Development and Characterization of Perovskites as Alternative Anode Materials for Intermediate Temperature Solid Oxide Fuel Cells

Department of Energy EngineeringSolid-oxide fuel cells (SOFCs) have the potential to meet the critical energy needs of our modern civilization and minimize the adverse environmental impacts from excessive energy consumption. They are highly efficient, clean and can run on a variety of fuel gases, including hydrocarbons and gasified coal or different types of ample carbonaceous solids. However, the conventional anode for an SOFC, a composite consisting of nickel metal and yttria-stabilized-zirconia (YSZ), is highly susceptible to carbon deposition and deactivation (poisoning) by sulfur contaminants commonly encountered in readily available fuels even in parts per million (ppm) levels. There is accordingly strong demand for development of alternative anode materials with tolerance to coking and sulfur poisoning. Among the novel anode electrodes, perovskite based materials (ABO3) are of great interest because they have been shown stable performance as redox stable anodes both in hydrocarbon and sulfur containing fuels. With an aim to improve the stability of perovskite related oxides and maximize the electrochemical performance, this dissertation focuses on perovskite-related oxide anode materials. The first part of this thesis describes the work done towards the development of a new Sc doped La0.8Sr0.2ScxMn1-xO3-δ and Y0.08Sr0.92Ti1-xFexO3-δ anode by infiltration on porous YSZ back bone. The composite anode exhibits enhanced electrochemical performance comparable to that of the conventional Ni-YSZ anode. Second, oxygen non stoichiometry and electrical conductivity of Sc doped La0.8Sr0.2MnO3-δ was measured by Coulometric titration at controlled temperature and oxygen partial pressure. The main goal of the work is to determine the oxygen vacancy formation and to provide a better understanding of the structural changes in perovskite related with a reduction of oxygen partial pressure. In this respect, a suitable defect chemical model is also proposed and verified with the oxygen non-stoichiometry in both oxygen excess and oxygen deficient regions in order to predict the oxygen defect formation. Also the correlation between defect formation and thermodynamic properties such as partial molar enthalpy and partial molar entropy of oxygen vacancy formation reaction is elaborated. Third, surfer tolerance of the conventional Ni-YSZ anode has been investigated by simple surface modification process. A single step infiltration of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) infiltration on Ni-YSZ anodes significantly improves the sulfur tolerance anode. The much-improved power output and sulfur tolerance of the BZCYYb modified Ni-YSZ anode were attributed to the adsorbed water uptake property of BZCYYb at microscopic levels and facilitated water-mediated sulfur removal reactions. The final part of this thesis outlined the work done towards the development of a new layered double perovskite PrBaMn2O5+δ anode materials. PrBaMn2O5+δ anode shows, superior electrochemical performance in both hydrogen and hydrocarbons fuels, with the high electrical conductivity in anode operating conditions. Transmission electron microscopy (TEM) analysis suggest that the most attractive properties of this material are the phase transition of disordered Pr0.5Ba0.5MnO3-δ perovskite, to A-site ordered PrBaMn2O5+δ perovskite, under SOFC anode operating condition, showing [MnO2] square sublattice is sandwiched between two rock salt layers, [PrO] and [BaO] layers, along the c axis. Towards the end, these findings contribute to understanding the electrochemical performance of perovskite anode materials in relation to oxygen non-stoichiometry and commercial viability of SOFCs that are driven by cost-effective and renewable fuels.ope

A perovskite oxide with high conductivities in both air and reducing atmosphere for use as electrode for solid oxide fuel cells

Electrode materials which exhibit high conductivities in both oxidising and reducing atmospheres are in high demand for solid oxide fuel cells (SOFCs) and solid oxide electrolytic cells (SOECs). In this paper, we investigated Cu-doped SrFe0.9Nb0.1O3−δ finding that the primitive perovskite oxide SrFe0.8Cu0.1Nb0.1O3−δ (SFCN) exhibits a conductivity of 63 Scm−1and 60 Scm−1 at 415 °C in air and 5%H2/Ar respectively. It is believed that the high conductivity in 5%H2/Ar is related to the exsolved Fe (or FeCu alloy) on exposure to a reducing atmosphere. To the best of our knowledge, the conductivity of SrFe0.8Cu0.1Nb0.1O3−δ in a reducing atmosphere is the highest of all reported oxides which also exhibit a high conductivity in air. Fuel cell performance using SrFe0.8Cu0.1Nb0.1O3−δ as the anode, (Y2O3)0.08(ZrO2)0.92 as the electrolyte and La0.8Sr0.2FeO3−δ as the cathode achieved a power density of 423 mWcm−2 at 700 °C indicating that SFCN is a promising anode for SOFCs

Achieving both high selectivity and current density for CO2 reduction to formate on nanoporous tin foam electrocatalysts

Currently, low catalytic activity, selectivity and stability are the biggest challenges which restrict the large scale applications of CO2 electrochemical reduction. Formic acid, one of the highest value-added products from electrochemical reduction of CO2, has gathered much interest. Here, we develop nanoporous tin foam catalysts which exhibit significantly high selectivity and faster production rate to formate. In a 0.1 M NaHCO3 solution, the maximum Faradaic efficiency for formate production reaches above 90% with a current density over 23 mA cm-2 , which are among the highest reported value to date under ambient conditions. The improved production rate can be attributed to the high surface area and porous structure. Moreover, the electrocatalysts are quite stable, namely, the Faradaic efficiency remains unchanged during 16 hour electrolysis. This is a promising technology to convert CO2 into useful hydrocarbons

