Operando Localized Probing of Model Electrodes for Solid Oxide Electrolysis/Fuel Cells

Abstract

The need for efficient large scale energy storage is becoming more and more pressing as entire nations are planning to move away from fossil fuels and intend to harvest more renewable energy sources. Electrochemical technologies such as solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC), collectively called solid oxide cells (SOCs), are being considered as potential technologies to be used to balance the grid and establish a sustainable transportation system[1–4]. However, SOCs remain economically unfeasible as a result of their fast degradation and short durability. Due to the complexity of the microstructure of the SOCs, operando localized probing of SOCs is necessary to further our understanding of the reaction mechanisms at the electrodes as well as their degradation. The purpose of this work was to investigate the electrochemical reactions that occurat the gas/electrode and electrode/electrolyte and to investigate the chemical and microstructural change of the different interfaces as a result of different conditions such as polarizations, atmosphere, temperature, impurity content etc. An operando localized probing approach was used to investigate model electrode systems by using a controlled atmosphere high temperature scanning probe microscope (CAHT-SPM) which was combined with electrochemical measurements at conditions close to the operating condition of SOCs. Scanning electrode microscopy (SEM) and time of flight secondary ion mass spectrometry (ToF-SIMS) were also used to investigate any surface microstructural and chemical changes. This work is divided into two parts, consisting of the investigation of the fuel electrode and the air electrode. The first part of this study was on the impact of strong cathodic polarization on Ni YSZ and Pt YSZ microelectrodes using a high temperature scanning probe microscope. Ni and PtIr sharp tips were used as working electrodes to perform electrochemical measurements at 650 ◦C in an atmosphere containing 3 % H2O ina 9 % H2 in N2 gas mixture. The influence of impurities was investigated by testing different samples which contained different amount of silica impurities. The impedance spectra under different DC cathodic polarizations revealed that electronic conductivity was introduced into the YSZ, in in both the Ni and PtIr cases, a decrease in the high frequency resistance was observed with increasing cathodic polarization. This was further supported by the appearance of high conductance regions close to the metalYSZ contact. The introduction of electronic conductivity also increased the active area where the water reduction reaction occurs. Additionally, the polarization influenced the distribution of impurities containing Si. With the purpose of investigating the microstructural changes of the metal YSZ interface, which was challenging dueto the small contact area between the tip and the YSZ sample, a similar study was performed using macro electrodes. The electrochemical measurements mainly consisted on cyclic voltammetry (CVs), chronoamperometry and a little of impedance spectroscopy. The second part consists of the investigation of the electrode surface reaction (La0.6Sr0.4)0.99FeO3−δ (LSF) and changes in its stochiometry. This study was performed on model electrodes which were fabricated by using pulsed laser deposition to deposit dense LSF films and photolithography to micro-pattern the film into microelectrodes with different diameters. The electrochemical measurements were performed in synthetic air at different temperatures starting from 500◦C up to 700◦C. The microstructure and surface chemistry were inspected after each step. The CV measurements indicate that at low temperature a change in the oxygen stochiometry occurs and as the temperature increases the oxygen evolution reaction becomes more dominant. These results were in agreement with the electrochemical impedance spectroscopy (EIS) measurements which indicate that the electrodes were completely blocking at low temperatures and the surfacereaction was a rate determining process.