In situ neutron diffraction on commercial batteries has been used to provide structural information of electrodes at various states of charge [179, 180], under overcharge (or overdischarge) conditions [181], with fresh and fatigued or used batteries [182, 183], and at different temperatures and electrochemical conditions (applied currents) [180, 183—187]. In situ NPD allows structural snapshots of electrodes within a battery to be obtained, and depending on the diffractometer, these snapshots can be extremely fast such that the time-resolved structural evo­lution can be captured. Notably, new aspects of the graphitic anode and LiCoO2 cathode that were commercialised in the 1990s are being discovered with this probe. Such insights include the existence of a small quantity of a spinel phase in
the previously-thought layered LiCoO2 cathode, an apparent lack of staged Li insertion into graphite to form LiC12 with low current rates [180], and the transfor­mation of the graphite anode to a wholly LiC6 anode with voltages around 4.5 V— above and beyond the recommend limits applied by manufacturers [181]. Figure 7.26 shows LiCoO2 and LixC6 reflections and their evolution as a function of time. Structural changes are a function of the applied charge/discharge rates, with faster structural evolution occurring at higher rates. Importantly, higher rates produce a lower capacity which is directly related to a lower quantity of the LiC6 (charged anode phase) being formed.

This information explains the processes in electrodes that are well known and in situ neutron diffraction can be used to explore a wide variety of battery-function parameters ranging from current, voltage, and lifetime. Kinetic processes in bat­teries can be probed with time-resolved data, where rates of structural changes are determined for electrode materials and related to the applied current. In most cases [184, 188, 189] the rate of structural change is directly proportional to the applied current. The rate of lattice expansion and contraction can be used to determine the

viability of electrode materials for higher power applications. However, the rela­tionship between kinetic structural parameters and the electrochemical capabilities of the battery are yet to be explored in detail. This is an active research area that may yield valuable information with more advanced experiments.

Lanthanum barium gallates, i. e. LaBaGaO4-based compounds, represent a novel class of proton-conducting oxides [89]. The structure of these materials consists of discrete GaO4 tetrahedra which are charge balanced by Ba/La ions, as shown in Fig. 9.18a. Increasing the Ba:La ratio results in the formation of oxygen vacancies, and similarly to perovskite-type oxides, such vacancies can be filled with — OH groups in a humid atmosphere [89]. In this regard, Kendrick et al. [89] raised the question of how the oxygen vacancies are accommodated in the structure, with the knowledge that the inclusion of oxygen vacancies may result in energetically unfavourable three-coordinated Ga atoms, which would limit the oxygen vacancy concentration and hence the degree of hydration. Using computer-modelling methods, the authors found that the oxygen vacancies are accommodated via con­siderable relaxation of neighbouring GaO4 tetrahedra, resulting in the formation of

Ga2O7 groups so that the Ga retains tetrahedral coordination (see purple unit in Fig. 9.18a) [89]. These computational results were supported by ND results which showed a splitting of the oxygen site, consistent with the presence of both GaO4 and Ga2O7 [89]. Hydration of the material can lead to the breaking of such units [89].

On the basis of these results, later investigations focussed on proton positions within the hydrated form of the lanthanum barium gallate structure, using ND coupled with difference Fourier-density analysis of protonated and deuterated samples. Notably, three stable proton positions, one located adjacent to the O3 site and two located adjacent to the O4 site, were found (Fig. 9.18a). The presence of several proton sites is in agreement with modelling work suggesting little difference in energy between proton sites for different oxygens [89]. Further, determination of Ga-O-H bond angles indicated that protons point almost perpendicularly to the Ga-O bond direction and that the protons experience a mixture of intra — and inter­tetrahedral hydrogen-bonding, as shown in Fig. 9.18b [91]. The highest degree of hydrogen bonding is found for O3-H — O3 linkages, in agreement with previous modelling work [91]. Modelling also suggests facile inter-tetrahedron proton-con­ducting pathways due to hydrogen bonding, whereas intra-tetrahedron pathways are found to be less favourable and hence rate-limiting for proton conduction [89].

Adsorption of hydrogen on graphite, or in zeolites, has been investigated at a molecular level for primarily fundamental reasons long before this became an urgent practical issue for the ultimate realization of a hydrogen economy. The use of neutron scattering techniques in both of these examples illustrates two basic requirements that can make neutrons a surface-sensitive probe, the first is that the adsorbate needs to have a significantly greater neutron scattering cross-section (e. g. H) than the host material (e. g. C or silica), and the second is that adsorbent must have a very large surface-area so that the total number of surface-bound molecules is sufficiently large. This is the case for most carbon materials, such as exfoliated graphite, which was used [94] to determine the two-dimensional structural phase diagram of monolayers of deuterium. The highest coverage was found for an incommensurate structure of 0.078 molecules A-2 which corresponds to about

1.3 x 10-5 mol. m-2. This number indicates a limit of the storage capacity for simple physisorption on carbons, namely that a material with a maximum surface area of 1,500 m2g-1 will bind close to 0.02 mol of H2 (0.04 g, 4 wt.%), which still falls short of what is required. Surface areas for the interior pores in zeolites are typically considerably less than that because of the high density of the silicate framework.

It has therefore long been recognized that surface areas have to be considerably increased, and this has been accomplished with the synthesis of a large number of metal-organic framework (MOF) materials with as much as 5,000 m2g 1 or more. Capacities around 8 wt.% have been achieved in these materials albeit under unfavourable conditions (77 K and pressures of approximately 60 bar), but the amount of hydrogen stored in MOFs at ambient temperature is far below those required for practical goals. The reason for this is, of course, that the binding energies for hydrogen provided by physisorption are too low (of the order of 5 kJmol 1) and hence vapour pressures are too high for the operation of such a storage system at ambient conditions.

Materials therefore need to be designed to not only have very high surface-areas but also binding energies for hydrogen at least three times as high as that for normal physisorption (e. g. on carbons) in order for such a system to operate at room temperature and modest pressures. Heats of adsorption must also be approximately constant over the entire range of hydrogen loading, i. e. of similar magnitude for all accessible binding sites.

An enormous effort to design and synthesize new and better porous materials guided by chemical or structural principles, including the use of combinatorial methods, has resulted in a huge variety of metal-organic materials not only for applications as hydrogen storage materials, but also for the capture of CO2, gas separation, and catalysis (see Chaps. 2 and 3). Binding energies of hydrogen molecules have more than doubled from that of simple physisorption in some of these materials, but are still quite far away from what is needed for a practical storage medium operating at ambient conditions.

