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Journal ArticleDOI

PEFC Electrode with Enhanced Three-Phase Contact and Built-In Supercapacitive Behavior

01 Jan 2009-Journal of The Electrochemical Society (The Electrochemical Society)-Vol. 156, Iss: 1
TL;DR: In this paper, the supercapacitive behavior of hydrous ruthenium oxide helps realize a fuel cell-supercapacitor hybrid, which is incorporated in the cathode electrocatalyst layer of the membrane electrode assembly for PEFCs.
Abstract: Hydrous ruthenium oxide, which exhibits both protonic and electronic conduction, is incorporated in the cathode electrocatalyst layer of the membrane electrode assembly for polymer electrolyte fuel cells (PEFCs). The supercapacitive behavior of ruthenium oxide helps realize a fuel cell–supercapacitor hybrid. Platinum (Pt) nanoparticles are deposited onto carbon-supported hydrous ruthenium oxide and the resulting electrocatalyst is subjected to both physical and electrochemical characterization. Powder X-ray diffraction and transmission electron microscopy reflect the hydrous ruthenium oxide to be amorphous and well-dispersed onto the catalyst. X-ray photoelectron spectroscopy data confirm that the oxidation state of ruthenium in Pt anchored on carbon-supported hydrous ruthenium oxide is Ru4+. Electrochemical studies, namely cyclic voltammetry, cell polarization, intrinsic proton conductivity, and impedance measurements, suggest that the proton-conducting nature of hydrous ruthenium oxide helps extend the three-phase boundary in the catalyst layer, which facilitates improvement in performance of the PEFC. The aforesaid PEFC operating with hydrogen fuel and oxygen as oxidant shows a higher power density (0.62 W/cm2 @ 0.6 V) in relation to the PEFC comprising carbon-supported Pt electrodes (0.4 W/cm2 @ 0.6 V). Potential square-wave voltammetry study corroborates that the supercapacitive behavior of hydrous ruthenium oxide helps ameliorate the pulse-power output of the fuel cell.

Summary (1 min read)

Experimental

  • -A chemical route reported elsewhere 11 was adopted to prepare hydrous ruthenium oxide.
  • The ruthenium oxide suspension was centrifuged and washed copiously with deionized water until no residual chloride was observed.
  • To this, 15 w/o PTFE suspension was added with continuous agitation.
  • The resulting ink was coated onto the gas-diffusion layer of the electrode.

Electrochemical characterization.-Polarization studies on

  • PEFCs.-MEAs were evaluated using a conventional 25 cm 2 fuel cell fixture with a parallel serpentine flow field machined on graphite plates ͑M/s Schunk Kohlenstofftechnik GmbH, Germany͒.
  • During the experiment, humidified hydrogen and humidified nitrogen were fed to the anode and cathode, respectively.
  • Capacitance measurements.-CVs were obtained to determine the capacitance of the fuel cell with samples 1 and 2 incorporated cathodes.
  • -A proton-conducting composite layer comprising carbon or carbonsupported ruthenium oxide with Nafion solution was inserted between a Nafion membrane and the cathode of PEFCs operating with hydrogen and oxygen, and their performance was examined.
  • The sample stubs were initially kept in the preparatory chamber overnight to desorb any volatile species at 10 −9 mbar and were introduced into the analysis chamber having a base pressure of 9.8 ϫ 10 −10 mbar for recording the spectra.

Results and Discussion

  • The structure of hydrous ruthenium oxide comprises a network of rutilelike clusters with extensions in the subnanometer range for highly hydrated samples, along with physi-and chemisorbed water in its grain-boundary regions.
  • This indicates that the discharged supercapacitor element at 0.6 V could be quickly recharged when the operating voltage of the fuel cell jumped back to 0.8 V.
  • The increased steady-state performance observed with a hydrous ruthenium oxide incorporated electrode is due to the increased proton conductivity in the catalyst layer.
  • PEFC with hydrous ruthenium oxide cathode shows a higher peak current than the one with Pt/C. Accordingly, it is conjectured that the rapid reaction kinetics and pseudocapacitance associated with hydrous ruthenium oxide ameliorates the peak current output of PEFC.

Conclusions

  • The incorporation of hydrous ruthenium oxide onto the carbon support of the platinum catalyst enhances the performance of PEFC.
  • An optimum loading of hydrous ruthenium oxide ͑10 w/o͒ is required to improve the performance of the PEFC.
  • In addition, the supercapacitive behavior of hydrous ruthenium oxide helps improve the pulse power output of the fuel cell.

