Electrochemical electrodes fabricated from a polymer precursor material and a method and a system for fabricating electrochemical electrodes using a deep defocused laser

Abstract

There is provided a system, method, and electrochemical electrodes fabricated from a polymer precursor material. Fabrication of the electrochemical electrodes includes applying a defocused laser pulse by a laser to a micro-scale area of the polymer precursor material, the laser positioned at a defocused distance relative to the polymer precursor material; and repeatedly, until a predetermined pattern is generated: moving the laser or the polymer precursor material; and subsequently or concurrently applying the defocused laser pulse to further micro-scale areas of the polymer precursor material, wherein the polymer precursor material with the predetermined pattern applied forms the electrochemical electrodes.

Description

ELECTROCHEMICAL ELECTRODES FABRICATED FROM A POLYMER PRECURSOR MATERIAL AND A METHOD AND A SYSTEM FOR FABRICATING ELECTROCHEMICAL ELECTRODES USING A DEEP DEFOCUSED LASER TECHNICAL FIELD

[0001] The present disclosure relates generally to the field of converting a polymer precursor into carbon material, and more particularly to electrochemical electrodes fabricated from a polymer precursor material and a method and a system for fabricating electrochemical electrodes using a deep defocused laser.BACKGROUND

[0002] Laser-induced graphitic carbon (LIGC) is a cost-effective alternative approach to directly pattern carbon material. A large number of miniaturized devices can be fabricated in a single step and in a short time. It is applicable for many applications, including energy storage in micro-supercapacitor (MSC). During graphitization of the carbonized substrate polymer, the laser parameters control electrical properties and morphology of the generated LIGC. However, MSCs fabricated using LIGC show limited low capacitance, which is a substantial bottleneck in the use of MSCs fabricated in this way.SUMMARY

[0003] In an aspect, there is provided electrochemical electrodes fabricated from a precursor material, the fabrication of the electrochemical electrodes comprising: applying a defocused laser pulse by a laser to a micro-scale area of a surface of the precursor material, the laser positioned at a defocused distance relative to the surface of the precursor material; and repeatedly, until a predetermined pattern is generated: moving the laser or the precursor material; and subsequently or concurrently applying the defocused laser pulse to further micro-scale areas of the surface of the precursor material, wherein the precursor material with the predetermined pattern applied forms the electrochemical electrodes.

[0004] In a particular case of the electrochemical electrodes, the precursor material comprises a carbonaceous material or a composite of a carbonaceous material with another material.

[0005] In another case of the electrochemical electrodes, the precursor material comprises a polymer.

[0006] In yet another case of the electrochemical electrodes, the polymer comprises one or more of polyimide, polyether ether ketone, polyetherimide, polysulfone, or lignin.

[0007] In yet another case of the electrochemical electrodes, the electrochemical electrodes are incorporated into one of a sensor, biosensor, battery, hydrogen generator, air-cathode, or anode.

[0008] In yet another case of the electrochemical electrodes, the electrochemical electrodes comprise graphitic carbon having a porous morphology.

[0009] In yet another case of the electrochemical electrodes, the defocused laser pulse converts the precursor material into graphitic carbon.

[0010] In yet another case of the electrochemical electrodes, the electrochemical electrodes comprise graphitic carbon having a porous morphology.

[0011] In yet another case of the electrochemical electrodes, successive laser pulses spatially overlap within the precursor material.

[0012] In yet another case of the electrochemical electrodes, the predetermined electrode pattern comprises an interdigitated electrode structure.

[0013] In another aspect, there is provided a method for fabricating electrochemical electrodes from a precursor material, comprising: applying a defocused laser pulse to a micro-scale area of the surface of the precursor material; and repeatedly, until a predetermined pattern is generated: moving a source of the defocused laser or the precursor material; and subsequently or concurrently applying the defocused laser pulse to further micro-scale areas of the surface of the precursor material, wherein the precursor material with the predetermined pattern applied forms the electrochemical electrodes.

[0014] In a particular case of the method, the defocused laser pulse is generated by positioning a laser at a defocused distance relative to the precursor material.

[0015] In another case of the method, successive laser pulses spatially overlap.

[0016] In yet another case of the method, the spatially overlapping laser pulses produce a porous graphitic carbon morphology.

[0017] In yet another case of the method, the laser is operated at a power level selected to avoid ablation of graphitic carbon during fabrication.

[0018] In yet another case of the method, wherein the predetermined electrode pattern comprises interdigitated electrodes for a micro-supercapacitor.

[0019] In another aspect, there is provided a system for fabricating electrochemical electrodes from a precursor material, comprising: a laser positioned at a defocused distance relative to the precursor material; and a processor in communication with a data storage to execute instructions via an instrument interface, the instructions comprising: directing the laser to apply a defocused laser pulse to a micro-scale area of the surface of the precursor material; repeatedly, until a predetermined pattern is generated: directing movement of the laser or the precursor material; and subsequently or concurrently directing the laser to apply the defocused laser pulse to further micro-scale areas of the surface of the precursor material, the precursor material with the predetermined pattern applied forms the electrochemical electrodes.

[0020] In a particular case of the system, the laser comprises a CO2laser.

[0021] In another case of the system, the processor provides instructions to cause spatial overlap of successive laser pulses at the precursor material.

[0022] In yet another case of the system, the laser operates at a power level selected to avoid ablation of graphitic carbon during fabrication.

[0023] These and other aspects are contemplated and described herein. It will be appreciated that the foregoing summary sets out representative aspects to assist skilled readers in understanding the following detailed description. Also, it will be appreciated that any of the above features can be combined.DESCRIPTION OF THE DRAWINGS

[0024] A greater understanding of the embodiments will be had with reference to the Figures, in which:

[0025] FIG. 1A shows a schematic diagram of laser-induced graphitic carbon (LIGC) engraving in accordance with example experiments of the present embodiments;

[0026] FIG. 1B shows high resolution scanning electron microscope (HRSEM) images of the LIGC materials engraved with 10% laser power at defocus position DF-1, in accordance with the example experiments of the present embodiments;

[0027] FIGS. 1C, 1D, and 1E shows HRSEM images of LIGC materials engraved with 23% laser power at defocus positions DF-4, DF-5, and DF-6, respectively, in accordance with the example experiments of the present embodiments, where scale bars of all SEM images and their insets represent 5 pm and 500 nm respectively;

[0028] FIGS. 1F and 1G shows transmission electron microscopy (TEM) images of the LIGC materials engraved with 10% laser power at DF-1 and with 23% laser power at DF-4, respectively, in accordance with the example experiments of the present embodiments, where scale bars of TEM images and their insets represent 50 nm and 2 nm;

[0029] FIG. 1H is a graph showing X-ray diffraction (XRD) of the LIGC materials engraved at DF-1 with 10% laser power and DF-4 with 23%, in accordance with the example experiments of the present embodiments;

[0030] FIG. 2A shows Raman spectra for LIGC materials engraved at (i) DF-1 position with 10% laser power, and (ii-iv) DF-4, DF-5, DF-6 defocus positions with 23% laser power, respectively, in accordance with the example experiments of the present embodiments;

[0031] FIG. 2B is a graph showing the ratio of integrated areas of the deconvoluted D1, D3, D4, and 2D peaks with respect to the G peak versus defocus position%, in accordance with the example experiments of the present embodiments;

[0032] FIG. 2C shows XPS C1 spectra with deconvoluted C=C, C-C&C-N, C-O, and 0=C-0 peaks for (i) PI material and LIGC material engraved at (ii) DF-1 position with 10% laser power and (iii-v) DF-4, DF-5, DF-6 defocus positions with 23% laser power, in accordance with the example experiments of the present embodiments;

[0033] FIG. 2D shows carbon, oxygen, and nitrogen percentages from XPS survey spectra and integrated areas of the deconvoluted C=C, C-C&C-N, C-O, and 0=C-0 peaks in the C1s peak plotted against material type and defocused positions, in accordance with the example experiments of the present embodiments;

[0034] FIG. 2E shows sheet resistance and specific capacitance of MSC devices for different defocus positions plotted against different engravable laser powers avoiding ablation, in accordance with the example experiments of the present embodiments;

[0035] FIG. 3A shows cyclic voltammetry curves of MSCs having electrodes engraved at in-focus position DF-1 with 10% laser power, and defocus positions DF-4, DF-5, and DF-6 with 23% laser power, in accordance with the example experiments of the present embodiments;

