Local Area Water Removal Analysis of a Proton Exchange Membrane Fuel Cell under Gas Purge Conditions

In this study, local area water content distribution under various gas purging conditions are experimentally analyzed for the first time. The local high frequency resistance (HFR) is measured using novel micro sensors. The results reveal that the liquid water removal rate in a membrane electrode assembly (MEA) is non-uniform. In the under-the-channel area, the removal of liquid water is governed by both convective and diffusive flux of the through-plane drying. Thus, almost all of the liquid water is removed within 30 s of purging with gas. However, liquid water that is stored in the under-the-rib area is not easy to remove during 1 min of gas purging. Therefore, the re-hydration of the membrane by internal diffusive flux is faster than that in the under-the-channel area. Consequently, local fuel starvation and membrane degradation can degrade the performance of a fuel cell that is started from cold

Nitrogen front evolution in purged polymer electrolyte membrane fuel cell with dead-ended anode

In this paper, we model and experimentally verify the evolution of liquid water and nitrogen fronts along the length of the anode channel in a proton exchange membrane fuel cell operating with a dead-ended anode that is fed by dry hydrogen. The accumulation of inert nitrogen and liquid water in the anode causes a voltage drop, which is recoverable by purging the anode. Experiments were designed to clarify the effect of N-2 blanketing, water plugging of the channels, and flooding of the gas diffusion layer. The observation of each phenomenon is facilitated by simultaneous gas chromatography measurements on samples extracted from the anode channel to measure the nitrogen content and neutron imaging to measure the liquid water distribution. A model of the accumulation is presented, which describes the dynamic evolution of a N-2 blanketing front in the anode channel leading to the development of a hydrogen starved region. The prediction of the voltage drop between purge cycles during nonwater plugging channel conditions is shown. The model is capable of describing both the two-sloped behavior of the voltage decay and the time at which the steeper slope begins by capturing the effect of H-2 concentration loss and the area of the H-2 starved region along the anode channel

Transport Resistance in Polymer Electrolyte Fuel Cells

Fuel cells offer the potential for high efficiency energy conversion with only water and heat as significant products of the electrochemical reaction. For a cost-competitive product, fuel cell researchers are exploring the limits of the Pt catalyst loading in parallel with performance and durability trade-offs. A significant portion of the performance loss in low-cost PEMFCs is associated with the partial pressure of oxygen (for an air cathode) at the Pt surface. This dissertation explores the main components of oxygen transport resistance which are associated with diffusion through partially saturated porous media and the ionomer coating in the catalyst layer. Under typical proton exchange membrane fuel cell (PEMFC) operating conditions, temperature gradients through the porous gas diffusion layer (GDL) can result in product water condensation. As a result, non-uniform partial saturation of the GDL changes the local effective porosity and tortuosity encountered by oxygen diffusing to the catalyst layer. This work establishes the impact of saturation on practical fuel cell system efficiency losses related to shutdown purge time and overall stack resistance. The transport resistance is further investigated in two-dimensions using limiting current experiments with simultaneous neutron imaging. The analysis of these data results in a diffusion coefficient vs. saturation relationship for two common GDL carbon fiber substrates. A significant oxygen transport limitation also occurs near the Pt surface. This is investigated here with loading studies that fix electrode thickness and bulk properties. The impact of Pt dispersion is probed by varying the average distance between Pt particles. Results elucidate how the electrode structure impacts local transport loss. It is demonstrated that local transport loss is not fully captured with a normalized Pt area. Additional geometric considerations that account for ionomer surface area relative to the Pt particles are required to resolve performance loss at low Pt loading as electrode structure varies. Furthermore, within this ionomer layer an interfacial resistance at both the gas and Pt interfaces is required to account for performance trends observed. These results demonstrate that residual performance loss associated with low cathode Pt loading can be mitigated by minimizing oxygen flux through the gas/ionomer interface

A comprehensive review of solutions and strategies for cold start of automotive proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) can be a significant eco-friendly alternative power source for vehicles. However, under subfreezing conditions, cell degradation and irreversible performance decay can occur because of ice formation and repetitive thaw/freeze cycles. These problems have limited the further commercialization of PEMFC in cold weather countries. Thus, many improvements have been made to repair the freeze protection and rapid cold startup problems in PEMFC vehicles. In this paper, a comprehensive review dedicated to engineers of the recent research progress on the PEMFC cold start problems is presented. Systems and methods for fuel cell shutdown are summarized and classified into two categories: purge solution and material to avoid freezing. Regarding the system and solutions for PEMFC cold startup, different heating solutions are classified into two main groups depending on their heating sources and categorized as internal and external heating methods. This paper concludes with a detailed review of cold startup strategies based on an exhaustive survey of journal papers and patents. © 2016 IEEE

Effects of an easy-to-implement water management strategy on performance and degradation of polymer electrolyte fuel cells

