Maher F. El-Kady

Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors

Infrared Route to Graphene Electrodes Electrochemical capacitors can deliver large amounts of power quickly, but have limited energy storage because only the surface regions of electrodes can store charge. Graphene represents an alternative to activated carbon electrodes because of their high conductivity and surface area, but graphene sheets tend to reassociate and lose surface area. El-Kady et al. (p. 1326; see the Perspective by Miller) show that graphite oxide sheets can be converted by infrared laser irradiation into porous graphene sheets that are flexible, robust, and highly conductive. Infrared laser reduction of graphene oxide creates a strong porous electrode with both high surface area and high conductivity. Although electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, charge and discharge faster than batteries, they are still limited by low energy densities and slow rate capabilities. We used a standard LightScribe DVD optical drive to do the direct laser reduction of graphite oxide films to graphene. The produced films are mechanically robust, show high electrical conductivity (1738 siemens per meter) and specific surface area (1520 square meters per gram), and can thus be used directly as EC electrodes without the need for binders or current collectors, as is the case for conventional ECs. Devices made with these electrodes exhibit ultrahigh energy density values in different electrolytes while maintaining the high power density and excellent cycle stability of ECs. Moreover, these ECs maintain excellent electrochemical attributes under high mechanical stress and thus hold promise for high-power, flexible electronics.

Scalable fabrication of high-power graphene micro-supercapacitors for flexible and on-chip energy storage

The rapid development of miniaturized electronic devices has increased the demand for compact on-chip energy storage. Microscale supercapacitors have great potential to complement or replace batteries and electrolytic capacitors in a variety of applications. However, conventional micro-fabrication techniques have proven to be cumbersome in building cost-effective micro-devices, thus limiting their widespread application. Here we demonstrate a scalable fabrication of graphene micro-supercapacitors over large areas by direct laser writing on graphite oxide films using a standard LightScribe DVD burner. More than 100 micro-supercapacitors can be produced on a single disc in 30 min or less. The devices are built on flexible substrates for flexible electronics and on-chip uses that can be integrated with MEMS or CMOS in a single chip. Remarkably, miniaturizing the devices to the microscale results in enhanced charge-storage capacity and rate capability. These micro-supercapacitors demonstrate a power density of ~200 W cm-3, which is among the highest values achieved for any supercapacitor.

Patterning and Electronic Tuning of Laser Scribed Graphene for Flexible All-Carbon Devices

Engineering a low-cost graphene-based electronic device has proven difficult to accomplish via a single-step fabrication process. Here we introduce a facile, inexpensive, solid-state method for generating, patterning, and electronic tuning of graphene-based materials. Laser scribed graphene (LSG) is shown to be successfully produced and selectively patterned from the direct laser irradiation of graphite oxide films under ambient conditions. Circuits and complex designs are directly patterned onto various flexible substrates without masks, templates, post-processing, transferring techniques, or metal catalysts. In addition, by varying the laser intensity and laser irradiation treatments, the electrical properties of LSG can be precisely tuned over 5 orders of magnitude of conductivity, a feature that has proven difficult with other methods. This inexpensive method for generating LSG on thin flexible substrates provides a mode for fabricating a low-cost graphene-based NO(2) gas sensor and enables its use as a heterogeneous scaffold for the selective growth of Pt nanoparticles. The LSG also shows exceptional electrochemical activity that surpasses other carbon-based electrodes in electron charge transfer rate as demonstrated using a ferro-/ferricyanide redox couple.

Engineering three-dimensional hybrid supercapacitors and microsupercapacitors for high-performance integrated energy storage

Significance Batteries run just about everything portable in our lives such as smartphones, tablets, computers, etc. Although we have become accustomed to the rapid improvement of portable electronics, the slow development of batteries is holding back technological progress. Thus, it is imperative to develop new energy storage devices that are compact, reliable, and energy dense, charge quickly, and possess both long cycle life and calendar life. Here, we developed hybrid supercapacitors that can store as much charge as a lead acid battery, yet they can be recharged in seconds compared with hours for conventional batteries. Supercapacitors now play an important role in the progress of hybrid and electric vehicles, consumer electronics, and military and space applications. There is a growing demand in developing hybrid supercapacitor systems to overcome the energy density limitations of the current generation of carbon-based supercapacitors. Here, we demonstrate 3D high-performance hybrid supercapacitors and microsupercapacitors based on graphene and MnO2 by rationally designing the electrode microstructure and combining active materials with electrolytes that operate at high voltages. This results in hybrid electrodes with ultrahigh volumetric capacitance of over 1,100 F/cm3. This corresponds to a specific capacitance of the constituent MnO2 of 1,145 F/g, which is close to the theoretical value of 1,380 F/g. The energy density of the full device varies between 22 and 42 Wh/l depending on the device configuration, which is superior to those of commercially available double-layer supercapacitors, pseudocapacitors, lithium-ion capacitors, and hybrid supercapacitors tested under the same conditions and is comparable to that of lead acid batteries. These hybrid supercapacitors use aqueous electrolytes and are assembled in air without the need for expensive “dry rooms” required for building today’s supercapacitors. Furthermore, we demonstrate a simple technique for the fabrication of supercapacitor arrays for high-voltage applications. These arrays can be integrated with solar cells for efficient energy harvesting and storage systems.

3D Freeze-Casting of Cellular Graphene Films for Ultrahigh-Power-Density Supercapacitors.

