Xiao-Zi Yuan

Molecular simulation of gas adsorption, diffusion, and permeation in hydrated Nafion membranes.

Molecular simulations were performed to characterize hydrated Nafion membranes in terms of gas adsorption, diffusion, and permeation. The experimental results validate the molecular model of Nafion with respect to material density, morphology, free volume, and water diffusivity. Nafions adsorption property is examined in terms of the solubility and adsorption isotherms for gases, including H(2), O(2), and N(2). The adsorption capacity of hydrated Nafion is shown to be strong for O(2) and N(2) but not for H(2). Due to the dilution effect, N(2) is able to suppress the loading of O(2) and protect the fuel cell from fuel crossover. The dynamic behaviors of H(2) and O(2) are represented by self-diffusion coefficients, with the results showing that H(2) diffusion in Nafion membranes is nearly 1 order of magnitude faster than O(2) diffusion. The effects of water content and the concentration of adsorbed gases were verified, and a close correlation of Nafion free volume to gas transport properties was revealed. On the basis of the solution-diffusion mechanism, the permeabilities of H(2) and O(2) in hydrated Nafion membranes are calculated and compared with corresponding experiments, and the permeability of H(2) is found to be approximately twice that of O(2).

Facile and Nonradiation Pretreated Membrane as a High Conductive Separator for Li-Ion Batteries.

Polyolefin membranes are widely used as separators in commercialized Li-ion batteries. They have less polarized surfaces compared with polarized molecules of electrolyte, leading to a poor wetting state for separators. Radiation pretreatments are often adopted to solve such a problem. Unfortunately, they can only activate several nanometers deep from the surface, which limits the performance improvement. Here we report a facile and scalable method to polarize polyolefin membranes via a chemical oxidation route. On the surfaces of pretreated membrane, layers of poly(ethylene oxide) and poly(acrylic acid) can easily be coated, thus resulting in a high Li-ion conductivity of the membrane. Assembled with this decorated separator in button cells, both high-voltage (Li1.2Mn0.54Co0.13Ni0.13O2) and moderate-voltage (LiFePO4) cathode materials show better electrochemical performances than those assembled with pristine polyolefin separators.

Synthesis of high-purity LiMn2O4 with enhanced electrical properties from electrolytic manganese dioxide treated by sulfuric acid-assisted hydrothermal method

Using sulfuric acid-assisted hydrothermal treatment, β-MnO2 particles were obtained from the electrolytic manganese dioxide (EMD). Via high-temperature solid-phase reactions, spinel lithium manganese oxides (LiMn2O4) were produced using the obtained β-MnO2 particles as precursor mixed with LiOH·H2O for the lithium-ion battery cathodes. Atomic absorption (AAS) shows that after the hydrothermal treatment, the contents of impurity ions, such as Na+, K+, Ca2+, and Mg2+, caused by the limitation of preparation technology of EMD are greatly reduced. X-ray diffraction and scanning electron microscopy show that β-MnO2 is highly alloyed consisting of nano sticks. Spinel lithium manganese (LiMn2O4) synthesized by the β-MnO2 precursor has high crystallinity with a well 111 face grow and presents a regular and micron-sized octagonal crystal. When used as cathode materials for lithium-ion batteries, LiMn2O4 synthesized by the β-MnO2 precursor has greater discharge capacity, better cycle performance, and better high-rate capability when compared with LiMn2O4 synthesized by the EMD precursor. Cyclic voltammetry and electrochemical impedance spectroscopy indicate that LiMn2O4 synthesized by the β-MnO2 precursor has better electrochemical reaction reversibility, greater peak current, higher lithium-ion diffusion coefficient, and lower electrochemical impedance.

Nanoscale Investigation of Nafion Membranes after Artificial Degradation

In this contribution we report on the nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes investigated by contact atomic force microscopy (AFM),conductive AFM, and pulsed force-mode (PFM)-AFM. The different techniques can provide complementary information on structure and conductivity. Conductive AFM gives information about the internal structure of ionic clusters during current flow. High resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature. The influence of H2O2 treatment as a method for artificial degradation is investigated. The analysis of adhesion forces demonstrates a significant change of thesurface properties with different membrane treatment. Topography and adhesion measurements clearly show materials changes with high resolution and correlate with changes in conductivity distributions.

Analysis of aged Polymer Electrolyte Fuel Cell (PEFC) components by non traditional methods

The ageing of microporous layers (MPL) of fuel cell gas diffusion layers has been quantitavely analyzed using a special atomic force microscopy technique, namely the so-called HarmoniX technique. From the change of mean adhesion force under dry and wet conditions an increased loss of polytetrafluoroethylene (PTFE) at the cathode was found. With ionic current measurement in tapping and contact mode by AFM, activated Nafion was investigated before fuel cell operation with high resolution and individual ionic channels were imaged in one cluster. These measurements were compared to the current distribution of membranes after 1600 h of fuel cell operation under OCV. Distinct current levels were found which demonstrate the existence of an interpenetrating ionic network with different branches not directly connected at the surface. SEM/EDX investigations of the specially designed fuel cells indicate an important role of platinum in degradation of membranes.

Atomic Force Microscopy Investigation of Polymer Fuel Cell Gas Diffusion Layers before and after Operation

The ageing of microporous layers (MPL) of fuel cell gas diffusion layers has been analyzed using a special atomic force microscopy technique. The mean local adhesion force on the surface as well as the energy dissipation on the same area has been used as a measure for changes of surface properties corresponding to ageing of the MPL. The samples have been studied before and after fuel cell operation. In all cases, the changes due to operation are stronger on the cathode compared to the anode. The results from measurements under comparably dry and wet conditions are consistent with an increased loss of polytetrafluoroethylene (PTFE) at the cathode which leads to a hydrophobicity loss.

FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Life-Limiting Considerations

This article reviews the published in the literature on performance degradation of and mitigation strategies for proton-exchange membrane fuel cells (PEMFCs). Durability is one of the characteristics most necessary for PEMFCs to be accepted as a viable product. In this chapter, a literature-based analysis has been carried out in an attempt to achieve a unified definition of PEMFC lifetime for cells operated under either steady-state or various accelerated conditions. Additionally, the dependence of PEMFC durability on different operating conditions is analyzed. Durability studies of the individual components of a PEMFC are introduced, and various degradation mechanisms are examined. Following this analysis, the emphasis of this review shifts to applicable strategies for alleviating the degradation rate of each component. The lifetime of a PEMFC as a function of operating conditions, component materials, and degradation mechanisms is then established. Finally, this article summarizes accelerated stress testing (AST) methods and protocols for various components, in an attempt to prevent the prolonged test periods and high costs associated with real lifetime tests. A statistical model correlating accelerated testing and real lifetime is also examined in detail in order to facilitate the establishment of AST protocols for PEMFC durability research.