Graphene Cathode Research Articles

Uranium extraction by a graphene-based asymmetric electrode configuration through combined complexation, electro-adsorption, and photocatalytic reduction

Recently, capacitive deionization has become a promising approach for uranium extraction owing to its energy-efficiency and environmental benign. However, theadsorbedchargeduranium(VI)(U(VI))wouldrepulsetheincomingU(VI)ions,andmightre-enterintosolution,vastlyimpedingtheimprovementofelectro-adsorptionperformance. Reducing the adsorbed U(VI) ions into U(IV) precipitation is an ideal way to address above issues. Here, a coupling approach of photocatalytic reduction and electro-adsorption (PEA) was proposed. An asymmetric electrode configuration, composed of phosphate functionalized graphene (GP) cathode and graphene/TiO2 nanocomposite (GT) anode was rationally designed, and originally proved its enhanced U(VI) extraction via the PEA method. The asymmetric electrode device enables the fast transport of photo-electron and the rapidly directed migration of U(VI) ion to the GP cathode. More importantly, it combines three synthetic mechanisms of complexation, electro-adsorption, and photocatalytic reduction to extract U(VI) ions. As a consequence, the as-designed PEA method shows a higher removal rate of 91.3% in comparison with conventional photocatalytic reduction (PA) and electro-adsorption (EA) methods. Meanwhile, its kinetics rate is 225% and 50% faster than PA and EA methods. Furthermore, an enhanced reduction efficiency of U(VI) to U(IV), good selectivity as well as reusability for PEA method were also obtained. The results provide a potential approach to combine multiple mechanisms for efficient U(VI) extraction from aqueous solution by purposeful design of asymmetric electrodes.

Increasing the efficiency and cycle life of Na-O2 batteries based on graphene cathodes with heteroatom doping

To overcome the challenges of Na-O2 batteries with respect to efficiency, capacity, and cycle life as well as to develop cheap, metal-free, and abundant electrocatalysts, we explored boron and nitrogen-functionalized graphene aerogels prepared by the hydrothermal self-assembly of graphene oxide with subsequent thermal reduction. The results showed an improve of both the cycling overpotential and the coulombic efficiency for the functionalized graphene aerogels. However, the nitrogen-containing cathode presented a shortened cycle life and decreased charging stability. The postmortem analysis of the full discharge, and the full discharge and charge cathodes demonstrated that nitrogen functionalization triggered the formation of solid parasitic products that passivate the cathode surface, thus resulting in a poorer electrochemical performance. By contrast, functionalization with boron-containing groups demonstrated to be a more promising strategy due to minimized parasitic products, leading to lower oxygen reduction and evolution overpotentials with a concomitantly enhanced cell efficiency vs. the undoped cathodes. This resulted in a cycle life of 70 cycles at a relatively high current density of 0.1 mA cm−2 with a capacity cut-off of 0.5 mAh cm−2. Our study underscores that functionalization with heteroatoms simultaneously alters multiple characteristics of graphene-based materials, including their chemistry, texture and morphology, which in turn presents a critical impact on the electrochemical response of the resultant Na-O2 cells.

ZIF derived PtCo alloys-based nitrogen-doped Graphene as cathode catalyst for proton exchange membrane fuel cell

The proton exchange membrane fuel cell (PEMFC) has gained a lot of attention due to its ability to provide electrical energy with high efficiency and minimal environmental pollution. However, platinum-based nanomaterials, as the most commonly used electrocatalysts, have very limited global reserves and high prices, posing significant barriers to widespread commercialization of PEMFC. Here, we present a novel method for preparing a Co-ZIF supported PtCo alloy catalyst for the oxygen reduction reaction (ORR). The precursor Co-ZIF doped graphene oxide (Co-ZIF@GO) is obtained by in-situ controlled growth of ZIF-67 nanocrystals on two surfaces of graphene oxide (GO) sheets with different nanoscale dimensions. PtCo alloys-based nitrogen-doped graphene (PtCoNG) cathode catalyst with well-dispersed PtCo alloy nanoparticles (∼5 nm) is obtained after pyrolysis and platinum loading. The optimal PtCoNG catalyst outperforms the commercial Pt/C catalyst in terms of performance and stability in acidic media, with a half-wave potential of up to 0.86 V vs reversible hydrogen electrode (RHE) and three times the mass activity. It demonstrates that this new synthesis method provides an effective strategy for developing active and stable Pt alloy catalysts by in-situ growing ZIF on graphene oxide.

