The progress of Li-ion batteries is severely hindered by the complexity of the chemical and physical processes and most importantly, the difficulty of observing these processes in situ during operation. Direct observation of Li ions in a battery in a non-destructive way is not possible by any conventional material analysis tech­nique. The consequence is that a large number of practical and fundamental questions remain including: how do Li-ion battery degradation mechanisms depend on battery conditions? How do the structural changes and electrochemical processes depend on the (dis)charge rate, and what actually determines the (dis)charge rate? An understanding of the interplay between structure, electrochemistry, and reaction mechanisms is required for battery design.

To answer these questions we require time-resolved and non-destructive struc­tural information including Li-ion positions and the Li distribution under operando conditions. These possibilities are offered by in situ neutron techniques that have been realized by the recent developments in neutron sources, detectors, and analysis methods. In situ neutron diffraction enables researchers to follow the structural changes and Li-positions upon all possible electrochemical manipulations in both the positive and negative electrodes. In situ neutron depth profiling determines Li — ion concentrations with high resolution in flat electrodes giving direct insight into the cylindrical batteries. Finally in situ neutron imaging allows a full three­dimensional picture of Li distribution in the battery to be determined. Recent advances in these three techniques will be discussed, including one of the major challenges, the cell design.

Hydrated alkali thio-hydroxogermanates, MxGeSx(OH)4_x-yH2O, where M = Na, K, Rb or Cs; 1 < x < 4; y * 1, represent a novel class of amorphous proton­conducting materials, which were first synthesized by Poling et al. [16, 82] at Iowa State University. The conductivities of these materials typically reach a maximum of the order of 10~2 Scm-1 in the intermediate-temperature range of 100-300 °C [16], which competes with even the best perovskite-type oxides. Karlsson et al. [85] reported a structural investigation of these materials using a combination of neutron diffraction and first-principles calculations. A key result from the experiment was that the neutron structure-factors of the hydrated and dehydrated materials (Fig. 9.13) are overall similar to each other, indicating that there are no dramatic structural changes such as phase transitions or structure degradation as the materials are dehydrated. In order to gain understanding for what such a structure may look like, the authors proceeded their analysis by generating a candidate three-dimen­sional structure of the Cs-based compound, by taking the orthorhombic crystal structure ofNa2GeS2(OH)25H2O as the starting point in the calculations, replacing the Na ions with Cs ions and reducing the number of water molecules from five to one in order to agree with the real composition. Snapshots of the generated structures of dehydrated and hydrated materials are shown in Fig. 9.14, whereas Fig. 9.15 shows a close up of the local configuration of the hydrated material.

In the hydrated state (Figs. 9.14a and 9.15), the calculations suggest a structure built of thio-hydroxogermanate anion dimers connected to each other via hydrogen bonds to water molecules located between the dimers. In the dehydrated state (Fig. 9.14b), the calculations suggest instead that the thio-hydroxogermanate anions form an extended network through the creation of inter-dimer hydrogen bonds, whereas the alkali ions are found to act as “space fillers” in “voids” formed by the thio-hydroxogermanate anion dimers in both the hydrated and dehydrated state. These generated structures are justified by comparing the experimental and calcu­lated pair-correlation functions, which are shown in Fig. 9.16. It can be appreciated that the experimental and calculated pair-correlation functions are overall similar

which suggests that the real structures are at least reasonably-well described by the structural models for both the hydrated and dehydrated materials. However, to obtain a complete picture of the structure and elucidate how the structure depends on the type and concentration of alkali ion, further investigations, such as reverse Monte-Carlo simulations of diffraction data, are necessary.

Borohydrides are also under investigation as potential hydrogen storage materials, particularly those with high hydrogen contents. For instance, LiBH4 can be reformed from the elements at 973 K and 150 bar H2 [31]. Conditions are slightly milder when LiBx precursors such as LiB3 or Li7B6 intermetallics are used. Other systems with limited reversibility include e. g. Mg(BH4)2 or Ca(BH4)2 which can be (partially) rehydrogenated at high pressures (90 MPa) and high temperatures (673 K) [32, 33]. Mg(BH4)2 would be an attractive storage material since it has a gravimetric storage capacity of 14.9 wt.% H2 and the reported enthalpy of reaction is 39 kJmol-1 for the reaction Mg(BH4)2 ^ MgH2 + B + 3H2 [34], so that the estimated equilibrium pressure at RT is 1 bar H2.

