Vicente B. Vert

Optimization of Oxygen Activation Fuel‐Cell Electrocatalysts by Combinatorial Designs

Solid oxide fuel cells (SOFC) are expected to contribute to low emission and highly efficient energy generation through the direct conversion of fossil and renewable fuels into electricity and highquality heat. Moreover, these solid state devices allow combining high power densities, fuel flexibility and long lifetimes while not demanding the use of expensive and sensitive precious metals as electrode catalysts. A major challenge in the development of SOFC devices is to achieve sufficient power density at intermediate temperature in the window 500-600oC, which will allow broadening the scope of application from stationary power generation to auxiliary power units in vehicles. The operation at low temperatures entails an important reduction in the electrolyte ionic conductivity 1 ] and a severe drop in the electrocatalytic activity of the electrodes, especially for the kinetics of oxygen reduction in the cathode. The PVD manufacture of supported thin-electrolytes in multilayer assemblies combining different fast ionic conductors as YSZ-CGO has enabled the minimisation of electrolyte ohmic losses whereas the improvement of electrode performances for oxygen activation and fuel oxidation at low temperature requires further experimental efforts. Cathode electrocatalysts are multifunctional materials generally made of multimetallic crystalline mixed oxides with balanced properties, i.e., solid state oxygen-ion and electronic conduction, adjusted macro and mesoporosity, catalytic activity for oxygen reduction and oxygen-ion incorporation into the oxide bulk. Perovskite oxides based on manganese, iron and cobalt have shown adequate electrochemical performance when used in classical SOFC configurations. The perovskite structure offers an enormous potential for the concurrent fine-tuning of the different electrode properties since this structure can accommodate in the lattice a very large variety of metals in different oxidation states while having a “flexible” anion sublattice. Consequently, this ideal system allows packing together all base metals and promoters with very different properties and functions in the crystal bulk and surface. In complex systems as solid catalysts made of several active metal oxides, it is usual that the discovery of synergetic effects in highly active materials is accompanied by certain degree of serendipity. A common practice to identify and understand cooperative non-linear interactions among catalyst components is the use of complex experimental designs supported by highthroughput methods and statistics / data mining computational techniques. Here, with the purpose of minimising the cathode polarisation, a screening of the simple-perovskite system A0.58Sr0.4Fe0.8Co0.2O3-δ was carried out by applying a quaternary mixture design (Figure 1a), in where up to four different elements where combined simultaneously in the A-site of the perovskite cation sublattice. This system was chosen due to the high performance of La0.58Sr0.4Fe0.8Co0.2O3-δ electrode at high temperature and the rather low cobalt content, warranting higher thermochemical stability while manufacturing and longer operation endurance. The four elements included in the screening were La, Pr, Sm and Ba and were selected taking into account the outcome of a previous screening of several lanthanides and Ba as single A-substituent in the A0.68Sr0.3Fe0.8Co0.2O3-δ compounds. When tested as cathodes on anode-supported fuel cells in the range 650-900oC, La and Pr-based compounds

Mixed Proton-Electron Conducting Chromite Electrocatalysts as Anode Materials for LWO-Based Solid Oxide Fuel Cells

Funding from the Spanish Government (ENE2011-24761 grant) and the European Union (FP7 Project EFFIPRO, Grant Agreement 227560) is acknowledged. The authors are indebted to S. Jimenez and M. Fabuel for sample preparation.

Compositional Improvement of Ln0.435Ba0.145Sr0.4Fe0.8Co0.2O3 − δ IT-SOFC Cathodes Performance by Multiple Lanthanide Substitution

A series of materials based on the perovskite Ln 0.435 Ba 0.145 Sr 0.4 Fe 0.8 C 0.2 O 3―δ system (Ln = La 1―x―y Pr x Sm y ) has been characterized as intermediate temperature solid oxide fuel cell (IT-SOFC) cathodes. Among the different La, Pr, and Sm combinations, those containing at a time Sm and La or alternatively Pr and La show the lowest polarization resistance values. Within the same substitution degree, praseodymium-based compositions have lower electrode resistance than samarium-based ones. Promising electrode compositions based on different ratios of Pr and La were tested in fully assembled fuel cells, comprising a Ni-yttria-stabilized zirconia (YSZ) anode and a YSZ/gadolinium-doped ceria bilayered electrolyte, and they were compared to a cell with a Pr 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3―δ cathode. A maximum power density of 0.45 W cm ―2 was achieved at 650°C for the most promising La 0.2175 Pr 0.2175 Bao 0.145 Sr 0.4 Fe 0.8 Co 0.2 O 3―δ cathode-based fully assembled fuel cell. A systematic electrochemical study based on electrochemical impedance spectroscopy is shown for the fully assembled cells as a function of pH 2 , pO 2 , applied current, and operating temperature.