Degradation of polycrystalline rhodium and rhodium nanoparticles

Article abstract of DOI:10.1016/j.electacta.2012.03.079

The electrochemical stability of polycrystalline rhodium and rhodium nanoparticles is quantitatively investigated in non-complexing sulfate electrolyte under potential cycling conditions. In situ measurements of the active surface area are complemented by

Full text of DOI:10.1016/j.electacta.2012.03.079

Electrochimica Acta 70 (2012) 355–359
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Degradation of polycrystalline rhodium and rhodium nanoparticles
a,b,∗
, Ioannis Katsounarosb,1, Sebastian O. Klemm , Josef C. Meierb,1, Karl J.J. Mayrhoferb,∗∗,1
b
Arndt Karschin
a
Insitut für Anorganische Chemie und Strukturchemie, Lehrstuhl I: Bioanorganische Chemie und Katalyse, Heinrich Heine Universität Düsseldorf, Universitätsstraße 1, 40225
Düsseldorf, Germany
b
Abteilung für Grenzflächenchemie und Oberflächentechnik, Max-Planck-Institut für Eisenforschung, Max-Planck-Straße 1, 40237 Düsseldorf, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemical stability of polycrystalline rhodium and rhodium nanoparticles is quantitatively
investigated in non-complexing sulfate electrolyte under potential cycling conditions. In situ measure-
ments of the active surface area are complemented by discrete elemental analysis of the electrolyte
solution. Rhodium electrodes are not stable and dissolve upon potential excursions into the oxide region
above approximately +0.5 VRHE; the higher the positive potential limit, the higher the mass loss rate per
cycle. Interestingly, the normalized catalyst mass loss is independent of the initial catalyst loading under
identical conditions. The dissolution of Rh and the concomitant structural changes have to be considered
in any further electrocatalytic study of this kind of electrodes.
Received 28 October 2011
Received in revised form 17 March 2012
Accepted 19 March 2012
Available online 27 March 2012
Keywords:
Rhodium
Noble metals
Stability
©
2012 Elsevier Ltd. All rights reserved.
Dissolution
Nanoparticles
1
. Introduction
decreased during cycling, by performing EQCM measurements. Fur-
thermore, it was shown that the rhodium dissolution is affected
by the scan rate as well as by the upper potential limit [16].
The extent of dissolution from EQCM studies can then be calcu-
lated indirectly from the frequency change between consecutive
cycles.
The electrochemical properties of polycrystalline rhodium
electrodes have been widely studied mostly using the cyclic
voltammetry technique [1–4]. The adsorption and desorption
processes of sulfate anions as well as of hydrogen and oxygen
species on rhodium surfaces during potentiodynamic scans are
well understood due to the aid of complementary techniques like
FTIR spectroscopy [5], electrochemical impedance spectroscopy
High surface area (HSA) nanocatalysts are more and more
in the focus of interest due to their very high mass-normalized
activities. In several reactions, rhodium alloys are used as nanopar-
ticulate electrocatalysts as for example, Pt/Rh in ethanol oxidation
[18–21] or Rh/Ir in borohydride oxidation [22]. Even though the
activity of the catalytic material is typically of primary inter-
est in many electrocatalytic studies, stability aspects are also
highly important, if not more important, in particular when deal-
ing with nanoparticulate catalysts [23–26]. Dissolution of noble
metal catalyst components can severely alter the surface com-
position and structure and thus their electrocatalytic properties.
In this paper we therefore study the stability of a polycrystalline
rhodium electrode and of HSA rhodium catalysts in sulfuric acid,
before investigating any electrocatalytic reaction thereon. Since
the EQCM is only applicable to planar electrodes, we instead
utilize a combined approach by following during electrochemi-
cal treatment both the decrease of the real (active) surface area
in situ as well as the concentration of dissolved rhodium in
the electrolyte by Inductively Coupled Plasma Mass Spectroscopy
(ICP-MS).
(
EIS) [6,7], infrared reflection adsorption spectroscopy (IRAS) [8,9],
radioactive labeling [4,10,11] or the electrochemical quartz crystal
microbalance (EQCM) [12–14]. Even though rhodium as a noble
metal catalyst was initially thought to be resistant to corrosion
in acid electrolytes, Rand and Woods have shown that rhodium
dissolution takes place during cyclic voltammetry up to suffi-
ciently positive potentials even in non-complexing acid solutions
like sulfuric acid [15]. Recently, Czerwi n´ ski and co-workers [16]
as well as Juodkazis et al. [17] showed that the electrode mass
∗ Corresponding author at: Insitut für Anorganische Chemie und Strukturchemie,
Lehrstuhl I: Bioanorganische Chemie und Katalyse, Heinrich Heine Universität Düs-
seldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany. Tel.: +49 211 6792 160;
fax: +49 211 6792 218.