[0036] FIG. 3B shows calculated capacitances of the MSCs plotted against scan rate, in accordance with the example experiments of the present embodiments;

[0037] FIG. 3C shows charge-discharge responses of MSCs of DF-1 with 10% and DF-6 with 23% laser power measured at a constant applied total current of 75 pA, in accordance with the example experiments of the present embodiments;

[0038] FIG. 3D shows galvanostatic charge-discharge (CD) specific capacitance of the MSCs at different total applied currents, in accordance with the example experiments of the present embodiments;

[0039] FIG. 3E shows capacitance retention cycling performance over 10,000 CD cycles measured at a total applied current of 3 mA to a MSC prepared at defocus position DF-4 and 23% laser power with calculated coulombic efficiency, in accordance with the example experiments of the present embodiments;

[0040] FIG. 3F shows energy density normalized by active area calculated at different power densities from CD results for DF-1, DF-4, DF-5, and DF-6 MSCs an Ragone plot, in accordance with the example experiments of the present embodiments;

[0041] FIG. 4 illustrates a schematic of an example laser engraving machine used in the example experiments with different parts of the laser, including CO2 laser tube, moveable laser carrier, and laser bed or platform;

[0042] FIG. 5A is a chart showing workable laser power windows for all defocus positions between threshold laser power and ablation limit, in accordance with the example experiments of the present embodiments;

[0043] FIG. 5B is a chart showing (top) thickness, and (bottom) width of all rectangular shape LIGC against laser power percentages in workable windows for the defocus positions, in accordance with the example experiments of the present embodiments;

[0044] FIG. 5C is a chart showing width, and FIG. 5D is a chart showing average diameter of the LIGC single pulse spot, in accordance with the example experiments of the present embodiments;

[0045] FIG. 5E is a diagram showing a schematic of the single pulse at the middle of 1000 pm width of the rectangular shape in which laser pulses sites are mapped out with resolution of 1200 dots per inch horizontally and vertically, in accordance with the example experiments of the present embodiments;

[0046] FIG. 5F is a chart showing number of pulse sites traced or overlap in this single pulse spot, in accordance with the example experiments of the present embodiments;

[0047] FIG. 5G is a chart showing thickness of selected samples, in accordance with the example experiments of the present embodiments;

[0048] FIG. 6A shows an optical top view of the rectangular shape LIGC material engraved under different laser power percentages at different defocus positions showing the pattern width, in accordance with the example experiments of the present embodiments;

[0049] FIG. 6B shows optical cross-sectional (thickness) images of the rectangular shape LIGC materials engraved at different defocus positions under different laser power percentages, in accordance with the example experiments of the present embodiments;

[0050] FIG. 7 shows a scanning electron microscope top view of an LIGC engraved at DF-1 with 10% laser power (image (a)), and engraved at DF-4 (image (b)), DF-5 (image (c)), DF-6 (image (d)) with 23% laser power, in accordance with the example experiments of the present embodiments;

[0051] FIGS. 8A and 8B are images from transmission electron microscopy (TEM) of LIGC engraved at DF1 with 10% laser power in FIG. 8A, and at DF-4 with 23% laser power FIG. 8B, in accordance with the example experiments of the present embodiments;

[0052] FIG. 9 is a chart of an XPS survey spectra of PI and LIGC engraved at different defocusing positions of DF-1, DF-4, DF-5, and DF-6, in accordance with the example experiments of the present embodiments;

[0053] FIGS. 10A to 10D show optical images of prepared MSG devices with in-focus DF-1 with 10% laser power shown in FIG. 10A, defocus DF-4 shown in FIG. 10B, DF-5 shown in FIG. 10C, and DF-6 shown in FIG. 10D, in accordance with the example experiments of the present embodiments;

[0054] FIG. 11 shows charts of cyclic voltammetry curves recorded at 5 mV / s scan rate of MSCs prepared at all defocus positions, in accordance with the example experiments of the present embodiments;

[0055] FIG. 12 is a chart showing cyclic voltammetry curves recorded at 5 mV / s scan rate of MSCs with electrodes prepared in-focus DF-1 with 10% laser power, and at defocus positions of DF-4, DF-5, and DF-6 with 23% laser power, in accordance with the example experiments of the present embodiments;

[0056] FIG. 13 shows cyclic voltammetry curves recorded at different scan rates between 1 to 100 mV / s of the MSCs prepared at normal focus DF-1 with 10% laser power, and defocus DF-4, DF-5, and DF-6 with 23% laser power, in accordance with the example experiments of the present embodiments;

[0057] FIG. 14 shows galvanostatic charge-discharge curves recorded at different total currents between 75 pA and 750 pA of the MSCs prepared with in-focus DF-1 with 10% laser power, and defocus DF-4, DF-5, and DF-6 with 23% laser power, in accordance with the example experiments of the present embodiments;

[0058] FIG. 15 is a diagram of a system for fabricating a micro-supercapacitor using a deep defocused laser, in accordance with an embodiment; and

[0059] FIG. 16 is a flowchart of a method for fabricating a micro-supercapacitor using a deep defocused laser, in accordance with an embodiment.DETAILED DESCRIPTION

[0060] Embodiments will now be described with reference to the figures. For simplicity and clarity of illustration, reference numerals may be repeated among the Figures to indicate corresponding or analogous elements. Specific details are set forth to provide a thorough understanding of the technical aspects and innovative features of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practised without these specific details, provided that the core innovative concepts are retained. In other instances, methods, procedures, and components which are known to a skilled person in the art have not been described in detail so as not to obscure the novel aspects of the embodiments described herein.

[0061] Various terms used throughout the present description should be read and understood as follows, unless the context indicates otherwise: “or” is inclusive, as though written “and / or”; singular articles and pronouns include their plural forms, and vice versa; similarly, gendered pronouns include their counterpart pronouns so that pronouns should not be understood as limiting anything described herein to use, implementation, or performance by a single gender; “exemplary” means “illustrative” or “exemplifying” and not necessarily “preferred” over other embodiments. Further definitions for terms are provided to ensure clarity and consistency in understanding the technical features and innovations described herein. These definitions apply to prior and subsequent instances of those terms as understood from reading the present description, ensuring that the novel and inventive aspects are consistently interpreted.

[0062] Any module, unit, component, server, computer, terminal, engine, or device exemplified herein that executes instructions described herein, may include access computer readable media, such as storage media, computer storage media, or data storage devices (removable and / or nonremovable) such as, for example, magnetic disks, optical disks, or tape. Computer storage media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information, such as computer readable instructions, data structures, program modules, or other data. Examples include RAM, ROM, EEPROM, flash memory or other non-transitory memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transitory medium that stores the desired data and / or information and can be accessed for execution of an application, module, or both. Any such non-transitory media may be part of the device or accessible thereto or may otherwise provide functionality to the system to enable efficient data storage and retrieval. Processors or controllers set out herein may be implemented as a single processor or a plurality of processors, which maybe arrayed or distributed, such as in a cloud-based model, in order to carry out processing functions individually or collectively. Methods, applications, or modules described herein may be implemented using computer readable / executable instructions that may be stored or otherwise held by such non-transitory media and executed by one or more processors to be used in the realization of the embodiments described herein.

[0063] Light-weight, low-cost electrochemical energy storage devices are in high demand in order to supply sufficient energy to run devices in applications such as smart Internet of Things (loT) devices. It is preferable to have a manufacturing process that is cost effective, clean, safe, and environment-friendly. Lithium-based batteries are generally not safe and environmentally friendly, have short lifespans, and offer low power density; however, such batteries offer high energy density in many large-scale applications. Therefore, in low current dependent small devices where long life of the energy storage device is required, Li batteries are preferably replaced with other efficient energy storage devices. An efficient supercapacitor provides a replacement for Li batteries because it offers a very long life, faster charge-discharge (i.e., high power), and is generally more environmentally friendly than existing Li-ion batteries.

