Development of Multi-scale Models for Transport Processes Involving Catalytic Reactions in SOFCs

Several physical phenomena appear in anode-supported solid oxide fuel cells (SOFCs), such as multi-component gas species/charge flow, thermal energy and mass transfer. Meanwhile, generation and consumption of gas- and surface-phase species together with electric current production are involved at the active sites in different length scales. Therefore, various reactions in SOFCs are strongly coupled with the transport processes making the physical phenomena more complicated. An effective anode is the one that correctly balances each of the transport processes and the reactions. To deeply understand the chemical-reacting transport processes in the porous anode, a fully three-dimensional numerical calculation method (CFD approach) is further developed. The considered domain includes the porous anode, fuel gas flow channel and the solid interconnects. By calculating surface-phase species, the gas-phase species/heat generation and consumption related to the internal reforming reactions and the electrochemical reactions have been employed. The variable thermal-physical properties and transport parameters of the fuel gas mixture have also been taken into account. Furthermore, the heat transfer due to the fuel gas diffusion is implemented into the energy balance based on multi-component diffusion models. A multi-step heterogeneous steam reaction scheme based on the micro and detailed reaction mechanisms of Ni catalyst is employed in this study. The surface reactions include 42 irreversible elementary ones, and they account for the steam reforming, the water-gas shift reforming and Boudouard reactions. This microscopic reaction model describes the adsorption and desorption reactions of 6 gas-phase species (H2, CO, CH4, CO2, H2O and O2) and surface reactions of 12 surface-adsorbed species (Nis, Hs, Os, OHs, HCOs, CHs, CH2s, CH3s, CH4s, COs, CO2s, H2Os). Simulation results are presented and discussed in terms of the gas-phase species and temperature distributions, the chemical reaction rates of the gas- and surface chemical species, the catalyst surface coverage and the effects on the transport processes

Numerical simulation of multi-scale transport processes and reactions in pem fuel cells using two-phase models

A numerical study for the cathode of a PEM fuel cell has been performed in this study. The results have been limited to cathode only because, in PEM fuel cells, the oxygen reduction reactions, ORRs, are considered the rate limiting reactions and govern the fuel cell performance. The modeling approach utilized the two-phase models involving water phase change for PEM fuel cells i.e. two-phase current (solid and membrane), two-phase flow (gas and liquid water) and two-phase temperature (fluid and solid). The catalyst layer has been modeled using the microscale agglomerate approach where diffusion of oxygen into the agglomerate structure was used to model the reaction rates. For comparison of the PEM fuel cell performance, detailed study was performed at load conditions of current densities of 0.22, 0.57 and 0.89 A/cm2 explicitly. A varying fuel cell performance was observed under different loads. At low current densities, the temperature, electro-osmotic drag, irreversible and losses are quite low but the membrane phase conductivity showed a decreasing pattern along the length of the cathode. At higher current density (0.89 A/cm2), a sharp decrease in the current was observed due to the mass limitation effects, and due to higher water content, the water flooding effect was observed as more prominent than at lower current densities. The maximum power density for the present case was observed at 0.55 V. By comparing the results of this study and previous study with single phase flow model, it can be seen that this model is more conservative and captures the mass limitation effects to a great extent and the maximum power density as predicted by the single phase models falls in the mass limitation zone

SOFC Modeling Considering Electrochemical Reactions at the Active Three Phase Boundaries

Abstract in UndeterminedIt is expected that fuel cells will play a significant role in a future sustainable energy system, due to their high energy efficiency and the possibility to use renewable fuels. A fully coupled CFD model (COMSOL Multiphysics) is developed to describe an intermediate temperature SOFC single cell, including governing equations for heat, mass, momentum and charge transport as well as kinetics considering the internal reforming and the electrochemical reactions. The influences of the ion and electron transport resistance within the electrodes, as well as the impact of the operating temperature and the cooling effect by the surplus of air flow, are investigated. As revealed for the standard case in this study, 90% of the electrochemical reactions occur within 2.4 mu m in the cathode and 6.2 mu m in the anode away from the electrode/electrolyte interface. In spite of the thin electrochemical active zone, the difference to earlier models with the reactions defined at the electrode-electrolyte interfaces is significant. It is also found that 60% of the polarizations occur in the anode, 10% in the electrolyte and 30% in the cathode. It is predicted that the cell current density increases if the ionic transfer tortuosity in the electrodes is decreased, the air flow rate is decreased or the cell operating temperature is increased

Three-Dimensional Design Optimization Of An Anode-Supported SOFC Using FEM

Abstract in UndeterminedSolid oxide fuel cells (SOFCs) are promising as energy producing devices, which at this stage of development will require extensive analysis and benefit from numerical modeling. A 3D model is developed based on the FEM for a single cell planar SOFC design optimization. Ion, electron, heat, gas-phase species and momentum transport equations are implemented and coupled to the kinetics of electrochemical reactions. High current density spots are identified, where the electron transport distance is short and the oxygen concentration is high. The relatively thin cathode results in a significant oxygen mole fraction gradient in the direction normal to the main flow direction. The electron transport especially within the cathode is found to be limiting for the electrochemical reactions at positions far from the channel walls (interconnect ribs). It is concluded that an increased pore size in the cathode support layer increases the current density more than an increased pore size in the anode support layer

Process Based Large Scale Molecular Dynamic Simulation of a Fuel Cell Catalyst Layer

In this paper, a large scale molecular dynamic method for reconstruction of the catalyst layers (CLs) in proton exchange membrane fuel cells is developed as a systematic technique to provide an insight into the self-organized phenomena and the microscopic structure. The proposed Coarse-Grained (CG) method is developed and applied to the step formation process, which follows the preparation of the catalyst-coated membranes (CCMs). The fabrication process is mimicked and evaluated in details with consideration of the interactions of material components at a large scale. By choosing three sizes of the unit box, the relevant configurations of the equilibrium states are compared and analyzed. Furthermore, the primary pores of 2-10 nm in the agglomerates mainly consist of the channel space, which acts as the large networks and could be filled with liquid water. Moreover, various physical parameters are predicted and evaluated for four cases. The active Pt surface areas are also calculated by the current model, and then compared with the experimental data available in the literature. Finally, the pair correlation functions are employed to predict the distributions and hydrophobic properties of the components, providing the information on phase segregation and microscopic structure of the CLs. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.028203jes] All rights reserved

LTNE approach to simulate temperature of cathode in a PEMFC

The solid phase and fluid phase temperature and species distribution have been calculated numerically in this study. The model considered here consists of catalyst layer, porous-transport layer and the current collector region (rib). Two energy equations approach has been employed in the porous transport layer and one energy equation is solved for the catalyst layer to simulate the temperature distribution. Full multi-component diffusion model and Knudsen effect have been included for the simulation of the species distribution in both catalyst and porous-transport layer. The agglomerate model has been used to simulate the catalyst layer. It has been found that the diffusion coefficient is low in the catalyst layer due to low permeability and porosity causing stagnation zones and the temperature rise is maximum in the stagnation zones causing local hot spots