F196
Journal of The Electrochemical Society, 155 ͑8͒ F196-F200 ͑2008͒
0013-4651/2008/155͑8͒/F196/5/$23.00 © The Electrochemical Society
The Synergistic Influences of OH− Concentration
and Electrolyte Conductivity on the Redox Behavior
of Ni„OH…2ÕNiOOH
,z
Chi-Chang Hu,a, Kuo-Hsin Chang,a,b and Tung-Yu Hsub
*
aDepartment of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan
bDepartment of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
The synergistic influences of the OH− concentration and electrolyte conductivity on the redox behavior of NiOOH/Ni͑OH͒ for
2
nickel oxide-coated graphite electrodes are clearly demonstrated by voltammetric and impedance analyses. The increase in the
OH− concentration and electrolyte conductivity effectively promote the utilization of active nickel species and the electrochemical
reversibility of NiOOH/Ni͑OH͒2, indicating the simultaneous involvement of OH− and cations in the redox transition. The upper
limit for utilizing Ni oxyhydroxides is mainly determined by the OH− concentration, which is facilely reached by increasing the
electrolyte conductivity ͑adding Na2SO4͒. The synergistic phenomena could be very important in the applications of Ni oxide-
based batteries, supercapacitors, sensors, electrochromic devices, and organic synthesis.
© 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2945911͔ All rights reserved.
Manuscript submitted February 13, 2008; revised manuscript received April 23, 2008. Available electronically June 27, 2008.
Nickel-based oxyhydroxides in various structures show distinc-
work demonstrates the enhancement in electrochemical kinetics of
tive electrochemical properties, which are widely studied for several
applications, e.g., nickel-based batteries,1,2 supercapacitors,3 electro-
catalysis in organic synthesis,4 sensors,4-6 and electrochromic
devices.7 To achieve a higher utilization of electroactive species,
Ni-based oxyhydroxides were widely investigated in concentrated
alkaline media ͑generally Ն1 M MOH, M: Li, Na, K͒.8,9 A model,
the so-called Bode diagram,10 was proposed to clarify the redox
transitions among ␣-Ni͑OH͒2, -Ni͑OH͒2, -NiOOH, and
␥-NiOOH. The exact oxidation states of these well-defined oxyhy-
droxides are unclear, although their structures seem to be clarified.
Actually, certain nonstoichiometric intermediates between these
well-defined structures were proposed,4,11,12 indicating the compli-
cated structures of various nickel oxyhydroxides.
the NiOOH/Ni͑OH͒ couple by adding a neutral supporting electro-
2
lyte ͑i.e., Na2SO4͒ into a dilute NaOH media, although this redox
transition is invisible in neutral solutions. The above synergistic in-
fluences of electrolyte conductivity and OH− concentration on the
redox behavior of NiOOH/Ni͑OH͒ could be very important in the
2
wide applications of Ni oxyhydroxides.
Experimental
For NiO synthesis via the modified sol–gel method,19
NiCl2·xH2O precursors were dissolved and agitated in a solution
with equal volumes of water and ethanol to form organometallic
species at room temperature for 0.5 h. A 1 M NaOH solution was
then added into the stirred solution until nickel oxide precipitates
were clearly found ͑ϷpH 10͒. Nickel oxide precipitates could be
easily obtained by means of a centrifuge, which was washed with
pure water several times until the pH was close to 7. The precipitates
were dried in a vacuum oven at room temperature for 8 h and then
annealed in air at 400°C for 1 h. The annealed oxide powders were
well dispersed in pure water in an ultrasonic bath for 20 min and
dropped onto pretreated graphite substrates without any binder.
These electrodes were dried in a vacuum oven at 85°C overnight.
The loading of NiO, 0.20 Ϯ 0.03 mg cm−2, is the weight difference
before and after the oxide coating, which can be effectively con-
trolled by the number of drops. The graphite substrates were first
abraded with ultrafine SiC paper, degreased with acetone and water,
then etched in a 0.1 M HCl solution at room temperature ͑ca. 26°C͒
for 10 min, and finally degreased with water in an ultrasonic bath.
The exposed geometric area of these pretreated graphite supports
was equal to 1 cm2, while the other surface areas were insulated
with polytetrafluorene ethylene coatings.
Electrochemical measurements were performed with an electro-
chemical analyzer system, CHI 633A ͑CH Instruments, Austin, TX͒
in a three-compartment cell. The impedance spectrum analyzer, IM6
͑Zahner͒, with the Thales software was employed to measure and
analyze the electrochemical impedance spectra ͑EIS͒ data. The po-
tential amplitude of ac was equal to 10 mV; meanwhile, its fre-
quency region was from 0.1 to 100 kHz. Before electrochemical
tests, 150 cyclic voltammetry ͑CV͒ cycles were performed in 1 M
NaOH between −0.15 and 0.7 V ͑vs Ag/AgCl͒ for every electrode to
obtain the so-called Ni͑OH͒2/G electrode ͑see Fig. 1A͒. A Ag/AgCl
electrode ͑Argenthal, 3 M KCl, 0.207 V vs standard hydrogen elec-
trode at 25°C͒ was used as the reference electrode. For convenient
comparisons, however, data measured in the electrolytes with differ-
ent pH values were referred to a reversible hydrogen electrode
͑RHE͒. A piece of platinum gauze with an exposed area of 4 cm2
It is well known that an increase in the OH− concentration can
effectively promote the utilization of electrochemically active nickel
species ͑estimated from voltammetric charges͒ and electrochemical
reversibility of NiOOH/Ni͑OH͒ ͑from the peak potential
2
difference͒,8,9 suggesting that this complicated redox reaction in-
volves the exchange of OH−. This action, however, results in an
increase in the electrolyte conductivity, probably favoring the redox
transition due to an increase in the cation concentration.13,14 Re-
cently, the redox transitions among various oxyhydroxides on
Ni͑OH͒ ultrathin films were systematically investigated by means
2
of the electrochemical quartz microbalance ͑EQCM͒.13-18 These
studies proposed several redox mechanisms involving the exchange
of cations, differing in whether H+ or OH− is transferred.18 How-
ever, contrary results were usually found when the mass of nickel
oxyhydroxides was varied,14,16,17 probably due to the influences of
electronic conductivity of oxyhydroxides and/or the diffusion issues
of ionic species involved in the redox transitions. Due to the much
higher loading of Ni͑OH͒ for sensors, batteries, supercapacitors,
2
and electrochromic devices than that studied in EQCM, the above
issues have to be carefully considered in these applications.
Based on the above viewpoints, the influences of electrolyte con-
ductivity and OH− concentration on the redox behavior of
NiOOH/Ni͑OH͒ with significant loading ͑ca. 0.20 mg cm−2͒ are
2
worth investigating. In this work, the electrochemical reversibility
and number of Ni oxyhydroxyl species active to the redox transi-
tions have been demonstrated to strongly depend on the concentra-
tions of OH− and Na2SO4. Moreover, the pH window for the redox
transition of NiOOH/Ni͑OH͒ has been effectively extended by
2
adding Na2SO4 in the dilute NaOH electrolytes. Furthermore, this
*
Electrochemical Society Active Member.
z E-mail: cchu@che.nthu.edu.tw
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