Purposeful Utilization of Carbonate Anions in Alkaline Media
Carbonate anions have long been perceived as unwanted impurity species in low temperature electrochemical devices. This perception is rooted in traditional alkaline fuel cells where the OH– anions in the electrolyte react with CO2 to yield CO32-. The carbonate further reacts with K+ from the electrolyte, precipitating the very low solubility K2CO3 on the electrode surface, reducing the electrochemically active area and impeding device performance. However, anion exchange membranes overcome this limitation since the cations are immobilized in the polymer matrix. This opens a new area of research for the purposeful utilization of carbonate in low temperature alkaline systems. Carbonate has several advantages in alkaline polymer systems. First, carbonate is a much weaker nucleophile, and membranes are far more stable in the presence of carbonate vs. hydroxide. Second, carbonate is also an efficient oxygen donor, which makes it ideal as a facilitator for oxidation reactions, including new chemistries not possible through hydroxide pathways.
Our group has spent a considerable effort in this area, developing cathode catalysts that can produce carbonate selectively through the direct electrochemical reduction of O2 and CO2; this includes the development of a new method to quantify the chemical vs. electrochemical production of carbonate in the cathode electrode layer. We have also investigated the reaction kinetics for several potential fuels with carbonate and hydroxide, as well as the stability of commercial membranes in simulated reacting environments.
Electrochemical Conversion of Methane to Fuels
The ability of carbonate anions to donate oxygen opens up a new possibility for activating methane at low temperatures through an insertion reaction. We are particularly interested in catalyst development and understanding the reaction mechanism for the partial oxidation of methane to methanol, formaldehyde and other low molecular weight oxygenates through carbonate pathways. The operating principles for such a device working on natural gas (a) and biogas (b) are shown below.
Our recent work has suggested that multifunctional electrocatalysts can be developed to activate methane at room temperature that utilize oxides as the terminal oxygen source that are replenished by OH- anions in alkaline media. This has opened up new possibilities for methane conversion to oxygenates in hydroxide environments, which were not possible previously. Our interest here lies in understanding the fundamental reaction mechanisms and scaleup from the lab scale to a bench-scale demonstration unit.
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