[0064] However, in addition to low charge storage ability, supercapacitors generally require the use of complicated small-scale electrode generation on expensive substrates. Such generation generally involves laborious and complex steps, such as lithography, or inkjet printing or screen printing with wet chemicals. In addition, such generation can include expensive electrolytes to cover the electrode and complex assembly. Therefore, micro-scale supercapacitors or micro-supercapacitor (MSC) have a number of substantial problems to overcome in order for widespread adoption. The present disclosure provides a method to fabricate low-cost MSC electrodes, that overcomes the substantial problems in the art, in order to arrive at an MSC with a significantly higher energy storage capacity than other approaches.

[0065] Graphene and its derivative materials have aroused tremendous research interest in material science since its discovery owing to their intriguing properties. Several routes have been proposed for their scalable fabrication for many applications, mainly classified into top-down and bottom-up. But complex, lengthy, and costly processing steps under harsh conditions for material deposition and patterning ultimately limit their use. Laser-induced graphitic carbon (LIGC) presents a scalable technique with the ability to generate and pattern graphitic carbon materials in a single step by laser engraving on certain polymer surfaces like widely available polyimide (PI) film (or Kapton).

[0066] The laser-induced graphitic graphene (LIGC) direct patterning of carbon material in conjunction with gel electrolytes is a cost-effective, one-step alternative technique to fabricate a large number of electrodes for micro-supercapacitor devices in a single step and a short time. However, its low charge storage ability (2-5 mF. cm-2) is a substantial obstacle for its widespread adoption and application. Generally, electrode modification is required to enhance energy storage ability to meet the application requirements. This adds additional steps and increases the preparation cost of the MSG. The presently described embodiments have advantageously achieved a large energy storage capacity of, for example, 133 mF. cm-2without necessitating further electrode modification and with a gel-electrolyte. In this way, the present embodiments provide a low-cost, safe, and environmentally friendly approach. Moreover, the present embodiments provide the ability for a large number of charge-discharge cycles. These substantial advantages were achieved by, at least, modification to a laser writing approach that provided control over the properties of the generated material and, ultimately, charge storage ability without significantly compromising any other device performance.

[0067] LIGC engraving generally consists of an interaction between a laser beam and a surface of the polymer substrate. Heat generation, depolymerization, carbonization, and graphitization of the small area of polymer transforms it into localized LIGC material. Such interaction can be manipulated with the help of different laser machine parameters, which is crucial for LIGC generation of different types. Changes in LIGC material properties can be accomplished with changes in laser parameters, such as laser carrier speed, pulse spacing, and laser power. In addition, morphological changes in LIGC can be accomplished as a result of laser defocusing. For example, it is known in the art that morphological changes in LIGC are produced by laser engraving of different substrates under defocus conditions due to enhanced overlap of the bigger laser pulse spot size due to defocusing. In an example, an enlarged laser pulse spot size of 300 pm compared to an in-focus spot of 175 pm is reported for a defocusing of just 1 mm compared with a focal length of 50 mm of the laser lens used in engraving. In another example, morphological changes in laser-induced graphene can be produced as a result of more defocus laser engraving than 1 mm of a flat polyimide surface placed at an angle of 45°. In both examples, a smaller focal length lens of 50 mm with maximum defocus of only 5% was used to carbonize the surface polymer during engraving. This effect of this shallow defocusing on the morphology was studied in the art, such as under a small defocusing distance under 3 mm compared to 50 mm focal distance and smaller in-focus spot size laser (i.e. , 125 pm) that is at most increased to 300 pm with maximum defocus. It leads to a maximum of 109 pulses overlapping for 1000 dotper inch (DPI) image resolution. DPI is a computer-controlled image resolution that controls the spacing between two consecutive laser pulses on a polyimide surface and controls the number of pulses that overlap in a unit area.

[0068] In contrast to the above, the present embodiments are directed to deep-defocus laser engraving using a lens of higher focal length (e.g., 100 mm) to fabricate micro supercapacitors, and 1200 DPI resolution instead of the above 1000 DPI resolution. In example embodiments, a defocus distance between, for example, a minimum of 15 mm can be used, and a larger in-focus spot size laser of, for example, 280 pm can be used; which can be increased to 860 pm at 15 mm defocus. This approach can lead to nearly 1300 laser pulses overlap, which is significantly higher than the above approach with 109 pulses overlapping. The present inventors determined that the emerging laser pulses out of higher focal length lens cause a deeper carbonizing effect into the polymer material than the surface.

[0069] FIG. 15 illustrates a diagram of a system 100 for fabricating electrochemical electrodes from a polymer precursor material, in accordance with an embodiment. The system 100 can include a number of physical and logical components, including a processor 52, a data storage 54, an instrument interface 56, a user interface 60, and a bus 80 enabling the components to communicate with each other. The data storage 54 stores any data and / or operating instructions to be used by the processor or other components. The communication interface 60 enables communication of the activities undertaken by the system 100 to be communicated to another system or to a user via a suitable device, and / or to receive instructions or information from the user or other system. In further cases, the communication interface 60 receives and / or stores such information in the data storage 54.

[0070] FIG. 16 illustrates a method 200 for fabricating electrochemical electrodes from a polymer precursor material, in accordance with an embodiment.

[0071] While the present disclosure generally describes electrochemical electrodes for use as part of a micro-supercapacitor, it should be appreciated that the electrochemical electrodes can be used as part of any suitable application; for example, as part of chemical sensors, biosensors, air cathodes, battery anodes, hydrogen generators, or the like. Additionally, while the present disclosure generally describes the polymer precursor material as being a polyimide polymer material, it should be appreciated that any suitable polymer precursor material can be used; for example, one or more of polyether ether ketone, polyetherimide, polysulfone, lignin, or the like.

[0072] At block 202, the communication interface 56 receives indication to begin fabrication of the electrochemical electrodes.

[0073] At block 204, the instrument interface 56 instructs the laser 80 to apply a defocused laser pulse of a given energy to a localized micro-scale area of a tape of precursor material 90. While the present disclosure generally refers to the preferred case of a polymer precursor material, it should be understood that the precursor material 90 can be comprised of a carbonaceous material or a composite of a carbonaceous material with another material. In the case of a polymer precursor material, the polymer can be comprised of, for example, one or more of polyimide, polyether ether ketone, polyetherimide, polysulfone, or lignin.

[0074] "Micro-scale" is to be given its meaning as understood by a person skilled in the art, which, as an example, can refer to a scale ranging from approximately 1 to 1000 micrometers (pm). The above application of the laser converts the surface polymers in the path of the laser into graphitic carbon due to localized heat generated owing to the laser absorption into the polymer. The instrument interface 56 controls parameters of the laser such that the generation of heat from the laser pulses carbonize and then graphitize the polymers of the localized micro-scale area of the polymer precursor material 90.

[0075] The system 100 generates a defocused engraving by having the laser parameters include a focal length lens of greater than 50 mm; and preferably, a focal length of 100 mm. In a particular case, the laser parameters further include a defocus distance of between approximately 5 to 30 mm; and preferably between approximately 15 to 30 mm (i.e. , 15% to 30% defocus compared to 100mm focal length of the lens). For the cases of a defocus distance between 15 to 30 mm, the laser power is set at 23%. It should be understood by a person skilled in the art that other suitable laser powers can also be used. In a particular case, the laser parameters further include a spot size of approximately 280 pm.

[0076] At block 206, the instrument interface 56 instructs the laser 80 to apply the laser pulse to another localized micro-scale area of the polymer precursor material 90 by moving the laser 80, the polymer precursor material 90, or both, via any suitable positioning mechanism. At block 208, the instrument interface 56 repeats the application of the laser pulse to a plurality of micro-scale areas of the polymer precursor material 90, each application subsequent or concurrent with the movement, until a predetermined pattern is generated. For example, a rectangular line pattern of 1 mm width and 15 mm length. In most cases, successive laser pulses can be overlapping.

[0077] At block 210, the communication interface 56 indicates that the fabrication of the electrochemical electrode is complete due to the polymer precursor material 90 having the completed predetermined pattern applied.

[0078] In some cases, by controlling laser parameters and laser writing, the instrument interface 56 can not only fabricate different microscale patterns of graphitic carbon but can also control the generated graphitic carbon morphologies and properties.

[0079] In some cases, the instrument interface 56 can fabricate porous graphitic carbon, instead of graphene, by controlling the substrate position. The porous graphitic carbon advantageously offers high energy storage ability. For the generation of porous graphitic carbon, other than laser pulse power and laser carrier speed, deep defocusing is causing it to be porous because of the significant increase in the laser pulses overlapping; for example, 1300 pulses in an example of the present embodiments compared to, for example, 109 in other approaches. Deep defocus changes pulse energy distribution on the surface, and hence, affects the carbonization and graphitization processes, which causes porous morphology.

