Proton transport in oxides can occur through the material's bulk when a sufficient bulk proton concentration is present, for example by dissociative hydration of oxygen vacancies (e.g. in well-established proton conductors such as Y-doped BaZrO₃). On the other hand, surface proton conductivity in adsorbed water layers is expected for a much larger range of materials, in particular at temperatures below ≈300°C. Water-oxide interaction and surface protonic conductivity have been investigated on several ceramic materials, see e.g. [1], [2]. Similar effects are anticipated also for mixed conducting oxides, and may be harnessed for porous electrodes of low temperature protonic ceramic fuel cells. Protonic carriers are expected to generate from water dissociation, followed by transport along the surface water layers and participation in the electrode reactions.

While preceding work focused on surface protonics of electronic insulators, the present study employs the Mg(Al_1-xFe_x)₂O₄ spinel solid solution (without oxygen vacancies) and praseodymium doped ceria Ce_1-xPr_xO_2-δ (with oxygen vacancies, but nevertheless negligible bulk hydration) as model materials to investigate the interplay of water adsorption, surface protonics and bulk electronic transport. Samples were synthesized by nitrate pyrolysis, and compacted by spark plasma sintering to pellets with relative densities of 50-90%. The water uptake is measured by thermogravimetry at different temperatures and pH₂O. Regimes of chemisorption (extending up to ≈600°C) and physisorption (significant below ≈150°C) can be identified, and modelled by mass action laws. Both Fe³⁺ and Pr⁴⁺ substitution affect the surface properties of the synthesized materials, which changes the enthalpy and the concentration of active sites for water molecular adsorption and dissociation.

Conductivity is investigated by impedance spectroscopy in an extended temperature and pH₂O range (up to 400 mbar). It depends sensitively on sample morphology (surface area and porosity). In humid gas, all porous samples show a characteristic upturn of conductivity below ≈250°C, which is attributed to surface proton conduction. This can increase the conductivity by more than 8 orders of magnitude relative to dry conditions. The increase of conductivity with the amount of adsorbed water is very steep. This emphasizes the important role of physisorbed water layers atop the chemisorbed layer, which facilitate water dissociation as well as proton mobility. For Mg(Al_1-xFe_x)₂O₄ as well as for Ce_1-xPr_xO_2-δ the surface proton conductivity is almost independent of the concentration of redox-active Fe or Pr cations and thus the bulk electronic conductivity. This is in strong contrast to the bulk proton transport of mixed conducting perovskites such as Ba(Zr,Y,Fe)O_3-d, for which the proton concentration and conductivity decreases significantly already for small amounts of redox-active iron ions [3].