Free-standing Reduced Graphene Oxide Research Articles

Electrochemical Response of 3D-Printed Free-Standing Reduced Graphene Oxide Electrode for Sodium Ion Batteries Using a Three-Electrode Glass Cell.

Graphene and its derivatives have been widely used to develop novel materials with applications in energy storage. Among them, reduced graphene oxide has shown great potential for more efficient storage of Na ions and is a current target in the design of electrodes for environmentally friendly Na ion batteries. The search for more sustainable and versatile manufacturing processes also motivates research into additive manufacturing electrodes. Here, the electrochemical responses of porous 3D-printed free-standing log-type structures fabricated using direct ink writing (DIW) with a graphene oxide (GO) gel ink are investigated after thermal reduction in a three-electrode cell configuration. The structures delivered capacities in the range of 50-80 mAh g-1 and showed high stability for more than 100 cycles. The reaction with the electrolyte/solvent system, which caused an initial capacity drop, was evidenced by the nucleation of various Na carbonates and Na2O. The incorporation of Na into the filaments of the structure was verified with transmission electron microscopy and Raman spectroscopy. This work is a proof of concept that structured reduced GO electrodes for Na ion batteries can be achieved from a simple, aqueous GO ink through DIW and that there is scope for improving their performance and capacity.

Mechanically flexible reduced graphene oxide/carbon composite films for high-performance quasi-solid-state lithium-ion capacitors

Practical applications of diverse flexible wearable electronics require electrochemical energy storage (EES) devices with multiple configurations. Moreover, to fabricate flexible EES devices with high energy density and stability, organic integration from electrode design to device assembly is required. To address these challenges, a free-standing reduced graphene oxide (rGO)/carbon film with a unique sandwich structure has been designed via the assistance of vacuum-assistant filtration for lithium-ion capacitors (LICs). The graphene acts as not only a binder to construct a three-dimensional conductive network but also an active material to provide additional capacitive lithium storage sites, thus enabling fast ion/electron transport and improving the capacity. The designed rGO/hard carbon (rGO/HC) and rGO/activated carbon (rGO/AC) free-standing films exhibit enhanced specific capacities (513.7 mA h g−1 for rGO/HC and 102.8 mA h g−1 for rGO/AC) and excellent stability. Moreover, the integrated flexible quasi-solid-state rGO/AC//rGO/HC LIC devices possess a maximum energy density of 138.3 Wh kg−1, a high power density of 11 kW kg−1, and improved cycling performance (84.4% capacitance maintained after 10,000 cycles), superior to the AC//HC LIC (43.5% retention). Such a strategy enlightens the development of portable flexible LICs.

Direct Laser Patterning of Free-Standing RGO Electrodes for Wearable Capacitive Pressure Sensors

Graphene-based flexible pressure sensors are promising for cutting-edge applications, for instance, e-skins, human-machine interaction, and wearable health-monitoring devices. However, the development of free-standing graphene electrodes with well-designed patterns is still challenging. Herein, we report the direct laser patterning of a pair of free-standing reduced graphene oxide (RGO) electrodes with complementary helical microstructures for developing high-performance capacitive pressure sensors (CPSs). A commercially available porous tape was sandwiched between the two RGO electrodes. The design and fabrication of the complementary helix-shaped RGO electrodes are essential for developing CPSs with improved sensing performance. The resultant CPSs enable dual-mode sensing detection (pressure and proximity) with a high sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 0.95$ </tex-math></inline-formula> kPa−1. As a proof-of-concept application, the RGO-based CPS has been employed as wearable electronics for finger motion monitoring.

Quantum-dot-like electrical transport of free-standing reduced graphene oxide paper at liquid helium temperatures

We present the results of a study of the electrical properties of macroscopic free-standing reduced graphene oxide paper (RGOP) with a 60 % fraction of sp 2 ‑carbons at liquid helium temperatures. Graphene domains form an array of quantum dots that determine material properties. At low temperatures, the electrical conductivity of the RGOP can be described by the model of tunneling of charge carriers through a disordered quasi-2D quantum dot array. In the free-standing RGOP at the temperature of 4.2 K, the charge carriers are localized in the quantum dots due to the Coulomb blockade. The effective localization length (~5–8 nm) of charge carriers coincides with the average size of the sp 2 ‑carbon regions (~6–8 nm). The transfer of charge carriers in the RGOP at liquid helium temperatures occurs due to tunneling between energetically close graphene domains, which are not nearest neighbors in space and are rather far from each other (up to ~100 nm). • Quantum-dot-like charge transfer in macroscopic free-standing RGO paper • Graphene domains array determines electrical properties of macroscopic RGO material. • Strong localization of charge carriers in free-standing RGO paper at low temperatures • Tunneling between energetically close graphene domains that are not nearest neighbors.

