Date of Award

8-9-2014

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

John W. Van Zee

Abstract

Off-the-shelf Balance of Plant (BOP) materials for proton exchange membrane fuel cell (PEMFC) systems may be more economical than custom-prepared materials. However, they pose a higher risk of decreased performance of a PEMFC by contaminating the electrodes. This dissertation contributes to the understanding of the mechanisms of electrode contamination by studying the effect of leachate extracts and model compounds from structural plastics and assembly aids that may be used as BOP materials. The effect of contamination was investigated by measuring the decrease in electrochemical surface area (ECA) of the Pt/C catalyst and the change in the oxygen reduction reaction (ORR) current using a thin film rotating disk electrode (TF-RDE) method. Experimental protocols were developed using several batches of electrodes with mass and specific activities within a narrow range of 250±10 mA/mgpt and 350±15 μA/cm2Pt respectively to ensure reproducibility and quality of contamination data. Preliminary data for screening of BOP materials showed the effect of the liquid phase contamination can be correlated with the chemistry of the constituents present in the leachates.

The organic constituents found in the leachates (detected by GCMS) were tested as model compounds with varying concentrations. The results from these individual organic compounds indicated different extent of contamination (on ECA and ORR) effects attributed to the poisoning of the catalyst or ionomer mirrored in the loss of Pt sites in the catalyst or the ORR currents. ECA losses due to the organic compounds were more than 50% in all cases at 20 mM. The experiments also offered insights on the poisoning mechanism (adsorption, absorption, parallel electrochemical reactions) by the contaminant molecules. Additional analyses were performed to measure and compare the peroxide formation during ORR experiments due to the contamination using a rotating ring disk electrode (RRDE) method. Recovery experiments were performed to assess the ability to restore the lost ECA and currents through potential cycling to higher voltages. Conclusions were drawn based on the severity of contamination (aromatics more than aliphatics) and recoverability of the electrodes.

Since the leachates were mixtures of organic and inorganic compounds, additional experiments were performed to demonstrate the effects of (1) mixture of organic compounds and (2) mixture of organic and anionic constituents identified in the leachates using TF-RDE method on Pt catalyst and ionomer. The effects of the mixtures on electrode were compared to the individual constituents’ effects. Data showed large and almost irreversible losses for a mixture of aromatic model compounds. In cases of mixtures of aliphatic and aromatic organic compound the contamination pattern resembled that of aromatic compounds. The anionic species demonstrated additional ECA loss compared to any other species, which could not be attributed to ionomer poisoning by absorption. It can be concluded that the aromatic compounds shows higher contamination features than aliphatics for both the ionomer and catalyst parts of the electrode.

Rights

© 2014, Mayukhee Das

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