**Abstract**

Proton-conducting perovskites are among the most promising electrolytes for Proton Ceramic Fuel Cells (PCFCs), electrolysers and separation membranes. Particularly, yttrium-doped barium cerate, BaCe1-xYxO3-<sup>δ</sup> (BCY), shows one of the highest protonic conductivities at intermediate temperatures (<sup>σ</sup> <sup>10</sup><sup>3</sup> S cm<sup>1</sup> at 400°C); values that are typically achieved under humidified atmospheres (*p*H2O <sup>10</sup><sup>2</sup> atm). However, BCY has commonly been discarded for such applications due to its instability in the presence of water vapour and carbonaceous atmospheres. A recent discovery has shown that BCY10 exhibits pure protonic conductivity under very low humidity contents (<sup>10</sup><sup>5</sup> –10<sup>4</sup> atm), owing to its very high equilibrium constant for hydration. This peculiar characteristic allows this material to retain its functionally as a proton conductor in such conditions, while preventing its decomposition. Hence, this chapter explores the electrochemical properties of the BaCe0.9Y0.1O3-<sup>δ</sup> (BCY10) composition, comprehensively establishing its limiting operation conditions through defect chemistry and thermodynamic analyses. Moreover, the importance of such conditions is highlighted with respect to potential industrially relevant hydrogenation/de-hydrogenation reactions at low temperatures under low humidity.

**Keywords:** perovskite, barium cerate, protonic conductivity, transport number, nominally dry conditions

### **1. Introduction**

Ceramic proton conductors have been highlighted for electrochemical synthesis, as potential membranes in hydrogenation and dehydrogenation reactions [1]. One of the best compositions for this role is that of the doped barium cerate, *e.g.* BaCe1-xMxO3-<sup>δ</sup> (M = Y3+, In3+, Gd3+, etc.), which can show very high levels of proton conductivity at intermediate temperatures (*i.e. <sup>σ</sup>* <sup>10</sup><sup>3</sup> S cm <sup>1</sup> at 400°C) [2–9]. This material belongs to the perovskite family with ABO3 ceramic oxide

structure, including a divalent alkaline earth element, such as Ba2+ (also, Sr2+ or Ca2+), in the A-cation site, while a tetravalent rare-earth element, Ce4+, is present in the B-cation site. The introduction of dopants in the B-site with suitable acceptor elements, such as Y3+, In+3 or Gd3+ trivalent cations, leads to the formation of charge compensating oxygen vacancies [9]:

$$\mathbf{BaO} + (\mathbf{1} - \mathbf{y})\mathbf{CeO}\_2 + \left(\frac{\mathbf{y}}{2}\right)\mathbf{M}\_2\mathbf{O}\_3 \Longrightarrow \mathbf{Ba}\_{\mathbf{Ba}}^\mathbf{x} + (\mathbf{1} - \mathbf{y})\mathbf{Ce}\_{\mathbf{Ce}}^\mathbf{x} + \left(\mathbf{y}\right)\mathbf{M}\_{\mathbf{Ce}}^\mathbf{f} + \left(\mathbf{3} - \frac{\mathbf{y}}{2}\right)\mathbf{O}\_\mathbf{O}^\mathbf{x} + \left(\frac{\mathbf{y}}{2}\right)\mathbf{V}\_\mathbf{O}^\mathbf{x} \tag{1}$$

In addition to potential oxide-ion conductivity, these acceptor-substituted materials are also capable of offering both protonic and electronic conductivity, depending on the temperature and atmospheric conditions. The protonic conductivity is the most significant characteristic of these materials that is usually associated with the existence of protonic defects (OH• O), upon filling of these oxygen vacancies in the presence of water vapour, as expressed by Eq. (2) [10–13]:

$$\rm H\_2O + V\_O^{\*\*} + O\_O^x \Leftrightarrow 2OH\_O^\* \tag{2}$$

Accordingly, the equilibrium constant for hydration, *K*w, is given by the following equation:

$$K\_{\rm w} \approx \frac{\left[\rm OH\_{0}^{\bullet}\right]^{2}}{p\_{\rm H2O}\left[V\_{\rm O}^{\bullet\bullet}\right]\left[O\_{0}^{\rm x}\right]}\tag{3}$$

