Micro-Modelling of IT-SOFC Electrodes Manufactured through Electrospinning

Intermediate Temperature-Solid Oxide Fuel Cell (IT-SOFC) electrodes manufactured through electrospinning are at the cutting edge of research since they offer the possibility of coupling good mechanical properties to specific geometric characteristics, such as a high surface/volume ratio [1]. This preparation technique offers advantages in terms of simplicity, efficiency, low cost and high degree of reproducibility of the obtained materials. Furthermore, suitable adjustment of the processing parameters allows to manufacture complex nanostructures, usually fiber-made, whose properties can be further tuned by the addition of nanoparticles, for instance through the infiltration method.

We consider infiltrated fiber-made electrodes from a modelling point of view, and in particular we address our attention towards MIEC (Mixed Ionic-Electronic Conductor) fibers. We consider that each MIEC fiber features two unconnected charge conduction paths, one for electrons and another one for oxygen-ions. Infiltrated dopant particles, adherent to the MIEC fibers, create contact points between the ionic and the electronic conductive paths, among which, otherwise, the charge transfer reaction would be negligible. Based on this picture of the doped MIEC fibers, a model is developed. The model includes the evaluation of i) electron and oxygen-ion conduction along the MIEC fiber, and ii) charge transfer reaction occurring at the doping particles and, possibly, at the electrode/electrolyte interface. The model relies on a detailed estimation of the model parameters, i.e. exchange current density and geometrical features, and includes an evaluation of the ionic and electronic electrode effective conductivities, accounting for percolation within the network of infiltrated particles.

We apply our model to different types of fibrous electrodes, anodes or cathodes [2-3], doped or undoped, manufactured through electrospinning but also through different techniques.

For example, we report here results about infiltrated cathodes based on fibrous LSCF scaffolds with different internal compositions. Simulation results are compared to literature experimental data, demonstrating good agreement (Fig. 1). In particular, the model captures very well the improvement of performance of the doped electrodes over the undoped ones, which can be five to ten fold or even more in some cases, and can bring the 1/R_p values to the order of magnitude of 10 S cm^-2 at 1000 K, which makes them good candidates for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. The model allows to investigate, in detail, the effect of morphological and geometrical parameters on the various sources of losses, which is the first step for an optimized electrode design. This sensitivity analysis shows that, when increasing the doping level, the simulated 1/R_pincreases up to a plateau, and this is confirmed by literature experimental data (Fig. 1). In addition, the model allows to identify the extent of the electrode thickness where the electrochemical reaction effectively occurs, which is 10-20 µm close to the electrode/electrolyte boundary, depending on the operating conditions and the doping level.

In parallel, we are developing an experimental campaign devoted to investigate electro-spun fibrous LSCF cathodes. Electrospinning is based on the application of an electric field to a drop of fluid polymer on the tip of a spinneret. When the applied electric field reaches a critical value, then a charged jet of the solution is then ejected with evaporation of the solvent and simultaneous formation of solidified, continuous, ultra-thin fibers. For electrospinning experiments, we use a custom design equipment (Spinbow s.r.l., BO, Italy) which contains a high electric voltage supplier connected to the stainless steel needle and a sample collector. A syringe pump connected to the needle controls the flow rate of the polymer solution. The instrument is also equipped with coaxial spinneret in order to made core-shell nanofibers as well as hollow nanofibers. As an example, we report in Fig. 2 SEM images of one of our electro-spun nanofibrous polymeric scaffolds.

By coupling the experimental and modelling approaches we aim at (i) additional model validation; (ii) further understanding of the mechanism of electrochemical promotion of dopants; and (iii) optimization of the electrochemical performance.

1. Cavaliere S., Subianto S., Savych I., Jones D. J. , Rozière J., 2011, Electrospinning: designed architectures for energy conversion and storage devices. Energy Environmental Science, 4, 4761-4785.
2. Enrico A., Costamagna P., 2014, Model of an infiltrated La_1-xSr_xCo_1-yFe_yO_3-δ cathode for intermediate temperature solid oxide fuel cells. Journal of Power Sources, 272, 1106-1121.
3. Enrico A., Costamagna P., 2015, Theoretical analysis of the electrochemical promotion of infiltrations in MIEC based electrodes for IT-SOFCs. Chemical Engineering Transaction, 43, in press.
4. Lou X., Wang S., Liu Z., Yang L., Liu M., 2009, Improving La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ cathode performance by infiltration of a Sm0.5Sr0.5CoO3−δ coating. Solid State Ionics, 180, 1285-1289.