Julie N. Renner

Carbonate dynamics and opportunities with low temperature, AEM-based electrochemical CO2 separators

Fossil fuel power plants are responsible for a significant portion of anthropogenic atmospheric carbon dioxide (CO2) and due to concerns over global climate change, finding solutions that significantly reduce emissions at their source has become a vital concern. When oxygen (O2) is reduced along with CO2 at the cathode of an anion exchange membrane (AEM) electrochemical cell, carbonate and bicarbonate are formed which are transported through electrolyte by migration from the cathode to the anode where they are oxidized back to CO2 and O2. This behavior makes AEM-based devices scientifically interesting CO2 separation devices or “electrochemical CO2 pumps.” Electrochemical CO2 separation is a promising alternative to state-of-the-art solvent-based methods because the cells operate at low temperatures and scale with surface area, not volume, suggesting that industrial electrochemical systems could be more compact than amine sorption technologies. In this work, we investigate the impact of the CO2 separator cell potential on the CO2 flux, carbonate transport mechanism and process costs. The applied electrical current and CO2 flux showed a strong correlation that was both stable and reversible. The dominant anion transport pathway, carbonate vs. bicarbonate, undergoes a shift from carbonate to mixed carbonate/bicarbonate with increased potential. A preliminary techno-economic analysis shows that despite the limitations of present cells, there is a clear pathway to meet the US DOE 2025 and 2035 targets for power plant retrofit CO2 capture systems through materials and systems-level advances.

Catalysts for nitrogen reduction to ammonia

AbstractThe production of synthetic ammonia remains dependent on the energy- and capital-intensive Haber–Bosch process. Extensive research in molecular catalysis has demonstrated ammonia production from dinitrogen, albeit at low production rates. Mechanistic understanding of dinitrogen reduction to ammonia continues to be delineated through study of molecular catalyst structure, as well as through understanding the naturally occurring nitrogenase enzyme. The transition to Haber–Bosch alternatives through robust, heterogeneous catalyst surfaces remains an unsolved research challenge. Catalysts for electrochemical reduction of dinitrogen to ammonia are a specific focus of research, due to the potential to compete with the Haber–Bosch process and reduce associated carbon dioxide emissions. However, limited progress has been made to date, as most electrocatalyst surfaces lack specificity towards nitrogen fixation. In this Review, we discuss the progress of the field in developing a mechanistic understanding of nitrogenase-promoted and molecular catalyst-promoted ammonia synthesis and provide a review of the state of the art and scientific needs for heterogeneous electrocatalysts. The artificial synthesis of ammonia remains one of the most important catalytic processes worldwide, over 100 years after its development. In this Review, recent developments in enzymatic, homogeneous and heterogeneous catalysis towards the conversion of nitrogen to ammonia are discussed, with a particular focus on how mechanistic understanding informs catalyst design.