Microtubular hydrogen electrode, a reference electrode for electrochemical analyses

Article abstract of DOI:10.1149/1.1874732

This paper describes a new type of miniature hydrogen electrode that is composed of a tubular polymer electrolyte and is suitable for use as the reference electrode in electrochemical measurements. The tubular hydrogen electrode was fabricated by supporting platinum black powder inside a perfluorinated polymer electrolyte tube and feeding hydrogen gas into the tube. It can be used as the reference electrode for electrochemical measurements both in acid electrolyte solutions and in a testing apparatus, for example, in polymer electrolyte fuel cells. The specific feature of the tubular hydrogen electrode is that it is so small that it can fit in microanalytical or electrochemical cell systems. The tubular hydrogen electrode can also work as both a counter electrode and a nonpolarizable reference electrode because of the very large specific surface area of the platinum particles deposited inside. This makes these tubular hydrogen electrodes usable in two-electrode cells, which is another advantage over conventional glass-type hydrogen reference electrodes.

Full text of DOI:10.1149/1.1874732

Journal of The Electrochemical Society, 152 ͑5͒ E161-E166 ͑2005͒
E161
0013-4651/2005/152͑5͒/E161/6/$7.00 © The Electrochemical Society, Inc.
Microtubular Hydrogen Electrode, a Reference Electrode
for Electrochemical Analyses
Masayuki Kunimatsu,b,c Huan Qiao,a and Tatsuhiro Okadaa,z
aNational Institute of Advanced Industrial Science and Technology, Ibaraki 305-8565, Japan
bNew Energy and Industrial Technology Development Organization, Kanagawa 212-8554, Japan
This paper describes a new type of miniature hydrogen electrode that is composed of a tubular polymer electrolyte and is suitable
for use as the reference electrode in electrochemical measurements. The tubular hydrogen electrode was fabricated by supporting
platinum black powder inside a perfluorinated polymer electrolyte tube and feeding hydrogen gas into the tube. It can be used as
the reference electrode for electrochemical measurements both in acid electrolyte solutions and in a testing apparatus, for example,
in polymer electrolyte fuel cells. The specific feature of the tubular hydrogen electrode is that it is so small that it can fit in
microanalytical or electrochemical cell systems. The tubular hydrogen electrode can also work as both a counter electrode and a
nonpolarizable reference electrode because of the very large specific surface area of the platinum particles deposited inside. This
makes these tubular hydrogen electrodes usable in two-electrode cells, which is another advantage over conventional glass-type
hydrogen reference electrodes.
© 2005 The Electrochemical Society. ͓DOI: 10.1149/1.1874732͔ All rights reserved.
Manuscript submitted September 30, 2004; revised manuscript received November 5, 2004.
Available electronically March 30, 2005.
Ordinary electrochemical measurements, such as steady-state
voltammograms or cyclic voltammograms ͑CVs͒ are done with
three-electrode cells consisting of a working electrode, reference
electrode, and counter electrode. The reference electrode assigns the
standard potential for the working electrode and is required for its
stability and reproducibility, as well as the ease of installation to the
measuring system.1-3 The standard electrode potential of hydrogen
electrodes has been defined as zero at all temperatures.4,5 Although
the hydrogen electrode is the best reference electrode for use in an
aqueous solution because of its stability and reproducibility in wide
temperature and pH ranges,1-3 the saturated calomel electrode or
silver silver-chloride electrode are frequently used as practical ref-
erence electrodes because of the ease of installation in the measuring
system.4
However, there are problems in such electrodes, e.g., occurrence
of the liquid junction potential and contamination of the measuring
system by salts leaching out of the inner reference solution of the
electrodes. The large size ͑ca. 1 cm diam and 8-10 cm length͒ is also
a disadvantage for microanalytical uses. A miniaturized and easy-to-
use hydrogen reference electrode would make the electrochemical
analyses easier and much more reliable.
