Search

The effects of water accumulation at the interface of gas diffusion layer (GDL) and gas supplying channel on the water distribution in polymer electrolyte membrane fuel cells (PEMFCs) is analyzed. The amount of water at the interface and in the GDL are quantified using X-ray. Quantitative analyses show that the value of the criterion of water accumulation that can affect the water distribution is in the water saturation range of 0.22–0.24. The amount of water in the GDL increases with the water accumulated at the beginning of the water generation cycle. However, it remains constant after the water accumulation exceeds a criterion value. The result shows that the water accumulation at the interface should be investigated to understand the water distribution in PEMFC. The water distribution in PEMFC cannot be analyzed based on the steady state concept but can be analyzed based on the concept of cyclic process. • Water accumulation at the interface GDL and gas supplying channel is visualized and analyzed quantitatively. • The relation between water accumulation at the interface and water distribution in PEMFC is analyzed quantitatively. • The criterion of water accumulation at the interface that can affect the water distribution in PEMFC is proposed. • The water distribution in PEMFC should be investigated not based on the steady state concept but based on the cyclic process. [ABSTRACT FROM AUTHOR]

In this study, the change of electrochemical parameters during the gradual water accumulation of a polymer electrolyte fuel cell (PEFC) are estimated using electrochemical impedance spectroscopy (EIS) measurements and an impedance model based on electrode theory during a two‐step oxygen reduction reaction (ORR). EIS measurements were carried out in a 5 cm2 H2/O2 PEFC operated under a dead‐ended configuration of the gas reactants and during gradual accumulation of water. Kramers‐Kronig evaluation demonstrated that the EIS measurements comply with stability and linearity properties and the inductive loops featured at low frequencies are attributed to electrochemical mechanisms within the PEFC and not to instability during gradual water accumulation. The impedance model has been reported in a previous study and simulates inductive loops at low frequencies which are attributed to platinum oxide formation during the ORR. The estimated parameters obtained from this EIS‐modeling analysis can provide an insight into the decrease in PEFC performance during flooding conditions. A further qualitative analysis considering the effect of long‐term water accumulation on the oxygen‐reduction charge transfer resistance is discussed. It is possible to have an insight into the physical mechanisms of the PEFC, by combining different experimental techniques and fundamental theory in a complimentary manner. [ABSTRACT FROM AUTHOR]

Water flooding at the cathode is a critical issue in polymer electrolyte membrane fuel cells, since it will block the oxygen transport, resulting in a large concentration loss. To address this issue, two tapered flow fields with varying height or width are proposed in this study, as the tapered channels can gradually increase flow velocity which is beneficial in water removal. To confirm the proposed function, both experiments and simulation are performed. The results prove that the tapered flow fields with high velocity at the downstream region can significantly enhance water removal in flow channels, which avoids the mass transport limitation and improves the cell performance at high current densities. Additionally, the tapered design can also enhance the under-rib convection between adjacent channels, which helps to remove accumulated water in the gas diffusion layer. [ABSTRACT FROM AUTHOR]

The intrinsic effect of properties of a self-supporting microporous layer (MPL) on the performance of proton exchange membrane fuel cells (PEMFCs) is identified. First, a self-supporting MPL is fabricated and applied to a gas diffusion layer (GDL) of a PEMFC, when the GDL is either an integrated sample composed of a gas diffusion backing (GDB, i.e., carbon paper) combined with MPL or a sample with only MPL. Cell performance tests reveal that, the same as the MPL fabricated by the coating method, the self-supporting MPL on the GDB improves the cell performance at high current density. Furthermore, the GDL composed only of the MPL (i.e., GDB-free GDL) shows better performance than does the integrated GDB/MPL GDL. These results along with literature data strongly suggest that the low thermal conductivity of MPL induces a high temperature throughout the GDL, and thus vapor diffusion is dominant in the transport of product water through the MPL. [ABSTRACT FROM AUTHOR]

PEMFC (Proton exchange membrane fuel cell) operated under dead-ended anode mode is an effective and simple device for high hydrogen utilization. Herein, the operation characteristics of PEMFC with dead-ended anode are systematically investigated under various operating temperature, anode inlet pressure, and cathode stoichiometry. In each operating parameter, the solenoid valve located at the anode outlet is closed all the time and the hydrogen utilization reaches 100%. It has been found that fuel cell with dead-ended anode can operate continuously for 26 h with slight decrease in output voltage under 600 mA cm −2 at 65 °C, 80% of RH (relative humidity) with hydrogen inlet pressure of 0.4 bar gauge and oxygen stoichiometry of 4.0. After accumulated 150 h operation under dead-ended anode mode, the degradation of according MEA (membrane electrode assembly) has been analyzed using CV (cyclic voltammetry) and cross-sectional morphology. The results show that the accumulation of water induced corrosion of catalyst layer is responsible for the decrease in ECSA (electrochemical surface area) of catalyst layer and the according degradation of cell performance. Moreover, the degradation of catalyst layer became much more serious at the vicinity of cathode outlet than the other regions. [ABSTRACT FROM AUTHOR]