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Figures (14)

Content maybe subject to copyright    Report

PEFC Electrode with Enhanced Three-Phase Contact
and Built-In Supercapacitive Behavior
G. Selvarani,
a
A. K. Sahu,
a
G. V. M. Kiruthika,
a
P. Sridhar,
a,
*
S. Pitchumani,
a
and A. K. Shukla
a,b,
*
,z
a
Central Electrochemical Research Institute, Karaikudi-630006, India
b
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560 012, India
Hydrous ruthenium oxide, which exhibits both protonic and electronic conduction, is incorporated in the cathode electrocatalyst
layer of the membrane electrode assembly for polymer electrolyte fuel cells PEFCs. The supercapacitive behavior of ruthenium
oxide helps realize a fuel cell–supercapacitor hybrid. Platinum Pt nanoparticles are deposited onto carbon-supported hydrous
ruthenium oxide and the resulting electrocatalyst is subjected to both physical and electrochemical characterization. Powder X-ray
diffraction and transmission electron microscopy reflect the hydrous ruthenium oxide to be amorphous and well-dispersed onto the
catalyst. X-ray photoelectron spectroscopy data confirm that the oxidation state of ruthenium in Pt anchored on carbon-supported
hydrous ruthenium oxide is Ru
4+
. Electrochemical studies, namely cyclic voltammetry, cell polarization, intrinsic proton conduc-
tivity, and impedance measurements, suggest that the proton-conducting nature of hydrous ruthenium oxide helps extend the
three-phase boundary in the catalyst layer, which facilitates improvement in performance of the PEFC. The aforesaid PEFC
operating with hydrogen fuel and oxygen as oxidant shows a higher power density 0.62 W/cm
2
@ 0.6 V in relation to the PEFC
comprising carbon-supported Pt electrodes 0.4 W/cm
2
@ 0.6 V. Potential square-wave voltammetry study corroborates that the
supercapacitive behavior of hydrous ruthenium oxide helps ameliorate the pulse-power output of the fuel cell.
© 2008 The Electrochemical Society. DOI: 10.1149/1.3005965 All rights reserved.
Manuscript submitted July 28, 2008; revised manuscript received September 29, 2008. Published November 13, 2008.
In the postoil energy economy, hydrogen-based fuel cells are
being perceived as a possible energy alternative. Hydrogen-based
polymer electrolyte fuel cells PEFCs are most promising as they
offer an order of magnitude higher power density than any other fuel
cell system. A PEFC is fed with hydrogen, which is oxidized at the
anode and oxygen that is reduced at the cathode. The protons re-
leased during the oxidation of hydrogen pass through the proton
exchange membrane to the cathode. The electrons released during
the oxidation of hydrogen travel through the external electric circuit,
generating an electric current. Owing to the high degree of irrevers-
ibility of the oxygen reduction, even under open-circuit condition,
the overpotential of the oxygen electrode in a PEFC happens to be
about 0.2 V. This represents a loss of about 20% from the theoret-
ical maximum efficiency for a PEFC. Accordingly, the PEFC cath-
ode electrocatalyst has to possess a high intrinsic activity for the
electrochemical reduction of oxygen at the cathode in order to attain
the maximum efficiency of the PEFC.
1,2
The activity of the cathode catalyst is reportedly improved by
design of the tailored catalyst with controlled composition and mi-
crostructure. Nevertheless, the high activity of the catalyst itself is
necessary, but not a sufficient condition for good fuel cell perfor-
mance. To ensure the optimum conditions for effective catalyst uti-
lization, an environment must be provided that allows for an ad-
equate supply of reactants as well as good connectivity of the active
sites to the electron and ion-conducting phases. The lack of any of
the aforesaid requirements will lead to a decrease in fuel cell
performance.
3-6
Conventionally, to extend the ion-conducting path in the catalyst
layer, the electrocatalyst needs to be dispersed with a proton-
conducting substance, such as Nafion. This has been shown to im-
prove the performance of the PEFCs, but the platinum in the catalyst
layer remains not fully utilized. Besides, Nafion often affects the
efficiency of the platinum electrocatalyst by blocking the active
sites, restricting the gas permeability of the catalyst layer as well as
its electronic conductivity.
6,7
Hence, there is a limit on the quantity
of Nafion that needs to be added without affecting the cell perfor-
mance. Furthermore, it is known that the quite rigid backbone of the
Nafion polymer is not able to penetrate into the small microspores.
Therefore, the catalyst situated in these microspores will have no
connectivity with the proton-conducting phase, and will conse-
quently remain electrochemically inactive. In recent years,
8-10
cer-
tain new approaches, such as the introduction of proton-conducting
agents in the catalyst layer or catalyst carbon support, have been
adopted to further the performance of PEFCs. However, a higher
loading of proton-conducting agents in the catalyst layer leads to a
