Internal Reforming of Methane Fuel within Proton-Conducting Ceramic Anode Supports

Abstract

The objective of this work is to advance our understanding of hydrocarbon internal-reforming processes within the anode supports of proton-conducting ceramic fuel cells. This work is centered on an exciting new class of protonic-ceramic materials that are emerging from the laboratory to play important roles in the commercial sector. While proton-conducting ceramics have been studied since the early 1980s, the unique properties of these materials are only now being harnessed to address societal challenges in electricity generation, energy storage, and chemicals synthesis. Protonic-ceramic fuel cells (PCFCs) have demonstrated remarkable power density and stable operation under hydrocarbon fuels at operating temperatures that are well below that of the more-well established oxygen-ion conducting ceramics. Performance of recent unit-cell PCFC stacks based on barium cerate-zirconates developed at the Colorado Fuel Cell Center exceeds 300 mW / cm2 at 0.78 V during operation under direct methane-steam fuel at 550 ºC. Such performance brings confidence that protonic ceramics can play a meaningful role in efficient electricity generation. These protonic-ceramic fuel cells have demonstrated remarkable stability under methane-steam fuel mixtures. Under these conditions, no upstream fuel processor is used to convert the methane fuel into synthesis gas (H2, CO, CO2, H2O, etc.). Rather, the methane-steam fuel feed is reformed within the PCFC anode support. This internal reforming brings many benefits, including higher efficiency, improved thermal uniformity, reduced thermal stress, and simpler, lower-cost balance-of-plant components. Such internal reforming is common is oxygen-ion conducting ceramics like yttria-stabilized zirconia (YSZ). However, YSZ-based devices operate at significantly higher temperatures (~ 800 ºC), where the chemical kinetics of methane-steam reforming are more favorable to high conversion. In contrast, PCFCs operate at temperatures at or below 550 ºC, where reaction kinetics are substantially slower. This lower-temperature operation under direct hydrocarbon fuels brings questions about the effectiveness of the internal reforming process. Inadequate reforming can lead to inefficient utilization of the parent hydrocarbon fuel, lower cell performance, and deleterious solid-carbon deposit formation. To explore these questions in more depth, we have developed unique experimentation that decouples the thermochemical phenomena of heterogeneous methane-steam reforming from the electrochemical processes underway during fuel-cell operation and electricity generation. The “Separated Anode Experiment” has been developed to study heterogeneous chemistry and gas-transport processes in solid-oxide fuel cell anodes. This experiment decouples gas-transport and hydrocarbon internal-reforming processes from electrochemistry by simulating operational conditions within an anode support in the absence of a dense electrolyte. The planar anode support is compression-sealed within two alumina manifolds into which flow channels have been machined. Feed gases cross-diffuse and heterogeneously react within the tortuous morphology of the anode. Exhaust-gas compositions are quantified to understand what gases reach the triple-phase boundaries at the anode-electrolyte interfaces, bringing insight to the conditions at which charge transfer occurs. A detailed computational model aids in interpretation of the experimental results. In this presentation, the performance of protonic-ceramic anode supports will be presented from the perspective of internal-reforming effectiveness. Cell morphology and operating conditions that promote efficient internal reforming will be presented. Figure 1