The Effects of Sr Content on the Performance of Nd1-xSrxCoO3-δ Air-Electrode Materials for Intermediate Temperature Solid Oxide Fuel Cells under Operational Conditions

The potential of the perovskite system Nd_1-xSr_xCoO_3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm^-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd_1/3Sr_2/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ present lower ASR values (0.10 Ω·cm² at 973 K in air) than composites prepared with Nd_2/3Sr_1/3CoO_3-δ and Ce_0.8Gd_0.2O_2-δ (0.34 Ω·cm²). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

Remarkable Effects of an Electrodeposited Copper Skin on the Strength and the Electrical and Thermal Conductivities of Reduced Graphene Oxide-Printed Scaffolds

Architected Cu/reduced graphene oxide (rGO) heterostructures are achieved by electrodepositing copper on filament-printed rGO scaffolds. The Cu coating perfectly contours the printed rGO structure, but isolated Cu particles also permeate inside the filaments. Although the Cu deposition conveys a certain mass augment, the three-dimensional (3D) structures remain reasonably light (bulk density ≅ 0.42 g·cm^-3). The electrical conductivity (σ_e) of the Cu/rGO structure (∼8 × 10⁴ S·m^-1) shows a notable increment compared to σ_e of the rGO structure (∼2 × 10² S·m^-1). The effect on the scaffold robustness is also notable with an increase of the compressive strength by nearly 10 times (from 20 kPa of the rGO scaffold to 150 kPa of the Cu/rGO structure) and cyclability as well. The improved thermal conductivity of the Cu-coated scaffolds (∼4 times higher), in addition to the σ_e and strength improvements, suggests that 3D Cu/rGO structures could be suitable assemblies for integration into thermal dissipation systems, particularly as thermal interface materials, for compact electronic devices.

Enhanced Intermediate-Temperature Electrochemical Performance of Air Electrodes for Solid Oxide Cells with Spray-Pyrolyzed Active Layers

The potential of interactive layers of mixed-conducting oxides for improving the performance of air electrodes of solid oxide cells in the intermediate-temperature range is demonstrated. Active layers of Ce_0.9Gd_0.1O_2-δ (CGO), Ce_0.8Pr_0.2O_2-δ (CPO), and SrFe_0.9Mo_0.1O_3-δ (SFM) with thickness in the range 200-400 nm are deposited on CGO-based electrolyte by spray pyrolysis, followed by deposition of a SFM/CGO composite air electrode by painting. The morphologies and phase composition of the active layers are examined by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy microanalysis. The electrochemical performance of the electrolyte-electrode assemblies is determined by impedance spectroscopy in the range 600-800 °C. Significant improvements in the performance of the electrode process and the geometrically normalized ohmic conductance are observed for the assembly with a CPO active layer with mixed-oxide-ionic-electronic conductivity, especially in the low-temperature range, attributable to extension of the surface path of the electrochemical reactions. The CGO intermediate layer also improves performance but to a lesser degree, most likely due to better ionic-current collection in comparison to the assemblies with either SFM as the active layer or no active layer.

Structures, Phase Fields, and Mixed Protonic-Electronic Conductivity of Ba-Deficient, Pr-Substituted BaZr0.7Ce0.2Y0.1O3-δ

The BaZr_0.7Ce_0.2Y_0.1O_3-δ-BaPrO_3-δ perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(Ce_0.2Zr_0.7)_{1- x}Pr _xY_0.1O_3-δ, 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3̅ c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of ∼0.5 S cm^-1 for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N₂) and at low Pr contents, conductivity is higher in humidified atmospheres (∼0.023 atm pH₂O) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N₂ due to the creation of protonic defects and the consumption of holes.

Synthesis of a 12R-type hexagonal perovskite solid solution Sr3NdNb3−xTixO12−δ and the influence of acceptor doping on electrical properties

The solid solution Sr₃NdNb_3−xTi_xO_12−δ (0 ≤ x ≤ 0.06) crystallises with a 12R-type hexagonal perovskite structure and exhibits increasing proton conductivity with x.