(677e) Electrochemical CO2 Capture with pH-Independent Redox Chemistry

Capturing anthropogenic carbon dioxide is a key strategy in reducing greenhouse gas emissions and mitigating climate change. However, current carbon capture technologies such as amine scrubbing incur high energy costs exceeding 100 kJ per mole of CO₂. This high energy cost, along with other challenges, limits commercial deployment. Considering that the thermodynamic energy input for concentrating a 15% CO₂ stream into 1 bar of pure CO₂ is only 4.7 kJ mol^-1, there is great potential for significant improvements. As an alternative, electrochemical carbon capture has garnered attention for its potential for higher energy efficiencies. Electrochemical activation specifically targets active materials, circumventing energy loss to substrate heating. A promising electrochemical method is the pH swing mechanism, which leverages the pH dependence of the solubility of inorganic carbon, including carbonates, bicarbonates and carbonic acid. Several different strategies including iipolar membrane electrodialysis (BPMED), Proton-coupled electron transfer (PCET) and gas looping have been explored. A fundamental property shared across these pH swing mechanisms is that H⁺ or OH^- is directly involved in the redox reaction. Whether it be a redox molecule for PCET or H₂ for BPMED and hydrogen looping, the reduced molecule releases a proton (or consumes a hydroxide ion) upon oxidation. Because H⁺ is a product in this oxidation reaction, a Nernstian potential gradient inherently arises from pH gradients,—typical ranges of 0.18-0.65 V that correspond to pH differences of 3-11,—which adds to the energy cost of CO₂ capture.

In this presentation, I will introduce a pH-independent redox chemistry for energy efficient CO₂ capture. We apply (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) for CO₂ capture and find that TEMPO-derivatives can modulate the pH through redox reactions. We demonstrate its potential as a CO₂ capture agent in an H-cell, where we observe the capture and release of CO₂ upon oxidation. Through in-situ Fourier transform infrared (FTIR) spectroscopy, we confirm the formation and departure of inorganic carbon through redox reactions. In particular, we find that the equilibrium redox potential remains constant across a wide pH range, an important property for minimizing thermodynamic energy input. Through an electrochemical flow cell composed of symmetrical TEMPO redox reactions, we demonstrate an equilibrium cell voltage close to zero and an operando cell voltage as low as 22 mV for CO₂ capture. We characterize the cell voltage, Faradaic efficiency and energy cost at various current densities, and confirm the stability of our system. We believe that optimization of the electrochemical cell and the chemistry could enable for further improvements in CO₂ capture performance, and we envision that this work could stimulate the discovery of other pH-independent redox chemistries for energy efficient CO₂ capture.