Low-temperature Thermal Reduction Research Articles

Metal cation crosslinked, partially reduced graphene oxide membranes with enhanced stability for high salinity, produced water treatment by pervaporative separation

Graphene oxide (GO)-based membranes hold significant promise for applications ranging from energy storage to protective coatings, to saline water and produced water treatment, owing to their chemical stability and unique barrier properties achieving a high selectivity for water permeation. However, unmodified GO membranes are not stable when submerged in liquid water, creating challenges with their commercial utilization in aqueous filtration and pervaporation applications. To mitigate this, we develop an approach to modify GO membranes through a combination of low temperature thermal reduction and metal cation crosslinking. We demonstrate that Zn2+–rGO and Fe3+–rGO membranes had the highest permeation flux of 8.3 ± 1.5 l m−2 h−1 and 7.0 ± 0.4 l m−2 h−1, for saline water separation, respectively, when thermally reduced after metal cross-linking; These membranes maintained a high flux of 7.5 ± 0.7 l m−2 h−1, and 5.5 ± 0.3 l m−2 h−1 for produced water separation, respectively. All the membranes had a salt rejection higher than 99%. Fe3+ crosslinked membranes presented the highest organic solute rejections for produced water of 69%. Moreover, long term pervaporation testing was done for the Zn2+–rGO membrane for 12 h, and only a minor drop of 6% in permeation flux was observed, while Zn2+–GO had a drop of 24%. Both modifiers significantly enhanced the stability with Fe3+–rGO membranes displaying the highest mechanical abrasion resistance of 95% compared to non-reduced and non-crosslinked GO. Improved stability for all samples also led to higher selectivity to water over organic contaminants and only slightly reduced water flux across the membrane.

Recovery of degraded LiCoO2 through a CO2 -assisted low-temperature thermal reduction approach

Recycling spent lithium-ion batteries (LIBs) using chemical-saving and energy-effective pathways has been pursued to minimize the secondary environmental footprints. In this study, a sealed stainless-steel reactor was selected to achieve CO2-assisted low-temperature thermal reduction of spent LiCoO2 by carbon at 500 °C. The thermal reduction was provoked by the in-situ generated CO that resulted from the reaction of carbon and CO2. More importantly, MgCO3 was used as a CO2 holder to provide CO2 at elevated temperatures and MgO can be used to absorb CO2 to regenerate MgCO3, enabling the CO2 as a clean additive in the reduction reaction. Using this low-temperature reduction coupled with the carbonate pyrolysis approach, the selective recovery of Li reached 94.21%. The spatial isolation of MgCO3 at the bottom layer, graphite at the middle layer, and LiCoO2 at the top layer in the same reactor avoids complex separation. In addition, LiCoO2 is directly regenerated by the obtained CoO/Co3O4 and Li2CO3, and regenerated LiCoO2 exhibits good electrochemical performance with a discharge capacity retention of 94.0% after 300 cycles. Overall, the carbonate pyrolysis in the close-reactor is a promising approach to achieving a low-temperature reduction reaction by using CO2 as a clean agent to recycle spent LIBs.

Characteristics of graphite oxide membranes with different thickness by low temperature thermal reduction for aqueous EDLC electrodes and hot activation phenomenon

Reduced graphite oxide membrane (RGOM) with different thickness were prepared and used as electric double layer capacitor (EDLC) electrodes with aqueous electrolyte. According to quantum and molecular dynamic simulations, it was difficult for K+ and OH− to diffuse into graphene sheets due to dense structure, which made the RGOM electrodes show poor EDLC behaviors. Electrochemical tests showed that controlling the thickness of electrodes and modifying the dense structure could improve the specific capacitance of EDLC. Further studies indicated that during galvanostatic charge/discharge processes, if the EDLC were heated at 70 °C for one cycle and then cooled down to room temperature, the specific capacitance could be increased further. Additionally, according to 10,000 times cycling tests at 5000 mA g−1, the improved capacitance could be maintained for long time (154–167 F g−1 during test process), indicating that the hot activation could make RGOM electrodes obtain good and stable EDLC performances.