Against this scenario, one common alternative is the use of the barium zirconates or compounds containing both Ce and Zr elements, where the introduction of Zr can significantly increase their chemical stability. Nonetheless, it has also been demonstrated that increased amounts of Zr negatively impact the total conductivity of these materials, due to an increase in their refractive nature and in their grain growth, which aggravate the problem of resistive grain boundaries. As such, much lower values of total conductivity are, typically, reported for the zirconate materials than for their cerate analogues, even though their bulk protonic conduc-

*Analysis of the Electrochemical Transport Properties of Doped Barium Cerate for Proton…*

*Carbonate formation temperature (blue) and the conductivity isotherm at 400°C of BaCe0.9M0.1O3-<sup>δ</sup> (M = Y, Tm, Yb, Lu, In or Sc) in moist H2 as a function of the ionic radius of the dopant. Adapted from [22].*

More recently, the work of Kim *et al.* [34] reported that the chemical instability of the barium cerates is due to the presence of a nanometre-thick amorphous phase found at the grain boundaries in proton-conducting BaCeO3 polycrystals, which not only leads to a reduced proton mobility, but also can act as a penetration path for H2O and CO2 gas molecules, facilitating chemical decomposition and collapse of the microstructure (**Figure 2a**). Furthermore, this effect could be minimised by controlling the composition to obtain Ba-deficient samples in which the intergranular amorphous layer could be minimised, leading to a mitigation of the reactivity with such gases (**Figure 2b**). The presence of an amorphous layer on the interfaces between grains has also been documented in barium zirconate-based compositions [26, 35], where this feature can exert significant complications during fabrication of

In summary, the high electrical conductivity and the facile processing of the doped barium cerates demands further investigation to succeed to overcome their limited stabilities. In fact, it is only very recently that research in these materials has moved towards a more fundamental and, yet, critical aspect, concerning a deeper understanding of the limiting atmospheric conditions that are necessary to retain their functionality. Taking this into account, Loureiro *et al*. [37] reanalysed the barium cerate stability limits by thermodynamic calculations, considering its decomposition products in the presence of water vapour and CO2 (**Figure 3**). According to this theoretical study, no degradation would be expected for humidity values of <sup>3</sup> <sup>10</sup><sup>2</sup> atm and temperatures higher than 500°C. However, when considering the formation of barium carbonate (**Figure 3**), the thermodynamics predict that much stricter conditions need to be applied, where only very low partial

tivities are actually greater [9, 26–33].

*DOI: http://dx.doi.org/10.5772/intechopen.93970*

**Figure 1.**

**53**

complete electrochemical cells [19, 36].

Due to the significant importance of humidity to promote protonic conductivity, most of the reported studies of barium cerate based materials have focused on highly wetted atmospheres with typical water vapour partial pressure *p*H2O � 3 � 10�<sup>2</sup> atm [14–18]. Unfortunately, these works also underline the tendency of this material for reacting with acidic gases, *viz.* carbon dioxide (CO2) and water vapour (H2O), leading to the formation of insulating carbonate or hydroxide phases, respectively, on the surface of the material. This complication impedes the ability of this material to be used in highly humidified and carbon-based fuels, thus, limiting its potential application range [3, 14–20]. The typical degradation reactions in such atmospheres include:

$$\text{BaCeO}\_{3(s)} + \text{CO}\_{2(g)} \to \text{BaCO}\_{3(s)} + \text{CeO}\_{2(s)}\tag{4}$$

$$\text{BaCeO}\_{3(s)} + \text{H}\_2\text{O}\_{(g)} \to \text{Ba(OH)}\_{2(g)} + \text{CeO}\_{2(s)}\tag{5}$$

The chemical stability of doped barium cerates is well documented in the literature and huge efforts have been made to explore the reasons behind its chemical instability, using both conventional and non-conventional techniques [21–25]. For instance, Matsumoto *et al.* [22] studied the effect of dopant M in BaCe0.9M0.1O3-<sup>δ</sup> (M = Y, Tm, Yb, Lu, In, or Sc) on the electrical conductivity in the temperature range 400–900°C and on the chemical stability with respect to CO2 by thermogravimetry (TG). Both the electrical conductivity (moistened H2 or O2, *<sup>p</sup>*H2O = 1.9 � <sup>10</sup>�<sup>2</sup> atm) and the stability against carbonate formation were shown to decrease with increasing ionic radius (**Figure 1**), corresponding to an increase in basicity. Nonetheless, all compounds were found to interact with pure CO2 at temperatures below 900°C, failing to succeed in the mitigation of the chemical instability in the doped barium cerate.

