Journal of The Electrochemical Society, 154 ͑12͒ A1077-A1082 ͑2007͒
A1077
0
013-4651/2007/154͑12͒/A1077/6/$20.00 © The Electrochemical Society
Electrochemical Behavior of Nanocrystalline Ru Me O
0
.8
0.2 2−x
„
Me = Fe, Co, Ni… Oxide Electrodes in Double-Layer
Region
,
z
K. Macounová, I. Jirka, A. Trojánek, M. Makarova, Z. Samec,* and P. Krtil*
J. Heyrovsky Institute of Physical Chemistry of ASCR, Prague, Czech Republic
Nanocrystalline oxides with average chemical composition corresponding to Ru0.8Me0.2O2−x ͑Me = Fe, Co, Ni͒ were prepared by
the sol-gel approach. All prepared materials were of single-phase character with rutile-type structure. The effect of nanocrystal size
and nature of doping cation on electrochemical supercapacitor behavior was studied by means of cyclic voltammetry and elec-
trochemical impedance. The specific capacitance of the Ru-based oxides increases by doping with lower-valence cation from ca.
2 F cm− of actual electrode surface area observed for pure RuO2 to 230 F cm in the case of Ni doped material. The
2
−2
2
improved capacitance behavior of the doped materials is ascribed to improvement of transport properties of the oxide structure
enabling easier diffusion of compensating protons.
©
2007 The Electrochemical Society. ͓DOI: 10.1149/1.2783774͔ All rights reserved.
Manuscript submitted March 30, 2007; revised manuscript received July 31, 2007. Available electronically September 28, 2007.
Transition metal oxides based supercapacitors represent an elec-
trochemical power source surpassing the power density and life
metal Me͒ remains the same as in the case of RuO synthesis. Start-
2
ing solutions were precipitated with aqueous solution ͑25 wt %͒ of
tetramethylammonium hydroxide ͑TMAH͒ ͑Fluka͒. Resulting col-
loidal solution of an amorphous precursor was first aged in a poly-
tetrafluethylene ͑PTFE͒-lined stainless steel autoclave at 100°C for
1,2
cycle of the current batteries. The attainable energy density and
stability of the metal oxide based supercapacitors are superior to
those achieved for systems based on carbon materials or
3
-5
polymers. The original concept of hydrous ruthenium dioxide
4
0 h and then filtered. The amorphous precursor was then washed
6
electrodes was later extended to other anhydrous materials of
with deionized water ͑Millipore MilliQ quality͒ and with 1 mL of
H O solution ͑Ͻ1%͒ and subsequently recrystallized at tempera-
7
8
9
rutile, perovskite, or pyrochlore structures with the aim to im-
prove the utilization of the employed noble metal. The specific sur-
2
2
tures between 400°C and 800°C to obtain nanocrystalline anhy-
drous oxides of various crystal size.
face charge densities of the anhydrous oxides are reported to be,
6
however, inferior to those of hydrous RuO . The superior behavior
2
The crystallinity and phase purity of the prepared samples was
checked using Bruker D8 Advanced powder X-ray diffractometer
with Vantec-1 detector and Cu K␣ radiation. Particle size distribu-
tion curves were obtained by analysis of scanning electron micro-
scope ͑SEM͒ micrographs recorded employing Hitachi S4800 scan-
ning electron microscope. The analysis was based on measurement
of 150 randomly selected particles.
of the hydrous oxides can be attributed to their structure, in which
the micro/nanoislands of crystalline anhydrous oxide coexist with
amorphous hydrous oxide. This coexistence increases significantly
the mobility of the protons in the solid phase, which may be con-
10
sidered to be the rate limiting process.
