A Stability Study Of Alkaline Polymer Electrolyte For Fuel Cell Application

The advent of“hydrogen future”has promoted the development of hydrogen fuel cell.Researchers have high expectations of hydrogen fuel cell,especially when it is used as vehicle power source and portable power source.As the most mature type of fuel cell,proton exchange membrane fuel cell?PEMFC?has implemented a small-scale application so far.However,the selection of catalyst is restricted among precious metal under strong acid condition.The high cost of catalysts and limited resource of precious metal hinder the widespread application of PEMFC.Alkaline polymer electrolyte fuel cell?APEFC?reserves the advantages of PEMFC,including room temperature startup,high power density and compact structure.Moreover,the alkaline media raise the possibility of employing nonprecious mental catalysts,which brings growing attention to APEFC.As an important component of APEFC,alkaline polymer electrolyte?APE?plays a vital role in the development of APEFC.Over the last decade,significant progresses have been acquired.There are a series of APEs,which possess the advantages of both high ionic conductivity and excellent chemical stability.However,a large part of the APEs exhibit poor fuel cell performance and bad durability of APEFC.This might be caused by the dynamic change of water uptake of membrane.As is well-known,the water generation and consumption is happening on the anode and cathode when the fuel cell is in function.As a result,the membrane swellings in high humidity and shrinks in low humidity.The frequent changes in size decrease the mechanical strength of the APE membrane,which has impact on fuel cell durability.Starting with poly?ether ether ketone??PEEK?which shows excellent durability of alkaline and good mechanical property,a construction of phase separation channel is carried out by polymer structure design to disentangle such a contradiction of high ionic conductivity and low swelling degree.A new approach named“water induced phase segregation”is employed.The molecular dynamics simulation and experiments are both used to verify the feasibility of this method.It turns out to be that introduction of little water can effectively induce the ordered arrangement of cation,which lead to an improvement of ionic conductivity.Although constructing phase separation channel is a good way to balance high ionic conductivity and low swelling degree,the swelling degree still can't meet requirements.Then,a kind of reinforcing material named sulfonated nano bamboo fiber?s-NBF?is introduced.The QAPEEK/s-NBF composite membrane shows low swelling degree at 7.5%,which remains constant as the temperature increases.Moreover,the sulfonic acid group attached to s-NBF makes the quaternary ammonium groups arranged partially orderly.It is beneficial to ionic conductivity of composite membrane,which turns out to be 120 mS cm^-11 at 80^oC.Meanwhile,it exhibits a good fuel cell performance at 0.93 W cm^-2,which is superior to other composite membrane.Then the chemical stability and durability of APEFC is tested.The result proves that the APE based on PEEK backbone is unstable.Upon using rigid backbone of engineering plastics,the chemical stability and flexibility of APEs are unsatisfactory,especially when the water uptake changes are unavoidable.Thus,the flexibility of membrane should be taken into consideration.As a block polymer which is extremely flexible and highly stable,SEBS is chosen to be the backbone of APE.However,the SEBS-C₆-TMA membrane has poor mechanical strength,which means it is unable to meet the requirement of APEFC.To address the problem,porous thin film?tetrafluoroethylene,PTFE?is applied.The SEBS-C₆-TMA@PTFE composite membrane has advantages in mechanical strength and flexibility,which makes it possible to manufacture ultrathin membrane.The reduce of thickness can drop the resistance of the membrane,in this way,the fuel cell performance and water transport are also improved.However,with the using of QAPPT ionomer,the cell performance is poor,only 0.6 W cm^-2 in peak power density.The chemical stability is confirmed by FT-IR and the changes of IEC,which turn out to be satisfactory.It seems that the reason of voltage decay lies in the structure of electrode and the surface of electrode and membrane.For ionomer study,the polystyrene is selected to be modified into Pst-C₆-TMA.The molecular weight of ionomer and preparation conditions of membrane electrode is adjusted to reconstruct the structure of catalyst layer.The ion exchange capacity?IEC?is changed to adjust the surface wettability of catalyst layer.The different mass ratio of catalyst and ionomer is carried out to construct better three-pahse interface.These factors are acknowledged to have effect on fuel cell performance.After optimizing of these factors,the fuel cell performance is up to 1.03 W cm^-2.Furthermore,the factor which caused the bad durability of fuel cell is revealed to be the losing of ionomer.To improve the durability of APEFC,the structure of membrane electrode should be maintained and the surface between catalyst layer and membrane should be well.Hot pressing is regarded as effective method to alleviate the surface problem.An appropriate temperature promotes the mobility of polymer chains,which is expected to increase the polymer interaction between ionomer and membrane.It is showed that the high frequency impedance of APEFC is reduced.Whereas,hot pressing makes the membrane electrode more compact,which directly hinder the transfer of gas and water.The performance and durability of APEFC isn't as good as expected.It seems that the losing of ionomer and the maintaining of membrane electrode structure should be taken into account at the same time.Then the fluoride acetylene black?F-ACET?is introduced to anode.F-ACET has potential in improving durability of APEFC,but the poor electrical conductivity has severely negative impact on cell performance.Afterward,acetylene black?ACET?is used instead.High electrical conductivity of ACET keeps the cell in good function and the durability of APEFC is significantly improved.This research has not only furthered the understanding of importance of membrane electrode structure,but also provides an efficient strategy for the study of APEFC durability.