Multiple-dimensioned defect engineering for graphite felt electrode of vanadium redox flow battery

The scarcity of wettability,insufficient active sites,and low surface area of graphite felt(GF)have long been suppressing the performance of vanadium redox flow batteries(VRFBs).Herein,an ultra-homogeneous multipledimensioned defect,including nano-scale etching and atomic-scale N,O codoping,was used to modify GF by the molten salt system.NH_(4)Cl and KClO_(3) were added simultaneously to the system to obtain porous N/O co-doped electrode(GF/ON),where KClO_(3) was used to ultra-homogeneously etch,and O-functionalize electrode,and NH4Cl was used as N dopant,respectively.GF/ON presents better electrochemical catalysis for VO_(2)+/VO_(2)+ and V3+/V2+ reactions than only O-functionalized electrodes(GF/O)and GF.The enhanced electrochemical properties are attributed to an increase in active sites,surface area,and wettability,as well as the synergistic effect of N and O,which is also supported by the density functional theory calculations.Further,the cell using GF/ON shows higher discharge capacity,energy efficiency,and stability for cycling performance than the pristine cell at 140 mA cm^(−2) for 200 cycles.Moreover,the energy efficiency of the modified cell is increased by 9.7% from 55.2% for the pristine cell at 260 mA cm^(−2).Such an ultra-homogeneous etching with N and O co-doping through“boiling”molten salt medium provides an effective and practical application potential way to prepare superior electrodes for VRFB.

O/N/S trifunctional doping on graphite felts:A novel strategy toward performance boosting of cerium-based redox flow batteries

The cerium-based redox flow battery(RFB)is regarded as a compelling gridscale energy storage technology to revolutionize the utilization of renewable energy by storing the energy in liquid electrolytes.However,its widespread implementation is impeded by the cerium redox reactions that exhibit slow kinetics on commercial graphite felt(GF)electrodes.Surface functionalization may be an available activation strategy to achieve a significant boost in the electrochemical performance of GFs.However,conventional chemical and/or electrochemical routes for the surface functionalization of GFs suffer from the issues of complication,and the deterioration of the resulting modified electrode surface over long-term cycle processes leads to catalytic activity decline.Here,we develop a facile and general strategy for introducing the functional groups to the electrode through the addition of L-cysteine into electrolytes.The-COOH,-NH_(2),and-SH groups in L-cysteine can induce oxygen/nitrogen/sulfur trifunctional doping on GF surfaces with lower deterioration rates,which enables the activated GFs to demonstrate a promising electrocatalytic activity toward cerium redox reactions and excellent durability when used as a cerium-based RFB electrode.This study proposes a rational strategy to overcome the intrinsic limitations of existing modification techniques for GFs and provides a potential pathway toward high-performance RFBs.

A heterogeneous double chamber electro-Fenton with high production of H_(2)O_(2) using La–CeO_(2) modified graphite felt as cathode

Hydrogen peroxide synthesis by electro-reduction of O_(2) to substitute the current anthraquinone process has attracted a great deal of attention. Low oxygen utilization rate and low hydrogen peroxide production remain obstacles to electro-Fenton application. In situ H_(2)O_(2) generated by electrochemical reaction depends on the electrochemical performance of the cathode and the structure of the reactor. Here, novel graphite felt(GF) modified by La-doped CeO_(2)(La-CeO_(2)) was developed as a cathode. A new double chamber electro-Fenton reactor was proposed, where an organic ultrafiltration membrane was used to prevent H_(2)O_(2) from spreading to the anode. The effects of hydrothermal temperature, time and urea concentration on the electrochemical properties of graphite felt were investigated. The accumulated concentration of H_(2)O_(2) on the modified cathode reached 218.4 mg·L^(-1)in 1 h when the optimal conditions of hydrothermal temperature 120 ℃ and urea concentration 0.55%(mass) in 24 h. The degradation rate of methyl orange reached 98.29%. The new electro-Fenton reactor can efficiently produce hydrogen peroxide to degrade various organic substances and has a high potential for treating wastewater in the chemical industry.

