Relationship between the Degree of Dealloying of Ptpb Ordered Intermetallic Nanoparticle Deposited on TiO2/ Cup-Stacked Carbon Nanotube and ORR Activity in Acidic Aqueous Media for Polymer Electrolyte Fuel Cells
Authors: Ando Fuma , Tanabe Toyokazu , Ohsaka Takeo , Matsumoto Futoshi 
Published: February 2020

The development of fuel cells that covert chemical energy to electric energy with high efficiency are one of key factor to solve energy issues. In order to widespread the fuel cells to our daily life, various technologies for the fuel cells have been developed with much effort of researchers and engineers. Now, electric cars (EV) powered by electricity which is produced by fuel cells are put on the market. However, it will take a long time before it becomes widespread because from the viewpoints of performance, durability and cost, many unsolved problems are still piled. One of the unsolved problems is sluggish kinetics of oxygen reduction reaction (ORR) even on the surface of platinum (Pt). Although, theoretically, the onset potential of ORR is 1.23 V vs. NHE, due to large overpotential for ORR, the onset potential can be seen around 1 V even on Pt surface which is the best ORR catalyst in acidic aqueous solutions among catalysts composed of single elements. Pt-based alloys, core-shell structures and Pt on metal oxide have been proposed as promising catalysts in many papers. Many results on the enhancement of ORR have been reported. The important principle at the bottom of the ORR enhancements is modification of the electric state of Pt atoms on the catalyst surfaces where oxygen molecules adsorb to start the ORR. In the discussion on the enhancement of ORR and electronic modification of Pt atoms, d-band center theory is often used. The theory has become a popular language in the community of electrocatalysis, especially ORR. The principle of this theory is that in the heterogeneous reaction, adsorption of reactants starts the catalytic reactions, and the binding energy of adsorbates to the catalysis surface is largely dependent on the catalytic activity. Moreover, the binding energy can be related to the electronic structure of the surface atoms of catalysis. For example, in the ORR, the interaction of the molecular oxygen (O2) 2p state with the platinum atom 5d state produces a filled, low-lying bonding and empty, high-lying antibonding molecular orbitals. The more the electrons are filled in the antibonding state, the weaker the Pt-O2 interaction become, resulting in lower catalytic activity because O2 cannot adsorb on the Pt surfaces. On the other hand, the less the electrons are filled in the antibonding state, the stronger the Pt-O2 interaction become, resulting in lower catalytic activity because oxygen intermediate species (or products) cannot desorb from the Pt surface. Therefore, the filling level of electrons in 5d state where adsorption of O2 and desorption of intermediate species (or products) moderately occur produce the highest activity for ORR on the catalyst surfaces. Because Pt bond O2 a little strongly, as mentioned above, the filling level of electron on 5d state is tuned by Pt-based alloys, core-shell structures and Pt on metal oxide. Many papers have reported excellent volcano plots between d-band center and ORR activity, indicating the importance of moderate binding energy between O2 and catalyst surfaces. However, there are several papers suggesting that the d-band center theory cannot predict perfectly the electrocatalytic activity. In order to confirm the value of the d-band theory, more and more experimental results and careful comparison of the data with the theory are needed. In this study, in each PtPb nanopartile coated electrodes treated by the potential cycling, ORR activity was measured with rotating ring-disk electrode method and change of the surface structure and the composition of Pb atoms on the PtPb NP surface treated by the potential cycling was analyzed with transmission electron microscope (TEM). In addition, the d-band center of the Pt atoms on the treated PtPb NPs was evaluated with X-ray photoelectron spectroscopy (XPS). The volcano plot mentioned above for ORR was tried to make with the ORR activities and d-band center obtained with the electrochemically treated PtPb NPs. It was made clear that the NP surfaces prepared by gradual electrochemical dissolution of Pb atoms exhibited the shift of d-band center and that the shift of d-band center controlled the ORR activity on the dealloyed Pt-Pb NP surfaces.