The hopping Hall effect in reduced graphene oxide

• Hall effect in RGO in Efros-Shklovskii variable range hopping conduction regime. • Hopping Hall mobility in wide temperature range up to room temperature. • Hall mobility decreases exponentially with decreasing temperature. This paper is the first study of the Hall effect in the Efros-Shklovskii variable range hopping conduction regime in the free-standing reduced graphene oxide paper (RGOP). It was found that the temperature dependence of the Hall mobility in RGOP agrees with the Hall effect theory for hopping conduction in disordered semiconductors. In the wide temperature range of 300–25 K, the Hall mobility decreases exponentially with decreasing temperature. The variable range hopping conduction channel is the main one contributing to the Hall effect in the RGOP even at room temperature. It was shown that the Hall effect in the RGO can have both positive and negative signs over the entire temperature range. The reduction of disorder and rise in the sp 2 -carbon fraction increase the hopping Hall mobility of charge carriers.

PdCu nanoparticles modified free-standing reduced graphene oxide framework as a highly efficient catalyst for direct borohydride-hydrogen peroxide fuel cell

A novel electrode of PdCu nanoparticles modified free-standing reduced graphene oxide (rGO) framework (PdCu@rGOF) is fabricated via electrodeposition and chemical reduction and employed to the direct sodium borohydride-hydrogen peroxide fuel cell (DBHPFC). The resultant catalyst reveals excellent catalytic performance towards hydrogen peroxide (H2O2) electroreduction reaction (HPRR) and sodium borohydride (NaBH4) electrooxidation reaction (BOR) along with low activation energy (Ea). Numerically, the PdCu@rGOF electrode delivers an HPRR current density of 455.2 mA cm−2 at 0 V (2 mol L−1 H2SO4+1.3 mol L−1 H2O2). Additionally, the current density of BOR reaches up to 1065.3 mA cm−2 at 0 V for 2 mol L−1 NaOH and 0.3 mol L−1 NaBH4. Meanwhile, the mechanism studies show that HPRR and BOR catalyzed by PdCu@rGOF mainly relates 2-electron-transfer process and quasi-8-electron-transfer process, respectively. Subsequently, PdCu@rGOF is assembled into a fuel cell, yielding the optimal power density of 188.9 mW cm−2. The open circuit potential (OCP) of DBHPFC is about 1.70 V at 293.15 K. The superior electrochemical performance of the optimized electrode can be attributed to the distinctive architecture and the strong adsorption and bond-breaking ability of Pd nanoparticles. This work indicates that the as-prepared PdCu@rGOF can be a promising catalyst for DBHPFC.

Investigation of the high-field transport, Joule-heating-driven conductivity improvement and low-field resistivity behaviour in lightly-reduced free-standing graphene oxide papers

Free-standing reduced graphene oxide (rGO) has been gaining popularity for its use in supercapacitors and battery applications due its facile synthesis, multi-layered structure, and high-current carrying capacity. Pertinent to the successful implementation of such applications, however, is the need to develop a thorough understanding of the electrical properties of such materials when subject to high applied electric fields. In this work, we undertake a detailed study of high-field electrical properties of mm-scale, lightly-reduced, rGO papers. Our results reveal that the I–V curves exhibit substantial nonlinearity with associated hysteresis that depends strongly on the applied electric field. The nonlinear behaviour which was interpreted using conventional transport models of Fowler–Nordheim tunnelling and space charge limited conduction revealed that while these models provided good qualitative fits to our data, they were quantitatively lacking, thus leaving the issue of high-field transport mechanisms in rGO open for debate. Careful I–V cycling experiments with measurement time-delay introduced between cycles revealed that the observed hysteresis contained recoverable and non-recoverable parts that we identified as arising from charge trapping and Joule heating effects, respectively. Time-dependent measurements showed that these effects were characterized by two distinct time scales. Importantly, the Joule heating was found to cause a permanent conductivity improvement in the rGO via the ‘current annealing’ effect by effectively eliminating oxygenated groups from the rGO. The analysis of the electrical breakdown in our samples resembled a thermal runaway-like event that resulted in premature damage to the rGO. Finally, we investigated the low-field resistivity in the 80 K–300 K temperature range. The reduced activation energy analysis revealed a robust power law behaviour below 230 K, while deviating from this trend at higher temperatures. For samples that received current annealing treatment, a reduced value for the power law exponent was obtained, confirming the effective lowering of disordered regions.

