Detailed structural analysis has been undertaken with the use of neutron diffraction on a variety of Li ionic conductors as demonstrated by studies of Li argyrodites. Using a combination of simulations and structural refinements against X-ray and NPD data, the structure of the Li-ion conducting argyrodites, Li6PS5X where X = Cl, Br, and I, were determined [151]. The Li content and location, along with the mixed occupancies of the X and Li or X and S sites was modelled. For X = Cl and Br, Cl or Br was found on two S sites, whilst I was found on an independent
wholly I-containing site. For X = Cl only one Li site was found, while for X = Br and I two Li sites were revealed with a distribution of Li that differs depending on X. This highlighted that the halide plays a critical role in the distribution of atoms, including the location and occupancy of Li. Ionic conductivity was found to be highest for the X = Br samples, suggesting the influence Br has on the atomic distribution to be favourable for this property [151].

These materials were also investigated using variable-temperature neutron dif­fraction, where the starting reagents were reacted under similar conditions to those used in the laboratory synthesis. This was to determine the optimal reaction tem­peratures and conditions for favourable properties and whether any intermediate highly-conducting phases were present. The notion was to explore whether syn­thesis temperatures could be lowered, potentially reducing manufacturing costs, or whether intermediate phases were able to provide superior ionic-conduction prop­erties. Analysis of neutron diffraction data showed that argyrodite formation begins at relatively low temperatures around 100 °C, well below the reported synthesis temperature of 550 °C, but at temperatures around 550 °C the reagents become amorphous or nano-crystalline with all reflections from the sample disappear (Fig. 7.17). Notably, on cooling the desired phase re-condenses and on inspection it is found that the anion ordering, leading to the most conductive phase, is actually found in the re-condensed phase rather than the initially-formed phase [152]. These types of systematic studies on bulk formation shed light on which phases and synthetic routines may provide the best ionic conduction.

Structurally disordered solid-state electrolytes have been investigated using INS. Work [153] exploring low-energy vibrational dynamics of the 11B2O3-7Li2O

Highly conductive phase formation

§m loss of long ■; range order

Anion ordering

Phase formation

20 (degrees)

Fig. 7.17 Collated NPD patterns of a heating and cooling sequence applied to Li6PS5Br. Although an argyrodite phase forms at relatively low temperatures, it is found to be less conducting than the phase formed after the loss of long-range order
system showed a boson peak between 2 and 10 meV. It was found that with increasing Li content the intensity and position of the boson peak changed, sug­gesting the presence of intermediate glass structures. More importantly, this information can be used to generate a master curve, which suggests a universal distribution of vibrational density-of-states that is composition independent, even though the structure changes markedly. Furthermore, increasing the Li content in these glasses results in chemical structure-induced densification, as fewer low- density B-containing groups are found. It is argued that the densification may arise from the same microscopic origin as the boson peak.

In order to make correct statements about the role of protons and oxygen vacancies on the structure and dynamics of proton-conducting oxides, an accurate measure of the proton concentration in the material is necessary. Determinations of proton concentrations are done routinely using thermogravimetric methods by measuring the weight change of the sample during dehydration on heating [26]. However, the use of thermogravimetric methods is not suitable for all types of materials. For example, hydrated perovskites containing elements that may change oxidation state upon heating may decrease in mass because of oxygen loss in addition to the evaporation of water molecules. An alternative technique for the analysis of proton concentration in such systems is neutron prompt-gamma activation analysis (PGAA), which can indeed be used for determining the presence and amount of elements in materials, irrespective of oxidation state.

An example of a PGAA study of a proton-conducting perovskite is the work by Jones et al. [75] on undoped and Y-doped BaPrO3. The PGAA spectra of dry and hydrated (saturated) BaY01Pr0.9O3_,5 are shown in Fig. 9.12a. The sensitivity to hydrogen and the effect of hydration are clearly visible. The proton concentration,

as derived from the PGAA spectra, is shown in Fig. 9.12b for both samples. A key result of this study is that the proton concentration in the hydrated Y-doped BaPrO3 sample is as much as three times larger than the dopant concentration. The unex- pectedly-high proton concentration in the hydrated sample is thought to occur as a result of the change of the Pr oxidation state from +4 to +3. This implies that the Pr ions act as self-dopants, which form intrinsic oxygen-vacancies that add to the oxygen vacancies formed by the replacement of Pr for Y. Hence, the material can accommodate more — OH groups and therefore take up more protons than is expected from the dopant concentration alone. This information is naturally of paramount importance for making the correct analysis and conclusions from data obtained in both structural and dynamical studies.