Selective dechlorination degradation of chlorobenzenes by dual single-atomic Fe/Ni catalyst with M-N/M-O active sites synergistic

Removal and detoxification of chlorobenzenes have attracted public concern, multiple active sites single-atom Fe and single-atom Ni composite nitrogen-doped graphene (FeSA/CN/NiSA) cathode catalyst supplied generation and adsorption capacity of hydrogen and hydroxyl active species. M-O active sites coupled with M-N improved activity and stability of the catalyst, and decreased bond breaking energy barrier of C-Cl, FeSA/CN/NiSA-NiF cathode showed superior removal performance of chlorinated aromatic hydrocarbons (monochlorobenzene: 98.9%, dichlorobenzene: over 90.4%, trichlorobenzene: over 85.7%) and selectivity. Chlorobenzenes were dechlorinated under low stepwise voltage on the FeSA/CN/NiSA-NiF cathode. The efficiencies of stepwise dechlorination reactions of chlorobenzenes were all above 76%, Faradaic efficiencies were above 71.8%. The FeSA/CN/NiSA-NiF cathode was not sensitive to the molecular structure and has overcome the high energy barrier of chlorobenzenes molecular structure. The electrophilic attack of H*ads formed hyperconjugation bond weakened the possibility of the Cl atom forming a bond with the benzene ring, and was favorable for the Cl position to achieve single-electron transfer dechlorination. The selective stepwise dechlorination degradation of chlorobenzenes by FeSA/CN/NiSA-NiF cathode with multiple active sites demonstrated the advantaged performance of M-O and M-N active sites coupled synergistic in electrochemical reduction and degradation, providing a strategy for product-selective degradation of chlorinated aromatic hydrocarbons.

An ion exchange membrane-free, ultrastable zinc-iodine battery enabled by functionalized graphene electrodes

The aqueous rechargeable zinc-iodine battery is a promising system due to its high theoretical capacity, zinc and iodine abundance, and safety of the aqueous electrolyte. However, several challenges need to be addressed for zinc-iodine batteries to be competitive, including self-discharge, sluggish kinetics, low practical energy density, and dendrite formation on zinc anodes. Here, we realize a high-performing zinc-iodine battery with long-term stability through a novel design of electrodes and electrolytes. A three-dimensional functionalized graphene cathode facilitates the iodide redox reactions as well as immobilizes the dissolved polyiodides, suppressing the detrimental shuttling effect. Additionally, we introduce a composite anode made of zinc coated with reduced graphene oxide. This design greatly enhances the performance and stabilizes the anode during repeated zinc stripping/plating preventing dendrite formation. The electrolyte is formulated in a way that allows the use of an economical and efficient glass fiber separator instead of the commonly employed ion-exchange membranes which are expensive and have a relatively high Ohmic resistance. Our optimized zinc-iodine batteries exhibit high current efficiencies of nearly 100%, along with stable specific capacities of 257, 186, 150, 84 mAh g−1 at corresponding current densities of 1, 2, 5, 10 A g−1. Furthermore, a long-term capacity retention of 96.7% at 5 A g−1 over 2000 cycles is achieved, outperforming many similar aqueous batteries. This work also provides valuable guidance in designing electrodes for other aqueous metal-halide energy storage systems.

