聚苯胺增强Pt/C催化剂活性和稳定性研究
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摘要
目的制备一种具有高稳定性和高催化活性的Pt/C@PANI"核/壳"结构催化剂。方法利用原位化学氧化聚合法制备聚苯胺修饰Pt/C@PANI"核/壳"结构催化剂,采用循环伏安加速寿命实验,结合电化学活性表面积、氧还原活性、X-射线光电子能谱和透射电镜,考察所制备催化剂的活性和稳定性,通过密度泛函理论探究PANI增强Pt/C催化剂活性和稳定性的量子化学本质原因。结果电化学测试表明, Pt/C@PANI催化剂的催化活性与聚苯胺包覆层含量和厚度密切相关,Pt/C@PANI(30%)催化剂具有最好的催化活性,其质量比活性和比表面比活性分别为商业化Pt/C催化剂的1.6和1.8倍。加速寿命实验表明,Pt/C@PANI(30%)催化剂具有很好的稳定性,经过1500圈CV扫描后,其电化学活性表面积仅下降了30%,而商业化Pt/C催化剂降低了83%。理论计算表明,PANI将电子转移给载体C,导致自身空穴增加,PANI部分氧化,导电性增强;PANI的存在使Pt/C@PANI体系的HOMO能级升高,减小了与氧气分子LUMO能级的差异,有利于电子从催化剂HOMO转移到氧分子的LUMO轨道,使得氧容易得到电子;PANI吸附后,Pt原子d带中心显著降低,利于中间物种的脱附,催化活性更高。结论 PANI包覆层抑制了Pt纳米粒子在载体表面的迁移、团聚长大和溶解/再沉积,有效地解决了Pt/C催化剂的Ostwald肿大,催化剂的活性和稳定性得到显著提升。
Objective To prepare a Pt / C@ PANI core-shell catalyst with high catalytic activity and durability. Methods The polyaniline( PANI)-decorated Pt / C@ PANI core-shell catalyst was prepared by in situ chemical oxidation polymerization method.The stability and activity of the Pt / C@ PANI catalyst prepared were evaluated using accelerated durability test( ADT),along with the monitoring of electrochemical surface area( ECSA),oxygen reduction reaction( ORR) activities,X-ray photoelectron spectrometer( XPS) and transmission electron microscopy( TEM),and the density functional theory( DFT) calculations were carried out to investigate the essential reasons for the enhanced catalytic activity and durability of Pt / C catalyst by PANI. Results The experimental results demonstrated that the activity for the ORR strongly depended on the PANI content and the thickness of the PANI shell,and that the prepared Pt / C@ PANI( 30%) catalyst with 5 nm PANI exhibited a Pt mass and a specific activity of nearly 1. 6and 1. 8 times higher than those of the commercial Pt / C catalysts. The accelerated stability test showed that the ECSA of Pt / C@PANI( 30%) catalyst was only decreased by ~ 30%,whereas the Pt / C catalysts lost ~ 83% of the initial ECSA after 1500 cycles of CV scanning. The DFT calculations disclosed that the number of holes in PANI increased with the electrons transfer from PANI to the supporting C,which caused partial oxidation of PANI and thus strengthened the electric conductivity of PANI. The lifted HOMO energy level and lowered d band center of Pt / C@ PANI compared with Pt / C were beneficial to the electron transfer between Pt / C@ PANI and O2 due to the reduced gap between the Pt / C@ PANI HOMO and the oxygen LUMO,and the desorption of intermediate species on the surface of the catalysts and releasing fresh catalytic sites for the subsequent reaction. Conclusion The PANI-decorated layer could enhance the Pt-support interaction and help to inhibit Pt dissolution / re-deposition,aggregation and ripening,which effectively solve the problem of Ostwald enlargement of the Pt / C catalyst,resulting in significant enhancement in the activity and stability of the catalyst.
Objective To prepare a Pt / C@ PANI core-shell catalyst with high catalytic activity and durability. Methods The polyaniline( PANI)-decorated Pt / C@ PANI core-shell catalyst was prepared by in situ chemical oxidation polymerization method.The stability and activity of the Pt / C@ PANI catalyst prepared were evaluated using accelerated durability test( ADT),along with the monitoring of electrochemical surface area( ECSA),oxygen reduction reaction( ORR) activities,X-ray photoelectron spectrometer( XPS) and transmission electron microscopy( TEM),and the density functional theory( DFT) calculations were carried out to investigate the essential reasons for the enhanced catalytic activity and durability of Pt / C catalyst by PANI. Results The experimental results demonstrated that the activity for the ORR strongly depended on the PANI content and the thickness of the PANI shell,and that the prepared Pt / C@ PANI( 30%) catalyst with 5 nm PANI exhibited a Pt mass and a specific activity of nearly 1. 6and 1. 8 times higher than those of the commercial Pt / C catalysts. The accelerated stability test showed that the ECSA of Pt / C@PANI( 30%) catalyst was only decreased by ~ 30%,whereas the Pt / C catalysts lost ~ 83% of the initial ECSA after 1500 cycles of CV scanning. The DFT calculations disclosed that the number of holes in PANI increased with the electrons transfer from PANI to the supporting C,which caused partial oxidation of PANI and thus strengthened the electric conductivity of PANI. The lifted HOMO energy level and lowered d band center of Pt / C@ PANI compared with Pt / C were beneficial to the electron transfer between Pt / C@ PANI and O2 due to the reduced gap between the Pt / C@ PANI HOMO and the oxygen LUMO,and the desorption of intermediate species on the surface of the catalysts and releasing fresh catalytic sites for the subsequent reaction. Conclusion The PANI-decorated layer could enhance the Pt-support interaction and help to inhibit Pt dissolution / re-deposition,aggregation and ripening,which effectively solve the problem of Ostwald enlargement of the Pt / C catalyst,resulting in significant enhancement in the activity and stability of the catalyst.