The structure of LiBH4 at low temperature is orthorhombic (LT) and changes near 380 K to a hexagonal high-temperature (HT) structure [35]. The four H atoms are bound covalently to the central B atom. Early structural studies found a strong asymmetry of the BH4-tetrahedron [36] in the LT phase, while later reports describe the geometry of the BH4-entities to be close to that of an ideal tetrahedron both below and above the structural phase transition [35]. The B-H distance was esti­mated to be dB-H = 1.16-1.26 A in the HT phase from synchrotron X-ray diffraction [35], while the B-D distances were determined to be dB-D = 1.18-1.20 A at 302 K in neutron diffraction experiments [37]. The structural phase transition is accompanied by increased thermal motion of the BH4-complexes, which is indicative of an order — disorder transition. The nature of the reorientations of the BH4-tetrahedron was investigated using quasielastic neutron scattering (QENS) for both the high tem­perature and low temperature phase. The type of reorientation was identified from the elastic incoherent structure-factor (EISF), which is proportional to the Fourier transform of the probability density of finding a hydrogen atom at a given position. The hydrogen mobility in both phases can be described by rotational jumps of the BH4-tetrahedra. The mean residence times т between successive jumps are on the order of a few picoseconds at temperatures near the phase transition [38]. While 120° rotations around one C3 symmetry axis of the tetrahedron could be identified in the LT phase [38], but there are also clear indications for two different dynamic processes with distinct activation energies both from NMR data [39], as well as from inelastic fixed window scans in QENS experiments [40]. The reported acti­vation energies are 162 ± 2 and 232 ± 11 meV, respectively. The BH4 dynamics becomes even faster above the phase transition [38] and the QENS data can then best be described by an orbit exchange-model [41] where three out of the four H atoms rotate fairly freely around a C3 axis while the axial H displays only occa­sional jumps as illustrated in Fig. 8.3.

The EISF obtained for HT LiBH4 may be compared (Fig. 8.3) with theoretical models for several reorientation mechanisms, such as 120° rotations around the C3 axis, tetragonal or cubic tumbling and the orbit exchange-model. It is apparent that it is essential to monitor as much of the reciprocal space as possible in the QENS exper­iments for the identification of the true nature of the motions, since a clear distinction

between different models can only be made above Q = 2.5 A 1. The best agreement between the prediction of a given reorientation model and the experimental data is obtained for the orbit exchange-model where frequent H jumps occur between sites on a circle around the C3 axis while the axial H only occasionally jumps (see Fig. 8.3, lines d and e). The model was calculated using either N = 6 or 12 equivalent positions on the circle, however already N =6 was sufficient to describe the data.

While reorientation of the BH4 — tetrahedra has been observed at temperatures as low as 175 K, translational diffusion of BH4 could only be seen above the melting transition [42] at 553 K. Exchange of atomic hydrogen between adjacent BH4 tetrahedra remains slow even at these temperatures and transport of H takes mainly place by way of the BH4- units. Li+ mobility, on the other hand, becomes much faster when going from the LT to the HT phase, which is indicated by a an increase of three orders of magnitude of the Li+ conductivity from 10-6 -10-8 Scm-1 in the LT phase to 10-3 Scm-1 in the HT phase [43]. Substitution of the BH4- units by I in LiBH4/LiI mixtures results in a stabilization of the HT phase at room temperature (RT). QENS measurements accordingly show significantly-enhanced dynamics of BH4 at RT compared to the LT phase (at RT) and a similar reorientation mechanism as in the pure LiBH4 HT phase [44]. The stabilization of the HT phase by I substitution also preserves the high Li+-conductivity at RT. It has been suggested that Li+ conductivity is supported by the BH4- movements [43] in a paddle-wheel like mechanism.

The alkaline borohydrides NaBH4 and KBH4 exhibit a structural phase-transi­tion associated with an order-disorder transition of the BH4 subunits, from a tetragonal LT phase to a cubic HT phase (at * 190 K for NaBH4 [45], and 65-70 K for KBH4 [46]). QENS data on NaBH4 suggest reorientational jumps around the C2 and C3 axis, which is in agreement with the crystal symmetry. Reorientation of the BH4 entities around a C4 axis have been reported for both NaBH4 and KBH4, so that the four hydrogens of the tetrahedron occupy the eight corners of a cube but with half occupancy [47]. Thermodynamic measurements provided evidence for an order-disorder transition in RbBH4 at 44 K and in CsBH4 at 27 K [48] but this could not be discerned from neutron powder diffraction down to low temperature [46]. However, vibrational spectroscopy of the torsion band indicates that the heavier alkali borohydrides also exhibit a similar order-disorder transition although the ordering of the BH4 seems to occur on a shorter length scale [47].

