DOI: 10.1002/cphc.200500646
Hydrogen Electrocatalysis**
Ludwig A. Kibler*[a]
A selection of recent theoretical and experimental studies on electrolytic hydrogen evolution is presented. It is demonstrated with
well-defined model surfaces that this reaction is a very structuresensitive process. Crystallographic orientation, defect density and
surface composition are parameters that determine the local
geometric and electronic surface structure, and are thus crucial
for the electrocatalytic activity as characterised by the exchange
current density. The observed trends can be understood within a
recent theory by J. K. Nørskov et al., which is based on density
functional calculations and which emphasises the impact of hydrogen chemisorption energies on the reaction rate, that is, on
the exchange current density. The particular electrocatalytic activities of ultrathin metal films and of nanostructures are addressed.
1. Introduction
laboratory related to the hydrogen reaction on various metal
surfaces of different structure and composition.
The reversible electrochemical reaction in Equation (1):
Hþ ðaqÞ þ e ! 1=2 H2ðgÞ
E 0 ¼ 0 V versus the standard hydrogen electrode ðSHEÞ
ð1Þ
needs to be catalysed by an electronic conductor, that is, it
takes place at an electrode surface. A huge variety of possible
materials exist, including metals, semiconductors and enzymes,
the active centres of the latter also contain metal atoms, such
as Ni or Fe.[1] It is well-known that the electrocatalytic activity
of an electrode, which determines the reaction rate at a certain
potential, depends strongly on its chemical composition. This
dependence has often been reduced to the question of electrode material. However, it has not been fully understood why
various materials show differences in activity and, moreover,
how structural and electronic properties of an electrode surface impact the reaction rate.
While progress in catalysis and electrocatalysis still often
relies on empirical laws and observations, a main current interest lies in a basic understanding of fundamental relations between an electrode’s structure (geometric and electronic) and
its activity for a given reaction, in order to set up systematic
trends. In order to get information at an atomic level, one of
the strategies is to combine theoretical with experimental
work for a reaction as simple as possible, taking place at the
interface of a clean and structurally well-defined electrode surface in contact with an electrolyte free from contaminants. The
hydrogen reaction, Equation (1), certainly belongs to the most
widely studied electrochemical reactions, first because of its
technological importance, and secondly because it is supposed
to be one of the apparently simple electrocatalytic reactions
that are suited for model studies, for example, with singlecrystal surfaces.
This Minireview shall give a survey on selected literature
studies, as well as on recent results obtained in the author’s
ChemPhysChem 2006, 7, 985 – 991
2. Reaction Kinetics
It is generally accepted that there are three possible reaction
steps in the hydrogen evolution reaction (HER) in acidic
media:[2]
1) Volmer or discharge reaction in Equation (2):
Hþ ðaqÞ þ e ! Had
ð2Þ
2) Tafel or combination reaction in Equation (3):
2 Had ! H2,ad
ð3Þ
3) Heyrowsky or ion + atom reaction in Equation (4):
Hþ ðaqÞ þ Had ! H2,ad
ð4Þ
The catalytic action of the surface is to adsorb hydrogen
atoms Had after reaction of protons from solution with electrons from the electrode, and to allow for the formation of molecular hydrogen gas—or vice versa for the hydrogen oxidation reaction (HOR). Free adsorption sites, which are not included in the Equations (2–4), are needed for binding hydrogen at the electrode surface. For constant activity of protons,
constant fugacity of molecular hydrogen and constant temper[a] Dr. L. A. Kibler
Abteilung Elektrochemie, Universitt Ulm
89069 Ulm (Germany)
Fax: (+ 49) 731-502-5409
E-mail: [email protected]
[**] Presented in part at the 3rd Gerischer Symposium, “Electrocatalysis: Theory
and Experiment”, held at the Harnack-Haus of the Max-Planck-Gesellschaft,
Berlin (Germany), July 2005.
