Vanessa K. Peterson and Gordon J. Kearley

Abstract Creating a global energy-system that is both environmentally and economically sustainable is one of the largest challenges currently facing scientific and engineering communities. Alternative energy-technologies and new materials have risen as a result of the combined needs for energy and environmental sustainability, with the focus moving increasingly away from fossil fuels. Neutron — based techniques of analysis play a role in almost all aspects of sustainable-energy materials research, and the chapters of this book will enlarge on these studies using examples and case studies to illustrate research approaches, methods, and outcomes.

1.1 Introduction

Research on renewable materials of relevance for environmentally benign energy — technologies is one of the most rapidly-growing research areas in materials science. The primary challenge in this research is the development of materials for such technologies that are viable in competition with existing energy-technologies, responding to application requirements such as efficiency, durability, and cost. Understanding the fundamental properties of materials and their functionality at the atomic and molecular level is crucial in addressing the global challenge of cleaner sources of renewable energy.

This book is divided into three main parts: materials for energy production, storage, and use. The central theme is identifying where the energy carrier is in the material and its interaction with its immediate environment so that these can be tailored to increase the concentration and/or transport of the carrier, which may be electrons, ions, atoms, or molecules. The theory of neutron scattering and analysis techniques, as well as the

V. K. Peterson (H) • G. J. Kearley

Australian Nuclear Science and Technology Organisation,

Lucas Heights, NSW 2234, Australia e-mail: vanessa. peterson@ansto. gov. au

G. J. Kearley

e-mail: gke@ansto. gov. au

© Springer International Publishing Switzerland 2015 1

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_1

associated instrumentation, are explained elsewhere in the “Neutron Scattering Applications and Techniques” book series [1] and we particularly refer the reader to Chaps. 2 and 3 , respectively, in the “Neutron Applications in Earth, Energy and Environmental Sciences” edition of this series [2] which are available on-line [3].

Crystallographic studies provide the time-averaged position of atoms, and whilst some insight into atom dynamics can be gained through analysis of the average structure and atomic displacement parameters, detailed information regarding the dynamics of the guest and host-guest system are better gained through other neu­tron-scattering methods. Inelastic neutron scattering (INS) combined with compu­tational calculations permit visualization of the atom dynamics and allow elucidation of the interaction between adsorbed guest molecules and the host.

Measurement of the interaction between CO2 molecules and the porous host is crucial to understanding the detailed binding mechanism and therefore the observed selectivity and guest-uptake properties of porous hosts. INS cannot directly detect the CO2 binding interaction within an adsorbent because the incoherent neutron­scattering cross section for these elements (for their naturally-abundant isotopes) is effectively zero, being 0.001 barns each. One approach to overcome this problem is to combine INS and DFT to visualize captured CO2 molecules within a porous host by investigating the change in the dynamics of the other atoms of the adsorbent structure. INS spectra can be calculated directly using DFT-based computations to obtain the force constants, and then making the harmonic approximation to obtain the eigenvectors and eigenvalues to determine the spectral intensities and fre­quencies, respectively [43]. INS and DFT-based calculations are a powerful com­bination in understanding the working mechanism of functionalized materials containing specific gas molecule binding-sites, probing directly the impact of functional groups, and other host features such as topology and pore shape and size on the orientation and type of binding of CO2 in the host.

An example of this is the application of INS and DFT to study CO2 in the material Al2(OH)2(bptc), where bptc = biphenyl-3, 3′,5, 5′-tetracarboxylate and also known as NOTT-300, where the neutron-scattering signal comes primarily from the H atoms in the Al2(OH)2(bptc) hydroxyl groups and benzene rings of the ligand, and the INS signal is perturbed by the binding of CO2 (Fig. 3.3) [44]. Al2(OH)2(bptc) is an Al-hydroxyl functionalized porous-solid exhibiting high chemical and thermal stability as well as high selectivity and uptake capacity for CO2 and SO2. The

material exhibits no apparent adsorption of H2 and N2, which is attributed to the slow diffusion of these gases through the narrow pore channels. In contrast, unusually high and selective CO2 and SO2 uptakes were observed, including at low-pressure. The INS spectra revealed two major increases in peak intensity upon adsorption of 1.0 CO2 into the formula unit: peak I at lower energy transfers (30 meV) and peak II at higher energy transfer (125 meV). Peaks in the range 100-160 meV were slightly shifted to higher energies in the CO2-adsorbed material, indicating a hardening of the motion of the Al2(OH)2(bptc) host upon CO2 adsorption.

