Proton-donating cations enable efficient and stable acidic CO₂ reduction in membrane electrode assemblies

Area of Science:

• Electrochemistry
• Catalysis
• Sustainable Chemistry

Background:

• Electrochemical CO2 reduction (CO2R) in acidic membrane electrode assemblies (MEAs) is promising for sustainable chemical production but faces challenges in selectivity, cell voltage, and stability.
• Current methods using alkali cations improve selectivity but suffer from high overpotential and precipitation due to water's weak proton-donating ability, leading to operational issues.
• Addressing these limitations is crucial for advancing CO2R technology towards practical applications.

Purpose of the Study:

• To introduce and evaluate ammonium (NH4+) as a dual-function cation and proton donor in acidic MEAs for electrochemical CO2 reduction.
• To demonstrate how NH4+ can simultaneously enhance selectivity, reduce overpotential, and improve operational stability.
• To overcome the limitations of traditional proton donors like water in CO2R processes.

Main Methods:

• Utilizing NH4+ as both a cation and proton donor in acidic MEAs with a CoPc@CNT-NH2 catalyst.
• Investigating the electromigration of NH4+ to the catalyst surface to stabilize intermediates and manage local proton concentration.
• Analyzing the proton-donating ability of NH4+ compared to water under limited proton transport conditions.
• Assessing the effect of NH4+ on bicarbonate decomposition and precipitate management.
• Operating the system under specific conditions (100 mA cm-2, 60°C) and evaluating performance over 110 hours.

Main Results:

• NH4+ demonstrated enhanced CO2 intermediate stabilization and reduced localized proton concentration, leading to high selectivity.
• The superior proton-donating ability of NH4+ decreased the protonation barrier, lowering the CO2R overpotential and cell voltage.
• NH4+ promoted efficient bicarbonate decomposition at lower temperatures, facilitating precipitate removal and enabling stable NH3/NH4+ recirculation.
• Achieved an average CO2-to-CO selectivity of 86% at 100 mA cm-2 and 60°C.
• Demonstrated stable operation for over 110 hours at an average cell voltage of 2.84 V, with 40.6% energy efficiency.

Conclusions:

• Ammonium (NH4+) effectively functions as a proton-donating cation in acidic MEAs, simultaneously improving CO2R selectivity, reducing overpotential, and enhancing operational stability.
• This strategy overcomes key limitations of water as a proton donor, paving the way for more efficient and practical electrochemical CO2 reduction.
• The findings represent a significant advancement in acidic MEA-based CO2R, moving the technology closer to real-world implementation.