Electrochemical investigation of free-standing reduced graphene oxide hydrogel

Free-standing reduced graphene oxide (RGO) hydrogel was synthesized through two-stepped hydrothermal reduction treatment of aqueous graphene oxide (GO) suspension. GO with high interface affinity for overlapping of cross-linking graphene sheets contributes to forming steady-state RGO hydrogel. RGO hydrogel shows higher B band/G band intensity ratio than GO suspension in Raman spectrum analysis, indicating well-restored hexagonal carbon rings with the enhanced crystal structure. Free-standing RGO hydrogel could be directly acted as supercapacitor electrode without using any substrate material. The specific capacitance decreases from 253.9 to 92.3 F g−1 in 1.0 M Na2SO4 electrolyte and from 275.3 to 125.9 F g−1 in 1.0 M KOH electrolyte when the current density increased from 0.3 to 4.0 A g−1, presenting the capacity retention of 36.4% and 45.7% respectively. Free-standing RGO hydrogel presents specific capacitance in alkaline KOH electrolyte rather than neutral Na2SO4 electrolyte. KOH electrolyte with superior ionic diffusion coefficient of hydroxyl ion leads to higher current response and electrical double-layer capacitance performance. The capacitance retention ratio of 97.8% at 4.0 A g−1 in 1.0 M KOH electrolyte after 1000 charge-discharge cycles presents superior cycling stability. Accordingly, free-standing RGO hydrogel applied well as superior supercapacitor electrode presents the promising energy storage application.

Facile preparation of graphene/polyaniline composite hydrogel film by electrodeposition for binder-free all-solid-state supercapacitor

The combination of graphene and polyaniline (PANI) as electrodes for supercapacitors has attracted dramatic interests due to their synergistic effects. However, the preparation of the electrode materials usually involves complicated process and other additives. In this work, we report the facile fabrication of the graphene/PANI composite hydrogel film as the symmetric all-solid-state supercapacitor electrode. The free-standing reduced graphene oxide (rGO) hydrogel film is hydrothermally synthesized from a GO solution layer, which is used for subsequent electropolymerization of PANI. The electrochemical measurements of the resulting rGO/PANI hydrogel film present the capacitance of 853.7 F g−1 at 1 A g−1 with good rate capability and cycling stability. In addition, the hydrogel composite film is assembled into the symmetric all-solid-state capacitor without any additives and attains the capacitance of 741.8 F g−1 with a high capacitance retention of 92.6% after 8000 cycles. These excellent electrochemical properties of the composite film are ascribed to the joint effort of superior conductivity and porous structure of rGO matrix with the outstanding pseudocapacitive property of PANI. The convenient method could be considered for the preparation of graphene-based functional composite materials.

Ice-Templated Free-Standing Reduced Graphene Oxide for Dendrite-Free Lithium Metal Batteries

The Li metal is increasingly receiving attention as the ultimate anode material for Li metal batteries on account of its superior capacity and low redox potential. Recent studies have suggested tha...

Patterning Large Free-Standing Reduced Graphene Oxide Films Assisted by Open-Area Copper Substrates for Graphene Drums

The growth of large free-standing reduced graphene oxide films induced by open-area copper substrates is analyzed. This phenomenon, which is initiated by the self-assembly and reduction of graphene...