[0080] Advantageously, in an example, the system 100 is able to achieve highly conductive granular nano-porous laser-induced graphitic carbon (LIGC) material on the surface of readily available polyimide (PI) or Kapton tape by defocusing the laser spot. Owing to the observed material-structural changes (e.g., granular nano-pores of minimum 5 nm diameter) in deep defocus engraved electrode material, MSCs engraved with deep defocus were determined to exhibit a very high specific capacitance of 133 mF / cm2, which is nearly 20 times higher compared with in-focus engraved MSC. Moreover, by changing the defocusing position, a change in sheet resistance of the LIGC, and corresponding change in capacitance, the present inventors were able to demonstrate rate capability and capacitance retention at high currents. The defocus engraved MSC exhibited capacitance that was improved by 25% after 10,000 cycles compared with its first cycle capacitance.

[0081] Generally, the porous morphology of the defocus LIGC provides more space for the present hydrogen ions in the gel electrolytes when compared to normal in-focus generated approaches; and hence, enhances the fabricated MSC in charge storage ability.

[0082] The MSC fabricated using the system 100, using unmodified LIGC as the electrode material, provides a comparable charge storage ability and lifespan when compared to other micro-supercapacitors manufactured with much more complex and expensive methods.Generally, other MSCs made of graphitic material rely on two approaches: (1) bottom-up, and (2) top-down. In the first approach, a very small amount of graphene material can be produced with the help of a very expensive apparatus, such as chemical vapor deposition (CVD) on a silicon substrate. This approach then further passes through a complex process of photo-lithography of patterning to prepare the graphene-based electrode of a specific shape and size. In the second approach, a large amount of graphite material is passed through a selective wet chemical-based exfoliation process, which requires process safety to be maintained, and then passed to a printing process. Both approaches are expensive and laborious because of the necessity to use expensive apparatuses and chemicals. Both approaches are time-consuming lengthy processes. Additionally, both approaches generate a lot of chemical waste that must be managed. In contrast, the present method 200 is much simpler and cheaper than the above approaches, significantly increasing the capacity of LIGC MSCs to become viable for a multitude of applications; such as being used in Internet-of-Things devices, autonomous small scale devices, micro-scale sensors, wireless sensor, wearable electronics, electronic watches, hearing aids, Bluetooth adapters, digital mobile network systems, flexible smartphones, microdevices, and the like.

[0083] The laser 80 and polymer substrate interaction can be manipulated with different laser 80 machine parameters, which permits LIGC generation of different types. An example of the laser 80 machine’s parts include (i) a laser beam tube (ii) a movable laser carrier, and (iii) a horizontal bed or platform. The laser tube produces a laser beam, and an on-off mechanism turns this beam into pulses of variable energy. With a reflector and concave lens, the moveable laser carrier reflects and focuses the beam perpendicularly at the focal point on the laser bed or platform. In an example, PI tape attached to a flat glass slide can serve as the substrate and placed at the focal point of the laser on the platform. The laser bed can be moved up and down to control the laser focus. The distance between the lens and the focal point is considered as the normal focus or in-focus distance (i.e. , which is position DF-1 in FIG. 1A). Other than the laser focus, other example laser parameters that govern the laser-PI interaction are (1) spacing between the laser pulses, (2) carrier speed, and (3) laser power. The laser pulses are scanned over the substrate by overlapping of these laser pulses to form the desired pattern. In a particular case, a rectangular line pattern of 1 mm width and 15 mm length can be used to prepare engraved samples and interdigitated structures to prepare MSCs. In an example, an infrared CO2 laser of wavelength 10.6 pm with a maximum power of 75 W can be used; however any suitable laser can be used. Power can be varied by pulse width modulation. A fixed laser carrier speed and spacing between pulses can also be used for laser engravings. Defocussed engraving can generally be achievedby bringing the substrate closer to the lens in, for example, 5 mm steps. Pulse laser engraving can be used with gaps between the pulses. Computer image resolution of, for example, 1200 dots per inches (DPI) or 47 dots per millimeter (DPmm) can be used. In an example, engraving 1 mm wide rectangle of 15 mm length results in nearly 47 lines horizontally and engraving 1 mm of each line results in 47 pulses in every 1mm length of this line.

[0084] In order to initiate the laser engraving process on PI, a minimum threshold laser power for a fixed laser carrier speed and pulse spacing is determined in order to produce the LIGC. Conversely, laser engraving with extremely high laser power ablates the LIGC. Ablation is the simultaneous generation and removal process of LIGC. Therefore, to obtain stable LIGC adhering to the PI substrate, power has to remain in a workable range between the threshold and the ablation level.

[0085] The present inventors determined that, advantageously, high laser power ablation is avoidable by defocussing the laser spot. In example experiments illustrating the present embodiments, other than an in-focus position referred to as ‘DF-1’, five defocusing positions were examined. Each position was separated by a constant distance of 5 mm, were used for LIGC material generation; referred to as ‘DF-2’, ‘DF-3’, ‘DF-4’, ‘DF-5’, and ‘DF-6’ respectively. Example experiments conducted by the present inventors determined that there is a shift in threshold laser power to high percentages under defocusing, e.g., 21% for defocus DF-6 compared with 10% for DF-1. The experiments also determined that there was an increase in the range of the workable laser power percentage window, e.g., a higher range from 17% to 25% power for deep defocus DF-4 position compared with 10% to 12% for normal focus DF-1. For the deep defocus engraving position of DF-6, this window was reduced again to 21%-25%. A change in the width and thickness of the LIGC line patterns engraved inside these power windows with different defocusing was observed. There was a recorded increase in thickness and width values with increasing laser power for each defocus position between DF-1 to DF-6. The highest 1940 pm width of DF-6 at 25% laser power was recorded.

[0086] The example experiments used a rectangular shape computer image of 1000 pm width. Width of the engraved LIGCs after engraving was found to be higher than 1000 pm and the rectangular shaped LIGC images showed variation with defocusing positions. With a width of 1860 pm of the rectangular LIGC engraved at defocus position DF-4, the LIGCs engraved at the same laser power of 23% at higher defocus positions showed smaller values. The Exampleexperiments determined that an engraved area of LIGC by each laser pulse varies with defocusing.

[0087] Energy E of a single laser pulse can be calculated using p power (J / s), v speed (m / s), andimage resolution t (dots per m) in the equation E = Therefore, calculated energy of a singlepulse of 23% laser power is 3.8 mJ. Similarly, 10% laser power pulse delivers 1.6 mJ energy. Considering single pulse energy, its average diameter, and average number of pulses in single LIGC pulse spot, the energy per pm2area in the middle of the spot of the in-focus engraving under 10 % laser power is 3.7 pJ / pm2. Engraving, even under higher laser power of 23%, exhibits 8.5 pj / pm2energy density without ablation. It is worth noticing that laser engraving at in-focus position under 23% will ablate the LIGC, but under defocusing conditions engraving under 23% laser power laser engraving will not ablate the LIGC. Defocussing can cause a larger decrease in thickness than the width of the rectangular LIGC shape using the same engraving conditions.

[0088] In order to investigate the structural changes for different focus positions, the example experiments examined LIGC materials at different resolutions using scanning electron microscopy (SEM), high-resolution SEM (HRSEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). Samples at DF-4, DF-5, and DF-6 were engraved using the same laser power of 23%. In-focus samples (DF-1) for comparison were engraved using 10% laser power, as LIGC was ablated at higher powers. The SEM image of the top surface of in-focus DF-1 LIGC material showed macro-scale craters of different sizes and thick nano-flakes at the edges of craters. Conversely, images of the defocussed LIGC (i.e., DF-4-6) showed a cover layer. HRSEM images of this top surface layer in DF-4 exhibited a large number of nano-scale particles. The particle density was decreased in HRSEM images of the more defocused LIGC DF-5 and DF-6. Similarly, SEM images of the inner surfaces of these deep defocus samples DF-4-6, with the top layer removed, also showed changes in defocused engraved LIGC compared to in-focus LIGC. Similar to the top, the inner surface of DF-1 also exhibited thick flakes of variable sizes at the edges of craters. However, the inner surface of the defocus LIGC at DF-4 showed flake-free craters of bigger size compared with DF-1. Crater size was further increased for DF-5 and DF-6.

