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

**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

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

*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).*

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

*Analytical Chemistry - Advancement, Perspectives and Applications*

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

degradation.

**54**

**Figure 3.**

**Figure 2.**

*(thin amorphous intergranular phase).*

oxygen non-stoichiometry (Δ*δ*), which can be associated with the reduction of Ce4+

<sup>O</sup> þ 2Ce<sup>0</sup>

Ce þ 1 2

p Oð Þ<sup>2</sup> 1 2 O2 (6)

Ce <sup>2</sup> (7)

Ce (8)

ð Þ 2Δδ (9)

<sup>O</sup> ⇔ V••

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

Ox O Cex

of Δ*δ* with oxygen partial pressure only at very high temperature, with a lower impact as temperature decreases. Thus, **Figure 6** demonstrates that very extreme reducing conditions and very high temperatures are required to produce appreciable increase in the oxygen-vacancy and electronic concentrations in BCY [47, 48]. These results contrast with those of fluorite-ceria-based materials which usually

The results of coulometric titration (**Figure 6**) [37] show considerable variations

Thus, to take the possibility of reduction into account, the methodology applied by Loureiro *et al.* [37] for the determination of reduction equilibrium follows the

and other mass and lattice position restrictions, on neglecting defect interactions

v0

*Oxygen non-stoichiometry as function of oxygen partial pressure (*p*O2) [37] (reproduced by permission of The*

Ce <sup>þ</sup> Ce<sup>0</sup>

The corresponding mass action constant (Eq. (7)) can be combined with the

O ≈ Y<sup>0</sup>

and assuming nearly ideal behaviour, with the following relations between the concentrations of relevant species, stoichiometric changes (Δ*δ*), and fraction of

> Ce<sup>0</sup> Ce <sup>¼</sup> <sup>Z</sup>

to Ce3+, following the equation:

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

electroneutrality condition:

trivalent additive (*x*):

**Figure 6.**

**57**

*Royal Society of Chemistry).*

2Ce<sup>x</sup>

show high reducibility under milder conditions [46, 49].

method reported elsewhere [50], as described below.

2 V•• O <sup>þ</sup> OH•

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

with the equilibrium constant for reduction reaction given by:

KR <sup>¼</sup> <sup>V</sup>•• O Ce<sup>0</sup> Ce <sup>2</sup>

#### **Figure 5.**

*(a) Temperature dependency of the total conductivity of BCY10 obtained in the temperature range 100–500°C in nominally dry conditions for H2, 10% H2-N2 and N2; (b) BCY10 total conductivity as function of hydrogen partial pressure (*p*H2) under nominally dry conditions in the temperature range of 350–600°C [37] (reproduced by permission of The Royal Society of Chemistry).*


#### **Table 1.**

*Comparison of literature studies of total conductivity of BCY10 in nominally dry and wet H2 at 400°C [37] (reproduced by permission of The Royal Society of Chemistry).*

sight, current results of total conductivity in nominally dry H2 are close to those corresponding to the available data in literature for humidified H2 (**Table 1**).

To analyse the contribution of electronic conductivity to this material in nominally dry conditions, the total conductivity was also analysed as a function of hydrogen partial pressure (*p*H2) [37], as shown in **Figure 5b**. A slight increase in total conductivity can be observed towards higher *p*H2 values.

To be able to understand this behaviour, firstly the potential for an electronic component to conductivity must be assessed. In reducing conditions (*e.g.* H2 containing atmospheres), the cerium cations from the B-site of the perovskite structure of BCY10 can reduce from a higher oxidation state, Ce4+, to a lower one, Ce3+, altering the contribution of the concentration of the electronic charge carriers. This phenomenon is well documented in the literature for various cerium-based compositions [37, 38, 44–47], being described as small-polaron electronic conductivity (*i.e.*, a localised, mobile electron, Ce<sup>0</sup> Ce). Due to the high mobility of electronic conductors, such electronic contribution can exceed that of the ionic, under very reducing conditions and high temperatures [37, 45–47]. However, in the case of BCY10, the extent of cerium reduction has been assessed by Loureiro *et al.* [37], who performed coulometric titration measurements to study the potential role of electronic contribution in BaCe0.9Y0.1O3-<sup>δ</sup> in reducing conditions as a function of temperature. This technique has been widely adopted to quantify the changes in the *Analysis of the Electrochemical Transport Properties of Doped Barium Cerate for Proton… DOI: http://dx.doi.org/10.5772/intechopen.93970*

