Vanadium redox reactions are considered a key factor affecting the energy efficiency of the all-vanadium redox flow batteries (VRFBs). Park M., Jeon I-.Y., Ryu J, Baek J-.B, Cho J*., “Exploring the Effective Location of Surface Oxygen Defects in Graphene-Based Electrocatalysts for All-Vanadium Redox-Flow Batteries,” Gev.
Introduction of flow batteries
- History
- Principle of flow battery and its types
- Materials for conventional vanadium redox flow batteries (VRFBs)
- Next-generation flow batteries
- Scope and organization of this dissertation
The negative electrolyte reservoir contains anodic redox-active materials dissolved in an electrolyte solution called the "anolyte" and the positive reservoir contains dissolved cathodic redox-active materials called the "catholyte." The total energy output depends on the volume of these reservoirs, which means that high-solubility catholytes and anolytes are preferred to achieve high volumetric energy densities. Alternatively, various porous polymer separators have been used to efficiently separate redox-active ions by pore size exclusion.
Synergistic effect of carbon nanotube/nanofiber catalyst for VRFBs
- Introduction
- Experimental detail
- Results and discussion
- Summary
Synthesis of CNF/CNT catalyst: A commercial carbon felt (PAN CF-20-3, Nippon carbon) was used as an electrode. CNF/CNT was synthesized on carbon felt (CF) electrode using acetylene gas at different temperatures ranging from 500°C to 800°C (hereafter referred to as CNF/CNT-T).
Corn-protein derived nitrogen-doped carbon nanoparticle catalysts for VRFBs
Introduction
A carbon-based material, such as pristine carbon felts, is considered one of the most promising electrode materials for VRFBs due to its low cost, high stability, high conductivity, and corrosion resistance.46, 54 Its poor kinetic reversibility and electrochemical reactivity have hindered its commercialization. VRFBs with low current density during the charge/discharge cycles.113, 114 Over the years, intensive efforts have been made to overcome the above drawbacks by introducing surface functionalities for abundant active sites or increasing electron conductivity.2. The metal-based catalysts (Pt,92 Bi,95 and Ir96 etc.) deposited on the carbon felt have been investigated to improve the cycle performance and catalytic activity for the vanadium redox reaction. However, this approach was discouraged due to the scarcity of metals or susceptibility to gas evolution.
Recently, the Pacific Northwest National Laboratory (PNNL) reported several new catalysts with low cost, excellent stability, and cell performance using a bismuth nanoparticle or niobium oxide nanorod.54,55 As an alternative strategy, modified carbon materials have been studied as an effective candidate for further broad applications of the VRFBs.115 Recently, we reported on the use of the CNT/CNF composite as a carbon-based electrocatalyst, leading to excellent cell performance.29 Significantly, N-doping in carbon materials, including CNTs, has been found that graphene , 31 and mesoporous carbon 117 enhance the vanadium redox reaction by facilitating the formation of defect sites for ion adsorption. This has been one of the critical issues for the commercial acceptance of the VRFBs. Bio-derived heteroatom-doped carbon materials have been extensively studied as electrode/catalyst materials for energy devices.118-122 Recently, to overcome these drawbacks, several amino acids such as alanine, cysteine, and glycine have been explored as a safe, non-polluting substance. -toxic and cheap heteroatom doping source for electrocatalysts.123.
Zein is the main protein of corn, which is mass-produced in the United States at a low cost (US$10/kg). This material mainly contains a high content of α-helix with β-sheet fractions.126 In addition, zein molecules have a unique structure with amphiphilic characteristics, 127, 128 which is one of the main driving forces for the formation of ordered film structures without external actions and their self-organization in two-dimensional periodic structures.129. Without supplying any external nitrogen-containing gases or metal seeds, the rearrangement of the zein molecules and the film-forming behavior facilitated the formation of heteroatom doping on the surface of the CB particles.
Experimental detail
Results and discussion
The HR-TEM image of the N-CB catalyst shows that all the CB nanoparticles are combined with the functionalized graphite layers (Figure 2d). The specific surface area of the N-CB catalyst was also measured in the value of ca. Cyclic voltammetric tests were performed on a three-electrode cell system to characterize the electrochemical properties of the N-CB catalyst.
Significantly, the reduction onset potential measured by the N-CB carbon sensing electrode was about 50 mV higher than that of the CB carbon sensing sample ( Figure 27a , inset). Additionally, the polarization-related peak potential separation of the N-CB carbon sensing electrode decreased to 146 mV at a scan rate of 5 mV s−1. We assembled single flow-type cells to evaluate the electrochemical performance of the untreated carbon electrode, CB and N-CB.
This indicates the electrochemical and chemical robustness of the N-CB catalyst in concentrated acid vanadium electrolytes. We have shown that the carbon felt electrode incorporated into the N-CB catalyst dramatically improves the electrochemical performance of the VRFB system. The experimental results of the oxidized carbon felt were obtained and compared with those of the N-CB carbon felt electrode, as shown in Figure 32.
Summary
Exploration of the effective location of graphene-based catalysts for VRFBs
- Introduction
- Experimental detail
- Results and discussion
- Summary
The unique advantage of the edge-functionalized graphene nanoplatelets lies in the presence of oxygen defects at the edge locations and better crystallinity of basal planes. It can be expected that the oxygen defects mainly occur at the edge locations of the E-GnP catalyst.139. These images show the nanostructure of the edge and basal plane of the rGO and E-GnP electrocatalysts (Figure 37).
