Simulation of electrochemical processes during oxygen evolution on Pb-MnO2 composite electrodes

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Journal Title
Journal ISSN
Volume Title
School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Date
2017
Major/Subject
Mcode
Degree programme
Language
en
Pages
28+6
Series
Electrochimica Acta, Volume 245
Abstract
The geometric properties of Pb-MnO2 composite electrodes are studied, and a general formula ispresented for the length of the triple phase boundary (TPB) on two dimensional (2D) compositeelectrodes using sphere packing and cutting simulations. The difference in the geometrical properties of2D (or compact) and 3D (or porous) electrodes is discussed. It is found that the length of the TPB is theonly reasonable property of a 2D electrode that follows a 1/r particle radius relationship. Subsequently,sphere packing cuts are used to derive a statistical electrode surface that is the basis for the earlierproposed simulations of different electrochemical mechanisms. It is shown that two of the proposedmechanisms (conductivity and a two-step-two-material kinetic mechanism) can explain the currentincrease at Pb-MnO2 anodes compared to standard lead anodes.The results show that although MnO2 has low conductivity, when combined with Pb as the metal matrix,the behaviour of the composite is not purely ohmic but is also affected by activation overpotentials,increasing the current density close to the TPB. Current density is inversely proportional to the radius ofthe catalyst particles, matching with earlier experimental results. Contrary to earlier SECM experiments,mass transport of sulphuric acid is not likely to have any influence, as confirmed with simulations.A hypothetical two-step-two-material mechanism with intermediate H2O2 that reacts on both the Pbmatrix and MnO2 catalyst is studied. It was found that assuming quasi-reversible generation of H2O2followed by its chemical decomposition on MnO2, results are obtained that agree with the experiments.If the quasi-reversible formation of H2O2 occurs near the peroxide decomposition catalyst, currentincreases, leading to an active TPB and to the current density that scales with 1/r. It is furtheremphasised that both the Pb matrix and MnO2 catalyst are necessary and their optimum ratio dependson the used current density. Yet, additional experimental evidence is needed to support the postulatedmechanism.
Description
Thesis advisor
Forsén, Olof
Keywords
Oxygen evolution on composite electrode, metal electrowinning, triple phase boundary length, two-step two-material mechanism, diffusion domain approach
Other note
Citation
Schmachtel, Sönke & Murtomäki, Lasse & Aromaa, Jari & Lundström, Mari & Barker, Michael H. 2017. Simulation of electrochemical processes during oxygen evolution on Pb-MnO2 composite electrodes. Electrochimica Acta. Volume 245. 28+6.