oxygen non-stoichiometry (Δ*δ*), which can be associated with the reduction of Ce4+ to Ce3+, following the equation:

$$2\mathbf{C}\mathbf{e}\_{\mathrm{Ce}}^{\mathrm{x}} + \mathbf{O}\_{\mathrm{O}}^{\mathrm{x}} \Leftrightarrow \mathbf{V}\_{\mathrm{O}}^{\bullet \bullet} + 2\mathbf{C}\mathbf{e}\_{\mathrm{Ce}}^{\prime} + \frac{1}{2}\mathbf{O}\_{2} \tag{6}$$

with the equilibrium constant for reduction reaction given by:

$$\mathbf{K}\_{\rm R} = \frac{\left[\mathbf{V}\_{\rm O}^{\star\star}\right]\left[\mathbf{C}\mathbf{e}\_{\rm Ce}^{\prime}\right]^{2}\mathbf{p}(\mathbf{O}\_{2})^{\frac{1}{2}}}{\left[\mathbf{O}\_{\rm O}^{\mathbf{x}}\right]\left[\mathbf{C}\mathbf{e}\_{\rm Ce}^{\mathbf{x}}\right]^{2}} \tag{7}$$

The results of coulometric titration (**Figure 6**) [37] show considerable variations of Δ*δ* with oxygen partial pressure only at very high temperature, with a lower impact as temperature decreases. Thus, **Figure 6** demonstrates that very extreme reducing conditions and very high temperatures are required to produce appreciable increase in the oxygen-vacancy and electronic concentrations in BCY [47, 48]. These results contrast with those of fluorite-ceria-based materials which usually show high reducibility under milder conditions [46, 49].

Thus, to take the possibility of reduction into account, the methodology applied by Loureiro *et al.* [37] for the determination of reduction equilibrium follows the method reported elsewhere [50], as described below.

The corresponding mass action constant (Eq. (7)) can be combined with the electroneutrality condition:

$$2\left[\mathbf{V\_{O}^{\*\*}}\right] + \left[\mathbf{OH\_{O}^{\*}}\right] \approx \left[\mathbf{Y\_{Ce}^{\prime}}\right] + \left[\mathbf{Ce\_{Ce}^{\prime}}\right] \tag{8}$$

and other mass and lattice position restrictions, on neglecting defect interactions and assuming nearly ideal behaviour, with the following relations between the concentrations of relevant species, stoichiometric changes (Δ*δ*), and fraction of trivalent additive (*x*):

$$\left[\text{Ce}'\_{\text{Ce}}\right] = \frac{\text{Z}}{\text{v}\_{\text{0}}} (2\Delta\delta) \tag{9}$$

#### **Figure 6.**

*Oxygen non-stoichiometry as function of oxygen partial pressure (*p*O2) [37] (reproduced by permission of The Royal Society of Chemistry).*

sight, current results of total conductivity in nominally dry H2 are close to those corresponding to the available data in literature for humidified H2 (**Table 1**).

*Comparison of literature studies of total conductivity of BCY10 in nominally dry and wet H2 at 400°C [37]*

*(a) Temperature dependency of the total conductivity of BCY10 obtained in the temperature range 100–500°C in nominally dry conditions for H2, 10% H2-N2 and N2; (b) BCY10 total conductivity as function of hydrogen partial pressure (*p*H2) under nominally dry conditions in the temperature range of 350–600°C [37]*

3.59 � <sup>10</sup>�<sup>3</sup> �10�<sup>5</sup> atm (dry H2) [37] 2.67 � <sup>10</sup>�<sup>3</sup> �10�<sup>2</sup> atm (wet H2) [14] 1.85 � <sup>10</sup>�<sup>3</sup> [17] 1.96 � <sup>10</sup>�<sup>3</sup> [15] 2.60 � <sup>10</sup>�<sup>3</sup> [16] 8.48 � <sup>10</sup>�<sup>4</sup> [18]

**) pH2O (atm) Reference**

nally dry conditions, the total conductivity was also analysed as a function of hydrogen partial pressure (*p*H2) [37], as shown in **Figure 5b**. A slight increase in

total conductivity can be observed towards higher *p*H2 values.

