Rotatng Dsk Electode -a hdodnamc method Fe Lu Ma 3, 0 ente fo Electochemcal Engneeng Reseach Depatment of hemcal and Bomolecula Engneeng
Rotatng Dsk Electode A otatng dsk electode RDE s a hdodnamc wokng electode used n a thee electode sstem. The RDE s one of the few convectve electode sstems fo whch the hdodnamc equatons and the convectve-dffuson equaton have been solved goousl fo the stead state. ente fo Electochemcal Engneeng Reseach, ho Unvest
onstuct of RDE Shaft bass Insulato Teflon Bush contact Dsk RDE Rotatng Dsk Electode RDE s constucted fom a dsk of electode mateal mbedded n a od of nsulatng mateal. The electode s attached to a moto and otated at a cetan fequenc. Insulatng mateal : Teflon, epox esn o othe plastc []. Bottom Vew ente fo Electochemcal Engneeng Reseach, ho Unvest 3
Veloct Pofle At Dsk Suface, 0, v 0, 0 and v. The soluton s dagged along at the suface of the dsk at the angula veloct ω. In the bulk soluton, v 0, v 0 and v U 0. Fo electochemcal studes, the mpotant veloct s v and v, Nea the suface of the otatng dsk 0, these veloctes ae gven b: 3/ / v 0.5 3/ / v 0.5 Fo, the lmt veloct n decton, / U 0 s: U lm v 0.88447 Hdodnamc bounda lae thckness / 3.6, v 0.8U 0, h 3.6 / ente fo Electochemcal Engneeng Reseach, ho Unvest v 0 h / 4 At
ente fo Electochemcal Engneeng Reseach, ho Unvest onvectve-dffuson Equaton 5 Stead-state convectve-dffuson equaton n tems of clndcal coodnates: Lmt cuent condton: Substtuton of the value of : D v v v dffuson convecton D v v / 3/.5 0 v
Soluton of onvectve-dffuson Equaton 0.5 Bounda condton: uent s the flux at the electode suface, that s: Unde lmt condtons: 3/ / Soluton of convectve-dffuson equaton: 0 0 nfad l, c l, c 6 D 0 /3 / /6 0. nfad ente fo Electochemcal Engneeng Reseach, ho Unvest 6
Dffuson Lae Thckness Levch equaton: Fo smple stead-state dffuson lae model: Fo RDE: l, c 6 /3 / /6 0. nfad D l, c nfam nfa m D 0.6D /3 / / 6 /3 / /6.6D ente fo Electochemcal Engneeng Reseach, ho Unvest 7
ente fo Electochemcal Engneeng Reseach, ho Unvest Geneal uent-potental uves at RDE 8 Bounda condton: uent s the flux at the electode suface, that s: Soluton of convectve-dffuson equaton: ombne wth Levch equaton: 3 0.50 D 0 0 0 nfad 0] [ 0.6 /6 / /3 nfad 0, c l
Kouteck-Levch Equaton Fo Levch equaton, s popotonal to / l c. A devaton of a / plot of vs. fom a staght lne ntesectng the ognal suggest a knetc lmtaton s nvolved n the electon-tansfe eacton. Fo totall vesble one-step one-electon eacton, the dsk cuent s : E 0 Whee : E s fowad eacton ate at E. k f, FAk FAk f f E l, c Wth eaangement and defnng: K FAk Kouteck-Levch equaton: K l, c K 0. 6nFAD f E /3 ente fo Electochemcal Engneeng Reseach, ho Unvest / / 6 9
Kouteck-Levch Equaton K l, c K 0. 6nFAD /3 / / 6 Vaaton of і wth ω / at an RDE fo an electode eacton wth slow knetcs ente fo Electochemcal Engneeng Reseach, ho Unvest 0
Advantages of RDE A stead state s attaned athe quckl and measuements can be made wth hgh pecson at RDE []. RDE gves good epoducblt and stable polazaton cuve. In stagnant soluton, dffuson lae thckness s the same as bounda lae thckness, t s eas to be dstub b extenal dstubance. But fo RDE: h.6 3.6 D / The dffuson lae thckness s fa less than bounda lae thckness, the dffuson lae s potect b bounda lae whch makes the polazaton cuve stable and has good epoducblt. [] L.H. Mendoza-Huza,.H. Ros Rees, M. Rvea and.a. Galán-Vdal, A Voltammetc Stud f The Undepotental Deposton f obalt nto A Glass abon Electode, AZojomo ISSN 833-X Volume 3 Janua 007 ente fo Electochemcal Engneeng Reseach, ho Unvest
Applcatons of RDE I. Measue Fe + /Fe 3+ oncentatons Fom the Levch equaton, Levch cuent the concentaton: l, c onsde a eacton:, 0.6nFAD a 0.6nFAD Fe 3 /3 /3 / /, s dectl popotonal to f the l c s known, the concentaton of Fe + /Fe 3+ can be calculated b lnea egesson usng the followng model [3] : / 6 / 6 e Fe /3 / l, c 0. 6nFAD l, c a a /6 b [3] Xn Jn, Geadne G. Botte, Electochemcal technque to measue FeII and FeIII concentatons smultaneousl, J Appl Electochem 009 39:709 77 ente fo Electochemcal Engneeng Reseach, ho Unvest
Applcatons of RDE II. Detemne Dffuson oeffcent Levch equaton: l, c 6 /3 / /6 0. nfad Plottng і l,s vs. ω / elds staght lne wth slope: slope /3 /6 0.6nFAD Dffuson coeffcent can be calculated b: Plots of і l,s vs ω /. Wokng soluton: K 3 FeN 6 0 mm + K 4 FeN 6 0 mm n Na S40. M at a vteous cabon RDE. D Slope / 6 0.60 nfa 3 ente fo Electochemcal Engneeng Reseach, ho Unvest 3
/ Applcaton fo cuent eseach III. Detemne knetc paametes 0. 0. 0.08 =.07x + 0.048 Kouteck-Levch equaton: 0.06 0.04 0.0 / K K 0.6nFAD /3 / / 6 0 0 0.0 0.04 0.06 0.08 / K s the ntecept at axs. ω -/ Plots of і - vs ω /. Wokng soluton: 0.33 M Uea and 5 M KH wth N base RDE at 0.5V. ente fo Electochemcal Engneeng Reseach, ho Unvest 4
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