Ir-based oxide electrodes: oxygen evolution reaction from mixed solvents
A. ROSSI
1
and J.F.C. BOODTS
1,2,
*
1
Chemistry Department, FFCLRP, University of Sa
˜
o Paulo, Av. Bandeirantes 3900, 14040-901, Ribeira
˜
o Preto,
SP, Brazil
2
Chemistry Institute, Federal University of Uberla
ˆ
ndia, Av. Joa
˜
o Naves de A
´
vila 2160, 38400-902, Uberla
ˆ
ndia,
MG, Brazil
(*author for correspondence, fax: þ55 16 633 8151, e-mail: jfcboodts@ufu.br)
Received 9 April 2001; accepted in revised form 16 April 2002
Key words: cosolvent influence, electrode kinetics, iridium dioxide, oxygen evolution reaction
Abstract
The influence of30% (v/v) organic cosolvent in 1.0 mol dm
À3
HClO
4
on the OER electrode kinetics, surface
properties and electrode stability ofIrO
2
-based electrodes was investigated by cyclic voltammetry and polarization
curves. The Tafel coefficients in the presence ofcosolvent are explained in terms ofthe change ofthe rate-
determining step (r.d.s.) ofthe OER electrode mechanism, coating dissolution and/or cosolvent oxidation. Ofthe
several cosolvents investigated t-BuOH and PC show less effects on the OER and electrode properties making them
the best choice for organic eletrosynthesis applications, in contrast to AN, which causes coating dissolution, and
DMF and DMSO which show an anticipation ofthe voltammetric current.
1. Introduction
The nature ofthe electrode material strongly affects the
efficiency for the OER [1]. Among the several electrode
materials available, one ofthe most efficient for the
OER are Ti-supported IrO
2
-based dimensionally stable
anodes, DSA
Ò
[2, 3]. The excellent electrocatalytic and
mechanical properties ofthis catalyst (IrO
2
) has stimu-
lated its application in electrochemical oxidation of
organic substrates, normally combined with the OER
[4, 5]. An important advantage ofthis kind ofelectrode
material, besides their good electrocatalytic and me-
chanical properties, is that the catalyst is immobilized
thus facilitating its recovery from the reaction mixture,
and reducing the number ofsteps necessary to isolate
the products.
Despite the several desirable properties, application of
IrO
2
-based (and similar) electrode materials to organic
electrosynthesis is still underexploited [6]. One ofthe
main problems is the blocking ofthe electrode surface
frequently observed with organic substrates, particularly
with aromatic substrates. Recently, it was shown that
the oxidation oforganics when carried out simulta-
neously with the OER significantly reduces the surface
blocking process [7]. One ofthe authors showed [8] that
blocking ofTi/RuO
2
and Ti/IrO
2
surfaces is mainly due
to surface film formation as a consequence of organic
radical dimerization/polymerization. Oxidation done
in the OER region can not only recover already spoiled
surfaces but also avoids blocking of the electrode
surface. This was attributed to the mechanical action
ofintense O
2
evolution, which favours the removal and/
or avoids fixation ofthe organic film, combined with
film oxidation by the several radical species involved in
the OER electrode process [5].
Because ofthe limited solubility ofnumerous organics
in water, the use ofan organic cosolvent is a frequently
encountered experimental situation in organic electro-
chemistry. Only a limited number ofpapers deal with
the investigation ofthe influence oforganic solvents on
the OER [7, 9]. Zanta et al. [7] investigated the surface
and catalytic properties ofTi/RuO
2
and Ti/IrO
2
elec-
trodes in non-aqueous media showing these properties
are significantly affected by both the cation ofthe
supporting electrolyte and the chemical nature ofthe
solvent. These authors also observed an anodic dis-
placement ofup to 300 mV (e.g., acetonitrile (AN) and
propylene carbonate (PC)) ofthe OER.
In this paper we report the influence ofmixed solvents
on the surface properties and the OER of TiO
2
stabilized IrO
2
-based electrodes.
2. Experimental details
2.1. Electrodes
Oxide layers ofnominal composition Ir
x
Ti
(1)x)
O
2
(x ¼ 0.3, 0.7 and 1.0) were deposited on both sides of
sand-blasted Ti supports (10 mm · 10 mm · 0.2 mm),
Journal of Applied Electrochemistry 32: 735–741, 2002.
735
Ó
2002 Kluwer Academic Publishers. Printed in the Netherlands.