New Routes Towards Nanoporous Carbon Materials for Electrochemical Energy Storage and Gas Adsorption

The chemical element carbon plays a key role in the 21st century. “The new carbon age” is associated with the global warming due to increasing carbon dioxide emissions. The latter are a major consequence of the continued combustion of fossil fuels for energy generation. However, carbon is also one key component to overcome these problems. Especially porous carbon materials are highly attractive for many environmentally relevant applications. These materials provide high specific surface area, high pore volume, thermal/chemical stability, and high electrical conductivity. They are promising candidates for the removal of carbon dioxide or other environmentally relevant gases from exhaust gas mixtures. Furthermore, porous carbons are used in electrochemical energy storage devices (e.g. batteries or electrochemical capacitors). The performance of the materials in these applications depends on their pore structure. Hence, precise control over the pore size and the pore geometry is important to achieve. Besides a high specific surface area (SSA) and a well-defined pore size, pore accessibility must be ensured because the surface must be completely available. If the porous carbons exhibit ink-bottle pores, the high surface area is useless because the guest species do not reach the pore interior. Therefore, carbon materials with hierarchical pore structure are attractive. They combine at least two different pore systems of different size which contribute with their individual advantages. While smaller pores provide large specific surface area, larger pores ensure efficient mass transport. Numerous methods for the targeted synthesis of carbide-derived carbon materials (CDCs) with hierarchical pore architectures were developed within this thesis (Figure 1). CDCs are produced by the extraction of metal- or semi-metal atoms from carbide precursors leading to the formation of a microporous carbon network with high specific surface area. PolyHIPE-CDCs with porosity on three hierarchy levels and total pore volumes as high as 8.5 cm3/g were prepared by a high internal phase emulsion technique. CO2 activation increases the SSA to values above 3100 m2/g. These materials are promising for the filtration of non-polar organic compounds from gas mixtures. CDC nanospheres with diameters below 200 nm were obtained from polycarbosilane-based miniemulsions. They show high capacitance of up to 175 F/g in symmetrical EDLCs in 1 M H2SO4 aqueous electrolyte. Besides such emulsion techniques, the hard-templating concept (also referred to as nanocasting) was presented as an efficient approach for the synthesis of CDC mesofoam powders and meso-macroporous CDC monoliths starting from silica templates and polycarbosilane precursors. As a wide range of pore sizes is approachable, the resulting materials are highly versatile in terms of application. Due to their high nanopore volume, well-defined mesopores and large SSA, they show outstanding properties as electrode materials in EDLCs or in Li-S batteries as well as high and rapid uptake in gas adsorption processes. CDC aerogels were produced by pyrolysis and high-temperature chlorine treatment of cross-linked polycarbosilane aerogels. These materials can be tailored for efficient CO2 adsorption and show outstanding performance in EDLC electrodes at high current densities of up to 100 A/g due to the very short electron diffusion pathways within the aerogel-type pore system. It was further shown that CDCs can be combined with mesopores by the sacrificial template method starting from PMMA particles as the pore-forming material. The use of highly toxic hydrofluoric acid for template removal and large amounts of organic solvents as typical for hard- and soft-templating approaches can be overcome. SSAs and total pore volumes of 2434 m2/g and 2.64 cm3/g are achieved ensuring good performance of PMMA-CDCs in Li-S batteries cathodes. Besides the characterization of CDCs in real energy storage devices and adsorption processes, their use as model substances in energy- and environmentally relevant applications was part of this thesis. The questions “How does it work?” and “What do we need?” must be clearly answered before any material can be tailored under the consideration of economic and ecological perspectives. The high potential of CDCs for this purpose was shown in this thesis. These carbons were used as model substances in combination with nuclear magnetic resonance (NMR) techniques to get a detailed understanding of the adsorption processes on porous carbon surfaces. However, such investigations require the use of model substances with a tailored and well-defined pore structure to clearly differentiate physical states of adsorbed species and to understand fundamental mechanisms. The characterization of the interaction of electrolyte molecules with the carbon surface was performed with solid-state NMR experiments. The materials were also studied in the high-pressure adsorption of 129Xe using an in-situ NMR technique. Both NMR studies enable the analysis of ions or gas atoms adsorbed on the carbon surface on an atomic level and experimentally demonstrate different strength of interaction with pores of variable size and connectivity. In addition, the novel InfraSORP technology was used for the investigation of the thermal response of CDCs and templated carbon and carbide materials during n-butane adsorption. These model systems lead to a more profound understanding of this technique for the rapid characterization of porous materials. The Kroll-Carbon (KC) concept is a highly attractive alternative for the synthesis of well-defined carbons on the large scale. In this technique, the porous materials are produced by the reductive carbochlorination reaction between oxidic nanoparticles and a surrounding carbon matrix. First KC materials were produced with high SSA close to 2000 m2/g and total pore volumes exceeding 3 cm3/g. This method was established with template particles of various dimensions as well as by using various types of oxides (silica, alumina, titania). Hence, porous carbon materials with various textural parameters are approachable. The first generation of KCs is promising for the use in Li-S battery cathodes and as electrode materials in EDLCs