[0089] To further see changes on the inner surfaces, HRSEM of DF-1, DF-4, DF-5, and DF-6 was performed in the example experiments. These images revealed LIGC flakes for in-focus LIGC (DF-1), and large craters with a porous surface for all other defocus LIGC material (i.e., DF-4-6). Porosity generally increased with defocusing. The morphology transforms from nano-flakes and small-size craters at the top and inside of in-focus (i.e., DF-1) engraved LIGC material into LIGCwith nanoparticles on the top surface and bigger craters with a porous surface inside the defocus LIGC (i.e. , DF-4-6). As nanoparticle density reduces, crater size increases, and porosity improves with deeper defocussing; i.e., moving from DF-4 to DF-6. Therefore, LIGC material exhibits a morphological surface change with deep defocusing from flakes to porous craters along with a strong thickness decrease.

[0090] To examine the difference in crystallinity between defocus LIGC material and in-focus LIGC, TEM was performed on DF-1 and DF-4 samples. DF-1 TEM images show LIGC with few layer thick nano-flakes. Conversely, the DF-4 TEM images show nano-pores and semi-spherical nano-crystallites. The corresponding HRTEM images further show a lattice diffraction image of the (002) planes of the graphene in one of the thick graphitic flakes for DF-1 and in one of the nano-crystallites for DF-4. The calculated lattice spacing of 3.4 A matches other reports for LIGC and is as expected for graphite. This diffraction pattern is the manifestation of the sp2hybridization of the graphitic carbon atoms during graphitization in LIGC nano-flakes. It was evidenced that there was formation of graphitized nano-scale crystallites in defocused LIGC compared with graphitized thick nano-flakes for in-focus LIGC. SEM and TEM results confirmed the structural change from sp2hybridized thick nano-flakes in the in-focus LIGC to sp2hybridized nanocrystalline porous LIGC with defocus.

[0091] FIGS. 1A to 1H illustrate the outputs of the above experiments. FIG. 1 A shows a schematic diagram of the LIGC engraving process with the top left inset shows a schematic of the six defocus positions DF-1 (in-focus) and DF-2 to DF-6 having equal separation of 0.5 cm. FIG. 1B shows HRSEM images of LIGC materials engraved with 10% laser power at DF-1. FIGS. 1 C, 1D, and 1E shows HRSEM images of LIGC materials engraved with 23% laser power at DF-4, DF-5, and DF-6, respectively; with corresponding zoom-in images shown in the respective insets. Scale bars of all SEM images in FIGS. 1B to 1E and their insets represent 5 pm and 500 nm, respectively. FIGS. 1F and 1G shows TEM images of the LIGC materials engraved with 10% laser power at DF-1 and with 23% laser power at DF-4, respectively; and corresponding HRTEM images are shown in their respective insets. Scale bars of TEM images in FIGS. 1F and 1G, and their insets, represent 50 nm and 2 nm, respectively. FIG. 1H is a graph showing XRD of the LIGC materials engraved at DF-1 with 10% laser power and DF-4 with 23%.

[0092] To further investigate the quality of sp2hybridization associated with graphitization and other material changes, Raman spectroscopy measurements were performed. The results of infocus LIGC DF-1 and defocus LIGC DF-4, DF-5, and DF-6 are shown in FIG. 2A (i to ivrespectively). These Raman spectroscopy results exhibit the G and D bands in the first-order Raman spectra region and the 2D band in the second-order region between Raman shifts of 1100 cm-1to 1700 cm-1and 2200 cm-1to 3300 cm-1, respectively, which are typical for graphitic carbon. A careful deconvolution of the D band revealed nested D1, D2, D3, and D4 peaks at Raman shift values of 1350 cm-1, 1620 cm-1, 1500 cm-1, and 1200 cm-1, respectively. The deconvolution of two-phonon overtone 2D band shows nested peaks at 2450 cm-1, 2700 cm-1, 2900 cm-1, and 3200 cm-1named 2D4, 2D1, D1+G, and 2D2, respectively. The G band peak (at 1580 cm-1Raman shift) in graphitic carbon originates from the stretching or breathing of sp2hybridized 0=0 bonded aromatic carbon atoms of the graphene planes, and its relative intensity reflects the amount of these bonds. However, many defects in these planes bring about a similar relative intensity rise of different nested peaks in D bands with peak broadening, e. g., D1, D2, D3, and D4 peaks correspond to in-plane defects, physical defects, out-of-plane defects, and aliphatic carbon 0-0 functional groups, respectively, and a decrease in G peak intensity. Therefore, the relative ratios of these D band peaks with the G band can be related to the quality of the graphitic material.

[0093] The areas of the above peaks were integrated, and the ratios IG / ID1, IG / ID3, and IG / ID4 were calculated and plotted in FIG. 2B. High ratios for in-focus DF-1 and its prominent G peak exhibited a high amount of 0=0 bonded graphene planes and, therefore, a better quality of nanoflakes. G peak intensity decreased with defocusing in DF-4, DF-5, and DF6, which shows that the amount of 0=0 bonds in crystallites in engraved LIGO decreases with defocusing. FIG. 2B shows a decrease in these values with defocusing, and DF-6 exhibits the lowest ratios in the plot (i.e., IG / ID1, IG / ID3, IG / ID4 ratios of 0.3, 0.9, 1.6, respectively). It can be inferred that defocusing causes more defects in nano-crystallites of engraved LIGO, and DF-6 is the most defective LIGO material. This result is also confirmed by an increase in the IG / 2D ratio with defocusing in the same plot in FIG. 2B. Compared to the G peak, the 2D band peak intensities diminish and broaden with defocusing in FIG. 2A (i-iv), which exhibit no 2D peak for deep defocus (i.e., DF-6) in FIG.2A (iv). This shows the increased disorder in the nano-crystallites of the LIGO for DF-6. The 2D band peaks 2D4, 2D1, D1+G, and 2D2 result from overtone (i.e., double resonance in neighboring planes) in two neighboring phonons of the D4, D1, D1 and G, and D2 peaks, respectively and the effect of related defects appears as their intensity decrease / broadening. The absence of a 2D band peak in the Raman spectrum of deep defocus DF-6 suggests that this LIGO possesses the most disordered nano-crystallites. Although graphitic nano-crystallites can be observed in LIGO under defocusing instead of nano-flakes for in-focus LIGO, the Raman peak ratio trends suggest diminishing crystallite quality of these nano-crystallites. The most deteriorated and disorderedcrystallites are found in deep defocus DF-6. Moreover, this suggests that the amount of doublebonded (C=C) carbon atoms decreases with defocusing and an increase in the amount of singlebonded (C-C) carbon atoms inside the engraved LIGC. This is further confirmed in terms of chemical changes with X-ray photoelectron spectroscopy (XPS).

[0094] XPS measurements were performed on polyimide substrate and LIGC samples engraved in-focus (i.e., DF-1) and under defocused (i.e., DF-4, DF-5, and DF-6) conditions. Their deconvoluted C1s peaks are shown in FIG. 2C (i-v). The carbon, nitrogen, and oxygen atomic percentages calculated from the C1s, N1s, and O1s peaks are plotted in FIG. 2D (i). The highest carbon atomic percentage of 90% is found at defocus DF-4, which is higher than in-focus DF-1 (87%) and PI (80%), and slightly higher than deep defocus DF-6 (89%). Nitrogen and oxygen percentage changes follow the opposite trend to carbon as expected, as these elements are removed during the laser process. This can be attributed to the formation of LIGC on PI during infocus laser engraving and morphological changes of the engraved LIGC under defocusing, i.e., from nano-flakes for in-focus DF-1 to porous LIGC with a granular top layer for DF-4 to highly porous LIGC for DF-6 as seen in the SEM images in FIGS. 1B to 1E.

