Electrochemical Impedance Study of Electrode‐Membrane Assemblies in PEM Fuel Cells: I. Electro‐oxidation of  H 2 and  H 2 /  CO  Mixtures on Pt‐Based Gas‐Diffusion Electrodes

Abstract

In situ EIS data are presented on the anodic process in proton exchange membrane (PEM) fuel cells and the influence of CO poisoning of the Pt gas-diffusion electrodes (GDE) is examined. A characterization of the effects of interfacial kinetics in determining polarization losses in the presence of CO is performed by comparing impedance patterns obtained for cells of the type H 2 /H 2 with H 2 /(H 2 ϩ CO). The impedance spectrum of the poisoned electrode is strongly dependent on potential and on the CO concentration in the gas feed. In the range between 0 and 0.3 V the impedance increases rapidly with the potential, while at potentials higher than 0.3 V a "pseudoinductive" behavior is observed, in which a positive loop at higher frequencies is accompanied by a low frequency (LF) loop in the fourth quadrant. The latter was assigned to a new rate-determining process, the oxidation of CO ads by Pt-H 2 O ads . As a critical potential V crit is attained, the diameters of the two loops become almost equal and the LF limit of the impedance (R o ) approaches the value for unpoisoned electrode, showing that the activity of the electrode activity has been restored. The value of V crit is 0.43 and 0.58 V for electrodes poisoned with 100 ppm and 2% CO, respectively. At very high potentials, where the oxidizing species are Pt-OH ads , the impedance pattern is reversed to the second and third quadrants. Stripping voltammetry and polarization curves recorded in situ, are used to support the conclusions obtained from impedance measurements.

In-situ investigation on the CO tolerance of carbon supported Pd–Pt electrocatalysts with low Pt content by electrochemical impedance spectroscopy

“…adsorbed CO on the Pt surface, which can release the active sites for hydrogen dissociative chemisorption, occur above a critical potential [22,23]. The absence of similar phenomenon for PdePt/C catalysts may imply that the CO coverage on the PdePt surface is lower than that on the Pt and PtRu.…”

“…New characteristic frequencies in the Bode plots of the single cells with Pt/C and PdePt/C catalysts are observed in the higher frequency region, indicating that the new process related to the poisoning CO is a faster process than ORR. Ciureanu et al attributed the faster process to the charge transfer of the adsorbed H atoms (H ads ) on the holes preexisting in the CO ads adlayer [22]. The diameter increase of large loops is attributed to the reduction of active sites for hydrogen adsorption due to the preferential adsorption of CO and the slow diffusion of the adsorbed species (H ads and CO ads ).…”

“…Furthermore, the R ct , C dl , L rex and O rex are introduced to the EC to account for the effect of CO on the anodic processes. These effects contain the reduction in active site for hydrogen adsorption/desorption due to the preferential adsorption of CO, the oxidation removal of CO ads and the charge transfer or the CO ads diffusion on the holes preexisting in the CO ads adlayer [22]. The lines in the Fig.…”

“…More in-situ information is required to illustrate the mechanism for the improved CO tolerance of the PdePt catalysts. Electrochemical impedance spectroscopy (EIS) is a powerful in-situ technique to explore the CO effect on the Pt and PtRu catalysts [22,23] due to its frequency domain characteristics. However, there are scarce EIS studies about the CO effect on the PdePt anodic catalyst.…”

Membranas híbridas basadas en estireno-metacrilato-sílice y ácido fosfowolfrámico obtenidas por sol-gel para pilas de combustible de intercambio protónico (PEMFC)

“…Por tanto, una de las áreas de mayor interés en estas pilas es el desarrollo de membranas de bajo coste para aplicaciones de alta temperatura (T> 100ºC) usando hidrógeno o metanol (DMFC) como combustible. Operar a alta temperatura tiene grandes ventajas como la mejora de las cinéticas de reacción de los electrodos, menor consumo de metal catalítico (Pt) y reducción del envenenamiento del platino con trazas de CO (3,5).…”