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Investigation of a direct methanol redox fuel cell with design simplification Ilicic, Alan Bartol

Abstract

A key objective of this work is to address a number of the central issues associated with the direct methanol fuel cell (DMFC) through the investigation of a redox flow battery (RFB) / DMFC hybrid fuel cell. The air cathode (Pt/carbon) of the DMFC is substituted by a Fe²⁺/Fe³⁺ redox couple cathode (carbon) with no platinum-group metal (PGM) catalyst. In this configuration, referred to as the direct liquid redox fuel cell (DLRFC), the Fe²⁺/Fe³⁺ redox couple cathode is selective to the redox couple reaction and fuel crossover does not cause cathode depolarization. A wide range of anolyte fuel concentrations were tested (2-24 M CH₃OH) and the best DLRFC performance was obtained at 16.7 M CH₃OH (equimolar CH₃OH / H₂O). A significant improvement in the DLRFC performance and catholyte charge density was obtained by switching from a sulfate-based iron salt to a perchlorate-based iron salt. This led to a greater than 150% increase in the solubility of the redox couple, greater than a 200 mV increase in the equilibrium half-cell potential of the redox couple and a greater than 200% improvement in the DLRFC peak power density (79 mW/cm² vs. 25 mW/cm²) relative to the sulfate-based system. The selective nature of the redox cathode enabled the demonstration and characterization of a novel mixed-reactant DLRFC (MR-DLRFC) where a mixed electrolyte containing methanol and the redox couple is fed to the cathode and fuel crossover is the mode of fuel supply. A non-optimized peak power density of 15 mW/cm² was obtained with this system. A novel in-situ redox couple regeneration approach was also demonstrated and characterized, which involved substituting the methanol anolyte by an air stream. This approach exploits the hybrid nature of the DLRFC and utilizes the PtRu catalyst at the DLRFC anode as an O₂ reduction cathode during regeneration. Eliminating the use of PGM catalysts at the fuel cell cathode and enabling the use of high fuel concentrations are decisive advantages of DLRFC technology. Furthermore, the ability to extend DLRFC technology to a mixed-reactant architecture where fuel crossover is desirable paves new ground for the future of fuel cell research.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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