[0095] A small decrease in carbon percentage from XPS for the deep defocus DF-6 compared with DF-4 corresponds to the more disordered nano-crystallites in porous defocus LIGC. This variation is also shown in the deconvolution of the XPS C1s peaks. The percentage of surface functional groups C=C, C-C&C-N, C-O, and 0=C-0 were calculated by integration of the deconvoluted peaks centered at 284.4 eV, 285.5 eV, 286.6 eV, and 288.6 eV, respectively (see FIG. 2D (ii)). A large change from PI to DF-1 is observed in C=C surface groups (36% to 70%) and C-C surface groups (49% to 16%). This can be attributed to the depolymerization of single bonded (C-C) carbon atoms of the PI polymer chains and their carbonization and graphitization to double bonded (C=C) carbon atoms during the engraving process, which results in LIGC nanoflakes in DF-1. This can also be seen as broad and sharp C1s peaks in FIG. 2C (i) and FIG. 2C (ii) for PI and DF-1 LIGC, respectively. The variation in the other surface-functional groups O=C-O and O=C is small. Although compared to the DF-1 C1s peak, the DF-4 C1s peak is not very different, the deep defocus C1s peak of the porous LIGC is broadened again with corresponding changes in C=C and C-C amount. The C=C percentage is decreased to 52% for very deep defocus engraved DF-6 compared with 69% for DF-4; similarly, the C-C percentage is increased to 37% from 17. The same trend is also observed in Raman results as a large variation in the amount of C=C and C-C atoms inside the nano-crystallites (see FIG. 2B).

[0096] Accordingly, although a large amount of sp2hybridized C=C bonds are created for in-focus engraved LIGC nano-flakes compared with PI, this amount decreases both inside and on the surface of the deep defocus porous graphitic LIGC (DF-4, DF-5, and DF-6). The lowest amount of C=C can be observed in the very deep defocus engraved DF-6. Conversely, the amount of single bonded C-C atoms increases with deep defocusing both inside and at the surface. These changes can be related to the corresponding morphological changes seen in FIGS. 1A to 1H.

[0097] The LIGC material’s structural disorder affects its sheet resistance. To monitor these changes, the sheet resistance of each defocus engraved material was measured using a four-point probe and plotted in FIG. 2E (i). Between threshold laser power percentage and ablation, the workable laser power window varies per sheet resistance. The sheet resistance decreases with laser power within each curve for each defocus condition as expected due to the improvement in LIGC material quality with increasing laser power. The decrease in sheet resistance for in-focus DF-1 is small. Deep defocus DF-6 exhibits a larger decrease. The resistivity of the materials was calculated from sheet resistance using the measured thicknesses. In order to investigate the effect of defocusing, the resistivity of the deep defocus LIGC DF-4-6 was plotted for the same laser power of 23% in FIG. 2E (inset). This is compared with in-focus DF-1 at 10% laser power, as the material is ablated with 23% laser power at DF-1. The results show low resistivity values for DF-1 and DF-4 of 0.9 mQ.m, but the deep defocusing resistivity values were high with DF-6 exhibiting resistivity of 4.6 mQ.m. These electrical resistivity results are consistent with the corresponding structural changes of the deep defocus engraved LIGC (DF-4-6) materials. The material quality of porous deep defocus engraved LIGC at DF-4 is as high as the nano-flakes at DF-1, but more defocusing results in porosity changes in engraved LIGC. The higher resistivity of DF-4 LIGC than DF-1 may be because of the morphological difference between these two LIGC materials. Deep defocus DF-6 is highly disordered exhibiting a low amount of C=C bonds and a high amount of C-C in both Raman and XPS measurements compared with DF-4, which may explain its high resistivity. However, the highly porous LIGC material at DF-6 has a high capacity to store electric charge.

[0098] FIG. 2A shows Raman spectra with deconvoluted G, D1, D2, D3, and D4 peaks in the D-band and D+D”, 2D, D+G and 2D’ peaks in the 2D-band for LIGC material engraved at (i) DF-1 position with 10% laser power, and (ii-iv) DF-4, DF-5, DF-6 defocus positions with 23% laser power, respectively. FIG. 2B shows a ratio of the integrated areas of the deconvoluted D1, D3, D4, and 2D peaks with respect to the G peak versus defocus position. FIG. 2C shows XPS C1 spectra with deconvoluted C=C, C-C&C-N, C-O, and 0=C-0 peaks for (i) PI material and LIGCmaterial engraved at (ii) DF-1 position with 10% laser power and (iii-v) DF-4, DF-5, DF-6 defocus positions with 23% laser power. FIG. 2D (i) carbon, oxygen, and nitrogen percentages from XPS survey spectra and FIG. 2D (ii) shows integrated areas of the deconvoluted 0=0, C-C&C-N, C-O, and 0=0-0 peaks in the 01 s peak plotted against material type and defocused positions. FIG.2E (i) shows sheet resistance and FIG. 2E (ii) shows specific capacitance of MSC devices for different defocus positions plotted against different engravable laser powers avoiding ablation. The inset of FIG. 2E shows specific capacitance and resistivity simultaneously for DF-1 with 10% laser power and DF-4, DF-5, and DF-6 with 23% laser power.

[0099] In order to investigate the electric charge storage capacity and rate capability, MSCs were prepared with different LIGO electrodes and a gel electrolyte before measuring their electrochemical performance. MSC fabrication involved engraving of eight identical rectangularshaped interdigitated LIGC electrodes, marking and separating the active area with Kapton tape, connecting the electrodes by silver paste, and keeping in a vacuum for 2 hours after drop casting PVA / H2SO4 gel electrolyte on the marked area. Capacitances were calculated using area under the CV curves as shown in FIG. 2E (ii). Except for DF-1 and DF-2, every curve showed a capacitance increase with laser power, then a drop after a peak. The decrease in sheet resistance with increasing power also levels off at the same laser power corresponding to the peak capacitance (FIG. 2E (i)). Previously, similar trends of sheet resistance and capacitance with laser power below the limit of ablation had been reported, which can be attributed to structural changes of the produced LIGC material with different laser powers. However, in these experiments, there were material structural changes in porous LIGC under defocusing, which leads to an increase in ablation limit and peak capacitance. Peak capacitance and corresponding resistivity of DF-4, DF-5, and DF-6 at 23% laser power compared with in-focus DF-1 at 10% laser power are shown in the inset of FIG. 2E.

[0100] All defocus peak capacitances were higher than in-focus DF-1 by more than an order of magnitude, and an improvement in peak capacitance with deeper defocusing can be seen. Moreover, the inset of FIG. 2E shows no significant resistivity difference between in-focus DF-1 and defocus-DF-4 LIGC. Nevertheless, the difference in peak capacitance is very high, i.e., 6.4 mF / cm2versus 63 mF / cm2for DF-1 and DF-4, respectively. This can be attributed to the difference in their LIGC morphologies. Very deep defocus LIGC (DF-6) exhibits a very high capacitance of 105 mF / cm2as well as the highest resistivity. Lower scan rate CVs at 1 mV / s in FIG. 3A exhibit higher capacitances, i.e., 7 mF / cm2, 70 mF / cm2, 85 mF / cm2, and 133 mF / cm2for DF-1, DF-4, DF-5, and DF-6, respectively. These are pseudo rectangular CVs with a shape that suggests a typicalelectrochemical double layer (ECDL) formation on the surface of LIGC during the anodic and cathodic process. The area under the DF-1 CV curve was minuscule compared to that of all defocus CVs. This increase in capacitance with defocusing can be attributed to changes in the morphology. DF-1 and DF-4 exhibit similar amounts of carbon double bonds (C=C) and similar conductivities but different morphologies, i.e., nano-flakes for in-focus DF-1 LIGC and a nano-porous surface for defocus DF-4. Deep defocus LIGC DF-6 exhibited a more porous surface as well as higher capacitance than DF-4.

[0101] The effect of these changes on the charge storage rate capability of DF-1, DF-4, DF-5, and DF-6 was investigated by measuring CV of MSCs at different scan rates between 1 mV / s and 100 mV / s. The capacitances calculated from these measurements are plotted in FIG. 3B. CV curves at different scan rates for DF-1 show an approximately proportional current increase with scan rate, and capacitances calculated using area under the CV curves showed only a small capacitance decrease from 6.7 mF / cm2to 5 mF / cm2, which highlights the excellent rate capacity of DF-1. DF-4 shows similar current proportionality and stable capacitance with scan rate, but the low-rate capacitance of 71 mF / cm2was ten times higher than DF-1. This can be attributed to their morphological differences, i.e., nano-flakes with 87% carbon content for in-focus DF-1 and nano-porous with 90% carbon content for DF-4. Conversely, deeper defocus DF-6 LIGC exhibited relatively high porosity, 89% carbon content with comparatively low amount of C=C bonded atoms and high amount of C-C bonded atoms, and high resistivity and sheet resistance compared with DF-4. Therefore, distorted CV curves at high scan rates of deep defocus DF-5-6 MSCs can be attributed to the large resistivity of the material, which leads to a large capacitance drop for DF-6 at high scan rates.

