Effect of Reversing-Current Operation on the Structure and Electrochemical Performance Evolution of Ni-YSZ Fuel Electrodes

Abstract

In reversible operation of solid oxide cells for energy storage applications, the current direction switches periodically as the cell alternates between fuel cell and electrolysis modes. Here we report the effect of this alternating current operation on Ni-YSZ fuel electrodes in 1000 h life tests at different current densities. Ni-YSZ/YSZ/Ni-YSZ symmetric cells were used because they allow relatively straightforward interpretation of electrochemical results. At constant cell current density values ≤ 0.4 A/cm 2 , the cell voltage and total resistance values were reasonably stable, with a slight reduction in R P . At current densities ≥ 0.6 A/cm 2 , voltage and total cell resistances were still approximately constant, but there was a steady increase in the R that was almost offset by a decrease in R P . The higher current densities resulted in damage to the electrode/electrolyte interface structure including delamination, explaining the R increase. Increasing the current density also produced more Ni and YSZ nanoparticles in the electrode, yielding the decrease of R P .

Effect of direct-current operation on the electrochemical performance and structural evolution of Ni-YSZ electrodes

“…The high frequency intercept increases more rapidly with time, indicating that the ohmic resistance R Ω increases with time while the polarization resistance R P decreases with time. The EIS data were fitted with an equivalent circuit model containing an inductance and a resistor in series with two R-CPE elements (presented in figure 2) as previously used to model Ni-YSZ electrodes [18]. [21,22] The fit agrees very well with the EIS data as shown in figure 2.…”

“…Furthermore, the steady decrease in R P with time during the life tests ( figure 3(b)) may be explained by an increase in TPB density associated with the nanoparticle formation. However, in longer-term SOEC operation the continued re-structuring of the Ni-YSZ electrode could lead to damage near the electrode/ electrolyte interface, similar to that observed during reversing current operation where nanoparticle formation is more extensive [18,29].…”

“…The symmetric cells were processed as described elsewhere [18]. A∼25-30 μm thick YSZ electrolyte was sandwiched between identical Ni-YSZ electrodes, each with an ∼20 μm thick functional layer with composition 50:50 wt.% NiO:YSZ and a∼150 μm thick support layer.…”

Effect of Nanoscale Ce0.8Gd0.2O2−δ Infiltrant and Steam Content on Ni–(Y2O3)0.08(ZrO2)0.92 Fuel Electrode Degradation during High-Temperature Electrolysis

“…However, the resistivity values remain in the range expected for an ∼8 μm thick YSZ electrolyte at 800 °C, with a value of ∼0.05 Ω cm 2 . The reasons for the gradual increase are not known, but it may be due in part to the breaking-in of the initial electrolysis cell performance that is commonly observed, 8,19 especially the faster degradation seen in the first 200 h. It may also be related to Ni migration or the loss of Ni particle percolation that has been observed previously in Ni−YSZ during electrolysis under high-steam conditions, 5,7,8 although there is no clear evidence of this in the SEM images in Figures 1 and S4. The faster degradation observed in the final 200 h at f = 10% for the Ni− YSZ electrode may be associated with the highly reducing conditions reached during SOEC operation at high current density 5,10 and low-steam conditions, 8 which have been previously shown to result in an increase of R Ω .…”

“…For the Ni−YSZ:GDC electrode, there was little R p degradation (correcting for the R p dependence on f) except for the first ∼200 h of the life test, which may be explained in part by the initial breaking-in often observed in SOECs over the first few hundred hours of operation. 8,19 Any degradation is substantially less than that for the Ni−YSZ electrode, which can be explained by the lower polarization resistance of the Ni−YSZ:GDC electrode and hence the lower overpotential.…”