Electrochemical behaviour of an electrodeposited rhodium electrode in alkaline solution

Prévia do material em texto

213 
J. Electroanal. Chem., 279 (1990) 273-282 
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands 
Electrochemical behaviour of an electrodeposited rhodium 
electrode in alkaline solution * 
Dunja &&man and Marijan Vukovit 
Laboratory of Electrochemistry and Surface Phenomena, Rudjer BoSkovii Institute, Zagreb, Croatia 
(Yugoslavia) 
(Received 27 June 1989; in revised form 6 October 1989) 
ABSTRACT 
The electrochemical properties of an electrodeposited rhodium electrode on a titanium substrate in 1 
mol dm- 3 NaOH solution have been investigated by potentiodynamic, potentiostatic and ac impedance 
measurements. The Tafel slopes of 125 mV/decade for the oxygen evolution reaction have been obtained 
on both electrodeposited and rhodium wire electrodes. The electrodeposited layer of rhodium behaves in 
a similar way as the rhodium wire electrode. The charge enhancement factor is, however, smaller on the 
electrodeposited electrode and so is the reversibility of the surface oxidation-reduction process. The 
stability of the electrodeposited layer under anodic polarization at + 0.8 V vs. SCE is good without any 
significant dissolution of the rhodium layer. 
INTRODUCTION 
Hydrous oxide films on noble metals have attracted much attention due to many 
interesting properties they exhibit in electrocatalysis, electrooptic display devices, 
corrosion and colloid chemistry [l]. A rhodium electrode, in alkaline solution, for 
example, exhibits enhanced oxide growth under continuous potentiodynamic cycling 
[2-41. The hydrous oxide layer formed with this procedure shows enhanced electro- 
catalytic activity in the oxygen evolution reaction (OER) [S]. 
While the electrochemistry of bright rhodium was the subject of interest in a 
number of papers [6-191 the electrodeposited layer of rhodium has been much less 
investigated from the electrocatalytic point of view. Homchenko et al. [20] reported 
the adsorptive properties of electrodeposited rhodium as a function of potential. 
Tyurin and Kossaya [21] investigated adsorption of hydrogen on electrodeposited 
rhodium as a function of pH in several buffer solutions. Podlovchenko and Aliua 
l Presented at the 38th ISE Meeting, Maastricht, The Netherlands, September 1987. 
0022-0728/90/$03.50 0 1990 Elsevier Sequoia S.A. 
274 
[22] showed that the electrodeposited rhodium layer exhibits similar electrochemical 
properties by a bright rhodium electrode. In a recent paper from this laboratory, the 
electrochemical properties of an electrodeposited rhodium electrode have been 
investigated in sulphuric acid solutions with emphasis on anodic stability [23]. It is 
of interest to investigate electrodeposited layers of noble metals from the fundamen- 
tal aspects of electrocatalysis as well as from the aspect of their possible application 
in electrochemical energy conversion (fuel cells, water electrolysis). Next to efficient 
electrocatalytic properties, good electronic conductivity and resistance to corrosion, 
the material of which the successful electrocatalyst is made must have an acceptable 
price. In connection with this it is desirable to prepare a relatively thin layer of 
noble metal on some conductive metallic substrate (DSA @ anodes based on RuO, 
on titanium, for example). In the present paper some electrochemical properties of 
rhodium electrodeposited on a titanium substrate in alkaline solution, are reported. 
EXPERIMENTAL 
The electrode preparation, cell, chemicals and the instrumental setup for elec- 
trodeposition, potentiostatic and potentiodynamic measurements have been de- 
scribed previously [23]. A titanium wire (Goodfellow Metals, 99.6% purity, 0.25 cm* 
geometrical area) was polished with emery paper and 1 pm alumina powder, washed 
with quadruply distilled water and treated potentiodynamically from - 1.1 to + 0.5 
V vs. SCE for 2 min at 2 Hz in 1 mol dme3 NaOH solution (Fluka, puriss.). 
AC impedance measurements were carried out using a Model 1250 frequency 
response analyzer and a Model 1286 electrochemical interface (Schlumberger- 
Solartron Co.) connected to a Hewlett-Packard 9816 computer. The frequency range 
was from 0.5 Hz to 2 kHz with a signal amplitude of 5 mV. 