[0102] The above capacitive behavior is further confirmed by the nearly triangular shape of galvanostatic charge-discharge (CD) curves obtained at different currents for the different LIGC MSCs. CD curves of DF-1 and DF-6 at the same constant current of 75 pA are shown in FIG. 3C. Calculated galvanostatic CD capacitances from the CD curves are plotted against the applied CD currents between 75 pA and 750 pA in FIG. 3D. Similar to CV scan rate response, DF-6 exhibits a higher capacitance drop than others from 95 mF / cm2to 35 mF / cm2, which is an effect of sluggish ECDL formation inside the porous MSC electrode at high currents due to high electrical resistance of LIGC engraved at a very deep defocus position of DF-6. Conversely, starting from 72 mF / cm2at low current, DF-4 showed a comparatively small capacitance drop to 44 mF / cm2at high current, implying a comparatively fast charge storing capability inside the nano-pores of DF-4 LIGC with lower sheet resistance than deeper defocus DF-5 and DF-6.

[0103] The cycling performance of DF-4 MSC was studied by charging and discharging 10,000 times at 3 mA. Percentage capacitance change compared to the capacitance of the first cycle and coulombic efficiency are shown in FIG. 3E. The capacitance increased by 25% after 10,000 cycles compared with the first cycle, which suggests a capacitance improvement after 10,000 charge-discharge cycles. The energy density normalized by active area was calculated at different power densities from CD results for DF-1, DF-4, DF-5, and DF-6 MSCs are shown in the Ragone plot of FIG. 3F. It shows DF-1 exhibits lower energy density compared to deep defocus DF-4-6 for the same power densities. All deep defocusing conditions showed more than 10 times higher energy densities than DF-1. At nearly 100 pW / cm2power density, the energy density of DF-6 is 12 pWh / cm2. Moreover, an improvement in rate capability was seen with defocus positions DF-4 and DF-5 compared with DF-6. These results were compared with other MSCs using pure LIGC material. Although different LIGC-electrode modifications could be used to increase the energy density, this increased the MSC fabrication cost. The deep defocus LIGC-based MSCs exhibited a higher energy density of up to 12 pWh / cm2than all previous reports of MSCs with unmodified LIGC electrodes prepared using a simple one-step laser engraving process.

[0104] FIG. 3A is a plot showing cyclic voltammetry curves of MSCs having electrodes engraved at in-focus position DF-1 with 10% laser power, and defocus positions DF-4, DF-5, and DF-6 with 23% laser power measured at 5 mV.s-1scan rate. FIG. 3B is a plot showing calculated CV capacitances of these MSCs plotted against scan rate. FIG. 3C is a plot showing chargedischarge responses of MSCs of DF-1 with 10% and DF-6 with 23% laser power measured at a constant applied total current of 75 pA. FIG. 3D is a plot showing CD specific capacitance of these MSCs at different total applied currents. FIG. 3E is a plot showing capacitance retention cycling performance over 10,000 CD cycles measured at a total applied current of 3 mA to a MSC prepared at defocus position DF-4 and 23% laser power with calculated coulombic efficiency. FIG. 3F is a Ragone plot of the devices of the present embodiments in comparison with other types of devices.

[0105] FIG. 4 illustrates a schematic of the laser engraving used in the example experiments with different parts of the laser, including CO2 laser tube, moveable laser carrier, and laser bed or platform.

[0106] FIG. 5A is a chart showing workable laser power windows for all defocus positions between threshold laser power and ablation limit. FIG. 5B is a chart showing (top) thickness, and (bottom) width of all rectangular shape LIGC against laser power percentages in the workablewindows for all defocus positions. FIG. 50 is a chart showing width, and FIG. 5D is a chart showing average diameter of the LIGO single pulse spot. FIG. 5E is a diagram showing a schematic of the single pulse at the middle of 1000 pm width of the rectangular shape in which laser pulses sites are mapped out with resolution of 1200 dots per inch horizontally and vertically. FIG. 5F is a chart showing number of pulse sites traced or overlap in this single pulse spot. FIG.5G is a chart showing thickness of the selected samples (DF-1 with 10% laser power, DF-4, DF-5, and DF-6 with 23% laser power). The same 1000x15000 pm rectangular shape computer image was used in each laser engraving process and pulse spacing remains the same (i.e., 21 pm).

[0107] FIG. 6A shows an optical top view of the rectangular shape LIGO material engraved under different laser power percentages at different defocus positions showing the pattern width; where scale bars in each image represent 1000 pm. FIG. 6B shows optical cross-sectional (thickness) images of the rectangular shape LIGO materials engraved at different defocus positions under different laser power percentages; where scale bars in each image represent 100 pm height.

[0108] FIG. 7 shows SEM top view of LIGO engraved at DF-1 with 10% laser power (image (a)), and engraved at DF-4 (image (b)), DF-5 (image (c)), DF-6 (image (d)) with 23% laser power. A closer view of the LIGO engraved in image (e) with DF-4 with 23% laser power and its respective HRSEM image (image (f)), HRSEM image of DF-5 (image (g)), and a closer view of DF-6 (image (h)). SEM of the LIGO material inside the film engraved at normal focus DF-1 (image (i)), defocus DF-4 (image (j)), DF-5 (image (k)), and DF-6 (image (I)).

[0109] FIGS. 8A and 8B are images from transmission electron microscopy (TEM) of LIGO engraved at DF1 with 10% laser power in FIG. 8A, and at DF-4 with 23% laser power FIG. 8B.

[0110] FIG. 9 is a chart of an XPS survey spectra of PI and LIGO engraved at different defocusing positions of DF-1, DF-4, DF-5, and DF-6.

[0111] FIGS. 10A to 10D show optical images of the prepared MSG devices with in-focus DF-1 with 10% laser power shown in FIG. 10A, defocus DF-4 shown in FIG. 10B, DF-5 shown in FIG.10C, and DF-6 shown in FIG. 10D; all with 23% laser power and a scale bar of 5mm.

[0112] FIG. 11 shows charts of cyclic voltammetry curves recorded at 5 mV / s scan rate of MSCs prepared at all defocus positions and all laser powers within the LIGC window.

[0113] FIG. 12 is a chart showing cyclic voltammetry curves recorded at 5 mV / s scan rate of MSCs with electrodes prepared in-focus DF-1 with 10% laser power, and at defocus positions of DF-4, DF-5, and DF-6 with 23% laser power.

[0114] FIG. 13 shows cyclic voltammetry curves recorded at different scan rates between 1 to 100 mV / s of the MSCs prepared at normal focus DF-1 with 10% laser power, and defocus DF-4, DF-5, and DF-6 with 23% laser power.

[0115] FIG. 14 shows galvanostatic charge-discharge curves recorded at different total currents between 75 pA and 750 pA of the MSCs prepared with in-focus DF-1 with 10% laser power, and defocus DF-4, DF-5, and DF-6 with 23% laser power.

[0116] The example experiments used a 125 pm thick PI sheet (i.e. , Kapton tape) stuck on a 1x2-inch glass slide as the substrate. Laser engraving followed to generate black LIGC. The pattern was defined by a 0.1x1.5 cm line image with 1,200 dot per inch image resolution corresponding to the pulse spacing. A CO2 pulse laser having 10.6 pm wavelength, normal or in-focus spot size of 250 pm, maximum laser power of 75 W, and a focal distance of 10 cm. Laser engraving was done using different laser power percentages that modulated the maximum power of 75 W using pulse width modulation having a fixed laser carrier speed. To prepare MSCs, an interdigitated electrode pattern was created using eight 0.1x1.5 cm lines that are spaced 1 mm apart. Engraving of this image under different laser powers and defocusing resulted in LIGC lines of different widths that changed the active area of the MSC; therefore, the experimentally measured area of engraved black LIGC was used for the specific capacitance calculation of an MSC. A gel electrolyte was drop cast over the interdigitated area. The device was exposed to vacuum for 2 hours and placed in ambient conditions for one day before electrochemical testing. The electrolyte was prepared by stirring 10 ml of deionized water, 1 ml of H2SO4, and 1 g of polyvinyl alcohol (PVA) for 1 h at 80 °C. Sheet resistance was measured for each rectangular LIGC electrode using the four-terminal method to eliminate confounding factors of contact and probe resistance. In order to supply a constant electric current of 1 pA and measure the voltage, four equally spaced contacts were made on rectangular LIGC with the help of silver paste.

