Identification of performance limiting electrode using asymmetric cell configuration in vanadium redox flow batteries

105

Electrochemical impedance spectroscopy and cyclic voltammetry were used to investigate the electrode kinetics of V II -V III and V IV -V V in H 2 SO 4 on glassy carbon, carbon paper, carbon xerogel, and carbon fibers. It was shown that, for all carbon materials investigated, the kinetics of V II -V III is enhanced by anodic, and inhibited by cathodic, treatment of the electrode; in contrast, the kinetics of V IV -V V is inhibited by anodic, and enhanced by cathodic, treatment. The potential region for each of these effects varied only slightly with carbon material. Rate constants were always greater for V IV -V V than for V II -V III except when anodized electrodes were compared, which may explain discrepancies in the literature. The observed effects are attributed to oxygen-containing functional-groups on the electrode surface. The considerable differences between the potentials at which enhancement of V II -V III and inhibition of V IV -V V occur indicates that they do not correspond to a common oxidized state of the electrode. Likewise inhibition of V II -V III and enhancement of V IV -V V do not correspond to a common reduced state of the electrode. It is possible that enhancement of both V II -V III and V IV -V V is due to the same (active) state of the electrode. There is considerable interest in vanadium flow batteries (VFBs), also known as vanadium redox flow batteries (VRFBs or VRBs), for storage of electrical energy particularly in conjunction with renewable energy sources such as wind and solar. [1][2][3][4][5][6] Active areas of research include cell design and modelling, [7][8][9] performance and state-of-charge monitoring, 10-16 coulombic and energy efficiencies, 5,17,18 electrolytes, [11][12][13][14][15][16]19,20 membranes, 4,21 and electrodes. Cells typically have porous carbon electrodes and electrode performance can depend strongly on electrode treatment. Various electrochemical, 22-27,36-41 chemical, 36,40,43,44 and thermal [45][46][47][48][49] treatments have been reported. These treatments often have the effect of oxidizing or reducing the surface, and the influence of surface oxygen species on electrochemical kinetics at carbon electrodes is recognized, 22,57-60 although often not well understood. Thermal 45-49 and chemical 36,40,43,44 treatments of electrodes for VFBs have been tested on a range of carbon-based electrodes and, in general, these treatments result in higher activities of the electrode toward the vanadium redox reactions. There are also a number of reports of the effect of electrochemical treatment of electrodes. Anodic treatment of carbon felt was reported 22,36 to cause a decrease in the kinetic rates of the V IV -V V redox couple. In contrast, there are also reports of enhancement of V IV -V V kinetics after electrochemical oxidation [38][39][40][41] (of graphite and carbon felt electrodes) and of V II -V III kinetics after potential cycling 61 (of highly-oriented-pyrolytic-graphite and glassy carbon electrodes). However, in considering the effects of anodization on a c...

Section: Comparison Of Vmentioning

confidence: 70%

Section: Comparison Of Vmentioning

confidence: 88%

mentioning

confidence: 99%

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Electrode Kinetics of Vanadium Flow Batteries: Contrasting Responses of V^II-V^IIIand V^IV-V^Vto Electrochemical Pretreatment of Carbon

Bourke

Miller

Lynch

et al. 2015

105

“…10. First, vanadium (IV) oxide sulfate hydrate (VOSO 4 · xH 2 O, Sigma Aldrich) was dissolved into a 4M solution of sulfuric acid and deionized water to achieve a final vanadium concentration of 1 M in the V(IV) oxidation state.…”

Section: Methods Of Approachmentioning

confidence: 99%

Enhancing Mass Transport in Redox Flow Batteries by Tailoring Flow Field and Electrode Design

