Effect of Pyrolysis Temperature on Electrochemical Performance of SU-8 Photoresist Derived Carbon Films As Lithium Ion Battery Anode Material

Introduction Since last decade, SU-8 negative photoresist is considered as a precursor to yield glassy carbon that comprises short range crystallites.1,2 Owing to its advantages in terms of patterning and ability to fabricate 3-D carbon electrodes, SU-8 photoresist derived carbon has been demonstrated as potential anode materials for lithium ion battery.2,3 We recently reported the use of stainless steel (SS) wafer instead of Si wafer as a substrate for preparing SU-8 derived planar carbon films which exhibited excellent reversible capacity as compared to previous available reports.4 While preparing the carbon films from SU-8 on SS wafer, we carried out the two step pyrolysis at 900 °C.4 However the effect of varying the pyrolysis temperature on structural properties of carbon films derived and subsequently on electrochemical characteristics is yet to be studied. In this work, we pyrolyzed SU-8 films on SS wafer at four different temperatures in the range of 700 °C and 1000 °C. As-fabricated SU-8 derived carbon films were structurally characterized by XRD, Raman and CHNS analysis. Electrochemical performance of as-prepared carbon films were tested using galvanostatic charge discharge experiments at 37.2 mA/g current density and the specific capacities were correlated with the H/C ratio and crystalline structure. Experimental: SU-8 2005 was spin coated on single side polished SS wafer after drying at 150 °C on hot plate for 10 min. Thus obtained films were crosslinked using C-MEMS process, and process conditions were optimized for SS wafer. These films were then pyrolyzed in tubular furnace in presence of N2 atmosphere. As-prepared carbon films were then used as working electrode to study their electrochemical properties whereas lithium foil was used as a counter electrode. Glass microfiber filter soaked with LP-30 electrolyte was used as a separator. Results and discussions: XRD and Raman spectra as shown in Fig. 1a and 1b confirms that prepared carbon films are hard carbons and increase in pyrolysis temperature has no significant effect on their crystallinity. However, samples pyrolyzed at different temperature showed significant change in carbon content and H/C ratio as shown in Fig.1c. H/C ratio increased with decreasing temperature. Fig. 1d shows the cyclic performance of the samples at 0.1 C and between 0.01 -3V. Galvanostatic charge discharge experiments reveals that though reversible capacity decreases, cyclic stability and initial cycle coulombic efficiency increases with increasing temperature. This behavior can be attributed to decrease in hydrogen content and increase in radius of gyration due to rearrangement of graphene layers which results in improved alignment with neighboring layers with increasing temperature.

Resorcinol-formaldehyde derived carbon xerogels: A promising anode material for lithium-ion battery

Organic gels obtained by sol-gel polycondensation reaction followed by subcritical drying in ambient conditions are termed as xerogels which are pyrolyzed to yield carbon xerogels. Resorcinol formaldehyde (RF) derived carbon xerogels have received considerable attention due to their higher carbon yield and ease of tuning their microstructure and therefore physiochemical properties. Recent advances in the synthesis of carbon xerogels have allowed porous as well as non-porous but large external surface area morphologies. Further efforts have been made about increasing the surface area by activation or changing the microstructure by doping with foreign elements. These advances in the area of carbon xerogels synthesis led to their use as high performance anode materials for Li ion batteries recently. This review summarizes these recent studies on electrochemical performance of carbon xerogels to clearly demonstrate their potential as high capacity anode material for Li ion batteries. Notably, given the potential not only for Li ion batteries but also for latest sodium-ion batteries and super-capacitors, this review provides a much needed attention of scientific community to so far unnoticed carbon xerogel materials

Fabrication of SU-8 Derived Three-Dimensional Carbon Microelectrodes as High Capacity Anodes for Lithium-Ion Batteries

It has been shown earlier that three-dimensional (3-D) electrode architecture facilitates higher energy and power density than the planar thin film based electrodes. In the present study, we fabricated SU-8 photoresist derived micro-patterned three-dimensional carbon-based electrodes using photolithography on stainless steel (SS) wafer used as a current collector. The preference of SS wafer over conventionally used silicon (Si) wafer is based on our previous study where use of SS wafer as a current collector enhanced the reversible capacity of thin carbon films to almost double as compared to the thin films prepared on Si wafer. As-fabricated 3-D carbon electrodes were then investigated for their electrochemical performance. At 0.1 C-rate, Li-ion reversible capacity was found to be nearly 600 mAh/g after 165 continuous charge/discharge cycles. Nearly 100% coulombic efficiency and excellent cyclic stability confirms the potential use of such 3-D micro-patterned carbon electrodes for next generation Li-ion batteries

Biomass (Neem leaves) Derived Carbon As Low Cost and High Capacity Anode Material for Lithium Ion Battery

Recently, biomass derived carbons received great attention as a renewable source of carbon with different morphologies[1,2]. Among various carbon material preparation methods such as carbonization of polymer precursors, arc discharge, chemical vapor deposition and chemical synthesis, carbonization / hydrothermal carbonization of biomass to yield carbon looks promising due to abundant availability of precursor. A variety of biomass derived carbon materials are well studied as an electrode material for energy storage devices like lithium ion battery, supercapacitors, lithium-sulphur battery and sodium ion batteries. [1] Rice husk, tea leaves, coconut fibers, coffee shell, neem seed etc., derived activated carbons shown promising anode material for lithium ion storage. Typically, these biomass derived carbons found to be hard carbons in nature but their morphology varied with precursor material. Neem (Azadirachta indica) derived carbon was used as electrode material for supercapacitor application and shows excellent performance.[3] However, there is no study available on lithium ion intercalation in need derived carbon to the best of our knowledge. In present work, we have investigated the electrochemical performance of neem leaves derived carbon as anode material for lithium ion battery application