A novel method for synthesizing Ti3C2Tx MXene nanosheets supported Si nanoparticles as lithium-ion batteries anode for electric vehicles applications

Titanium carbide MXene (Ti3C2Tx MXene) supported Si nanoparticles composite is a promosing lithium ion batteries (LIBs) anode material for electric vehicles applications due to its high specific capacity and stable cycling performance. However, past fabrication of Si/Ti3C2Tx MXene composite involves complex processes which limits its practical application. Herein, we propose a simple and fast method for synthesizing Ti3C2Tx MXene nanosheets supported Si nanoparticles (Si/TMNSs) anode composite. The Si/TMNSs composite is prepared by a low-temperature aluminum thermal reduction of mechanical mixture of TMNSs and commercial SiO2. TMNSs can accommodate Si expansion during cycling and enhance electronic conductivity. Thus Si/TMNSs electrode exhibits outstanding electrochemical performances (a stable capacity of 868 mAh g−1 at high current density of 5 A g−1 after 1000 cycles with only 0.033% capacity loss per cycle over 1000 cycles). Overall this work demonstrates a novel method for large scale preparation of LIBs Si/MXene anode material for electric vehicles applications.

Microwave-triggered low temperature thermal reduction of Zr-modified high entropy oxides with extraordinary thermochemical H2 production performance

Solar-driven two-step thermochemical H2O splitting has emerged as a promising strategy for hydrogen production, but the conversion efficiency of solar-to-fuel has to be improved to make it more economically viable. Here, we present insights on microwave-triggered low temperature (600 °C) oxygen exchange for water splitting based on using Zr-modified high entropy oxides. The as-synthesized nanoparticles were characterized by XRD, FTIR, SEM, TEM, BET and EIS. The characterization results showed that the introduction of Zr4+ enlarged the Metal-Oxygen bond length of the spinel phase of FeMgCoNiOx and produced more oxygen vacancies with the increase of Zr4+ concentration, which greatly improved the thermochemical performance of FeMgCoNiOx. Water was splitted via reaction with FeMgCoNiOx/Zry that was previously reduced by microwave irradiation. By tuning the level of Zr4+ content from 0.0 to 1.0, FeMgCoNiOx/Zr0.6 was found to show the best trade-off, giving rise to a H2 yield of 4.84 mmol/g that was two times higher than that of FeMgCoNiOx (2.35 mmol/g) and outstanding thermodynamic energy efficiency (50 %). This work demonstrates that designing materials with abundant oxygen vacancies is a very efficient strategy to improve their thermochemical H2 evolution activity. Moreover, the energy efficiency will be further improved through tighter control of microwave reduction energetics.

Nickel-incorporated electrochemical activated carbon cloth for efficient and robust water oxidation activity

Carbon cloth is treated with a moderate electrochemical activation strategy to enrich its surface functional groups. In result, the hydrophilicity and intrinsic activity of carbon cloth (CC) can be enhanced. Furthermore, the activated carbon cloth (ACC) can adsorb massive transition-metal like nickel ions with a wet chemistry method. Combined with a low temperature thermal reduction under the atmosphere of Ar/H2, nickel-incorporated activated carbon cloth (Ni-ACC) can be obtained. In comparison with bare CC and nickel-incorporated carbon cloth (Ni-CC), the Ni-ACC displays more superior activity and durability of water oxidation in alkaline environment. The overpotential at the current density of 10 mA cm−2 (η10) is maintained at 245 mV after chronopotentiometry (CP) test for 24 h. The electrochemical activated method can serve as a simple approach for fabricating various metal-incorporated ACC.

A Novel Approach to Synthesize Reduced Graphene Oxide (RGO) at Low Thermal Conditions

