Effects of support material and electrolyte on a triphenylamine substituted cobalt porphyrin catalytic oxygen reduction reaction

The interfacial charge transfer conditions are crucial for understanding the catalytic mechanism and thus developing high performing electrocatalysts. To justify the effects of support material and electrolyte on metalloporphyrin catalytic oxygen reduction reaction (ORR), we synthesize a symmetric triphenylamine (TPA) substituted cobalt porphyrin, nominated as s-TPA-CoP, and select carbon black and multi-walled carbon nanotubes as the supports to fabricate composite catalysts. The ORR performances are measured in both acidic and alkaline solutions. While the support material affects negligibly on reduction potentials, the stronger 7C-7C stacking in s-TPA-CoP/CNT, as revealed by X-ray photoelectron spectroscopy measurements, leads to greater limiting current densities and larger electron transfer numbers as compared to s-TPA-CoP/XC in either electrolyte. Both composites perform better ORR activity but poorer 4-electron selectivity in base than in acid, whose origins are discussed from the aspects of interfacial charge states and inner- / outer-sphere electron transfer mechanisms. Strikingly, s-TPA-CoP/CNT exhibits an electron transfer number of 3.8 in acid, demonstrating the privilege of TPA as the substitution on monometallic cobalt porphyrins in chasing 4-electron transfer oxygen reduction.

Automated summary

The interfacial charge transfer conditions are crucial for understanding the catalytic mechanism and developing high performing electrocatalysts. To investigate the effects of support material and electrolyte on metalloporphyrin catalytic oxygen reduction reaction (ORR), a symmetric triphenylamine (TPA) substituted cobalt porphyrin, s-TPA-CoP, was synthesized, and carbon black and multi-walled carbon nanotubes were selected as the supports to fabricate composite catalysts. The ORR performances were measured in both acidic and alkaline solutions. The results showed that the support material had negligible effects on reduction potentials, but the composite catalyst with stronger 7C-7C stacking in s-TPA-CoP/CNT exhibited greater limiting current densities and larger electron transfer numbers compared to s-TPA-CoP/XC in either electrolyte. Both composites showed better ORR activity but poorer 4-electron selectivity in alkaline solution than in acid, and the origins of these differences were discussed in terms of interfacial charge states and inner-/outer-sphere electron transfer mechanisms. Notably, s-TPA-CoP/CNT demonstrated an electron transfer number of 3.8 in acid, highlighting the advantage of TPA as a substitution on monometallic cobalt porphyrins for achieving 4-electron transfer oxygen reduction.