∗
∗ Corresponding author. Tel.: +49 211 6792 160; fax: +49 211 6792 218.
E-mail addresses: akarschin@gmail.com (A. Karschin), mayrhofer@mpie.de
(
K.J.J. Mayrhofer).
1
ISE Member.
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.03.079

3
56
A. Karschin et al. / Electrochimica Acta 70 (2012) 355–359
2
. Experimental
under an Argon atmosphere. After stirring for 30 min at room
temperature the solution was purged with hydrogen gas for 24 h,
which resulted in a black solution of dissolved rhodium NPs.
High Resolution Transmission Electron Microscopy (HR-TEM)
analysis proved the existence of particles with a mean diameter of
2.2 ± 0.8 nm (see Fig. S2 in Supporting Information). The rhodium
2
.1. Electrochemical measurements
All electrochemical experiments were carried out in a three
compartment Teflon cell (see Fig. S1 in the Supporting Information)
at room temperature using a Reference 600 potentiostat (Gamry,
USA) and a rotating disk electrode (RDE) setup with an analytical
rotation controller (Radiometer, France). A polycrystalline rhodium
disk or rhodium nanoparticles (NPs) attached on a glassy car-
bon disk (both 0.196 cm2 geometrical area and 3 mm thickness)
were used as working electrodes. The polycrystalline rhodium and
the glassy carbon disks were ground prior to the measurements
with SiC grinding paper (2500 and 4000 grit) and polished with a
−
1
content of the prepared solution was 130 mg mL . In a standard
procedure, 40 ␮L of the rhodium nanoparticle solution were
pipetted on the glassy carbon disk, which corresponds to a loading
−
2
of 26.6 ␮gRh cm . The solvent was thereafter removed by vacuum
evaporation in a desiccator. For higher loadings this procedure
was repeated accordingly after removing the solvent from the
electrode surface.
3
␮m and a 1 ␮m diamond suspension. A graphite rod and a satu-
3
. Results and discussion
rated Ag/AgCl electrode (Metrohm, Germany) placed into a Tschurl
modification [27] were used as counter and reference electrodes,
respectively. However, all potentials in this paper are expressed
with respect to the reversible hydrogen electrode (RHE) potential.
The potentiostat, the rotator and the gas flow were automati-
cally controlled and programmed using an in-house built control
software based on LabVIEW [28]. All cyclic voltammograms were
3.1. Degradation of a polycrystalline rhodium electrode
A typical cyclic voltammogram of a polycrystalline rhodium
electrode in 0.1 M H SO in the potential region between +0.07 VRHE
2
4
and +1.07 VRHE is shown in Fig. 1A. In agreement with previ-
ous investigations in sulfuric acid electrolyte [3,16,35] the cyclic
voltammogram consists of five main regions, noted in the figure:
−1
recorded with a scan rate of 0.1 V s
with a negative potential
limit of always +0.07 VRHE. The experiments were performed in
(a) hydrogen desorption (HUPD region); (b) double-layer region; (c)
−
.1 M H SO solution under continuous Argon flow (200 mL min ).
2 4
1
0
surface oxide formation region; (d) surface oxide reduction and (e)
hydrogen adsorption (HUPD region). The cyclic voltammogram is
qualitatively similar with that of a polycrystalline platinum sur-
face [36], the only difference being the less defined double-layer
region particularly in the negative scan due to the overlapping of
the surface oxide reduction with the hydrogen adsorption region
Electrolyte solutions were freshly prepared using ultrapure water
®
(
18 Mꢀ, PureLabs ) and concentrated sulfuric acid (98%, Merck).
All gases were of 99.999% purity (AirLiquide, France). The rotation
rate of the working electrode during the degradation cycles was set
to 100 rpm to ensure homogeneous mixing of dissolved rhodium
species in the electrolyte. The solution resistance was compensated
by positive feedback and the residual uncompensated resistance
was less than 3 ꢀ in all experiments. Samples of the electrolyte
[
10,16,37]. The oxide reduction is extending even into the HUPD
region, thus the absolute currents in the negative scan are some-
what higher than in the positive scan. The response was not affected
by the rotation rate, thus it was confirmed that the downward-shift
of the cyclic voltammogram in the HUPD region is only due to a non-
faradaic process, i.e. the superimposed oxide reduction current. The
hysteresis observed between the surface oxide formation (c) and
reduction (d) region is an indication of irreversible oxide formation;
that is, the mechanisms of surface oxide formation and reduction
are different [38]. It should be noticed that the increase of the upper
potential limit shifts the oxide reduction peak toward even more
negative potentials [1,35], due to more extensive irreversible oxide
formation.
