Using Host-Guest Chemistry to Examine the Effects of Porosity and Catalyst-Support Interactions on CO₂ Reduction

Bis-porphyrin nanocages (M₂BiCage, M = FeCl, Co, Zn) and their host-guest complexes with C₆₀ and C₇₀ were used to examine how molecular porosity and interactions with carbon nanomaterials affect the CO₂ reduction activity of metalloporphyrin electrocatalysts. The cages were found to adsorb on carbon black to provide electrocatalytic inks with excellent accessibilities of the metal sites (≈50%) even at high metal loadings (2500 nmol cm^-2), enabling good activity for reducing CO₂ to CO. A complex of C₇₀ bound inside (FeCl)₂BiCage achieves high current densities for CO formation at low overpotentials (|j_CO| >7 mA cm^-2, η = 320 mV; >13.5 mA cm^-2, η = 520 mV) with ≥95% Faradaic efficiency (FE_CO), and Co₂BiCage achieves high turnover frequencies (≈1300 h^-1, η = 520 mV) with 90% FE_CO. In general, blocking the pore with C₆₀ or C₇₀ improves the catalytic performance of (FeCl)₂BiCage and has only small effects on Co₂BiCage, indicating that the good catalytic properties of the cages cannot be attributed to their internal pores. Neither enhanced electron transfer rates nor metal-fullerene interactions appear to underlie the ability of C₆₀/C₇₀ to improve the performance of (FeCl)₂BiCage, in contrast to effects often proposed for other carbon nanosupports.