Keywords

Fuel cell, Durability, Conductivity

Abstract

Membrane durability is one of the limiting factors for proton exchange membrane fuel cell (PEMFC) commercialisation by limiting the lifetime of the membrane via electrochemical / mechanical / thermal degradation. Lower internal humidity in the membrane at high temperature ( > 100 °C) and low relative humidity (25-50 %RH) operating conditions leads to increased resistance, lowering of performance and higher degradation rate. One of the promising candidates is composite proton exchange membranes (CPEMs) which have heteropoly acid (HPA) e.g. Phosphotungstic acid (PTA) doped throughout the Nafion® matrix. HPA is primarily responsible for carrying intrinsic water which reduces the external water dependence. The role of relative humidity during membrane casting was studied using surface analysis tools such as Xray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermo-gravimetric analysis (TGA), and Scanning electron microscopy (SEM) / Energy dispersive spectrometer (EDS). Membrane casting at lower relative humidity (30% approx.) results in finer size, and better PTA incorporation in the composite membrane. The effect of increase in PTA concentration in the Nafion matrix was studied with regards to conductivity, performance and durability. In-plane conductivity measurements were performed at 80 oC and 120 oC. During theses measurements, relative humidity was varied from 20% to 100% RH. Membrane conductivity invariably increases on increasing the relative humidity or operating temperature of the cell. Membrane conductivity increases with increasing PTA content from 3% to 25% PTA but never reaches the conductivity of membrane with 0% PTA. Possible explanation might be the role of cesium in PTA stabilisation process. Cesium forms a complex compound with PTA inside host matrix, rendering the PTA incapable of holding water. In plane conductivity measurements only measure surface conductivity, hence another reason might be the existence of a PTA skin on the membrane surface which is not truly representative of the whole membrane. XRD revealed that the structure of the composite membrane changes significantly on addition of PTA. Membrane with 3% PTA has structure similar to Nafion® and does not exhibit the characteristic 25o and 35o 2Ө peaks while membrane with 15% PTA and 25% PTA have strong characteristic PTA peaks. Also the membrane structure with 25% PTA matches well with that of PTA.6H2O. By applying the Scherer formula, PTA particle size was calculated from Full width half maximum (FWHM) studies at 17o 2Ө peak of the membranes. Particles coalesce on increasing the PTA concentration in the membrane leading to larger particles but still all particles were in nanometer range. Also the FWHM of membranes decreased at 17o 2Ө peak on increasing the PTA concentration, leading to higher crystallinity in the membrane. Structure analysis by FTIR indicated increase in PTA signature intensity dips, as the PTA concentration in membrane increases from 0-25%. Also by FTIR studies, it was found that some PTA is lost during the processing step as shown by comparison of as cast and protonated spectra. Possible reasoning might be that some amount of PTA does not gets cesium stabilized which gets leached away during processing. TGA studies were performed which showed no signs of early thermal degradation (temperature > 300 °C); hence the assumption that all membranes are thermally robust for intended fuel cell applications. The membranes with different amounts of PTA were then catalyst coated and tested for 100-hour at open circuit voltage (OCV), 30% RH and 90 oC. By increasing the PTA in the host Nafion® matrix, the percent change in fuel crossover decreases, percent change in ECA increases, cathode fluoride emission rate decreases, and percent change in OCV decreases after the 100 hour test. Possible reasons for decreasing percentage of fuel crossover might be the increased internal humidity of the membrane due to increasing PTA incorporation. It is reported that during higher relative humidity operation, there is decrease in fuel crossover rate. Increasing ECA percentage loss might be due to the fact that HPA in the membrane can get adsorbed on the catalyst sites, rendering the sites inactive for redox reaction. Decrease in cathode fluorine emission rate (FER) might be due to the fact that there is more water available internally in the membrane as compared to Nafion®. It is reported that at higher relative humidity, FER decreases. ECA and crossover both contribute to the OCV losses. Higher component of OCV is crossover loss, which results in mixed potentials. Hence decreasing percentage of crossover might be the reason behind the decreasing OCV loss. Initial performance of fuel cell increases with increasing PTA concentration, but after the 100 hour test, higher PTA membrane exhibited highest performance loss. Increasing initial fuel cell performance can be due to the lowering of resistance due to PTA addition. Increasing ECA losses might be responsible for the increasing performance losses on adding more PTA to host membrane.

Notes

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Graduation Date

2008

Advisor

Fenton, James

Degree

Master of Science (M.S.)

College

College of Engineering and Computer Science

Department

Mechanical, Materials, and Aerospace Engineering

Degree Program

Materials Science and Engineering

Format

application/pdf

Identifier

CFE0002486

URL

http://purl.fcla.edu/fcla/etd/CFE0002486

Language

English

Release Date

December 2008

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

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