Low CO2 mass transfer promotes methanol and formaldehyde electrosynthesis on cobalt phthalocyanine

Abstract

Cobalt phthalocyanine supported on carbon nanotubes (CoPcCNT) usually catalyzes the electroreduction of CO₂ (CO₂RR) to CO, although several reports have also indicated methanol formation. Herein, by analyzing the effects of CoPc loading and CO₂ partial pressure on the CO₂RR, we show that a lower rate of CO₂ mass transfer to each CoPc favors methanol formation, while a higher rate of CO₂ mass transfer favors CO evolution. The ratio of the production rates of methanol and CO is related to the average CO₂ mass transfer rate by a power function with a negative exponent. Hence, methanol can only be formed when the supply of CO₂ feed is low. This mass transfer effect is supported by supplementary experiments and computational modelling, which show that CO binding to CoPc is weaker than that of CO₂, in agreement with previous studies. Consequently, *CO may only be reduced to methanol when the supply of CO₂ is low and the dwelling time of CO is long. We further provide a quantitative guideline for the design of methanol-selective catalysts. At −0.86 V vs. RHE, enhanced CO mass transfer boosts the CORR to methanol with a faradaic efficiency up to 70% at a total current density of −19 mA cm⁻². The production of formaldehyde, a reaction intermediate from CO reduction to methanol, is also boosted with a faradaic efficiency of up to 7%. We pinpoint CoPc containing Co(I) as the active CO₂RR site.