Protonic defects in oxides

From the formation reaction of protonic defects in oxides (eq 23), it is evident that protonic defects coexist with oxide ion vacancies, where the ratio of their concentrations is dependent on temperature and water partial pressure. The formation of protonic defects actually requires the uptake of water from the environment and the transport of water within the oxide lattice. Of course, water does not diffuse as such, but rather, as a result of the ambipolar diffusion of protonic defects (OH and oxide ion vacancies (V ). Assuming ideal behavior of the involved defects (an activity coefficient of unity) the chemical (Tick s) diffusion coefficient of water is... 

The introduction of protonic defects in oxides in hydrogen-containing ambients has been reported frequently, and has recently been reviewed by Colomban and Novak and Strelkov etal. ... 

Exactly the opposite occurs, namely the conversion of an ex situ parameter to an in situ one, if foreign components become sufficiently mobile. The corresponding incorporation reaction then becomes reversible. Under such conditions it is natiurally better to speak of solubility equihbria. Important examples are segregation equilibria of impurities at very high temperatures, another refers to the incorporation of protonic defects in oxides by the dissolution of H2O. Materials interesting in this respect are CaO-doped Z1O2 [196] or acceptor-doped perovskites [197], such as the Fe-doped SrTiOs discussed above. (As before we regard the acceptor dopant... 

Simple Cubic Perovskites. Since the work of Stotz and Wagner in 1966, the existence of protonic defects in wide-band-gap oxides at high... 

Figure 5. Time-averaged structure of a protonic defect in perovskite-type oxides (cubic case), showing the eight orientations of the centrai hydroxide ion stabiiized by a hydrogen-bond interaction with the eight next-nearest oxygen neighbors. ...

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

In the 1960s, all the tools needed to treat hydrogen defects in oxides [41-43] were fundamentally developed by Wagner and collaborators. However, up to the end of the 1970s, real developments in the field did not take place. During the last years of this decade, some significant studies were carried out [44-48], After that, it was realized that the introduction of defects in some perovskite structures determine the protonic conductivity of these materials. In this regard, Iwahara and... 

For more simple systems, the predictive character of ab initio and quantum MD simulations has already made possible the directed identification of improved proton- conducting materials. A prominent example is yttria-doped BaZr03, an oxide with the perovskite structure forming highly mobile protonic defects in the presence of water vapor [22,23]. Quantum MD simulations [24,25] have revealed details of the proton-conduction mechanism, including the critical interactions, and electronic structure calculation have helped identify the best possible dopant [26]. 

Hydrogen may in principle dissolve in the form of different species as neutral atoms (H), hydride ions (H ), and protons (H+). In oxides the commonly observed dissolved hydrogen species are the protons, often termed interstitial protons, H, but in reality always associated with oxygen ions as hydroxide groups. If that oxygen sits on a normal lattice site we thus get substitutional hydroxide defects, OHq. ... 

Sintering governed by lattice diffusion will be dependent upon the concentration of point defects in oxides. Accordingly, sintering rates of an oxide can be optimised by close control of impurities or dopants and the ambient partial pressures of oxygen and of water vapour in cases where proton defects affect the defect structure of the oxide. 

We have in the present version of the treatment of electrochemical transport omitted many cases. These include a more full coverage of proton transport in oxides and transport by other hydrogen species and other foreign species. Moreover, we have left out the case of solid-solid reactions. Also, creep and sintering were given more phenomenological than defect-chemical treatments. 

All in all, proton conductivity in oxides is a matter of compromise in composition and temperature between high concentration of protons (favorable hydration kinetics), high proton mobility, and chemical robustness. In this contribution, we concentrate on a description of protonic defects and their thermodynamics in various perovskite-related oxides, give an overview of the resulting proton... 

J.H. Yu, J.-S. Lee, J. Maier, Formation of protonic defects in perovskite-type oxides with redox-active acceptors case study on Fe-doped SrTiOs. Phys Chem Chem Phys, 7, 3560-3564 (2005)... 

In the first part of this short chapter, it is discussed where protons find energetically favored sites in oxides with the perovskite structure, before the mechanisms of proton diffusion via these sites are described in detail. While this discussion is restricted to structure and dynamics of protonic defects in ideal (cubic) perovskites, the following section addresses complications such as symmetry reduction and the effect of the presence of dopants, before, finally, a few implications for the development of proton-conducting electrolytes for fuel cell applications are discussed. 

As shown by DFTB and CPMD simulations, the principal features of the transport mechanism are rotational diffusion of the protonic defect and proton transfer toward a neighboring oxide ion. That is, only the proton shows long-range diffusion, whereas the oxygens reside in their crystallographic positions. Both experiments " " and quantum-MD simulations, have revealed that rotational diffu-... 

This is the change in the active masses volume (active mass breathing). Oxidized and reduced forms do not usually have the same volumetric density. This may cause a defect in electronic percolation and loss of active mass. The lower the difference in density, the higher the cyclability. Usually, proton and lithium intercalation compounds have low volumetric variation on cycling. 

When interest was rekindled in the 1980s, the method commonly used to synthesise PAni was the oxidative coupling of aniline with ammonium persulphate in aqueous HC1. This produces partially protonated EM salt which can be deprotonated with ammonia to form EM base. The molecular mass can then be determined in dilute LiCl solution in N-methylpyrrolidone (NMP) typical values correspond to 160 of the repeat units shown in Fig. 9.6. The reaction shows the characteristics of a living polymerisation, since the molecular mass increases if further monomer and oxidant are added to the reaction mixture, when the chain length increases to a maximum of about 240 repeat units. NMR studies of the LM form show about 5% defects in the polymer backbones. When the reaction is performed at 248 K the polymer is produced in nearly 100% yield, with higher molecular mass, typically over 500 repeat units, and greatly reduced defect content (Adams et ai, 1996). Other oxidising... 

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