Study of pH dependent Pt electrodeposition for hydrogen production in PV assisted water electrolysis system

Article abstract of DOI:10.1149/1.3582766

The electrodeposition of Pt film is carried out in an electrolytic bath of different pH (pH varied from 2 to 4) at constant potential - 0.35V for 30 min. The prepared Pt film is studied by different analyzing techniques as well as used as a cathode electrode in photovoltaic (PV) cell assisted water electrolysis system for hydrogen production. The film properties show strong dependence on pH of electrolytic solution. Pure Pt film with low mass loading (2.44 10^-5 g cm^-2) is obtained at pH 2, which subsequently undergo oxidation and mass loading also increases with pH. The earth worm like surface morphology is observed in a film deposited at pH 2, which is changed into thorny ball like morphology with pH. The active surface area and the activity for hydrogen evolution show linear relation and decreases with pH. Thus, the highly active Pt film for hydrogen production is obtained at pH 2, which shows about 25 higher hydrogen production and 3500 times lower Pt loading (from 0.07 g cm^-2 to 2.44 10^-5 g cm^-2) than those of commercial Pt mesh.

Full text of DOI:10.1149/1.3582766

Journal of The Electrochemical Society, 158 (7) E73-E77 (2011)
0
E73
013-4651/2011/158(7)/E73/5/$28.00 VC The Electrochemical Society
Study of pH Dependent Pt Electrodeposition for Hydrogen
Production in PV Assisted Water Electrolysis System
z
Jyotiprakash B. Yadav, Sang-Youn Chae, and Oh-Shim Joo
Clean Energy Research Center, Korea Institute of Science and Technology (KIST), Seongbuk-gu, Seoul 130-650, Korea.
The electrodeposition of Pt film is carried out in an electrolytic bath of different pH (pH varied from 2 to 4) at constant potential
ꢀ
0.35V for 30 min. The prepared Pt film is studied by different analyzing techniques as well as used as a cathode electrode in
photovoltaic (PV) cell assisted water electrolysis system for hydrogen production. The film properties show strong dependence on
ꢀ
5
ꢀ2
pH of electrolytic solution. Pure Pt film with low mass loading (2.44 ꢁ 10 g cm ) is obtained at pH 2, which subsequently
undergo oxidation and mass loading also increases with pH. The earth worm like surface morphology is observed in a film depos-
ited at pH 2, which is changed into thorny ball like morphology with pH. The active surface area and the activity for hydrogen evo-
lution show linear relation and decreases with pH. Thus, the highly active Pt film for hydrogen production is obtained at pH 2,
ꢀ
2
ꢀ5
ꢀ2
)
which shows about 25% higher hydrogen production and 3500 times lower Pt loading (from 0.07 g cm to 2.44 ꢁ 10 g cm
than those of commercial Pt mesh.
VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3582766] All rights reserved.
Manuscript submitted December 7, 2010; revised manuscript received March 1, 2011. Published May 3, 2011.
ꢀ
1
Rising energy demand, dwindling conventional energy sources
and global warming, are the major challenges facing the energy
future, which are forcing mankind to search for alternative energy
sources. Among all alternatives, hydrogen is an attractive one
because of its advantageous features such as; high flammability,
richest in energy per mass unit, light weight, environmental compat-
ible and direct conversion into thermal, mechanical and electrical
evolution performance for this film was 533 ml h , wherein, mate-
rial loading was reduced by 1000 times and performance improved
ꢀ
1
by 26% higher than the commercial Pt mesh (418 ml h ). How-
ever, this technique needed post heat treatment that possibly
increases the cost of film preparation. Hence, our intention is to
search for a technique, which has no need to do post treatment and
can possibly deposit Pt film with minimum loading and high cata-
lytic activity for hydrogen evolution.
1–3
energy.
Several production methods including steam reforming,
bioconversion and water electrolysis have been studied with impres-
Among the several Pt film deposition techniques, electrodeposi-
tion has the advantages of being economic, high control over film
4
–6
sive results.
Among them, 96% of hydrogen is produced by
16
reforming from fossil fuels and remaining from water electrolysis.