9 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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L. A. Kibler
ature, the electrocatalytic activity is crucially dependent on
composition and surface structure of the electrode, and it is
usually characterised by the exchange current density, j0, which
is proportional to the reaction rate. The exchange current density refers to the anodic or cathodic current density flowing
under conditions of dynamic equilibrium, and is defined within
the famous Butler–Volmer equation, Equation (5),[3] which relates the reaction current density j to the overpotential h:
aFh
ð1 aÞFh
j ¼ j0 exp
exp
RT
RT
“thermoneutral”, that is, DadG0 is near zero. This is especially
true for bare platinum and, interestingly, for the active centres
in hydrogen-producing enzymes, as shown in Figure 1 with
ð5Þ
In the following, we want to illuminate the physical origins of
apparent j0 values ranging over several orders of magnitude
(see Table 1), rather than analysing the reaction mechanism
and the meaning of the symmetry factor a (which is usually
around 0.5) in greater detail.
Figure 1. Free-energy diagram for the HER at various metal surfaces and
enzyme models, under standard and equilibrium conditions. Electrocatalysts
with free energies of hydrogen adsorption close to zero belong to the more
active ones. Courtesy of J. K. Nørskov et al.[11]
3. Volcano Plots
As for many other reactions, kinetic data for the HER were empirically found to correlate with thermodynamic properties of
related systems in a simple way. For example, a logarithmic
plot of j0 for various electrodes versus the respective metal–hydrogen bond strength reveals a so-called volcano curve.[7] The
shape of such a volcano plot has been rationalised independently by Gerischer and Parsons,[8, 9] who deduced that j0 is related to the standard free-energy for hydrogen adsorption DadG0.
The two linear branches of the volcano are symmetric to
DadG0 = 0. This is an example of a so-called linear free-energy
relationship, since the logarithm of the reaction rate given by
log(j0) is proportional to the free enthalpy of activation, according to transition-state theory. For experimental volcano plots,
however, data for adsorption enthalpies from the gas phase reaction have been used, rather than free adsorption enthalpies
in solution. In addition, kinetic data taken under various experimental conditions just allow the demonstration of a qualitative
trend. Consistent data for DadG0 from both theory and experiment and carefully measured j0 values still give a volcano
curve (Section 6.1 and Figure 6) that provides a more quantitative relationship.
A consistent set of hydrogen chemisorption energies DEH for
0.25 hydrogen coverage on various metal surfaces was recently
obtained by Nørskov et al. by density functional calculations
on periodically repeated metal slabs.[10] The free energies of hydrogen adsorption DadG0 were estimated to be given by Equation (6):
Dad G0 ¼ DE H þ DE ZPE T DSH ¼ DE H þ 0:24 eV
ð6Þ
where DEZPE is the difference in zero point energy between adsorbed and gas phase hydrogen and DSH is the entropy of hydrogen adsorption. With the assumption that the presence of
solvent molecules does not affect the given trends, the authors
demonstrated that the very active electrocatalysts for the HER
bind the intermediate atomic hydrogen not too strongly and
not too weakly, and allow the overall reaction to be quasi
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models for a hydrogenase and a nitrogenase. Starting from
these observations, these authors predicted a specific MoS2
structure to be of high catalytic activity, and this was successfully shown in a subsequent experiment with MoS2 nanoparticles.[11]
Moreover, these authors developed a simple kinetic model,
which explains the two branches of a volcano curve, which are
clearly separated at DadG0 = 0 (Figure 2). The reaction rate
Figure 2. Measured (top) and calculated (bottom) j0 for HER on various
metals as a function of the calculated hydrogen chemisorption energy DEH
(top) or of the free energy of hydrogen adsorption DadG0 (bottom). Courtesy
of J. K. Nørskov et al.[10]
under equilibrium conditions at pH 0 was taken to be proportional to a rate constant k1 and proportional to (1q), where q
is the hydrogen coverage. With Equation (7):
K¼
q
Dad G0
¼ exp
1q
kT
9 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ð7Þ
ChemPhysChem 2006, 7, 985 – 991
Hydrogen Electrocatalysis
and the assumption that DadG0 can be also viewed as activation energy for proton reduction, j0 can be written as Equation (8):
j0 ¼ eNk0
K
ðDad G0 0Þ
1þK
ð8Þ
where e is the elementary charge, k0 = 200 s1 site1 is the only
parameter fitted to experimental values, and N is the density
of sites. Equation (8) explains the branch on the right of the
volcano for positive values of DadG0 in Figure 2. The branch on
the left is obtained for zero activation energy, leading to k1 = k0
and Equation (9):
j0 ¼ eNk0
1
ðDad G0 < 0Þ
1þK
ð9Þ
In essence, the free energy of hydrogen adsorption is wellsuited for a correlation with exchange current densities.