Reduced graphene oxide (RGO) or graphene as it is commonly referred to is currently the most promising nanomaterial with potential applications. Synthesis of RGO starts with oxidation of graphite to graphene oxide (GO) which is further reduced either by chemical or thermal means. However, chemical reduction of GO to RGO involves the use of toxic chemical reagents which are not environmental friendly. Hence, in this work, low-temperature thermal reduction has been utilized to obtain high-quality RGO from GO effectively at a temperature of only 50 °C. The precursor of RGO which is GO is synthesized by modification of Improved Hummer’s method (Marcano et al. in ACS Nano 4(8):4806–4814, 2010), a non-toxic and non-explosive method of GO production. The highly exothermic reactions in producing GO are controlled by using ice baths with magnetic stirring. The prepared samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, Scanning electron microscopy and Energy-dispersive X-ray spectroscopy. All four characterizations confirm the efficient oxidation and reduction that has taken place to produce GO and RGO-4. (RGO formed on the 7th day of the reduction process.) XRD peak of GO at 10.46° corresponding to (001) plane indicating an interplanar spacing of 0.80 nm confirms the proper oxidation of graphite to GO. However, after thermal reduction at 50 °C the 2θ peak of GO at 10.46° shifts to 2θ = 24.15° with an interplanar spacing of 0.36 nm that confirms the formation of RGO-4 with retention of most of the sp2 structures by proper reduction of the oxygenated functional groups of GO. The GO produced is hydrophilic in nature due to existence of large number of oxygen-containing functional groups as shown in FTIR analysis. Raman results show that after reduction of GO to RGO-4 at 50 °C, the ID/IG ratio decreased significantly from 1.93 of GO to 1.00 of RGO-4 indicating highly reduced defects density in RGO-4.

Fabrication, structural evolutions and properties of large-area orientation reduced graphene oxide films by self-assembly at the air–water interface and thermal treatment

A very facile and efficient method was supplied to fabricate the large-area orientation RGO films by the interface self-assembly and low-temperature thermal reduction. The structural evolutions of RGO films were systematically investigated by AFM, FT-IR, Raman scattering and XPS spectra. The conductivity and wettability of RGO films with temperature changes were quantitatively evaluated by the four-probe measurement and contacted angle testing. As a result, the RGO films could be well reduced in the 200 ~ 300 °C for 2 h under air atmosphere.

Insight into the effect of graphite grain sizes on the morphology, structure and electrical properties of reduced graphene oxide

One of the most promising ways of obtaining graphene on a large scale is a two-stage oxidation-reduction process. It is worth mentioning, however, that the quality, structure and properties of the obtained graphene are highly dependent not only on conditions of the oxidation and reduction process but also the type of graphite from which the graphene (rGO) is obtained. This paper presents the influence of graphite grain size on the morphology, structure, and electrical properties of reduced graphene oxide. The tested materials were obtained in the process of low-temperature thermal reduction by micro-explosion. The research clearly shows that, although the type of graphite does not directly impact the reduction process itself, it nevertheless significantly affects parameters such as the degree of structure defect, size of crystallites or oxygen content in the sample. The work also proved that the smaller size of graphite grains facilitates the formation of oxygen-containing functional groups located in the plane of the layer. The differences in morphology and structure result in different electrical properties of the reduced graphene oxides obtained from different graphites.

Similar “relay race” capacitance behaviors of folded graphene films based high-performance supercapacitors

Graphene film has a great planar electron transport characteristic, yet its vertical electron transport efficiency is limited due to the two-dimensional stacked structure, which restricts the wide implementation of graphene-based films in energy storage field, such as supercapacitors (SCs). Herein, we present a novel folding strategy to tackle this issue and enable preparation of high mass loading graphene film possessing high-performance. Folded reduced graphene oxide (F-rGO) films via low-temperature thermal reduction show the folded paper-like structure and an interesting electrochemical law. Briefly, by combining the capacitance performance of different fold films, the resulting films exhibit a capacitance retention behavior of similar “relay race”, rendering that their gravimetric capacitance always maintain high level value (>180 F g−1) at the range of 2–9 mg cm−2, and the volumetric capacitance can achieve ~232 F cm−3 even at 11.3 mg cm−2. Meanwhile, their areal capacitance also exhibits a relay-like and linear growth with increasing mass loading and eventually achieve ~3400 mF cm−2 at 17 mg cm−2. In addition, flexible all-solid state SCs assembled by the F-rGO films also present excellent electrochemical and structural stability under bending states. Therefore, we believe that folding technique is a meaningful way to produce high-performance graphene based electrodes.

Solution-processed graphene oxide electrode for supercapacitors fabricated using low temperature thermal reduction.

