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Platinum Metals Rev., 2002, 46, (3), 117

Catalysis for Low Temperature Fuel Cells

PART II: The Anode Challenges

  • By T. R. Ralph
  • M. P. Hogarth
  • Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K.
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Article Synopsis

In the first part of this three part paper, published in January 2002, we dealt with enhancing the performance of low temperature proton exchange membrane fuel Cells by improvements to the platinum-based cathode materials and to the cathode design. In this second part of the paper we shall discuss the improvements in the reformate (CO and CO2) tolerance at the anode and in extending the MEA durability. Improvements have been achieved by advances in platinum/ruthenium electrode design, in the application of bilayer anodes for durable air bleed operation, and in the addition of a water electrolysis electrocatalyst to the anode for Cell reversal tolerance. In the third part of the paper the particular challenges presented by the direct methanol fuel Cell will be discussed.

The proton exchange membrane fuel Cell (PEMFC) operating on air (or oxygen for space applications) and on selected fuels, is being developed as an electrochemical power source for a variety of applications (1). While at the cathode, the choice of oxidant is either air or oxygen, the selection of the fuel is not so straightforward and a number of options are being pursued.

Fuel Selection


The simplest and highest performing PEMFC systems employ pure hydrogen as the fuel. For example, DaimlerChrysler and Ballard Power Systems are manufacturing transit buses with a 250 kW PEMFC engine that can provide a range of 250 miles using compressed hydrogen stored in the roof space, with overnight refuelling at the depot. This hydrogen system is envisaged as an early market entry for fuel Cells. For the automotive market, using the currently available storage technology, there is not sufficient space in a car to store hydrogen as a gas, While for stationary power generation the hydrogen supply infrastructure does not exist. In an automobile requiring 75 kW of fuel Cell power, a liquid methanol feed is normally employed, although gasoline is now being examined. For stationary power generation in the home 3 to 5 kW fuel Cell systems are required, While 250 kW power plants are being bulk for larger industrial or residential sites. In this case natural gas is used as the fuel since it is readily available and there are significant worldwide reserves.

The methanol, gasoline or natural gas is reformed using steam, partial oxidation or autothermal reforming to produce the reformate — a hydrogen-rich gas stream (H2, 40-70%) containing carbon dioxide (CO2, 15—25%), carbon monoxide (CO, 1-2%) and small quantities of inert gases, such as nitrogen and water vapour. However, at current PEMFC stack operating temperatures of around 800C the membrane electrode assemblies (MEAs) within the stack cannot tolerate such high CO levels. Therefore, the reformate must be passed from the reformer to a shift reactor and then to a catalytic preferential oxidation (CPOX) reactor to reduce the CO content to less than 100 ppm, and in some cases down to a few ppm. The reformate can then enter the stack and react in the anode electrocatalyst layer of the MEA. The additional reformer, shift and CPOX reactors significantly complicate and add extra cost to the PEMFC system (see (1) and (19)).

Hydrogen Electrooxidation


The electrochemical oxidation of hydrogen is extremely facile on a wide range of metals (20). The Platinum group metals (pgms) such as Platinum (Pt) and Palladium (Pd) show a key advantage: they have the required stability to operate in the acidic environment inside the MEA. The high activity of Pt for hydrogen electrooxidation is shown by an exchange current density of 10-3 A cm-2 Pt at ambient temperature. This is some 107 to 109 times more facile than oxygen reduction at the cathode. As a consequence, when operating with pure hydrogen, at practical current densities, the anode potential is typically less than + 0.1 V (vs. RHE (reversible hydrogen electrode)), which is only some 0.1 V from the standard reversible potential for hydrogen electrooxidation (Reaction (i)):


(i)

where NHE is the normal hydrogen electrode. Under such operating conditions the Cell potential is only slightly lower than the cathode potential and the MEA performance essentially reflects the cathode operation.

CO Poisoning on Pt


The mechanism of hydrogen electrooxidation on Pt in acid electrolytes is thought to proceed by rate-determining dissociative electrosorption of molecular hydrogen (Reaction (ii)) followed by facile electron transfer (Reaction (iii)) (21):


(ii)

(iii)

At the typical MEA operating temperature of 800C, the CO in the reformate stream binds very strongly to the Pt electrocatalyst sites in the anode layer. Even at ppm levels of CO, the CO coverage is above 0.98 (22). The adsorbed CO prevents the dissociative electrosorption of hydrogen (Reaction (ii)) and dramatically lowers the Cell potential produced by the MEA since a much higher anode potential is required to sustain the rate of hydrogen electrooxidation (23). This is demonstrated in Figure 9 by the effect of ppm levels of CO on an MEA with a Pt anode. The anode contains 0.25 mg Pt cm-2, and is based on a 20 wt.% Pt/Vulcan XC72R carbon-supported electrocatalyst.

Fig 9.

Progressive poisoning from 10, 40 and 100 ppm CO on pure Pt and Pt0.5Ru0.5 alloy anodes. Increased CO tolerance is shown by the Pt0.5Ru0.5 alloy anodes. The anodes are prepared from 20 wt.% Pt/Vulcan XC72R or 20 wt.%Pt, 10 wt.% Ru/Vulcan XC72R at a loading of 0.25 mg Pt cm-2.

The cathode uses 40 wt. % Pt/Vulcan XC72R at a loading of 0.6 mg Pt cm-2. The MEAs are based on catalysed substrates bonded to Nafion NE-115 membrane. The Ballard Mark 5E single Cell is operated at 800C, 308/308 kPa, 1.3/2 stoichiometry with full internal membrane humidification

Progressive poisoning from 10, 40 and 100 ppm CO on pure Pt and Pt0.5Ru0.5 alloy anodes. Increased CO tolerance is shown by the Pt0.5Ru0.5 alloy anodes. The anodes are prepared from 20 wt.% Pt/Vulcan XC72R or 20 wt.%Pt, 10 wt.% Ru/Vulcan XC72R at a loading of 0.25 mg Pt cm-2.  The cathode uses 40 wt. % Pt/Vulcan XC72R at a loading of 0.6 mg Pt cm-2. The MEAs are based on catalysed substrates bonded to Nafion NE-115 membrane. The Ballard Mark 5E single Cell is operated at 800C, 308/308 kPa, 1.3/2 stoichiometry with full internal membrane humidification

As the CO content is increased from 10 to 100 ppm there is a progressive lowering in the Cell potential from that achieved with pure hydrogen. At such low Pt loadings on the anode, the Cell potential losses are significant even at low current densities below 200 mA cm-2. But at higher current densities the Cell potential losses are more dramatic. Eventually the Cell potential is reduced to ∼ 0.3 V, at which the electrooxidation of CO to CO2 can occur (Reaction (iv)).


(iv)

Under these circumstances the anode potential reaches ∼ +0.45 V (vs. RHE), some 0.55 V anodic of the standard reversible potential for Reaction (iv). This is sufficient to drive CO electrooxidation and some of the poisoned Pt sites are now free to take part in hydrogen electrosorption and electrooxidation (Reactions (ii) and (iii)). The Cell potential then levels out.

PtRu alloy Electrocatalysts


It is well documented (2427) that in the PEMFC at 800C platinum-ruthenium (PtRu) alloys are much more tolerant to CO poisoning than pure Pt electrocatalysts. This has made them the electrocatalysts of choice for reformate operation.

For example, Figure 9 shows the large improvement in CO tolerance for an MEA with an anode based on a Pt0.5Ru0.5 alloy electrocatalyst prepared as 20 wt.% Pt, 10 wt% Ru/Vulcan XC72R. The Cell potentials are significantly higher at all current densities compared with the MEA based on pure Pt, for operation with hydrogen containing 10, 40 and 100 ppm CO. This improved CO tolerance has resulted in much effort to optimise the PtRu electrocatalyst for reformate operation.

For industrial-scale manufacture of PtRu alloy electrocatalysts Johnson Matthey use an aqueous slurry route with chemical reduction to form the metal alloy particles (3). Table III shows physical characterisation data of an unsupported Pt0.5Ru0.5 alloy black electrocatalyst and two high metal-loaded Pt0.5Ru0.5 alloys supported on Vulcan XC72R carbon black at 40 wt% Pt, 20 wt% Ru and at 20 wt.% Pt, 10 wt% Ru.


