The accelerated degradation in the front cells of a polymer electrolyte membrane fuel cell(PEMFC)stack seriously reduces the reliability and durability of the whole stack.Most researches only focus on the size and configuration of the gas intake manifold,which may lead to the maldistribution of flow and pressure.In order to find out the mechanisms of the accelerated degradation in the front cells,an extensive program of experimental and simulation work is initiated and the results are reported.It is found that after long-term lifetime tests the accelerated degradation in the front cells occurs in all three fuel cell stacks with different flow-fields under the U-type feed configuration.Compared with the rear cells of the stack,the voltage of the front cells is much lower at the same current densities and the membrane electrode assembly(MEA)has smaller active area,more catalyst particle agglomeration and higher ohmic impedance.For further investigation,a series of three dimensional isothermal numerical models are built to investigate the degradation mechanisms based on the experimental data.The simulation results reveal that the dry working condition of the membrane and the effect of high-speed gas scouring the MEA are the main causes of the accelerated degradation in the front cells of a PEM fuel cell stack under the U-type feed configuration.Several mitigation strategies that would mitigate these phenomena are presented:removing cells that have failed and replacing them with those of the same aging condition as the average of the stack;choosing a Z-type feed pattern instead of a U-type one;putting several air flow-field plates without MEA in the front of the stack;or exchanging the gas inlet and outlet alternately at a certain interval.This paper specifies the causes of the accelerated degradation in the front cells and provides the mitigation strategies.
LI PengchengPEI PuchengHE YonglingYUAN XingCHAO PengxiangWANG Xizhong
Sn-doped TiO_2 nanoparticles with high surface area of 125.7 m^2·g^(-1) are synthesized via a simple one-step hydrothermai method and explored as the cathode catalyst support for proton exchange membrane fuel cells.The synthesized support materials are studied by X-ray diffraction analysis,energy dispersive X-ray spectroscopy and transmission electron microscopy.It is found that the conductivity has been greatly improved by the addition of 30 mol%Sn and Pt nanoparticles are well dispersed on Ti_(0.7)Sn_(0.3)O_2 support with an average size of 2.44 run.Electrochemical studies show that the Ti_(0.7)Sn_(0.3)O_2 nanoparticles have excellent electrochemical stability under a high potential compared to Vulcan XC-72.The as-synthesized Pt/Ti_(0.7)Sn_(0.3)O_2 exhibits high and stable electrocatalytic activity for the oxygen reduction reaction.The Pt/Ti_(0.7)Sn_(0.3)O_2 catalyst reserves most of its electrochemically active surface area(ECA),and its half wave potential difference is 11 mV,which is lower than that of Pt/XC-72(36 mV) under 10 h potential hold at 1.4 V vs.NHE.In addition,the ECA degradation of Pt/Ti_(0.7)Sn_(0.3)O_2is 1.9 times lower than commercial Pt/XC-72 under 500 potential cycles between 0.6 V and 1.2 V vs.NHE.Therefore,the as synthesized Pt/Ti_(0.7)Sn_(0.3)O_2 can be considered as a promising alternative cathode,catalyst for proton exchange membrane fuel cells.
Yuan GaoMing HouZhigang ShaoChangkun ZhangXiaoping QinBaolian Yi
Supercapacitors(SCs) have attracted much attention as one of the alternative energy devices due to their high power performance,long cycle life,and low maintenance cost.Graphene is considered as an innovative and promising material due to its large theoretical specific surface area,high electrical conductivity,good mechanical properties and chemical stability.Herein,we report an effective strategy for elaborately constructing rationally functionalized self-standing graphene(SG) obtained from giant graphene oxide(GGO) paper followed by an ultrarapid thermal-processing.This treatment results in both the exfoliation of graphene sheets and the reduction of GGO by elimination of oxygencontaining groups.The as-prepared SG electrode materials without additive and conducting agent provide an excellent combination of the electrical double layer capacitor(EDLC) and pseudocapacitor(PC) functions and exhibit superior electrochemical performance,including high specific capacitance,good rate capability and excellent cycling stability when investigated in three-electrode electrochemical cells.
Pt/WO3/C nanocomposites with parallel WO3 nanorods were synthesized and applied as the cathode catalyst for proton exchange membrane fuel cells(PEMFCs). Electrochemical results and single cell tests show that an enhanced activity for the oxygen reduction reaction(ORR) is obtained for the Pt/WO3/C catalyst compared with Pt/C. The higher catalytic activity might be ascribed to the improved Pt dispersion with smaller particle sizes. The Pt/WO3/C catalyst also exhibits a good electrochemical stability under potential cycling. Thus, the Pt/WO3/C catalyst can be used as a potential PEMFC cathode catalyst.
Meiling DouMing HouZhilin LiFeng WangDong LiangZhigang ShaoBaolian Yi
We differentiated the effects of Cu films deposited on single crystalline a-,r-,and c-plane sapphire substrates upon graphene films synthesized with atmospheric pressure chemical vapor deposition(CVD).The data illustrate that the realization of high-crystalline Cu film is dependent not only on the crystallinity of underlying substrate,but also on the symmetric match of crystallographic geometry between metal film and substrate.We also systematically investigated the effects of PMMA removal on the Raman ID/IG and IG/I2D values of transferred graphene.The results reveal that different PMMA removal methods do not alter the ID/IG values;instead,the residue of PMMA increases the IG/I2D values and the thermal decomposition of PMMA leads to higher IG/I2D values than the removal of PMMA with acetone.The effects of PMMA removal on variations of the Raman spectra are also discussed.
HU BaoShanWEI ZiDongAGO HirokiJIN YanXIA MeiRongLUO ZhengTangPAN QingJiangLIU YunLing
