A. Fuchs, K.D. Kreuer, W. M¨unch and J. Maier
The generation of protonic defects in acceptor-doped oxides and the occurrence of proton conductivity are experimentally well established phenomena. Both are of high significance for a fundamental understanding of proton transport in solids as well as for electrochemical applications. We have shown over the years that the proton transport in these oxides is due to a phonon assisted proton hopping rather than to (OH ) hopping or proton tunneling.
It is generally believed that high stability and high proton conductivity cannot be achieved simultaneously, and hence application of such proton conducting oxides in fuel cells is not promising. However, we will outline that this pessimistic view ignores substantial entropy effects which stabilize protonic defects and it is indeed possible to combine high conductivity with sufficient thermodynamic stability. Consequently we propose that Y-doped BaZrO3is a promising candidate. Our treatment relies on a comprehensive study of the formation and transport of protonic defects by experimental and simulation techniques.
Generation of protonic defects
The major reaction leading to the generation of protonic defects in oxides is the dissociative dissolution of water involving oxygen vacancies (VO) according to:
H2O+VO +OxO2OHO: (9)
Two internal (OH) groups (OHO) are formed in this reaction, and, therefore, correlations are expected between the enthalpy of this exothermic reaction and the basicity of the cor-responding oxides. Such trends are indeed observed for several systems. In addition the thermodynamic data we obtained from the hydration isobars of a variety of cubic, acceptor-doped perovskite-type oxides reveal an unexpectedly high influence of the reaction entropy on the thermodynamic stability of such defects.
The isobars shown in Fig. 26 demonstrate that for BaCeO3the increase in the concentration of Y-acceptor dopants thermodynamically destabilizes protonic defects, although the heat of hydration becomes more negative. Even more surprising is the high stability of protonic defects in significantly less basic Y-doped BaZrO3. The hydration enthalpy is about half of that of BaCeO3, but the significantly less negative hydration entropy stabilizes protonic defects to quite high temperatures.
Proton conduction mechanism and transport rates
The long range diffusion of protonic defects in oxides involves proton transfer reactions to neighboring oxygens and subsequent reorientation. The latter is brought about by rapid rotational diffusion, which has been evidenced by quasi-elastic neutron scattering (QNS) and muon spin resonance (-SR) experiments and quantum molecular dynamics (QMD) simulations. On the other hand, strong hydrogen bonding between the protonic defect and the oxygen neighbors, as indicated by the IR spectra, suggests a stronger orientation con-finement. A detailed analysis of QMD simulation data, reveals some unexpected features of the hydrogen bond in this unusual environment. The hydrogen bonds are indeed found to be very strong, but the free energy decrease of the system due to the hydrogen bonding is almost compensated by the effect of the lattice distortion (the OH/O separation has to be reduced significantly to form a hydrogen bond) over a wide range of OH/O separations
d O / O / p m the vicinity of a protonic de-fect in BaZrO3as obtained from a quantum molecular dynamics simulation. Top: The free en-ergy of the system as a function of the hydrogen bond separation (OH:::O) is extremely flat as a result of the balance of the con-tributions from hydrogen bond pro-tonic defect and one of the eight oxygen neighbors.
The corresponding free energy curves of the system as a function of the length of the tran-sient OH/O bond shows a very shallow minimum. In other words the lattice is ‘softened’
in the vicinity of the protonic defect, and the resulting ‘liquid-like’ dynamics corresponds to high defect entropies and produce configurations, which favor defect reorientation, and configurations which are favorable for proton transfer reactions (Fig. 27).
( 1 0 0 0 / T ) / K - 1 wa-ter partial pressure of 23 hPa.
The oxide ion conductivity of a 9% Y-doped zirconia (YSZ) is shown for comparison.
The situation is found to be particularly advantageous for the amphoteric, cubic perovskite-type Y-doped BaZrO3 (Y : BaZrO3). For this system, not only the highest mobility of protonic defects is measured, but due to the high defect concentration (see above) also the highest proton conductivity in such oxides is observed (Fig. 28).
High proton conductivity and phase instability
Proton conducting oxides with high conductivities contain earth alkaline metals and are generally considered to degrade in CO2 containing atmospheres (as in high temperature fuel cells) to form carbonates. Their instability versus acidic gases, such as CO2and SO3, was especially thought to be the price paid for a sufficiently large concentration of protonic charge carriers.
The observation that high ground state entropies of protonic defects which thus stabilize compounds also stabilize the charge carriers and can even promote their mobility in ‘am-photeric’ oxides, is hence of great significance for the application of such compounds in electrochemical cells, such as fuel cells and electrochemical sensors.
These considerations point toward BaZrO3-based compounds as most promising: They are thermodynamically stable at the CO2-levels of air at temperatures above about 300ÆC. We could show that up to about 700ÆC, the protonic conductivity of Y-doped BaZrO3 is even higher than that of Y-stabilized zirconia (YSZ, see also Fig. 28), the standard electrolyte material for many electrochemical applications.