The basal plane of the rGO catalyst also showed a similar defect structure with low crystallinity (Figure 37c). This agrees well with the above TEM analysis due to the defect-free basal plane of the E-GnP catalyst. Remarkably, the redox peak current density of the E-GnP catalyst is higher than that measured with the RGO catalyst.
Compared with the rGO and E-GnP catalyst, the enhanced catalytic activity of E-GnP toward vanadium redox couples can contribute to the defect-free basal plane for fast electron transfer and abundant edge oxygen functional groups. Importantly, we found that the slope of the E-GnP catalyst is 36% higher than that of rGO, which means that the edge-selectively functionalized structure is more favorable for the transport of vanadium ions. The enhancement of vanadium redox reactions can be attributed to the unique structure of edge-selectively functionalized graphene materials with a defect-free basal plane.
Catalytic activity of halogen-doped graphene-based catalysts for VRFBs
- Introduction
- Experimental detail
- Results and discussion
- Summary
TEM images of a) F-GNP, b) Cl-GNP and c) Br-GNP together with their elemental mapping of carbon, oxygen and halogens, respectively. The possibilities of initiation of oxidation and reduction reactions (V2+/V3+) in the negative half-cell test were similar to those in Cl- and Br-GNP samples. However, in a positive half-cell assay, the Br-GNP sample was found to have higher electrocatalytic activity toward vanadium redox couples than the Cl-GNP sample (Figure 48b).
As shown by the CV test, the redox current densities of the X-GNPs in positive half-cells were obtained in the order: Br-GNP > Cl-GNP > F-GNP. The slope of Br-GNP showed 28% and 31% higher values than that of F-GNP in oxidation and reduction reactions, respectively. Plot of the peak current densities of oxidation and reduction versus the square root of the scan rates for the positive vanadium redox reaction of the F-GNP, Cl-GNP, and Br-GNP catalyst.
These rate tests suggested that the electrical conductivity of Br-GNP at high current densities was much better than F-GNP and Cl-GNP. Thus, Br-GNP greatly promoted the vanadium redox reactions at high charge/discharge current densities, presumably by reducing cell polarization. Based on the experimental results, Br-GNP was shown to be effective electrocatalyst in the VRFB system.
Organic redox couples for lithium-based flow batteries
Introduction
Clean, renewable and sustainable energy storage systems have been extensively researched to meet the demands of modern society177, 178. Among various redox-active organic electrode materials, such as organosulfur compounds, nitroxide radical-bearing polymers and conductive polymers, the carbonyl group-containing molecules have been emphasized for their superior electrochemical performance89. Quinone-based molecules are one of the promising candidates for lithium-based batteries due to their molecular structural diversity with relatively fast electron and ionic transfer kinetics89, 179-181.
In contrast, the soluble redox-active organic molecules are highly required for the development of catholytes (dissolved organic cathode materials in liquid electrolytes) in flow batteries. For example, recent studies have shown that liquid catholytes can provide reliable capabilities and high reversibility185-187, as demonstrated by a modified anthraquinone185, 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB )186 , and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)187. Although extensive studies have focused on the highly soluble catholytes, the electrochemical properties affected by molecular structures in liquid catholytes have, to our knowledge, rarely been discussed.
In this study, to investigate the structure-dependent electrochemical performance of redox-active organic catholytes, we present new redox-active catholyte molecules based on 5,12-naphthacenequinone (NAQ) and 1,2-benzanthraquinone (BAQ). The newly discovered structural dependence of quinone isomers in the liquid catholyte is largely related to redox potentials, voltage plateaus, and electrochemical properties. More interestingly, flexible rechargeable lithium batteries containing our organic catholyte have successfully demonstrated stable electrochemical properties even under severe bending/stretching deformations.
Experimental detail
The electrochemical impedance spectrum (EIS) was measured on a single potentiostat (Ivium) by applying an alternating voltage of 5 mV over the frequency ranging from 10-2 to 105 Hz.
Results and discussion
Bismuth nanoparticles decorate graphite felt as a high-performance electrode for an all-vanadium redox flow battery. Sulfonated poly(etheretherketone)/functionalized carbon nanotube composite membrane for vanadium redox flow battery applications. Research on active electrodes modified with platinum/multi-walled carbon nanotubes for vanadium redox flow batteries.
Investigation of Ir-modified carbon felt as the positive electrode of an all-vanadium redox flow battery. Modified multi-walled carbon nanotubes as electrode reaction catalyst for all-vanadium redox flow battery. Strategies for improving the electrochemical activity of carbon-based electrodes for all-vanadium redox flow batteries.
Nitrogen-doped carbon nanotube/graphite felts as advanced electrode materials for vanadium redox flow batteries. PbO2-modified graphite felt as the positive electrode for a full vanadium redox flow battery. Nitrogen-doped graphene: Effects of nitrogen species on the properties of the vanadium redox flow battery.
Summary
Challenge facing flow batteries
Improved performance of Bi-modified graphite felt as positive electrode of vanadium redox flow battery.
