PLATINUM METALS REVIEW · 2016. 1. 28. · Cabot Vulcan XC72R carbon black. Central to the...

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PLATINUM METALS REVIEW

A quarterl-y surve?, of research on the p la t inum metals and of developments in their application i n industry

VOL. 41 JULY 1997

Contents

Proton Exchange Membrane Fuel Cells By T. R. Ralph

Platinum Increases Hydrogen Photoproduction

Autocatalyst Manufacture in Malaysia By Kala Shanmugam

Optical Oxygen Sensors By Andrew Mills

Platinum Nanorods in Carbon Nanotubes

Automotive Catalytic Pollution Control By R. A. Searles

Platinum 1997

Zeolite-Encapsulated Rhodium Catalysts - The Best of Both Worlds? By J. M. Andersen

Palladium Catalysts in Modern Organic Synthesis ByM. V. Twigg

Abstracts

New Patents

NO. 3

102

113

114

115

127

128

131

132

141

142

148

Communications should be addressed to The Editor, Susan V . Ashton, Platinum Metals Review

Johnson Matthey Public Limited Company, Hatton Garden, London ECl N 8JP

Proton Exchange Membrane Fuel Cells PROGRESS IN COST REDUCTION OF THE KEY COMPONENTS

By T. R. Ralph Johnson Matthey Technology Centre

Proton exchange membrane f u e l cells operating o n hydrogenlair are being considered as high efficiency, low pollution power generators for stationary and transportation applications. There have been many successful demonstrations of this technology in recent years. However, to penetrate these markets the cost of the fuel cell stack must be reduced. Th i s report details the progress made on reductions in the stack cost by lowered plat inum catalyst loadings in the latest stack designs, the development of lower cost membrane electrolytes, the design of alternative bipolarflow field plates, and the introduction of mass production technolou. Despite such advances, there i s still a need for further reductions in the stack cost, through improvements in the performance of the membrane electrode assembly. However, improved stack performance must be demonstrated not only with pure hydrogen fuel but also, moreparticularly, with reformate fuel , where tolerance to poisoning by carbon monoxide and carbon dioxide needs to be improved. Advances that are required in the ancillary sub-systems are also briefly considered here.

As with all fuel cells, the operating principles of the proton exchange membrane fuel cell (PEMFC) are straightforward. The chemical energy present in a fuel, usually hydrogen, and an oxidant, oxygen, is cleanly, quietly and efficiently converted directly into electrical energy. The hydrogen is oxidised at the anode and the oxygen reduced at the cathode of a single cell. The power requirement in fuel cell technology is achieved by combining a number of single cells in series to produce a fuel cell stack, and a number of stack modules are then combined in series to produce the power plant.

The membrane electrode assembly (MEA) is at the heart of the single cell. It is the critical component of the PEMFC, since it is the site of the fuel cell reactions. The MEA, see Figure 1, is less than a millimetre thick and consists of a solid polymer, proton conducting membrane electrolyte, with a layer of platinum-based catalyst and a gas-porous electrode support material on both sides of it, forming the anode and cathode of the cell. The membrane electrolyte is typically bonded to the electrodes by

hot pressing at a temperature above the glass transition temperature of the membrane. Membranes currently employed are based on perfluorinated sulfonic acid (for example the Nafione range of materials produced by Du Pont) which must be kept in a fully hydrated state during fuel cell operation in order to realise the maximum proton conductivity. This requirement currently limits the temperature of operation of the PEMFC to below 1 OO'C, at mod- erate pressures.

Hydrogen fuel, supplied either in a pure form fiom high pressure vessels or metal-hydride stor- age materials, or diluted with carbon dioxide and other components from a fuel reformer, and the oxidant, usually air, are supplied to the MEA from the flow field plates, see Figure 1. To-date most precommercial stacks have used pure graphite flow field plates with channels machined into their surfaces. The raised land- ing areas make electrical connection with the electrode support and the channels provide gas access to, and water removal from, the MEA. In most cases the flow field plates are designed

Platinum Metals Rev., 1997,41, (3), 102-1 13 102

Fig. 1 The fuel cell stack, showing the structure of a typical single eeU. The MEAs are located between earbon flow field platea which feed humidi6ed fuel (Hz or reformate) and air to the MEA. The reactants permeate through the carbon based electrode support structures into the platinum catalyst layers where they react to generate electricity, heat and pure water. The heat is removed via eooling plates, although since the current stacks operate at 80 to 90"C, it cannot be usefully employed. Sufficient MEAs are combined in series to produce stacks which deliver 1 to 25 kW. The stacks are then combined in series to give enough power to drive a ear (50 kW), a bus (200 kW) or to provide stationary power (< 1 M W )

Motor

Bipolar flow field plate 3mm

- Anode catalyst

Membrane electrolyte

recirculation

air(H20)g

Cathode catalyst layer 25pm

Heat

Electrode suppwt 2 5 0 p

to be bipolar, where one side of the plate is the anode of one cell, and the other side is the cathode of the adjacent single cell. The cells are con- nected in series via the plates. The gases pass from the bipolar flow field plate through the electrode support material to the catalyst layer where they react. Hydrogen is consumed at the anode, yielding electrons to the anode and producing hydrogen ions:

Hz = 2H' + 2e- E" = 0.OOOV (i)

The hydrogen ions are conducted through the proton conducting membrane electrolyte, and are combined at the cathode with electrons and the oxygen &om the air supply to generate water:

(ii)

Thus, the PEMFC combines hydrogen and oxygen to generate electrical power. Pure water and heat are the only by-products:

%02 + 2H' + 2e- = HzO E" = 1.234 V

Hz + %Oz = Hz0 Eoccll = 1.234 V (iii)

As long as the single cell is supplied with fuel and oxidant it will generate electrical energy in the form of a direct current. Thus, a fuel cell is better regarded as an electrochemical engine,

where fuel is continuously oxidised by air to produce power, rather than as a form of battery, in which energy is stored.

Cost, Performance and Operating Lifetime Requirements

In addition to space, military and low power (< 1 kW) applications (l), the PEMFC is being considered for two major markets. One essential feature of the PEMFC - its ability to generate much higher power densities at lower tem- peratures than other fuel cell types -has opened up the potential of fuel cell technology to provide reliable, low cost power for a new generation of non-polluting automotive engines. It is also being considered for more traditional stationary power generation, for distribution or on- site requirements, such as for hotels, hospitals and the home. The drive to reduce pollution levels and to increase the fuel efficiency of the internal combustion engine and of conventional power stations is aiding the adoption of low or zero emission fuel cell technology, and indeed there have now been a significant number of successful demonstration programmes worldwide. But, for this technology to become widely

Platinum Metals Rev., 1997,41, (3) 103

commercialised cost and, to a lesser extent, performance need to be improved.

In this regard the automotive application is more demanding. The size, weight and cost of the stack are all key issues. Stack power density and energy density targets of 1 .O kW 1-l and 1.0 kW kg-’, respectively, a t a cost of less than U.S.$50 kW’ have been quoted for passenger cars (2,3). To meet this cost target requires both a reduction in material costs and the development of high volume, low cost manufacturing processes for the stack components. Increased power density, achieved by operating at higher current densities while maintaining cell potentials at reasonable levels, will, besides reducing the size and weight of the stack, also have the effect of lowering the stack material costs.

For stationary power applications the size and weight of the stack are generally less of a con- cern and the cost target is also less demanding at around U.S.$500 kW’ (3). Improved energy efficiencies, that is higher cell potentials, are more important in this application.

While cost reductions and performance improvements in the PEMFC stack are important, they must be maintained over the operating lifetimes. In this respect, the transportation market is less exacting, with operating lifetimes of some 5000 hours being adequate for a passenger car, compared with operating lifetimes of at least 40,000 hours demanded by stationary power plants.

Progress in Reducing Stack Costs Over the past five years considerable progress

has been made in lowering the cost of PEMFC stacks. This has been achieved largely by reducing the costs of the three critical cell components: the electrodes, the membrane electrolyte and the flow field plate. Some groups are also addressing the cost reductions available from volume manufacture of the stack components.

Lower Platinum Loadings on Electrodes The leading developer of the PEMFC is the

Canadian company Ballard Power Systems. Since the early 1990s several hundred stacks have been produced for a wide range of demon-

stration programmes. Most of these stacks have been rated at a maximum power output of 5 kW (the Mark V type). The loading of platinum black catalyst on the electrodes has typically been 4.0 mg cm-’ (4). At operating cell potentials of 0.75 to 0.65 V, the platinum black loading corresponds to 26.7 down to 16.0 g Pt k W ‘ of power output. A comparison with the tar- geted stack costs shows quite clearly that these platinum loadings have to be reduced if the PEMFC is to be developed as a commercially viable product.

