Independent of the direction of future research, a major leap in the development of next-generation fuel cells depends on the exploration of new classes of materials and a better understanding of those already known. This chapter has given a flavour of past and current research in the area of proton-conducting ceramics, targeted as electrolytes for future intermediate-temperature fuel-cell technology, and demon­strated the important role neutron scattering plays in elucidating the fundamental science of these materials. In particular, a collection of contemporary neutron studies on proton-conducting perovskite type oxides, hydrated alkali thio-hydrox- ogermanates, solid acids, and gallium-based oxides, using a range of different neutron methods, has been reviewed in order to illustrate the breadth of information that can be obtained.

In the future, it is clear that there exists great scope for further neutron studies to explore and understand the basic science of structural and dynamical aspects of such classes of proton-conducting oxides. In particular, I foresee an increasing use of PDF analysis and reverse Monte-Carlo modelling [92, 93] of neutron total­scattering data for the investigation (and re-investigation) of local-structural details, such as bond distances and angles, proton sites, and oxygen vacancy and/or cation ordering, for example, of both traditional and new materials. The influence of interactions between oxygen vacancies and dopant atoms on the conductivity of oxide-ion conducting yttria-doped zirconia has been observed [94, 95] and their extension to the broader class of proton-conducting perovskite-structured analogues is an interesting direction of research. In parallel, QENS will play an increasingly important role in elucidating the detail of the proton-conduction mechanism and how it depends on the local-structural details as explored with diffraction methods. For this purpose, I foresee an increasing use of the neutron spin-echo technique in particular, which offers the twin advantages of reaching the long time-scales needed to observe the translational proton-diffusion on an atomic length-scale whilst covering a very large time-range, so that it may be possible to observe and analyse different types of proton motions in a single measurement.

The importance of exploring nanoionic and thin film phenomena is also noted, as nanostructuring and thin film properties may be, and often are, different from the properties of the bulk. In this context, the use of neutron reflectivity, a technique which, to the best of my knowledge, thus far has been neglected in studies of proton-conducting oxides, offers unique possibilities to obtain information about surface and near-surface states and may yield information such as the properties of interfaces and distribution of protons across a single electrolytic-membrane or membrane-electrode assembly. Such information would certainly help in under­standing the role of interfaces and, in particular, the reason for the reduced proton conductivity across grain boundaries (GBs). Two explanations for low GB con­ductivity have been put forward, the first being a structural misalignment in the GB region, and the second being the appearance of a space-charge layer around the GB core, leading to Schottky barriers and the depletion of mobile protons. Presently, the latter explanation predominates research [96, 97], however, details of the GB core are neither well understood nor sufficiently explored.

From a more technical point of view, the recent development of in situ con­ductivity and humidification cells for ND now allow relatively small features in conductivity to be related to concurrent changes in structure and/or level of hydration [98]. In the near future, the development of in situ cells may also enable investigations of materials under conditions that mimic those under operating fuel­cell conditions and therefore also present the potential to bridge the gap between fundamental scientific problems and applied research. In the longer term, this research can expect also to benefit from the development of completely-new instrumental concepts. An example of this is the recent demonstration of using dynamic nuclear-polarization techniques [99] coupled with ND, where the Bragg peaks can be enhanced or diminished significantly and the incoherent background is reduced [100]. This method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in ND patterns of hydrogenous matter, including proton-conducting oxides.