Paola Costamagna

Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells

Water loss and the coincident increase in membrane resistance to proton conduction are significant barriers to high performance operation of traditional proton exchange membrane fuel cells at elevated temperatures where the relative humidity may be reduced. We report here approaches to the development of high temperature membranes for proton exchange membrane fuel cells; composite perfluorinated sulfonic acid membranes were prepared to improve water retention, and non-aqueous proton conducting membranes were prepared to circumvent the loss of water. Experimental results of composite membranes of Nafion and zirconium phosphate show improved operation at elevated temperatures. Imidazole impregnated membranes poisoned the electrocatalysts. Cesium hydrogen sulfate membranes were not able to produce appreciable current. A brief analysis of temperature requirements for CO tolerance and a framework for understanding water loss from fuel cell membranes are presented.

Micro-modelling of solid oxide fuel cell electrodes

Abstract Porous composite solid oxide fuel cell (SOFC) electrodes formed by a mixture of electronic conductor and ionic conductor particles of small size have been studied by means of an analytical simulation model. The model takes into account electronic and ionic transport together with the electrochemical reaction, while mass transport phenomena in the macropores of the electrode have been demonstrated to be negligible. The morphological properties of the electrode have been considered with reference to percolation thresholds, in terms of effective conductivity for both phases and contact area between the phases. The results agree with the literature experimental data. Simulation shows that for thick electrodes, the optimum performance is obtained for a composition near to the percolation threshold of the electron conducting phase, while for thin electrodes the optimum performance is obtained in correspondence to the composition that ensures the maximum contact area between the two phases. The electrode thickness has to be chosen following optimisation criteria. For an electrode composition very near to the percolation threshold of the electronic conductor the reciprocal electrode resistance shows a maximum as a function of the electrode thickness; the maximum appears for a thickness that is in the typical range for SOFC electrodes. For an electrode composition far from the percolation threshold of the electronic conductor the electrode resistance is almost constant for an electrode thickness larger than a certain value and within the typical values for SOFC electrodes.

Transport phenomena in polymeric membrane fuel cells

Transport phenomena of mass, energy, momentum and electrical charges play a significant role in proton exchange membrane fuel cells (PEMFCs). In this study the transport and balance equations are the basis of a simulation model which allows the evaluation of the distribution of the physico-chemical parameters within the structure of a PEMFC reactor. Model validation is presented; the validated model is then used to investigate the behaviour of the reactor, with particular attention to critical operating conditions. Critical conditions arise in a number of cases: flooding, membrane drying and degradation due to temperature peaks are discussed in this paper.

Characterisation of composite SOFC cathodes using electrochemical impedance spectroscopy. Analysis of Pt/YSZ and LSM/YSZ electrodes

The performance of composite electrodes as a function of the electrode composition is investigated experimentally through electrochemical impedance spectroscopy (EIS). In particular, two different composite cathodes are analysed, i.e. Pt/YSZ and LSM/ YSZ. Typical impedance spectra have been obtained and used to evaluate the overall electrode polarisation resistance Rp. The qualitative features are similar for both cathodes; in particular, in both cases the electrode performance (expressed in terms of R p 1 ) shows a dome-shaped curve as a function of the electrode composition. Nevertheless, the quantitative aspects are very different; in particular, the maximum value of R p 1 is much higher for the LSM/YSZ electrode than for the Pt/YSZ one (22 vs. 1.5 S cm 2 ). Also, the composition which gives the maximum performance and the experimental values of the percolation thresholds are different. The reasons for the discrepancies are analysed and discussed. # 2002 Elsevier Science Ltd. All rights reserved.

Modeling and experimentation of molten carbonate fuel cell reactors in a scale-up process

Modeling and experimental activities are presented for molten carbonate fuel cell electrochemical reactors, starting from the local kinetic study to the overall stack analysis. The inclusion of the micro-model of the electrode-kinetics in the macroscopic model and the study of the effects of gas cross-over phenomena through the electrolyte are among the original features of this work. The model gives new insights into the phenomenological behavior of the process. A good agreement between simulation and experimental results is presented and discussed.

