电化学论文proposal代写范文

这篇电化学proposal出自EssayPhD团队英国化学PhD大神Lin之手，研究的问题是can PEDOT:PSS modified 3D graphene material be used to improve the performance of lithium- sulfur battery，涉及到石墨烯材料、锂硫电池、PEDOT:PSS等高新领域，专业水平一览无余。选择EssayPhD为您代写化学方面的论文，是您明智的选择！

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PEDOT:PSS modified 3D graphene material for Lithium- sulfur battery

Research question

The research question in this proposal is, can PEDOT:PSS modified 3D graphene material be used to improve the performance of lithium- sulfur battery?

Lithium- sulfur (Li-S) battery is a rechargeable battery that is known for its high capacity. Li-S battery has been used since 1940s(Zhu, Yang, Yin, Yan, & Zhang, 2014). The element lithium (Li) and sulfur (S) undergo a redox reaction that leads to a capacity of 1672 mAh g−1 battery with energy of 2600 Wh kg−1, in theory. The chemical reaction is described as S8+16 Li = 8 Li2S

The other advantages of Li-S battery include its light weight and low cost, thanks to the abundance of Li and S in nature. Despite the obvious potential benefits gained from Li-S battery, the existence of some major challenges stand in the way of industrialization and commercialization of Li-S battery. The biggest problem lies in the nature of element sulfur. S has a low conductivity. Using S without any modification significantly lowers down the material efficiency(Yang et al., 2011b). The solubility of intermediate polysulfide anions in the charge-discharge process is another problem. During cycling, insulate products Li2S2 or Li2S  can cause fast capacity loss(Manthiram, Fu, & Su, 2013; Yang et al., 2011a). Also, the large expansion of sulfur leads to 80% volumetric increasement in the process of charging and discharging, that will ultimately compromise the cycle life (Yang et al., 2011a).

To improve the performance of Li-S battery, one possible way is to encapsulate sulfur in a carbon matrix.  The conductivity of S cathode will be enhanced and soluble polysulfide anions can be trapped. A 3D porous structure  can solve these problems. This research proposal aims to achieve a better cycling behaviour and stable capacity of Li-S battery by utilizing a mixed material of PEDOT:PSS and macroporous graphene. The potential outcome from this study is to achieve the goal of producing Li-S batteries with better behaviours.

Literature review

Graphene

Graphene is a composed by sp2-bonded carbon atoms that formed a lattice honeybomb 2D structure(Cabana, Monconduit, Larcher, & Palacin, 2010). Such unique structure provides graphene with high surface area, moderate flexibility and high electric conductivity. It has been widely used in medicine, chemistry and applied physics(Geim & Novoselov, 2007). To achieve larger surface area, 2D graphene sheets are translated to 3D well defined macroscopic structure. Such structure is achieved by combination of porous structure and graphene sheets. 3D structure also exhibit other advanced features, such as durable mechanical strength, and satisfying mass and electron transportation(Li & Shi, 2012). Zhao et al developed a 3D graphene/single-walled carbon nanotube (SWCNT) material which enables a high electrical conductivity. The spacious room between the graphene layers and SWCNTs is used for sulfur storage, which eventually provides a  650 mAh g−1 capacity after 100 cycles (Zhao et al., 2012). In addition, Zhou and Cheng’s group has also successfully attach sulfur nanocrystals with graphene fibers that expressed high capacity and electric transportation (Zhou et al., 2013). It is proved that 3D porous structure is suitable as coating for Li-S battery.

