直接甲醇燃料电池耐甲醇氧还原电催化剂的研究

燃料电池以其高效、环境友好的发电方式，被誉为21世纪的能源技术。其中，直接甲醇燃料电池（DMFC）更以燃料甲醇来源丰富，价格低廉，储存、携带方便而成为近年的研究热点。目前DNDFC存在的一个主要问题是"甲醇透过"，即甲醇从阳极穿过固体电解质膜进入到阴极，而阴极催化剂一般是Pt/C，因此在阴极会同时发生甲醇氧化和氧还原，严重降低了电池的库仑效率和电压效率。此夕卜甲醇及其氧化中间产物还会使P口C中毒。虽然试验了一些低甲醇透过率的电解质膜，但仍无法完全消除甲醇透过。因此研制对氧还原催化活性高而对甲醇氧化没有活性，即耐甲醇的氧还原电催化剂是一个十分重要的课题。本论文主要从催化剂的组成、热处理、制备方法和载体等方面进行了相关研究，此外，还开展了生物燃料电池阴极电催化剂的研究。具体结果如下：1．热处理对电催化剂性能的影响（1）首次研究了炭载铂（Pt/C）对氧还原和甲醇氧化的催化活性与热处理温度的关系。发现P"C的催化活性随热处理温度的升高而降低，其原因是热处理使R／C中Pt的结晶度提高、粒径变大、表面浓度降低。但是，热处理使PUC催化甲醇氧化活性的降低程度远大于催化氧还原活性的降低程度。该研究提供了一种有效改善P口C催化剂耐甲醇性能的简便方法。（2）研究了炭载四狡基酞著钻（CoPcTc／C）和炭载四苯基铁叶琳（FeTPP／C）对氧还原和甲醇氧化的催化活性与热处理温度的关系。发现800℃热处理的CoPcTc／C对氧还原的催化活性最高；XPS和XRD分析表明，其活性位主要为含CoN4结构的物质。FeTPP／C催化剂与CoPcTc／C类似，700℃热处理的对氧还原催化活性最高。二者对甲醇氧化都没有活性。（3）首次研究了炭载四苯基铁叶琳一铂（FeTPP-Pt/C）复合催化剂对氧还原和甲醇氧化的催化活性与热处理温度的关系。发现热处理使FeTPP-Pt/C对氧还原的催化活性提高，并且优于相应P灯C，这是因为复合催化剂对氧还原的催化活性来源于FeTPP和Pt两部分。另外，FeTPP-Pt／C对甲醇氧化的催化活性随热处理温度的升高而降低，降低幅度大于相应Pt/C，这是因为在复合催化剂中，FeTPP在Pt/C表面的分散会降低甲醇与R的接触。700℃热处理的FeTPP-Pt/C对氧还原的催化活性最高，并且耐甲醇能力很强，非常适合作为DMFC阴极电催化剂。（4）首次研究了FeTPP-TiO2/C复合催化剂对氧还原的催化活性与热处理温度的关系。发现70。℃热处理的FeTPP-TiO2／C对氧还原的催化活性最高，并且稳定性好；复合催化剂提高了氧还原的电子转移数。这是因为TIOZ能够将FeTPP催化氧还原过程中产生的H2O2及时分解为O2和H2O，再重新被FeTPP还原。TIOZ的加入有望改善过渡金属大环化合物催化剂的长程稳定性。此夕卜该复合催化剂对甲醇氧化没有活性。2．制备方法对电催化剂性能的影响（1）首次同时研究了Pt/C对氧还原和甲醇氧化的催化性能，讨论了影响Pt/C催化活性的主要因素。XRD、XPS和TEM分析表明，无定型Pt含量高的Pt/C对氧还原的催化活性较高，表面氧化物含量高的Pt/C对甲醇氧化的催化活性较高。为制备耐甲醇能力强、催化氧还原活性高的Pt/C催化剂提供了理论参考。（2）比较了平衡吸附法和强制沉积法制备的FeTPP-Pt/C催化剂的性能，发现前者对氧还原和甲醇氧化的催化活性都高于后者，这是因为由强制吸附法制备的复合催化剂，FeTPP将一部分Pt覆盖，使其无法发挥活性。3．活性炭载体对Pt/C电催化剂性能的影响利用多种分析手段，系统比较了VulcanXC-72炭和上海松木炭的物理、化学性质对Pt/C电催化剂性能的影响。发现孔径适当、电导率高、灰分和表面含氧基团较少的活性炭作载体时，制得的P口C催化剂的性能较好。为PEMFC中电催化剂载体的选择提供了一些理论依据。4．生物燃料电池阴极电催化剂的研究首次制备了炭载微过氧化物酶-11（MP-11／C）电催化剂，通过循环伏安法、线性扫描法和旋转圆盘电极技术研究发现，MP-11／C对O2还原具有较高的催化活性，并且稳定性好，为生物燃料电池的研制提供了一种较好的酶固定方法。