The variety of structural phases is especially rich for the alkaline earth borohy — drides Mg(BH4)2 and Ca(BH4)2. Both of these can be synthesized in solvent free form, but the crystal structure depends strongly on the preparation conditions. Two polymorphs were initially identified for Mg(BH4)2: an а-phase, which transforms irreversibly into P-Mg(BH4)2 at temperatures above 490 K. Both polymorphs have unexpectedly-complex crystal structures which differ from numerous theoretical predictions [49-52]. Crystal structure determination from powder data proved dif­ficult especially concerning the hydrogen positions but eventually, the structure of the а-phase was identified as hexagonal with space group P6122 [50]. Adjacent BH4- groups are coordinated by a central Mg via two opposite edges of the tetrahedron, but the resulting Mg-H2BH2-Mg bridges are not exactly planar. a-Mg(BH4)2 has 6.4 % unoccupied voids (amounting to 37 A2 per unit cell) whereas P-Mg(BH4)2 which has an orthorhombic structure (Fddd) and is much more dense with no unoccupied voids. The а and P-polymorphs are built up of a corner sharing network of Mg2+[BH4 ]4 tetrahedra. A highly porous form, y-Mg(BH4)2, was synthesized more recently, and found to possess a cubic structure (Id 3a) with a network of interconnected channels and 33 % porosity, which could be utilized for the adsorption of molecular hydrogen (or nitrogen) at low temperatures [52]. Applica­tion of pressure (1-1.6 GPa, diamond anvil cell) causes the structure to collapse and results in tetragonal 6-Mg(BH4)2 [52]. Synchrotron X-ray diffraction studies iden­tified yet another polymorph, e-Mg(BH4)2, during the decomposition y-Mg(BH4)2 [53]. DFT calculations find that many structural models for Mg(BH4)2 are nearly degenerate in energy, which can explain difficulties with the correct prediction of crystal structures and decomposition pathways [54, 55]. Moreover it is likely that more polymorphs of Mg(BH4)2 exist depending on preparation conditions (and/or impurity content).

The reorientational motion of the BH4-tetrahedra in P-Mg(BH4)2 have been studied using QENS on two instruments with different energy resolution and therefore different timescales. Two thermally-activated reorientation processes have been observed on these timescales in the temperature range from 120-437 K, and have been identified as rotations around the C2||-axis (which connects the two Mg atoms in the Mg-H2BH2-Mg bridge) or around the C3-axis of the tetrahedron [56] (Fig. 8.4).

Fig. 8.4 Left The measured and modelled EISFs for p-Mg(BH4)2: Using the mica analyser high resolution time-of-flight backscattering spectrometer (MARS) at the Paul Schemer Institute: (red plus) 120 K, (blue x) 150 K, (red triangle) 180 K, (green asterisk) 210 K, and (black hexagon) 240 K. Using the spectrometer for high energy resolution (SPHERES) at the Heinz Maier-Leibnitz Zentrum: (red square) 318 K, (green triangle) 365 K, and (blue circle) 473 K. The calculated EISF are for the reorientation models: (—) C2n (MARS data) and C3 (SPHERES data) rotational diffusion-model and hindered rotation diffusion-model (—). Right: BH4 reorientation as hindered rotation around C2||. Reprinted with permission from (P. Martelli, D. Blanchard, J. B. Maronsson, M. D. Riktor, J. Kheres, D. Sveinbj, E. G. Bardaji, J. Phys. Chem. C 116, 2013 (2013)) [56]. Copyright (2013) American Chemical Society

The activation energies were determined to be 39 ± 0.5 meV and 76 ± 5 meV for the C2|| rotation and 214 ± 4 meV for the C3 axis rotation, respectively [55]. Low — frequency vibrational spectroscopy using Raman scattering revealed a complex structure of both internal and external modes in a mixed a/p-Mg(BH4)2 sample which is considerably modified when passing through the phase transition [57]. Neutron spectroscopy revealed an additional band which was attributed to BH4 librations [56] but more spectroscopic work along with DFT calculations is needed for a more comprehensive understanding of Mg(BH4)2.

