Modeling Elementary Heterogeneous Chemistry and Electrochemistry in Solid-Oxide Fuel Cells

Abstract

This paper presents a new computational framework for modeling chemically reacting flow in anode-supported solid-oxide fuel cells ͑SOFC͒. Depending on materials and operating conditions, SOFC anodes afford a possibility for internal reforming or catalytic partial oxidation of hydrocarbon fuels. An important new element of the model is the capability to represent elementary heterogeneous chemical kinetics in the form of multistep reaction mechanisms. Porous-media transport in the electrodes is represented with a dusty-gas model. Charge-transfer chemistry is represented in a modified Butler-Volmer setting that is derived from elementary reactions, but assuming a single rate-limiting step. The model is discussed in terms of systems with defined flow channels and planar membrane-electrode assemblies. However, the underlying theory is independent of the particular geometry. Solid oxide fuel cells ͑SOFC͒ can be operated with a variety of fuels, including hydrogen, CO, hydrocarbons, or mixtures of these. This is possible because of the relatively high operating temperatures, and, at least in conventional SOFC anodes, the use of transition metal catalysts that promote the water-gas-shift reactionand steam reforming, which for methane may be written globally asIf sufficient steam is produced electrochemically at the anode/ electrolyte interface by the reactionthen reforming and shifting can, in principle, lead to full ͑if indirect͒ electrochemical oxidation of a hydrocarbon fuel. However, competing reaction pathways catalyzed by transition metals may also lead to solid carbon deposition, which can quickly destroy the anode. For this reason, some degree of upstream fuel processing, eiher by catalytic partial oxidation or by steam reforming, is usually used to produce a fuel stream that is rich in H 2 and CO and dilute in residual hydrocarbons before reaching the SOFC. Because upstream processing adds to the complexity, size, and cost of the overall plant, it is of considerable interest to minimize or even eliminate the need for it. There is evidence that mixing some oxygen with a hydrocarbon fuel can deliver good performance.1 In this case there must be partial oxidation within the anode structrue. Another promising alternative to utilize hydrocarbon fuels "directly" in SOFCs is to use a ceria oxidation catalyst instead of a transition metal. 2Whether an SOFC uses a "reforming anode" with a transition metal catalyst, or a "direct oxidation" anode with a ceria-based catalyst, or perhaps uses a different, novel anode design, optimizing the system to run efficiently on hydrocarbon or hydrocarbon-derived fuels is a very challenging problem, due to the complex, coupled physico-chemical processes involved. When significant CO and/or hydrocarbons are present in the fuel, models must also account for the in situ production of hydrogen through reforming and shifting reactions within the anode, as well as solid-carbon formation.Many questions of interest for optimization studies cannot currently be answered easily. For example, for...

SOFC modeling considering hydrogen and carbon monoxide as electrochemical reactants

“…The current density is calculated separately at the anodic side for hydrogen and carbon monoxide according to the approach developed by Suwanwarangkul et al [31], and presented in eqns (46)- (48). The electrochemical oxidation of methane (eqns (7)- (8)) is neglected, because the electrochemical oxidation rate of methane is neglible, compared to the ones for hydrogen and carbon monoxide [9,27].…”

“…Carbon monoxide can be oxidized in the electrochemical reaction, but also reacts with water in the WGSR [48]]. Note that the WGSR is in general much faster than the carbon monoxide electrode reaction [47][48][49]. Methane reacts with steam in the MSR.…”

Progressive activation of degradation processes in solid oxide fuel cells stacks: Part I: Lifetime extension by optimisation of the operating conditions

“…The steam-methane reforming rate is computed by the kinetic approach of Achenbach and Riensche [33] and the wateregas shift reaction is assumed at equilibrium in the anode. Only hydrogen is electrochemically converted at the interface, following the set of elementary processes proposed by Zhu et al [34].…”

Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells

“…3 Even the mechanism of simple hydrogen electro-oxidation and water electrolysis on oxide surfaces of solid oxide electrochemical cells remains controversial with regard to the presence or absence of a double layer (dipole layer) at the gas−solid interface and the rate limiting processes on the oxide surfaces. 11 In previous publications, we showed how ambient pressure X-ray photoelectron spectroscopy can be used to monitor changes in Ce oxidation states in CeO 2−x electrodes, measure local surface potentials across an entire solid oxide electrochemical cell (SOC), and measure local overpotentials on operating SOC devices in far-from-equilibrium conditions. 12,13 These studies laid the groundwork for performing in situ mechanistic studies designed to "observe" charge separation at an SOC gas−solid interface and monitor changes in chemical intermediates involved in the electrochemical process.…”