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 ...

In a hydrogen-bromine (H 2 -Br 2 ) fuel cell, the Br 2 reactions don't require precious metal catalysts, hence porous carbon gas diffusion media (GDM) are widely used as electrodes. However, the specific surface areas of the commercial carbon gas diffusion electrodes (GDEs) are quite low and need to be enhanced. In order to improve the active surface area of carbon GDEs, a study was conducted to grow multi-walled carbon nanotubes (MWCNTs) directly on the carbon electrode fiber surface. Both constant and pulse current electrodeposition techniques were used to deposit Co nanoparticles to catalyze the MWCNT growth. The MWCNTs were grown in the presence of a mixture of acetylene, argon, and hydrogen gases using the chemical vapor deposition process. Based on the results obtained from SEM, TEM, and EDX analysis, MWCNT growth following the tip model was confirmed. The results from the multi-step chronoamperometry study have shown that the synthesized carbon GDEs with MWCNTs have 7 to 50 times higher active surface area than that of a plain GDE. The performance of a single layer of the best MWCNT GDE measured in a H 2 -Br 2 fuel cell was found to be equal or slightly higher compared to that obtained using a three-layer plain carbon electrode. Electrical energy storage is required to address the increasing use of intermittent energy sources. The regenerative hydrogen-bromine (H 2 -Br 2 ) fuel cell was identified as a promising candidate for large scale electrical energy storage due to the rapid kinetics of the H 2 and Br 2 reactions translating to its higher energy conversion efficiency and power density capability.1-8 Moreover, the abundance of active materials used in this system is an added advantage. The electrochemical reactions associated with the H 2 -Br 2 fuel cell system are shown below. The bromine electrode in the H 2 -Br 2 fuel cell can be replaced by alternative materials such as vanadium, cesium, chromium or iron. However, the H 2 -Br 2 system is more attractive than the hydrogen-vanadium 9 because its active material (HBr) is inexpensive, its electrode reaction kinetics are much faster, and it has much higher energy density (higher HBr/Br 2 concentrations and no supporting electrolyte required because HBr serves as both the reactive material and electrolyte). On the other hand, cesium is a rare earth element and is very expensive ($10/gram or $300/ounce) making it unsuitable for large scale energy storage. Also, the cesium, chromium, and iron systems have not been explored with a negative hydrogen electrode.While the H 2 reactions currently require the use of precious metal catalysts like platinum, the bromine reactions don't since reasonable exchange current densities can be obtained with carbon materials. 6 Even though the exchange current density of Br 2 reactions on platinum (Pt) is two orders of magnitude higher than that on carbon materials, the corrosivity and toxicity of hydrobromic acid (HBr) and * Electrochemical Society Student Member.* * Electrochemical Society Active Member. * * * Electrochem...

The commercially available carbon gas diffusion electrodes (GDEs) with low specific active area but high permeability are often used as Br 2 electrodes in the H 2 -Br 2 fuel cell. In order to increase the specific active surface area of the existing carbon GDEs, a study was conducted to grow multi-wall carbon nanotubes (MWCNTs) directly on the surface of carbon fibers of a commercial carbon electrode. Experimental fixtures were developed to promote the electrodeposition of cobalt and the growth of MWCNTs on the carbon GDE. The MWCNT growth across the carbon electrode was confirmed by SEM. The carbon GDE with a dense distribution of short MWCNTs evaluated in a H 2 -Br 2 fuel cell has 29 times higher active surface area than a plain carbon electrode and was found to be highly durable at an electrolyte flow rate of 10 cc/min/cm 2 . The performance of the best single layer MWCNT GDE measured at 80% discharge voltage efficiency in a H 2 -Br 2 fuel cell was found to be 16% higher compared to that obtained using three layers of plain carbon electrodes. Finally, the preliminary material cost analysis has shown that the MWCNT-based carbon electrodes offer significant cost advantages over the plain carbon electrodes. The regenerative hydrogen-bromine (H 2 -Br 2 ) fuel cell is one of the potential candidates for large scale electrical energy storage. [1][2][3][4][5][6][7][8]11 However, there are three major material related challenges that need to be addressed in order to make the H 2 -Br 2 fuel cell system more viable. First, a membrane that restricts the crossover of unwanted bromine species (Br 2 , Br − , and Br 3 − ) from the bromine electrode to the hydrogen electrode is required. Second, an active H 2 electrocatalyst that is stable in hydrobromic acid (HBr)/bromine (Br 2 ) is needed. Third, bromine electrodes with high active surface area are desired. Prior works conducted in this area have explored novel H 2 electrocatalyst and membrane materials that could possibly replace the conventionally used platinum (Pt)/carbon (C) catalyst and Nafion membrane used in the H 2 -Br 2 fuel cell system. 9-13 Also, a previous study conducted by our group has introduced multi-wall carbon nanotube (MWCNT)-based carbon electrodes with high specific active surface area as an alternative to the widely used plain carbon Br 2 electrodes. 14,15 Several research investigations reported in the literature have successfully grown MWCNTs directly on the carbon electrode fiber surface. [16][17][18][19] These MWCNT-based electrodes were successfully employed in polymer electrolyte membrane (PEM) fuel cells. The MWCNTs were grown on the surface of the GDE, and subsequently platinum (Pt) was electrodeposited in order to improve the catalyst utilization in the PEM fuel cell. 16 The electronic pathways were secured by depositing Pt nanoparticles directly on the MWCNTs, thus improving the catalyst utilization. On the other hand, the bromine reactions in H 2 -Br 2 fuel cells don't require expensive metal catalysts and hence plain carbon GDEs with high...

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