Molten base carbonisation and activation of biomass for supercapacitor application

Abstract

Energy conservation and environmental sustainability has become global concern. Therefore, energy sources, their associated sustainable conversion and storage technologies to maximize energy and minimize pollution is being looked for. Biomass materials has shown ability for energy generation and storage. Due to their abundant sources and renewability, large amount undergo spoilage readily because of their high moisture content resulting to increased amount of CO2 released to the environment. Consequently, efficient conversion of these biomass materials to value-added materials will help various industries and save the environment.
Conversion of biomass into porous carbon material has been a costly and energy-intensive process as it has involved multi-stage thermal process above 900 ˚C. Several conversion methods, such as carbonisation and activation, hydrothermal and activation, and other templating strategies has been explored to overcome the above-mentioned challenges. However, the process of first obtaining biochar from biomass, followed by the biochar activation with chemicals such as ZnCl2, KOH or removal of templates results in carbon that falls short of several applications especially supercapacitor applications. Therefore, developing a single-stage thermal route for biomass to carbon for supercapacitor application will be a significant advancement.
This PhD research has focused on optimising a single-stage thermal process route for biomass materials conversion to functional (porous) carbon using the molten base carbonisation and activation (MBCA) process.
The novelty used pristine wet biomass with a minimal ratio of the desired base to achieve depolymerization of biomass constituents, intercalation of metallic ion through nucleophilic substitution and bond breaking of biomass monomers, dehydration, deacetylation, and recombination of reactive O- species before drying, followed by elevated temperature (carbonisation) process. This process route has not been reported.
The process mechanism and reactions are summarised as follows: At room temperature the interaction of KOH, Biomass (components) and H2O caused oxidation, dehydration, deacetylation, decarboxylation and decarbonylation through bond cleavages and nucleophilic substitution to generate potassium organic compounds. The potassium organic compounds are also formed because of the organic acid interactions with base via neutralisation reactions. At elevated temperatures: the generated potassium organic compound and oxygen active species enhanced dehydration, deoxygenation, decarboxylation, decarbonylation, dehydrogenation and aromatisation reactions coupled with KOH activation mechanism for in situ activation of carbon with the release of volatiles.
Moso bamboo shoots and Radish were selected as biomass precursors based on their rich carbon source, short maturity cycle and sustainability. They were subjected to proximate analysis and ultimate analysis to determine their organic composition and elemental composition. Also, thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC), Xray-diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), were conducted on samples mixed with KOH to understand the biomass transformation and thermal behaviour. The derived carbon samples were characterised in terms of their thermal stability, elemental composition, morphology, surface functional groups and composition, specific surface area (SSA), pore structure, crystallographic and graphitic structure, and electrical conductivity before application as alternative electrode active material in EDL capacitor.
Preliminary results on feasibility studies for MBCA showed biomass to KOH ratio affected the carbon properties in relation to morphology, SSA, porous structure, and graphitic structure. Optimum biomass to KOH ratio was revealed to be 15:1 with SSA of 1367.82 m2 g-1 and specific capacitive performance of 327.23 F g-1. Also, the least biomass to KOH ratio (5:1) had the largest SSA of 1452.45 m2 g-1 with specific capacitive performance of 277.08 F g-1. This result revealed that high specific capacitance was achieved with large SSA combined with a hierarchical porous structure.
Further investigation on the effect of cation with Moso bamboo shoots with optimum ratio, revealed that the hydroxide cation used for MBCA affected the properties of derived carbon. The potassium hydroxide (KOH) derived carbon via MBCA had higher SSA of 1367.82 m2 g-1 than 955.36 and 872.62 m2 g-1 for lithium hydroxide (LiOH.H2O) and sodium hydroxide (NaOH) mediated processes, respectively. This revealed that cation and carbon reactivity are different irrespective of same temperature condition and biomass source. The calculated specific capacitance at 20 mV s-1 was 306.99, 176.22, 186.98 F g-1 for KOH, NaOH and LiOH.H2O respectively.