The use of neutrons makes it possible to employ a systematic, molecular-level approach to the development of improved hydrogen storage materials, particularly when combined with advanced computational analysis. Such experiments can provide the location of the adsorbed hydrogen molecules at different sites (neutron diffraction) and give some measure of the strength of the interaction with the host material at each site (inelastic neutron scattering), whereas conventional thermo­dynamic measurements (adsorption isotherms) only give average properties for all sites that are occupied at a given loading. Results of such experiments should then serve as the basis for detailed computational analysis, which can rationalize both the locations of the hydrogen molecules and their binding strength in terms of the interatomic guest-interactions. This type of information should then in principle be useful in the design and synthesis of improved storage materials.

A detailed knowledge of the binding sites of hydrogen molecules in crystalline porous materials can be obtained by neutron diffraction methods. Adsorbed hydrogen molecules cannot be “seen” in single crystal X-ray diffraction experi­ments even at very low temperature, but this method has been used to obtain a very detailed picture of the adsorption and binding sites of Ar and N2, i. e. molecules with many electrons, in the prototypical metal-organic material Zn4O(bdc)3 (where bdc = 1,4-benzenedicarboxylate), also known as MOF-5. The primary binding site at 30 K for both types adsorbates was found to be in the pocket of the ZnrO cluster in the corner of the framework (a-site) [95]. A number of other binding sites were clearly identified, including ones that are triply or doubly bridging the carboxylate oxygens (P and у sites, respectively), which connect the cluster the organic linker. Interaction with the benzene ring leads to two binding sites, one near the two aromatic hydrogens in a side-on configuration, and the other on top of the ring.

Ideally a similar approach with single-crystal neutron diffraction should provide similar detail for hydrogen binding sites. This has, however, been accomplished just once, namely for hydrogen in MOF-5 [96], since the sizes of MOF single crystals are usually too small for the relatively-low intensities in neutron diffraction experiments. This experiment was carried out on a crystal that barely was of adequate size. Nonetheless, the most favourable binding site for H2 in MOF-5 at 5 K was clearly identified to be in the pocket of the Zn4O cluster (a-site) with one end of the molecule on the three-fold axis pointing at the central oxygen atom. The H-H bond of the molecule is at an angle relative to the three-fold axis of the crystal so that the other end is disordered over three equivalent orientations (Fig. 8.7) in a similar manner as N2. This site is essentially fully occupied at 5 K. Hydrogen was also located on the P-site, but could not be refined as two separate atoms because of rather large atomic displacement parameters.

Because of the aforementioned difficulties in obtaining sufficiently-large crystals of MOFs, a number of studies to characterize hydrogen binding sites have been carried out by neutron powder diffraction, including also on MOF-5 [97]. For neutron powder diffraction it is highly advantageous, although not always neces­sary, to have all Hs in the system replaced by deuterium (D) in the synthesis of the material, which can, of course, be rather difficult and expensive, and to use D2 as the adsorbate. The strong background of the incoherent scattering by H can thereby

be reduced. Diffraction patterns can be collected at different loadings of D2 and interpreted under the assumption that sorption sites become occupied roughly in order of their respective binding energies. The molecule itself cannot, however, be refined as two separate atoms, so that only its centre-of-mass is found. In addition to the locations of the D2 molecules the degree that these sites are occupied is refined in the analysis, along with isotropic atomic displacement parameters all within the symmetry of the parent structure.

Cu3(btc)2 (where btc = 1,3,5-benzenetricarboxylate) and also known as HKUST-1, was one of the earliest MOFs reported and has been extensively characterized since then, including several neutron diffraction and inelastic neutron-scattering studies, as well as computational studies. Perhaps the most interesting structural feature of this material is the well-known dicopper acetate “paddlewheel” unit, which has subse­quently been used in many MOFs. The apical ligand on the Cu usually is a weakly bound water molecule, which can easily be removed, and thereby opens up a metal site for presumably stronger adsorption of hydrogen. The most favourable binding site in HKUST-1 was indeed found to be, using neutron powder diffraction, adjacent to the open Cu site (D2(1) in Fig. 8.8) [98, 99]. This would be expected on the basis of the relatively-strong interaction of the hydrogen molecule with the metal, so that these sites are largely responsible for the improved heats of adsorption found in this material. The hydrogen molecule was located [98] at a distance of 2.39(1) A from the Cu site, which is, however, far too great to allow for a direct electronic interaction with the metal. Additional binding sites at various points in the host structure were iden­tified in this study, some of which are shown in Fig. 8.8. Site 2, for example, is adjacent to the benzene rings of the btc unit, while D2 in site 3 is close to six btc oxygen atoms in the small window leading to the 5 A pores.

The enhanced interaction of H2 with such open-metal sites has been demonstrated in a number of other MOFs, most notably MOF-74 [100, 101], also known as CPO — 27 [102], for which a number of isostructural variants with different metal centres

(Zn, Mg, Mn, Co, Ni) can be synthesized [102—104]. The materials consist of a linear chain of inorganic metal clusters connected with organic linkers to form hexagonal channels, M2(dhtp), (dhtp = dihydroxyterephthalate). H2 binding energies indeed depend strongly on the type of metal centre, and do reach about 13 kJ/mol for Ni, but only until all these sites are filled. A powder neutron diffraction study [137] on the Zn analog finds that the D2 molecule is close enough (d(Zn-D2) * 2.6 A) to be affected by polarization from the metal, but again too far for a direct electronic interaction. A somewhat closer metal-H2 distance of 2.27 A was found [105] by neutron powder diffraction in the Mn-based MOF formed from Mn4Cl clusters and 1,3,5-benzenetristetrazolate bridging ligands. Desolvation of the parent material opens up one of the coordination sites on Mn2+, which in turn interacts rather strongly with H2 (10.1 kJ/mol near zero-loading), but again, not by direct electronic interaction, which requires H2-mteal distances in the order of 1.8 A.