When pure hydrogen is used as the fuel almost all of the performance losses occur at the cathode. This is because of the slower kinetics of oxygen reduction, Equation (ii). Despite many studies of this reaction in the PEh4FC and other types of fuel cell, platinum-based electrocata- lysts remain the only practical catalyst material, since they combine both activity and stability in the fuel cell environment. At the anode, losses in cell potential are usually small (less than 50 mV at 2.0 A cm-2) due to the extremely facile nature of the hydrogen oxidation reaction, Equation (i), on platinum-based catalysts.

Many groups have recently demonstrated very high cell performances from PEMFCs operating on hydrogedair at substantially lower platinum cathode loadings than the 4 mg cm-’ typical of the early prototype stacks. This has been achieved by the successful exploitation of higher surface area carbon supported catalysts, such as 20 weight per cent platinum supported on Cabot Vulcan XC72R carbon black. Central to the development of these electrodes has been the need to impregnate the active catalyst layer with a soluble form of the polymer electrolyte. This significantly increases the protonic contact between the electrolyte and the surface of the platinum catalyst within the porous carbon structure, thus permitting lower platinum loadings on the electrodes to be utilised more effectively. Leading investigators have been the groups at Los Alamos National Laboratories (LANL) (5, 6) and Texas A&M University (TAMU) (7), although similar approaches to electrode design have subsequently been reported by other academic groups around the world (8-10).


Two basic approaches to electrodes have been developed by these groups.

At LANL thin film catalyst layers comprising the carbon supported catalyst mixed with the soluble form of the electrolyte (5 weight per cent Nafion" EW1100 in propan-2-01 and water) are deposited onto the membrane electrolyte. A microporous gas diffusion backing layer, employing a carbon cloth based electrode sub- strate, is compressed against the catalyst layer in the single cell to provide efficient removal of the product water from the cathode.

At TAMU the catalyst layer is first deposited onto the electrode support s t c u m e &om a mix- ture of carbon supported catalyst and polyte- trafluoroethylene (PTFE), which binds the layer together. Subsequently, the catalyst layer surface is impregnated with the membrane electrolyte solution.

Using these approaches LANL and TAMU have reported performances comparable to platinum black based cathodes, but with cathode catalyst loadings as low as 0.12 mg Pt cm-' and 0.05 mg Pt cm-2, respectively. Assuming a neg- ligible anode loading (reasonable for pure hydrogen operation) the noble metal usage is reduced, from 13.4 down to 8.0 g Pt kW' for platinum black cathodes to less than 0.4 g Pt kW-' of power output at operating cell potentials of 0.75 to 0.65 V. Based on a platinum price of about U.S.312 g-', this represents a cost of less than U.S.35 k W ' for the noble metal, which is compatible with all PEMFC applications.

Other methods of achieving high performance cathodes with low platinum loadings have been demonstrated. Notable is work by PSI Technol- ogy using cathodes formed from uncatalysed carbon black impregnated with a soluble form of the membrane electrolyte and then electro- deposited with platinum at loadings as low as 0.05 mg cm-2 (1 1). The aim of this approach was to deposit platinum only in regions of the electrode which were both in electronic contact with the carbon support and protonic contact with the membrane electrolyte, again ensuring efficient use of very low platinum loadings.

Other groups have deposited the platinum catalyst directly into the polymer membrane

electrolyte by chemical reduction of platinum salt solutions, see for instance (1 2). Loadings deposited in this way tend to be higher than with other methods (> 1 mg Pt cm-'). It appears that platinum can be isolated within the membrane in regions where it cannot be contacted electronically. This, coupled with the much lower platinum dispersions achieved on the membrane, has meant that the effective catalyst surface areas are inferior to those in electrodes employing carbon supported platinum catalysts.

Electrodes Manufactured in Volume for Advanced Stack Designs

The above studies have largely been at the lab- oratory scale. Electrodes have been fabricated using manual techniques which do not neces- sarily lend themselves to scale-up to high volume production and MEAs have been evaluated in small (< 50 cm2 active area) single cells. Cell performances have often been measured in cells functioning at high gas stoichiometries. This cannot be justified economically and performances can be raised on air operation, particularly at higher current densities ( 1 3). In addition, there have been few evaluations of operating lifetime data. Both LANL and TAMU are now, however, beginning to address these factors. LANL are examining the principle of ink jet printing the electrodes by modifying a chart recorder (1 4) and TAMU have investigated spraying and rolling the catalyst layers (1 5). Performances are being evaluated in larger single cells (50 to 100 cmz active area). The suc- cess in reducing the platinum loadings has led other academic and industrial groups to focus on mass production techniques suitable for electrodes and MEAs. However, testing has mostly been restricted to single cells, often with only 5 cm2 active area, see for example (1 6).

One major target of a joint programme between Ballard Power Systems and Johnson Matthey has been the development, for full size PEMFC stack hardware, of low platinum loading electrodes prepared in high volume. An electrode manufacturing process based on screen printing has been developed (17) and a small pilot plant facility has been established at Johnson

Platinum Metals Rev., 1997, 41, (3) 105

+ Unsupported Pt black MEA, e omg Ptcm-’ - LOW catalyst loading MEA. 062mg Ptcri’

Internal humidification

0.24 I 0 203 400 600 800 1000 1200 1400

CURRENT DENSITY, mA

Fig. 2 Cell potentiallcurrent density plots for high loading unsupported platinum black and low platinum loading carbon supported catalyst based MEAs. The membrane electrolyte was the experimental Dow XUS- 13204.10 material

Matthey. The plant currently produces around 50,000 electrodes per annum, but has the potential for scaling-up as demand rises. Electrodes have been produced from inks comprising carbon supported platinum catalysts and solutions of the polymer electrolyte. For high volume production it was deemed unsuitable to use the organic solutions of Nafion“ employed by LANL, TAMU and others to impregnate the catalyst (5-1 1). Besides not wanting to handle large vol- umes of organic solvents in a production environment, there is a possibility of interaction between the solvents and the high surface area platinum catalysts, which could result in the production of organic by-products in the inks and, with high solvent concentrations, possibly combustion. Thus, a method was developed to remove the alcohols from 5 weight per cent Nafion’ solution to give an essentially ‘aqueous’ Nafion” polymer solution (18), which has been used for routine manufacture of electrodes with catalyst loadings of 0.1 to 1 .O mg Pt cm-’.

Comparison of Electrodes Figure 2 shows the “beginning of life” cell

potential versus current density plot of a low catalyst loading MEA (0.62 mg Pt cm-’), compared with the higher loading unsupported platinum black MEA (8.0 mg Pt ern-') in a Ballard Mark V single cell of 240.2 cmz active area. The performances are comparable. The cathode comprises 40 weight per cent platinum on

Vulcan XC72R at a loading of 0.37 mg Pt cm-’ and the anode 20 weight per cent platinum-10 weight per cent ruthenium (Pt/Ru) on Vulcan XC72R at a loading of 0.25 mg Pt cm-’. At a cell potential of 0.75 to 0.65 V the MEA gen- erates 0.4 to 0.8 A ern-', equivalent to 0.3 to 0.5 W cm-’. This translates to a much reduced catalyst requirement of 2.0 down to 1.2 g Pt kW’ compared to 26.7 to 16.0 g Pt k W 1 for the platinum black MEA. This reduced loading shows considerable progress towards a platinum catalyst cost compatible with the stack cost targets for stationary and vehicle applications.