The discovery of reversible hydrogen uptake and release from sodium alanate (NaAlH4) doped with titanium by Bogdanovic and Schwickardi [1] in 1997, brought about a considerable increase in research activities in the field of hydrogen storage in complex hydrides. Many of the fundamental properties of complex hydrides had not been adequately investigated up that point, and a great number of important ques­tions remain to be answered to this date. Neutron scattering investigations should be an indispensable tool in this effort because of their great sensitivity to hydrogen. The interest in complex hydrides mainly stems from the high hydrogen content of light complex hydrides (e. g. 18.4 wt.% H2 in LiBH4 [2, 3]), which make them attractive as potential solid-state hydrogen storage materials. Complex hydrides may simply be viewed as salt-like compounds of the type An+[XHM]n where A is either an alkaline, alkaline earth, or early transition metal and X typically is B, Al, or N. The bond between hydrogen and X has covalent character and the resulting complex is a rather stable entity which fulfills the “18 electron” rule, i. e. the hydrogen containing entity such as [AlH4] has a closed-shell electronic structure. While complex hydrides do have high hydrogen-content, only a few systems exhibit reversible hydrogen uptake and release, and the thermodynamic and kinetic factors which govern the hydrogen exchange reaction are not yet fully understood. Hydrogen release and uptake is typically accompanied by a solid-state reaction which involves long-range diffusion of atoms heavier than hydrogen, which potentially limits fast reaction-rates. The covalent bond of hydrogen with the transition metal also appears to be very stable, so that the mechanisms for bond splitting (and reformation during hydrogen charging) still need to be fully understood.

Neutron scattering experiments can address virtually all fundamental atomic — level questions on the function of complex hydrides, such as the crystallographic and dynamic properties of compounds which are often not well characterized, materials development such as the mapping of the reaction pathway during hydrogen exchange, or technology development such as using imaging techniques to monitor the H uptake in prototype tanks. The focus of most investigations has been on structure determination using neutron powder diffraction and character­ization of the H dynamics using vibrational spectroscopy and quasielastic neutron scattering (QENS). The latter process gives access to stochastic motions of H, i. e. translational and rotational diffusion inside the crystal lattice, while vibrational spectroscopy can be used to determine the H density of states and has become an essential tool to benchmark first-principles calculations. In the following sections we will describe a few examples of neutron scattering experiments for the char­acterization of complex hydrides and their development as solid-state H storage materials. Apart from fundamental insights into these materials, which in many cases lack adequate characterization, phase transitions during hydrogen absorption and desorption can be monitored and thus giving insight into the reaction mecha­nism, intermediate reaction steps, and kinetic limitations.

In the early 1980s Volino and Dianoux et al. used QENS to characterize the water dynamics in a hydrated Nafion® film [49, 50]. The Nafion® sample was 1200 EW and the water content was kept at approximately 15 % by mass. Experiments were carried out at 25 °C on the multichopper time-of-flight spectrometer (IN5) at the Institut Laue Langevin using three different neutron wavelengths of approximately 10, 11, and 13 A, corresponding to an energy resolutions (Лю) of 18.5, 14, and 9 peV, respectively. At each energy resolution, data was collected over the Q-range

0. 4—1.1 A-1. Along with the hydrated films, dry films were measured as a control. In early work, the quasi-elastic broadening was characterized using a Lorentzian to describe the long-range self-diffusion of the water molecules in the hydrated membrane. The dynamic scattering-function, S(Q, m), for diffusion of an incoherent — scattering particle inside an impermeable sphere of radius, a, was presented in the original work. Considering the broadening to be due to only long-range diffusion,

1. e. a ^ to, the scattering function reduces to:

1 dq2

p (DQ2)2+x2

where a Lorentzian function is used with full-width at half-maximum of Лю = 2DQ2 and D is the long-range diffusion coefficient.

It was determined that this model did not sufficiently describe the QENS spectra, and therefore inadequately described the water motions in Nafion®. Given the fact that a single Lorentzian function, as used in Eq. (10.1), could not completely describe the quasi-elastic broadening, a second model was proposed that considered the rotational motions of the water molecules within the membrane. This particular model assumes that the water molecules diffuse on a sphere of radius, p, with a rotational-diffusion coefficient, Dr. Using the distance between the centre-of-mass and the protons (0.95 A) as p, the researchers were unable to fit the data using any reasonable value for Dr. With p as adjustable parameter, they found reasonable fits with p ^ 3 A, but because this value did not correspond to any reasonable length — scale in the water molecule the analysis approach was not pursued further. Therefore, it was determined that the measured QENS spectra could not be suffi­ciently described by the rotational diffusion of water alone.

With the failure of the two simple models described above, a third model com­bining translational self-diffusion with rotational diffusion was proposed. Through various initial attempts at determining Dr, p, and D, it was found that the general form of the model could sufficiently describe the QENS data, particularly with p ^ 3 A. Again, given that this value did not match any reasonable length-scale for water rotation, it was determined that although the mathematical description was sufficient, the physical interpretation was lacking. Therefore, it was decided that the mathe­matical form of the model would be preserved and would be one in which the water molecules (viz. the protons) are restricted to local diffusion, Db inside a sphere of radius, a, combined with long-range translational diffusion, Dt, between the spheres. The best-fit curves were calculated using D[ = 1.8 x 10-5 cm2s-1, a = 4.25 A, and Dt = 1.6 x 10 6 cm2s 1. Although this model proved to provide a reasonable explanation, and fit, to the QENS data in the Q-range 0.4—1.1 A 1, it was suggested that more sophisticated models could be proposed and were being pursued. How­ever, this was not thought to change the overall conclusions of the work. This work showed that on a local size-scale (ca. 10 A) the water molecules diffuse with a self­diffusion coefficient that is similar to bulk water (Dwater = 2.2 x 10 5 cm2s :), but that the long-range self-diffusion is restricted due to the morphology of material. Essentially, water is relatively free to move within a water domain, or ionic aggregate, but motion between domains over large length-scales suffers. Subsequent measurements on oriented membranes use more sophisticated models describing diffusion within a cylinder instead of a sphere.