Catalytically Graphitized Nanostructured Carbon Xerogels as High Performance Anode Material for Lithium Ion Battery

Resorcinol formaldehyde (RF) xerogel is an organic precursor to hard carbon which is catalytically graphitized at 1100 °C in nitrogen atmosphere with two different catalyst loading (2 and 5 wt.%). The effect of catalyst loading during graphitization and later its removal is studied in terms of physiochemical properties of RF xerogel derived hard carbon using XRD, Raman spectroscopy and FESEM. Electrochemical performance of the as-prepared catalytically graphitized RF xerogel derived nanostructured carbon has been studied using cyclic voltammogram and galvanostat/potentiostat experiments. For 5 wt.% catalyst loading, a reversible capacity of 192 mAh/g was maintained with nearly 99% coulombic efficiency

Enhanced catalytic graphitization of resorcinol formaldehyde derived carbon xerogel to improve its anodic performance for lithium ion battery

Resorcinol-formaldehyde (RF)–derived carbon xerogels are hard carbons, which are not explored thoroughly as anodes in lithium-ion batteries as they exhibit poor crystallinity, conductivity and significant capacity fading during cycling. Graphitization of hard carbons, using metal catalysts at elevated temperatures, may be an effective method to enhance their crystallinity. In this study, RF xerogels were graphitized using an iron catalyst at a moderate temperature (1100 °C) in nitrogen atmosphere. Three different catalyst loadings (2, 5, and 10 wt. %) were used to study the extent of graphitization and thereby the effect on physiochemical properties of these RF-derived carbon xerogels. X-ray diffraction and Raman spectroscopy revealed an increase in the degree of graphitization and crystallinity, while high-resolution transmission electron micrographs showed an enhancement in the structural ordering of graphene layers with increase in catalyst loading. Interestingly, when tested electrochemically, the 5 wt.% catalyst loaded RF carbon xerogel was found to exhibit the best anodic performance as ever reported. A specific reversible capacity of 470 mA h/g at 0.2 C rate was retained even after 100 continuous charge/discharge cycles, with nearly 100% columbic efficiency. Furthermore, even at high C-rates (1C), a reasonably high reversible capacity of 348 mA h/g was maintained during cycling

Template Assisted Micro-Patterned Electrospun Nanofibrous Mat As a Potential Carrier for Controlled Drug Release

Controlled drug release by manipulating the surface wettability and degradability of the drug carrier is essential for medical treatments.1 Usually, the wettability of a surface can be altered by surface coatings and by changing the surface roughness. In this work we have fabricated micro-patterned cellulose acetate electrospun nanofibrous mats using template assisted electrospinning (Figure 1A). This approach produces mats with tunable wettability. Nylon meshes (Figure 1B) with different grid spacing (50 µm, 100 µm and 200 µm) were used as a template to fabricate micro-patterned nanofabric over a large area (10 × 10 cm2) in a single step. This non-conductive nylon mesh in strong electrostatic field polarizes and generates a negative static charge on the nylon mesh by static induction and polarization.2 Further, the nylon grid attracts fibers onto grid lines and forms micropatterned fiber mats as shown in Figure 1 (A). Initially, fibers are preferably deposited on nylon grid lines. After 10 min of deposition, the fibers repel incoming material resulting in a random deposition to yield micro-patterned nanofabric surfaces as shown in Figure 1C. Water contact angle measurements were performed on these micro-patterned nanofibrous mats fabricated with different spacing to study the effect of micropatterning on wettability. As shown in Figure 1C, these micro-patterned surfaces form non-communicating air gaps and air trapped in this gaps minimizes the solid-water interface and opposes wetting by capillary action. This non-wetting pressure plays a key role in the wettability of non-commuting air trapped systems.3 The Water Contact Angle (WCA) changes from 30° for the non-patterned surface to 138° for a micro-patterned surface with a 50 µm spacing. This change in WCA can be attributed to capillary pressure increases with decreasing spacing. WCA measurements are summarized in Figure 1D. Further, the effect of surface wettability on drug release kinetics was investigated by loading Diclofenac sodium into cellulose acetate precursor solutions prior to electrospinning. Transdermal drug releases study shown a significant change in drug release kinetics with a change in surface wettability. Micro-patterned surfaces with a WCA of 138° showed zero order release kinetics up to 12 hours, whereas non-patterned hydrophilic (30°) samples only showed controlled drug release for 1 hr

Cellulose Acetate Derived Free-Standing Electrospun Carbon Nanofibrous Mat As an Anode Material for Rechargeable Lithium-Ion Battery

Electrospun polymer derived carbon nanofibers have received attention in recent years as an anode material for energy storage devices due to their enhanced surface area, single step synthesis and shorter diffusion length.[1] Due to these enhanced surface properties, carbon nanofibers showed promising performance even at high C-rate applications.[2] More often, carbon derived from Polyacrylonitrile and its composites are explored as anode material for energy storage devices due to high carbon yield.[1,3,4] However, some other polymeric carbon precursors like polypyrrole, polyvinyl alcohol, and more recently, SU-8 negative photoresist were also explored as an electrode material for a rechargeable Lithium-ion battery. Use of SU-8 facilitates fabrication of three-dimensional electrodes as it can be patterned using photolithography, but it is an expensive source for carbon. The present study is the first report, where cellulose acetate derived carbon fibers have been used as an anode for lithium-ion battery. Cellulose acetate is an inexpensive polymeric carbon precursor. Moreover, our group has recently developed template-assisted electrospinning technique to produce patterned three-dimensional cellulose acetate nanofibrous structures [5] which can be explored further for higher surface area and enhanced performance