Experimental
Fabrication of microtubular hydrogen electrodes.—Flemion
tubes ͑inner diam 0.3 mm, outer diam 0.6 mm, perfluorosulfonic
acid polymer electrolyte membrane, Asahi Glass Engineering͒ were
used as polymer electrolytes, which were cleansed by successive
boiling in 3 wt % H2O2, deionized ͑DI͒ water, 1 mol/L H2SO4,
and DI water, each for 1 h. To prepare a hydrogen electrode,
the platinum electrode layer was made by injecting Pt black
catalyst paste into the electrolyte tube containing a current-
conducting platinum wire of 0.2 mm diam. The catalyst paste
was made by mixing Pt black powder ͑Johnson Matthey, Inc.͒
and 5 wt % Nafion solution ͑Aldrich͒ and stirring until the mixtures
became a smooth paste. The tubular hydrogen electrodes were
dried at room temperature and then heat-treated at 135°C for
3 min. A picture of a tubular hydrogen electrode is shown in
Fig. 1.
Another type of fabrication was tested where the catalyst layer
inside the tube was made by the chemical plating method ͑see the
Appendix͒. Both Pt plating and a mixture of PtRu plating were
tested as hydrogen electrodes.
Measurements with microtubular hydrogen electrodes.—The
prepared tubular hydrogen electrode was evaluated in the
electrochemical system shown in Fig. 2, by measuring the following
characteristics in comparison with a normal hydrogen electrode.
The items evaluated were ͑i͒ potential indication of the tubular
hydrogen electrodes, ͑ii͒ effect of sulfuric acid concentrations
of the outside solution, ͑iii͒ temperature dependence of the potential,
͑iv͒ time response, and ͑v͒ polarization characteristics. Hydrogen
gas of more than 99.995% purity was fed inside the tubular
Recently, we have developed a micro fuel cell with tubular poly-
mer electrolytes and have made steady progress in our research and
development.6,7 In this microtubular fuel cell, the fuel chamber is
the inside space of the tube, and there is no leakage of fuels, which
has been a major problem for planar-type fuel cells. Thus, tubular
electrolytes may offer an attractive basis for fabricating a new type
of hydrogen reference electrode.
In this paper, we report the test results of a newly developed
hydrogen electrode consisting of microtubular polymer electrolytes
applied to electrochemical systems. We evaluated the following cri-
teria: the effect of the acid concentrations of electrochemical sys-
tems on the potential, temperature-dependence characteristics, dy-
namic characteristics, especially response time to the condition
change, and polarization characteristics for unexpected current flow.
The viability of a two-electrode system is also examined, because a
large platinum surface area is obtained by the use of platinum black
particles inside the tube. As a practical example, the electrochemical
measurement is carried out in a sulfuric acid solution while using a
tubular hydrogen electrode as a reference electrode. The perspective
of using a tubular hydrogen electrode as a reference electrode in
testing polymer electrolyte fuel cells ͑PEFCs͒ is also presented.
c Present address: Kanagawa Industrial Technology Research Institute, Kanagawa,
243-0435, Japan.
z E-mail: okada.t@aist.go.jp
Figure 1. Photograph of a tubular hydrogen electrode.
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Journal of The Electrochemical Society, 152 ͑5͒ E161-E166 ͑2005͒
Figure 4. Indication potential of tubular hydrogen electrode at various
temperatures.
Figure 2. Schematic of electrochemical cell used in this study.
neutral and alkaline solutions. The indication potentials of tubular
hydrogen electrodes in KOH and KCl solutions are shown in Fig. 5.
The potentials considerably fluctuate from RHE in both solutions
and do not stabilize. These potential fluctuations are probably due
to the exchange of hydrogen ions in the polymer electrolyte tube by
potassium ions in the solution. Cations except hydrogen ions
were not to be used in the electrolyte solution. Therefore, it is con-
cluded that tubular hydrogen electrodes that are defined as H+ re-
sponding electrodes cannot be used in neutral and alkaline electro-
lyte solutions.
electrode at 1 atm pressure. Temperatures of the electrochemical
cell were controlled by thermostat.