Free-standing reduced graphene oxide (rGO) membrane for salt-rejecting solar desalination via size effect

Abstract Interfacial solar vapor generation has revived the solar-thermal-based desalination due to its high conversion efficiency of solar energy. However, most solar evaporators reported so far suffer from severe salt-clogging problems during solar desalination, leading to performance degradation and structural instability. Here, we demonstrate a free-standing salt-rejecting reduced graphene oxide (rGO) membrane serving as an efficient, stable, and antisalt-fouling solar evaporator. The evaporation rate of the membrane reaches up to 1.27 kg m−2 h−1 (solar–thermal conversion efficiency ∼79%) under one sun, out of 3.5 wt% brine. More strikingly, due to the tailored narrow interlayer spacing, the rGO membrane can effectively reject ions, preventing salt accumulation even for high salinity brine (∼8 wt% concentration). With enabled salt-antifouling capability, flexibility, as well as stability, our rGO membrane serves as a promising solar evaporator for high salinity brine treatment.

Ultra-broadband self-powered reduced graphene oxide photodetectors with annealing temperature-dependent responsivity

Self-powered ultra-broadband photothermoelectric photodetectors based on free-standing reduced graphene oxide (RGO) films annealed under various temperatures (200–1000 °C) are developed for the first time. The preparation process of the free-standing RGO films is simple, low-cost, and scalable. An interesting phenomenon was found that with increasing annealing temperature, responsivities of RGO-based photodetectors decreased across the ultra-broadband range from ultraviolet (375 nm) to terahertz (118.8 μm) regions. The dependence of device responsivity on annealing temperature is derived from differences in the oxygen functional groups present on the RGO films. The negative sheet Hall coefficients of the annealed RGO films confirmed that they behaved as n-type semiconductors, indicating that electron-donating groups dominated in all the annealed free-standing RGO films. The content of electron-donating groups gradually decrease as the annealing temperature rose. The response times of our devices (<50 ms) are approximately one to three orders of magnitude faster than the reported ultra-broadband photodetectors based on supported RGO films and silicon-based heterojunctions. Since our photodetectors are macro-electronic devices, the response time could be further decreased by reducing the channel length. Considering the properties of self-powered, low cost, easy preparation, and ultra-broadband response, our RGO photodetectors have potential applications for integrated optoelectronics.

Flexible and Free-Standing Reduced Graphene Oxide and Polypyrrole Coated Air-Laid Paper-Based Supercapacitor Electrodes

Nowadays, the needs of bendable energy storage components are growing to match with the development of wearable devices. However, it is still a great challenge to fabricate flexible electrodes to k...

The Influence of Dispersion State of Graphene Sheets on the Microstructure and Thermal Conductivity of Free-Standing Reduced Graphene Oxide Films

Reduced graphene oxide (rGO) aqueous dispersions were synthesized from GO suspensions with hydrazine as reducing agent via a microwave-hydrothermal process in ammonia solution. The rGO films were fabricated by vacuum filtration of above dispersions. When the weight ratio of hydrazine to GO (R[Formula: see text]) increases from 0.125 to 9, the dispersion stability of rGO nanosheets in hydrazine/ammonia solution degrades gradually, the surfaces of resultant films become rough from smooth and large interspaces turn up in the transverse section. With the increasing of R[Formula: see text], the C/O atomic ratio and orientation degree of (002) plane of rGO films increase, and the in-plane thermal conductivity also increases, achieving a maximum value of 1056[Formula: see text]W/m K when R[Formula: see text] is 3.375, then decreases because large interspaces in the rGO film with higher reduction degree greatly limit the thermal transport due to the severe phonon scatterings at the sheets boundaries.

A free-standing reduced graphene oxide aerogel as supporting electrode in a fluorine-free Li2S8 catholyte Li-S battery

We report on a novel, simple, and environmentally benign synthesis route for a free-standing reduced graphene oxide (r-GO) aerogel and its application as supporting electrode for the electrochemical redox reaction of sulphur in a catholyte-based lithium-sulphur battery. A mesoporous matrix is formed by a layers of r-GO, providing sites for electrochemical reactions and a highly conducting pathway for electrons. The highly porous structure is easily infiltrated by a catholyte solution providing a homogeneous distribution of the sulphur active material in the conductive graphene matrix and ensuring efficient electrochemical reactions. This is demonstrated by a high capacity, 3.4 mAh cm−2, at high mass loading, 3.2 mg cm−2 of sulphur in the cathode and in total the sulphur loading in the Li-S cell is even double (6.4 mg cm−2). Additionally, the presence of oxygen groups in the r-GO aerogel structure stabilizes the cycling performance and the Li-S cell with the fluorine free catholyte shows a capacity retention of 85% after 350 cycles.