decrease in electron conductivity as well as flooding of the elec-
trode.
Hydrous ruthenium oxide RuO
2
·xH
2
O is reported to be a
mixed electron and proton conductor, making it a potential mate-
rial for fuel cells. Hydrous ruthenium oxide is also a known material
in pseudocapacitors, because it has a high stability, high specific
capacitance, and rapid faradaic reaction. However, to improve its
utilization, hydrous ruthenium oxide needs to be supported on a
high-surface-area carbon.
11,12
The proton conductive behavior of ruthenium oxide is due to the
solid-state surface redox transitions of ruthenium species, schemati-
cally written as
RuO
x
+ H
+
+ e
RuO
x
OH
0 艋␦艋x 1
Interestingly, the transitions proceed reversibly over the whole po-
tential region of fuel cell operation.
In the literature,
13-16
a few studies are reported to demonstrate
the suitability of hydrous ruthenium oxide as an anode catalyst sup-
port for direct methanol fuel cells. Although hydrous ruthenium ox-
ide has been found to increase proton conductivity and catalyst uti-
lization in a membrane electrode assembly MEA, its influence on
the promotion of carbon monoxide and methanol oxidation is lim-
ited. In the case of a fuel cell operated with hydrogen and oxygen,
Wang et al.
17
reported the incorporation of a supercapacitive
RuO
2
·xH
2
O sublayer between the electrocatalyst layer and Nafion
membrane. Their study shows a higher peak power than that of a
conventional fuel cell system, but the nominal power of the hybrid
system is found slightly inferior in relation to the conventional sys-
tem due to additional proton transport resistance of the supercapaci-
tive layer. D’Souza et al.
18
have also reported that the addition of
hydrous ruthenium oxide into the catalyst layer shows a high peak-
power performance, but there is little improvement in the steady-
state performance of the fuel cell. The reason for not achieving
significant steady-state performance could be attributed to the
method of preparation, incorporation, and the need for optimizing
hydrous ruthenium oxide loading in the catalyst layer. Besides, the
aforesaid studies lack in the characterization of hydrous ruthenium
oxide and its mechanistic investigation.
*
Electrochemical Society Active Member.
z
E-mail: akshukla2006@gmail.com
Journal of The Electrochemical Society, 156 1 B118-B125 2009
0013-4651/2008/1561/B118/8/$23.00 © The Electrochemical Society
B118
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In the present study, hydrous ruthenium oxide is prepared onto
the carbon support followed by impregnation of the Pt catalyst. The
optimum amount of ruthenium oxide loading in the cathode catalyst
layer is determined through cyclic voltammetry CV and cell po-
larization measurements. Cell polarization, intrinsic proton conduc-
tivity, and impedance studies show that the hydrous ruthenium oxide
incorporated fuel cell exhibits a superior performance in relation to
fuel cell employing carbon-supported platinum electrodes. Further-
more, the capacitance and potential square-wave voltammetry stud-
ies show that the hydrous ruthenium oxide incorporated fuel cell
possesses a high-energy storage capacity, which helps to improve
the pulse power of the fuel cell system.
Experimental
Preparation of hydrous ruthenium oxide.— A chemical route re-
ported elsewhere
11
was adopted to prepare hydrous ruthenium oxide.
In brief, RuCl
3
·xH
2
O Alfa Aesar was dissolved in equal volumes
of water and methanol under ultrasonication for 1 h to form orga-
nometallic species at room temperature. 0.1 M NaOH solution was
then added to the solution with continued mechanical stirring until a
pH value 7 was attained to form a turbid suspension of ruthenium
oxide. The ruthenium oxide suspension was centrifuged and washed
copiously with deionized water until no residual chloride was ob-
served. The ruthenium oxide thus obtained was dispersed in aqueous
ammoniacal solution and sonicated for 2 h to form a metastable
colloidal solution of RuO
2
·xH
2
O. To prepare the carbon-supported
RuO
2
·xH
2
O, the required amount of colloidal solution of
RuO
2
·xH
2
O and carbon were ultrasonicated for 1 h, and the result-
ant mixture was heated at 60°C in vacuum to remove the solvent.
Finally, the carbon-supported ruthenium oxide was annealed at
150°C in air for 2 h.
Preparation of supported platinum catalyst.— A sulfito-complex
route was adopted
9
to anchor 40 wt % w/o platinum on carbon.
The required amount of carbon-supported hydrous ruthenium oxide
was suspended in distilled water and agitated in an ultrasonic water
bath Vibronics, 300 W, 250 kHz to form a slurry. The required
amount of Na
6
PtSO
3
4
was dissolved in 1 M H
2
SO
4
and diluted
adequately with distilled water. The solution was added dropwise to
the carbon slurry with constant stirring at 80°C followed by the
addition of 30% H
2
O
2
with the temperature maintained at 80°C.
The solution was further stirred for 1 h. Subsequently, platinum on
carbon was obtained by adding 10% formic acid solution, which