The reference hydrogen electrode ͑RHE͒ consisted of a glass
tube with hydrogen inlet and outlet, platinum mesh to which the
hydrogen gas was supplied in contact with sulfuric acid solution and
the electric lead to the platinum mesh. The indication potentials
were also examined in neutral and alkaline solutions. As applica-
tions, the tubular hydrogen electrodes were evaluated as reference
electrodes not only in electrolyte solutions but also in fuel cell test-
ing systems.
Dynamic response of tubular hydrogen electrode.—Figure 6
shows the CV of a tubular hydrogen electrode as a working
electrode. This CV shows characteristic peaks of polycrystalline
platinum, and the interference due to the impurity is not observed.
The platinum surface area estimated from the adsorption or
desorption peaks of hydrogen was 100 cm2, approximately 700
times larger than the geometrical surface area. This fact indicates
that a two-electrode system may be designed for electrochemical
measurements by using the single tubular hydrogen electrode.
In this case, a tubular hydrogen electrode works as both a reference
electrode and a counter electrode.
As a test experiment of tubular hydrogen electrodes, the CV
of a platinum plate was measured in sulfuric acid solutions.
Figure 7 shows CVs measured with two-electrode and three-
electrode cells when a tubular hydrogen electrode was used as the
reference electrode and also as the counter electrode. The same re-
sults occurred with both the two-electrode and three-electrode cells,
indicating the applicability of tubular hydrogen electrodes in simple
two-electrode cells.
Results
Hydrogen electrode potential at tubular hydrogen electrode.—
Figure 3 shows the time variation of the indication potentials
of the tubular hydrogen electrode in sulfuric acid solutions of
various concentrations. The stable potentials, which are within
±0.2 mV of RHE, are attained in sulfuric acid in about 5 min.
It is seen in Fig. 3 that the tubular hydrogen electrode completely
works as an RHE.
Temperature dependences of potentials at tubular hydrogen
electrode.—Figure 4 shows the potential of the tubular hydrogen
electrode measured over time at each temperature. It indicates that a
temperature increase leads to a negative shift of the indication po-
tentials. The potential shifted in negative direction by 1.0 mV at
50°C and by 4.0 mV at 80°C.
Testing in neutral and alkaline solutions.—In addition to the
testing in acid solutions as shown, testing was also carried out in
Figure 3. Indication potential of tubular hydrogen electrode in sulfuric acid
Figure 5. Indication potential of tubular hydrogen electrode in alkaline and
solutions.
neutral solutions.
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Journal of The Electrochemical Society, 152 ͑5͒ E161-E166 ͑2005͒
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Figure 6. CV of tubular hydrogen electrode.
Figure 8. Polarization curves of tubular hydrogen electrode.
the cell voltage were recorded separately. The tubular hydrogen
electrodes performed perfectly and conveniently as the reference
electrode in fuel cells.
Figure 8 shows the polarization characteristics when a current is
forced to flow to the tubular hydrogen electrode. A gradient of
1.3 mA/͑cm2 mV͒ was obtained in a 0.5 mol/L sulfuric acid solu-
tion. When the ohmic drop was compensated for the electrolyte
solution resistance with the current interruption method, a gradient
of 2.9 mA/͑cm2 mV͒ was obtained. As such, tubular hydrogen elec-
trode may be used in two-electrode electrochemical cells because
the potential shift is so small that it can be neglected for measure-
ments in a current range of a few milliamperes ͑i.e., it can be re-
garded as a nonpolarizable electrode͒. In addition, hysteresis of the
potential during the sweep is negligible.
Figure 9 shows the current response when potential steps are
applied to the tubular hydrogen electrode. The potential immediately
recovers to the initial potential when the current flow is stopped. No
problems are observed in the response characteristics as long as the
tubular hydrogen electrodes are used in normal electrochemical
measurements.
