**3.2 Methanol electrooxidation**

Cyclic voltammograms for a high-surface-area Pt-coated Au disk electrode in 0.5 mol dm-3 KOH solution containing 0.5 mol dm-3 methanol as a function of temperature (Fig. 5) clearly show that the onset potentials of substantial methanol electrooxidation are negatively shifted by increasing temperature. Because this prominent negative potential shift with increasing temperature is not caused by the potential shift of the reference electrode, it is reasonably believed that the electrooxidation of methanol is substantially accelerated. To assess the kinetics of methanol electrooxidation, we have compared the onset potentials for methanol oxidation and H2 oxidation (Fig. 3) under similar conditions. The value of their onset potential difference is significantly decreased with increasing temperature. At 150 °C, a typical value is approximately 60 mV. It is well accepted that the hydrogen oxidation is highly facile on Pt, this value indicates that highly facile electrooxidation of methanol can be achieved on single-element Pt electrocatalyst in aqueous alkaline solution in the intermediate-temperature range.

Investigations of Intermediate-Temperature Alkaline Methanol

0.01

electrooxidation could be written as follows (13):

0.1

**Current density / A cm-2**

**150 o C 130 o C 105 o C**

1

other metal elements.

Fuel Cell Electrocatalysis Using a Pressurized Electrochemical Cell 169

0.20 V when the temperature is increased from 20 °C to 150 °C. Although these values are higher compared to those measured at Pt electrode under the similar conditions, the value of 0.20 V would suggest that Pd is highly active toward the methanol oxidation. It is therefore prospective to replace Pt with Pd in alkaline methanol fuel cells for decreased cost since Pd is normally three times cheaper than Pt. Moreover, the activity of Pd-based catalysts could be further improved by introducing metal oxides to Pd or alloying Pd with

133

mV dec



120

mV dec


125

mV dec


**20 o C** 123

mV dec


131 mV dec

0.1 0.2 0.3 0.4 0.5 0.6 0.7

**Potential / V vs RHE**

**60 o C**

*Tafel* plots for the methanol electroxidation at the high-surface-area Pt electrode in a solution of 0.5 mol dm-3 KOH and 0.5 mol dm-3 CH3OH as a function of temperature are shown in Fig. 7. The values of measured *Tafel* slopes range from 120 mV dec-1 to 133 mV dec-1 in the intermediate-temperature range over 20 to approximately 150 °C. These values are in agreement with literature results obtained under similar conditions at a platinized Pt electrode and single crystal Pt(110) and Pt(111) electrodes (Tripković et al., 1998 & 2002). It has been proposed that the chemical reaction between the surface intermediate HCO*ad* and OH*ad* is the rate-determining step and the overall rate equation for the methanol

Fig. 7. Tafel plots for methanol electrooxidation at a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol dm-3 CH3OH as a function of

3

°C could be obtained, which fits with the experimental data shown in Fig. 7.

where A is a constant and other terms have their normal meanings. From this expression, a *Tafel* slope ranging from 120 to 164 mV dec-1 as the temperature is increased from 20 to 150

0.5 0.5 exp( ) *CH OH OH <sup>F</sup> j Ac c RT*

(5)

reaction temperature with data extracted from the positive-scans in Fig. 5.

Fig. 5. Temperature dependence of cyclic voltammograms for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol dm-3 CH3OH at a scan rate of 10 mV s-1.

Fig. 6. Temperature dependence of cyclic voltammograms for a high-surface Pd-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol dm-3 CH3OH at a scan rate of 10 mV s-1.

Cyclic voltammograms for a high-surface-area Pd-coated Au disk electrode in 0.5 mol dm-3 KOH solution containing 0.5 mol dm-3 methanol as a function of temperature are shown in Fig. 6. Similar temperature dependence is observed. Increasing temperature significantly shifts the onset overpotential for methanol electrooxidation to more negative potentials. The value of the onset overpotential is decreased from 0.52 V to approximately

0.2 0.4 0.6 0.8

0.2 0.4 0.6 0.8

**Potential / V vs RHE**

Fig. 6. Temperature dependence of cyclic voltammograms for a high-surface Pd-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol dm-3 CH3OH at a scan rate