Pivovar and Pivovar [51] also used QENS to investigate the water dynamics in Nafion® over a range of hydration levels from X = 2 to 16 in order to determine how the local dynamical behaviour of water is correlated with proton conductivity, particularly in the low-X regime. The QENS data were collected at a time-of-flight instrument at NIST. The data were collected at 295 K over a Q-range from 0.4 to 2.0 A 1 with neutron wavelength of 6 A. The energy resolution was between 55 and 70 peV. Assuming that the QENS data were the result of both elastically and quasi­elastically scattered neutrons arising from immobile and mobile protons, respec­tively, the experimental results were successfully modelled using the following function:

Sexp(Q, X = I(Q)(x • R(x) + (1 — x)R(rn) ® Цю)) + Bo (10.2)

where I(Q) is the scaled intensity at a given value of Q, R(oj) is the instrument resolution function, L(ffl) is a Lorentzian distribution, and Bo is a flat background. The quantity, x, is the elastic incoherent structure-factor (EISF), which is the ratio of the elastic scattering-intensity to the total scattering-intensity from both the quasi­elastic and elastic components. The EISF can yield information as the length-scale of the proton motions occurring within the sample. The fits to the experimental data based on Eq. 10.2 were interpreted using two different models. For Q < 0.7 A-1, the Lorentzian broadening was analysed according to a model where the incoherent scattering particles, i. e. protons, were thought to undergo continuous diffusion within a sphere as in the model proposed by Volino and Dianoux [55, 56]; above this Q-value a random unconstrained jump-diffusion model was used. From these two models the characteristic length-scales over which motion occurs, along with local (Dlocal) and jump (Djump) diffusion coefficients, were determined. In the context of the diffusion in a sphere model, the measured EISF was modelled with the following equation

x = EISF = fND + (1 — /nd)( jfl) (10.3)

where fND is the fraction of protons not diffusing in the timescale of the mea­surement, R is the characteristic radius of the sphere within which the protons are diffusion and j1 is the first-order Bessel function. The results of this analysis over the entire hydration range can be seen in Fig. 10.7.

The radius of the dynamic sphere is characterized by a linear increase from about 2 to 3.5 A in the X range from 1 to 7. Above a X of ca. 7 an asymptotic value of 3.68 A is reached for fully saturated membranes. Correspondingly, within the time­scale of the measurement, fND decreases with increasing water content. These results indicate that at low X values the hydrophilic domains are small and water molecules are likely to have strong interactions with the acid sites (i. e. the confining surface). These water molecules are effectively bound. As the domains swell, a smaller fraction of the protons feel the confining effects of these interactions. There is, however, inconsistency between the proposed size of these confined hydrophilic domains and the size as determined by SANS measurements. This can be accounted for by limitations in the time resolution of the instrument.

Following the work by Pivovar and Pivovar, further advancements in modelling of the QENS obtained from time-of-flight and backscattering spectrometers were made by Perrin and co-workers [52,53]. Their efforts aimed at developing a single model to account for bound-continuous diffusion, long-range diffusion, and atomic granularity (i. e. jump diffusion). This model used Gaussian statistics to describe the diffusion of the scattering particles within a restricted geometry with ill-defined boundaries. Essentially, the model accounted for two populations of diffusing protons: a popu­lation of fast protons and a second population of slower-moving protons.

Fig. 10.7 a The dynamic sphere radius, R, from a model fit to the EISF as a function of the water content, X, where the dashed lines demark the slope in the low-swelling regime of the asymptotic value at high hydration-level and b the fraction of non-diffusion hydrogen atoms, fND, as determined from the model fit as a function of water content, where the dashed line is the theoretically-calculated value offND assuming a single, non­diffusing hydronium ion for each sulfonic acid site. Reprinted with permission from (A. A. Pivovar,

B. S. Pivovar, J. Phys. Chem. B 109, 785 (2005)) [51]

© 2009 American Chemical Society

As described by Perrin, the slow protons are, for all intents and purposes, related to the hydronium ion formed when water abstracts a proton from the sulfonic acid site, but it is also likely that these are tightly bound or restricted, water molecules similar to those described by Pivovar and Pivovar [53]. The timescale of motion of this population becomes shorter with increasing hydration level and the characteristic length-scale of motions increases. No long-range diffusion was reported. With increased hydration, the “extra” water molecules comprise the fast protons whose motions are characterized over a similar length-scale as the slower protons along with long-range diffusion between adjacent confining domains. In general it was shown that for both populations of protons, the length-scale increases and the timescale of motions decrease with increasing hydration level. As with earlier studies, an asymptotic value was reached for these values at X of about 10.