Table III

Physical Characterisation of PtRu Alloy Electrocatalysts

Electrocatalyst XRD crystallite size, nm Calculated metal area, m2 g-1 PtRu CO chemisorption metal area, m2 g-1 PtRu XRD lattice parameter, nm
67 wt.% Pt, 33 wt.% Ru (unsupported) 2.9 114 77 0.388
40 wt.%Pt, 20wt.%Ru supported on XC72R 2.5 131 104 0.388
20 wt.%Pt,10wt.%Ru supported on XC72R 1.9 150 139 0.388

Particularly interesting is the high metal dispersion of all of the electrocatalysts, especially at high metal loadings. The X-ray diffraction (XRD) crystallite sizes are 2.9 nm (PtRu black), 2.5 nm (40 wt.% Pt, 20 wt.% Ru) and 1.9 nm (20 wt,% Pt, 10 wt% Ru). These are much lower than the corresponding values of 5.8 nm (Pt black), 4.5 nm (60 wt.% Pt) and 2.8 nm (30 wt% Pt) for the corresponding pure Pt electrocatalysts. The high degree of PtRu dispersion is most probably a reflection of the surface characteristics of the materials. although the Pt0.5Ru0.5 electrocatalysts were well alloyed the contraction in the lattice parameter shown in Table III suggests all were slightly Pt rich (that is Pt0.59Ru0.41). The lack of any crystalline Ru-rich phases in the XRDs suggests that the unalloyed Ru is present as an amorphous phase. cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) studies performed at Johnson Matthey indicate that the surface of the electrocatalysts were rich in amorphous Ru oxide. This amorphous Ru oxide may play an important role in the codeposition of the PtRu particles and in reducing the degree of sintering during the electrocatalyst manufacturing process.

It is also interesting to compare the measured CO chemisorption metal areas with the calculated metal areas from the XRD crystallite sizes (assuming spherical particles of density 18.2 g cm-3). Table III shows the measured CO chemisorption metal areas of 77 m2 g-1 PtRu (PtRu black), 104 m2 g-1 PtRu (40 wt.% Pt, 20 wt% Ru) and 139 m2 g-1 PtRu (20 wt% Pt, 10 wt% Ru) are 48, 26 and 8% lower, respectively, than the calculated values. The measured values become progressively lower than the calculated metal areas as the metal loading on the electrocatalyst increases. This probably reflects the increased clustering and fusing of the PtRu alloy crystallites as the metal loading increases.

Figure 10 shows the transmission electron micrographs (TEMs) of the 20 wt.% Pt, 10 wt.% Ru/Vulcan XC72R and of the unsupported PtRu black electrocatalysts. For the carbon-supported electrocatalyst, the highly dispersed PtRu alloy crystallites are clearly evident on the primary carbon black particles that are fused together to form the aggregate and the larger agglomerate structure typical of fuel Cell supports. This is in contrast to the unsupported electrocatalyst where a large number of small PtRu alloy particles form an extended array of PtRu aggregates and agglomerates. The morphology of the unsupported electrocatalyst is very different from the supported materials.

Fig 10.

TEMs of 20 wt.% Pt.l0 wt.% Ru/Vulcan XC72R and unsupported PtRu alloy black electrocatalysts. Left: Highly dispersed PtRu alloy crystallites on the surface of the primary carbon black particles. Right: The PtRu black particles clump together to form small aggregates and larger agglomerate structures

TEMs of 20 wt.% Pt.l0 wt.% Ru/Vulcan XC72R and unsupported PtRu alloy black electrocatalysts. Left: Highly dispersed PtRu alloy crystallites on the surface of the primary carbon black particles. Right: The PtRu black particles clump together to form small aggregates and larger agglomerate structures

Effect of PtRu Composition


For the volume manufacture of PtRu electrocatalysts it is important to understand the effect of the composition on the MEA performance. Using a 20 wt.% Pt, 10 wt.% Ru/Vulcan XC72R electrocatalyst, the effect of alloy formulation on CO tolerance was examined. Electrocatalysts were prepared, both as fully alloyed and as completely unalloyed materials, with a nominal composition of Pt0.5Ru0.5.

The unalloyed Pt0.5Ru0.5 was prepared by depositing Ru onto a pre-reduced Pt electrocatalyst XRD confirmed that there was no alloying from the Pt lattice spacing of 0.392 nm, that is equivalent to pure Pt. In addition there was no significant crystalline Ru, suggesting the Ru was present as amorphous Ru oxide. Half-cell studies were performed in sulfuric acid (H2SO4) at 80°C using typical PEMFC anode structures containing Nafion perfluorinated sulfonic acid. cyclic voltammetry provided insight into the nature of the electrocatalyst surface. Figure 11 shows typical cyclic voltammograms for a Pt0.5Ru0.5 alloy and a pure Pt electrocatalyst recorded at a lower temper-ature of 25°C.

Fig 11.

Cyclic voltammograms recorded in 1 mol dm-3 H2SO4 at 25°C for Pt0.75Mo0.25, Pt0.5Ru0.5 and pure Pt electrodes. For the Pt0.75Mo0.25 alloy, CO electrooxidation activity starts at +0.15 V (vs. RHE). For the Pt0.5Ru0.5 alloy, the peak potential for CO electrooxidation is at the lowest electrode potential. The electrodes are prepared from 35 wt. % Pt, 5 wt. % Mo/Vulcan XC72R. 40 wt.%Pt, 20 wt.% Ru/Vulcan XC72R or 40 wt. % Pt/Vulcan XC72R at a loading of 0.25 mg Pt cm-2. The electrodes were placed in a 3-compartment Cell with a Palladium/carbon electrode purged with hydrogen to act as the RHE and a Pt counter electrode. The electrolyte was first purged with pure CO to poison the electrode at +0.15 V (vs. RHE) and then with nitrogen to remove any solution phase CO. The electrode potential was swept at 10 m V s-1 from +0.15 to +1.0 V (vs. RHE) then back to 0 V (vs. RHE). The second (dashed lines) and subsequent potential sweeps are between 0 V and +1.0 V (vs. RHE)

Cyclic voltammograms recorded in 1 mol dm-3 H2SO4 at 25°C for Pt0.75Mo0.25, Pt0.5Ru0.5 and pure Pt electrodes. For the Pt0.75Mo0.25 alloy, CO electrooxidation activity starts at +0.15 V (vs. RHE). For the Pt0.5Ru0.5 alloy, the peak potential for CO electrooxidation is at the lowest electrode potential. The electrodes are prepared from 35 wt. % Pt, 5 wt. % Mo/Vulcan XC72R. 40 wt.%Pt, 20 wt.% Ru/Vulcan XC72R or 40 wt. % Pt/Vulcan XC72R at a loading of 0.25 mg Pt cm-2. The electrodes were placed in a 3-compartment Cell with a Palladium/carbon electrode purged with hydrogen to act as the RHE and a Pt counter electrode. The electrolyte was first purged with pure CO to poison the electrode at +0.15 V (vs. RHE) and then with nitrogen to remove any solution phase CO. The electrode potential was swept at 10 m V s-1 from +0.15 to +1.0 V (vs. RHE) then back to 0 V (vs. RHE). The second (dashed lines) and subsequent potential sweeps are between 0 V and +1.0 V (vs. RHE)

The alloyed Pt0.5Ru0.5 had features that are pre-dominandy Ru in character with no resolvable Pt hydride peaks. The unalloyed Pt0.5Ru0.5 showed predominately Pt character with resolvable Pt hydride peaks, although there was strong evidence of surface interaction between the Pt and the Ru oxide. The peak potential for CO electrooxidation (Reaction (iv)) occurred at +0.44 V (vs. RHE), close to the +0.37 (vs. RHE) for the Pt0.5Ru0.5 alloy and much lower than the +0.58 V (vs. RHE) for the pure Pt electrocatalyst. The peak potentials are much lower than in Figure 11, due to the much higher electrolyte temperature employed, although the relative peak positions are comparable.

With hydrogen containing 100 ppm CO as reactant the specific activity for hydrogen electrooxidation was measured. At +0.010 V (vs. RHE) the specific activity increased from 8 mA cm-2 Pt for pure Pt to 25 mA cm-2 PtRu for the unalloyed Pt0.5Ru0.5 and to 60 mA cm-2 PtRu for the Pt0.5Ru0.5 alloy. This suggests that the alloying process which incorporates the Ru into the Pt lattice (and reduces the Pt lattice spacing) is important for improving the CO tolerance. Besides enhancing the intimate contact between the Pt and the Ru, extended X-ray absorption fine structure (EXAFS) analysis has shown that alloying removes electron density from the Pt (28). Both these effects are capable of promoting a lower CO coverage on Pt and increasing the hydrogen electrooxidation activity of the anode.