Cyclic voltammograms for a high-surface-area Pd-coated Au disk electrode in 0.5 mol dm-3 KOH solution containing 0.5 mol dm-3 methanol as a function of temperature are shown in Fig. 6. Similar temperature dependence is observed. Increasing temperature significantly shifts the onset overpotential for methanol electrooxidation to more negative potentials. The value of the onset overpotential is decreased from 0.52 V to approximately

 20 <sup>o</sup> C 60 <sup>o</sup> C 105 <sup>o</sup> C 130 <sup>o</sup> C 150 <sup>o</sup> C

**Potential / V vs RHE**

Fig. 5. Temperature dependence of cyclic voltammograms for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol dm-3 CH3OH at a scan rate

> 20 <sup>o</sup> C 60 <sup>o</sup> C 105 <sup>o</sup> C 130 <sup>o</sup> C 150 <sup>o</sup> C

0.0

0.00

0.05

0.10

**Current density / A cm-2**

0.15

0.20

0.25

0.2

0.4

**Current density / A cm-2**

of 10 mV s-1.

of 10 mV s-1.

0.6

0.8

0.20 V when the temperature is increased from 20 °C to 150 °C. Although these values are higher compared to those measured at Pt electrode under the similar conditions, the value of 0.20 V would suggest that Pd is highly active toward the methanol oxidation. It is therefore prospective to replace Pt with Pd in alkaline methanol fuel cells for decreased cost since Pd is normally three times cheaper than Pt. Moreover, the activity of Pd-based catalysts could be further improved by introducing metal oxides to Pd or alloying Pd with other metal elements.

Fig. 7. Tafel plots for methanol electrooxidation at a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol dm-3 CH3OH as a function of reaction temperature with data extracted from the positive-scans in Fig. 5.

*Tafel* plots for the methanol electroxidation at the high-surface-area Pt electrode in a solution of 0.5 mol dm-3 KOH and 0.5 mol dm-3 CH3OH as a function of temperature are shown in Fig. 7. The values of measured *Tafel* slopes range from 120 mV dec-1 to 133 mV dec-1 in the intermediate-temperature range over 20 to approximately 150 °C. These values are in agreement with literature results obtained under similar conditions at a platinized Pt electrode and single crystal Pt(110) and Pt(111) electrodes (Tripković et al., 1998 & 2002). It has been proposed that the chemical reaction between the surface intermediate HCO*ad* and OH*ad* is the rate-determining step and the overall rate equation for the methanol electrooxidation could be written as follows (13):

$$j = A c\_{CH\_3OH}^{0.5} c\_{OH}^{0.5} \exp(\frac{aF}{RT}\eta) \tag{5}$$

where A is a constant and other terms have their normal meanings. From this expression, a *Tafel* slope ranging from 120 to 164 mV dec-1 as the temperature is increased from 20 to 150 °C could be obtained, which fits with the experimental data shown in Fig. 7.

Investigations of Intermediate-Temperature Alkaline Methanol

normally triggered by the formation of surface oxide.

0.0

0.2

0.4

**Current density / A cm-2**

0.6

0.8

poison.

temperature.

Fuel Cell Electrocatalysis Using a Pressurized Electrochemical Cell 171

temperature range. Fig. 9 shows the temperature dependence of cyclic voltammograms for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH equilibrated with 300 psi CO. The voltammetric characters for the oxidation of dissolved CO in the alkaline media are significantly different from those reported for adsorbed CO. The CO oxidation commences at around 0.23 V vs RHE at 60 °C. The oxidation current initially increases with increasing potential until a maximum current is reach at approximately 0.65 V. Further increase in the potential results in sudden oxidation current decrease. These facts interestingly indicate that the oxidation of CO proceeds on oxide-free Pt surface and the formation of the surface oxide inhibits the CO oxidation since the formation of the surface oxide on Pt commences at around 0.65 V (Fig. 2). These are quite different from literature results for the oxidation of adsorbed CO on Pt in acidic and alkaline media since the latter is