3D cellular graphene films with open porosity, high electrical conductivity, and good tensile strength, can be synthesized by a method combining freeze-casting and filtration. The resulting supercapacitors based on 3D porous reduced graphene oxide (RGO) film exhibit extremely high specific power densities and high energy densities. The fabrication process provides an effective means for controlling the pore size, electronic conductivity, and loading mass of the electrode materials, toward devices with high energy-storage performance.

Designing 3D Highly Ordered Nanoporous CuO Electrodes for High-Performance Asymmetric Supercapacitors

The increasing demand for energy has triggered tremendous research efforts for the development of lightweight and durable energy storage devices. Herein, we report a simple, yet effective, strategy for high-performance supercapacitors by building three-dimensional pseudocapacitive CuO frameworks with highly ordered and interconnected bimodal nanopores, nanosized walls (∼4 nm) and large specific surface area of 149 m(2) g(-1). This interesting electrode structure plays a key role in providing facilitated ion transport, short ion and electron diffusion pathways and more active sites for electrochemical reactions. This electrode demonstrates excellent electrochemical performance with a specific capacitance of 431 F g(-1) (1.51 F cm(-2)) at 3.5 mA cm(-2) and retains over 70% of this capacitance when operated at an ultrafast rate of 70 mA cm(-2). When this highly ordered CuO electrode is assembled in an asymmetric cell with an activated carbon electrode, the as-fabricated device demonstrates remarkable performance with an energy density of 19.7 W h kg(-1), power density of 7 kW kg(-1), and excellent cycle life. This work presents a new platform for high-performance asymmetric supercapacitors for the next generation of portable electronics and electric vehicles.

Vapor-Phase Polymerization of Nanofibrillar Poly(3,4-ethylenedioxythiophene) for Supercapacitors

Nanostructures of the conducting polymer poly(3,4-ethylenedioxythiophene) with large surface areas enhance the performance of energy storage devices such as electrochemical supercapacitors. However, until now, high aspect ratio nanofibers of this polymer could only be deposited from the vapor-phase, utilizing extrinsic hard templates such as electrospun nanofibers and anodized aluminum oxide. These routes result in low conductivity and require postsynthetic template removal, conditions that stifle the development of conducting polymer electronics. Here we introduce a simple process that overcomes these drawbacks and results in vertically directed high aspect ratio poly(3,4-ethylenedioxythiophene) nanofibers possessing a high conductivity of 130 S/cm. Nanofibers deposit as a freestanding mechanically robust film that is easily processable into a supercapacitor without using organic binders or conductive additives and is characterized by excellent cycling stability, retaining more than 92% of its initial capacitance after 10,000 charge/discharge cycles. Deposition of nanofibers on a hard carbon fiber paper current collector affords a highly efficient and stable electrode for a supercapacitor exhibiting gravimetric capacitance of 175 F/g and 94% capacitance retention after 1000 cycles.

Direct Laser Writing of Graphene Electronics

One of the fundamental issues with graphene for logic applications is its lack of a band gap. In this issue of ACS Nano, Shim and colleagues introduce an effective approach for modulating the current flow in graphene by forming p-n junctions using lasers. The findings could lead to a new route for controlling the electronic properties of graphene-based devices. We highlight recent progress in the direct laser synthesis and patterning of graphene for numerous applications. We also discuss the challenges and opportunities in translating this remarkable progress toward the direct laser writing of graphene electronics at large scales.

Free-standing composite hydrogel films for superior volumetric capacitance

High volumetric capacitance is vital for the development of wearable and portable energy storage devices. We herein introduce a novel simple route for the fabrication of a highly porous, binder-free and free-standing polyaniline/reduced graphene oxide composite hydrogel (PANi/graphene hydrogel) as an electrode with a packing density of 1.02 g cm−3. PANi played critical roles in gelation, which include reduction, crosslinking, creation of pseudocapacitance and as a spacer preventing graphene sheets from stacking. The composite hydrogel film delivered a volumetric capacitance of 225.42 F cm−3 with a two-electrode supercapacitor configuration, which was enhanced to 592.96 F cm−3 in a redox-active electrolyte containing hydroquinone. This new strategy will open a new area for using conducting polymer derivatives in the development of flexible graphene electrodes towards many applications such as batteries, sensors and catalysis.

High-performance supercapacitors using graphene/polyaniline composites deposited on kitchen sponge

We in this study used a commercial grade kitchen sponge as the scaffold where both graphene platelets (GnPs) and polyaniline (PANi) nanorods were deposited. The high electrical conductivity of GnPs (1460 S cm(-1)) enhances the pseudo-capacitive performance of PANi grown vertically on the GnPs basal planes; the interconnected pores of the sponge provide sufficient inner surface between the GnPs/PANi composite and the electrolyte, which thus facilitates ion diffusion during charge and discharge processes. When the composite electrode was used to build a supercapacitor with two-electrode configuration, it exhibited a specific capacitance of 965.3 F g(-1) at a scan rate of 10 mV s(-1) in 1.0 M H2SO4 solution. In addition, the composite Nyquist plot showed no semicircle at high frequency corresponding to a low equivalent series resistance of 0.35 Ω. At 100 mV s(-1), the supercapacitor demonstrated an energy density of 34.5 Wh kg(-1) and a power density of 12.4 kW kg(-1) based on the total mass of the active materials on both electrodes. To demonstrate the performance, we built an array consisting of three cells connected in series, which lit up a red light emitting diode for five minutes. This simple method holds promise for high-performance yet low-cost electrodes for supercapacitors.