# GDC-Based Infiltrated Electrodes for Solid Oxide Electrolyzer Cells (SOECs)
## 2 Materials and Methods  
8 mol% Y\({}_{2}\)O\({}_{3}\)-doped ZrO\({}_{2}\) (8 YSZ) powders purchased from Tosoh were used as an electrolyte. The powders were uniaxially pressed into pellets and sintered at 1500 \({}^{\circ}\)C for 3 h using a heating rate of 2 \({}^{\circ}\)C/min. The density of the sintered pellets, measured with a hydrostatic balance, was 5.86 g/cm\({}^{3}\) (about 97% of the theoretical value). Thick electrodes were screen-printed onto both sides of the pellets. Scandium-doped Zirconia (ScSZ, Pi-Kem Ltd.--Tamworth, UK), YSZ (Tosoh--Tokyo, Japan), and Lanthanum Strontium Manganite (LSM, Pi-Kem Ltd.--Tamworth, UK) were employed for anode preparation and Lanthanum Strontium Chromium Manganite (LSCM, Pi-Kem Ltd.--Tamworth, UK) for the cathode. Inks for deposition were prepared by ball milling each powder with a dispersant (Polyester/Polyamide Copolymer Dispersant, KD-1) and organic binders (Terpineol and Decoflux WB41) [32]. Glassy carbon and graphite were used as a pore former. A total of 30 wt% of pore former was necessary to get sufficient porosity for infiltration. Solutions (2 M) of 10% mol Gd-doped CeO\({}_{2}\) (GDC10) were prepared for the cathode impregnation. Proper amounts of Ce(NO\({}_{3}\))\({}_{3}\):6H\({}_{2}\)O (Sigma-Aldrich--Steinheim, Germany) and Gd(NO\({}_{3}\))\({}_{3}\):6H\({}_{2}\)O (Sigma Aldrich--Steinheim, Germany), i.e., 19,537 and 2256 g for cerium nitrate and gadolinium nitrate, respectively, were dissolved into 25 mL of either distilled water (GDCW solution) or into pure ethanol (GDCE solution). Thus, the two solutions used for infiltration are named: GDCW and GDCE.  
For the cathode fabrication, a thin first layer of YSZ was screen-printed onto the YSZ electrolyte pellet to favor a good adhesion with the overlaying LSCM layer. The deposited layers were soaked at 800 \({}^{\circ}\)C for 1 h to remove any undesired carbon residues and then co-fired at 1300 \({}^{\circ}\)C for 2 h. For the anode fabrication, a ScSZ layer was firstly screen-printed and then an LSM layer was successively added. Each layer was soaked at 800 \({}^{\circ}\)C for 1 h and the bilayer was co-fired at 1100 \({}^{\circ}\)C for 2 h using a heating rate of 2 \({}^{\circ}\)C/min. The thickness of both bilayer electrodes was about 30-40 \(\upmu\)m.  
The LSCM/YSZ backbone was infiltrated using two GDC precursor solutions. The infiltration steps were carried out at room temperature and at atmospheric pressure. After each infiltration step, a thermal treatment at 500 \({}^{\circ}\)C for 45 min was performed to let the nitrates decompose. After each firing, the cell was weighted in order to get the final GDC loading equal to 50% of the YSZ/LSCM bilayer weight. A final calcination step of 1 h at 1100 \({}^{\circ}\)C was performed.  
Structural and microstructural characterizations of infiltrated cathodes were performed. X-Ray Diffraction (XRD) analysis was carried out with a X-Pert Pro 500 diffractometer (Philips--Amsterdam, Nederlands) to evaluate the presence of the undesirable crystalline phases. Field emission-scanning electron microscopy (FE-SEM) using a Leo Supra 35 (Zeiss--Oberkochen, Germany) was used to investigate the grain size and distribution of the GDC particles infiltrated into the LSCM/YSZ cathode.  
For electrochemical measurements, the cells were fastened onto an alumina tube using a gas-tight ceramic paste (curing treatment of 150 \({}^{\circ}\)C for 3 h). Au paste and wires were used as current collectors. Electrochemical impedance spectroscopy (EIS) measurements at open circuit voltage (OCV) were collected using a frequency response analyzer (FRA, Solartron 1260--Solartron Analytical--Leicester, UK) coupled with a dielectric interface (Solartron 1296--Solartron Analytical--Leicester, UK) with a frequency range of between 0.1 Hz and 1 MHz and an AC voltage amplitude of 100 mV. The measurements were carried out in the 700-800 \({}^{\circ}\)C temperature range, exposing the cathode to a 30/70 volume ratio of CO/CO\({}_{2}\) and the anode to static air.