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 achieved high current densities for CO formation at low overpotentials (|jCO| > 7 mA cm-2, η = 320 mV; > 13.5 mA cm-2, η = 520 mV) with ≥ 95 % Faradaic efficiency (FECO), and Co2BiCage achieved high turnover frequencies (~1300 h-1, η = 520 mV) with 90 % FECO. In general, blocking the pore with C60 or C70 improved the catalytic performance of (FeCl)2BiCage and had 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.