Low CO2 Mass Transfer Promotes Methanol and Formaldehyde Electrosynthesis on Cobalt Phthalocyanine

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 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 works. 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 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 to methanol, is also boosted with a Faradaic efficiency of up to 7 %. We pinpoint CoPc containing Co(I) as the active CO2RR site.