High Temperature Polymer Electrolyte Membrane Fuel Cells - Performance and Durability Studies

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Abstract

High temperature proton exchange membrane fuel cells based on phosphoric acid doped polybenzimidazole membranes are a technology featured by simplified construction and operation with possible integration with e.g. methanol reformers. Further improvement of the performance and durability is a key to promote the technology. The present thesis addresses these challenges by focusing on the anode performance in the presence of fuel impurities e.g. CO and catalyst degradation during dynamic potential cycling.

The small poisoning effect by up to 2% CO in pure hydrogen is verified with electrodes of high Pt loading, ca. 12 mV at 0.2 A/cm2. This is however increased to 24 mV for electrodes of 0.3 mgPt/cm2 and to 63 mV for electrodes of 0.15 mgPt/cm2 . On the other hand, the hydrogen dilution by an inert gas (e.g. N2 or Ar) in the absence of CO is found to deteriorate the anode performance. Switching from 100% to 60%H2 leads to a lowered performance by ca. 25 – 30 mV for electrodes of Pt loadings from 0.15 to 1.3 mgPt/cm2.

When CO is present in the diluted hydrogen steam, the combined effects of the hydrogen dilution and CO poisoning becomes substantially high especially for low Pt loading electrodes. For anodes of 0.3 mgPtcm-2, 2.0% CO in a 40% H2 fuel stream results in an anode voltage loss by 130 mV. This combined effect is well correlated to the ratio [CO]/[H2], which in turn determines the Pt surface coverage by CO and hence the fraction of the active sites available for the hydrogen oxidation. A strong dependence of the anode performance loss on the [CO]/[H2] ratio is found to be in an exponential form, which is highly related to the Pt loading of the electrode.

The hydrogen oxidation and evolution reaction kinetics is of fundamental as well as practical significance in the presence of CO, and has been studied in acidic electrolytes at temperatures of up to 80°C. The study is extended to temperatures of up to 180 °C in the absence and presence of carbon monoxide using acid doped polybenzimidazole electrolytes. The study is made by means of a hydrogen pumping cell with gas diffusion electrodes of ultralow Pt loadings of 1.5-6.5 μgPt/cm2. The ultralow Pt loading allows for minimizing the mass transport and eliminating the ohmic resistance contributions to the kinetic behaviors. Fitting with the Butler–Volmer equation obtains an HOR/HER exchange current density of 200 to 500 mA/cm2 based on the electrochemically active surface area of platinum. These numbers are comparable to those obtained at 40-80°C in perfluorosulfonic acid electrolytes. The activation energy in a temperature range from 120 to 180°C is found to 49.6 kJ/mol. It is shown that the HOR, but not HER, rate is limited by the Tafel step.

In the presence of CO severely asymmetric Tafel plots for HER/HOR are observed. The strong adsorption of CO on the Pt surface hampers the HOR kinetics by further limiting the slow dissociative hydrogen adsorption. At an overpotential of 0.1 V, the HOR kinetic current density was decreased from 600 mA/cm2 for pure hydrogen to 2.9 mA/cm2 at the CO concentration of 0.25%. In term of exchange current densities of HOR the presence of CO at a concentration of 0.25-2.0% results in a decrease of i° by about 2 orders of magnitude.

The performance degradation of HT-PEMFC technology is studied by potential cycling at 160°C. The membrane-electrode-assemblies are made of acid-doped polybenzimidazole membranes and PtCox catalysts on the cathode. While the upper potential limit is fixed at 0.95 V the degradation is stressed by varying the low-end potential from 0.8 to 0.65 V and hold duration from 5 to 60 sec. During the cycling tests, little degradation of the membrane, in terms of ohmic resistance and open circuit voltage is observed. A significant performance loss is attributable to the mass transport due to the degradation of electrode hydrophobicity. A major catalyst degradation mechanism is the platinum particle growth via the electrochemical Ostwald ripening. No evidence of the cathode dealloying was detected during the potential cycling. The dissolution of platinum or the surface oxides of platinum during the upper potential hold at 0.95 V is limited by the available acid in the PBI membrane. The low-end potential and the hold duration seem critical where the reduction and re-deposition of platinum occur, leading to formation of larger particles and hence loss of the active surface area.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages144
Publication statusPublished - 2023

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  • HT-PEM Durability and Mitigation

    Celenk, S. (PhD Student), Seland, F. (Examiner), Steenberg, T. (Examiner), Li, Q. (Main Supervisor), Cleemann, L. N. (Supervisor) & Jensen, J. O. (Supervisor)

    01/12/201930/10/2023

    Project: PhD

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