Abstract: Water distributions of a polymer electrolyte fuel cell (PEFC) with 9-parallel channels during operation were visualized using a neutron radiography facility at B4 port in KUR (Kyoto University Research Reactor). An imaging system with a neutron image intensifier (I.I.) was employed for reducing the exposure time, and the water distributions in the in-plane and through-plane directions in the PEFC were alternately obtained every 20sec. The accumulation processes from the GDL to the channels were confirmed. Water accumulated in the GDL at the cathode and evacuation into the channels started around 5min. Water tended to accumulate at the edge of the ribs, and accumulated as water drops in the channels. The size of the water drops grew up to 1mm which was the same size as the channel width and height, and the cell voltage was decreased because the liquid drops disturbed the air supply. [Copyright &y& Elsevier]

Abstract: Longer channels within serpentine flow fields are highly effective at removing liquid water slugs and have little water accumulation; however, the long flow path causes a large pressure drop across the cell. This results in both a significant concentration gradient between the inlet and outlet, and high pumping losses. Parallel flow fields have a shorter flow path and smaller pressure drop between the inlet and outlet. This low pressure drop and multiple routes for reactants in parallel channels allows for significant formation of liquid water slugs and water accumulation. To investigate these differences, a polymer electrolyte membrane fuel cell parallel flow field with the ability to modify the length of the channels was designed, fabricated, and tested. Polarization curves and the performance, water accumulation, and pressure drop were measured during 15 min of 0.5 A cm−2 steady-state operation. An analysis of variance was performed to determine if the channel length had a significant effect on performance. It was found that the longer 25 cm channels had significantly higher and more stable performance than the shorter 5 cm channels with an 18% and an 87% higher maximum power density and maximum current density, respectively. Channel lengths which result in a pressure drop, across the flow field, slightly larger than that required to expel liquid water slugs were found to have minimal water accumulation and high performance, while requiring minimal parasitic pumping power. [Copyright &y& Elsevier]

Abstract: In a proton exchange membrane fuel cell (PEMFC), flow field design is an important factor that influences the distributions of current density and water accumulation. The segmented model developed in prior study is used to investigate the effect of flow field patterns on current density distribution. This model predicts the distributed characteristics of water content in the membrane, relative humidity in the flow channels, and water accumulation in the gas diffusion layers (GDLs). Three single cells with different flow field patterns are designed and fabricated. These three flow field designs are simulated using the segmented model and the predicted results are compared and validated by experimental data. This segmented model can be used to predict the effect of flow field patterns on water and current distributions before they are machined. [ABSTRACT FROM AUTHOR]

Abstract: Time-dependent measurements of pressure drop in different flow fields on the cathode of a PEM fuel cell with different operating conditions of mass flow rates and cell temperatures on current distribution were conducted. The results show that, among four flow fields studied herein, the interdigitated flow channel has the biggest pressure drop and least influence on current distribution at an early phase (⩽30min) compared to those of the other three channels. In addition, the effect of pressure drop on water accumulation was noted. Similarly, the effects of mass flow rates as well as the cell temperature were also examined and discussed. [ABSTRACT FROM AUTHOR]

Abstract: A simplified, miniaturized polymer electrolyte fuel cell without gas diffusion layers was investigated under operation by neutron radiography. By visualizing liquid water, it was possible to identify limiting effects, which are directly related to the simplified construction principle. Depending on the operation conditions, undesired water accumulation either in particular micro-channels or on the cathode catalyst layer as well as drying of the anode catalyst layer was observed. As a consequence, the design of a fuel cell without gas diffusion layers must take into account these limitations visualized by neutron radiography. [Copyright &y& Elsevier]

• A comprehensive review of researches on PEMFC operates in dead-end anode. • Performance deterioration due to hydrogen starvation, water accumulation and nitrogen crossover. • Purging strategy to recover cell and stack performance are evaluated. • Alternatives method to recover stack performance are analyzed. • Future research directions to improve dead-end anode operation are discussed. To improve fuel utilization and reduce complexity of polymer electrolyte fuel cell especially for automotive application, dead-end anode operation is desirable. In this operating mode, the anode outlet is closed to achieve nearly 100% fuel utilization. Despite its great potential, operating the fuel cell in a dead-end anode mode brings consequence of nitrogen crossover and liquid water back diffusion which accumulate in the anode, hindering contact between hydrogen fuel with the catalyst inducing fuel starvation. This fuel starvation not only deteriorates fuel cell performance but also degrades the cell integrity by inducing carbon corrosion. To address these issues and achieve optimum operation conditions for the fuel cell, numerous studies on the performance of the dead-end anode fuel cell have been conducted, several key parameters have been evaluated and various mitigation strategies have been proposed. However, the dead-end anode fuel cell has not reached its mature commercialization stage and more research and development is required. To assist further research and development of the dead-end anode fuel cell and expedite its mass application, it is imperative to grasp and discuss the main findings of the previously reported studies. At the moment, no review paper on the dead-end anode fuel cell is available. Therefore, this paper is presented to comprehensively review the development and advancement of dead-end anode fuel cells. In addition, the required research and development for further advancements of the field are also outlined and discussed. [ABSTRACT FROM AUTHOR]