A possible consequence of these findings is that low Ru contents might not show such high CO tolerance in the PEMFC. As an interesting exercise the Ru content in the PtRu alloy was reduced to match the output ratio of the pgms from the refinery. Thus the Ru content in a fully alloyed 20 wt.% Pt, Ru/Vulcan XC72R electrocatalyst was lowered from Pt0.5Ru0.5 to the mined ratio of Pt0.7Ru0.3, and then examined in the PEMFC The XRD lattice spacing of 0.390 nm confirmed a Pt0.7Ru0.5 alloy was formed. For hydrogen containing 10, 40 and 100 ppm CO MEAs prepared with Pt0.7Ru0.3 matched the performances in Figure 9 for the Pt0.5Ru0.5 electrocatalyst at an identical anode loading of 0.25 mg Pt cm-2. This confirmed that using the Johnson Matthey manufacturing route at an atomic ratio of Pt0.7Ru0.3 there is sufficient Ru incorporated into the Pt lattice to adequately modify the CO tolerance of Pt

These findings were in agreement with the fundamental kinetic work of Gasteiger et al. (29) who used 1000 ppm CO in hydrogen and built, planar Pt0.5Ru0.5 and Pt0.9Ru0.1 alloys. The findings also agree with the more applied results in the PEMFC of Iwase and Kawatsu (26) for alloys of composition Pt0.85Ru0.15 to Pt0.15Ru0.85 all based on 20 wt.% Pt, 10 wt.% Ru/Vulcan XC72R electrocatalysts. They found that the CO tolerance was significantly lower only at Ru contents below Pt0.8Ru0.2 or above Pt0.2Ru0.8. However, there have also been a few reports of lower CO tolerance at lower Ru levels including Pt0.7Ru0.3 (27). This highlights the importance of the manufacturing route on the degree of PtRu alloying and on the nature of the electrocatalyst surface. This variability probably accounts for the significant spread in CO tolerances reported for PtRu anode electrocatalysts (24).

Mechanism of CO Tolerance on PtRu


While it is clear that PtRu provides considerably enhanced CO tolerance, an understanding of the mechanism operating at the anode is needed for the development of improved electrocatalysts. Based on early work by Watanabe and Motoo (30) (methanol electrooxidation) a bifunctional mechanism involving water activation by Ru (Reaction (v)) and subsequent CO electrooxidation on a neighbouring Pt atom (Reaction (vi)) has been postdated by a number of groups (26, 27, 29, 31-33).


(v)

(vi)

To investigate the mechanism of CO tolerance of PtRu, half-cett studies in H2SO4 at 800C with PEMFC anode structures containing Nafion were performed using pure CO as the reactant. The half-cell outlet gas was analysed using in-line mass spectrometry for evidence of CO2 — to examine PtRu for CO electrooxidation activity (Reaction (vi)). The results indicated that CO electrooxidation was occurring on PtRu at above +0.25 V (vs. RHE). This is some 0.2 V less anodic than required for pure Pt electrocatalysts and reflects the greater ability of the Ru to adsorb water (Reaction (v)) compared with Pt Figure 11 shows that, in the cyclic voltammograms measured in H2SO4 at 25°C, the promotional effect from PtRu is also reflected in a lower peak potential for CO electrooxidation at the electrodes: PtRu (+0.5 V (vs. RHE)) compared with Pt (+0.8 V (vs. RHE)). Both the half-cell and the cyclic voltammetry studies confirmed that at anode potentials above +0.25 V (vs. RHE) the improved CO tolerance of PtRu is due to the enhanced electrocatalysis of CO electrooxidation to CO2 (Reactions (v) and (vi)).

At lower anode potentials, where the more efficient anode electrocatalysts function during PEMFC operation (i.e. < +0.2 V (vs. RHE)), the situation is open to debate. In half-cell/mass spectrometry studies at low anode potentials of 0.0 to +0.2 V (vs. RHE), there was no measurable CO electrooxidation current or detection of CO2 product in the half-cell outlet gas. While it has been calculated that currents as low as 10-9 A cm-2 Pt can remove sufficient CO from the electrocatalyst to give effective CO tolerance (34), it is reasonable to argue that CO removal is more likely due to a weakening of the Pt–CO bond strength. As discussed, EXAFS has shown (28) that alloying does reduce the Pt electron density which should weaken the Pt–CO bond strength and decrease the CO coverage on PtRu. This will increase the number of electrocatalyst sites available for hydrogen elec-trosorption and electrooxidation. The rate of Reactions (ii) and (iii) would then increase. This mechanism would also be consistent with the observed importance of PtRu alloying on the CO tolerance discussed earlier. Particularly relevant is the observation that the unalloyed PtRu material shows comparable CO electrooxidation activity in the cyclic voltammograms, although it is not as CO tolerant Thus, it seems more probable that the mechanism of CO tolerance at practical anode potentials is due to a weakening in the Pt–CO bond strength rather than to the promotion of CO electrooxidation to CO2.

CO2 Poisoning in the PEMFC


While it has been known for many years that CO poisons Pt-based electrocatalysts, it has only Recently been discovered that CO2 also acts as a poison in the PEMFC (25, 35). As shown in Figure 12, the effect on a Pt anode of a high CO2 concentration of 25%, typical of many reformate streams, is modest. Only at the highest current densities do Cell potential losses become significant. In fact, comparing the performance of an MEA with an identical Pt anode in the presence of 10 ppm CO (Figure 9) and 25% CO2 (Figure 12) shows that the Cell potentials are much lower with 10 ppm CO under the particular single-cell operating conditions examined.

Fig 12.

Poisoning effect of 25% CO2 on Pt and Pt0.5Ru0.5 alloy anodes. The plots show the increased CO2 tolerance of the Pt0.5Ru0.5 alloy anodes. The anodes and Pt loading are as described in Fig. 9. The cathode and MEA are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9. except that here it is using hydrogen or (hydrogen with 25% CO2)/ai

Poisoning effect of 25% CO2 on Pt and Pt0.5Ru0.5 alloy anodes. The plots show the increased CO2 tolerance of the Pt0.5Ru0.5 alloy anodes. The anodes and Pt loading are as described in Fig. 9. The cathode and MEA are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9. except that here it is using hydrogen or (hydrogen with 25% CO2)/ai

CO2 poisoning can come from two sources both of which generate CO which then poisons the Pt electrocatalyst sites for hydrogen electrooxidation. The CO sources are: the reverse water gas shift reaction (RWGSR, Reaction (vii)) and the electrochemical equivalent of the RWGSR (Reaction (viii)).


(vii)

(viii)

From thermodynamics it has been calculated that at 800C for reformate containing 25% CO2 in hydrogen, 100 to 200 ppm CO could be generated by the RWGSR (36). The exact amount of CO produced is however highly dependent on the water concentration. In practice, as shown by comparing Figures 9 and 12, with the Johnson Matthey anodes much less than 10 ppm CO is produced. This suggested that with these anode designs, the RWGSR may not be the major mode of poisoning.

CO2 Electroreduction


Consequently, at a Pt and a Pt0.5Ru0.5 alloy electrode in H2SO4 at 70°C, an examination of the electrochemical poisoning by CO2 was performed. By holding the electrode potential constant and saturating the H2SO4 with pure CO2 it was shown that both the pure Pt and the Pt0.5Ru0.5 alloy electrocatalysts form a ‘Pt–CO'-like poison at electrode potentials below + 0.4 V (vs. RHE). In this potential range there is sufficient Pt-Hads available to promote CO2 electroreduction. Indeed, the ‘Pt–CO’ coverage correlates well with the Pt-Hads coverage.

The ‘Pt–CO’ poison was removed by sweeping the electrode to higher potentials. The characteristic CO electrooxidation peak was evident in the cyclic voltammograms at peak potentials of ∼ +0.4 V (vs. RHE) for the Pt0.5Ru0.5 alloy electrode, and ∼ +0.6 V (vs. RHE) for the pure Pt electrode. It was therefore concluded that at operating anode potentials in the PEMFC, CO2 poisoning occurs predorninantly by the electrochemical reduction reaction Reaction (viii)), with the ‘Pt–CO’ poison removed at much higher anode potentials.

Most significantly, as shown in Figure 13, the potential-dependent coverage by the ‘Pt–CO’ poison, calculated from the CO electrooxidation current in the cyclic voltammograms, is much higher for the Pt than for the Pt0.5Ru0.5 alloy electrocatalyst For example, the maximum coverage recorded at +0.0.5 V (vs. RHE) is ∼ 0.9 on the pure Pt electrode and ∼ 0.5 on the Pt0.5Ru0.5 alloy. The lower ‘Pt–CO’ coverages on the Pt0.5Ru0.5 alloy electrocatalyst suggested that it should provide an improved CO2 tolerance in the PEMFC.

Fig 13.

Lower ‘Pt-CO’ coverage due to CO2 electroreduction at a Pt0.5Ru0.5 alloy electrode compared to pure Pt. as a function of the electrode potential. The electrodes and Pt loading are as described in Fig. 9 for the anode.

