Numerical analysis of air-cooled proton exchange membrane fuel cells with various cathode flow channels

Highlights

• A validated three-dimensional model is built to investigate air-cooled fuel cells.

• Larger channel width induces uneven distribution of RH and oxygen concentration.

• Rib-channel ratio around 3.0 is preferred in order to enhance the performance.

• RH rises with regular fluctuation along the direction of cathode channel.

• Channels with curved features improve mass transfer and cell performance.

Abstract

Air-cooled proton exchange membrane (PEM) fuel cells simplify fuel cell design by combining oxygen supply and air cooling in open cathode channels. Their performance is sensitive to the structure of cathode channels, which significantly affects distribution of temperature, relative humidity and mass transfer in the cells. This study offers a three-dimensional air-cooled fuel cell model with consideration of electrochemistry simulation to investigate the effect of cathode channel design. Experiments are conducted to validate the model. It is observed that obvious gradient in the distributions of temperature, humidity and oxygen concentration lies in the membrane exchange assembly (MEA) between the channel and rib owing to air dual functions in distributing oxygen and cooling the stack. For models with fixed rib-channel ratio of 1.0, the performance is better when channel width is smaller. Considering the effect of contact resistance when the ratio is small, rib-channel ratio within a reasonable range of around 3.0 is preferred in order to enhance the performance. Channels with curved features improve the mass transfer from channel to catalyst layer, thus increasing the cell performance. This study is helpful for enhancing our understanding of the relationship between cell performance and cathode channel design in the air-cooled fuel cell.

Introduction

In proton exchange membrane (PEM) fuel cells, the energy is released by electrochemical reaction of hydrogen and oxygen, and output in the form of electric energy. They can be operated at low temperature and are not limited by the Carnot cycle in the process of energy conversion. During the working process, only water is produced, which leads to truly zero pollution [[1], [2], [3]]. In terms of cooling manners, PEM fuel cells can be classified as air-cooled fuel cells and water-cooled fuel cells. Air-cooled fuel cells combine the cooling function with the cathode air flow to reduce both the weight and complexity of the fuel cell system effectively. They are promising energy conversion technologies for a broad range of applications including portable power supply and unmanned aerial vehicle (UAV) due to their low cost, easy operation and simplified design.

Status of temperature, humidity and mass transfer is a critical issue to the performance of PEM fuel cells [[4], [5], [6]]. Loss of water in fuel cells will cause the drying in membrane, which leads to insufficiency of the electrochemical reaction. Too much water accumulation induces flooding in the gas diffusion layer (GDL) and catalyst layer (CL), thus affecting the mass transfer [7]. High temperature will decrease the lifetime of membrane exchange assembly (MEA) or even cause material failure, while low temperature reduces the cell performance. In addition, the cell temperature affects the saturation pressure of water, hence the local water content and mass flow. As a result, it is important to keep a balance among these coupled physical quantities. Moreover, for air-cooled fuel cells, cooling medium and reactant oxygen are simultaneously supported by the cathode flow channels of bipolar plate (BPP) which contribute significantly to the oxidant supply, produced water removal and operating temperature control and so have a significant influence on the energy conversion of fuel cells. Understanding the effects of cathode channels is beneficial to the improvement of air-cooled fuel cell performance.

Many studies have been conducted in an effort to investigate the inner phenomena such as the transport of water, gas, heat and electrons in water-cooled PEM fuel cells [8], [9], [10], [11], [12], [31]. For example, a sensitivity investigation of electrochemistry models was performed based on multi-parametric sensitivity analysis to define the relative importance of these factors to fuel cells by Min et al. [13]. Feng et al. [14] studied the interaction between water and thermal transport by defining various temperature and humidity conditions in the boundary of numerical models. Flow channels have also been optimized and decorated to promote the oxygen transport and water removal. For example, Zhu et al. [15] numerically studied the effects of channel geometry on the evolution and motion of water droplets, flow resistance, water saturation and coverage ratio. Tiss et al. [16] found that partial blocks inserted in the gas channel improved the fuel cell performance by progressing reactant distribution in the GDL. Similar enhancement in performance has also been observed by Ghanbarian et al. [17], Kuo et al. [18]’s and Ashorynejad et al. [19], in which effects of the blockages at various sizes and gaps were discussed. This is because the pressure pulsations would be generated on the flow and concentration fields in both the GDL and channel by the blockages which are conducive to the enhancement of the oxygen transfer to the cathode surface. The produced water may be more easily discharged under the influence of these features. Afshari et al. [20] built a novel three-dimensional flow channels to obtain better performance in oxygen transport and liquid water removal by adding baffles. In general, these researches are carried out with respect to the reaction efficiency and channel optimization of water-cooled fuel cells, in which cooling and reactant are separately managed during cell operation.

Nevertheless, air-cooled fuel cells exhibit water, thermal and mass transport behaviors completely different from water-cooled fuel cells. Air-cooled fuel cells do not utilize a compressed flow of oxidant, but instead absorb ambient air to simultaneously act as both the coolant and reactant in the cathode side. Their performance is more sensitive to surrounding environment and channel geometry. Lower operating temperature and output power are usually designed for this kind of fuel cell due to its low cooling efficiency and self-breathing operation. In addition, the open cathode flow channels are perpendicular to the anode ones, which is different from water-cooled fuel cell [21]. As a result, the inner temperature, humidity and mass transport behavior are likely in the ways of distinguishing. However, limited studies have been devoted to the systematic development of air-cooled fuel cells. You et al. [22] pointed out that temperature of air-cooled fuel cells gradually went up along with the increasing load and an optimal temperature should be controlled to achieve stable performance. Adazakpa et al. [23] found that in contrast to water-cooled fuel cells, the temperature non-uniformity in the air-cooled cell was shown to be very high, reducing the overall cell performance. The significant temperature difference was also reported by Shahsavari et al. [24] and Wu et al. [25]. This is attributed to the intrinsic poor cooling efficiency and reaction consumption of the air in cathode channels. It can be seen that there is significant difference between the air-cooled and water-cooled cells. Two issues remain unclear for air-cooled fuel cells: how the inner temperature, humidity and mass distributes in the cell with combining oxidant and coolant flow, and how the cathode channel geometry affects the cell performance. It is necessary to understand the mechanism of correlation between these coupled physical quantities and dimensions of cathode channels.

Therefore, and in light of the aforementioned factors, a three-dimensional air-cooled fuel cell model with consideration of electrochemistry simulation is established to predict the inner phenomena in the cell and investigate the effect of cathode channel design. Modelling results are validated by experimental data from practical air-cooled fuel cell operation. The characteristics of temperature, relative humidity (RH) and mass distribution are observed. Influences of cathode channel parameters are systematically discussed based on the numerical model.