Ion Bridging by Carbon Dioxide Facilitates Electrochemical Energy Storage at Charged Carbon–Ionic–Liquid Interfaces

Abstract Solvent free ionic liquid (IL) electrolytes facilitate high‐voltage supercapacitors with enhanced energy density, but their complex ion arrangement and through that the electrochemical properties, are limited by strong Coulombic ordering in the bulk state and like‐charged ion repulsion at electrified interfaces. Herein, a unique interfacial phenomenon resulting from the presence of carbon dioxide loaded in 1‐Ethyl‐3‐methylimidazoliumtetrafluorborate electrolyte that simultaneously couples to IL ions and nitrogen‐doped carbonaceous electrode is reported. The adsorbed CO 2 molecule polarizes and mitigates the electrostatic repulsion among like‐charged ions near the electrified interface, leading to an ion “bridge effect” with increased interfacial ionic density and significantly enhanced charge storage capability. The unpolarized CO 2 possessing a large quadrupole moment further reduces ion coupling, resulting in higher conductivity of the bulk IL and improved rate capability of the supercapacitor. This work demonstrates polarization‐controlled like‐charge attraction at IL–electrode–gas three‐phase boundaries, providing insights into manipulating complex interfacial ion ordering with small polar molecule mediators.Solvent‐free ionic liquid (IL) electrolytes enable high‐voltage and high‐energy‐density applications. The proposed bridge effect with CO 2 loaded in an IL electrolyte acting as a mediator on the nitrogen‐doped electrode surface, alleviates the undesired Coulombic ordering and interfacial like‐charged ion repulsion. By breaking the limitation of interfacial ion packing density, this effect results in a significant enhancement of charge storage capability. imag

Application of Thermal Response Measurements to Investigate Enhanced Water Adsorption Kinetics in Ball-Milled C2N-Type Materials

Sorption-based water capture is an attractive solution to provide potable water in arid regions. Heteroatom-decorated microporous carbons with hydrophilic character are promising candidates for water adsorption at low humidity, but the strong affinity between the polar carbon pore walls and water molecules can hinder the water transport within the narrow pore system. To reduce the limitations of mass transfer, C2N-type carbon materials obtained from the thermal condensation of a molecular hexaazatriphenylene-hexacarbonitrile (HAT-CN) precursor were treated mechanochemically via ball milling. Scanning electron microscopy as well as static light scattering reveal that large pristine C2N-type particles were split up to a smaller size after ball milling, thus increasing the pore accessibility which consequently leads to faster occupation of the water vapor adsorption sites. The major aim of this work is to demonstrate the applicability of thermal response measurements to track these enhanced kinetics of water adsorption. The adsorption rate constant of a C2N material condensed at 700 °C remarkably increased from 0.026 s−1 to 0.036 s−1 upon ball milling, while maintaining remarkably high water vapor capacity. This work confirms the advantages of small particle sizes in ultramicroporous materials on their vapor adsorption kinetics. It is demonstrated that thermal response measurements are a valuable and time-saving method to investigate water adsorption kinetics, capacities, and cycling stability

Textural Characterization of Micro- and Mesoporous Carbons Using Combined Gas Adsorption and n-Nonane Preadsorption

Porous carbon and carbide materials with different structures were characterized using adsorption of nitrogen at 77.4 K before and after preadsorption of n-nonane. The selective blocking of the microporosity with n-nonane shows that ordered mesoporous silicon carbide material (OM-SiC) is almost exclusively mesoporous whereas the ordered mesoporous carbon CMK-3 contains a significant amount of micropores (25%). The insertion of micropores into OM-SiC using selective extraction of silicon by hot chlorine gas leads to the formation of ordered mesoporous carbide-derived carbon (OM-CDC) with a hierarchical pore structure and significantly higher micropore volume as compared to CMK-3, whereas a CDC material from a nonporous precursor is exclusively microporous. Volumes of narrow micropores, calculated by adsorption of carbon dioxide at 273 K, are in linear correlation with the volumes blocked by n-nonane. Argon adsorption measurements at 87.3 K allow for precise and reliable calculation of the pore size distribution of the materials using density functional theory (DFT) methods

Improving the Stability of Supercapacitors at High Voltages and High Temperatures by the Implementation of Ethyl Isopropyl Sulfone as Electrolyte Solvent

