Half Cell Data

The development of the DMFC was pioneered in the 1960s and 1970s by Shell and Exxon-Alsthom using liquid sulfuric acid and alkaline electrolytes, respectively (see Chapter 2). These programs failed to produce stacks with sufficiently high power densities because of poor electrode kinetics and severe fuel crossover between the electrodes. In sulfuric acid electrolytes, methanol crossover was a particular problem since both the anode and cathode catalysts were based on platinum. This is demonstrated in Fig. 7.6, which shows the change in performance of an oxygen cathode as a result of methanol crossover. Clearly, the electrode efficiency is considerably reduced even when low methanol concentrations are used in the cell.

In recent years, however, significant progress has been made in the development of the DMFC operating with solid polymer electrolyte materials. These polymer materials have extended the operating temperature of the cell above the boiling point of sulfuric acid and have helped reduce fuel crossover. Electro-catalyst issues have centered on the need for stable materials with higher intrinsic activities for methanol electro-oxidation. A number of important half-cell studies have shown progressive improvement in the anode performance.

A major concern in the development of the current Pt/Ru-based catalyst materials is whether they can be improved to a level where the DMFC can become a viable alternative to the current H2- PEMFC/ reformer technology. Recent work at the Johnson Matthey Technology Centre has shown that it is possible to improve Pt/Ru-based catalytic systems further for them to suit DMFC applications. Fig. 7.7 compares the half-cell electrochemical activities of electrodes fabricated from 20 wt% Pt and 20 wt% Pt/10 wt% Ru catalysts supported on Vulcan XC-72R carbon black, at 80°C in 0.5 M H2SO4 and 2 M methanol. The electrodes consisted of a thin catalyst layer bonded to a Nafion® 117 membrane and a current-collecting substrate.

One important figure of merit in the determination of catalyst activity, which is rarely considered in the literature, is the measurement of activity in terms of real metal surface area (mAcm-2 Pt). This is determined by the electrosorption of a monolayer of carbon monoxide on the metal surface. This layer is then electro-chemically oxidized from the surface to produce a charge that can be equated to the total electrochemical metal area (ECA). This technique allows catalyst activity to be characterized independently of surface area and clearly distinguishes materials that possess higher intrinsic activity for methanol electro-oxidation. This is demonstrated in Fig. 7.8, which clearly shows that the Pt/Ru materials possess substantially higher activities than Pt, with the Type II Pt/Ru being significantly more active than the standard Type I Pt/Ru.

1000 900 w 800 DC 700

w 600

Effect of Methanol on the Oxygen Reduction Performance of 20% Pt / XC-72R

0.5M Sulphuric Acid at 80°C Ambient Oxygen, 1M methanol

Effect of Methanol on the Oxygen Reduction Performance of 20% Pt / XC-72R

0.5M Sulphuric Acid at 80°C Ambient Oxygen, 1M methanol

- No methanol 1M methanol added to electrolyte

0.5mg Pt/cm2

0 20 40 60 80 100 120 140 160 180 200 Current Density (mA/cm2)

FIGURE 7.6 Current/potential curves for an oxygen cathode in the presence of methanol, demonstrating the effects of methanol crossover.

Comparison of 20 wt.% Pt / XC-72R with 20 wt.% Pt 10 wt.% Ru / XC-72R

Comparison of 20 wt.% Pt / XC-72R with 20 wt.% Pt 10 wt.% Ru / XC-72R

Specific Activity (mA/cm2 Pt)

FIGURE 7.7 This graph compares the half-cell electrochemical activities of electrodes fabricated from 20 wt% Pt and 20 wt% Pt/10 wt% Ru catalysts supported on Vulcan XC-72R carbon black, at 80°C in 0.5 M H2SO4 and 2 M methanol. The electrodes consisted of a thin catalyst layer bonded to a Nafion 117 membrane and a current-collecting substrate.

Specific Activity (mA/cm2 Pt)

FIGURE 7.7 This graph compares the half-cell electrochemical activities of electrodes fabricated from 20 wt% Pt and 20 wt% Pt/10 wt% Ru catalysts supported on Vulcan XC-72R carbon black, at 80°C in 0.5 M H2SO4 and 2 M methanol. The electrodes consisted of a thin catalyst layer bonded to a Nafion 117 membrane and a current-collecting substrate.

Comparison of 20 wt.% Pt / XC-72R with Type I and Type II 20 wt.% Pt 10 wt.% Ru / XC-72R

Comparison of 20 wt.% Pt / XC-72R with Type I and Type II 20 wt.% Pt 10 wt.% Ru / XC-72R

Specific Activity (mA/cm2 Pt)

FIGURE 7.8 This figure clearly shows that the Pt/Ru materials possess substantially higher activities than Pt, with the Type II Pt/Ru significantly more active than the standard Type I Pt/Ru.

Specific Activity (mA/cm2 Pt)

FIGURE 7.8 This figure clearly shows that the Pt/Ru materials possess substantially higher activities than Pt, with the Type II Pt/Ru significantly more active than the standard Type I Pt/Ru.

In terms of current densities, the half-cell performance has improved considerably since the 1980s, from current densities in the range of 20-25 mAcm-2 at 0.4 V (Cameron et al., 1987) to the present day state-of-the-art electrodes capable of more than 200 mAcm-2 at 0.3 V (Hogarth et al., 1995).

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