Nowadays, water electrolysis is attracting more scientific attention
because of its simple, non-polluting and high purity hydrogen pro-
duction features. Technologically, basics for hydrogen production
via water electrolysis have long been known. However, there are
some disadvantages such as electricity cost, which is two third of its
growth and complex surface deposition ability. Herein, the electri-
cal parameters are main controller of film thickness, morphology
and composition. Type of precursor, composition, temperature and
pH of electrolytic bath are also play a vital role in electrochemical
deposition. There are several reports on Pt film deposited at different
16,17
precursor solution, composition and temperature.
Only few
7
18
operation cost. Hence the electrolysis particularly powered by
renewable energy has got a great attention.
reports are available on pH dependent study of Pt film preparation.
Thus by keeping in mind all these aspects, an attempt has been
made for Pt films electrodeposition at different pH and characterize
those films by different analytical techniques.
In present work, we have used a PV assisted water electrolysis
system for hydrogen production, which simply consists of an elec-
trolytic cell and silicon solar cell. In schematic of PV assisted water
electrolysis system, silicon solar cell is placed outside the electro-
lytic cell, which provides biasing current to cathode-anode electrode
Experimental
8
assembly (in electrolytic cell) for water splitting. In the cell, the
Pt films were electrodeposited on a nickel (Ni) substrate (9 ꢁ 9
cm) through potentiostat (IviumStat technology, Netherland) by
using chrono-amperometric method. The electrolytic bath was pre-
pared by adding 0.5 mM H PtCl . 5.7 (H O) in distilled water along
hydrogen evolution efficiency greatly depends on the overpotential
for cathodic and anodic reactions, and on internal resistance of elec-
trolytic cell. These factors depend on catalytic activity of electrode
9
2
6
2
material and its electrochemical stability.
with continuous stirring, which resulted in yellow transparent solu-
tion of pH 3. The pH (2–4) of electrolytic bath was adjusted by
using H SO4 and NaOH solution. The electrolytic solution was
2
2
deoxygenated by bubbling N gas prior to use. The film deposition
was carried out on Ni plate for 30 min, in a single cell three electro-
A principal focus of the present work is to design cathode elec-
trode with desirable electrochemical stability and improved electro-
catalytic activity towards water splitting reaction. According to
10
Stojic et al., the electrocatalytic activity of various metals for
hydrogen evolution is a function of outer shell electronic configura-
tion. It means catalytic activity increases with d-electrons and
reaches to maximum at nearly filled d-orbital. Among all literature
des system at a constant potential (ꢀ 0.35V) and at room tempera-
ture, wherein Pt plate and Ag/AgCl (saturated NaCl) were used as
counter electrode and reference electrode, respectively. Finally, as
prepared Pt film was rinsed with distilled water and dried under the
flow of argon gas. The resulted films were characterized by different
analyzing techniques such as scanning electron microscopy (SEM-
HITACHI S-4100 model) and X-ray photoelectron spectroscopy
techniques (XPS-ESCA, PHI-5800 model). The XPS analysis was
carried out at excitation energy of 1486.6 eV and a scan step of 0.1
eV. Auger electron spectroscopy (AES-ULVAC-PHI-700) was used
for depth profile measurement at beam voltage 5 kV and sputtering
9
known d-orbital transition metals, platinum (5d electronic configu-
ration) has an aggressive electro-catalytic activity, biocompatibility
and electrochemical stability, which makes it a promising cathode
Only the cost of
1
1–14
electrode material for water splitting reaction.
Pt is a serious problem. Hence during past few decades, research in-
terest is focused on cost reduction by depositing highly active Pt
film on a low cost metal substrate with minimum Pt loading or intro-
ducing economic film preparation method.
In previous report, we studied low Pt loading electrospray depo-
sition technique for hydrogen evolution electrode. The hydrogen
ꢀ
1
˚
1
5
rate 150A min . The PV assisted water electrolysis system was
used for hydrogen evolution study, wherein the silicon solar cell
was used for powering the electrochemical cell. The Pt film on Ni
substrate was used as a cathode (hydrogen electrode) and stainless
z
E-mail:joocat@kist.re.kr
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E74
Journal of The Electrochemical Society, 158 (7) E73-E77 (2011)
steel (SS) plate as an anode in the electrolytic bath containing 1 M
NaOH solution. The silicon solar cell was irradiated by Xenon
lamp, whose intensity was fixed at one sun condition by using a ref-
erence solar cell (PVM-153). The electrochemical products such as
oxygen and hydrogen were measured using a wet-test meter for 5 h
with 1 h time lapse.
