PEDOT:PSS self-assembled films to methanol crossover reduction in Nafion (R) membranes

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Development of plurimetallic electrocatalysts prepared by decomposition of polymeric precursors for EtOH/O2 fuel cell

This work aimed to develop plurimetallic electrocatalysts composed of Pt, Ru, Ni, and Sn supported on C by decomposition of polymeric precursors (DPP), at a constant metal:carbon ratio of 40:60 wt.%, for application in direct ethanol fuel cell (DEFC). The obtained nanoparticles were physico-chemically characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). XRD results revealed a face-centered cubic crystalline Pt with evidence that Ni, Ru, and Sn atoms were incorporated into the Pt structure. Electrochemical characterization of the nanoparticles was accomplished by cyclic voltammetry (CV) and chronoamperometry (CA) in slightly acidic medium (0.05 mol L-1 H2SO4), in the absence and presence of ethanol. Addition of Sn to PtRuNi/C catalysts significantly shifted the ethanol and CO onset potentials toward lower values, thus increasing the catalytic activity, especially for the quaternary composition Pt64Sn15Ru13Ni8/C. Electrolysis of ethanol solutions at 0.4 V vs. RHE allowed determination of acetaldehyde and acetic acid as the main reaction products. The presence of Ru in alloys promoted formation of acetic acid as the main product of ethanol oxidation. The Pt64Sn15Ru13Ni8/C catalyst displayed the best performance for DEFC.

PtSnIr/C anode electrocatalysts: promoting effect in direct ethanol fuel cells

This study investigates the promoting effect of PtSnIr/C (1:1:1) electrocatalyst anode, prepared by polymeric precursor method, on the ethanol oxidation reaction in a direct ethanol fuel cell (DEFC). All of the materials used were 20% metal m/m on carbon. X-ray photoelectron spectroscopy (XPS) analysis showed the presence of Pt, PtOH2, PtO2, SnO2 and IrO2 at the electrocatalyst surface, indicating a possible decorated particle structure. X-ray diffractometry (XRD) analysis indicated metallic Pt and Ir as well as the formation of an alloy with Sn. Using the PtSnIr/C electrocatalyst prepared here with two times lower loading of Pt than PtSn/C E-tek electrocatalyst, it was possible to obtain the same maximum power density found for the commercial material. The main reaction product was acetic acid probably due to the presence of oxides, in this point the bifunctional mechanism is predominant, but an electronic effect should not be discarded.

Mordenite-incorporated PVA-PSSA membranes as electrolytes for DMFCs

Composite membranes with mordenite (MOR) incorporated in poly vinyl alcohol (PVA)–polystyrene sulfonic acid (PSSA) blend tailored with varying degree of sulfonation are reported. Such a membrane comprises a dispersed phase of mordenite and a continuous phase of the polymer that help tuning the flow of methanol and water across it. The membranes on prolonged testing in a direct methanol fuel cell (DMFC) exhibit mitigated methanol cross-over from anode to the cathode. The membranes have been tested for their sorption behaviour, ion-exchange capacity, electrochemical selectivity and mechanical strength as also characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis. Water release kinetics has been measured by magnetic resonance imaging (NMR imaging) and is found to be in agreement with the sorption data. Similarly, methanol release kinetics studied by volume-localized NMR spectroscopy (point resolved spectroscopy, PRESS) clearly demonstrates that the dispersion of mordenite in PVA–PSSA retards the methanol release kinetics considerably. A peak power-density of 74 mW/cm2 is achieved for the DMFC using a PVA–PSSA membrane electrolyte with 50% degree of sulfonation and 10 wt.% dispersed mordenite phase. A methanol cross-over current as low as 7.5 mA/cm2 with 2 M methanol feed at the DMFC anode is observed while using the optimized composite membrane as electrolyte in the DMFC, which is about 60% and 46% lower than Nafion-117 and PVA–PSSA membranes, respectively, when tested under identical conditions.