Ca(BH4)2 is another interesting candidate for storage application with a gravi­metric hydrogen storage density of 11.6 wt.% H2 or 130 kgm-3 of H2. There are four known polymorphs: a, a’, and у-phase, which are considered LT phases while P-Ca (BH4)2 is an HT phase. The a and у modification have an orthorhombic structure (Fddd [58, 59] or F2dd [60] for a-Ca(BHr)2 and Pbca [58] for y-Ca(BH4)2) whereas the a’-modification is tetragonal (142d [59]). Above 400 K, a tetragonal (P4) high temperature phase P-Ca(BH4)2 is observed [57, 58]. Each Ca is surrounded by 6 boron atoms in the a, P, and у-phases forming a CaB6 octahedron [58] and the polymorphism results from different arrangements and connections of the CaB6 octahedra. The energy differences of the different polymorphs are small [57, 61] which explains why frequently more than one crystalline phase is found in the same sample batch. Low energy spectroscopic measurements indicate that the phase stability is linked to the librations of the BH4- units [62] and it was suggested that entropic contributions are driving the phase transitions. Moreover, the different polymorphs exhibit different decomposition properties [63], hence a thorough understanding of the bare materials is essential for their potential application in solid-state hydrogen storage. The hydrogen dynamics in (predominantly) P-Ca(BH4)2 have been investigated by QENS [64]. Two different thermally-acti­vated rotational-reorientation processes have been identified in the temperature range from 100 to 260 K, which were attributed to rotations around the C2 and C3 axis, respectively. Translational diffusion with a jump length of 2.5 A was also observed, which the authors attributed to the H2 diffusion of trapped impurities, while no long-range diffusion of the BH4 units was observed at these temperatures.

Besides the pure alkaline and alkaline-earth metal borohydrides, mixtures combining different cations have been investigated [65-67] in an attempt to modify the reaction enthalpy for hydrogen desorption. The decomposition temperature for borohydrides correlates with the electronegativity of the cation [68, 69] and thus dual cation borohydrides could be ideal candidates for tuning the thermodynamic properties.

In addition to the studies of water dynamics in PEM materials, inelastic scattering techniques have been used to examine the dynamics of ions in neutralized mem­branes as well as the polymer-chain dynamics. The aim of these studies is to gain a deeper fundamental insight into the mechanisms of charge transport and to examine the correlation between ion motion and polymer-chain dynamics. In the earliest studies of this kind, Rollet and co-workers [74, 75] used QENS to probe the dynamics of N(CH3)+ ions in hydrated (D2O) Nafion® membranes. Nafion® and D2O both have very low incoherent scattering cross-sections, and hence the mea­sured scattering in these studies is dominated by the hydrogenated counter-ion, allowing for directionality of ion motions, as a function of ion concentration, to be determined. In order to extend the time-range over which the ion dynamics could be probed, nuclear magnetic resonance and radiotracer experiments were also per­formed. For the given set of experimental conditions outlined, they found that at short time-scales the self-diffusion of the ions within the water domains of Nafion® was similar to that found in non-confined solutions. With increasing electrolyte concentration the self-diffusion coefficient of N(CH3)+ was found to decrease, a phenomenon thought to arise from viscosity effects. For long-range diffusion, the transport of ions was found to be limited by the tortuosity of the diffusion path which is in large part determined by the channels connecting the water domains.

Page and co-workers [76—79] have also used inelastic neutron methods to study the molecular dynamics of Nafion® as part of an effort to further the fundamental understanding of the role of electrostatic interactions in relaxation phenomena observed in these materials. QENS was used to measure the dynamics of the counter-ions in perfluorosulfonate ionomers (PFSIs) neutralized with various alkyl ammonium ions. While the work by Rollet and co-workers focused on measuring the counter-ion dynamics at room temperature in hydrated systems, Page and co-workers measured the counter-ion dynamics in dry systems over a range of temperatures in the vicinity of the corresponding alpha-relaxation temperature, which is highly dependent of the choice of counter-ion. Transitions in the counter­ion dynamics were correlated with bulk mechanical-relaxations in an effort to directly observe the ion-hopping process thought to be the mechanism for long — range diffusive motions of polymer chains and ions in these systems. This work explicitly showed that the counter-ion dependent alpha-relaxation, observed in thermo-mechanical analysis, is linked to the onset of mobility of the counter-ions on the length-scale of 2-3 nm. These data, taken together with other studies, dem­onstrate that the motions of the ions and the polymer chains are highly correlated in these systems (Fig. 10.11).

