Direct Carbon Fuel Cell With Stable Molten Hydroxide Catholyte

In the mid-1990’s, SARA developed and patented (Patent Number 6,200,697) a Direct Carbon-Air Fuel Cell (DCFC) which uses a molten hydroxide electrolyte in a cell design that is characteristic of what are commonly known as metal-air fuel cells. This technology forms the basis of the Direct Coal to Electricity Conversion system that is being developed at SARA with support from American Electric Power and the Electric Power Research Institute. The main feature of the cell which uses molten hydroxide electrolyte is the design simplicity in which the cathode is a simple iron container sparged with air. The drawback of this design however is chemical instability of the electrolyte due to its reaction with anode product CO₂ resulting in hydroxide to carbonate conversion that lessens the cell performance and shorts the cell operation duration. Researchers at SARA are exploring various means to prevent or reduce the carbonate formation. One of the means is based on the use of high water content in the electrolyte that will shift the equilibrium of hydroxide to carbonate conversion to the left resulting in low $CO32−$ ion concentration. Another means to prevent conversion of hydroxide melt into carbonate according to the literature [1–2] is based on the use of oxide additives such as SiO₂, As₂O₃, and MgO as well as oxyanions such as pyrophosphate and persulfate that decompose carbonate and therefore these compounds together with water might help in preventing conversion of hydroxides into carbonates. Unfortunately neither water content nor oxide additives exerted substantial reduction of carbonate formation at temperatures up to 650° C. Much higher temperatures are needed for these effects to be significant. Since the beginning of 2004, SARA has been performing experiments with a new generation of DCFC. The results of those experiments have permitted much longer term operation of the DCFC than was possible in earlier experiments. This has led SARA to a new cell configuration with a porous separator that separates electrolyte in the anode compartment (anolyte) from the electrolyte in the cathode compartment (catholyte) and prevents hydroxide to carbonate conversion in the catholyte. In this cell design the anolyte is carbonate melt whereas the catholyte is hydroxide melt. Consequently the electrochemical activity of carbon anodic dissolution is not as high as in hydroxide electrolyte, whereas the high activity of oxygen cathode and subsequently its simple design is retained. This new configuration has several advantages over the older cell configuration: (1) we can use particulate carbon directly in the cell, (2) the CO₂ that is produced by the cell comes out in a form that can be easily sequestered, and (3) the electrolyte is stable for long term operation. The starting electrolyte in both cell compartments is a mixture of NaOH and LiOH (1:1 by mol). During the cell operation the anolyte is being converted to carbonate, the anode potential is getting less negative and at certain point it reaches the plateau. At this point gaseous CO₂ starts leaving the cell and the electrolyte is stabilized showing no further changes during cell operation. No effects of $CO32−$ ions on O₂ cathode performance was observed over 500 h of operation indicating little or no $CO32−$ transport through the separator. Time required converting hydroxide anolyte into carbonate one depends on the cell current, but the cell operation can also start with carbonate anolyte. In any case the amount of CO₂ determined in anode off gas is proportional to the cell current indicating that CO₂ is formed as a result of electrochemical reactions at the carbon anode.