RESULTS AND DISCUSSION 
Current efficiency and the thickness of the rhodium layer 
The cathodic efficiency was determined using a titanium plate of 2 cm2 area and 
the same experimental conditions (40 mA cm-’ current density, 15 min deposition 
time) as were used in electroplating the titanium wire electrode. The amount of 
electrodeposited rhodium at 2 cm2 titanium plate was 4.4 X 10d4 g, giving 2.3% 
current efficiency and 1.3 X 10” atoms/cm*; the rest of the current was consumed 
in the hydrogen evolution reaction. As a number of 1.3 X lOi atoms per cm* in one 
layer of platinum has been suggested as a standard for other noble metals [6] this is 
equivalent to 1000 layers of electrodeposited rhodium. The true surface area of the 
galvanostatically electrodeposited rhodium was previously determined by measuring 
the voltammetric charge of hydrogen deposition [23]; 0.25 cm’ of geometrical area 
had 198 cm’ real data. The number of rhodium atoms calculated from this value is 
1.0 X lOi atoms/cm’ of titanium substrate. Therefore, 77% of the electroplated 
rhodium atoms were available for hydrogen deposition, which is not surprising in 
215 
the case of such porous electrodes. The voltammetric charge, however, as a surface 
process, cannot always be used as a measure of metal loading, except under certain 
experimental conditions. It has been shown [23] that the charge of hydrogen 
deposition is proportional to the deposition time up to 15 min with 40 mA cm-* 
current density, i.e. it is proportional to the metal loading. 
Potentiodynamic and ac impedance characterization of the electrodes 
Typical surface oxidation-reduction processes, which characterize the elec- 
trodeposited rhodium in a potentiodynamic experiment, are presented in Fig. 1. 
Hydrogen ionization is followed by oxide formation after -0.65 V and by a family 
of reversible surface oxidation-reduction curves from -0.65 to -0.2 V. The oxide 
layer, formed at the most positive potentials (curves 10-15) is reduced irreversibly, 
the last one overlapping completely with hydrogen deposition. Oxide formation 
starts immediately after hydrogen ionization at -0.65 V and it is evident that a 
double layer charging region is absent, contrary to the potentiodynamic behaviour 
of bright rhodium [8] and electrodeposited rhodium [23] electrodes in sulphuric acid. 
As can be seen, the electrodeposited layer of rhodium in an acidic solution behaves 
like platinum in sulphuric acid, while in the alkaline solution it is more similar to 
the potentiodynamic profile of ruthenium in sulphuric acid [24,25]. The potentio- 
dynamic profiles of a potentiostatically electrodeposited rhodium electrode (Fig. 2) 
as well as the profile of a rhodium wire electrode (bright rhodium) (Fig. 3) are 
similar to the potentiodynamic profile of a galvanostatically deposited rhodium 
electrode (Fig. 1). The main differences are in the current values due to the different 
surface roughnesses of the three types of rhodium electrode. The true surface areas 
were determined by measuring the charge of hydrogen deposition in 0.5 mol dme3 
H,SO, and found to be 2.5 cm2, 18.7 cm2 and 198 cm* for the rhodium wire 
-60 
-1.2 -0.8 -0.4 0 0.4 
E/V vs. SCE 
Fig. 1. Cyclic voltammogram of a galvanostatically deposited rhodium electrode on a titanium substrate 
in 1 moldm- 3 NaOH recorded at a sweep rate of 50 mV s-’ in 100 mV increments in the positive 
direction. 
-2.0 
-0.8 -0.4 0 OX 
E/V vs. SCE 
Fig. 2. Cyclic voltammogram of potentiostatically deposited rhodium on a titanium substrate (- ) 
and the same electrode activated by square-wave pulses at 2 Hz from - 1.1. V to +0.45 V for 5 min (- 
- -_). 1 mol dm-’ NaOH. 
electrode, the potentiostatically electrodeposited rhodium electrode and the galvano-statically deposited rhodium electrode, respectively [23]. It was also shown that the 
surface roughness depends on the current density and deposition time. The same 
experimental procedures were carried out in electrode preparation in this work. The 
currents are expressed versus the geometrical area of a titanium substrate. 
The ac impedance measurements were carried out at the characteristic potentials 
of the cyclic voltammogram of a rhodium electrode in 1 mol dmP3 NaOH. The 
results are presented in the form of complex plane impedance plots (Fig. 4) and 
complex admittance plots (Fig. 5). The electrolyte resistances were determined as 
, 
0.4 I 
-0q , , , , , 
-1.2 -0.8 -0.4 0 0.4 
E/V vs. SCE 
Fig. 3. Cyclic voltammogram of a rhodium wire electrode in 1 mol dme3 NaOH. Sweep rate 50 mV s-‘. 
