The Study Of Catalysis And Device For Alkaline Polymer Electrolyte Fuel Cells

The proton exchange membrane fuel cells(PEMFCs)are currently the most mature type of fuel cells,with the advantages of high power density and good stability.But the acidic working environment makes it difficult to get rid of the dependence on precious metal catalysts,which will hinder its large-scale application.The alkaline polymer electrolyte fuel cells(APEFCs)provide the possibility to solve this problem,therefore being brought into focus.Recently,some good progress has been made in the research of alkaline polymer electrolytes(APEs),whose ion conductivity has been comparable to that of Nafion.However,in the aspect of the devices performances,the gap between APEFC and PEMFC is still large.The anode and cathode reaction formulas are quite different between APEFC and PEMFC,explaining the different states of water production and consumption.Water management issues may be one of the causes of that gap.The factors,such as the anode and cathode humidifying conditions,and the thickness of the membrane and the catalyst layer,which effects on the APEFC performances,were investigated.The results showed that water flooding in anode and water depletion in cathode,may be problems for APEFCs.For alleviating these problems,some strategies should been adopted to enhance the water transport ability of the APE membrane,including thinning the thickness of membrane and improving the intrinsic water diffusion coefficient of membrane.This study offered new ideas for the APE materials design.After water management optimization,the performance of APEFC was still significantly inferior to that of PEMFC.According to some literatures,this gap may be caused by the disparity of the hydrogen oxidation reaction(HOR)kinetics.Under alkaline condition,the HOR catalytic activity of PtRu is superior to that of Pt.By replacing the anode catalyst from Pt/C to PtRu/C,the peak power density of APEFC was nearly doubled,boosted to 1.0 W cm-2,the highest in the reports of same period.To clarify the debate of HOR mechanism in alkaline media,CO-stripping experiment was carried out,indicating that the improvement of PtRu could hardly be explained as the bifunctional effect.Rather,revealed by the voltammetric(CV)behavior and DFT calculations,the incorporation of Ru had posed an electronic effect on weakening the Pt-Had interaction,which thus benefitted the HOR kinetic in alkaline media.These findings cast new light on the exploration of better anode catalyst for APEFCs.In the short term,it is still a big challenge for APEFC anodes to use non-precious metal catalysts.Nevertheless,the situation is much better for APEFC cathodes,in which some kinds of non-precious metal catalysts have been verified suitable.Among them,the materials of N-doped carbon exhibited excellent activity in the rotating disk electrode(RDE)tests of catalyzing alkaline ORR.However,when used as the APEFC cathode catalysts,the performances of them were far from expected.Ag/C and CoO/C were synthesized,and then been used as the cathode catalysts of APEFCs,with the peak power densities of 0.50 W cm-2 and 0.43 W cm-2,respectively,not inferior to that of N-doped carbon materials.In the APEFC tests,Ag/C and CoO/C showed the advantages of conductivity and catalytic activity,respectively.To combine their advantages,Ag/CoO/C composite catalyst was synthesized,and then been used in APEFC cathode,with the peak power density close to 0.7 W cm-2.This performance demonstrated the good potential for using non-precious metal catalysts in APEFC cathode.In the RDE tests of catalyzing alkaline ORR,Ag/CoO/C composite catalyst also exhibited better activity than Ag/C and CoO/C.The CV behaviors,TEM and XPS characterizations,pointed out that CoO could change the electronic structure of Ag,whose adsorption strength for oxygen species was enhanced,to promote the ORR activity improved.Considering that Ag belong to noble metal,and transition metal oxides had exhibited good ORR activity under alkaline condition,manganese cobalt spinel(MCS)was attempted as APEFC cathode catalyst.The Mn/Co ratio in the MCS synthesis was optimized.When that ratio was 1,the performance of MCS catalyst was best.In the RDE tests,MCS performed significantly inferior to Pt.However,in the APEFC tests,under the conditions of low inlet humidity or high current density,the MCS exhibited better performance than Pt and reached a higher peak power density of 1.1 W cm-2,which never achieved by APEFCs with non-precious cathode catalysts.Kinetic isotope experiment showed that the electron transfer rate and the proton transfer rate both made effects on the ORR kinetic under alkaline condition.That performance contrast was caused by the different contents and states of H2O in RDE and APEFC tests.Characterizing the crystal structure and surface properties of MCS,and combined with DFT calculations,the different roles of Mn and Co sites on the MCS surface were pointed out.When catalyzing ORR under alkaline condition,the Mn sites function in binding O2,the Co sites in close proximity active H2O so as to facilitate the proton-transferred reduction of O2.Such a synergistic bifunctional mechanism was pivotal to the high rate of ORR,in particular under water depletion conditions.These findings were not only of fundamental significance to the understanding of ORR catalysis and relevant material design,but also a solid step towards the development of APEFC technology.In order to study the alkaline ORR catalytic mechanism of metal oxides more thoroughly,the lithium manganese oxygen spinel(LMO)catalysts were adopted,whose crystal structures were flexible.The electrochemical tests exhibited good alkaline ORR activity of LMO,which had never been reported.The LMO materials with different Mn/Li ratio were prepared.Through structure characterizations and activity tests,combined with DFT calculations,the structure-property-activity relationship of LMO catalyzing alkaline ORR was established.Using the LMO of best activity as APEFC cathode catalyst,the peak power density was up to 0.8 W cm-2,an excellent level in the literatures.In the RDE tests,the activity of LMO was slightly superior to that of MCS.But used in the APEFC cathodes,its performance was significantly inferior to MCS.This contrast added a new experiment evidence for the synergistic bifunctional mechanism of MCS catalyzing alkaline ORR.