tivity (*i.e.*, a localised, mobile electron, Ce<sup>0</sup>

*(reproduced by permission of The Royal Society of Chemistry).*

*Analytical Chemistry - Advancement, Perspectives and Applications*

*(reproduced by permission of The Royal Society of Chemistry).*

**Figure 5.**

**Table 1.**

**56**

**Conductivity (S cm**�**<sup>1</sup>**

To analyse the contribution of electronic conductivity to this material in nomi-

To be able to understand this behaviour, firstly the potential for an electronic component to conductivity must be assessed. In reducing conditions (*e.g.* H2 containing atmospheres), the cerium cations from the B-site of the perovskite structure of BCY10 can reduce from a higher oxidation state, Ce4+, to a lower one, Ce3+, altering the contribution of the concentration of the electronic charge carriers. This phenomenon is well documented in the literature for various cerium-based compositions [37, 38, 44–47], being described as small-polaron electronic conduc-

conductors, such electronic contribution can exceed that of the ionic, under very reducing conditions and high temperatures [37, 45–47]. However, in the case of BCY10, the extent of cerium reduction has been assessed by Loureiro *et al.* [37], who performed coulometric titration measurements to study the potential role of electronic contribution in BaCe0.9Y0.1O3-<sup>δ</sup> in reducing conditions as a function of temperature. This technique has been widely adopted to quantify the changes in the

Ce). Due to the high mobility of electronic

*Analytical Chemistry - Advancement, Perspectives and Applications*

$$\left[\mathbf{V\_{O}^{\*\*}}\right] = \frac{\mathbf{Z}}{\mathbf{v\_{O}}} \left(\Delta \mathbf{\delta} + \frac{\mathbf{x}}{2}\right) \tag{10}$$

$$\mathbb{E}\left[\mathbf{C}\mathbf{e}\_{\mathrm{Ce}}^{\mathrm{x}}\right] = \frac{\mathrm{Z}}{\mathbf{v}\_{0}}(\mathbf{1} - \mathbf{x} - 2\Delta\delta) \tag{11}$$

$$\mathbb{E}\left[\mathbf{O}\_{\rm O}^{\rm x}\right] = \frac{\mathbf{Z}}{\mathbf{v}\_{0}} \left(\mathbf{3} - \frac{\mathbf{x}}{2} - \Delta\delta\right) \tag{12}$$

where Z is the number of atoms per unit cell and n0, the unit cell volume. Substitution in Eq. (7) leads to the values of the equilibrium constant for reduction (KR) from the entire range of values of Δ*δ versus p*O2 at a given temperature *T*:

$$\mathbf{K}\_{\rm R}(\mathbf{T}) = \frac{4\Delta\delta^2 \left(\Delta\delta + \frac{\mathbf{x}}{2}\right) \mathbf{p}\_{\rm O2}^{1/2}}{\left(\mathfrak{d} - \frac{\mathbf{x}}{2} - \Delta\delta\right) \left(\mathbf{1} - \mathbf{x} - 2\Delta\delta\right)^2} \tag{13}$$

The following equation was then determined to describe the temperature dependence of *K*R, from the results of oxygen-nonstoichiometry shown in **Figure 6**:

$$\mathbf{K}\_{\rm R}(\rm T) = 4.47 \cdot 10^{14} \exp\left(-7.85 \cdot 10^4/\rm T\right) \text{atm}^{1/2} \tag{14}$$

2 V•• O � � <sup>þ</sup> OH•

with Eqs. (3), (15), (16), *K*<sup>w</sup> can be reformulated as

*K*<sup>w</sup> ¼ exp

*(reproduced by permission of The Royal Society of Chemistry).*

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

OH• O � � <sup>¼</sup> <sup>3</sup>*:K*<sup>0</sup> �

**Figure 7.**

impurities in the feed stream:

**59**

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

*<sup>p</sup>*H2O *<sup>S</sup>* � OH•

*Concentration protonic defects obtained from TG in N2 and from the simulation performed in [37]*

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

9*K*<sup>0</sup> � 6*K*<sup>0</sup>

where *K*<sup>0</sup> = *K*w.*p*H2O and S = [YCe']. Because the formation of protonic defects is usually accompanied by a significant weight increase, the concentration of protonic defects as a function of temperature and water vapour partial pressure is generally measured by thermogravimetric analysis (TG). From **Figure 7**, one can observe an increase in the concentration of protonic species as a function of decreasing temperature, even in nominally dry 10%H2/N2. This factor is most likely related to the intrinsic formation of water vapour under the presence of hydrogen and oxygen

and then, the concentration of protonic defects is given by

H2 gð Þ þ

1 2

reducibility of cerium cations measured by coulombic titration (**Figure 6**).

