A Multi‐Scale Dynamic Mechanistic Model for the Transient Analysis of PEFCs

Franco, Alejandro A.; Schott, Pascal; Jallut, Christian; Maschke, Bernhard

doi:10.1002/fuce.200500204

Cited by 145 publications

(215 citation statements)

References 57 publications

(77 reference statements)

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“…To describe the kinetics of the Faradaic reaction, we apply the generalized Frumkin-Butler-Volmer equation, which, for a one-electron reaction, can be represented in dimensional form as [47][48][49][50][51][52][53][60][61][62][63][64][65][66][67][68][69][70][71][72][73] …”

Section: Faradaic Reactions In the Dlsmentioning

confidence: 99%

Diffuse charge and Faradaic reactions in porous electrodes

2011

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Porous electrodes instead of flat electrodes are widely used in electrochemical systems to boost storage capacities for ions and electrons, to improve the transport of mass and charge, and to enhance reaction rates. Existing porous electrode theories make a number of simplifying assumptions: (i) The charge-transfer rate is assumed to depend only on the local electrostatic potential difference between the electrode matrix and the pore solution, without considering the structure of the double layer (DL) formed in between; (ii) the charge-transfer rate is generally equated with the salt-transfer rate not only at the nanoscale of the matrix-pore interface, but also at the macroscopic scale of transport through the electrode pores. In this paper, we extend porous electrode theory by including the generalized Frumkin-Butler-Volmer model of Faradaic reaction kinetics, which postulates charge transfer across the molecular Stern layer located in between the electron-conducting matrix phase and the plane of closest approach for the ions in the diffuse part of the DL. This is an elegant and purely local description of the charge-transfer rate, which self-consistently determines the surface charge and does not require consideration of reference electrodes or comparison with a global equilibrium. For the description of the DLs, we consider the two natural limits: (i) the classical Gouy-Chapman-Stern model for thin DLs compared to the macroscopic pore dimensions, e.g., for high-porosity metallic foams (macropores >50 nm) and (ii) a modified Donnan model for strongly overlapping DLs, e.g., for porous activated carbon particles (micropores <2 nm). Our theory is valid for electrolytes where both ions are mobile, and it accounts for voltage and concentration differences not only on the macroscopic scale of the full electrode, but also on the local scale of the DL. The model is simple enough to allow us to derive analytical approximations for the steady-state and early transients. We also present numerical solutions to validate the analysis and to illustrate the evolution of ion densities, pore potential, surface charge, and reaction rates in response to an applied voltage.

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Section: Faradaic Reactions In the Dlsmentioning

confidence: 99%

Diffuse charge and Faradaic reactions in porous electrodes

2011

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“…The ionic groups of polyelectrolyte gels are essentially immobile (and thus are treated as uniform fixed charges), and, in contrast, their counterions freely diffuse in the entire volume of the system; our model thus extends the treatments of these ions that have been developed in soft matter physics [17][18][19][20][21][22][23] to electrochemical systems. Analogous models are used to treat electrolyzers [13][14][15]24,25 and fuel cells [26][27][28] that use polymer electrolyte membranes. We treat the cases in which counterions are not the reactants or products of electrochemical reactions.…”

Section: Resultsmentioning

confidence: 99%

“…Our theory may not be effective for the cases in which electrochemical reactions at the electrodes are limited by the transport of reactants (for example, for the cases that counterions are the reactants or products of the electrochemical reactions 10,11,[24][25][26][27] ). Second, polyelectrolyte gels that are used to prepare PGDs are stiff enough; polyelectrolyte gels that do not have enough stiffness may locally collapse to neutralize electric charges on the electrodes 20,21,36 .…”

Section: Discussionmentioning

confidence: 99%

“…In practice, exchange current densities J 0 may be sensitive to the electrostatics and chemistry of polyelectrolyte gels and the metals that are used for electrodes [24][25][26][27] . Because water molecules in the solvent of polyelectrolyte gels are relatively abundant, compared with products, we neglect backward reactions that produce water molecules; equations (7) and (8) are only effective for DV40 and DVo0, respectively.…”

Section: Methodsmentioning

confidence: 99%

“…We treated hydrogen and hydroxide ions as symmetric ions; this is to highlight the roles played by the electric charges of polyelectrolyte gels and their counterions in the ion current rectification of PGDs. D hyd is the diffusion constant of hydrogen and hydroxide ions (this constant depends on the volume fraction and the density of the ionic groups of polyelectrlyte gels for the cases in which these parameters are relatively large 22 and thus should be measured by separate experiments or predicted by using multiscale modelling [24][25][26][27][28] Poisson equation. Electrostatic potentials are generated by the electric charges of hydrogen ions, hydroxide ions, the polyelectrolyte gels and their counterions and are derived by using the Poisson equation that has the from @ 2 @z 2 cðzÞ ¼ À 4pl B n H ðzÞ À n OH ðzÞ þ n pol ðzÞ þ n c þ ðzÞ À n c À ðzÞ…”

Section: Methodsmentioning

confidence: 99%

See 2 more Smart Citations

Electrochemical mechanism of ion current rectification of polyelectrolyte gel diodes

Yamamoto

Doi

2014

Nat Commun

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Polyelectrolyte gel diodes that are double layers of two oppositely charged polyelectrolyte gels, sandwiched by two symmetric electrodes, are emergent ionic devices. These diodes are designed to rectify ion currents with a physical mechanism that is analogous to conventional semiconductor diodes-the asymmetry in the permeability of ions across the interfaces between the two oppositely charged gels. Here we show that polyelectrolyte gel diodes indeed rectify steady currents with a physical mechanism that is very different from conventional diodes by using a simple electrochemical model; electric currents are limited by electrochemical reactions that are driven by potential drops at electrodes and these potential drops markedly change with changing the direction of applied voltages due to the redistribution of non-reactive counterions, leading to rectified ion currents. This concept is relatively generic and thus may provide insight in the physics of analogous ionic and biomimetic systems that show electrochemical reactions.

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Dynamic characteristics of internal current during startups/shutdowns in proton exchange membrane fuel cells

2019

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Summary Studying dynamic characteristics of proton exchange membrane fuel cells (PEMFCs) during startups/shutdowns is of great importance to proposing strategies to improve fuel cell performance and durability. In this study, internal current during startup and shutdown processes in PEMFC is investigated, and effects of gas supply/shutoff sequences and backpressure are analyzed by measuring local current densities and the cell voltage in situ. The experimental results show that when reactants were fed/shut off, internal current occurs and variation patterns of local current densities along the flow channel are different. During startups, local current densities in the downstream drop to negative values and internal current can be eliminated when air is first supplied into the cell. While during shutdowns, the results show that negative currents occur in the upstream, and if hydrogen is shut off first, all local current densities remain constant at zero, indicating the effectiveness of gas shutoff sequence in eliminating/mitigating internal current in PEM fuel cells. Further experimental results show that the magnitude of internal current increases with the pressure difference between the anode and the cathode.

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A Multi‐Scale Dynamic Mechanistic Model for the Transient Analysis of PEFCs

Cited by 145 publications

References 57 publications

Diffuse charge and Faradaic reactions in porous electrodes

Diffuse charge and Faradaic reactions in porous electrodes

Electrochemical mechanism of ion current rectification of polyelectrolyte gel diodes

Dynamic characteristics of internal current during startups/shutdowns in proton exchange membrane fuel cells

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