Low energy carbon capture via electrochemically induced pH swing with electrochemical rebalancing

Anthropogenic CO2 emissions are the primary driver of climate change. While transitioning to renewable energy sources is crucial, fossil fuels will remain important for a considerable time. Carbon capture, from point sources or directly from the air (DAC), is therefore gaining prominence. Existing methods like wet amine scrubbing and alkaline solutions utilize temperature swing cycles, requiring significant thermal energy (∼100 kJ molCO2−1 for point source capture and >150 kJ molCO2−1 for DAC). These methods also raise environmental concerns due to sorbent volatility, toxicity, and corrosivity. Ocean-based CO2 removal methods face challenges related to high water handling requirements. Electrochemically mediated separation presents a more attractive alternative due to decreasing renewable electricity costs and mild operating conditions. However, most electrochemical methods operate at low current density (<5 mA cm−2) due to high overpotentials and corresponding energy costs, leading to high capital costs. This research builds upon a previously demonstrated pH swing cycle for CO2 separation electrochemically driven by proton-coupled electron transfer (PCET) of redox-active organic molecules. This new study presents a proof-of-concept point-source CO2 separation system using DSPZ, achieving an energy cost of only 61.3 kJ molCO2−1 at 20 mA cm−2. Addressing the sensitivity of reduced DSPZ (DSPZH2) to chemical oxidation by oxygen, the researchers introduce an electrochemical rebalancing method to restore the initial electrolyte composition.

The paper reviews existing carbon capture technologies, highlighting the limitations of thermal-swing methods such as wet amine scrubbing and alkaline solutions for both point source and direct air capture (DAC). These methods suffer from high energy requirements (approximately 100 kJ/mol CO2 for point source and >150 kJ/mol CO2 for DAC) and environmental concerns related to sorbent properties. Ocean-based CO2 removal is also discussed, noting the challenges associated with significant water handling. The authors then position electrochemical methods as a promising alternative due to their potential for lower energy consumption and operation under ambient conditions. However, they acknowledge the limitations of existing electrochemical approaches, specifically their operation at low current densities, leading to high capital costs. The paper specifically cites prior work on electrochemically driven pH swing cycles using proton-coupled electron transfer (PCET) of redox-active organic molecules as the foundation for the current research.

The research employs a Fe(CN)6/DSPZ carbon capture flow cell. The upstream gas composition is controlled by CO2 and N2 mass flow controllers (MFCs). Downstream, the gas is dried, and the total flow rate and CO2 partial pressure are measured. A pH probe monitors the negolyte pH, enabling real-time tracking of total alkalinity (TA) and dissolved inorganic carbon (DIC). The four-state pH swing carbon capture cycle involves: (1) electrochemical reduction of DSPZ to DSPZH2, producing hydroxide which reacts with CO2; (2) switching from inlet to exit pressure (p1 to p3, p3 always set to 1 bar); (3) electrochemical oxidation of DSPZH2 consuming hydroxide and releasing CO2; and (4) switching from exit to inlet pressure (p3 to p1). The authors detail the processes quantitatively, noting the impact of DSPZH2 oxidation by atmospheric oxygen, which leads to a decrease in cell capacity. This oxygen-induced imbalance is addressed by an electrochemical rebalancing method that expels oxygen and restores the initial electrolyte composition. A single carbon capture cycle with p1 = 0.1 bar and p3 = 1 bar at 40 mA cm−2 is analyzed in detail, examining changes in current density, voltage, total alkalinity, pH, gas composition, and flow rate. The absorbed and released CO2 volumes are calculated, and various methods for calculating DIC are presented (using flowmeter and CO2 sensor, pH measurements with and without assuming gas-solution equilibrium). The calculation of molar cycle work is explained, combining deacidification and acidification work. Productivity is evaluated by dividing the change in DIC by the sum of absorption and desorption times. Further experiments explore cycles with p1 = 0.1–0.5 bar at 40 mA cm−2 to assess the relationship between electrical work and inlet pCO2. The estimation of molar cycle work at p1 = 0.4 mbar (simulating DAC conditions) is achieved by extrapolating from higher p1 values, considering the non-linear relationship between ΔDICflow,3−1 and p1. Finally, the impact of current density (20–150 mA cm−2) on molar cycle work is investigated. The electrochemical rebalancing method is evaluated by comparing the cell capacity and performance under nitrogen and air, before and after the rebalancing process. The method involves cathodic reduction of K3Fe(CN)6 in the posolyte and anodic oxygen evolution in the negolyte, leading to a restoration of the cell’s initial composition and capacity.

The study achieves a molar cycle work of 61.3 kJ molCO2−1 for CO2 separation at 20 mA cm−2 when capturing CO2 from a 0.1 bar inlet and releasing to a 1 bar outlet. Extrapolating to a more challenging direct air capture (DAC)-relevant condition of a 0.4 mbar inlet, the estimated molar cycle work ranges from 121 to 237 kJ molCO2−1 at 20 mA cm−2. This range depends on the initial alkalinity of the electrolyte. Higher initial alkalinity leads to higher energy costs. The study found that the molar cycle work increases with increasing current density due to increased overpotentials. The CO2 absorption and desorption times are significantly affected by inlet pressure, with longer absorption times at lower pressures. A novel electrochemical rebalancing method effectively mitigates the capacity fade caused by oxygen, restoring the cell's performance to near its original state. The productivity, measured in molCO2 h−1, shows a positive correlation with current density, demonstrating the potential for higher throughput at higher current densities. The electrochemical rebalancing process itself requires an additional energy cost, which could add up if frequent rebalancing is required; however, the cost is deemed insignificant if rebalancing is needed only infrequently.

The achieved molar cycle work of 61.3 kJ molCO2−1 at 20 mA cm−2 for the separation of CO2 at 0.1 bar inlet and 1 bar outlet significantly outperforms other electrochemical methods reported in the literature for flue gas capture. This improvement is attributed to the use of a higher current density, which generally necessitates larger overpotentials and higher energy consumption. The successful extrapolation to estimate the molar cycle work at 0.4 mbar inlet demonstrates the potential scalability of this technology for direct air capture applications. The electrochemical rebalancing technique addresses a critical challenge—the oxygen-induced degradation of the cell's performance. By effectively restoring the electrolyte's composition, the method shows promise for enhancing the long-term viability and economic competitiveness of electrochemical carbon capture systems. The study’s findings have direct implications for the development of cost-effective and environmentally benign carbon capture technologies.

This research successfully demonstrates a low-energy electrochemical carbon capture system based on electrochemically induced pH swings using DSPZ molecules. The system achieves a competitive molar cycle work, especially at lower current densities. The introduction and validation of the electrochemical rebalancing method provides a crucial solution for addressing the capacity fade issues resulting from oxygen exposure, leading to increased system longevity. Future research could focus on developing oxygen-insensitive molecules and optimizing the cell design for enhanced mass transport and gas-liquid contacting, leading to even greater efficiency and productivity.

The study's extrapolation to low CO2 partial pressures (0.4 mbar) relies on theoretical calculations and may not perfectly reflect real-world conditions. The relatively small scale of the flow cell experiments limits the direct translation to industrial-scale applications. The electrochemical rebalancing method, while effective, adds an energy cost to the overall carbon capture process. Furthermore, the effect of long-term operation on the stability and performance of the system was not fully addressed, necessitating further investigation into system durability under prolonged use.