Real-time characterisation of a new miniature-honeycomb fuel cell shows its outstanding properties

02-04-2019

A team from Imperial College has designed a miniature ceramic solid oxide fuel cell with excellent properties and together with scientists from the University College London, the company Finden and the ESRF, they characterised the cell as it works on beamline ID15A, confirming the great performances of the new device.

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Ceramic fuel cells are considered as one of the most promising technologies for sustainable energy generation thanks to  their interesting features, such as higher efficiency compared to conventional combustion-based power plants, high operating temperatures (600 - 1000 °C) that generate high-grade waste heat, and superior fuel flexibility that allows the direct utilization of hydrocarbons.

To date, ceramic fuel cells are used in a wide range of applications, including stationary power supply, combined heat and power system (CHP), auxiliary power units (APU), etc., and will continue receiving attention as shale gas and biofuels are becoming the premium fuel choices thanks to their low carbon footprint.

However, the state-of-the-art ceramic fuel cell technology still has a long way to go in terms of volumetric power output, long-term stability of the system and most importantly, the cost. For instance, two major types of designs, namely planar and tubular, both have some critical bottlenecks yet to be solved. For the planar design, the most commonly used, the sealing between layers becomes unreliable at high temperatures. This has always been an issue, despite the high-performance it offers.

On the other hand, the tubular design has the advantage of a simplified sealing, but its volumetric power output is several folds lower compared its planar competitor. The attempts to enhance the power output by going small and miniaturizing the ‘tubular’ into ‘micro-tubular’ range have resulted in an inevitable compromise of mechanical reliability. Researchers are therefore on a quest to develop the “perfect” design, which combines the features from both planar and tubular designs.

A team from Imperial College London, led by Professor Kang Li, has developed a new conceptual design to tackle the current limitations. The so-called ‘micro-monolithic’ design is like a honeycomb structure whose mechanical reliability has been well proved in different industrial applications. The cell has been prepared using their custom-developed extrusion set-up, assisted by the phase-inversion process. Such phase-inversion process is evolved from the commercial manufacturing process of polymeric hollow fibre membrane. This process introduces a hierarchical micro-structure in the final completed fuel cells to improve the mass transport property and also helps to miniaturize the overall dimension down to the millimetre scale.

 The team joined forces with the University College London (UCL), Finden Ltd UK and the ID15A beamline at the ESRF to use the state-of-the-art facility to perform real-time characterizations of the new fuel cell. “This is the first time the X-ray Diffraction Computed Tomography (XRD-CT) technique has been applied on a complete solid oxide fuel cell as it is functioning and the results speak for themselves”, explains Marco Di Michiel, scientist in charge of ID15A, the beamline where the real-time characterisations took place.

This holistic design, featuring both excellent power output and superior mechanical reliability, is a success. An exceptional power density of 1.27 W cm-2 at 800 °C has been obtained, which is one of the highest reported. Due to the efficient miniaturization of cell diameter down to the millimetre scale, such design is estimated to produce a volumetric power density exceeding 10 W cm-3, which is even superior to the performance of planar designs. “We have, for the first time, developed a micro-monolith with a unique cross-sectional structure, which enable us to miniature solid oxide fuel cell systems with excellent power densities. The state-of-the-art experiments at the ESRF have contributed enormously to the study, which represents a step change in commercialising portable ceramic solid oxide fuel cell systems with the highest volumetric power, fast thermal cycling and excellent mechanical stabilities”, explains Professor Kang Li, corresponding author of the research.

Promising future for XRD-CT

Synchrotron XRD-CT, which has been applied to investigate the thermal cycling performance of this real-life SOFC, showed the excellent chemical/structural heterogeneity the micro-monolithic SOFC possessed during the cycling. “This work not only demonstrates the feasibility to perform spatially-resolved studies of full cells, but also adds a very powerful characterization tool for the SOFC community to exploit and perform in situ and indeed operando chemical tomographic experiments,” adds Di Michiel.

ID15A has played a major role in revolutionising X-ray powder diffraction tomography: data collection speed and data quality have improved enormously in recent years thanks to developments in optics and detectors.  The future on this beamline looks even brighter, as the new EBS machine at the ESRF will provide a massive increase in photon flux.  Chemical imaging via XRD-CT can therefore take another giant leap forward.

Reference:

Tao Li, et al, Nature Communicationsvolume 10, Article number: 1497 (2019).

Top image: Micro-computed tomography and X-ray diffraction computed tomography images. XRD-CT maps of LSM (green), YSZ (red) and NiO (blue) have been overlaid on top of a micro-CT image collected at the same z position. The scale bar corresponds to 0.5 mm. Copyright: Tao Li.