Neutrons and the future of energy generation

Materials World magazine
,
5 Jan 2016

Dr Stéphane Rols explains the role of neutrons in the development of the next generation of energy materials and the work being done by the Institute Laue-Langevin, France, in this area.

Developments in materials science are heralding a new generation of energy materials. According to the Director of Science at the Institut Laue-Langevin (ILL), Dr Helmut Schober, ‘Physical and chemical processes are at the heart of the energy problem – whether in solar cells, nuclear reactors, or modern batteries. In order to optimise current technology or to develop new techniques, it is essential to understand the processes and the evolution of materials at the atomic level.' Neutron scattering is one of the best analytical probes available for gathering new information of this type. This is especially true if the materials contain elements that neutrons will highlight, like hydrogen or lithium.

Inside an operating fuel cell

Fuel cells are one of the key green-energy technologies being developed as an alternative to fossil fuels. They convert chemical energy – derived from the oxidation of a fuel such as hydrogen – into electricity and heat. The proton exchange membrane fuel cell (PEMFC) is one such electrochemical device, and is an ideal power source for electric vehicles, because its components are relatively light, it is fast-starting at room temperature, and has a high power-density. 

The PEMFC has a complicated layered system. It converts hydrogen and oxygen to water using catalytic electrodes separated by a polymer-membrane electrolyte. Increasing the PEMFC’s performance and longevity, as well as reducing its cost, are crucial issues to address for the large-scale application of fuel cells – and require a deep understanding of the system’s components and behaviour. One of the main issues affecting the power output, stability and lifetime is
the amount and distribution of water within the cell. The water distribution in the active areas should be as homogeneous as possible. Moreover, a critical problem for operation is maintaining the balance of water within the membrane – keeping the right level of hydration while avoiding drying out or flooding the electrodes. 

New results from experiments conducted by ILL’s Lionel Porcar, in collaboration with CEA-LITEN scientists Arnaud Morin, Gérard Gebel and Sandrine Lyonnard, provide invaluable information that can optimise the design of the next generation of high performance fuel cells. Small-angle neutron scattering (SANS), when used on a specially constructed neutron-transparent fuel cell, has proved to be a non-intrusive, highly-sensitive proton probe. SANS measures the deviation at small angles of a neutron beam due to structures of small size in the sample. ‘Small’ means dimensions of a few tenths to about 100 nanometres, such as clusters in alloys and polymers. They found that it was the only method that could measure simultaneously the variation in water content in both vertical and horizontal planes throughout the cell. They have now carried out several SANS experiments at the ILL on the D22 diffractometer – the high neutron flux and the flexibility of its setup make D22 an instrument particularly suited for real-time experiments and weakly scattering samples. 

These experiments were used to systematically screen the impact of operating conditions on local water distribution. The experimental team varied the membrane thickness (20–200µm), gas composition, temperature (-10–80°C), current density (up to 0.8 A.cm-2), pressure (up to 300Kpa) and relative humidity of the fed gases (from 0–100%), and investigated transient regimes during on/off cycles. They were able to record a series of 3D water-distribution maps with unprecedented spatial and temporal resolutions. After developing a method to analyse the SANS data in a working cell, they could precisely correlate the water content and distribution to both the operating conditions and cell design. 

The tests showed good agreement between the performance and the average water content inside the membrane as well as outside. It is possible to estimate the membrane resistance from the water profile and the knowledge of proton conductivity as a function of water content. 

Making the most of waste heat

Today, power generation and consumption rely on inefficient processes, creating high energy losses through waste heat. The development of more efficient thermoelectric materials, which convert heat into electricity, is resulting in renewed interest in using them for power generation. A promising way of converting waste heat to useful energy is offered by thermoelectric materials (TEMs). The principle is that an electric current is induced when one side of a slab of the material is heated (for example, by waste heat) and the other side is kept cold. 

Electrical energy is propagated from one side to the other, and can then be harvested. To achieve the highest electric currents requires maintaining the steepest thermal gradient. This means the inevitable, accompanying heat flow across the gradient must be suppressed as much as possible. 

This heat is transported in two ways. Firstly, via the actual flow of electrons and, secondly, via the vibrations of the atoms forming the crystal lattice of the TEM – the acoustic phonons. The aim is therefore to identify materials in which heat transport by acoustic phonons is kept to a minimum, while maintaining the electron flow. Semiconductors are the most efficient TEMs because their electrical conductivity increases with temperature, and the heat flow mediated by phonons can be minimised by tailoring their vibrational states. 

There are several strategies to achieve the highest efficiencies in TEMs. In general, the more complex the crystal structure is, the fewer of the heat-carrying excitations are present, and the more likely they are to be scattered and stopped from propagation. Research led by ILL scientist Michael Marek Koza shows that host–guest materials such as cobalt- and iron-antimonide-based skutterudites are proving to be of particular interest. These materials are characterised by having voids in their host structures, which can accept heavy rare-earth atoms as guests. These guests act as ‘rattlers’ and dissipate the vibrations, but do not obstruct the electrical current. 

