Size and Electronic Effects on the Performance of (Corrolato)cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis

Abstract

Considering the worldwide efforts for designing catalysts that are not based on platinum group metals while still reserving the many advantages thereof, this study focused on the many variables that dictate the performance of cathodes used for fuel cells, regarding the efficient and selective reduction of oxygen to water. This was done by investigating two kinds of porous carbon electrodes, modified by molecular cobalt(III) complexes chelated by corroles that differ very much in size and electronwithdrawing capability. Examination of the electronic effect uncovered shifts in the Co II /Co III redox potentials and also large differences in the affinity of the cobalt center to external ligands. Spontaneous absorption of the catalysts was found to depend on the size of the corrole's substituents (C 6 F 5 ≫ CF 3 ≫ H) and the metal's axial ligands (PPh 3 versus pyridine), as well as on the porosity of the carbon electrodes (BP2000 > Vulcan). The better-performing cobalt-based catalysts were almost as active and selective as 20% platinum on Vulcan in terms of the onset potential and the only 2−10% undesirable formation of hydrogen peroxide. Durability was also addressed by using the best-performing modified cathode in a proper anion-exchange membrane fuel cell setup, revealing very little voltage change during 12 h of operation.

Surprising Route to a Monoazaporphyrin and Full Characterization of Its Complexes with Five Different 3d Metals

“…The transition metal complexes of the new MAP were briefly examined for exploring their potential utility as electrocatalysts for the heterogeneous oxygen reduction reaction (ORR), frequently considered as most crucial for the future of fuel cells. − A common procedure for such testing is to adsorb the potential catalysts onto porous carbon materials that are conductive, robust, have a high surface area, and guarantee efficient mass and charge transport. − Vulcan XC-72R (Vulcan) and Black Pearls 2000 (BP2000) were employed here, noting that the latter has a much bigger surface area (1537 versus 235 m 2 /g) and ten times more mesopores . The process started by dissolving the molecular catalyst in 2-propanol and mixing it with the insoluble carbon.…”

“…Both iron complexes were excluded from this study because 1-FeCl was not stable in isopropanol and 1-Fe 2 O was not soluble in it. In contrast with meso -aryl substituted porphyrins and corroles, whose adsorption onto BP2000 is much more significant than onto Vulcan, 1-Co , 1-Ni , and 1-Cu were fully adsorbed on both carbons (Figure ) when mixed in a 0.8/10 mg ratio. This phenomenon may be confidently attributed to the small size of meso -CF 3 relative to meso -aryls: the latter are perpendicular to the macrocycle and hence interfere with its π-stacking interactions, which are responsible for the spontaneous physisorption onto the carbon surface.…”

Lignin-Derived Precious Metal-Free Electrocatalysts for Anion-Exchange Membrane Fuel Cell Application

“…To evaluate the performance of FeNC­(Mg) and FeCoNC­(Mg) catalysts in AEMFC conditions, gas diffusion electrodes (GDEs) of 2.25 cm 2 were prepared in a manner similar to previous publications. ,,,,,− For the anodes, ∼22 mg of powdered anion-exchange ionomer (Fumion, Fumatech) was ground in a mortar for 3 min, after which 61 mg of PtRu/C catalyst (40% Pt and 20% Ru on carbon black HiSPEC 10000, Alfa Aesar) was added to the mortar along with 30 mg of carbon black (Vulcan XC-72) to increase the porosity and hydrophobicity of the catalyst layer and thereby avoid flooding . After that, 1 mL of deionized water and 9 mL of 2-propanol were added to the mortar and further ground for 10 min to obtain a low-viscosity catalyst ink.…”