Proton conductivity grain boundaries

Haile, S. M., Staneff, G., and Ryu, K. H. Non-stoichiometry, Grain Boundary Transport and Chemical Stability of Proton Conducting Perovskites, Journal of Materials Science, 36, 1149 (2001). 

Surfaces of crystals and the grain boundaries may have disordered regions where proton transport is facilitated. Adsorbed water or electrolyte solution trapped between the grains will also increase the conductivity and give rise to false conclusions concerning the solid-state nature of conductivity. Surface or liquidlike conductivity has a low activation energy ( a 0.1-0.4eV), approaching the of aqueous acidic... 

There is no standard procedure for the measurement of proton conductivity and experimental techniques as well as structural and chemical considerations have to be adapted to the material under investigation. There are some peculiar features common to most solid proton conductors which make it difficult to identify the proton as the charge carrier, to determine its accurate conductance (self diffusion) and to separate bulk proton transport from artefacts (such as transport along surfaces, grain boundaries, domain walls, dislocations, second phases). How to cope with these will be discussed in the following section. 

Fig. 26.1. Different contributions from the protonic conductivity of a powder sample bulk conductivity (1), grain boundary conductivity in series (2) and parallel (3) to the grains. A simple equivalent circuit including simulated impedance spectra for two different cases is shown. The bulk resistance (Rj) can be extracted only from spectrum (b).

Recently, there have been several attempts to increase the stability of BaCeOs-based proton conductors by forming solid solutions with BaZrOs, which is thermodynamically much more stable [208], For Gd- and Nd-doped materials, a decrease of the proton conductivity with increasing Zr content is generally observed. This is, however, only true for the bulk conductivity of Ce-rich compositions [209, 210[. For Ce-poor compositions, however, the observed decrease of the total conductivity [211 [ most likely reflects the behavior of grain boundaries rather than the bulk of the solid solutions, as discussed by Kreuer [212[ for BaZrOs-based oxides. 

This material conducts protons under a wet environment, as mentioned above, and is known to have very high grain-boundary resistivity. The displacement of atoms in the grain boundary was previously attributed as the cause because. 

Shirpour M, Lin CT, Merkle R, Maier J (2012) Nonlinear electrical grain boundary properties in proton conducting Y-BaZrOa supporting the space charge depletion model. Phys Chem Chem Phys 14 730-740... 

For perovskite-type oxides, the proton solubility decreases, but the stability to CO2 increases as the basicity of B-site ions decreases in the order of Ce Zr Sn Nb Ti [156]. The best compromise for high proton conductivity and chemical stability is found for Y-doped BaZr03, but unfortunately, this material exhibits large grain boundary resistance, and it stiU reacts slightly with CO2 under certain conditions [157]. Chemical stability against CO2 has been demonstrated recently with BaCeo,3Zro,5Yo.203 8 by Fabbri et al. [158]. 

In order to address the protonic conductivity of the catalyst layer in conjunction with the support effect, hydrous ruthenium oxide was evaluated as support for Pt [302]. Ru02 xH20 is a mixed electronic-protonic conductor, the latter being generated by the water adsorbed in the grain boundary regions. 

C. Kjolseth, H. Fjeld, P.I. Dahl, C. Estournes, R. Haugsrud, T. Norby, Space Charge theory applied to the grain boundary impedance of proton conducting BaZro.9Yo.i03. under publication. 

H. Fjeld, R. Haugsrud, A.E. Gunnaes, T. Norby, Proton and oxide ion conductivity in the grain interior and grain boundaries of Ca-doped Er2Ti207 with Si impurities. Solid State Ionics, 179(33-34), 1849-1853(2008)... 

High bulk proton conductivity, high stability, and a wide ionic domain [47] therefore make Y-doped BaZrOs an interesting parent compound for the development of proton-conducting electrolytes for SOFC applications. Unfortunately, the unfavorable brittleness, the grain boundary impedance, and the increasing phase instability with increasing Y-dopant level remain problems to be solved. The addition of small amounts of BaCeOs or a compromise in the choice of the kind of dopant may help to reduce these problems. 

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