Tunable Metal-Free Imidazole-Benzimidazole Electrocatalysts for Oxygen Reduction in Aqueous Solutions

Acid Strength Effects on Dimerization during Metal-Free Catalytic Dioxygen Reduction

Development of earth-abundant catalysts for the reduction of dioxygen (ORR) is essential for the development of alternative industrial processes and energy sources. Here, we report a transition metal-free dicationic organocatalyst (Ph2Phen2+) for the ORR. The ORR performance of this compound was studied in acetonitrile solution under both electrochemical conditions and spectrochemical conditions, using halogenated acetic acid derivatives spanning a pKa range of 12.65 to 20.3. Interestingly, it was found that under electrochemical conditions, a kinetically relevant peroxo dimer species forms with all acids. However, under spectrochemical conditions, strong acids diminish the kinetic contribution of this dimer to the observed rate due to lower catalyst concentrations, whereas weaker acids were still rate-limited by the dimer equilibrium. Together, these results provide insight into the mechanisms of ORR by organic-based, metal-free catalysts, suggesting that balancing redox activity and unsaturated character of these molecules with respect to the pKa of intermediates can enable reaction tuning analogous to transition metal-based systems.

Geometric and Electronic Engineering in Co/VN Nanoparticles to Boost Bifunctional Oxygen Electrocatalysis for Aqueous/Flexible Zn-Air Batteries

Modulating metal-metal and metal-support interactions is one of the potent tools for augmenting catalytic performance. Herein, highly active Co/VN nanoparticles are well dispersed on three-dimensional porous carbon nanofoam (Co/VN@NC) with the assistance of dicyandiamide. Studies certify that the consequential disordered carbon substrate reinforces the confinement of electrons, while the coupling of diverse components optimizes charge redistribution among species. Besides, theoretical analyses confirm that the regulated electron configuration can significantly tune the binding strength between the active sites and intermediates, thus optimizing reaction energy barriers. Therefore, Co/VN@NC exhibits a competitive potential difference (ΔE, 0.65 V) between the half-wave potential of ORR and OER potential at 10 mA cm-2, outperforming Pt/C+RuO2 (0.67 V). Further, catalyst-based aqueous/flexible ZABs present superior performances with peak power densities of 156 and 85 mW cm-2, superior to Pt/C-based counterparts (128 and 73 mW cm-2). This research provides a pivotal foundation for the evolution of bifunctional catalysts in the energy sector.

Metal-Free Homogeneous O2 Reduction by an Iminium-Based Electrocatalyst

The oxygen reduction reaction (ORR) is important for alternative energy and industrial oxidation processes. Herein, an iminium-based organoelectrocatalyst (im+) for the ORR with trifluoroacetic acid as a proton source in acetonitrile solution under both electrochemical and spectrochemical conditions using decamethylferrocene as a chemical reductant is reported. Under spectrochemical conditions, H2O2 is the primary reaction product, while under electrochemical conditions H2O is produced. This difference in selectivity is attributed to the interception of the free superoxide intermediate under electrochemical conditions by the reduced catalyst, accessing an alternate inner-sphere pathway.