**4. Comparison between reducing and oxidising conditions under low humidity**

As discussed previously, to maximise the protonic conductivity is necessary to maintain a minimum level of humidity in the order of 10<sup>4</sup> –10<sup>5</sup> atm. It is also important to emphasise that, while this level of humidity is intrinsically formed in nominally dry hydrogen-containing atmospheres, in the case of oxidising atmospheres this level must be externally supplied. A comparison of the partial conductivities in all cases is shown in **Figure 11**, for *<sup>p</sup>*H2O <sup>10</sup><sup>4</sup> atm. At temperatures

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

#### **Figure 11.**

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

*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*

As discussed previously, to maximise the protonic conductivity is necessary to

important to emphasise that, while this level of humidity is intrinsically formed in nominally dry hydrogen-containing atmospheres, in the case of oxidising atmospheres this level must be externally supplied. A comparison of the partial conductivities in all cases is shown in **Figure 11**, for *<sup>p</sup>*H2O <sup>10</sup><sup>4</sup> atm. At temperatures

–10<sup>5</sup> atm. It is also

**4. Comparison between reducing and oxidising conditions**

maintain a minimum level of humidity in the order of 10<sup>4</sup>

*Analytical Chemistry - Advancement, Perspectives and Applications*

the oxygen vacancies.

*permission from Elsevier.*

**Figure 10.**

**62**

**under low humidity**

*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 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 (Eq. (20)).

Overall, BCY10 is shown to be a predominant protonic conductor in both reducing and oxidising atmospheres at sufficiently low temperatures ≤500°C, even under relatively low water vapour partial pressures (*p*H2O <sup>10</sup><sup>4</sup> –10<sup>5</sup> atm). Moreover, the level of conductivity measured at 400°C in these conditions is high, *e.g.* <sup>10</sup><sup>3</sup> S cm<sup>1</sup> . The origin of protonic conductivity is due to a high equilibrium 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, due to their much lower values of *K*w.

low temperatures *i.e.* < 400°C, even in nominally dry atmospheres with negligible

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

In the other hand, in oxidising conditions, the same behaviour can only be

low temperatures ≤500°C. At higher temperatures, at this low humidity, the onset of hole conductivity can be noted at higher oxygen partial pressures due partial

proton conductivity in both reducing and oxidising atmospheres at low temperatures ≤500°C, are highly interesting as they highlight the possibility of using this composition in applications where low humidity levels and temperatures are required, such as the suggested de-hydrogenation/hydrogenation chemical reac-

The authors acknowledge Fundação para a Ciência e Tecnologia (FCT) for the PhD grants – PD/BDE/142837/2018, SFRH/BD/130218/2017, and PD/BDE/114353/ 2016. The authors also acknowledge the projects UID/EMS/00481/2019-FCT and CENTRO-01-0145-FEDER-022083 - Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement,

Laura I.V. Holz, Vanessa C.D. Graça, Francisco J.A. Loureiro\* and Duncan P. Fagg Mechanical Engineering Department, Centre for Mechanical Technology and

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

The present discussion shows the importance of controlling the humidity levels in order to maximise the protonic conductivity of BCY under operation. The very

–10<sup>5</sup> atm) at

–10<sup>5</sup> atm), to ensure predominant

obtained by externally supplying humidity in the range (*p*H2O <sup>10</sup><sup>4</sup>

oxide-ion/electronic influence.

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

low levels of humidity required (*p*H2O <sup>10</sup><sup>4</sup>

tions, while maintaining its stability against decomposition.

through the European Regional Development Fund (ERDF).

Automation, University of Aveiro, Aveiro, Portugal

provided the original work is properly cited.

\*Address all correspondence to: francisco.loureiro@ua.pt

material dehydration.

**Acknowledgements**

**Author details**

**65**

**Figure 12.** *Equilibrium constant for hydration of several perovskite proton conductors. Adapted from [9].*


#### **Table 3.**

*Examples of dehydrogenation/hydrogenation reactions that can occur at very low humidity conditions.*

This is a very exciting result since it opens a wide range of possibilities for using the BCY material, in particular, in different applications that involve very low humidity levels and low temperatures of operation. The most well-known is that of ammonia electrochemical synthesis [39–41], although many other processes concerning hydrogenation and de-hydrogenation reactions can also be considered that agree with these operating conditions (**Table 3**).

#### **5. Conclusions**

The current chapter highlights that the transport properties of BaCe0.9Y0.1O3-<sup>δ</sup> (BCY10) in very low humidity conditions are dependent on the nature of the surrounding atmosphere and on the temperature, being significantly different in reducing and oxidising conditions and at high and low temperatures. In reducing conditions, BCY10 shows a very high protonic conductivity (*e.g.* � <sup>10</sup>�<sup>3</sup> S cm�<sup>1</sup> ) at *Analysis of the Electrochemical Transport Properties of Doped Barium Cerate for Proton… DOI: http://dx.doi.org/10.5772/intechopen.93970*

low temperatures *i.e.* < 400°C, even in nominally dry atmospheres with negligible oxide-ion/electronic influence.

In the other hand, in oxidising conditions, the same behaviour can only be obtained by externally supplying humidity in the range (*p*H2O <sup>10</sup><sup>4</sup> –10<sup>5</sup> atm) at low temperatures ≤500°C. At higher temperatures, at this low humidity, the onset of hole conductivity can be noted at higher oxygen partial pressures due partial material dehydration.

The present discussion shows the importance of controlling the humidity levels in order to maximise the protonic conductivity of BCY under operation. The very low levels of humidity required (*p*H2O <sup>10</sup><sup>4</sup> –10<sup>5</sup> atm), to ensure predominant proton conductivity in both reducing and oxidising atmospheres at low temperatures ≤500°C, are highly interesting as they highlight the possibility of using this composition in applications where low humidity levels and temperatures are required, such as the suggested de-hydrogenation/hydrogenation chemical reactions, while maintaining its stability against decomposition.