flat gold surfaces in acidic solution. A further advantage of
studying HER on gold single-crystal surfaces is the rather detailed knowledge of local structural properties, as obtained by
in situ scanning tunnelling microscopy (STM).[13] However, available literature values for j0 of HER on the low-index faces of
gold show a large scatter.[15] Although different acidic solutions
have been used by various groups, the presence of anions like
sulphate or perchlorate should not play an important role in
the hydrogen evolution region. However, the details of the surface structure were often not characterised or considered.
Since electrode preparation is crucial for the quality of the surface and the distribution of various active sites, it is important
to carefully control and specify the surface structure, as will be
demonstrated in the following.
Figure 3 shows current–potential curves at 5 mV s1 for the
HER at room temperature on AuACHTUNGRE(100), AuACHTUNGRE(111) and AuACHTUNGRE(110),
measured under clean conditions and for thermally recon-
4. Measurements of Exchange Current Density
It is still a challenge to measure reliable values of j0 for clean
and structurally uniform electrode surfaces comparable to the
model surfaces used in calculations. Two main experimental
problems make a correlation with theory difficult. First, the
presence of defects, for example, monoatomic high steps,
leads to certain structural and electronic heterogeneity of the
surface. As will be shown below (Section 6.2), differences in
the metal–hydrogen bonding for each type of adsorption site
might be important for the overall activity. As a consequence,
variation of the defect density leads to changes in activity for
the HER/HOR. Secondly, mass transport is a critical issue for the
very active surfaces, especially for HOR. It has recently been
shown by the use of small platinum particles, sub-micrometer
in size, that j0 of platinum may often been underestimated by
more than one order of magnitude.[12] Catalytic effects observed with nanoparticles are discussed in Section 7. Besides
the use of microstructures, rotating disk electrodes or flow
cells are used by various research groups to avoid mass-transport limitations and to ensure the determination of kinetic
data for the hydrogen reaction. In any case, it is reasonable to
compare kinetic data, which are gathered by the same methodology, in order to obtain reliable trends that are at least
qualitative. The j0 values presented in the following were extracted from simple voltammetric scans performed at room
temperature and at a slow scan rate, by fitting the data obtained at overpotentials negative of 0.15 V to the cathodic
term of Equation (5) and correcting for the solution resistance.
Effects of diffusion have been neglected, because varying the
sweep rate leads to similar results.
5. Single-Crystal Surfaces
Gold does certainly not belong to the very active materials for
the hydrogen reaction, which is in contrast to platinum, rhodium and iridium. However, just for that reason, there should be
no big problems with mass-transport limitation for the HER on
ChemPhysChem 2006, 7, 985 – 991
Figure 3. Current–potential curves for HER on various reconstructed Au
single-crystal electrode surfaces: AuACHTUNGRE(100), AuACHTUNGRE(111) and AuACHTUNGRE(110), in 0.1 m
H2SO4 at room temperature. Scan rate v = 5 mV s1.
