燃料电池实车应用背景动态工况耐久性和稳态工况耐久性结果对比与在线分析

动态工况和稳态工况相比，稳态工况的最终性能一定更优么？

PEMFC Durability Test under Specific Dynamic Current Solicitation， Linked to a Vehicle Road Cycle

F． Harel

X． François

D． Candusso

M．－C． Péra

D． Hissel

J．－M． Kauffmann

Abstract

The Laboratory of Electrical Engineering and Systems （L2ES） is concerned with fuel cell system durability and reliability． Since the lab is predominantly involved in the field of transport applications， one of the objectives is to study PEMFC ageing in different specific environments linked to electric vehicle operation．

The first phase of the durability program was devoted to the reference test of a 100 W three cell stack， operated in a stationary regime at roughly nominal conditions for 1，000 hours． The second phase， on which this paper is focused， is dedicated to the durability test of another 100 W stack placed under dynamical current constraints． The current cycle load applied is first computed from a standardized transportation mission profile （time vs． speed of vehicle） and then adapted to the power of the fuel cell tested． The gas supply strategy is defined and the work presented in this paper shows the importance of the gas flow management in providing highly dynamical current profiles． The experimental methodology is based on polarization curves as well as on electrochemical impedance spectroscopy measurements performed regularly throughout the ageing． The experimental results showing the degradation of the fuel cell performances are presented and analyzed．

Fig． 1 Control interface of the 1 kWtest bench – Dynamical mission profile configuration

Table 1 Technical specifications of the fuel cell．

Fig． 2 Evolution of the main physical quantities related to the fuel cell over the road cycle．

Fig． 3 Stack voltage vs． time．

Fig． 4 Display of polarization curve set recorded for 2／5 anode／cathode
stoichiometry rates．

The curves ”Hinit” and ”Hinit bis” correspond to the two polarization
curves performed before the start of the durability test．

H标记的是第几个小时。

Fig． 5 Display of polarization curve set using three dimensional shaded
surface （2／5 anode／cathode stoichiometry rates）．

Fig． 6 2D display of polarization curve set recorded for 2／5 anode／cathode stoichiometry rates．

Fig． 7 2D display of polarization curve set recorded for 2／4 anode／cathode stoichiometry rates．

Fig． 8 2D display of polarization curve set recorded for 2／3 anode／cathode stoichiometry rates．

Fig． 9 2D display of polarization curve set recorded for 1．5／3．5 anode／
cathode stoichiometry rates．

Fig． 10 For comparison， 2D display of polarization curve set recorded for 1．5／3．5 anode／cathode stoichiometry rates in the framework of the first ageing test （performed at 50 A constant current）．

Fig． 11 ‘Beginning of life’ impedance spectrum （“Hinit bis“ spectrum）．

交流阻抗的四段含义，图和解释比较清楚：

1an inductive part present at high frequencies （between 30 kHz and 5．5 kHz approximately）， which is due to the various FC connection elements and electric wires．
2a first capacitive arc （in the 5．5 kHz–4 Hz frequency range） that mainly corresponds to fast charge transfer phenomena （electrons and protons）．
3a second capacitive arc （for frequencies between 4 Hz and 0．05 Hz） related to slow mass transport （ions in the gaseous phase） and water transport． The size of this arc changes dramatically with the air stoichiometry rate．

4another inductive part （for frequencies below 0．05 Hz）．This inductive part may mainly be linked with the difficulty in – obtaining stable operation at very low frequencies（for instance， the impact of water temperature control over the FC voltage should be considered）． It may also be due to the presence of parallel impedances coming from the combined monitoring of the three cell voltages． This inductive part can also be seen and is theoretically explained in some electrochemical studies

Fig． 12 Set of impedance spectra recorded during the second durability test （under dynamical current solicitation）．

Fig． 13 Display of impedance spectra recorded during the second durability test using a three dimensional shaded surface．