was filtered, washed copiously with hot distilled water, and dried in
an air oven at 80°C for 2 h. In the subsequent text, carbon-
supported platinum catalyst without ruthenium oxide is referred to
as sample 1 and platinum anchored on carbon-supported hydrous
ruthenium oxide catalyst is referred to as sample 2.
Fabrication of MEAs.— Both the anode and cathode comprise a
backing layer, a gas-diffusion layer, and a reaction layer. A Teflo-
nized 15 w/o poly tetrafluoroethylene, PTFE carbon paper
Toray-TGP-H-120 of 0.35 mm thickness was employed as the
backing layer in these electrodes. To prepare the gas-diffusion layer,
Vulcan-XC72R carbon was suspended in cyclohexane and agitated
in an ultrasonic water bath for 30 min. To this, 15 w/o PTFE sus-
pension was added with continuous agitation. The resultant slurry
was spread onto a Teflonized carbon paper and sintered in a furnace
at 350°C for 30 min. To prepare the catalyst layer, the required
amount of the catalyst samples 1 or 2 was suspended in isopropyl
alcohol. The mixture was agitated in an ultrasonic water bath and 30
w/o of Nafion DuPont solution was added to it with continuous
agitation for 1 h. The resulting ink was coated onto the gas-diffusion
layer of the electrode. Both the anode and cathode contain the plati-
num loading of 0.5 mg cm
−2
active area 25 cm
2
that is kept iden-
tical for all MEAs. To establish effective contact between the cata-
lyst layer and the polymer electrolyte, a thin layer of Nafion solution
5w/o diluted with isopropyl alcohol in 1:1 ratio was spread onto
the surface of each electrode. MEAs were obtained by hot-pressing
the cathode and anode on either side of a pretreated Nafion-1135
membrane at 60 kg cm
−2
at 130°C for 3 min.
Electrochemical characterization.— Polarization studies on
PEFCs
.— MEAs were evaluated using a conventional 25 cm
2
fuel
cell fixture with a parallel serpentine flow field machined on graph-
ite plates M/s Schunk Kohlenstofftechnik GmbH, Germany. After
equilibration, the single cells were tested at 60°C with humidified
gaseous hydrogen at a flow rate of 1 L per min at the anode and
humidified gaseous oxygen at a flow rate of 1 L per min at the
cathode at atmospheric pressure. While using air in place of oxygen,
the flow rate was kept at 1.5 L/min. Measurements of cell potential
as a function of current density were conducted galvanostatically
using an LCN100-36 electronic load procured from Bitrode Corpo-
ration, U.S.A.
CV measurements: Electrochemical surface-area measure-
ments.— CV measurements were conducted to determine the elec-
trochemical surface area using a potentiostat Autolab-PGSTAT 30
with its leads for reference and counter electrodes connected to the
cell anode, and its lead for working electrode connected to the cell
cathode. During the experiment, humidified hydrogen and humidi-
fied nitrogen were fed to the anode and cathode, respectively. The
voltammograms were recorded after a run time of 1 h. During the
first hour, the electrode was cycled between 0 and 1 V at a sweep
rate of 50 mV/s to attain stable and reproducible voltammograms.
The electrochemical surface area ESA of Pt catalyst was estimated
from the equation
9
ESAcm
2
/gPt = Q
H
mC/cm
2
/0.210mC/cm
2
electrode loadinggPt/cm
2
兲兴 2
In Eq. 2, Q
H
represents the charge of hydrogen desorption and
210 C/cm
2
is the charge required to oxidize a monolayer of H
2
on
smooth platinum surface.
Capacitance measurements.— CVs were obtained to determine the
capacitance of the fuel cell with samples 1 and 2 incorporated cath-
odes. The capacitance was calculated from the following equation
19
C = Q/V兲关3
In Eq. 3, C is the capacitance, Q is the charge obtained by current
integration during the discharge scan, and V is the voltage scan.
The capacitance values obtained were converted into gravimetic ca-
pacitance by dividing the capacitance values by mass. All voltam-
mograms were recorded between 0.5 and +0.5 V at a scan rate of
5mV/s. During the experiment humidified nitrogen was fed to both
the anode and cathode chambers.
Potential square-wave voltammetry study.— The peak power of the
fuel cell with samples 1 and 2 was measured under humidified hy-
drogen at the anode and air at the cathode or humidified nitrogen on
either side using a Solartron analytical model 1480 electrochemical
interface, under the pulse voltage condition using potential square-
wave voltammetry. The potential was stepped between 0.8 and
0.4 V, and the corresponding current was measured.
Comparison of proton conductivity in catalyst layers of PEFCs.— A
proton-conducting composite layer comprising carbon or carbon-
supported ruthenium oxide with Nafion solution was inserted be-
tween a Nafion membrane and the cathode of PEFCs operating with
hydrogen and oxygen, and their performance was examined. The
performance of the PEFC employing MEA without the composite
layer was also obtained for a comparison, as suggested in the
literature.
20
Figure 1a provides the schematic diagram of the fuel
cell with the MEA comprising sample 1 as cathode, while Fig. 1b
and c represent the fuel cells akin to Fig. 1 but with additional
