Single-Molecule Study on the Catalytic Role of Co-O-2 Binding in ORR by In Situ ECSTM

We report here the in situ electrochemical scanning tunneling microscopy (ECSTM) study of O-2 adsorption and reduction on cobalt porphyrin (CoOEP) and phthalocyanine (CoPc). Electrochemical measurements reveal higher ORR activity of CoOEP than CoPc. Self-assembled CoOEP and CoPc monolayers are fabricated on Au(111) substrates and molecularly resolved by ECSTM. The direct molecular imaging demonstrates that O-2 adsorbs on Co-II sites and barely on Co-III sites. The CoOEP-O-2 and CoPc-O-2 complexes exhibit high contrast in ECSTM images due to O-2 binding. The surface coverage of CoOEP-O-2 is ca. 1.4 times higher than that of CoPc-O-2. The dynamics of O-2 adsorption is measured quantitatively by fitting data from potential step experiments to obtain rate constants for O-2 adsorption (kad) and dissociation (k(di)) on Co centers. kad and kdi are higher on CoOEP than on CoPc, indicating higher dynamics of O-2 binding and lower barriers to Co-O dissociation on CoOEP.