MODELING AND EXPERIMENTAL STUDIES TO OPTIMIZE THE PERFORMANCE OF A HYDROGEN – BROMINE FUEL CELL

Abstract

The regenerative Hydrogen-Bromine (H2-Br2) 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, highly reversible reactions and low capital cost. The preliminary performance of a H2-Br2 fuel cell using both conventional as well as novel materials (Nafion and electrospun composite membranes along with platinum and rhodium sulfide electrocatalysts) was discussed. A maximum power density of 0.65 W/cm2 was obtained with a thicker Br2 electrode (780 μm) and cell temperature of 45◦C. The active area and wetting characteristics of Br2 electrodes were improved upon by either pre-treating with HBr or boiling them in de-ionized water. On the other hand, similar or better performances were obtained using dual fiber electrospun composite membranes (maximum power densities of 0.61 W/cm2 and 0.45 W/cm2 obtained with 25 μm and 65 μm electrospun membranes at 45◦C) versus using Nafion membranes (maximum power densities of 0.52 W/cm2 and 0.41 W/cm2 obtained with Nafion 212 and Nafion 115 membranes at 45◦C). The rhodium sulfide (RhxSy) electrocatalyst proved to be more stable in the presence of HBr/Br2 than pure Pt. However, the H2 oxidation activity on RhxSy was quite low compared to that of Pt. In conclusion, a stable H2 electrocatalyst that can match the hydrogen oxidation activity obtained with Pt and a membrane with low Br2/Br- permeability are essential to prolong the lifetime of a H2-Br2 fuel cell. A 1D mathematical model was 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 H2-Br2 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 validated that for the electrode properties and operating conditions selected, increasing the thickness of the Br2 electrode beyond a certain value does not have any effect on the discharge performance of the fuel cell. The reactant concentration available inside the Br2 electrode was greatly increased by operating the fuel cell at higher feed flow rates. Finally, the fuel cell configuration involving a thinner Br2 electrode with higher specific active surface area was found to be the optimal choice for generating high performance. The commercially available carbon gas diffusion electrodes (GDEs) were commonly used as Br2 electrodes in the H2-Br2 fuel cell. However, the specific surface area of commercial carbon GDEs is quite low and needs to be enhanced. In order to improve the active surface area of carbon GDEs, a study was conducted where multi-walled carbon nanotubes (MWCNTs) were grown directly on the carbon electrode fiber surface. The results from multi-step chronoamperometry study have shown that the synthesized carbon GDEs with MWCNTs have 7 to 75 times higher active surface area than that of a plain GDE. The carbon GDE with a dense distribution of short MWCNTs evaluated in a H2-Br2 fuel cell has 29 times higher active surface area than that of a plain carbon electrode and was found to be highly durable at an electrolyte flow rate of 10 cc/min/cm2. The performance of the best MWCNT GDE (1 layer) measured at 80% discharge voltage efficiency in a H2-Br2 fuel cell was found to be 16 times 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.