We present a low temperature and solution-based fabrication process for reduced graphene oxide (rGO) electrodes for electric double layer capacitors (EDLCs). Through the heat treatment at 180 °C between the spin coatings of graphene oxide (GO) solution, an electrode with loosely stacked GO sheets could be obtained, and the GO base coating was partially reduced. The thickness of the electrodes could be freely controlled as these electrodes were prepared without an additive as a spacer. The GO coating layers were then fully reduced to rGO at a relatively low temperature of 300 °C under ambient atmospheric conditions, not in any chemically reducing environment. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that the changes in oxygen functional groups of GO occurred through the heat treatments at 180 and 300 °C, which clearly confirmed the reduction from GO to rGO in the proposed fabrication process at the low thermal reduction temperatures. The structural changes before and after the thermal reduction of GO to rGO analyzed using Molecular Dynamic (MD) simulation showed the same trends as those characterized using Raman spectroscopy and XPS. An EDLC composed of the low temperature reduced rGO-based electrodes and poly(vinyl alcohol)/phosphoric acid (PVA/H3PO4) electrolyte gel was shown to have high specific capacitance of about 240 F g−1 together with excellent energy and power densities of about 33.3 W h kg−1 and 833.3 W kg−1, respectively. Furthermore, a series of multiple rGO-based EDLCs was shown to have fast charging and slow discharging properties that allowed them to light up a white light emitting diode (LED) for 30 min.

(Invited) Large Low Temperature Thermoelectric Power Factor from Nanostructured Carbon-Based Nanocoatings

In an effort to create a paintable/printable thermoelectric materials, comprised exclusively of organic components, polyaniline (PANi), graphene, and double-walled carbon nanotubes (DWNT) were alternately deposited from aqueous solutions using the layer-by-layer assembly technique. Graphene and DWNT are stabilized with an intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). A 1 µm thick film, composed of 80 PANi/graphene-PEDOT:PSS/PANi/DWNT-PEDOT:PSS quadlayers (QL) exhibits electrical conductivity (σ) of 1.88 X 105 S/m and a Seebeck coefficient (S) of 120 µV/K, producing a thermoelectric power factor (S 2∙σ) of 2710 µW/(m∙K2). This is the highest value ever reported for a completely organic material measured at room temperature. Furthermore, this performance matches or exceeds that of commercial bismuth telluride. Air-stable n-type organic thermoelectric nanocomposites were achieved by depositing layers of double-walled carbon nanotubes (DWNT), stabilized with polyethylenimine (PEI), and graphene oxide (GO) in a layer-by-layer fashion from aqueous solutions. A 30 bilayer film (~ 610 nm thick), comprised of this DWNT-PEI/GO sequence, exhibits electrical conductivity of 27.3 S/cm and Seebeck coefficient of -30 µV/K, producing a power factor of 2.5 µW/(m∙K2). Low temperature thermal reduction (150 °C for 30 min) of this composite thin film significantly improves its thermoelectric performance. An electrical conductivity of 460 S/cm and Seebeck coefficient of -93 µV/K are achieved. A 30 BL DWNT-PEI/reduced graphene oxide (rGO) film (~480 nm thick) exhibits a power factor as large as 400 µW/(m∙K2), which is one of the highest values reported for an organic n-type material. The combination of water-based processing, air stability and high power factor is a major step toward producing efficient thermoelectric devices on flexible substrates (e.g. textiles for clothing). For the first time, there is a real opportunity to harness waste heat from unconventional sources, such as body heat to power devices in an environmentally-benign way. Figure 1

Thermally Reduced Graphene Oxide as a Counter Electrode Material for Dye-Sensitized Solar Cells.

In this present study, a highly conductive thermally reduced graphene oxide (TRGO) was synthesized by a low temperature thermal reduction method using RF heating, under an argon-hydrogen atmosphere. The crystallinity and morphology were examined by X-ray diffraction, Raman spectroscopy and TEM analysis. The chemical structure including the functional groups present on TRGO was studied using X-ray photoelectron spectroscopy and FTIR analysis. The studies reveal that thermal reduction of graphene oxide was successful under the experimental conditions and that the TRGO had high crystallinity. Further, the performance of the as-prepared TRGO was tested as a counter electrode (CE) in a dye-sensitized solar cell (DSSC). The maximum power conversion efficiency (PCE) obtained was 4.86% for TRGO under one sun illumination, which is comparable to that of a platinum CE-based DSSC (5.24%). The electrocatalytic activity and electron transfer kinetics were examined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel measurements. The series resistance (Rs) and charge transfer resistance (Rct) values were found to be 35.4 Ωcm-2 and 56.40 Ωcm-2 for TRGO. The results reveal that the TRGO had an electrocatalytic performance similar to that of Pt, making TRGO-CEs promising alternatives to the conventional Pt-CEs in DSSCs.

