The development of fuel electrodes for high temperature solid oxide cells

Our energetic matrix is currently based on finite fossil fuels, leading to climate change and increasing hazardous air pollutants. Nevertheless, solid oxide cells have emerged as a feasible and profitable route for energy generation. Solid oxide electrolysis cells can convert the excess electrical energy into chemical energy, thereby decoupling the transport fuels and chemicals production from today's fossil fuels, while solid oxide fuel cells can convert chemical energy back into electricity, thus balancing energy availability and demand. Solid oxide electrolysis cells afford an opportunity for upgrading biogas through the internal dry reforming of biogas and carbon dioxide electrolysis, producing hydrogen and carbon monoxide. Solid oxide electrolysis cells with conventional Ni-YSZ cermet fuel electrode and yttria stabilized zirconia electrolyte were constructed and tested on the direct feed of simulated biogas mixture (i.e. CH₄/CO₂ = 60/40, 50/50 and 40/60) at 850 °C. Cell performance and outlet gases measurements were carried out under open-circuit and closed-circuit conditions. The current densities at 1.8 V are -0.448, -0.678 and -0.876 A cm⁻² for the gas mixtures of CH₄/CO₂= 60/40, 50/50 and 40/60, respectively. The short term durability tests were performed in these three gas mixtures at 850 °C and 1.4 V. The cell fed with high CO₂ content demonstrates stable performance. No carbon deposition was observed on the Ni-YSZ fuel electrode surface, which might be due to not reaching the thermodynamic equilibrium and the reverse Boudouard reaction. Nonstoichiometric perovskites with active metal nanoparticles exsolved on the surface have been proposed as the promising fuel electrode in solid oxide cells. Here, La₀.₄₀Ca₀.₄₀TiO₃ and La₀.₄₃Ca₀.₃₇MχTi₁-χO₃-γ (M = Ni₀.₀₅, Ni₀.₁₀, Mn₀.₁₀, Co₀.₁₀, Ni₀.₀₅Mn₀.₀₅, and Ni₀.₀₅Co₀.₀₅) perovskite oxides were synthesized. The in-situ exsolution of Ni, Co and NiCo metal nanoparticles from the perovskite oxide parents was successfully according to the X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The results demonstrate that the exsolved metal nanoparticles can enhance the electrical conductivity, catalytic activity toward the hydrogen oxidation end carbon dioxide reduction. The cell performance can be improved by employing high voltage electrochemical reduction and extending the electrochemical reduction time. The best cell performance in 3% H₂O/H₂ was achieved by La₀.₄₃Ca₀.₃₇Ni₀.₁₀Ti₀.₉₀O₃-γ, exhibiting the maximum power density of 1.50 W cm⁻² at 900 °C. La₀.₄₃Ca₀.₃₇Co₀.₁₀Ti₀.₉O₃-γ based solid oxide electrolysis cell displays the highest current density of 0.856 A cm⁻² at 1.4 V.