carbon and Nafion solution admixture as the composite layer, and
carbon-supported ruthenium oxide with Nafion solution as the com-
posite layer, respectively.
Impedance measurements.— An impedance analyzer Autolab-
PGSTAT 30 was employed to measure the resistance of the MEA at
the operating cell voltage of 0.8 V. The reference and counter elec-
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trode leads were connected to the hydrogen electrode, and the work-
ing electrode lead was linked to the air electrode. Impedance mea-
surements were conducted in the frequency range between 100 mHz
and 5 kHz while imposing a sine wave of 10 mV amplitude.
Physical characterization.— Physical characterization of sam-
ples 1 and 2 was carried out using powder X-ray diffraction XRD
obtained from an X-ray diffractometer Philips Pan Analytical, and
the morphology of ruthenium oxide and platinum was examined
through a Tecnai 20 G2 transmission electron microscope TEM.
X-ray photoelectron spectra XPS of the samples were recorded
on a MultiLab 2000 Thermofisher Scientific, UK fitted with a twin
anode X-ray source using Mg K radiation 1253.6 eV. For re-
cording the spectrum, the powder samples were pressed onto a con-
ducting carbon tape pasted onto the indium-coated stainless steel
stubs. The sample stubs were initially kept in the preparatory cham-
ber overnight to desorb any volatile species at 10
−9
mbar and were
introduced into the analysis chamber having a base pressure of
9.8 10
−10
mbar for recording the spectra. High-resolution spectra
averaged over five scans with a dwell time of 100 ms in steps of
0.02 eV were obtained at a pass energy of 20 eV in constant ana-
lyzer energy mode. Experimental data were curve fitted with a
Gaussian and Lorentzian mixed product function after subtracting
the Shirley background. Spin-orbit splitting and the doublet intensi-
ties were fixed as described in the literature.
9
The relative intensity
of the species on the surface was estimated from the respective areas
of the fitted peaks.
Results and Discussion
The structure of hydrous ruthenium oxide comprises a network
of rutilelike clusters with extensions in the subnanometer range for
highly hydrated samples, along with physi- and chemisorbed water
in its grain-boundary regions. Proton transport is facilitated by the
hydrous-grain-boundary regions, while electron transport takes
place inside the network of RuO
2
clusters. Although proton conduc-
tivity is optimized with high water content, the electron conductivity
of these samples happens to be low, as most of the RuO
2
clusters are
electronically insulated by the hydrous grain boundaries. As water is
removed, the RuO
2
clusters grow in size and the grain-boundary
regions become narrower until they eventually lead to the formation
of interconnections. It is reported that amorphous ruthenium oxide
with annealing at a critical temperature close to its crystalline tem-
perature 共⬃150°C shows optimum protonic and electronic
conductivity.
18,19,21
Accordingly, in the present study, hydrous ruthe-
nium oxide annealed at 150°C is used.
Powder-XRD patterns for samples 1 and 2 are shown in Fig. 2.
Diffraction peaks at ca. 40, 47, and 68° are due to 111, 200, and
220 planes of the face-centered cubic structure for Pt metal, re-
spectively. The XRD peak positions for samples 1 and 2 remain
identical, indicating that the modification procedure has little effect
on the crystallinity of Pt. An estimation of mean size of Pt in
samples 1 and 2 is performed from the half-width at full maximum
for the Pt111 Bragg reflection peak using the Debye–Scherrer
equation. The mean particle size of Pt in samples 1 and 2 is 4.2 and
3 nm, respectively. A decrease in particle size is observed in hydrous
ruthenium oxide incorporated catalyst, which may be due to the
prevention of agglomeration of Pt particles by amorphous hydrous
ruthenium oxide. In addition, the XRD experiments are carried out
Figure 1. Schematic diagrams of PEFCs
with MEAs comprising a sample 1 with-
out composite layer, b carbon and
Nafion solution admixture as composite
layer, and c carbon-supported ruthenium
oxide with Nafion solution admixture as
composite layer.
20 30 40 50 60 70 80 90
0
50
100
150
200
0
50
100
150
200
(b)
2 Theta
(
de
g
rees
)
(a)
Inten s ity (c/s)
Figure 2. Powder XRD patterns for a sample 1 and b sample 2 contain-
ing 10 w/o hydrous ruthenium oxide.
B120 Journal of The Electrochemical Society, 156 1 B118-B125 2009B120
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for hydrous ruthenium oxide without carbon support, with carbon
support, and after annealing at 150°C. These XRD patterns are akin
to that reported in the literature.
14,16,19
TEM images for samples 1 and 2 are shown in Fig. 3a and b. In
Fig. 3a, it is seen that the nanometer-sized Pt particles are not in
proper shape and less homogeneously dispersed throughout the cata-
lyst. Figure 3b shows the TEM image for sample 2. Amorphous
ruthenium oxide is barely visible due to the high platinum loading
