Efficient Catalytic Electrode for CO₂ Reduction Realized by Physisorbing Ni(cyclam) Molecules with Hydrophobicity Based on Hansen’s Theory

An electrochemical electrode physisorbed with Ni­(cyclam) complex molecules containing tetraphenylborate ions (BPh₄^–) as counteranions shows catalytic activity for the reduction reaction of CO₂ to CO in an aqueous electrolyte, superior to that of an electrode physisorbed with conventional [Ni­(cyclam)]­Cl₂ complex molecules. The BPh₄^–-containing Ni­(cyclam) is inferred as having high hydrophobicity based on its Hansen solubility parameter (HSP), with an interaction sphere excluding HSPs of water in a three-dimensional vector space. The high hydrophobicity of BPh₄^–-containing Ni­(cyclam) molecules inhibits their dissolution into aqueous electrolyte and retains their immobilization onto the electrode surface, which we believe to result in the improved catalytic activity of the electrode physisorbed with them. HSP analysis also provides an optimized mixing ratio of solvents dissolving BPh₄^–-containing Ni­(cyclam) molecules.