ABSTRACT
The proton species is considered peculiar, thanks to its very small size that lies between the electron and the smallest primary Li+ion. Consequently, the physics and chemistry of protons-and of materials containing protons-are unique and are commonly named
“protonics” for being present between the electronics and ionics domain frontiers [1]. At present, proton-conducting polymer-based technology benefits a rather mature establishment of their basic transport and diffusion properties through the organic matrix via the proton-hopping “Grotthuss” mechanism using proton-hosting mobile species (water, heterocycles, etc.) [2]. On the contrary, the wide panel of proton-conducting ceramic materials merged since 1980s suffers to some more complex physical and chemical behaviors against polymer-based proton-conducting materials far from being totally understood. In particular, significant parasitic transport of other species, physicochemical issues such as material incompatibility, low tolerance with respect to variations of the operating conditions, and fuel efficiencies are still significantly lower than theoretical. So, strategies for the development of novel materials and devices containing protons require the identification of such species, their nature, and their location in the hosting framework, the influence of partial water pressure, the role of defects and surface phenomena, and the measurement of their short-and long-range dynamics/kinetics. In such a context, the first part of the chapter deals with the description of the fundamental formation and transport principles of proton elements through the solid oxide matrix at high temperature. In particular, the influence of thermodynamic parameters for the reactions determining how the concentration and mobility of proton elements depends on temperature and atmospheric conditions will be discussed and the understanding thereof is of outmost importance. A second section will point out the main electrochemical, physical, and structural tools commonly used to characterize it. 1.1 Thermodynamics of Hydration
It is well known that oxides, phosphates, borates, and silicates may conduct protons at intermediate and high temperatures, typically
above 300°C. But, protons are not structural elements in these compounds and are associated with oxide ions as hydroxide defects, with one positive effective charge denoted as OH∑O in Kröger-Vink notation. It is generally recognized that most proton-conducting oxides are oxygen deficient, with oxygen vacancies accommodated in the structure in the dry state. These oxygen vacancies may be intrinsically present as part of a stoichiometric defect pair, for example, Schottky defects, or formed by defect reactions in equilibrium with the surrounding atmosphere. The chemical reactions implied can be represented as O O (g)O Ox v e= + +ii 2 1 2 2¢ (1.1)and O h O (g)O O 2x v+ = +2 1 2i ii (1.2) under reducing and oxidizing conditions, respectively. The two reactions represent reduction and oxidation, and it is required that at least one of the cations at hand is multivalent. Oxides are reduced also when protons dissolve from hydrogen, being charge-compensated by electrons: H (g) 2O 2OH 2 2 O O /+ = +x ei (1.3) where the equilibrium constant is expressed as
[OH ][O ]O 2 2H O 22K np x= i (1.4) There are not many examples in the literature where this reaction has been proven to be of any importance, for example, ZnO and TiO2 [4]. So, when protons are one of the majority point defects, they are more commonly introduced by interaction between oxygen vacancies and water vapor according to H O (g) O 2OH 2 O O O+ + =v xii i (1.5) and the equilibrium constant expressed as
K S R
H RT v px
= - =exp exp [OH ][ ][O ]O 2O O H O2D D0 0 iii (1.6) where ΔS0 and ΔH0 represent the standard molar hydration entropy and enthalpy, respectively. These expressions are central when
addressing proton dissolution, or water uptake in oxides. The reaction is often referred to in the literature as the hydration reaction and its thermodynamics, hydration thermodynamics. Protonic defects dissolved in the oxides may also be charge-compensated by formation of oxygen interstitials or metal vacancies, which again requires oxidation of the material. The concentration of point defects in oxides can generally be altered by changing the cation ratio of the constituents (e.g., Ba3Ca1+xNb2-xO3-δ) or extrinsically by the addition of foreign aliova-lent cations (e.g., Y3+-doped BaZrO3). When the cationic substituents are effectively negative and, as such act as acceptors, they must be charge-compensated either by a reduction in the number of other negative defects or by the formation of positive defects, for example, electron holes, oxygen vacancies, or protons. Substitution of an acceptor, A, can be exemplified for a general oxide MO with oxygen vacancies (dry conditions) as the charge-compensating defect as M A A M vM M
x xO + O = 2 + + 2O + 2 O O¢ ii (1.7) In the presence of water vapor, these vacancies can be hydrated, and acceptor doping, accordingly, increases the concentration of protons. So far, the literature of high-temperature proton conductors is dominated by systems that are to some extent acceptor doped. More recent advances have shown that also materials containing disordered inherent vacancies on the oxygen sublattice, typically oxides with fluorite-related structures (see other chapters of this book), dissolve significant amounts of water and become proton conductors. The behavior of these materials can be described by a Kröger-Vink-derived nomenclature [5] but where the principle of hydration corresponds to the standard reaction in the Eq. 1.1. Therefore we do not treat hydration of these materials separately. 1.1.2 Thermodynamic ParametersThe entropy and enthalpy of hydration are invaluable parameters to classify materials and may form the bases for empirical relations between concentration of protons and physical and/or structural properties. Figure 1.1 shows the influence of these thermodynamic parameters to the concentration of protons as a function of tempera-ture with several features:
(i) Due to the exothermic nature of the hydration reaction, the concentration of protonic defects increases with decreasing temperature. (ii) The concentration of protons is constant when the material is saturated, that is, when the material is fully hydrated. (iii) The concentration of protons at saturation in principle corresponds to the concentration of acceptor doping in the compound, (|Acc/|=|OH∑O|), in this case 0.05 (site fraction). (iv) The material starts to dehydrate (lose protons) above a certain temperature, and this temperature decreases with increasing enthalpy and/or decreasing entropy. One may note that a similar concentration profile can be calculated from two different ΔS and ΔH pairs (see the green and blue curves in Fig. 1.1), which means that they can be correlated factors during fitting routines to experimental data and one must be careful to comment on one parameter without the other.