Thermally Enhanced n‐Type Thermoelectric Behavior in Completely Organic Graphene Oxide‐Based Thin Films

AbstractAir‐stable n‐type organic thermoelectric (TE) materials with high power factor are needed to produce efficient, lightweight devices that could be self‐powered by harnessing waste heat. Here, a completely organic n‐type TE nanocomposite is achieved by depositing layers of double‐walled carbon nanotubes (DWNT) stabilized with polyethylenimine (PEI) and graphene oxide (GO) in a layer‐by‐layer fashion from aqueous solutions. A 30 bilayer (BL) film (≈610 nm thick), comprised of this DWNT‐PEI/GO sequence, exhibits electrical conductivity of 27.3 S cm−1 and a Seebeck coefficient of −30 µV K−1, producing a power factor of 2.5 µW m−1 K−2. Low temperature thermal reduction (150 °C for 30 min) of this composite thin film significantly improves its thermoelectric performance. An electrical conductivity of 460 S cm−1 and Seebeck coefficient of −93 µV K−1 are achieved. A 30 BL DWNT‐PEI/reduced graphene oxide (rGO) film (≈480 nm thick) exhibits a power factor as large as 400 µW m−1 K−2, which is one of the highest values reported for an organic n‐type material. By depositing layers containing montmorillonite clay on top, these n‐type nanocomposites exhibit excellent air stability. This combination of air stability and high power factor could enable efficient thermoelectric devices on flexible substrates (e.g., clothing).

Revealing hidden endotherm of Hummers' graphene oxide during low-temperature thermal reduction

Graphene oxide (GO) is a promising precursor material for fabricating graphene-like structures. However, the detailed kinetics and dynamics of GO thermal reduction still need to be revealed. Here, we present a new insight into low-temperature thermal reduction of Hummers' GO. During the low-temperature thermal treatment, the decomposition process of functional groups of Hummers' GO is found to consist of 4 steps (Step 1: below 160 °C, Step 2: between 160 and 210 °C, Step 3: between 210 and 300 °C, Step 4: above 300 °C). We discovered that Step 3 is an endothermic process. This finding differs from the common view that the thermal reduction of Hummers' GO is only an exothermic event. In addition, gaseous aromatic by-products form below 300 °C, implying that the basal carbon network could start to decompose at such low temperatures. This is in contrast to the former experimental finding that the GO carbon plane decomposes only at higher temperature (above 350 °C). Finally, based on the combined kinetic and compositional measurements, we observe the evolution of functional groups during heat-treatment, and thereby, reveal the mechanism of low temperature reduction of Hummers' GO.

Studies on NH3 gas sensing by zinc oxide nanowire-reduced graphene oxide nanocomposites

ZnO nanowire-reduced graphene oxide nanocomposites have been prepared and used to detect ammonia at room temperature successfully. The nanocomposites are synthesized through hybridization of ZnO nanowires and graphene oxide sheets, which have been produced in large scale through mechanical mixing and low-temperature thermal reduction process. The micro ammonia sensors based on the as-prepared nanocomposites are subsequently constructed by dip-dropping method. The resultant device exhibits excellent response (∼19.2%) to NH3 at room temperature, which is much better than pure reduced graphene oxide based gas sensors. It is revealed that ZnO nanowires can provide pathways for electron transfer on reduced graphene oxide sheets, thus the ZnO nanowire-reduced graphene oxide nanocomposites based sensors show excellent overall sensing performance with high response, short response and recovery time, good stability and excellent selectivity. Moreover, the sensor has a very small size and low power consumption, which are critical for system integration and portable equipment. Our work can hold a great potential for realistic applications in ammonia detection fields.

