Controlling Methanol Selectivity on Immobilized Cobalt Phthalocyanine on Carbon Nanotubes for Photoelectrochemical and Electrochemical CO₂ Reduction

Electrochemical CO₂ reduction (CO₂R) using heterogenized molecular catalysts usually yields 2-electron reduction products (CO, formate); recently, it has been reported that certain preparations of immobilized cobalt phthalocyanine (CoPc) produce methanol (MeOH), a 6-electron reduction product. Here, we demonstrate the significance of the role mass transport plays in CoPc selectivity to different CO₂R products. Specifically, a moderate linear flow velocity of 8.5 cm/min has the highest MeOH selectivity at 35%, with higher flow rates increasing CO selectivity and lower flow rates increasing HER, suggesting that CO is a free intermediate. We use a simple, physically mixed, polymer free preparation of CoPc on multiwalled carbon nanotubes (MWCNT) to achieve moderate MeOH selectivity in near-neutral aqueous conditions at -1.2 V vs RHE. An onset potential of -0.8 V vs. RHE for MeOH was observed with increasing Faradaic efficiency until it reaches a relative maximum at -1.2V vs. RHE and lowering as the reductive potential increases. The catalyst was made compatible with Au planar substrates using a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) adhesion layer. Using the PEDOT:PSS adhesion layer, we integrated the CoPc catalyst onto a multijunction GaInP/GaAs three terminal tandem (3TT) solar cell, that could operate two separate electrodes at different potentials. We achieved 4% FE toward MeOH on the 3TT solar cell at +0.2 V vs RHE, demonstrating that the 3TT can be used as a platform for future study of tandem photoelectrochemical systems.