Electrochemical behavior of diverse vanadium ions at modified graphite felt electrode in sulphuric solution

PAN-based graphite felt （PGF） treated in 98% sulphuric acid for 5 h and then kept at 450 ℃ for 2 h was evaluated for their electrochemical performance as electrodes of vanadium redox battery （VRB）. Structure and characteristic of treated PAN-based graphite felt （TPGF） were determined by means of Fourier Transform Infi-ared Spectroscopy, Scanning Electron Microscopy, Brunauer-Emmett-Teller surface area analysis and VRB test system. The results show that the acid and heat synergistic effect increase the number of --COOH functional groups on the PGF surface, and the PGF is eroded by sulphuric acid oxidation, resulting in the surface area increases from 0.31 m^2/g to 0.45 m^2/g. The V（ Ⅱ ）/V（Ⅲ） redox reaction is electrochemically reversible on the TPGF electrode, while the V（Ⅳ）/V（Ⅴ） couple is a quasi reversible process. The diffusion coefficients of the oxidation for V（Ⅳ）/V（Ⅴ） obtained from the scope of peak current Ip vs scan rate v^1/2 is 4.4×10^-5 cm^2/s. The improvement of electrochemical activity for the electrode is mainly ascribed to the increase of the number of ---COOH groups on the TPGF, which behaves as active sites catalyzing the vanadium species reactions and accelerating electron transfer reaction and oxygen transfer.

CoFe-LDH nanowire arrays on graphite felt: A high-performance oxygen evolution electrocatalyst in alkaline media

Developing non-noble-metal oxygen evolution reaction(OER) electrocatalysts with high performance is critical to electrocatalytic water splitting. In this work, we fabricated Co Fe-layered double hydroxide(LDH) nanowire arrays on graphite felt(Co Fe-LDH/GF) via a hydrothermal method. The Co Fe-LDH/GF, as a robust integrated 3 D OER anode, exhibits excellent catalytic activity with the need of low overpotential of 252 and 285 mV to drive current densities of 10 and 100 mA/cm^(2) in 1.0 mol/L KOH, respectively. In addition, it also maintains electrochemical durability for at least 24 h. This work would open up avenues for the development of GF like attractive catalyst supports for oxygen evolution applications.

High electro-catalytic graphite felt/Mn O2composite electrodes for vanadium redox flow batteries

A mild and simple synthesis process for large-scale vanadium redox flow batteries(VRFBs)energy storage systems is desirable.A graphite felt/Mn O_2(GF-MNO)composite electrode with excellent electrocatalytic activity towards VO^(2+)/VO_2^+redox couples in a VRFB was synthesized by a one-step hydrothermal process.The resulting GF-MNO electrodes possess improved electrochemical kinetic reversibility of the vanadium redox reactions compared to pristine GF electrodes,and the corresponding energy efficiency and discharge capacity at 150 m A cm^(-2)are increased by 12.5%and 40%,respectively.The discharge capacity is maintained at 4.8 A h L^(-1)at the ultrahigh current density of 250 m A cm^(-2).Above all,80%of the energy efficiency of the GF-MNO composite electrodes is retained after 120 charge-discharge cycles at 150 m A cm^(-2).Furthermore,these electrodes demonstrated that more evenly distributed catalytic active sites were obtained from the Mn O_2particles under acidic conditions.The proposed synthetic route is facile,and the raw materials are low cost and environmentally friendly.Therefore,these novel GF-MNO electrodes hold great promise in large-scale vanadium redox flow battery energy storage systems.

Electro-catalytic activity of CeO_(x) modified graphite felt for carbamazepine degradation via E-peroxone process

E-peroxone(EP)was one of the most attractive AOPs for removing refractory organic compounds from water,but the high energy consumption for in situ generating H_(2)O_(2) and its low reaction efficiency for activating O_(3) under acidic conditions made the obstacles for its practical application.In this study,cerium oxide was loaded on the surface of graphite felt(GF)by the hydrothermal method to construct the efficient electrode(CeO_(x)/GF)for mineralizing carbamazepine(CBZ)via EP process.CeO_(x)/GF was an efficient cathode,which led to 69.4%TOC removal in CeO_(x)/GF-EP process with current intensity of 10 mA in 60 min.Moreover,CeO_(x)/GF had the flexible application in the pH range from 5.0 to 9.0,TOC removal had no obvious decline with decrease of pH.Comparative characterizations showed that CeO_(x)could enhance surface hydrophilicity and reduce the charge-transfer resistance of GF.About 5.4 mg/L H_(2)O_(2) generated in CeO_(x)/GF-EP process,which was 2.1 times as that in GF-EP process.The greater ozone utility was also found in CeO_(x)/GF-EP process.More O_(3) was activated into hydroxyl radicals,which accounted for the mineralization of CBZ.An interfacial electron transfer process was revealed,which involved the function of oxygen vacancies and Ce^(3+)/Ce^(4+)redox cycle.CeO_(x)/GF had the good recycling property in fifth times'use.

