Influence of the Pore Shape and Size Distribution in Hierarchically Porous Electrodes on Energy and Power Densities of Electrochemical Devices

Abstract

The microporous-mesoporous structure of nanoporous carbon and modified carbon electrodes has a strong influence on the electrochemical characteristics of the devices like supercapacitors, polymer electrolyte fuel cells and electrolysers, gas storage devices, as well as of Li-ion and Na-ion batteries. For that reason, it is crucial to have a good understanding of the porosity and hierarchical structure of carbon and modified carbon materials used [1]. As a standard method, the nitrogen adsorption measurements and various analysis theories are used. These models need the pore shape as an input parameter for detailed calculations, however, often the slit-shaped pore model is postulated without additional verification [1]. This assumption is also very often transferred to the behaviour of carbon based materials in different applications leading to some serious misinterpretations. For cost-effective hydrogen and methane based economy, efficient methods for hydrogen and methane storage are urgently needed. To improve the gas storage properties, the fundamental understanding of the confinement of the gas in microporous-mesoporous materials is inevitable. In particular, the role of confinement dimensions in the mass transfer of the hydrogen, methane as well as oxygen molecules has huge impact on the fuel cell characteristics. Our research focuses on detailed estimation of pore shape and size in various carbide-derived carbons and other carbon materials using various standardized and novel methods, like nitrogen, hydrogen and carbon dioxide adsorption measurements. All carbon samples were dried under vacuum at 350 оC at least for 24 h prior to each measurement. N2 sorption analysis at -196 оC 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 [2,3]. 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” [4,5] (demo software 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) [6,7], the results generally support the findings from N2 sorption isotherms. Raman spectra analysis, small-angle neutrons scattering and small angle X-ray scattering combined with the FIB-SEM and high resolution TEM data have been used. The results obtained are compared to the behaviour of these materials in different energy storage/conversion related devices like supercapacitors [8,9] and polymer electrolyte fuel cells 10]. As a result, the importance of hierarchical porous structure and pore shape along with pore size (pore size distribution) of used carbon materials in the energy technology applications will be addressed and demonstrated. The hydrogen sorption parameters will be compared with the methane sorption characteristics as well as correlated with PEM fuel cell and supercapacitor characteristics. Acknowledgments Authors would like to thank HZB and PSI for the allocation of neutron radiation beamtime on instruments V16 and FOCUS, respectively. 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) for financial support.