引文
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[13]SHAO M H,LIU P,ZHANG J L,et al.Origin of Enhanced Activity in Palladium Alloy Electrocatalysts for Oxygen Reduction Reaction[J].Journal of Physical Chemistry B,2007,111:6772—6775.
[14]XIN H L,ADAM H,SULJO L.Predictive Structure Reactivity Models for Rapid Screening of Pt-based Multimetallic Electrocatalysts for the Oxygen Reduction Reaction[J].ACS Catalysis,2012,2:12—16.
[15]STAMENKOVIC V,MUN B S,MAYRHOFER K J J,et al.Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure[J].Angewandte Chemie International Edition,2006,45:2897—2901.
[16]LI L,WEI Z D,ZHANG Y,et al.DFT Study of Difference Caused by Catalyst Supports in Pt and Pd Catalysis of Oxygen Reduction Reaction[J].Science in China Series B—Chemistry,2009,52:571—578.
[2]FERREIRA P J,LA O'G J,HORN Y S,et al.Instability of Pt/C Electrocatalysts in Proton Exchange Membrane Fuel Cells:A Mechanistic Investigation[J].Journal of the Electrochemical Society,2005,152(11):A2256—A2271.
[3]ZHANG J,SASAKI K,SUTTER E,et al.Stabilization of Platinum Oxygen-reduction Electrocatalysts Using Gold Clusters[J].Science,2007,315:220—222.
[4]YU X W,YE S Y.Recent Advances in Activity and Durability Enhancement of Pt/C Catalytic Cathode in PEMFC:Part II:Degradation Mechanism and Durability Enhancement of Carbon Supported Platinum Catalyst[J].Journal of Power Sources,2007,172(1):145—154.
[5]SHAO Y Y,YIN G P,Gao Y Z.Understanding and Approaches for the Durability Issues of Pt-based Catalysts for PEM Fuel Cell[J].Journal of Power Sources,2007,171:558—566.
[6]CHEN Z W,WAJE M,LI W Z,et al.Supportless Pt and PtPd Nanotubes as Electrocatalysts for Oxygen-reduction Reactions[J].Angewandte Chemie International Edition,2007,119(22):4138—4141.
[7]CHEN S G,WEI Z D,LI H,et al.High Pt Utilization PEMFC Electrode Obtained by Alternative Ion-exchange/Electrodeposition[J].Chemical Communications,2011,47:10984—10986.
[8]DONTHU S,CAI M,RUTHKOSKY M,et al.Carbon-Titania Composite Substrates for Fuel Cell Catalyst Applications[J].Chemical Communications,2009,45:4203—4205.
[9]SHAO Y Y,YIN G P,WANG J J,et al.Durability Study of Pt/C and Pt/CNTs Catalysts under Simulated PEM Fuel Cell Conditions[J].Journal of the Electrochemical Society,2006,153:A1093—A1096.
[10]RYU J K,PARK C B.Synthesis of Diphenylalanine/Polyaniline Core/Shell Conducting Nanowires by Peptide Self-assembly[J].Angewandte Chemie International Edition,2009,48:4820—4823.
[11]CHEN R J,ZHANG Y G,WANG D W,et al.A Self-healing Oxygen-evolving Catalyst[J].Journal of the American Chemical Society,2001,123:3838—3939.
[12]KANGASNIEMI K H,CONDIT D A,JARVI T D,et al.Characterization of Vulcan Electrochemically Oxidized under Simulated PEM Fuel Cell Conditions[J].Journal of the Electrochemical Society,2004,151:E125—E132.
[13]SHAO M H,LIU P,ZHANG J L,et al.Origin of Enhanced Activity in Palladium Alloy Electrocatalysts for Oxygen Reduction Reaction[J].Journal of Physical Chemistry B,2007,111:6772—6775.
[14]XIN H L,ADAM H,SULJO L.Predictive Structure Reactivity Models for Rapid Screening of Pt-based Multimetallic Electrocatalysts for the Oxygen Reduction Reaction[J].ACS Catalysis,2012,2:12—16.
[15]STAMENKOVIC V,MUN B S,MAYRHOFER K J J,et al.Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure[J].Angewandte Chemie International Edition,2006,45:2897—2901.
[16]LI L,WEI Z D,ZHANG Y,et al.DFT Study of Difference Caused by Catalyst Supports in Pt and Pd Catalysis of Oxygen Reduction Reaction[J].Science in China Series B—Chemistry,2009,52:571—578.