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Three-electrode solid-state fuel cell for evaluating working electrodes
Solid State Ionics 6 (1982) 337-339 North Holland Publishing Company
THREE-ELECTRODE SOLIDSTATE FUEL CELL FOR EVALUATING WORKING ELECTRODES 0. NAKAMURA, I. OGINO Government Industrial Research Institute, O&Q, Ikeda, Osaka563, Japan
and R.N. CASTELLANO, J.B. GOODENOUGH Inorganic Chemistry Laboratory, South Parks Road, Oxford OXI 3QR, UK Received
Development of a commercti’solid-state fuel cell depends on identification of suitable catalytic electrodes to replace platinum. A three-electrode test cell for electrode evaluation is reported. The solid protonic electrolyte used was dodecamolybdophosphoric acid, H3MolzP04,-,*29HzO, and a thin platinum wire inserted into the electrolyte served as the third electrode. Reproducibility and insensitivity to third-electrode position were demonstrated. The third electrode measures separately the anode and cathode interfacial resistances, thus providing a direct measure of the relative catalytic ao tivity of a given test electrode. Application of the technique is illustrated.
1. Introduction
The discovery [I] that dodecamolybdophosphoric 29H.$ (12MPA), and dodecaacid, H,Mo,~PO~~ l tungstophosphoric acid, H3 W,,PO40 29H20 (12WPA), have a high proton conductivity (0.2 mho cm-l at 25°C) has led to an evaluation of these materials as solid electrolytes in an H2-0, fuel cell. Current densities of 100 mA/cm2 have been obtained with platinum-black catalytic electrodes. However, construction of a commercial cell depends on the development of alternative electrodes to replace platinum. Evaluation .of candidate electrode materials requires separation of three resistances connected in series in the test cell: the anode, electrolyte and cathode resistances. We report here a three-electrode system that permits this separation provided the electrolyte resistance is kept relatively small. Since the resistances of the two proton electrolytes 12-MPA and 12-WPAare sensitive to the degree of hydration, and therefore temperature and environmental humidity, we used a humidity-control device specifically developed [2] to maintain a constant resistance for l
0 167-2738/82/0000-0000/$02.75
0 1982 North-Holland
these electrolytes. The application of this cell is illustrated,with an evaluation of a composite electrode consisting of dispersed platinum supported on a nominal Pb2Ta207 substrate mixed with carbon. The Pb,TazO, crystallites having about 0.5 at.% platinum-on the surface exhibit high catalytic activity toward the,conversion of CO to CO,.
2. Experimental The experimental cell consisted of three components: a solid protonic electrolyte containing a thin platinum wire extending outside of it, a test electrode and a platinum black counter. electrode. The solid electrolyte was prepared from about 5 g of crystalline 12.MPA that had been obtained by lowering the temperature of a saturated solution of dodecamolybdophosphoric acid from 25°C to 22°C. The crystalline precipitate was crushed into a powder under an atmosphere of 85-95% relative humidity. A pellet 18 &in diameter was obtained under a pressure of 1000 kg/cm2 with a separable mold made of glass-fiber-reinforced’epoxy resin. A steel mold
0. Nakamura et al. / Three-electrode solid-statefuel cell
338
3. Results and discussion
PI Solid
electrolyte
Third
electrode
Mold
_ A--&&
/
Fig. 1. Assembly for fabrication of solid-electrolyte with third-electrode wire.
pellet
could not be used because it reacted with the Mo(Vl) ions. A thin metal wire could be inserted into the pellet before higher pressures were applied by means of the separable mold illustrated in fig. 1. Both platinum and gold wires gave good reproducible data. In the experiment reported here, a platinum wire 0.2 mm in diameter was used as platinum exhibits a larger exchange-current density. A platinum-black hydrogen electrode was pressed against the electrode on one side, the test oxygen electrode was pressed against the opposite side of the solid-electrolyte pellet. A platinum-black oxygen electrode was used as a standard for comparison. Since the composite test electrode reported here used graphite as the current collector, a flake-graphite electrode was also used as a calibrant oxygen electrode. The composite test electrode was formed from pyrochlore crystals of nominal PbzTa207 grown from a PbO flux in platinum crucibles. Excess flux was removed by leaching in hot aqueous acetic acid to isolate the cubic crystallites of approximately 200 pm dimensions. ESCA measurements indicate about 0.5 at.% platinum at the surfaces of the crystallites, and an excellent catalytic activity for CO oxidation can be attributed to the platinum on the surface. To investigate whether such a dispersed-platinum catalyst could be used as a catalytic electrode, 20 mg of this material was mixed with 10 mg of flake graphite to form an electronically conducting composite. This composite electrode was used as the test oxygen electrode. A constant current was maintained across the cell with a Takeda Riken TR6141 constant-current generator, and the voltage responses between the third electrode and each working electrode were recorded simultaneously with a two-pen recorder.
