ABSTRACT
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.
ABSTRACT
Electrochemical techniques, coupled with in situ scanning tunneling microscopy, have been used to examine the mechanism of CO oxidation and the role of surface structure in promoting CO oxidation on well-ordered and disordered Pt(111) in aqueous NaOH solutions. Oxidation of CO occurs in two distinct potential regions: the prepeak (0.25-0.70 V) and the main peak (0.70 V and higher). The mechanism of reaction is Langmuir-Hinshelwood in both regions, but the OH adsorption site is different. In the prepeak, CO oxidation occurs through reaction with OH that is strongly adsorbed at defect sites. Adsorption of OH on defects at low potentials has been verified using charge displacement measurements. Not all CO can be oxidized in the prepeak, since the Pt-CO bond strength increases as the CO coverage decreases. Below theta(CO) = 0.2 monolayer, CO is too strongly bound to react with defect-bound OH. Oxidation of CO at low coverage occurs in the main peak through reaction with OH adsorbed on (111) terraces, where the Pt-OH bond is weaker than on defects. The enhanced oxidation of CO in alkaline media is attributed to the higher affinity of the Pt(111) surface for adsorption of OH at low potentials in alkaline media as compared with acidic media.