Increasing temperature substantially increases the CO oxidation at the oxide-free Pt surface and diminishes the onset overpotential. At temperature higher than 130 °C, the onset overpotential is approximately 0.15 V. This value is lower than those potentials used for the chronoamperometric measurements for the methanol oxidation on Pt (Fig. 8). Therefore, the current decays are less likely to be caused by the formation of surface CO as a catalyst

> 60 <sup>o</sup> C 80 <sup>o</sup> C 105 <sup>o</sup> C 130 <sup>o</sup> C 150 <sup>o</sup> C

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Fig. 9. Variation of cyclic voltammograms for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH equilibrated with 300 psi CO as a function of reaction

We have also investigated the oxidation of dissolved CO at the Pd electrode. Fig. 10 shows the temperature dependence of cyclic voltammograms for a high-surface Pd-coated Au disk electrode of 0.5 mm diameter in CO-saturated 0.5 mol dm-3 KOH. The temperature dependence of the CO oxidation is more complicated at the Pd electrode than at the Pt electrode. At temperature lower than 105 °C, the oxidation of dissolved CO is very slow. A broad oxidation wave can be seen in the double layer region. However, the CO oxidation is

**Potential / V vs RHE**

Fig. 8. Dependence of chronoamperometric curves at a controlled potential at 150 °C for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol dm-3 CH3OH upon polarization time.

The activity of high-surface-area Pt toward the methanol electrooxidation in aqueous KOH solution at 150 °C is evaluated using chronoamperometry. The current-time transients at controlled potentials are shown in Fig. 8. The mass activity was estimated from measured pseudo steady-state chronoamperometric current upon a polarization of 300 s. This figure shows that pseudo current densities can be attained even at 0.18 V vs RHE. This indicates that methanol can be dominantly oxidized to non-poisoning products in alkaline media even at low overpotentials. The activity of Pt is rather high, characterized by a mass activity of 14.9 A g-1 and a specific area activity of 0.05 mA cm-2 at an overpotential of 0.18 V. This fact is important for the improvement of the anodic reaction kinetics. In this figure, the activity decays are slow at low overpotentials and they become fast at high overpotentials. The slow decays may be caused by the surface blocking of Pt electrode owing to the adsorption of surface poisons (Matsuoka et al., 2005). Analogous to methanol oxidation in acidic media, CO has been also proposed as the predominant surface poison formed in the methanol oxidation in alkaline media. Additionally, progressive carbonation of the solution caused by CO2 produced by oxidation reaction may decrease the pH value of the solution close to the electrode surface, leading to a decrease in reactivity. It is reasonable to attribute the progressive carbonation for the fast activity decays.

#### **3.3 CO electrooxidation**

Adsorption and oxidation of CO on Pt surfaces in aqueous electrolytes has been extensively studied, primarily because of the importance of CO as a catalyst poison and also as a reaction intermediate (Garcia & Koper, 2011; Spendelow et al., 2006). The onset of CO oxidation on Pt occurs at lower overpotentials alkaline media than in acidic media. However, all these studies have been performed at low temperature. Here we have investigated the oxidation of dissolved CO in alkaline media in the intermediate

0 50 100 150 200 250 300 350

Fig. 8. Dependence of chronoamperometric curves at a controlled potential at 150 °C for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH + 0.5 mol

The activity of high-surface-area Pt toward the methanol electrooxidation in aqueous KOH solution at 150 °C is evaluated using chronoamperometry. The current-time transients at controlled potentials are shown in Fig. 8. The mass activity was estimated from measured pseudo steady-state chronoamperometric current upon a polarization of 300 s. This figure shows that pseudo current densities can be attained even at 0.18 V vs RHE. This indicates that methanol can be dominantly oxidized to non-poisoning products in alkaline media even at low overpotentials. The activity of Pt is rather high, characterized by a mass activity of 14.9 A g-1 and a specific area activity of 0.05 mA cm-2 at an overpotential of 0.18 V. This fact is important for the improvement of the anodic reaction kinetics. In this figure, the activity decays are slow at low overpotentials and they become fast at high overpotentials. The slow decays may be caused by the surface blocking of Pt electrode owing to the adsorption of surface poisons (Matsuoka et al., 2005). Analogous to methanol oxidation in acidic media, CO has been also proposed as the predominant surface poison formed in the methanol oxidation in alkaline media. Additionally, progressive carbonation of the solution caused by CO2 produced by oxidation reaction may decrease the pH value of the solution close to the electrode surface, leading to a decrease in reactivity. It is reasonable to attribute