The electrode was placed in a 3-compartment Cell containing I mol dm-3 H2SO4 at 70°C with a Palladium/ carbon electrode purged with hydrogen, to act as the RHE and a Pt counter electrode. The electrolyte was purged with pure CO2 to poison the electrode at the selected rest potential, then with nitrogen to remove any solution phase CO2. The electrode potential was swept at l0 mV s-1 from the rest potential to +1.0 V (vs. RHE) then back to 0V (vs. RHE). The ‘Pt-CO’ electrooxidation current was used to determine the Pt-CO’ coverage

Lower ‘Pt-CO’ coverage due to CO2 electroreduction at a Pt0.5Ru0.5 alloy electrode compared to pure Pt. as a function of the electrode potential. The electrodes and Pt loading are as described in Fig. 9 for the anode.  The electrode was placed in a 3-compartment Cell containing I mol dm-3 H2SO4 at 70°C with a Palladium/ carbon electrode purged with hydrogen, to act as the RHE and a Pt counter electrode. The electrolyte was purged with pure CO2 to poison the electrode at the selected rest potential, then with nitrogen to remove any solution phase CO2. The electrode potential was swept at l0 mV s-1 from the rest potential to +1.0 V (vs. RHE) then back to 0V (vs. RHE). The ‘Pt-CO’ electrooxidation current was used to determine the Pt-CO’ coverage

CO2 Performance at PtRu Anodes


Figure 12 shows that for single-cell operation on 25% CO2 in hydrogen, the Pt0.5Ru0.5 alloy has improved the CO2 tolerance compared to the pure Pt anode. The clear benefit from the Pt0.5Ru0.5 alloy becomes progressively greater as the current density increases - because the CO2 losses on pure Pt increase. Indeed, at a current density of 1 A cm-2 the Cell potential of the Pt0.5Ru0.5 alloy-based MEA is some 0.2 V higher. This is a significant benefit.

The better CO2 tolerance of the PtRu anode is probably due to a weakening in the ‘Pt-CO’ bond strength that is believed to be the source of the improved CO tolerance of PtRu below +0.2 V (vs. RHE). While the precise nature of the adsorbed CO intermediate(s) from CO2 electroreduction are not clear (37-39) the cyclic voltammograms for the electrooxidarion of the ‘Pt–CO’ poison indicate that they are very similar to those produced by pure CO in this potential range.

Anode Design for Reformate Tolerance


As with the development of improved cathodes for the PEMFC discussed in Part I of this paper (40), the optimum anode for reformate operation requires not only a PtRu electrocatalyst but also an optimised anode electrocatalyst layer.

The importance of the anode design in promoting the CO tolerance of some Pt0.5Ru0.5 anodes is highlighted in Figure 14. This shows for single- Cell operation at 538 mA cm-2, the cell potential loss (relative to pure hydrogen) with fuel containing from 10 to 100 ppm CO. While anode loadings of 1.0 mg Pt cm-2 are not economical for the majority of PEMFC applications, the performance of unsupported PtRu black layers at this high loading shows the importance of the anode layer thickness and the in situ anode electrode Pt surface area (EPSA), measured using cyclic voltammetry (4). The Cell potential losses in Figure 14 are only 5 mV (at 10 ppm CO), 30 mV (at 40 ppm CO) and 40 mV (at 100 ppm CO). This is because the anode layer thickness is less than 10 μm and the measured in situ anode EPSA is high at close to 770 cm2 PtRu cm-2.

Fig 14.

Reduction in the Cell potential loss at 538 mA cm-2 (relative to pure hydrogen operation) due to 10. 40 and 100 ppm CO as the anode EPSA is increased for Pt0.5Ru0.5 alloy anodes. The anodes are prepared from 40 wt. % Pt, 20 wt. % Ru/Vulcan XC72R or PtRu black at a loading of 0.35 mg Pt cm-2 and 1.0 mg Pt cm-2, respectively. The cathode and MEA are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9 except that here it is operating at 1.5/2 stoichiometr

Reduction in the Cell potential loss at 538 mA cm-2 (relative to pure hydrogen operation) due to 10. 40 and 100 ppm CO as the anode EPSA is increased for Pt0.5Ru0.5 alloy anodes. The anodes are prepared from 40 wt. % Pt, 20 wt. % Ru/Vulcan XC72R or PtRu black at a loading of 0.35 mg Pt cm-2 and 1.0 mg Pt cm-2, respectively. The cathode and MEA are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9 except that here it is operating at 1.5/2 stoichiometr

It is important to maximise the anode EPSA as this measures the maximum PtRu surface area available for hydrogen electrooxidation in the absence of CO and CO2 poisoning. As the anode layer is thin, the whole anode layer thickness is active and all of the EPSA can therefore be utilised during PEMFC operation. Proton conductivity and hydrogen permeability do not limit the active layer thickness of the anode. This is particularly important at high current densities (41).

In contrast, even using a high loaded 40 wt.% Pt, 20 wt.% Ru/Vulcan XC72R-supported electrocatalyst at 1.0 mg Pt cm-2, the anode layers are much too thick. As a consequence, much of the depth of the anode layer is redundant and the Cell potential losses are much higher than shown in Figure 14 for the unsupported PtRu black anode layer at 1.0 mg Pt cm-2.

Significantly, however, at more economical electrode Pt loadings of 0.35 mg Pt cm-2 the anode layer is sufficiently thin for the carbon-supported electrocatalysts to ensure all of the anode thickness is used during PEMFC operation. In this case, the PtRu black anode cannot achieve the same degree of CO tolerance as a well designed anode layer using 40 wt.% Pt, 20 wt% Ru/Vulcan XC72R. This is because the CO metal area is lower for the PtRu black (see Table III) and the anode EPSA is reduced.

For carbon-supported electrocatalysts to achieve a high anode EPSA it is necessary to pay attention to the interaction between the PtRu sites on the carbon support (shown in the TEM in Figure 10) and the protons in the perfluorinated sulfonic acid solution used to provide proton conduction within the anode layer. With the traditional aqueous-based electrocatalyst inks employed at Johnson Matthey to deposit the anode layer, the initial in situ anode EPSAs for the 40 wt% Pt, 20 wt.% Ru/Vulcan XC72R layers were low. Initial measurements at an anode loading of 0.35 mg Pt cm-2 indicated that the anode EPSA was below 200 cm2 PtRu cm-2. This was much lower than the 364 cm2 PtRu cm-2 potentially available.

Therefore, improved aqueous-based PtRu inks were manufactured which enhanced the interaction between the electrocatalyst and the Nafion proton conducting polymer solution. With the improved inks, the in situ anode EPSA of the resulting anode layers was raised much closer to the maximum 364 cm2 PtRu cm-2. This produced the significantly lower Cell potential losses in Figure 14 for the 40 wt.% Pt, 20 wt.% Ru/Vulcan XC72R anodes. The increasing anode EPSA produces Cell potential losses at 538 mA cm-2 that are reduced by 40 mV (at 10 ppm CO), 70 mV (at 40 ppm CO) and 60 mV (at 100 ppm CO), respectively.

Comparable improvements in the CO2 tolerance of the MEA are also observed for the better PtRu anode designs. Figure 15 shows, for single Cell operation at 754 mA cm-2, the relationship between the Cell potential loss (relative to pure hydrogen) with 25% CO2 in hydrogen and the in situ anode EPSA. As the EPSA is increased from 160 to 600 cm2 PtRu cm-2 for the 40 wt.% Pt, 20 wt.% Ru/Vulcan XC72R and the unsupported PtRu black-Nafion ink layers, the CO2 tolerance is improved. In Figure 15, the Cell potential loss is reduced from 65 mV to as little as 10 mV.

Fig 15.

The reduction in Cell potential loss at 754 mA cm-2 (relative to pure hydrogen operation) due to 25% CO2 as the anode EPSA is increased for Pt0.5Ru0.5 alloy anodes. The anodes and Pt loadings are as described in Fig. 14. The cathode is as described in Fig. 9. The MEAs are based on catalysed substrates bonded to Dow XUS13204.10 membrane. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9. except that here it is operating at 1.5/2 stoichiometr

The reduction in Cell potential loss at 754 mA cm-2 (relative to pure hydrogen operation) due to 25% CO2 as the anode EPSA is increased for Pt0.5Ru0.5 alloy anodes. The anodes and Pt loadings are as described in Fig. 14. The cathode is as described in Fig. 9. The MEAs are based on catalysed substrates bonded to Dow XUS13204.10 membrane. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9. except that here it is operating at 1.5/2 stoichiometr

Further work has shown the importance of ensuring that the in situ anode EPSA does not fall below the values in Figure 15. For an anode layer containing 0.25 mg Pt cm-2, and based on 20 wt.% Pt, 10 wt.% Ru/Vulcan XC72R, removing the Nafion from the catalyst ink (by using an alternative binder) dramatically lowers the CO2 tolerance. For single-cell operation at 538 mA cm-2 with 25% CO2 in hydrogen at the operating conditions detailed in Figure 12, the Cell potential was nearly 150 mV lower. The reduction in CO2 tolerance is due to a very low in situ anode EPSA below 100 cm2 PtRu cm-2. It seems that the Pt not in contact with Nafion can catalyse the RWGSR (Reaction (vii)) to generate levels of CO approaching 100 ppm.

