Designed Structured Carbon Based Materials for Energy Storage and Conversion Applications

結構化碳基材料的設計及其在能源存儲與轉化的應用

Student thesis: Doctoral Thesis

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Award date4 Jun 2020

Abstract

The increasing global energy demand and environmental pollution issues stimulate intense research to develop clean and sustainable energy conversion and storage systems even featuring with flexibility and wearability characteristics. The design of advanced electrode materials is key to develop these energy conversion and storage devices with high energy and power density, prolonged stability, safety, and low cost. Different energy systems possess different working mechanism which requires different designs of electrode materials. The basic of the electrode materials design require high surface areas and good electronic conductivity. Carbon based materials are the most abundant and versatile electrode materials used in energy conversion and storage devices. However, most of bare carbon-based materials are intrinsic electrochemical inert while their surface areas and electronic conductivity are needed to be further improved when served as electrodes materials for batteries or supercapacitors. Component modifying on bare carbon materials, such as doping with heteroatoms or transition metal with heteroatoms will tailoring the electronic structure of bare carbon, resulting in breaking the electrochemical inert property of bare carbon. Carbon based materials designed with suitable nanostructure can improve their electronic conductivity and specific surface area. Therefore, structure and component modifying on carbon-based materials with promising batteries or supercapacitors performance are highly desirable. Here, we have focused on structuring and component modifying the carbon-based materials with favorable properties for energy system via different clean and facile strategies.

As a crucial family of carbon materials, hollow carbon sphere (HCS), comprised of carbon shells and interior voids, provide them with many unique features such as low specific density, high surface area, and tunable porosity, that are desirable as electrode materials for supercapacitors. Therefore, we synthesize hollow carbon spheres via a facile method by directly carbonization of polyaniline-co-polypyrrole (PACP) as electrode materials for supercapacitor applications. Conventional supercapacitors play an important role in energy storage market owing to their high power density and cycling stability; however, their low energy density originated from the conventional energy storage mechanism have hindered their widespread applications. Interestingly, the emerging zinc ion hybrid supercapacitors, combining the energy storage mechanism of conventional supercapacitors with fast anions adsorbing/desorbing process, and batteries with Faradaic ion intercalating/de-intercalating process, have improved the energy density while maintain the power density; however, the flexible designs of zinc ion hybrid supercapacitors are not available currently. Therefore, we introduce a safe and flexible solid-state zinc ion hybrid supercapacitor (ZHS) based on the synthesized hollow carbon spheres as cathode, polyacrylamide (PAM) hydrogel as electrolyte and deposited-Zn on carbon cloth as anode. Owing to the high surface area of the HCS and the hollow structure which improve the ions adsorption and desorption kinetics of the cathode, the flexible solid-state ZHS delivers a highest capacity of 86.8 mAh g-1 and a maximum energy density of 59.7 Wh kg-1 with the power density of 447.8 W Kg-1. These devices present excellent cycling stability with 98% capacity retention over 15,000 cycles at a current density of 1.0 A/g. Moreover, the solid-state ZHS is flexible enough to sustain various deformations including squeezing, twisting and folding due to the use of flexible electrodes and electrolyte.

Next, as the developed hollow carbon spheres possess high surface area and the hollow structure is ideal to improve ions adsorption and desorption kinetics. We supposed that these materials would also be good electrocatalysts for zinc-air batteries (ZABs). For such applications however, it is important to overcome the lack of active sites which has hindered the practical applications of hollow carbon sphere materials as electrocatalysts. Taking advantage of current efforts to use transition metal-nitrogen doped carbon material (M-N/C) as electrocatalysts for ZABs due to synergetic M and N co-doping effects; we introduce cobalt and nitrogen coordinated active sites into hollow carbon spheres via carbonization of Co-polymer after soaking with different concentration of cobalt acetate solution. The prepared cobalt and nitrogen co-doped hollow carbon spheres have been denoted as Co-NHCs. We have been able to show that 0.1-Co-NHCs catalyst presents suitable Co doping content and exhibits favorable ORR catalytic activity (onset potential of 0.99 V and half-wave potential of 0.81 V vs. RHE), comparable to that of the commercial Pt-C (onset potential of 1.02 V and half-wave potential of 0.83 V vs. RHE) while surpassing Pt-C in terms of cycling stability. The excellent performance of the catalyst is attributed to the synergetic effect of Co and N doping with high total ratio of active sites, high surface area and good conductivity of the material. More impressively, the assembled rechargeable ZABs based on the 0.1-Co-NHCs catalyst outperforms those afforded by commercial Pt-C.

We further explore the advantages of structure and components modification of carbon based materials and also compare the structure modification effect and components modification effect of carbon based materials on electrochemical performance by designing a uniform virus-like cobalt and nitrogen co-doped carbon materials (Co-N-Cs) via facile carbonization of a Prussian blue analog (PBA) precursor. The obtained virus-like Co-N-Cs exhibit excellent bifunctional catalytic activity and cycling stability when applied as oxygen reduction and evolution reactions (ORR and OER) owing to the favorable virus-like structure with a rough surface that results in a relatively large surface area, high electronic conductivity, and synergetic Co and N co-doping effects which are desirable electrode materials characteristics for rechargeable ZABs as confirmed by our results. When compared with the optimized 0.1-Co-NHCs with favorable structural design for a higher surface area and electronic conductivity obtained in the last section, the virus-like Co-N-Cs with favorable component engineering for a higher intensity of active species, exhibits better intrinsic activity, which suggests that the component modification on carbon materials effect is larger than that of structural effect on electrochemical performance. Lastly, we seek to endow the rechargeable ZABs with flexibility and stretchability as this is highly desirable for flexible and wearable electronic devices with the coming of artificial intelligence era. However, lack of highly stretchable solid-state electrolyte with good alkaline-tolerance has hindered the development of wearable ZABs. Therefore, we synthesized a solid-state dual-network sodium polyacrylate and cellulose (PANa-cellulose) based hydrogel electrolyte with good alkaline-tolerance that has been recently reported in the literature. Excellent performance is enabled by cellulose and N, N"-methylenebis-acrylamide (MBAA)-assisted toughening, hydrogen bond cross-linking, and carboxyl groups neutralized by hydroxyl, as well as cellulose acting as an alkaline stabilizer. We have thus fabricated a solid-state fiber-shaped ZAB using this hydrogel electrolyte, the virus-like Co-N-Cs air cathode, and a zinc spring anode. The fabricated device displayed excellent stretchability, up to 500% strain without damage, and outstanding electrochemical performance, 128 mW cm-2 peak power density, and good cycling stability > 600 cycles at 2 mA.

In summary, the structure and component modification of carbon based materials with favorable properties for ZHS and ZAB applications have been studied in this thesis. The developed ZHS and ZABs based on the synthesized modified carbon materials exhibit promising electrochemical performance. Furthermore, novel fascinating features, such as flexibility and stretchability have been incorporated in the developed ZHS and ZABs in this thesis. It is believed that the studies and strategies presented here can shed light on developing clean, sustainable, and flexible energy storage and conversion devices based on modified carbon electrode materials.