|dc.description.abstract||Polymer electrolyte membrane fuel cells (PEMFC) have the potential to deliver high power density with a lower weight and volume compared to other fuel cells. However, some of the barriers to the successful commercialization of PEMFCs include problems associated with durability, stability and cost. Fuel cell defects that arise and propagate in the membrane electrode assembly (MEA) components during manufacturing and subsequent operation are the biggest factors limiting their durability and stability, leading to shortened lifetimes, reduced performance or cell failure. Defects in the production line must be minimized if PEMFCs are to become reliable electrochemical energy devices on a commercial scale.
A conventional PEMFC electrode consists of layers (CL) of nanoscale Pt catalyst particles mixed with an ionomer on a high surface area carbon support deposited on the polymer electrolyte membrane (PEM) and sandwiched between gas diffusion media (GDM). The defects in these components originate from the raw materials used in the catalyst layers, process conditions during catalyst mixing, coating techniques, drying process, thickness variations in the casting substrate and the temperature and humidity of the processing environment. These defects can lead to reduced performance and can increase fuel cell degradation, specifically in the MEA components. Understanding the MEA component defects that affect fuel cell performance and lifetime is integral to the successful development of an on-line quality control strategy.
Previous research studies have been conducted on defects in catalyst-coated membranes (CCMs) and gas diffusion layers (GDLs) with various dimensions that have been introduced artificially at specific locations, which does not satisfactorily mimic the situation with real manufacturing defects. Very few studies on real defects have been reported to date with limited work on localized effects on CL defects such as loss of catalyst, the morphology of defect growth or the effect of defect location within the CCM on the resulting cell performance. This has limited our fundamental and comprehensive understanding of the nature of defects in the beginning-of-life (BOL) state and the manner in which they may or may not propagate during PEMFC operation. The focus of this research is to analyze real catalyst layer defects and membrane pinholes on commercial CCMs that are developed during mass production.
Specifically, the objectives of this study are to: (i) develop a non-destructive method to identify and quantify defects in CCM electrodes, (ii) implement a defect analysis framework to age CCMs using open-circuit voltage(OCV)- accelerated stress tests (AST), (iii) characterize the electrochemical performance of CCM/MEAs with varying extent of manufacturing defects (catalyst layer thickness, degree of catalyst non-uniformity) and compare this to a baseline, defect-free CCM/MEA using ASTs as well as in-situ and ex-situ methods and (iv) investigate defects on GDL-microporous layer (MPL) using infrared (IR) imaging and surface conductivity measurements.
The first set of quality control experiments were performed on CCMs by using optical microscopy to characterize catalyst layer defects. Defects such as micro/macro cracks, catalyst clusters, missing catalyst layer defects (MCLDs), void/empty areas, CL delamination and pinholes in the CCM were characterized in terms of areal dimension (size, shape, and orientation) prior to electrochemical analysis. The OCV-AST protocol was developed to age defected CCMs in a custom-designed test cell and track defect propagation and behavior during aging. The geometric features of the defects were quantified and their growth measured at regular time intervals from beginning-of-life (BOL) to end-of-life (EOL) until the OCV had dropped by 20% from its initial value (as per the DOE-designed protocol). Overall, two types of degradation were observed: surface degradation caused by catalyst erosion and crack degradation caused by membrane mechanical deformation. Furthermore, the catalyst layer defects formed during the decal transfer process exhibited a higher growth rate at middle-of-life (MOL-1) before stabilizing by EOL. The results of the crack propagation analysis during AST showed that the defected area covered under cracks increased from 2.4% of the total CL area at BOL to 10.5% by EOL with a voltage degradation rate of 2.55mV/hr. This type of analysis should provide manufacturers with baseline information that will allow them to select and reject CCMs, increasing the lifetime of fuel cell stacks.
Once the CCM defects were analyzed comprehensively, research was carried out on the MEA stack. MEAs containing defected CCMs (incomplete catalyst layer defects-MCLD), pinhole across sealant and artificial pinholes at inlet/middle/outlet were investigated using a cyclic open-circuit voltage (COCV)-AST. Different RH cycling periods from 80% RH to 20% RH with time delays from 5 mins to 30 mins were applied to the cathode to study the propagation of defects and their effect on overall cell performance. In-situ analysis included the measurement of polarization curves, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) to measure electrode degradation. Non-destructive ex-situ analysis using IR thermography was conducted every 100 cycles to monitor the evolution of defects in the MEA. The growth of pinholes was studied on the basis on hydrogen crossover curves. Sealing defects were found to have a major impact on performance loss compared to catalyst layer defects. It was also observed that MCLDs degraded within a short period of time and developed pinholes although the extent of this degradation depended on defect thickness. The MCLD defects were unstable and observed to continually grow due to gradual loss of catalyst particles inside the defected areas that accelerated pinhole formation in CCMs. This effect was clearly reflected in the continuous decay of OCV during the fuel cell operation. Therefore, CCMs leaving the production line with missing and /or thin portions of CL are not recommended for MEA fabrication as they ultimately affect the long-term stability of PEMFC.
The last set of quality control experiments was conducted on GDL-MPL defects in samples that were being aged by RH cycling in a custom-design test cell. Thermal image analysis using IR thermography was carried out by passing DC current through the GDL sheet mounted on a porous vacuum stage to identify hot and cold spots reflecting defective areas. The morphological features and surface conductivity of MPL cracks were characterized using optical microscopy and four-point probe conductivity measurements. Interestingly, the nature of defects/cracks propagation in the GDL-MPL was found to affect cell performance in the mass transfer region at high currents. Crack propagation in GDL-MPL increased mass transport losses due to water flooding on the cathode, which was clearly observed in the polarization curves.
Finally, the overall effects of catalyst layer defects, membrane pinholes and GDL defects on cell performance were compared. MEA sealant defects (pinholes) had such a negative effect on cell performance that EOL was reached after only ~ 50 hours of COCV operation at 80% - 20% RH cycling. Thus, the detection of such a defect in a CCM should be sufficient cause to reject it for use in a commercial stack. We also observed that CCMs with defects that led to 70% reduced thickness of the CL failed faster than those with the same type of defects that had resulted in 30% reduced thickness of the CL, presumably due to less available catalyst for electrochemical reactions. Clearly, CL defects should be given high priority in quality control inspection strategies devised by CCM electrode manufacturers and PEMFC operators.||en