An Electrochemical Dealloying of Ordered Intermetallic Platinum-Lead (PtPb) Nanoparticles Supported on TiO2-Deposited Cup-Stacked Carbon Nanotube for Highly Efficient Catalysis Toward Oxygen Reduction Reaction in Acidic Media

Abstract

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 vehicles (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 to our daily life 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 moderately occur produce the highest activity for ORR on the catalyst surfaces. Because Pt bond O2 a little strongly, the filling level of electron on 5d state is tuned by Pt-based alloys, core-shell structures and Pt on metal oxide. As the experimental factor of the degree of filling level of electron on 5d state, d-band center was used in the evaluation of the relationship between the ORR activity and the electronic state of Pt atoms. 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 our previous paper, we have reported that dealloyed PtPb ordered intermetallic nanoparticles (NPs) deposited on TiO2-coated cup-staked carbon nanotube (CSCNT) enhance the ORR. Although as-prepared PtPb NPs on TiO2/CSCNT exhibits lower activity for ORR, potential cycling for ORR between 0.05 and 1.1 V (vs. NHE) improved the ORR curves with the shift of ORR curves to the positive direction. The improvement of ORR curves was caused by dissolution of Pb elements from the surface of PtPb NPs. The ORR improvement was stopped in the middle of potential cycling and then, in reverse, potential cycling depressed the ORR activity even with the dissolution of Pb atoms from the NP surfaces. These results indicate that there is the best composition of Pt-Pb alloy for ORR enhancement. In this study, in each electrodes treated by the potential cycling, ORR activity was measured 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 TEM. The d-band center of the Pt atoms on the treated PtPb NPs was evaluated with X-ray photoelectron spectroscopy (XPS). The volcano plot for ORR was tried to make with the ORR activities and d-band center obtained with the electrochemically treated PtPb NPs.