*Analysis of the Electrochemical Transport Properties of Doped Barium Cerate for Proton… DOI: http://dx.doi.org/10.5772/intechopen.93970*

#### **Figure 1.**

structure, including a divalent alkaline earth element, such as Ba2+ (also, Sr2+ or Ca2+), in the A-cation site, while a tetravalent rare-earth element, Ce4+, is present in the B-cation site. The introduction of dopants in the B-site with suitable acceptor elements, such as Y3+, In+3 or Gd3+ trivalent cations, leads to the formation of charge

Ba <sup>þ</sup> <sup>1</sup> � <sup>y</sup> Cex

In addition to potential oxide-ion conductivity, these acceptor-substituted materials are also capable of offering both protonic and electronic conductivity, depending on the temperature and atmospheric conditions. The protonic conductivity is the most significant characteristic of these materials that is usually associ-

vacancies in the presence of water vapour, as expressed by Eq. (2) [10–13]:

<sup>O</sup> <sup>þ</sup> <sup>O</sup><sup>x</sup>

Accordingly, the equilibrium constant for hydration, *K*w, is given by the

*<sup>K</sup>*<sup>w</sup> <sup>≈</sup> OH•

most of the reported studies of barium cerate based materials have focused on highly wetted atmospheres with typical water vapour partial pressure *p*H2O � 3 � 10�<sup>2</sup> atm [14–18]. Unfortunately, these works also underline the tendency of this material for reacting with acidic gases, *viz.* carbon dioxide (CO2) and water vapour (H2O), leading to the formation of insulating carbonate or hydroxide phases, respectively, on the surface of the material. This complication impedes the ability of this material to be used in highly humidified and carbon-based fuels, thus, limiting its potential application range [3, 14–20]. The typical degradation reactions in such

*<sup>p</sup>*H2O <sup>V</sup>•• O Ox O

Due to the significant importance of humidity to promote protonic conductivity,

<sup>O</sup> ⇔ 2OH•

O <sup>2</sup>

BaCeO3 sð Þ þ CO2 gð Þ ! BaCO3 sð Þ þ CeO2 sð Þ (4)

BaCeO3 sð Þ <sup>þ</sup> H2Oð Þ<sup>g</sup> ! Ba OH ð Þ2 gð Þ <sup>þ</sup> CeO2 sð Þ (5)

The chemical stability of doped barium cerates is well documented in the literature and huge efforts have been made to explore the reasons behind its chemical instability, using both conventional and non-conventional techniques [21–25]. For instance, Matsumoto *et al.* [22] studied the effect of dopant M in BaCe0.9M0.1O3-<sup>δ</sup> (M = Y, Tm, Yb, Lu, In, or Sc) on the electrical conductivity in the temperature

range 400–900°C and on the chemical stability with respect to CO2 by

thermogravimetry (TG). Both the electrical conductivity (moistened H2 or O2, *<sup>p</sup>*H2O = 1.9 � <sup>10</sup>�<sup>2</sup> atm) and the stability against carbonate formation were shown to decrease with increasing ionic radius (**Figure 1**), corresponding to an increase in basicity. Nonetheless, all compounds were found to interact with pure CO2 at temperatures below 900°C, failing to succeed in the mitigation of the chemical

H2O <sup>þ</sup> <sup>V</sup>••

Ce þ y M<sup>0</sup>

Ce <sup>þ</sup> <sup>3</sup> � <sup>y</sup>

O), upon filling of these oxygen

(3)

<sup>O</sup> (2)

2 

Ox <sup>O</sup> <sup>þ</sup> <sup>y</sup> 2 V•• O (1)

compensating oxygen vacancies [9]:

2 

ated with the existence of protonic defects (OH•

M2O3¼)Ba<sup>x</sup>

*Analytical Chemistry - Advancement, Perspectives and Applications*

BaO <sup>þ</sup> <sup>1</sup> � <sup>y</sup> CeO2 <sup>þ</sup> <sup>y</sup>

following equation:

atmospheres include:

instability in the doped barium cerate.