It is therefore clear that to achieve a high degree of reformate tolerance it is important to maximise the in situ anode EPSA by increasing the Pt dispersion of the electrocatalyst and by maximising the proton contact to the PtRu sites from the proton conducting polymer in the anode. Equally important to the in situ anode EPSA is the production of thin electrocatalyst layers. This reduces the possibility of the proton conductivity and the hydrogen permeability in the anode layer from limiting the performance of the MEA, particularly at higher current densities (41).

Alternative Reformate Tolerant Electrocatalysts


There has been a significant effort over several decades to mitigate the effects of CO poisoning at the anode by developing electrocatalysts that are superior to PtRu (36, 42, 43). The electrocatalysts generally fall into two categories.

(i) Intrinsic Mechanism


The first category is the wide range of Pt alloys that have been examined in an attempt to modify the CO and hydrogen electrosorption properties of Pt — to reduce the CO coverage and increase the rate of hydrogen electrooxidation. Much study has focused on PtRu, PtRh and PtIr alloys. These were originally examined in the 1960s by General Electric (e.g. see 44-46) as unsupported metal blacks at high anode loadings of 30 mg Pt cm-2. More Recently (47) they have been prepared on carbon supports and examined at lower anode loadings of 0.5 mg Pt cm-2. It was shown that at below 1000C none of the Pt alloys was superior to PtRu but that the CO tolerance of PtRh approached that of PtRu. Iwase and Kawatsu (26) also found that at 800C a large number of Pt alloys supported on Vulcan XC72R (PtRh, PtIr, PtPd, PtV, PtCr, PtCo, PtNi, PtFe, PtMn) all gave inferior CO tolerance compared to PtRu.

However, there is a considerable drive to raise the operating temperatures of the PEMFC to above 100°C. This would raise the system efficiency and dramatically improve the CO tolerance of Pt-based electrocatalysts. For example, the CO coverage on Pt drops from in excess of 0.98 at 80°C to ∼ 0.5 at 1600C (43). At this temperature a Pt-based anode can function in the presence of 1 to 2% CO rather than with ppm levels of CO, eliminating the need for the CPOX reactor. If the temperature can be raised to 160°C, studies in the phosphoric acid fuel Cell (PAFC) with 1 to 2% CO indicate that PtRh and PfNi may offer superior CO tolerance (48, 49).

Indeed, at 2000C pure Pt is the favoured electrocatalyst in the PAFC At higher temperatures PtRu may not be the electrocatalyst of choice in the PEMFC. There is currently much research (50) aimed at developing membranes capable of proton conduction at 120 to 2000C

(ii) Promoted Mechanism


The second category of electrocatalysts involves modifying Pt with metal oxides to catalyse the electrooxidation of CO to CO2 (Reaction (iv)) at operating anode potentials. This would reduce the CO coverage and increase the number of Pt electrocatalyst sites available for hydrogen electrooxidation. Again the most promising group of materials date from the early General Electric work (44, 51). Based on these studies, carbon-supported Platinum-molybdenum (PtMo), PtCoMo, PtWO3 and PtCoWO3 have Recently been investigated. The electrocatalysts were shown to have enhanced CO tolerance properties compared to carbon-supported PtRu electrocatalysts (47, 52).

In particular, it was Recently reported (53) that a carbon-supported Pt0.8Mo0.2 alloy electrocatalyst produced, in a small PEMFC single Cell operating at 85°C, only a 50 mV loss in Cell potential for operation with hydrogen containing 100 ppm CO. In contrast, the carbon-supported Pt0.5Ru0.5 alloy electrocatalyst showed a Cell potential loss of 160 mV. The promising PtMo system has been examined at Johnson Matthey.

PtMo alloy Electrocatalysts


A Pt0.75Mo0.25 alloy was prepared as 20 wt.% Pt, 3 wt.% Mo by sequentially depositing Pt oxide and Mo oxide followed by thermal reduction at high temperature. half-cell measurements in H2SO4 at 800C using Nafion-containing electrodes were performed to determine the specific activity for hydrogen electrooxidation in the presence of 100 ppm CO. Figure 16 shows the resulting plots of anode potential versus the specific activity for the Pt0.75M00..25 alloy electrode compared to the corresponding Pt0.5Ru0.5 alloy electrode and a pure Pt electrode. As expected, due to the greater level of CO poisoning it is only at the pure Pt electrode that anode potentials in excess of +0.0.5 V (vs. RHE) are required to sustain hydrogen electrooxidation. Most significantly, at anode potentials below +0.025 V (vs. RHE) a higher specific activity for hydrogen electrooxidation is measured for the Pt0.75M00.25 alloy electrode compared to the Pt0.5Ru0.5 alloy electrode.

Fig 16.

Specific activity as a function of anode potential for hydrogen electrooxidation in the presence of 100 ppm CO and for pure CO electrooxidation at pure Pt, Pt0.5Ru0.5 alloy and Pt0.75Mo0.25 alloy electrodes. At low anode potentials the Pt0.75Mo0.25 alloy shows a higher specific activity for hydrogen and CO electrooxidation. The electrodes and Pt loading are as described in Fig. 11.

The electrodes were located in a half-cell with a hydrogen-purged platinised Pt electrode as the RHE and a Pt counter electrode. The half-cell was placed in 1 mol dm-2 H2SO4 at 80°C and the electrolyte deoxygenated with nitrogen.

The steady-state current was measured at selected electrode potenlials from 0 V (vs. RHE) using hydrogen with 100 ppm CO and then pure CO. The electrode potential was returned to 0 V (vs. RHE). Cyclic voltammograms were recorded at 10 mV s-1 between 0 and +1.0 V(vs. RHE) to obtain the CO electrooxidation currents used to calculate the EPSAs and the specific activities

Specific activity as a function of anode potential for hydrogen electrooxidation in the presence of 100 ppm CO and for pure CO electrooxidation at pure Pt, Pt0.5Ru0.5 alloy and Pt0.75Mo0.25 alloy electrodes. At low anode potentials the Pt0.75Mo0.25 alloy shows a higher specific activity for hydrogen and CO electrooxidation. The electrodes and Pt loading are as described in Fig. 11.  The electrodes were located in a half-cell with a hydrogen-purged platinised Pt electrode as the RHE and a Pt counter electrode. The half-cell was placed in 1 mol dm-2 H2SO4 at 80°C and the electrolyte deoxygenated with nitrogen.  The steady-state current was measured at selected electrode potenlials from 0 V (vs. RHE) using hydrogen with 100 ppm CO and then pure CO. The electrode potential was returned to 0 V (vs. RHE). Cyclic voltammograms were recorded at 10 mV s-1 between 0 and +1.0 V(vs. RHE) to obtain the CO electrooxidation currents used to calculate the EPSAs and the specific activities

The source of the improved hydrogen electrooxidation activity was confirmed by using pure CO as the reactant and monitoring the half-cell exhaust for product CO2using mass spectrometry (Reaction (iv)). Figure 16 shows that, for the Pt0.5Mo0.5 alloy electrode with pure CO, there is considerable specific activity for CO electrooxidation starting from anode potentials of +0.0.5 V (vs. RHE). Mass spectrometry of the exhaust gas confirmed that the electrooxidation current was due to product CO2 formation.

This indicated that at anode potentials below +0.2 V (vs. RHE) (needed for efficient PEMFC operation), the CO tolerance of the PtMo electrocatalyst comes from promoting CO electrooxidation (Reaction (iv)). The low electrode potentials required for CO electrooxidation were also evident in the cyclic voltammograms. Figure 11 shows for the Pt0.75Mo0.25 alloy electrode a broad oxidation current starting at +0.15V (vs. RHE) that is complicated by Mo surface redox reactions (54).

These results are in direct contrast to those from both the pure Pt and the Pt0.5Ru0.5 alloy electrodes. There is no detectable specific activity for CO electrooxidation below +0.2 V (vs. RHE) in Figure 16 and no product CO2 was detected in the half-cell exhaust gas. Figure 11 confirmed that there was no electrooxidation activity measured at these low electrode potentials for either the Pt0.5Ru0.5 alloy or the pure Pt electrodes in the cyclic voltammograms.

