Electrical and electrochemical properties of architectured electrodes based on perovskite and A₂MO₄-type oxides for Protonic Ceramic Fuel Cell

Two mixed ionic-electronic conducting oxides (MIEC) have been investigated as potential cathode materials for protonic ceramic fuel cell (PCFC): the perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and the Ruddlesden Popper Pr₂NiO_4+δ (PrN). Their electrical properties have been studied over a large range of water vapour partial pressure. All compounds exhibit high electronic conductivities (σ ≥ 40 S.cm⁻¹ at 600 °C) whatever the pH₂O of the surrounding atmosphere. Electrochemical characterizations have been performed as a function of pH₂O, under zero dc conditions and under dc polarization using symmetrical cells based on BaCe_0.9Y_0.1O_3-δ (BCY10) as electrolyte. For this purpose, two electrode architectures have been elaborated: a single phase electrode and a composite cathode/BCY10 architectured electrode. All electrodes showed pH₂O-dependence with promising polarization resistance values lower than 0.8 Ω cm² at 600 °C under air whatever the gas humidification rate. The use of architectured electrodes led to a significant decrease of the polarization resistance with values as low as 0.23 and 0.19 Ω cm² for PrN and BSCF respectively, at 600 °C and pH₂O = 0.20 bar. Concerning the oxygen reduction reaction (ORR) mechanisms, rate determining steps involving protons have been identified. They have been respectively assigned to the proton interface transfer and to the water formation and/or desorption for single phase and architectured electrodes. This change has been attributed to an extent of the electrochemically active area and to an enhancement of the protonic transport properties in the architectured electrodes. However electrodes performances seem to be governed by the dissociative adsorption of oxygen species and/or the charge transfer. Concerning performances under dc current, cathodic polarization is reduced when architectured electrodes are used. An enhancement of the electrodes performances has been also evidenced with water content increasing which corroborates the protonic conduction process into the cathode materials