Since such a three-electrode test cell has not been reported previously for a solid protonic electrolyte, we first established the reproducibility of our data with both gold and platinum electrodes and then investigated the sensitivity of the data to the position within the electrolyte of the third electrode. For this latter purpose, we placed the third electrode at distances of 1.6, 3.2, 4.8, and 6.5 mm from a working electrode and measured the voltage differences between it and two flake-graphite working electrodes with a constant current of 1 mA through the solid electrolyte. For each third-electrode position, the voltage difference between anode and third electrode was 0.59 V between cathode and third electrode was 0.57 V, see fig. 2 and discussion below. Moreover, the sum of the voltage differences was equal to the voltage difference between the two working electrodes. Thus the measured voltages were shown to be insensitive to the third-electrode position, which implies that the voltage drop across the electrolyte is small relative to the observed voltages of 0.59 V and 0.57 V. Moreover, these measured voltages then reflect the rate constants of the electrochemical reactions at the electrode interfaces, and it is apparent that these reaction rates determine the inner resistance of the cell. The measured ohmic resistance of the solid electrol te, about 2.5 Q/cm for an electrode area of 2.54 cm Y, is consistent with this conclusion. With 1 mA current through the cell, the voltage drop between working electrodes across the solid electrolyte is only 2.4 mV; this voltage drop is negligible compared to the measured interelectrode voltages of 0.59 V and 0.57 v.
-0.6
Fig. 2. Time dependence of the voltage between the anode and third electrode (---) and between the cathode and third electrode (-), following current inversion at t = 0, for flake paphite electrode.
0. Nakamwa et al / Three-electrode solid-statefuel cell
339
400 02
200
0
P
electrode
1
-o-0-0-0.0 ‘0
‘9
IO
E
3 i ImA
02 electrode
I
\
I
3
>
200 -
100 err-’
I
> E
i ImA
cm -2
10
>
H2 ------_~--_-
-6001
electrode 0-O
-400 I
I- 4 H2 electrode a_
Fig. 3. Current density (i) versus terminal voltage (V) at each electrode of a solid-state fuel cell with platinum-black electrodes.
Fig. 2 shows a typical result for the two-flakegraphite working electrodes. An equilibrium current of 1 mA was established between the electrodes and switched at r = 0. Fig. 2 displays the resulting voltagetime plots for each electrode separately as measured with the platinum-wire third electrode. Interpretation of the complex electrochemical changes taking place at the electrodes is not attempted; the point to notice is simply the measurement of a separable ohmic voltage drop, and hence electrode resistance, for each half-circuit. Fig. 3 shows typical polarization curves for a fuel cell with two platinum-black working electrodes. It is apparent that the cell is electrode-limited and that the cathode (oxygen electrode) performs more poorly than the anode (hydrogen electrode), which is why candidate oxygen electrodes are of greatest immediate interest. The composite electrode (2 : 1 PbzTa207, Ptsfc : carbon), if used as a test oxygen electrode, gives a maximum current density of only 3 mA/cm2 with-
a-a-~-
-600 ’
Fig. 4. Current density (r) versus terminal voltage (V) at each electrode of a solid-state fuel cell with a platinum-black hydrogen electrode and two test oxygen electrodes, graphite (0) and composite (0).
out voltage inversion, which is little better than a pure flake-graphite oxygen electrode, see fig. 4. The influence of either the dispersed platinum or the oxide substrate on the catalytic activity of the electrode is minimal. Nevertheless, this result demonstrates not only how candidate catalytic electrodes can be empirically tested, but also that the thirdelectrode technique is a powerful tool for estimating the catalytic activity of electrodes in contact with 12.MPA or 12-WPA solid protonic electrolytes.
References [l] 0. Nakamura, T. Kodama, I. O&no and Y. Miyake, Chem. Letters (1979) 17. [ 21 0. Nakamura, I. Ogino and T. Kodama, Rev. Sci. Instrum. 50 (1979) 1313.