Low-temperature thermal reduction of suspended graphene oxide film for electrical sensing of DNA-hybridization

A reduced graphene oxide (RGO) based capacitive real time bio-sensor was presented. Suspended graphene oxide (GO) film was assembled electrophoretically between the source and drain electrodes of a transistor and then reduced by annealing in hydrogen/nitrogen forming gas to optimize the surface functional groups and conductivity. The resonance frequency of the transmission coefficient (S21) of the transistor was observed to shift with deoxyribonucleic acid (DNA)-hybridization, with a detecting limit of ~5nM. The advantages of the bio-sensing approach include low-noise frequency output, solution based real time detection and capable of on-chip integration.

Preparation and characterization of graphene derived from low-temperature and pressure promoted thermal reduction

Abstract The reduction of graphene oxide was promoted remarkably under pressure via low temperature thermal treatment. Traditionally, graphene oxide is usually reduced in a preheated high temperature environment as a precondition of the thermal reduction. We report a pressure promoted method for low temperature thermal reduction and exfoliation of graphene oxide in large quantity at 260 °C. The physicochemical properties of parent graphite, as well as the microstructure and physicochemical properties of graphene oxide and resultant graphene were investigated by Raman spectrometer, thermograviment analyzer (TGA), transmission electron microscope (TEM), X-ray diffractometer (XRD) and Fourier transform infrared spectroscopy (FT-IR). Results show that graphene oxide was reduced to graphene with less stack via low-temperature pressure promoted thermal treatment, meanwhile, the degree of disorder reduced: the ratio of ID/IG in Raman spectrum decreases from 0.64 to 0.56. Moreover, graphene derived from low-temperature pressure promoted treatment exhibit better thermal stability than graphene oxide, and oxygen functional groups were removed with a high level. All of results exhibit improved comprehensive properties than graphene synthesized via traditional thermal reduction at 1000 °C.

Low-temperature thermal reduction of graphene oxide films in ambient atmosphere: Infra-red spectroscopic studies and gas sensing applications

Low-temperature (≤300°C) thermal treatment of graphene oxide (GO) films in ambient air is examined. In particular, the role of low to moderate heating temperatures, to the evolution of the original functional groups anchored on the GO skeleton, is closely investigated by Fourier transform infra red (FT-IR) spectroscopy. The study shows that, contrary to vacuum or inert ambient heating, heating under ambient atmosphere triggers concomitant reduction and oxidation reactions. Hydroxyl and epoxy groups are progressively eliminated, but at the same time newly formed carbonyls appear due to oxidation. Electrical measurements indicate that despite the presence of oxygen containing groups in the restored graphene sp2 network, the conductivity enhances. The process, therefore, lends itself to the production of conductive reduced GO with increased functionalities suitable for application in gas sensor fabrication. The concept is evaluated with a humidity sensor where thermally reduced GO is prepared at different reduction temperatures. The evaluation unveils that a critical reduction temperature exists where sensor sensitivity is optimized.

Electrochemical capacitors based on the composite of graphene and nickel foam

An improved Hummers method was developed for the simple and efficient production of high-quality graphene oxide (GO), and the composite of GO and nickel foam (NF) (GO/NF) was fabricated by ultrasonication-vacuum-assisted deposition of an aqueous solution of GO on NF. After chemical or thermal reduction, the composite of reduced GO and nickel foam (rGO/NF) was obtained. The electrochemical capacitance performance of rGO/NF was investigated using cyclic voltammetry and galvanostatic charge/discharge measurements. The chemically reduced rGO/NF composite (C-rGO/NF) exhibited high specific capacitance of 379 F/g at 1.0 A/g and 266.5 F/g at 10 A/g. We also prepared thermally reduced graphene oxide at 473 K in order to illuminate the difference in effect between the chemical and low-temperature thermal reduction methods on electrochemical properties. The cycling performance of thermally reduced rGO/NF composite (T-rGO/NF) and C-rGO/NF had ~91% and ~95% capacitance retention after 2000 cycles in a 6 mol/L KOH electrolyte, respectively. Electrochemical experiments indicated that the obtained rGO/NF has very good capacitive performance and could be used as a potential application of electrochemical capacitors. Our work revealed high electrochemical capacitor performance of rGO/NF composite and provided a facile method of rGO/NF preparation.