Leveling Transient and Spatial Gradients in Planar Solid Oxide Cells Using Spatially Varied Microstructures: A Numerical Study

Abstract

Sharp spatial and transient gradients of overpotential, current density, gas species and temperature within planar solid oxide cells (SOCs) can lead to increased degradation rates, decreased active area utilization and high thermal stresses. Such issues will lead to decreased cell efficiency and functional lifetime while increasing the risk of sudden cell failure due to cracking or delamination. Spatial gradients occur in part due to the interconnect geometry. Commonly, the electrochemical reaction rate within SOC electrodes is disproportionately higher near the inlet regions of the fuel or steam channels and decreases towards the outlets as the concentrations of fuel or steam decreases for fuel cell and electrolysis modes, respectively. Transient gradients occur on short time scales due to rapid fluctuations in loading during ramp up or ramp down and for reversible operation when the current direction is reversed. It might be possible to greatly reduce both transient and spatial gradients by controlling the reaction rate distribution within SOC electrodes through the strategic manipulation of the electrode active layer microstructures. Advances in additive manufacturing techniques now facilitate the design of engineered electrodes with precisely controlled microstructure properties such as pore size, porosity and volume ratios of active layer materials (i.e. Ni or YSZ). The purpose of the present study is to explore the impact and possible benefits of applying functionally graded or regionally altered microstructural properties in the active layer of planar SOCs using a previously developed transient 3D multiphysics performance model of commercial sized planar SOCs. Percolation theory is used to estimate properties such as triple phase boundary (TPB) density and tortuosities from the applied variations of particle sizes, volume fractions and pore former properties as shown in Figure 1(a). It is shown that indeed both spatial and transient gradients can be significantly decreased by engineering gradients in composition and microstructure. For example, under the extreme conditions of ramping up from 0 to 0.75 A/cm2 current density and 75% H2 utilization in fuel cell mode, the multiphysics performance model predicts a nearly 300K temperature increase in approximately 100s for the baseline case as shown in Figure 1(d). Applying linear gradients of porosity and Ni/YSZ ratios such that TPB density increases from the fuel inlet to outlet significantly decreases this temperature gradient by 25-100K. However, there may be a tradeoff in terms of overall performance with decreasing steady state cell voltage corresponding with decreased temperature gradients. This is partly associated with the decreased temperature itself as the reaction kinetics are assumed to increase with increasing temperature resulting in decreased overpotential at the TPBs. Additional impacts of these and other functionally graded microstructures include decreased current density gradients and more uniform oxygen reduction in the oxygen electrode compared to the baseline case. Results indicate that functionally graded electrode microstructures could be designed based on the desired operating conditions to reduce degradation and thermal stresses within cell and stack assemblies. Such electrode designs have the potential to significantly reduce cell failure rate and increase cell lifetime particularly in transient loading and/or high current density applications. Figure 1