ChemInform Abstract: Advances in Cathode Materials for Solid Oxide Fuel Cells: Complex Oxides Without Alkaline Earth Metal Elements

This project was conducted under the Co-operative Agreement No. DE-FE0027584 with the US Department Energy to developed advanced Solid Oxide Fuel Cell (SOFC) Technologies. The overall objective of this project was to advance SOFC technology at the cell and stack level to enhance cell robustness and durability, increase performance, and reduce balance-of-plant (BOP) requirements. By reducing system complexity combined with the increases in power density and efficiency, the ultimate goal of the project was to increased reliability and to reduce capital and operating costs of installed systems. The project was focused on pathways that will reduce the cost of the SOFC cell and stack, including the following areas: Robust, redox tolerant cell technology Lower cost cell manufacturing through advances in cell design, which will reduce the amount of material, energy and time used in the fabrication of SOFCs High performance, low temperature electrolyte based on improvement of established materials Innovative SOFC stack architecture which truly integrates Balance of Plant functionality into the stack level design Thermal management of the fuel cell stack for increased durability and expanded window of operation Novel stack design amenable for use in sub-MW to multi-MW-scale power plants and having low replacement cost The incorporation of balance-of-plant (BOP) equipment into the stack platform increased the economic viability of smaller scale systems. The project objectives were met by a multi-prong approach, including new cell design complemented with modifications to existing cell technology, as well as a new stack design incorporating components typically included in the BOP, such as heat exchangers, oxidizer, fuel reformers, and recycle systems. The project culminated with demonstration of a stack test validating the viability of the cell and stack improvements. A cost model was also developed to estimate costs for the advanced stack technology at high volume manufacturing levels. The net outcome of the project is SOFC cell and stack technology with costs significantly below current DOE targets without compromising and, in some cases, improving on the performance and degradation rate demonstrated with the current state-of-the-art stack design. The results of this project advanced the reliability, robustness, and endurance of low-cost SOFC technology that ultimately are ready to be deployed in coal power systems with greater than 60 percent efficiency (based on higher heating value of fuel) and the capability for ≥97% CO2 capture at a cost-of-electricity that is approximately 40 percent below presently available Integrated Gasification Combined Cycle systems.

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.

Solid Oxide Fuel Cell (SOFC) cathode materials rely on the catalytic processes at their surfaces to reduce and incorporate oxygen. For many common cathode materials surface passivation occurs through 'A site' segregation at common operating temperatures [1]. These processes have a negative effect upon the oxygen reduction rates and therefore overall cell performance. Quantifying the true extent these segregation effects have on the exchange properties has not been fully explored but it is clear that the properties of the surface are essential. Many techniques have been employed to study cell properties in-situ, however, surface studies are typically restricted to post mortem or ex-situ analysis due to the necessity of a high vacuum. In this study High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) has been employed to analyse the morphology changes of common SOFC cathode materials in-situ. Samples of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and La0.6Sr0.4CoO3-x (LSC) were annealed from room temperature to 1000oC at pressure ranges between 3 – 6mbar in atmospheres of Oxygen and Water. Using secondary electron imaging during the thermal annealing cycle morphological and compositional changes were able to be documented at temperature for the first time (Figure 1). This technique has enabled detailed analysis of the surface degradation that occurs during SOFC fuel cell operation. Secondary phase precipitation rate and formation behaviour has been quantified over a range of temperatures and time scales furthering the understanding of SOFC cathode surfaces significantly. [1] M. Kubicek, A. Limbeck, T. Fromling, H. Hutter, and J. Fleig, "Relationship between cation segregation and the electrochemical oxygen reduction kinetics of La0.6Sr0.4Co0.2Fe0.8O3 thin film electrodes" Journal of The Electrochemical Society, vol. 158, no. 6, pp. B727–B734, 2011. Figure 1 - Showing in-situ growth of secondary phase particulates on a polished surface of LSCF Figure 1

Solid oxide fuel cell (SOFC) technology requires further maturations to become commercialized broadly. Many existing problems are expected to be solved by intermediate temperature SOFC (IT-SOFC). On the other hand, electrochemical resistances of cathode materials increase rapidly at lower temperature. So it is necessary to understand the underlying mechanisms of oxygen reduction reaction and develop new materials. Although many experimental and theoretical researches for this purpose are in progress, traditional trial-and-error materials search must be inefficient and take too long time. Considering the above situation, we explore critical factors that can explain the oxygen exchange rate of perovskite oxide cathode in SOFCs. It has been said that the bulk ionic conductivity is strongly related to the oxygen exchange rate, while the electronic conductivity is not as far as it is high enough. However, we found that the combination of ionic and electronic conductivities shows much stronger correlation than ionic conductivity alone while analyzing reported experimental data on 18 materials in figure 1. Materials which have high oxygen exchange rate can be explained by the multiplication of bulk ionic conductivity and electronic conductivity while others explained by the ionic conductivity. Fig. 1 Relationship between oxygen exchange rate and bulk electronic conductivity, ionic conductivity, and the model function with both conductivities. Figure 1