Effects of treatment in different atmosphere on Pt3Sn/C electrocatalysts for ethanol electro-oxidation

Pt3Sn/C catalyst was prepared by a modified polyol process and treated in air, H-2/Ar, and Ar atmosphere, respectively. XRD analyses indicate that all of these catalysts have face-centered cubic (fcc) crystal structure. Temperature-programmed reduction (TPR) experiments show that more Sn exists in zero-valence in the Ar-treated PtSn catalyst than in the others. Cyclic voltammetry (CV), chronoamperometry (CA) experiments, and the performance tests of direct ethanol fuel cell (DEFC) indicate that the catalytic activity of PtSn/C for ethanol oxidation was affected significantly by the chemical state of Sn in catalyst particles. The as-prepared PtSn/C gives the higher power density, while Ar-treated PtSn/C shows the lower cell performance. It seems that the multivalence Sn rather than the zero-valence Sn in the PtSn catalyst is the favorable form for ethanol oxidation. Energy dispersion X-ray analysis (EDX) of the PtSn/C-as prepared and PtSn/C (after stability test) shows the active species (platinum, tin, and oxygen) composition changed to a different extent. Further attempt to improve the catalyst stability is needed.

Preparation of highly active Pt/C cathode electrocatalysts for DMFCs by an improved aqueous impregnation method

An improved aqueous impregnation method was used to prepare 40 wt% Pt/C electrocatalysts. TEM analysis of the samples showed that the Pt particles impregnated for a short time have a very narrow size distribution in the range of 1-4 nm with an average size of 2.6 nm. UV-vis spectroscopy measurements verified that the redox reaction between PtCl62- and formaldehyde took place with a slow rate at ambient temperature via a two-step reaction path, where PtCl42- serves as an intermediate. The use of the short-time-impregnated 40 wt% Pt/C as cathode electrocatalysts in direct methanol fuel cells yields better performance than that of commercial 40 wt% Pt/C electrocatalyst. Experimental evidence provides clues for the fundamental understanding of elementary steps of the redox reactions, which helps in guiding the design and preparation of highly dispersed Pt catalyst for fuel cells.

The mechanism studies of ethanol oxidation on PdO catalysts by TPSR techniques

The paper studies the direct oxidation of ethanol and CO on PdO/Ce0.75Zr0.25O2 and Ce(0.75)Zr(0.2)5O(2) catalysts. Characterization of catalysts is carried out by temperature-programmed desorption (TPD), temperature-programmed surface reaction (TPSR) techniques to correlate with catalytic properties and the effect of supports on PdO. The simple Ce0.75Zr0.25O2 is in less active for ethanol and CO oxidation. After loaded with PdO, the catalytic activity enhances effectively. Combined the ethanol and CO oxidation activity with CO-TPD and ethanol-TPSR profiles, we can find the more intensive of CO2 desorption peaks, the higher it is for the oxidation of CO and ethanol. Conversion versus yield plot shows the acetaldehyde is the primary product, the secondary products are acetic acid, ethyl acetate and ethylene, and the final product is CO2. A simplified reaction scheme (not surface mechanism) is suggested that ethanol is first oxidized to form intermediate of acetaldehyde, then acetic acid, ethyl acetate and ethylene formed going with the formation of acetaldehyde, acetic acid, ethyl acetate; finally these byproducts are further oxidized to produce CO2. PdO/Ce0.75Zr0.25O2 catalyst has much higher catalytic activity not only for the oxidation of ethanol but also for CO oxidation. Thus the CO poison effect on PdO/Ce0.75Zr0.25O2 catalysts can be decreased and they have the feasibility for application in direct alcohol fuel cell (DAFC) with high efficiency.

Carbon nanotubes decorated with Pt nanocubes by a noncovalent functionalization method and their role in oxygen reduction

Combined with polymer wrapping and layer-by-layer techniques, a noncovalent functionalization method is developed to disperse Pt nanocubes (NCs) onto carbon nanotubes (CNTs). By adjusting the relative ratio of Pt NCs to CNTs, nanotubes with different Pt NC loadings are produced. The composites exhibit excellent electrocatalytic activity towards oxygen reduction.

