Improving Gas Diffusion in Solid Oxide Cells Through Laser-Ablated Electrode Supports

Abstract

Improving solid oxide cell power density will enable cheaper commercial-scale SOFCs and SOECs. In Ni-YSZ electrode-supported cells, gas diffusion through the electrode support layer can be a major limitation at high H2 or H2O utilization and high temperature. Conventional methods of improving diffusion through the electrode support include increasing the support porosity and reducing the support thickness, but these can reduce the cell’s structural integrity. Freeze casting1 and 3D printing2 have also been explored to enhance diffusion. Here we explore cells in which the Ni-YSZ supports have macroscopic channels, produced by laser ablation, that reduce the average gas diffusion length.To test the difference in mass transport through patterned and pristine electrode supports, symmetric electrode-supported Ni-YSZ cells are patterned on one side, which enables comparison of the electrochemical performance of two otherwise identical electrodes (Figure 1a). Cells with varying pattern geometry, pore geometries, Ni/YSZ ratios, and Ni-YSZ particle sizes were created to fully understand how the support layer microstructure and macrostructure affects the cell performance. Patterned and pristine cells were tested together at three temperatures (600C, 700C, and 800C) in a 97%H2:3%H2O environment, chosen to produce a clear gas diffusion limitation in H2O electrolysis. j-V sweeps and electrochemical impedance spectroscopy (EIS) were carried out on each cell at each test condition, and the microstructure of patterned and control supports was characterized through SEM imaging.Upon removing the ohmic portion and fitting the j-V data to Equation 1 (see Figure 1), patterned electrodes consistently demonstrated higher limiting current density and lower mass transport losses than the control (Figure 1c-d). Three-point-bend mechanical testing revealed that the mean flexural fracture strengths of pristine cells and patterned cells are 36.0 ± 11.4 MPa and 33.0 ± 8.9 MPa respectively.Combining the equations for limiting current density with diffusion in a system with prismatic channels, we determined the ratio of the limiting current density for a patterned and pristine electrode using Equation 2 (see Figure). For the cell data shown in Figures 1c-1d, we expect an average 33% increase (with a standard deviation of 3.5%) in limiting current density based on microscopy measurements of channel and support layer thicknesses. In 1c, we see that the limiting current density of the patterned electrode is 32% higher than the pristine electrode, which matches our expectation. Furthermore, the Nyquist plot in Figure 1d demonstrates that patterned symmetric cells have similar impedance to pristine cells in the high-frequency regime with less impedance in the low-frequency regime, which is consistent with our expectations that the patterned cells have reduced mass transport losses with similar ohmic and activation losses. These j-V and EIS results suggest that macroscopic support patterning is a promising method for improving performance without compromising structural integrity. Figure 1: a) Ni-YSZ symmetric cell cross-section, laser-patterned with 600 µ m by 600 µ m channels and 200 µ m channel spacing. b) Comparison of the flexural fracture strength for pristine and patterned cells, under 3-point-bend testing. c) IR-free current density-voltage profiles of a patterned Ni-YSZ symmetric cell with a predicted current density improvement of 33%, at 700C in a 97%H2:3%H2O environment. Two distinct j-V responses are observed, based on whether the pristine electrode or the patterned electrode is under a cathodic bias. d) Unbiased potentiostatic EIS (1 MHz - 100 mHz) of a pristine cell versus two patterned cells at 700 C in 97%H2:3%H2O. The cells all have similar Ohmic resistances, but the impedance of the patterned cells is significantly lower in the low-frequency regime, due to the reduction in mass-transport losses. Equation 1: a is a constant, α is the transfer coefficient, j is current density, and jlim,H2O is the limiting current density for H2O diffusion. Equation 2: jlim/jlim,0 is the limiting current density ratio, A is the fraction of the cell surface that is patterned, and tc is the ratio of the channel depth to the patterned electrode thickness.