Elucidating the charge storage mechanism of high-performance vertical graphene cathodes for zinc-ion hybrid supercapacitors

Aqueous zinc-ion hybrid supercapacitors (ZHSs) are gaining enormous attention due to intrinsic safety, low cost and potential for acquiring both high energy density and high power density, but their electrochemical properties are realistically restricted by improper physicochemical characteristics and ambiguous charge storage mechanism of carbon cathodes. Herein, we report an advanced carbon cathode of activated vertical graphene (A-VGN) for high-performance ZHSs. For the A-VGN cathode, vertical graphene array morphology and hierarchically porous structure effectively shorten ion diffusion distance, and meanwhile, large specific surface and oxygen doping offer abundant active sites for ion adsorption. Consequently, the A-VGN cathode-based ZHS presents exceptional electrochemical performance such as a high capacity of 246 mAh/g, fast charge/discharge capability, superior cycling stability with 97.4% capacity retention over 10,000 charge/discharge cycles, as well as preponderant energy density among various carbon cathode-based ZHSs. Furthermore, in-operando Raman spectroscopy and in-operando pH monitoring techniques, being supplemented by ex-situ methods, are applied to investigate the charge storage mechanism of the A-VGN cathode. Dynamic adsorption/desorption behaviors of different ions on the carbon cathode during charge/discharge processes are elucidated. This work is expected to facilitate the development of ZHSs by providing new insights into the design and charge storage mechanism of carbon cathodes.

Multi-layered fluorinated graphene cathode materials for lithium and sodium primary batteries

Multi-layered fluorinated graphene cathode materials for lithium and sodium primary batteries

Freeze-tolerant gel electrolyte membrane for flexible Zn-ion hybrid supercapacitor

At low temperatures, the working capability of Zn-ion energy storage devices at low temperatures is becoming a huge challenge in the field of electrochemical energy storage. This is primarily due to the slow kinetic process of ions along with the failure of hydrogel electrolytes under subzero temperatures. Herein, a flexible anti-freezing Zn-ion hybrid supercapacitor is developed with the use of freeze-tolerant polyacrylamide‑sodium alginate (PAM-SA) gel electrolyte membrane combined with graphene cathode and Zn foil anode. The freeze-tolerant PAM-SA gel electrolyte membrane is synthesized by a simple solvent displacement process and it shows an excellent ionic conductivity of ∼15.29 mS cm−1 at room-temperature (RT) with an anti-freezing ability of 4.17 mS cm−1 at −25 °C. Also, the prepared electrolyte membrane demonstrates an outstanding long-term stability over 900 h without the formation of Zn dendrites. The anti-freezing Zn-ion hybrid supercapacitor displays a sound electrochemical performance with an excellent capacity of 210.6 F g−1 in conjunction with the capacitance retention of 65 % even after charge-discharge cycling for 40,000 times at RT. Also, the device possesses an excellent anti-freezing performance. It can still deliver a capacity of 79.60 F g−1 and the capacitance retention of 80 % after cycling for 15,000 times even at a low temperature of −25 °C. These results indicate that the present work may offer a feasible way of constructing Zn-ion energy storage devices applicable in a subzero temperature environment.

Ultrahigh Energy Density and Long-Life Cyclic Stability of Surface-Treated Aluminum-Ion Supercapacitors.

In this study, aluminum-graphene supercapacitors (denoted as aluminum-ion supercapacitors; ASCs), consisting of a battery-type aluminum anode, a capacitor-type graphene cathode, and ionic liquid 1-ethyl-3-methylimidazolium chloride (EMImCl) and aluminum chloride (AlCl3) electrolyte, were prepared. This study primarily aimed to investigate the enhanced electrochemical performance of ASCs arising from changes in the surface oxide layer and morphology via electrochemical surface treatments, including electropolishing and electrodeposition of aluminum anodes. The ASC devices based on an electrodeposited anode at a current density of 3 A g-1 exhibited a high specific capacity of 211 F g-1 compared to that of the electropolished anode (∼186 F g-1); these were 20 and 5.7%, respectively, higher than that of the pristine aluminum anode. In particular, the electrodeposited ASC delivered an energy density of 151 W h kg-1 at a power density of 3,390 W kg-1. Furthermore, a maximum power density of 11,104 W kg-1 was achieved at an energy density of 124.3 W h kg-1. These values are among the best as compared to those of previously reported aluminum-based supercapacitors, suggesting the potential feasibility of these ASCs with outstanding energy and power densities for next-generation energy storage devices.

High performance lithium oxygen batteries based on a phosphorous-doped holey graphene cathode

High performance lithium oxygen batteries based on a phosphorous-doped holey graphene cathode

Charge Transfer Dynamics of Doped Graphene Electrodes for Organic Light-Emitting Diodes.

