• Energy Research

Cold-start (subzero startup) capability of polymer electrolyte fuel cell (PEFC) is of great importance. In this paper, the effects of the micro-porous-layer (MPL), varying start-up temperatures, start-up current density variation on the cold-start operation are investigated. We found PEFC with anode micro-porous-layer (AMPL), compared with one with CMPL, has higher possibility of successful cold start. By analyzing the anode and cathode pressure revolution of PEFC, the effect of MPL on the super cooled water removal and ice formation at different temperatures (-7,-10,-15 and-20℃) are discussed . In addition, we investigate the ice distribution in MEA through the X-ray device, by comparing the initial image when fuel cells shut down (before the produce the water) and final image after failed cold-start. We also explore the negative voltage phenomenon in the initial process of cold-start operation.

Abstract A 3D (three-dimensional) multi-phase model of PEMFC (proton exchange membrane fuel cell) is developed, in which the Eulerian-Eulerian model is utilized to solve the gas and liquid two-phase flow in channels, while the two-fluid model is adopted in porous electrodes. Hence, the surface tension, wall adhesion and drag force in channels are all included. Besides, the gravity effects in the whole PEMFC are also taken into consideration. It is found that the water vapor concentration in cathode channel at high inlet humidity (e.g. 1.0) will be much higher than the saturation concentration if neglecting the water vapor condensation. In addition, the liquid water condensed from vapor in channel is mainly blown to side walls, rather than only exist on the bottom surface. The effect of water condensation and evaporation in channel is found to be lower than that in porous electrodes because of the much higher gas velocity in channel. Besides, the partial low temperature is likely to cause local liquid water accumulation in both anode and cathode channels and porous electrodes. Meanwhile, the wave-like channel is found to be able to remove the liquid water effectively due to the enhanced convection effect. Meanwhile, the simulation results in this study show that the serpentine flow field is much more beneficial to the liquid water removal and reactant gas distribution than parallel flow field, which results in the much higher performance.

Abstract Alkaline anion exchange membrane (AAEM) fuel cell is becoming more attractive because of its outstanding merits, such as fast electrochemical kinetics and low dependence on non-precious catalyst. In this study, a three-dimensional multiphase non-isothermal AAEM fuel cell model is developed. The modeling results show that the performance is improved with more anode humidification, but the improvement becomes less significant at higher humidification levels. The humidification level of anode can change the water removal mechanisms: at partial humidification, water is removed as vapor; and for full humidification, water is removed as liquid. Cathode humidification is even more critical than anode. Liquid water supply in cathode has a positive effect on performance, especially at high current densities. With more liquid water supply in cathode, liquid water starts moving from channel to CL, rather than being removed from CL. Liquid water supply in cathode is needed to balance the water amounts in anode and cathode. Decreasing the membrane thickness generally improves the cell performance, and the improvement is even enhanced with thinner membranes, due to the faster water diffusion between anode and cathode, which reduces the mass transport losses.

Abstract The effects of the operating conditions on the performance of metal hydride hydrogen storage tanks are complicated and need detailed investigations for further optimization. In this study, a mathematical model is developed to understand the effects of the various operating conditions on the hydrogen absorption processes in a LaNi 5 metal hydride tank. The numerical results indicate that the quickest charging process occurs within the first 20 s, and the quickest charging rate and duration are mainly affected by the charging pressure and initial temperature, respectively. The effect of cooling level on this process is insignificant. For both the short-time charging (2 min) and long-time charging, the hydrogen fueling performance is significantly affected by the cooling level (the heat transfer coefficient and surrounding temperature) and charging pressure. In order to ensure sufficiently quick hydrogen charging, the charging pressure needs to be kept enough higher than the equilibrium pressure, and due to the fast heating of the metal hydride, the influence of the initial temperature is less significant than the cooling condition. The general distributions of the absorbed hydrogen fraction and temperature are similar under the different operating conditions.