A numerical model of a liquid-feed solid polymer electrolyte DMFC and its experimental validation

A one-dimensional, biphasic, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymeric electrolyte membrane has been developed for a liquid-feed solid polymer electrolyte direct methanol fuel cell (SPE- DMFC). The model employs three important requisites: (i) implementation of analytical treatment of nonlinear terms to obtain a faster numerical solution as also to render the iterative scheme easier to converge, (ii) an appropriate description of two-phase transport phenomena in the diffusive region of the cell to account for flooding and water condensation/evaporation effects, and (iii) treatment of polarization effects due to methanol crossover. An improved numerical solution has been achieved by coupling analytical integration of kinetics and transport equations in the reaction layer, which explicitly include the effect of concentration and pressure gradient on cell polarization within the bulk catalyst layer. In particular, the integrated kinetic treatment explicitly accounts for the nonhomogeneous porous structure of the catalyst layer and the diffusion of reactants within and between the pores in the cathode. At the anode, the analytical integration of electrode kinetics has been obtained within the assumption of macrohomogeneous electrode porous structure, because methanol transport in a liquid-feed SPE- DMFC is essentially a single-phase process because of the high miscibility of methanol with water and its higher concentration in relation to gaseous reactants. A simple empirical model accounts for the effect of capillary forces on liquid-phase saturation in the diffusion layer. Consequently, diffusive and convective flow equations, comprising Nernst-Plank relation for solutes, Darcy law for liquid water, and Stefan-Maxwell equation for gaseous species, have been modified to include the capillary flow contribution to transport. To understand fully the role of model parameters in simulating the performance of the DMCF, we have carried out its parametric study. An experimental validation of model has also been carried out. (C) 2003 The Electrochemical Society.

直接甲醇燃料电池阳极催化剂及甲醇氧化稀土助催化剂剂研究

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。但是，DMFC在其实用化之前还需要解决一些重要问题，其中的关键之一就是高性能的贵金属催化剂的研究。我们知道，甲醇的电化学活性要低于氢气三个数量级；而且甲醇在R表面进行电化学氧化时，其中间解离吸附产物会造成贵金属催化剂中毒，显著降低了催化剂的活性。因此，要使DMFC具有相当高的电流密度和运行稳定性，就需要对贵金属催化剂制备进行不断的研究和改进。在本文的工作中，主要从Pt/C催化剂的制备方法、新型碳纳米管载体、稀土助催化剂等三个方面进行了研究和探索，取得的具体结果如下：1．Pt／C催化剂制备方法的研究与改进（1）在本组已有的研究结果基础上，对预沉积还原法进行了一些改进，采用原子吸收光谱（AAS）进行表征，发现Pt的利用率得到了明显的提高。采用X射线衍射。（RD）、透射电子显微镜（TEM）和BET表征铂的粒径、晶态结构和催化剂特性，分析表明，经过改进的预沉积还原法制备的催化剂仍然具有良好的分散性、较小的粒径、较低的晶态结构和良好的催化剂特性，电化学测试证明其性能要优于同等的E-TEK催化剂。（2）借鉴冶金学中的相关技术，提出了一种新的Pt/C催化剂制备方法一程序升温焙烧法。该方法的具体步骤增强了金属催化剂粒子和碳载体之间的相互作用力，提高了碳载体的导电性，并且形成了部分有利于催化反应进行的活性晶态结构。得到的R／C催化剂获得了近似E-TEK催化剂的催化活性，在具体方法上仍有改进的潜力。采用了同（1）的催化剂表征方法。2．甲醇电化学氧化稀土助催化剂研究在直接甲醇燃料电池Pt/C催化剂的研究过程中，一个重要的方面就是助催化剂的研究，并且已经得到了较好的结果。本工作选用了稀土元素为研究对象，因为稀土元素属于过渡金属，具有丰富的d电子轨道，易于和金属形成强的类化学键的吸附作用，并且能够和有机小分子形成多种配位化合物。经过初步的工作，发现了有些稀土离子如Sm3+能够在Pt表面吸附并且对甲醇电化学氧化具有较稳定的促进作用，采用循环伏安法，计时电流，交流阻抗等电化学方法进行了表征。根据实验结果，对其反应机理进行了初步的探讨。3．碳纳米管（CNTs）作为贵金属催化剂载体的研究碳纳米管（CNTs）由于其结构上的特殊性（径向尺寸为纳米量级，轴向尺寸为微米量级）而表现出典型的一维量子材料，同时具有较高的机械强度和超常的电学性能，能够为化学反应提供纳米级的反应场所，因此受到了化学界包括电化学研究人员的极大关注。已经在作为贵金属催化剂载体方面进行了一些研究，本工作的主要内容就是针对Pt/CNTs催化剂对碳纳米管的要求，对其预处理方法进行了改进，采用了如（1）中的催化剂表征方法和（2）中的相关电化学方法进行测定，发现碳纳米管作为贵金属催化剂载体时，对它的纯化处理方法的不同明显地影响了其载体性质和催化剂的活性。

