(Invited) Electrochemical Separation of Carbon Dioxide Using Anion Exchange Membranes

Abstract

Anion exchange membranes provide a useful platform for electrochemical separation of carbon dioxide from dilute gas streams. Electrochemically-driven acid gas separations require a cathode reaction that generates hydroxide and an anode reaction that consumes hydroxide. Hydroxide generated at the cathode reacts with CO2 to make carbonate, which is transported across the membrane to the anode. The reverse process at the anode drives release of CO2. The reactions must be compatible with the gas mixtures present and must function over a pH range of about 7-12 for anode and 12-14 for cathode. These requirements allow flexibility for a range of electrode reactions. Four cells can be made from the reactions of hydrogen, oxygen, and water: the fuel cell1, electrolyzer, hydrogen pump, and oxygen pump2. Additionally, certain battery electrodes, such as nickel hydroxide / oxyhydroxide can be used to eliminate the gaseous reactants or byproducts.We will present experimental and model results for electrochemical separators using various electrode reactions. Applications include direct air capture of CO2 and scrubbing of air to >99% CO2 removal for use in hydroxide exchange membrane fuel cells. To date, the best studied system is the separator using fuel cell chemistry in a scrubbing application. Recent progress is shown in Figure 1. In the past three years, an order of magnitude improvement in air scrubbing capacity has been achieved through careful optimization of mass transport.Electrochemical separation of CO2 is well described by standard electrochemical models and much insight can be gained into the methods to improve performance. The role of membrane resistance provides a clear example. A pH gradient must be maintained between anode and cathode, both to create the thermodynamic driving force to transport CO2 from low partial pressure to high and to increase the kinetics of CO2 capture by providing excess hydroxide at cathode. Anion diffusion tends to reduce this pH gradient, while migration strengthens the gradient. As a result, the CO2 capture rate at constant current density tends to increase with increasing membrane resistance1, and the best fuel cell membranes have too high of conductivity to make optimized electrochemical separators. Figure 2 shows an experimental validation of the effect of membrane resistance. CO2 separation is greatly improved as membrane resistance increases, both in the case of intentionally degraded membranes and fresh membranes of reduced conductivity. The performance difference is greatest at the lowest current density, where the potential gradient across the membrane is smallest. References Matz, S. et al. Demonstration of Electrochemically-Driven CO 2 Separation Using Hydroxide Exchange Membranes . J. Electrochem. Soc. 168, 014501 (2021).Rigdon, W. A. et al. Carbonate Dynamics and Opportunities With Low Temperature, Anion Exchange Membrane-Based Electrochemical Carbon Dioxide Separators. J. Electrochem. Energy Convers. Storage 14, 020701 (2017). Figure 1: Progression of electrochemical carbon dioxide separation performance from 2018-2020 using 25 cm2 single cells with fuel cell chemistry. Air with 400 ppm CO2 was fed to the cathode. Cell temperature & relative humidity: 70 °C, 80% (2018-1, 2018-2) or 60 °C, 70% (2019-1, 2019-2, 2020); current density: 20 mA/cm2 (2018-1, 2018-2, 2019-1, 2019-2) or 35 mA/cm2 (2020); outlet pressure: 100 kPaa (2018-1, 2018-2, 2019-1), 175 kPaa (2019-2), or 250 kPaa (2020). Anode hydrogen flow rate was 50 sccm (2018-1, 2018-2), 7 sccm (2019-1, 2019-2), or 12 sccm (2020). EDCS:FC ratio is the active area of electrochemical separator divided by the active area of the fuel cell that it could supply based on the air flow rate, assuming 1.5 A/cm2 fuel cell current density and 1.5x cathode stoichiometry. Figure 2: Effect of membrane area-specific resistance on CO2 separation performance. Conditions: 60 °C cell temperature, 90% relative humidity, 20 sccm H2 flow at anode, 1000 sccm air (w/ 400 ppm CO2) at cathode, atmospheric pressure. Membrane details: A) PAP-TP-85 membrane B) Same as A, but slightly degraded to increase resistance (140 h dry exposure in 80 °C oven). C) Same as A, but moderately degraded to increase resistance (96 h dry exposure in 97 °C oven). D) Fumasep FAA-HM membrane from FuMA-Tech Gmbh. Figure 1