Valdecir A. Paganin

Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells

Electrochemical studies on low catalyst loading gas diffusion electrodes for polymer electrolyte fuel cells are reported. The best performance is obtained with an electrode formed from 20 wt% Pt/C, 0.4 mg Pt cm−2 and 1.1 mg Nafion® cm−2 in the catalyst layer and 15% PTFE in a diffusion layer of 50 µm thickness, for both the cathode and the anode. However, it is also observed that the platinum requirement can be diminished to values close to 0.2 mg Pt cm−2 in the cathode and 0.1 mg pt cm−2 in the anode, without appreciably affecting the good characteristics of the fuel cell response. The experimental fuel cell data were analysed using theoretical models of the electrode structure and of the fuel cell system. It is seen that most of the electrode systems present limiting currents and some also show linear diffusion components arising from diffusion limitations in the gas channels and/or in the thin film of electrolyte covering the catalyst particles.

Methanol electro-oxidation on gas diffusion electrodes prepared with PtRu/C catalysts

This work presents results of the study of methanol electro-oxidation on gas diffusion electrodes with PtRu catalysts supported on carbon. The catalysts were prepared by reduction with formic acid and their performance compared with Pt/C and Pt50Ru50/C E-TEK commercial catalysts, in half cell and single DMFC experiments. The results show that: (i) the activity of the catalysts in a half-cell depends on the concentration of methanol, the maximum being observed close to 2 mol l−1; (ii) atomic contents of Ru of 25% and above increase the tolerance of the catalyst to larger methanol concentrations; (iii) the catalyst Pt75Ru25/C is the one that exhibits the larger activity for methanol oxidation. Gas diffusion electrodes built with this catalyst showed potentials 0.14 V higher than with Pt/C E-TEK when tested in single DMFC.

Catalysts for DMFC: relation between morphology and electrochemical performance

Abstract In this work Pt–Ru/C alloys were prepared by chemical reduction using various modifications of the formic acid method with the objective of improving the performance. The supported alloys were characterized by XRD, backscattering and TEM analyses, and the active areas were determined by CO adsorption. The materials showed similar particle sizes and different morphological characteristics. The performance of the supported alloys for methanol electrooxidation was evaluated by linear sweep voltammetry experiments and curves of cell potential vs. current density in a single direct methanol fuel cell. It is shown that the difference in the morphology of the catalysts in nano- and microscale is the main effect in the different performances.

Effect of water transport in a PEFC at low temperatures operating with dry hydrogen

In this work, an experimental study of the polarisation response of a H2/O2 PEFC was carried out at low temperatures as a function of the membrane thickness. Results were analysed using a simplified model describing the transport of water in the membrane. It is seen that the model equations lead to good fittings of experimental current–potential data for polymer electrolyte fuel cells (PEFC) working with dry hydrogen and with Nafion® 115 and 117 membranes. For these membranes, limiting effects due to oxygen diffusion are minimal and the water transport through the membrane is a limiting effect for the cell behaviour at high current densities. With the Nafion® 112 membrane, oxygen diffusion effects dominate the characteristics of the current–potential curve.

Effect of thermal treatment on the performance of CO-tolerant anodes for polymer electrolyte fuel cells

Abstract In this work, carbon-supported Pt:Ru electrocatalysts, mainly for application in polymer electrolyte fuel cells, have been prepared by different methods. The materials were tested in single cells with respect to the hydrogen–oxidation reaction in the presence of CO, and the performances, before and after thermal treatments, compared to that of state-of-the-art, commercial E-TEK catalysts. In most cases, it was found that the performance of the anode improves substantially after the thermal treatment of the material. Of the several methods considered, the preparation of the catalyst via the formation of a sulfite complex, followed by a thermal treatment, gave the best results, comparable to those obtained with the state-of-the-art catalyst.

Nonelectrochemical Pathway of Methanol Oxidation at a Platinum-Catalyzed Oxygen Gas Diffusion Electrode

Potentiostatic investigations of the methanol oxidation in the absence and presence of oxygen at the three-phase boundary of a Pt-catalyzed gas diffusion electrode were carried out at several electrode potentials. In the presence of oxygen, a purely chemical reaction between oxygen and methanol was observed, with the products being CO 2 and water. A study of the rate of CO 2 formation and of the electrochemical currents for methanol oxidation showed that the rate of fuel consumption at the electrode is strongly increased by this chemical pathway. This is accompanied by a considerable decrease in the electrochemical currents for oxygen reduction. In the absence of oxygen and for potentials above 1.1 V vs. RHE, current oscillations were observed during the anodic process, which were attributed to the formation of an unstable layer of surface platinum oxides.