Adsorption and oxidation of CO on Pt surfaces in aqueous electrolytes has been extensively studied, primarily because of the importance of CO as a catalyst poison and also as a reaction intermediate (Garcia & Koper, 2011; Spendelow et al., 2006). The onset of CO oxidation on Pt occurs at lower overpotentials alkaline media than in acidic media. However, all these studies have been performed at low temperature. Here we have investigated the oxidation of dissolved CO in alkaline media in the intermediate

**Time / s**

0

20

40

**Mass activity / A g-1**

0.38 V 0.33 V 0.28 V 0.23 V 0.18 V 60

80

0.0

the progressive carbonation for the fast activity decays.

**3.3 CO electrooxidation** 

0.1

0.2

**Specific activity / mA cm-2**

dm-3 CH3OH upon polarization time.

0.3

temperature range. Fig. 9 shows the temperature dependence of cyclic voltammograms for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH equilibrated with 300 psi CO. The voltammetric characters for the oxidation of dissolved CO in the alkaline media are significantly different from those reported for adsorbed CO. The CO oxidation commences at around 0.23 V vs RHE at 60 °C. The oxidation current initially increases with increasing potential until a maximum current is reach at approximately 0.65 V. Further increase in the potential results in sudden oxidation current decrease. These facts interestingly indicate that the oxidation of CO proceeds on oxide-free Pt surface and the formation of the surface oxide inhibits the CO oxidation since the formation of the surface oxide on Pt commences at around 0.65 V (Fig. 2). These are quite different from literature results for the oxidation of adsorbed CO on Pt in acidic and alkaline media since the latter is normally triggered by the formation of surface oxide.

Increasing temperature substantially increases the CO oxidation at the oxide-free Pt surface and diminishes the onset overpotential. At temperature higher than 130 °C, the onset overpotential is approximately 0.15 V. This value is lower than those potentials used for the chronoamperometric measurements for the methanol oxidation on Pt (Fig. 8). Therefore, the current decays are less likely to be caused by the formation of surface CO as a catalyst poison.

Fig. 9. Variation of cyclic voltammograms for a high-surface Pt-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH equilibrated with 300 psi CO as a function of reaction temperature.

We have also investigated the oxidation of dissolved CO at the Pd electrode. Fig. 10 shows the temperature dependence of cyclic voltammograms for a high-surface Pd-coated Au disk electrode of 0.5 mm diameter in CO-saturated 0.5 mol dm-3 KOH. The temperature dependence of the CO oxidation is more complicated at the Pd electrode than at the Pt electrode. At temperature lower than 105 °C, the oxidation of dissolved CO is very slow. A broad oxidation wave can be seen in the double layer region. However, the CO oxidation is

Investigations of Intermediate-Temperature Alkaline Methanol

shown in Fig. 9 at the same temperature.

processes (Demarconnay et al., 2004).


0.5 mol dm-3 KOH equilibrated with 300 psi O2.



**Current density / A cm-2**


0.00

**3.4 Oxygen electroreduction** 

Fuel Cell Electrocatalysis Using a Pressurized Electrochemical Cell 173

*COOH OH e CO H O ad* 2 2

A theoretical *Tafel* slope of around 167 mV dec-1 will be expected at 150 °C if the electrochemical formation of COOHad is the rate-determining step. This value is close to our experimental value of 138 mV dec-1 measured from the polarization region over 0.2 to 0.35 V

Pt is the most used and active catalyst for the oxygen reduction reaction (*orr*) and all of the Pt-group metals reduce oxygen in alkaline media according to the 4-electrode process (Lima & Ticianelli, 2004). Silver has been studied as a potential replacement of Pt due to its high activity for the *orr* and its low cost. The *orr* occurs with the participation of 2 and 4-electron processes, depending on its oxidation state and electrode potential (Kotz & Yeager, 1980) Moreover, the size of the Ag particles affects the different catalytic activity for these two

0.6 0.8 1.0 1.2

Fig. 11. Temperature dependence of polarization curves at a scan rate of 10 mV s-1 for a high-surface Pt (in gray) and Ag (in black) coated Au disk electrode of 0.5 mm diameter in

efficient and concentration in the aqueous solution at higher temperature.

