Electrode surface area directly affects the hydrogen reaction rate in fuel cells
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Peer-reviewed literature demonstrates that modifying catalyst morphology to increase electrochemical surface area directly enhances hydrogen oxidation and evolution reaction activity in fuel cells.
Abstract Ni-CeO2 composites with CeO2 of different morphologies (nanorods and nanocubes) are prepared and used as the hydrogen electrode of reversible solid oxide cells (RSOCs). Ni-CeO2 nanorods have larger surface area and more surface oxygen than Ni-CeO2 nanocubes. When the RSOCs are used as solid oxide fuel cells (SOFC), the maximum power densities (Pmax) of the cells with Ni-CeO2 nanorods and Ni-CeO2 nanocubes as the anodes are 530.5 and 390.2 mW cm−2, respectively, at 700 °C with H2 as the fuel. The superficial diffusion of adsorbed H atom on Ni-CeO2 composites anode is the rate-determining step (RDS) for H2 electrochemical oxidation reaction and the diffusion rate of the hydrogen atoms adsorbed on Ni-CeO2 nanorods are larger than that of hydrogen atoms adsorbed on Ni-CeO2 nanocubes. Furthermore, when the RSOCs are used as solid oxide electrolysis cells (SOECs), the electrolysis performances of the cell with Ni-CeO2 nanorods as the hydrogen electrode are better than those of the cell with Ni-CeO2 nanocubes as the hydrogen electrode at 700 °C under various H2O concentrations.
A RhxSy/C catalyst with high mass-specific electrochemical surface area (ECSA/mass), high hydrogen oxidation reaction (HOR)/hydrogen evolution reaction (HER) activity, and high Nafion® ionomer-affinity was synthesized and evaluated. A new sulfur source, Na2S instead of (NH4)2S2O3, was applied to prepare the rhodium sulfide precursor Rh2S3 that resulted in a RhxSy catalyst with higher HOR/HER catalytic activity after thermal treatment. The higher activity was attributed to the higher quantity formation of the more active phase Rh3S4, in addition to the other active Rh17S15 phase, in the RhxSy catalyst. Using this new sulfur source, carbon substrate functionalization, and the mass-transfer-controlled nanoparticle growth process, the average particle size of this catalyst was reduced from 13.5 nm to 3.2 nm, and its ECSA/mass was increased from 9.3 m2/g-Rh to 43.0 m2/g-Rh. Finally, by applying the Baeyer–Villiger and ester hydrolysis process to convert the Nafion® ionomer-unfriendly ketone group on the carbon support surface to the Nafion ionomer-friendly carboxylic group, which increases the Nafion® affinity of this catalyst, its use in the hydrogen electrode of an H2-Br2 fuel cell resulted in a performance that is 2.5× higher than that of the fuel cell with a commercial RhxSy catalyst.
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