The INS spectrum derived from DFT calculations show good agreement with the experimental spectrum and confirm that the adsorbed CO2 molecules are located end-on to the hydroxyl groups. The O-H distance between the CO2 molecule and the hydroxyl group is 2.335 A, indicating a moderate to weak hydrogen bond, with the optimized C-O bond distances in CO2 being 1.183 A at the hydrogen-bonded end and 1.178 A at the free end. The CO2 is linear with a O-C-O bond angle of 180°. Each adsorbed CO2 molecule is found to be surrounded by four aromatic C-H groups, forming weak cooperative supramolecular interactions between the O(5-) of the CO2 and the H(5+) of the — CH (where O-H = 3.029 and 3.190 A and each occurs twice). Peak I in the INS spectrum was assigned to the O-H group wag, occurring perpendicular to the Al-O-Al direction and attributed to the presence of the CO2. Peak II in the spectrum was assigned to the wag of the four aromatic C-H groups on four benzene rings adjacent to each CO2, in conjunction with the OH group wag. Hence, in this work the direct visualization of host-guest interactions through INS and DFT calculations was crucial in rationalizing the material’s high selectivity for CO2 and in understanding the detailed binding mechanism of CO2 in the material. The low H2 uptake of the material was rationalised in a similar manner, with the contribution from H2 in the material to the INS data, consistent with that expected for liquid H2, indicating a weak interaction with the material.

Chalcopyrite-based thin-film solar cells with the semiconductor Cu(In, Ga)(S, Se2) (CIGSe) as absorber material show the highest efficiencies among thin-film photovoltaics in the laboratory as well as in module production (see Part 1). This indium based thin-film technology has a huge potential for low-cost photovoltaic production, but the scarcity of indium (indium’s abundance in the continental crust is estimated to be approximately 0.05 part per million). Indium can be jointly refined from trace concentrations in leading ores as zinc, copper, and lead or the material comes from recycled scrap. Increasing prices of indium could easily limit the production growth. There is also an increased demand for indium for use in other technologies, for example items such as: flat panel displays, solders, thermal-
interface materials, batteries, compound semiconductors and light-emitting diodes. On the other hand, the worldwide reserve of viable indium is approximately 16 kton.[1] Nevertheless, the price of indium has varied dramatically in recent years from US$94/kg in 2002 to over US$1,000/kg in 2006. The price in 2012/2013 was about US$580/kg [35]. This reflects the scarcity of supply and the dependence on the small number of production facilities worldwide.

In order to secure the long term development of compound semiconductor based thin-film solar cells, the search for the replacement of indium is advisable. Semi­conductors suitable as absorber materials in thin-film solar cells should fulfil a number of requirements, the most important amongst them being a high absorption-coefficient (in the range of 105 cm 🙂 and a band-gap energy in the optimal range of about 1.4 eV. Here multinary compounds, like solid solutions between non-isotype binary II-VI and ternary I-III-VI2 compounds, are potential candidates. Moreover, there are a variety of chalcogenide minerals available as a source of novel, indium-free absorber materials. Replacing indium in CuIn(S, Se)2 by the abundant elements zinc and tin yields the quaternary compound Cu2ZnSn(S, Se)4, which is also a direct band-gap p-type semiconductor with an absorption coefficient higher than 104 cm-1 [36].

Function is of great importance to the study of energy materials. At the heart of the application of neutron-based techniques of analysis to the study of energy materials is the understanding of structure — and dynamic-function relations. Comprehending the working mechanism, at the atomic and molecular scale, is the key to progressing alternative and sustainable-energy technologies, and fundamental to this is the study of the materials during operation. As such, in situ and even operando studies are commonplace and necessary in energy materials research. The in situ technique, often applied to materials under equilibrium, has been extended in recent years to operando studies, where the materials are studied under non-equilibrium conditions whilst performing their function. The advent of new-generation reactor and spall­ation neutron sources, as well as associated faster instrumentation, has greatly assisted in facilitating such research.

Steels are important structural materials in modern nuclear-power systems. Com­mon alloy systems include ferritic materials which have high resistance to radiation induced swelling, nickel-based alloys for high-temperature applications, and au­stenitic (stainless) steels which typically offer superior corrosion resistance. Typical applications are piping, pressure vessels, heat exchangers, steam generators, and general structural components. Some reactor vessels are made from a ferritic shell which is then clad with stainless steel for corrosion resistance. Structural integrity issues which have been investigated by neutron methods include stress corrosion cracking, weld and cladding cracking, and loss of creep strength and embrittlement due to exposure to temperature and radiation.

