A new cell designed to mimic the photosynthetic processes of plants to convert carbon dioxide into carbonaceous products and oxygen at high efficiency, has an improved configuration using a polymer membrane electrolyte and an alkaline medium. This increases efficiency of the artificial photosynthetic process, achieves high conversion rates, permits the use of inexpensive catalysts, and widens the range of products generated by this type of process.
The alkaline membrane electrolyte allows for the continuous generation of sodium formate without the need for any additional separation system. The electrolyte type, pH, electro-catalyst type, and cell voltage were found to have a strong effect on the efficiency of conversion of carbon dioxide to formate. Indium electrodes were found to have higher conversion efficiency compared to lead. Bicarbonate electrolyte offers higher conversion efficiency and higher rates than water solutions saturated with carbon dioxide. pH values between 8 and 9 lead to the maximum values of efficiency. The operating cell voltage of 2.5 V, or higher, ensures conversion of the carbon dioxide to formate, although the hydrogen evolution reaction begins to compete strongly with the formate production reaction at higher cell voltages.
Formate is produced at indium and lead electrodes at a conversion efficiency of 48 mg of CO2/kilojoule of energy input. This efficiency is about eight times that of natural photosynthesis in green plants. The electrochemical method of artificial photosynthesis is a promising approach for the conversion, separation and sequestration of carbon dioxide for confined environments as in space habitats, and also for carbon dioxide management in the terrestrial context.
The heart of the reactor is a membrane cell fabricated from an alkaline polymer electrolyte membrane and catalyst-coated electrodes. This cell is assembled and held in compression in gold-plated hardware. The cathode side of the cell is supplied with carbon dioxide-saturated water or bicarbonate solution. The anode side of the cell is supplied with sodium hydroxide solution. The solutions are circulated past the electrodes in the electrochemical cell using pumps. A regulated power supply provides the electrical energy required for the reactions. Photovoltaic cells can be used to better mimic the photosynthetic reaction. The current flowing through the electrochemical cell, and the cell voltage, are monitored during experimentation. The products of the electrochemical reduction of carbon dioxide are allowed to accumulate in the cathode reservoir. Samples of the cathode solution are withdrawn for product analysis. Oxygen is generated on the anode side and is allowed to vent out of the reservoir.
This work was done by Sri Narayan, Brennan Haines, Julian Blosiu, and Neville Marzwell of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
JPL
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Refer to NPO-45777, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
High-Efficiency Artificial Photosynthesis Using a Novel Alkaline Membrane Cell
(reference NPO-45777) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing advancements in high-efficiency artificial photosynthesis using a novel alkaline membrane cell. This research aims to develop a method for converting carbon dioxide and water into organic compounds and oxygen, mimicking the natural process of photosynthesis. The technology has significant implications for both space exploration and addressing climate change by reducing carbon emissions from fossil fuels.
Central to the electrochemical process is a membrane electrode assembly (MEA), which consists of electrodes coated with catalysts and bonded to an ion-conducting membrane. The study highlights the production of formate from carbon dioxide, demonstrating a conversion efficiency that is eight times greater than that of natural plants. While plants typically convert 5-8 mg of carbon dioxide per kilojoule of solar energy, the developed electrochemical reactor achieved efficiencies of up to 48 mg of carbon dioxide per kilojoule of electrical energy input.
The research identified key factors influencing conversion efficiency, including the type of electrode catalyst, operating cell voltage, the composition of the cathode solution (water vs. bicarbonate), and the duration of the experiment. Indium and lead were tested as catalysts, with indium showing a conversion efficiency four times higher than lead under similar conditions. In carbon dioxide-saturated water solutions, the reactor achieved conversion values of 28 mg/kJ with indium and 7 mg/kJ with lead after one hour of operation at 2.5 V.
Spectrophotometric measurements confirmed the production of formate at specific voltages, validating the reactor's performance. The findings suggest that an artificial photosynthetic pathway with high conversion efficiencies is feasible, paving the way for future applications in energy production and carbon capture.
Overall, this document outlines a significant step forward in the development of sustainable technologies that could play a crucial role in mitigating climate change and supporting long-term space missions. The research reflects NASA's commitment to innovative solutions that leverage advanced materials and processes to address pressing global challenges.