MBCA conducted on Moso bamboo shoots and Radish at 700 ˚C for 3 hrs had hierarchical porous structure with large SSA of 1367.82 and 1172.83 m2 g-1 respectively, compared with 0.58 and 0.12 m2 g-1 for pristine Moso bamboo shoots and Radish with only carbonisation. This result revealed that low amount of KOH could be used to convert different biomass sources to porous carbon via MBCA. These results in terms of SSA are comparable to commercial activated carbon with 1425.89 m2 g-1 derived from multi-stage thermal processes (carbonisation and activation).
Furthermore, MBCA process revealed 800 ˚C to be the best temperature to achieve maximum SSA of 1948.85 and 1571.92 m2 g-1 for Moso bamboo shoots and Radish, respectively. However, they did not show the best electrochemical performance which could be linked to their pore structure. This result showed that the MBCA process can be used to obtain large SSA carbon for diverse applications that would require large surface area. Though samples obtained at higher carbonisation temperatures had better electrical conductivity compared to those obtained at low temperatures. It further confirmed that MBCA process is temperature and ion dependent.
Moreso, the residence time for MBCA process affected carbon properties. At 700 ˚C for 1 hr, the least SSA of 1190.51 and 984.56 m2 g-1 was obtained for Moso bamboo shoots and Radish respectively. At 3 hrs, maximum SSA of 1367.82 and 1172.83 m2 g-1 was obtained for Moso bamboo shoots and Radish respectively. They had better graphitic structure and conductivity. These results revealed that MBCA process could yield a porous carbon within a short residence time, but some other properties would suffer some deficiency.
Furthermore, the specific capacitance of various derived carbon evaluated in symmetric electrochemical double-layer (EDL) capacitor at 5 mV s-1 in 6 M KOH was 327.23 and 257.03 F g-1 for Moso bamboo shoots and Radish respectively, compared to 142.44 F g-1 for the commercial carbon used for most supercapacitors. After 10,000 cycles, the symmetric cells showed outstanding cycling stability, keeping 95% and 90% of their initial specific capacitance. The performance was attributed to their high SSA, hierarchical porous structure, and rich surface functional (oxygen and nitrogen) groups, which supplied an efficient electrode area and improved contact for improved performance.
Further investigation on the scale-up of the MBCA process using Moso bamboo shoots was conducted with different reactor designs (horizontal and vertical) at 700 ˚C for 1 hr. The SSA for horizontal reactor (H-1) and vertical reactor (V-1) was 1190.51 and 1198.47 m2 g-1, respectively. This further confirms that using MBCA, process time for porous carbon production can be maximized to save energy. The differences in graphitic structure and conductivity of carbon samples reflected in their specific capacitance of 164.75 and 160.26 F g-1 for the vertical and horizontal reactors respectively, at 10 mV s-1 in 1 M TEABF4/AN (which is an organic electrolyte used in most commercial devices). Both carbons displayed better specific capacitance compared to commercial carbon (143.08 F g-1) at same condition. Furthermore, at a specific current of 0.1 A g-1, V-1 displayed a better cycling stability of 10,000 cycles which proved its actual ability to serve as an alternative electrode material.
Compared material, energy, and cost analysis for the conventional carbonisation and activation process using the pre-dried Moso bamboo shoots and the MBCA of wet bamboo shoots with KOH was conducted. The MBCA approach had a better overall yield of 8.6 % compared to 5.93 % for the conventional approach. The MBCA process proved to be 36.98 % of the cost for conventional process. The cost saving is from the cost of activation reactor and the amount of KOH used in the existing conventional process.
In conclusion, this study has shown that the conversion of biomass to porous functional carbon suitable for electrode material for energy storage can be conducted in a single-stage thermal process at low temperatures, with little resources, and at a cheap cost. The findings of this study suggested that using wet biomass through MBCA might be a practical way to lower the cost of producing functional carbon from biomass. It would also reduce the environmental problems associated with wet biomass, particularly agricultural wastes that are easily decomposable. The innovative technique presented here offers an easy pathway for the large-scale production of carbon for supercapacitor use, decreasing biomass waste while contributing to energy storage.

Date of Award	Oct 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Di Hu (Supervisor)