Browsing by Person "Zimmermann, Michael"

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Activated carbon from corncobs doped with RuO2 as biobased electrode material
(2021) Hoffmann, Viola; Rodriguez Correa, Catalina; Sachs, Saskia; del Pilar Sandoval-Rojas, Andrea; Qiao, Mo; Brown, Avery B.; Zimmermann, Michael; Rodriguez Estupiñan, Jenny Paola; Cortes, Maria Teresa; Moreno Pirajan, Juan Manuel Carlos; Titirici, Maria-Magdalena; Kruse, Andrea
Bio-based activated carbons with very high specific surface area of >3.000 m² g−1 (based on CO2 adsorption isotherms) and a high proportion of micropores (87% of total SSA) are produced by corncobs via pyrolysis and chemical activation with KOH. The activated carbon is further doped with different proportions of the highly pseudocapacitive transition metal oxide RuO2 to obtain enhanced electrochemical properties and tune the materials for the application in electrochemical double-layer capacitors (EDLC) (supercapacitors). The activated carbon and composites are extensively studied regarding their physico-chemical and electrochemical properties. The results show that the composite containing 40 wt.% RuO2 has an electric conductivity of 408 S m−1 and a specific capacitance of 360 Fg−1. SEM-EDX, XPS, and XRD analysis confirm the homogenous distribution of partly crystalline RuO2 particles on the carbon surface, which leads to a biobased composite material with enhanced electrochemical properties.
Hydrothermal carbonization of fructose—effect of salts and reactor stirring on the growth and formation of carbon spheres
(2021) Jung, Dennis; Duman, Gözde; Zimmermann, Michael; Kruse, Andrea; Yanik, Jale
Hydrothermal carbonization (HTC) has become a promising technology for the production of hydrochar and carbon spheres. Several studies indicate a strong dependency of the reaction conditions on the sphere diameter. The usage of additives, such as salts, is one possibility to increase the size of the spheres. However, the growth mechanism which leads to larger particles is not fully understood. In this work, kinetic studies of HTC with fructose were performed with different salts as additives. The growth of the particles (the increase in size) has been compared to the formation rates (increase in yield) of hydrochar by using the reaction rate constants from the kinetic model. The results indicate that the acceleration of the growth rate is independent of the formation rate. It is therefore assumed that coagulation, as a growth mechanism, took place. With longer reaction times, the particles reached a stable particle size, independently from the added salts; therefore, it was assumed that the particles underwent some sort of solidification. The state of matter can therefore be described as an intermediate state between liquid and solid, similar to mesophase pitch. Experiments with a stirrer resulted in squashed particles, which supports the model, that the particles exhibit emulsion-like behavior.