THREE-ELECTRODE SOLIDSTATE FUEL CELL FOR EVALUATING WORKING ELECTRODES 0. NAKAMURA, I. OGINO Government Industrial Research Institute, O&Q, Ikeda, Osaka563, Japan
and R.N. CASTELLANO, J.B. GOODENOUGH Inorganic Chemistry Laboratory, South Parks Road, Oxford OXI 3QR, UK Received
Development of a commercti’solid-state fuel cell depends on identification of suitable catalytic electrodes to replace platinum. A three-electrode test cell for electrode evaluation is reported. The solid protonic electrolyte used was dodecamolybdophosphoric acid, H3MolzP04,-,*29HzO, and a thin platinum wire inserted into the electrolyte served as the third electrode. Reproducibility and insensitivity to third-electrode position were demonstrated. The third electrode measures separately the anode and cathode interfacial resistances, thus providing a direct measure of the relative catalytic ao tivity of a given test electrode. Application of the technique is illustrated.
1. Introduction
The discovery [I] that dodecamolybdophosphoric 29H.$ (12MPA), and dodecaacid, H,Mo,~PO~~ l tungstophosphoric acid, H3 W,,PO40 29H20 (12WPA), have a high proton conductivity (0.2 mho cm-l at 25°C) has led to an evaluation of these materials as solid electrolytes in an H2-0, fuel cell. Current densities of 100 mA/cm2 have been obtained with platinum-black catalytic electrodes. However, construction of a commercial cell depends on the development of alternative electrodes to replace platinum. Evaluation .of candidate electrode materials requires separation of three resistances connected in series in the test cell: the anode, electrolyte and cathode resistances. We report here a three-electrode system that permits this separation provided the electrolyte resistance is kept relatively small. Since the resistances of the two proton electrolytes 12-MPA and 12-WPAare sensitive to the degree of hydration, and therefore temperature and environmental humidity, we used a humidity-control device specifically developed [2] to maintain a constant resistance for l
0 167-2738/82/0000-0000/$02.75
0 1982 North-Holland
these electrolytes. The application of this cell is illustrated,with an evaluation of a composite electrode consisting of dispersed platinum supported on a nominal Pb2Ta207 substrate mixed with carbon. The Pb,TazO, crystallites having about 0.5 at.% platinum-on the surface exhibit high catalytic activity toward the,conversion of CO to CO,.
2. Experimental The experimental cell consisted of three components: a solid protonic electrolyte containing a thin platinum wire extending outside of it, a test electrode and a platinum black counter. electrode. The solid electrolyte was prepared from about 5 g of crystalline 12.MPA that had been obtained by lowering the temperature of a saturated solution of dodecamolybdophosphoric acid from 25°C to 22°C. The crystalline precipitate was crushed into a powder under an atmosphere of 85-95% relative humidity. A pellet 18 &in diameter was obtained under a pressure of 1000 kg/cm2 with a separable mold made of glass-fiber-reinforced’epoxy resin. A steel mold
0. Nakamura et al. / Three-electrode solid-statefuel cell
338
3. Results and discussion
PI Solid
electrolyte
Third
electrode
Mold
_ A--&&
/
Fig. 1. Assembly for fabrication of solid-electrolyte with third-electrode wire.
pellet
could not be used because it reacted with the Mo(Vl) ions. A thin metal wire could be inserted into the pellet before higher pressures were applied by means of the separable mold illustrated in fig. 1. Both platinum and gold wires gave good reproducible data. In the experiment reported here, a platinum wire 0.2 mm in diameter was used as platinum exhibits a larger exchange-current density. A platinum-black hydrogen electrode was pressed against the electrode on one side, the test oxygen electrode was pressed against the opposite side of the solid-electrolyte pellet. A platinum-black oxygen electrode was used as a standard for comparison. Since the composite test electrode reported here used graphite as the current collector, a flake-graphite electrode was also used as a calibrant oxygen electrode. The composite test electrode was formed from pyrochlore crystals of nominal PbzTa207 grown from a PbO flux in platinum crucibles. Excess flux was removed by leaching in hot aqueous acetic acid to isolate the cubic crystallites of approximately 200 pm dimensions. ESCA measurements indicate about 0.5 at.% platinum at the surfaces of the crystallites, and an excellent catalytic activity for CO oxidation can be attributed to the platinum on the surface. To investigate whether such a dispersed-platinum catalyst could be used as a catalytic electrode, 20 mg of this material was mixed with 10 mg of flake graphite to form an electronically conducting composite. This composite electrode was used as the test oxygen electrode. A constant current was maintained across the cell with a Takeda Riken TR6141 constant-current generator, and the voltage responses between the third electrode and each working electrode were recorded simultaneously with a two-pen recorder.
