Dual Salt Cation-Swing Process for Electrochemical CO2 Separation.

Electrochemical CO2 separations, which use electricity rather than thermal energy to reverse sorption of CO2 from concentrated point sources or air, are emerging as compelling alternatives to conventional approaches given their isothermal, ambient operating conditions, and ability to integrate with renewable energy inputs. Despite several electrochemical approaches proposed in previous studies, further explorations of new electrochemical CO2 separation methods are crucial to widen choices for different emissions sources. Herein, we report an electrochemical cation-swing process that is able to reversibly modulate the CO2 loading on liquid amine sorbents in dimethyl sulfoxide (DMSO) solvent. The process exploits a reversible carbamic acid-to-carbamate conversion reaction that is induced by changing the identity of Lewis acid cations (e.g. K+, Li+, Ca2+, Mg2+, and Zn2+) coordinated to the amine-CO2 adduct in the electrolyte. Using ethoxyethylamine (EEA) as a model amine, we present NMR-based speciation studies of carbamic acid-to-carbamate conversion as a function of amine/salt concentrations and cation identity. The reaction is further probed using gas-flow reaction microcalorimetry, revealing the energetic driving forces between cations and the amine-CO2 adduct that play a key role in the described re-speciation. A prototype electrochemical cell was further constructed comprising a Prussian white (PW) potassium (K+) intercalation cathode, zinc (Zn) foil anode, and EEA/DMSO electrolyte containing a dual KTFSI/Zn(TFSI)2 salt. A low CO2 separation energy of ∼22-39 kJ/mol CO2 (0.1-0.5 mA cm-2) was achieved with a practical CO2 loading delta of ∼0.15 mol CO2/mol amine. Further optimizations in electrolyte design and cell architectures toward continuous CO2 capture-release are expected to enhance rate performance while retaining favorable separation energies.

Electrochemical reduction of CO2 in the captured state using aqueous or nonaqueous amines.

CO2 capture and its electrochemical conversion have historically developed as two distinct technologies and scientific fields. Each process possesses unique energy penalties, inefficiencies, and costs, which accrue along the mitigation pathway from emissions to product. Recently, the concept of integrating CO2 capture and electrochemical conversion, or "electrochemically reactive capture," has aroused attention following early laboratory-scale proofs-of-concept. However, the integration of the two processes introduces new complexities at a basic science and engineering level, many of which have yet to be clearly defined. The key parameters to guide reaction, electrolyte, electrode, and system design would, therefore, benefit from delineation. To begin this effort, this perspective outlines several crucial physicochemical and electrochemical considerations, where we argue that the absence of basic knowledge leaves the field of designing metaphorically in the dark. The considerations make clear that there is ample need for fundamental science that can better inform design, following which the potential impacts of integration can be rigorously assessed beyond what is possible at present.

Pyrazine-Linked 2D Covalent Organic Frameworks as Coating Material for High-Nickel Layered Oxide Cathodes in Lithium-Ion Batteries.

The high specific capacity in excess of 200 mAh g-1 and low dependence on cobalt have enhanced the research interest on nickel-rich layered metal oxides as cathode materials for lithium-ion batteries for electric vehicles. Nonetheless, their poor cycle life and thermal stability, resulting from the occurrence of cation mixing between the transition-metal (TM) and lithium ions, are yet to be fully addressed to enable the widespread and reliable use of these materials. Here, we report a two-dimensional (2D) pyrazine-linked covalent organic framework (namely, Pyr-2D) as a coating material for nickel-rich layered cathodes to mitigate unwanted TM dissolution and interfacial reactions. The Pyr-2D coating layer, especially the 2D planar morphology and conjugated atomic configuration of Pyr-2D, protects the electrode surface effectively during cycling without sacrificing the electric conductivity of the host material. As a result, Pyr-2D-coated nickel-rich layered cathodes exhibited superior cyclability, rate performance, and thermal stability. The present study highlights the potential ability of 2D conjugated covalent organic frameworks to improve the key electrochemical properties of emerging battery electrodes.

Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst.

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although they exhibit various catalytic activities and stabilities during water oxidation, Na2CoP2O7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential (∼0.42 eV). Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry.

Partially Oxidized Sub-10 nm MnO Nanocrystals with High Activity for Water Oxidation Catalysis.

The oxygen evolution reaction (OER) is considered a major bottleneck in the overall water electrolysis process. In this work, highly active manganese oxide nano-catalysts were synthesized via hot injection. Facile surface treatment generated Mn(III) species on monodisperse 10 nm MnO nanocrystals (NCs). Size dependency of MnO NCs on OER activity was also investigated. Surprisingly, the partially oxidized MnO NCs only required 530 mV @ 5 mA cm(-2) under near neutral conditions.