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（青岛大学浮山校区 山东青岛 266071）
ABSTRACT：Proton exchange membrane fuel cells (PEMFC) are promising in energy efficiency,high energy density, no liquid electrolyte corrosion, low temperature quick starting,compacting structure which made it has a wide application prospect in electric vehicles,underwater submarine and communication power. This paper is mainly reviewed the the working principle of the proton exchange membrane fuel cell . The test method of durability and the degradation mechanism are analyzed. More research needs to be done to study the durability of PEMFC.
Keywords: proton exchange membrane fuel cells ; durability; degradation mech
1 Introdutcion
The Proton exchange membrane fuel cell(PEMFC) is considered as an environmentally friendly power generation device.Compared with some conventional powder devices, it has high current density, high energy efficiency, and the product is water, so it has no pollution to the environment and is considered to be environmentally friendly.PEMFC has a wide range of application,both as a power source for automotive, aerospace, unmanned aerial vehicles, as well as for power generation, home heat supply, backup power, etc. However, the durability of PEMFC batteries has always been a hindrance to the commercial production of fuel cells. For PEMFC to meet commercial needs, life expectancy needs to meet certain conditions. When used as a power source, the minimum life expectancy of PEMFC is 5000h. When used for distributed generation, the life expectancy must be at least 4000h.
In order to achieve the maximum benefits, some scholars have been investigating the improvement durability of PEMFC.This paper is a review of recent work done on accelerated stress tests in the study of PEMFC fuel cell durability, with a primary focus on the main components of the membrane electrode assembly (MEA). zhang reported the recent work of accelerated stress tests in the study of PEM fuel cell durability,n, different degradation mechanisms proposed in recent publications are briefly reviewed. Litster focused on the catalyst layer in a very constructive manner but presented a limited discussion on the gas diffusion layer. Xiao reviewed test protocols of polymer electrolyte membrane fuel cell. Wang analyzed the durability of Nafion as electrolyte. The results show that scanning 1000circle CV in acidic solution environment is an ideal testing method of life.
The intent of this paper is to review the durability of the proton exchange membrane fuel cell. The working principle of PEMFC is summarized in this paper. The test method of durability and the degradation mechanism are analyzed. At present, the research focus of fuel cell durability mainly includes key components such as exchange membrane, electrode, catalyst and gas diffusion layer.
2 Principle of PEMFC
PEMFC is a device that directly converts chemical energy into electrical energy, using hydrogen as fuel and oxygen as oxidant. When the PEMFC is working, hydrogen diffuses through the gas diffusion layer to the anode catalytic layer. Under the catalytic action of the catalyst, hydrogen loses electrons to form H+, H+ passes through the proton exchange membrane, and passes from the anode to the cathode. The electrons first diffuse through the catalytic layer to the gas. The diffusion layer is transferred to the bipolar plate, and finally the external wire connected to the collector plate migrates to the cathode; at the cathode, oxygen diffuses through the gas diffusion layer to the cathode catalytic layer, and under the catalytic action of the catalyst, oxygen is obtained from the anode. The migrated electrons and H+ react to form water, which is discharged with the reaction tail gas. Electronic migration outside the PEMFC and internal H+ migration ensure proper battery operation.
Usually, the output voltage of a single PEMFC is only about 0.7V. In order to meet the needs, in practice, multiple PEMFCs are connected in series or in parallel to form a battery stack. In this way, tens or even hundreds of MEAs can be stacked together. Since this is a modular structure, it is easy to achieve various capacity requirements, and maintenance and maintenance.The output of the PEMFC battery stack is DC. When powering the AC equipment, it needs to be inverted into AC.
3 Durability test method
Due to the numerous factors affecting the degradation of PEMFC performance, the degradation mechanisms of the various components involved are different, a large number of experiments has been done both at home and abroad.
