Kinetics Of The Voltage Reversal Process In Proton Exchange Membrane Fuel Cells

The voltage reversal caused by H2 starvation in the anode will seriously reduce the performance and durability of a fuel cell,which is considered to be an important reason preventing the cell from reaching the long-life target.Therefore,it is urgently expected to reduce the damage of this accident by improving the voltage reversal tolerance of membrane electrode assembly(MEA).The water electrolysis plateau(WEP)time is the most important indicator to evaluate the voltage reversal tolerance of the MEA,therefore the industry has focused on improving the voltage reversal tolerance of the MEA by extending the time of the WEP.Generally,a certain amount of oxygen evolution(OER)catalyst(such as IrO2)is added to the anode catalyst layer to prepare a so-called reversal-tolerant anode(RTA).However,whatever type of RTA,the voltage will decrease significantly without warning after a while in the plateau area during the voltage reversal,which is called the failure of the WEP.Nowadays,there are three mainstream views about this failure mechanism:(1)lack of water in the anode catalyst layer,(2)physical deactivation of OER catalyst,and(3)chemical deactivation of OER catalyst.However,these three views are constantly controversial due to a lack of sufficient evidence,and there is no conclusion yet.Therefore,the further R&D of RTA is still facing huge challenges.In this work,a self-designed new test system is used to synchronously track the cell internal resistance(IR)changes during the process of voltage reversal,thereby monitoring the change of internal state inside the cell during the process of this accident,and further explore the failure mechanism of the WEP.The results have indicated that the main reason for the failure of the WEP is the significant increase in IR(from～180 mΩ·cm2 to～6360 mΩ·cm2)caused by carbon corrosion at the end of the WEP,rather than the three mainstream views mentioned above.To further understand the voltage reversal process and guide the R&D of RTA,a kinetic theoretical model of the WEP process is established using the charge conservation equation combined with the carbon oxidation and oxygen evolution equations based on the above studies.The main applications and conclusions are as follows:1.Since the WEP time is the most important indicator to evaluate the voltage reversal tolerance of an MEA,the factors affecting the WEP time are theoretically discussed based on the kinetic of the WEP process.The results have shown that:1)The WEP time can be significantly prolonged by increasing the OER catalyst loading during voltage reversal,which is due to the significantly reduced carbon corrosion rate during this process.2)The WEP time can be significantly reduced by increasing the operating temperature,which is due to the significantly increased carbon corrosion rate during this process.3)For conventional MEA,there has little effect on the extension of WEP time by increasing the graphitization degree of the anode carbon support.If it is an RTA with a certain amount of OER catalyst added to the anode catalyst layer,the use of higher graphitized carbon support can increase the WEP time,but the Uθ(critical kinetic potential of corrosion)of support must exceed 1.6 V.4)Under high current density or low H2 stoichiometry,the time of WEP is greatly shortened.2.Based on the kinetic of the WEP process and related experiments,it is found that even in the WEP,the carbon support still has a certain degree of corrosion,which in turn leads to large performance decay.The longer the time,the more obvious the decline.Therefore,the WEP time cannot be used as the only indicator to evaluate the voltage-reversal tolerance of the MEA.3.Based on the above research,the new evaluation indicator of an RTA is systematically proposed:WEP time,performance degradation rate,and cost.The kinetic of the WEP process have suggested that the breakthrough point for the development of low-cost and robust RTA at this stage is maximizing the utilization rate of IrO2.The utilization rate of IrO2 can be effectively improved by spraying an ultra-thin IrO2 layer on the anode microporous layer.Only 15μg·cm-2 IrO2 is used,the WEP time exceeds 250 min,and the voltage decay is only 4.7%(@90 min and@1000 mA·cm-2)by utilizing this technology.