This result emphasises the existence of protonic conductivity in nominally dry hydrogen-containing atmospheres, as even trace amounts of oxygen can form water vapour, potentially contributing to the hydration of the BCY10 material. Hence, the partial conductivities can be obtained by combining the results from both coulombic titration and TG experiments using a defect chemistry methodology [37].

**Figure 8** shows the partial conductivities of all species (protons, oxide-ions and electrons) obtained at the temperature range (350–600°C) in nominally dry H2 (*p*H2O � <sup>10</sup>�<sup>4</sup> atm). One can observe a dominance of the ionic charge carriers over the electronic carriers in the whole temperature range, corroborating the negligible

4 OH• O � �<sup>2</sup>

O � � � � <sup>6</sup> � *<sup>S</sup>* � OH•

O

� � � � ! (17)

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

*:S* þ *K*<sup>0</sup> *:S*<sup>2</sup> <sup>þ</sup> <sup>24</sup>*<sup>S</sup>* � <sup>4</sup>*S*<sup>2</sup> <sup>p</sup>

� � <sup>¼</sup> <sup>3</sup> (16)

O

*<sup>K</sup>*<sup>0</sup> � <sup>4</sup> (18)

O2 gð Þ \$ H2Oð Þ<sup>g</sup> (19)

with an enthalpy for reduction, Δ*H*<sup>R</sup> = 804.99 kJ mol�<sup>1</sup> . This value is significantly higher than those obtained by other authors for fluorite ceria-based materials (**Table 2**) [46, 49], underscoring the low reducibility of BCY10 in such conditions from intermediate to low temperatures.

On the basis of these results, the potential rehydration of the BCY10 material was then assessed by thermogravimetric experiments [37]. **Figure 7** depicts the concentration of protonic charge carriers as a function of temperature, calculated from the following methodology.

By expressing the equilibrium constant for water incorporation reaction (Eq. (3)) in terms of entropy, Δ*S*w, and enthalpy, Δ*H*w:

$$K\_{\rm w} = \exp\left(\frac{\Delta S\_{\rm w}}{R}\right) . \exp\left(-\frac{\Delta H\_{\rm w}}{RT}\right) \tag{15}$$

where *T* and R have usual meanings. Given Eq. (1) and knowing that the number of oxygen sites per formula unit of barium cerate is restricted to 3, implying the site restriction relationship:


**Table 2.**

*Enthalpy (Δ*H*R) for reduction of different ceria-based based solid solutions materials.*

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

**Figure 7.**

V•• O <sup>¼</sup> <sup>Z</sup> v0

v0

v0

<sup>3</sup> � <sup>x</sup>

where Z is the number of atoms per unit cell and n0, the unit cell volume. Substitution in Eq. (7) leads to the values of the equilibrium constant for reduction (KR) from the entire range of values of Δ*δ versus p*O2 at a given temperature *T*:

The following equation was then determined to describe the temperature dependence of *K*R, from the results of oxygen-nonstoichiometry shown in **Figure 6**:

cantly higher than those obtained by other authors for fluorite ceria-based materials (**Table 2**) [46, 49], underscoring the low reducibility of BCY10 in such conditions

On the basis of these results, the potential rehydration of the BCY10 material was then assessed by thermogravimetric experiments [37]. **Figure 7** depicts the concentration of protonic charge carriers as a function of temperature, calculated

By expressing the equilibrium constant for water incorporation reaction

Δ*S*<sup>w</sup> R 

where *T* and R have usual meanings. Given Eq. (1) and knowing that the number of oxygen sites per formula unit of barium cerate is restricted to 3, implying

BaCe0.9Y0.1O3-*<sup>x</sup>*/2-Δ*<sup>δ</sup>* 805 [37] Ce0.9Gd0.1O2-*<sup>x</sup>*/2-Δ*<sup>δ</sup>* 410-420 [46]

Ce0.8Gd0.2O2-*<sup>x</sup>*/2-Δ*<sup>δ</sup>* 430 [46]

Ce0.9Sm0.1O2-*<sup>x</sup>*/2-Δ*<sup>δ</sup>* 400 [52] Ce0.8Sm0.2O2-*<sup>x</sup>*/2-Δ*<sup>δ</sup>* 385 [52]