One of the more important aspects of experimental efforts to locate the adsorbed hydrogen molecules in these systems is that this sort of information is critical for benchmarking the extensive theoretical studies of these systems that have been undertaken. A neutron powder diffraction study on hydrogen adsorbed in the zeolitic imidazolate framework ZIF-8 [106] was the first to rationalize the experi­mental findings with parallel DFT calculations of binding energies at various sites. The preferred adsorption site in this material in fact is not near the metal cluster but on top of the metal imidazolate linker. Most computational characterization of MOFs for hydrogen adsorption is accomplished by grand-canonical Monte Carlo simulations, or by molecular dynamics, both of which readily yield pair-distribution functions (PDFs) for the adsorbate molecules. These can be directly compared with diffraction experiments without any model dependent structure refinement, if the diffraction data are converted to direct space, i. e. to PDF form, as has been reported for D2 in IRMOF-1 [107]. This approach would be especially important for those MOFs, where the adsorption sites are poorly defined, MOFs which have defect structures, or are in fact amorphous.

A much greater number of neutron scattering studies on sorbent hydrogen storage materials has been carried out by inelastic scattering from the bound hydrogen molecules, perhaps because of the relative ease with which these can be carried out, and because of the sensitivity of the method. A detailed interpretation of such data is, however, very challenging, and ultimately requires extensive com­putational studies. The rotational dynamics of the adsorbed hydrogen molecule are a highly sensitive measure of its interactions with the host material and are readily accessible by the inelastic scattering of neutrons. The reason for this is that the rotational energy levels of the quantum rotor H2, E, are strongly perturbed from those of the free quantum rotor (E = BJ(J + 1), where B is the rotational constant of the molecule and J is the quantum number for total rotational angular momentum) by the barrier to rotation resulting from the influence of the adsorbent.

Transitions between these hindered-rotor levels can be directly observed by incoherent INS and give an indication of the height and shape of the barrier to rotation. Such a quantitative analysis of these measurements must however employ a phenomenological model for this barrier, whose parameters may be determined from the pattern of observed transitions.

The simplest such model is one of a double-minimum potential with the two angular degrees of freedom for the H2 molecule,

V(в, Ф) = 1/2V2(1 — cos20)

The energy levels for this potential can then be obtained by solving the Schrodinger equation:

for various values of V2, with the result that the observed transition(s) yield a value for the height of the barrier. It is important to note that the energy-level diagram (Fig. 8.9) shows that the spacing between the two lowest levels decreases with increasing barrier height, which is indicative of rotational tunnelling, where the tunnelling probability has an approximately-exponential inverse dependence on the barrier height. Therein lies the great sensitivity of this type of measurement.

A number of more elaborate phenomenological models for the H2 potential — energy surface (PES) have also been used to interpret the INS data, and these include separate barriers to in plane and out of plane reorientation (or some ratio between the two) plus a separable centre-of-mass vibration perpendicular to the surface [109]. Such models, however, require the observation of several transitions in order to determine the parameters in the form of the potential. While the use of such a phenomenological potential is essential for the assignment of the observed rotational transitions to hydrogen molecules absorbed at different sites, it does not, however provide direct insight into the atomic level origins of the rotational bar­riers. What we would like to learn from these experiments is the origin and nature of the interatomic interactions at each binding site that give rise to the PES, which

in turn gives rise to the observed rotational energy levels. This can, of course, be done by ab initio computational studies as long as the structural details of the binding sites are known, preferably from a neutron diffraction experiment. The dynamics of the hydrogen molecule within such a theoretical PES then has to be treated exactly, i. e. with quantum dynamics for both the rotational and (centre of mass) translational excitations. This has been accomplished for H2 in clathrates, various kinds of fullerenes, and just one MOF, namely MOF-5, which will be described below.

The seminal work of this type [110] on H2 adsorbed in MOF-5 helped to rationalize the large storage capacity of this material in terms of the availability of several different types of binding site. The INS spectra are collected at 10 K (or less) as a function of loading (carried out at 77 K) in a direct relationship to the numbers of equivalent sites of particular types available to facilitate a tentative identification of specific spectral features with binding sites (Fig. 8.10) by making use of the phenomenological PES described above. Three additional sites could be inferred from the INS spectra, which may now be matched with information available from the diffraction experiments as well as computational studies, i. e. the в, у, and 5 sites.

Similar experiments have been carried out on hydrogen on various forms of carbon, some of which were at one time considered to be promising hydrogen storage materials. The INS spectra, however, generally show just one broad peak centred near the free rotor value of 14.7 meV, which is representative of a broad

Fig. 8.11 INS spectra of H2 adsorbed on (top to bottom) two forms of activated carbon, nanotubes, and nanofibers. Reprinted from (H. G. Schimmel, G.

J. Kearley, M. G. Nijkamp, C. T. Visser, K. P. Dejong, F. K. Mulder, Chem. Eur. J. 9, 4764 (2003)) [111] with permission distribution of binding sites with very low barriers to rotation, or weak interaction with the host. These spectra clearly show that there is very little difference in the interaction of H2 and various forms of carbons, be it activated carbon, or nanotubes [111, 112] (Fig. 8.11).

These results may be contrasted with those for MOF-5, where the INS spectrum exhibits a number of sharp transitions indicative of different, well-defined binding sites, some of which have higher barriers to rotation of H2 than do carbons.

A considerable number of metal-organic materials (MOMs) have been investi­gated using rotational-tunnelling INS spectroscopy of the adsorbed H2 in an effort to determine which structural features may improve the observed heats of adsorption. Here we utilize the fact that a smaller rotational tunnel splitting implies a higher barrier to rotation, which in turn should, in general, signal a stronger interaction between the adsorbed H2 and the host and a particular binding site. This type of information can only be extracted if particular spectral features can be associated with H2 a known sites, which can be quite difficult in the presence of simultaneous multiple-site occupancy, as will be described below.

Previous studies of H2 adsorption in zeolites [113] had shown the strong effect of a charged framework along with the presence of charge-balancing extra framework cations on the binding energies of hydrogen molecules. The vast majority of MOMs, however, possess neutral frameworks, but some MOFs with anionic or cationic frameworks have indeed been characterized. Zeolitic MOFs, ZMOFs, are such a class of anionic MOFs built from metal centres with imidazole dicarboxylate ligands to form zeolitic structures (e. g. rho-ZMOF, Fig. 8.12) with much larger pores than their SiO2 analogs. This structural similarity arises from the fact that the Si-O-Si bond angle in the zeolite is nearly the same as the angle between the two N atoms in the imidazole dicarboxylate. The negatively-charged framework is compensated by exchangeable extra framework cations, as in zeolites. In the case of ZMOFs, how­ever, these cations are hydrated and bound to the framework by H-bonds from the water ligands to carboxylate oxygens. INS studies on rho-ZMOFs with different

cations revealed rather broad spectra with few well-defined peaks, which do not appear to depend very much on the nature of the cation. Much of the intensity in these spectra at lower loadings may be found well below 12 meV, the main peak in the spectrum for H2 in MOF-5 and therefore indicate stronger interaction (higher barriers to rotation) with the charged framework (Fig. 8.13). A wide range of binding sites seem to be occupied by hydrogen at all loadings, which appears to be characteristic of this environment of large, hydrated cations in an anionic framework. The isosteric heats of adsorption are indeed some 50 % greater than in the neutral framework MOF-5 at low loadings, but virtually independent of the nature of the cation.