Graphene-based material

It is important to have rational design of a graphene-based electrode materials to achieve the full potential of Li-S battery. Sulfur and carbon has strong interaction. A good method to deposit sulfur onto graphene sheets is to use melted sulfur. This method successfully increases the electrical conductivity. However, the polysulfides intermediates are not well constrained and their mobility is observed in the irregular pores from aggregated graphene. It compromises the cycling stability.
In making efficient 3D porous graphene structure, chemical modification is necessary. To achieve better performance of Li-S battery, the interaction between graphene and sulfur or polysulfide materials needs to be enhanced. A system of nano-sulfur graphene oxide (GO) sheets was designed by a group of researchers using chemical reaction on an emulsion system. This strategy solves the problem of soluble polysulfides intermediates. When sulfur is attracted by C-C bonds, polysulfides species are generated. The epoxy and hydroxyl chemical groups on the surface  of this innovate sheets can trap sulfur and stop the formation of polysulfides. Such system exhibits a capacity of 954 mAh g−1 for 50 cycles. Capacity fading is kept on a very low level (Zhang, Zhang, Dong, & Wang, 2012). A method of synthesizing nanoparticles on a micro-sized sulfur scale enveloped by the reduced graphene oxide (rGO) was achieved by a hydrothermal oxidation process(X. L. Ji & Nazar, 2010), which shows a higher loading amount of sulfur deposited onto the rGO sheets. Polysulfides are restrained on a higher level by an efficient conductive pathway from a strong hydrophilic to hydrophilic interactions. More densed nanopores that prevent the mobility of sulfur were designed from a modified chemical activation of hydrothermally rGO. It was also proved to have stable cycling (Zhang et al., 2012). The disadvantage of this method is the incomplete oxidation in the charging process, which leads to the formation of insoluble sulfurpolymers on the microsized cells. It gives rise to unstable cycling and obvious capacity fading within 50 cycles(Evers & Nazar, 2012).

PEDOT:PSS and porous graphene hybrids
International Union of Pure and Applied Chemistry (IUPAC) has categorized the size of porous structures by diameters.  In diameter, micropores are pores smaller than 2 nm; macropores are pores bigger than 50 nm; mesopores are pores with dimension in between (Rouquerol et al., 1994).

It has been reported that mesoporous carbon is effective at trapping polysulfides intermediates because of their relatively small pore diameter. However, polysulfides can still escape from their large surface area (X. Ji, Lee, & Nazar, 2009). For the first 20 cycles, a 10% capacity decay was observed in sulfur/mesoporous graphne material. Poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate)(PEDOT:PSS) is proved to be a good solution. It has been tested as a candidate synthetizing mesoporous hybrids (Zhan et al., 2008). PEDOT:PSS  has physical property of being stable and rigid in an electrochemical environment. The capacity retention of sulfur electrode is improved from 70%/100 cycles to 80%/100 cycles with an additional 10% increase in delivered discharge capacity in the PEDOT:PSS hybrid compared to mesoporous graphene alone as coating (Zhan et al., 2008).

The advantages of macroporous graphene materials are that they can provide larger surface area with integrated 3D structures, high conductivity and low weight density. As a result, macroporous graphene-based hybrids can transfer the features of the individual components to a macroscopic scale. It also keeps the unique properties of graphene nanostructures. This proposal is to test the properties of PEDOT:PSS/ macroporous graphene hybrids and decide whether they are suitable in Li-S batteries.

Template Methods

Template synthesis is the most effective method of producing micro- or meso- porous 3D graphene materials. The size and the surface structure of the pores are easily adjusted. Inorganic or organic nanostructures are the unit cells or templates in template method. Porous graphene materials are achieved by either the soft- or hard-template approaches, depending on the usage. Hard-template approach is suitable in making rigid structures(Liang, Li, & Dai, 2008).

Methodology

In the proposed research, rapid freezing method by processing PSS-G liquid dispersions, using ISISA is used to make macroporous graphene material, as described by Vickery et al (Vickery, Patil, & Mann, 2009).

