The Impact of Strong Cathodic Polarization on SOC Electrolyte Materials

Abstract

One of the most promising reversible energy conversion/storage technologies is that of Solid Oxide Fuel/Electrolysis Cells (SOFC/SOEC, collectively termed SOC). Long term durability is typically required for such devises to become economically feasible, hence considerable amount of work has and is being done on the degradation and long term durability of these systems. When using a SOC in electrolysis mode, it is economically beneficial to operate the cell at high current density, since it increases the fuel production rate. However, several degradation phenomena, such as segregation of impurities at the grain boundaries, electrode poisoning, delamination or cracks of the electrolyte etc., have been observed in cells operated at such conditions, lowering the lifetime of the cell1,2. High polarizations are observed at the electrolyte/cathode interface of an electrolysis cell operated at high current density. In case of a cell voltage above 1.6 V, p-type and n-type electronic conductivity are often observed at the anode and cathode respectively3. Hence, a considerable part of the current is lost as leakage through the electrolyte, thus lowering the efficiency of the cell considerably. The presented work aims to study the impact of short term strong cathodic polarization in SOC model systems PtIr/YSZ and Ni/YSZ. The study is mainly focused on electrolyte materials such as yttria stabilized zirconia (YSZ). Since impurity segregation is one of the suggested degradation mechanisms, electrolyte materials with silica additions are investigated likewise. The electrochemistry of the electrolytes is studied in a Controlled Atmosphere High Temperature Scanning Probe Microscope (CAHT-SPM)4,5, using PtIr (8:2) or Ni probes as working electrodes. Different cathodic polarizations ranging from -0.1 V to -2 V vs. a macroscopic platinum reference electrode in the same atmosphere are applied to the YSZ electrolyte at 650 °C through the well-defined tip of the probe. The atmosphere used for these measurements is a mixture of 9% H2 in N2 saturated with water vapor at room temperature. The electrochemical impedance spectroscopy measurements, in addition to the expected change in the DC resistance, reveal a decrease in the series resistance (Rs) with increasing cathodic polarization (Figure 1a, 1b). Such decrease in the ohmic resistance of the electrolyte indicates the introduction of electronic conductivity in the electrolyte material. The measurements presented in Figure 1 are obtained from stepwise cathodic polarization ranging from -0.2 V to -2 V with a conditioning period of 120 s before each impedance scan on a dense YSZ electrolyte using a nickel probe as working electrode. Similar behavior of the Rshas been obtained using PtIr probes. In addition to the electrochemical study, the chemistry, microstructure and surface conductance of the region impacted from the polarization are investigated using Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Scanning Electron Microscopy (SEM) and CAHT-SPM. The surface conductance measurements for the PtIr-YSZ model system show that the impact of the polarization is larger than the tip-electrolyte contact area. The presence of the high and low conductance regions is attributed to the introduction of electronic conductivity, the segregation of impurities/low conducting phases and the microstructural changes in the material. The affected area is strongly dependent on the time and strength of the polarization. Such trends are also observed in the chemistry and microstructure of the surface. The nickel ion image obtained with ToF-SIMS (Figure 1c) gives an indication of the tip-sample contact area, since the formation of Ni-Zr compounds has been previously observed. The siliconion image on the other hand, demonstrates that silicon species segregate in a much larger region surrounding the contact area. The silicon ion signal is attributed to silica, since the polarization applied is lower than the silica reduction potential. Thus, by combining all of the above techniques, valuable information is obtained about multiple degradation mechanisms caused from strong cathodic polarization.