Recycled Energy Storage Materials for the Supercapacitors from Waste Coffee Sludge
Authors: Kang Jihyeon , Kwak Soyul , Jang Seohyeon , Kwon Ohhyun , Choi Seyoung , Eom Hojong , Nam Inho 
Published: December 2020

Coffee is one of the largest agricultural products generally used in beverages. The international production of coffee is approximately 120 billion bags per year (60 kg per bag) from the International Coffee Organization. It corresponds to an annual production of approximately 8 million metric tons of coffee; therefore, coffee can be considered as a significant agricultural commodity. However, the majority of the produced coffee is disposed of waste sludge by beverage manufacturers. Herein, we report the use of graphitic porous carbon materials that have been derived from waste coffee sludge for developing an energy storage electrode based on a hydrothermal recycling procedure. Waste coffee sludge is used as a carbonaceous precursor for energy storage due to its lower cost, greater abundance, and easier availability as compared to other carbon resources. In addition, excluding the oil components, coffee sludge is mainly composed of cellulose-based materials with many heteroatoms (e.g., nitrogen, oxygen, and sulfur). It is considered good precursors for the fabrication of functionalized carbon materials. The unique graphite porous carbon production by hydrothermal carbonization of coffee sludge is particularly attractive as it addresses waste handling issues, offers a cheaper recycling method, and reduces the requirement for landfills. Therefore, we recycled the coffee slurry into hydrated carbon via sequential hydrothermal carbonization (HTC) as an activation process. The HTC reaction converts the cellulose-based biomass into hydrochar, bio-liquid, and gas at a relatively low temperature range (180°C~230°C). Hydrochar has various surface functional groups; so, it has been investigated for alternative fuels, carbon dioxide sequestration, and heavy metal immobilization for environmental remediation. During sequential hydrothermal activation, a porous and graphitic structure of the hydrochar can be developed, where these features are the most important characteristics of materials for electron accumulation. The porous and graphitic property of carbonaceous materials promotes electron accumulation for capacitive reactions. The electrical double-layer capacitive (EDLC) reaction has attracted considerable interest for next-generation energy storage systems such as supercapacitors. Our investigations revealed that the graphitic porous carbon, which is derived from spent coffee sludge, was employed in EDLCs by manipulating its unique microstructure which has a high hierarchical porous nature and graphitic edges. To activate the synthesized material, the dried hydrochar was mixed with 1 M KOH at a mass ratio of 1:3.5. The mixed sample was also heated at 700°C for 2 h in the hydrothermal reactor. After that, the activated hydrochar (AHC) was washed with pure water until a pH value of 7 was reached; subsequently, it was dried in an oven at 105°C for 24 h. After activation process, to gain deeper insights into the microstructure of HC and AHC regarding the specific surface area and pore size distribution, N2 adsorption/desorption isotherms were conducted. In Figure c, the TEM image of AHC has graphitized structure, which indicates easy pathways for ion and electron transport. AHC provides a higher electrochemical performance as an EDLC electrode than that demonstrated by the HC because of the high surface area (~1067 m2 g−1) and a hierarchical multi-porous structure of AHC as shown in Figure d. In addition, the conductivity of AHC is superior to that of HC; that is expected because AHC has a connected graphitized structure, which is not observed for HC, as confirmed by the TEM data (Figure a-c). The tangent to the curve in the low frequency region for AHC is higher than that for HC in Figure f. This indicates AHC has a better ion propagation and more ideal supercapacitive performance than HC. Figure a-c show the morphological differences due to the activation reaction, suggesting the existence of a well-developed hierarchical porous structure in AHC as compared to that the structure of HC. When the porous electrode is dipped into the electrolyte, the interconnections between the macropores not only provide sites for localization of the electrolyte, but also form pathways. The specific capacitance of the AHC based supercapacitors was 140 F g-1 in Figure e. Also, it demonstrated a good cyclic performance with 3% reduction over 1500 cycles at current density of 0.3 A g-1. Thus, AHC from waste coffee sludge could be an environmentally friendly technique associated with energy storage systems.
This research was funded and conducted under the Competency Development Program for Industry Specialists of the Korean MOTIE, operated by KIAT (No. P0012453, Next-generation Display Expert Training Project for Innovation Process and Equipment, Materials Engineers).

Figure 1