Modeling of a Single Repeating Unit for Protonic Ceramic Cell Applications
Authors: Ferrero Domenico , Dealberti Gabriele , Anelli Simone , Baggio Andrea , Schmider Daniel , Dailly Julian , Smeacetto Federico , Santarelli Massimo 
Published: September 2023

Protonic Ceramic Cells (PCCs) are a promising alternative to oxygen-ion conducting solid oxide cells (SOCs), offering benefits for hydrogen separation, electrolysis and fuel cell applications. The main advantages are the lower operating temperature and the capability of using/producing a pure hydrogen stream due to the protonic conductor. Recent advancements in PCCs at the laboratory level are encouraging, and the current development stage is focusing on the scale-up of the technology to cross the gap between single cells and stacks. An intermediate stage for this process is the development of single repeating units (SRU), composed of cells, sealings and interconnects. The design of the SRU is the first step of development, which consists in the selection of the optimal geometry of the interconnect-cell assembly for a peculiar cell and application which allows achieving a homogeneous temperature field and an even distribution of the reactants to the electrodes during the operation. In order to optimize the design of the geometry, a common approach is to develop numerical models to simulate the fluidic distribution in the SRU and the thermal-electrical response of the integrated interconnect-cell assembly.
A numerical model for three-dimensional, steady-state simulations of an SRU geometry designed for a circular, anode-supported 40 cm2 PPC, composed of a BCZY electrolyte, NiO/BCZY anode, and a BCZY-BGLC air electrode has been developed. The numerical model implemented applies the equations for the conservation of mass, energy and momentum in fluid and porous domains coupled with activation kinetic equations in the active electrodes. In the solid domains, the model applies energy and charge conservation equations. The model calculated the distribution of chemical species and temperature, and the pressure and velocity fields in the fluid domains. In the solid domains, the distribution of temperature, electronic/ionic potentials and current is evaluated. The cell model has been first calibrated using experimental data from single cell polarizations.
The structure considered for the SRU is shown in Figure 1 (cross-section). In the configuration simulated, the cell is sealed with a glass-ceramic seal placed between the electrolyte of the cell and a metallic frame (Figure 1b and Figure 1c). Metallic meshes/foams are placed in contact with the electrodes to improve the electrical contact between them and interconnect plates of the system (Figure 1d). Compressive seals (Figure 2e) are utilized between the frame and interconnects. Finally, interconnects are at the bottom and on top of the system.
To optimize the cell operation, we focused on the design of the interconnects, either with channels or porous foams to improve the gas distribution. The area in which the reactant has to be distributed and the thickness of layers is imposed by the cell geometry, but it is important to properly choose a gas flow distribution geometry (flow field) of the interconnect plate to guarantee high performance of chemical reactions in the cell electrodes. Hence, the geometrical parameters selected for the optimization were those of the channels (number, length, ribs dimensions parameters).

Figure 1: Main structure of the SRU with a single cell. In the sequential schematics one can observe, the cell (a), the glass sealing (b), the metallic frame (c), the metallic meshes (d), the compressive sealing (e) and eventually the interconnect with channels (f).

The objective functions selected for the optimization of the SRU geometry were the maximum temperature gradient in the cell volume and the voltage losses.
The model has been applied to the simulation of both electrolysis and fuel cell operation and the interconnect geometry has been optimized for these two applications. The use of a porous metallic foam in place of the channels configuration has been also evaluated. Results show that complex channel configurations could optimize the flow fields for specific operating points, but simpler geometries are more flexible for the development of a SRU platform for general PCC applications.

Figure 1