In order to study the degradation of a polycrystalline rhodium
electrode over time, 1200 cyclic voltammograms within a fixed
potential window were recorded continuously, with a scan rate
of 0.1 V s . Thereby the concentration of dissolved rhodium in the
electrolyte during the treatment was monitored by discrete sam-
pling and elemental analysis using ICP-MS after certain amount of
cycles. The lower potential limit was the same in all experiments
(
1 mL) were withdrawn at specific times using a syringe through
a small hole in the cap of the Teflon cell (main compartment –
see Supporting Information, Fig. S1). The concentration of dissolved
rhodium in the samples was determined by an ICP-MS system (Nex-
ION 300X, Perkin Elmer), which was equipped with a cyclonic spray
chamber and a Meinhard nebulizer operating at a pump rate of
around 300 ␮L min . An internal standard ( In, 5 ppb) was added
to each sample and measured parallel to 1 Rh to compensate for
instrument fluctuations.
−1
115
03
2.2. Real surface area determination
The real surface area of the Rh samples was determined based
on the integration of the area of desorption of the underpotentially
deposited hydrogen (so-called HUPD) during the cyclic voltammo-
grams. For this, the saturation coverage of hydrogen on Rh (0.59 ML
−
1
[
29,30]) and the charge during the adsorption/desorption of a full
−2
monolayer of HUPD (221 ␮C cm ) [3,30] were taken into account.
The HUPD adsorption on Rh is influenced by the co-adsorption of
electrolyte anions [4,8,9,11,31] and the charge integration is more
prone to systematic error [32,33]; however, it is directly accessi-
ble from every degradation cycle without any interruption of the
degradation experiment. Despite the fact that the exact quantita-
tive determination may not be fully reliable, the systematic error
remains constant so that this approach is sufficiently accurate to
monitor relative changes in the surface area following the HUPD
response.
(
(
+0.07 VRHE) whereas three different upper potentials were used
+0.87, +1.07 and +1.27 VRHE) in the three measurements that were
performed using the same procedure. The mass of rhodium m(Rh)
in the electrolyte (in ␮g) was calculated by
m(Rh) = c(Rh) · V
(1)
where c(Rh) is the concentration of Rh as measured by ICP-MS (in
−
3
3
␮g cm ), and V is the volume of the electrolyte (in cm ) in the
main compartment of the electrochemical cell. Fig. 1B shows the
mass of Rh in the electrolyte vs. the number of cycles. The mass of
rhodium in the electrolyte increases linearly with the number of
cycles, implying a constant rate of dissolution over time. Thereby
the real surface area of the polycrystalline electrode estimated from
the HUPD is not changing significantly. The rate of dissolution cal-
culated from the slope of the linear fit of the measured rhodium
2.3. Nanoparticle synthesis and film preparation
RhCl ·3H O
(10 mg, 38 ␮mol)
and
sodium(2-
3
2
(
[
diphenylphosphino)ethyl) phosphonate (11.4 mg, 38 ␮mol)
34] were dissolved in a 100 mL flask filled with water (30 mL)

A. Karschin et al. / Electrochimica Acta 70 (2012) 355–359
357
Fig. 1. (A) Cyclic voltammogram of a polycrystalline rhodium electrode in 0.1 M H2SO4. Scan rate: 0.1 V s−1; rotation rate: 100 rpm. (B) Mass of dissolved rhodium species
in the electrolyte vs. the number of cycles during the degradation of a polycrystalline rhodium electrode using three different positive potential limits: +0.87 VRHE (black
squares); +1.07 VRHE (red circles); +1.27 VRHE (blue triangles). The degradation rate shown in the inset corresponds to the slope of the linear fit for each experiment. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
mass increased almost linearly as the positive potential limit
leading to a gradual decrease of the real surface area of the elec-
trode. In order to study the stability of the prepared rhodium NPs,
the same degradation tests as for the polycrystalline rhodium elec-
trode were performed. In Fig. 3 the charge in the HUPD region is
plotted over the number of cycles, with the inset graph contain-
ing the complementary data on the mass of rhodium dissolved into
the electrolyte (calculated from Eq. (1)) vs. the number of cycles,
using different upper potential limits. During the first 100 cycles a
very fast increase in the HUPD charge is observed, which is related
to the cleaning of the particles due to oxidation of the inorganic
ligand. However, already at this stage a second competitive pro-
cess, namely the dissolution of the particles causing a decrease of
the real surface area, starts to occur in parallel and becomes dom-
inating after full surface utilization is reached. In the case of the
−
1
increases, i.e. in the order +0.87 VRHE (0.66 ng cycle ) < +1.07 VRHE
− −1
1
(
1.24 ng cycle ) < +1.27 VRHE (2.24 ng cycle ). This is related to the
fact that the application of higher potentials causes more extensive
irreversible oxide formation, as it has been shown by cyclic voltam-
metry for noble metals [38], which obviously increases the rate of
dissolution. The exact mechanism of dissolution and the relation
to the oxidation/reduction of the surface, however, is still not clear
and will require further investigations. For the uppermost potential
limit, taking into account the roughness factor of the electrode (1.3)
−1
−1
the rate of 2.24 ng cycle corresponds to a loss of 0.04 ML cycle ,
which is slightly higher from that calculated from Czerwi n´ ski and
co-workers under similar conditions [16].