While work on dry, neutralized forms of Nafion® has helped to further the fundamental understanding of the complex molecular-relaxation behaviour, i. e., the relationship between electrostatic interactions, counter-ion dynamics, and polymer chain relaxations, these model studies are less useful in understanding the inter­dependence of water dynamics/transport and polymer-chain motions in the more device-relevant, acid-form hydrated Nafion® membranes. More recently, Page and co-workers have begun using NSE spectroscopy to probe the relationship between water content and polymer-chain dynamics in Nafion®. NSE enables the study of slow relaxation processes in polymeric systems [80] and for Nafion® samples hydrated with D2O, the polymer-chain dynamics can be measured by monitoring the intermediate-scattering function, I(Q, t), at a Q corresponding to the length-scale associated with the distance between the individual polymer chains. An example of the results from an ongoing study can be found in Fig. 10.12. The I(Q, t) decay was fit with a Kohlrausch-Williams-Watts (KWW) function (Fig. 10.12) and it was determined that with increasing water content and temperature, the relaxation time for chain motions decreased (i. e. chain motions became faster). Interestingly, the timescale associate with the polymer-chain motions plateaus at value near a k of 6, which is also where other studies have shown a transition in the dynamics and transport of water [51]. This application of NSE indicates that there is a degree of coupling between the local water dynamics/transport and the polymer-chain dynamics. This influence of water on the polymer-chain dynamics provides a molecular-level understanding of the observed decrease in Young’s modulus with increasing humidity and temperature. The increased mobility in molecular relax­ations induced by the presence of water points to the molecular origins of the temperature — and humidity-dependent softening mechanisms in Nafion® and other poly (perfluorosulfonic acid) membrane materials.

The highest proton-conductivities in polycrystalline samples of perovskite-struc — tured oxides are generally found in barium cerates (BaCeO3-based compounds), however, these materials react with CO2 and/or H2O at intermediate temperatures to form BaCO3 (or Ba(OH)2) and CeO2 and therefore degrade with time. Hence, they are poorly suited for use in fuel cells [26]. Strategies to increase the chemical stability of such materials include substitution (doping) of different types of atoms, but improvements are generally small. In comparison, barium zirconates (BaZrO3- based compounds) show excellent chemical stability in CO2 and H2O-containing atmospheres and are in this respect more suitable for use in fuel cells from an application point of view. The conductivity of one specific barium zirconate, namely 10 % Y-doped BaZrO3 (included in Fig. 9.2), indeed exhibits the highest bulk proton-conductivity reported for any oxide material [26]. However, barium zirconates are difficult to sinter, which implies that barium zirconate samples contain a relatively-large volume of grain boundaries, which decrease the total conductivity [26, 35-38]. The sinterability of barium zirconates may be enhanced with the use of sintering aids [36, 39] or by the introduction of a second dopant at the B site of the perovskite [40, 41], however, the bulk conductivity is then typically lowered.

9.1 Neutron Scattering of Proton-Conducting Perovskites

The development of new perovskite structures with improved conductivity, thermodynamical stability, and sinterability, depends on the exploration of new classes of compounds as well as an increased understanding of the basic science of those materials already known. Such investigations should elucidate key material detail such as crystal structure, proton sites, proton concentrations, hydrogen­bonding interactions, and the mechanics of proton dynamics on different time — and length-scales, as well as to clarify how these details correlate with each other. For this purpose, neutron scattering offers the unique potential to access simultaneously information in both space and time, through the momentum (hQ) and energy (hE) transferred in the scattering event, respectively. This combination makes neutron scattering a powerful tool for investigating structures (using neutron diffraction (ND)), vibrational dynamics and hydrogen-bonding interactions (using inelastic neutron scattering (INS)), and diffusional dynamics (using quasielastic neutron scattering (QENS)). Neutrons can also be used to obtain details concerning the proton concentration in the sample through prompt-gamma activation analysis (PGAA). Therefore, neutrons offer good opportunities to advance the understanding of state-of-the-art proton-conducting perovskites. In this context, the remainder of this chapter aims to give a flavour of the important role that neutron methods play in providing a deep insight into the functionality of these materials. Emphasis is put on barium zirconates, due to their great promise for fuel-cell applications.

Examples of recent neutron work on proton-conducting perovskites, which are here briefly reviewed, include studies of proton sites and local structures using ND (Sect. 9.3.1), studies of vibrational proton dynamics and hydrogen-bonding inter­actions using INS (Sect. 9.3.2), studies of proton diffusion using QENS (Sect. 9.3.3), and studies of proton concentrations using PGAA (Sect. 9.3.4). No attempt has been made to be complete in this work, with the aim rather to highlight the different types of information that can be obtained using neutrons.

Historically, in situ NPD has seen comparatively fewer applications to Li-ion battery research relative to in situ X-ray or synchrotron diffraction (see Ref. Brant et al. [177] for further details regarding in situ X-ray-based studies). This is due to a variety of factors, including the inherent complexities of the measurement tech­nique, sample requirements, and the number of neutron diffractometers available for such experiments. However, in recent years neutron diffractometers and research trends have overcome the perceived difficulty of such complex experiments.

