Electrochemistry Communications 60 (2015) 208–211
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Potentiodynamic hydrogen permeation on Palladium-Kelvin probe
compared to 3D printed microelectrochemical cell
Gabriela Schimo a,b, Wolfgang Burgstaller a,c, Achim Walter Hassel a,b,c,⁎
a
b
c
Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria
CEST Competence Center for Electrochemical Surface Technology, Viktor Kaplan Str. 2, 2700 Wiener Neustadt, Austria
Christian Doppler Laboratory for Combinatorial Oxide Chemistry, Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria
a r t i c l e
i n f o
Article history:
Received 17 July 2015
Received in revised form 5 September 2015
Accepted 7 September 2015
Available online 12 September 2015
Keywords:
Scanning Kelvin probe
Hydrogen permeation
Palladium
3D printing
a b s t r a c t
A specially designed flow cell, fabricated via rapid prototyping (3D printing), was used to perform in-situ electrochemical hydrogen loading and cyclic voltammetry on a Pd foil in alkaline solution during scanning Kelvin probe
(SKP) measurements. SKP was successfully employed for hydrogen detection on the exit side of the sample, including determination of hydrogen diffusion coefficient in Pd to 3.32 ⋅ 10−7 cm2⋅s−1 at 23 °C. Convection of electrolyte
allowed hydrogen charging even under H2-forming conditions without surface blockage by evolving gas bubbles at
very negative potentials. Comparison with electrochemical hydrogen detection under the same conditions, allowed
a more comprehensive interpretation of SKP results including determination of trapping effects on measurement of
diffusion coefficient. In this manner, the potentiodynamic hydrogen loading technique combined with SKPH-detection was utilized to determine the effective hydrogen diffusion coefficient (Deff).
© 2015 Published by Elsevier B.V.
1. Introduction
In the last few years, scanning Kelvin probe (SKP) microscopy is
gaining in importance as a tool for hydrogen detection in metals, due
to its high sensitivity towards hydrogen as well as its spatially resolved,
non-destructive measurement principle. Determination of hydrogen
diffusion coefficients and hydrogen concentrations in metals and
metal alloys is of great interest not only for hydrogen embrittlement
studies, but also in terms of hydrogen storage. SKP can be employed
for qualitative and quantitative hydrogen detection. Qualitative hydrogen detection was reported for pure metals and metal thin films of palladium [1,2], and iron [3,4], but also on alloys [5] and steels [1,6–8].
Hydrogen loaded spots show a lowered work function compared to
hydrogen-free surrounding areas [9]. The hydrogen content can also
be determined indirectly by, for example, investigating the reduction
of surface oxides by hydrogen as it was shown for iron [3] or silver
[10]. For quantitative hydrogen determination, Pd can be used because
its work function shows logarithmical dependence on the hydrogen
amount absorbed in the metal lattice and protons in the nanoscopic
water layer on the sample surface, similarly as it can be calculated via
the Nernst equation for a Pd:H electrode [1,2,11,12]. For in-situ
hydrogen charging, SKP setups were reported to be inverted with the
electrochemical cell on top and the bottom side investigated by the
⁎ Corresponding author at: Institute for Chemical Technology of Inorganic Materials,
Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria. Fax: +43 732
2468 8905.
E-mail address: achimwalter.hassel@jku.at (A.W. Hassel).
http://dx.doi.org/10.1016/j.elecom.2015.09.005
1388-2481/© 2015 Published by Elsevier B.V.
SKP. This inversion of the setup hinders gas bubble accumulation at
the entrance side [2]. The aim of this work is to present a solution for
facilitating the use of existing SKP setups for simultaneous hydrogen
loading and unloading avoiding surface blockage by gas bubbles [13],
as well as to prove its functionality by performing in-situ cyclic
voltammetry (CV) on the entrance side of a Pd foil in alkaline solution.
Moreover, by performing potentiodynamic hydrogen loading and
unloading, the ability of SKP to determine H-diffusion coefficients without delaying effects, arising from interaction of hydrogen with trap sites
in the cold-rolled Pd membrane [14–16], is studied.
2. Materials and methods
Pd foil (Goodfellow, as rolled, 99.95%) with a thickness (L) of 0.1 mm
was used for all described measurements. It was ultrasonically cleaned
in acetone and subsequently in ultrapure deionized water for 10 min
prior to the measurement. For electrochemical H-loading of Pd as well
as for electrochemical H-detection, 0.1 M NaOH solution was employed
as electrolyte, which was deaerated by 2 h Ar purging before use.
All measurements were performed at a SKP measurement chamber
temperature of 23 ± 0.5 °C.
SKP measurements have been carried out in an in-house developed
setup based on a commercial SKP (Wicinski & Wicinski GbR) equipped
with a Cr-Ni probe tip of 300 μm diameter. The tip–sample distance was
kept constant during all measurements at 110 μm. The SKP measurement chamber was completely closed and flushed with dry nitrogen
to set up stable environmental conditions of relative humidity of
5.3 ± 0.2% RH and an oxygen content of 0.8 ± 0.2 Vol.%. SKP contact
G. Schimo et al. / Electrochemistry Communications 60 (2015) 208–211
potential difference (CPD) calibration was performed by probing a
liquid surface of saturated CuSO4 solution [17].
