(656a) Carbonate Fuel Cells: Scaling-up the Magic Carbon Capture Box

To economically sequester or utilize CO₂ from dilute sources such as industrial emissions or power generation, technologies for concentrating CO₂ are required. Unfortunately, commercially available technologies are capital and energy intensive, reducing their effectiveness in limiting emissions. The advantage of the carbonate fuel cell is that it can effectively concentrate CO₂ and simultaneously produce electricity and hydrogen. This example of process intensification takes advantage of the electrochemical reaction to drive a desirable separation. Although carbonate fuel cells have been deployed commercially for power generation, strenuous conditions of carbon capture are much more challenging. A joint development program has been created to reengineer the fuel cell stack for carbon capture, utilizing model-guided and parallel development strategies.

Work on single cells has shown that the use of dilute CO₂ feeds from flue gas sources can have a significant effect on molten carbonate fuel cell behavior beyond increased cathode overpotentials. Specifically, testing has shown that the electrolyte can behave as a dual ion conductor passing both carbonate and non-carbonate ions through the electrolyte. Kinetic testing has helped elucidate mechanistic reaction pathways and suggest that the non-carbonate ions are likely OH^- formed from the catalysis of H₂O and O₂. In addition, mass transport effects are found to be of increased importance at these conditions and a leading factor in controlling fuel cell ionic selectivity. These learnings highlight the need for several design modifications which can lead to improved fuel cell performance in carbon capture applications.

A kW scale technology stack with commercially sized repeat components is being used to evaluate the scale-up challenges of mass transfer and thermal management. Insights from the lab scale fundamentals research have been applied to design a cathode current collector that reduced the CO₂ mass transfer limitations in the fuel cell cathode. With a redesigned current collector, the stack ionic selectivity was improved by >15% while simultaneously increasing voltage by >40 mV. Additionally, because of the exothermic nature of the electrochemical reactions and electrical resistivity, thermal management is critical. Endothermic steam methane reforming is used to heat balance the fuel cell stack and provide hydrogen fuel. Carefully co-locating the heat generation and consumption reactions is critical to reducing thermal gradients rather than exacerbating them. A modeling approach was utilized to design a methane reforming catalyst pattern that could reduce thermal gradients by ~40% within the fuel cell stack and allow for improved electrochemical performance.

The carbonate fuel cell carbon capture technology is able to reduce parasitic energy demands of carbon capture while actually co-producing hydrogen and electricity. The deployment of this unique reactive separation technology, however, relies on careful engineering to properly manage the heat and mass transfer during scale-up. Fundamentals based models have been key in guiding the scale up by highlighting the key mass and heat transfer issues. Going forward, the model guided design approach will be key in matching process conditions and hardware design to specific carbon capture applications.