**52**

*Carbonate formation temperature (blue) and the conductivity isotherm at 400°C of BaCe0.9M0.1O3-<sup>δ</sup> (M = Y, Tm, Yb, Lu, In or Sc) in moist H2 as a function of the ionic radius of the dopant. Adapted from [22].*

Against this scenario, one common alternative is the use of the barium zirconates or compounds containing both Ce and Zr elements, where the introduction of Zr can significantly increase their chemical stability. Nonetheless, it has also been demonstrated that increased amounts of Zr negatively impact the total conductivity of these materials, due to an increase in their refractive nature and in their grain growth, which aggravate the problem of resistive grain boundaries. As such, much lower values of total conductivity are, typically, reported for the zirconate materials than for their cerate analogues, even though their bulk protonic conductivities are actually greater [9, 26–33].

More recently, the work of Kim *et al.* [34] reported that the chemical instability of the barium cerates is due to the presence of a nanometre-thick amorphous phase found at the grain boundaries in proton-conducting BaCeO3 polycrystals, which not only leads to a reduced proton mobility, but also can act as a penetration path for H2O and CO2 gas molecules, facilitating chemical decomposition and collapse of the microstructure (**Figure 2a**). Furthermore, this effect could be minimised by controlling the composition to obtain Ba-deficient samples in which the intergranular amorphous layer could be minimised, leading to a mitigation of the reactivity with such gases (**Figure 2b**). The presence of an amorphous layer on the interfaces between grains has also been documented in barium zirconate-based compositions [26, 35], where this feature can exert significant complications during fabrication of complete electrochemical cells [19, 36].

In summary, the high electrical conductivity and the facile processing of the doped barium cerates demands further investigation to succeed to overcome their limited stabilities. In fact, it is only very recently that research in these materials has moved towards a more fundamental and, yet, critical aspect, concerning a deeper understanding of the limiting atmospheric conditions that are necessary to retain their functionality. Taking this into account, Loureiro *et al*. [37] reanalysed the barium cerate stability limits by thermodynamic calculations, considering its decomposition products in the presence of water vapour and CO2 (**Figure 3**). According to this theoretical study, no degradation would be expected for humidity values of <sup>3</sup> <sup>10</sup><sup>2</sup> atm and temperatures higher than 500°C. However, when considering the formation of barium carbonate (**Figure 3**), the thermodynamics predict that much stricter conditions need to be applied, where only very low partial

#### **Figure 2.**

*Schematic representation of microstructural changes upon reaction with water and carbon dioxide: (a) Ba-stoichiometric compositions (thick amorphous intergranular phase); (b) Ba-deficient compositions (thin amorphous intergranular phase).*

that of the BCY stability limit. These results demonstrate that, for example, at 400° C, these values should range between the values of 10<sup>3</sup> <sup>&</sup>lt; *<sup>p</sup>*H2O <sup>&</sup>lt; <sup>10</sup><sup>4</sup> atm in order to avoid decomposition of the perovskite phase, for the potential hydrocarbon

*Thermodynamic equilibrium for the formation of carbon dioxide from a hydrocarbon-based mixture and water*

*Analysis of the Electrochemical Transport Properties of Doped Barium Cerate for Proton…*

Nonetheless, one of the requirements for operating in such low water vapour partial pressures is that the protonic conductivity must be maintained in order to ensure the functionality of the electrolyte membrane in these applications. In this respect, protonic conductors are complex materials as they are capable to offer mixed conductivity (protonic, oxide-ion and electronic), depending on the temperature and on the nature of the surrounding atmosphere [37, 38]. One of the most promising compositions for this type of application is that of the yttrium-doped barium cerate, BaCe1-xYxO3-<sup>δ</sup> (BCY), which has very high protonic conductivity at lower temperatures under humidified atmospheres (*e.g.* <sup>10</sup><sup>3</sup> S cm<sup>1</sup> at 400°C,

