Proton Exchange Membrane Fuel Cells Model: Comparison
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Please note this is a comparison between Version 2 by Fanny Huang and Version 1 by zhiming zhang.

PEMFCs (Proton Exchange Membrane Fuel Cells) are commonly used in fuel cell vehicles, which facilitates energy conversation and environmental protection. The fuel cell electrochemical performance is significantly affected by the contact resistance and the GDL (Gas Diffusion Layer) porosity due to ohmic and concentration losses.

  • PEMFC
  • assembly force
  • temperature
  • humidity

1. The Need for Proton Exchange Membrane Fuel Cells Model

According to the IEA (International Energy Agency), the transport sector emitted 7980 million tons of CO2 globally in 2022, accounting for 21.7% of total CO2 emissions from fuel combustion in the world [1]. The transportation field has become an important focus in reducing oil consumption and carbon emissions [2]. To develop new energy vehicles as a breakthrough to comprehensively promote low-carbon transportation is one of the most important ways to get rid of oil dependence and reduce CO2 emissions [3]. Hydrogen fuel cell electric vehicles have the advantages of zero emission, high efficiency, and long range, which is an important direction for future transport development. At present, PEMFCs are the most commonly used in fuel cell vehicles with the main advantages being low operating temperature, fast start-up, high efficiency, etc. [4].
The fuel cell stack is a key part of the fuel cell electric vehicle. As a continuous power generation source, it is responsible for providing energy to drive the vehicle. Improving the energy efficiency and electrochemical performance of the fuel cell stack is an important goal of fuel cell research. A fuel cell stack usually consists of hundreds of single cells. A single cell includes BPPs (Bipolar Plates) and a MEA (Membrane Electrode Assembly), which is made up of GDLs (Gas Diffusion Layers), CLs (Catalyst Layers), and a PEM (Proton Exchange Membrane). The MEA and the BPP are assembled together via a certain assembly force.
The assembly force of the fuel cell stack is generally applied using point load, line load, or surface load [5]. The assembly force acting on the GDLs will affect the porosity of GDLs and the contact resistance between the GDLs and BPPs. In addition, stress concentration will occur in the GDL under the action of assembly force [6]. Different porosities will affect the dual-function of gas supply and liquid water removal capacity in fuel cells, which directly affects the concentration losses of the fuel cell performance [7,8][7][8]. The different contact resistance will affect ohmic losses of the fuel cell [9]. However, the balance between the GDL porosity and the contact resistance on the fuel cell performance under assembly force is difficult to predict and optimize. Therefore, it requires us to study the compression mechanisms of GDL involved in an effective numerical model and experimental validation. In addition, due to the coupled complexity of the operating conditions (assembly force, working temperature, relative humidity, gas supply, etc.) in fuel cells, the electrochemical performance is difficult to predict and identify, not to mention also optimize.
Due to the limitation of experimental methods and technology, it is difficult to measure the detailed data of the fuel cell components during the fuel cell working operation, such as the porosity and permeability of GDLs and the distribution of internal water content and its current density. With the development and popularization of the numerical simulation method, which has become a practical assistant to analyze these complex problems, its high efficiency, convenience, low cost, and other characteristics attract attention as more and more research is applied to analyzing fuel cell performance. How to build a fuel cell model, combined with the complex behaviors of assembly force, working temperature, relative humidity, etc., and how to facilitate the optimization of fuel cells is the focus of the current research.