Discussion
The fixed capacity of the ion-exchange groups, or the hydrogen
ion concentration, in the polymer electrolyte corresponds to approxi-
mately pH 0 ͑ca. 1 mol/L H+ concentration in the polymer electro-
lyte͒. Then it is expected that the microtubular hydrogen electrode
would work as a standard hydrogen electrode ͑SHE͒ regardless of
the H+ concentration in the electrolyte solutions, if this solution has
no other cations. In the case where there are other cations, the H+
concentration at the inner surface of the tube would be regarded as
constant in the experimental time until the cation penetrates the tube
completely. If this is true, then the potential of the tubular hydrogen
electrode in reference to RHE ͑glass electrode͒ should be expressed
as follows
RT
F
RT
F
Application to fuel cell measurements.—A tubular hydrogen
electrode was installed in a PEFC ͑see the Appendix͒. The electro-
lyte membrane of the PEFC and a tubular hydrogen electrode were
connected together by gluing with 5 wt % Nafion solution and heat-
treated at 135°C. The potential difference between a tubular hydro-
gen reference electrode and a PEFC at both the anode and the cath-
ode supplied with H2 falls within ±0.2 mV. This means that the
tubular hydrogen electrode functions as a reference electrode in the
PEFCs.
E = −
ln aH
+
͑s͒ = −
ln cH
+
͑s͒
͓1͔
where the activity aH+͑s͒ of H+ in the solution is replaced by the H+
concentration cH+͑s͒.
The indication potentials in sulfuric acid solutions of various
concentrations are shown in reference to SHE ͑Fig. 11͒. Contrary to
expectations, the performance as shown in Fig. 11 indicates that the
tubular hydrogen electrode performs like an RHE.
RHE indicates the potential of a hydrogen reaction between H+
ion in the electrolyte solution and the H2 gas absorbed on the plati-
Figure 10 shows the result measured in a direct methanol fuel
cell ͑DMFC͒ where the anode potential, the cathode potential, and
Figure 7. CVs of Pt plate measured with three-electrode and two-electrode
systems using tubular hydrogen electrode.
Figure 9. Response characteristics of tubular hydrogen electrode.
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Journal of The Electrochemical Society, 152 ͑5͒ E161-E166 ͑2005͒
Figure 12. Indication potential vs. SHE with pH contour lines.
Figure 10. Polarization characteristics of a DMFC measured by using a
tubular hydrogen electrode.
increase in water content by polymer swelling, while the size of the
polymer electrolytes increases due to a temperature increase.
Figure 12 gives a pH contour line obtained from Fig. 4 for the
temperature dependence of the potential. The pH contour is drawn
from the following equation
num surface contacting the same electrolyte solution. In the tubular
hydrogen electrode, the hydrogen reaction occurs at the interface
between the polymer electrolyte and the platinum particles contact-
ing it, without direct contact to the solution outside of the tube.
From the preceding results, it is conceived that the indicated poten-
tial may involve the junction potential between the polymer electro-
lyte and the outside electrolyte solutions,8 because a hydrogen reac-
tion always occurs on the platinum particle at the polymer
electrolyte inside the tube. This junction potential Ejunc is assumed
to take the form
RT
F
RT
F
E = E0 +
ln aH
+
= −
pH
͓4͔
In Fig. 12, the apparent pH of the electrolyte increases with the
temperature increase. This apparent pH increase would be in part
due to the polymer swelling and decrease in the average concentra-
tion of ion-exchange sites. The potential of the tubular hydrogen
electrodes shifts to the negative direction with the temperature in-
crease ͑the potential decreases by 1.0 mV at 50°C͒ so that correction
is necessary for accurate analyses.
u+ − u− RT
u+c+͑s͒ + u−c−͑s͒
u+c+͑m͒ + u−c−͑m͒
Ejunc
=
ln
͓2͔
ͭ
ͮ
u+ + u−
F
Here u+ and u− are the mobility of cations and anions, respectively,
that can transfer through the membrane/solution interface. m and s
stand for the membrane phase and the solution phase at this inter-
face. For cation exchange membranes like in the present case, u−
= 0 and Eq. 1 reduces to the Nernst potential
There are several advantages in using a microtubular hydrogen
electrode. First, the size of the tube is so small that it can be used in
microanalytical cells and miniature-sized electrochemical systems.