Carbon-supported Pd nanocatalyst modified by non-metal phosphorus for the oxygen reduction reaction

A carbon-supported palladium catalyst modified by non-metal phosphorus(PdP/C) has been developed as an oxygen reduction catalyst for direct methanol fuel cells.The PdP/C catalyst was prepared by the sodium hypophosphite reduction method. The as-prepared Pd nanoparticles have a narrow size distribution with an average diameter of 2 nm. Energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) results indicate that P enters into the crystal lattice of Pd and forms an alloy.

Preparation of Pd/C catalyst for formic acid oxidation using a novel colloid method

A novel colloid method using (WO3)(n)center dot xH(2)O as colloidal source was developed to prepare Pd/C catalyst for formic acid oxidation. Transmission electron microscopy image shows that the Pd/C nanoparticles have an average size of 3.3 nm and a narrow size distribution. Electrochemical measurements indicate that the Pd/C catalyst exhibits significantly high electrochemical active surface area and high catalytic activity with good stability for formic acid oxidation compared with that prepared by common method.

Pd nanoparticles supported on WO3/C hybrid material as catalyst for oxygen reduction reaction

Pd nanoparticles supported on WO3/C hybrid material have been developed as the catalyst for the oxygen reduction reaction (ORR) in direct methanol fuel cells. The resultant Pd-WO3/C catalyst has an ORR activity comparable to the commercial Pt/C catalyst and a higher activity than the Pd/C catalyst prepared with the same method. Based on the physical and electrochemical characterizations, the improvement in the catalytic performance may be attributed to the small particle sizes and uniform dispersion of Pd on the WO3/C, the strong interaction between Pd and WO3 and the formation of hydrogen tungsten bronze which effectively promote the direct 4-electron pathway of the ORR at Pd.

Novel Hybrid Electrocatalyst with Enhanced Performance in Alkaline Media: Hollow Au/Pd Core/Shell Nanostructures with a Raspberry Surface

In this paper, a hollow Au/Pd core/shell nanostructure with a raspberry surface was developed for methanol, ethanol, and formic acid oxidation in alkaline media. The results showed that it possessed better electrocatalyst performance than hollow Au nanospheres or Pd nanoparticles. The nanostructure was fabricated via a two-step method. Hollow Au nanospheres were first synthesized by a galvanic replacement reaction, and then they were coated with a layer of Pd grains. Several characterizations such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) were used to investigate the prepared nanostructures.

High proton conductive advanced hybrid membrane based on sulfonated Si-SBA-15

Composite membranes based on Sulfonated poly(ether ether ketone) (SPEEK) and sulfonated organically modified Si-SBA-15 (S-SBA-15) were investigated with the purpose of increasing the proton conductivity. The novelty of the composite membranes was attributed to two special structures and different ion exchange capacities (IEC) of S-SBA-15 fillers, which were embedded in membranes. The typical hexagonal channels array of S-SBA-15 was confirmed by XRD and TEM. The regular vermiculate and amorphous structures of the inorganic fillers were proved by SEM. Composite membranes were prepared through common solvent casting method. SEM images indicated that the inorganic filler with regular structure dispersed homogeneously in the composite membranes, but the amorphous filler caused an agglomeration phenomenon at the same loading content.

The size-controlled synthesis of Pd/C catalysts by different solvents for formic acid electrooxidation

The size-controlled synthesis of Pd/C catalyst for formic acid electrooxidation is reported in this study. By using alcohol solvents with different chain length in the impregnation method, the sizes of Pd nanoparticles can be facilely tuned; this is attributed to the different viscosities of the solvents. The results show that a desired Pd/C catalyst with an average size of about 3 nm and a narrow size distribution is obtained when the solvent is n-butanol. The catalyst exhibits large electrochemically active surface area and high catalytic activity for formic acid electrooxidation.

Nanoelectrocatalyst based on high-density Au/Pt hybrid nanoparticles supported on a silica nanosphere

A high-efficiency nanoelectrocatalyst based on high-density Au/Pt hybrid nanoparticles supported on a silica nanosphere (Au-Pt/SiO2) has been prepared by a facile wet chemical method. Scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy are employed to characterize the obtained Au-Pt/SiO2. It was found that each hybrid nanosphere is composed of high-density small Au/Pt hybrid nanoparticles with rough surfaces. These small Au/Pt hybrid nanoparticles interconnect and form a porous nanostructure, which provides highly accessible activity sites, as required for high electrocatalytic activity. We suggest that the particular morphology of the AuPt/SiO2 may be the reason for the high catalytic activity. Thus, this hybrid nanomaterial may find a potential application in fuel cells.