on carbon. However, platinum is homogeneously dispersed on car-
bon. In addition, the size of the Pt particle is smaller than that in
sample 1.
Figure 4 shows XPS Pt 4f core level region for samples 1 and
2. Pt 4f regions for samples 1 and 2 can be fitted into two sets of
spin-orbit doublets. For sample 1, the Pt 4f
7/2,5/2
peaks at 71.09,
72.10 eV and 74.39, 75.17 eV have been assigned to Pt
0
and Pt
2+
,
respectively.
9,22,23
The full-width at half-maximum fwhm values
for Pt
0
and Pt
2+
peaks are 1.33 and 2.06 eV. Pt 4f
7/2,5/2
peaks for
sample 2 at 71.09, 72.38 eV and 74.39, 75.43 eV have been as-
signed to Pt
0
and Pt
2+
, respectively, and their corresponding fwhm’s
are 1.37 and 2.13 eV. The relative intensities of the different species
obtained from the respective area are shown in Table I. Pt
0
is found
to be the predominant species in both samples 1 and 2. The Pt
0
percentage in sample 2 is 77, which is higher than its value in
sample 1 60%. This is due to the oxophilic nature of ruthenium in
sample 1.
22
The XPS spectrum of the C 1s peak entirely covers the Ru
3d
3/2
and partially overlaps with Ru 3d
5/2
. Hence, a quantitative
estimation of the oxidation state of ruthenium is not possible from
these spectra.
22
Therefore, the Ru 3p
1/2,3/2
region has been studied
to examine different ruthenium species, and the deconvoluted spec-
trum of sample 2 is shown in Fig. 5. Spectral parameters obtained
from the analysis of the Ru 3p
1/2,3/2
region are listed in Table II.
The main Ru 3p
3/2
peak in RuO
2
, is deconvoluted into a compo-
nent observed at 463.1 eV and a higher-energy component observed
at 465.5 eV, which are ascribed to Ru
4+
and hydrous Ru
4+
species,
respectively.
24,25
This suggests that the oxidation state of ruthenium
in Pt anchored on carbon-supported ruthenium oxide is Ru
4+
.
The O 1s spectrum for sample 1 shown in Fig. 6a could be
deconvoluted into three components. Peak 1 530.7 eV is ascribed
to O
2−
of PtO; peak 2 532.3 eV is due to adsorbed OH-species,
and peak 3 533.8 eV is due to physisorbed water.
26-28
The O 1s
spectrum for sample 2 shown in Fig. 6b could be deconvoluted into
five components. Peaks 1, 2, and 3 are similar to sample 1 but for
the higher intensity for peak 3 in sample 2; peaks 4 529.1 eV and
5 535.3 eV are due to O
2−
in RuO
2
and free water,
respectively.
29,30
The presence of higher-intensity peaks 3 and 5 in
sample 2 clearly reflects the hydrated nature of sample 2.
The enhanced catalytic nature of the fuel cell with hydrous ru-
thenium incorporated cathode is further examined through CV, po-
larization, and impedance studies. CV studies are carried out to
quantify the variation of ESA and percentage of platinum utilization
with varying hydrous ruthenium oxide loading in sample 2 and com-
pared with sample 1. A typical CV is shown in Fig. 7. The peak
between 0.0 and 0.4 V provides the information on the hydrogen
(a)
(b)
Figure 3. Electron micrographs for a sample 1 and b sample 2 containing
10 w/o hydrous ruthenium oxide.
6
6
8
8
7
7
0
0
7
7
2
2
7
7
4
4
7
7
6
6
7
7
8
8
0
1
0
1
2
2
0
2
0
2
4
6
4
6
(b)
2
2
1
I
I
n
n
t
t
e
e
n
n
s
s
i
i
t
t
y
y
/
/
(
(
C
C
P
P
S
S
x
x
1
1
0
0
4
4
)
)
B
B
i
i
n
n
d
d
i
i
n
n
g
g
E
E
n
n
e
e
r
r
g
g
y
y
/
/
e
e
V
V
1
(a)
2
2
1
1
Figure 4. Color online XPS spectra for Pt 4f region in a sample 1 and
b sample 2 containing 10 w/o hydrous ruthenium oxide. The solid line
represents the fitted XPS spectra, and broken line represents the peaks due to
platinum metal and its oxides 1,2 correspond to Pt
0
and Pt
2+
species, respec-
tively.
Table I. Binding energy (BE), fwhm, and relative intensity values
for different Pt species as observed from Pt (4f) spectra for
sample 1 and sample 2.
Catalyst Pt species
BE eV
fwhm
eV
Relative intensity
%4f
7/2
4f
5/2
Sample 1
Pt
0
71.09 74.39 1.33 60
Pt
2+
72.1 75.17 2.06 40
Sample 2
Pt
0
71.09 74.39 1.37 77
Pt
2+
72.38 75.43 1.81 23
B121Journal of The Electrochemical Society, 156 1 B118-B125 2009 B121
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adsorption/desorption. The coulombic charge for oxidation of hy-
drogen, corresponding to the area under the anodic peak minus the
double-layer charge, increases with increased loading of ruthenium
oxide in sample 2 with a maximum at 10 w/o, and subsequently
decreases as the ruthenium oxide loading is increased further.
ESA and adsorbed hydrogen charge Q
H
are calculated from CV
data and the results are presented in Table III. An increase in plati-
num utilization is found after incorporation of hydrous ruthenium
oxide onto the carbon support. This is due to the extension of proton
and electron conducting paths in the catalyst layer. The optimum
loading is found to be 10 w/o in sample 2. However, a further
increase in ruthenium oxide loading to 15 w/o decreases Pt utiliza-
tion, possibly due to the increased hydrophilic nature, which results
in reduced mass transport to the catalyst sites.
The potential range of 0.4–0.55 V in CV is responsible for the
double-layer charge. The hydrous ruthenium incorporated system
shows a higher double-layer charge than the one without it. This is