Lu investigated a a voltage degradation model for PEMFC tack used in a fuel cell bus.Phase I onsists of idling running, dynamic load running, partial power running and phase II consists of full power running and overload running. One driving cycle consists of four phase I and one phase II, and the idling condition and full power condition account for a higher proportion , and the idling condition and full power condition account for a higher proportion. Hou introduced a 60KW fuel cell engine developed in DICP. In order to get higher efficiency , the FCE was operated under different pressure. Hou conducted a study with a 20-cycle test under this conditions, the engine performance is stable by comparing the voltage values at several output powers. The voltage standard deviation is 2.17 volts at rated output power. Huang studied the durability the PEMFC under open circuit voltage operation and relative humidity cycling operation. The influence of proton exchange membrane on thed durability of membrane electrode assembly (MEA) has investigated under OCV operation and the mechanical degradation of the membrane has studied under relatibe humdity cycling operation. Finally , the influence of back pressure on the durability of MEA has investigated under OVC operation. The results indicates the durability of PEMFC has been improved.
Standard durability assessment procedures are important for research in PEMFC , and advanced research in the United States has driven research into the durability of fuel cells and their critical components.In 2007, the US Department of Energy released the PEMFC's durability testing standard test procedure, with the main purpose of testing the results of its hydrogen energy program. This standard process is divided into two parts, one for the fuel cell as a whole and the other for testing the various components of the fuel cell. Based in the powder demands from a car driving , the dynamic stress test was developed in order to assess the long -time durability of PEMFC. CAFE is accept on the automotive industry which is a fuel emission and economy standard of USA. The highway driving cycle and urban driving cylces constitute the CAFE. Testing the durability of PEMFC stack with CAFE can examine the fuel cells degradation under different typical conditions, such as dynamic load conditions, start-stop conditions, idle state, full power conditions.
4 The degradation mechanism
The study of degradation mechanisms is a prerequisite for us to mitigate performance degradation. The study of degradation mechanism is mainly based on the degradation mechanism of various components, mainly degradation of proton membrane, degradation of catalyst and degradation of electrodes.
4.1 Proton exchange membrane degradation
The role of the proton exchange membrane is to conduct protons while sequestering H2 and oxidizing agents. Proton exchange membranes have excellent conductivity, low fuel permeability, high thermal stability, good mechanical properties and electrical insulation. The commonly used proton exchange membrane is the perfluorosulfonic acid (PFSA) proton exchange membrane developed by DuPont Company.The PFSA proton exchange membrane consists of a PFSA resin with a hydrophobic polytetrafluoroethylene as the backbone and a hydrophilic sulfonic acid group as a branch.The PFSA proton exchange membrane has excellent proton conductivity and low fuel permeability. However, the sulfonic acid group in the PFSA proton exchange membrane is unstable when the temperature is too high, and the sulfonic acid group in the SA proton exchange membrane is easily attacked by free radical attack during the actual use of the PEMFC, and the durability of the PEMFC is lowered. Sex. Therefore, the development of a new composite proton exchange membrane based on PFSA resin has become the focus of research.Mcdonald's study found that after proton exchange membranes undergo 350 temperature cycles of 80 ° CC to -40 ° C, the proton exchange membrane has a decreased proton conductivity, a large gas permeability, and a decrease in mechanical strength. When the proton exchange membrane forms a pinhole due to mechanical attenuation, the permeate flux of the reaction gas is increased, and the reaction gas is directly burned, resulting in enthalpy. The local high temperature on the surface of the catalytic layer or the chemical decay of the proton exchange membrane accelerates the attenuation of the MEA.
4.2 Catalytic layer attenuation
The attenuation of the catalytic layer includes the attenuation of the catalyst in the catalytic layer, the corrosion of the catalyst carbon carrier, and the degradation of the fion resin in the catalytic layer. The attenuation of the three portions leads to a decrease in the electrochemical reaction surface of the catalytic layer and a decrease in the performance of the PEMFC.
