Mitigating Carbon Formation in SOFCs with Aluminum Titanate Doped NiO-YSZ Anodes

Abstract

Solid Oxide Fuel Cells (SOFCs) are high temperature energy conversion devices that produce electricity efficiently and sustainably. Because of the need to electrochemically reduce molecular oxygen and because of the relatively high activation energy required for oxide ions to diffuse through the dense, solid-state electrolyte, SOFCs typically operate at temperatures in excess of 650˚C. High operating temperatures endow SOFCs with many advantages including fuel flexibility and high conversion efficiencies, distinguishing them from many other types of fuel cells. In order to benefit from this role SOFCs must be able to operate well with a variety of fuels including H2, CH4, syngas, biogas, and higher molecular weight hydrocarbons. This requirement necessitates keeping the SOFC anodes clear from excessive carbon deposition or “coking” when exposed to hydrocarbon fuels. Standard SOFC anodes composed of a Ni-YSZ cermet are highly susceptible to coking due to Ni’s high catalytic activity towards activating hydrogen-carbon bonds. Deposited carbon can be detrimental to SOFCs as it can block catalytic sites and impede gas transport, limiting overall cell performance. One way to mitigate this effect is to introduce steam into the fuel feed to remove carbon via oxidation. However, as steam also risks oxidizing the catalytically active Ni metal, materials-focused solutions are often preferred. Modifying the Ni based anodes with secondary materials via infiltration is one such solution that yields significant improvements to the anode stability with minimal impacts to overall SOFC fabrication procedures. Aluminum titanate (Al2TiO5 or ALT) has previously been shown to increase electrochemical performance, mechanical strength, and resilience toward redox cycling in cells operated with H2. We have shown that the addition of this same material can also help limit carbon formation on both the anode surface and within the bulk when operating with CH4 as a fuel. The combination of ALT and NiO-YSZ leads to the formation of secondary phases, including a nickel aluminate spinel (NiAl2O4) and a zirconium titanate (Zr5Ti7O24), that via multi-pathway reaction mechanisms improve the Ni-YSZ anode tolerance to hydrocarbon fuels. These studies have used a combination of operando Raman spectroscopy and electrochemical measurements to identify and quantify carbon formation as a function of applied polarization and temperature. Results show that ALT doped anodes form ~25% less carbon on the surface under open circuit conditions. Additionally, an increase of polarization beyond 50% of maximum cell current results in close to no carbon depositing on the ALT doped surface. The effect of long term exposure to CH4 was studied using spectrochronopotentiometry methods. These experiments suggest that carbon build up within the bulk of the ALT doped cells is minimal over longer CH4 exposures which suggest that the secondary phases formed not only help prevent carbon deposits through a barrier layer mechanism but also function as internal reformers. Results provide evidence that ALT is effective at preventing carbon from accumulating to an extent that would hinder overall cell performance during the exposure to CH4 and that small changes to SOFC anode processing can have substantial effects on the performance.