Predicting optimal geometries of 3D-printed solid oxide electrochemical reactors

Solid oxide electrochemical reactors (SOERs) may be operated in fuel cell (SOFC) or electrolyser (SOE) modes, at temperatures > 800 K, depending on electrolyte and electrode materials. In electrolyser mode, current densities of ≥ ca. 104 A m−2 are achievable at potential differences ideally at the thermoneutral values of 1.285 V for steam splitting or 1.46 V for CO2 splitting at 750 °C. As for large scale chemical processes in general, such reactors are required to be energy efficient, economic, of scalable design and fabrication, and durable ideally over ≥ ca. 10 years. Increasing densities of electrode | electrolyte interfacial areas (and electrode | electrolyte | pore triple phase boundaries) of solid oxide fuel cells or electrolysers offers one means of increasing performance, reproducibility, durability and potentially decreasing cost. Three-dimensional structuring of those interfaces can be achieved by 3D printing, but modelling is required to optimise geometries. Using kinetic parameter values from the literature, COMSOL Multiphysics® finite element software was used to predict effects of 3D geometries, increasing interfacial to geometric area ratios, on SOER performances for YSZ ((ZrO2)0.92(Y2O3)0.08) oxide ion conducting electrolyte and Ni-YSZ electrode based cells, relative to corresponding planar structures with < 10 μm thick planar YSZ electrolyte. For the negative electrode, electrolyte and electrode layers were inkjet printed on Ni(O)-YSZ substrate precursors, then sintered. For the positive electrode, porous lanthanum strontium manganite (LSM: La0.8Sr0.2MnO3-δ) was brush-coated over the (gas-tight) YSZ, then sintered to produce complete SOERs: H2O-H2 | Ni(O)-YSZ | YSZ-YSZ pillars | YSZ-LSM | LSM | O2. Results are reported showing that, in the case of solid YSZ pillars, despite interfacial electrode | electrolyte areas being up scaled by factors of 10–150 depending on height (10–150 μm), current densities were predicted to increase by only ca. 1.14 in electrolysis mode and peak power densities were predicted to increase by ca. 1.93 in fuel cell mode. This was due to increased ionic current path length along the pillars, increasing ohmic potential losses relative to faradaic impedances; as expected, such predictions depend strongly on electrode kinetic parameter values. After sintering the porous Ni(O)-YSZ pillars and their subsequent reduction with H2 to nickel, they were assumed to constitute equipotential surfaces, depending on current collector design. Predicted current densities were up to 1011 mA cm−2, far greater than in solid YSZ pillars, ultimately limited by reactant or product mass transport through porous pillars of increasing height.