Coherent structures in oscillatory boundary layers

The dynamics of the vortex structures appearing in an oscillatory boundary layer (Stokes boundary layer), when the flow departs from the laminar regime, is investigated by means of flow visualizations and a quantitative analysis of the velocity and vorticity fields. The data are obtained by means of direct numerical simulations of the Navier–Stokes and continuity equations. The wall is flat but characterized by small imperfections. The analysis is aimed at identifying points in common and differences between wall turbulence in unsteady flows and the well-investigated turbulence structure in the steady case. As in Jimenez & Moin (1991), the goal is to isolate the basic flow unit and to study its morphology and dynamics. Therefore, the computational domain is kept as small as possible. The elementary process which maintains turbulence in oscillatory boundary layers is found to be similar to that of steady flows. Indeed, when turbulence is generated, a sequence of events similar to those observed in steady boundary layers is observed. However, these events do not occur randomly in time but with a repetition time scale which is about half the period of fluid oscillations. At the end of the accelerating phases of the cycle, low-speed streaks appear close to the wall. During the early part of the decelerating phases the strength of the low-speed streaks grows. Then the streaks twist, oscillate and eventually break, originating small-scale vortices. Far from the wall, the analysis of the vorticity field has revealed the existence of a sequence of streamwise vortices of alternating circulation pumping low-speed fluid far from the wall as suggested by Sendstad & Moin (1992) for steady flows. The vortex structures observed far from the wall disappear when too small a computational domain is used, even though turbulence is self-sustaining. The present results suggest that the streak instability mechanism is the dominant mechanism generating and maintaining turbulence; no evidence of the well-known parent vortex structures spawning offspring vortices is found. Although wall imperfections are necessary to trigger transition to turbulence, the characteristics of the coherent vortex structures, for example the spacing of the low-speed streaks, are found to be independent of wall imperfections.

Some more considerations on the optimization of cermet solid oxide fuel cell electrodes

Limiting behaviour and optimization considerations are discussed on the basis of an analytical model (Costamagna, Costa and Antonucci, Electrochim. Acta, 43, 375 (1997); Costamagna, Antonucci, Costa and Arato, Proc. IEA Workshop, Les Diablerets, Switzerland, January 1997, p. 196) for cermet solid oxide fuel cell electrodes. The results show that it is not possible to optimize the overall electrode conductivity, taking polarization and ohmic effects into account (Costamagna, Costa and Antonucci, Electrochim. Acta, 43, 375 (1997)), and the electrode utilization at the same time. Under conditions of optimal conductivity, the electrochemical reaction is unevenly distributed and only a fraction of the electrode thickness effectively supplies electrical current; on the contrary, thicknesses small enough for a uniform current distribution to result are sub-optimal from the point of view of the electrode conductivity.

Effect of composition on the performance of cermet electrodes. Experimental and theoretical approach

This work aims to analyse the behaviour of cermet electrodes as a function of their composition, i.e. the ratio between ionic and electronic conducting particles. This is an important parameter to be considered to obtain maximum performance from this type of electrode, which is currently under study for application in oxygen sensors and solid oxide fuel cells. Experimental results of overall electrode resistance, including both ohmic and activation polarisation effects, have been obtained through electrochemical impedance spectroscopy measurements of Pt/YSZ electrodes in air. The results compare favourably with the theoretical predictions for several compositions above the percolation threshold of the electronic conductor. For this reason, the model is a useful tool for the design of optimised cermet electrodes; in particular, the experimental data show that maximum performance is attained for compositions very close to the percolation threshold of the electronic conductor, and this is in very good agreement with the modelling results.

Fluid dynamic study of fuel cell devices: simulation and experimental validation

Abstract The paper is concerned with the mass flow distribution in fuel cell stacks. In particular, the flow through the manifold system connected to the parallel arrangement of the cell channels is modelled and numerically treated. The numerical results are recognized to be more realistic than those obtained by means of an approximate analytical solution since more detailed effects could be accounted for. This evidence is confirmed by experiments carried out at a stack model device consisting of 100 cells. Pressure and velocity distributions were measured for various Reynolds numbers and geometrical shapes of the manifolds. The agreement between the experimental and numerical results is good.

A Hybrid System Based on a Personal Turbine (5 kW) and a Solid Oxide Fuel Cell Stack: A Flexible and High Efficiency Energy Concept for the Distributed Power Market

In this paper a high efficiency and flexible hybrid system representing a new total energy concept for the distributed power market is presented. The hybrid system is composed of a very small size (5 kW) micro gas turbine (named personal turbine-PT) presented in a companion paper by the authors coupled to a small size solid oxide fuel cell (SOFC) stack. The power of the whole system is 36 kW depending on the design parameters assumed for the stack. The design and off-design performance of the hybrid system have been obtained through the use of an appropriate modular code named HS-SOFC developed at the University of Genoa and described in detail in this paper. The results of the simulation are presented and discussed with particular regards to: choice of the hybrid system (HS) design point data, HS design point performance, off-design performance of PT and SOFC stack, and off-design performance of the whole HS. Some preliminary economic results are also included based on different fuel and capital cost scenarios and using the cost of electricity as the parameter for comparison between PT and HS.