The “beginning of life” performances of the low platinum loading MEAs have been retained in the advanced Ballard stack technology being developed for submarine, bus, automotive and utility applications. With pure hydrogen fuel and air as oxidant, stable long term performance has been demonstrated in 4 and 8 cell sub-stacks and in full stacks. Figure 3 shows the performance of Dow membrane MEAs with anode and cathode catalyst loadings of 0.25 mg Pt cm-’ and 0.50 mg Pt cm-’, respectively, in an 8 cell stack representative of the Phase 2 bus stack hardware. This hardware is being developed for the forthcoming precommercial transit bus fleet trials in Chicago and Vancouver. Transit buses offer an early market opportunity for fuel cell power plants as cost targets are less demanding than for small vehicles and there is space to store sufEcient hydrogen fuel on-board to give a range

Platinum Metals Rev., 1997, 41, ( 3 ) 106

z W

P z W

P

6 Fig. 3 Durability of low platinum loading MEAs in an 8 cell Ballard stack, wing Dow XUS-13204.10 membrane > electrolyte. The stack J., construction is typical of the f hardware employed in the f) precommercial transit buses being supplied to Chicago and Vancouver

g a 4 z lJ

3

- Stack potential a t 0.646 A c m 2 t Stack potential at 1.076 A cm-*

I 200 400 600 BOO 1000 1200

TIME, hours

comparable to current diesel-fuelled engines. After 1000 hours at 0.646 A ern-', with 5-hour excursions to 1.076 A mi*, the degradation rate of below 4 pV h-' was well within target levels.

Reproducible Cell Performance Particularly important in volume manufacture

is reproducible performance. The low catalyst loading MEAS exhibit excellent cell to cell repro- ducibility in full stacks. In Figure 4 consistent

cell potentials produced by the individual cells in an 80 cell stack built for an air-independent submarine propulsion application are shown. The MEAs comprise cathodes with 0.6 mg Pt ern-', anodes with 0.25 mg Pt cm-' and Nafion" 1 17 membrane. The minimal variation in cell performance achieved over a range of current densities represents the highest level of cell to cell consistency that has recently been reported in the literature for PEMFC stacks, see for

0.2 4 I 0 10 20 30 4 0 50 60 70 80

NUMBER OF CELL

Fig. 4 Individual cell potentials in a full 80 cell stack for submarine propulsion. The low platinum loading MEAS contain Nafion" 117 membrane electrolyte. The high consistency of the cell potentials is a reflection of the control of operating conditions through the stack and the ability of the pilot plant at Johnson Matthey to manufacture reproducible electrodes. Uniformity in cell performance is of prime importance in the development of PEMFC technology


z W

P

z W

P

z W

P

example the 50 cells in (19). This highlights the excellent degree of consistency achieved in MFL4 production and the efficient control of operating conditions throughout the stack.

Based on the large volume of data now available, it is clear that achieving economical platinum loadings on electrodes is no longer a bar- rier to the commercialisation of PEMFCs. The reduced metal loadings are clearly compatible with the cost targets for stationary power generation and are close to the levels required for passenger cars. Indeed, rather than pursuing further cost reductions by lowering the electrode platinum loadings in stacks to the lowest levels studied in small cells (0.05 mg Pt cm-2), greater attention should be paid to improving cell performance at slightly higher, total MEA loadings (about 0.2 to 0.35 mg Pt cm-’ for automotive applications, and 0.5 to 1.0 mg Pt cm-* for stationary power plants). This would reduce the cost of all other stack materials and improve stack efficiencies.

Improved cell performance may be achieved by developing electrode structures in which the platinum catalyst is still more efficiently utilised. Even with the high performances discussed here, there are still performance losses due to oxygen and proton diffusion limitations in the electrode structures. Modelling and practical studies have indicated that even in the low platinum loading electrodes, only a limited thickness of the catalyst layer is being utilised. LANL have estimated this to be as little as 5 p in their structures (20) and TAMU estimate a 10 pm thick active layer for their electrodes (2 1). While this suggests there is scope for maintaining performances as catalyst loadings are reduced - to the order of 0.1 mg Pt cm-’ and below - it also indicates enhanced performance is attainable.

Low Platinum Loading Anodes for Reformate Operation

There is no great technical challenge in lowering platinum loadings on anodes operating with pure hydrogen fuel; anode catalyst loadings as low as 0.025 mg Pt cm-’ are sufficient for high current density operation (22). However, for stationary power plant and light

duty vehicle applications, the anode fuel will almost certainly be impure hydrogen (reformate) produced via reforming carbon-containing fuels, such as natural gas, methanol, petrol or diesel. Achieving satisfactory performance and durability with these fuels is much more difficult, because the anode is poisoned by trace levels of carbon monoxide present in the reformate. Carbon dioxide poisoning and dilution effects also reduce performance. But, even using impure hydrogen and with lowered anode platinum loadings the tolerance to poisoning has been increased. Unsupported platinum black catalyst has been replaced by higher surface area and more poison tolerant PtJRu catalyst supported on Vulcan XC72R, and the catalyst has been impregnated with the soluble form of the polymer electrolyte to increase the number of active catalyst sites in the anode.

The favoured catalyst for tolerance to both carbon monoxide and carbon dioxide is PtJRu at 50:50 atomic per cent (23-25). Wilkinson and colleagues have confirmed that with PtJRu catalysts a higher catalyst dispersion and an increased protonic contact between the platinum and the membrane electrolyte improves the poison tolerance by increasing the number of non-poisoned catalyst sites within the anode (23). The best results have come from Iwase and Kawatsu, who showed that with Pt/Ru anodes containing 0.4 mg Pt cm-’ there is com- plete tolerance to 100 ppm carbon monoxide in hydrogen, at 80°C and 7 psig (24).

The long term performance of a 4 cell Ballard sub-stack, with Johnson MattheyBallard Power Systems low platinum loading MEAs is shown in Figure 5 . The MEAs contain PtJRu at 0.25 mg Pt cm-’ in the anode, Pt at 0.55 mg cm ’ on the cathode and Nafion” 117 membrane electrolyte. The stack is operating at a constant current density of 0.4 A cm-’ under typical stationary power plant testing conditions. The oxidant is air and the reformate fuel contains 70 per cent hydrogen, 25 per cent carbon dioxide and smaller quantities of other contami- nants, including 10 ppm carbon monoxide. A 2 per cent air bleed was also directly passed into the anode chamber to promote gas-phase


0 . 7

i 5

--Q--- Cell 1 Reformate - Cell 1 Hydrogen - Cell 2 Hydrogen - -O- - - Cell 2 Reformate -A- Cell 3 Hydrogen --A- Cell 3 Reformate

---0-- Cell 4 Reformate --t Cell 4 Hydrogen

0.4 4 I 0 500 1000 1 5 0 0 2000 2500 3000

OPERATION ON REFORMATE FUEL, hours

Fig. 5 Durability of low platinum loading MEAs in a 4 cell stack employing Nafione 117 membrane electrolyte. The stack is operating at 0.4 A cm" under stationary power plant conditions with air as oxidant and reformate fuel with a 2 per cent air bleed

oxidation of carbon monoxide to carbon dioxide (26). Compared with pure hydrogen operation, the loss in performance at a current density of 0.4 A cm-' is less than 30 mV. This loss is due principally to the effect of diluting the fuel to 70 per cent. By thus combining the effects of the intrinsically more poison resistant Pt/Ru anode electrocatalyst and a low level air bleed, tolerance to carbon monoxide and carbon dioxide poisoning has been achieved. Significantly, after 3000 hours of continuous operation an acceptable decay rate of only 4 pV h-' was measured at this current density, see Figure 5. This durability has now been extended to over 5000 hours of reformate operation, demonstrating that stable performance can be achieved with reformate fuel using a WRu catalyst, even in the presence of an air bleed.

At current densities much higher than 0.4 A cm-' there are performance losses due to the effects of carbon dioxide, which are not overcome, principally because the poisoning does not respond to the air bleed technique. A typical loss is 50 mV at 1 A cm-'. This suggests that while these Pt/Ru anode loadings may be acceptable for stationary applications where the reformate fuel is cleaned by the water-gas shift

reaction and selective oxidation, a more poison tolerant anode catalyst may be required for automotive use to achieve high power densities at the required platinum loadings.

In both applications, an anode catalyst that is much superior to Pt/Ru would allow lower anode platinum loadings and put less demand on the fuel processing system. Ideally, removing or sim- plifying the selective oxidation reactor would benefit the overall system design, but anode tolerance would need to be increased to a level close to 1000s of ppm of carbon monoxide. Such improved anode catalyst tolerance could also be used to eliminate the need for the air bleed, which adds. to the complexity of the system and raises concerns over safety, heat management and losses in efficiency because of the possibility of hydrogen and oxygen recombining.

