Journal of Applied Electrochemistry

, Volume 41, Issue 9, pp 1053–1064 | Cite as

Thin-film flooded agglomerate model for silver-based oxygen depolarized cathodes

Original Paper

Abstract

A mathematical model for a porous, silver-based electrode for the oxygen reduction in alkaline solutions, based on the thin film flooded agglomerate theory, was developed. These electrodes are employed in the energy-efficient chlor-alkali electrolysis with oxygen depolarized cathodes. The model parameters were determined from overpotentials at different oxygen concentrations obtained in half-cell measurements. For the description of the reaction kinetics, it was necessary to introduce two Tafel equations, which might be explained by a change of the adsorption isotherm of the intermediate species during oxygen reduction. The model allows for a successful description of the overpotentials in the region of industrially relevant current densities. The analysis of the oxygen concentration distribution in the liquid electrolyte reveals that massive diffusion limitations occur although the calculated size of the agglomerates is only in the range of a few micrometers.

Keywords

Oxygen reduction Ag Chlor-alkali electrolysis Modeling 

List of symbols

A

Constant describing the exchange current density (A m mol−1)

c

Molar concentration of oxygen dissolved in NaOH solution at boundary film–agglomerate (mol m−3)

c*

Molar concentration of oxygen dissolved in NaOH solution at boundary gas–film (mol m−3)

ci

Molar concentration of species i (mol m−3)

Dij

Binary Maxwell–Stefan diffusion coefficient for species i and j (m2 s−1)

Di,K

Knudsen diffusion coefficient for species i (m2 s−1)

E°

Equilibrium potential (V)

Echange

Potential of change in Tafel slope (V)

F

Faraday constant (= 96485.3399(24) C mol−1)

H

Henry constant (Pa m3 mol−1)

j

Current density (A m−2)

kc

(Chemical) reaction rate constant (s−1)

L

Characteristic length (m)

m

Molality of NaOH solution (mol kg−1)

Mi

Molar mass of species i (kg mol−1)

Ni

Flux of species i (mol m−2 s−1)

Pi

Partial pressure of species i (Pa)

Pm

Saturation partial pressure of water above NaOH solution (Pa)

R

Universal gas constant (= 8.314472(15) J mol−1 K−1)

r

Radius (m)

rag

Radius of agglomerates (m)

r

Reaction rate (mol m−3 s−1)

Stf

Specific surface area of thin film per total volume of reaction layer (m2 m−3)

Ts

Tafel slope (V decade−1)

T

Temperature (K)

z

Length domain in reaction layer (m)

\(z^{\prime}\)

Length domain in gas diffusion layer (m)

\(z^{\prime\prime}\)

Length domain in boundary layer (m)

Greek letters

δtf

Thickness of thin film (m)

\(\Updelta\varphi\)

Potential difference between solid electrode and electrolyte (V)

\(\varepsilon\)

Porosity (dimensionless)

γ±

Mean molal activity coefficient of NaOH solution (dimensionless)

γ

Thiele modulus (dimensionless)

ηeff

Effectiveness factor (dimensionless)

η

Overpotential (V)

κ

Electronic/ionic conductivity (S m−1)

μi

Chemical potential of species i (J mol−1)

π

\(=3.14159265\ldots\)

ρNaOH

Density of NaOH solution (kg m−3)

τ

Tortuosity (dimensionless)

\(\varphi\)

Potential (V)

Subscripts

b

Boundary layer

e

Electronic, solid phase of agglomerates

g

Gas chamber

i

Ionic, liquid phase of agglomerates

n

Agglomerate phase in reaction layer

s

Gas diffusion layer

T

Total

t

Gas phase in reaction layer

tf

Thin film

H2O

Total water (gaseous + liquid)

Superscripts

\({-\!\!\!\!\circ}\)

Standard state

eff

Effective

g

Gas phase

l

Liquid phase

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Copyright information

© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  1. 1.Institute of Chemical Process EngineeringClausthal-ZellerfeldGermany

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