Tracking the evolution of a single composite particle during redox cycling for application in H2 production

Composite materials consisting of metal and metal oxide phases are being researched intensively for application in various energy conversion and storage technologies. In these applications, composites are often expected to operate under redox conditions at elevated temperature. The understanding of the dynamics of composite phase and morphology evolution during redox cycling is still very limited, yet critical to maximising performance and increasing durability. Here we track the microstructural evolution of a single composite particle over 200 redox cycles for hydrogen production by chemical looping, using multi-length scale X-ray computed tomography. We show that redox cycling triggers a centrifugal redispersion of the metal phase and a centripetal clustering of porosity, both seemingly driven by the asymmetric nature of oxygen exchange in composites. We show that initially the particle develops a large amount of internal porosity which boosts activity, but on the long term this facilitates structural and compositional reorganisation and eventually degradation. We also correlate the microstructural data with phase and activity analysis to identify structure-property correlations which not only provide valuable insight into the evolution of composite materials under redox conditions but also for the design of new composite materials with enhanced durability.