Unraveling the Capacitive Charge Storage Mechanism of Nitrogen-Doped Porous Carbons by EQCM and ssNMR.

Fundamental understanding of ion electroadsorption processes in porous electrodes on a molecular level provides important guidelines for next-generation energy storage devices like electric double layer capacitors (EDLCs). Porous carbons functionalized by heteroatoms show enhanced capacitive performance, but the underlying mechanism is still elusive, due to the lack of reliable tools to precisely identify multiple N species and establish clear structure property relations. Here, we use advanced analytical techniques such as low-temperature solid-state NMR (ssNMR) and electrochemical quartz crystal microbalance (EQCM) to relate the complex nitrogen functionalities to the charging mechanisms and capacitive performance. For the first time, it is demonstrated at a molecular level that N-doping strongly influences the electroadsorption mechanism in EDLCs. Without N-doping, anion (SO42-) adsorption-desorption dominates the charging mechanism, whereas after doping, Li+ electroadsorption plays a key role. With the help of EQCM, it is demonstrated that SO42- is strongly immobilized on the N-doped surface, leaving Li+ as the main charge carrier. The smaller size and higher concentration of Li+ compared to SO42- benefit a higher capacitance. Amine/amide N is responsible for high capacitance, but surprisingly the pyridinic, pyrrolic, and graphitic N groups have no significant influence. 2D 1H-15N NMR spectroscopy indicates that the conversion from pyridinium to pyrrolic N gives rise to a slightly decreased capacitance. This work not only demonstrates ssNMR as a powerful tool for surface chemistry characterization of electrode materials but also uncovers the related charging mechanism by EQCM, paving the way toward a comprehensive picture of EDLC chemistry.

Electrochemical Characterization of Single Layer Graphene/Electrolyte Interface: Effect of Solvent on the Interfacial Capacitance.

The development of the basic understanding of the charge storage mechanisms in electrodes for energy storage applications needs deep characterization of the electrode/electrolyte interface. In this work, we studied the charge of the double layer capacitance at single layer graphene (SLG) electrode used as a model material, in neat (EMIm-TFSI) and solvated (with acetonitrile) ionic liquid electrodes. The combination of electrochemical impedance spectroscopy and gravimetric electrochemical quartz crystal microbalance (EQCM) measurements evidence that the presence of solvent drastically increases the charge carrier density at the SLG/ionic liquid interface. The capacitance is thus governed not only by the electronic properties of the graphene, but also by the specific organization of the electrolyte side at the SLG surface originating from the strong interactions existing between the EMIm+ cations and SLG surface. EQCM measurements also show that the carbon structure, with the presence of sp2 carbons, affects the charge storage mechanism by favoring counter-ion adsorption on SLG electrode versus ion exchange mechanism in amorphous porous carbons.

Nanoporous carbon for electrochemical capacitive energy storage.

The urgent need for efficient energy storage devices has stimulated a great deal of research on electrochemical double layer capacitors (EDLCs). This review aims at summarizing the recent progress in nanoporous carbons, as the most commonly used EDLC electrode materials in the field of capacitive energy storage, from the viewpoint of materials science and characterization techniques. We discuss the key advances in the fundamental understanding of the charge storage mechanism in nanoporous carbon-based electrodes, including the double layer formation in confined nanopores. Special attention will be also paid to the important development of advanced in situ analytical techniques as well as theoretical studies to better understand the carbon pore structure, electrolyte ion environment and ion fluxes in these confined pores. We also highlight the recent progress in advanced electrolytes for EDLCs. The better understanding of the charge storage mechanism of nanoporous carbon-based electrodes and the rational design of electrolytes should shed light on developing the next-generation of EDLCs.

Charge Storage Mechanisms of Single-Layer Graphene in Ionic Liquid.

Graphene-based carbon materials are promising candidates for electrical double-layer (EDL) capacitors, and there is considerable interest in understanding the structure and properties of the graphene/electrolyte interface. Here, electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM) are used to characterize the ion fluxes and adsorption on single-layer graphene in neat ionic liquid (EMI-TFSI) electrolyte. It is found that a positively charged ion-species desorption and ion reorganization dominate the double-layer charging during positive and negative polarizations, respectively, leading to the increase in EDL capacitance with applied potential.

Ultrafast Synthesis of Calcium Vanadate for Superior Aqueous Calcium-Ion Battery.

Recently, multivalent aqueous calcium-ion batteries (CIBs) have attracted considerable attention as a possible alternative to Li-ion batteries. However, traditional Ca-ion storage materials show either limited rate capabilities and poor cycle life or insufficient specific capacity. Here, we tackle these limitations by exploring materials having a large interlayer distance to achieve decent specific capacities and one-dimensional architecture with adequate Ca-ion passages that enable rapid reversible (de)intercalation processes. In this work, we report the high-yield, rapid, and low-cost synthesis of 1D metal oxides MV3O8 (M = Li, K), CaV2O6, and CaV6O16·7H2O (CVO) via a molten salt method. Firstly, using 1D CVO as electrode materials, we show high capacity 205 mA h g-1, long cycle life (>97% capacity retention after 200 cycles at 3.0 C), and high-rate performance (117 mA h g-1 at 12 C) for Ca-ion (de)intercalation. This work represents a step forward for the development of the molten salt method to synthesize nanomaterials and to help pave the way for the future growth of Ca-ion batteries.

ZnO nanoparticle based highly efficient CdS/CdSe quantum dot-sensitized solar cells.

20 nm ZnO nanoparticles are used to fabricate the mesoporous photoanode of the CdS/CdSe quantum dot-sensitized solar cells by the simple doctor blade method. A maximum power conversion efficiency of 4.46% has been achieved, which indicated exciting prospects for ZnO nanoparticle based quantum dot-sensitized solar cells.