Proton Exchange Membrane Electrolytes

The most widely used membrane electrolytes are based on Nafion@ type since these are known to offer a long lifetime in aggressive environ- ments (50,000 hours). Nafion' is a perfluorinated sulfonic acid polymer consisting of a flu- oropolymer backbone, similar to Teflon, to

Plazinurn Metals Rew., 1997,41, (3) 109

z W

P

z W

P

which sulfonic acid groups are chemically bonded (27). The sulfonic acid groups are fixed in position, but the associated protons are free to provide conduction. These membranes are commercially available for around U.S.$ 700 m-’. Figure 6 shows the performances of a range of Nafion‘ membranes and of the experimental Dow XUS-13204.10 membrane in a Ballard Mark V single cell. The electrodes, from Johnson Mattheymallard Power Systems, have low platinum loadings. The membrane resistance controls the single cell performance in the pseudo- linear region of the plot. The thinner Nafion’ membranes give enhanced performance particularly at higher current densities (Nafion” 1 12 > Nafion’ 1 15 > Nafion’ 1 1 7) although the thinner membranes are less durable and more diacult to handle in mass production. The Dow membrane is close to the thickness of Nafion* 115, but gives superior performance due to a lower equivalent weight (defined as: gram of polymer/mole of sulfonic acid group).

Based on the superior performances of the Na60nY 1 12 and Dow membranes the electrolyte costs translate to about U.S.$ 135 k W ‘ at 0.65 V and U.S.$230 kW’ at 0.75 V, which is closer to the operating potential of a stationary power plant. This is perceived to be too costly, particularly for widespread application of PEMFCs in passenger cars.

It has been quoted, however, that with an increase in demand of two orders of magnitude - a market of 1 million cars per year - membrane costs could be reduced by an order of magnitude (2). But, from a commercial view- point, it is likely that a range of products and suppliers will be necessary to drive the costs down, particularly to the levels required for automotive applications.

It is also likely that different membrane electrolytes will be employed in automotive and stationary power plants. For stationary application, where 40,000 hours of operating lifetime is required but high power densities are less important, a thicker more durable membrane may be used, for instance a Nafion’ type membrane which, with cost reductions from volume demand and increased competition, could be

compatible with the cost target for this use, particularly with improved performance. In contrast, a thinner higher power density membrane, which may be less durable than the perfluorinated sulfonic acid membranes but have an operating lifetime of 5,000 hours, will probably be used in passenger cars. Steck has reviewed studies on preparing intrinsically lower cost membranes, many of which are hydrocarbon based or partially fluorinated (27). Most do not have the necessary stability or the low specific resistance shown by NafionR membranes.

New Ballard and Other Membranes Recently Ballard Advanced Materials have

developed a membrane material that is not fully fluorinated and which may be suitable for automotive applications (3, 27). The membrane shows at least equivalent “beginning of life” performance to the Dow XUS-13204.10 and Nafion’ 112 membranes in a Ballard Mark V single cell, Figure 6, (27). The Ballard membrane has demonstrated this performance for over 4500 hours - almost suficient for an automotive application (3). Since the preparation of this membrane is cheaper and since it produces more quantitative yields of partially fluorinated polymer, it is anticipated that with volume demand the membrane costs will be reduced to as little as U.S.$50 m-’. This translates to U.S.$ 10 kW-’ at 0.65 V and U.S.$ 17 k W ’ at 0.75 V with current performance levels, based on the low projected costs with volume demand.

In an alternative approach to preparing electrolytes for a wide range of applications, W. L. Gore & Associates have produced low platinum loading electrodes and MEAs based on very thin (5 to 40 pm) composite membranes, by impreg- nating a PTFE support with perfluorinated sulfonic acid membrane electrolyte solution. The PTFE support is claimed to provide mechanical strength and dimensional stability in the thin membranes (28). Studies at LANL have shown that these Gore-SelectTM membranes give high performance both with LANL “thin film” electrodes and Gore’s own cathode, in small (5 cmz active area) single cells operating on hydrogen and air (28). The performance is not, however,

Platinum Metals Rm., 1997,41, (3) 110

1 0

> 0.8

i Q z W

0 06

-I 1 W U

0 4

0 2

t Nafion 117 178pm nominally -s- Nation 115,127 pm nominally

I + Nation 112, 51 prn nominally -e- Dew, 125pm nominally

X EAM3G. taken from (27)

HI. air, 30 30psig, 15 20stoichiometry T C ~ I at 80-C

Internal humiditiutim

1 200 400 600 800 1000 1200 1.

CURRENT DENSITY, mA cn i ’

0

Fig. 6 Cell potentidcurrent density plots for a range of membrane electrolytes showing the importance of the membrane thickness and membrane type on performance

as high as projected for a homogeneous Nafione membrane of similar thickness, since the specific resistance of the Gore membranes is higher. At 0.75 to 0.65 V, power densities of 0.3 to 0.5 W cm-’ are generated, similar to the power density produced by existing precommercial stacks. Potential cost savings are still available by using these thinner membrane materials. A material cost reduction to around U.S.$ 100 m-’ for a 20 pm thick membrane, based on the list price of the membrane electrolyte solution for the chloroalkali industry, has been quoted (29).

Recently, the performance of Gore MEAs was examined in a 50 cell stack manufactured by Energy Partners ( 19). The lower power density than observed in small single cells (0.24 W cm-’ at 0.6 V) was limited by a few cells. This was attributed to difficulty in controlling the water content in the MEAs resulting in flooding and gas diffusion limitations at the cathode.

Flow Field Plates The bipolar plate is the third high-cost com-

ponent in the fuel cell stack. The machined graphite flow field plates currently used are

estimated to account for as much as 60 per cent of present stack costs (29). As the plates are the heaviest and thickest stack components - and there are over two plates per MEA when addi- tional cooling and humidification plates are considered - their replacement offers great potential for reductions in stack size and weight. Two main approaches to fabricating flow field plates are being examined: thin metal plates and composite carbon plates; but there is little infor- mation in the literature as much of the work is industry led. While both approaches could lead to cheaper and lighter plates, there are problems with maintaining low specific resistances in alternative materials. This will require much thinner plates to minimise ohmic potential losses, particularly for an automotive application.

Stainless steel plates have been examined by ECN (30) and LANL (14), and LANL have also investigated metal meshes; in general, cell potentials were lower due to higher contact resistances probably as a result of surface oxide films. LANL also noted that the cost of machining the metal plates was prohibitively high (1 4). Several com- panies are investigating the use of thin metallic


plates fabricated fkom a low cost base metal with a thin protective coating to provide the necessary stability to oxidation in the fuel cell environment. With such substrates it may be possible to press or stamp out the flow fields.

The second approach is to fabricate moulded carbon black-polymer composite flow field plates. International Fuel Cells hold a patent on this concept (31). Thin, flexible graphite sheets with flow fields punched or stencilled in place have also been patented by Ballard (32, 33).

Remaining Technical Challenges and A Look Forward

While there has been considerable progress in recent years in reducing the stack costs, a number of technical challenges remain. Improvements in performance are necessary to increase cell efficiency and power density at total MEA loadings of < 0.35 mg Pt cm-' for passenger cars and slightly higher for stationary power plants. At the cathode, improved oxygen reduction catalysts and improved cathode designs to utilise the platinum catalyst more fully are desirable. At the anode, besides greater utilisation of the catalyst in improved electrode designs, greater intrinsic catalyst tolerance to carbon monoxide and carbon dioxide is needed.

The development of membranes which can operate above 100°C would be of significant benefit in reducing carbon monoxide poisoning, improving the electroche@cal kinetics and mass transport processes in the MEA, and allow- ing water removal in the vapour phase. A wider selection of membrane electrolytes from different suppliers is needed to reduce costs, particularly for automotive use. Membranes with resistances lower than those of NafionO 1 12 or Dow XUS-13204.10, and with low gas transfer rates and durability of at least 5000 hours would clearly be beneficial for an automotive stack.

MEAs will have to be manufactured using high volume technology, and cheaper bipolar plate materials need to be developed. The resultant low cost MEAs and flow field plates then have to demonstrate the required operating lifetimes in the advanced stack hardware.

Among the stack ancillaries, the air delivery

sub-system is a major drain on power. Air com- pressors consume 10 to 15 per cent of the fuel cell output (2). Consequently there is a trade- off between improved stack performance at higher air pressures and the power required for compression. A low cost highly efficient air compressor is needed. In fact, many groups are developing electrodes, MEAs and stacks that function at ambient pressure (3), generally employing passive water and thermal management. Although small ambient pressure systems are attractive for low power applications it remains to be seen if compact ambient pressure systems can be developed for major multi-kilo- watt applications.