直接甲醇燃料电池甲醇替代燃料的研究

论文主要研究了直接甲醇燃料电池（DMFC）中三种甲醇替代燃料，二甲氧基甲烷（DMM）、乙醇和乙二醇及导致阳极催化剂中毒的吸附CO（COad）在光滑R电极及几种新的R基催化剂电极上的电氧化行为。结合对催化剂的X射线光电子能谱（xPS）、X衍射(XRD)、扫描电子显微镜（SEM）和热重分析（TG）表征，初步探讨了几种新催化剂对三种甲醇替代燃料的电催化活性要高于碳载R（PtC）催化剂的原因。另外，还研究了用表面活化处理来提高阳极催化剂对甲醇氧化的电催化活性的方法。本论文得到的主要结果如下：1．在研究DMM在不同条件下，在不同R基催化剂电极上的电化学氧化行为的基础上，发现碳载R和TiO2（Pt-TiO2／C）复合催化剂对DMM氧化的电催化活性要优于Pt/C电极。而Pt-TiO2／C催化剂在吸附Ho3+(Pt-TiO2-Ho3+/C)或Eu3＋（Pt-TiO2-Eu3＋/C）后，对DMM氧化的电催化活性比Pt-TiO2／C电极高。表明TiO2、Eu3+和Ho3+对DMM的氧化都有很好的促进作用，这主要是它们都能为DMM的氧化在电极表面提供更多的含氧物种。由于DMM本身在这些催化剂电极上的氧化性能不好，而且DMM容易在酸性溶液中水解，生成甲醇和甲醛，因此，DMM不是一种好的甲醇替代燃料。2．无论在中性介质中还是在酸性介质中，Eu3＋和Ho3＋对乙醇在R／C电极上的电化学氧化反应都有较好的促进作用，而Eu3＋的促进作用要大于H3+，Eu3＋和H3+在酸性溶液中的促进作用要大于在中性溶液中。无论是中性溶液还是酸性溶液中，吸附CO（COad）在Pt／C催化剂电极上在较正的电位处有一个很大的氧化峰，而在R-Eu3＋／C或R-H3＋／C催化剂电极上在较负电位处有两个小的氧化峰，表明吸附的Eu3＋和Ho3+对cood在R／C催化剂电极上的氧化都有很好的促进作用，主要表现在使Cood的吸附强度降低和吸附量减少。XPs测量表明，当R／C电极表面吸附了Eu3＋或H了十后，使催化剂中Pt和c土的电子云密度变小，因此使Pt对coad的吸附强度减弱。由于Eu3＋或H03＋在电极上吸附不是物理吸附，而是化学吸附，因此，它们与Pt的结合具有相对的稳定性。乙醇电氧化的中间产物，如COad等能强烈地吸附在R上，因此会使R中毒。而Eu3＋或H矿"能降低Coad在R上地吸附强度，因此，Eu3＋或H3+能促进乙醇在Pt／C电极上的电化学氧化反应。Pt-TiO2／C催化剂对乙醇氧化的电催化活性要高于R／C催化剂，表明TiO2对乙醇在R／C电极上的电化学氧化反应也有较好的促进作用。XPS的测量表明，TiO2的加入并不改变Pt的电子状态，因此，TiO2能促进乙醇电氧化反应的主要原因是TIOZ能为乙醇氧化提供含氧物种。实验结果表明，Eu3＋或H3+对乙醇在R-TiO2／C电极上的电化学氧化反应也有一定的促进作用。这是由于Eu3＋或H3+改变了R的电子状态，降低了乙醇电氧化中间产物，COod在Pt上的吸附强度，而TIOZ提供了COod的氧化所必须的含氧物种。3．无论是酸性溶液中还是中性溶液中，乙二醇在R-TiO2／C电极上的氧化活性比在R／C电极上高。这表明TIOZ能促进乙二醇在Pt上的电氧化反应。进一步的实验表明，TIOZ对COad在Pt催化剂电极上氧化的促进作用并不明显。XPS测量表明，这是由于TIOZ并不改变R的电子状态。所以，TiO2对乙二醇在Pt上的电氧化的促进作用只是基于提供乙二醇电氧化所需的含氧物种。无论是酸性溶液中还是中性溶液中，乙二醇在R-WO3／C电极上的氧化活性都比在R／C电极上高。这表明W03能促进乙二醇在R上的电氧化反应。进一步的实验表明，R-WO3／C电极对Coad氧化的电催化活性也要高于R／C电极，XPS测量表明，WO3会降低Pt的电子云密度。所以，WO3对乙二醇在R上的电氧化的促进作用除了提供含氧物种外，还由于它能降低R的电子云密度而降低了乙二醇电氧化中间产物。4．用四氢吠喃和丙酮混合溶液浸泡法对电极进行表面处理后能使Pt／C和Pt-WO3／C电极对乙醇和乙二醇氧化的电催化活性有很大的提高。其原因可能是Pt／C和Pt一w03／C电极在经表面处理后，在电极制备过程中带来的表面活性剂等杂质由于溶解在混合溶液中而被除去，Nafion也可能在用混合溶液浸泡后会发生一定程度的结构变化，因此，使活性中心的位点增加，从而增加了催化剂的电催化活性。另外，经表面处理后，Pt／C和Pt-WO3／C电极的活性中心的结构有一定的变化，使COad的吸附强度降低而容易氧化，降低了COad对催化剂的毒化作用，因而提高了电极对乙醇和乙二醇氧化的电催化活性。

