Corrugated Electrode/Electrolyte Interfaces in SOFC: Theoretical and Experimental Development
Authors: Chesnaud Anthony , Delloro Francesco , Geagea Maya , Abellard André-Pierre , Ouyang Jian , Li Di , Shi Tang , Chi Bo , Ihringer Raphaël , Cassir Michel , Thorel Alain 
Published: February 2020

The development routes to improve the performances of SOFCs are globally well identified and lead to shift the characteristic i/v curve towards higher potentials and current densities, an effect that is consistent with a lowering of the three overpotentials (activation, resistance and concentration). An increase of the Triple Phase Boundary (TPB) density at the electrodes, a reduction of the materials thickness, the development of materials with improved performances, the optimization of electrodes microstructure all serve as guidelines that aim to this goal. Taking as an example the specific design of battery electrodes, we have explored how the corrugation of electrode/electrolyte interfaces at a mesoscopic scale in a SOFC could improve the electrochemical performances, from theoretical and experimental points of view.

First, an electrochemical model was established and implemented in the COMSOL Multiphysics software, taking into account masses and charges conservation, gas transport and electrochemical reaction kinetics. The model results demonstrated that the presence of a periodic pattern (parallelepipeds, pyramids, ellipsoids) at the electrolyte/electrode interfaces, along with an electrolyte the thickness of which is significantly smaller than the dimensions of the pattern, could lead to a strong increase of the exchange surface, hence to higher exchange currents and cell performances. To that extent, it is theoretically shown that the patterns must have concave and convex singularities so as to confine the cathode material on the cathode side and the anode material on the anode side, hence the active layers on both sides, so that a much larger number of TPB are solicited and involved in the chemical reactions, reducing then the activation overpotential, as compared with a flat surface. It is also shown that the geometrical feature can be chosen so that it minimizes the concentration overpotential. In addition, the modeling provided us with key information regarding the characteristics of the patterns (shape, width and depth, distance between them, …) that should lead to a large gain in exchange current throughout the interface. For example, an increase of the exchange current of about 60% was calculated for a parallelepipedic pattern with 70 μm width and 100 μm depth, each geometric feature separated from the other by a few hundred microns.

With the use of laboratory standard ceramic processes (tape casting, screen printing, bar coating, cold pressing, cold stamping), we have implemented such mesoscopic architectures on green self supported anodes (YSZ + Ni) on top of which a thin layer of electrolyte (YSZ) was deposited. As anticipated the electrical testing and impedance spectroscopy results show that such corrugation at anode/electrolyte interfaces improves significantly the electrochemical performances of the cell even though the sintering tends to alter and blunt the geometrical features of the pattern. Based on the electrochemical performances and modeling, the geometry of the pattern and its evolution during sintering is discussed in terms of activation and concentration overpotentials.