[0117] A field emission SEM was used to observe the morphology of the LIGC with different resolutions. TEM images were obtained on a Talos L120C TEM using a LaB6 filament operating at 120 kV. Images of the LIGC were acquired with a Ceta 4k x 4k CMOS camera. A Dispersive Raman Microscope was used for Raman spectroscopy having 1.5 pm spot diameter, 10 mWpower, and 532 nm wavelength laser. Peak intensities, i.e., IG and ID1, ID3, ID4, and ID5, were the integrated area of the peaks defined by Gaussian curve fitting. XPS was performed using Al Ka X-ray radiation. The survey spectra and elemental spectra were recorded using 1 eV step size, and 0.02 eV step size, respectively. XRD was conducted on a powder XRD system with Cu K-alpha radiation (wavelength 1.54 A). Each wide-range XRD scan was conducted using 2e values between 15° and 60°.

[0118] Electrochemical cyclic voltammetry (CV) and Galvanostatic constant current chargedischarge (CD) measurements were performed using a potentiostat / galvanostat. Specific area capacitance from CV curves was calculated using Equation (1):where S is total active electrode area of a device in cm2; v is the voltage sweep rate in V. s-1; I(V) is voltammetric current in amperes; Vfand V, are initial (0 V) and final (1 V) potentials of the CV curves and l(V)dVis the integrated area of the CV curves.

[0119] Specific Galvanostatic CD capacitance normalized by active area of the interdigitated LIGCs CAfrom galvanostatic discharging curves was calculated from equation (2):where i is discharging current in amperes; S is active electrode area; Vf, V;, tfand t, are final and initial values of potentials V and time t, respectively.

[0120] Specific energy EAand power PAdensities used in the Ragone plot were calculated by using Equations (3) and (4), respectively:PA= ^ X 3600 (4)where AV is the potential difference of final and initial potentials Vf- V;during discharging, and At is discharging time in seconds.

[0121] The example experiments show that the LIGC micro-supercapacitors have a high specific capacitance by using a defocused laser approach. The morphological and material structural changes due to defocusing were examined. Defocusing was created by changing the polyimide substrate distance to the laser carrier in steps of 0.5 cm at six different defocus positions named DF-1 to -6. The workable window of laser powers between threshold and ablation was shifted with defocusing. Without defocusing, the LIGC material exhibited an open nano-flake morphology in SEM. Conversely, the morphology of deep defocus engraved LIGC exhibited craters containing nano-porous crystallites and a top cover layer of nano-grains. Both morphologies shared the sp2hybridized (002) planes and 3.4 A layer spacing in the TEMs, further confirmed by XRD. For deeper defocusing positions (DF-4 to -6) at 23% laser power, the interior craters’ surface porosity changed, the number of nano-grains in the top cover layer decreases in SEM, and drastic material structural changes was observed. Increasing defocus from DF-4 to DF-6 lead to a decrease in the overall percentage of carbon content and C=C bonded carbon atoms, an increase in the amount of C-C bonded atoms and defects, as seen in XPS and Raman spectroscopy. Very deep defocus engraved LIGC (DF-6) exhibited very disordered LIGC.

[0122] In the example experiments, changes in the LIGC sheet resistance and MSC capacitance were observed with laser power for different defocusing positions. Increasing deep defocus from DF-4 to DF-6 caused both sheet resistance and capacitance to increase. The highest capacitance at a CV scan rate of 5 mV / s of 105 mF / cm2 was observed for the deepest defocus DF-6. This is nearly 16 times higher than the capacitance of 6.4 mF / cm2 for in-focus engraved LIGC. For a low scan rate of 1 mV / s, this was further increased to 133 mF / cm2, which is a very high capacitance for LIGC MSCs fabricated in a single step without any LIGC modification. Moreover, a CD capacitance of 100 mF / cm2 was achieved at 75 pA applied current. However, the deepest defocus DF-6 shows CV and CD capacitance dropping with increasing scan rate and current, respectively. Conversely, the rate capability of MSCs fabricated with less defocused LIGC (DF-4) was improved with a low capacitance drop starting from nearly 75 mF / cm2. Furthermore, a 25% capacitance increase was observed after 10,000 CD cycles tested at 3mA applied current.

[0123] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.

Claims

CLAIMS1. Electrochemical electrodes fabricated from a precursor material, the fabrication of the electrochemical electrodes comprising:applying a defocused laser pulse by a laser to a micro-scale area of a surface of the precursor material, the laser positioned at a defocused distance relative to the surface of the precursor material; andrepeatedly, until a predetermined pattern is generated:moving the laser or the precursor material; andsubsequently or concurrently applying the defocused laser pulse to further microscale areas of the surface of the precursor material,wherein the precursor material with the predetermined pattern applied forms the electrochemical electrodes.

2. The electrochemical electrodes of claim 1, wherein the precursor material comprises a carbonaceous material or a composite of a carbonaceous material with another material.

3. The electrochemical electrodes of claim 1, wherein the precursor material comprises a polymer.

4. The electrochemical electrodes of claim 1, wherein the polymer comprises one or more of polyimide, polyether ether ketone, polyetherimide, polysulfone, or lignin.

5. The electrochemical electrodes of claim 1, wherein the electrochemical electrodes are incorporated into one of a sensor, biosensor, battery, hydrogen generator, air-cathode, or anode.

6. The electrochemical electrodes of claim 1 , wherein the formed electrochemical electrodes comprises a porous morphology with a plurality of pores having nanoscale dimensions.

7. The electrochemical electrodes of claim 1 , wherein the defocused laser pulse converts the precursor material into graphitic carbon.

8. The electrochemical electrodes of claim 1, wherein the electrochemical electrodes comprise graphitic carbon having a porous morphology.

9. The electrochemical electrodes of claim 1, wherein successive laser pulses spatially overlap within the precursor material.

10. The electrochemical electrodes of claim 1, wherein the predetermined electrode pattern comprises an interdigitated electrode structure.

11. A method for fabricating electrochemical electrodes from a precursor material, comprising:applying a defocused laser pulse to a micro-scale area of the surface of the precursor material; andrepeatedly, until a predetermined pattern is generated:moving a source of the defocused laser or the precursor material; and subsequently or concurrently applying the defocused laser pulse to further microscale areas of the surface of the precursor material, wherein the precursor material with the predetermined pattern applied forms the electrochemical electrodes.

12. The method of claim 11 , wherein the defocused laser pulse is generated by positioning a laser at a defocused distance relative to the precursor material.

13. The method of claim 11 , wherein successive laser pulses spatially overlap.

14. The method of claim 12, wherein the spatially overlapping laser pulses produce a porous graphitic carbon morphology.

15. The method of claim 11 , wherein the laser is operated at a power level selected to avoid ablation of graphitic carbon during fabrication.

16. The method of claim 11, wherein the predetermined electrode pattern comprises interdigitated electrodes for a micro-supercapacitor.

17. A system for fabricating electrochemical electrodes from a precursor material, comprising:a laser positioned at a defocused distance relative to the precursor material; and a processor in communication with a data storage to execute instructions via an instrument interface, the instructions comprising:directing the laser to apply a defocused laser pulse to a micro-scale area of the surface of the precursor material;repeatedly, until a predetermined pattern is generated:directing movement of the laser or the precursor material; and subsequently or concurrently directing the laser to apply the defocused laser pulse to further micro-scale areas of the surface of the precursor material, the precursor material with the predetermined pattern applied forms the electrochemical electrodes.

18. The system of claim 17, wherein the laser comprises a CO2laser.

19. The system of claim 17, wherein the processor provides instructions to cause spatial overlap of successive laser pulses at the precursor material.

20. The system of claim 17, wherein the laser operates at a power level selected to avoid ablation of graphitic carbon during fabrication.

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