Guadalupe Ramos-Sanchez

Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction

In this study, low-mass loadings (ca. 5 wt%) Pt/C catalysts were synthesized using the carbonyl chemical route allowing for the heterogeneous deposition of Pt nanoparticles on different carbon-based substrates. N-doped reduced graphene oxide, reduced graphene oxide, graphene oxide, graphite and Vulcan XC-72 were used for the heterogeneous deposition of Pt nanoparticles. The effect of the chemical nature of the carbon-based substrate on the Oxygen Reduction Reaction (ORR) kinetics at Pt nanoparticles surfaces was investigated. XPS results show that using N-doped reduced graphene oxide materials for the deposition of Pt nanoparticles leads to formation of Pt–N chemical bonds. This interaction between Pt and N allows for an electronic transfer from Pt to the carbon support. It is demonstrated that ca. 25% of the total amount of N atoms were bound to Pt ones. This chemical bond also revealed by the DFT analysis, induces changes in the oxygen adsorption energy at the platinum surface, engendering an enhancement of the catalyst activity towards ORR. In comparison with Vulcan XC-72, the mass activity at 0.9 V vs. RHE is 2.1 fold higher when N-doped reduced graphene oxide is used as substrate. In conjunction with the experimental results, DFT calculations describe the interaction between supported platinum clusters and oxygen where the support was modelled accordingly with the carbon-based materials used as substrate. It is demonstrated that the presence of N-species in the support although leading to a weaker O2 adsorption, induces elongated O–O distances suggesting facilitated dissociation. Additionally, it is revealed that the strong interaction between Pt clusters and N-containing substrates leads to very slight changes of the cluster–substrate distance even when oxygen is adsorbed at the interfacial region, thus leading to a lower resistance for electron charge transfer and enabling electrochemical reactions.

Influence of sp3–sp2 Carbon Nanodomains on Metal/Support Interaction, Catalyst Durability, and Catalytic Activity for the Oxygen Reduction Reaction

In this work, platinum nanoparticles were impregnated by two different techniques, namely the carbonyl chemical route and photodeposition, onto systematically surface-modified multiwalled carbon nanotubes. The different interactions between platinum nanoparticles with sp(2)-sp(3) carbon nanodomains were investigated. The oxidation of an adsorbed monolayer of carbon monoxide, used to probe electronic catalytic modification, suggests a selective nucleation of platinum nanoparticles onto sp(2) carbon nanodomains when photodeposition synthesis is carried out. XPS attests the catalytic center electronic modification obtained by photodeposition. DFT calculations were used to determine the interaction energy of a Pt cluster with sp(2) and sp(3) carbon surfaces as well as with oxidized ones. The interaction energy and electronic structure of the platinum cluster presents dramatic changes as a function of the support surface chemistry, which also modifies its catalytic properties evaluated by the interaction with CO. The interaction energy was calculated to be 8-fold higher on sp(3) and oxidized surfaces in comparison to sp(2) domains. Accelerated Stability Test (AST) was applied only on the electronic-modified materials to evaluate the active phase degradation and their activity toward oxygen reduction reaction (ORR). The stability of photodeposited materials is correlated with the surface chemical nature of supports indicating that platinum nanoparticles supported onto multiwalled carbon nanotubes with the highest sp(2) character show the higher stability and activity toward ORR.

Influence on the Electrocatalytic Water Oxidation of M2+/M3+ Cation Arrangement in NiFe LDH: Experimental and Theoretical DFT Evidences

AbstractThis contribution reports the effect of the iron content and M2+/M3+ ratio cation arrangement-distribution on the oxygen evolution reaction (OER) catalyzed by layered double hydroxides. The electrocatalysts, containing variable contents of Ni and Fe, were successfully prepared through a homogeneous precipitation method. The formation of LDH structure was verified by powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Other properties were determined such as specific surface area, electrical conductivity, and surface basicity. First-principles DFT+U calculations complemented and supported the electrochemical results. According to both the electrochemical and simulation results, the increase of the catalytic activity for the OER on the presence of Fe3+ is closely related with the configuration and distribution of Fe and Ni cations in the brucite layer structure. The effect of iron is indirect, favoring the electron hopping on the Ni sites for certain local configuration. Graphical Abstractᅟ

Activity and Durability of PEFCs Alloy Core-Shell Catalysts: Role of Surface Oxidation

Low temperature fuel cells are one of the most promising systems for the transformation of fuels into electricity in an efficient, silent, and environmentally friendly manner. In this paper we show the advances accomplished in the synthesis and a theoretical-experimental analysis of the changes induced by the Ni@Pt structure and the presence of the almost unavoidable NiO species. The synthesis of core-shell nanoparticles is described and then physical and electrochemical characterizations confirm the presence of core-shell nanoparticles with a high electrochemical activity towards the Oxygen Reduction Reaction. Periodic density functional theory calculations are used to analyze the shift in the oxidation potential for Pt, Ni@Pt and NiO@Pt with different number of layers in the shell. The changes in the electrochemical activity towards oxygen reduction are evaluated by allowing oxygen to adsorb on the surface of the nanoparticle and alloys. It is found that only the first and second layers of Pt are being affected by the presence of the Ni or NiO core.

Multi-scale Simulation Study of Pt-Alloys Degradation for Fuel Cells Applications

Low-temperature fuel cells are one of the most promising systems for the transformation of fuels in an efficient, silent, and environmentally friendly manner. The requirements for the electrocatalyst are essentially three: the highest possible catalytic activity and the longest life cycle at the lowest cost. Sometimes, we can obtain one at expenses of the other. In this chapter, we review the simulation methods used in our group to study the degradation of catalysts for fuel cell applications: Density functional theory (DFT), classical molecular dynamics (CMD), Ab initio molecular dynamics (AIMD), and kinetic Monte Carlo (KMC). In the first part, we employ DFT, AIMD, and CMD to address the importance of the oxygen concentration on the surface of the catalysts and its influence on the “buckling” of Pt atoms and the role of the subsurface atoms. Then we analyze the temporal evolution of shape and composition of Pt/Ni nanoalloys by KMC simulations at various overall compositions and applied voltages. Finally, using DFT we study the effect that the presence of oxygen in the subsurface has on the buckling of Pt skin/PtCo structures by varying the oxygen coverage factor. The different methods and time scales used for the simulations permit us to fathom the factors governing the stability of electrocatalysts for fuel cells applications.