Research overview
We're a chemistry research group working across the areas of solid-state chemistry and heterogeneous catalysis, with the overarching goal of developing materials and processes that will help enable society’s transition to a sustainable global energy system. We have expertise in the synthesis, characterisation and understanding of novel inorganic systems, principally metal-nitrogen-hydrogen (M-N-H) materials.
In all our work we seek to understand the chemistry which underpins the desired function of a material. We do this using a variety of structural and spectroscopic probes, with a particular emphasis on in situ or in operando techniques, where we can examine the material in its active state. We also aim to be engaged in projects which range from fundamental science to technology demonstration.
See below for some brief introductions to some of our current research areas.
Metal-nitrogen-hydrogen materials
Our research is largely based on the synthesis, characterisation and application of inorganic metal-nitrogen-hydrogen materials, incorporating a range of NHx anions in the solid state. Challenges with synthesis and handling of these materials has hindered their investigation in the past, but with recent advances, significant opportunities exist for the development of functional materials based on M-N-H compounds.
The Li-N-H system is an excellent exemplar of the chemical diversity which is possible in these materials. The ternary phase diagram shows the known materials which form with increasing hydrogen content from lithium nitride (Li3N): lithium nitride hydride (Li4NH), lithium imide (Li2NH), lithium amide (LiNH2) and lithium tetra-ammoniate (Li(NH3)4). Four different anions, five different crystal structures and a range of functional properties. Solid solutions between nitride-hydride, imide and amide also highlight the potential for facile modification of the anion array and formation of mixed-anion materials.
Synthesis of M-N-H materials generally requires air-sensitive sample handling, and synthetic approaches using solid state techniques, mechanochemistry and reactions with hydrogen, nitrogen and gaseous, liquid and supercritical ammonia. We seek to synthesise new M-N-H materials, exploring their structure and functional properties.
NHx species and examples of the Li-N-H system.
Li-N-H ternary phase diagram.
Ammonia synthesis and decomposition catalysis
Ammonia has the potential to be an elegant solution to the challenge of hydrogen storage. Its high volumetric energy density – over 20 times that of commercial lithium-ion batteries – makes it attractive for longer range/heavy transport and large-scale (e.g. inter-seasonal) energy storage. It benefits from a mature synthesis process and straightforward transportation through liquefaction.
However, traditional catalytic cracking of ammonia to release hydrogen is only achieved at very high temperatures with expensive rare metal catalysts (e.g. ruthenium). Recent research has shown that light metal amides and imides are cheap and effective alternatives, with some giving a superior performance to state-of-the-art ruthenium catalysts with a far cheaper and more earth-abundant catalyst formulation.
Additionally, the development of catalysts for ammonia synthesis which facilitate milder reaction conditions may help enable small-scale and intermittent ammonia synthesis, for example when coupled to renewable electricity installations.
Our work in this area focuses on exploring this new family of catalysts in order to understand what governs their catalytic activity and so ultimately design better-performing catalysts. This understanding is generally developed through the use of in situ X-ray and neutron scattering experiments, where we can watch the structural evolution of the catalyst as it decomposes ammonia.
Key publications: J. Am. Chem. Soc., 2014, 136(38), 13082-13085; Chem. Sci., 2015, 6(7), 3805-3815; J. Power Sources, 2016, 329, 138-147, Int. J. Hydrog. Energy, 2019, 44(15), 7746-7767.
Ammonia decomposition performance of metal amides compared with ruthenium.
Ammonia decomposition with in situ neutron powder diffraction.
Ionic conductivity: hydrogen storage and battery materials
In the hydrogen storage reactions of the Li-N-H system, in situ synchrotron X-ray diffraction experiments have revealed that the facile reversible conversion between lithium amide and imide derives from their pseudo-topotactic structures, resulting in the formation of solid solutions of amide and imide during the reactions In this way, the Li-N-H system and its derivatives have achieved hydrogen storage reversibility that is unparalleled among complex-anion hydrogen storage materials. This structural transformation is fundamentally driven by lithium ion diffusion, a phenomenon that is also critical in battery technology.
Key publications: Phys. Chem. Chem. Phys., 2014, 16(9), 4061-4070; J. Phys. Chem. C, 2017, 121(22), 12010-12017.
Dehydrogenation of lithium amide to lithium imide through a solid solution, monitored by in situ synchrotron X-ray powder diffraction.