Study of sintered stainless steel fiber felt as gas diffusion backing in air-breathing DMFC

Adoption of a sintered stainless steel fiber felt was evaluated as gas diffusion backing in air-breathing direct methanol fuel cell (DMFC). By using a sintered stainless steel fiber felt as an anodic gas diffusion backing, the peak power density of an air-breathing DMFC is 24 mW cm(-2), which is better than that of common carbon paper. A 30-h-life test indicates that the degraded performance of the air-breathing DMFC is primarily due to the water flooding of the cathode. Twelve unit cells with each has 6 cm(2) of active area are connected in series to supply the power to a mobile phone assisted by a constant voltage diode. The maximum power density of 26 mW cm(-2) was achieved in the stack, which is higher than that in single cell. The results show that the sintered stainless steel felt is a promising solution to gas diffusion backing in the air-breathing DMFC, especially in the anodic side because of its high electronical conductivity and hydrophilicity. (C) 2004 Elsevier B.V. All rights reserved.

The preparation of high activity DMFC Pt/C electrocatalysts using a pre-precipitation method

It is suggested that a Pt/C cathodic catalyst for the direct methanol fuel cell (DMFC) can be prepared with a pre-precipitation method, in which, H2PtCl6 is precipitated onto the carbon black as (NH4)(2)PtCl6 before H2PtCl6 is reduced to Pt. The electrocatalytic activity of this Pt/C-A catalyst for oxygen reduction is excellent because the Pt/C catalyst prepared with this pre-precipitation method possesses a small average particle size, low relative crystalinity and a large electrochemically active surface area. In addition, the pre-precipitation method is simple and economical and it can be used to prepare a Pt/C catalyst on a large scale.

Structure-activity relationship of surfactant for preparing DMFC anodic catalyst

Three kinds of surfactants as stabilizer were applied to the preparation of electrocatalysts for direct methanol fuel cell (DMFC). The catalysts have been characterized by examining their catalytic activities, morphologies and particle sizes by means of cyclic voltammetry, chronoamperometry, X-ray diffraction and transmission electron microscopy (TEM). It is found that the surfactants with different structures have a significantly influence on the catalyst shape and activity. The catalysts prepared with non-ionic surfactants as the stabilizer show higher activity for direct oxidation of methanol. The structure-activity relationship (SAR) analysis has been explored and the effect of hydrophile-lipophile balance (HLB value) has also been discussed.