due to the capacitance behavior of hydrous ruthenium oxide.
The capacitive behavior of PEFC containing Pt/C–RuO
2
·xH
2
O
10 w/o electrode is measured using CV in nitrogen environment to
investigate the influence of hydrous ruthenium oxide. For clarity, the
capacitive behavior of PEFC with Pt/C electrode is also tested under
the same conditions. CV curves exhibit pseudocapacitive behavior
between 0.5 and +0.5 V. For a comparison, CV curves for fuel
cells containing sample 2 limited to 10 w/o of hydrous ruthenium
oxide and sample 1 are shown in Fig. 8. CVs are recorded at a scan
rate of 5 mV/s. The hydrous ruthenium oxide incorporated system
exhibits a higher capacitance than the one without it. The superior
capacitive performance of the ruthenium incorporated system is due
to the pseudocapacitive nature of ruthenium oxide. For the PEFC
without hydrous ruthenium oxide, the observed capacitance is attrib-
uted to the double-layer capacitance of carbon black that is used as
a supporting material for the Pt electrocatalyst.
It is well-known that hydrous ruthenium oxide has a good proton
conductivity, high specific power density, rapid charge/discharge be-
havior, and is capable of sustaining several charge–discharge cycles
without any perceptible capacity decay. Hence, the fuel cell catalyst-
4
4
6
6
0
0
4
4
7
7
0
0
4
4
8
8
0
0
4
4
9
9
0
0
-
-
1
1
0
1
2
0
1
2
3
4
5
3
4
5
2
2
1
1
Ru3p
1/2
I
I
n
n
t
t
e
e
n
n
s
s
i
i
t
t
y
y
/
/
C
C
P
P
S
S
x
x
1
1
0
0
3
3
B
B
i
i
n
n
d
d
i
i
n
n
g
g
E
E
n
n
e
e
r
r
g
g
y
y
/
/
e
e
V
V
Ru3p
3/2
Figure 5. Color online XPS spectra for Ru 3p region for sample 2 con-
taining 10 w/o hydrous ruthenium oxide. The solid line represents the fitted
XPS spectra, and broken line represents the peaks due to ruthenium IV
oxides 1,2 correspond to RuO
2
and RuO
2
·xH
2
O, respectively.
Table II. BE, fwhm, and relative intensity values for different Ru
species as observed from Ru 3p
3Õ2
spectra for sample 1.
Catalyst Ru species
BE of Ru 3p
3/2
eV
fwhm
eV
Relative
intensity %
Sample 2
RuO
2
463.1 3.45 76
RuO
2
·xH
2
O
465.5 3.5 24
0
8
16
525 530 535 54
0
0
2
4
(a)
3
Intensity/(CPS x10
3
)
1
2
5
(b)
4
3
2
1
Bindin
g
Ener
gy
/eV
Figure 6. Color online O 1s region in a sample 1 and b sample 2
containing 10 w/o hydrous ruthenium oxide. The solid line represents the
fitted XPS spectra, and broken line represents the peaks due to various forms
of oxides.
Figure 7. Color online CVs for samples 1 and 2 with varying amounts of
hydrous ruthenium oxide 25 cm
2
single cell at 25°C; N
2
and H
2
streams at
cathode and anode, respectively; scan rate = 50 mV/s.
Table III. Hydrogen adsorption charge and electrochemical sur-
face area of Pt with different w/o hydrous ruthenium oxide onto
the catalyst-carbon support.
Catalyst Q
H
a
mC ESA
b
m
2
/gofPt
Sample 1 66.30 63.1
Sample2with5w/o
hydrous ruthenium oxide
74.04 70.5
Sample 2 with 10 w/o
hydrous ruthenium oxide
80.58 76.7
Sample 2 with 15 w/o
hydrous ruthenium oxide
72.69 69.3
a
Q
H
: Experimentally determined adsorbed hydrogen charge.
b
ESA: Electrochemically determined active surface area from Q
H
as-
suming 0.210 mC/cm
2
for Q
H
on smooth platinum.
B122 Journal of The Electrochemical Society, 156 1 B118-B125 2009B122
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Citations
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TL;DR: In this article, the electrocatalytic activity of the catalysts toward oxygen reduction reaction (ORR), both in the presence and absence of methanol, was evaluated for application in direct methanoline fuel cells (DMFCs).
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01 Jan 2009
TL;DR: In this paper, a carbon-supported Pt-TiO2/C catalysts with varying at. wt ratios of Pt to Ti, namely, 1:1, 2:1 and 3:1 were evaluated for direct methanol fuel cells DMFCs.
Abstract: Carbon-supported Pt–TiO2 Pt–TiO2/C catalysts with varying at. wt ratios of Pt to Ti, namely, 1:1, 2:1, and 3:1, are prepared by the sol–gel method. The electrocatalytic activity of the catalysts toward oxygen reduction reaction ORR, both in the presence and absence of methanol, is evaluated for application in direct methanol fuel cells DMFCs. The optimum at. wt ratio of Pt to Ti in Pt–TiO2/C is established by fuel cell polarization, linear sweep voltammetry, and cyclic voltammetry studies. Pt–TiO2/C heattreated at 750°C with Pt and Ti in an at. wt ratio of 2:1 shows enhanced methanol tolerance, while maintaining high catalytic activity toward ORR. The DMFC with a Pt–TiO2/C cathode catalyst exhibits an enhanced peak power density of 180 mW/cm2 in contrast to the 80 mW/cm2 achieved from the DMFC with carbon-supported Pt catalyst while operating under identical conditions. Complementary data on the influence of TiO2 on the crystallinity of Pt, surface morphology, and particle size, surface oxidation states of individual constituents, and bulk and surface compositions are also obtained by powder X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive analysis by X-ray, and inductively coupled plasma optical emission spectrometry.