Results and Discussion
Theoretical growth mechanism.— The common method of Pt
electrodeposition involves an electrochemical bath containing Pt(IV)
complexes (complexed with chloride, ammine, sulfide, nitride and
hydroxyl group). Most of the researchers prefer economic Pt(IV)
chloride complex for Pt film deposition, which is followed by a
chemical Reaction 1 (Ref. 19)
2
ꢀ
2
ꢀ
½
PtCl ꢂ þ2e ! ½PtCl ꢂ þ2Cl
[1]
6
4
2ꢀ
2ꢀ
Initially, [PtCl
6
]
4
ion reduces to [PtCl ]
by gaining two electrons
from the cathode, which further reduces to Pt(0) metal by Reaction 2
PtCl ꢂ þ2e ! Ptð0Þ þ 4Clꢀ
2
ꢀ
[2]
½
4
The magnitude of Pt film deposition depends on reduction rate of
both [PtCl4] and [PtCl6] ions. However, in strongly acidic solu-
tion, the high concentration of Cl ion inhibits the electro-reduction
2ꢀ
2ꢀ
ꢀ
2
ꢀ
2ꢀ
19
rate of [PtCl4] ion as compared to [PtCl6] ion. Equation (1)
and (2) are the two steps of single reaction, leading to the formation
of Cl ions that significantly reduces the rate of Pt deposition. In
case of weakly acidic solution (pH > 2), Cl ions in the coordination
sphere are replaced by OH ions and it forms intermediate ions of
ꢀ
ꢀ
ꢀ
2
ꢀ
[
film.
Pt(OH) Cl
]
, which contribute to the electrodeposition of Pt
m
6ꢀm
2
0
PtCl6ꢂ þ mðH OÞ ꢀ ½PtðOHÞ Cl6ꢀmꢂ þ mHþ
2
ꢀ
2ꢀ
[3]
½
2
m
In equation (3), the value of ‘m’ increases (from 0 to 6) with con-
centration of OH ions or pH of electrolytic bath. Thus, the pH of
ꢀ
2
electrolytic bath play a crucial role in the formation of [PtCl ] and
intermediate ions of [Pt(OH) Cl ]
growth mechanism of Pt film. In this way, in strong acidic solution,
ꢀ
6
Figure 1. (A) The deposition current vs time (for initial 10) plot for Pt film
deposited at different pH. (B) A plot of current measured at time 1000 S dur-
ing the deposition process at different pH.
2
ꢀ
that strongly affect the
m
6ꢀm
Pt deposition follows the Reactions 1 and 2, but in case of pH > 2,
Cl6ꢀm
2ꢀ
the intermediate ions of [Pt(OH)
the Reaction 4
m
]
contribute and follow
current density of Pt film deposited at pH 2 is high but it decreases
up to pH 3.5 and show small rise in case of pH 4. The high current
observed at pH 2 is concerning for combined contribution from
hydrogen evolution on working electrode during film growth and Pt
salt reduction. That current subsequently decreases with pH because
PtðOHÞCl5ꢂ ꢀþ 4eꢀ ! Pt þ ðOHÞ þ5Cl
2
ꢀ
ꢀ !
½
[4]
2
1
20
Growth mechanism.— According to Pierce et al., the first stage
of electrodeposition involves the establishment of suitable boundary
layer at the electrode surface, which thereafter starts to grow a thin
film in second stage. The formation of initial boundary layer is very
important stage for film growth, which is responsible for morpho-
logical change in the film. Therefore, it is needed to analyze the ini-
tial current density response for short time interval (10 s) that is
shown in Fig. 1A. The current density show strong dependence on
pH of electrolytic bath. Initially, current shows sudden rise and fall
within short time interval that finally reaches to a plateau. The initial
rise in current is due to accumulation of ions or formation of double
of reduced hydrogen evolution. According to Hubbard et al., the
2ꢀ
reduction potential for [PtCl6] takes place near to ꢀ 0.25 V and it
shifts to a higher potential (ꢀ 0.30 V & onward) for intermediate
2
ꢀ
), which is depending on value of ‘m’.
ions of ([Pt(OH)
m
Cl6ꢀm
]
The concentration of intermediate ion increases with pH that may
increase the deposition current at pH 4.
