Deciphering the active site of the hydrogen evolution reaction on metal phthalocyanine-carbon nanotube hybrid catalysts

Electrochemical reduction of CO₂ to methanol is a sustainable method for synthesizing a liquid fuel and feedstock chemical from a cheap and abundant starting material. This reaction can be catalyzed by the hybrid molecular catalyst of cobalt phthalocyanine (CoPc) on carbon nanotubes (CNTs). However, the negative potential needed to produce methanol also gives rise to the hydrogen evolution reaction (HER), compromising Faradaic efficiency and limiting the methanol productivity. Further improvement is constrained by the lack of understanding whether the CoPc molecules or the CNT support is the major active sites for HER. In this work, we distinguish the activity of CoPc for HER by systematically varying the mass loading of CoPc on CNTs. This series of materials, along with materials of nickel phthalocyanine (NiPc) and iron phthalocyanine (FePc) molecules loaded on CNTs, were measured for their HER activity at potentials relevant to CO₂ reduction. At −0.94 V vs. reversible hydrogen electrode (RHE), CoPc molecules are one order of magnitude more active for HER than the CNT support or NiPc/FePc molecules. This demonstrates that the undesirable HER activity of CoPc/CNT is mainly from the CoPc molecules.