A dynamic infiltration technique to synthesize nanolayered cathodes for high performance and robust solid oxide fuel cells

The high-temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen-based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C<T<800 °C) in SOFCs in order to extend their life span, thermal compatibility, cost-effectiveness, and ease of fabrication. However, the major challenges in developing cathode materials for low/intermediate temperature SOFCs include structural stability, catalytic activity for oxygen adsorption and reduction, and tolerance against contaminants such as chromium, boron, and sulfur. This research aims to provide an updated review of the perovskite-based state-of-the-art cathode materials LaSrMnO3 (LSM) and LaSrCOFeO3 (LSCF), as well as the recent trending Ruddlesden-Popper phase (RP) and double perovskite-structured materials SOFCs technology. Our review highlights various strategies such as surface modification, codoping, infiltration/impregnation, and composites with fluorite phases to address the challenges related to LSM/LSCF-based electrode materials and improve their electrocatalytic activity. Moreover, this study also offers insight into the electrochemical performance of the double perovskite oxides and Ruddlesden-Popper phase materials as cathodes for SOFCs.

SrCo0.4Fe0.4Zr0.1Y0.1O3-δ, A new CO2 tolerant cathode for proton-conducting solid oxide fuel cells

Overcoming phase instability of RBaCo2O5+δ (R=Y and Ho) by Sr substitution for application as cathodes in solid oxide fuel cells

Exergoeconomic evaluation of an ethanol-fueled solid oxide fuel cell power plant

Nanomaterials with oxygen mobility for catalysts of biofuels transformation into syngas, SOFC and oxygen/hydrogen separation membranes: Design and performance

Enhancing the oxygen reduction reaction activity of SOFC cathode via construct a cubic fluorite/perovskite heterostructure

The reaction pathways on Pt for the oxygen reduction reaction (ORR) in acid and alkaline solution are compared with those on metallic transition‐metal oxides. These reactions are distinguished from those that occur at high temperature at an oxide/air interface of an oxygen‐permeation membrane or the cathode of a solid oxide fuel cell (SOFC). Distinctions are made between polaronic, vibronic, and metallic electronic conductivity in oxides, and the influences of counter cation on the conductivity are pointed out for oxides like the AMO 3 perovskites and A 2 M 2 O 6 O′ pyrochlores, where M is the transition‐metal atom responsible for electron transport. Mixed oxide‐ion and electronic conductors (MIECs) are used as permeation membranes or as cathodes of a SOFC. The criteria for oxide‐ion conductivity in an oxide‐ion electrolyte are reviewed; the same criteria apply for MIECs. The ORR takes place at the cathode of an electrochemical cell, and gas‐diffusion cathodes operating with aqueous electrolytes contain as catalyst Pt or an oxide dispersed on teflon‐bonded, stabilized carbon. Dispersed Pt is compared with Pb 2 [Ir 2− x Pb x ]O 6 O′ 1− y pyrochlore for an acidic electrolyte. Perovskites are used in alkaline solution, and a bifunctional ORR and oxygen evolution reaction (OER) is used in secondary metal/air batteries. Performance curves for perovskites and pyrochlores are compared. Finally, the criteria for choosing a MIEC as a SOFC cathode or an oxygen‐permeation membrane are given and the performances of several of them are compared.

&lt;div class="htmlview paragraph"&gt;Delphi Automotive Systems and BMW are jointly developing Solid Oxide Fuel Cell (SOFC) technology for application in the transportation industry primarily as an on-board Auxiliary Power Unit (APU). In the first application of this joint program, the APU will be used to power an electric air conditioning system without the need for operating the vehicle engine.&lt;/div&gt; &lt;div class="htmlview paragraph"&gt;The SOFC based APU technology has the potential to provide a paradigm shift in the supply of electric power for passenger cars. Furthermore, by supplementing the conventional fuel with reformate in the internal combustion engine, extremely low emissions and high system efficiencies are possible. This is consistent with the increasing power demands in automobiles in the new era of more comfort and safety along with environmental friendliness.&lt;/div&gt; &lt;div class="htmlview paragraph"&gt;Delphi Automotive Systems and BMW were successful in demonstrating an Auxiliary Power Unit (APU) based on Solid Oxide Fuel Cell (SOFC) technology in February, 2001. A SOFC APU generates power using hydrogen and carbon monoxide reformed from fuels such as gasoline, diesel, or natural gas. The proof-of-concept unit and the advantages of using a SOFC APU will be activities in the development of a second generation APU. This development has been targeted towards resolving the fundamental issues with the following key subsystems: fuel cell stack, fuel reformers, and energy and thermal management. Major focus has also been directed at system integration challenges to make a more robust and efficient product.&lt;/div&gt;

Thermodynamic model and parametric analysis of a tubular SOFC module

Doubling the diffusivity measurement efficiency in solid oxide fuel cells (SOFCs) via a bi-sensor electrochemical cell