All projects were slightly retarded by the COVID situation and the problems it brought for hirements and especially exchange, but the employment and exchange structure is now fully established, operative, and partly already very productive. The following executive report is organized the single subprojects and uses a general language.
A) Tuning porous carbon structure and surface chemistry to boost their electrochemical performance
Using electrochemical impedance spectroscopy and gravimetric electrochemical quartz crystal microbalance (EQCM) to study carbon-supercapacitors reveals that solvents enhance charge carrier density at the solid-liquid interface. In the absence of solvent, ionic liquids show constant charge density, but negative surface charge and double layer formation through cation reorientation without ion movement. Further, nitrogen-doping in porous carbons, drawing on the noble carbons concept, improves capacitive performance by changing charging mechanisms in a Li2SO4 solution, shifting from anion adsorption-desorption to lithium ion electro-adsorption. These results are presented in E. Zhang et al.'s 2022 publication in the Journal of the American Chemical Society.
B) Surface modified of 2D Metal carbide materials with improved electrochemical performance
In a collaborative project, the impact of MXene surface functionalities on their electrochemical properties was investigated using HRTEM and in-situ analysis. Dr. Liyuan Liu from Toulouse did anexchange stay at MPI, leading to two published papers. The findings indicate that Cl-terminated MXenes lacked electrochemical activity, whereas N-doped Ti3C2Tx exhibited remarkable capacitive performance in sulfuric acid, achieving over 300 F g-1 at 2 V s-1. Moreover, addition of oxygen group to MXenes through ammonium persulfate treatment improved pseudocapacitive responses in non-aqueous electrolytes, reaching up to 240 mAh g-1 with high-rate capability. This highlights the role of surface chemistry in enhancing electrode performance for energy storage.
C) New operando electrochemical in-plane impedance spectroscopy technique
The Toulouse group introduced a method combining operando and in-plane impedance to study electrode materials' electronic and ionic conductivities, crucial for battery and supercapacitor efficiency. Utilizing dual potentiostats, they analyzed in-plane impedance on YP50F porous activated carbon and MXene Ti3C2 across different potentials. YP50F showed capacitive behavior in EMITFSI electrolyte, while Ti3C2 MXene exhibited low resistance at its redox peak in H2SO4, suggesting structural changes otherwise only accessible by larger instruments. This novel approach promises to significantly improve knowledge of electrode material properties.
D) Design of “synthetic” SEI-like passive layers on Zn anode
Zinc metallic anodes, ideal for eco-friendly batteries due to their low cost and safety, face short lifetimes from uncontrolled processes. An artificial solid electrolyte interface (a-SEI) was created to improve zinc deposition, hinder dendrite growth, and enhance overpotential for hydrogen evolution. This a-SEI proved its efficiency in Zn/Zn cells, achieving stable, dendrite-free operation for 3000 hours at high current, far surpassing the lifespan of unprotected cells. This breakthrough is still unpublished.
E) Noble Porous covalent materials
The MoMa-STOR project's method of carbonizing organic monomers with high HOMO levels and using salt melts for porosity customization has led to the creation of Oxocarbons. These materials boast superior traits like high electrical conductivity, a broad electrochemical window for better stability, and enhanced interactions with salts and solvents, essential for supercapacitors. However, Jiaxin Li's in her Toulouse internship for in-situ QCM analysis found delamination during charging, pinpointing a crucial issue related to the material's hydrophilicity and the resulting dispersion of electrodes. To solve this issue, monolithic materials and pressure cells must be applied for advancing energy storage solutions.
F) Design of “synthetic” SEI-like passive layers.
Our study focused on enhancing battery performance by coating carbon electrodes with nitrogen-rich semiconductors for a synthetic SEI, aiming for safer Na-metal deposition. We applied C3N4 layers to Na-anode carbon, boosting electrochemical metrics with minimal added weight. Further, we investigated biomass-derived materials for eco-friendly sodium-ion batteries, with hexose derivatives proving superior for energy storage. This approach yielded a 30% capacity increase over glucose carbons, reaching energy densities up to 217 Wh kg-1 at a 0.2 C rate and retaining 75% capacity at a 2C rate, demonstrating both long-term stability and efficiency.
G) New Sulfur cathodes for Na-S batteries
Sodium-sulfur batteries, promising for sustainable storage, suffer from short lifetimes due to sulfur migration. The MoMaStor project improved this by embedding elemental sulfur in a natural terpene based carbon matrix, achieving 99% sulfur utilization and a record electrode capacity. This approach, using subnano-confined sulfur pores, keeps 80% capacity after 1500 cycles and 50% capacity even at high charging rates (15 A/g), significantly enhancing NaS-battery performance and durability.
H) Theory and Modelling: ILs in confined space
In collaboration with Barbara Kirchner's team at Universität Bonn, Leonard Dick was recruited as a Post-Doc to study ionic liquids in confined spaces, examining their impact on structural, energetic, and electrical properties to discover new supercapacitor storage techniques. Despite the research's early phase and complexity, significant advancements have been achieved. Simulations show that ionic liquids adjust their structure within confined spaces, leading to different patterns such as "quantum fluidic lines" in smaller pores and intermittent structures in larger ones, enhancing energy storage and suggesting faster response capabilities for supercapacitors.
I) In situ HRTEM experiments
Commercial electrochemical liquid cell holders have been found to have voltage discrepancies due to nanowiring resistive losses. Efforts to improve batteries involve a new holder design that aligns electrodes perpendicularly, using graphene layers to support electrodes up to 5 nm thick, thus reducing electrode distance from 20 microns to 100 nm to lessen polarization and resistive losses. This new setup, still without a cover electrode, was used to analyze an ionic liquid in CMK-3 material, and enabled detailed analysis of the electronic structure around a nitrogen atom in the IL with 5 nm resolution. This highlights the differences between bulk and quantum confined ILs.