Second, there is no disturbance to the measuring system caused by
hydrogen bubbling into the solution, which sometimes becomes a
problem when conventional-type hydrogen electrodes are utilized.
Third, the response time is short enough to attain a steady potential,
and also the drift is very small. Fourth, the large surface area of the
platinum particles makes it possible to use the tubular hydrogen
electrode both as a reference electrode and as a counter electrode at
the same time. A practical reference electrode was proposed where
the hydrogen evolution electrode by forced current from the outer
circuit is made the reference electrode.9 But the shift of potential
from RHE was inevitable depending on the solution composition.
When there are cations other than H+ in the solution, this would
cause a Donnan potential at the interface between the polymer elec-
trolyte and the outside solution.5 According to the estimation, the
deviation of the indication potential due to the Donnan potential
EDon when there are H+ and Na+ is expressed as follows10
RT
F
c+͑s͒
c+͑m͒
Ejunc
=
ln
͓3͔
Summing up Eq. 1 and 3 and setting c+͑m͒ = 1, the net potential
becomes zero, which is what we observed in Fig. 3.
The indication potentials of conventional hydrogen electrodes
should follow Nernst’s equation for temperature changes. For the
tubular hydrogen electrode, polymer electrolyte membranes may ex-
pand with the temperature increase so that it is necessary to calibrate
the effect of the temperature. It seems that the concentration of the
ion-exchange group of the polymer electrolyte decreases due to an
RT
F
␥+c+͑m͒
ͩ ͪ
k+c+͑s͒
EDon = −
ln
͓5͔
where ␥+ is the activity coefficient of the cation ͑H+ or Na+͒ in the
membrane, and k+ is the hypothetical partition coefficient of cation
between solution and membrane phases, in the absence of sulfonic
acid groups. Equating EDon for H+ and Na+, one obtains
−
cSO ͑m͒
RT
F
1 + rc
3
EDon = −
ln
͓6͔
ͩ
kH
kNa
rc
␥
Na
ͪ
2−
2cSO ͑s͒
+
4
␥
H
Figure 11. Indication potential of tubular hydrogen electrode vs. SHE.
When there is no Na+, Eq. 6 becomes
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Journal of The Electrochemical Society, 152 ͑5͒ E161-E166 ͑2005͒
E165
−
cSO ͑m͒
Table A-I. Estimated shift of Donnan potential ⌬EDon from
H2SO4 system to H2SO4 + M2SO4 system.
RT
F
␥
H
3
EDon͑H+͒ = −
ln
͓7͔
ͩ ͪ
2−
2cSO ͑s͒ kH
4
M
Li
KHM
0.586
rc
⌬EDon͑mV͒
−
Here cSO ͑m͒ is the concentration of sulfonic acid groups in the
25°C
40°C
80°C
membran3e, cSO ͑s͒ is that of SO4 ͑analytical͒ in the solution, and
−
2−
1
10
1000
1
10
1000
1
−6.0
−12.1
−13.7
2.2
3.9
4.2
20.7
30.3
32.0
−6.3
−12.7
−14.4
2.3
4.1
4.5
21.8
31.8
33.6
−7.1
−14.4
−16.2
2.6
4.6
5.0
24.5
35.9
37.9
rc is the ratio cN4 a+͑s͒/cH+͑s͒. Equation 7 is the same as the junction
potential as shown in Eq. 3. The deviation of the Donnan potential
due to the presence of Na+ is written as follows
Na
K
1.18
3.84
RT
F
1 + rc
⌬EDon ϵ EDon − EDon͑H+͒ = −
ln
͓8͔
␥
k
ͩ ͪ
H
1 +
Na rc
10
1000
␥
kH
Na
The value of ͑␥H/␥Na͒͑kNa/kH͒ is estimated as 1.18 from the
11
literature value of KHNa
.