structed surfaces. The catalytic activity increases in the given
order (j0 = 0.05, 0.12 and 0.4 mA cm2, respectively), although all
three surfaces have a hexagonally densely packed surface. This
proves the HER is a very structure-sensitive reaction. It is important to note that these kinetic data are strongly dependent
on the quality of each individual crystal, and are only meaningful for careful surface preparation. Three structural modifications shall be discussed, which give rise to a significant change
in activity. 1) After formation of the (1 K 1)-structures and subsequent complete potential-induced reconstruction,[13] the activities were about twice as high as those of the (freshly prepared) thermally reconstructed surfaces. Since the unit cells are
identical for each surface, it is obvious that the smaller domains and the presence of domain boundaries enhance the
HER activity. 2) The use of stepped AuACHTUNGRE(111) surfaces shows that
the density of defects, such as monoatomic high steps, plays
an important role as well. A systematic increase in the defect
density by taking surfaces with a miscut leads to larger j0
values, which approach the activity of AuACHTUNGRE(110). 3) It is not easy
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987
L. A. Kibler
to investigate the HER activity of the unreconstructed Au surfaces, because of potential-induced reconstruction at negative
potentials. However, preliminary potential step experiments
with AuACHTUNGRE(100) show that the (1 K 1) structure of AuACHTUNGRE(100) has a
HER activity about one order of magnitude higher than the
thermally reconstructed surface (compare the different surface
structures for the STM images in Figure 4).
various authors.[6, 18] As will be shown below (Section 6), it can
be helpful to take H UPD into account for explaining HER activities.
Marković has determined kinetic parameters for HER over
the three low-index faces of platinum by measuring close to
the equilibrium potential with a rotating disk electrode configuration.[6] The j0 values at room temperature in 0.05 m H2SO4
were found to be around 350, 450 and 900 mA cm2 for PtACHTUNGRE(111),
PtACHTUNGRE(100) and PtACHTUNGRE(110), respectively. Provided that mass-transport
limitation does not play an important role (see ref. [12] for a
detailed discussion), it remains an open issue how surface defects contribute to the overall activities. Nevertheless, the
impact of crystallographic orientation on the HER activity has
been clearly demonstrated for Pt electrodes.
6. Palladium Electrodes
Figure 4. STM images of a AuACHTUNGRE(100) electrode surface in 0.1 m H2SO4 revealing
a) the (hex)-reconstruction at 0.2 V and b) the island-covered unreconstructed surface at 0.35 V. Courtesy of Dakkouri and Kolb.[14]
An amazing observation is the fact that the reconstructed
AuACHTUNGRE(111) surface is more than double as active as the reconstructed AuACHTUNGRE(100) surface (see j0 values in Table 1), although the
arrangement of surface atoms is practically the same; a fact
which is also reflected in the similar values for the potential of
zero charge.[16] It would be interesting to learn more about the
local electronic structures of these surfaces, including the
effect of the underlying Au bulk structure and the consequences on hydrogen binding energies.
In contrast to gold electrodes, hydrogen adsorbs already at
potentials more positive than the equilibrium potential on
platinum and palladium surfaces and thus belongs to the socalled underpotential deposition (UPD) phenomena. This
Volmer-type reaction, see Equation (2), is known to be extremely structure-sensitive for many noble metal surfaces, and
often serves as to characterise the metal/electrolyte interface
in terms of surface quality and cleanliness.[17] The relevance of
UPD hydrogen on platinum has recently been discussed by
While platinum is supposed to be the more prominent catalyst
material, one of the advantages of palladium for model studies
is the easy fabrication of monolayers,[19] sub-monolayer islands
and nanoclusters,[20] the HER activities of which shall be discussed in the following. These various structures are especially
interesting because of the absence of hydrogen absorption.
This reaction often hampers the use of massive Pd electrodes,
especially in the case of single crystals, because their bulk
structure may suffer from lattice expansion during hydrogen
incorporation. The direct participation of absorbed hydrogen
in the HER mechanism shall not be addressed here.