Atomically thin graphene has attracted immense attention as a future transparent electrode for flat-panel displays owing to its excellent conductivity, optical transparency, and flexibility. In particular, a graphene doping process is essential for implementing graphene-based high-performance devices, and the development of a transparent cathode with a low work function is required to simplify the integration process of thin-film transistors and organic light-emitting diodes (OLEDs) into active matrix displays. In this study, a transparent n-doped graphene cathode is proposed for implementing inverted OLEDs through two types of cesium (Cs)-based doping techniques: a dipping method using wet chemicals and an evaporation method under a vacuum atmosphere. The changes in the chemical structures and work functions of the n-doped graphene electrodes, as well as their surface morphologies and transmittances, were systematically investigated. The n-type doping mechanism of graphene was investigated, and a close relationship between the electrical charge transfer characteristics of graphene transistors and the formation of C-O-Cs complexes was revealed. Finally, an effective Cs-doped graphene electrode was developed, exhibiting a dramatically decreased work function while maintaining high transmittance; therefore, the Cs-doped graphene cathode was successfully integrated with inverted OLEDs with a bottom-light emission structure that exhibited enhanced external quantum efficiency of graphene cathode-based OLEDs. Thus, our findings provide a better understanding of the doping strategies and potential of n-doped graphene as a transparent cathode for developing high-performance future displays.

Microwave-Reduced Graphene Oxide for Aluminum Batteries

Recently, rechargeable graphene-based aluminum-ion batteries (AIBs) as an attractive energy storage system has been studied. Owing to the requirements such as high conductivity and low defects, the graphene cathode used in AIBs is typically fabricated using chemical vapor deposition (CVD). Here, we utilize solution-processable microwave-reduced graphene oxide (MWrGO) that has been shown to be of very high quality with low defect density as a cathode in AIBs. Our results show that it is possible to increase the AlCl4– ion storage with MWrGO, relative to thermally reduced graphene oxide (TrGO). The MWrGO film cathode for AIBs shows a stable discharge capacity of 87.7 mA h g–1 at 0.2 A g–1 with a coulombic efficiency of >93% and favorable rate capability that can withstand more than 4500 cycles without capacity decay at a high current density of 2 A g–1. The relationship between AlCl4– ion storage capability and microwave time has been examined to elucidate the fundamental relationship between AlCl4– intercalation and defects in graphene. We found that improved quality and lowered defects lead to higher diffusion-controlled contribution arising from the faradaic redox reaction, also leading to increased AlCl4– ion storage capability, whereas higher defects of graphene result in increased electrical double-layer capacity contribution but reduced AlCl4– ion storage capability.

Flexible aqueous asymmetric capacitor with SiC-decorated boron-doped graphene cathode and core–shell SiC@graphene anode

• Core-shell SiC@graphene (Gr) nanoparticles were synthesized as an anode material. • B-doped graphene (BGr) nanosheets embedded with SiC nanoparticles were synthesized. • SiC-BGr electrocatalyzed the NiOOH/NiO redox reaction to improve pseudocapacitance. • SiC-BGr cathode showed the maximum specific capacitance of 1803 mF cm −2 at 4 mA cm −2 . • Aqueous SiC-BGr//SiC@Gr capacitor showed high working voltage and stability. Flexible aqueous pseudocapacitors have the advantages of high specific capacitance, high safety, low cost, and fast ion diffusion. However, the development of such devices is a challenge because of the low stability of the active material and the high interface resistance of the current collector-active material. In this study, we used silicon carbide (SiC)-boron-doped graphene (BGr) nanosheets and core–shell SiC@Gr nanoparticles as the cathode and anode, respectively, and constructed a flexible aqueous asymmetric capacitor (ASC) with a 3 M NaOH/carboxymethyl cellulose hydrogel as the electrolyte. In an aqueous NaOH solution, the optimized SiC-BGr cathode showed a specific capacitance of 1384.3 mF cm −2 (461.4 F g −1 ) at the current density of 3 mA cm −2 (1 A g −1 ), and the core–shell SiC@Gr anode showed a wide potential window of −1.0–0 V. The as-assembled ASC device had a high working voltage of 1.4 V, delivered an excellent capacitance of 337.1 mF cm −2 , and retained 86% of its initial capacitance after 10 000 cycles. Furthermore, the ASC also showed high energy density (57.3 μWh cm −2 ), high power density (6536.9 μW cm −2 ), high bending stability, and applicability at a low temperature of −20 °C. Thus, the preparation of novel SiC-Gr materials can promote the development of flexible aqueous capacitors with high working voltage and good temperature adaptability for application in the field of wearable electronic devices with safety, strong environmental adaptability, and reliable performance.