277 
Fig. 4. Complex plane impedance plot for an electrodeposited rhodium electrode on a titanium substrate 
in 1 mol dm-3 NaOH. 
high frequency intercepts of Z,&, and were subtracted from the measured cell 
impedance. The impedances are almost linear lines, with a phase angle greater than 
45 O, indicating that the capacitive component is predominant. The complex admit- 
6 
0 
0 2 L 6 8 
Fig. 5. Complex plane admittance plot for an electrodeposited rhodium electrode on a titanium substrate 
in 1 mol dm-3 NaOH. 
278 
tance plot (Fig. 5) shows a circular shape followed by a linear part, indicating two 
parallel processes [26] on the porous electrode surface, one of which is the adsorp- 
tion of hydroxyl ion. 
Oxide growth and charge enhancement factor 
The enhanced growth of oxide on bright rhodium in alkaline solution under 
potentiodynamic cycling is a well known phenomenon [2-41. Electrodeposited 
rhodium behaves in a similar way, with some differences, however, as far as the 
reversibility of the surface reaction and the charge enhancement factor (CEF) are 
concerned. The charge enhancement factor is defined by Mozota and Conway [27] 
as the ratio of the voltammetric charge of an oxide monolayer to the charge of a 
grown oxide film. Figure 2 shows the cyclic voltammogram of a potentiostatically 
electrodeposited rhodium electrode and the cyclic voltammogram when the same 
electrode was activated with a square-wave pulse from - 1.1 to +0.45 V vs. SCE at 
2 Hz for 5 min. As in potentiodynamic cycling, this procedure also leads to oxide 
growth and electrode activation. Inspection of Fig. 2 shows that two processes occur 
simultaneously during this treatment. There is a decrease of the voltammetric charge 
in the hydrogen region, probably due to rhodium dissolution and decrease of the 
real surface area. The second process is a change in the chemistry of the oxide film 
due to electron transfer between hydrated oxohydroxide species of Rh(II1) and 
Rh(IV) [3]. In the positive region around + 0.4 V (Fig. 2), there is an increase in the 
voltammetric charge of the activated electrode as a result of this change in the 
oxidation state. The change in the oxidation state towards higher oxides favours also 
the electrocatalysis of the OER. Hoare [28] and Tseung and Jasem [29] have 
proposed as a general rule, that if there is more than one form of the oxide present, 
the better electrocatalyst for the OER will be the one whose potential is closer to the 
potential of an oxygen electrode in the same solution. The enhanced electrocatalytic 
activity for the OER of the square-wave-treated electrode is evident in Fig. 2, where 
the OER current at +0.5 V is a factor of two higher than the current at the 
unactivated electrode. This process was also investigated by Burke and O’Sullivan 
[5] as a function of the charge capacity of the oxide film. Figure 6 shows cyclic 
voltammograms of a rhodium wire electrode treated in the same way as the 
potentiostatically electrodeposited rhodium in Fig. 2. Figure 6 is a typical example 
of a reversible surface reduction of oxide characterized by the sharp transition from 
anodic currents after switching the potential. The cyclic voltammogram of the 
potentiostatically electrodeposited and activated electrode (Fig. 2) shows less re- 
versible reduction of the oxide but is still more reversible than that of the bright but 
unactivated rhodium wire electrode (Fig. 3). Part of the oxide is reduced in a rather 
broad peak from +0.4 to +0.2 V (Fig. 2, dashed line) while at the rhodium wire 
electrode the majority of the oxide is reduced near the hydrogen region. It is also 
evident in Fig. 2 that the largest amount of oxide is reduced at the potentials from 
-0.4 to -0.9 V. Figures 2 and 6 also show that the CEF is considerably higher on 
the rhodium wire electrode than on the potentiostatically electrodeposited one. The 
CEF values are 14 in the case of the rhodium wire electrode and 2 in the case of the 
219 
Fig. 6. Cyclic voltammogram of a rhodium wire electrode activated by square-wave pulses of 2 Hz from 
- 1.1 V to +0.45 V for 5 min. Sweep rate 50 mV s-’ with 100 mV increments in the positive direction; 1 
mol dme3 NaOH. 
potentiostatically electrodeposited rhodium electrode after the pretreatment condi- 
tions given in Figs. 2 and 6. The CEF is absent in the case of the galvanostatically 
electrodeposited electrode. The latter has the highest surface roughness and it seems 
that the compactness of the metal lattice is responsible for oxide growth under such 
conditions. The porous nature of the electrodeposited layers is not so sensitive to the 
activation process which leads to oxide growth. This activation, however, changes 
the galvanostatically electrodeposited rhodium layer as far as the reversibility of the 
surface process is concerned. The oxide reduction is more reversible than at an 
untreated electrode. 