Suppose that rc = 1, and Eq. 8 gives
⌬EDon = 2.2 mV at 25°C and 2.6 mV at 80°C. Even in the case
rc = 10,000, Eq. 8 gives ⌬EDon = 4.3 mV at 25°C and 5.0 mV at
80°C. This result means that the deviation becomes larger than sev-
eral mV only when the Na+ concentration in the solution reaches
10,000 times that of H+ in the solution. For other cations, details are
given in the Appendix.
tacting any components when they are necessary. In this case, the
electrolyte membrane of each cell must be exposed outside to con-
tact the tubular hydrogen electrode.
Conclusions
In this work, a new microtubular hydrogen electrode has been
developed and evaluated as a reference electrode. Tubular hydrogen
electrodes are easy to use as reference electrodes in both acid solu-
tions and PEFCs. The advantage of tubular hydrogen electrodes over
conventional glass body hydrogen electrodes are that they ͑i͒ have
very small size with reversible and stable potential, ͑ii͒ are nonpo-
larizable and have no hysteresis, ͑iii͒ work as RHE for different pH
electrolyte solutions, and ͑iv͒ are usable for two-electrode cells.
However, tubular hydrogen electrodes cannot be used in electrolyte
solutions containing cations other than H+ like neutral or alkaline
solutions. Tubular hydrogen electrodes require attention for the po-
tential shift that occurs with temperature change. In fundamental
studies, newly developed hydrogen electrodes are to be applied to
evaluate diffusion rates of metal ions in polymer electrolyte mem-
branes and also to elucidate phenomena at liquid junction interfaces.
When there is very little H+ in the solution, the higher the ten-
dency for the cation to exchange with H+ in the polymer electrolyte,
the larger the error would be in the measured potential. This is a
disadvantage of the present electrode, and precautions must be ad-
dressed when the electrochemical system deviates from an acidic
environment. But as long as the electrochemical measurements are
conducted in solutions of fixed composition, the tubular hydrogen
electrode can still be used as a reference electrode.
The temperature affects the potential of the tubular hydrogen
electrode in two ways. First, the dependence through the Nernst
equation causes a several millivolt positive shift when it is used at
70-80°C, but this can be corrected through a Nernstian fit. The sec-
ond and the large factor in potential shift is due to the polymer
swelling, which may shift the potential to the negative direction by
an activity change of H+ in the polymer. All these effects should be
corrected by preparing calibration curves before the electrochemical
measurements.
One of the promising features of the tubular hydrogen electrode
is the application in fuel cell testing.12 This is made possible due to
its gastight structure, miniaturized size, and capability of sticking to
small portions of the measuring system. An effective application of
the tubular hydrogen electrode may be in fuel cell stacks. It is de-
sired for the optimal design of fuel cell stacks to obtain knowledge
about the potential distribution within a stack during the operation
test. However, it is difficult to install reference electrodes in each of
the cells comprising a fuel cell stack. Tubular hydrogen electrodes
may fit into the cells to measure anode/cathode potentials by con-
Acknowledgment
M.K. acknowledges the New Energy and Industrial Technology
Development Organization ͑NEDO͒ fellowship program for its
financial support.
The National Institute of Advanced Industrial Science and Technology
assisted in meeting the publication costs of this article.
Appendix
Supporting information on the microtubular hydrogen electrode developed for
microanalytical and electrochemical purposes is described here. The fabrication of
the Pt layer is done here by the chemical plating method with successful results.
The Donnan potential corrections for H+ and alkaline cation systems are discussed.
Details of application of the microtubular hydrogen electrode to the fuel cell system are
presented.