*Enthalpy (Δ*H*R) for reduction of different ceria-based based solid solutions materials.*

Ce<sup>x</sup> Ce <sup>¼</sup> <sup>Z</sup>

*Analytical Chemistry - Advancement, Perspectives and Applications*

Ox O <sup>¼</sup> <sup>Z</sup>

KRð Þ¼ T

with an enthalpy for reduction, Δ*H*<sup>R</sup> = 804.99 kJ mol�<sup>1</sup>

(Eq. (3)) in terms of entropy, Δ*S*w, and enthalpy, Δ*H*w:

**Compound δ***h***<sup>r</sup> (kJ mol**�**<sup>1</sup>**

*K*<sup>w</sup> ¼ exp

from intermediate to low temperatures.

from the following methodology.

the site restriction relationship:

**Table 2.**

**58**

<sup>Δ</sup><sup>δ</sup> <sup>þ</sup> <sup>x</sup> 2 

<sup>2</sup> � <sup>Δ</sup><sup>δ</sup> 

> 2 p

KRð Þ¼ <sup>T</sup> <sup>4</sup>*:*<sup>47</sup> � 1014 exp �7*:*<sup>85</sup> � 104*=*<sup>T</sup> atm1*<sup>=</sup>*<sup>2</sup> (14)

*:* exp � <sup>Δ</sup>*H*<sup>w</sup>

R*T* 

**) Reference**

438 [51]

385 [51]

375 [49]

1*=*2 O2

<sup>2</sup> � <sup>Δ</sup><sup>δ</sup> ð Þ <sup>1</sup> � <sup>x</sup> � <sup>2</sup>Δ<sup>δ</sup> <sup>2</sup> (13)

. This value is signifi-

<sup>3</sup> � <sup>x</sup>

<sup>4</sup>*Δ*δ<sup>2</sup> <sup>Δ</sup><sup>δ</sup> <sup>þ</sup> <sup>x</sup>

ð Þ 1 � x � 2Δδ (11)

(10)

(12)

(15)

*Concentration protonic defects obtained from TG in N2 and from the simulation performed in [37] (reproduced by permission of The Royal Society of Chemistry).*

$$2\left[\mathbf{V\_O^{\bullet}}\right] + \left[\mathbf{OH\_O^{\bullet}}\right] + \left[\mathbf{O\_O^{\mathbf{x}}}\right] = \mathbf{3} \tag{16}$$

with Eqs. (3), (15), (16), *K*<sup>w</sup> can be reformulated as

$$K\_{\rm w} = \exp\left(\frac{4\left[\mathbf{OH}\_{\rm O}^{\star}\right]^2}{p\_{\rm H2O}\left(\mathbf{S} - \left[\mathbf{OH}\_{\rm O}^{\star}\right]\right)\left(\mathbf{6} - \mathbf{S} - \left[\mathbf{OH}\_{\rm O}^{\star}\right]\right)}\right) \tag{17}$$

and then, the concentration of protonic defects is given by

$$\mathbb{E}\left[\mathbf{OH}^{\bullet}\_{\mathrm{O}}\right] = \frac{\mathbf{3.}K^{\prime} - \sqrt{\mathbf{9}K^{\prime} - \mathbf{6}K^{\prime}\mathbf{S} + K^{\prime}\mathbf{S}^{2} + 2\mathbf{4S} - 4\mathbf{S}^{2}}}{K^{\prime} - 4} \tag{18}$$

where *K*<sup>0</sup> = *K*w.*p*H2O and S = [YCe']. Because the formation of protonic defects is usually accompanied by a significant weight increase, the concentration of protonic defects as a function of temperature and water vapour partial pressure is generally measured by thermogravimetric analysis (TG). From **Figure 7**, one can observe an increase in the concentration of protonic species as a function of decreasing temperature, even in nominally dry 10%H2/N2. This factor is most likely related to the intrinsic formation of water vapour under the presence of hydrogen and oxygen impurities in the feed stream:

$$\text{H}\_{2(g)} + \frac{1}{2}\text{O}\_{2(g)} \leftrightarrow \text{H}\_2\text{O}\_{(g)}\tag{19}$$

This result emphasises the existence of protonic conductivity in nominally dry hydrogen-containing atmospheres, as even trace amounts of oxygen can form water vapour, potentially contributing to the hydration of the BCY10 material. Hence, the partial conductivities can be obtained by combining the results from both coulombic titration and TG experiments using a defect chemistry methodology [37].

