Preparation and characterization of Ni/ScSZ as composite electrode material for solid oxide fuel cell and electrolysis cell applications
Solid oxide fuel cell (SOFC) and electrolysis cell (SOEC) are recognized as key enabling technologies as we progress towards an economy that is fully dependent on clean renewable energy production and storage. An electrolysis cell stores energy, and a fuel cell converts chemical energy into electric...
|Solid oxide fuel cell (SOFC) and electrolysis cell (SOEC) are recognized as key enabling technologies as we progress towards an economy that is fully dependent on clean renewable energy production and storage. An electrolysis cell stores energy, and a fuel cell converts chemical energy into electrical energy. The composite of Ni and yttria stabilized zirconia, known as Ni/YSZ cermet, is still considered as the state-of-the-art fuel electrode for solid oxide electrochemical cells (SOC). Nevertheless, widespread application of SOCs has been hampered partly due to the unsolved drawbacks of the Ni/YSZ cermet electrode including carbon deposition, sulfur poisoning, and redox instability. Hence, there is considerable interest in the investigation of alternative electrodes. Less attention has been given to scandia-doped zirconia (ScSZ) as the ceramic phase of the Ni-based cermet even if it was established that ScSZ has the highest ionic conductivity among the zirconia solid solutions. Furthermore, Ni-based cermets have been typically prepared using the solid-state reaction method. In this work, composite powder of 50 wt. percent Nio and 50 wt. percent Zr0.84SC0.1601.92 was prepared via a single-step glycine nitrate combustion process (GNP). The as-combusted NiO/ScSZ powders were calcined at 1000°C for 6 h, compacted using uniaxial press, and then sintered at 1300°C for 6 h in air. Sintered NiO/ScSZ were subsequently reduced under an Ar/5 % H2 gas flow to form the porous Ni/ScSZ electrode. The effect of the initial glycine-to-nitrate (g/n) molar ratios on the structural, morphological, and electrical properties were evaluated. Three different g/n ratios were investigated: fuel lean (g/n = 0.27), stoichiometric (g/n = 0.54), and fuel rich (g/n = 1.1). Even before any heat treatment, X-ray diffraction (XRD) of the as combusted products revealed the successful synthesis of cubic phases of Nio, and SSZ via GNP. XRD patterns of the as-reduced samples showed complete reduction of NiO to Ni after 2 h of exposure to Ar/5 percent H2 gas flow at 700°C. Scanning electron microscopy (SEM) revealed that highly porous microstructures were achieved even without using pore formers. Moreover, g/n ratio remarkably influenced the sinterability of NiO/ScSZ with the fuel lean condition resulting to the densest microstructure. Upon reduction to Ni/ScSZ, SEM revealed a further increase in porosity due to the removal of oxygen from Nio. Energy-dispersive X-ray spectroscopy (EDS) revealed homogeneous doping of Sc in the ZrO2 lattice as well as the good dispersion of Ni and ScSZ phases. Conductivity of unreduced NiO/ScSZ and reduced Ni/ScSZ were determined using electrochemical impedance spectroscopy (EIS). Conductivity of Nio/ScSZ was successfully improved by an order of magnitude just by simply changing the amount of glycine by a few millimoles. From a conductivity of 3.2 mS/cm for the g/n ratio of 1.1, the conductivity jumped to 12 mS/cm for the sample prepared with g/n ratio of 0.27 when measured at 700°C under O2 gas flow with activation energy (Ea) of 0.57 eV indicating mixed ionic and electronic conduction. Upon reduction to Ni/ScSZ, conductivity further increased to 76.6 mS/cm for g/n ratio of 0.27 when measured at 700°C under Ar/5 percent H2 gas flow with an Ex of 0.02 eV indicating dominance of electronic conduct
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