Many of the original MOFs were noted for their very large pore sizes and hence extremely-low densities. This property is not, however, optimal for storing large amounts of hydrogen under ambient conditions because much of the interior of the pores may only bind H2 to other H2 molecules. If, however, the pore size of a MOM were to be reduced to values not much bigger than that the size of a hydrogen molecule, it could interact with many framework atoms all around rather than just those on a surface. We can therefore readily conclude that a MOM with uniformly small pores would be highly beneficial to hydrogen binding. This was convincingly demonstrated in recent work by Pham et al. [115] on a MOM, which occurs in two forms, one with interpenetrating identical frameworks, the other in a non-inter­penetrating form. SiF6 units and bipyridyl acetylene (bpa) linkers connect the Cu centres in this structure (Fig. 8.14). We note that the channel dimensions of the interpenetrating structure of 3.5 x 3.5 A are only slighty larger than the size of the hydrogen molecule of 2.9 A, which would suggest that H2 will have appreciable interaction with many of the surrounding framework atoms.

The INS spectrum in fact is rather unique for H2 in pores in that it only exhibits one strong peak at all loadings with a shoulder on the low energy side (Fig. 8.15), both of which simply increase intensity until the pores are filled. Moreover, this peak occurs at a lower energy (6.3 meV) than that for hydrogen near open-metal sites and hence indicates an appreciably-stronger overall interaction with the framework. The two similar binding sites found in a computational study [115] are

adjacent to each other in the narrow channels, and hence differ little in their interactions with framework atoms.

The analogous binding sites in the non-interpenetrating form are open towards the large pore, so that binding energies and barriers to rotation are much lower, and hence the rotational-tunnel transitions in the usual range of MOFs without open — metal sites, i. e. above 10 meV. The isosteric heat of adsorption [115] in the interpenetrating structure (8.3 kJmol-1) is nearly twice that of the non-interpene­trating structure and constant over most of the range of gas loading. The latter is an
important requirement for an adsorption-based hydrogen storage system, and one that few such materials to date can satisfy.

A direct electronic interaction of H2 with metal centres is, of course, known from inorganic and organometallic chemistry to be very strong and does in fact lead to dissociative binding of H2 in most cases. The ability to create coordinatively — unsaturated metal sites in a MOM by removal of a weakly-bound ligand might therefore have been expected to significantly enhance hydrogen binding-energies. INS studies of H2 adsorbed at such sites in a variety of MOFs have indeed revealed stronger interaction by the observation of larger barriers to rotation with smaller rotational-tunnelling splittings. The effect is not, however, nearly as large as anticipated, since the H2 molecule does not approach the metal site very closely. This was shown directly by powder neutron diffraction experiments on HKUST-1 and on MOF-74 [98, 99, 104, 137].

INS spectra of H2 in HKUST-1 [111] do show evidence for the stronger inter­action with this open-metal site, with a peak at about 9 meV (Fig. 8.16), which can then be associated with sorption near the open Cu site. This value is appreciably lower (and hence the barrier to rotation higher) than in MOF-5, which in turn reflects the respective isosteric heats of adsorption of 6.8 and 4.8 kJmol-1, respectively, at the lowest loadings.

Additional features in the INS spectrum have been attributed to secondary binding-sites in accordance with the structural information, although this process is not unambiguous. For example, the peak at 12.5 meV may be attributed to hydrogen at the so-called small pore site, while at higher loadings a peak near the free-rotor transition may be indicative of several binding sites farther inside the pores. The precise attribution of these spectral features has, however, been subject to different interpretations, which can be difficult to resolve.

The Cu paddlewheel structural building unit (Fig. 8.8, left) can be found in many MOFs, some of which have been investigated by INS on adsorbed H2. The rota­tional transitions for H2 at this open Cu site vary somewhat from that in HKUST-1 depending on structural details of the environment, as, for example, the close proximity of another paddlewheel unit in Cu6(mdip)3 (mdip = 5,5’-methylene diisophthalate), also known as PCN-12 [119], where these are observed 7.7 and

8.6 meV.

A remarkable new class of ‘designer MOFs’ have structures with so-called rht net, which are synthesized from metal ions coordinated to a C3 symmetric ligand with three coplanar isophthalate moieties. Each isophthalate group is linked to metal ions to form a square paddlewheel cluster. In rht-MOF-1 a trigonal Cu3O trimer is liked to three 5-tetrazolylisophthalic acid ligands (Fig. 8.17). This com­pound therefore contains at least two different open Cu sites, which were identified from the rotational-tunnelling spectra and computational studies [120]. In this case the transition associated with the Cu paddlewheel was found to be at 9.7 mev, compared with about 7 meV for binding sites near the Cu3O trimer.

The increased interaction strength of hydrogen with these open-metal sites arises primarily from polarization, but falls far short of what one might expect if the molecule were to coordinate to the metal as in organometallic complexes. To date this has definitively been achieved only in the post-synthesis modified zeolite NanAlnSi96-nOi92 where 0 < n < 27, also known as ZSM-5. Highly undercoordi­nated Cu sites can be created by deposition of Cu in some form from the vapour or

solution in the pores of the material, whereby Cu-ZSM-5 is created. Cu (II) ions evidently coordinate to framework O atoms and can be reduced to Cu(I) by thermal activation. H2 can coordinate to these open Cu ions and form a dihydrogen complex similar to some organometallic compounds [121], which tend to show very small rotational tunnelling splittings (Fig. 8.18a) because of the chemical bond formed with the metal centre. This also requires that the H2 be able to approach the metal to within at least 1.7—1.8 A as is indeed shown in the computational model developed for the open Cu site using the zeolite chabazite (Fig. 8.19) [122].

