A review of polymer electrolyte membrane fuel cell stack testing

This paper presents an overview of polymer electrolyte membrane fuel cell (PEMFC) stack testing. Stack testing is critical for evaluating and demonstrating the viability and durability required for commercial applications. Single cell performance cannot be employed alone to fully derive the expected performance of PEMFC stacks, due to the non-uniformity in potential, temperature, and reactant and product flow distributions observed in stacks. In this paper, we provide a comprehensive review of the state-of-the art in PEMFC testing. We discuss the main topics of investigation, including single cell vs. stack-level performance, cell voltage uniformity, influence of operating conditions, durability and degradation, dynamic operation, and stack demonstrations. We also present opportunities for future work, including the need to verify the impact of stack size and cell voltage uniformity on performance, determine operating conditions for achieving a balance between electrical efficiency and flooding/dry-out, meet lifetime requirements through endurance testing, and develop a stronger understanding of degradation.     

Stochastic modeling of polymer electrolyte membrane fuel cell gas diffusion layers – Part 1: Physical characterization

Stochastic modeling of GDL structures requires a detailed characterization of the constituent elements of the material. In this work, a variety of imaging methods, including optical microscopy, microscale computed tomography, and scanning electron microscopy were used to characterize seven commercially available gas diffusion layers (GDLs). The result is a catalogue of the following geometrical characteristics: fiber diameter, fiber pitch and co-alignment, areal weight and volume, and microporous layer (MPL) crack size and frequency. This catalogue, when combined with previous GDL characterizations, is expected to provide enough information to create representative, predictive, stochastic models of the GDL.     

Effect of compression on liquid water transport and microstructure of PEMFC gas diffusion layers

This work explores how the degradation of the gas diffusion layer (GDL) under compression contributes to the formation of preferential pathways for water transport. Fluorescence microscopy is used to provide ex situ visualization of liquid water transport through the GDL placed beneath an optically transparent clamping plate. Transient image data obtained with a CCD camera indicates that areas of compression in the GDL coincide with preferential pathways for water transport and break-through. Preferential flow of water through the smaller pores resulting from GDL compression is contrary to the expected behaviour in a hydrophobic medium, and this suggests a loss of hydrophobicity. Scanning electron microscopy (SEM) is used to investigate the effect of compression on the morphology of the GDL. These SEM images show that compressing the GDL causes the breakup of fibers and, indeed, deterioration of the hydrophobic coating.     

Humidity and Temperature Cycling Effects on Cracks and Delaminations in PEMFCs

Temperature and relative humidity (hygrothermal) cycles during PEM fuel cell operation can lead to the introduction and exacerbation of micro‐scale mechanical defects. We developed a two‐dimensional finite element model based on cohesive zone theory to describe the delamination propagation at the cathodic membrane/catalyst layer interface due to temperature and hygrothermal duty cycles. Particularly, the effects of hygrothermal cycle amplitudes, relative humidity (RH) distribution profiles, and gas flow channel position were studied. It was found that doubling the hygrothermal cycle amplitude resulted in a 6‐fold increase in fatigue stresses, and a defect length growth to 0,1 mm before reaching the end of the fuel cell life (40,000 cycles). A counter intuitive result was also observed, whereby a crack located within the membrane was found to grow faster than a delamination located at the catalyst layer/membrane interface. When introducing an anode/cathode channel offset, a 2‐fold increase in the rate of delamination propagation was found compared to the case with the aligned anode and cathode channels.     

Stochastic modeling of polymer electrolyte membrane fuel cell gas diffusion layers – Part 2: A comprehensive substrate model with pore size distribution and heterogeneity effects

A stochastic modeling algorithm was developed that accounts for porosity distribution, fiber diameter, fiber co-alignment, fiber pitch, and binder and/or polytetrafluorethylene fractions. Materials representative of a commercially available GDL were digitally generated based on empirical measurements of these various properties. Materials made with varying fiber diameters and binder/fiber volume ratios were compared with a generated reference material through porosity heterogeneity calculations and mercury intrusion porosimetry simulations. Fiber diameters and binder/fiber ratios were found to be key modeling parameters that exhibited non-negligible impacts on the pore space. These key parameters were found to positively correlate with heterogeneity and mean pore diameter and exhibit a complementary relationship in their impact on the pore space. Because both parameters directly impacted the number of fibers added to the domain, modeling techniques and parameters pertaining to fiber count must be considered carefully.     

Identifying in operando changes in membrane hydration in polymer electrolyte membrane fuel cells using synchrotron X-ray radiography

The cost of the polymer electrolyte membrane (PEM) fuel cell must undergo significant reductions before the widespread adoption of PEM fuel cell powered automotive drivetrains can be achieved. Eliminating the need for active anode humidification is one strategy for reducing the cost and system size of the PEM fuel cell. In this study, we investigated the impact of anode gas inlet relative humidity (RH) on membrane hydration and the associated electrochemical performance of the PEM fuel cell. The anode gas inlet RH was varied to study the impact on fuel cell potential, during which simultaneous in operando visualizations were performed using synchrotron X-ray radiography, and electrochemical impedance spectroscopy was used to gain an understanding of the membrane hydration and water dynamics. The thickness of a Nafion^® N115 membrane expanded by up to 26 μm (20% of nominal thickness) compared to the manufacturer specification, as a result of changes in membrane hydration. Through this work, we present the utility of synchrotron X-ray radiography for tracking changes in membrane hydration of an operating PEM fuel cell.