In order to support this consideration, we have measured thick-
ness of Pt film from depth profile plot of AES and values are given
in Table I. The film thickness for a film deposited at pH 2 is ꢃ 30
nm and increases linearly up to 90 nm with pH. The film deposited
at pH 2 shows high current (Fig. 1B) and low thickness (Table I),
which reveals that hydrogen evolution also contributes to the depo-
sition current. Above pH 2, the film thickness increases and deposi-
tion current decreases due to decreasing hydrogen evolution
contribution.
2
2
layer charge on substrate surface, which is concerned with pres-
ence of electroactive nuclei in electrolytic bath. The double layer
charging current decreases with pH, which reduces the electroactive
nuclei and thus affects on film growth mechanism. This means elec-
troactive nuclei decreases with pH of electrolytic bath. After double
layer charge regime, current starts to reduce and reaches to plateau
with steady state deposition behavior that concerns for continuous
nucleation and growth of Pt film.
The amount of Pt loading is calculated from the thickness by
ꢀ
3
assuming bulk density of Pt as 21.45 g cm and plotted with
respect to pH. The roughness factor is calculated from the cyclic
voltammeter plot by using hydrogen adsorption and desorption
23
The plot of deposition current density (measured at 1000 s) vs
pH of electrolytic bath is shown in Fig. 1B. It gives information
about growth rate of Pt film on substrate surface. The deposition
curve. The amount of Pt loading was used for material loading
and all the values of roughness factor and material loading are given
in Table I. The amount of Pt loading shows strong dependence on
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Journal of The Electrochemical Society, 158 (7) E73-E77 (2011)
E75
1
showing peak a at binding energy around 71.6 eV, which is slightly
Table I. The thickness, roughness factor and amount of Pt load-
ing values.
decreased to 71.4 and 71.3 eV in a film deposited at pH 2.5 and 3,
respectively. The observed small peak shift is negligible and corre-
0
sponds to pure Pt film. The binding energy shift becomes more
pH of
electrolyte
solution
Amount of
Pt loading
Thickness
(nm)
Roughness
factor
Bulk density
3
prominent above pH 3, that observes at 71.7 (þ 0.5) eV and 72.1
(þ 0.9 eV) for pH 3.5, and pH 4, respectively. This binding energy
shift may be prominant because of interaction with contaminated
species inside the film material. The films deposited above pH 3
ꢀ
(ꢁ 10ꢀ5g cm
ꢀ2
of Pt (g cm
)
)
2
30
63
68
75
90
2.68
2.19
2.18
1.09
0.62
21.45
21.45
21.45
21.45
21.45
2.44
6.16
6.68
14.75
—
2
3
3
4
.5
.5
1
have a newly growing peak b that corresponds to oxidized Pt,
whose intensity is found to increases with pH. The intensity ratio of
peak b /a1 gives additional information about percentage of oxi-
1
dized Pt in a film surface area. It turned out that about ꢃ 20% of
film surface area exists in oxidized Pt form in the film deposited at
pH 3.5, which subsequently increases to ꢃ 37% in the film deposited
at pH 4. As we discussed above in growth mechanism, above pH 2
ꢀ
5
ꢀ2
pH of electrolytic bath. Low amount is observed (2.44 ꢁ 10 g cm
)
in a film deposited at pH 2 and the amount increases linearly up to
2
ꢀ
ꢀ
5
2
the [PtCl6]
ions (from electrolytic bath) undergo hydrolysis and
1
4.75 ꢁ 10 g cm with pH. The film deposited at pH 4 is not 100%
2
ꢀ
form intermediate ions of [Pt(OH)
3
ions of [Pt(OH)
m
Cl6ꢀm
. Hence, above pH 3 the probable contribution of the intermediate
]
according to equation
covered {that’s observed in SEM image (Fig. 2E)}; hence Pt loading
for this film has not been calculated.