The Pt particles are easily affected by impurity gases in the reaction gas, such as gases such as CO and SO2. Gases such as CO and SO2 poison the Pt particles and reduce the reactivity of Pt, resulting in reversible decay or irreversible decay of Pt, and even sintering, migration, and shedding. J.Xie found that after the MEA section running 1000h, the particle size of the Pt particles at the interface increases, and the shape of the EF particles changes. The change in the shape of the particles may be caused by the migration of ions of Pt to the interface between the film and the catalytic layer. It is also observed that the Pt particles in the interior of the film are also recrystallized from the migration of Pt ions into the film .The carbon carrier is a commonly used catalyst carrier, and the carbon carrier can raise the specific surface area of the catalyst, reduce the agglomeration of the catalyst particles, and improve the utilization rate of the catalyst. Typically, the carbon support is a spherical particle obtained by pyrolysis of different classes, having a diameter of less than 30 nm. Theoretically, when the potential is greater than 0.207V, the carbon carrier will corrode.Nafion resin is an important part of the MEA catalytic layer, usually accounting for 30%-40 %wt and its content has a critical influence for MEA performance. Xiao used sulfonated polyetheretherketone as a proton exchange membrane to avoid proton exchange.The decay of the membrane changes to produce fluoride ions, which has an effect on the degradation of Nafion in the catalytic layer. It is found that Nafion in the catalytic layer will also degrade to generate fluoride ions due to free radicals, resulting in a decrease In the ECSA of the MEA and a decrease in performance.
4.3Attenuation of the gas diffusion layer
The gas diffusion layer is generally composed of two layers of bonding, one layer of which is highly conductive and porous.
The dielectric layer, the other layer is a microscopic pore layer composed of carbon particles and Teflon tape, which is a key component of electron conduction and water transfer. Many researchers have recognized that in the life of fuel cell experiments, the gas diffusion layer is intrinsically linked to changes in the state of the water (mainly micron-nanoscale changes). During the experiment, the operating environment of the fuel cell gradually changed the GDL from hydrophobic to hydrophilic, which in turn affected the operation of the fuel cell. For example, the convection of gas and the diffusion of gas in the later stage of the experiment interfered with each other. Pierre found that after running 1000h, the hydrophobicity of the gas diffusion layer is lowered. Borup reported that The higher the operating temperature of PEMFC, the faster the hydrophobicity of the gas diffusion layer decreases. It is reported that the decrease in hydrophobicity. Wood immersed the gas diffusion layer in liquid water of different oxygen contents. The results show that the higher the oxygen content and temperature of the water, the worse the hydrophobicity after the gas diffusion layer is immersed for a period of time.of the gas diffusion layer is mainly due to the attenuation of the microporous layer.
5 conclusions
To conclude, the working principle of PEMFC is summarized in this paper. The test method of durability and the degradation mechanism are analyzed.
The durability study of PEMFC is mainly paving the way for its commercialization. More research needs to be done to improve the durability of PEMFC. The degradation mechanism of the PEMFC is the focus of the future research.
References：
[1]Dai W , Wang H , Yuan X Z , et al. A review on water balance in the membrane electrode assembly of proton exchange membrane fuel cells[J]. International Journal of Hydrogen Energy, 2009, 34(23):9461-9478.
[2]Ralph T R , Hards G A , Keating J E , et al. Low cost electrodes for proton exchange membrane fuel cells: Performance in single cells and Ballard stacks[J]. Cheminform, 1998, 29(10):3845-3857.
[3]Yuan X Z , Li H , Zhang S , et al. A review of polymer electrolyte membrane fuel cell durability test protocols[J]. Journal of Power Sources, 2011, 196(22):9107-9116.
[4]Park J, Oh H, Ha T, et al. A review of the gas diffusion layer in proton exchange membrane fuel cells: Durability and degradation[J]. Applied Energy, 2015, 155:866-880.
作者简介：张昊 1993.06.29 男 汉 黑龙江省牡丹江市 硕士 青岛大学 SOFC电池连接体
论文作者:张昊
论文发表刊物:《科技研究》2018年10期
论文发表时间:2019/1/4
标签：青岛大学论文; 浮山论文; 牡丹江市论文; 期刊论文; 黑龙江省论文; 尽在论文; 校区论文; 《科技研究》2018年10期论文;