Effect of the degree of alloying of PtRu/C (1:1) catalysts on ethanol oxidation

The effect of alloying degree on the ethanol oxidation activity of a PtRu/C catalyst with a Pt/Ru atomic ratio of 1:1 was investigated by measurements in a half-cell and in a single direct ethanol fuel cell. The increase of the amount of Ru alloyed from one third to two thirds of the total Ru content in the catalyst clearly resulted in a decrease of the ethanol oxidation activity. As the amount of the highly active hydrous ruthenium oxide was near the same, the lower activity of the PtRu/C catalyst with higher alloying degree was mainly ascribed to the presence of an excessive number of Ru atoms around Pt active sites, hindering ethanol adsorption on Pt sites. The reduced ethanol adsorption could be also related to the decreased Pt–Pt bond distance and to the electronic effects by alloying.

Development of small polymer electrolyte fuel cell stacks

Abstract The results on the research and development of small polymer electrolyte fuel cell stacks, including the assembly of single cell. 6-cell and 21-cell modules, are described. The important characteristics of the systems are: (i) membrane and electrode assemblies were made with Nafion ® 115 and 117 membranes and particularly low catalyst loading electrodes presenting a geometric area of 20 cm 2 and a catalyst loading of 0.4 mg Pt/cm 2 : (ii) bipolar plates were fabricated using a nonporous graphite material in which a series/parallel flow field was machined out: (iii) external distribution of gases to the cells was done using parallel manifolding; (iv) cooling systems were tested employing water/air cooling plates distributed every three cells throughout the stack; (v) the reactant gases were externally humidified using temperature controlled humidification bottles. Testing of the stacks was conducted in a specially designed test station employing nonpressurized H 2 /O 2 reactants and measuring the individual and the overall cell voltage vs. current under several conditions for the overall system operation.

Studies of the CO Tolerance for Anodes of PtFe/C in PEMFC

Introduction Proton exchange membrane fuel cells (PEMFCs) are gaining attention for electric power generation, as they offer high energy density with low pollution. One of the major problem of PEMFC is related to the Pt catalyst poisoning by low levels of carbon monoxide, when reformatted hydrogen is used as the anode reactant. In order to reduce this poisoning problem, a common approach consists in the utilization of a second metal in Pt-based catalysts, able to form oxygenated species (metal-OH) at potentials lower than for pure Pt [1,2]. Watanabe et al. found that Pt-Fe exhibits excellent CO tolerance for H2 oxidation, similar to that of the Pt-Ru alloy [3]. For measurements of the electrocatalytic activity authors prepared thin film Pt alloy electrodes by sputtering Pt and Fe targets simultaneously onto a glass substrate. In this work, we will discuss if the reported effects of Pt-Fe also occur on highly dispersed catalysts under operating conditions of a PEM fuel cell. Measurements of differential electrochemical mass spectrometry (DEMS) were performed for understanding the CO oxidation process is occurring in the electrodes.

CO Tolerance and Stability of Proton Exchange Membrane Fuel Cells with Nafion® and Aquivion® Membranes and Mo‑Based Anode Electrocatalysts

This work presents a CO tolerance study of PtMo/C (70:30, Pt:Mo) and Pt/MoO2-C anode catalysts on proton exchange membrane fuel cells (PEMFC) with Nafion and Aquivion ionomer membranes. Results denote improved activity for the hydrogen oxidation reaction (HOR) in the presence of CO on both anode catalysts at 85 °C. Experiments of identical location transmission electron microscopy evidenced very good stability of the Pt particles along an accelerate stress test (AST) applied to the Mo-based anode catalysts. However, a crossover of degradation Mo products originated in the anode and going to the cathode takes place along this anode AST, causing a PEMFC performance decay. A very little effect of the nature of the membrane, Nafion or Aquivion, is observed over these crossover phenomena. Results also denote that when operating at appropriate high temperatures (105 °C for Nafion and 125 °C for Aquivion), there is no need of incorporating unstable oxophilic transition metals on Pt to achieve improved CO tolerance on PEMFCs, when the CO level in the hydrogen fed is of the order of 100 ppm.