David L. Jacobson

Neutron Imaging Technique for In Situ Measurement of Water Transport Gradients within Nafion in Polymer Electrolyte Fuel Cells

Water transport is an important consideration in the optimization of polymer electrolyte fuel cell (PEFC) performance, affecting both internal resistance and cathode polarization losses. Novel experiments are described using neutron radiography to measure water gradient profiles within Nafion{reg_sign} in an operating PEFC. Preliminary neutron intensity gradients show qualitative agreement with the expected response of membrane water content to changes in feed gas humidification and fuel cell current. Previous experimental measurements of similar water gradients have generally relied on integral measurements such as ac impedance spectroscopy which cannot probe details of the gradient within the membrane. This is one of the first differential measurements of water gradients within Nafion.

In Situ High-Resolution Neutron Radiography of Cross-Sectional Liquid Water Profiles in Proton Exchange Membrane Fuel Cells

High-resolution neutron radiography was used to image an operating proton exchange membrane fuel cell in situ. The cross-sectional liquid water profile of the cell was quantified as a function of cell temperature, current density, and anode and cathode gas feed flow rates. Detailed information was obtained on the cross-sectional water content in the membrane electrode assembly and the gas flow channels. At low current densities, liquid water tended to remain on the cathode side of the cell. Significant liquid water in the anode gas flow channel was observed when the heat and water production of the cell were moderate, where both water diffusion from the cathode and thermal gradients play a significant role in determining the water balance of the cell. Within the membrane electrode assembly itself, the cathode side was moderately more hydrated than the anode side of the assembly from 0.1 to 1.25 A cm -2 . The total liquid water content of the membrane electrode assembly was fairly stable between current densities of 0.25 and 1.25 A cm -2 , even though the water in the gas flow channels changed drastically over this current density range. At 60°C, the water content in the center of the gas diffusion layer was depleted compared to the membrane or gas flow channel interfaces. This phenomenon was not observed at 80°C where evaporative water removal is prevalent.

Real-Time Imaging of Liquid Water in an Operating Proton Exchange Membrane Fuel Cell

Neutron imaging experiments were carried out to measure the water content of an operating proton exchange membrane fuel cell (PEMFC) under varying conditions of current density and temperature. It was found that the water content of the PEMFC is strongly coupled to the current density and temperature of the cell. These measurements indicate that changes in water content lag changes in current density by at least 100 s, both when the current density was increased and decreased. Less liquid water was measured in the cells when operating at 80°C than at 40°C. At 60°C cell temperature, a peak in water content was observed around 650 mA/cm 2 and the water content was found to decrease with increasing current density. This is explained in the context of cell heating by performing a simple thermal analysis of an operating PEMFC so as to yield quantitative information on the waste heat and its effects on the liquid water contained in the cell.

Measurement of Liquid Water Accumulation in a PEMFC with Dead-Ended Anode

2active area, Nafion 111-IP membrane, and carbon cloth gas diffusion layer. Even though dry hydrogen is supplied to the anode via pressure regulation, accumulation of liquid water in the anode gas distribution channels was observed in most tested conditions. Moreover, the accumulation of liquid water in the anode channels is followed by a significant voltage drop. Anode purges and cathode surges are also used as a diagnostic tool for differentiating between anode and cathode water flooding. The rate of accumulation of liquid water, and its impact on the rate of cell voltage drop is shown for a range of temperature, current density, cathode inlet RH, and air stoichiometric conditions. Operating the fuel cell under dead-ended anode conditions offers the opportunity to observe water dynamics and measured cell voltage during large and repeatable transients.

Imaging: Phase radiography with neutrons

The interaction of neutrons with matter enables neutron radiography to complement X-ray radiography in analysing materials. Here we describe a simple quantitative method that provides a new contrast mechanism for neutron radiography and allows samples to be imaged at low radiation doses. Large phase shifts can be measured accurately from detailed structures not amenable to conventional techniques.

Understanding Liquid Water Distribution and Removal Phenomena in an Operating PEMFC via Neutron Radiography

A proton exchange membrane fuel cell (PEMFC) was imaged using neutron radiography under pseudo steady-state operating conditions to determine the total liquid water content of the cell and the liquid water content distribution across the active cell area as a function of cell temperature, current density, and cathode air flow rate. A simple cathode-based model was formulated to rationalize the observed dry inlet regions which were most strongly influenced by temperature and current density. Between temperatures of 40 and 80°C and current densities of 0.5 and 1.5 A cm -2 , the outlet gas temperature was measured to be 1-5°C greater than the cell bulk temperature. This small temperature difference was enough to account for drying of 20-40% of the cell area, depending on the bulk cell temperature. For the cell construction used in this work, the temperature and cathode stoichiometric flow had a marginal effect on the polarization curve performance but had a large effect on the liquid water content and distribution within the cell.

Neutron Imaging of Lithium Concentration in LFP Pouch Cell Battery

This paper shows how neutron radiography can be used for in situ quantification of the lithium concentration across battery electrodes, a critical physical system state. The change in lithium concentration between the charged and discharged states of the battery causes a change in number of detected neutrons after passing through the battery. Electrode swelling is also observed during battery charging. The experimental setup and the observations from testing a pouch cell with LFP cathode and graphite anode are reported here. The bulk Li concentration across the electrode and folds of the pouch cell is quantified at various states of charge. To interpret the measurements, the optics of the neutron beam (geometric unsharpness) and detector resolution are considered in order to quantify the lithium concentration from the images due to the thinness of the electrode layers. The experimental methodology provides a basis for comprehensive in situ metrology of bulk lithium concentration.

Precision neutron interferometric measurements and updated evaluations of the n p and n d coherent neutron scattering lengths

We have performed high-precision measurements of the coherent neutron scattering lengths of gas phase molecular hydrogen and deuterium using neutron interferometry. After correcting for molecular binding and multiple scattering from the molecule, we find

Phase radiography with neutrons.

{b}_{\mathrm{np}}=(\ensuremath{-}3.7384\ifmmode\pm\else\textpm\fi{}0.0020)\mathrm{fm}

Quantification of Temperature Driven Flow in a Polymer Electrolyte Fuel Cell Using High-Resolution Neutron Radiography

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