For in-situ hydrogen permeation experiments, the sample was
galvanostatically polarized at cathodic potentials at 1 mA⋅ cm−2 until
a steady-state of measured CPD was reached. CV was recorded by scanning the potential between 0.6 and −0.9 V vs. standard hydrogen electrode (SHE) with scan rates of 2 and 10 mV⋅s−1 plus 20 and 50 mV⋅s−1
in case of electrochemical H-detection, which is achieved by applying a
constant potential of 0.04 V (SHE) in 0.1 M NaOH solution, allowing
rapid oxidation of permeated hydrogen. All electrochemical measurements were performed with IVIUM CompactStat potentiostats operating in floating ground mode.
3. Results and discussion
Electrochemical H-charging and cyclic voltammetry were performed with a flow cell, as schematically depicted in Fig. 1(a), which
were specifically designed for applications where removal of evolving
gas bubbles is of decisive importance. The cell was designed in a CADsoftware and fabricated via stereolithography similar to cells produced
before [18]. Stereolithographic fabrication route was chosen due to
complexity of flow channel geometry, which is not producible by conventional mechanical production techniques. The cell consists of three
electrolyte channels, each of them offering space for insertion of a counter electrode (CE), realized by a gold-plated stainless-steel thread rod,
and a micro-reference electrode (RE). As RE, custom-made Hg/HgO/
0.1 M NaOH μ-reference electrodes with a potential of 0.154 V (SHE)
were used. Each flow channel is equipped with a breaker (1.3 mm
distance to sample), forcing the electrolyte solution to pass by close to
the sample surface entraining possibly formed gas bubbles. Depending
on the measurement task, three measurement areas of different geometrical shape and size are available. For scanning applications, the largest
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area with rectangular shape (5 × 15 mm) can be preferably chosen,
whereas for point measurements all three areas (3.5 mm, 2.5 mm
diameter) can be used. Electrolyte is pumped from the electrolyte tank
through the V-shaped flow channels of the cell. For electrochemical
hydrogen permeation measurements, a second flow cell of smaller size
with U-shaped electrolyte channel is pressed on top of the SKP-flow cell
fitting exactly the centrally located measurement area (Fig. 1(b)). The
threefold measurement arrangement contributes to the high flexibility
of the cell, enabling investigation of even small spots on the sample
using various techniques without removal from the cell, hence ensuring
uniform measurement conditions. As it was reported [19], flowing of electrolyte can affect the hydrogen entry and permeation rate. Therefore, the
same flow rate of 75 ml⋅min−1 was used in all experiments.
A representative measurement for determination of hydrogen diffusion coefficient from SKP measurement is shown in Fig. 2(a). The
recorded CPD changes drastically towards the negative direction when
hydrogen reaches the exit side of the membrane. Onset- or response
time values for this potential drop were used to determine the average
diffusion coefficient via the formula depicted in Fig. 2(a). Calculation resulted in a value of 96 s for the response time and 3.32 ⋅ 10−7 cm2⋅s−1
for the hydrogen diffusion coefficient in Pd at 23.2 °C, in agreement with
literature [16,20–22]. Possible H-trapping is not considered, therefore
the calculated value can be perceived as apparent diffusion coefficient
(Dapp).
Investigation on dynamic H-loading and unloading was done by
applying a triangular potential waveform to the Pd membrane.
Resulting voltammograms for scan rates of 2 and 10 mV ⋅ s−1 are
presented in Fig. 2(b) featuring slightly different shape compared to
formerly published results [21] for Pd foil in alkaline solution due to
convection of electrolyte. Towards negative potentials, below approximately − 0.5 V (SHE), hydrogen adsorption, absorption as well as
hydrogen evolution processes appear and gain in importance in this
Fig. 1. Schematic of the used 3D printed electrochemical flow cells for (a) SKP measurements and (b) for electrochemical measurements, as well as an overview over the corresponding
reactions for H-loading and unloading.
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G. Schimo et al. / Electrochemistry Communications 60 (2015) 208–211
Fig. 2. H-permeation measurement performed with SKP plus calculation of (a) Dapp from
the measured onset time (tOnset) and (b) CV including the identified starting potentials for
H-loading (−0.477 V (SHE)) and unloading (−0.392 V (SHE)) as well as the potential of
maximum H-entry (−0.796 V (SHE)) for Pd in alkaline solution.
sequence [23]. In the anodic direction, hydrogen is oxidized from potentials of about −0.4 V (SHE) as depicted in Fig. 2(b).