Dennison

Ağar

Aküzüm

et al. 2015

Self Cite

161

116

In this study, we investigate the mass transport effects of various flow field designs paired with raw and laser perforated carbon paper electrodes in redox flow batteries (RFBs). Previously, we observed significant increases in peak power density and limiting current density when perforated electrodes were used in conjunction with the serpentine flow field. In this work, we expand on our earlier findings by investigating various flow field designs (e.g., serpentine, parallel, interdigitated, and spiral), and continuously measuring pressure drop in each configuration. In all cases, these perforated electrodes are found to be associated with a reduction in pressure drop from 4% to 18%. Flow field designs with a continuous path from inlet to outlet (i.e., serpentine, parallel, spiral) are observed to exhibit improved performance (up to 31%) when paired with perforated electrodes, as a result of more facile reactant delivery and resulting greater utilization of the available surface area. Conversely, flow fields with discontinuous paths which force electrolyte to travel through the electrode (e.g. interdigitated), are adversely affected by the creation of perforations due to the high permeability 'channels' in the electrode. These results demonstrate that mass transport can significantly limit the performance of RFBs with carbon paper electrodes. Redox flow batteries (RFBs) are widely considered to be a promising technology for grid-scale electrical energy storage, in applications such as buffering renewable energy sources and time-shifting energy from periods of high supply to periods of high demand (known as peak shaving).1-5 The unique aspect of RFBs, as compared with conventional secondary batteries, is that energy capacity and power output are decoupled from each other, such that the energy capacity depends on the volume of the electrolyte storage tanks and the concentration of active species in solution, while the power capacity depends on the total cell area and the number of cells in a stack. Although numerous redox chemistries have been proposed and studied, the all-vanadium chemistry is perhaps the most thoroughly characterized. [6][7][8] Regardless of the redox chemistry, the costs associated with these flow battery systems should be minimized to enable their widespread implementation. Previously, it has been shown that the costs associated with the cell stack constitute a substantial portion of the overall system cost.9 This has prompted numerous efforts to increase the power density, thereby decreasing the stack size needed for a given power output, and thus lowering the cost of the stack.10-24 Previous works have primarily focused on cell geometry, 19-24 electrode materials and functionalization, 10-24 and membrane selection. 20In 2012, significant improvements in power density were demonstrated by Mench and co-workers by utilizing carbon paper electrodes instead of more conventional carbon felt electrodes.19-22 Carbon papers tend to be much thinner than conventional carbon felt electrodes, and thus a...

“…Much efforts have been devoted to diminishing the activation loss by modifying the carbon fiber surface. [8][9][10][11][12] In addition, ohmic loss and mass transfer loss also lead to the reduction of the voltage efficiency. It is generally recognized that supplying high flow rate of electrolyte for the RFB is the most feasible method to minimizing mass transfer loss, but more pumping power is inevitably required for increasing flow rates.…”

mentioning

confidence: 99%

The Dependence of Mass Transfer Coefficient on the Electrolyte Velocity in Carbon Felt Electrodes: Determination and Validation

You

Cheng

2017

In a flow battery, the salient impact of the electrolyte velocity on the mass transfer coefficient in carbon felt electrodes is demonstrated and quantified. A lab-scale flow battery, fed with identical electrolyte solutions containing Fe 2+ /Fe 3+ as active substances in both the anode and the cathode, is used to realize stable tests free from side reactions in a broad range of current densities. The electrolyte velocities ranging from 2.5 to 15 mm s −1 are selected in this work, which are typical in flow through electrodes in most flow batteries. By measuring limiting currents at various flow rates, a correlation between the mass transfer coefficient and the velocity in dimensionless form is obtained as Sh = 1.68 Re 0.9 . Meanwhile, a 2-D numerical model incorporating this correlation and the experimentally measured electrolyte conductivity is proposed. Voltage losses of the battery fed with adequate reactants at different velocities are both experimentally measured and numerically simulated. The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents. Owing to the exclusive advantage of decoupling power generation and energy storage, redox flow batteries (RFBs) have been considered as a critical candidate for large-scale electrical energy storage (EES). [1][2][3][4] Unlike the electrodes of conventional secondary batteries, the electrodes of RFBs do not participate in reactions but only provide electrochemical reaction sites for active ions and support electrons transfer. For numerous kinds of developed RFBs, the porous carbon felts, composed of randomly interlaced carbon fibers, are generally selected as electrodes. [5][6][7] Obtaining high voltage efficiency at high current densities is one of the major challenges in the commercialization of RFBs. Much efforts have been devoted to diminishing the activation loss by modifying the carbon fiber surface. [8][9][10][11][12] In addition, ohmic loss and mass transfer loss also lead to the reduction of the voltage efficiency. It is generally recognized that supplying high flow rate of electrolyte for the RFB is the most feasible method to minimizing mass transfer loss, but more pumping power is inevitably required for increasing flow rates. Tang et al. 6 and Ma et al. 7 carried out special research on optimizing the operating strategy of electrolyte flow rate for kilowatts class allvanadium flow battery (VRFB) systems, in which the significant impact of flow rate on cell voltage was detected during the whole charge/discharge process. In a carbon felt electrode, the mass transfer loss is caused by the species transport between the bulk solution in the pore and the carbon fiber surface, which is usually quantified by the mass transfer coefficient k m . 13 For cells operating under high current densities, the mass transfer loss also contributes a significant proportion to the voltage loss, especially when approaching the end of charge/discharge process. Considerin...