Other studies have shown similar results. Paciaroni and co-workers [54, 55] studied the dynamics of the confined water in Nafion® films at low hydration levels (ca. X = 6) over a temperature range from 200 to 300 K. They characterized the motion according to random jumping inside a confining region, which they associate with the boundaries of the ionic aggregates, or clusters, of Nafion®. They also observed a transition in the water dynamics at approximately 260 K, which was reported to be related to an increase in the available degrees of freedom of the water created by a melting-like
process of the bound water. Their results concerning the local-diffusion coefficient of the water are similar to those previously reported [51]. Paciaroni et al. [55] also studied a composite membrane comprised of Nafion® with zirconium phosphate nano-filler. Again, the QENS data were modelled according to random jump-diffusion inside a confining spherical region. Ultimately, this work demonstrated that the local water dynamics were unaffected by the presence of the nano-filler.

Researchers have used QENS to look at novel PEM materials and to investigate the fundamental relationship between structure and transport. Peterson and co-workers used QENS to investigate the water dynamics in novel plasma-poly­merized PEMs [56, 57]. The membranes were synthesized using a pulsed-plasma enhanced chemical vapour-deposition system. The resulting membrane was a cross- linked polystyrene containing trifluoromethanesulfonic-like acid sites. The ion — exchange capacity of this novel membrane was shown to be somewhat less than that of Nafion® (0.7 mequiv/gm compared with 0.91 mequiv/gm) with a signifi­cantly lower water content. The quasi-elastic broadening was modelled using a Lorentzian. Plots of the half-width at half-maximum (HWHM) of the Lorentzian as a function of Q2 were used to calculate the diffusion coefficients. The water in the plasma produced PEM showed similar diffusion behaviour compared to Nafion®. However, they did observe an increase in HWHM for the plasma produced PEM at low Q values compared to that measured for Nafion®, but no explanation was given for this observation. A later, more detailed, QENS study from this group revealed interesting behaviour in the plasma-polymerized PEM (PP-PEM) [57]. They found that the PP-PEMs studied had a proton conductivity, as measured by impedance spectroscopy that was ^21 % higher than that of Nafion®. A careful QENS study, and subsequent fitting, of the proton motions revealed that the quasi-elastic broadening could be described by two components, or types of protons. One component was due to motions arising from a dispersed relaxation in the frequency domain, which is equivalent to a stretched exponential in the time domain, as proposed by Bergman [58]. These protons were labelled as type 2 and considered to diffuse much like the protons in Nafion® membranes. The other, faster component was described by a broad Lorentzian and accounted for 41(3) % of the protons. The Q2 dependence of the two types of protons was used to determine self-diffusion coefficients for the fast and slow diffusing protons. The fast protons, which accounted for 41(3) %, of the protons in the system, had a self-diffusion coefficient of 2.8(1) x 10-4 cm2s-1. This is an order of magnitude faster than that measured for the Nafion®-like protons (3.0(2) x 10 5 cm2s :). These results help to support and give an explanation as to the increased proton conductivity measured for the PP — PEMs, despite having a lower water uptake. It was proposed that the superfast proton diffusion is likely due to the arrangement of the sulfonates in PP-PEM. While detailed structural information was not presented, a simple model was pro­posed which involve the hydronium ion being passed between adjacent trifluoro — methanesulfonate groups. The local environments of these groups are thought to allow for rapid, relatively long-range proton transfer as compared to Nafion® [57]. Lyonnard et al. [59] have also used QENS to study the role of structure and confinement on water mobility. This study, however, was not carried out on PEMs, but on a perfluorinated surfactant (PFOS) bearing a similarity to the side-chain found in Nafion®. By varying the ratio of PFOS to water, they were able to create both lamellar and hexagonal phases. They showed that the water motions are spatially confined and, more importantly, that the geometry of the confinement affects the diffusion behaviour. In the hexagonal phase, the water dynamics were found to be almost bulk-like while for the lamellar phase there were serious restrictions in water mobility. This information could play a vital role in the rationale design of future, novel membrane materials.

While it is not in the scope of this chapter, it is worth mentioning that QENS has also been used to study the motions of hydrogen gas, the fuel in a fuel cell, on PEM fuel-cell catalyst supports. Such studies demonstrated that the interactions between H2 and the carbon support play a significant role in reactant transport in the PEM fuel cell [60, 61].

The aforementioned studies demonstrate the efficacy of QENS for understanding the water transport in PEM materials. Several key pieces of information concerning the fundamental nature of water mobility in PEMs have emerged thanks to detailed and clever analyses carried out by the researchers mentioned. In summary, one can say that the water motions in PEMs, especially Nafion®, have the following characteristics:

1. Water motions occur in confined domains, the nature of which is largely dictated by the nanoscale morphology of the material.

2. Within this confined geometry the water motions are influenced largely by the number of water molecules present. As the number of water molecules decreases, the dynamics are increasingly restricted.

3. With a sufficient level of hydration, the local dynamics are not unlike the motions that occur in bulk water, but the long-range motions are restricted due to the material morphology.

This body of information has contributed significantly to our current under­standing of these materials and will continue to illuminate the path toward the rational design of new PEM materials.

Loading protons into the perovskite structure depends on acceptor doping at the B site [26]. Acceptor doping, such as when In is substituted for Zr in BaZrO3, creates an oxygen-deficient structure, which under elevated temperature and humidity absorbs water dissociatively so that oxygen vacancies fill with hydroxyl groups and the remaining protons bind to other oxygens in the structure [26]. In the Kroger — Vink notation [27] this reaction is written

H2O + Vo + O*, 2(OHO), (9.1)

where VO denotes an oxygen vacancy, O* denotes a lattice oxygen, and OHO denotes a proton bound to a lattice oxygen (the superscripts • and x denote positive and neutral charges, respectively). The protonation process is shown schematically in Fig. 9.3b. At elevated temperature, the protons are not stuck to any particular oxygen but move rather freely from one oxygen to another, as described further below, resulting in a high conductivity of the material.