6.1. Palladium Monolayers
It is plausible that Pd monolayers deposited on foreign substrates do not absorb hydrogen into their bulk structure. It was
shown that this reaction only occurs for ultrathin deposits with
a thickness larger than two monolayers.[19a] A systematic study
of hydrogen adsorption on Pd monolayers deposited onto several hexagonally close-packed noble metal surfaces has revealed that the adsorption potential is strongly influenced by
the chemical nature of the substrate (Figure 5).[21] The fact that
the properties of such monolayers are different from the properties of the bulk material is explained within the so-called dband model introduced by Hammer and Nørskov.[22] Accordingly, an up-shift of the centre of the d-band leads to a stronger interaction with adsorbates
like hydrogen, or vice versa.[23]
Both geometric and electronic
Table 1. Values of j0 for HER measured over noble metal single-crystal surfaces at room temperature.
effects may contribute to such a
d-band shift.[24] For Pd monolayElectrolyte
Reference
Electrode
j0 [mA cm2]
ers, which often grow pseudothermally reconstructed
0.12
0.1 m H2SO4
[4]
Au(111)
morphically, it has been ob[4]
potential reconstructed
0.20
0.1 m H2SO4
[4]
thermally reconstructed
0.05
0.1 m H2SO4
served that lateral compression
Au(100)
[4]
potential reconstructed
0.10
0.1 m H2SO4
leads to a systematic lowering of
[4]
AuACHTUNGRE(110)
thermally reconstructed
0.4
0.1 m H2SO4
the characteristic hydrogen ad[4]
RuACHTUNGRE(0001)
3
0.1 m H2SO4
sorption peak.[21] These adsorp[5]
AgACHTUNGRE(111)
10
0.1 m H2SO4
PtACHTUNGRE(111)
350
tion potentials, and accordingly
[6]
PtACHTUNGRE(100)
450
0.05 m H2SO4
the experimentally obtained free
PtACHTUNGRE(110)
900
adsorption energies for UPD hy-
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ChemPhysChem 2006, 7, 985 – 991
Hydrogen Electrocatalysis
monolayer on a PtRuACHTUNGRE(111) surface, might be underestimated
by experiment due to diffusion
limitation. This is because j0
values were extracted from
simple
voltammetric
scans,
which is not an ideal method for
highly active surfaces. It has also
to be pointed out that the presence of surface defects may lead
to variations in the HER activity.
Still, Figure 6 highlights the fact
that j0 can be deduced from a
calculated set of free energies
for hydrogen adsorption.[25] The
systematic variation of these free
energies for hydrogen on the
various Pd monolayers was also
found in the experiment.[21]
6.2. Palladium Multilayers
By increasing the overlayer’s
thickness, the electronic modification of the Pd surface by the
Figure 5. Top) Current–potential curves for hydrogen adsorption on Pd monolayers deposited on various subunderlying substrate should dis1
strates in 0.1 m H2SO4 ; v = 10 mV s . Bottom) Scheme for the d-band model shows a down-shift or an up-shift of
appear, that is, the so-called
the d-band centre for compressive or tensile strain within the pseudomorphic Pd monolayers, respectively.[21]
ligand effect vanishes. If the Pd
multilayer is still pseudomorphic,
altered properties should arise solely from lateral strain. This
drogen, show a linear relation with the calculated shift of the
was theoretically demonstrated for hydrogen adsorbed on Aud-band centre of the Pd surface.[21]
ACHTUNGRE(111) and on AuACHTUNGRE(100) by Roudgar and Gross,[26] who calculated
The effect of tuning the hydrogen binding energy on the
HER activity is shown in Figure 6, which very much resembles
variations in hydrogen adsorption energy with increasing Pd
a volcano, as described above (Section 3).[25] The agreement
film thickness. Maximum binding energies were found for an
overlayer thickness of two monolayers. Similar changes were
between theory and experiment is remarkably good. It is seen
that the activity of the apparently most active system, a Pd
recently observed for hydrogen adsorption on Pd overlayers
