Electron Transfer Kinetics at Co Porpyrin-Modified Electrodes with Controlled Interfaces
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/PdML) was prepared by the underpotential deposition (UPD) method. The Au and Au/PdML 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/PdML. 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/PdML 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/PdML electrodes. (c) Tafel plots for the ORR on TPDI-CoTPP monolayer formed on Au and Au/PdML electrodes.