Acquiring a comprehensive understanding of interfacial reactions in batteries is essential for the design of new materials [4]. However, due to the difficulty in probing the interfaces, relatively little is known about the relevant processes. What is known is that the stability of electrolytes at the electrode interface plays a key role in determining the cycle life and safety of batteries. The most studied interface is between the carbon negative electrode and carbonate organic electrolytes. The instability of carbonate electrolytes with respect to the carbon chemical-potential results in deposition of the electrolyte Li-containing inorganic and organic decomposition products on the electrode surface, otherwise known as the SEI layer. This reduces the amount of active Li available to the cell and degrades the elec­trolyte. Typically electrolytes contain two components, one for the Li-salt disso­lution, and one that assists in the formation of a protective layer on the anode preventing continuous electrolyte-reduction and self-discharge, e. g. ethylene car­bonate. This requires the formation of a stable SEI film showing good ionic — conductivity and poor electronic-conductivity. In this way the SEI can passivate against further electrolyte decomposition without severely influencing battery performance. The SEI formation and maintenance during further cycling is expected to play an essential role in the cycle life and stability of batteries, however, the growth mechanism under variable battery-conditions is largely unexplored. In addition, the charge-transfer reaction at the interface, most likely influenced by the SEI development [169], is an essential parameter that in many cases limits the power of batteries. Also, interfaces within the electrode material can establish upon phase transitions during (de)lithiation. From an applications point of view these transitions have the favourable property of being associated with a constant potential that is independent of the composition, but the disadvantage of being associated with volumetric changes that may restrict the cycle life. Probing such interfaces under in situ conditions is possible using neutron reflectometry.

One of the first reported neutron-reflectometry studies on a Li-ion battery system determined the Li insertion and extraction mechanism in thin film anatase TiO2, a negative electrode material operating around 1.7 V versus Li/Li+. Lithiation of the tetragonal anatase TiO2 leads, via a first-order phase transition, to the orthorhombic Li05TiO2 Li-titanate phase. The aim of the neutron-reflectometry study was to discover the phase-evolution scheme in this electrode material, which is of more general value for electrode materials undergoing first-order phase transitions. Pre­vious studies suggested the establishment and movement of a diffusion-controlled phase boundary, parallel to the electrode surface, between the Li-rich Li-titanate and the Li-poor anatase phase [170]. This is in contrast to, for instance, a perco­lation scheme where the Li-titanate phase would penetrate the original anatase layer only at certain regions of the thin film. Further intercalation would increase both the perpendicular and the lateral dimension of these percolation paths, eventually leading to a homogeneously-intercalated film. Van de Krol et al. suggested a specific scheme in order to explain the more facile Li-ion extraction rate compared to the insertion rate [170]. Based on the assumed faster Li-diffusion in the Li — anatase phase [171] one might expect fast depletion of Li in the near-surface region of the Li-titanate phase containing electrode, which is in contact with the electro­lyte. As a result, during Li extraction, the Li-anatase phase starts to grow at the electrolyte surface into the layer at the expense of the Li-titanate phase.

The contrast difference between the Li0.5TiO2 Li-titanate phase and the TiO2 anatase phase for neutrons should make it possible for neutron reflectometry to determine the phase-evolution scheme both during Li insertion and extraction.

An approximately 25 nm smooth anatase TiO2 electrode was deposited on a thin *20 nm Au current collector on a 10 mm thick single-crystal quartz block that served as the medium for the incoming and reflected neutron beam. The latter is practically transparent to thermal neutrons allowing approximately 70 % trans­mission over 10 cm path length. The TiO2 electrode is exposed to a 1 M solution of LiClO4 in propylene carbonate electrolyte using Li metal both as counter and as reference electrode. Li was galvanostatically inserted in two steps and extracted in two steps using 10 mA (C/5) in the same voltage window. Neutron-reflection experiments were performed after each step when a constant equilibrium-potential was achieved. The results, including the fit and the associated scattering-length density (SLD) profiles are shown in Fig. 7.23. The profound change observed in the neutron reflection from the virgin state and the state after the electrochemistry can be explained by the formation of a SEI layer on the TiO2 surface.

For the half lithiated state the best fit of the neutron reflection data was achieved by assuming a Li-rich Li-titanate phase (Li05TiO2) in contact with the electrolyte. As a result of the negative coherent neutron-scattering length of Li the SLD of lithiated TiO2 being is smaller than that of pure TiO2. Neutron reflectometry proved the

establishment of a phase boundary parallel to the electrode surface rather than a percolation model, which would lead to a homogeneous change of the SLD. The fully-lithiated state was fitted with a single electrode layer with the SLD corre­sponding to the composition Li052TiO2. The neutron reflection data after half de — lithiation indicated that the phase front moves back via the way in which it came, with the Li-titanate phase being in contact with the electrolyte. This is in contrast to the expected Li depletion during extraction, which should lead to TiO2 formation at the electrolyte interface. This symmetric phase-front movement does not immedi­ately explain the difference in insertion and extraction rate. However, nuclear magnetic resonance experiments show the diffusion over the phase boundary to be the rate-limiting step [172, 173], giving a rationale for the more sluggish lithiation of TiO2, which as opposed to delithiation, requires diffusion over the phase boundary.

