Interestingly enough, introducing sterically hindered substituents on the ligand was found to suppress catalyst aggregation and increase catalytic activity in the case of immobilized cobalt complex, despite the decrease in the coverage of catalyst. Prolonged catalytic lifetime was observed in the immobilized cobalt octaalkoxyphthalocyanine (CoPc) catayzed CO2 reduction. CoPc was anchored on a graphene sheet via π-π stacking, showing that the alkoxy substitution on the ligand improved the catalytic activity to over 30 hours (Figure ). The opposite impacts of sterically hindered substituents on the metal complex observed in the two hybrid systems: iron complex modified silver electrode catalyzed oxygen reduction reactions and cobalt complex modified electrode catalyzed CO2 reduction pose the challenges remain on predicting the role of the substituents for optimal design. FIGURE 27. Representations of iron porphyrin complexes with electron-withdrawing, electron-donating, positively, and negatively charged substituents on porphyrin, as well as substituents that possibly introduce H-bonding formation. Homogenous iron complex with thiophenyl created a relatively dense molecular layer on electrode surface, whereas bulky mesityl substituents resulted in less stable interaction with the electrode surface and created less dense surface coverage. To disentangle the through-bond inductive and throughspace effects, Savéant and coworkers took advantage of computational methods for separate evaluations. Realizing the importance of electrostatic stabilization in heterogeneous catalysis, especially for energy related transformations such as CO2 reduction, is beneficial to the logic design of the heterogenized catalyst. Cobalt porphyrin was chosen again and immobilized on carbon paper. Selective production of CO from CO2 reduction was achieved with a Faradaic efficiency of 95%. Investigation of peripheral functionalities on the porphyrin ligand indicated an increase in electron density of the cobalt center by introducing electron-donating substituents on the ligand that, in turn, increased CO2 adsorption, thus facilitating the reaction (Figure ). Quantification of electronegativity using density functional theory (DFT) via Mulliken population analysis on hydrogen charge densities was conducted. Along with experimental results, further proved the association between the inductive effect of the substituents and the electronic structure at the cobalt center. Cationic functionalities, on the other hand, were found to form a favored electrostatic stabilization with the key intermediate in the rate-limiting step and also increase the activity. The opposite catalytic behavior was observed when anionic functionalities were investigated, supporting the electrostatic hypothesis. Once again, considering electrostatic effects in addition to inductive electronic effects for ligand substitutions is essential for a rational design of a molecularly modified electrochemical system.