The coordinated H2 is located at 1.653 A from the Cu centre, and the H-H bond has been activated as shown by an elongated H-H distance of 0.804 A. Heats of adsorption for binding to Cu(I) in the zeolite have been observed and calculated [122, 123], to be as high as 70 kJmol 1, more than ten times the typical H2 physisorption energies. The rotational-tunnelling splitting of about * 1 cm 1 for coordinated H2 is about 1/100 of that of hydrogen physisorbed on the walls (*11.5 meV) in the same zeolite [124]. However, only about 40 % of the Cu sites in the zeolite appear to be able to molecularly chemisorb H2, so that the overall isosteric heat of adsorption decreases rapidly when the H2 loading increases much beyond that. Nonetheless, this result may be viewed as a proof of principle in that it demonstrates that this type of molecular chemisorption offers the greatest hope to boost binding energies in sorption-based systems to the point where they can store adequate amounts of hydrogen for ambient temperature operation.

For MOMs to enable this type of binding the metal sites must be more open than what is provided by just having one vacant binding site. This may be more easily achieved in the presence of large numbers of defects, or in an amorphous material, and efforts to this end are well underway [125].

A hybrid type of hydrogen storage medium, which combines sorption of H2 and binding of H in the solid utilizes the so-called hydrogen spillover effect [126]. While neutron scattering methods would again seem to offer the best approach for the study of this phenomenon, experiments carried out to date have encountered difficulties because of the complexity of such media, e. g. metal particles on carbon supports, and the spillover process itself. The approach taken has been to monitor changes in the rotational transition band for H2 on the carbon support, and attempt to detect the dissociated H by INS vibrational spectroscopy [127].

One of the great strengths of neutron scattering methods is that the experimental results can be connected in a very direct manner to computational studies on account of the simple form of the interaction of neutrons with the atomic nuclei. This aspect does not play a role when it comes to determining to location of the adsorbate molecules, as the experimental probe does not enter the picture, but it does so in spectroscopy. In this case one may compare actual intensities for the various transitions or entire spectral profiles with experiment, in addition to their frequencies.

A number of approaches have been taken in the present case of adsorbed H2, the simplest of which may be that of rotational barriers. Energies are calculated for stepwise rotation of the hydrogen molecule with fixed centre-of-mass to obtain the rotational barrier height. This value is compared with a barrier height given by a assuming a phenomenological potential which is adjusted to account for the experimentally observed transitions. For H2 in Cu-ZSM-5 the barriers are 15 and 9 kJmol-1, respectively.

It is, of course, preferable to avoid having to choose a phenomenological potential for interpreting the experimental transition-frequencies by calculating those directly from a theoretical potential and making the comparison on the basis of frequencies, instead of derived barrier-heights. This requires not only an extremely accurate PES for the adsorbed H2, but also a full quantum dynamical treatment of all of its five degrees of freedom (two rotational and three translational). A simplification of this rather challenging computation can be achieved if one makes the assumption that the rotational degrees of freedom are separable from the centre-of-mass translational motions. This approach has been taken a number of times, including for HKUST-1 and MOF-74. Translational motion is often treated separately using a harmonic oscillator model as an approximation. For H2 in MOF-74, for example, the PES was derived from a periodic DFT calculation using a van der Waals density functional method [128], from which the calculated rotational transitions for the main binding sites near the open metal, as well as two other sites, were obtained in reasonable agreement with experiment. Translational excitations were calculated separately in a harmonic model with anharmonic corrections.

The computational methodology for the full five-dimensional, coupled transla­tional-rotational quantum dynamics for a bound H2 was developed by Bacic and collaborators [129] and originally applied to cases where the H2 was essentially trapped in a cage, and where the interaction potentials are well known, such as H2 in C60 [130], and in ice clathrates [131]. In the more general case for molecules with six degrees of freedom, such as methane, the Schrodinger equation with the fol­lowing Hamiltonian is solved numerically:

h=-Ь (I?+b+@?)+8,2+V (xyzфv) (8-8)

where the potential V has to be determined either from empirical forces, or high — level ab initio calculations, or some combination which includes all relevant interactions between the adsorbed molecule and the host. To do this for all five or six degrees of freedom is, however, a lengthy undertaking for many reasons, not the least of which is the fact the H2 interacts very weakly with its surroundings.

The first application of this methodology to hydrogen adsorbed in a MOF used PESs for MOF-5 derived in two different ways [132]. While both potentials reproduced the structural aspects (binding geometries) fairly well, they were found that a low barrier could facilitate translational motion from the a to the neigh­bouring у sites. This is, however, in disagreement both with diffraction experiment, and INS, which show single site occupancy at low temperature and low loadings. The full five-dimensional PES was therefore calculated with the у sites blocked by other H2 molecules, and translational-rotational transitions calculated within the PES surface shown (Fig. 8.20). The transition energies obtained agree well with the experimental. spectrum, and do confirm the conclusion reached from infrared experiments [133] that the lowest energy peak at 10 meV in the INS spectrum is in fact a translational excitation. Translational and rotational excitations were found to be coupled only for transitions to higher levels, as has generally been assumed in the analysis of the INS spectra described above at low frequencies.

While the comparison based on observed and calculated transition frequencies described above is indeed rather powerful, the ideal connection between experiment and theory is one where the computation is extended to produce what is actually observed in an experiment, namely in this case the INS spectrum itself. This is quite commonly done for INS vibrational spectroscopy, but has only recently been accomplished [135] for the full five-dimensional translational-rotational quantum dynamics of H2, in this case when trapped in the small cage of clathrate hydrate (Fig. 8.21).

This is a relatively more favourable case for application of this methodology that for H2 on interior surface sites in MOF-5, as the molecule is trapped in a well — defined site, and its interaction with the host, i. e. ice, is rather better known than with those of MOFs. Nonetheless, this approach holds significant promise for extracting the maximum amount of information on the interaction of H2 with the host material from the INS spectra. The significant computational effort involved in obtaining accurate, multi-dimensional PESs for each binding site, along with solving the five-dimensional quantum dynamics problem for the energy levels and displacements does, however, limit its application to select cases at the present time.