The recent progress in developing lower cost PEMFC stack components has created confi- dence that the remaining challenges can be successfully overcome. This view is supported by the increased attention being paid to fuelling and fuel infrastructure issues for vehicle applications. While in the longer term the production of hydrogen fuel for PEMFCs from renew- able sources, such as solar energy, allied to the development of more efficient hydrogen stor- age materials is clearly desirable, early commercialisation will require hydrogen from fossil fuel sources. There is a major need to produce high performance, compact, fast start up, respon- sive, and fuel flexible, reformer systems, and Johnson Matthey, besides working to develop lower cost, higher performance catalysts, electrodes and MEAs, have produced a compact (0.25 1) autothermal reformer (Hotspot'") (34). This converts methanol selectively to hydrogen to produce a high output per unit volume - 2.5 kW of fuel cell power per litre of reactor - by a combination of steam reforming and partial oxidation. Heat transfer between the two reactions occurs over microscopic distances within the catalyst, avoiding the need for complicated heat-exchange engineering. Scale-up is achieved by feeding the appropriate number of individual reactors from a central manifold.

Thus all aspects of fuel cell development are under intensive investigation and with the savings achieved in the key components successful commercialisation is much more certain.

Platinum Met& Rev., 1997, 41, (3) 112

References 1 D; S. Watkins, in “Fuel Cell Systems”, eds. L. J.

M. J. Blomen and M. N. Mugerwa, Plenum Press, New York, 1993, Chapter 1 1

2 C. E. Borroni-Bird,J. h e r Sources, 1996,61,33 3 K. Prater,J Power Sources, 1994, 51, 129 4 K. PraterJ Power Sources, 1990,29, 239 5 M. S. Wilson and S . Gottesfeld, J. Electrochem.

SOL., 1992, 139, L28 6 M. S. Wilson, J. A. Valerio and S. Gottesfeld,

Electrochim. Acta, 1995,40,355 7 C. Ferreira and S. Srinivasan, Extended Abstracts

Electrochemical Society Spring Meeting, San Francisco, 94-1, p. 969, The Electrochem. SOC., Pennington, New Jersey, 1994

8 G. Sasikumar, M. Raja and S. Parthasarathy, Electrochim. Acta, 1995,40, 285

9 P. G. Driven, W. J. Engelen and C. J. M. Van Der Poorten,J. Appl. Electrochem., 1995, 25, 122

10 M. Uchida, Y. Aoyama, N. Eda and A. Ohta, J. Electrochem. SOC., 1995,142,463

1 1 E. J. Taylor, E. B. Anderson and N. R K. Vilambi, J. Electrochem. Soc., 1992, 139, L45

12

13

14

15

16

17

P. Aldebert, F. Novel-Cattin, M. Pineri, P. Millet, C. Doumain and R. Durand, Solid State Zonics, 1989,35,3 T. R. Ralph, G. A. Hards, J. E. Keating, S. A. Campbell, D. P. Wilkinson, M. Davis, J. St-Pierre and M. C. Johnson, J. Electrochem. SOC., submit- ted for publication C. Zawodzinski, M. S. Wilson and S. Gottesfeld, Proc. First Int. Symp. on Proton Conducting Membrane Fuel Cells, eds. S. Gottesfeld, G. Halpert and A. Landgrebe, Vol. 95-23, p. 57, The Electrochem. SOC., Pennington, New Jersey, 1995 M. Wakizoe, 0. A. Velev and S. Srinivasan, Electrochim. Acta, 1995,40, 335 1 1 th World Hydrogen Energy Conference, 1996, 23-28th June, Stuttgart, reviewed by M. L. Doyle, Plannum Metals Rev., 1996,40, (4), 175 T. R. Ralph, G. A. Hards, D. Thompsett and J. M. Gascoyne, Extended Abstracts Fuel Cell Seminar, San Diego, 1994,199

18 J. Denton, J. M. Gascoyne and D. Thompsett, European Appl. 731,52OA, 1996

19 F. Barbir, F. Markin, B. Bahar and J. A. Kolde, Extended Abstracts Fuel Cell Seminar, Orlando, 1996,505

20 T. E. Springer, M. S. Wilson and S. Gottesfeld, J. Electrochem. SOC., 1993, 140,3513

21 E. A. Ticianelli, J. G. Berry and S. Srinivasan, J. Appl. Electrochem., 1991,21,597

22

23

24

25

26

27

28

29

30

31 32

33

G. A. Hards, T. R Ralph and S. J. Cooper, ETSU (of the UK DTI) Report No. ETSUIFCW002, “High Performance, Low Cost Membrane Electrode Assemblies for Solid Polymer Fuel Cells”, December 1992 D. P. Wilkinson, M. Bos, S. Knights, D. Thompsett, G. A. Hards and T. R. Ralph, to be published M. Iwase and S. Kawatsu, op.cit., (Ref. 14), Vol. 95-23, p. 12, The Electrochem. SOC., P e d g t o n , New Jersey, 1995 H. F. Oetjen, V. M. Schmidt, U. Stimming and F. Trila, 3. Electrochem. SOC., 1996, 143, 3838 S. Gottesfeld and J. Pafford, 3 Electrochem. SOC., 1988,135,2651 A. E. Steck, Proc. First International Symposium on New Materials For Fuel Cell Systems, Montreal, 1995,74 J. A. Kolde, B. Bahar, M. S. Wilson, T. A. Zawodzinski and S. Gottesfeld, op.cit., (Ref. 14), Vol. 95-23, p. 193, The Electrochem. SOC., Pennington, New Jersey, 1995 B. Bahar, Conference “Commercialising Fuel Cell Vehicles”, Chicago, 1996 R. K. A.M. Mallant, F. G. H. Koene, C. W. G. Verhoeve and A. Ruiter, Extended Abstracts Fuel Cell Seminar, San Diego, 1994, 503 R. J. Lawrence, US. Patent 4,214,969; 1980 D. P. Wilkinson, G. J. Lamont, H. H. Voss and C. Schwab, WorldAppl. 95/16287A; 1995 K. B. Washington, D. P. Wilkinson and H. H. Voss, US. Patenr 5,300,370; 1994

34 N. Edwards, S. R Ellis, J. C. Frost, S. E. Golunski, M. I. Petch and J. G. Reinkingh, European Fuel Cell News, 1996, 3, (2), 17

Platinum group metal complexes find exten- sive use as sensitisers, photocatalysts and photo- electrodes in harvesting sunlight to produce electricity or hydrogen by the electrolysis of water (1). Hydrogen is usually photoproduced from water using semiconductors and catalysts, and polymer semiconductors combined with ruthenium chloride can produce hydrogen effectively.

Now scientists from the Tokyo Institute of Technology (2) report a photocatalytic system which produces hydrogen from the electrolysis of water using a semiconducting chelating n-conjugated polymer poly(2,2‘-bipyridine-5,5’- diyl), which has a strong affinity towards metals,

Platinum Increases Hydrogen Photoproduction and [Pt(bpy)~] (NO& (bpy = 2,2‘-bipyridyl) as photocatalyst in aqueous NEt3 solution.

When irradiated with light, the system produced hydrogen with a turnover number of &:[Pt(bpy)$’ of 490 and maintained this acuv- ity over several cycles. This is thought to be related to the ability of the polymer to trap platinum as [Pt(bpy)]’+ on its surface.

1 Referencea

M. Gratzel, Platinum Metals Rev., 1994, 38, (4), 151; M. LDoyle,PlatinumMetaLF Rev., 1996,40,

T. Maruyama and T. Yamamoto, J. Phys. Chem. B, 1997, 101, (19), 3806

(413 175 2


Autocatalvst Manufacture in Malavsia J J

A JOHNSON MATTHEY JOINT VENTURE TO CONTROL VEHICLE EMISSIONS IN SOUTH-EAST ASIA

Following the establishment of a new joint venture, Johnson Matthey HICOM Sdn Bhd, the Johnson Matthey Catalytic Systems Division is poised to make a significant contribution to South-East Asian automotive markets. The new facility, Johnson Matthey’s seventh autocatalyst manufacturing plant, is located in Nilai, one of the fastest expanding industrial zones in Malaysia.. From here Johnson Matthey HICOM will start to serve regional markets, such as Thailand and Indonesia, possibly as early as mid-1998. In the meantime the facility produces emission control catalysts for vehicles produced by PROTON, the Malaysian automobile conglomerate, for the export market. One factor that influ- enced the choice of Nilai was the securing of a deal to supply PROTON with catalyst units. These units comply with the requirements of EC Stage UII standards.

