Systematically controlling AEMFC electrode structure and water leads to record 1.9 W cm−2 performance with ETFE membranes/ionomers and PtRu/Pt catalysts.

High-resolution neutron radiography was used to image an operating proton exchange membrane fuel cell in situ. The crosssectional 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.

Neutron imaging experiments were carried out to measure the water content of an operating proton exchange membrane fuel cell ͑PEMFC͒ 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.Understanding liquid water content and its distribution within an operating proton exchange membrane fuel cell ͑PEMFC͒ is critical to designing high-performance systems and formulating rational models for simulating PEMFC behavior. The generation, transport, and removal of liquid water are key phenomena that occur in a PEMFC. Effective water transport through and removal from the membrane electrode assembly ͑MEA͒ is crucial to achieving high current density and maintaining PEMFC performance. In the design and optimization of PEMFCs, it is important to be able to quantify the water content in an operating cell in order to gain insight into the dominant phenomena or processes that influence liquid water transport and removal. This work is concerned with the measurement of liquid water in an operating PEMFC under various temperatures, relative humidities, and current densities. Neutron imaging, or radiography, is a useful tool for gaining qualitative and quantitative insight into liquid water content and distribution in near real-time ͑temporal resolution ϳ1 s͒.Both Tuber et al. 1 and Yang et al. 2 used optical methods for imaging water in PEMFCs under a range of operating conditing. In order to use optical techniques, a transparent fuel cell must be fabricated. Optical imaging is capable of high spatial and temporal resolutions for the elucidation of dynamic processes, but optical techniques suffer from fogging of the transparent window under humidified conditions and it is more difficult ͑though possible͒ to obtain quantitative information. Furthermore, optical investigations are limited to studying liquid water in the gas flow channels because that is the only visible water in the fuel cell; liquid water inside the gas diffusion layers ͑GDLs͒ cannot be imaged using optical techniques. Tuber et al. 1 were able to correlate the appearance of water in the cell with a drop in current density, although they did not quantify the liquid water in the cell. Yang et al. 2 focused on the appearance and dynamics of liquid water droplet formation and breakup in the gas flow channels. Their work ...

Neutron imaging has been demonstrated to be a powerful tool to measure the in situ water content of commercial proton exchange membrane fuel cells (PEMFCs) in two and three dimensions. The National Institute of Standards and Technology neutron imaging facility was designed to produce a high intensity, highly collimated neutron imaging beam to measure the water content of operating fuel cells. The details of the neutron optics and neutron detection are discussed in terms of the random uncertainty in measuring the liquid water thickness that is typical of operating PEMFCs.

There is a need to understand the water dynamics of alkaline membrane fuel cells under various operating conditions to create electrodes that enable high performance and stable, long-term operation. Here we show, via operando neutron imaging and operando micro X-ray computed tomography, visualizations of the spatial and temporal distribution of liquid water in operating cells. We provide direct evidence for liquid water accumulation at the anode, which causes severe ionomer swelling and performance loss, as well as cell dryout from undesirably low water content in the cathode. We observe that the operating conditions leading to the highest power density during polarization are not generally the conditions that allow for long-term stable operation. This observation leads to new catalyst layer designs and gas diffusion layers. This study reports alkaline membrane fuel cells that can be operated continuously for over 1000 h at 600 mA cm −2 with voltage decay rate of only 32-μV h −1the best-reported durability to date.

The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. An image-processing procedure resolved a previously reported discrepancy between the measured and predicted membrane water content. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeder's paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. At open circuit and during fuel cell operation, the measured water content indicates that the membrane is operating between the vapor- and liquid-equilibrated states.

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