Triple-conducting oxides (TCOs) enable simultaneous transport of three charge carriers (h⁺/O^2-/H⁺), and as a result are highly effective as positive electrode materials in a variety of electrochemical devices. Among prominent TCOs, the perovskite BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY4411) has garnered heightened interest due to its convenient synthesis and stability. Recent studies emphasize that the electronic/ionic transport behavior and electrocatalytic activity of this material can be further tailored through the Co/Fe composition. The Co and Fe doping not only modulates the ionic and electronic conduction behavior, but also enhances the catalytic activity for the oxygen reduction and oxygen evolution reactions (ORR/OER).

In a previous study, we investigated composition-dependent trends in the oxygen ion kinetics in the BCFZY system by adjusting the Co/Fe ratio [1]. We found that high Co content increases both the bulk oxygen ion diffusivity and the oxygen surface exchange kinetics. However, the effect of the Co/Fe composition on bulk proton diffusion and surface exchange remains unresolved. Although the proton kinetic behavior in BCFZY4411 has been partially examined, studies have been limited to electrical conductivity relaxation (ECR) investigations [2-4], which cannot unambiguously isolate the proton dynamics due to the presence of multiple charge carriers. Therefore, limiting assumptions and complex analyses are generally required to decouple the dynamics of oxygen ions and protons when using the ECR technique.

Here, we combine a series of alternative characterization techniques, including isotope-labeled time-of-flight secondary ion mass spectrometry (TOF-SIMS), isotope-contrast powder neutron diffraction (ND), and solid-state nuclear magnetic resonance (SSNMR) to untangle the proton local environments and dynamics in the BCFZY system. First, the isotope exchange technique was used to determine tracer proton bulk and surface kinetic properties by selectively exchanging isotopic species (H₂O/D₂O) in dense BCFZY pellet samples across a range of temperatures. ToF-SIMS was used to collect data on OH^-/OD^- species, and the isotope diffusion profiles were fitted by the diffusion solution to obtain proton tracer diffusivities and surface exchange coefficients as a function of BCFZY composition and temperature. Secondly, temperature-resolved ND experiments coupled with pair distribution function (PDF) analysis was used to analyze the crystallographic features of dried, hydrated, and deuterated BCFZY samples. Proton locations and concentrations in the BCFZY structure were established using Fourier difference maps, and activation energies for proton migration in the local structure were estimated based on bond valence sum analysis. Finally, SSNMR was employed to quantify the proton content in the bulk and surface regions of hydrated BCFZY samples. While the bulk proton concentration decreased with increasing Co content, the surface proton content showed the opposite trend. Wide-line analysis was utilized to study proton mobility, and a higher Co-content exhibited lower proton mobility. This study provides a comprehensive insight into proton kinetics in the BCFZY system through combined characterization approaches and can guide the understanding and design of other Co and Fe-containing TCOs.