The main synthetic approach used in the rutile structural family
to increase the noble metal utilization is the substitution of Ru or Ir
in the cationic sublattice. From the synthetic point of view, substi-
tuted single-phase oxides with rutile structure may be prepared ei-
The chemical analysis of the surface of prepared samples was
based on X-ray photoelectron spectroscopy ͑XPS͒. The photoelec-
tron spectra of the Ru0.8Me0.2O2−x samples were measured using
spectrometer ESC A 3 Mk II ͑VG͒ equipped with hemispherical
analyzer in a fixed transmission mode. Photoelectrons were excited
using nonmonochrornatized Al K␣1,2 X-rays. Vacuum level main-
tained during experiments was better than 10− Torr. Samples were
measured in the powder state using a double-sided Scotch tape.
Surface concentration of Me was expressed in terms of Me/Ru
atomic ratio calculated from the intensities of Ru 4p ͑Eb
1
1
12
11,13
11,14,15
ther by homovalent substitution ͑Ti , Zr , Ce
or by heterovalent substitution.
, or Sn
͒
16-18
For both types of substitution,
an increase in attainable specific surface charge was observed. The
actual mechanism of this improvement and the role of the nature of
the doping cation remains unknown.
9
This paper summarizes the capacitive behavior of several nano-
crystalline doped Ru-based oxides conforming to the summary for-
mula Ru0.8Me0.2O2−x, where Me represents iron, cobalt, or nickel.
The cyclic voltammetry and electrochemical impedance measure-
ment data are related to the diffraction and microscopic characteris-
tics of the electrode materials to describe the effects of the doping
cation nature and nanocrystal size on the pseudocapacitive behavior
of these materials.
ϳ 45 eV͒ and Co 3p ͑E ϳ 60 eV͒, Ni 3p ͑E ϳ 68 eV͒, Fe 3p
b
b
͑Eb ϳ 56 eV͒, and normalized on the pertinent values of photoion-
1
9
ization cross sections. Binding energy E values were calibrated
b
using E of C 1s photoelectron line ͑284.8 eV͒ of the adventitious
b
carbon.
The electrochemical characterization of the prepared materials
was performed in a single-compartment glass cell with RuO2 or
RuO0.8Me0.2O2−x based working electrode, saturated calomel refer-
ence electrode and Pt counter electrode. The reference electrode
͑containing KCl͒ was separated by a salt bridge filled with
Experimental
Nanocrystalline anhydrous RuO , and Ru0.8Me0.2O2−x ͑where
2
Me stands for Co, Ni, or Fe͒ materials were prepared by a sol-gel
synthesis analogue with that reported in Ref. 16. Ru͑NO͒͑NO3͒3
Alfa Aesar͒ was dissolved in a 1:1 ͑v/v͒ mixture of 2-propanol and
ethanol ͑both Aldrich, ACS grade͒ to obtain a starting solution with
Ru concentration of 0.03 mol/L. In the case of Ru1−xMe O ma-
͑
0.1 M NaNO . The potential control was achieved using PAR 263A
3
potentiostat. The electrochemical impedance spectroscopy measure-
ments were carried out in the same three-electrode arrangement us-
ing Autolab P30 potentiostat in the frequency range from 50 kHz to
0.1 Hz with an ac amplitude of 10 mV ͑peak to peak͒. Impedance
data were analyzed using Z-Plot/Z-View software ͑Scribner Associ-
x
2−y
terials, the above starting solution was complemented by adding
Ni͑NO ͒ ·6H O, Co͑NO ͒ ·6H O, and Fe͑NO ͒ ·9H O ͑Lachema,
3
2
2
3 2
2
3 2
2
p.a. grade͒, respectively, to obtain the solution with Ru:Me ratio of
4:1. The overall concentration of cations ͑i.e., of Ru and doping
ates͒. All experiments were carried out in 0.1 M HClO ͑Aldrich
4
p.a.͒. The RuO /RuO0.8Me0.2O2−x working electrodes were prepared
2
on Ti mesh substrate ͑open area 20%, Goodfellow͒ by sedimentation
of synthesized nanocrystalline powders from an aqueous suspension
͑5 g/L͒. The deposition of the oxides on the Ti substrate proceeded
*
Electrochemical Society Active Member.
E-mail: Petr.Krtil@jh-inst.cas.cz
z