(Invited) Selective and Economical CO2 Reduction in Acid
Authors: Ha Tae Hyeon , Oh Jihun 
Published: December 2024

The increasing urgency to address climate change has propelled the exploration of innovative technologies capable of mitigating its effects. Among these, the electrochemical reduction of CO2 stands out as a promising solution, leveraging the conversion of CO2—a principal greenhouse gas—into valuable chemicals and fuels using electricity. This process is not only pivotal for reducing atmospheric CO2 levels but also plays a crucial role in steering the global economy towards sustainability and circularity.

Unfortunately, there are some critical problems due to the intrinsically produced hydroxide during electrochemical CO2 reduction. This hydroxide reacts with the reactant CO2, leading to various issues. Foremost among these issues is the phenomenon known as carbonate crossover, wherein carbonate ions permeate through the anodic side. The permeated carbonates contaminate and drop the pH of the anolyte, which cause the additional electrolyte regeneration process in alkaline condition. In the neutral condition, carbonates are regenerated to CO2 at the anodic side and then emitted with O2 from oxygen evolution reaction (OER), which incurs an additional gas separation process. Furthermore, carbonates remaining at the cathode precipitate as salts, resulting in overall performance degradation. To overcome these issues, CO2 reduction in acidic conditions can serve as a viable alternative. In acidic conditions, hydroxide and (bi)carbonates can revert to CO2, especially at cathodic side. Therefore, carbonate crossover is significantly reduced, which in turn eliminates the theoretical limitation of single-pass conversion (SPC) previously capped at 50% in alkaline and neutral conditions. While acidic conditions offer this advantage, they necessitate effective suppression of the competing hydrogen evolution reaction (HER).

In this research, we propose a set of design principles aimed at enhancing the selective production of carbon monoxide at high current densities within acidic electrolytic environments, drawing on insights into both the mass transfer dynamics of CO2 and H+, and the electro-kinetic interactions at the interface. Through a combination of theoretical and empirical studies, it has been determined that the electrochemical reduction of CO2 in a gas-fed flow cell inherently favors the production of CO over H2, primarily due to the reduction of protons in acidic conditions. However, it has also been observed that the reduction of water (H2O) adversely affects the selectivity for CO, a challenge that can be substantially mitigated by employing a catalyst that possesses inherently strong activity for CO production.

For this purpose, nickel single-atom catalysts supported on carbon nanotubes (Ni-SAC-CNTs) were employed as an exemplary system, demonstrating exceptional performance in CO production. These catalysts achieved a CO Faraday efficiency exceeding 90% under conditions of –100 mA cm-2 and at a pH of 2.0. Furthermore, the introduction of weakly hydrated cations into the electrolyte has been shown to enhance CO selectivity in acidic settings significantly, by facilitating CO2RR while concurrently inhibiting the kinetics of the HER.

A detailed analysis of the carbon balance during the electrolysis process revealed that the use of Ni-SAC-CNTs not only promotes highly selective CO formation but also substantially diminishes the formation of carbonate ions. This dual effect leads to achieving the highest single-pass conversion of CO2 to CO, which was recorded at approximately 77%. It is notable that it was achieved without the need for adjusting local reaction conditions within the electrochemical system.

This study not only demonstrates the effectiveness of Ni-SAC-CNTs in improving the selectivity and efficiency of electrochemical CO2 reduction in acidic environments but also provides a blueprint for further development of catalyst systems that could be adapted for industrial applications. The insights gained into the interaction between catalyst properties, electrolyte composition, and operational conditions open up new avenues for the deployment of CO2 reduction technologies in sectors where acidic waste streams are prevalent.

Moving forward, transitioning CO2 reduction from merely lab-scale experiments to practical applications necessitates its implementation in membrane electrode assembly (MEA) systems. In MEA operations, challenges such as carbonate crossover and salt formation emerge as significant obstacles. However, by applying acidic conditions within MEA systems and employing strategic system engineering, these issues can be effectively addressed. We will also present our current research process, which has demonstrated efficiency levels that surpass those of studies published to date, showcasing the potential of our approach in real-world applications.

Through this, we hope our study contributes valuable insights within MEA systems, offering a viable pathway to overcome the limitations of current technologies. By enhancing the scalability, efficiency, and stability of electrochemical CO2 reduction, our work not only advances the field but also aligns with global efforts to combat climate change and foster a sustainable future

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