2ꢀ
m
Cl6ꢀm
and the films become more oxidized at pH 3.5 and 4.
]
increases that may entrap oxygen ions
Surface morphological study.— The scanning electron micros-
copy (SEM) images of Pt film deposited at different pH are shown
in Fig. 2. From figure it is seen that, all films show morphological
changes from earth worm like structure to thorny ball with pH of
electrolytic bath. Subhramannia et al and Whalen et al have previ-
ously reported morphological changes in electrodeposited Pt film
Evolution of active surface area by electrochemical
process.— The catalytic activity of Pt film depends on available
27
active surface area of the film for electrochemical reaction. Hence,
it needs to calculate the active surface area of Pt film that is calcu-
2
4,25
2 2 4
lated from cyclic voltammetry in N purged H SO solution. The
with respect to deposition potential.
We also found the changes
2
active surface area of Pt film (dimension 1 cm ) is measured by inte-
grating hydrogen desorption curve of CV. During the reduction pro-
cess on Pt film, proton from the acidic solution adsorbs on the sur-
face of electrode while in case of oxidation, desorption takes place
according to Reaction 5
but with respect to pH of electrolytic bath. As seen from the figure,
an earth worm like structure is observed in a film prepared at pH 2
(
Fig. 2A), which is changed into dense and uniform surface mor-
phology with pH (up to pH 3) (Fig. 2B) and (Fig. 2C), respectively.
Highly agglomerated spherical grains with uniform structure are
observed in a film prepared at pH 3.5 (Fig. 2D), which are subse-
quently changed into scattered thorny ball like structures with
decreased surface coverage at pH 4 (Fig. 2E). The surface coverage
is continuously decreased above pH > 4 and below pH < 2, hence
these are not summarized in the present study.
þ
ꢀ
Had ! H þ e
[5]
After measuring the number of liberated electrons during oxida-
tion, the number of hydrogen atoms desorbed from the Pt surface
can be calculated. This defines the active surface of the film (Ar),
2
ꢀ1 22
)
XPS studies.— X-ray photoelectron spectrum (XPS) of Pt ele-
ment in a film deposited at different pH of electrolytic bath is shown
which is used for calculation of specific catalyst area, S(m g
in Fig. 3. The films deposited up to pH 3 show only two peaks (a &
a ), while at higher pH, it splits into three peaks of different inten-
2
0
H
1
Ar ¼ Q =Q
[6]
[7]
H
sity. All these peaks are related to Pt4f energy band, which after
deconvolution show four peaks that correspond to the presence of
pure Pt (a & a ) and oxidized Pt (b & b ). The peaks a and b
S ¼ ðArÞ=ðAgWÞ
0
26
1
2
1
2
1
1
where,
–total charge associated with hydrogen desorption after dou-
ble layer charge correction.
reveals for the Pt4f7/2 and its position gives information about pres-
ence of oxidized form of Pt. The pH of electrolytic bath varies the
Q
H
0
0
oxidation state of Pt film. Below pH 3, the film is in pure Pt form
but it undergoes oxidation with pH. The film deposited at pH 2 is
H
Q —theoretical charge associated with plane Pt surface (210
2
lC/cm )
Figure 2. SEM images of Pt film depos-
ited at different pH (A) pH 2 (B) pH 2.5
(
C) pH 3 (D) pH 3.5 (E) pH 4.
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E76
Journal of The Electrochemical Society, 158 (7) E73-E77 (2011)
Figure 3. (Color online) XPS spectra of
Pt film deposited at different pH (A) pH 2
(
B) pH 2.5 (C) pH 3 (D) pH 3.5 (E) pH 4.
W–Pt loading (given in Table I)
Ag–geometric surface area (1 cm ).
this is due to surface oxide formation and decrease in Pt surface area
coverage.
2
Hydrogen production study.— The hydrogen evolution amount
is measured 5 h with 1 h time lapse by using Pt film deposited at dif-
ferent pH as cathode electrode in photovoltaic (PV) cell assisted
water electrolysis system. The average hydrogen evolution amount
is plotted with pH of electrolytic bath, which is shown in Fig. 5.
