Fuel cells are devices that produce electric power by means of the chemical reaction of oxygen and fuels more efficiently than current technologies, and they are expected to become a cleaner source of energy. [1][2][3][4] Despite considerable recent advances, especially in polymer electrolyte membrane fuel cells (PEMFCs), existing technology still has drawbacks, including kinetic limitations on the oxygen reduction reaction [5,6] and the instability of Pt catalysts and polymer membranes nearby, in particular during startup-shutdown cycles. [4,[7][8][9] The chemical reactions are uneven throughout the reaction field and not well understood. A central issue in fuel cell research is the measurement of the parameters that determine performance during cell operation, a difficult task owing to the structure of these devices. The distribution of liquid water in operating fuel cells has been measured and imaged through neutron radiography, [10,11] by NMR spectroscopy, [12,13] and by X-ray microtomography, [14] and temperatures have been recorded with a thermograph.[15] Hydrogen cross-over from the anode to cathode has been studied by mass spectrometry and magnetic resonance imaging in an operating PEMFC.[16] Oxygen consumption and H 2 O 2 formation, as well as the local catalytic activity of a catalyst have been investigated and visualized in solution with a scanning electrochemical microscope.[17] However, to improve the performance and durability of PEMFCs, [18][19][20] it is crucial to understand distributions in real time not only of liquid water but also of reactants and products (oxygen, fuel, CO 2 , water vapor, etc.) throughout the cell. Here we present a laboratoryuse, nondestructive system for visualizing oxygen distribution in the interior of the operating fuel cells which relies on dye films [21,22] painted on the transparent gas flow field. Oxygen partial pressures were successfully visualized with spatial and time resolutions of 300 mm and 500 ms, respectively. We found that the oxygen distribution in PEMFC is not in accordance with that expected based on the current, which suggests a significant contribution from water. This imaging system is applicable to other important parameters such as water, carbon monoxide, and temperature, and should help in the design of new fuel cell separators and a reaction field called a membrane-electrode assembly (MEA).An oxygen-sensitive porphyrin, tetrakis(pentafluorophenyl)porphyrinatoplatinum (PtTFPP), was used in the visualization system. This dye complex was dispersed in an oxygenpermeable polymer matrix, poly(1-trimethylsilyl-1-propyne) (pTMSP), [22,23] for making a thin, water-insoluble dye film. To understand the properties of the dye film, we placed the film in an environment with controlled oxygen partial pressure (mixtures of oxygen, nitrogen, and water vapor at 0-26 kPa of a total of approximately 101.3 kPa), temperature, and humidity, and irradiated it with a laser light at a wavelength of 407 nm. The emission from the film was filtered (> 600 nm), and the inte...