Fast and homogeneous ion regulation toward a 4 V, high-rate and dendrite-free potassium metal battery

The uneven distribution and slow diffusion kinetics of potassium ions (K+) result in the formation and growth of K dendrites as well as poor rate capabilities, restricting the development of K metal anodes (KMAs). Paired with a Mn-based Prussian blue (PB) cathode, the KMA-based full battery exhibits a low average discharging voltage of ≈3.6 V and a high overpotential. Herein, a N-doped graphene nanomesh (NGM) acts as a conductive and potassiophilic host to solve this issue. First, the NGM enables K+ ions to rapidly transport its nanopores and edges to reach the whole electrode, offering unblocked transport channels for K+ ions and thus acquiring an enhanced rate capability. Second, the N atoms and defects within NGM reduce the nuclear energy barrier of K+ ions and induce their homogeneous deposition, avoiding the formation of K dendrites. Third, the nano-/micro-pores within NGM can accommodate the volume exchange of KMA during cycles, improving its cycling stability. Benefiting from these merits, the symmetric K@NGM cell delivers an ultra-low overpotential of 12.4 mV after 1700 h. Matched with a K1.7Mn[Fe(CN)6]0.9/graphene cathode, a 4 V and dendrite-free KMnHCF/G‖K@NGM full cell is designed, which exhibits excellent cycling stabilities and rate capabilities.

The influence of electrolyte concentration and solvent on operational voltage of Li/CFx primary batteries elucidated by Nernst Equation

Fluorinated carbons (CFx) have the highest energy density as the cathode of lithium primary battery. They demonstrate distinct behaviors in different electrolytes. While, the reaction mechanism is still unclear. In this work, the influences of electrolyte in aspects of Li-ion concentration, anion species, and solvent species on the performances of Li/CFx batteries are explored. Contrary to conventional wisdoms that electrolytes play a role as a Li-ion conductor and only limit the kinetics, we found that the electrolyte can determine its thermodynamic behavior. In detail, a lower Li-salt concentration and a solvent solvating Li+ with more molecular numbers provide higher operational voltages in Li/CFx battery. This can be well elucidated by Nernst Equation. Interesting, in Li-free electrolyte (0 M DMSO), fluorinated graphene cathode can still work, offering an impeccable specific discharge capacity of 738 mAh g−1 with a specific energy of 1968 Wh kg−1 at 0.1C.

High energy-power density Zn-ion hybrid supercapacitors with N/P co-doped graphene cathode

By combining the large power density of supercapacitors with the high energy density of ion batteries, a novel zinc-ion hybrid supercapacitor (ZHSC) is proposed and considered a promising candidate. But there are still challenges in the selection and application of reliable cathode materials. Herein, we used a mature and safe one-step hydrothermal method to synthesize N/P co-doped graphene (NPG), which exhibited a unique three-dimensional structure and a large specific surface area, showing great advantages as cathode to assemble Zn//NPG ZHSC with zinc foil. In the 1 M ZnSO 4 solution, the ZHSC shows superior electrochemical performance, which presents an excellent specific capacitance of 210.2 F g −1 under a wide working voltage of 1.8 V, and it achieves both a maximum energy density of 94.6 Wh kg −1 and a large power density of 4500 W kg −1 . More importantly, the capacitance retention rate remains 82% and a coulomb efficiency of ∼100% is achieved after 15,000 cycles. Overall, the results of this study highlight the efficient synthesis and excellent performance of NPG cathode materials, and also promote the rapid development and great application of high-performance Zn-ion hybrid capacitors. • The N/P co-doped graphene shows a high specific capacitance in ZnSO 4 solution. • Zn-ion hybrid supercapacitor is assembled by Zn//ZnSO 4 //NPG. • The excellent energy and power density of Zn//ZnSO 4 //NPG are achieved.

