ABSTRACT
Ru@Pt core shell nanoparticles possess optimal catalytic properties that facilitate the anodic oxidation reaction of H2 with decreased Pt loading in hydrogen fuel cells. Moreover, since they preferentially oxidize CO, Pt poisoning is considerably reduced, which significantly improves the stability of the cell. The Ru cores used in this system are usually synthesized by dissolving a RuCl3*H2O precursor in an ethylene glycol-carbon black-NaOH mixture. However, the possibility that remnant Cl and Na from the synthesis process are present in the Ru nanoparticles has not been extensively studied. Therefore, due to the challenges in detecting impurities with traditional characterization methods, here correlative atom probe tomography (APT) with scanning transmission electron microscopy ((S)TEM) techniques were implemented. The capabilities of APT to obtain chemical information with high sensitivity at the nanoscale, in combination with the high spatial resolving power of (S)TEM, provide the necessary resolution to fully characterize the structure and chemical makeup of Ru nanoparticles.
ABSTRACT
Ru@Pt core-shell nanoparticles are currently being explored as carbon monoxide tolerant anode catalysts for proton exchange membrane fuel cells. However, little is known about their degradation under fuel cell conditions. In the present work, two types of Ru@Pt nanoparticles with nominal shell thicknesses of 1 (Ru@1Pt) and 2 (Ru@2Pt) Pt monolayers are studied as synthesized and after accelerated stress tests. These stress tests were designed to imitate the degradation occurring under fuel cell operating conditions. Our advanced (scanning) transmission electron microscopy characterization explains the superior initial electrochemical performance of Ru@1Pt. Moreover, the 3D reconstruction of the Pt shell by electron tomography reveals an incomplete shell for both samples, which results in a less stable Ru metal being exposed to an electrolyte. The degree of coverage of the Ru cores provides insights into the higher stability of Ru@2Pt during the accelerated stress tests. Our results explain how to maximize the initial performance of Ru@Pt-type catalysts, without compromising their stability under fuel cell conditions.
ABSTRACT
The operation related degradation processes of high temperature polymer electrolyte membrane fuel cell operated with hydrogen-rich reformate gas are studied. CO impurities from the reformate gas are strongly adsorbed by the catalyst surface, leading to poisoning and thus, reduction of the overall performance of the cell. Most of the studies are performed in a laboratory set-up by applying accelerated stress tests. In the present work, a high temperature polymer electrolyte membrane fuel cell is operated in a realistic configuration for 12 000 h (500 days). The fuel cell contains as electrocatalyst Pt in the cathode and a Pt-Ru alloy in the anode. The study of the degradation occurring in the functional layers, i.e. in different regions of cathode, anode and membrane layer, is carried out by scanning electron microscopy, (scanning) transmission electron microscopy and energy dispersive X-ray spectroscopy. We observed a thinning of the functional layers and a redistribution of catalyst material. The thinning of the cathode side is larger compared to the anode side due to harsher operation conditions likely causing a degradation of the support material via C corrosion and/or due to a degradation of the catalyst via oxidation of Pt and Ru. A thinning of the membrane caused by oxidation agents is also detected. Moreover, during operation, catalyst material is dissolved at the cathode side and redistributed. Our results will help to design and develop fuel cells with higher performance.
ABSTRACT
Supported metal catalysts with partial encapsulation resulting from strong metal-support interactions show distinctive structural features which strongly affect their functionalities. Yet, challenges in systematic synthesis and in-depth characterization for such systems limit the present understanding of structure-property relationships. Herein, the synthesis and characterization of two Pt/TiO2 models are conducted by a simple change of the synthesis order, while keeping all other parameters constant. They differ in containing either bare or encapsulated Pt nanoparticles. The presence of an extremely thin and inhomogeneous TiO2 layer is clearly demonstrated on 2-3 nm sized Pt nanoparticles by combination of imaging, energy dispersive X-ray spectroscopy and electron energy loss spectroscopy performed in a transmission electron microscope. The two Pt/TiO2 systems exhibit differences in morphology and local structure which can be correlated with their electrochemical activity and stability using cyclic voltammetry experiments. Beyond enhanced particle stability, we report an increase in H+ intercalation on titania and reduced Pt activity due to partial encapsulation by TiO2. Finally, the growth of an encapsulation layer as a result of cyclic voltammetry measurements is discussed. These results shed light on the in-depth structure-property relationship of catalysts with strong metal-support interactions which leads to enhanced functional materials for electrochromic devices and energy applications.
ABSTRACT
The structural changes of copper hexacyanoferrate (CuHCF), a Prussian blue analogue, which occur when used as a cathode in an aqueous Zn-ion battery, are investigated using electron microscopy techniques. The evolution of Znx Cu1-x HCF phases possessing wire and cubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCF is revealed locally using scanning transmission electron microscopy. A substitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn-ion battery cycling. The present study demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathode in high electrolyte concentration.
ABSTRACT
The search for suitable materials for solar water splitting is addressed with combinatorial material science methods. Thin film Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron cosputtering and subsequent annealing in air. The design of the libraries comprises a combination of large compositional gradients (from Fe10V90O x to Fe79V21O x) and thickness gradients (from 140 to 425 nm). These material libraries were investigated by high-throughput characterization techniques in terms of composition, structure, optical, and photoelectrochemical properties to establish correlations between composition, thickness, crystallinity, microstructure, and photocurrent density. Results show the presence of the Fe2V4O13 phase from â¼11 to 42 at. % Fe (toward low-Fe region) and the FeVO4 phase from â¼37 to 79 at. % Fe (toward Fe-rich region). However, as a third phase, Fe2O3 is present throughout the compositional gradients (from low-Fe to Fe-rich region). Material compositions with increasing crystallinity of the FeVO4 phase show enhanced photocurrent densities (â¼160 to 190 µA/cm2) throughout the thickness gradients whereas compositions with the Fe2V4O13 phase show comparatively low photocurrent densities (â¼28 µA/cm2). The band gap energies of Fe-V-O films were inferred from Tauc plots. The highest photocurrent density of â¼190 µA/cm2 was obtained for films with â¼54 to 66 at. % Fe for the FeVO4 phase with â¼2.04 eV for the indirect and â¼2.80 eV for the direct band gap energies.
