Abstract
This study investigates the production of biobased carbon materials from potato waste and its application in energy storage systems such as supercapacitors. Three different categories of carbons were produced: hydrochar (HC) from hydrothermal carbonization (HTC) at three different temperatures (200 °C, 220 °C, 240 °C) and two different duration times (two hours and five hours), pyrolyzed hydrochar (PHC) obtained via pyrolysis of the HTC chars at 600 °C and 900 °C for two hours and pyrochar from the pyrolysis of biomass at 600 °C and 900 °C for two hours. The carbon samples were analysed regarding their physico-chemical properties such as elemental composition, specific surface area, bulk density and surface functionalities as well as their electrochemical characteristics such as electric conductivity and specific capacity via cyclic voltammetry. N- and O-enriched carbon materials with promising specific surface areas of up to 330 m2 g−1 containing high shares of microporosity were produced. Electric conductivities of up to 203 S m−1 and specific capacities of up to 134 F g−1 were obtained. The presence of high contents of oxygen (4.9–13.5 wt.%) and nitrogen (3.4–4.0 wt.%) of PHCs is assumed to lead to considerable pseudocapacitive effects and favor the high specific capacities measured. These results lead to the conclusion that the potential of agricultural biomass can be exploited by using hydrothermal and thermochemical conversion technologies to create N- and O-rich carbon materials with tailored properties for the application in supercapacitors.
Highlights
IntroductionRising greenhouse gas emissions combined with the depletion of fossil fuels lead to increased pressure on innovation within energy storage systems to create a suitable alternative for combustion engines in the mobility sector.The electric double layer capacitor (supercapacitor) (EDLC) is regarded as a key technology in this context due to its enormous power density in combination with its large number of charging and discharging cycles and its broad temperature compatibility [1] and has already successfully established itself in some fields of application such as in the automotive industry (regenerative braking systems), in hybrid electric vehicles, and in consumer electronics [2,3].It is suitable for the application areas mentioned, because it has a significantly higher power density compared to batteries, its energy density is usually significantly lower.Depending on the design, up to 500,000 charging and discharging cycles with high charging and discharging energy densities can be achieved [4].Energies 2020, 13, 2406; doi:10.3390/en13092406 www.mdpi.com/journal/energiesThe energy storage mechanism in an EDLC takes place by electrostatic adsorption of electrolyte ions on the surface of an electrically conductive porous electrode material [1]
It can be stated that the carbon content in the carbon increases with temperatures as well as with reaction time
−1 carbon content and the best electrochemical performance of up to 134.15 F g. This relatively high capacity is explained by pseudocapacitive effects due to the high O-content in the carbon (8.9 wt.%)
Summary
Rising greenhouse gas emissions combined with the depletion of fossil fuels lead to increased pressure on innovation within energy storage systems to create a suitable alternative for combustion engines in the mobility sector.The electric double layer capacitor (supercapacitor) (EDLC) is regarded as a key technology in this context due to its enormous power density in combination with its large number of charging and discharging cycles and its broad temperature compatibility [1] and has already successfully established itself in some fields of application such as in the automotive industry (regenerative braking systems), in hybrid electric vehicles, and in consumer electronics [2,3].It is suitable for the application areas mentioned, because it has a significantly higher power density compared to batteries, its energy density is usually significantly lower.Depending on the design, up to 500,000 charging and discharging cycles with high charging and discharging energy densities can be achieved [4].Energies 2020, 13, 2406; doi:10.3390/en13092406 www.mdpi.com/journal/energiesThe energy storage mechanism in an EDLC takes place by electrostatic adsorption of electrolyte ions on the surface of an electrically conductive porous electrode material [1]. The electric double layer capacitor (supercapacitor) (EDLC) is regarded as a key technology in this context due to its enormous power density in combination with its large number of charging and discharging cycles and its broad temperature compatibility [1] and has already successfully established itself in some fields of application such as in the automotive industry (regenerative braking systems), in hybrid electric vehicles, and in consumer electronics [2,3] It is suitable for the application areas mentioned, because it has a significantly higher power density compared to batteries, its energy density is usually significantly lower. Electrode materials with very high specific surfaces can achieve very high capacities [5]
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