Hydrogen can be absorbed into metals, particularly at high temperatures. This interstitial hydrogen can significantly affect the strength and ductility of the material, leading to reduced structural integrity. Hydrogen flows along gradients of interstitial spacing, particularly the variations caused by temperature and stress. Thus it will flow to hot areas associated with welding, and to stress concentrations associated with crack tips. As a strong incoherent scatterer of neutrons, the hydrogen distribution in metals can be investigated with radiography and prompt — gamma neutron activation analysis.

A new class of steels, the oxide-dispersion strengthened steels, have recently been developed specifically for improved radiation tolerance. This material contains a fine dispersion of nano-sized, precipitate-like features which improve high-tem­perature creep properties and act as sinks for transmutation-produced helium, provide better void-swelling resistance and promote recombination of irradiation — induced point defects. Small-angle neutron scattering has proved to be a particularly useful technique for bulk characterisation of the nano-cluster distribution, volume fraction, shape, interface and size.

As discussed in Sect. 6.2, one of the loss mechanisms to which OPVs might be subject can be via the strong electron-phonon coupling inherent to the molecular nature of the device, which limits the efficiency of charge separation. Upon photo­excitation, strongly bound exciton-states are formed that first need to dissociate before charge-transport to the electrodes can occur. Dissociation takes place at the DA interface, where intermediate CT states are formed with the hole on the donor, and electron at the acceptor molecule. It is established that charge separation is mediated by the higher lying vibronic-states of the CT manifold [38, 39]. As introduced in Sect. 6.3.1.3, fundamental knowledge about the electronic and vibrational properties of the excited-state levels and relaxation pathways is a key topic in further improving the working of OPVs.

For self-assembled aggregates such as DLCs and DLC-CT complexes, the characterization of photo-induced electron-transfer and relaxation processes is in its infancy. The addition of electron acceptors has been shown to increase the con­ductivity of DLCs. On the other hand, it has been proposed that recombination processes limit the hole photocurrent in such DLC-CT compounds [40].

An important step towards the characterization of the influence of molecular vibrations on the charge-carrier relaxation in self-assembled DLCs can be made by studying the prototypical discotic electron-donating discoid HAT6 and its 1:1
mixture with electron acceptor TNF, combining synergistically Raman, resonant Raman, UV-visible and nuclear magnetic resonance techniques, as illustrated in reference [41]. From this one can draw the following conclusions:

(i) The lowest electronic-transition in the CT complex is due to charge-transfer from the HAT6 core to TNF, with a strong involvement of the nitro groups.

(ii) There are strong indications for a weak ground-state electron-transfer in the HAT6-TNF complex.

(iii) Both nuclear magnetic resonance chemical-shift changes and Raman fre­quency-shifts are seen and are consistent with a weak electron-transfer from the HAT6 core to TNF leading to a delocalized redistribution of the charge on TNF.

(iv) The resonant Raman spectra of both HAT6 and the CT-complex show a strong enhancement of the modes related to the benzenes forming the tri- phenylene core. This is characterized by a doubly-degenerate electronic — state which is vibronically coupled (pseudo-Jahn-Teller) to a nondegenerate state. It should be noted that this Jahn-Teller mode provides a significant contribution to the reorganization energy of HATn, being a limiting factor for hole transport along the columnar stacks [42].

(v) The hot-carrier relaxation processes in the CT-band in the visible-light region are relatively slow compared to the fast relaxation within the original UV absorption band of pure HAT6, which will be relevant concerning the efficient separation of charge in organic PV-devices.

6.4 Conclusions

Inorganic photovoltaics possess a major advantage over organic solar cells by pro­viding the highest power-conversion efficiencies. But in terms of the fabrication flexibility, production costs and market accessibility, organic photovoltaics are also potential candidates for the future generation of solar cells. This requires under­standing the dynamic and electronic properties so that optimization routes can be devised that will increase their efficiency and boost their performance. This chapter illustrated how neutron scattering, in combination with other techniques, can be used to extract this fundamental aspect of organic photovoltaics working principles. In this chapter different structural and dynamical aspects of an organic molecular-model for photovoltaic applications are presented, including an evaluation of the known limi­tations. The chapter provides an example of how the logical sequential advance of experiment, theory and understanding lead towards improvements. Different exper­imental techniques and theoretical methods are used, in a quite complementary way— if not synergetic. Neutron-scattering techniques are shown to play an important role in this field by probing efficiently structure and dynamics in the presented hydrogenated materials. Numerical simulations are almost mandatory in this field to aid the inter­pretation and analysis of the different measurements including the other experimental techniques (IR, UV, Raman, and nuclear magnetic resonance methods).