on AuACHTUNGRE(100).[27] From the above considerations with Pd monolayers, it was expected that the HER activity for the two monolayer system would be lowered, owing to the larger hydrogen
binding energy. This was indeed observed in the experiment.[27]
However, j0 for HER on the Pd/AuACHTUNGRE(100) surfaces did not increase for Pd overlayers thicker than two monolayers
(Figure 7), although hydrogen was again found to adsorb
more weakly. This behaviour can be explained by the abovementioned hydrogen absorption reaction, which occurs for Pd
coverages above two monolayers. Apparently, the presence of
hydrogen in the Pd bulk lattice lowers the activity for hydrogen evolution. It is shown in Figure 7 that j0 does not change a
lot for Pd coverages on AuACHTUNGRE(100) between two and 100 monolayer equivalents. However, for Pd overlayers deposited onto
PtACHTUNGRE(100), the HER activity drops down continuously with increasing coverage, and is still much higher for a 100-monolayer
film compared with the same amount of Pd on AuACHTUNGRE(100)
Figure 6. j0 for hydrogen evolution at pH 0 as a function of the differential
free-energy of hydrogen adsorption. The curve shows the theoretical predic(Figure 7). It is not very probable that the substrate influences
tion based on a simple theoretical model:[10] ^ correspond to the predictions
the surface properties of such a relatively thick overlayer, but it
for the various pseudomorphic Pd monolayers PdML/X, where X denotes the
is assumed that lattice expansion due to hydrogen absorption
respective hexagonally closed-packed substrate; * represent experimentally
leads to defects in the Pd multilayer. This could explain the
obtained j0 values. Courtesy of J. K. Nørskov and co-workers.[25]
ChemPhysChem 2006, 7, 985 – 991
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989
L. A. Kibler
Figure 7. j0 for HER over Pd multilayers on AuACHTUNGRE(100) and on PtACHTUNGRE(100) as a function of coverage.
higher HER activities of Pd on PtACHTUNGRE(100), because lattice destruction might partially expose the very active first Pd layer.[6a]
Figure 9. j0 for HER over Pd sub-monolayer deposits on AuACHTUNGRE(111) as a function of coverage. Contributions from various active sites in dependence of
the Pd coverage are included: 1) uncovered AuACHTUNGRE(111) surface (black), 2) Pd islands (red), 3) Pd/Au sites at the perimeter of the Pd islands (blue) and
4) Pd/Pd step sites (green). The presence of these active sites can only explain the activities of Pd coverages above 0.2 monolayers (violet), while extremely high activities per Pd atom are seen at lower coverage. Experimentally obtained data points (squares) are connected with a dotted line as
guide for the eye.
6.3. Palladium Sub-Monolayers
The HER activity of Pd monolayers deposited on Au substrates
was found to be about two orders of magnitude higher than
that of the bare substrates.[27, 28] This is illustrated by the current–potential curves shown in Figure 8. In addition, it is seen
1) the uncovered unreconstructed AuACHTUNGRE(111) surface, the activity of which decreases with increasing Pd coverage qPd, Equation (10):
ðiÞ
Auð111Þ
j0 ¼ j0
Auð111Þ
ð1 qPd Þ with j0
1 mA cm2
ð10Þ
2) Pd islands, the activity of which is assumed to be proportional to qPd, Equation (11):
ðiiÞ
j0 ¼ j0PdML qPd
with j0PdML 20 mA cm2
ð11Þ
3) Pd/Au sites at the perimeter of the Pd islands. The
number of these sites was calculated on the basis of progressive nucleation and growth as seen by in situ STM[19c] to yield
Equation (12):
ðiiiÞ
2
j0 ¼ j0PdAu ð1 qPd Þð lnð1 qPd ÞÞ =3
with j0PdAu 100 mA cm2
ð12Þ
Figure 8. Current–potential curves for hydrogen evolution in 0.1 m H2SO4 on
AuACHTUNGRE(111) and a Pd monolayer deposited onto the same, well-ordered AuACHTUNGRE(111)
surface and onto a AuACHTUNGRE(111) surface with a 68 miscut; v = 1 mV s1.