Therefore, the current chapter will be focus on the electrochemical transport properties of the BaCe0.9Y0.1O3-d (BCY10) in reducing and oxidising conditions when operating in very low humidity levels. The aim of this chapter is to comprehensively explain the working limits of BCY10 and to assess its applicability as an electrolyte membrane for fuel cell, electrolysers and other electrochemical-based applications, with special focus on operation under low water vapour partial

**2. Electrochemical properties of BCY10 in nominally dry reducing**

**Figure 5** depicts the total conductivity of BCY10 analysed by impedance spectroscopy between 100 and 500°C in H2, 10%H2-N2 and N2, highlighting that no significant differences can be observed in the conductivity measured under these atmospheres. In addition, at the higher temperature range, a notable decrease of the activation energy is observed in all cases, as a result of the exsolution of protons from the structure of BCY10, and the concomitant decrease of the protonic contribution to the electrical transport [37]. Interestingly, and also surprisingly at first

atmospheres of CH4, C3H8 or C6H6 [43].

*DOI: http://dx.doi.org/10.5772/intechopen.93970*

*at 400°C [38] (reproduced by permission of The Royal Society of Chemistry).*

*<sup>p</sup>*H2O <sup>10</sup><sup>2</sup> atm) [1, 38].

pressures.

**55**

**Figure 4.**

**conditions**

#### **Figure 3.**

*Thermodynamic stability of carbon dioxide partial pressure (*p*CO2) and water vapour partial pressure (*p*H2O) as function of temperature considering the equilibrium of BaCeO3 and its decomposition products (*i.e. *BaCO3 and Ba(OH)2) [38] (reproduced by permission of The Royal Society of Chemistry).*

pressures of CO2 (*e.g. p*CO2 <sup>&</sup>lt; <sup>10</sup><sup>8</sup> atm at 400°C) are able to avoid barium cerate degradation.

For this reason, only very few reports can be found on successful applications of BCY membranes for chemical reactions. Most of these have concerned, ammonia synthesis [39–41], or the conversion of propane to propylene [42]. In these cases, no chemical instability has been reported and the survival of the BCY material is likely to be related to the effective absence of CO2 or significant water vapour in these operations. To understand this further, **Figure 4** presents the maximum water vapour partial pressure (*p*H2O) that could be tolerated in different carbonaceous atmospheres to provide an equilibrium partial pressure of CO2 that remains below

*Analysis of the Electrochemical Transport Properties of Doped Barium Cerate for Proton… DOI: http://dx.doi.org/10.5772/intechopen.93970*

#### **Figure 4.**

*Thermodynamic equilibrium for the formation of carbon dioxide from a hydrocarbon-based mixture and water at 400°C [38] (reproduced by permission of The Royal Society of Chemistry).*

that of the BCY stability limit. These results demonstrate that, for example, at 400° C, these values should range between the values of 10<sup>3</sup> <sup>&</sup>lt; *<sup>p</sup>*H2O <sup>&</sup>lt; <sup>10</sup><sup>4</sup> atm in order to avoid decomposition of the perovskite phase, for the potential hydrocarbon atmospheres of CH4, C3H8 or C6H6 [43].

Nonetheless, one of the requirements for operating in such low water vapour partial pressures is that the protonic conductivity must be maintained in order to ensure the functionality of the electrolyte membrane in these applications. In this respect, protonic conductors are complex materials as they are capable to offer mixed conductivity (protonic, oxide-ion and electronic), depending on the temperature and on the nature of the surrounding atmosphere [37, 38]. One of the most promising compositions for this type of application is that of the yttrium-doped barium cerate, BaCe1-xYxO3-<sup>δ</sup> (BCY), which has very high protonic conductivity at lower temperatures under humidified atmospheres (*e.g.* <sup>10</sup><sup>3</sup> S cm<sup>1</sup> at 400°C, *<sup>p</sup>*H2O <sup>10</sup><sup>2</sup> atm) [1, 38].

Therefore, the current chapter will be focus on the electrochemical transport properties of the BaCe0.9Y0.1O3-d (BCY10) in reducing and oxidising conditions when operating in very low humidity levels. The aim of this chapter is to comprehensively explain the working limits of BCY10 and to assess its applicability as an electrolyte membrane for fuel cell, electrolysers and other electrochemical-based applications, with special focus on operation under low water vapour partial pressures.