While quasielastic neutron-scattering (QENS) studies have been crucial in the extensive efforts to reach an understanding of the function of complex hydrides such experiments have rarely been carried out on sorption based systems. Translational diffusion of hydrogen in MOFs can readily be observed by QENS [136], but this may not be of sufficient interest for these system, as it is not in any way a rate — limiting factor in the operation of a typical sorption-based storage medium. Inves­tigations of materials with very small pores, or those including hydrogen spillover, may well require detailed QENS studies in the future for their understanding.

Knowledge about proton sites is essential to understand the properties of proton­conducting oxides, whether it be the local proton-dynamics, hydrogen-bonding interactions, or macroscopic proton-conductivity. Some early examples of ND measurements on proton-conducting perovskites in this area include by Knight [42] who reported on BaCe0.9Y01O2.95, Sata et al. [43] who reported on Sc-doped SrTiO3, Sosnowska et al. [44] who reported on Ba3Ca1.18Nb182O9_,5, and Kendrick et al. [45] who reported on La06Ba04ScO2 8. More recently, Ahmed et al. [46] reported on BaZr050In050O3_d, whilst Azad et al. [47] reported on BaCe0.4Zr0.4Sc0.2O2.9. One may note that most of these studies were performed on samples which were deuterated rather than hydrated, in order to reduce the inco­herent scattering and transform the scattering into a useful signal, which increases the chance of locating the positions of protons even in systems with low proton- concentration.

To give a representative example of the previous work we turn to the neutron powder diffraction measurements by Ahmed et al. [46] on BaZr0 50In0 50O3_d. As pointed out by the authors, the Rietveld analysis of the ND patterns (Fig. 9.4a) indicated no departure from cubic Pm3m symmetry. However, further analysis showed that the material is phase-separated into a deuterium-rich and non-deuter — ated phase with phase fractions of 85 and 15 %, respectively.

To determine the deuterium positions in the deuterium-rich phase, the authors calculated the Fourier-difference maps by taking the difference between simulated (with no deuterium in the structural model) and experimental data in reciprocal space; the Fourier-difference map taken at z = 0 is shown in Fig. 9.4b. Here, the positive peak at approximately (0.5, 0.2, 0) in Fig. 9.4b is consistent with the positive scattering-length of deuterium, suggesting that this is the deuterium site. However, from a closer analysis of the neutron data, it was found that the missing scattering density is distributed anisotropically within the ab plane, which suggests instead delocalization of the deuterium atom at the 24k site. It follows that there are eight equivalent deuterium-sites around each oxygen, which are tilted towards a neighbouring oxygen [46]. Such tilting increases the tendency for the formation of a strong hydrogen-bond between the deuterium and the oxygen towards which it is

tilted (c. f. Fig. 9.5a) and is consistent with results obtained both from first-principles calculations and infrared spectroscopy [48].

Further structural refinements based on the diffraction patterns revealed highly- anisotropic atomic displacement parameters (ADPs) of the oxygen atoms, as illustrated in Fig. 9.5b, as well as the deuterium site occupancy. The large ADPs

reflect static, localized, displacements of the oxygen ions around their ideal site in a cubic structure and most likely result from the structural disorder as induced by the difference in size between Zr4+ and In3+ [46]. Similarly, large ADPs have been obtained elsewhere for the composition BaZr0 33In0 67O3_a and those results were also attributed to static-disorder effects [49]. To conclude, the authors not only succeeded in determining the location and concentration of protons (deuterons) in the perovskite structure, but also revealed the presence of pronounced short-range structural distortions of the average cubic perovskite-structure, which are likely to affect considerably the proton transport in the material. More detailed information about the local structure, however, needs the use of more local probes, such as PDF analysis of neutron total-scattering data, which is highlighted below.

Maximising the signal from materials of interest and maintaining acceptable elec­trochemical performance has been the overriding factor in designing neutron — friendly batteries. Initial designs were plagued by the need for large quantities of electrode materials and the associated requirement to use low current to ensure the reaction of the bulk of the electrode, for example studies of LiMn2O4 electrodes used 5 g of material as shown in Fig. 7.27 (left) [190, 191]. This design has evolved to designs shown in Fig. 7.27 (right) [178] which increasingly resemble their commercial equivalents, allowing high current to be used and a more direct com­parison with commercial performance. For most of these examples the polyethylene separator is replaced with a separator containing a smaller amount of H, e. g. polyvinylidene difluoride, and the electrolyte solution is replaced with deuterated equivalents. By using the design in Fig. 7.27 it was possible to show the loss of long-range order of the MoS2 anode during its first discharge [178], the composite nature of the TiO2/Li4Ti5O12 anode [192], relaxation phenomena in LiCo0.16Mn184O4 cathodes [193], evolution of LiMn2O4 structure [190, 191], and the reaction mechanism evolution of LiFePO4 [188, 194].

Alternate designs have been developed for in situ neutron diffraction experiments and these include coin-type cells [195-197] which still feature relatively-thick electrodes but have been used to successfully investigate Li4Ti5O12, graphite, and LiFePO4. Similarly, pouch-type cells with alternate layers of cathode and anode — coated current collectors are applicable for investigating full cells, as opposed to the use of Li metal in the majority of the previous examples. Studies have been con­ducted on Li[Ni1/3Mn1/3Co1/3]O2llgraphite, Li[Li0.2Ni018Mn0.53Co01]O2Hgraphite [198], and LiNi0 5Mn15O4lLi4Ti5O12 [199] full cells.

The motivation for designing these neutron-friendly cells is that any electrode material can be tested in situ in a real cell. Effectively, some of these designs can be manufactured using relatively-small electrode sizes (0.5-1 g) allowing a variety of materials to be investigated, and the interplay between structure, electrochemistry, and reaction mechanism can be elucidated. This information can then be used to direct the choice of future electrode-materials.

Some of these cells have been used to extract time-dependent information which reveals the rate of reactions as a function current applied, relating structural per­turbations to electrochemical factors [188, 193]. Of particular note has been the study of the reaction mechanism of LiFePO4 [188]. The evolution of LiFePO4, either by a single-phase solid-solution reaction, or a two-phase reaction, during charge/discharge has been extensively discussed in the literature (see [188]). Some parameters that lead to a particular type of reaction mechanism being favoured have been detailed. However, there was a lack of time-resolved infor­mation concerning bulk-electrode behaviour in a commercially-equivalent cell to definitively establish the working mechanism of LiFePO4. Time-resolved in situ NPD data showed the evolution of the reaction mechanism of LiFePO4 during charge/discharge processes. This is significant because the experiment probed the material under real working-conditions at the bulk-electrode scale. It should be noted that the LiFePO4 sample used was expected to have only two-phase behaviour, and this work revealed a solid-solution reaction mechanism region during charge/discharge which is followed by a two-phase reaction mechanism. Moreover, the transition between the ‘competing’ reaction mechanisms was iden­tified and characterized to be a gradual transition with solid-solution reactions persisting into the two-phase reaction region, rather than an abrupt transition. Figure 7.28 details this evolution and the co-existing reaction mechanism region.