Unlike conventional neutron measurements where, in most cases, only the material under study is in the beam, with in situ methods, everything comprising the device can be in the beam, and thus contribute to the observed signal. For dif­fraction, H can be particularly problematic in the analysis of batteries [178] as the separator (e. g. polyethylene), electrolyte solutions, and the binder are often H-rich. Adding further to the background signal is the liquid or paste-like electrolyte. Therefore, attention has been devoted to custom-made cells for in situ neutron diffraction studies.

Commercial batteries are often produced with minimal quantities of electrolyte to maximise lifetime and avoid the issue of electrolyte leakage. Additionally, the electrodes are often coated on both sides of current collectors, and the overall of quantity of electrodes is significantly larger than that achieved in custom-made batteries. Furthermore, these batteries can be cycled at relatively high rates and are used in “real-world” applications. These considerations can outweigh the detri­mental contribution to the background that H-containing components make and yield significant information on the evolution of electrode structure.

As the challenges in battery design and construction are investigated, we also look toward the best instruments for this task by considering the neutron flux, detector, and acquisition time.

Proton-conducting solid acids are compounds, such as KHSO4 and CsHSO4, that feature spectacular phase transitions during heating for which the proton conduc­tivity increases by several orders of magnitude [81, 86, 87]. CsHSO4, for example, has a phase-transition temperature of 414 K [81]. Below this temperature, CsHSO4 has a monoclinic structure in which the number of protons is equal to the number of proton sites. Consequently, the hydrogen atoms are localized within rigid hydrogen bonds between SO4 tetrahedra and hence their mobility is low. In the high-tem­perature phase, the SO4 tetrahedra can rotate rather freely between crystallo — graphically identical positions, creating six times as many possible proton sites as there are protons available. As a result, an almost isotropic and dynamic hydrogen­bonding network between the different sulfate groups is created, where all oxygens are involved in hydrogen bonding. In this hydrogen-bonded network, proton dif­fusion is a fast process which occurs through proton jumps between neighbouring SO4 groups, as assisted by rotational motion of these groups.

Chan et al. [88] addressed the question of how the addition of nanoparticles, such as SiO2 and TiO2, impacts on the CsHSO4 phase-transition temperature and proton conductivity. Using QENS the authors showed that nanostructuring has the twin effect of lowering the superprotonic phase-transition temperature and increasing the local diffusional-dynamics in the superprotonic phase. The results are summarized in Fig. 9.17, which shows (a) the QENS spectra of bulk CsHSO4 and nanocomposite CsHSO4 with SiO2 (7 nm), (b) the quasielastic width, and (c) the fraction of mobile protons as derived from the quasielastic intensity as a function of temperature. As can be seen in Fig. 9.17b, the phase-transition temperature for the nanostructured

samples is reduced to a least 360 K. As suggested by Chan et al. [88], this behaviour may be linked to the creation of space-charge layers between the conducting phases and the nanoparticles. The creation of such space-charge layers would lead to an increase of the number of vacant sites for the protons to move to, hence allowing a larger fraction of the protons to become as mobile as they are in the superprotonic phase. Indeed, Fig. 9.17c shows that up to 25 % of the protons are mobile in the nanocomposite sample below the phase-transition temperature of 414 K, whereas no sign of proton mobility can be seen in the bulk sample at those temperatures. The results suggest that nanostructuring may be a rewarding research direction in the pursuit of optimized proton-conductivity.

Lithium amides have also been studied in some detail for hydrogen-storage applications following the discovery of Chen et al. that mixtures of LiNH2 + 2 LiH can be reversibly cycled according to the following reactions [70]:

LiNH2 + LiH $ Li2NH + H2

6.5 wt.% H2; AH = 45 kJmor1

Li2NH + LiH $ Li3N + H2

5.5 wt.%H2; AH = 161 kJmor1

Experimentally 11.5 wt.% H2 (relative to the hydrogen-free Li3N) are reversibly stored in the mixture, and temperatures of 473 K and 593 K are required for the first and second reaction step, respectively. Pure LiNH2 decomposes by evolving ammonia in contrast to the reaction system LiNH2 + LiH. Some of the Li can be substituted by Mg which leads to lower desorption-temperatures and a lower reaction enthalpy [71]. During the first desorption 2 LiNH2 + MgH2 undergoes a metathesis reaction [72-74] and the resulting reversible reaction is modified to:

