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On Photo-enhanced Transport in Ionically Conducting Solids

Periodic Reporting for period 1 - OPTICS (On Photo-enhanced Transport in Ionically Conducting Solids)

Reporting period: 2021-09-01 to 2023-08-31

The ability of certain crystalline oxides to conduct ions at a significant rate forms the basis for a range of electrochemical devices such as solid oxide fuel cells, solid oxide electrolyser cells, and batteries. These devices are key for the EU Energy 2050 long-term strategy, hence there is an increasing demand for the development of faster and more stable ionic conducting materials.

Ionic transport in oxides requires the long-range motion of crystalline defects and is limited by one or more of the following processes: bulk diffusion, incorporation/excorporation into the material, or transport across grain boundaries or interfaces. Fundamentally, these limits are caused by either the concentration of ionic or electronic defects, migration barriers (according to the bond strengths and steric constraints of the ions), interactions between defects, or the formation of space charge regions that deplete charge carriers. Traditionally, materials development has been based on either searching for new oxides or tuning the chemical composition and microstructure of current materials to maximise ionic transport. But progress has been slow, with newly developed materials failing to meet the strict requirements to replace the state-of-the-art materials at a commercial level.

The OPTICS project aims to investigate and exploit new methods for enhancing ionic transport in electroceramic materials, namely the use of above-bandgap radiation. Several recent reports have suggested that the concentration and effective mobility of ionic defects may be varied by UV light, but currently this effect is poorly understood. Light-enhanced ionic transport has the potential to rapidly progress beyond current state-of-the-art in technologically relevant ionically conducting oxides. While experientially and computationally non-trivial, demonstrating light-enhanced ionic transport would represent significant progress from an academic standpoint, but crucially, would also have the potential to usher in a new class of opto-ionic fuel cells, electrolysers, and batteries.
A custom set-up was designed and constructed for tracer diffusion studies and electrochemical impedance spectroscopy measurements under a UV light source. This included temperature and pressure control with electrical feedthroughs for sample contacting. Thin films of rare-earth substituted ceria were fabricated using pulsed laser deposition (PLD) on various substrates and deposition conditions to systematically vary the microstructure and distribution of grain boundary types. The crystal structure and texture of the thin films were comprehensively characterised used X-ray diffraction. The conductivity of the thin films was successfully measured under UV light, demonstrating an enhancement of the conductivity of up to 50% higher compared to ‘dark’ conditions. Conductivity measurements were taken as a function of temperature for a range of films with different microstructures, to provide deeper insight into the underlying mechanisms for UV-enhanced conductivity. Preliminary findings were presented at the Power of Interfaces meeting in Palma De Mallorca. A manuscript detailing the findings is currently under preparation for publication.

The surface exchange of Fe-substituted strontium titanate (STF) was investigated using electrochemical conductivity relaxation (ECR) measurements on porous bars. Rather than study the effect of UV radiation, the role of binary oxide impurities was on the surface was studied. It was demonstrated that the surface exchange could be modified by over an order of magnitude, via the infiltration of CaO and SiO2 species. The change in the surface exchange correlated to the acidity of the infiltrated species. These results were presented at the 23rd International Conference on Solid State Ionics in Boston, USA, the Workshop on Mixed Ionic and Electronic Conductors for Energy Applications, University of Cambridge, UK, the Symposium on Materials For Emerging Energy Technologies, Madrid, Spain, and the Bunsen Colloquium New Horizons in Solid State Ionics, RWTH Aachen, Germany. A manuscript detailing the findings of this part of the project is also under preparation for publication.
Electrochemical devices, such as solid oxide fuel cells, solid oxide electrolyser cells, and all solid-state batteries, are currently not widely commercialised despite potential for the global decarbonation effort. Improvement in the ionic transport of materials used in these devices will accelerated their roll-out. The OPTICS project has made progress in two major ways in this direction.

Firstly, demonstrating the enhancement in the conductivity of rare-earth substituted ceria combining with new insights into the role of different grain boundary types and the underlying mechanism is a significant step towards enhanced electrochemical devices and opto-ionic devices.

Secondly, the ability to change the surface exchange of STF through the use of binary oxides on the represents significant progress to realizing enhanced surface exchange on technologically-relevant perovskite-structured materials. Our findings represent a deeper understanding of the underlying mechanisms and open new routes to materials engineering.
Enhancement in the conductivity of a Gd:CeO2 film under UV illumination.