CO Tolerance of PtMo MEAs


Based on the promise offered by the half-cell results, the Pt0.75Mo0.25 alloy electrocatalyst was examined in the PEMFC. The MEA performances confirmed the much improved CO tolerance for hydrogen containing from 40 to 100 ppm CO. For example, Figure 17 shows that for operation with hydrogen containing 46 ppm CO, at an electrode loading of 0.25 mg Pt cm-2, the Pt0.75Mo0.25 alloy provides much higher performance than the Pt0.5Ru0.5 anode. The performance benefit increases progressively from 400 mA cm-2 and at 1.0 A cm2 the Pt0.75Mo0.25 alloy is operating at some 0.2 V above the Pt0.5Ru0.5 alloy.

Fig 17.

The ability of Pt0.75Mo0.25 and Pt0.5Ru0.5 alloy anodes to tolerate 46 ppm CO in hydrogen and 25% CO2 in hydrogen. The Pto.75Mo0.25 alloy is more CO tolerant but much less CO2 tolerant. The PtMo and PtRu anodes and their Pt loading are as described in Fig. 11. The cathode and MEAs are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 1.5/2 stoichiometry

The ability of Pt0.75Mo0.25 and Pt0.5Ru0.5 alloy anodes to tolerate 46 ppm CO in hydrogen and 25% CO2 in hydrogen. The Pto.75Mo0.25 alloy is more CO tolerant but much less CO2 tolerant. The PtMo and PtRu anodes and their Pt loading are as described in Fig. 11. The cathode and MEAs are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 1.5/2 stoichiometry

However, as the CO concentration is reduced the benefit due to the Pt0.75Mo0.25 alloy electrocatalyst is reduced, until at 10 ppm CO, the Pt0.5Ru0.5 alloy electrocatalyst is the more CO tolerant. The difference in reaction order for the Pt0.75Mo0.25 and the Pt0.5Ru0.5 alloys is perhaps indicative of the distinct mechanisms of CO-tolerance operating at the electrocatalysts.

CO2 Tolerance of PtMo MEAs


However, a much larger issue than the MEA performance at low ppm levels of CO was found for the Pt0.75Mo0.25 alloy. When operating on hydrogen with 25% CO2, the Pt0.75Mo0.25 alloy shows poor CO2 tolerance. As shown in Figure 17 the Pt0.75Mo0.25 alloy shows a similar performance to hydrogen containing 46 ppm CO. This contrasts with the Pt0.5Ru0.5 alloy where the single-cell performance in Figure 17 with hydrogen containing 25% CO2 is much higher than with hydrogen containing 46 ppm CO. Indeed, comparison with Figure 9 shows the cell potential loss with 25% CO2 is less than with 10 ppm CO. As a consequence, for full reformate operation with hydrogen containing both 25% CO2 and 40 ppm CO, the Pt0.5Ru0.5 alloy is much more tolerant than the Pt0.75Mo0.25 alloy. Figure 18 indicates that at 538 mA cm-2, compared with the hydrogen performance, the Pt0.5Ru0.5 alloy shows a Cell potential loss of 100 mV but the Pt0.75Mo0.25 alloy shows a much higher Cell potential loss of 215 mV. As expected, the pure Pt-based anode provides the lowest performance and is operating at 390 mV below its performance with pure hydrogen. Thus, today, with typical reformate feeds that contain CO2, PtRu remains the electrocatalyst of choice for PEMFC operation below 1000C

It is worth considering the reason for the poor CO2 tolerance of the PtMo electrocatalyst and the impact on the anode design for the PEMFC The poor CO2 tolerance of the PtMo system reflects an increased ability of the electrocatalyst to promote the electroreduction of CO2 (Reaction (viii)). This produces much higher coverages of the ‘Pt–CO’ poison. The source of the CO tolerance shown by the PtMo electrocatalyst, namely its ability to electrooxidise CO (Reaction (iv)) also results in an ability to catalyse the reverse electroreduction reaction (Reaction (viii)). The relative magnitude of the electrooxidation of CO and the electroreduction of CO2 will depend strongly on the concentrations of each reactant and on the PEMFC operating conditions. At present, for operation below 100°C, the reformate streams usu-ally contain much more CO2 than CO. Figure 18 indicates in this situation the rate of CO2 electroreduction is dominant This implies that the search for alternative electrocatalysts to provide improved CO tolerance by electrooxidising CO at low anode potentials may be fundamentally flawed for current PEMFC stack operating conditions.

Fig 18.

The reformate tolerance of pure Pt, Pt0.75Mo0.25 alloy and Pt0.5Ru0.5 alloy anodes. The change in Cell potential with time at 538 mA cm-2 on switching the anode fuel from hydrogen to hydrogen with 40 ppm CO and 25% CO2 shows the much higher tolerance of the Pt0.5Ru0.5 alloy anode. The anodes and Pt loading are as described in Fig. 11. The cathode and MEAs are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 1.5/2 stoichiometr

The reformate tolerance of pure Pt, Pt0.75Mo0.25 alloy and Pt0.5Ru0.5 alloy anodes. The change in Cell potential with time at 538 mA cm-2 on switching the anode fuel from hydrogen to hydrogen with 40 ppm CO and 25% CO2 shows the much higher tolerance of the Pt0.5Ru0.5 alloy anode. The anodes and Pt loading are as described in Fig. 11. The cathode and MEAs are as described in Fig. 9. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 1.5/2 stoichiometr

The work to develop new membranes to allow operation of the stack at higher temperatures, above 1000C could change the situation. For example at 1600C much greater CO concentrations of 1 to 2% can be fed to the MEA Alternatively, membrane purification (55) could be adopted to reduce CO2 to ppm levels. Using membrane purification would allow PtMo to show a clear benefit over PtRu electrocatalysts at CO concentrations greater than 10 ppm.

The Air Bleed Technique


At present, for PEMFC operation below 100°C, PtRu is still the favoured electrocatalyst for full reformate operation. However, even with PtRu in a thin anode layer designed to have a high EPSA there are still losses in Cell potential with full reformate operation that lower the MEA's performance. The losses can be largely mitigated by injecting a small air bleed (≤ 2%) just ahead of the PEMFC stack (23) or within the stack itself (56). This air removes the CO from the PtRu electrocatalyst sites by gas phase dissociative chemisorption of oxygen on the CO-free Pt catalyst sites, followed by bimolecular surface reaction between Pt–CO and Pt–O to form CO2 (Reaction (ix)) (57, 58):


(ix)

The gas-phase catalytic oxidation reduces the CO coverage on the PtRu electrocatalyst and increases the hydrogen electrooxidation activity. Normally temperatures in excess of 1000C are required for catalytic oxidation of CO, but since the CO levels are low (10—100 ppm) there are sufficient free Pt sites to promote Reaction (ix) at 800C It seems that the presence of hydrogen and water in the PEMFC has little impact on the catalytic oxidation. In fact, complete tolerance to CO levels of up to 100 ppm can be achieved in well-designed PtRu anode structures.

Typically, however, with PtRu anodes the small loss in Cell potential due to CO2 poisoning, shown in Figure 12, is not completely recovered. This is probably because there are some PtRu electrocatalyst sites not in contact with the proton conducting polymer that are free to promote the RWGSR (Reaction (vii)). There have been a few recent reports (24) that the CO2 losses can be completely mitigated in very thin anode layers at electrode loadings below 0.2 mg Pt cm-2. In this case, all of the PtRu electrocatalyst is probably in contact with the proton conducting electrolyte.

Durability Issues


While the air bleed technique is capable of recovering the Cell potential losses due to CO poisoning and most of the losses due to CO2 poisoning there are still issues to be addressed. For instance, in addition to the catalytic oxidation of CO to CO2 (Reaction (ix)) the air can recombine with the hydrogen fuel in the presence of Pt-based catalysts (Reaction (x)):


(x)

Recombination removes a little of the hydrogen fuel but the major concern is that both the CO oxidation and the recombination reactions are highly exothermic. This increases the local temperature within the MEA, and indeed, temperatures exceeding 1000C have been recorded in MEAs that are functioning in PEMFC stacks operating at 80°C.

The increased temperature can result in sintering of the PtRu electrocatalyst and eventually to pin-holing of the membrane and failure of the MEA and the stack. Figure 19 shows, for example, the performance with time of a standard Johnson Matthey anode based on a Pt0.5Ru0.5 alloy electrocatalyst. The MEA is operating at 754 mA cm-2 and 700C on reformate containing 25% CO2 and 40 ppm CO. Initially a 3% air bleed is sufficient to sustain the Cell potential but this has to be increased progressively with time to 4, 5 and then 7% to maintain the MEA's performance. cyclic voltammetry in situ in the PEMFC indicated that the anode EPSA was decreasing with time. XRD confirmed that this was due to sintering of the Pt0.5Ru0.5 alloy electrocatalyst.

Fig 19.