3.2. Degradation of rhodium nanoparticles
In the case of nanoparticle catalysts, no rhodium features are vis-
ible during the first cycles, because the metal surface is not exposed
to the electrolyte. The rhodium particles are covered by the inor-
ganic ligand sodium(2-(diphenylphosphino)ethyl)phosphonate
that is used in the synthesis procedure. Electrochemical oxidation
of the ligand cleans the surface so that rhodium features start to
increase gradually during the initial potential cycles and maximize
after around 50–100 cycles, depending on the positive potential
limit (see Fig. S3 in Supporting Information). Thus, the curve (1) in
Fig. 2 shows the cyclic voltammogram of a rhodium nanoparticle
−2
catalyst (loading: 53.2 ␮gRh cm ) after 100 cycles in the potential
region between +0.07 VRHE and +1.07 VRHE. All typical rhodium fea-
tures as explained above for the polycrystalline electrode (Fig. 1A
regions a–e) are visible. Note, however, that the measured cur-
rents for the high surface area (HSA) catalyst are approximately one
order of magnitude higher compared to that for the polycrystalline
surface, due to the substantially higher roughness of the former.
When cycling the HSA catalyst for longer times, the measured
currents are becoming lower and typical rhodium features are
decreasing (Fig. 2), which is attributed to rhodium degradation
Fig. 2. Cyclic voltammograms of rhodium NPs (53.2 ␮gRh cm−2) attached on glassy
carbon in Ar-saturated 0.1 M H2SO4 solution after (1) 100, (2) 175, (3) 250, (4) 375,
−
1
(5) 500 and (6) 1000 cycles. Scan rate: 0.1 V s ; rotation rate: 100 rpm.

3
58
A. Karschin et al. / Electrochimica Acta 70 (2012) 355–359
Fig. 3. Charge in the HUPD region vs. the number of cycles during the degradation
tests using different positive potential limits. The inset graph shows the measured
mass of rhodium in the electrolyte vs. number of cycles, and the expected maximum
mass after complete Rh dissolution (5.2 ␮gRh). The curves in the inset serve as a
Fig. 4. Charge in the HUPD region vs. number of cycles using different catalyst load-
ings. The inset graph shows the mass of rhodium in the electrolyte measured with
ICP-MS vs. the number of cycles. For each measurement, the final measured mass
of rhodium is given, as well as the expected mass (in brackets) if the entire catalyst
was dissolving. The upper potential limit was +1.07 V in all measurements.
−2
guide to the eye. Rh-loading: 26.6 ␮gRh cm ; Electrolyte: 0.1 M H2SO4; scan rate:
0
.1 V s−1; rotation rate: 100 rpm. Abrupt decreases of the HUPD charge for +0.87 VRHE
labeled with “film detachment” in the figure (not reproducible between replicates)
are intentionally shown to raise the awareness for the effect of film detachment as
a major technical difficulty in thin-film RDE measurements.
with the number of cycles, as for the bulk electrode (Fig. 1A) because
the real surface area of the electrode (i.e. the surface roughness)
decreases during the treatment.
uppermost potential limit (+1.27 VRHE) the highest HUPD charge is
obtained already after 30 cycles after a steep initial increase of the
HUPD charge, much earlier compared to the other upper potential
limits. This implies that the increase of the upper potential limit
has also a strong effect on the rate of the oxidation of the inorganic
ligand, besides the accelerated dissolution of rhodium particles.