Figure A-2. Indication potential of tubular hydrogen electrode in mixture of
H2SO4 and Na2SO4.
Figure A-1. CVs of Pt plate using tubular hydrogen electrode with Pt and
PtRu plating in 1 M H2SO4 at room temperature. Sweep rate: 100 mV s−1
.
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Journal of The Electrochemical Society, 152 ͑5͒ E161-E166 ͑2005͒
Figure A-4. Indication potential of tubular hydrogen electrode in PEFC.
tivity coefficients in Ref. 11 do not change with temperature. Estimated ⌬EDon values
are shown in Table A-I as a function of the ratio rc ϵ cM+/cH+ and temperature. From
the table, it is recommended that as the supporting electrolyte in the acid solution, Na+
solution should be used to minimize the Donnan potential shift, because for other
systems using, e.g., Li+ and K+, the shift becomes substantially large. Experimental
results with H2SO4 + Na2SO4 system are shown in Fig. A-2, which indicates that as far
as the solution is acidic, Na+ with large amounts does not interfere much with the
indication potential.
Figure A-3. Schematic picture of PEFC single cell with a tubular hydrogen
electrode.
Preparation of Pt or PtRu by chemical plating method.— For the chemical
plating method ͑impregnation-reduction method͒, tetraammineplatinum͑II͒ chloride
͓͑Pt͑NH3͒ ͔Cl2͒ and pentaamminechlororuthenium͑III͒ chloride ͓͑RuCl͑NH3͒ ͔Cl2͒,
both from Wako Pure Chemical Industries, Ltd., were used as precursors for Pt and Ru,
respectively. Cation exchange was carried out by immersing the tubular electrolyte
membrane into a Pt complex solution or a mixture of Pt and Ru complex solutions
͑Pt:Ru atom % = 1:1͒ at room temperature. Subsequently, an NaBH4 ͑90%, Wako Pure
Chemical Industries Ltd.͒ alkali solution was injected inside the Flemion tube at 323 K
to precipitate the Pt or PtRu layers. Residual precursors were removed by dipping in 1
M H2SO4 and DI water at 353 K.
The hydrogen electrode was made by injecting the mixing paste of carbon powder
͑Vulcan XC-72R, Cabot͒ and 5 wt % Nafion solution ͑Aldrich Chemical Company, Inc.͒
into the Pt or PtRu deposited tubular electrolyte containing a current-conducting Pt wire
͑⌽ 0.1 ϫ 50 mm, Nilaco Corp.͒.
Figure A-1 shows CVs of the Pt plate electrode measured using tubular hydrogen
reference electrodes with Pt and Pt/Ru plating layers inside the tube. Both Pt and Pt/Ru
mixed catalyst layers worked perfectly as reference electrodes in 1 mol/L sulfuric acid
solutions. The indication potential was within ±0.5 mV in reference to RHE in
0.5 mol/L to 1 mol/L sulfuric acid solutions, respectively.
Application of tubular hydrogen electrodes to PEFC measurement.— Tubular
hydrogen electrode was glued to the membrane of a membrane electrode assembly
͑MEA͒ of a PEFC as shown in Fig. A-3, using 5 wt % Nafion solution ͑Aldrich͒.
Humidified H2 gas produced by bubbling it through a water pot was fed into the tube.
When H2 gas was supplied to the anode or the cathode of the PEFC, the potential stayed
within 0.2 mV in reference to the tubular hydrogen electrode in a temperature range
between 20 and 80°C, as shown in Fig. A-4.
4
5
References
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Equation
8
RT
F
1 + rc
⌬EDon ϵ EDon − EDon͑H+͒ = −
ln
͓A–1͔
ͩ
ͪ
1 + KMH rc
The shift of the Donnan potentials from the pure H2SO4 system to H2SO4 + M2SO4
system ͑M = Li, Na, K͒ is estimated using Eq. A-1, with the assumption that the selec-
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