Decarbonising Hydrogen Production with Catalytic Sonochemical Reaction Engineering

Green hydrogen (H2) will play a key role in global net-decarbonisation, whether used as a fuel to overcome the intermittency challenges of renewable energy or as a reducing agent substitute for fossil fuels in the steel industry. The electrolysis of water is currently the only commercial method of producing green H2, accounting for 5% of worldwide production. There is little existing research in liberating H2 from atypical sources, including seawater, wastewater, and aqueous ammonia, in order to minimise the energy consumption and cost requirements of any upstream desalination or purification processes. Sonochemistry is a potential solution for low-energy, carbon-free decomposition of these sources into H2. When ultrasound waves are applied to a liquid medium, the resulting pressure oscillations from the periodic rarefaction and compression of the sound waves can induce the formation, growth, and collapse of microbubbles, known as acoustic cavitation. The resulting extreme temperatures and pressures can enhance chemical activity, namely sonochemistry, and thus initiate H2 carrier decomposition without emitting carbon.
Conventional sonochemical reactors have both low space-time yield (STY) and energy-to-H2 efficiency, thus preventing industrial scale-up. This project aims to expand from the novel sonochemical reactor created by the Physical Acoustics Laboratory (PacLab) at the University of Oxford, which currently uses cheap materials, catalysts, and acoustic beamforming to amplify STY by orders of magnitude. With the objective of improving energy-to-H2 efficiency (to at least 1%) and determining how efficiency metrics scale-up, experimental studies will initially consist of implementing unique acoustic fields to a H2 carrier medium. This will be achieved by designing and testing the ultrasound sequencing pattern generated by the transducer arrangement within the reactor. Parameters to vary include the wave type (e.g. continuous or pulse based), pressure amplitude, phase, and frequency of the ultrasound input. There is also scope for exploring the effect of catalyst presence, composition, structure, size, concentration, and reusability.
Novelty will also derive from characterising the acoustic field using cavitation density distributions (CDD) and passive acoustic mapping (PAM) imaging, as used in the biomedical field. This will aid in optimising the acoustic field, predicting how similar fields can be induced in larger reactors, and determining whether factors, such as reactor geometry and its interaction with the acoustic field, impose a limit on the maximum energy-to-H2 efficiency.
H2 production rates and selectivity will be detected using methods including gas chromatography mass spectrometry and ultraviolet-visible spectroscopy. The resulting analysis will indicate whether alternative reaction pathways (for example, hydrogen peroxide from water sources or hydrazine from aqueous ammonia) are limiting the energy-to-H2 efficiency and therefore, which parameters can be tuned to prevent them.
This project falls within the EPSRC hydrogen and alternative energy vector research area, under both the 'engineering' and 'energy and decarbonisation' themes. Collaboration with EDF Energy will provide advice on overall research direction and determining the feasibility of industrial scale-up.