Activation protonic conductors

M. Makri, A. Buekenhoudt, J. Luyten, and C.G. Vayenas, Non-Faradaic Electrochemical Modification of the Catalytic Activity of Pt using aCaZr09ln0 03.a Proton Conductor, Ionics 2, 282-288 (1996). 

The coordination chemistry of the trichalcogenophosphonates is very undeveloped when compared to the analogous metal organophosphonates (RP032), which have been extensively studied owing to their potential and practical applications as ion exchangers, sorbents, sensors, proton conductors, nonlinear optical materials, photochemically active materials, catalysts and hosts for the intercalation of a broad spectrum of guests.145... 

Dr. Hui has worked on various projects, including chemical sensors, solid oxide fuel cells, magnetic materials, gas separation membranes, nanostruc-tured materials, thin film fabrication, and protective coatings for metals. He has more than 80 research publications, one worldwide patent, and one U.S. patent (pending). He is currently leading and involved in several projects for the development of metal-supported solid oxide fuel cells (SOFCs), ceramic nanomaterials as catalyst supports for high-temperature PEM fuel cells, protective ceramic coatings on metallic substrates, ceramic electrode materials for batteries, and ceramic proton conductors. Dr. Hui is also an active member of the Electrochemical Society and the American Ceramic Society. 

More recently, H. Iwahara et al. [20] reported that some compounds having the perovskite structure (see Section 2.7.3) become proton conductors if hydrogen is introduced into the crystal, and the solubility of water and proton mobility in perovskites are now actively researched topics [21]. The perovskites which can be tailored to exhibit high protonic conductivity have compositions of the type... 

In noncubic perovskites, whilst some of the 0—0 distances become shorter, any continuous path must also include longer 0-0 distances. The cubic perovskites should, therefore, be better proton conductors, and a shortening of the 0—0 distances should be dynamic, due to thermal motion. The H — M" repulsion contributes to the activation energy of the proton transport. Indeed, A + + O3-based materials are much poorer proton conductors than A + M + 03-based phases. Consequently, the A + O3 perovskites appear to show promise ]15,21], although... 

Kurita, N., Fukatsu, N., Miyamoto, S., Sato, F., Nakai, H., Irie, K. and Ohashi, T. (1996) The measurement of hydrogen activities in molten copper using oxide protonic conductor. Metall. Mater. Trans. B, -a, 929-35. 

Electrodes were fabricated with catalyst layers containing platinum-ruthenium alloys and platinum-ruthenium oxide. Membrane electrode assemblies were fabricated with such cells, and the performance was evaluated in a full cell configuration. Although ruthenium oxide is a proton conductor and is expected to enhance the rate of proton transport from the interface during methanol oxidation, no noticeable improvement in activity of the catalyst layer was observed by addition of ruthenium oxide. The role of other metal oxides such as tungsten oxide will be investigated next year, along with evaluation of non-noble metal catalysts based on nickel, titanium, and zirconium. 

Table 2.1 provides the value of the total electrical conductivity of various perov-skite-type proton conductors at 900 °C in dry or wet H2 atmospheres. It also lists the activation energies of the electrical conduction in the range 800-950 °C. The Arrhenius plots of the conductivity for some of these perovskite-type ceramics are presented in Figure 2.3. These ceramics become almost pure protonic conductors... 

Three-phase contact between metal, solid electrolyte and gas is known to act as an active site for electrochemical reactions (half cell reaction). If the solid electrolyte is a proton conductor, for instance, the following reaction takes place in the presence of H2, and the half cell equilibrium is expressed by the following Nernst equation ... 

Fig. 3.1. Relationship between the pre-exponential factor

Table 3.3. Room temperature conductivity ((Trx) and activation energy (E ) of protonic conductors classified according to their method of synthesis... 

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