Antonio Bertei

Mathematical Modeling and Simulation for Optimization of IDEAL-Cell Performance

The IDEAL-Cell is an innovative SOFC concept, comprising the anodic part of a proton conducting fuel cell (i.e., anode and protonic electrolyte) and the cathodic part of a solid oxide fuel cell (i.e., cathode and anionic electrolyte), connected through a porous composite central membrane of proton conducting and anion conducting materials where water recombination reaction between protons and oxygen ions occurs. A mathematical model for the description of transport phenomena and reactions in steady-state conditions is presented. The model is based on charge and mass balances in a continuum approach. Simulations are performed considering negligible polarization resistances due to electrochemical activations in order to evaluate the maximum performance of the cell. Simulations show that the IDEAL-Cell performance is comparable to that provided by the current state of the art for proton conducting fuel cells, and it may be further improved by reducing ohmic losses with thinner layers.

Microstructural Degradation: Mechanisms, Quantification, Modeling and Design Strategies to Enhance the Durability of Solid Oxide Fuel Cell Electrodes

Electrode microstructure is one of the main factors determining the performance and durability of solid oxide fuel cells (SOFCs). The degradation is intimately linked to the microstructure, which in turn depends upon manufacturing and operation conditions. In this chapter we discuss the main causes for degradation of electrodes, concentrating mainly on the anode and present the techniques—both typical and state-of-the-art to follow these changes. We emphasize the need to quantitatively link the microstructural properties (e.g., triple-phase boundaries, porosity, and tortuosity) with the electrochemical responses measured and, most importantly, to link the change in microstructure to the performance degradation via suitable models. The knowledge gained must then be used to design new electrodes that can extend the lifetime of SOFCs once the critical parameters have been identified.

Engineered electrode microstructure for optimization of solid oxide fuel cells

This paper presents a mathematical model for the description of transport and reaction phenomena in porous composite electrodes for solid oxide fuel cell (SOFC) applications. The model is based on charge and mass balances, describing transport of charged and gas species along with the electrochemical reaction occurring at the solid/gas phase interface. Effective properties of the porous media are evaluated on numerically reconstructed microstructures. The correlation between electrode microstructure and electrochemical performance is investigated. In particular, the study focuses on how a distribution of particle size within the thickness may improve the air-electrode efficiency. The results show that distributing smaller particles at the electrolyte interface reduces the sensitivity of the cathode efficiency to the electrode thickness, with clear advantages from the manufactory point of view. However, the conditions for which this advantage is relevant, that is, particle size smaller than 0.10 μm and porosity in the order of 15 %, are not technically achievable at the present.

Uncovering the mechanisms of electrolyte permeation in porous electrodes for redox flow batteries through real time in situ 3D imaging

Increasing energy demands have expedited the need for grid-scale energy storage solutions. High power densities have been achieved using carbon-based electrodes in all-vanadium redox flow batteries. However, fundamental limitations must be overcome to improve viability. This study addresses the infiltration of vanadium solutions into dry electrodes using time-resolved 3D X-ray tomography, thus relating the rate of permeation and other wetting phenomena to microstructural characteristics and surface properties of carbon fibers. It is shown that electrolyte infiltration proceeds according to a non-uniform progression front, with small channel-like pores filled in first and remaining larger pores filled only after 6 h. The vanadium concentration affects the infiltration rate, being faster for 0.5 M VOSO4 while slower for both higher and lower concentrations. The infiltration mechanism is strongly correlated with the temporal decrease in the contact angle, revealing that fibers become more hydrophilic as they are treated by the electrolyte solution. Modification of carbon surface groups through plasma treatment boosts the infiltration process. The results reveal counter-intuitive behaviors of the electrolyte flow whereby the capillary driven flow is found to be secondary to the primary wetting mechanisms. Uncovering these physical phenomena is essential for operational procedures in flow batteries and avoiding cell degradation.

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

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 sponge-like 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−2) and wider current density window for maximum power density than the conventional design.