A 1D Mathematical Model of a H₂/Br₂Fuel Cell

Abstract

Hydrogen-Bromine (H 2 -Br 2 ) fuel cells are considered to be one of the viable systems for large scale energy storage because of their high energy conversion efficiency, flexible operation and low capital cost. A 1D mathematical model is developed to serve as a theoretical guiding tool for the experimental studies. The impact of convective and diffusive transport and kinetic rate on the performance of a H 2 -Br 2 fuel cell is shown in this study. Of the two flow designs (flow-by and flow-through) incorporated in this study, the flow-through design demonstrated better performance, which can be attributed to the dominant convective transport inside the porous electrode. Both experimental and simulated results validate that for the electrode properties and operating conditions selected, increasing the thickness of the Br 2 electrode beyond a certain value does not have any effect on the discharge performance of the fuel cell. The reactant concentration available inside the Br 2 electrode is greatly increased by operating the fuel cell at higher feed flow rates. Finally, the fuel cell configuration involving a thinner Br 2 electrode with higher specific active surface area is found to be the optimal choice for generating high performance.Electrochemical energy storage devices are the most promising candidates for large scale electrical energy storage because of their ability to convert electrical energy directly into chemical energy and vice versa at high efficiency. Significant advantages such as low capital cost, highly reversible reactions, flexible operation, and their ability to isolate energy and power components make flow batteries or reversible fuel cells an attractive alternative to conventional rechargeable batteries. 1-4 (Note that while a reversible fuel cell is essentially a flow battery, what differentiates a fuel cell from a flow battery is that the reactions at one or both of the fuel cell electrodes require a catalyst). The fact that the electrodes of these cells act as the conversion sites for the reactants and are not part of the reactant/product system implies these systems have much longer cycle life than conventional rechargeable batteries. The energy level in these fuel cells is determined by the concentration and volume of the reactants whereas the power level is determined by the electrode size and number of cells in the stack.Even though the Hydrogen-Oxygen (H 2 -O 2 ) fuel cells are known for their higher gravimetric energy density and long cell life, their performance is affected by the sluggish oxygen reduction kinetics. In the case of the Hydrogen-Bromine (H 2 -Br 2 ) fuel cells, faster reaction kinetics reduces the activation losses and enhances the electricto-electric energy conversion efficiency. A recent study by Cho et al. shows that rapid bromine reaction kinetics could be obtained on plain carbon substrates without any catalyst coating. 5 However, proper safety measures are required to handle the toxicity of Br 2 in the H 2 -Br 2 fuel cell. A recent study by our group comparing t...

Theory of Flow Batteries with Fast Homogeneous Chemical Reactions

“…− . 17 Unlike recent models of H 2 -Br 2 flow batteries (Figure 1a), 17,24,38 we here account for the complexation reaction occurring in the catholyte, Br 2 + Br − ⇔ Br − 3 , and the reduction of Br 3…”

“…[20][21][22] While it is well-known that homogeneous complexation reactions play a crucial role in halogen batteries, recent theoretical models of halogen flow batteries neglected the presence of such reactions. 17,23,24 In these latter models, the diatomic halogen molecule (Br 2 or Cl 2 ) is assumed to participate only in the heterogenous electrochemical reaction at the cathode. However, it can be reasonably expected that the predicted battery performance would be significantly affected by accounting for complexation, as polyhalide ions behave differently than the halogen molecule.…”

A Review of Hydrogen/Halogen Flow Cells

“…A separator is required to prevent shorting and product or reactant mixing . Several types of membrane separators have been applied to the hydrogen/halogen electrochemical system, including a perfluorinated sulfonic acid (PFSA) membrane for H 2 /Br 2 and H 2 /Cl 2 systems, nanoporous proton‐conducting material, and phosphoric acid doped polybenzimidazole (PBI) membrane . The vast majority of cells use an ion‐conducting membrane, of which Nafion is the most common due to its good transport properties and thermomechanical and chemical stability.…”

A Comprehensive Study of an Acid-Based Reversible H2-Br2Fuel Cell System

“…32 An alternate approach that can increase the active surface area without affecting the transport related morphological properties like porosity and tortuosity of the electrode is to grow nanotubes directly onto the electrode fiber surface to create high active surface area. 38 This is a more suitable option compared to using a multi-layered carbon electrode approach because the electrode surface area can be improved without affecting the porosity/tortuosity or the electrode thickness.…”