$ = 1.13 [65]) could be the cause of the changes in the local structure and in the interatomic distances due to the additional repulsion effects. A similar effect was observed for another doped composition based on BaLaInO₄ [59,60], and the presence of ions with different electronegativity in the same sublattice can be considered as the reason of the changes in the crystal lattice size.

The morphology of the obtained sample was checked using scanning electron microscopy. Composition BaLa_0.9Pr_0.1InO₄ consists of grains (~3–5 μm) agglomerated in the particles with the size ~10 μm (Figure 1b,c). The elemental composition was proved via EDS analysis. The average element ratios for BaLa_0.9Pr_0.1InO₄ composition are presented in

Table 2. The results of the energy-dispersive analysis for the composition BaLa_0.9Pr_0.1InO₄ (theoretical values in atomic % are provided in the brackets).

Metal	Barium	Lanthanum	Praseodymium	Indium
Content	33.4	29.9	3.2	33.5
	(33.3)	(30.0)	(3.3)	(33.4)

. The good agreement between theoretical and experimental values was confirmed.

The amount of water uptake was measured via thermogravimetry (TG) coupled with the differential scanning calorimetry (DSC) method. The results are presented in Figure 2. As we can see, the initially hydrated composition loses mass due to water release that was confirmed by MS-results. No other volatile components were detected. The main mass loss happens at ~ 200–400 °C, which is confirmed by the signal on the DSC-curve (green line in Figure 2). The dissociative water intercalation into the crystal structure of layered perovskites is possible due to the placement of hydroxyl groups in the rock salt space of the layered framework [62]:

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_{\mathrm{o}}^{\mathrm{x}}\iff {(\mathrm{OH})}_{\mathrm{o}}^{•}+{(\mathrm{OH} )}_{\mathrm{i}}^{’}$$

(1)

Consequently, the increasing of the crystal lattice size should lead to the increasing of the water uptake [62]. As we can see, the water uptake for the Pr-doped composition is about 1 mol water per formula unit (Figure 2), which is bigger than 0.62 mol registered for undoped BaLaInO₄ composition [62]. In other words, a good correlation between the changes of the geometric characteristics of the unit cell and water uptake is observed.

The electrotransport properties of the Pr-doped composition were investigated via the impedance spectroscopy method. Nyquist plots under dry and wet air at 400 °C are presented in Figure 3a as an example of collected data. The calculation of conductivity values was made using the resistance value obtained by extrapolating the high-frequency semicircle to the abscissa axis (capacitance ~10⁻¹² F/cm). The effect of variation in oxygen partial pressure to the conductivity values is presented in Figure 3b. As we can see, the electrical conductivity is mixed hole ionic at dry oxidizing conditions. The share of oxygen ionic transport does not change at the temperature variation, and it is around 25%, which is comparable with the value (20%) for undoped BaLaInO₄ composition [56]. We can suggest that the dopant concentration of 0.1 mol is not enough to have a meaningful impact on the conductivity nature. At the same time, the nature of dopant is also a possible reason for the absence of significant changes in the nature of conductivity. However, a significant increase in the conductivity values (~1.5 orders of magnitude) during doping is observed (Figure 3c,d). The most probable reason for the increase in the mobility of oxygen ions is due to the increase in the size of the crystal lattice and space for ionic transport. It should be noted that the conductivity values from the electrolytic area (oxygen ionic conductivity, pO₂ < 10⁻⁵) do correlate well with the conductivity values obtained at the Ar atmosphere. This allows us to consider the values obtained in argon as ionic conductivity values.

The humidity of the atmosphere affects the conductivity values below 600 °C. The proton concentration increases with temperature decreases. This is because of the increase of the conductivity under wet conditions in comparison with dry conditions (Figure 3b,c). The proton conductivity was calculated as the difference between conductivity values obtained under wet Ar and dry Ar:

$${\sigma }_{H^{+}}={\sigma }_{wet Ar}-{\sigma }_{dry Ar}={\sigma }_{wet}^{ion}-{\sigma }_{dry}^{ion}$$

(2)

and its temperature dependencies are shown in Figure 4a. As we can see, the protonic conductivity for the Pr-doped composition is higher than the undoped sample by ~1.5 orders of magnitude. This increase is due to the increase in both the proton concentration and proton mobility (Figure 4b). The proton conductivity value for doped BaLa_0.9Pr_0.1InO₄ composition is 5.0 × 10⁻⁶ S/cm (T = 400 °C) in comparison with 2.7 ×∙10⁻⁷ S/cm for BaLaInO₄ composition that is 18 times greater. It can be suggested that the change in the dopant concentration and dopant nature can lead to significant changes in the nature of electrical conductivity.

4. Conclusions

The layered perovskite BaLa_0.9Pr_0.1InO₄ was obtained and investigated as a protonic conductor for the first time. The possibility of proton transport is shown. It was proved that isovalent doping Pr³⁺ → La³⁺ leads to an increase in the crystal lattice size, proton concentration and proton mobility. The proton conductivity value for the doped BaLa_0.9Pr_0.1InO₄ composition is 5.0 ×∙10⁻⁶ S/cm (T = 400 °C), in comparison with the 2.7∙× 10⁻⁷ S/cm for BaLaInO₄ composition that is 18 times greater. Further research for a dopant capable of a significant change in its electrical conductivity nature is relevant. Layered perovskites based on BaLaInO₄ are promising materials for application in proton conducting electrochemical devices.