Since such a three-electrode test cell has not been reported previously for a solid protonic electrolyte, we first established the reproducibility of our data with both gold and platinum electrodes and then investigated the sensitivity of the data to the position within the electrolyte of the third electrode. For this latter purpose, we placed the third electrode at distances of 1.6, 3.2, 4.8, and 6.5 mm from a working electrode and measured the voltage differences between it and two flake-graphite working electrodes with a constant current of 1 mA through the solid electrolyte. For each third-electrode position, the voltage difference between anode and third electrode was 0.59 V between cathode and third electrode was 0.57 V, see fig. 2 and discussion below. Moreover, the sum of the voltage differences was equal to the voltage difference between the two working electrodes. Thus the measured voltages were shown to be insensitive to the third-electrode position, which implies that the voltage drop across the electrolyte is small relative to the observed voltages of 0.59 V and 0.57 V. Moreover, these measured voltages then reflect the rate constants of the electrochemical reactions at the electrode interfaces, and it is apparent that these reaction rates determine the inner resistance of the cell. The measured ohmic resistance of the solid electrol te, about 2.5 Q/cm for an electrode area of 2.54 cm Y, is consistent with this conclusion. With 1 mA current through the cell, the voltage drop between working electrodes across the solid electrolyte is only 2.4 mV; this voltage drop is negligible compared to the measured interelectrode voltages of 0.59 V and 0.57 v.
-0.6
Fig. 2. Time dependence of the voltage between the anode and third electrode (---) and between the cathode and third electrode (-), following current inversion at t = 0, for flake paphite electrode.
0. Nakamwa et al / Three-electrode solid-statefuel cell
339
400 02
200
0
P
electrode
1
-o-0-0-0.0 ‘0
‘9
IO
E
3 i ImA
02 electrode
I
\
I
3
>
200 -
100 err-’
I
> E
i ImA
cm -2
10
>
H2 ------_~--_-
-6001
electrode 0-O
-400 I
I- 4 H2 electrode a_
Fig. 3. Current density (i) versus terminal voltage (V) at each electrode of a solid-state fuel cell with platinum-black electrodes.
Fig. 2 shows a typical result for the two-flakegraphite working electrodes. An equilibrium current of 1 mA was established between the electrodes and switched at r = 0. Fig. 2 displays the resulting voltagetime plots for each electrode separately as measured with the platinum-wire third electrode. Interpretation of the complex electrochemical changes taking place at the electrodes is not attempted; the point to notice is simply the measurement of a separable ohmic voltage drop, and hence electrode resistance, for each half-circuit. Fig. 3 shows typical polarization curves for a fuel cell with two platinum-black working electrodes. It is apparent that the cell is electrode-limited and that the cathode (oxygen electrode) performs more poorly than the anode (hydrogen electrode), which is why candidate oxygen electrodes are of greatest immediate interest. The composite electrode (2 : 1 PbzTa207, Ptsfc : carbon), if used as a test oxygen electrode, gives a maximum current density of only 3 mA/cm2 with-
a-a-~-
-600 ’
Fig. 4. Current density (r) versus terminal voltage (V) at each electrode of a solid-state fuel cell with a platinum-black hydrogen electrode and two test oxygen electrodes, graphite (0) and composite (0).
out voltage inversion, which is little better than a pure flake-graphite oxygen electrode, see fig. 4. The influence of either the dispersed platinum or the oxide substrate on the catalytic activity of the electrode is minimal. Nevertheless, this result demonstrates not only how candidate catalytic electrodes can be empirically tested, but also that the thirdelectrode technique is a powerful tool for estimating the catalytic activity of electrodes in contact with 12.MPA or 12-WPA solid protonic electrolytes.
References [l] 0. Nakamura, T. Kodama, I. O&no and Y. Miyake, Chem. Letters (1979) 17. [ 21 0. Nakamura, I. Ogino and T. Kodama, Rev. Sci. Instrum. 50 (1979) 1313.