Fig. 3 presents the course of the CPD change (ΔCPD) while applying
the triangular potential waveform to the Pd sample for both investigated potential scan rates. At the lower scan rate of 2 mV⋅s−1 (Fig. 3(a)),
larger ΔCPD values were obtained, simply because more hydrogen is
inserted in the Pd and the time interval, in which H-atoms are able to
diffuse to the exit side of the metal before being drawn back towards
the entry side is longer than for the higher scan rate of 10 mV ⋅ s− 1
(Fig. 3(b)). After subtracting the time delay, which results from hydrogen
diffusion through the membrane, following potentials can be determined:
Firstly, the potential, at which absorption of hydrogen, subsequently
permeating through the metal, starts. Secondly, the potential, at which
hydrogen is quantitatively oxidized and therefore removed from the
membrane. In a similar way, maximum permeation rates from electrochemically recorded transients can be used to calculate the potentials of
maximum H-insertion (Fig. 2(b)).
To compare results from SKP measurements with those from
electrochemical H-detection, the modified experimental conditions
and their consequences have to be considered (Fig. 1). While hydrogen
removal from Pd is occurring relatively slowly, partly by release of protons into the nanoscopic water layer on the surface, which is present
even at dry conditions, and reaction with residual oxygen in the SKP
chamber atmosphere as well as recombination of adsorbed hydrogen
followed by desorption from the surface, electrochemical H-detection
implies an H-concentration of zero at the exit side of the sample [20].
The slow removal of hydrogen at the exit side during SKP measurements, leads to accumulation of hydrogen in the membrane, reaching
much higher hydrogen concentrations. When reaching potentials at
the entry side, which enable H-unloading, hydrogen situated close to
Fig. 3. SKP measurement and resulting change of CPD (starting value: ΔCPD = 0 ≡ 0.554 V
(SHE)) during CV performed at the H-entry side (x = 0) of the Pd foil with potential scan
rates of (a) 2 mV⋅s−1 and (b) 10 mV⋅s−1. (c) H-permeation transient electrochemically
measured during CV on the entry side at varying potential scan rates.
the surface will be oxidized and removed primarily. Hydrogen atoms
close to the exit surface will diffuse back to the other sample side,
which will take approximately as long as the calculated response time.
This is the main reason why CPD is not adopting its original value.
While the time period between stopping and starting of H-loading
is large enough for potential scan rates of 2 and 10 mV⋅s−1 in the case
of electrochemical measurement to completely remove hydrogen
from the sample, removal is incomplete in the case of SKP for both
scan rates, because there is simply too much hydrogen left as to be
oxidized during the H-unloading interval.
Because of the accumulation of hydrogen at the exit side of the
membrane, the reduction of the H-entry for electrochemical measurements with partial rise and decay transients [24] is not realizable in
the reported way for SKP H-detection. Another possibility is to partly
reduce the amount of diffusible hydrogen at the exit side of the membrane for a certain time interval. Exactly this was done by performing
G. Schimo et al. / Electrochemistry Communications 60 (2015) 208–211
a CV at the entry side. If the scan rate is high enough, only diffusible
hydrogen is removed from the membrane, as it will arrive at the exit
side earlier. On the other hand the scan rate has to be sufficiently low
to introduce an adequate amount of hydrogen to fill the traps as well
as to remove it during the anodic branch of the CV in order to obtain satisfactory ΔCPD curves allowing a proper evaluation. This condition is
achieved for both low scan rates in the case of SKP measurements and
for high scan rates for electrochemical H-detection (Fig. 3(c)) showing
incomplete peak separation. With knowledge of the point in time at
which starting potentials of hydrogen insertion is exceeded, Dapp can
be determined from the onset times of the SKP signal during the first
cycles of CV at 2 and 10 mV ⋅ s− 1, while the real diffusion coefficient
(Deff), without delaying trapping effects, can be calculated from the
subsequent cycles as traps will remain filled and only the amount of diffusible hydrogen is changed during these cycles. In this manner, a value
of 4.9 ⋅ 10− 7 cm2⋅ s−1 was calculated. This value, which surpasses as
expected Dapp, was also obtained from the response times between
reaching maximum H-insertion potentials and maxima in permeation
transients for electrochemical measurements in case of 20 and
50 mV ⋅ s− 1 scan rates.
4. Conclusions
Computer-aided design coupled to rapid prototyping manufacturing
process allowed development of a low-cost electrochemical flow cell
setup for integration in a conventional SKP chamber, enabling hydrogen
charging of samples even under strong hydrogen evolution conditions.
Hydrogen permeation measurements with SKP as tool for hydrogen
detection were performed on Pd foil showing the use of SKP for determination of hydrogen diffusion coefficient. Potentiodynamic H-loading and
unloading in terms of cyclic voltammetry at the H-entry side allowed
exclusion of retarding trapping effects and calculation of starting potentials of H-insertion and removal. Naturally, with the knowledge of suited
potentials and H-loading/unloading intervals, the described method can
be modified into a simple switching between H-loading and H-unloading
potentials for determination of H-diffusion coefficient.
Conflict of interest
There is no conflict of interest.
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Acknowledgments
Financial support of the Austrian Research Promotion Agency (FFG)
within the COMET framework and financial support of Lower Austria is
appreciated. The financial support by the Austrian Federal Ministry of
Science, Research and Economy and the National Foundation for Research, Technology and Development through the Christian Doppler
Laboratory for Combinatorial Oxide Chemistry (COMBOX) is gratefully
acknowledged. The authors are indebted to the voestalpine steel for
the support.
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