**3. Electrochemical properties of BCY10 in low humidity oxidising conditions**

The transport numbers of BCY10 in oxidising atmospheres were firstly studied by Oishi *et al.* [54] and by Grimaud *et al.* [55]. Later, Lim *et al.* [56] determined the concentration of charge carriers in BCY10 by thermogravimetric analysis (TGA) under two different humidity conditions (dry and wet, *<sup>p</sup>*H2O <sup>10</sup><sup>5</sup> and 10<sup>3</sup> atm, respectively). More recently, Loureiro *et al.* [38] focused on the determination of the transport properties of this composition at temperatures below 600°C and under very low humidity levels (*p*H2O ≤ 10<sup>4</sup> atm).

In oxidising conditions, the absence of hydrogen species, shifts the water formation reaction, Eq. (19), away from the water product, leading to a lower intrinsic water vapour partial pressure that can, in turn, decrease the protonic transport number [38]. Therefore, at the intermediate temperature range, 350–600°C, it is necessary to externally add humidity to guarantee a sufficient level of protonic conductivity. Moreover, BCY10 is known to possess p-type electronic conductivity in oxidising atmospheres, which can importantly impact the total conductivity in these conditions [38], as expressed by

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

$$\frac{1}{2}\text{O}\_{2(g)} + \text{V}\_{\text{O}}^{\bullet\bullet} \longleftrightarrow \text{O}\_{\text{O}}^{\bullet} + 2\text{h}^{\bullet} \tag{20}$$

with the following mass action constant

$$K\_{\rm O} \approx \frac{\left[\rm h^{\star}\right]^{2}}{\left[\rm V\_{\rm O}^{\star}\right] \cdot p\_{\rm O2}^{1/2}}\tag{21}$$

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

$$2\mathbf{O} \mathbf{H}\_{\bullet}^{\star} \Leftrightarrow \mathbf{H}\_{2}\mathbf{O} + \mathbf{V}\_{\bullet}^{\star\star} + \mathbf{O}\_{\bullet}^{\rm x} \tag{22}$$

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 comparison to the other studied atmospheres.

**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

#### **Figure 9.**

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

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

*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*

*Analytical Chemistry - Advancement, Perspectives and Applications*

**3. Electrochemical properties of BCY10 in low humidity oxidising**

The transport numbers of BCY10 in oxidising atmospheres were firstly studied by Oishi *et al.* [54] and by Grimaud *et al.* [55]. Later, Lim *et al.* [56] determined the concentration of charge carriers in BCY10 by thermogravimetric analysis (TGA) under two different humidity conditions (dry and wet, *<sup>p</sup>*H2O <sup>10</sup><sup>5</sup> and 10<sup>3</sup> atm, respectively). More recently, Loureiro *et al.* [38] focused on the determination of the transport properties of this composition at temperatures below 600°C and

In oxidising conditions, the absence of hydrogen species, shifts the water formation reaction, Eq. (19), away from the water product, leading to a lower intrinsic water vapour partial pressure that can, in turn, decrease the protonic transport number [38]. Therefore, at the intermediate temperature range, 350–600°C, it is necessary to externally add humidity to guarantee a sufficient level of protonic conductivity. Moreover, BCY10 is known to possess p-type electronic conductivity in oxidising atmospheres, which can importantly impact the total conductivity in

of protons from the structure (**Figure 7**).

under very low humidity levels (*p*H2O ≤ 10<sup>4</sup> atm).

these conditions [38], as expressed by

**conditions**

**60**

**Figure 8.**

*Society of Chemistry).*

**Figure 10.**

*Partial conductivities obtained in wet and low humidity conditions in (a) and (b) N2, and (c) and (d) O2. The activation energy values, Ea, were calculated in the temperature range 350–500°C. Reproduced from [38] with permission from Elsevier.*

below 450°C, the total conductivity is dominated by protonic conductivity, with the oxide-ion conductivity taking a negligible role. In contrast, at higher temperatures (*T* > 450°C), the oxide- ion conductivity dominates the total conductivity with a simultaneous decrease of protonic conductivity. With respect to the electronic conductivity, this term increases as *p*O2 increases, being only relevant in oxidising conditions and/or high temperatures. This can be explained due to the creation of electronic holes, which become more relevant with increasing *p*O2 and temperature

*Temperature dependence of partial conductivities in at* <sup>p</sup>*H2O <sup>10</sup><sup>4</sup> atm: (a) H2, (b) N2 and (c) O2. Activation energy values, Ea, calculated in the temperature range 350-500°C. Reproduced from [38] with*

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

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

Overall, BCY10 is shown to be a predominant protonic conductor in both reducing and oxidising atmospheres at sufficiently low temperatures ≤500°C, even

Moreover, the level of conductivity measured at 400°C in these conditions is high,

constant for water absorption that allows this material to offer high bulk protonic conductivity at intermediate temperatures in these very low humidity conditions. From **Figure 12**, one can immediately envisage that this is a particular behaviour of BCY10 that cannot be obtained in other competing proton-conducting perovskites,

. The origin of protonic conductivity is due to a high equilibrium

–10<sup>5</sup> atm).

under relatively low water vapour partial pressures (*p*H2O <sup>10</sup><sup>4</sup>

(Eq. (20)).

**63**

**Figure 11.**

*permission from Elsevier.*

*e.g.* <sup>10</sup><sup>3</sup> S cm<sup>1</sup>

due to their much lower values of *K*w.

atmospheres. In the case of hole conductivity, the activation energies obtained were found to be lower at low humidity conditions (0.61–1.03 eV,*T* = 350–500°C) in comparison with those obtained in wet conditions (1.29–1.75 eV,*T* = 350–500°C). This can be explained by the creation of electronic defects (Eq. (20)), upon filling the oxygen vacancies.