Flexible and free-standing reduced graphene oxide thick films with PMMA stabilized silver nanoparticles, as a potential probe for cancer thermal therapy

Thermal therapy is an efficient, cost-effective method for treating different types of cancers and hence flexible, light weight, biocompatible heating materials with low power consumption are of utmost importance. Partially reduced flexible, robust and bio-compatible thick graphene oxide films with, mostly edge functionalized Polymethyl methacrylate (PMMA) and in situ grown silver nanoparticles were prepared using simple soak and dry strategy. Larger distribution of silver nanoparticles with a mean diameter of 2.5 nm were anchored towards the edges of these thick films. The electrothermal (Joule heating) performance were found to increase in the presence of silver nanoparticles due to decrease in sheet resistance (0.33 × 104 Ωm) and increase in electron-phonon interaction. These free-standing, flexible films exhibited stable Joule heating behavior under repeated voltage on/off cycles. In-vitro studies performed on breast cancer cells in the presence of the prepared films, with an external applied voltage of 10 VDC, shows 67% cell death. This work proves the potentiality of thick heterogeneous (partially) reduced graphene oxide films towards site specific destruction of solid tumors by utilizing its Joule heating properties.

Comparative study on chemical reduction of free-standing flexible GO films and their cyclic voltammetry performance

Chemical reduction of graphene oxide (GO) is one of the most practical methods to recover the properties of graphene. However, these properties vary with the reduction agents. Understanding these differences is fundamental to applications related to reduced graphene oxide (RGO). In this work, six different reduction agents were used to make flexible and free-standing RGO films, resulting in different properties such as surface morphology, inner structure, surface C/O ratio, electrical conductivity, and wettability. We found that not all the six reduction agents were suitable for making flexible and free-standing films. Then, we further studied the electrochemical behavior of these samples and found that the electrochemical properties of the RGO films depend on all the properties but electrical conductivity played the most important role in determining the pseudocapacitance behavior.

Flexible Hydrogen Peroxide Sensors Based on Platinum Modified Free-Standing Reduced Graphene Oxide Paper

In this article, we report a facile method to fabricate free-standing reduced graphene oxide paper (rGOP) by vacuum filtration. Pt nanoparticles were electrodeposited on reduced graphene oxide paper to construct a sensitive and flexible hydrogen peroxide (H2O2) sensor. The properties of Pt/rGOP were characterized by scanning electron microscopy, energy dispersive X-ray, X-ray photoelectron spectroscopy and Raman spectroscopy. The electrochemical characterizations of the resulting sensor were carried out by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The Pt/rGOP hybrid electrode had an excellent electrochemical property, with superb electrocatalytic activity, a large electrochemical active surface area, flexibility and high stability. The linear ranges of the as-prepared sensor for H2O2 detection were divided into two linear sections: 0.2 &mu;mol/L to 2.0 mmol/L and 2.0 to 8.5 mmol/L, with a detection limit of 100 nmol/L (S/N = 3) and a response time of less than 5 s. The proposed sensor has great potential to become a reliable and flexible tool in biosensor and point-of-care medical devices.

Fabrication of flexible free-standing reduced graphene oxide/polyaniline nanocomposite film for all-solid-state flexible supercapacitor

Graphene and polyaniline (PANI) are considered as promising electrode active materials. A rational design of composite structure for graphene and PANI is essential and prerequisite for obtaining excellent properties. In this paper, a flexible free-standing reduced graphene oxide (rGO)/PANI nanocomposite film has been successfully prepared through self-assembly and in situ polymerization of anilines in graphene oxide (GO) sheets. Owing to the formation of PANI nanoparticles, the electrochemical capacitance of rGO/PANI nanocomposite film is significantly enhanced. To evaluate electrochemical properties, an all-solid-state flexible supercapacitor is further fabricated through tailoring and assembling the flexible free-standing rGO/PANI nanocomposite film. The nanocomposite film shows high specific capacitance of 0.92 F/cm2 (>1314.3 F/cm3) and its specific capacitance exhibits no obvious fading under bending state or after bending 200 times. The excellent electrochemical performance can be readily ascribed to the synergistic effect between two-dimensional rGO and PANI nanoparticle in the nanocomposite film. This study demonstrates an efficient approach to prepare flexible free-standing active materials for flexible energy storage devices.