We have compared the activities of Pt and Ag for the *orr* in alkaline media in the intermediate temperature range. Fig. 11 shows the temperature dependence of voltammograms for high surface area Ag and Pt coated gold disk electrode of 0.5 mm in diameter in 0.5 M KOH solution equilibrated by 300 psi O2. At both Pt and Ag electrodes, the values of the current density at lower overpotentials are substantially increased with increasing temperature with obvious positive shift of the onset potential. At higher overpotentials, a limiting current plateau is observed and it is increased with increasing temperature. The increase of the limiting current is probably caused by higher O2 diffusion

**Potential / V vs RHE**

(9)

 20 <sup>o</sup> C, Pt 60 <sup>o</sup> C, Pt 105 <sup>o</sup> C, Pt 130 <sup>o</sup> C, Pt

 20 <sup>o</sup> C, Ag 60 <sup>o</sup> C, Ag 105 <sup>o</sup> C, Ag

 130 <sup>o</sup> C, Ag

inhibited at more positive potentials. This behaviour becomes more pronounced at 130 °C. The onset of the oxidation occurs at 0.20 V. Further increasing temperature causes substantial changes in the voltammograms. At 150 °C, the onset potential for the CO oxidation is shifted negative to approximately 0 V vs RHE. This value is much lower than the onset potential (0.2 V) for the methanol oxidation under similar conditions (Fig. 6). Moreover, the oxidation current is increased with increasing potentials even in the oxide formation region. These strongly suggest that the CO oxidation is more facile than the methanol oxidation at the Pd electrode at 150 °C. Therefore, it is highly likely that methanol can be oxidized to non-poisoning products rather than surface poison CO under our conditions.

Fig. 10. Variation of cyclic voltammograms for a high-surface Pd-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH equilibrated with 300 psi CO as a function of reaction temperature.

It is well accepted that the oxidation of the adsorbed CO at Pt and Pd follows a Langmuir-Hinshelwood mechanism with the reaction between adsorbed CO and surface OH as the rate-determining step as follows (Spendelow et al., 2004):

$$
\text{OH}^-\text{-}e^- \to \text{OH}\_{\text{ad}}\tag{6}
$$

$$\text{CO}\_{ad} + \text{OH}\_{ad} \to \text{COOH}\_{ad} \tag{7}$$

This mechanism suggests that the substantial oxidation of adsorbed CO occurs in the oxideformation potential region. However, cyclic voltammograms in Figs. 9 and 10 show that the CO oxidation occurs at the oxide-free electrodes and is inhibited by the oxide formation. The CO oxidation is also likely to proceed via the following mechanism:

$$\text{C}\_{\text{ad}} + \text{OH}^- - e^- \rightarrow \text{COOH}\_{\text{ad}} \tag{8}$$

$$\text{COOH}\_{\text{ad}} + \text{OH}^- - \text{e}^- \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{9}$$

A theoretical *Tafel* slope of around 167 mV dec-1 will be expected at 150 °C if the electrochemical formation of COOHad is the rate-determining step. This value is close to our experimental value of 138 mV dec-1 measured from the polarization region over 0.2 to 0.35 V shown in Fig. 9 at the same temperature.

#### **3.4 Oxygen electroreduction**

172 Electrochemical Cells – New Advances in Fundamental Researches and Applications

inhibited at more positive potentials. This behaviour becomes more pronounced at 130 °C. The onset of the oxidation occurs at 0.20 V. Further increasing temperature causes substantial changes in the voltammograms. At 150 °C, the onset potential for the CO oxidation is shifted negative to approximately 0 V vs RHE. This value is much lower than the onset potential (0.2 V) for the methanol oxidation under similar conditions (Fig. 6). Moreover, the oxidation current is increased with increasing potentials even in the oxide formation region. These strongly suggest that the CO oxidation is more facile than the methanol oxidation at the Pd electrode at 150 °C. Therefore, it is highly likely that methanol can be oxidized to non-poisoning products rather than surface poison CO under our

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Fig. 10. Variation of cyclic voltammograms for a high-surface Pd-coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH equilibrated with 300 psi CO as a function of reaction

It is well accepted that the oxidation of the adsorbed CO at Pt and Pd follows a Langmuir-Hinshelwood mechanism with the reaction between adsorbed CO and surface OH as the

*OH e OHad*

This mechanism suggests that the substantial oxidation of adsorbed CO occurs in the oxideformation potential region. However, cyclic voltammograms in Figs. 9 and 10 show that the CO oxidation occurs at the oxide-free electrodes and is inhibited by the oxide formation.

