Intermediate Temperature Proton Research Articles

Intermediate temperature proton conducting salt–oxide composites

Compared with high temperature proton conducting oxides, proton conducting salts and salt–oxide composites (SOCs) may be called intermediate temperature (300 to 700°C) proton conductors. SOCs can be solid or solid-like with a soft plastic or molten salt phase (MSOCs). Three types of SOCs (MSOCs) are discussed in this paper, oxyacid-based SOCs, chloride- and fluoride-based SOCs and hydroxide- or nitrate-based MSOCs. The MSOC is a new concept for materials science, which is based on the typical character of superionic conductors. Chlorides, fluorides and relevant SOCs have opened a new promising area for the investigation of proton conducting materials, since all previously studied proton conductors are always oxygen containing materials. It is a highly interesting academic subject why and how proton conduction occurs in chlorides, fluorides and relevant SOCs. Some proton conducting mechanisms in these new materials are under discussion.

Solid-state protonic conductors: principles, properties, progress and prospects

A brief overview is given of the types and principles of solid state protonic conductors. Their properties are summarised in terms of protonic conductivity and operation temperature. A few trends and highlights of the current scientific advances are given for some of the main classes of protonic conductors. Despite our progress, a crucial point still seems to be the ability and computer power required to understand and model the complex quantum mechanical proton transfer process. Many of the commercial prospects are closely connected with the need for cleaner energy technologies and the use of hydrogen as an energy carrier. In particular, polymer-based proton conductors are quickly approaching the market for small fuel cells and electrolysers: from bench-scale demonstration units for hydrogen energy to automobiles. There is also emerging interest in mixed protonic–electronic conductors for use as hydrogen permeable membranes in hydrogen separation technologies. This will also be helpful in the much needed development of electrodes for high- and intermediate-temperature proton conductors.

Status and future prospects of intermediate, high temperature proton conductor fuel cell

The paper describes the features of fuel cells using proton conductors as electrolyte. Comments on the properties of some intermediate and high temperature proton conductors and their applications for fuel cells are given. Finally, further research areas including the search for new proton conductors and new electrode materials compatible to the electrolytes as well as the development of preparation techniques are proposed.

Strategies and recent trends for proton conduction at intermediate temperatures

Intermediate and high temperature proton conductors contain no structural protons. They only become proton conductors in water vapor or hydrogen containing atmospheres. The reaction of these materials with the water from the atmosphere was first presented by Wagner [1]. These reactions will be discussed in detail with doped earth alkaline cerates as an example.

Tin-based stabilizers

Crystalline polyoxometalate (POM)–polyethylene glycol (PEG) composites aimed as non-humidified intermediate-temperature proton conductors were synthesized and characterized by single crystal and powder XRD, solid state MASNMR, and TG-DTA measurements. Among the POM–PEG composites, Cs2.7H0.3[PW12O40]·1.2PEG1000 (CsHPW-PEG1000) possessed one-dimensional channels with diameters of ca. 6 and 8 Å, where PEG probably resided, and showed the best performance as a proton conductor (1.2×10−5 S cm−1 at 443 K). Proton conductivities of POM–PEG composites decreased by the increase in molecular weights of PEG (CsHPW-PEG 12,000) or anion charges (CsHSiW-PEG1000). Variable contact time 13C-CP (cross polarization) MASNMR revealed that local mobility (i.e., segmental motion) of PEG is related to the trends in proton conductivities. These results show that amount of acidic protons (H+) is not the primary factor in proton conduction and that segmental motion of PEG assists the proton hopping among POMs in the crystal lattice of POM–PEG composites.