A Hydrogen‐Bromine Cell for Energy Storage Applications

187

183

The electrochemical behavior of a promising hydrogen/bromine redox flow battery is investigated for grid-scale energy-storage application with some of the best redox-flow-battery performance results to date, including a peak power of 1.4 W/cm 2 and a 91% voltaic efficiency at 0.4 W/cm 2 constant-power operation. The kinetics of bromine on various materials is discussed, with both rotating-disk-electrode and cell studies demonstrating that a carbon porous electrode for the bromine reaction can conduct platinumcomparable performance as long as sufficient surface area is realized. The effect of flow-cell designs and operating temperature is examined, and ohmic and mass-transfer losses are decreased by utilizing a flow-through electrode design and increasing cell temperature. Charge/discharge and discharge-rate tests also reveal that this system has highly reversible behavior and good rate capability. The environmental concerns and limited resources of fossil fuels have stimulated research for renewable energy sources such as wind and solar energy. Globally, there is 94 GW of electricity-generating wind power as of 2007, and it is estimated to reach 474 GW by 2020. 1The electricity from solar photovoltaics is growing at 40% per year worldwide, and the United States has targeted 100 GW of solar power by 2020.1 However, the electricity from these and other renewable resources is not constant and reliable due to their sensitive response to local weather conditions. To level out the variable generation of energy, large-scale electrical-energy storage (EES) is required. For the energy storage and load leveling, redox-flow batteries (RFB) have been considered as promising candidates due to their independently controllable power and energy, rapid response time, and high energy efficiency. Extensive research has been performed on RFB systems, including iron-chromium, all-vanadium, sodium-polysulfide, etc. 2-5However, due to the challenging issues such as low cell performance, power density, durability, and high electrolyte cost, their wide-spread adoption has not been realized. For example, the all-vanadium system, which is considered one of the closest to commercialization, utilizes a relatively expensive reactant and achieves power densities that are on the order of 0.2 to 0.7 W/cm 2 with relatively low efficiency. A hydrogen/bromine system is proposed as the reactants are earth-abundant and inexpensive and, as will be shown, high performance with high efficiency is obtainable.Yeo and Chin first investigated the hydrogen/bromine flow battery and reported excellent electric-to-electric efficiency, introducing it as a promising RFB system for energy-storage applications.6 The operating principle of the H 2 /Br 2 RFB can be described with a typical cell structure as in Figure 1. During discharge, a solution of Br 2 in HBr (aq) is fed into the cathode compartment where bromine reacts with protons supplied from the anode side and is reduced to bromide, generating the theoretical electric potential of 1.098 V at 25• C. The redu...

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confidence: 99%

High Performance Hydrogen/Bromine Redox Flow Battery for Grid-Scale Energy Storage

Cho

Ridgway

Weber

et al. 2012

187

183

“…Cation-exchange membranes like Nafion may suppress, but do not eliminate, crossover of Br anions. [18][19][20][21] Br species that enter the negative electrode can be separated from the negative electrolyte and returned to the positive electrolyte without causing permanent decay, in a similar fashion to vanadium in VRB. 8,21 Transport of ions in RFB membranes has been investigated previously, although the literature focuses predominantly on diffusion.…”

