Activated carbons as emerging classes of porous materials have gained tremendous attention because of their versatile applications such as gas storage/separations sorbents, oxygen reduction reaction (ORR) catalysts and supercapacitor electrodes. This diversity originates from fascinating features such as low-cost, lightweight, thermal, chemical and physical stability as well as adjustable textural properties. More interestingly, sole heteroatom or combinations of various elements can be doped into their framework to modify the surface chemistry. Among all dopants, nitrogen as the most frequently used element, induces basicity and charge delocalization into the carbon network and enhances selective adsorption of CO2. Transformation of a task-specific and single source precursor to heteroatom-doped carbon through a one-step activation process is considered a novel and efficient strategy. With these considerations in mind, we developed multiple series of heteroatom doped porous carbons by using nitrogen containing carbon precursors. Benzimidazole-linked polymers (BILP-5), benzimidazole monomer (BI) and azo-linked polymers (ALP-6) were successfully transformed into heteroatom-doped carbons through chemical activation by potassium hydroxide. Alternative activation by zinc chloride and direct heating was also applied to ALP-6. The controlled activation/carbonization process afforded diverse textural properties, adjustable heteroatom doping levels and remarkable gas sorption properties. Nitrogen isotherms at 77 K revealed that micropores dominate the porous structure of carbons. The highest Brunauer-Emett-Teller (BET) surface area (4171 m2 g-1) and pore volume (2.3 cm3 g-1) were obtained for carbon synthesized by KOH activation of BI at 700 °C. In light of the synergistic effect of basic heteroatoms and fine micropores, all carbons exhibit remarkable gas capture and selectivity. Particularly, BI and BIPL-5 derived carbons feature unprecedented CO2 uptakes of 6.2 mmol g-1 (1 bar) and 2.1 mmol g-1 (0.15 bar) at 298 K, respectively. The ALP-6 derived carbons retained considerable amount of nitrogen dopants (up to 14.4 wt%) after heat treatment owing to the presence of more stable nitrogen-nitrogen bonds compared to nitrogen-carbon bonds in BILP-5 and BI precursors. Subsequently, the highest selectivity of 62 for CO2/N2 and 11 for CO2/CH4 were obtained at 298 K for a carbon prepared by KOH activation of ALP-6 at 500 °C.

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The underlying motivation for the work carried out for this thesis was the growing need for more sustainable technologies. The aim was to synthesize a “palette” of functional nanomaterials using the established technique of hydrothermal carbonization (HTC). The incredible diversity of HTC was demonstrated together with small but steady advances in how HTC can be manipulated to tailor material properties for specific applications. Two main strategies were used to modify the materials obtained by HTC of glucose, a model precursor representing biomass. The first approach was the introduction of heteroatoms, or “doping” of the carbon framework. Sulfur was for the first time introduced as a dopant in hydrothermal carbon. The synthesis of sulfur and sulfur/nitrogen doped microspheres was presented whereby it was shown that the binding state of sulfur could be influenced by varying the type of sulfur source. Pyrolysis may additionally be used to tune the heteroatom binding states which move to more stable motifs with increasing pyrolysis temperature. Importantly, the presence of aromatic binding states in the as synthesized hydrothermal carbon allows for higher heteroatom retention levels after pyrolysis and hence more efficient use of dopant sources. In this regard, HTC may be considered as an “intermediate” step in the formation of conductive heteroatom doped carbon. To assess the novel hydrothermal carbons in terms of their potential for electrochemical applications, materials with defined nano-architectures and high surface areas were synthesized via templated, as well as template-free routes. Sulfur and/or nitrogen doped carbon hollow spheres (CHS) were synthesized using a polystyrene hard templating approach and doped carbon aerogels (CA) were synthesized using either the albumin directed or borax-mediated hydrothermal carbonization of glucose. Electrochemical testing showed that S/N dual doped CHS and aerogels derived via the albumin approach exhibited superior catalytic performance compared to solely nitrogen or sulfur doped counterparts in the oxygen reduction reaction (ORR) relevant to fuel cells. Using the borax mediated aerogel formation, nitrogen content and surface area could be tuned and a carbon aerogel was engineered to maximize electrochemical performance. The obtained sample exhibited drastically improved current densities compared to a platinum catalyst (but lower onset potential), as well as excellent long term stability. In the second approach HTC was carried out at elevated temperatures (550 °C) and