After undergraduate studies at Flinders University of South Australia, I came to Oxford in 2010 to undertake a DPhil in Inorganic Chemistry on hydrogen storage under the supervision of Professor Bill David and Professor Peter Edwards. After completing my doctorate, I worked as a Project Scientist at the Rutherford Appleton Laboratory before starting my current position at St John’s College. My laboratory work is completed through visiting positions at the Inorganic Chemistry Laboratory and the Rutherford Appleton Laboratory.
The provision of a sustainable, affordable and secure supply of energy to the world population is arguably the defining challenge of the coming century. In the transition to an energy system with high proportions of wind and solar power, as well as vehicles which are not fuelled by petrol or diesel, energy storage technologies are of critical importance. My research focus is on the development and understanding of sustainable energy storage materials, with a particular emphasis on hydrogen-based fuels.
Hydrogen has long been pursued as a sustainable fuel due to its high natural abundance and the fact that its use only produces water as an emission. However, difficulties associated with its storage detract from its usefulness. Ammonia is one possible solution to this challenge; it contains very high density hydrogen, and is easily stored and transported as a liquid. In this form, its energy density by volume is roughly 40% that of petrol (and around 10 times that of the lithium ion battery), which makes it highly attractive as a transportation fuel and for inter-seasonal energy storage.
The main technical hurdle to the use of ammonia – in addition to real and perceived safety issues – is that releasing its stored hydrogen requires very high temperatures, and the best catalysts for the reaction use ruthenium, an expensive and rare metal. My work involves the design and analysis of novel metal-nitrogen-hydrogen materials which are active catalysts for ammonia decomposition.
Our experiments have shown that metal amide and imide catalysts based on abundant metals like sodium, lithium and calcium, can have even higher catalytic activity than ruthenium. We synthesise new catalysts, test their activity, and use structural methods to observe changes in the catalyst under reaction conditions. Our intention is to use this information to understand their function and thus help design more active catalysts, improving the viability of ammonia as a sustainable fuel.
Fig 1: Variable-temperature ammonia conversion performance of light metal amide catalysts in comparison to supported transition metals catalysts.
Solid materials are another method of storing hydrogen. While there has been much success in synthesising solids which can store large amounts of hydrogen (up to 20 weight percent), making materials which can undergo multiple hydrogen storage and release cycles has been very challenging. The lithium amide-lithium hydride system is unusual in that it shows easily reversible hydrogen storage in the solid state under relatively mild conditions. Our experiments have shown that this reversibility is underpinned by a conservation of topology, which is facilitated by lithium ion conductivity. I am exploring the structure-conductivity-hydrogen storage relationships in these materials, to understand how high-density, reversible hydrogen storage is possible in the solid state.
Fig 2: Contour plot of X-ray diffraction data showing migration of lithium ions through the lithium amide-imide structure as hydrogen is released.
Makepeace JW and David WIF ‘Structural Insights into the Lithium Amide-Imide Solid Solution’ Journal of Physical Chemistry C, 2017, 121(22), 12010-12017
Makepeace JW, Hunter HMA, Wood TJ, Smith RI, Murray CA and David WIF ‘Ammonia decomposition catalysis using lithium-calcium imide’ Faraday Discussions, 2016, 188, 525-544
Makepeace JW, Wood TJ, Hunter HMA, Jones MO and David WIF ‘Ammonia decomposition catalysis using non-stoichiometric lithium imide’ Chemical Science, 2015, 6, 3805-3815
David WIF, Makepeace JW, Callear SK, Hunter HMA, Taylor JD, Wood TJ and Jones MO ‘Production of hydrogen from ammonia using sodium amide’ Journal of the American Chemical Society, 2014, 136 (38), 13082-13085
Makepeace JW, Jones MO, Callear SK, Edwards PP and David WIF ‘X-ray Powder Diffraction Studies of Hydrogen Storage and Release in the Li-N-H system’ Physical Chemistry Chemical Physics, 2014, 16, 4061-4070
See here for a full publications list.