Determination of ionic resistance and optimal composition in the anodic catalyst layers of DMFC using AC impedance

In the present study, a method based on transmission-line mode for a porous electrode was used to measure the ionic resistance of the anode catalyst layer under in situ fuel cell operation condition. The influence of Nafion content and catalyst loading in the anode catalyst layer on the methanol electro-oxidation and direct methanol fuel cell (DMFC) performance based on unsupported Pt-Ru black was investigated by using the AC impedance method. The optimal Nafion content was found to be 15 wt% at 75 degrees C. The optimal Pt-Ru loading is related to the operating temperature, for example, about 2.0 mg/cm(2) for 75-90 degrees C, 3.0 mg/cm2 for 50 degrees C. Over these values, the cell performance decreased due to the increases in ohmic and mass transfer resistances. It was found that the peak power density obtained was 217 mW/cm(2) with optimal catalyst and Nafion loading at 75 degrees C using oxygen. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Tubular cathode prepared by a dip-coating method for low temperature DMFC

A novel tubular cathode for the direct methanol fuel cell (DMFC) is proposed, based on a tubular titanium mesh. A dip-coating method has been developed for its fabrication. The tubular cathode is composed of titanium mesh, a cathode diffusion layer, a catalyst layer, and a recast Nafion® film. The titanium mesh is present at the inner circumference of the diffusion layer, while the recast Nafion® film is at the outer circumference of the catalyst layer. A DMFC single cell with a 3.5 mgPt cm tubular cathode was able to perform as well, in terms of power density, as a conventional planar DMFC. A peak power density of 9 mW cm was reached under atmospheric air at 25 °C. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

Simulação da nanoestruturação de nanofolhas de óxido de grafeno reduzido através de dinâmica molecular

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Desenvolvimento de conjunto membrana-eletrodos para célula a combustível de metanol direto passiva

Direct methanol fuel cells (DMFCs) without external pumps or other ancillary devices for fuel and oxidant supply are known as passive DMFCs and are potential candidates to replace lithium-ion batteries in powering portable electronic devices. This paper presents the results obtained from a membrane electrode assembly (MEA) specifically designed for passive DMFCs. Appropriated electrocatalysts were prepared and the effect of their loadings was investigated. Two types of gas diffusion layers (GDL) were also tested. The influence of the methanol concentration was analyzed in each case. The best MEA performance presented a maximum power density of 11.94 mW cm^-2.

Comportamiento electroquímico y en pila de combustible de cátodos fabricados mediante evaporacion electrónica de Pt frente a electrodos comerciales

(SPA) Con el objetivo de disminuir el coste de los electrodos utilizados en las pilas de combustible de membrana polimérica se ha llevado a cabo un estudio de la influencia sobre la respuesta electroquímica y en ensayos en monocelda de pila de combustible de alcohol directo, de la deposición por evaporación electrónica de platino sobre tela de carbón. Se han estudiado en las mismas condiciones dos electrodos comerciales con distinta carga de catalizador y dos electrodos preparados por evaporación electrónica de platino. Se encuentra que la evaporación electrónica de platino sobre tela de carbón ahorra carga de catalizador, aumenta la superficie electroactiva y permite alcanzar rendimientos comparables a los de los electrodos comerciales, mejorando mucho la potencia obtenida por unidad de masa de material catalítico. (ENG) In order to diminish the cost of the electrodes used in polymer membrane fuel cells a study of the influence on the electrochemical response and essays in a single direct methanol fuel cell of the deposition by electronic evaporation of Platinum on carbon cloth has been carried out. Two commercial electrodes with different catalyst loading and two electrodes prepared by electronic evaporation of Platinum have been studied in the same conditions. It can be concluded that electronic evaporation of Platinum on carbon cloth saves catalyst load, increases the electroactive surface area and reaches fuel cell performances comparable with those obtained using commercial electrodes, improving clearly the power obtained per unit mass of catalytic material.