A particularly promising, yet challenging, clean-energy technology is the solid oxide fuel-cell (SOFC) [1-9]. At the heart of this device is an oxide-ion or proton­conducting oxide electrolyte, which is sandwiched between two porous electrodes (the anode and cathode). The working principle of the SOFC is based on the chemical reaction of hydrogen (at the anode) and oxygen (at the cathode) to pro­duce electricity and water, working temperatures being in the range * 800-1,000 °C. The electrolyte does not conduct electrons, but is permeable to the diffusion of either oxide ions or protons. Schematics of SOFCs based on oxide-ion and proton-conducting electrolytes are shown in Fig. 9.1.

M. Karlsson (H)

Chalmers University of Technology, 412 96 Goteborg, Sweden e-mail: maths. karlsson@chalmers. se

© Springer International Publishing Switzerland 2015 243

G. J. Kearley and V. K. Peterson (eds.), Neutron Applications in Materials for Energy, Neutron Scattering Applications and Techniques,

DOI 10.1007/978-3-319-06656-1_9

In the 1990s the Siemens Westinghouse SOFC, consisting of an oxide-ion con­ducting yttria-stabilized zirconia (YSZ) electrolyte, a lanthanum strontium manga — nite cathode, and a nickel-YSZ cermet anode, in a unique tubular design, was developed and is now commercially available [1, 10, 11]. Thanks to its high oper­ation temperature, which offers the advantages of fuel flexibility (existing fossil fuels can be used) and high energy-conversion efficiency, the SOFC is particularly attractive for use in combined heat and power applications or efficiently coupled with gas turbines. However, the high operation-temperature also has disadvantages, such as a long startup time, durability issues, and the need for relatively expensive component materials. Therefore, in more recent years, much research has focused on trying to reduce the operation temperature of the SOFC to the so-called intermediate temperature range between approximately 200 and 500 °C. Such a reduction in operating temperature would have a beneficial impact on the total cost and the durability of the fuel cell, as problems associated with thermal cycling and perfor­mance degradation would be reduced and it would be possible to use cheaper materials in interconnects and heat exchangers [12]. Lowering the temperature would also shorten the startup time of the fuel cell, which is of particular importance for mobile applications. Further, in comparison to low temperature (<100 °C) polymer-electrolyte membrane fuel cells (PEMFCs) [13], a temperature of 200 °C is high enough in order to allow for the use of smaller amounts of expensive platinum to catalyze the electrode reactions [12]. In fact, intermediate-temperature fuel-cell technology has the unique potential to be used both for stationary combined heat and power systems and for hybrid and plug-in hybrid vehicles and are indeed expected to produce energy densities per volume and specific energy per weight significantly larger than state-of-the-art Li ion and Ni metal hydride batteries [14].

However, it is discouraging that any decrease of the operation temperature of SOFCs results in a decreased power density of the device, mainly due to a lowering of the ionic conductivity of the electrolyte. Targeted conductivities exceed

7TC

800 500 400 300 200

1000/r (K ’)

Fig. 9.2 Conductivity of state-of-the-art electrolytes over a wide temperature range. Note the gap of highly-conducting materials in the temperature range * 100-500 °C. The figure is redrawn and modified from Ref. [15], copyright Wiley, 2002. The conductivity of Cs2GeS2(OH)2^yH2O is taken from Ref. [16]

0.01 Scm V and in this respect proton-conducting ceramics emerge as the main candidate electrolytes for SOFCs. However, despite intense research, the conduc­tivities of even the best proton-conducting ceramics are still one to two orders of magnitude below the target. This can be appreciated from Fig. 9.2, which displays the conductivities for the state-of-the-art ionic conductors over a wide temperature range, including electrolytes for low temperature PEMFCs and high-temperature SOFCs, i. e. the more mature fuel-cell technologies. A significant advancement in relation to the conductivity of present day proton-conducting ceramics is therefore critical to future breakthroughs in the development of next-generation fuel-cell technology, operating in the intermediate temperature range. Such an enhancement depends on increasing the understanding of the fundamental science of key mate­rials aspects such as crystal structure and proton-conduction mechanisms, in the most promising classes of materials, and the exploration of novel and completely new systems. For this purpose, neutron scattering is a powerful tool that has been applied successfully to studies of both oxide-ion and proton-conducting materials, and its role in this area of research is vast.

The aim of this chapter is not to give an exhaustive account for all the neutron scattering studies in this field, nor is it aimed at providing a review of SOFC technology, as excellent reviews can be found elsewhere [1-9]. Rather, this chapter centres on proton-conducting ceramics as separator materials for intermediate — temperature fuel cells and aims to give a flavour of how a variety of neutron methods can be used to reveal the details of structures and proton dynamics in this class of energy-relevant materials. In particular, the scope of contemporary structural and dynamical studies of perovskite-type oxides, known as the most promising class of proton-conducting ceramics for intermediate temperature fuel-cell applications, will be reviewed. This is followed by a concise review of selected examples of studies of other classes of promising candidate materials, such as hydrated alkali thio-hy — droxogermanates, solid acids, and lanthanum gallates, to illustrate the breadth of information that can be obtained. The chapter finishes with my personal thoughts on prospectives for future work in this timely area of research. The chapter may be viewed as an extended version of my recent publication in Dalton Trans. [17].