Oxygen eoolution and the stability of the oxide film 
Figure 7 shows potentiostatic polarization data for the OER in 1 mol dme3 
NaOH on three types of rhodium electrode. The increase in current at the same 
potentials is not due to an increase in electrocatalytic efficiency, but is the result of 
an increase in the roughness factor, as was already discussed earlier in this paper. 
The same Tafel slopes indicate that the mechanism of the OER is the same for all 
types of rhodium electrode. The Tafel slope of 125 mV/decade, close to 2RT/F, 
suggests that the electrochemical oxide path proposed by Bockris [30] which was 
also observed by Damjanovic et al. [lo] in the case of a rhodium wire electrode in 1 
mol dm- 3 KOH, holds also for the electrodeposited rhodium electrode. 
Resistance of the electrode material against corrosion is one of the main 
requirements for a good electrocatalyst. The stability of the electrodeposited rhodium 
layers on a titanium substrate was monitored at a fixed potential of + 0.8 V vs. SCE 
in 1 mol dm- 3 NaOH as a function of time. Figure 8 illustrates the change in 
current during such a polarization of the galvanostatically electrodeposited rhodium 
electrode and, for comparison, of the rhodium wire electrode and the rhodium oxide 
electrode. The latter was prepared by heating the galvanostatically deposited rhodium 
at 923 K in air for 1 h. This is the temperature of the Rh to Rh,O, transition [31]. 
280 
E/V 
SCE 
1.0 
0.9 
0.6 
0.7 
0.6 
0.5 
0.4 
0.3 
J 
0.001 0.01 0.1 1 10 100 
i/mAcme2 
Fig. 7. Potentiostatic polarization curves for the OER in O,-saturated 1 mol drne3 NaOH solution on a 
rhodium wire electrode (0); on a Rh electrode potentiostatically deposited on Ti (X); and a Rh electrode 
galvanostatically deposited on Ti (v). Data were taken after 2 min at each potential value and were 
corrected for the IR-drop using current interruption. 
60 
r 
r \ 
-0 0 0 
X X 
/ 
1 
I I I I 
2 3 L 5 
t/h 
ol- 
Fig. 8. The OER current at +0.8 V in 1 mol dm -3 NaOH on a rhodium wire electrode ( x ), on a 
rhodium electrode galvanostatically deposited on titanium (0) and on a thermally treated (923 K) 
galvanostatically deposited rhodium electrode (v). 
281 
The currentis higher on the electrodeposited electrode, but only as a result of 
surface roughness. A recalculation of the currents to the true surface area would 
show that the most efficient electrode reaction occurs on the rhodium wire elec- 
trode. This was also the case for the OER on rhodium electrodes in sulphuric acid 
[23]. This is not surprising, since the release of oxygen bubbles which block the 
surface of the electrode is much faster from the more compact metal lattice of the 
bright rhodium than from the porous layer of the electrodeposited rhodium. What is 
of importance, however, is the stability of the electrodeposited rhodium layer (Fig. 
8). The decrease in current is not due to electrode dissolution. Cyclic voltammo- 
grams recorded before and after this polarization did not show any significant 
change of the voltammetric charge of the hydrogen region as a result of changes in 
the real surface area or electrode dissolution. The voltammetric charge, as a surface 
process, cannot, in principle, be used as a measure of electrocatalyst loading. In the 
case of such porous electrodes, as was already said before, the voltammetric charge 
is proportional to the electrocatalyst loading. In contrast, the dissolution of such a 
porous layer would lead to a decrease in the voltammetric charge. This was not the 
case with the electrodeposited rhodium in this work. These findings differ from 
those for the stability of electrolytically grown oxide films on iridium [32] or of 
electrodeposited ruthenium films [33] which dissolve under anodic polarization in 
sulphuric acid and require heat treatment to stabilize the oxide films [32,34]. 
ACKNOWLEDGEMENTS 
This work was supported by the Self-managed Authority for Scientific Research, 
SR of Croatia. A Technical Assistance Grant of the International Atomic Energy 
Agency (IAEA 512-C2-YUG-4.023) is gratefully acknowledged. We thank Dr. 
KreSirnir Kvastek and Mr. Dalibor Hodko for helpful discussions and Mr. Momir 
Milunovic and Mr. SreCko KaraSic for technical assistance. 
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