*CO OH e COOH ad ad*

The CO oxidation is also likely to proceed via the following mechanism:

(6)

*CO OH COOH ad ad ad* (7)

(8)

**Potential / V vs RHE**

0.0

rate-determining step as follows (Spendelow et al., 2004):

0.2

0.4

**Current density / A cm-2**

0.6

 105 <sup>o</sup> C 130 <sup>o</sup> C 150 <sup>o</sup> C

0.8

conditions.

temperature.

Pt is the most used and active catalyst for the oxygen reduction reaction (*orr*) and all of the Pt-group metals reduce oxygen in alkaline media according to the 4-electrode process (Lima & Ticianelli, 2004). Silver has been studied as a potential replacement of Pt due to its high activity for the *orr* and its low cost. The *orr* occurs with the participation of 2 and 4-electron processes, depending on its oxidation state and electrode potential (Kotz & Yeager, 1980) Moreover, the size of the Ag particles affects the different catalytic activity for these two processes (Demarconnay et al., 2004).

Fig. 11. Temperature dependence of polarization curves at a scan rate of 10 mV s-1 for a high-surface Pt (in gray) and Ag (in black) coated Au disk electrode of 0.5 mm diameter in 0.5 mol dm-3 KOH equilibrated with 300 psi O2.

We have compared the activities of Pt and Ag for the *orr* in alkaline media in the intermediate temperature range. Fig. 11 shows the temperature dependence of voltammograms for high surface area Ag and Pt coated gold disk electrode of 0.5 mm in diameter in 0.5 M KOH solution equilibrated by 300 psi O2. At both Pt and Ag electrodes, the values of the current density at lower overpotentials are substantially increased with increasing temperature with obvious positive shift of the onset potential. At higher overpotentials, a limiting current plateau is observed and it is increased with increasing temperature. The increase of the limiting current is probably caused by higher O2 diffusion efficient and concentration in the aqueous solution at higher temperature.

Investigations of Intermediate-Temperature Alkaline Methanol

0.0

0.0

0.2

0.4

**Cell voltage / V**

0.6

0.8

0.2

0.4

**Cell volatge / V**

120 SCCM O2.

feed: 120 SCCM O2.

0.6

0.8

Fuel Cell Electrocatalysis Using a Pressurized Electrochemical Cell 175

0 100 200 300 400

intermediate-temperature alkaline methanol fuel using Pt/C for both anode and cathode and operated at 120 °C. Anode feed: 2 mol dm-3 CH3OH + 2 mol dm-3 KOH; cathode feed:

Fig. 13. Dependence of cell voltage and power density on current density for an

**Current density / mA cm-2**

0 100 200 300 400

To evaluate the potential of using non-Pt catalysts in the novel fuel cell, the performance of the fuel cell utilizing Pd/C anode and Ag/C cathode has been measured and is shown in Fig. 14. It is characterized by a peak power density of around 75 mW cm-2 seen at around 260 mA cm-2. This performance is comparable to that of the fuel cell utilizing Pt catalysts. Therefore, significant cost reduction would be expected for the novel fuel cell without the

Fig. 14. Dependence of cell voltage and power density on current density for an intermediate-temperature alkaline methanol fuel using Pd/C for anode and Ag/C for cathode operated at 140 °C. Anode feed: 2 mol dm-3 CH3OH + 2 mol dm-3 KOH; Cathode

**Current density / mA cm-2**

0

0

20

40

**Power density / mW cm-2**

60

80

20

40

**Power density / mW cm-2**

60

80

100

Fig. 12. Temperature dependence of onset polarization potential difference for the *orr* on Pt and Ag with data taken from Fig. 11.