La and Sr Composite Oxides-modified Graphite Felt for Aqueous Organic Redox Flow Batteries

In this study,aqueous organic redox flow batteries(AORFBs)with NaCl as supporting electrolyte were investigated.In AORFBs,the chlorine evolution reaction should be retarded,not the hydrogen evolution reaction.To enhance the catalytic activity of the graphite fel(GF)electrode,the metal oxides were proposed to decorate on the GF surface.Among the loading oxides,significant enhancement of the mass transfer and reaction activity was obtained by the presence of LaSrOx nanoparticles.X-Ray photoclectron spectroscopy and contact angle measurements revealed that the content of oxygen-containing groups and the hydrophilicity were remarkably inereased.After the electrode assem-bled in the battery,the LaSrO/GF electrode presented huge enhancement of the battery performance,obviously increasing in the battery capacity and efficiency.At a current of 50 Am/cm^(2),the energy efficiency(EE)of the battery increased from 54.76%to 61.37%by the LaSrO/GF electrode.Furthermore,the cyclability of the system tested that no obviously fading was observed after 100 cycles,signifying the excellent stability of the LaSrOx/GF electrode.

ZIF-derived holey electrode with enhanced mass transfer and N-rich catalytic sites for high-power and long-life vanadium flow batteries

Electrode materials with good redox kinetics,excellent mass transfer characteristics and ultra-high stability play a crucial role in reducing the life-cycle cost and prolonging the maintenance-free time of the vanadium flow batteries(VFB).Herein,a nitrogen-doped porous graphite felt electrode(N-PGF)is proposed by growing ZIF-67 nanoparticles on carbon fibers and then calcinating and acid etching.The multi-scale structure of“carbon fiber gap(electrolyte flow),micro/nano pore(active species diffusion)and Nitrogen active center(reaction site)”in N-PGF electrode effectively increases the catalytic sites and promotes mass transfer characteristics.Reasonable electrode design makes the battery show excellent rate performance and ultra-high cycling stability.The peak power density of the battery reaches 1006 mW cm^(-2).During 1000 cycles at 150 mA cm^(-2),the average discharge capacity and average discharge energy of N-PGF increase substantially by 11.6%and 23.4%compared with the benchmark thermal activated graphite felt,respectively.More excitingly,after ultra-long term(5000 cycles)operation at an ultra-high current density(300 mA cm^(-2)),N-PGF exhibits an unprecedented energy efficiency retention(99.79%)and electrochemical performance stability.

Broad temperature adaptability of vanadium redox flow battery-part4:Unraveling wide temperature promotion mechanism of bismuth for V^(2+)/V^(3+) couple

Vanadium flow battery （VFB） is a fast going and promising system for large-scale stationary energy stor- age. However, drawbacks such as low power density and narrow temperature window caused by poor catalytic activity of graphite felt （GF） electrodes limit its worldwide application. In this paper, bismuth, as a low-cost, no-toxic and high-activity electrocatalyst, is used to modify the thermal activated GF （TGF） via a facile hydrothermal method. Bismuth can effectively inhibit the side reaction of hydrogen evolution in wide temperature range, while promoting the V2＋/V3＋ redox reaction. As a result, the VFB assembled with Bi/TGF as negative electrode demonstrates outstanding rate performance under the current density up to 400 mAcm-2, as well as a long-term stability over 600 charging/discharging cycles at a high cur- rent density of 150mA cm-2. Moreover, it also shows excellent temperature adaptability from -10 ℃ to 50 ℃ and high durability for life test at the temperature of 50 ℃.

Electrode modification and electrocatalysis for redox flow battery(RFB) applications

The electrode is one of the main components in redox flow batteries(RFBs), as it provides the reactions sites for redox couples and can influence the cell performance through its effect on cell voltage losses associated with activation overpotential, concentration overpotential and ohmic losses. Extensive research has thus been carried out on material selection, structural design and modification of electrodes as well as electrocatalysis for redox reactions. This review provides an historical overview of the screening and modification of electrode materials together with recent progress in novel electrode architectures, electrode modification and electrocatalysis methods. RFB systems such as iron/chromium, polysulfide/bromine and all vanadium batteries are discussed in detail.

Electrochemical Aziridination of Tetrasubstituted Alkenes with Ammonia

Ammonia(NH3)is an ideal nitrogen source in terms of availability,reactivity,safety,atom economy,environmental compatibility,and ease of isolation.However,its utility for amine synthesis is limited by its high bond dissociation energy,its strong coordination ability,and the difference between its reactivity and that of the product amines.Herein,we reported the first electrochemical protocol for direct syntheses of unprotected tetrasubstituted aziridines with NH3 and alkenes in the absence of an oxidant,which are highly challenging to achieve by other methods.The combination of graphite felt as the anode material and MeOH as the solvent was the key to the success of the protocol,and the effects of these factors were investigated by means of cyclic voltammetry and density functional theory calculations.