Future production levels are expected to rise significantly by early 1998 when newly approved emission control legislation will be implemented in Malaysia, requiring catalytic converters to be fitted to all new models of petrol-fuel14 vehicles. As PROTON holds 65 per cent of the domestic market share, with 180,000 vehicles sold in 1996, the opportunity for catalyst sales by the new joint venture is substantial. By 1998 it is expected that all other South-East Asian

counmes will face enforcement of emission controls, to EC Stage I standards, on petrol- fuelled vehicles, and at the turn of the cen- tury, regulations requiring petrol-engined cars to meet EC Stage I1 standards are expected to be in place in Malaysia and most other South-East Asian countries. Catalysts are also likely to be compulsory on motorcycles and commercial vehicles, including diesel-fuelled vehicles.

Clearly this joint venture demonstrates the continuing commitment of Johnson Matthey to the improvement of air qual- ity worldwide, and will move Malaysia to the forefront in the use of platinum metals catalysts to control emissions from hydrocarbon-fuelled vehicles.

KALA SHANMUGAM

Plarinurn Metah Rev., 1997,41, (3), 114 114

Optical Oxygen Sensors UTILISING THE LUMINESCENCE OF PLATINUM METALS COMPLEXES

By Professor Andrew Mills Department of Chemistry, University of Wales Swansea

Oxygen is a n immensely important chemical species - essential for lije. The need to determine levels of oxygen occurs in many diverse fields. In environ- mental analysis, oxygen measurement provides a n indispensable guide to the overall condition of the ecology and it i s routine practice to monitor oxygen levels continuously in the atmosphere and in water. In medicine, the oxygen levels in the expired air or in the blood of apatient are keyphysiologicalpara- meters for judging general health. Such parameters should ideally be monitored continuously, which may present problems. Determining oxygen levels in blood requires blood samples which m a y be difficult to take or impossible to take regularly - the elderly suffer f rom collapsed veins, while babies may only have 125 cm3 of blood. The measurement of oxygen levels is also essential in indus- tries which utilise metabolising organisms: yeast for brewing and bread mak- ing, and the planzs and microbes that are used in modern biotechnology, such as those producing antibiotics and anticancer drugs. Here, the background to oxygenmeasurements is described and work to develop new optical oxygen sensors which utilise the luminescence of plat inum metals complexes is discussed.

Due to its widespread importance, the quantitative determination of oxygen, in the gas phase or dissolved in a liquid phase, is a cornerstone piece of analysis that has been dominated for the last three decades by electrochemical sensors, such as the amperomemc Clark cell or the galvanic Mancy cell (1). These sensors are robust and quite reliable when used correctly. However, they are bulky and not readily miniaturised (without great expense); they also suffer from electrical interference and, since they consume oxygen, can easily generate misleading data. Thus, there is a constant interest in new, superior techniques for oxygen detection, and optical oxygen sensors represent one of the new hopefuls in this area (2,3). Optical oxygen sensors are cheap, easily miniaturised and simple to use. Additionally, they do not suffer from electrical interference or consume oxygen. Optical sensors for other analytes, including pH (4), carbon dioxide (5) and biological analytes, such as glucose (6), have also been developed. By coupling optical sensors for a range of

analytes onto the distal end of a fibre optic it is possible to perform remote, non-perturb- ing, multianalyte analysis in very confined spaces, such as blood vessels (7).

Early Optical Oxygen Sensors and Their Basic Operating Principles

Most optical oxygen sensors respond specifically and usually reversibly to molecular oxygen by a change in the intensity of luminescence emitted from a probe molecule. The luminescence is reversibly quenched by molecular oxygen. The luminescent probe molecules are usually encapsulated in a gas permeable, but ion imper- meable, material such as silicone rubber, to create the thin-film oxygen sensor.

The field of optical oxygen sensors is dominated by platinum metals complexes, which are used as the oxygen-quenchable lumophoric probe, and in particular by the following three luminescent platinum metals complexes:

tris(4,7-diphenyl-l,lO-phenanthroline)- ruthenium(II), represented by [Ru(dpp)$';

Platinum Metals Rev., 1997,41, (3) , 115-127 115

a, cc Lc

C 0

x '= x 2

Q m -

I

tris( 1,lO-phenanthroline)ruthenium(II),

tris(2,2'-bipyridyl)ruthenium(TI), [R~(bpy)~]". Their structures are illustrated in Figure 1.

These are very photostable dyes with long excited-state lifetimes and high quantum yields of luminescence. They are readily quenched by oxygen, as indicated by the data in Table I. The longer lifetime of the [Ru(d~p)~]" species has made it a particular favourite in recent attempts to develop optical oxygen sensors of greater sensitivity than earlier ones based on [Ru(bpy)3IZ'.

In an homogeneous medium, such as an aqueous solution, quenching the luminescence of ruthenium diimine complexes, including those in Figure 1, by oxygen has been found to obey the Stern-Volmer equation:

[R~(phen)~]~+ and

Ion = t,/t = 1 + Kw.pO, (9

where I.(t.) and I(t) are the intensities (lifetimes) of the luminescence in the absence and presence, respectively, of oxygen at partial pressure, pOz, and IGv is the Stern-Volmer constant. The Stern-Volmer constant depends directly upon the rate constant for the diffusion of oxygen, the solubility of oxygen and the natural lifetime of the electronically-excited state of the lumophore in the plastic medium. In much of the work on optical oxygen sensors, it is the intensities, L and I, that are studied, rather than the lifetimes, to and t. Certainly, in any commercial device incorporating such sensors it will be much simpler and cheaper to measure the intensities instead of the excited state lifetimes. Lifetime measurements however, have an advantage over intensity measurements, since they are not usually affected by processes which result in loss of the complex, such as leaching or pho- todegradation. Figure 2(a) illustrates the variation in the emission spectrum of [R~(bpy)~]" dissolved in water under atmospheres of nitrogen, air and oxygen.

In most of the luminescent oxygen sensors developed to-date, where the lumophore is incorporated into a polymer matrix, such as silicone rubber, it has been observed that, in contrast to homogeneous solutions, the Stern-Volmer plot of I,/I versus p 0 ~ has a downward curvature.


Tabl

e I

Phot

oche

mic

al C

hara

cter

istic

s of t

he R

uthe

nium

Com

plex

es, [

Ru(

bpy)

3I2'

, [R

~(ph

en)~

]*' an

d [R

u(dp

p)3I

2' i

n W

ater

*

Dye

Lu

min

esce

nce

A,,, M

olar

hm

ax

@L

ka(0

z).

To.

ps

nm

1o4d

m3 m

ol 'c

m '

nm

yiel

d of

lif

etim

e,

(abs

orpt

ion)

, ab

sorp

tivity

, (e

mis

sion

),

Qua

ntum

1

09

dm

3m

01

1~

1

lum

ines

cenc

e

IRu(

bpyb

I2'

0.60

(4

23)s

h. 4

52

1.46

.

61 3,

627

0.04

2 3.

3

[R~

(ph

en

)~]"

0.

92

447,

421

1.83

. 1.9

0 60

5,62

5 0.

080

4.2

in 2

-but

anon

e

[R~~

PP)~

I''

5.34

46

0 2.

95

61 3,

627

- 0.3

0 2.

5 I

in w

ated

etha

nol

I in

met

hano

l I

I I

in m

etha

nol

I I

pO,(S

= %

) R

efs.

. in

silic

one

rubb

er, R

TV 1

18,

torr

(8)

376.

8 9.

10,

11

111.

3 11

.12

11, 1

3, 1

4 29

.8

I I

sh is

sho

ulde

r is

the

bim

olec

ular

que

nchi

ng ra

te c

onst

ant o

f the

ele

ctro

nica

lly e

xcite

d st

ate

of t

he lu

min

esce

nt d

ye b

y ox

ygen

sh

is s

houl

der

unle

ss s

tate

d o

thew

ise

I

ri

LN

bipyridyl

A

1,lO-phmanthroline

2 +

Fig. 1 Structures of the major ruthenium(I1) diimine lumophores used in optical ox ~ensors, namely: [Ru(bpy),]”, [Ru(pheu),] and Pen [R~(dPP)4*+

Two models: the multisite model and the non- linear solubility model, have been used to account for this phenomenon (8, 15). In the multisite model, it is suggested that the sensor molecule can exist at two or more sites, each with its own characteristic quenching constant. The non-linear solubility model assumes that deviation from linearity is due to the non-linear solubility of oxygen in the polymer. Both models predict the following modified form of the Stern-Volmer equation:

L A = 1 + A.pO2 + B.p02/(1 + b.pO2) (ii) where A, B and b are constants relating to the parameters in the kinetic and solubility equa- tions associated with the respective models.