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References
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Book
12 Oct 1992
TL;DR: In this paper, the authors used the carbon C 1s peak at 285 eV as a reference for charge correction in XPS analyses of samples prepared outside the high vacuum chamber relatively thick carbon layers are formed on the surfaces.
Abstract: Analysis of XPS spectra of Fe2 and Fe3 ions in oxide April 15th, 2019 Carbon is ubiquitous and is present on all surfaces for XPS analysis It is common practice to use the carbon C 1s peak at 285 eV as a reference for charge correction In routine XPS analyses of samples prepared outside the high vacuum chamber relatively thick carbon layers are formed on the surfaces and the corrected XPS peak positions are independent of the apparent or experimentally

3,450 citations

BookDOI
18 Feb 2003
TL;DR: The second edition of Fuel Cell Systems Explained presents a full and clear explanation of the operation of all the major fuel cell types, and an introduction to possible future technology, such as biological fuel cells.
Abstract: Building on the success of the first edition Fuel Cell Systems Explained presents a balanced introduction to this growing area. "In summary, an altogether satisfying book that puts within its covers the academic tools necessary for explaining fuel cell systems on a multidisciplinary basis." Power Engineering Journal. "An excellent book well written and produced." Journal of Power and Energy. Fully revised and updated, the second edition: Provides an essential guide to the principles, design and application of fuel cell systems. Includes full and updated coverage of fuel processing and hydrogen generation and storage systems. Presents a full and clear explanation of the operation of all the major fuel cell types, and an introduction to possible future technology, such as biological fuel cells. Features a new chapter on the direct methanol fuel cell. Now includes examples of the modelling, design and engineering of real fuel cell systems. A clear overview of fuel cell operation and thermodynamics. Coverage of the complete fuel cell system including compressors, turbines, and the electrical and electronic sub-systems such as regulators, inverters, grid inter-ties, electric motors, and hybrid fuel cell/battery systems.

1,724 citations

Journal ArticleDOI
08 Jan 1999-Langmuir
TL;DR: In this paper, it was shown that although practical Pt−Ru blacks have diffraction patterns consistent with an alloy assignment, they are primarily a mix of Pt metal and Ru oxides plus some Pt oxides and only small amounts of Ru metal.
Abstract: Pt−Ru is the favored anode catalyst for the oxidation of methanol in direct methanol fuel cells (DMFCs). The nanoscale Pt−Ru blacks are accepted to be bimetallic alloys as based on their X-ray diffraction patterns. Our bulk and surface analyses show that although practical Pt−Ru blacks have diffraction patterns consistent with an alloy assignment, they are primarily a mix of Pt metal and Ru oxides plus some Pt oxides and only small amounts of Ru metal. Thermogravimetric analysis and X-ray photoelectron spectroscopy of as-received Pt−Ru electrocatalysts indicate that DMFC materials contain substantial amounts of hydrous ruthenium oxide (RuOxHy). A potential misidentification of nanoscale Pt−Ru blacks arises because RuOxHy is amorphous and cannot be discerned by X-ray diffraction. Hydrous ruthenium oxide is a mixed proton and electron conductor and innately expresses Ru−OH speciation. These properties are of key importance in the mechanism of methanol oxidation, in particular, Ru−OH is a critical component ...

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Journal ArticleDOI
01 Jan 1995-Carbon
TL;DR: In this article, the surface composition of the as-received fibers is heterogeneous and undergoes significant changes upon nitric acid exposure, and the net effect of these surface treatments appears to be the systematic and preferential formation of carbonyl and carboxyl groups.

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Frequently Asked Questions (2)
Q1. What have the authors contributed in "Pefc electrode with enhanced three-phase contact and built-in supercapacitive behavior" ?

The aforesaid PEFC operating with hydrogen fuel and oxygen as oxidant shows a higher power density 0. 62 W/cm2 @ 0. 6 V in relation to the PEFC comprising carbon-supported Pt electrodes 0. 4 W/cm2 @ 0. 6 V. Potential square-wave voltammetry study corroborates that the supercapacitive behavior of hydrous ruthenium oxide helps ameliorate the pulse-power output of the fuel cell. 

The enhanced catalytic nature of the fuel cell with hydrous ruthenium incorporated cathode is further examined through CV, polarization, and impedance studies. The potential range of 0. 4–0. 55 V in CV is responsible for the double-layer charge. D study suggests that the high proton-conducting nature of hydrous ruthenium oxide extends the proton-conduction path in the catalyst layer.