Deuterium is a special case for neutrons since it has comparable coherent and incoherent cross-sections. With D2 molecules, it is then possible to measure simultaneously Ds and Dt. Such an experiment was performed for different con­centrations of D2 adsorbed at 100 K in the NaX zeolite [19]. The self-diffusivities were checked from the broadenings measured at the same loadings for H2, after correcting from the mass difference between the two isotopes.

The values of Ds are plotted in Fig. 2.4. The slight increase of the self-diffusivity which is observed when the concentration increases was initially attributed to an interaction with the sodium cations [19]. This was later confirmed by atomistic computer simulations on the basis of the loading dependence of the partial molar configurational internal energy of the sorbate molecules, which indicated the existence of low-energy sites which are preferentially filled at low occupancy [20]. Molecules residing in these sites tend to exhibit lower translational mobility than molecules sorbed elsewhere in the intracrystalline space at higher occupancy.

The values of Dt, obtained at the same loadings, are also reported in Fig. 2.4. The width of the coherent scattering was found to show a minimum corresponding to the maximum of the structure factor. This line narrowing is characteristic of quasi­elastic coherent scattering and was first predicted by de Gennes [21].

It appears that at low D2 concentration, the self — and transport diffusivities are similar, but for higher loadings the transport diffusivity increases rapidly and exceeds the self-diffusivity, as expected from the contribution of the thermody­namic factor, Eq. (2.1). This means that the lattice gas model, which predicts that the transport diffusivity does not depend on the concentration, does not apply for this system. Only close to the saturation of the zeolite does the transport diffusivity

start to decrease, indicating that collective motions become affected by the packing density. The corrected diffusivity, D0, was obtained from Dt and from the ther­modynamic factor calculated by fitting a Langmuir isotherm to the adsorbed quantities. It is clear from Fig. 2.4 that for D2 in NaX the corrected diffusivity is not constant, this assumption being often made in the interpretation of macroscopic measurements.

Neutrons are particularly important in the imaging of nuclear-fuel rods which are strong у-sources (X-ray background) and are made from heavy metal elements i. e. uranium or lead with a high attenuation of X-rays. A resolution of 50 pm or better can be accomplished and this has been utilized to characterize nuclear materials, e. g. fuel rods [43] or cladding materials [42], see the section on hydrides above. Recent detector developments allowing for spatially and time-resolved neutron detection [59] are likely to open new avenues of characterization of nuclear materials and nuclear fuels in particular as they allow for isotope-sensitive imaging via neutron resonance absorption [60].

Despite the superficial similarity of the application of neutrons with those of photons, neutrons have some key differences to photons that enable neutron-based techniques to play a particularly important role in the sustainable-energy area. A full account of neutron theory and techniques is given elsewhere in this series [1] with key methods prevalent in this book outlined here. For our purposes we can regard all of the advantages and disadvantages of neutron scattering as having a single origin: neutrons interact with, and are sometimes scattered by, atomic nuclei. In all neutron-scattering methods this leads the following advantages:

i. The scattering characteristics of each type of isotopic nucleus are well known, but vary almost randomly from one isotope type to another (scattering-lengths are shown in Fig. 1.1). This provides considerable scope for measuring light nuclei in the presence of very heavy nuclei, and also changing the scattering length by using a different isotope of the same element. For materials such as lithium-ion battery electrodes, where lithium must be probed in the presence of transition metals (see Chap. 7), this is a considerable advantage over other techniques where the scattering arises from electron density.

ii. Incoherence arises when scattering from the nuclei do not interfere construc­tively. The random relation between the nuclear spins of hydrogen and neighbouring atoms contributes to the extreme incoherent neutron-scattering cross-section of the 1H nucleus, which can be turned off by simple deuteration. Hydrogen is probably the most important element in sustainable energy — materials and it is very convenient that neutron scattering provides this selectable sensitivity for this element.

iii. Neutron-scattering cross sections are in general quite small, so neutrons are relatively penetrating, where measurements (reflection is an exception) occur for the bulk of the sample. This penetration alleviates the need for special window materials in difficult sample environments and in situ studies. There are good examples of this in Chap. 7 for lithium-ion batteries in which the com­position of the electrodes can easily be measured in an operating battery.

iv. Neutron-absorption cross sections are also normally quite low, contributing to the high penetration of neutrons, but some isotopes have very large absorption. Lithium is perhaps the second most important element in sustainable-energy materials, so it is again convenient that 6Li has high absorption, whilst 7Li does not. Consequently, either radiography (and other bulk techniques) or normal neutron-scattering experiments can be made using the appropriate isotopic composition.