Information related to the structure and composition of the SEI layers is mostly based on ex situ spectroscopic and microscopic studies [174, 175], but because of the reactive and delicate nature of these layers, in situ analysis is essential to improve our understanding. Being relatively sensitive to the light organic and inorganic species present in the SEI and to the surface layers ranging from a few to hundreds of nanometers, neutron reflectometry is an exceptionally suitable tech­nique for in situ studies of the growth, composition, and the structure of the SEI.

Owejan et al. [176] used neutron reflectometry to study the formation and structure of the SEI layer in a Li battery. A requirement for neutron reflectometry is a flat and smooth surface as it probes the average in-plane SLD profile. A Li half-cell was configured with Cu as the ‘counter’ electrode to prevent Li reaction with the electrode, so that all electrochemical charge can be attributed to decomposition of the electrolyte and SEI layer formation. The use of a non-intercalating electrode, such as Cu, as model electrode for electrolyte decomposition appears to be justified by the similarity of the SEI layers formed by C materials at low potentials in Li-salt con­taining electrolytes [46, 47]. Additionally, the thermodynamics of electrolyte reduction appear to be governed by the cation that is used in the electrolyte [43]. The scattering contrast of the electrolyte was increased by preparation of a 1 M LiPF6 solution in a 1:2 (v/v) ratio of deuterated ethylene carbonate and isotopically-normal diethyl carbonate. The deuterated ethylene carbonate also offers the opportunity to identify the possible preferential decomposition of cyclic (ethylene) over acyclic (diethyl) carbonates. By deuterating selected components in the electrolyte solution researchers can access which component contributes or forms the SEI layer.

In Fig. 7.24 the neutron reflectivity versus Q is shown for the pristine Cu electrode immersed in the electrolyte at the open-cell potential. This electrode underwent 10 cyclic voltammogram sweeps between 0.05 and 3 V at a 10 mVs-1 rate, followed by holding the potential at 0.25 V versus Li/Li+ (potentiostatic reducing conditions). A clear difference between the peak amplitudes and oscilla­tions (positions) in the reflectivity of the fresh and electrochemically-cycled elec­trode is observed. Initially, at the Cu-electrolyte interface, copper carbonate/ hydroxide ligand-containing layers appear to be present which are removed after the cyclic voltammetry. Interestingly, after 10 cyclic voltammetry sweeps and the potential-hold step under reducing conditions, a 4.0 nm thick SEI layer at the

interface had developed with a SLD much lower than that of the electrolyte. A further 10 additional cyclic voltammetry sweeps led to only a small growth of the SEI layer. For completeness, the authors then took a number of data sets at different potentials by slowly ramping the potential at 10 mVs-1 to the next potential value and holding the potential during neutron reflectometry data collection. The results

are summarized in Fig. 7.25. During the first two points d and e, using an oxidation current, a small decrease in the SEI layer thickness was observed and the SLD suggests a shrinking of the SEI layer due to solubility of SEI components. How­ever, at the next point, holding at a reducing current the SEI layer grows signifi­cantly up to 8.9 nm. Most of the neutron reflection measurements indicate rather homogeneous SLD profiles with little roughness, in contrast to proposed structures in literature. After point f even the lowest potentials do not lead to further SEI layer growth, illustrating the passivating nature of the SEI layer. The systematic decrease of the SLD at lower potentials indicates that the SEI is increasingly composed of low SLD elements, which indicate Li-rich molecules.

Further insight into the composition of the SEI layer was obtained by combining X-ray photoelectron spectroscopy-derived compositions with the neutron reflec — tometry results for the SEI layer. This indicated an increase in LiOH and LiF molecules, and the decrease of lithium alkyl carbonates at the lower reducing — potentials. This study demonstrated the advantage of neutron reflectometry in giving direct insight into the growth and composition of the SEI layer and its relationship to the electrochemical conditions.

It is clear that research on proton-conducting ceramics continues to be focused on archetypical ABO3-type perovskites. However, a variety of other classes of proton­conducting ceramics which show appreciable proton-conductivities at intermediate to high temperatures have been developed, and are receiving increased attention. Examples of these include more complex perovskites of the form Ba3Ca1.1gNb1.g2Og.72 and Sr3CaZr05Ta15Og.75 [19—21], which possess cation ordering leading to a doubling of the unit-cell, and perovskite-related phases such as brownmillerite-structured oxides (e. g. Ba2In2O5 [49]). Other examples include gallium-based oxides (e. g. LaBaGaO4 [76]), pyrochlores (e. g. La2Zr2O7 [23, 24]), phosphates (e. g. LaPO4 [77, 78]), niobates and tantalates (e. g. LaNbO4 [79]), tungstates (e. g. La6WO12 [80]), solid acids (e. g. CsHSO4 [81]), hydrated alkali thio-hydroxogermanates (e. g. Cs2GeS2(OH)2yH2O [16, 82]), tungsten-bronze titanate/niobate systems (e. g. Ba06Mg0067Nb0.933O3 [83]), and cupsidine systems (e. g. La4(Ga2_xTixO7+x/2)O2, x = 0-2 [84]).

