NASA has investigated and demonstrated the simultaneous production of methane (CH4) and oxygen (O2) via the electrolysis of carbon dioxide (CO2) and water (H2O) in one or more ionic liquids (ILs). In order to improve the likelihood of methane and oxygen production, and to reduce the likelihood of unwanted side-product formation, several innovative approaches were investigated. Research has shown thousands of options for ionic liquids that can be used in the electrochemical process; however, care must be taken to choose an ionic liquid that has high carbon dioxide solubility, limited change in viscosity due to carbon dioxide absorption, and chemical stability during the electrochemical process.
Research was focused on using the commercially available EMIM and BMIM ionic liquids with either tetrafluoroborate or hexafluorophosphate anions. A copper cathode and a platinum anode were chosen for the initial electrochemical cell design. One of the limitations described in the research was the production of unwanted side-products during the electrochemical process. In many cases, it was hypothesized that the electrochemical cell design could be a contributing factor in the production of unwanted side-products, and that a different cell design could minimize the number of unwanted side-products and increase the conversion of carbon dioxide and water into methane and oxygen.
Multiple custom electrochemical cells were designed and fabricated for use in studying the feasibility of producing methane and oxygen from the simultaneous electrolysis of carbon dioxide and water. Initially, two cell designs (one small-volume, one larger volume) were fabricated for the project. These first two cells were two-chamber cells fabricated from polycarbonate. The small-volume, two-chamber cell was designed to provide a very short working distance between the working and counter electrodes (approximately 0.11”) and was designed to hold a total volume of 8 ml of each electrolyte (ionic liquid and water). The larger-volume, two-chamber cell was fabricated by modification of a commercially available Gamry Paracell. The larger cell was designed to hold a total volume of approximately 35 ml of each electrolyte for larger-scale methane production.
In both cells, a proton-exchange membrane (typically a Nafion membrane) was used to separate the two chambers of each cell. During early testing, the small-volume cell was damaged, and a new cell was designed and fabricated that was similar to the large-volume, two-chamber cell but with a smaller electrolyte capacity (approximately 10 ml of each electrolyte). Test results obtained from electrolysis experiments performed using the new small-volume cell were unexpected; the copper working electrode was poisoned, and the presence of calcium and carbonate were detected on the surface of the electrode. Further investigation led to the conclusion that the source of the calcium and carbonate was the polycarbonate used for cell fabrication. To eliminate the presence of calcium and carbonate, an exact replica of the 10-ml cell was fabricated from high-density polyethylene (HDPE). An electrochemical experiment conducted using the HDPE cell did not show the production of any methane, but did show the production of carbon monoxide and hydrogen. Additionally, no calcium carbonate was detected.
Another cell that was evaluated was a commercially available, two-chamber glass cell with a glass frit separating the two halves of the cell. Several experiments were conducted using this cell, but several issues were identified, including gas leakage. Novel caps were fabricated for the cell to eliminate this issue, but the cell was damaged while preparing for an experiment, and no methane production data was collected for this cell design.
The two-chamber cell design provides the opportunity to minimize the production of unwanted side-products by minimizing the interaction between the aqueous electrolyte and the ionic liquid electrolyte. Additionally, the two-chamber cell design provides the opportunity to circulate each electrolyte independently, which allows the purification and removal of unwanted products in each electrolyte by whatever means is necessary for each individual electrolyte (without worrying about the other electrolyte). This is a huge advantage when dealing with long-term use, especially from an ISRU perspective where consumables are limited.