The list above shows that neutron scattering is well suited to the study of sustainable-energy materials, but there are other important considerations to be made before undertaking a neutron experiment. Firstly, neutrons are difficult to produce in high quantities with the correct energies for the studies in this book, and consequently, virtually all experiments require a central facility and associated logistics. Secondly, even at the most powerful sources the neutron beams are weak when compared with photons or electrons, so the samples and the counting times for neutrons are correspondingly greater. Consequently, neutron scattering is rarely used when the desired information can be obtained by another means, and maxi­mum use of complementary techniques (most frequently X-ray) and computer­modelling methods is common.

Application of quasielastic neutron scattering (QENS) in tandem with molecular dynamics (MD) simulations brings insights into the dynamics and transport of CO2 in porous media [45]. Whilst the self-diffusion of molecules containing atoms that have appreciable incoherent neutron-scattering cross sections can be measured directly using QENS, the neutron-scattering cross section of CO2 is coherent and so the coherent QENS approach must be used to monitor the CO2 and the transport diffusivity extracted from this. Comparisons between MD simulation and QENS experiments involve the diffusing molecule’s self-diffusivity, however, in practical separations and catalytic applications, it is the transport diffusivity that is of greater importance. The transport diffusivity involves the response to a chemical potential gradient and its direct determination calls for non-equilibrium experiments. At the molecular level the dependence of the transport diffusivity and the so-called cor­rected diffusivity needs to be resolved. Linear response theory allows the transport

and corrected diffusivity to be determined directly under equilibrium conditions, where chemical potential gradients are absent. Chapter 2 contains details of the relationship between the self-diffusivity, as measured using incoherent QENS, and the transport diffusivity, Dt, as derived from coherent neutron-scattering. Experi­mentally, this objective can be accomplished by using coherent QENS, which probes the collective motion of guest molecules at equilibrium [46]. The same objective can be accomplished using equilibrium MD simulations.

The joint MD-experimental QENS approach in which simulated CO2 dynamic properties are validated allows further details of the CO2 dynamics and transport in the host to be obtained, such as first demonstrated for CO2 in the NaX and NaY faujasite zeolites [47]. The joint coherent QENS-MD approach was first applied to MOFs in the study of the isostructural Cr(OH)(bdc) and V(O)(bdc) materials and also known as MIL-53(Cr) and MIL-47(V), respectively [48, 49]. This work built on the study of H2 self-diffusion in these materials as detailed in Chap. 2. V(O)(bdc) contains corner-sharing V4+O4O2 octahedra connected by bdc linker groups yielding one-dimensional channels (Fig. 3.4). Consequently, V(O)(bdc) has a rel- atively-high working capacity for CO2 uptake with moderate selectivity for CO2 in the absence of functional groups within the channel wall. The concentration- dependent self corrected (calculated from MD) and transport diffusivities (measured using QENS) of CO2 in the rigid V(O)(bdc) material were determined [48], revealing the three-dimensional diffusion of CO2 through the channels.

Cr(OH)(bdc) differs from V(O)(bdc) by the substitution of p2-O groups located at the M-O-M links in V(O)(bdc) by ^-OH groups in Cr(OH)(bdc). This

difference makes Cr(OH)(bdc) highly selective for CO2, in a specific pressure range, which is a consequence of the large-scale breathing modes exhibited by the material. This allows it to switch from a large pore (LP) form to a narrow pore (NP) structure upon CO2/CH4 adsorption, with the NP structure able to trap CO2 and not CH4. As with the V(O)(bdc) study, the combined coherent QENS-MD approach was used to study the concentration-dependent self- and transport diffusion of CO2 in Cr(OH)(bdc). This work found a single-file diffusion regime in the material at high CO2 loading, a phenomenon not previously shown for any MOF (Fig. 3.5).