Towards High-Performance Supercapacitor Electrodes via Achieving 3D Cross-Network and Favorable Surface Chemistry

Vibrational Property Tuning of MXenes Revealed by Sublattice N Reactivity in Polar and Nonpolar Solvents

MXenes, a family of two-dimensional (2D) materials based on transition metal carbides and nitrides, have desirable properties, such as high conductivity, high surface area, and tunable surface groups, for electrocatalysis. Nitride MXenes, in particular, have shown excellent electrocatalytic performance for the nitrogen and oxygen reduction reactions, but a fundamental understanding of how their structures evolve during electrocatalysis remains unknown. Equally important and yet unknown is the effect of the reactivity of the lattice nitrogen on the vibrational behavior of nitride MXenes and the resulting implications in electrocatalysis. Here, we investigate the reactivity of lattice nitrogen and the vibrational properties of titanium nitride MXenes in relevant electrocatalytic solvents using confocal Raman spectroscopy. We found that the vibrational modes of titanium nitride MXenes are attenuated in polar solvents, which is revealed through the alteration of the Raman scattering in solvents. Contrary to polar solvents, the vibrational modes remain unchanged in nonpolar solvents like hydrocarbons due to the inactivity of the lattice nitrogen. We found that this behavior is unique to nitrides because the Raman characteristics of carbides and sulfides are unaffected by the solvent types. However, the inclusion of nitrogen into the carbide structure does exhibit Raman-solvent behavior similar to that of nitrides, suggesting that replacing carbon with nitrogen affects MXene-light interactions. We demonstrated a proof-of-concept utilizing lattice nitrogen reactivity to enhance the electrocatalytic nitrogen reduction reaction for ammonia production. In summary, we elucidate the vibrational properties of nitride MXenes in solvents and demonstrate the tunability of MXene vibrational properties via lattice atom substitution, which in turn can be exploited to advance the applications of MXenes in electrocatalysis.

A Ferricyanide Anion-Philic Interface Induced by Boron Species within Carbon Framework for Efficient Charge Storage in Supercapacitors

Carbon materials with hierarchical porous structures hold great potential for redox electrolyte-enhanced supercapacitors. However, restricted by the intrinsic inert and nonpolar characteristics of carbon, the energy barrier of anchoring redox electrolytes on the pore walls is relatively high. As such, the redox process at the interface less occurs, and the rate of mass transfer is impaired, further leading to a poor electrochemical performance. Here, a ferricyanide anion-philic interface made of in situ inserted boron species into carbon rings is constructed for enhanced charge storage in supercapacitors. Profiting from the unique component-driven effects, the polar anchoring sites on the pore wall can be built to grasp the charged redox ferricyanide anion from the bulk electrolyte and promote the redox process; the dynamics process is fastened correspondingly. Especially, the boron atoms in BC2O and BCO2 units with higher positive natural bond orbital values in the carbon skeleton are pinpointed as intrinsic active sites to bind the negatively charged nitrogen atoms in the ferricyanide anion via electrostatic interaction, confirmed by density functional theoretical calculations. This will suppress the shuttle and diffusion effects of the ferricyanide anion from the surface of the electrode to the bulk electrolyte. Finally, the well-designed PC-3 with high content of BC2O and BCO2 units can reach 1099 F g-1 at 2 mV s-1, which is a more than 2-fold increase over boron-free units of carbon (428 F g-1). The work offers a novel version for designing high-performance carbon materials with unique yet reaction species-philic effects.

Surface chemistry and structure manipulation of graphene-related materials to address the challenges of electrochemical energy storage

Energy storage devices are important components in portable electronics, electric vehicles, and the electrical distribution grid. Batteries and supercapacitors have achieved great success as the spearhead of electrochemical energy storage devices, but need to be further developed in order to meet the ever-increasing energy demands, especially attaining higher power and energy density, and longer cycling life. Rational design of electrode materials plays a critical role in developing energy storage systems with higher performance. Graphene, the well-known 2D allotrope of carbon, with a unique structure and excellent properties has been considered a "magic" material with its high energy storage capability, which can not only aid in addressing the issues of the state-of-the-art lithium-ion batteries and supercapacitors, but also be crucial in the so-called post Li-ion battery era covering different technologies, e.g., sodium ion batteries, lithium-sulfur batteries, structural batteries, and hybrid supercapacitors. In this feature article, we provide a comprehensive overview of the strategies developed in our research to create graphene-based composite electrodes with better ionic conductivity, electron mobility, specific surface area, mechanical properties, and device performance than state-of-the-art electrodes. We summarize the strategies of structure manipulation and surface modification with specific focus on tackling the existing challenges in electrodes for batteries and supercapacitors by exploiting the unique properties of graphene-related materials.