The application of hierarchical structures in energy devices: new insights into the design of solid oxide fuel cells with enhanced mass transport

Abstract

a Mass transport can significantly limit the rate of reaction and lead to concentration polarisation in electrochemical devices, especially under the conditions of high operating current density. In this study we investigate hierarchically structured micro-tubular solid oxide fuel cells (MT-SOFC) fabricated by a phase inversion technique and quantitatively assess the mass transport and electrochemical performance improvement compared to a conventional tubular SOFC. We present pioneering work to characterise the effective mass transport parameters for the hierarchically porous microstructures by an integrated computed fluid dynamics simulation, assisted by multi-length scale 3D X-ray tomography. This has been historically challenging because either imaging resolution or field of view has to be sacrificed to compensate for the wide pore size distribution, which supports different transport mechanisms, especially Knudsen flow. Results show that the incorporation of radially-grown micro-channels helps to decrease the tortuosity factor by approximately 50% compared to the conventional design consisting of a spongelike structure, and the permeability is also improved by two orders of magnitude. When accounting for the influence of Knudsen diffusion, the molecule/wall collisions yield an increase of the tortuosity factor from 11.5 (continuum flow) to 23.4 (Knudsen flow), but the addition of micro-channels helps to reduce it down to 5.3. Electrochemical performance simulations using the measured microstructural and mass transport parameters show good agreement with the experimental results at elevated temperatures. The MT-SOFC anode displays 70% lower concentration overpotential, 60% higher power density (0.98 vs. 0.61 W cm À2 ) and wider current density window for maximum power density than the conventional design. Broader contextHierarchical materials are a popular choice not only for electrochemical energy devices such as fuel cells and batteries, but also for a range of functional materials including catalysts, diffusion media and membranes for waste processing. This study qualitatively and quantitatively illustrates the effectiveness of hierarchically structured pores in reducing gas transport resistance, providing new insights into advanced microstructure optimisation. Moreover, it is a common mistake in the modelling of hierarchical materials to assume the flow to be governed by continuum physics irrespective of the length scale, whereas in the region of finest structure, the molecule/wall collisions become dominant, which cannot be addressed by continuum physics. It is also impossible to define a relevant average pore size to describe the mass transport in these materials due to the wide pore size distribution. The integrated computed fluid dynamics (I-CFD) technique proposed by us for the first time herein, aims to overcome this problem and accurately characterise the effective mass transport in these complex hierarchical structures. The concept and techniques used in this study are believed to be of wide inter...

Developments in X-ray tomography characterization for electrochemical devices

“…Finally, similar to the work conducted on PEFCs, modelling across multiple length scales can also be a powerful tool for the investigations of SOFCs. For instance, gas transport modelling can be applied to various structures and geometries [166,167] and may even be applied to real structures obtained from X-ray CT [168]. Grew et al concisely review multiscale modelling for SOFCs [169].…”

Multi-length scale microstructural design of micro-tubular Solid Oxide Fuel Cells for optimised power density and mechanical robustness

“…The MT-SOFC sample was previously fabricated using phase inversion technique. The detailed information of the material preparation procedure and the scanning parameters of the multi-scale Xray computed tomography (X-ray CT) can be found here [36]. Fig.…”

“…These algorithm-based morphologies are less representative of the typical 3D porous pathways and the roughness of the particle surface in the electrode than the reconstructed 3D volume. This study will be a significant extension building on our previous study which presented the developed methodology in the extraction of Knudsen mass transport parameters using DSMC technique [35]. Here, we aim to integrate this methodology for the first time to elucidate the interplay of electrochemical performance of MT-SOFCs, with the multi-length scale microstructural characteristics, such as size, percolation, compactness and radial distribution of the macroscopic micro-channels, as well as local pore size/porosity and tortuosity in the spongy substrate.…”

Design of next-generation ceramic fuel cells and real-time characterization with synchrotron X-ray diffraction computed tomography

“…As such, there is a need for in situ non-destructive material characterization techniques that permit investigation of SOFCs in real time and provide direct insight into thermal cycling processes. Although absorption and phase contrast computed tomography (CT) techniques have been applied extensively to SOFC materials 15–18 , the study of full cells remains limited 19–21 . Moreover, XRD has been used extensively in the characterization of SOFC materials but spatially-resolved chemical tomography methods such as XRD-CT remain relatively emergent techniques.…”