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Lignin-derived carbon fiber flexible supercapacitors

Periodic Reporting for period 1 - LIGNOCAP (Lignin-derived carbon fiber flexible supercapacitors)

Reporting period: 2018-06-01 to 2020-05-31

i. Lignin-Derived Carbon Fiber Flexible Supercapacitors [LIGNOCAP]
Compared to supercapacitors (SCs), research on flexible supercapacitors (FSCs) are still in its infancy due to their high cost, low energy density, low capacitance, poor flexibility, safety problems, leakage, toxicity and scarcity of some nanomaterials, and poor cycling stability, which continuously force us to search for alternative sustainable and high-performance electrode materials. To overcome these obstacles, clearer coalitions between fiber thickness and modulated structure, porosity, functionality, effect of dopant, electrochemical performance and cyclability are needed. We have correlated the physical properties of carbon fiber composite, with their electrochemical properties by using a combination of synthetic approaches and in depth characterization studies. These insights will provide a great support for the design of the next generation advanced FSCs and will accelerate their commercialization.
ii. Na-ion Batteries
Similarly, to overcome the challenges of lithium (Li) batteries like less availability of Li resources, sodium (Na) batteries based on the abundant Na resources have gradually become a promising alternative to Li batteries. Among numerous anode materials, metallic sodium (Na) has been widely regarded as the ultimate anode material for next-generation Na batteries on account of its ultra-high theoretical capacity of 1166 mAh g-1 and a lowest negative electrochemical potential (Na+/Na = −2.714 V vs standard hydrogen electrode (SHE)). However, metallic sodium deposition is considered to be quite random process, resulting in severe sodium dendrite formation during cycling. Several studies have employed carbon fabrics as a functional skeleton to guide the metallic deposition based on uniform nucleation at the very first cycle. Most studies have claimed that the porosity would benefit the metallic deposition behaviour, but in fact, the specific effect of defective structures in carbon matrix haven’t been carefully considered, whose functions definitely differ from those of the pore structures. It has never been clearly proven whether the defect or pore is indeed affecting the metallic deposition as well as entire electrochemical performance because most porous carbon materials possess defective structures at the same time.
Objectives: The goal is to accelerate the commercialization of our high performance flexible supercapacitors (FSCs) by interacting closely with the energy conversion/storage industry. We also carried out an additional research on Na-ion batteries by using our as-prepared carbon fibers from lignin. The electrospinning allows producing the desirable carbon fibers with interesting features of structure, porosity, thickness and morphology. Also, we have investigated the synthesis of various carbon fibers not only from Lignin, but also from biomass. HTC carbon spheres were also synthesized from fructose to incorporate the Fe NPs and encapsulate within the carbon fibers to enhance the stability of Fe NPs.
• Materials Synthesis and Characterization of CFC
• Electrochemical evaluation
• Results and Exploitation
1. Highly stable Fe-N- doped HTC spheres encapsulated Carbon Fibers for Flexible Energy Devices (Fig. 1)
• Electrospinning ordered and flexible Carbon Fibers (CFs) from Lignin.
• Encapsulating Fructose-derived HTC spheres (~ 500 nm) within Carbon Fibers.
• Using N and Fe rich Potassium Ferrocyanide as precursor for high Fe and N (>10%) doping.
• Decorating the Potassium Ferrocyanide (Prussian Blue) micelles over the HTC spheres.
• Encapsulate Fe NPs anchored HTC Spheres within the CF for enhancing the stability.
• Fe-Fe3C crystalline phase with rich N doping for supercap and ORR.
• Electrochemical testing with flexible and free-standing CF electrodes.
2. Homogenous Metallic Deposition on Sustainable Skeletons with Rich Defects for Sodium Metal Batteries (Fig. 2&3)
Here, sustainable carbon skeletons with defective structures but low porosity were rationally designed by one-step annealing of electrospun organosolv lignin mats and then employed as three-dimensional (3D) hosts for Na metal anodes. Operando and ex situ characterization as well as multi-scale modelling from atomic to engineering scale was conducted to help understand the deposition behaviour of metallic Na on the skeletons and relative solid electrolyte interface (SEI) components in different electrolytes including typical ester- and ether-based electrolytes. We have successfully proved that the defects play a more important role in benefitting the nucleation of metallic Na than pore structures. Compared with carbonate-based electrolytes, diglyme (DME) has lower ionic conductivity and higher lowest unoccupied molecular orbital (LUMO) energy which both contribute to the uniform deposition. Coupling with various cathodes (Prussian blue (PB) crystals, sulfur (S), etc.), high-performance Na batteries were fabricated, where carbon skeletons show great importance on improving electrochemical performance, especial on reducing electrochemical polarization. Our carbon skeletons were first demonstrated in terms of overall sustainability by an aqueous electrospinning of bioresource - lignin, followed by a low-temperature thermal pyrolysis (700 oC) method. To obtain defect and pore structures, series of carbon skeletons were prepared under higher temperatures (1100 and 1500oC), which were named as L700, L1100 and L1500, and their corresponding physico.
The review paper entitled “Lignin: A Rising Sustainable Precursor for Nanostructured Carbons for Supercapacitors” has been written for the special issue to be published in the RSC journal, Sustainable Energy & Fuels. In this review, we present a broad overview of the lignins structure, classification and their performances in electrochemical energy storage applications, particularly in supercapacitors with recent advances (Fig. 4).
It has never been proven whether the defect or pore is affecting the metallic deposition as well as electrochemical performance because most porous carbon materials possess defective structures. In addition, most commercial carbon fabrics are derived from fossil fuels which is in shortage. Considering the limitations of the state-of-the-art, sustainable carbon skeletons with defective structures but with low porosity were rationally designed by one-step annealing of electrospun lignin mats and used as three-dimensional (3D) hosts for Na metal anodes. In case of FSCs, carbon fibers were functionalized and composited with HTC spheres and abundant inorganic metal oxide pseudocapacitative nanoparticles (Fe NPs). The fabricated FSCs and Na-ion batteries have great potential for commercialization.
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Figure 4
Figure 2