Therefore, in situ NPD not only provides information on the evolution of electrode structure, but also on the evolution of the (de)lithiation reaction mechanisms of the electrode. This information can be time-dependent and as a function of the electrochemical process, and can be used to design alternative electrodes that avoid, or undergo, certain reaction mechanisms to enhance battery performance.

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.

The use of neutron-scattering methods in atomic, and molecular level character­ization of virtually all types of potential hydrogen storage materials has provided remarkably fine detail of their structures and the associated hydrogen dynamics which would be impossible to obtain by any other single experimental technique. Hydrogen storage materials are, of course, ideal subjects for neutron scattering studies on account of the neutron’s sensitivity to hydrogen, the fact that both diffraction and spectroscopic investigations can readily performed, and that this can be done under a wide range of experimental conditions (pressure, temperature, hydrogen content) with relative ease.

In the case of complex and chemical hydrides it is the knowledge of hydrogen positions in the various crystal structures, identification of the nature of the H-species, in part by INS studies together with computation, nature of diffusive processes, either self-diffusion or aided by reorientations, which have helped to develop some tentative picture of possible discharge-processes in a number of important systems, and in some cases regeneration paths as well. A large number of important questions remain to be answered in detail, most notably perhaps, the role of the catalyst where it is necessary for the function of the material.

Neutron scattering studies of sorption-based storage systems have provided a wealth of detail of the mainly weak interactions of hydrogen molecules with the various host systems. Perhaps the most noteworthy aspect of these investigations is the remarkable sensitivity of the rotational tunnelling transition of the adsorbed H2 molecules, that can readily be observed by INS, and are found to range over more than two orders of magnitude. While we typically observe a higher barrier to rotation for a more strongly-bound molecule, we note that these barriers are not directly related to the centre-of-mass binding energies of the hydrogen molecule, but to angle-dependent interactions with the host. Nonetheless, these can be coupled by computational studies, which include the development of multi-dimensional potential energy surfaces. The effort to obtain such PESs and to understand hydrogen sorption in MOFs and other porous media by computational analysis has therefore spawned an extensive theoretical work to improve treatment of non­bonded, or mainly dispersive, interactions in DFT methods, which have long been overdue in the study of many other weakly-interacting systems.

Much of the detailed interpretation and computational analysis of some of the results described above depend on the fact the vast majority of the materials under investigation are crystalline so that the positions of the atoms can be known. These must be known, for example, as a starting point for most theoretical modelling studies. Difficulties do arise from the presence of disorder or various types, and the very large zero-point and thermal motion of the weakly bound hydrogen molecules in sorption-based systems. This problem becomes less and less tractable for storage systems that are amorphous, or for the liquid carriers, including those, which have chemical hydrides in suspension. Structural studies of amorphous and liquid sys­tems by the well-established pair distribution function (PDF) approach are therefore likely to become more common, as these approaches to hydrogen storage gain in importance. Structural results from these techniques (preferably a combination of neutron PDF for H and other light atoms, and X-ray PDF for heavier atoms) are probabilistic in nature rather than giving accurate positional parameters, but do bear a direct relationship to molecular dynamics simulations by way of comparisons of observed and calculated pair-correlation function. Spectroscopic studies by INS will suffer from inhomogeneous broadening of the lines from the disordered structures, and the fact that it is usually best done at low temperatures. Quasielastic neutron scattering may become more important in these cases as well, but may depend even more critically on molecular-dynamics simulations.

We can certainly expect that neutron scattering methods in conjunction with computational analyses will continue to play the pivotal role in the ultimate development and design of a practical, materials-based hydrogen storage system for mobile applications, which at this time is viewed as a long-range solution to this problem. In the near term, however, highly compressed hydrogen gas at room temperature is the storage medium of choice for most of the experimental, and commercial (starting in 2015) hydrogen powered fuel-cell cars, including battery hybrids, on the road.

The “extra” scattering between Bragg peaks adds information about the structure on a local scale and is therefore of high importance for structural studies of proton­conducting perovskites when the material is not fully periodic, such as when dis­order is present. In particular, the pair-correlation function G(r) is sensitive to key structural details, such as bond distances and angles, the symmetry of structural distortions, and oxygen and/or cation ordering, for example. The initial neutron total-scattering experiments coupled to PDF analysis of proton-conducting per — ovskites were done by Malavasi et al. [50, 51] on undoped and Y-doped BaCeO3. For the undoped material, the neutron total-scattering data suggests no difference between the long-range orthorhombic Pnma structure as determined by Rietveld refinement and the short-range structure as determined from PDF analysis, regardless if the sample is hydrated or not [51].

The good agreement between the long-range and short-range structures may be appreciated from Fig. 9.6, which, for the dry undoped material, shows the good fit of the Pnma model obtained from Rietveld refinement to G(r). For the dry Y-doped material, however, the fit of the Pnma model is less satisfactory, c. f. Fig. 9.7a. For this material, the authors instead found an excellent agreement between the G(r) and a structural model based on the lower-symmetry space group P21 (see Fig. 9.7b), indicating local regions around the Y dopant of such symmetry [51]. Moreover, it was found that the Y-doped material returns to the orthorhombic Pnma structure upon hydration, suggesting that the source of local structural distortion is mainly due to Y-induced oxygen vacancies and not linked directly to the substi­tution of cations [51]. Since the local structure around the proton can be expected to correlate strongly with the mechanistic detail of proton dynamics, such local structural information is of high interest and indeed crucial for the tailoring of new materials with higher proton-conductivities. The importance of understanding the details of local structure may be exemplified by the 10 % Y — and Sc-doped BaZrO3 materials, which both exhibit an average cubic structure, but for which the proton conductivity differs by several orders of magnitude [52].