Durability of a standard and a bilayer anode (selox layer) structure for hydrogen/air and reformate (70% hydrogen, 5% nitrogen, 25% CO2, 40 ppm CO)/air operation at 754 mA cm-2 with an air bleed. The standard anode fails after 1000 hours but the bilayer anode is still functioning after 2000 hours of continuous operation with a 3% air blee.

The anodes are based on 20 wt.% Pt, 10 wt.% Ru/Vulcan XC72R at a loading of 0.25 mg Pt cm-2. The bilayer anode contains a catalytic oxidation layer (selox layer) prepared from 20 wt.% Pt/Shawinigan at a loading of 0.1 mg Pt cm-2. The cathode is as described in Fig. 9. The MEAs are as described in Fig. 15. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 70°C at stoichiometries of l.5/2

Durability of a standard and a bilayer anode (selox layer) structure for hydrogen/air and reformate (70% hydrogen, 5% nitrogen, 25% CO2, 40 ppm CO)/air operation at 754 mA cm-2 with an air bleed. The standard anode fails after 1000 hours but the bilayer anode is still functioning after 2000 hours of continuous operation with a 3% air blee.  The anodes are based on 20 wt.% Pt, 10 wt.% Ru/Vulcan XC72R at a loading of 0.25 mg Pt cm-2. The bilayer anode contains a catalytic oxidation layer (selox layer) prepared from 20 wt.% Pt/Shawinigan at a loading of 0.1 mg Pt cm-2. The cathode is as described in Fig. 9. The MEAs are as described in Fig. 15. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 70°C at stoichiometries of l.5/2

The resulting reduction in the number of active electrocatalyst sites was balanced by increasing the air bleed, to sustain the number of CO-free PtRu electrocatalyst sites. However, Figure 19 shows that the pure hydrogen performance was not affected as only a few electrocatalyst sites are required in this case. half-cell studies have shown that in the absence of reformate poisoning, as little as 0.025 mg Pt cm-2 of a 20 wt.% Pt/Vulcan XC72R electrocatalyst is sufficient to sustain hydrogen electrooxidation below +0.0.5 V (vs. RHE). Eventually at a very high air bleed level of 7%, applied after 1000 hours of continuous operation, the MEA based on the standard anode failed due to pin-holing of the membrane.

Bilayer Anodes


Johnson Matthey and Ballard Power Systems have significantly extended anode lifetimes by introducing a bilayer anode structure. In this structure, a carbon-supported Pt-based catalyst layer is inserted between the PtRu electrocatalyst layer and the anode substrate (59). As the function of the additional layer is to promote gas-phase CO oxidation, it does not contain a proton conducting ionomer — in contrast to the electrocatalyst layer. The ionomer can hinder the distribution of the air bleed.

Figure 19 shows that the bilayer anode structure (that is the selox layer) achieved a comparable MEA performance to the standard anode when operating both on reformate and on pure hydrogen. More importantly, the bilayer anode did not require an increase in the level of the air bleed beyond 3% to sustain the MEA performance, even after 2000 hours of continuous operation. The heat generated by Reactions (ix) and (x) has been largely removed from the electrocatalyst layer and the electrocatalyst layer-membrane interface. This minimises the degree of electrocatalyst sintering and significantly reduces the possibility of membrane pin-holing. The anode gases and the stack cooling can more effectively remove the heat generated in the gas-phase selox layer.

Using the bilayer anode has resulted in stable MFA performances over many thousands of hours of continuous operation. For example, for an MEA with a bilayer anode containing 0.35 mg Pt cm-2 the reformate performance with a 2% air bleed was stable after in excess of 8000 hours of continuous operation. This bilayer approach may produce the required MEA lifetimes for PEMFC applications which need an air bleed to sustain reformate operation.

Cell Reversal Tolerant Anodcs


The use of an air bleed is not the only durabili-ty issue evident at the anode. During operation it is possible that one or more MEAs in a stack, or even a complete stack in a multi-stack system, will show a reverse in polarity. Such Cell reversal incidents reduce the power output from the system and can irreversibly damage the stack. There can be several sources of Cell reversal but most damaging is reactant starvation of the MEA, especially at the anode. Fuel and oxidant starvation can arise from a lack of system control during a sudden change in reactant demand. Alternatively, a flow field channel may be blocked due to stack material debris or by water flooding and inert gas blanketing of the MEA.

Air Starvation


If the MEA is starved of oxidant, then rather than oxygen reduction sustaining the current flowing through the cathode, hydrogen evolution takes place (Reaction (xi)):


(xi)

The anode reaction and the anode potential remain unchanged and the PEMFC acts as a hydrogen pump. The Cell potential at 500 mA cm-2 is typically close to -0.1 V since both hydrogen electrooxidation and hydrogen evolution are facile on Pt-based electrocatalysts. Such a condition does raise reliability concerns since hydrogen is produced in the cathode chamber and significant heat is generated in the MEA With a lack of water produced at the cathode, this can damage the membrane.

Fuel Starvation


However, more dramatic effects arise if the MEA is starved of fuel. Studies at Ballard Power Systems (60) have shown that the electrochemical processes in this case can be monitored by follow-ing the Cell potential with time and by gas chromatographic analysis of the anode outlet gas for oxygen and CO2 production. Figure 20 shows the typical response from an MEA with a carbon-supported PtRu anode. In this case the cathode reaction and the cathode potential remain unchanged and the change in Cell potential reflects the anode potential. As the anode is starved of fuel the anode potential increases until water electrolysis occurs (Reaction (xii)):


(xii)

Figure 20 shows that at 40 mA cm-2 this corresponds to a Cell potential of -0.6 V. After falling quickly to -0.6 V, the Cell potential gradually decays to -0.9 V corresponding to an anode potential of +1.4 to +1.7 V (vs. RHE). That the resulting water electrolysis plateau in Figure 20 is due to Reaction (xii) was confirmed by the high current efficiency for oxygen evolution. At such a high anode potential, a small degree of carbon corrosion (Reaction (xiii)) accompanies water electrolysis as shown in Figure 20 by the small quantity of CO2 detected in the anode outlet gas:


(xiii)

If the supply of water to the anode electrocatalyst runs out or if the Pt-based anode electrocatalyst is deactivated for water electrolysis, the Cell potential moves to more negative potentials. Figure 20 shows a carbon corrosion plateau as the cell potential reaches —1.4 V, corresponding to an anode potential of +2.1 V (vs. RHE). At this stage the current is sustained increasingly by carbon corrosion rather than by water electrolysis, as shown in Figure 20 by the increase in the rate of CO2 production and the decrease in the rate of oxygen production in the anode chamber. Significant irreversible damage to the MEA occurs since the anode electrocatalyst carbon support, the anode gas diffusion substrate and the anode flow field plate (if carbon-based) all corrode. After a few minutes in this condition the MEA is normally electrically shorted due to the significant amount of heat generated in the membrane.

Fig 20.

The change in the cell potential at 40 mA cm and the rate of oxygen and CO2 produced at an MEA during Cell reversal due to fuel starvation. The anode is preparedfrom 20 wt. % Pt, 10 wt.% Ru/Shawinigan at a loading of 0.25 mg Pt cm-2. The cathode uses Pt black at a loading of 4.0 mg Pt cm-2. The MEAs are based on catalysed substrates bonded to Nafion NE-1135 membrane. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here H is operating with nitrogen at the anode (replicating fuel starvation)/air, 200/200 kPa, 1.5/2 stoichiometry. The oxygen and CO2 were detected in the anode outlet gas using gas chromatography

The change in the cell potential at 40 mA cm and the rate of oxygen and CO2 produced at an MEA during Cell reversal due to fuel starvation. The anode is preparedfrom 20 wt. % Pt, 10 wt.% Ru/Shawinigan at a loading of 0.25 mg Pt cm-2. The cathode uses Pt black at a loading of 4.0 mg Pt cm-2. The MEAs are based on catalysed substrates bonded to Nafion NE-1135 membrane. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here H is operating with nitrogen at the anode (replicating fuel starvation)/air, 200/200 kPa, 1.5/2 stoichiometry. The oxygen and CO2 were detected in the anode outlet gas using gas chromatography

Water Electrolysis Electrocatalysts


The effects of Cell reversal can be mitigated by using diodes to carry the stack current past each MEA or by monitoring the Cell potential of each MEA and shutting the stack down if a Cell potential is low. However, such measures are complex and expensive to implement. A more practical approach to protect the carbon-based components at the anode is to incorporate an additional electrocatalyst in the anode catalyst layer to sustain the rate of water electrolysis. This approach has been jointly investigated by Johnson Matthey and Ballard Power Systems (60).