The above equation successfully fits the observed data associated with most optical oxygen sensors. Figure 2(b) shows the straight-line variation of I./I versus pOz for [R~(bpy)~]~’ in

aqueous solution, predicted from the data in Table I and Equation (i), and the downward- curving variation found when the same complex is encapsulated in silicone rubber. In the silicone rubber medium the Stern-Volmer plot is curved and less steep, indicating that it is generally less oxygen sensitive. This is a common feature of encapsulation, not usually due to a variation in the natural lifetime of the probe, which is largely unchanged on going from an aqueous medium to an encapsulating solid medium. Instead, it is due to a combination of a lower solubility of oxygen and a lower bimol- ecular quenching rate constant - the latter is usually diffusion controlled. A useful quick, though rough, guide to the sensitivity of any

-- I I

350 425 500 575 650 725 800 WAVE LENGTH, A, nm

F%2(4’ * spedra of PwPY>s”(~l in aqueous solution saturated with (top down- wards): nitrogen, air, oxygen. The insert diagram is the Stern-Vohner plot of L/l versus PO, of the data in the main diagram, gradient = 0.0037 tom-’

100 200 300 400 500 600 700 800 po2, torr

Fq. 2(b) Stern-Volmer plot of L/I versus Po, for [R~(bpy).~(Cl-).] in: (i) aqumw solution,calcu- lad from data in Table I, (solid line) and (ii) in a silicone rubber fh, (broken line); data from (8)

Platinum Metals Rew., 1997,41, (3) 117

oxygen optrode at low oxygen levels, is the value of the partial oxygen pressure, p02, when the intensity of luminescence, I, has dropped to a value of IJ2, that is pOz(S = %). In an homogeneous medium, where quenching is expected to obey the Stern-Volmer equation (Equation (i)) the parameter pOz(S = %) is given by:

pOn(S = %) = 1 K ” (iii)

However, for the majority of optical oxygen sensors, the variation in luminescence as a function of pOz is more likely to be described by Equation (ii) - the modified form of the Stern- Volmer equation - and parameter pOz(S = %) is described by the following expression:

pOn(S = %) = [- (A+B-b) f d(A+Bb)’ + 4Ab] (iv) 2Ab

Table I lists the pOz(S = %) values for the [Ru(dpp)3] ’+, [Ru(phen)s] ’+ and [R~(bpy)~] ’+ lumophores encapsulated as their perchlorate salts in silicone rubber. These results show that the most oxygen sensitive films of all the ruthenium(1I) diimine complexes tested incorporate the [R~(dpp)~]” species.

The common chloride and perchlorate salts of the ruthenium diimine complexes listed in Table I are hydrophilic and thus not readily soluble in the hydrophobic encapsulating polymer medium (8). Early optical oxygen sensors were made by soaking the silicone rubber in a dichloromethane solution of a hydrophilic l d - nescent salt, [R~(dpp)~(ClO&]. The silicone rubber swells up in dichloromethane and the ruthenium complex can then penetrate the film and become trapped after the highly volatile solvent evaporates (8). Such sensors still have problems, such as dye leaching and fogging, especially when exposed to humid air or aqueous solution, and low luminescence, due to the low solubility of the hydrophilic complex salt (8).

Work by us (1 6) and others (1 7, 18) has shown that hydrophilic cationic luminescent species, such as [Ru(bpy)?], [Ru(phen)?] and [Ru(dpp)?], can be readily solubilised into a hydrophobic polymer medium, by coupling the cation with a hydrophobic anion, such as tetraphenyl borate or dodecyl sulfate, to

create a hydrophobic ion-pair, for example [R~(dpp),2’(Ph,B-)~]. Using this approach, a range of thin-film luminescence oxygen sensors can be produced with much greater stability to dye leaching and film fogging, and with better response times, luminescence intensities and stabilities than prior oxygen sensors. Table I1 describes some optical oxygen sensors based on the ruthenium diimine cations.

Tuning the Sensitivity of Optical Oxygen Sensors

The sensitivity of an optical oxygen sensor depends mainly upon the ability of oxygen to quench the luminescence emitted by the probe. This depends, in turn, upon: [a] The natural lifetime of the excited luminescent state in the absence of oxygen, to; long lived excited states favour the creation of sensitive oxygen sensors. [b] The rate of diffusion and solubility of oxygen in the encapsulating medium; combining these two parameters is the permeability of the medium towards oxygen, and the greater this value the more sensitive the oxygen sensor. [c] The efficiency of quenching: the optimum situation is when the excited state of a luminescent probe molecule is quenched whenever it encounters an oxygen molecule, thus the process is diffusion-controlled. This is usually the case for most optical oxygen sensors.

Oxygen sensors with different sensitivities can be created by varying [a] to [c]. Factors [a] and [c] depend largely upon the nature of the luminescent probe dye. Thus, sensors of low sensitivity (pOz(S = %) = 377 torr) and high sensitivity (p02(S = %) = 29.8 tom) towards oxygen have been made using the ruthenium diimine cations: [R~(bpy)~]” (z, = 0.6 ps) and [Ru(dpp)J2’ (to = 5.3 ps), respectively (€9, see Table I. The variation in sensitivity is primarily due to the difference in to of the two probe dyes.

Early Work with Silicone Rubber It is also possible to alter the sensitivity of an

optical oxygen sensor by using different encapsulating media which have different values for factor [b]. This is most dramatically illustrated


.- 6 r" U

0) c m 3 In P m V c w

.- c -

Platinum Metals Rev., 1997,41, (3)

by the results in Table HI, which lists the oxygen sensitivities, as measured by the value of pOz(S = %) for a series of films of [Ru(dpp)?(PhS encapsulated in different polymers; the lower the value of pOz(S = %) the greater the oxygen sensitivity of the sensor. Silicone rubber is clearly preferred as the encapsulating medium, see Table 11. This is because silicone rubber has very high permeability, 100 times greater than for any other organic polymer (26), high chemical and mechanical stability, and it is very hydrophobic. The latter min- imises dye leaching and interference quenching by any ionic species in the test medium.

Although different commercial silicone rubbers have been used as the encapsulating medium, it is not always clear which characteristics of the final cured encapsulating medium determine the overall sensitivity of the resultant oxygen sensor. The situation is complicated by the propriety nature of the silicone rubbers - some commercial silicone rubber formulations have added silica filler, full details of which are not readily available. Elegant work carried out by Demas and co- workers has demonstrated that the presence of silica in a silicone rubber can considerably alter the sensitivity of the oxygen sensor (27).

Plasticised Polymers At present, tuning the sensiuv-

ity of an oxygen sensor which utilises silicone rubber is often crude, empirical and lacking in dynamic range. However, unlike silicone rubber, polymers such as

119

Lum

ines

cent

Ion-

Pair

17.74

2.48

2.43

9.03

4.22

5.63

9.93

1.92

15.88

4.52

1.13

3.92

20.98

Tabl

e II

Cha

ract

erist

ics o

f Typ

ical

Rut

heni

um D

iimin

e Oxy

gen

Lum

ines

cent

Sen

sors

126

377

124

111 57.3

28.0

54:O

29.8

18.3

32.6

495

138 33.9

Enc

apsu

latin

g M

ediu

ma

Cel

lulo

se a

ceta

te +

TBP

plas

ticis

er (200 p

hr)

Sili

cone

with

sili

ca fi

ller (RTVll8; GE

) D

ye s

uppo

rted

on k

iese

lgel

dis

pers

ed in

sili

cone

(E43; Wac

ker)

S

ilico

ne w

ith s

ilica

fille

r (RTVI 18; GE

) D

ye s

uppo

rted

on k

iese

lgel

dis

pers

ed in

sili

cone

(E43; Wac

ker)

P

MM

A +

TBP

plas

ticis

er (133 p

hr)

Sili

cone

(Em

; Wac

ker)

S

ilico

ne w

ith s

ilica

fille

r (RTV118; GE

) S

ilico

ne w

ith s

ilica

fille

r (RTVI 18; GE)

Sili

cone

(RTV 732; D

ow C

orni

ng)

Pol

ysty

rene

P

olyv

inyl

chlo

ride

Sili

cone

(RTV

732; D

ow C

orni

ng)

A,

0.00

1

7.05

1.15

1.46

3.94

13.83

15.0

5.39

21.95

35.05

12.96

1.28

6.30

1.93

B.

