Pt_(x)Cu_(y)/C电催化剂甲醇氧化反应性能及机理研究

Performance and Mechanism of Pt_(x)Cu_(y)/C Electrocatalyst for Methanol Oxidation

直接甲醇燃料电池(DMFC)是一种将甲醇燃料的化学能直接转化为电能的能量转换装置,具有能量转化效率高、环境友好、燃料来源丰富等优势,在移动电源等领域具有广泛应用前景,但阳极铂基电催化剂的性能及成本制约着DMFC的发展。本论文通过简单的液相浸渍还原法,制备了系列PtCu/C纳米电催化剂,电化学性能测试结果表明,电催化剂对甲醇氧化反应(MOR)活性顺序为商品Pt/C<Pt_(3)Cu/C<PtCu_(4)/C<PtCu/C<PtCu_(3)/C,且活性最高的PtCu_(3)/C电催化剂表现出较为优异的电化学稳定性。结合物相表征、电化学测试及DFT计算,阐释了PtCu_(3)/C催化剂中存在的少量CuO相能够促进水分子解离产生*OH,通过双功能机制促进类CO反应中间物种氧化为CO_(2)。因此,相比于商品Pt/C,虽然PtCu_(3)/C电催化剂的ECSA不足其一半,但质量比活性和面积比活性分别提高1.88倍和3.74倍。

Direct methanol fuel cell(DMFC),which directly converts the chemical energy of methanol fuel into electrical energy,has the advantages of high energy conversion efficiency,environmental friendliness,and abundance of fuel sources.DMFC is considered as the promising substitutes in the field of portable devices,military applications,and stationary power stations,while the broad application is severely hindered by the sluggish kinetic of methanol oxidation reaction(MOR)in the anode and the high cost of platinum(Pt)-based anodic electrocatalysts.Herein,a series of carbon supported Pt_(x)Cu_(y)(Pt_(x)Cu_(y)/C)binary metal electrocatalysts,featured with high activity and low consumption of precious metal,were prepared under the ambient environment by a simple liquid phase impregnation reduction method using ethanol as the solvent and sodium borohydride as the reducing agent.Uniform distribution of Pt_(x)Cu_(y)nanoparticles in the range of 2~4 nm was achieved by rationally optimizing the dropping rate,stoichiometric ratio,and reaction time.As revealed by XRD and TEM characterizations,adding way of the reducing agent into the metal salt precursor carbon slurry had a significant impact on the morphology.Compared with dumping,adding the reducing agent in a dropwise way was beneficial for obtaining nanoparticles with a smaller size and uniform distribution.By adjusting the amount of the reducing agent up to 20 times the molar amount of metal precursors,the complete reduction,signifying a higher loading and less waste of Pt precursors,can be safely ensured.When increasing the content of Cu precursors,those metal nanoparticles tended to connect into worm-like structures and the individual CuO phase was observed in PtCu_(3)/C and PtCu_(4)/C samples.MOR activity of as-prepared electrocatalysts was determined by systematically electrochemical measurements and an activity order of commercial Pt/C<Pt_(3)Cu/C<PtCu_(4)/C<PtCu/C<PtCu_(3)/C was revealed.In particular,the specific area activity of PtCu_(3)/C was 2.86 mA·cm-2,which was 3.74 times higher than that of commercial Pt/C(0.94 mA·cm-2),while the electrochemical active area(ECSA)was only half of commercial Pt/C.It indicated the enhanced performance stemmed from the accelerated reaction process instead of the increased reaction site.This was further confirmed by density functional theory(DFT)calculations that the introduction of Cu as well as the formation of CuO phase can promote the hydrolysis reaction,and the subsequent produced*OH can promote the oxidation of CO-like intermediate species into CO_(2) through the bifunctional mechanism.The current work opens a new avenue for the convenient and controllable synthesis of binary Pt-Cu alloy electrocatalysts on MOR and facilitates the development of high-efficient and low-cost DMFC devices.