Research On Dynamic Measurements And Performance Optimization Of Proton Exchange Membrane Fuel Cell

The high cost and the poor durability are two main barriers to the large-scale commercialization of proton exchange membrane fuel cells.In order to improve the cell performance and enhance its durability,it is necessary to study the dynamic behavior and failure mechanism of fuel cells furtherly and put forward practical countermeasures to alleviate the degradation of cell performance and life as much as possible.At present,domestic and foreign researchers have conducted a large number of studies on the internal behaviors of small single fuel cells,however,without studying the coupling behavior in the fuel cell simultaneously.In particular,there is no published literature report on in-situ measurements of multi-physical parameters in a commercial-size fuel cell or fuel cell stack simultaneously.In this dissertation,an in-situ dynamic measurement system that suitable for current density,temperature,relative humidity and pressure measurements simultaneously in commercial-size fuel cells was developed.Adopting the in-situ measurement system,the effect of operating conditions on the fuel cell performance was studied,the internal behavior and failure mechanisms of the fuel cell were revealed,and the corresponding mitigation strategies were put forward.The main research work of this dissertation are as follows:The detection requirements of the fuel cell dynamic measurement system was identified based on the research purposes.The commercial fuel cell with an active area of 200 cm² was developed.The segmented in-situ measurement bipolar plates were developed and the corresponding detection principles for current density,temperature humidity and pressure measurements were presented in detail.In addition,a high-precision data acquisition system for the fuel cell was developed,which provides a research basis for the subsequent dynamic measurements and performance optimization of the fuel cell.In addition,the influences of different operating conditions on the cell performance were researched via in-situ measurements.The hydrothermal management effect and coupling characteristics inside the cell were analyzed under different gas arrangements and gas relative humidities?RHs?.The distribution characteristics of current density,temperature,relative humidity and pressure under different operating conditions were revealed and studied.Besides,the operating conditions under different operating modes were optimized by adopting the orthogonal experiments.The results show that the optimal operating temperature and gas RH of the cell are 70 ? and 80%,respectively.Under low current density,the cell with counter-flow mode has a better performance and internal consistency.However,the cell in co-flow mode has a better performance and hydrothermal management effect under high current density.Under start-up operation,the optimal level of the fuel cell temperature is 60 ?,the optimal level of RH of the inlet gases is 60% and the optimal level of air stoichiometric ratio is 2.0.Under other operating modes,the optimal level of the fuel cell temperature is 70 ?,the optimal level of inlet gases RH is 80% and the optimal level of air stoichiometric ratio is 2.5.Moreover,the dynamic process of voltage reversal was studied systematically by employing the in-situ dynamic measurement system.The failure mechanism of reversal tolerant anode?RTA?was revealed through a series of rigorous experiments.Besides,the relative humidity that benefits the RTA durability during voltage reversal process was optimized.The optimization design method of RTA membrane-electrode assembly?MEA?was proposed based on the research findings for enhancing the cell durability.It was found that the failure of the RTA MEA was caused by the destruction of the electron conduction pathway,rather than the deactivation of IrO₂.The catalyst coated membrane,gas diffusion layer and bipolar plate on the anode side were corroded in different degrees after the voltage reversal testing.An RH over 55% adversely affects the durability of RTA when hydrogen starvation occurs.While maintaining the same content of IrO₂ catalyst,increasing the content of IrO₂ in the high RH area by gradient design of RTA MEA can improve the voltage reversal durability of fuel cells effectively.Finally,the shutdown processes of the fuel cell were studied deeply adopting the in-situ dynamic measurement system.The shutdown processes under three kinds of shut-down loads were compared.The influences of different shut-down loads on the cell internal consistency and durability were evaluated.Also,the combined shutdown strategy was proposed for improving the cell internal uniformity and prolonging cell life.The results show that adopting the combined shut-down strategy of constant power load and constant current load can not only shortens the shut-down time,but also reduces the reverse current and cathode high potential in the cell effectively.Besides,the fuel cell also obtained a good internal uniformity when adopting the combined shut-down strategy.