K. Hemmes

Distributed Generation: Toward a New Energy Paradigm

This paper discusses distributed generation which is emerging as a new paradigm to produce on-site highly reliable and good quality electrical power. Thus, the DG systems are presented as a suitable form to offer highly reliable electrical power supply. The concept is particularly interesting when different kinds of energy resources are available, such as photovoltaic (PV) panels, fuel cells (FCs), or wind turbines.

A Comparative Study of NiO ( Li ) , LiFeO2, and LiCoO2 Porous Cathodes for Molten Carbonate Fuel Cells

Porous cathodes of NiO(Li), Co-doped LiFeO[sub 2], and LiCoO[sub 2] for the molten carbonate fuel cell (MCFC) were examined in a comparative study using electrochemical impedance spectroscopy at temperature of 923, 973, and 1023 K. Using this technique the contributions of charge transfer and diffusion to the impedance could be separated. The impedance results as a function of gas composition were compared with theoretical predictions using the thin-film model leading to the conclusion that the most predominant diffusing species in porous MCFC electrodes are molecular oxygen and carbon dioxide. The reaction mechanism is probably the same for all three cathodes involving either the reduction of peroxy-carbonate or the reduction of dissociated oxygen. The remaining difference in gas dependencies can then be explained by assuming a low coverage of oxide ions on LiFeO[sub 2] while NiO(Li) and LiCoO[sub 2] have intermediate coverage by oxide ions. From the temperature dependence of the impedance an estimate may be given of the activation energies of the polarization processes.

Electrochemical oxidation of carbon in a 62/38 mol % Li/K carbonate melt

The electrochemical gasification of coal to CO in a direct carbon fuel cell (DCFC) has thermodynamical advantages, including the conversion of heat into power at a reversible efficiency of 100%. Molten carbonate fuel cell (MCFC) technology may form the basis for constructing DCFCs. Here the electrochemical oxidation of carbon in a 62/38 mol % Li/K carbonate melt is studied using impedance spectroscopy (IS) and cyclic voltammetry (CV). A set of equilibria is introduced which fully describes the electrochemical equilibrium of the system. From IS it is shown that for temperatures lower than 700 °C, charge transfer is the slowest step, while at higher temperatures a second unidentified step also contributes significantly to the d.c. resistance of the electrode. The d.c. resistance is 100 to 220 Ω cm2 at 650 °C and 12 to 60 Ω cm2 at 750 °C, depending on the carbon surface roughness.

Effect of alloying elements on the contact resistance and the passivation behaviour of stainless steels

Abstract The effects of alloying elements (Cr, Mo) on the passivation and the transpassive transition behaviour of various commercial stainless steels, which are candidate materials for the bi-polar plates in polymer electrolyte membrane fuel cell, were investigated via a contact electric resistance (CER) technique with graphite counter part. The contact resistance, too high for a semi-conductor film with high donor density, is not the inherent ohmic resistance of the passive film. The CER potentials decrease linearly with the pitting resistance equivalent number (PREN). The passive films of stainless steels with high PREN are more readily removed compared to those with low PREN in terms of the charge necessary to remove the passive films. The change in transpassive transition behaviour of the stainless steels from that of iron to that of chromium went on steeply with an increase in the PREN.

A Three‐Phase Homogeneous Model for Porous Electrodes in Molten‐Carbonate Fuel Cells

In this paper a new model for porous electrodes in molten-carbonate fuel cells (MCFC) is presented. The model is based on an averaging technique commonly used in porous-media problems. Important disadvantages of the existing agglomerate model caused by geometric assumptions and restrictions are eliminated in this new model. Unlike the agglomerate model, the new model is suitable for studying three-dimensional and anisotropic problems and incorporating the degree of electrolyte fill. Different reaction mechanisms can easily be incorporated. The validity of the new model is checked and compared with the agglomerate model by fitting the two models to ac-impedance spectra recorded from porous MCFC cathodes.

An extensive treatment of the agglomerate model for porous electrodes in molten carbonate fuel cells—I. Qualitative analysis of the steady-state model

The agglomerate model for porous electrodes in molten carbonate fuel cells (MCFC) is subjected to an extensive analytical study. It is shown that for cathodes in MCFCs the agglomerates may be viewed as equivalent diffusion volumes instead of real geometric representations. In this view the shape of the agglomerates (eg cylindrical or planar) is less relevant. As a consequence, this paper considers the easiest-to-handle shape of semi-infinite slabs. Using analytical mathematical tools, the model is shown to be mathematically and physically well-posed. Indications for optimal electrode thickness and agglomerate size are given, based on general problem properties and analytic solutions for special cases. Of all process parameters a variation of the ionic conductivity was shown to have the largest impact on the overall polarization. The impact of the exchange current density is comparable with or even larger than found for the diffusion coefficients, even though the electrode process is probably diffusion-controlled. The diffusion coefficients have to be increased simultaneously for a significant improvement of performance. While the exact nature of the agglomerates is not very well defined, the predicted polarization depends strongly on the width that is chosen. This is an important disadvantage of the agglomerate model.