Mg(NH2)2+ 2LiH $ Li2Mg(NH)2+ 2H2

8.5

5.6 wt.% H2 AH = 39 kJmor1

The reaction takes place at 473 K and in total 5.6 wt.% H2 can be reversibly exchanged, but hydrogen release takes place in distinct reaction steps. The high reaction temperature suggests the presence of kinetic barriers hampering the hydrogen exchange reactions and research efforts are focused on the understanding of the reaction steps and the mitigation of the kinetic limitations. Neutron powder

diffraction was used to determine the structure of the hydrogen-rich components LiNH2 [70, 75], Mg(NH2)2 [76] as well as of the hydrogen-depleted imide forms Li2NH [77, 78], a-Li2Mg(NH)2, P-Li2Mg(NH)2 [79], and MgNH [80], all of which are observed as intermediate decomposition-products. With the exception of MgNH, the experimentally-observed crystal structures exhibit characteristic simi­larities, i. e. they are related to the antifluorite structure where the N atoms build an approximate face-centred cubic lattice with the Li or Mg ions (and vacancies) residing in tetrahedral interstitials (see Fig. 8.5). Substitution of Li+ with Mg2+ leads to vacancy ordering on the tetrahedral sites, which results in an orthorhombic structure of a-Li2Mg(NH)2 with an approximately doubled unit cell relative to cubic Li2NH [73]. First-principles calculations identified the hydrogen and cation local arrangement as the structural building blocks of the amide/imide structure [81] and their energetics is consistent with the high degree of disorder that is observed in the mixed imide Li2Mg(NH)2, especially when going from the low temperature а-modification to the high temperature P-phase.

The structural phase evolution during hydrogen absorption and desorption of Mg(NH2)2 + 2LiH has been monitored in situ using neutron powder diffraction (Fig. 8.6). Several studies found that the reaction passes through an intermediate

reaction step, however, the composition of the intermediate phase was not solved unambiguously. Weidner et al. suggested an intermediate phase of composition Li2Mg2(NH)3 [82] which was subsequently also identified in systems with LiH excess [83].

Aoki et al. [84] reported an intermediate phase with composition Li4Mg3(NH2)2(NH)4 for the system Mg(NH2)2 + 6LiH which upon further hydrogen release exhibits a continuous transition towards Li2Mg(NH)2 by way of solid- solution-like compounds of the form Li4+xMg3(NH2)2-x(NH)4. Intermediate phases with non-stoichiometric composition have also been observed experimentally during the decomposition of LiNH2 + LiH [85, 86] and possible intermediates have been investigated using first-principles calculations [87, 88]. The defect structure and vacancy ordering for both systems, LiNH2 + 2LiH and Mg(NH2)2 + 2LiH, has a significant influence on energy landscapes of the systems. Further efforts on theory and experiment are needed, however, to fully comprehend this system.

Additions of LiBH4 to the Mg(NH2)2 + 2LiH system have been reported to improve the hydrogen-exchange reaction because of the intermittent formation of Li4(BH4)(NH2)3 [89]. Neutron powder diffraction on annealed samples of 1LiBD4 + 3LiNH2 was used to characterize the structure.

The beneficial effect of the LiBH4 on the desorption properties is thought to stem from a combination of factors, including an altered reaction-pathway from the removal of intermittent LiNH2 from the mixture, improved recrystallization of Mg (NH2)2, and the exothermic heat-effect during the formation of the quaternary compound Li4(BH4)(NH2)3 which has body-centred cubic symmetry [90].

Mixtures of LiNH2/LiBH4 with varying composition have also been investigated as hydrogen storage materials. More than 10 wt.% H2 is released in an exothermic process [91-93] for the stoichiometric composition (LiNH2)0.67(LiBH4)0.33.

While not an extensive review of the field, the work described here shows the critical role that neutron techniques play in the research and development of PEM fuel cells. Interestingly, neutron techniques can serve researchers at several points along the process of developing a working and high-performance PEM fuel cell. At the earliest stages, neutron scattering can inform chemists of the structures that develop given the choices made in molecular architecture. In turn, it can then be determined how these structures may impact important materials properties such as water and ion transport. Finally, state-of-the-art neutron methods can be used to monitor and analyse operating PEM fuel cells and aid in determining operating conditions for optimum fuel-cell performance. Therefore, researchers have the tools necessary to correlate materials chemistry and structure to overall device perfor­mance. This can deliver critical information and serve as a powerful tool to a chemist or materials developer, by providing them with a general set of fundamental design parameters with which they can move forward in membrane development. It is reasonable to consider that neutron-based techniques will continue to serve the PEM fuel-cell community by aiding in the characterization and establishment of fundamental membrane structure-property-performance relationships.