**Figure 8** shows the partial conductivities of all species (protons, oxide-ions and electrons) obtained at the temperature range (350–600°C) in nominally dry H2 (*p*H2O � <sup>10</sup>�<sup>4</sup> atm). One can observe a dominance of the ionic charge carriers over the electronic carriers in the whole temperature range, corroborating the negligible reducibility of cerium cations measured by coulombic titration (**Figure 6**).

1 2

with the following mass action constant

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

p-type electronic conductivity [57, 58] (see Eq. (20)):

parison to the other studied atmospheres.

*Reproduced from [38] with permission from Elsevier.*

**Figure 9.**

**61**

2OH•

O2 gð Þ <sup>þ</sup> <sup>V</sup>••

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

O⟷Ox

*<sup>K</sup>*<sup>O</sup> <sup>≈</sup> <sup>h</sup>• ½ �<sup>2</sup> V•• O *:p* 1*=*2 O2

**Figure 9** shows the total conductivity of BCY10 measured in the temperature range 350–600°C in wet and low humidity O2 and N2. From **Figure 9**, this expected decrease in the concentration of protonic species is corroborated, as in both, N2 and O2, total conductivity is shown to be higher in wet conditions (*p*H2O � <sup>10</sup>�<sup>3</sup> atm) than in low humidity conditions (*p*H2O � <sup>10</sup>�<sup>7</sup> atm). It is also possible to observe that low humidity N2 (*p*H2O � <sup>10</sup>�<sup>7</sup> atm) the total conductivity is lower in the whole measured temperature range in comparison to wet N2 (*p*H2O � <sup>10</sup>�<sup>3</sup> atm), as a result of dehydration of the sample according to Eq. (22). In contrast, in O2, the total conductivity in low humidity and wet conditions are similar, particularly at higher temperatures, a factor that can be explained due to the presence and dominance of

<sup>O</sup> <sup>⇔</sup> H2O <sup>þ</sup> <sup>V</sup>••

In agreement, the presence of p-type electronic conductivity can explain the slightly higher activation energy registered in low humidity O2, 0.49 eV, in com-

*Total conductivity of BCY10 in wet (*p*H2O* � *<sup>10</sup>*�*<sup>3</sup> atm) and low humidity (*p*H2O* � *<sup>10</sup>*�*<sup>7</sup> atm) N2 and O2.*

**Figure 10** illustrates the partial conductivities obtained in wet (*p*H2O � <sup>10</sup>�<sup>3</sup> atm) and low humidity (*p*H2O � <sup>10</sup>�<sup>7</sup> atm) conditions in N2 and O2. **Figure 10a** and **<sup>b</sup>** show that in moderate wet conditions (*p*H2O � <sup>10</sup>�<sup>3</sup> atm) the protonic conductivity is dominating in both atmospheres with activation energies similar to that obtained for the protonic conduction (�0.4–0.5 eV) [16, 17]. In contrast, in low humidity conditions (**Figure 10c** and **d**) a drop on protonic conductivity with increasing temperature is observed, due to predominant oxide-ion conductivity in both

<sup>O</sup> <sup>þ</sup> Ox

<sup>O</sup> <sup>þ</sup> 2h• (20)

<sup>O</sup> (22)

(21)

#### **Figure 8.**

*Total (experimental and calculated) and partial conductivities* vs. *temperature. Data obtained in the temperature range 350–600°C in nominally dry conditions [37] (reproduced by permission of The Royal Society of Chemistry).*

Furthermore, at the low temperature range (350–400°C), the dominance of protonic conductivity is related to the high equilibrium constant for water incorporation in BCY10, allowing a significant hydration even at *p*H2O values as low as <sup>10</sup><sup>4</sup> atm [53], as confirmed by TG (**Figure 7**). This behaviour also explains the slight *p*H2 dependence of conductivity shown in **Figure 5b** that is due, not to electronic behaviour, but to changes in the effective water vapour partial pressure arising from Eq. (19) and subsequent slight increase in ionic conductivity due to a higher level of hydration Eq. (18). In contrast at higher temperatures in the (550– 600°C) range, oxide-ion conductivity starts to become dominant at due to the loss of protons from the structure (**Figure 7**).
