La1.5Sr1.5Mn1.5Ni0.5O7 ±δ Vs La1.5Sr1.5Co1.5Ni0.5O7 ±δ: Exsolved Materials as Anodic Layers for Direct Biogas-Fueled SOFC

Abstract

Currently, one of the main problems in commercial SOFC systems fueled directly with hydrocarbon compounds, such as biogas, is the high risk of blocking the active phase on the anode side ( e.g. Ni-YSZ ) due to carbon formation/deposition during the conversion of fuel into power energy. This issue causes a lowering in the overall performance and durability of the cell. Therefore, to overcome this deactivation mechanism, it was successfully demonstrated that an additional anodic active layer in commercial SOFC using materials based on exsoluted perovskites could be an interesting and viable alternative. This asseveration is based on the fact that using this kind of material is possible to get heterogeneous surface systems with highly stable and electrocatalytically active embedded nanoparticles uniformly distributed on the surface with a high carbon coking tolerance in a hydrocarbon fuel atmosphere. This study aimed to evaluate and compare the electrocatalytic behaviour of two new catalytic materials with Ni exsolution for direct dry biogas-fueled SOFC. The starting materials were Ruddlesden-Popper-type based on a nickel manganite (La 1.5 Sr 1.5 Mn 1.5 Ni 0.5 O 7±δ or LSMN ) and nickel cobaltite (La 1.5 Sr 1.5 Co 1.5 Ni 0.5 O 7±δ or LSCN ). Both materials have been synthesized by the Pechini method using stoichiometric amounts of precursors as nitrates. Once the respective gels have been formed, they were treated in the air at two dwell temperatures, 300°C for 2 h and 500°C for 3 h, to ensure the total elimination of the organic compounds. Finally, the resulting powders were treated in air at 1300°C for 12h and then, physicochemically characterized. For the electrochemical characterization, the as-treated powders were mixed individually with Gd 0.1 Ce 0.9 O 2 ( CGO ) in a weight ratio of 70:30 using a ball milling for 6h. Finally, to get the slurry for the coating layer, each mixture ( LSMN+CGO and LSCN+CGO ) was ground for an additional 2h in the presence of 8 wt % of triethanolamine, 2 wt % polyvinyl butyral resin (BUTVAR B-98) and 2-propanol. Commercial button SOFC cells by InDEC® (anode-supported cell Ni-YSZ/YSZ/LSM) were painted on the anode side getting an active area of 2 cm 2 . The experiments were carried out at 800°C with pre-conditioning using diluted H 2 and then, with simulated dry biogas. A Biologic tool was used as a device for the electrochemical measurements. The purpose of this communication is to present the results of experiments with two button cells derived from the same large area commercial cell (anode supporting cell) coated with the two electrocatalysts developed in this work. The electrochemical test carried out for more than 200 h demonstrated that this external functional layer on the anode side contributes to getting a stable potential within the whole working time at the selected galvanostatic conditions (500 mA cm −2 ). By comparing the results of these two tests, the exsolved LSCN layer showed better perfomances, because of the the B-site doped with Co instead of Mn which is responsible for an enhanced O 2- transport within the crystal structures. Finally, post-reaction characterization revealed minimum/null carbonous species in both cases which suggests that the classical risks caused by the direct use of hydrocarbon fuels in commercial SOFC can be suppressed with this kind of approach which would lead us to think about the use of this type of materials in a next level or further electrochemical applications. Acknowledgements The authors acknowledge the project entitled "Direct utilisation of bio-fuels in solid oxide fuel cells for sustainable and decentralised electric power production and heat (DIRECTBIOPOWER)" Grant Agreement number: 2017FCFYHK. The authors acknowledge the Ministry of Ecological Transition of Italy (MiTE) for funding this research through the AdP "Piano Operativo di Ricerca (POR) sull'idrogeno verde Figure 1