Scandium Doping Effect on a Layered Perovskite Cathode for Low-Temperature Solid Oxide Fuel Cells (LT-SOFCs)

Layered perovskite oxides are considered as promising cathode materials for the solid oxide fuel cell (SOFC) due to their high electronic/ionic conductivity and fast oxygen kinetics at low temperature. Many researchers have focused on further improving the electrochemical performance of the layered perovskite material by doping various metal ions into the B-site. Herein, we report that Sc3+ doping into the layered perovskite material, PrBaCo2O5+ (PBCO), shows a positive effect of increasing electrochemical performances. We confirmed that Sc3+ doping could provide a favorable crystalline structure of layered perovskite for oxygen ion transfer in the lattice with improved Gold-schmidt tolerance factor and specific free volume. Consequently, the Sc3+ doped PBCO exhibits a maximum power density of 0.73 W cm(-2) at 500 degrees C, 1.3 times higher than that of PBCO. These results indicate that Sc3+ doping could effectively improve the electrochemical properties of the layered perovskite material, PBCO

Review on exsolution and its driving forces in perovskites

Exsolution is a promising method to design metal nanoparticles for electrocatalysis and renewable energy. Metal nanoparticles exsolved from perovskite oxide lattices have been utilized as catalysts in many energy fields because of their high durability and excellent electro-catalytic properties. Although this method has received much attention in recent years, a comprehensive understanding is still lacking because of difficulties in finding a rational combination of driving forces and perovskite supports. Thus, the aim of our work here is to recapitulate the principles of exsolution and collect various exsolution studies by categorizing the driving forces of exsolution and the structural characteristics of perovskite supports. These classifications provide guidelines for selecting suitable materials groups and remodeling existing materials, thereby exploring applications of catalysts using exsolution that are applicable to academic and industrial fields

Electrochemical performance of YST infiltrated and fe doped YST infiltrated YSZ anodes for IT-SOFC

Donor doped and donor-acceptor co-doped strontium titanate perovskite are investigated for intermediate temperature solid oxide fuel cells (IT-SOFCs) anodes. Y0.08Sr0.88TiO3-delta and Y0.08Sr0.92Ti1-xFexO3-delta (x = 0.2, 0.4) anodes were prepared by infiltration in 65% porous yttria stabilized zirconia (YSZ) scaffolds. The microstructure and electrical conductivity of Y0.08Sr0.88TiO3-delta and Y0.08Sr0.92Ti1-xFexO3-delta strongly depends on Fe content. The conductivity of Y0.08Sr0.88TiO3-delta andY(0.08)Sr(0.92)Ti(1-x)Fe(x)O(3-delta); decreases with increasing Fe content in humidified H-2. Y0.08Sr0.88TiO3-delta, Y0.08Sr0.92Ti0.8Fe0.2O3-delta, and Y0.08Sr0.92Ti0.6Fe0.4O3-delta, anodes with a Pd/CeO2 catalyst show peak power density of 298, 421, and 321 mW cm(-2), respectively, in wet H-2 at 1073 K.open0

Advances in reforming and partial oxidation of hydrocarbons for hydrogen production and fuel cell applications

One of the most attractive routes for the production of hydrogen or syngas for use in fuel cell applications is the reforming and partial oxidation of hydrocarbons. The use of hydrocarbons in high temperature fuel cells is achieved through either external or internal reforming. Reforming and partial oxidation catalysis to convert hydrocarbons to hydrogen rich syngas plays an important role in fuel processing technology. The current research in the area of reforming and partial oxidation of methane, methanol and ethanol includes catalysts for reforming and oxidation, methods of catalyst synthesis, and the effective utilization of fuel for both external and internal reforming processes. In this paper the recent progress in these areas of research is reviewed along with the reforming of liquid hydrocarbons, from this an overview of the current best performing catalysts for the reforming and partial oxidizing of hydrocarbons for hydrogen production is summarized

Development of fuel electrode for solid oxide fuel cells through in situ exsolution of metal nanoparticles in layered perovskite

Solid oxide fuel cells (SOFCs) have attracted attention as a promising electrochemical energy conversion device due to their excellent energy conversion efficiency, low environmental pollution, and high fuel flexibility. Generally, Ni-based composite anode has been widely used as the SOFC anode material because Ni has high catalytic activity and electrical conductivity in H2 fuel. However, the Ni-based anode materials suffer from several drawbacks in hydrocarbon fuels, such as low tolerance to redox cycling, sulfur poisoning, and carbon coking. In this regard, extensive efforts have been devoted to develop Ni-free ceramic materials and surface modification technologies. Among the surface modification methods, an exsolution has been received attention because of its special properties. The catalytically active transition metals are incorporated on the B-site of perovskite oxide (ABO3) during synthesis in air, and then the metals are exsolved from the perovskite oxide in reducing atmosphere. The exsolution evenly makes electro-catalytic nanoparticles on the electrode surface, which can suppress carbon coking and increase redox stability. Here, we selected PrBaMn2O5+ (PBMO) as the parent layered perovskite oxide, and doped its B-site with various transition metals. The exsolution of the transition metals is confirmed by density functional theory (DFT) calculations and a transmission electron microscopy (TEM) analysis