The onset potential difference for the *orr* at Ag and Pt as a function of reaction temperature is shown in Fig. 12. Their potential difference is obviously decreased with increasing temperature. At room temperature, the difference is approximately 0.10 V. It falls in a range of 0.02 to 0.03 V when the temperature is higher than 130 °C. This indicates that Ag is a very promising electrocatalyst for the *orr* in the intermediate temperature range. Although the stability of Ag in alkaline media is questioned, a few strategies have been proven efficient to this issue.

#### **3.5 Single cell performance**

Based on the above fundamental studies, an intermediate-temperature alkaline methanol fuel has been developed and its performance is measured using a single-cell system with temperature control, gas flow rate and pressure control, and liquid flow rate and pressure control.

The performance of the novel fuel cell utilizing commercial Pt/C as both anode and cathode catalysts under optimized operating conditions is demonstrated in Fig. 13. The peak power density seen at around 280 mA cm-2 reaches 90 mW cm-2. This value is much higher than a typical value of around 50 mW cm-2 of the state-of-the-art DMFCs using Nafion-based proton-conducting membranes and PtRu anode catalysts (Dillon et al, 2004).

0 20 40 60 80 100 120 140 160

**Temperature / o**

Fig. 12. Temperature dependence of onset polarization potential difference for the *orr* on Pt

The onset potential difference for the *orr* at Ag and Pt as a function of reaction temperature is shown in Fig. 12. Their potential difference is obviously decreased with increasing temperature. At room temperature, the difference is approximately 0.10 V. It falls in a range of 0.02 to 0.03 V when the temperature is higher than 130 °C. This indicates that Ag is a very promising electrocatalyst for the *orr* in the intermediate temperature range. Although the stability of Ag in alkaline media is questioned, a few strategies have been proven efficient to

Based on the above fundamental studies, an intermediate-temperature alkaline methanol fuel has been developed and its performance is measured using a single-cell system with temperature control, gas flow rate and pressure control, and liquid flow rate and pressure

The performance of the novel fuel cell utilizing commercial Pt/C as both anode and cathode catalysts under optimized operating conditions is demonstrated in Fig. 13. The peak power density seen at around 280 mA cm-2 reaches 90 mW cm-2. This value is much higher than a typical value of around 50 mW cm-2 of the state-of-the-art DMFCs using Nafion-based

proton-conducting membranes and PtRu anode catalysts (Dillon et al, 2004).

**C**

0.02

0.04

0.06

**Onset potential difference / V**

and Ag with data taken from Fig. 11.

**3.5 Single cell performance** 

this issue.

control.

0.08

0.10

0.12

Fig. 13. Dependence of cell voltage and power density on current density for an intermediate-temperature alkaline methanol fuel using Pt/C for both anode and cathode and operated at 120 °C. Anode feed: 2 mol dm-3 CH3OH + 2 mol dm-3 KOH; cathode feed: 120 SCCM O2.

Fig. 14. Dependence of cell voltage and power density on current density for an intermediate-temperature alkaline methanol fuel using Pd/C for anode and Ag/C for cathode operated at 140 °C. Anode feed: 2 mol dm-3 CH3OH + 2 mol dm-3 KOH; Cathode feed: 120 SCCM O2.

To evaluate the potential of using non-Pt catalysts in the novel fuel cell, the performance of the fuel cell utilizing Pd/C anode and Ag/C cathode has been measured and is shown in Fig. 14. It is characterized by a peak power density of around 75 mW cm-2 seen at around 260 mA cm-2. This performance is comparable to that of the fuel cell utilizing Pt catalysts. Therefore, significant cost reduction would be expected for the novel fuel cell without the

Investigations of Intermediate-Temperature Alkaline Methanol

2007), pp. 49-77, ISSN: 1463-9076.

2002), pp. 964-967, ISSN: 00280836

2075-2077, ISSN: 00368075

186, ISSN: 09263373

657, ISSN: 00223654

1090, ISSN: 00134686

2007), pp. 205-216, ISSN: 00220728

Fuel Cell Electrocatalysis Using a Pressurized Electrochemical Cell 177

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