(Invited) Surface Reactivity and Ionic Transport of Mixed Conductors in Wet Atmospheres
Authors: Téllez Helena , Druce John , Ishihara Tatsumi , Kilner John 
Published: February 2020

Proton ceramic fuel cells (PCFC) for electrochemical energy conversion target operation in a temperature range between 300-600 °C, reducing the degradation rates and the materials cost in comparison with high-temperature oxide ion conducting fuel cells (SOFCs). In both cases (SOFC and PCFC), one of the main limitations for cell performance is the kinetics of the cathode reaction which takes place at the cathode-electrolyte interface when the cathode materials shows only electronic conductivity (i.e. at the triple phase boundary (TPB) where cathode surface, electrolyte and gas phase are in contact). Conversely, in mixed ionic-electronic conducting cathodes (MIECs), the oxygen reduction reaction sites are extended to the whole effective surface of the air electrode, providing high electrocatalytic activity even at the intermediate and low temperature range.

Recently, MIEC cathode materials used for SOFC have been proposed as promising candidates for air electrodes on proton conducting electrolytes for PCFC. Among these oxygen-conducting materials (O-MIECs), single perovskite and double perovskites have been suggested (e.g. Ba0.5Sr0.5Co0.8Fe0.2O3-δ and NdBaCo2O5+δ, respectively) (1, 2). The assumption for the use of these O-MIECs in PCFCs is that sufficient proton conductivity is present, i.e. there are three mobile carriers through the bulk of the cathode (electrons, protons and oxygen ions). Although these materials work effectively on proton conducting electrolytes, there are few direct studies on the specific ionic carriers involved in the bulk transport.

On the other hand, the chemical stability of the cathode material can be compromised, especially in steam or CO2-containing atmospheres (3). Many of the suggested cathode materials are doped with aliovalent cations in order to increase their MIEC character. Nevertheless, even at the low working temperatures of PCFC devices, the segregation of the substitutional cations lead to a specific surface termination. For instance, Ba-containing double perovskites, such as GdBaCo2O5+δ, shows fast segregation of the Ba cations at 400°C, leading to a Ba-rich outer surface after annealing in an oxygen atmosphere (e.g. 15 minutes) (4, 5). The dynamics of the segregation of the divalent cations are likely to be affected by the annealing atmosphere, as well as the surface reactivity of the segregated species in the presence of CO2 and H2O.

In this work, the surface composition and the ionic transport properties of MIEC air electrodes in wet atmospheres will be discussed. The relative importance of the oxygen and proton conductivities in MIEC materials is directly measured by a combination of stable isotope exchange experiments (18O and 2D2O tracers) and Time-of-Flight Secondary Ion Mass Spectrometry depth profiling (ToF-SIMS), while the surface composition of the material is studied by Low-Energy Ion Scattering (LEIS).

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