The Nanoporous RP-20 Carbon Electrode As a Model for Energy Storage and Conversion Systems – Studied with µ CT, SAXS and SANS Methods
Authors: Härk Eneli , Kardjilov Nikolay , Hilger André , Jafta Charl , Petzold Albrecht , Risse Sebastian , Vaas Ingrid , Goerigk Günther , Ballauff Matthias , Lust Enn 
Published: February 2020

There is no doubt that the electrochemical energy storage and conversion devices are the key components in future’s smart energy grids enabling on-demand and efficient energy usage. During the last thirty years, various carbon based materials have been studied and used in the energy storage and conversion devices: supercapacitors, fuel cells and batteries. Despite the efforts made so far, there is still a strong need for better understanding of processes related to the mass transport properties of electrolyte ions inside of the porous electrode materials driven by their morphology [1,2]. It is obvious that electrochemical performance (durability/efficiency) of such a device depends on the rate of the charging and discharging processes inside the electrodes that in turn are strongly influenced by the properties of the carbon material and electrolyte used (i.e., the specific surface area, pore shape, size/volume, wettability, ionophilicity/ionophobicity, dielectrical permittivity and polarizability, ect.) [3,4]. It has been recognized that different carbon materials exhibit different properties such as the surface area, hierarchical porosity, electrical and ionic conductivity, water uptake rate as well as the electrocatalytic activity [5,6].
The scientific goal of this study is in depth understanding of the fundamental properties of the carbon materials and their functionality at the molecular and atomic level under non-polarized conditions. For this purpose the nanoporous carbon RP-20 [7] as a main electrode component has been used and correlation between morphology (i.e., pore shape, size of the pores) and instance wetting (filling density) of the carbon material matrix with the solvent was determined. Keeping in mind real application requirements, the primarily used 1M NaClO4 solution in dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1 wt%) solvent mixture as an electrolyte [3] has been studied. Several independent methods – x-ray microcomputed tomography (µCT) [8], small-angle neutron scattering (SANS) [9], and small-angle x-ray scattering (SAXS) [10-12] were applied for characterization of the following systems: RP-20 1MNaClO4 in EC:DMC; RP-20 in EC:DMC or RP-20 1MNaClO4 in D2O. It is rather difficult to simultaneously determine/distinguish the structure or dynamic effects on the efficiency and durability of the electrochemical devices (i.e., inhomogeniously/randomly distributed pores filled with electrolyte contributing during the charge and discharge cycling processes), therefore the above listed methods have contributed to an improved awareness of the nature of complex systems in steady state (non-polarized) conditions. Based on the µCT measurements, the linear attenuation coefficient values and filling densities were obtained and calculated. To determine the shape and size of the pores for nanoporous RP-20 electrode and to observe the influence/interaction of the electrolyte ions dissolved in aqueous and non-aqueous solvent (i.e., differences in filling of the pores) SANS method was applied. SAXS measurements were performed in order to collect quantitative morphological and structural information for nanoporous RP-20 powder and RP-20 electrode material (e.g., surface area, pore size, pore structure, binder effect).

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Acknowledgments

The Authors would like to thank HZB for the allocation of neutron radiation beamtime on instrument V4. This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under the NMI3-II Grant number 283883, 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 TK117 and, TK141).