In this method, macroporous graphene structures are achieved from the deposition of graphene sheets on inorganic or organic particles with a diameter bigger than 50 nm or template lamination resulted from in-situ growth on metallic porous frameworks. ISISA is ice-segregation-induced self-assemble, which are amphiphilic polymers such as polyvinyl alcohol or chitosan. Rapid freezing of dispersions with ISISA is used to produce porous structures with functional and well defined structures such as previously mentioned SWCNT or multi-walled carbon nantubes (MWCNT). Self-supporting monoliths composed by internally aligned macroporous structures are achieved by immersion of dispersions of polystyrene sulfonate-stabilized graphene (PSS-G) sheets and GO/PVA or PVA with different weight ratios in a freeze-drying cycle using liquid nitrogen. Other materials that can be served as hard templates to fabricated 3D graphene materials include polystyrene and polymethyl methacrylate (Gutierrez, Ferrer, & del Monte, 2008).

PEDOT:PSS-Coated PSS-G hybrids is fabricated by the method described by Yang et al (Yang et al., 2011a).

Materials and Methods

Preparation of PSS-G:
Chemical reduction of exfoliated GO sheets is conducted to prepare dispersed PSS-G. Graphite is used to prepare aqueous suspensions of exfoliated GO, which is later dialyzed and then sonicated for one hour. To remove non-exfoliated GO sheets, the suspension is later centrifuged. The GO supernatant will be then diluted to achieve a stable dispersion of the exfoliated GO sheets. Hydrazine hydrate and PSS will be mixed into the previous product and heated. Repeat the centrifugation and washing cycles to get rid of excess polymer and hydrazine hydrate. A redispersion of the PSS-stabilized graphene sheets will be performed later in deionized water.

Preparation of PEDOT:PSS-Coated PSS-G.
PEDOT:PSS solution is prepared adding dimethyl sulfoxide into pre-prepared solution. It will be later then diluted in deionized water (volume ratio of 1:10). Additional ethanol is added into the solution. To achieve PEDOT:PSS coated PSS-G sheets,  about 8 mg of PSS-G powder is added into 1 mL PEDOT:PSS solution, which will be later on under sonication.

Electrode Fabrication and Electrochemical Measurement.
To make sulfur electrode, aluminium foil is used as the base for casting solution. Dry the cast under vacuum. Bake the product from previous step. PSS-G sheet will be prepared in the same way. Put the samples into the wells. Lithium will be used as a counter electrode.

Data collection and analysis

Characterization: A carbon-coated copper grid is used as a base for dispersion to TEM images of GO and PSS-G sheets. TEM microscopy is conducted. Zeta potential measurements are recorded. SEM images of PSS-G coated beads are obtained and recorded. PXRD patterns can be achieved from using a Bruker D8 Advanced X-ray diffractometer. Stress versus strain data was recorded and plotted at a compression rate of 0.06 mm s-1. Samples are approximately 4 mm in length and radius, and should be collected from different areas of the main structure.

Scanning electron microscope (SEM) will be used to examine the structure. X-ray photoelectron spectroscopy (XPS) is used for characterization of the coating layer on the surface. Transmission electron microscopy (TEM) is used to test the behaviour of PEDOT:PSS-Coated PSS-G. Atomic force microscope (AFM) images can be used to study the structure as well. Cyclic voltammetry (CV) is used to study the electrochemical characters of the hybrid. The effect of PEDOT:PSS on PSS-G coating will be tested by electrochemical impedance spectroscopy (EIS). The comparison of the characters of PSS-G sheets and PEDOT:PSS-Coated PSS-G hybrids.

Anticipated results

After the comparison of electrochemical properties between PEDOT:PSS-Coated PSS-G and PSS-G sheet, the behaviour of PSS-G should be enhanced by adding the PEDOT:PSS coating. For example, the solubility of intermediate polysulfides in the charge-discharge process is prevented. During cycling, the capacity loss is decreased. Also, cycle life should be enhanced. The purpose of this research is to design an  appropriate coating for sulfur in Li-S batteries. Since theoretically the proposed coating will enhance the overall behaviour of macroporous graphene material, which has been proposed as a potential coating for Li-S battery, the PEDOT:PSS-Coated PSS-G should be expected to be more efficient.

Bibligraphy

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