2. The Effect of Assembly Force, Temperature, and Humidity on Fuel Cell Electrochemical Performance

During the PEMFC operation process, the internal operating conditions in fuel cells involve multiple physical fields, such as the force field, thermal field, and humidity field, which are coupled with each other. The distribution of temperature, humidity, component deformations, and contact pressure directly affects the GDL porosity and contact resistance, even the electrochemical performance output of fuel cells [10].
The effect of assembly force on fuel cell stacks is mainly reflected in the porosity of GDLs and the contact pressure which affects the contact resistance between the GDLs and BPPs. The contact resistance in fuel cells is one of the main challenges to be overcome during the commercialization of fuel cells. GDL compression behavior is the key factor to controlling the contact resistance. The contact pressure on the GDL plays an important role in the performance improvement of PEM fuel cells via reducing ohmic and concentration losses [11]. The contact resistance is affected by several mechanical parameters, such as assembly force, the porosity of GDLs, and the dimensions of the GDLs and BPPs [12].
In PEMFC, the non-uniform distribution of contact pressures on the GDL leads to an increase in contact resistances, which increases ohmic losses, reduces porosity, and leads to the reaction gas concentration [13]. Therefore, it is essential to optimize the consistency and uniformity of the contact pressure of the GDL [14]. By optimizing the geometric parameters of the fuel cell assembly deign, the uniformity of the contact pressure distribution on the GDL can be increased [15]. The uniformity of the contact pressure on the GDL can be also improved by optimizing the position and size of assembly force [16].
With the increase in assembly force, the porosity and contact resistance of GDLs are continuously reduced [17]. Atyabi et al. [9] simulated a 3D multiphase model of a PEM fuel cell to investigate the effect of assembly force on the contact resistance between the GDL and the BPP. The results show that the increase in assembly force is related to the decrease in contact resistance between the GDL and the BPP. In addition, it was found that the distribution of the electric potential and oxygen concentration is more uniform at a higher assembly force.
Assembly force also affects the electrochemical reaction concentration inside the PEMFC, which changes the output’s current density. Peng et al. [18] measured the current distribution inside the fuel cell using the BPP with current sensors. The higher current passes through the reactive region with the higher contact pressure, and the current density is lower in the area with poor contact behavior. For higher output currents, the current density has a large deviation.
Under the effect of assembly force, the non-uniform porosity distribution exists in the GDL [19]. The gas flow channel will be intruded when the GDL is under the action of assembly force, which causes the GDL porosity changes [20].
The GDL porosity will also affect the transport of gas and produced water, which affects fuel cell performance. Suitable deformation for the GDL is conducive to the flow of liquid water to the channel, which reduces the accumulation of liquid water under the ribs. At the same time, the GDL porosity will affect its permeability, which is not conducive to the mass transfer in the GDL [21].
The contact resistance between the GDL and the BPP existing in PEMFC is responsible for ohmic losses during the electrochemical reactions [22]. When the assembly force increases, the contact resistance becomes smaller, but the mass transmission of fuel cells becomes weaker, leading to the increase in concentration losses [13]. An optimum PEMFC performance can be achieved by choosing an appropriate assembly force to balance the relationship between them.
Factors affecting fuel cell performance are not only assembly force, but also temperature and humidity [23]. A high fuel cell performance requires appropriate temperature and humidity [24]. During the operation of fuel cells, the temperature and humidity distribution are non-uniform and changes under different operating conditions [25]. Under the condition of low-current densities, the distribution of temperature and humidity are relatively uniform, while for high-current densities, the non-uniform distribution temperature and humidity will increase [26]. Temperature and humidity also interact with each other, which are competitive, and humidity especially has a greater impact on the temperature distribution at high-current densities [27]. At the same time, the PEMFC performance increases with the increase in temperature and humidity [28,29][28][29].
The variance in temperature and humidity will affect the MEA (Membrane Electrode Assembly) mechanical properties and cause MEA thermal and hydrated deformation [30]. By studying the force-temperature coupled model of fuel cells, it was found that the contact pressure between the MEA and the BPP increases due to thermal variance, which affects the stress distribution inside the MEA [31].
The relationship among temperature, humidity, and assembly force is not independent. For example, high temperature with high humidity obviously leads to stress concentration on the MEA; assembly force also affects the humidity distribution [32]. Liu et al. [33] established a two-dimensional fuel cell model to study the stress distribution under assembly force and found that stress concentration exists in the joint area between the gaskets and GDLs, and the maximum stress is mainly related to temperature and humidity. Under the combined action of temperature and humidity, the stress of the MEA increases, which can affect the PEM durability [34]. Studies have also shown that stress is higher at low assembly force and high humidity and temperature, which affects the fuel cell performance [35].
Ouaidat et al. [36] used a multi-physics model to optimize the fuel cell performance. A 3D finite element analysis including a full coupling of thermal-electrical-mechanical model is proposed to predict the electrical resistance of fuel cells. Zhang et al. [23] studied the performance of fuel cells using a coupled two-dimensional PEM fuel cell deformation model and a three-dimensional CFD model. The membrane deformation and PEM fuel cell performance under different RHs (Relative Humidities) were also investigated, and medium RH values were found to achieve good fuel cell performance if the membrane swelling effect was considered. Some studies are listed in Table 1.
Table 1. The comparison of models.
Author Dimension Influence Factor Research Content
Liu [33] 2D assembly force stress distribution
Bograchev [34] 2D assembly force, temperature, humidity stress distribution
Mehrtash [35] 2D assembly force, humidity, current density stress distribution
Mert Taş [28] 3D temperature, humidity performance optimization
Shen [27] 3D temperature, humidity, current density performance optimization
Ouaidat [36] 3D thermal, electrical, mechanical performance optimization
Zhang [23] 3D humidity, deformation performance optimization
Based on these analyses, it can be seen that temperature, humidity, and assembly force all greatly affect the fuel cell electrochemical performance, which are coupled together to act on the contact pressure and the GDL porosity. However, the current studies on these factors do not take the non-uniform distribution and the dynamic variance of temperature and humidity into account. There are certain differences between the actual situation and the numerical model results, which cannot fully reflect the complex state inside the fuel cell, not to mention also optimization.

References

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