Ultimately, the goal of in situ neutron diffraction is to track the Li content and location in crystalline electrode materials as a function of charge and discharge. This is a difficult task, that until recently was only demonstrated at limited battery states of charge [191] with long collection times and effectively under equilibrium conditions. Other studies have inferred Li content via electrochemical approxima­tions (amount of charge transferred) and the evolution of reflection intensities [196]. If the Li composition of an electrode can be reliably determined as a function of discharge/charge this would give a direct measure of the capacity of the battery, or more accurately, the capacity of the battery that is stored in the crystalline com­ponent of the electrode.

Recently, the ability to track the Li location and content as a function of time (and charge/discharge) has been demonstrated using commercial Li1+yMn2O4 cathodes [189]. Arguably, this represents the most Li-centric view of a Li-ion battery during operation. The Li evolution is found to differ at a structural level during charge/discharge (Fig. 7.29) accounting for the ease of discharging these types of cathodes, relative to charging. Additionally, the Li evolution is shown to progress from one to two crystallographic sites during the charge/discharge pro­cesses. The lattice parameter follows a linear relationship with Li content during single Li site processes (Vegard’s law) and during processes involving two Li sites the relationship between the lattice parameter and the Li occupancy and site is a linear combination of the individual single site processes (Fig. 7.30). This work provided unparalleled insight into the function of the cathode and is used to understand the origins of how the electrode functions. Further studies on structural permutations may provide insight on how these electrodes can be improved from the perspective of the Li.

LiNi05Mn15O4 is attracting significant attention for cathode applications due to the high-voltage redox couple during battery function, and Li4Ti5O12 is attracting attention for anode applications due its small volume change during Li insertion and extraction. By specifically constructing a neutron-friendly cell made of this elec­trode-combination it was possible to study the structural evolution of these mate­rials using time-resolved in situ neutron diffraction [199]. This highlights another advantage of using custom-made cells for in situ neutron diffraction experiments, where research is not limited to commercially-available materials. In this case, it was possible to determine the evolution of Li occupation in the cathode and indi­rectly infer the Li occupation in the anode (Fig. 7.31) in addition to determining the reaction mechanism evolution for the electrodes. It was found that a solid-solution reaction occurred at the cathode with the Ni2+/Ni3+ redox couple at *3.1 V and a two-phase reaction with the Ni3+/Ni4+ redox couple at * 3.2 V. Thus, the extraction of Li from the cathode and insertion of Li into the anode during charge was directly determined, again in real-time. This opens up a way to evaluate a range of materials used as electrodes in Li-ion batteries, where how Li is extracted and inserted while a battery functions can be determined.

Fig. 7.29 a The evolution of Li at the 8a (black) and 16c sites and in a formula unit of Li1+yMn2O4 (lower plot) during charge/discharge. The discharge process shows an increase in Li content, whilst charge shows a decrease. b A representation of the sites described in (a) for Li1+yMn2O4. Reprinted from (N. Sharma, D. Yu, Y. Zhu, Y. Wu, V. K. Peterson, Chem. Mater. 25, 754 (2013)) [189]
Fig. 7.30 The relationship between Li site occupancies and the lattice parameters of Li1+yMn2O4. Single-site regions are shown in black and red and mixed-site regions are in blue and purple. Reprinted from (N. Sharma, D. Yu, Y. Zhu, Y. Wu, V. K. Peterson, Chem. Mater. 25, 754 (2013)) [189]
8.22
8.20
to 8.14
a 8.12 —
8.10 —
8.08 —
8.06 —
Lithium content in the formula unit

Kirt A. Page, Joseph A. Dura, Sangcheol Kim, Brandon W. Rowe and Antonio Faraone

Abstract Polyelectrolyte membranes (PEMs) have been employed as solid elec­trolytes in fuel-cell technologies as early as the 1950s, when they were used in NASA’s Gemini program. However, PEM materials have only gained wide-spread attention in the last two decades due to advancements in membrane electrode­assembly (MEA) formation and the synthesis of new and interesting materials. Over the past several decades, various neutron techniques have played an instrumental role in measuring the structure and transport properties of PEMs in order to develop a deeper understanding of structure-property and performance relationships in PEM materials for fuel-cell applications.

10.1 Introduction

Proton-exchange (or polyelectrolyte) membrane fuel-cells (PEMFCs) have received increasingly more attention over the last two decades and is the principle subject of this chapter [1]. More specifically, this chapter presents an overview of how neutron techniques have been used to study polyelectrolyte membrane (PEM) materials. For an historical perspective on the use of polymers in fuel-cell technologies, the reader is encouraged to consulting the existing body of literature on the matter.

Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

K. A. Page (H) • S. Kim • B. W. Rowe

Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA e-mail: kirt. page@nist. gov

J. A. Dura • A. Faraone

Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg,

MD 20899, USA

© Springer International Publishing Switzerland 2015 273

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_10

It is generally understood that one of the key material properties influencing the conduction of protons through the PEM is the morphology of the material. The size — scale of the morphological features typically present in PEMs make small-angle neutron scattering (SANS) an ideal tool for probing the morphology. Additionally, researchers have recently shown an increasing interest in probing the structures that are present in PEM materials at interfaces, as these materials are used as binders in the catalyst layer and can be confined at interfaces with various types of material surfaces. For this, researchers have employed neutron reflectometry (NR). While the morphology of the material serves to provide a pathway for proton conduction, it is also understood that the polymer dynamics can play a role in charge transport. Moreover, the presence of water and water transport/dynamics is critical for optimal fuel-cell performance, and is therefore vital to elucidate the role and interdependent relationship that polymer and water dynamics have on charge transport in PEM fuel cells. Researchers have turned to neutron spectroscopic techniques such as quasi­elastic neutron scattering (QENS) and neutron spin-echo spectroscopy (NSE) to investigate the polymer and water dynamics in hydrated PEM materials.

The following chapter is an overview of the efforts to use neutron-scattering methods to study the structure and transport/dynamics in PEM materials and is divided into three sections. The first section gives a very brief overview of the characteristics of PEMs and highlights the material most studied using neutron techniques. The second section summarizes the structural studies on PEMs to date and demonstrates how techniques such as SANS and NR have aided in characterizing the bulk morphology and structures at interfaces, respectively. The third section focuses on the transport and dynamics in these materials, specifically describing how neutron spectroscopic techniques have been used to study the ion and water dynamics in hydrated PEMs. This chapter is not a comprehensive review of PEM materials and neutron techniques, but is intended to provide the reader with a demonstration of the many ways in which neutron measurements can aid in the understanding of structure — property and performance relationships in PEM materials.