From figure it is seen that, the average hydrogen evolution amount
The specific catalyst area of Pt film is calculated from equation 7
and plotted with respect to pH, which is shown in Fig. 4. From fig-
ure it is seen that, the specific catalyst area of Pt film is decreased
2
ꢀ1
with pH. High active surface area (98.5 m g ) is observed for a
film deposited at pH 2 and subsequently decreases in a film depos-
2
ꢀ1
2
ꢀ1
ited at pH 2.5 (35.49 m g ) and pH 3 (32.48 m g ). The surface
morphology (from SEM Figs. 3A–3C) seems to be changed from
rough surface (earth worm like structure) to highly uniform surface
with pH. The surface area and surface roughness are relevant terms
and directly proportional to each other. This is the reason for
decreasing active surface area of Pt film with pH. The film deposited
at pH 3.5 shows sudden drop down in active surface area (7.412
ꢀ
1
decreases linearly with pH. It is observed about 525.6 ml h in the
ꢀ
1
film deposited at pH 2 that subsequently decreases to 515.2 ml h
ꢀ
in the film deposited at pH 2.5), 511.8 ml h (in the film deposited
1
at pH 3) and down to 427.2 ml h (in the film deposited at pH 4)
with pH. According to Pournaghi–Azar et al, the catalytic activity
of Pt film depends on available active surface area for electrochemi-
1
(
ꢀ
27
2
ꢀ1
cal reaction, which increases with active surface area. It means the
m g ) due to surface passivation by oxide formation (XPS Fig.
4
2
ꢀ1
D). In case of pH 4, the surface area is very small (2.294 m g ),
ꢀ
1
production (ml h ) for Pt film
Figure 4. A plot of specific catalyst area (S) of Pt film deposited at different
pH.
Figure 5. A plot of average amount of H
deposited at different pH.
2
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Journal of The Electrochemical Society, 158 (7) E73-E77 (2011)
E77
active surface area is directly proportional to hydrogen evolution
performance. Almost similar trend for both active surface area (Fig.
) and hydrogen evolution performance (Fig. 5) is observed for Pt
Acknowledgment
This work was funded by the “Hydrogen Energy R&D Center”,
one of the 21 century frontier R&D programs, funded by Ministry
of Science and Technology of Korea.
4
st
28
films deposited at different pH. According to Conway et al., Pt has
more unpaired electron in d band, which interacts strongly with
electron donating atoms or molecules such as hydrogen. Thus,
hydrogen adsorbs strongly on Pt surface and forms electron pair
with unpaired d electrons. The hydrogen evolution reaction is
involving desorption of adsorbed hydrogen atom. It means rate of
hydrogen evolution depends on rate of hydrogen desorption. In oxi-
dized Pt film unpaired electrons form pairing with oxygen atom,
which causes to reduce unpaired electrons for hydrogen adsorption
and probably reducing the rate of hydrogen evolution. In case of
film deposited at pH 3.5 and 4, the part of Pt is observed in oxidized
form. This is another reason to decrease the hydrogen evolution per-
formance. Thus, the Pt film deposited at pH 2 shows high hydrogen
evolution performance and lower Pt loading than those of commer-
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[
Pt(OH)
m
Cl6ꢀm
]
increased with pH. The pH of electrolyte bath
also affected on the surface morphology, elemental composition and
catalytic activity of the Pt film. It was observed as earth worm like
structure in a film deposited at lower pH 2, which slowly changed
into spherical grains with smooth surface up to pH 3.5 and subse-
quently changed into non-uniformly distributed thorny ball like
structure in a film deposited at pH 4. The surface elemental compo-
sitional study showed that pure Pt film was obtained upto pH 3 and
above this pH it undergo oxidation. The 20% surface area of the
film prepared at pH 3.5 was in oxidized Pt form, which subsequently
increased up to 37% in a film prepared at pH 4. The active surface
area and hydrogen evolution performance showed almost linear
relation and both proportionally decreased with pH. Thus, pure Pt
19. L. Y. Lau and T. Hubbard, J. Electroanal. Chem., 24, 237 (1970).
2
2
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2
2
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2
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2
ꢀ
5
ꢀ2
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2
face area (98.5 m g ) and high hydrogen production (525.6 ml
ꢀ1
2
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ꢀ
1
h
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