Abstract Two of the main weaknesses of modern electric double‐layer capacitors are the rather limited ranges of operating voltage and temperature in which these devices do not suffer from the occurrence of irreversible decomposition processes. These parameters are strongly interconnected and lowering the operating voltage when increasing the temperature is unavoidable, so as to protect the electric double‐layer capacitor from damage. With the aim to maintain the operating voltage as high as possible at elevated temperatures, in this study, the application of ethyl isopropyl sulfone as an electrolyte solvent for electric double‐layer capacitors is presented. It is shown that ethyl isopropyl sulfone‐based electrolytes display excellent thermal and electrochemical stability enabling high capacitance retention after floating tests for 500 h at 60 and 80 °C, e.g. 68% at 3.4 V at 60 °C. A possible reason for the above‐average stability is that decomposition products of ethyl isopropyl sulfone can deposit on the electrode surface which may act as a passivation layer and prevent further degradation.Ethyl isopropyl sulfone is an interesting electrolyte solvent for electric double‐layer capacitors in high‐temperature surroundings. Forming a protective passive layer on the electrode surface, ethyl isopropyl sulfone offers high thermal and electrochemical stability for supercapacitors at temperatures of up to 80 °C and voltages of up to 3.4 V. imag

Hydrophilic non-precious metal nitrogen-doped carbon electrocatalysts for enhanced efficiency in oxygen reduction reaction

Exploring the role of surface hydrophilicity of non-precious metal N-doped carbon electrocatalysts in electrocatalysis is challenging. Herein we discover an ultra-hydrophilic non-precious carbon electrocatalyst, showing enhanced catalysis efficiency on both gravimetric and areal basis for oxygen reduction reaction due to a high dispersion of active centres

Hydrophilic non-precious metal nitrogen-doped carbon electrocatalysts for enhanced efficiency in oxygen reduction reaction

Exploring the role of surface hydrophilicity of non-precious metal N-doped carbon electrocatalysts in electrocatalysis is challenging. Herein we discover an ultra-hydrophilic non-precious carbon electrocatalyst, showing enhanced catalysis efficiency on both gravimetric and areal basis for oxygen reduction reaction due to a high dispersion of active centres

Effects of Carbon Pore Size on the Contribution of Ionic Liquid Electrolyte Phase Transitions to Energy Storage in Supercapacitors

Recent research on ionic liquid electrolyte-based supercapacitors indicated the contribution of phase transitions of the electrolytes at high cell voltages to the energy stored. This mechanism can be exploited to significantly increase the energy density of supercapacitors, which up to now remains their major drawback. It was found that these ordering transitions require the presence of mesopores within the carbon electrode materials and that porosity in general is a key factor to trigger them, but details of the mechanism remains unexplained. To get a more profound understanding of this phenomenon, carbon materials with different pore diameters and volumes were synthesized and the effect of those properties on the phase transitions in the ionic liquids was studied by means of cyclic voltammetry. A clear correlation between the peak current and the mesopore volume is revealed and an optimal pore diameter was determined, exceeding which does not improve the phase transition behavior. These findings are useful as guidelines for the rational design of carbon mesopores in order to utilize the new energy storage modes which are neither fully capacitive, nor redox-based

Micro‐blooming: hierarchically porous nitrogen‐doped carbon flowers derived from metal‐organic mesocrystals

Synthesis of 3D flower‐like zinc‐nitrilotriacetic acid (ZnNTA) mesocrystals and their conformal transformation to hierarchically porous N‐doped carbon superstructures is reported. During the solvothermal reaction, 2D nanosheet primary building blocks undergo oriented attachment and mesoscale assembly forming stacked layers. The secondary nucleation and growth preferentially occurs at the edges and defects of the layers, leading to formation of 3D flower‐like mesocrystals comprised of interconnected 2D micropetals. By simply varying the pyrolysis temperature (550–1000 °C) and the removal method of in the situ‐generated Zn species, nonporous parent mesocrystals are transformed to hierarchically porous carbon flowers with controllable surface area (970–1605 m2 g−1), nitrogen content (3.4–14.1 at%), pore volume (0.95–2.19 cm3 g−1), as well as pore diameter and structures. The carbon flowers prepared at 550 °C show high CO2/N2 selectivity due to the high nitrogen content and the large fraction of (ultra)micropores, which can greatly increase the CO2 affinity. The results show that the physicochemical properties of carbons are highly dependent on the thermal transformation and associated pore formation process, rather than directly inherited from parent precursors. The present strategy demonstrates metal‐organic mesocrystals as a facile and versatile means toward 3D hierarchical carbon superstructures that are attractive for a number of potential applications