Xin Zhong,Chao Li,Yuewen Shao,Lijun Zhang,Shu Zhang,Xun Hu
Bio-oil, a major product from biomass pyrolysis, is a renewable source for production of functional carbon materials such as activated carbon (AC). Bio-oil can be activated directly via mixing with an activator or be pre-carbonized followed by subsequent activation. These two processes feature with distinct reaction network and might affect pore characteristics in different ways. This was investigated herein by direct activation of bio-oil or biochar from pre-carbonization of bio-oil using K 2 C 2 O 4 or KOH as the activator at 800 ºC. The results indicated that bio-oil was pre-carbonized into biochar at 500 ºC improved yields of AC (83.6 % versus 17.5 % from direct activation of bio-oil with K 2 C 2 O 4 ) through enhanced aromatic degree and resistivity to cracking. This also diminished specific surface area of resulting AC (716.3 versus 1035.5 m 2 g −1 from direct activation). Much more intensive cracking reactions in direct activation of bio-oil with K 2 C 2 O 4 were confirmed with in-situ IR technique. This not only formed AC of more developed pore structures, especially more mesopores/macropores, but also generated gases as dominate product (yield: 74.5 %). Similar result was observed from activation with KOH, but KOH was more effective than K 2 C 2 O 4 for cracking. This, however, did not result in AC of more developed pore structures, as the K 2 CO 3 derived from KOH tended to be wrapped with organics in direct activation of bio-oil and was difficult to be washed away, blocking pores generated.
Aqueous zinc-ion hybrid supercapacitors (ZHSs) are attracting increased attention as emerging electrochemical energy storage systems. However, the design of high-performance carbon cathodes for ZHSs remains a challenge. Herein, we report the synthesis of heteroatom-rich carbon cathodes based on a biomass precursor of yeast and a hydrothermal pre-carbonization strategy, realizing high-performance ZHSs. The yeast is composed of polysaccharide chains containing abundant O/N heteroatoms, and a hydrothermal pre-carbonization process is conducive to preserving these heteroatoms in the high-specific-surface-area carbon materials obtained by carbonizing-activating the yeast precursor. As a result, the synthesized carbon materials are endowed with high O/N heteroatom contents (exceeding 13.9 at%), and present superior electrochemical performance in ZHSs, including a high specific capacity of 132 mAh/g, a high energy density of 94.4 Wh/kg and outstanding cycling stability with ∼100% capacity retention after 7000 cycles at 5 A/g. Besides, the heteroatom-rich carbon cathodes show a high capacity retention of 85.3% when their mass loading increases from 3.8 to 12.2 mg/cm 2 , demonstrating promising application for practical ZHSs. Electrochemical analysis reveals that the O/N heteroatoms promote ion chemical adsorption and thus the electrochemical properties of the carbon cathodes. Furthermore, flexible ZHS devices constructed with the heteroatom-rich carbon cathodes and a biodegradable ZnSO 4 /dough solid-state electrolyte exhibit excellent flexibility (as reflected by almost unchanged capacity under different bending states and 85% capacity retention after 500 bending cycles) as well as good repairability after dehydration under abnormal environments. This study offers new thinking in designing high-performance carbon cathodes and promotes nonflexible/flexible ZHSs moving towards practical applications.
Chao Li,Kai Sun,Yifan Sun,Yuewen Shao,Guoming Gao,Lijun Zhang,Shu Zhang,Xun Hu
Pre-carbonization is one frequently used pre-treatment for production of activated carbon (AC) from activation of biomass, which impacts not only nature of AC but also composition of bio-oil and gases in activation of biomass. In this study, activation of sawdust/cellulose/lignin directly or via pre-carbonization were conducted, with a particular focus on effects of pre-carbonization on evolution of the various products and environment. The results indicated that the pre-carbonization removed significant portion of aliphatic structures, creating difficulty for developing pores from the resulting biochar. This, however, enhanced overall yields of AC and gases at expense of bio-oil with K 2 C 2 O 4 , CO 2 or H 2 O as activators. The high yield and high specific surface area of AC could not be obtained simultaneously and needs to be balanced in application. In converse, ZnCl 2 or H 3 PO 4 as activators generated more AC with developed pore structures via direct activation of biomass, as they catalyzed the dehydration of aliphatic structures in biomass, but not in biochar. The life cycle assessment (LCA) indicated that the pre-carbonization led to a higher impact to environment and energy consumption than the direct activation. ZnCl 2 and CO 2 showed the smallest influence on the fossil fuel depletion and global warming. Direct activation exhibited lower environmental impact in the production of AC with more developed pore structures, rendering it a more desirable route for production of AC.
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