Fig． 14 Display of impedance spectra recorded during the first durability test （performed at 50 A constant current） using a three dimensional
shaded surface．

Fig． 15 Some particular points of the impedance spectrum diagrams

Fig． 16 Evolution of FC internal resistance as a function of time．

Fig． 17 Evolution of FCinternal resistance vs． time（first ageing test， performed at constant current）．

Fig． 18 Evolution of FC polarization resistance as a function of time．

Fig． 19 Evolution of FC polarization resistance （first ageing test）．

Fig． 20 Modulus of points related to first arc vertexes．

Conclusion and Perspectives

The lifetime of existing stacks can be extended through the control of the various FC operational conditions． The scientific program that is carried out at the L2ES laboratory aims at evaluating the impact of the transport environment and determining the optimum FC system
operating conditions in order to enhance durability， while maintaining good performance． In particular， this project should result in lifetime improvements of PEMFC by seeking operational conditions that will prevent stack failure．
In this paper， an ageing test performed on a 100 W stack under specific dynamical load transients linked to a vehicle road cycle， has been described． The characterization methodologies and tools initially developed for the reference tests performed at constant nominal conditions ［1］ have been used within the framework of this latter experiment．

The work has shown the importance of gas flow management to provide high dynamical current profiles． Moreover，the FC durability is seen to be strongly linked to the flow strategy adopted． For example， in dynamical conditions， pressure drops can create a mechanical stress on the MEAs and the uneven flow and water distributions in the cells may lead to the appearance of hot spots． In reality， during the experiment，both the characterization of the FC investigated and also the proposed gas management strategy have been achieved． However， the early comparisons made between the first two ageing tests suggest that the average value of the current over the cycle has a very significant impact on the ageing process． Some additional tests and comparisons must be undertaken in order to better evaluate the respective effects of current dynamics and current average value on the FC ageing．

Future work will focus on durability and reliability studies of identical three cell PEMFC considering the influences of other physical factors （e．g．， stack temperature） or other gas management strategies （defined in real time if the mission profile is not known a priori）． The influences of current and flow dynamics， as well as the impacts of environment temperature， gas polluting and vibrating conditions will all be
studied separately thanks to the set of facilities available on the Belfort test platform． The laboratory equipment （1 kW test bench， impedancemeter） allows the characterization of the FC performance with accuracy and enables the indoor reproduction of solicitations linked to transport operating conditions（e．g．， climatic caisson and vibrating table）． Lifetime improvements of PEMFC will also be accomplished through stack testing and by conducting research that elucidates membrane failure mechanisms under transportation operating conditions． This will be achieved in particular， by the observation of electrical records performed on failed membranes， to discern the mode of failure or electrical signature corresponding to a failure．

Fundamental understanding of physical ageing mechanisms is a task for further experimental and theoretical work， which can be undertaken in collaboration with chemists or electrochemists．Indeed， a good knowledge of materials and powerful instrumentation are needed to perform the analyses （e．g．，microscopy， advanced x－ray or magnetic
resonance imaging） and the post－mortem characterizations of MEAs． The correlation between characterization data on virgin and aged stacks should lead to more focused material development strategies．
The development of accelerated ageing tests and lifetime predictor
procedures is another task for the future， especially for bus and light railway applications since the required 20，000 hours or more of testing， are often not obtainable during shorter length projects．

in the case of the first ageing test， performed at 50 A constant current

This difference between the degradation rates observed for the two ageing tests is probably due to the difference between the average current levels of 12．5 A and 50 A， respectively， for the first and second tests．。

稳态工况是作者的会议论文，这篇文章关于动态工况。后面这段话写反了。动态工况和稳态工况700hr相比，稳态工况的性能更差一些，动态工况1．6V＠50A，稳态工况1．6V＠40A。

作者也说：

Some additional ageing tests have to be performed in the future for different current dynamics and current average values in order to clearly conclude on the relative effects of these two parameters
on the FC ageing．