Figure 21 shows the performance in Ballard stack hardware of three different cell-reversal-tolerant (i.e. water electrolysis) electrocatalysts prepared by Johnson Matthey. In this case the MEAs have received significant prior cell-reversal periods to drive the anodes to the limit of their tolerance. The impact that inclusion of a water electrolysis electrocatalyst has on the ability of the anode to sustain water electrolysis is evident. At the standard 40 wt.% Pt, 20 wt.% Ru/Shawinigan carbon black-based anode the water electrolysis plateau is so short it is difficult to detect. Only by using a cell-reversal-tolerant electrocatalyst in the anode are water electrolysis plateaux evident in Figure 21.

Fig 21.

The change in Cell potential at 200 mA cm during Cell reversal due to fuel starvation at a Pt0.5Ru0.5 alloy anode containing different water electrolysis electrocatalysts. The MEAs were first subjected to Cell reversal at 200 mA cm-2 for 5 minutes then for 3 sets of 30 pulses (10 seconds on, 10 seconds off) with 15 minute recovery periods on hydrogen/air and a final overnight recovery period. The anodes are as described in Fig. 20 but at a loading of 0.1 mg Pt cm-2. The water electrolysis electrocatalysts were prepared from 20 wt. % RuO2 /Shawinigan, 20 wt. % RuO2. TiO2 (Ru0.9Ti0.1)/Shawinigan or 20 wt.% RuO2, IrO2 (Ru0.9lr0.1)/Shawinigan and added at an anode loading of 0.1 mg Ru cm-2. The cathode is as described in Fig. 9. The MEAs are as described in Fig. 20. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 75°C using nitrogen at the anode (replicating fuel starvation)/air, 160/160 kPa. with stoichiometries of 1.3/1.5

The change in Cell potential at 200 mA cm during Cell reversal due to fuel starvation at a Pt0.5Ru0.5 alloy anode containing different water electrolysis electrocatalysts. The MEAs were first subjected to Cell reversal at 200 mA cm-2 for 5 minutes then for 3 sets of 30 pulses (10 seconds on, 10 seconds off) with 15 minute recovery periods on hydrogen/air and a final overnight recovery period. The anodes are as described in Fig. 20 but at a loading of 0.1 mg Pt cm-2. The water electrolysis electrocatalysts were prepared from 20 wt. % RuO2 /Shawinigan, 20 wt. % RuO2. TiO2 (Ru0.9Ti0.1)/Shawinigan or 20 wt.% RuO2, IrO2 (Ru0.9lr0.1)/Shawinigan and added at an anode loading of 0.1 mg Ru cm-2. The cathode is as described in Fig. 9. The MEAs are as described in Fig. 20. The Ballard Mark 5E single Cell operating conditions are as described in Fig. 9, except that here it is operating at 75°C using nitrogen at the anode (replicating fuel starvation)/air, 160/160 kPa. with stoichiometries of 1.3/1.5

As a result, the degree of carbon corrosion is significantly reduced by the cell-reversal-tolerant electrocatalysts. The typical PtRu anode electrocatalyst layer shows carbon corrosion after only 15 seconds of operation under the final cell-reversal conditions of Figure 21. This is extended to 4.5 minutes by adding RuO2 to the electrocatalyst layer, to 24 minutes using RuO2/TiO2 and to 48 minutes with RuO2/IrO2 in the anode layer. While the carbon corrosion plateaux are not clear in Figure 21, the relative rate of production of CO2 and oxygen confirmed the time-scale required for significant levels of carbon corrosion.

After this extended cell-reversal testing, returning the more cell-reversal-tolerant anodes (optimised for PEMFC operation) to normal fuel Cell operation indicated that there was negligible loss in the MKA power output. This contrasts with MEA failure in the absence of the cell-reversal-tolerant electrocatalyst. In addition, incorporation of the water electrolysis electrocatalyst in the favoured anode structures adds very little extra pgm to the MEA (< 0.1 mg cm-2). Thus, this safeguard to the stack durability comes at little additional MEA cost.

Conclusions


At the current MEA operating temperature of 800C there are significant reformate poisoning issues at the anode. The most tolerant electrocatalyst is still PtRu alloy. While unalloyed PtRu does promote CO electrooxidation above +0.25 V (vs. RHE) the alloying process is important for maximising the CO tolerance below +0.20 V (vs. RHE). This is probably because it weakens the Pt–CO bond strength. With the Johnson Matthey manufacturing process the Ru content can be reduced from Pt0.5Ru0.5 to Pt0.7Ru0.3 without any loss in anode performance, presumably because there is sufficient Ru incorporated into the Pt lattice.

To reailse the highest reformate tolerance from PtRu it is important to maximise the in situ anode EPSA and to produce thin anode layers. The high in situ EPSA increases the number of poison-free PtRu sites and minimises CO production from the RWGSR Thin layers are produced by using high loadings of 40 wt.% Pt, 20 wt.% Ru with carbon-supported materials, and high in situ anode EPSAs from good PtRu ink preparations. Unsupported PtRu black functions well in PEMFC applications that employ high anode Pt loadings.

Efficient anodes operate at electrode potentials below +0.2 V (vs. RHE). At such anode potentials the PtRu alloy provides CO and CO2 tolerance by weakening the ‘Pt-CO’ bond strength. Only at much higher anode potentials does water adsorption on Ru promote significant levels of CO electrooxidation to CO2. The search for electrocatalysts capable of electrooxidising CO at much lower anode potentials has led to PtMo alloys that are capable of promotion below +0.1 V (vs. RHE). Consequently, in PEMFC testing above 10 ppm CO, carbon-supported Pt0.75Mo0.25 alloy shows an increased level of CO tolerance com pared to PtRu.

However, the CO2 tolerance of the Pt0.75Mo0.25 alloy is very poor. The electrocatalyst also promotes the electroreduction of CO2 to a ‘Pt–CO’ poison. The result is much poorer reformate tolerance compared to PtRu using typical anode feeds containing CO and CO2. application of a membrane purifier to significantly reduce the CO2 content and raising the MEA operating temperature to 1600C might allow the improved CO tolerance of the Pt0.75Mo0.25 alloy to be realised.

Even with PtRu in a well-designed anode layer there are losses in Cell potential, with reformate operation, that are not recovered. The air bleed technique must be employed to introduce ≤ 2% air into the fuel stream to catalyse the oxidation of CO to CO2. This recovers the Cell potential losses at 100 ppm CO and most of the small loss due to 25% CO2. However, in standard anode designs the air bleed introduces durability issues, due to significant local heat generation within the anode from the catalytic oxidation of CO and the recombination of hydrogen and oxygen. The PtRu electrocatalyst sinters and the membrane eventually pin-holes. To extend the anode lifetime it is necessary to adopt a bilayer anode structure using a pgm-based gas-phase catalytic oxidation layer between the gas diffusion substrate and the PtRu electrocatalyst layer. This moves the heat generation away from the the PtRu electrocatalyst layer and the membrane.

An additional major durability issue arises at the anode if the MEA is starved of fuel. The MEA changes polarity due to Cell reversal. Initially water electrolysis sustains the power output but after some 20 minutes at 200 mA cm-2 the PtRu electrocatalyst is deactivated for water electrolysis and significant rates of carbon corrosion are detected. This quickly destroys the carbon-supported PtRu electrocatalyst, the gas-diffusion substrate and the flow field plate in contact with the anode. However, adding a RuO2/IrO2 water electrolysis electrocatalyst into the anode layer can provide a high degree of Cell reversal tolerance. This sustains the rate of water electrolysis for an additional 40 minutes. Rather than MEA failure after 20 minutes the MEA can be returned to normal PEMFC operation after experiencing 60 minutes under the conditions of Cell reversal due to fuel starvation.

The approach of using a bilayer anode for air bleed operation and of including a water electrolysis electrocatalyst for cell-reversal tolerance can be provided at little additional cost. With continued development of the concepts, the pgm levels used can probably be thrifted to below 0.0.5 mg cm-2.

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Acknowledgements

Part of this paper is based on a keynote lecture by T. R. Ralph at the 17th North American Chemical Society Meeting held at the Westin Coast Harbour Castle Hotel, Toronto, Ontario, Canada, from the 3rd June to 8th June 2001.

The Authors

Tom Ralph is Product Development Manager at the Johnson Matthey Technology Centre, responsible for MEA design. He has been working with PEMFCs since 1991 and with developing all aspects of the MEA: electrocatalyst, electrocatalyst layer, gas diffusion substrate and Solid polymer membrane, and in integrating MEAs into customer hardware.

Martin Hogarth is a Senior Scientist at the Johnson Matthey Technology Centre and has worked in the area of DMFCs since 1992. His main interests are the development of new catalyst materials and high-performance MEAs for DMFCs. More recently his interests have expanded into novel high-temperature and methanol-impermeable membranes for the PEMFC and DMFC, respectively.

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