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confidence: 99%

The Influence of Electric Field on Crossover in Redox-Flow Batteries

Darling

Weber

Tucker

et al. 2015

152

Transport of active species through the ion-exchange membrane separating the electrodes in a redox-flow battery is an important source of inefficiency. Migration and electro-osmosis have significant impacts on the crossover of reactive anions, cations, and neutral species. In this paper, these phenomena are theoretically and experimentally explored for commercial cation-exchange membranes. The theoretical analysis indicates that plotting the cumulative Coulombic mismatch between charge and discharge as a function of time can be used to assess crossover rates. The relative importance of migration and electro-osmosis over diffusion is quantified and shown to increase with increasing current density and membrane thickness because the contributions of migration and electro-osmosis to ionic flux are independent of membrane thickness and proportional to current density, while diffusion is inversely proportional to membrane thickness and independent of current density. Redox-flow batteries (RFBs) possess compelling attributes for stationary energy storage. In particular, the inherent decoupling of energy and power in RFBs enables the cost effective use of active materials with low energy density.1,2 Energy is stored in redox-active molecules and power is generated by oxidizing and reducing different redox couples at the positive and negative electrodes.3-5 Two promising technologies are the all-vanadium RFB (VRB) and the hydrogen/bromine RFB, where good performances have been demonstrated recently. [6][7][8][9][10][11][12][13][14] Protons in a cation-exchange membrane (CEM) typically carry charge between the two electrodes in these two RFBs. In both systems, movement of reactant or product species from one electrode to the other through the membrane results in inefficiency and reversible capacity loss. 8,15,16 For these reasons it is worthwhile to investigate the causes of crossover, and examination of the two chosen RFBs allows for the study of transport of active cations, anions, and neutral species.The reactions at the negative and positive electrodes in a VRB are:andrespectively. Protons are the primary ionic charge carriers. Because the active species at both electrodes are vanadium ions, transfer from one electrode to the other is less detrimental than it is in other RFBs that have dissimilar active materials at the two electrodes. Vanadium that traverses the separator reacts to form a discharged ion at the opposite electrode. 17 Although imbalances that occur after repeated cycling can be recovered by mixing the electrolytes, vanadium that crosses the membrane represents an inefficiency that is detrimental in applications like grid-scale energy storage where high efficiency is required for economic success.For the hydrogen/bromine RFB, the reactions at the negative and positive electrodes areBr 2 (aq) + 2e 18-21 Br species that enter the negative electrode can be separated from the negative electrolyte and returned to the positive electrolyte without causing permanent decay, in a similar fashion to vanadium in V...

“…[1][2][3][4][5][6][7] The charge and discharge reactions occurring are as follows: The active reactant material, hydrobromic acid (HBr) is also used as the supporting electrolyte. If the energy-storage system is commissioned in the discharged state, which is the most common case, HBr is the only chemical that is required.…”

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confidence: 99%

Advanced Hydrogen-Bromine Flow Batteries with Improved Efficiency, Durability and Cost

Lin

Chong

Yarlagadda

et al. 2015

The hydrogen/bromine flow battery is a promising candidate for large-scale energy storage due to fast kinetics, highly reversible reactions and low chemical costs. However, today's conventional hydrogen/bromine flow batteries use membrane materials (such as Nafion), platinum catalysts, and carbon-paper electrode materials that are expensive. In addition, platinum catalysts can be poisoned and corroded when exposed to HBr and Br 2, compromising system lifetime. To reduce the cost and increase the durability of H 2 /Br 2 flow batteries, new materials are developed. The new Nafion/ polyvinylidene fluoride electrospun composite membranes have high perm-selectivity at a fraction of the cost of Nafion membranes; the new nitrogen-functionalized platinum-iridium catalyst possesses excellent activity and durability in HBr/Br 2 environment; and the new carbon-nanotube-based Br 2 electrodes can achieve equal or better performance with less materials when compared to baseline electrode materials. Preliminary cost analysis shows that the new materials reduce H 2 /Br 2 flow-battery energy-storage system stack and system costs significantly. The resulting advanced H 2 /Br 2 flow batteries offer high power, high efficiency, substantially increased durability, and expected reduced cost. The active reactant material, hydrobromic acid (HBr) is also used as the supporting electrolyte. If the energy-storage system is commissioned in the discharged state, which is the most common case, HBr is the only chemical that is required. During charge, hydrobromic acid is electrolyzed to generate hydrogen and bromine, which are stored in separate tanks. Bromine has a moderate solubility in water which can be greatly enhanced by the presence of Br − via complexation to form Br 3 − or Br 5 − . 8,9 The gas phase H 2 electrode also simplifies the separation and recovery of crossover catholyte, which can be returned back to the catholyte tank.The H 2 /Br 2 flow battery technology has been under investigation since the 1960s. Brief literature reviews can be found in recent publications by Cho et al., 4 Kreutzer et al. 5 and Tolmachev. 6 H 2 /Br 2 flow batteries share the same cell architecture as proton-exchange-membrane fuel cells (PEMFCs). Therefore, H 2 /Br 2 flow batteries are also referred to as regenerative or reversible H 2 /Br 2 fuel cells. Similar to PEMFCs, * Electrochemical Society Active Member. * * Electrochemical Society Student Member. * * * Electrochemical Society Fellow.z E-mail: gygylin@gmail.com membrane-electrode assemblies (MEAs) are the most crucial components in the H 2 /Br 2 flow batteries. In today's state-of-the-art H 2 /Br 2 flow batteries, MEAs are commonly made of commercial perfluorosulfonic acid (PFSA) membranes such as Nafion, platinum catalysts, and plain carbon papers. The PFSA membrane in a H 2 /Br 2 flow battery is used to physically separate the positive and negative electrodes, and prevent mixing of hydrogen and bromine/bromides while allowing proton transport between the electrodes. The membrane resistance has ...