C. M. Rangel

Review on micro-direct methanol fuel cells

Fuel cells have unique technological attributes: efficiency, minimization of moving parts and low emissions. The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. With the advance of micromachining technologies, miniaturization of power sources became one of the trends of evolution of research in this area. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices, so, MicroDMFC is an emergent technology. There has been a growing interest in the development of this type of micro cells in the last years, resulting both in experimental studies (operating conditions, cell design and new materials) and in modeling studies. Despite the increase in the knowledge acquired, many challenges are still to be reached. This paper provides a detailed comprehensive review both on fundamental and technological aspects of micro-direct methanol fuel cells. Special attention is devoted to systematization of published results on experimental area since to date and also to a special section dedicated to modeling studies.

RenH 2 - Stand-Alone Energy System Supported by Totally Renewable Hydrogen Production

This paper describes a Stand-Alone Energy System Supported by Totally Renewable Hydrogen Production. The system was conceived for off-grid operation and is composed by solar panels, a wind turbine, a fuel-cell, an electrolyzer, hydrogen tanks and power electronics converters. The basic control strategy for the overall system considers the pressurized hydrogen gas storage as the energy buffer. The basic logic is that the exceeding renewable energy (solar and wind) is used to accumulate hydrogen, while the fuel cell uses this hydrogen to produce electrical energy when there is insufficient solar/wind energy.

Lanthanide-Based Conversion Coatings for Aluminium

The demand for new protection systems for aluminium alloys that might inhibit corrosion to the performance level of chromate, has driven the search for replacement candidates. Lanthanide compounds represent a promising alternative incorporating an environmental issue of significant impact, since chromate treatments are highly toxic and still used on both civil and military aircraft. A Ce based treatment was developed for use on structural alloy Al 2024-T3 implemented as a dipping process, without the use of external polarisation. The formation time was shortened by means of addition of a catalytic agent, hydrogen peroxide. When prior to the conversion coating formation, a chemical pre-treatment and a desmutting procedure was performed, no significant improvement was found in the susceptibility to pitting. This is thought to be probably due to the formation of a copper rich layer, as a result of etching, and the attack suffered by intermetallics during the chemical pre-treatment.

Effects of NaBH4 Additions on Hydrogen Absorption by Nanostructured FeTi Powders

FeTi intermetallic powders are very promising media for reversible hydrogen storage. However, difficult activation treatments including annealing at elevated temperatures in high pressure H2 gas atmosphere are mandatory. In the present work nanostructured FeTi powders were produced and activated in situ at room temperature using mechanical alloying/milling (MA/MM) of pure metallic constituents, Fe and Ti, added with sodium borohydride. The resultant powders, FeTiHx, already H2 pre-charged, absorbed a significant amount of H2 but require optimization for reversible absorption/desorption. This system has one of the highest volumetric storage capacities and can be produced at low cost. Several parameters of the as-milled powders were controlled. The phase constitution of the reaction products was characterized by X-ray diffraction and scanning electron microscopy and the absorption isotherms of the activated powders were determined.

Modeling of catalytic hydrogen generation from sodium borohydride

This paper present a dynamic model compiled in gPROMS that describes the hydrogen production via the catalytic hydrolysis reactions of sodium borohydride (NaBH4) solutions. It extends our previous work for the self-hydrolysis of NaBH4 [1], with the addition of a modified version of Davis et al. [2] empirical correlation to describe the reaction mechanism limiting step rate. The kinetic parameters were estimated using the gPROMS Parameter Estimation tool supported by experimental data in terms of produced gas volume and solution pH. The results have shown that the developed model accurately describes, not only the catalytic hydrolysis but also the self-inhibited self-hydrolysis and the alkaline storage of NaBH4. This model allows the prediction of hydrogen volume, pressure, rate of release and solution stability for this hydrogen generation reaction.

Novel data-driven methodologies for parameter estimation and interpretation of fuel cells performance

