Assembly and Electrochemical Characterization of Nanometer-Scale Electrode/Solid Electrolyte Interfaces

A technique is herein described for the assembly and characterization of nanometer-scale metal electrode/solid electrolyte interfaces of variable dimensions. The specific system examined in this work involves a sharp Pt tip attached to the piezo-driven head of a scanning tunneling microscope (STM) allowing the tip to be inserted into (or retrieved from) a Nafion membrane placed normal to the direction of tip travel. The actual Pt/Nafion area of contact was determined by coulometric analysis of the characteristic voltammetric features of Pt, using the tip as the working electrode and a much larger Pt gauze attached to the other side of the Nafion as a counter-reference electrode, yielding for some of the interfaces examined values equivalent to as low as 35 000 Pt surface atoms. This rather versatile arrangement allows experiments to be performed in both inert (Ar) and reactive atmospheres, such as oxygen or hydrogen on either or both sides of the membrane, under controlled humidity conditions, and thus sheds light into such phenomena as changes in the overall faradaic currents induced by plastic deformations of the Nafion as well as fundamental aspects of mass transport at reactant gasjPtjNafion three-boundary interfaces of relevance to polymer electrolyte fuel cells (PEFCs)

X-ray Line Profile Analysis of Nanoparticles in Proton Exchange Membrane Fuel Cell Electrodes

We present a method to extract X-ray diffraction patterns from a multiphase system and analyze the particle size distribution of each phase. The method is demonstrated for crystalline nanoparticles in the electrodes of proton exchange membrane fuel cells (PEMFCs), where it is particularly useful to determine particle size distributions without destroying the device. The structure of the electrodes has a considerable influence on the power and durability of a fuel cell and can be further optimized, for example with respect to the durability of the cell. Since the membrane electrode assembly (MEA) contains multiple and partially X-ray transparent layers, the individual catalyst signals from the anode (platinum-ruthenium alloy) and the cathode (platinum) can be extracted from the diffraction patterns recorded of either side of the MEA using the technique presented in this article. By analysis of the platinum (220) reflection by fitting a pseudo-Voigt function, the individual particle size distributions are determined for the anode and the cathode. The catalyst surface area loss due to particle growth is studied in long-term experiments during the operation of a single model cell for 2100 h and, for comparison, during the storage in different gas atmospheres (Ar, H2, and O2) for 6500 h. With respect to the single cell operation, approximately one-third of the surface is lost in the storage experiment with a slight influence from the gas atmosphere and the catalyst type. The comparison with transmission electron micrographs shows that the size distributions have a similar shape and width but differ in absolute sizes

Imaging and Reactivity Measurements by Surface Probe Microscopy Techniques with relevance to Fuel Cells

The structure of gas diffusion electrodes in fuel cells is complex: The porous electrode often consists of platinum nanoparticles, carbon support, the ionomer (e.g. Nafion) and hydrophobic additive (e.g. Teflon). The reaction is assumed to take place at the three-phase boundary between gas, catalyst surface and ionomer. However, the details of the structure/activity relationship of the electrode remain unknown. Especially, it is very interesting to investigate the structure and distribution of the catalyst particles as well as the extent of ionomer coverage of the particles. The local electrode structure is probably very important for the ORR activity. In order to provide insight into these relationships, a novel method to analyse the local reactivity of a catalyst/electrode interface is being developed with a high lateral resolution. The method is based on a scanning tunneling microscope working in an electrochemical cell. Using STM the structure of the electrode surface can be imaged on the nanoscale. The novel research strategy is to combine local structural and reactivity measurements by using the STM tip as an oxygen generator and subsequently as a sensor electrode for the ORR. First results of the surface structure of carbon supported commercial catalysts will be presented as well as first results on local reactivity at various catalysts concerning the ORR. In order to investigate the fundamental interaction between platinum and the ionomer a method is herein described that enables the sharp, conically-shaped tip of a short Pt wire attached to a conventional scanning tunneling microscope (STM) head to be gradually inserted into a Nafion solid polymer electrolyte membrane. A comparatively much larger Pt electrode placed on the underside of the membrane allows for the area of the Pt tip in contact with the Nafion to be determined by coulometric analysis of the cyclic voltammetric features. Preliminary results have shown that this novel tactic makes it possible to examine clean Pt surfaces involving as few as ca. 94,000 active metal sites to contact Nafion, opening new prospects for probing the Pt|Ionomer|Gas three-phase interface under conditions which replicate those found in fuel cells

Delayed diagnosis of tuberculosis in persons living with HIV in Eastern Europe : associated factors and effect on mortality-a multicentre prospective cohort study

Background: Early diagnosis of tuberculosis (TB) is important to reduce transmission, morbidity and mortality in people living with HIV (PLWH). Methods: PLWH with a diagnosis of TB were enrolled from HIV and TB clinics in Eastern Europe and followed until 24 months. Delayed diagnosis was defined as duration of TB symptoms (cough, weight-loss or fever) for ≥ 1 month before TB diagnosis. Risk factors for delayed TB diagnosis were assessed using multivariable logistic regression. The effect of delayed diagnosis on mortality was assessed using Kaplan-Meier estimates and Cox models. Findings: 480/740 patients (64.9%; 95% CI 61.3-68.3%) experienced a delayed diagnosis. Age ≥ 50 years (vs. < 50 years, aOR = 2.51; 1.18-5.32; p = 0.016), injecting drug use (IDU) (vs. non-IDU aOR = 1.66; 1.21-2.29; p = 0.002), being ART naïve (aOR = 1.77; 1.24-2.54; p = 0.002), disseminated TB (vs. pulmonary TB, aOR = 1.56, 1.10-2.19, p = 0.012), and presenting with weight loss (vs. no weight loss, aOR = 1.63; 1.18-2.24; p = 0.003) were associated with delayed diagnosis. PLWH with a delayed diagnosis were at 36% increased risk of death (hazard ratio = 1.36; 1.04-1.77; p = 0.023, adjusted hazard ratio 1.27; 0.95-1.70; p = 0.103). Conclusion: Nearly two thirds of PLWH with TB in Eastern Europe had a delayed TB diagnosis, in particular those of older age, people who inject drugs, ART naïve, with disseminated disease, and presenting with weight loss. Patients with delayed TB diagnosis were subsequently at higher risk of death in unadjusted analysis. There is a need for optimisation of the current TB diagnostic cascade and HIV care in PLWH in Eastern Europe