Abstract Catalyst layer (CL) is the core electrochemical reaction region of proton exchange membrane fuel cells (PEMFCs). Its composition directly determines PEMFC output performance. Existing experimental or modeling methods are still insufficient on the deep optimization of CL composition. This work develops a novel artificial intelligence (AI) framework combining a data-driven surrogate model and a stochastic optimization algorithm to achieve multi-variables global optimization for improving the maximum power density of PEMFCs. Simulation results of a three-dimensional computational fluid dynamics (CFD) PEMFC model coupled with the CL agglomerate model constitutes the database, which is then used to train the data-driven surrogate model based on Support Vector Machine (SVM), a typical AI algorithm. Prediction performance shows that the squared correlation coefficient (R-square) and mean percentage error in the test set are 0.9908 and 3.3375%, respectively. The surrogate model has demonstrated comparable accuracy to the physical model, but with much greater computation-resource efficiency: the calculation of one polarization curve will be within one second by the surrogate model, while it may cost hundreds of processor-hours by the physical CFD model. The surrogate model is then fed into a Genetic Algorithm (GA) to obtain the optimal solution of CL composition. For verification, the optimal CL composition is returned to the physical model, and the percentage error between the surrogate model predicted and physical model simulated maximum power densities under the optimal CL composition is only 1.3950%. The results indicate that the proposed framework can guide the multi-variables optimization of complex systems.

A digitally-assisted method is proposed to accelerate the structure design of large-size proton exchange membrane fuel cells, including backward engineering and forward design.

Abstract A comprehensive 3D (three-dimensional) multiphase model of PEMFC (proton exchange membrane fuel cell) is developed, in which the gas and liquid two-phase flow in channel and porous electrodes are investigated in detail. In the simulation of gas and liquid two-phase flow in channels, the effect of surface tension, wall adhesion and gravity is taken into account, including the influence of pressure difference between the inlet and outlet on inlet reactant gas concentration; while in porous electrodes, the anisotropy of GDL (gas diffusion layer) and liquid saturation jump at the interface of two different porous layers (e.g. GDL and MPL (micro-porous layer)) are also considered in this model. It is found that the amount of liquid water in channels increases with the increment of current density. In addition, increasing the contact angle at GDL/channel interface is found to be able to improve the performance of PEMFC by facilitating the water removal process in channels. Moreover, it can be concluded that adding baffles in cathode channel not only increases the oxygen concentration in porous electrodes but also facilitates the water removal process, both of which prevent PEMFC from concentration loss effectively.

Effective water removal from the proton exchange membrane fuel cell (PEMFC) surface exposed to the flow channel is critical to the operation and water management in PEMFCs. In this study, the water removal process is investigated numerically for a novel flow channel formed by inserting a hydrophilic needle in the conventional PEMFC flow channel, and the effect of the surface wettability of the membrane electrode assembly (MEA) and the inserted needle on the water removal process is studied. The results show that the liquid water can be more effectively removed from the MEA surface for larger MEA surface contact angles and smaller needle surface contact angles. The pressure drop for the flow in the channel is also examined and it is seen to be indicative of the liquid water flow and transport in the flow channel, suggesting that pressure drop is a useful parameter for the investigation of water transport and dynamics in the flow channel.

Abstract Alkaline anion exchange membrane fuel cell (AEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary application in recent years. To ensure high ionic conductivity and efficient reactants delivery, water management is regarded as one of the most critical issues for AEMFC. In this study, an analytical model is formulated to investigate the effect of electrode wettability on the water transport and resultant AEMFC performance. The pressure continuity method is considered to simulate liquid saturation jump on the interfaces of adjacent electrode layers. The results show that decreasing the cathode catalyst layer (CL) contact angle improves the performance because more water can be kept in the cathode CL decreasing polarization losses. The anode micro porous layer (MPL) is generally helpful, by forcing the liquid water to back-diffuse to the cathode. However, cathode MPL hinders the water transport to the cathode CL, leading to a lower reaction rate and membrane conductivity. The liquid water injection into the cathode has great potential to further improve the performance of AEMFC, however it may cause flooding in the flow channel and GDL. The cathode reaction kinetics should be considered as one of the most significant factors dragging the cell performance.

The dynamic performance of proton exchange membrane fuel cell (PEMFC) is critically important, especially for automotive applications. In this study, the dynamic behavior of the local current density distribution inside PEMFC has been investigated experimentally by using the segmented flow field plate and printed circuit board (PCB) method. The effect of different values of air stoichiometry ratios on the dynamic characteristics of local current density has been tested. It has been found that the fluctuation of the local current density increases as the cell voltage is reduced, corresponding to the increase in the average cell current density or cell loading. Increasing the stoichiometry ratio of the cathode supply gas significantly reduces the fluctuations of the current density. The fluctuation is also lower near the flow inlet than near the flow exit. The phenomena observed are closely related to the water and reactant distributions and transports in the cell structure.