Public Acceptance of Hydrogen in the Netherlands: Two Surveys That Demystify Public Views on a Hydrogen Economy

Interest in a hydrogen economy has grown significantly in the past decade. However, the success of old technologies that are being re-engineered to work on hydrogen, as well as the creation of new hydrogenbased technologies, hinges upon public interest in and demand for such technologies. With increasing investments in the research and development of hydrogen technologies, there exists a need to monitor public attitudes toward hydrogen to ensure its success as an alternative energy carrier. To address this need, two surveys were conducted. The results of the first survey give insights about how colored information influences the public acceptance of hydrogen, and the results of the second survey elucidate what trade-offs the public is willing to accept during the transition to a hydrogen economy. Furthermore, the research highlights how hydrogen knowledge and acceptance differ among age, education, and gender groups.

CO2 reduction in molten 62/38 mole% Li/K carbonate mixture

The product of the square root of the diffusion constant and the solubility of CO 2 , S√D, is an important parameter for the correct interpretation of electrochemical results on the complicated mechanism of the oxygen reduction in molten carbonates, in which CO 2 participates. We studied the CO 2 reduction with linear sweep voltammetry (LSV) and chronoamperometry (CA) to obtain S√D values for CO 2 , independent of the oxygen reduction. The measured LSV voltammograms and CA responses are well described by the theoretical response of a simple charge transfer step of the form Ox + e - = Red with the Red species initially not present. S√D values for CO 2 in the 62/38 mole% Li/K carbonate mixture were determined from the LSV responses in a straightforward way using little assumptions in a (small) temperature interval between 575°C and 650°C. These values obeyed Arrhenius law and a combined activation energy for S√D of 32.20 kJ/mole was determined. In an earlier study Nishina et al. showed that the oxygen reduction is rate-limited by the CO 2 diffusion [1]. They measured S√D values in the 62/38 mole% Li/K melt in a temperature range between 615°C and 700°C which they could ascribe to CO 2 by making various assumptions. Since these values for S√D, and moreover their combined activation energy for S√D, are very close to our values, our study supports Nishinas interpretation of his results and his conclusion that the oxygen reduction is rate-limited by the CO 2 diffusion.

Identification and electrochemical characterization of in situ produced and added reduced oxygen species in molten Li2CO3 + K2CO3

Abstract In two different laboratories, electrochemical experiments have been carried out in order to investigate the nature and the mechanism of the oxygen reduction process at a gold electrode in 62/38 mol% and 42.7/57.3 mol% Li 2 CO 3 + K 2 CO 3 . An original aspect of these experiments is that cyclic voltammetric measurements were performed in which potentials higher than the positive stability limit of the molten carbonate were applied. It has been shown that the carbonate ions were oxidized at the electrode to molecular oxygen. Subsequently, the oxygen reacted with carbonate ions in the diffusion layer forming in-situ one or more reduced oxygen species. The effect of the positive limit potential, preelectrolysis time at this potential, scan rate, acidity of the melt, temperature, gas composition and peroxide and superoxide additions, in the form of Li 2 O 2 and KO 2 , on the reduction process was thoroughly analyzed. The gas dependency of the reduction wave was also investigated in conditions where the start potential was the open-circuit potential. Under acidic conditions superoxide species, with a low stability in the bulk of the melt, is probably reduced according to a net three electron reversible process. Under basic conditions peroxide species, well stabilized in the melt, undergoes a two-electron reversible reduction. It has also been deduced from experimental data that a peroxycarbonate mechanism is not likely in this melt. The agreement between the results in both laboratories. ENSCP and TU Delft, although not complete, was such that similar conclusions could be obtained from them.

A search for suitable coating materials on separator plates for molten carbonate fuel cells

Abstract Stainless steel separator plates in molten carbonate fuel cells need to be protected against corrosion, preferably by a coating. We select a suitable coating material by an elimination procedure. First we argue that non-oxide metal/non-metal ceramic coatings are preferred. Then we select the metallic and non-metallic components for the ceramic coating. Selected non-metallic components of the most promising ceramics are B, C, N, Si and P. Metals eliminated in the elimination procedure have oxides or mixed oxides with eutectics below the MCFC operating temperature of 700 °C. Metals selected as components of the most promising ceramics are Ti, Y, Nb, La, Ce, Nd, Hf and Ta. Thus the borides, carbides, nitrides, suicides or phosphides of these selected metals may be suitable coating materials. Stability experiments show that TiN, TiC and Ce-based ceramics are promising coating materials.