Preparation and Electrochemical Performance of a Graphene Cathode Catalyst Decorated With Pt by Prolysis

For fuel cells, to produce high-quality and low-platinum catalyst is a pressing technical problem. In this study, graphene cathode catalysts with controllable platinum content were decorated by pyrolyzing chloroplatinic acid under various process parameters to obtain a high catalytic activity and durability. The results show that platinum particles generated by pyrolyzing chloroplatinic acid are uniformly loaded on graphene without agglomeration. The average particle size of platinum particles is about 2.12 nm. The oxygen reduction reaction catalytic activity of catalyst samples first increases, then decreases with increasing platinum loading in cyclic voltammetry and LSV. Compared with the commercial Pt/C (20 wt% Pt) catalyst, the initial potential and the current density retention rate of the catalyst decorated with 8% platinum are 55 mV and 23.7% higher, respectively. From i-t curves, it was found that the stability of the catalyst prepared in this paper was improved compared with the commercial Pt/C catalyst. The catalysts prepared in the present research exhibits superior catalytic activity and stability.

Boron-doping-induced defect engineering enables high performance of a graphene cathode for aluminum batteries

Defect engineering using boron doping endows aluminum–graphene batteries with high capacity and outstanding long-term stability of more than 10 000 cycles.

High-density three-dimensional graphene cathode with a tailored pore structure for high volumetric capacity zinc-ion storage

Zinc-ion hybrid supercapacitors (ZHCs), based on the multivalent ion storage principle, are considered to be a promising candidate for large-scale energy storage devices. However, with the increasing demands for portable and miniaturized energy storage devices, how to obtain sufficient energy output in a limited volume is particularly critical, so that requires high volumetric energy density with no sacrifice of power density. Herein, we designed a compact three-dimensional porous graphene (3D-PG-1) cathode for ZHCs to balance the porous structure and high density by drying the graphene hydrogel with a short period of capillary evaporation. As for 3D-PG-1, the pore size range is 0.6–10 nm, where the micropore is for high capacity and the mesopore helps Zn2+ rapid transport, and the high density of 1.38 g cm−3 guaranteed its high volumetric capacitance. Therefore, the ZHC based on this 3D-PG-1 cathode delivered an excellent volumetric performance of 299 F cm−3 at 0.1 A g−1, excellent long cycle stability (225 F cm−3 at 5 A g−1, 85% retention after 30000 cycles) and extremely high volumetric energy density up to 118 Wh L−1 at 116 W L−1, which retained 61 Wh L−1 at the power density as high as 23.2 kW L−1.

Concept for Realizing High Output Power Density Thermionic Energy Convertor by Field-Assisted Thermionic Emission Using a Direct-Tunneling Metal–Insulator–Graphene Cathode

The usage of waste heat is an effective way to satisfy the great demand for energy in modern human society. A thermionic energy convertor (TEC) can directly produce electricity from thermal energy with high efficiency. However, high working temperature is one of the major challenges in TEC. In this work, a concept based on field-assisted thermionic emission (FTE) has been proposed to solve this problem. Basically, the field-assisted thermionic energy convertor (FTEC) uses a metal-insulator-graphene (MIG) structure as the cathode to produce direct-tunneling FTE current for the conversion. The fundamental performance of FTEC has been theoretically studied. The penetration field within the MIG structure that controls both the FTE current and the hot electron energy is the key for the performance optimization. By engineering the MIG cathode band structure to balance the above mentioned effects of the penetration field, the ideal efficiency of ~10% and the corresponding output power density of ~5 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> can be obtained simultaneously under 700 K. Further comparison between FTEC and conventional TEC indicates that the output power density can be improved by at least four orders of magnitude under a mild temperature (600-700 K). These results provide a universal approach for realizing compact high output power density TEC and its combined system.