A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies

This paper analyzes the small signal ac impedance of electron diffusion and recombination in a spatially restricted situation with application in systems such as porous TiO2 nanostructured photoelectrodes and intrinsically conducting polymers. It is shown that the diffusion−recombination model with the main types of boundary conditions assumes a finite set of possible behaviors in the frequency domain, which are classified according to relevant physical parameters. There are four possible cases:  (i) the impedance of finite diffusion with reflecting boundary, (ii) the impedance of finite diffusion with absorbing boundary, (iii) the impedance of diffusion-reaction in semiinfinite space or Gerischer impedance, and (iv) the impedance that combines Warburg response at high frequency and a reaction arc at low frequency. The generality of the approach is discussed in terms of the spatial distribution of the electrochemical potential or quasi-Fermi level and also in terms of the transmission line representation....

Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer

MES (multi-energy systems): An overview of concepts and evaluation models

The production of functional nanostructured materials starting from cheap natural precursors using environmentally friendly processes is a highly attractive subject in material chemistry today. Recently, much attention has been focused on the use of plant biomass to produce functional carbonaceous materials, encompassing economic, environmental and social issues. Besides the classical route to produce activated carbons from agricultural side products, the hydrothermal carbonization (HTC) process shows clear advantages in that it can generate a variety of cheap and sustainable carbonaceous materials with attractive nanostructure and functionalization patterns for a wide range of applications. In this tutorial review we present the latest developments in this traditional but recently invigorated technique. It will be shown that HTC does not only access carbonaceous materials under comparatively mild hydrothermal conditions, but also replaces the more technical and structurally well-defined charring by a controlled chemical process. It will be shown that this makes it possible to tailor the final structure with the tools of colloid and polymer science, leading to very different morphologies with miscellaneous applications, including modern carbon nanocomposites and hybrids.

Chemistry and materials options of sustainable carbon materials made by hydrothermal carbonization

This paper presents an approach for combined optimization of coupled power flows of different energy infrastructures such as electricity, gas, and district heating systems. A steady state power flow model is presented that includes conversion and transmission of an arbitrary number of energy carriers. The couplings between the different infrastructures are explicitly taken into account based on the new concept of energy hubs. With this model, combined economic dispatch and optimal power flow problems are stated covering transmission and conversion of energy. A general optimality condition for optimal dispatch of multiple energy carriers is derived, and the approach is compared with the standard method used for electrical power systems. Finally, the developed tools are demonstrated in examples

Optimal Power Flow of Multiple Energy Carriers

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.

Distributed Generation: Toward a New Energy Paradigm

Towards multi-source multi-product energy systems

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.

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

Analytical fuel cell modeling

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 DCFC's. 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.

Kas Hemmes

Papers

Distributed Generation: Toward a New Energy Paradigm

Towards multi-source multi-product energy systems

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

Analytical fuel cell modeling

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