Influence of Hierarchical Porosity of Electrodes on the Energy and Power Densities of Electrochemical Devices
Authors: Lust Enn , Janes Alar , Nerut Jaak , Thomberg Thomas , Monerjan Eneli , Jaeger Rutha , Oll Ove , Romann Tavo , Koppel Miriam , Kalder Laura , Ers Heigo , Paalo Maarja , Härmas Riinu , Härmas Meelis , Siinor Liis , Grozovski Vitali , Väärtnõu Mart , Pikma Piret , Lust Karmen , Valk Peeter , Möller Egert , Tuul Kenneth , Kanarbik Rait 
Published: December 2024

Influence of specific micropore-, mesopore and total surface areas and pore volumes and other porosity characteristics on the electrochemical parameters and power density of electrochemical devices will be discussed [1-3]. Many ionic liquids (EtMeImBF4, Et4B(CN4)4, etc.) and organic (Et3MeNBF4, Et4NBF4, Et3PrNBF4, etc.) and inorganic salts (LiBF4, NaBF4, LiClO4, NaClO4, LiPF6, NaPF6, Rb2SO4, Li2SO4, Na2SO4, LiI, NaI, NaBr, LiBr, LiCl, NaCl, CsBF4, Cs carborane, etc.) based electrolytes in various solvents (H2O, acetonitrile, ethyl methyl carbonate, propylene carbonate, diethyl carbonate, acetone) and their mixtures with specific chemical additions (fluoroethyl carbonate, propylene sulfite, diethyl sulfite) will be discussed. 1M 3-ethyl-methyl-ammonium tetrafluoroborate (Et3MeNBF4) solution in acetonitrile and on 1-ethyl-3-methylimidazolium tetrafluoroborate (Et3MeImBF4) will be analyzed for all carbon materials as reference data.

The pore size distribution calculated from N2, CO2 and Ar adsorption isotherms using mainly Carbon 2D non-local density functional theory for heterogeneous surface (2D-NLDFT-HS) model will be compared with crystallographic characteristics obtained by Raman, X-ray diffraction, focused ion beam scanning electron microscopy (FIB-SEM), high-resolution transmission electron microscopy, (HR-TEM) combined with low electron energy diffraction (EELS) and selected area electron diffraction (SAED) methods. Small angle and wide-angle X-ray scattering (SAXS and WAXS), small-angel neutron diffraction (SANS), quasielastic (QENS) and inelastic neutron scattering (INES) etc. data will be analyzed. Electrochemical temperature dependent XRD will be used for analysis of influence of temperature and electrode potential on the structural parameters at various temperatures and electrode potentials.

For surface analysis, electrochemical oxidation and reduction processes at electrode | vacuum and electrode | ionic liquid interfaces have been studied using synchrotron radiation photoelectron spectroscopy (Sr-XPS) and near ambient pressure dual chamber synchrotron beam XPS (Sr-NAP-DC-XPS) methods.

Post–mortem analysis of electrodes applied under high temperature and high electrode overpotential conditions was conducted using FIB-TOF-SIMS (secondary ion mass spectroscopy), FIB-SEM –EDX and thermogravimetry methods to determine the effect of electrode polarization and temperature on the intercalation layer formation kinetics (mass-transfer depths) for electrodes with different micro-meso-macro porosity. For selected flatter electrodes, the in situ STM and AFM measurements combined with Raman spectroscopy surface analysis have been used. Surface structures of adsorbed and formed interfacial layers will be simulated by computation analysis using different DFT, molecular dynamic etc. methods.

Based on systematic analysis on results collected, it will be demonstrated that the chemical composition and crystallographic structure of precursor material, chemical composition of activators, synthesis and activation conditions have decisive influence on the shape of the pores (spherical, cylindrical or slit-shape pores, mixed cylindrical and slit-shape, etc.) and on the hierarchically porous structure and electrical conductivity of carbon, activated carbon and complex oxide materials. For carbon materials with mixed cylindrical-slit shape hierarchical porosity, very high series and parallel capacitances from 130 to 168 F g-1 in 1M Et3MeNBF4+ acetonitrile and up to 155 F g-1 in EtMeImBF4 ionic liquid have been established. Very high gravimetric power density (up to 150kW kg-
1) has been obtained for optimized EDLCs with sol-gel method fabricated electrodes and moderate gravimetric and volumetric energy densities for d-glucose and Estonian peat derived carbons based electrodes [1-4]. The very short characteristic charging/discharging times (lower than 0.3 s for Et3MeNBF4 +AN and 1.0 s for EtMeImBF4) can be achieved for optimized EDLCs. Complicated adsorption kinetics of halide ions at micro-mesoporous carbon electrodes has been observed and explained by the strong chemical adsorption of halides. Very strong influence of hierarchical porosity and micropores inner surface roughness on the hydrogen adsorption parameters has been established by SANS/SAXS methods [5,6]. Strong influence of preparation conditions of Pt-free electro-catalysts for polymer membrane and alkaline fuel cells and electrolysis cells will be demonstrated.

Acknowledgements. This work was supported by the European Regional Development Fund (TK141), Estonian Ministry of Education and Research (TK210), Personal Research Grant PRG676, by the project „Increasing the knowledge intensity of Ida-Viru entrepreneurship” co-funded by the European Union (ÕÜF12, ÕÜF1).

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