Pd-TiO_(2)/C催化剂中TiO_(2)对直接碱性燃料电池中丙三醇氧化的影响

Effects of TiO_(2) in Pd-TiO_(2)/C for glycerol oxidation in a direct alkaline fuel cell

以Vulcan XC-72炭为载体,采用硼氢化钠还原法制备了Pd-TiO_(2)/C电催化剂用于直接丙三醇燃料电池阳极材料,并用能量色散谱(EDX)、X射线衍射(XRD)、透射电子显微镜(TEM)、循环伏安法(CV)、计时电流法(I-t)和全反射-傅里叶变换红外光谱(ATR-FTIR)对催化剂进行了表征。EDX结果表明,Pd-TiO_(2)/C中Pd和Ti含量接近名义原子比。Pd-TiO_(2)的X射线衍射结果表明,Pd具有面心立方fcc结构,同时TiO_(2)展现出四方结构的锐钛矿相的峰特征。TEM图像显示Pd和TiO_(2)纳米颗粒在碳载体中分布均匀并存在一些簇状区域,纳米颗粒大小为7.0-8.0 nm。循环伏安显示在1 mol/L KOH电解液中催化剂Pd-TiO_(2)/C在约-0.7 V vs Ag/AgCl有明显的析氢行为和电容增加现象,表明催化剂在TiO_(2)修饰后活性明显增加。通过循环伏安对碱性体系的丙三醇电催化氧化表明,Pd-TiO_(2)/C(70∶30)显示了非常好的催化活性:起始氧化电位也变得更负达到-0.4V vs Ag/AgCl,氧化电流达到14.91 mA/cm,优于Pd-TiO_(2)/C(90∶10)的9.37 mA/cm和Pd-TiO_(2)/C(50∶50)的4.88 mA/cm,明显高于Pd/C和TiO_(2)/C的1.88和0.55 mA/cm。I-t实验表明1000 s之前Pd-TiO_(2)/C展现出最高的电流密度,但在1000 s后电流密度有一定程度的下降,表明催化剂的催化活性很高但长期稳定性方面需要进一步提升。在碱性直接丙三醇燃料电池中进行的实验表明,所制备的Pd-TiO_(2)/C(50∶50)、Pd-TiO_(2)/C(70∶30)和Pd-TiO_(2)/C(90∶10)对甘油的电化学氧化性能均优于Pd/C电催化剂,其中,Pd-TiO_(2)/C(70∶30)的催化活性最好:开路电压达到817 mV,最大功率密度达到12.5mW/cm,高于Pd-TiO_(2)/C(50∶50)的12.4mW/cm、Pd-TiO_(2)/C(90∶10)的7.9mW/cm、Pd/C的3.7mW/cm以及TiO_(2)/C的2.25 mW/cm,这表明,TiO_(2)对Pd/C催化剂有很好的促进作用,这种协同效应可以归因为TiO_(2)对Pd的电子效应和双功能机理,同时Pd和TiO_(2)比例为70∶30时,其催化性能达到最优。ATR-FTIR结果表明,反应后所有Pd-TiO_(2)/C电催化剂表面均有甘油醛、羟丙酮酸、甲酸等高附加值产物生成,为直接丙三醇燃料电池电-化学品联产提供一定的依据。

The Pd-TiO_(2)electrocatalysts were synthesized via sodium borohydride reduction and characterized by X-ray diffraction(XRD),transmission electron microscopy(TEM),cyclic voltammetry,chronoamperometry and attenuated total reflectance-Fourier transform infrared(ATR-FTIR).The X-ray diffraction experiments of the Pd-TiO_(2)showed peaks associated with Pd face-centered cubic(fcc)structure and peaks characteristics of TiO_(2)(anatase phase)with a tetragonal structure.The TEM images showed that the Pd and TiO_(2)nanoparticles were well distributed in the carbon support showing some clustered regions with nanoparticle sizes between 7 and 8 nm.Cyclic voltammograms showed an increase in current density values after the glycerol adsorption process.Experiments in alkaline direct glycerol fuel cells at 60℃showed a higher power density for Pd-TiO_(2)/C(70∶30)in comparison to the commercial Pd/C electrocatalyst indicating that the use of the Ti O_(2)co-catalyst with Pd nanoparticles had a beneficial behavior.This effect can be attributed to the electronic effect or to the bifunctional mechanism.Molecules with high-value added glyceraldehyde,hydroxypyruvate and formate were identified as electrochemical reaction products of glycerol on all prepared electrocatalysts.