• Energy Research

Abstract In this study, the gas liquid two-phase flow of low-temperature fuel cells is simulated to study the water removal process in the anode channel utilizing VOF (volume of fluid) method. It is found that the water removal process in the cathode channel is easier compared to that in the anode under the same operating condition and inlet velocity. Increasing the humidification rate and contact angle is helpful for water removal. Moreover, further simulations show that in the semicircle-U channel, the water can be removed successfully, but it will be stuck in the corner in the rectangular-U channel, which can be prevented by changing the contact angle of specific walls into extremely hydrophilic or hydrophobic. Extremely hydrophilic wall would make the entire water pave on the wall and smash into little droplets which is not only helpful in water removal and evaporation but also prevents the water from blocking the holes on the Gas Diffusion Layer (GDL). And different hydrophilic/hydrophobic levels also influence the water removal.

Aligned nano-sponges accommodate only non-freezable water and facilitate efficient water retention in the membrane, even under low relative humidity conditions.

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.apenergy.2018.08.125 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/

Abstract Water dynamics in the flow channel of a proton exchange membrane fuel cell is significantly important to water management and removal. In this study, volume-of-fluid method is used to investigate numerically the three-dimensional water dynamics in a flow channel with a hydrophilic needle. It is found that water transport and dynamics in this novel flow channel are quite different from the conventional channel. Liquid water droplet, introduced on the electrode surface, is removed through capillary effect once touching the hydrophilic needle. This is desirable since the electrode surface becomes free of liquid water, avoiding the flooding and blockage of reactant gas transport into the electrode. Increasing the contact area between the water droplet and needle, through an increase in the diameter or length of the needle, can facilitate water removal from the electrode surface because of greater capillary effect, but it also increases the pressure drop in the channel due to greater blockage by the needle. Overall, the pressure drop in the modified channel is still small compared to the pressure drop in a serpentine flow channel, making the present approach viable for use in the conventional parallel flow channels for proton exchange membrane fuel cells.

In the application of power converters, the ambient temperature (Ta) experiences significant fluctuations. For the case-to-ambient resistance (Rca), apart from the influence of the material’s inherent properties, factors such as heat dissipation structure, working environment, and operational state can all have an impact on the Rca. Notably, while there are a limited number of models that consider environmental changes, existing models for calculating the Rca predominantly overlook the influence of varying working conditions or boundary conditions on the self-thermal resistance. Based on simulation and experimental analyses, the methods to calculate the Rca are outlined, and the key factors, inclusive of the coupling effects that influence the Rca in the thermal modeling of power devices, are thoroughly discussed.

Abstract A three-dimensional (3-D) model for planar, anode-supported, solid oxide fuel cell (SOFC) is developed to investigate the effect of operating pressure on cell characteristics. The results show that the elevated operating pressure can improve cell performance by increasing open circuit voltage and reducing activation overpotential, and enhance the electrochemical reaction in the vicinity of electrolyte. Besides, the high pressure can also change the distributions of species and internal reforming reactions. Compared to the case using syngas as fuel, the operating pressure has more significant effects on temperature gradient along flow direction when partly pre-reformed gas is supplied. In addition, efficient control of cell temperature could be achieved by decreasing fuel utilization in the case of partly pre-reformed gas, but this is achieved at the expense of cell efficiency, especially under high pressure condition. Another way to reduce the temperature gradient is to adopt higher air ratio. Moreover, when partly pre-reformed gas is used, the counter-flow configuration has a better performance due to the higher overall temperature.

Abstract Proton exchange membrane fuel cell (PEMFC), as a representative of fuel cell technology, has attracted much attention for its huge advantages in transportation application. Water management has a significant impact on the lifetime and performance of PEMFC. This study numerically investigates the movement of a water droplet in a single serpentine flow channel of PEMFC with different U-turn designs. The transport characteristics of the water droplet are obtained under both low and high airflow speeds, respectively. It is found that the droplet can pass through the U-turn for large fillet radius, while it is stuck in the corners of the U-turn for small fillet radius. The water droplet is transported as a whole without breakup under low airflow speed, but it is split into small droplets under high airflow speed at the U-turn. Hydrophobic channel surface promotes water removal at the U-turn of the channel, but its effect is weakened under high airflow speed.

Abstract Catalyst layer structural changes in polymer electrolyte membrane fuel cells have significant impact on the cell performance and durability. In this study, ex-situ experiments are designed to investigate the effect of humidity and/or thermal cycles on the structural changes of catalyst layers. The relative humidity and temperature are controlled by an environmental chamber and the catalyst layer structure is characterized by scanning electron microscopy and optical microscopy. The experimental results indicate that crack growth and development, catalyst agglomerate detachment, and surface bulges are the main structural changes of the catalyst layers. Applying relative humidity and thermal cycling simultaneously causes the most significant crack growth, while applying thermal cycling alone causes no appreciable changes. This indicates that the absolute humidity is the key parameter for the crack growth. Through cyclic voltammetry analysis, it is shown that the electrochemical active surface area decreases from 64.1 m2 g−1 to 49.1 m2 g−1 after 500 combined relative humidity and thermal cycles. Analyses of electrochemical impedance spectroscopy show that the charge transfer resistance and ohmic resistance increase significantly after 500 combined relative humidity and thermal cycles, causing the cell performance degradation.