In this section, we follow some examples of recent neutron-scattering studies of hydrated alkali thio-hydoxogermanates, solid acids, and gallium-based oxides, to further highlight the breath of information that can be obtained with neutron scattering.

The archetypical example NaAlH4 decomposes (and reabsorbs) hydrogen in two distinct reaction steps as follows:

3NaAlH4 $ Na3AlH6 + 2Al + 3H2

3.7 wt.%, AH = 38 kJmoP1 (8Л)

Na3AlH6 $ 3NaH + Al + 1.5 H2

3 6 2 (8.2)

1.7 wt.%, AH = 47 kJmoP1

Additives such as TiCl3, ScCl3, or CeCl3 enable nearly complete reversible conversion and rehydrogenation at 373 K and 100 bar H2. Undoped NaAlH4, on the other hand, exhibits negligible reversibility under these conditions. The function of the catalyst is still being debated, and while numerous different materials have been identified to be able to catalyze these reactions [4—7], a unified picture for their activity is still lacking.

Neutron powder diffraction measurements have identified the structure of NaAlD4 as body-centred tetragonal I41/a (NaAlH4) [8]. The intermediate decomposition product, Na3AlH6 occurs in two modifications, i. e. low temperature monoclinic a-Na3AlH6 (P21/n) [9, 10], and P-Na3AlH6 with a cubic structure (Fm 3m) [11] is observed above 525 K. The reversible hydrogen exchange reaction is greatly facil­itated by Ti additives and numerous attempts have been made to localize the additive and to unravel its catalytic mode of operation. Among the suggested mechanism are: (a) Ti acts as a surface catalyst that facilitates the splitting of the AlH4/AlH3 bonds [12, 13], (b) Ti initiates vacancy formation and hence promotes H diffusion [11, 14], (c) Ti weakens the Al-H bond [15], or (d) Ti acts as a grain refiner [16]. While neutron powder diffraction measurements did not show any indication of a solid solution with Ti-based additives directly after ball milling, the formation of Al1-xTix species were observed upon cycling [17, 18]. Synchrotron X-ray diffraction studies of Ti-doped NaAlH4 suggested that Ti could substitute in Al-sites [19] while density functional theory (DFT) calculations show that both the substitution of Na or Al in the NaAlH4 structure should be possible in the bulk [20-23] or small clusters [24]. The influence of Ti-doping on the native vacancy formation was also investigated by DFT methods. Ti-doping in NaAlH4 can yield the formation of hydrogen vacancies and interstitials [25]. In contrast, the same paper suggests that in Ti-doped LiBH4 only a reorientation of the BH4 units occurs. Since structural characterization of the Ti sites proved to be difficult, the localization of the additive and/or its induced vacancy diffusion mech­anisms was studied by neutron spectroscopy techniques. QENS data on Ti-doped NaAlH4 at temperatures below 350 K indicate that in both NaAlH4 and Na3AlH6, the hydrogen mobility is vacancy mediated [26, 27] but the relative amount of mobile hydrogen at these temperatures is less than 1 % in NaAlH4 even at 390 K and there is no significant difference on the bulk diffusion induced by TiCl3 additions. Similarly, neutron vibrational spectroscopy showed no distinct differences for pure NaAlH4 and Ti-doped NaAlH4 [24, 28] while significant changes of the vibrational density-of- states were observed after thermal treatment of both NaAlH4 and Na3AlH6 [29]. These were attributed to the presence of different ionic species resulting from the partial decomposition of the samples. Neutron spectroscopy data (Fig. 8.2) suggest the formation of AlH3 and its oligomers (AlH3)n [28] during reabsorption of the H depleted mixture NaH/Al (after 0.5 h at 140 bar H2, 403 K).

Here, the measured inelastic neutron scattering (INS) spectra are compared with calculated spectra of AlH3 and of Al4H12. Volatile species of this kind had been suggested to be an intermediate species responsible for mass-transfer processes during the hydrogen exchange reaction [30] and neutron spectroscopy is a unique tool to detect these intermediate species.

Although historically used as a tool to probe the structure of PEM materials, recently neutron techniques and sample environments have been developed to probe the transport of water in these materials by enabling structural changes to be monitored as a function of time. Kim and co-workers developed an in situ vapour sorption apparatus for SANS that is capable of controlling the vapour pressure of a given solvent and have employed it to investigate the effects of water vapour sorption in Nafion® films [62, 63]. A French group has developed an in situ, in operando SANS experiment and analysis method to observe the structure of Nafion® and determine the water profile across the thickness of the PEM [64—69]. This technique has also

been used by this group and others to investigate the behaviour of water in a working fuel-cell environment and will be discussed in greater detail below [70, 71]. More recently, Gebel and co-workers [72] used a similar cell and have demonstrated the ability to measure the kinetics of water sorption in Nafion® along with the water concentration-profiles across the thickness of the membrane using neutron scattering.