Intermittent switching between wet and dry reactant gases during operation in a polymer electrolyte fuel cell (PEFC) can improve performance stability, alleviating the effects of flooding by controlling the water content within the system. However, lifetime durability may be affected due to membrane electrode assembly (MEA) boundary delamination and membrane damage. Two relative humidity (RH) control strategies were investigated, using electrochemical performance and MEA degradation as critical indicators. It was found that intermittent switching between wet and dry gases does not accelerate fuel cell degradation if the duration of the dry gas period is set reasonably (dry gases stops before the voltage reaches the apex of the hump). Additionally, current and temperature distribution mapping was utilised to capture the dynamic response between these transitional stages. The switching of dry gases first makes the current density distribution homogeneous, and the maximum current density is reduced subsequently. Then, the current density near the inlet keeps decreasing. Intermittent switching between wet and dry reactant gases is easy to implement and overcomes limitations in mass transfer at medium and high current densities

Measurement of Liquid Water Accumulation in a Proton Exchange Membrane Fuel Cell with Dead-Ended Anode

The operation and accumulation of liquid water within the cell structure of a polymer electrolyte membrane fuel cell (PEMFC) with a dead-ended anode is observed using neutron imaging. The measurements are performed on a single cell with 53 square centimeter active area, Nafion 111-IP membrane and carbon cloth Gas Diffusion Layer (GDL). Even though dry hydrogen is supplied to the anode via pressure regulation, accumulation of liquid water in the anode gas distribution channels was observed for all current densities up to 566 mA cm-2 and 100% cathode humidification. The accumulation of liquid water in the anode channels is followed by a significant voltage drop even if there is no buildup of water in the cathode channels. Anode purges and cathode surges are also used as a diagnostic tool for differentiating between anode and cathode water flooding. The rate of accumulation of anode liquid water, and its impact on the rate of cell voltage drop is shown for a range of temperature, current density, cathode relative humidity and air stoichiometric conditions. Neutron imaging of the water while operating the fuel cell under dead-ended anode conditions offers the opportunity to observe water dynamics and measured cell voltage during large and repeatable transients

Visualization of Thawing and Desaturation in Frozen Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells via Synchrotron X-ray Computed Tomography

Proton exchange membrane fuel cells (PEM fuel cells) are one of the promising clean energy solutions to replace fossil fuel in applications such as automobiles and stationary power systems. Though significant research progress has been made, there are still some key technical challenges to be solved including the water management and cold-start problems, hindering large scale commercialization of this technology. A successful water management requires the amount of water content in a PEM fuel cell system to be kept at an optimal level. A poor water management would lead to membrane dehydration or liquid water flooding, which would cause temporary or permanent losses in performance and durability. The water flooding problem would become more serious in the subzero temperature during the cold-start process, which could lead to irreversible damages on cell components, or even cell failure in some extreme cases. Due to opaque nature of PEM fuel cell components, visualization and understanding of water transport behavior remains a challenge. Therefore, thawing and desaturation processes of gas diffusion layers (GDLs) under cold-start operating conditions were studied in this research via synchrotron X-ray computed tomography (CT) imaging techniques. The high speed and high resolution CT scan made it possible to capture the dynamic water behavior during the thawing and desaturation process for both qualitative and quantitative analyses. The experiments were performed on a half cell (cathode side) with a 40 mm serpentine channel, where Sigracet® 35AA and 35BA graphite GDLs were selected in different trials, with the superficial gas velocity of the purging air set to 2.88 m/s, 4.26 m/s, 5.98 m/s and 9.02 m/s. A similar desaturation pattern was observed in both global and local GDL regions; however, heterogeneity in water transfer was found over the entire GDL domains, both in-plane and through-plane. It was also found that the air purging rate, purging distance, and flow field geometry would affect the desaturation pattern, while the GDL hydrophobicity would mainly affect the initial saturation level. These data provide valuable information for future experimental and modeling studies that involve the thawing process in the GDL, and could be used to optimize the cell design and develop the cold-start protocols

Nitrogen Blanketing and Hydrogen Starvation in Dead-Ended-Anode Polymer Electrolyte Fuel Cells Revealed by Hydro-Electro-Thermal Analysis

Dead-ended anode operation has a number of practical advantages that simplify system complexity and lower cost for polymer electrolyte fuel cells. However, dead-ended mode leads to performance loss over time which can only be reversed by performing intermittent purge events. This work applies a combined hydro-electro-thermal analysis to an air-cooled open-cathode fuel cell, presenting experimental functional maps of water distribution, current density and temperature. This approach has allowed the identification of a 'nitrogen blanketing' effect due to nitrogen cross-over from the cathode and a 'bypass' effect where a peripheral gap between the gasket and the GDL offers a hydrogen flow 'short circuit' to the border of the electrode. A consequence of high local current density at the margin of the electrode, and resulting high temperatures, may impact the lifetime of the cell in dead-end mode