1 To ensure that the total internal resistance (electrolyte + electrodes) of a fuel cell is sufficiently small, the target value for the areal specific-resistivity of the electrolyte is set at 0.15 Xcm2. Oxide films can be reliably produced using conventional ceramic fabrication routes at thicknesses down to * 15 im. It follows that the specific conductivity of the electrolyte must exceed 0.01 Scm^1 [1].

[1] Mineral Commodities Summary 2008, United States Geological Survey (2008); later versions

(2009-2012) do not give a number for In reserves.

[3] A difference is marked in the literature between “stacked” and “tandem” PVs. The former refers to the case of layers made from the same material, whereas in the latter there are two different materials.

[4] Calculations based on standard semi-local, hybrid or meta functionals lead to a repulsive PES, i. e. no minimum is found. The combination of hybrid and meta contributions in PBE1KCIS is presently adequate for treating weak interactions. For reliability, detailed benchmark calculations were made for several non-bonded systems and are reported in [20—22] (and references therein). It is worthwhile to note that DFT-based methods can nowadays be improved in an efficient way like wave function-based approaches [23].

[5] The quantitative fragment-orbital approach and a symmetry-adapted linear combination of the HOMOs of the two HAT6 molecules are used. The corresponding computational procedure [24, 25] can be summarized as follows: Molecular orbitals (MOs) are firstly calculated for each single HAT6 forming the dimer in the specific orientation. Subsequently, MOs of the two stacked molecules are then expressed as a linear combination of the MOs of the monomer HAT6 (frag­ment), фi, leading to the overlap matrix S, the eigenvector matrix C, and the eigenvalue vector E. The relation hKS = SCEC-1 provides CTIs, < фі|ЬК5|ф/ > . This procedure allows exact and direct calculations of J as the diagonal elements of the Kohn-Sham Hamiltonian hKS. A second approach is adopted to estimate J, which is called the dimer approach and is based on a zero spatial overlap assumption and can be used in some limited cases where the overlap between the interacting individual units forming a stacked system is negligible. It consists of the use of the half energy splitting between the HOMO and HOMO-1 to get, qualitatively, the effective CTI.

[6] Results of the fragment approach are compared to those obtained using the dimer approach. There is a significant difference between the methods due to the non-zero overlap neglected in the latter.

[7] Neutron powder diffraction on HAT6 and HAT6D was performed using the D16 diffractometer at the Institut Laue Langevin in France, using wavelength of 4.54 A to get a good compromise between d-spacing range and angular resolution.

[8] The MD force field employed is COMPASS which is a second-generation force field that generally achieves higher accuracy by including cross terms in the energy expression to account for such factors as bond, angle, or torsion distortions caused by nearby atoms. A periodic hex­agonal super-cell consisting of 72 HAT6D molecules (10,368 interacting atoms) was used. The 72 molecules were arranged in 12 columns, each column consisting of 6 molecules (Fig. 6.11c).

[9] The dynamics on the picosecond timescale were determined by directly comparing the QENS spectra of the two MD models TWIST25 and TWIST60 with the experiments [27]. QENS has the advantage that neutrons follow both the temporal and spatial characteristics of atomic motion via a well-characterized interaction with the atomic nuclei. Consequently, it is fairly straightforward to calculate the expected spectral profiles by using the atomic trajectories from the MD simulations. If these are in acceptable agreement with the observed spectra, the time scales of motion can be assigned to the underlying mechanisms. Further details about the experiment and the theoretical basis can be found in reference [27].

[10] A skeletal density of 1.08 ± 0.01 g cm 3 is observed for HAT6D-TNFD, corresponding to an increase in density of about 7 % with respect to HAT6.

[11] Research exists in conversion-alloying electrodes of Li containing compounds, but this chapter is focused on the insertion materials.

[12] tG = (Ra + Ro)//2(Rb + RO), where RA is the ionic radius of the A ion, RB is the ionic radius of the B ion, and RO is the ionic radius of oxygen [25].

[13] Assuming that the two-state model is true and the proton spends an average time, t1, in the defect — free region and a time in a trap, tj + t0, then the diffusivity is scaled by a factor, tj/(tj + t0) [32]. Increasing the concentration of traps then leads to a decrease of t1. It implies that in the two-state model the diffusivity depends on the concentration of traps but not on the activation energy.

ND has been widely applied to study proton-conducting perovskites. More fre­quently, Rietveld refinement using conventional neutron powder diffraction data is applied, and in some cases, is used to understand how the structure may change with temperature and/or surrounding atmosphere. ND is of paramount importance for studies of proton-conducting perovskites due to its ability to determine proton sites, with both Rietveld and pair-distribution function (PDF) analysis of neutron total-scattering data being applied.