Koza et al’s study results give unequivocal evidence of essentially temperature-independent lattice dynamics with well-defined phase relations between guest and host dynamics. The vibrational modes of the heavy ion fillers are coherently coupled with the host-lattice dynamics and associated with eigenmodes of low energy owing to the heavy mass of these atoms. 

These conclusions are in disagreement with the ‘phonon glass’ paradigm based on individual ‘rattling’ of the guest atoms and have had an essential impact on the design and improvement of thermoelectric materials and on the development of microscopic models needed for these efforts. 

Another successful approach is to create selective disorder in the crystalline lattice, forming random scattering channels for the acoustic phonons. In research by Voneshan et al, an Einstein-like rattling mode at low energy was directly observed, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2.

Optimising the efficiency of such TEMs requires a comprehensive understanding of their microscopic dynamics. Inelastic neutron scattering (INS) is a unique tool for meeting this requirement, where the intensity of the scattered neutrons is analysed with respect to the momentum () and the energy () exchanged between the neutrons and the scattering system. The characteristic energies and momentum of neutrons in INS experiments perfectly match the kinematics of lattice vibrations in TEMs. In this way, not only can the distribution of vibrational states be measured, but also the specific modes that work against the overall lattice thermal conductivity. We can determine whether the vibrational excitations are of collective heat-carrying character, as well as shed light on the material’s velocity of sound, elastic properties and heat capacity. The energy resolution of modern neutron spectrometers renders the measurement of the lifetime of relevant excitations feasible, and allows us to discern the effectiveness of TEMs in atomic detail. 

Improving liquid-crystal solar cells

Sunlight is the most abundant energy source, and a great deal of research is going into photovoltaic devices that harness incident solar energy. They employ materials in which a charge separation is induced by photons to create a flow of charge carriers (negative electrons and the positive ‘holes’ they leave behind). Thanks to their low cost, inherent flexibility and relative ease of processing, photovoltaics composed of organic materials are potential candidates for the next generation of solar cells.

One particular group of organic materials that is of interest is discotic liquid crystals (DLCs). These have a molecular structure consisting of a planar core composed of several conjoined, electron-rich hexagonal (aromatic) carbon rings, to which are bound a symmetrical arrangement of hydrocarbon tails that spread out from the core. The resulting disc-like molecules self-assemble into stable columnar superstructures, which possess both solid and liquid-like properties arising from the rather stiff aromatic cores and ‘floppy’ hydrocarbon tails respectively. The columns act as one-dimensional ‘molecular wires’ that allow charges to ‘hop’ across overlapping electron-rich cores when combined with an electron acceptor. The charge separation results in an electric current when connected to an external circuit.

However, organic photovoltaics tend to suffer from a poor dissociation of charges, limiting their solar conversion efficiency. In the case of DLCs, the overall conductivity is strongly affected by the local conformation of the molecules, structural irregularities in the columns, and the disorderly motions induced by the fluidic tails. Knowledge of how each of these factors limits the hopping of the charge carriers along the columnar stack is valuable for the design of discotic compounds. This calls for a careful study of structure-versus-dynamics relationships at the microscopic level. 

Neutron scattering is a convenient tool for studying this in molecular organic systems – along with neutron diffraction, these techniques can reveal the structural arrangements within the system, and probe molecular motions on the required picosecond (ps) timescale. In this way, one could elucidate the morphology and motions in a prototype discotic liquid crystal, hexakis(alkyloxy)triphenylene (HAT6), and determine the effect of the disorder on its conductivity. 

An international collaboration between ILL’s Mohamed Zbiri and Mark Johnson with representatives from Delft University of Technology, the Netherlands, and ANSTO, Australia, has uncovered the fundamental mechanisms that influence conductivity in a new type of solar-energy material. Using the time-of-flight spectrometer IN6, motions on two timescales were observed from the QENS spectra, which the team assigned to molecular translations (0.2 ps), and tilt-and-twist motions (7 ps) of the whole HAT6 molecule. They indicated that the motion of the hydrocarbon tails was driven by the core dynamics. The diffraction data, obtained with the diffractometer D16, highlighted considerable conformational disorder, which caused displacements of the planar aromatic cores along the stacking axis. These displacements act as structural traps for the charge carriers because they persist for several tens of picoseconds, which is longer than the characteristic timescale for the charge-hopping. 

It turned out that the large disorder in the core-to-core distances is the major factor limiting the conductivity of HAT6. The charge-hopping rate decreases exponentially as adjacent cores get further apart. Using larger discotic molecules, which have higher conductivities, as a benchmark, the team found that the structural defects resulting from variations in core-to-core distances reduce conductivity by a factor of about 100. 

The future

Manipulating the resources of energy provided by nature empowers us to change our environment. All through human history, progress in exploiting energy resources has triggered important societal changes. To maintain the current model of civilisation, we have no other option than to make energy sustainable. 

In the universe, energy is not in short supply – the problem is harvesting and properly distributing power in a way that does not jeopardise our delicate terrestrial biosphere. The possibilities offered by neutron science could help us do so. 

Dr Stéphane Rols is an Instrument Scientist and carbon nanotube specialist at the Institut Laue-Langevin. For more information on the ILL, please visit www.ill.eu