Within the two experimental data sets of +0.87 and +1.07 VRHE
in Fig. 3, there are some small but important differences, even
though the surface area decrease seems quite similar over the first
In order to investigate further the influence of the surface
roughness on the rate of mass loss, additional degradation exper-
iments where performed within a potential window of +0.07 and
+1.07 VRHE using different catalyst loadings of the rhodium NPs. As
it is shown in Fig. 4 the charge in the HUPD region and the mass of
rhodium in the electrolyte (calculated from Eq. (1)) both depend
on the loading as well. Note that again the measured final mass of
rhodium in the electrolyte is always slightly (5–10%) lower than
the expected one (see value in brackets in the inset), if we assume
that no catalyst is remaining on the glassy carbon after the treat-
ment. Regardless of the loading, the catalyst was fully degraded
after about 1000 cycles, in line with the measurement shown in
Fig. 3 for the same potential window.
When the measured mass of rhodium in the electrolyte was nor-
malized to the corresponding mass of rhodium that was initially
deposited on the glassy carbon substrate for all three loadings, then
the percentage of the rhodium that was dissolved vs. the number
of cycles was the same, regardless of the loading (see Fig. 5). The
same is also true for the loss of rhodium surface area normalized
to the area after 60 cleaning cycles (see inset in Fig. 5). This proves
that the rate of catalyst mass loss is directly proportional to the Rh
surface in contact with electrolyte, and explains the profile of the
dissolved mass vs. number of cycles for the case of NPs. In addition,
it shows that a single degradation rate cannot be deduced for the
nanoparticles from these data, since it continuously changes over
time.
The investigations of the stability of this kind of NP electrodes
now set the basis for upcoming electrocatalytic investigations. It is
clear that under the conditions presented in this manuscript, the
cluster network undergoes severe morphological changes and that
different approaches have to be employed accordingly in order to
retrieve reproducible and meaningful data on activity. Further stud-
ies will also try to reveal the microscopic structure that develops
during the potential cycling treatment, which is not clear at the
moment but will additionally help to understand the NP electrode
degradation. Most importantly, utilizing our novel direct coupling
of an electrochemical flow cell to the ICP-MS we will try to shed
more light onto the exact dissolution mechanism of Rh electrodes
6
00 cycles. In the case of an upper potential limit of +0.87 VRHE an
abrupt decrease of the measured HUPD charge from one CV to the
next occurs, indicating catalyst film detachment from the carbon
electrode. Taking this artifact into account, the true rate of mass
loss is much lower for the +0.87 VRHE degradation, as can also be
anticipated from the behavior beyond 600 cycles where no film
detachment deteriorates the HUPD decrease. Note that detachment
of single particles might additionally take place. This might also be
the case for the other two measurements, even though this is not
visible as for the above mentioned measurement. The NPs would
then precipitate on the bottom of the cell and are not sampled for
the analysis, which could explain why the final mass of rhodium
measured by ICP-MS is always slightly lower than the expected
mass of rhodium assuming that all particles are dissolved into the
electrolyte (see inset of Fig. 3). Moreover, this deviation can par-
tially also be explained by the small amounts (<5%) of rhodium
species detected in the reference electrode compartment due to
diffusion through the Teflon hose connector. Although they are dif-
ficult to quantify, these potential systematic errors are the main
determinants for the accuracy of the rate of mass loss.
In agreement with the degradation results on a polycrystalline
rhodium electrode, the degradation rate on NPs again increases as
the positive potential limit increases, as it can be seen by the faster
decrease of the HUPD charge and the faster increase of the rhodium
mass for the higher potential limit experiment. For instance, in the
case of an upper potential limit of +1.27 VRHE, a complete degra-
dation of the nanoparticle catalyst occurs already after 400 cycles,
whereas in the case of an upper potential limit of +0.87 VRHE the
degradation process takes more than 1000 cycles. The increase of
the mass of rhodium in the electrolyte does not increase linearly

A. Karschin et al. / Electrochimica Acta 70 (2012) 355–359
359
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2012.03.079.
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2
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We thank Andrea Mingers for the measurement of all ICP-MS
data. J.C.M. acknowledges financial support by the Kekulé Fellow-
ship from the Fonds der Chemischen Industrie (FCI).
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