Electron Transfer Kinetics at Co Porpyrin-Modified Electrodes with Controlled Interfaces

Electrocatalysis has been extensively studied in the fields of both fundamental research and applications such as fuel cells.  Chemical modification of an electrode surface is quite beneficial to non-catalytic metals to introduce catalytic activity for a particular electrochemical reaction.  For example, metalloporphyrin-modified electrodes are known to act as electrocatalysts for oxygen reduction reaction (ORR). Chemical design of molecular catalysts is clearly one of key issues to realize highly efficient electrocatalysts.  However, the electron transfer rate at such a chemically modified electrode is significantly affected by electronic coupling between the metal and molecular catalysts.  In this work, a gold electrode surface was self-assembly modified with Cobalt porphyrin (CoTPP) having ORR activity.  To control the connection between Au and CoTPPs, bridge monolayers were formed on the electrode using various thiol- or isocyanide-molecules, and CoTPPs were then immobilized on top of the bridge layers.  For isocyanide-monolayers, the electrode-molecule junctions were also controlled by forming Pd- or Pt-atomic layers on the Au surface.  The electron transfer rate for ORR was thoroughly investigated for such various CoTPP-modified electrodes using the rotating disk electrode (RDE) method. 

The junction between the electrode and the molecular monolayers was controlled as shown in Fig. (a).  The Pd monoatomic layer on an Au surface (Au/Pd_ML) was prepared by the underpotential deposition (UPD) method.  The Au and Au/Pd_ML electrodes were self-assembly covered with TPDI (4,4’-terphenyl diisocyanide) molecules.  As shown in Fig. (b), the adsorption geometry of TPDIs, measured by PM-IRRAS, was on-top configuration on Au and bridge on Au/Pd_ML.  The 2120 cm^-1 peak found on both electrodes was attributed to be the unbound NC streching mode.  Figure (c) shows Tafel plots for ORR on the TPDI-CoTPP monolayer formed on each electrode.  Electron transfer rate on the Au/Pd_ML was much faster than that on the Au.  The elementary step of the electron transfer between O₂molecule and CoTPP is not affected much by the presence of the Pd monolayer, as the Tafel slop is nearly identical for both plots.  Therefore, this result indicates that the electron transfer rate at the electrode-molecule interface was significantly enhanced by controlling the adsorption geometry, leading to the enhancement of the ORR activity of the electrode.

Figure (a) Schematic illustration of TPDI-CoTPP monolayers on metal electrodes. (b) PM-IRRAS spectra of TPDI-SAMs on Au and Au/Pd_ML electrodes. (c) Tafel plots for the ORR on TPDI-CoTPP monolayer formed on Au and Au/Pd_ML electrodes.