Fuel cell based power generation systems are expected to become more widespread in the near future. Stationary fuel cells may be used as an uninterruptible or back-up power supply, or to supply micro-grids. In particular, proton exchange membrane fuel cells (PEMFC) are an attractive technology due to its high energy density, rigid and simple structure, low operating temperature and fast start-up characteristics. The power quality assessment of fuel cells as a viable power sources requires a good understanding of the fuel cell performance characteristics. This paper presents two novel data-driven methodologies for the identification of the main steady state (polarization curve) and the dynamic (impedance response) characteristics for fuel-cells allowing the development of rapid, accurate and empirical models based on the experimental data. M-NMF is a modified non-negative matrix factorization technique developed for the analysis of polarization curve data that allows to identify the three main contributions for the fuel-cell power degradation, while for impedance spectroscopy data, this paper proposes the use of fractional order transfer functions (FC-FOTC) to describe the main dynamic modes present in the fuel-cell. A brief description of these two approaches is presented, together with the analysis of a real experimental dataset obtained from a 12W open cathode PEMFC stack to illustrate their potential and scope. While the former is instrumental for the deconvolution of the fuel cell polarization curves into its major components, the latter enables the estimation of the parameters related to the inherent transport and kinetic phenomena, thus opening the way, in both cases, for the interpretation of the effect of the operating conditions on the relative dominance and magnitude of these components and phenomena.

Fractional-order transfer functions applied to the modeling of hydrogen PEM fuel cells

Abstract This paper presents a new model formulation based on fractional order transfer functions (FOTF) for the modeling frequency response data (impedance) of hydrogen polymer electrolyte membrane fuel cells. Due to non-convexity of the minimization problem associated with the identification of the model parameters and the possible outliers contaminating the experimental data-set, a novel optimization strategy is proposed to reduce the risk of poor parameter estimation. This stochastic strategy is a modified random consensus sampling algorithm (RANSAC) which uses the Levenberg-Marquardt (LM) method to solve the inner optimization problem. The new model formulation and the optimization strategy were tested using an experimental data-set composed of three frequency response curves recorded from a PEMFC operating at different conditions. The results show that the proposed model formulation can be adjusted with a high accuracy to the experimental data, thus allowing the estimation of good quality parameters from experimental impedance data.

Selective Catalytic Reduction of NOx over Zeolite-Coated Cordierite-Based Ceramic Foams: Water Deactivation

The reactors used for Selective Catalytic Reduction (SCR) of NOx require low pressure drop structured catalyst packing. Structured packings, such as ceramic foams, are gaining increasing interest for application in low pressure drop reactors, membrane reactors and catalytic distillation units. In this work, cobalt ion exchanged mordenite (Co-HMOR)-coated cordierite-based foams produced by the replication method were evaluated for catalytic reduction of NOx with methane. The addition of 0.3 wt.% Pd to 2 wt.% Co-HMOR leads to a material that can convert 50 % NOx to N2 at 450 °C in a reaction mixture containing 2000 ppm CH4, 1000 ppm NOx, 5 % O2 and balance helium, at GHSV=17000 h-1. Although in an early stage of development, an efficient coating procedure was explored and different ways of exchange of Co and Pd cations into mordenite (Si/Al=10) were studied. Additions of 2 wt.% fumed silica enhanced adhesion of the zeolite onto the ceramic foam. Pd-exchanged Co-HMOR showed to be very sensitive to steam. A 50 % decrease in NOx conversion to N2 was observed after Pd/Co-HMOR samples were exposed at 450 °C to a reaction mixture containing 2 vol% H2O. Although further research is needed to ascertain the mechanism of this deactivation behaviour, agglomeration of Pd forming PdO particles is envisaged.

Modified titania in the photo-assisted oxidation of chloroform

Degussa P-25 TiO2 powder was used as a catalyst in the photocatalytic oxidation of organics. With the objective of promoting a more effective electron-hole separation upon irradiation, the catalyst was modified with small amounts of platinum. Characterisation of the powders was done by X-Ray diffraction and SEM. Chloroform was used as a model molecule and its degradation followed using selective electrodes, with excellent results for the modified titania. Langmuir-Hinselwood kinetic was used to describe the heterogeneous oxidation of chloroform on TiO2, for low solute concentrations.

Growth of Anodic Oxides on Sputtered Al-Nb Alloys

The growth of amorphous anodic films on Al-Nb alloys, with 21 to 75 at% Nb has been examined with a focus on the composition and morphology of the resultant oxides and their relation to the electrochemical impedance response. The films were formed by anodising at 5mAcm -2 to 150 V in 0.1 M ammonium pentaborate, which resulted in growth at close to 100% current efficiency. The composition of films were determined by Rutherford backscattering spectroscopy, revealing oxides containing units of Nb2O5 and Al2O3 in proportion to the alloy composition. From impedance data, the dielectric constant of the film materials increased with increasing niobium concentration in the film. The growth mechanisms of the film, involving counter-migration of anions and cations, results in growth of the film material at the alloy/film and film/electrolyte interfaces, with different relative mobilities of Nb 5+ and Al 3+ ions, leading to the presence of a thin alumina layer at the outer surface formed on alloys of highest aluminium contents.