Metal Composite Nano-Catalyst Enhanced Solid Oxide Fuel Cell Anodes for Improved Performance and Stability with Hydrocarbon Containing Fuels

Abstract

Implementation of nano-catalyst materials into solid oxide fuel cell (SOFC) electrodes to improve performance and stability has been widely studied. Addition of the nano-catalysts into an electrode structure serves to enhance the electrochemical performance of the SOFC by increasing the Triple Phase Boundary (TPB) area, improving redox stabilization, and modifying reaction kinetics of hydrocarbon gasses that cause anode degradation due to carbon deposition.SOFCs operating upstream of a reformer need to exhibit good tolerance to any hydrocarbon components that may make their way to the stack. Typical Ni-based cermet anodes suffer from anode deactivation due to carbon build up under hydrocarbon flows. Carbon builds up and covers the TPB area which results in poor electrochemical performance. Larger carbon deposits can block pores within the anode microstructure which leads to gas diffusion issues. Carbon buildup also causes mechanical stresses to the electrode due to volumetric changes which can lead to fracture and complete failure of the cell.This work studied nano-catalyst decoration of the Ni-based cermet anodes with catalysts that promote internal reforming to protect against coking. Addition of active metal components (such as Co, Ge, Sn), and ceramic reforming promoters (such as CeO2, MgO) were investigated. Multi-component systems with several catalysts were also examined.Uniform incorporation of nano-catalyst into the anode microstructure was achieved through a patented liquid phase surfactant assisted process (using various catechol surfactants). Deposition loading densities and distribution of nanoparticles was controlled by altering the surfactant and catalyst solution concentrations.Nano-catalyst depositions were characterized through Scanning Electron Microscope (SEM) for imaging, Atomic Force Microscopy (AFM) for topographical analysis, and Energy-Dispersive X-ray Spectroscopy (EDS) for chemical characterization. Figure 1 shows SEM imaging of nano catalyst distribution of cerium oxide (CeO2) and cobalt oxide (CoO) co-deposition within an anode structure. A uniform distribution of nano-sized catalyst materials is observed.Accelerated evaluation of nano-catalysts for SOFCs was completed through symmetrical anode tests, where a symmetrical anode cell was subjected to hydrocarbon impurity at SOFC operating temperatures and electrochemical impedance spectroscopy (EIS) characterization was done over time. The best catalyst systems were down selected and long-term SOFC tests were completed with current-voltage-power (I-V-P) and EIS evaluation. Post-mortem microstructure and chemistry characterizations were also used in analysis.Uniform nano catalyst distribution of CeO2 and CoO within the anode structure demonstrated greater than 50% sustained improvement in harsh environment, long-term tests where the cell was subjected to 40% CH4 for 50+ h. Figure 1