that the activity of a full monolayer increases with a higher surface-defect density for Pd on AuACHTUNGRE(111), by using single-crystal
surfaces of different miscut. Figure 9 shows that j0 for electrodeposited Pd sub-monolayer amounts on AuACHTUNGRE(111) is practically
constant between 0.1 and 1 monolayers and exhibits a maximum below that range of coverage.[28] Based on a simple
model, which takes various active sites into consideration, the
HER activity can be understood for coverages higher than 0.2
monolayers. These active sites are each related with a characteristic activity and involve:
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4) Pd/Pd step sites over monoatomic Au steps, where a
normal distribution was assumed, since these steps are only
present near qPd = 1. These four contributions can be predicted
from independent measurements. The only two parameters
fitted to the experiments are j0PdAu and the activity of Pd/Pd
step sites, which was obtained from measurements with Pd
monolayers deposited on stepped AuACHTUNGRE(111) electrodes, as
shown in Figure 8.
The various contributions are plotted into Figure 9 together
with their sum. It was assumed that each active site for this bimetallic surface is related to a specific hydrogen binding
energy, giving rise to a certain activity for hydrogen evolution,
as demonstrated above (Section 3). It is clearly observed that
mixed Pd–Au step sites show an enhanced activity compared
to terrace sites.
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Hydrogen Electrocatalysis
Exchange current densities for HER of several hundreds of
mA cm2 were measured for Pd coverages below (!) 0.1 monolayers on Au single-crystal electrodes (not shown here).[27, 28]
These extreme electrocatalytic activities were observed after
just dipping the Au electrodes into a PdSO4 solution at positive
potentials. It is assumed that adsorption of Pd2 + and its subsequent reduction in a Pd2 + -free solution leads to the formation
of special Pd/Au sites, the structure of which is still unknown.
7. Palladium Nanostructures
Extraordinarily high electrocatalytic activities for HER have also
been reported for single Pd nanoparticles supported on a AuACHTUNGRE(111) substrate as measured with an STM tip.[29] On the basis of
a kinetic model, it was concluded that molecular hydrogen
may spill over from the Pd particle and diffuse to the bare Au
surface.[30] An alternative explanation for the high activity
might be that hydrogen is bound more weakly at a Pd nanocluster on AuACHTUNGRE(111), as calculated by Roudgar and Groß[31] However, it is not clear if the STM tip as used in the experiment is
suited for determining the catalytic activity of a nanoparticle.[29]
Meanwhile, it is possible to generate large cluster fields of several thousands of Pd particles on AuACHTUNGRE(111) by STM tip-induced
cluster formation.[20b] It might be interesting to use the well-defined geometry of a microelectrode in a scanning electrochemical microscope (SECM) to obtain reliable data on the promising electrocatalytic HER activity of Pd nanostructures.[32]
8. Summary and Outlook
It has been demonstrated by the use of well-defined singlecrystal surfaces that the HER is an extremely structure-sensitive
process. The free energy of hydrogen adsorption is a key parameter in relating surface structure with electrocatalytic activity. The overall activity of a more complex electrode surface is
supposed to be determined by the ability of various active
sites to chemisorb hydrogen. Defects, such as monoatomic
high steps, are often found to be more active than terrace
sites.
The use of pseudomorphic palladium monolayers allows for
a tuning of HER activity over several orders of magnitude,
which can be understood with a simple kinetic model. It still
remains important to determine experimentally the coverage
of hydrogen participating in the HER. A combination of theory
and experiment is believed to be necessary for understanding
general trends in electrocatalysis.
Of special interest are 1) the electrocatalytic activities of
nanostructures from both theoretical and experimental points
of view and 2) new experimental methods for studying the kinetics of very active surfaces in order to extract reliable values
for j0. In the end, a deep understanding of basic principles in
electrocatalysis shall allow for the efficient design of new materials. Further studies will be necessary to demonstrate the
relevance of findings with single-crystal surfaces for more realistic nanoparticle electrode materials.
ChemPhysChem 2006, 7, 985 – 991
Acknowledgements
This work was supported by the Fonds der Chemischen Industrie.
The author thanks Professor D. M. Kolb for his invaluable help.
Keywords: electrochemistry · hydrogen
nanostructures · surface chemistry
·
interfaces
·
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Received: November 22, 2005
Published online on April 11, 2006
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