Abstract
In a hydrogen-bromine (H2-Br2) fuel cell, the Br2 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 H2-Br2 fuel cell was found to be equal or slightly higher compared to that obtained using a three-layer plain carbon electrode.
Highlights
Electrical energy storage is required to address the increasing use of intermittent energy sources
The regenerative hydrogen-bromine (H2-Br2) fuel cell was identified as a promising candidate for large scale electrical energy storage due to the rapid kinetics of the H2 and Br2 reactions translating to its higher energy conversion efficiency and power density capability.[1,2,3,4,5,6,7,8]
The nucleation step is where nuclei or seeds are formed on the surface of the carbon fibers which act as sites for the growth of Co nanoparticles
Summary
The specific active surface area (intrinsic property) of commercially available untreated carbon GDEs is quite low (0.65 m2/g compared to 100 m2/g of Pt/C electrodes).[11] One of the common approaches reported in the literature to improve the active surface area of a Br2 electrode was to employ multiple layers of carbon GDEs.[6,8] A major disadvantage of this approach is that the thickness (bulk property) of the Br2 electrode increases with the number of carbon GDE layers used resulting in longer ionic, electronic, and molecular diffusion pathways This could lead to mass transport-limited performance at higher current densities.[12] Several previous studies have investigated high surface area carbon materials such as carbon nanotubes, graphene-based nanoplatelets, and activated carbon powders for battery and electrochemical capacitor applications.[13,14] these high surface area carbon materials are normally blended with a polymeric binder of some kind in order to fabricate solid carbon electrodes for fuel cell applications. Durable high surface area carbon materials with high porosity and tortuosity that can withstand high liquid flow rates are required
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