Highly efficient, coking-resistant SOFCs for energy conversion using biogas fuels

Solid oxide fuel cells (SOFCs) afford an opportunity for the direct electrochemical conversion of biogas with high efficiency; however, direct utilisation of biogas in nickel-based SOFCs is a challenge as it is subject to carbon deposition. A biogas composition representative of a real operating system of 36% CH4, 36% CO₂, 20% H₂O, 4% H₂ and 4% CO used here was derived from an anode recirculation method. A BaZr_0.1Ce₀.₇Y_0.1Yb_0.1O_3−δ (BCZYYb) infiltrated Ni-YSZ anode was investigated for biogas conversion. The infiltration of BCZYYb significantly promoted the electrochemical reactions and the cells exhibited high power output at the operational temperatures of 850, 800 and 750 °C. At 800 °C, supplied with a 20 ml min⁻¹ biogas, the cell with a BCZYYb-Ni-YSZ anode, generated 1.69 A cm⁻² at 0.8 V with an optimal amount of 0.6 wt% BCZYYb, whereas only 0.65 A cm⁻² was produced with a non-infiltrated Ni-YSZ in the same conditions. At 750 °C, a maximum power density of 1.43 W cm⁻² was achieved on a cell with a BCZYYb-Ni-YSZ anode, a 3 μm dense YSZ film electrolyte, a Gd_0.1Ce_0.9O₂ (GDC) buffer layer and a La_0.6Sr_0.4Co_0.2Fe0.8O₃–Gd_0.1Ce_0.9O₂ (LSCF-GDC) composite cathode. The cell remained stable, while operating at 0.8 V for 50 hours with a current density of 1.25 A cm⁻². A well-designed cell structure and selected components made it possible to obtain excellent performance at good fuel utilisation. The analysis of gases in open-circuit conditions or under various current loads suggested that the prevalent reaction was reforming of methane without coking. This study demonstrates that the BCZYYb-Ni-YSZ is a promising electrode for carbon-containing fuel.