Kim and co-workers were able to measure structural changes in Nafion® under various relative humidity conditions, ranging from dry to hydrated, using an in situ vapour-sorption SANS (iVSANS) cell. Scattering intensity was measured over the Q range 0.1-0.3 A-1 as a function of time as shown in Fig. 10.8. The position and intensity of the ionomer peak were determined from the scattering profiles to measure the structural evolution of pretreated and as-received Nafion® films after being exposed to water vapour. The humidity changes investigated included dry to 20, 35, 50, 65, 80, and 95 % relative humidity. The results for the as-received Nafion® film can be seen in Fig. 10.9 for a humidity change from dry to 95 % relative humidity at 23 °C. Over the course of the sorption experiment, the ionomer peak increases in intensity and the position of the peak shifts to lower Q, as detailed in Fig. 10.9. The macroscopic scattering intensity (differential cross-section on an absolute scale), d2/dG, of the ionomer peak was correlated with the water uptake (Fig. 10.9a) and increased rapidly during the early stages of sorption and levelled off upon reaching equilibrium for each of the target humidity values. The rate of water sorption and the intensity, both related to the water uptake, were found to increase with increasing relative humidity. The ionomer peak position was found to follow the same trend, with the domain spacing increasing with time during the early stages and plateauing at later times. The equilibrium spacing increases with increasing relative humidity. The time-resolved (kinetic) data of the time-evolution

of the macroscopic scattering intensity was modelled with a solution to Fick’s second law to determine the diffusion coefficient for both as-received and pretreated Nafion® membranes. More recently, Gebel and co-workers performed a similar set of experiments [72]. In addition to obtaining kinetic data to determine the diffusion coefficient, Gebel and co-workers used an established technique to determine the water concentration-profile across the membrane during the sorption process [64—68, 70, 71]. Scattering data from Nafion® equilibrated at various relative humidity values, were recorded and served as a reference to reconstruct the scat­tering obtained during the equilibration process. It is thought that the scattering data taken during the sorption process could be considered as a sum of slices with varying thickness and water contents. These slices can be considered to be repre­sented by the recorded reference spectra and the total in situ scattering from the

membrane in the operating fuel-cell can be recreated by a linear combination of said reference spectra.

One of the most innovative uses of SANS to date has been the in situ, operando technique developed by a group in France [64—68, 70, 71]. In a working PEM fuel

cell, the membrane is not uniformly hydrated across the thickness of the cell and there is usually a water gradient from the anode to the cathode, with the water concentration being higher at the cathode. Ideally, one would want to know the water concentration profile through the membrane thickness as a function of the operating conditions in order to optimize fuel-cell performance and water man­agement. The group in France was the first to develop a fuel cell that was neutron — transparent, which enabled them to measure the scattering from the membrane during cell operation. The premise behind this technique is the water gradient across the fuel cell and the varying membrane nanostructure as a consequence of the different amounts of water. The features of the neutron-scattering data that arise from the nanostructure of Nafion® (i. e. the ionomer peak, incoherent background, etc.) are sensitive to the amount of water in the membrane. Thus, the membrane in the working fuel cell was considered to consist of a series of slices, each with different water content. Reference spectra were obtained for Nafion® membranes that were considered to be at equilibrium with respect to swelling over a range of water contents. The shape of the ionomer peak at a given X served as a reference of the scattering for that particular water content. The scattering intensity obtained during operation, Itotal(Q), was considered to be a linear combination of the scat­tering intensity of the reference spectrum, lYf(Q), where the weight (or coefficient) associated with each individual reference spectrum, a,, was directly correlated with the thickness of the corresponding hydration layer.

Itotai (Q) . агТ[е/ (Q) + k; with^a, = 1 (10.4)

The cell, the reference spectra, and a typical measured water profile can be seen in Fig. 10.10. This technique has been used to study the water gradient profiles during fuel-cell operation as a function of current density, H2/O2 gas ratio, and with differing gas-diffusion layers and gas-flow configurations. Details of this technique and the neutron-transparent fuel cell can be found in the literature [64—67, 73].

The dynamical picture of the proton-conduction mechanism in hydrated perovskites started to emerge in the mid-nineties and is mainly based on results obtained from molecular-dynamics simulations [28-31] and quasielastic neutron scattering [32, 33]. On a local scale, the protons jump between neighbouring oxygens, with an intermediate reorientational motion of the — OH group in between jumps (a sche­matic is shown in Fig. 9.3c), whereas at the longer length-scale the protons diffuse as a series of such jumps and reorientations. However, the effect of dopant atoms on the local chemistry and structure, as well as the resultant symmetry reduction and proton-defect interactions, complicate the description of the proton conductivity and such effects are not completely understood for even the simplest perovskite systems.

A key parameter for long-range proton diffusion is the hydrogen bonding experienced between a proton and a neighbouring oxygen, since the transfer between neighbouring oxygens is a hydrogen-bond mediated process, whereas the reorientational motion of the — OH group requires the breaking of such bonds. The proton diffusivity is further affected by the vibrational dynamics of the proton. More specifically, the proton performs localized O-H stretch and O-H wag vibrations, which may be seen as precursors to the transfer and reorientational step, respec­tively. There is also a coupling of the proton motions to the phonons of the perovskite host lattice [30, 34]. Hence, the hydrogen bonding, vibrational dynamics, and long-range proton diffusion are, most likely, strongly correlated.