pressure (50 bar), corresponding to the superheated vapor regime (htHTC). It was demonstrated that the carbon materials obtained via htHTC are distinct from those obtained via ltHTC and subsequent pyrolysis at 550 °C. No difference in htHTC-derived material properties could be observed between pentoses and hexoses. The material obtained from a polysaccharide exhibited a slightly lower degree of carbonization but was otherwise similar to the monosaccharide derived samples. It was shown that in addition to thermally induced carbonization at 550 °C, the SHV environment exhibits a catalytic effect on the carbonization process. The resulting materials are chemically inert (i.e. they contain a negligible amount of reactive functional groups) and possess low surface area and electronic conductivity which distinguishes them from carbon obtained from pyrolysis. Compared to the materials presented in the previous chapters on chemical modifications of hydrothermal carbon, this makes them ill-suited candidates for electronic applications like lithium ion batteries or electrocatalysts. However, htHTC derived materials could be interesting for applications that require chemical inertness but do not require specific electronic properties. The final section of this thesis therefore revisited the latex hard templating approach to synthesize carbon hollow spheres using htHTC. However, by using htHTC it was possible to carry out template removal in situ because the second heating step at 550 °C was above the polystyrene latex decomposition temperature. Preliminary tests showed that the CHS could be dispersed in an aqueous polystyrene latex without monomer penetrating into the hollow sphere voids. This leaves the stagnant air inside the CHS intact which in turn is promising for their application in heat and sound insulating coatings. Overall the work carried out in this thesis represents a noteworthy development in demonstrating the great potential of sustainable carbon materials. / Das Ziel der vorgelegten Arbeit war es, mit Hilfe der Hydrothermalen Carbonisierung (HTC) eine Palette an verschiedenen Materialien herzustellen, deren physikalische und chemische Eigenschaften auf spezifische Anwendungen zugeschnitten werden können. Die Motivation hierfür stellt die Notwendigkeit, Alternativen zu Materialien zu finden, die auf fossilen Brennstoffen basieren. Dabei stellen vor allem nachhaltige Energien eine der größten Herausforderungen der Zukunft dar. HTC ist ein mildes, nachhaltiges Syntheseverfahren welches prinzipiell die Nutzung von biologischen Rohstoffen (z. B. landwirtschaftlichen Abfallprodukten) für die Herstellung von wertvollen, Kohlenstoff-basierten Materialien erlaubt. Es wurden zwei verschiedene Ansätze verwendet, um hydrothermalen Kohlenstoff zu modifizieren. Zum einen wurde HTC unter „normalen“ Bedingungen ausgeführt, d. h. bei 180 °C und einem Druck von etwa 10 bar. Der Zucker Glukose diente in allen Fällen als Kohlenstoff Vorläufer. Durch Zugabe von stickstoff und /oder schwefelhaltigen Additiven konnte dotierte Hydrothermalkohle hergestellt werden. Dotierte Kohlenstoffe sind bereits für ihre positiven Eigenschaften, wie verbesserte Leitfähigkeit oder erhöhte Stabilität, bekannt. Zusätzlich zu Stickstoff dotierter Hydrothermalkohle, die bereits von anderen Gruppen hergestellt werden konnte, wurde in dieser Arbeit zum ersten Mal Schwefel in Hydrothermalkohle eingebaut. Außerdem wurden verschiedene Ansätze verwendet, um Oberfläche und definierte Morphologie der dotierten Materialien zu erzeugen, welche wichtig für elektrochemische Anwendungen sind. Schwefel- und/oder stickstoffdotierte Kohlenstoff Nanohohlkugeln sowie Kohlenstoff Aerogele konnten hergestellt werden. Mit Hilfe von einem zusätzlichen Pyrolyseschritt (d. h. Erhitzen unter Schutzgas) konnte die Leitfähigkeit der Materialien hergestellt werden, die daraufhin als Nichtmetall-Katalysatoren für Wasserstoff-Brennstoffzellen getestet wurden. Im zweiten Ansatz wurde HTC unter extremen Bedingungen ausgeführt, d. h. bei 550 °C und einem Druck von ca. 50 bar, welches im Wasser Phasendiagram dem Bereich des Heißdampfes entspricht. Es konnte gezeigt werden, dass die so erhaltene Hydrothermalkohle ungewöhnliche Eigenschaften besitzt. So hat die Hochtemperatur-Hydrothermalkohle zwar einen hohen Kohlenstoffgehalt (mehr als 90 Massenprozent), enthält aber auch viele Wasserstoffatome und ist dadurch schlecht leitfähig. Da damit elektrochemische Anwendungen so gut wie ausgeschlossen sind, wurde die Hochtemperatur-Hydrothermalkohle für Anwendungen vorgesehen, welche chemische Stabilität aber keine Leitfähigkeit voraussetzen. So wurden beispielsweise Hochtemperatur-Kohlenstoff-Nanohohlkugeln synthetisiert, die großes Potential als schall- und wärmeisolierende Additive für Beschichtungen darstellen. Insgesamt konnten erfolgreich verschiedenste Materialien mit Hilfe von HTC hergestellt werden. Es ist zu erwarten, dass sie in Zukunft zu nachhaltigen Technologien und damit zu einem weiteren Schritt weg von fossilen Brennstoffen beitragen werden.