Influence of the Pore Shape and Size Distribution in Hierarchically Porous Electrodes on Energy and Power Densities of Electrochemical Devices
Authors: Lust Enn , Härmas Riinu , Härmas Meelis , Pohl Maarja , Väli Ronald , Thomberg Thomas , Tallo Indrek , Jänes Alar , Romann Tavo , Palm Rasmus , Kurig Heisi , Härk Eneli , Kanarbik Rait , Kruusma Jaanus 
Published: February 2020

The microporous-mesoporous hierarchical structure of nanostructural carbon and modified carbon materials has a strong influence on the electrochemical characteristics of the electrical energy conversion and storage devices [1-4]. To improve the mass transfer rate, the understanding of the accumulation mechanisms of the ions in microporous-mesoporous materials is inevitable having huge impact on the power and energy density characteristics of supercapacitors, Li- and Na-ion batteries as well as fuel cells. Therefore, it is crucial to have a good understanding of the graphitization level, porosity and hierarchical structure of carbon and modified carbon materials used [1-5]. Therefore, systematical analysis of the gas adsorption, Raman, X-ray diffraction, scanning electron microscopy, photoelectron spectroscopy (including synchrotron beam based method), high-resolution transmission electron microscopy (with EELS and SAED) and small-angle neutron scattering data for various carbon materials prepared by high-temperature chlorination, hydrothermal carbonization, etc. methods will be given. Comparison with small-angle X-ray scattering data [6] will be given.

The data obtained by gas-phase analysis methods will be compared with cyclic voltammetry, constant current charge/discharge, electrochemical impedance and constant power discharge data. Influence of electrolyte chemical composition, solvent characteristics (aqueous and non-aqueous) and ionic liquids with and without non-aqueous solvent additions will be discussed in combination with hierarchical porosity, graphitization level and particle size analysis data.

All carbon samples were very carefully reduced with hydrogen at 800 оC, thereafter dried under vacuum at 350 оC at least for 24 h priori each measurement. N2 sorption analysis at -196 оC, Ar at -186 oC and CO2 sorption analysis at 0 оC were carried out on an ASAP 2020 instrument (Micromeritics, USA). The experimental data were treated with classical models (BET and t-plot) as well as different NLDFT models available through 3Flex and SAIEUS (Micromeritics, USA) software [7,8]. The results for pore size distribution (PSD) calculations from N2 isotherms using a slit shape pore model and model “Carbon-N2, 2D-NLDFT Heterogeneous Surface” [9,10] (SAIEUS, Micromeritics, USA) will be reported. The pore size distribution data calculated from CO2 sorption isotherms using a slit shaped pore model and model “CO2-DFT Model” (software 3Flex, Micromeritics, USA) will be discussed. Although, the pore size distributions calculated from CO2 have high level of roughness and several model artefacts (e.g. around 0.7 nm) [11,12], the results generally support the findings from N2 sorption isotherms. In addition, Raman spectra analysis, small-angle neutrons scattering and small angle X-ray scattering combined with the FIB-SEM and HR-TEM data will be discussed. The results obtained will be compared to the behavior of these materials in different energy storage/conversion related devices like supercapacitors [1-6] and Li- and Na-ion battery negatively charged electrodes [4]. Thus, the importance of graphitization level, hierarchical porous structure and pore shape along with pore size distribution data for carbon materials energy and power densities will be addressed and demonstrated.

Acknowledgments

Authors would like to thank HZB and PSI for the allocation of neutron radiation beamtime on instruments V16 and FOCUS, respectively; and The Estonian Ministry of Education and Research (institutional research project IUT20-13, personal research grant PUT55) and European Regional Development Fund (The Centres of Excellence 3.2.0101–0030 and 2014-2020.4.01.15-0011) for financial support.

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