0.001

2.88

2.91

8.59

10.14

4.49

29.0

20.18

12.28

25.47

20.33

1.15

1.46

47.19

b.

1 %I.

0.001

Ref

.

17 8

19 8 6

16

12 8 20

15

21

22

23

a U

nles

s st

ated

oth

erw

ise

all s

ilico

nes

refe

rred

to h

ere

wer

e ge

nera

ted

from

one

-com

pone

nt,

acet

lc a

ad

rele

asin

g, s

ilica

ne p

repo

lym

er w

ith

no a

dded

sili

ca fi

ller:

phr

= p

arts

per

hun

dred

resi

n M

ost on/ e

n op

tical

sen

sors

giv

e S

tern

-Vol

mer

plo

ts (

IJ ve

rsus

po

d w

hich

dev

iate

from

line

arity

and

obe

y E

quat

ion

(ii).

For e

ach

oxyg

en s

enso

r the

orig

inal

dat

a re

porte

d in

the

asso

ciat

ed a

rticl

e w

ere

fitte

d to

the

mod

ified

&er

n-V

olm

er

equa

tion.

Equ

atio

n (ii

)and

the

valu

es fo

r A.

B an

d b.

whi

ch p

rovi

de th

e op

timum

fit,

wer

e us

ed to

cal

cula

te th

e va

lues

for

p02(

S = %

) re

porte

d in

this

Tab

le u

sing

Equ

atio

n (iv

). All

the

oxyg

en

sens

ors

refe

rred

to in

this

Tab

le a

re th

e m

ost s

ensi

tive

for w

hich

Ste

rn-V

olm

er d

ata

wer

e gi

ven

in th

e as

soci

ated

arti

cle

Encapsulating polymer A, I O - ~ B. lo3

Silicone rubber 21.9 12.3

Polyvinyl chloride

Cellulose acetate 0.0376 3.27 Polymethyl methacrylate 0.1 99 1.61

- -

cellulose acetate (CA), polymethyl methacrylate (PMMA) and polyvinyl chloride (PVC) can be obtained in a well defined, reproducible form. As shown in Table 111, such polymers have poor oxygen diffusiodsolubility characteristics and are of limited use by themselves as the encapsulating medium. However, these three polymers are also readily plasticised, with such species as tributyl phosphate (TBP) and dioctyl phthalate, to create films which have much better oxygen diffusionlsolubility characteristics than the pure polymer (28). As a result, plasticisation of a polymer encapsulating medium, such as CA, PMMA or PVC, with TBP for example, allows the sensitivity of an optical oxygen sensor to be increased markedly in a well- defined manner.

The variation in the Stern-Volmer plots for a series of [Ru(dpp)?(PhaB )4 in PMMA films

b, pOz(S = Yz), Ref. torr

1.92 29.8 a 0.594 368 24 0.683 a06 16

1000 25 -

which contain increasing amounts of TBP plasticiser is shown in Figure 3. The sensitivity of the optical oxygen film increases with plasticisation, and so do the 90 per cent response and recovery times of the film, when exposed to an alternating atmosphere of 100 per cent oxygen (response) and nitrogen (recovery). This is shown in Figure 4 for the same series of films of [R~(dpp),Z’(Ph,B-)~l in PMMA.

Plasticisation of a polymer increases its work- ability, flexibility and distensibility and, most importantly for the oxygen optical films, the mobility of polymer segments. The latter leads to an increase in gas diffusion coeficients ( 17). It is common practice to refer to the value of the solubility parameter, 6, of the plasticiser and the polymer in order to identify compatible plas- ticker-polymer combinations.

In general, it has been suggested that the

2 0

15 145

121

73

48

- . -* 1 0

5

0

Fig. 3 Stern-Volmer plots of 1.n versus pOr for a series of [Ru(dpp)Jt’(PhS),] -based, cellulose acetate oxygen sensor films in which the plasticiser, TBP, was varied over the range 0 to 194 phr (values of phr on the right hand side of the traces) and neat TBP.

squares lines of best fit and were calculated using Equation(ii) and optimiaed values for A, B and b. Reprioted from Mills m d Williarus (24) with

The solid lines represent the least

Platinum Metals Rm., 1997,41, (3) 120

Table 111

Variation in the Sensitivity of Non-Plasticised [Ru(dpp)32’(PhS)*] Films as a Function of Different Polymer Encapsulating Media

- -” Fig. 4 Plot of pO,(S = %) versus TBP concentrations derived from data shown in Figure 3 for the [Ru(dpp)?(PU-)~]-based, cel- 3oo. lulose acetate oxygen sensor films, k containing different amounts of 2 plasticiser. The (0) point repre- sent8 the value for pOl(S = +) obtained for the dye dissolved in yr neat TBP plasticiser. The insert - 100 200

[TBP], phr diagram shows the observed vari- g ation in the 90 per cent response (open circles) and recovery (black circles) times, t(90), recorded for the films. Data from (24); phr = parts per hundred resin (weight

to the polymer)

M).

per Cent of plasticiser with respect 40 80 120 160 200 [ T B P l , Phr

difference in these solubility parameters:

(A6 = 6(polymer) - G(plasticiser)}

should be less than 3.7 (J cm-’)liZ (29). Table IV lists the solubility parameters and A6 values for a series of films of polymers plasticised with TBP (6 = 17.5 (Jcm-’)”*) containing [R~(dpp),”’(PhS)~]. From the values of pOz(S = %), determined for the corresponding [R~(dpp)~~’(Ph~B-)~] -in-polymer-plasticised- with-TBP (30 phr) films, it appears that, in general, the greater the polymer-plasticiser com- patibility, that is the smaller A6, the greater is the oxygen sensitivity of the film. The latter observation is not surprising given that the sensitivity of the optical oxygen film sensor depends on both the rate constant for the diffusion of oxygen through the film and also the solubility of oxygen in the film. Both of these parameters are expected to improve with plasticisation.

Optical Oxygen Sensors Based on Platinum Metals Porphyrins

Much of the early work on optical oxygen sensors was driven by their possible medical use; in particular, as part of the development of remote, cheap, continuous bedside monitoring of a patient’s condition. The oxygen levels that were of interest centred on the amount in air, namely 159 torr or 21 per cent and the ruthenium(I1) diimine complexes, when encapsulated in silicone rubber or a plasticised polymer,

are quite well suited for operating at or around this level. However, as optical oxygen sensors have developed, interest in them has grown together with a realisation that more sensitive ones could find application elsewhere. One example is in modified-atmosphere food packaging; many foods, especially meats, are pack- aged in the absence of oxygen to minimise bac- terial growth. Carbon dioxide is usually the packaging gas, but vacuum packaging is com- monplace for supermarket foods. Therefore the incorporation of a cheap, disposable optical oxygen sensor as a label in the packing, which would operate at, say, the 15 ton; 2 per cent oxygen level, would provide an ideal check that the packaging is intact. Other possible areas of application are in the control of anaerobic processes and low vacuums.

The development of more sensitive optical oxygen sensors has been dominated by the use of platinum metals porphyrins as the luminescent probes. Such probes absorb and emit light in the visible spectrum (typically, at 540 nm and 655 nm, respectively) have long excited state lifetimes (usually > 10 ps) and in many cases are commercially available. Some of the platinum metals porphyrin structures used as luminescent probes in optical oxygen sensors are shown in Figure 5; invariably the metal in such complexes is platinum or palladium.

Porphyrins of platinum and palladium do not usually fluoresce, rather they phosphoresce and

Platinum Metals Rev., 1997, 41, ( 3 ) 121

Tabl

e IV

Cha

ract

erist

ics o

f Som

e Diff

eren

t Pol

ymer

s Use

d to

Cre

ate

[R~(

dpp)

32'(

Ph.S

)~] Fi

lms

of D

iffer

ent O

xyge

n Se

nsiti

vity

, Pla

stic

ised

with

TB

P (3

0 ph

r)

~

Oxy

gen

sens

itivi

ty c

hara

cter

istic

s of

[Ru(

dpp)

;'(Ph

&)2]

in

film

s co

mpr

isin

g di

ffere

nt p

olym

ers,

pla

stic

ised

with

TBP

(30

phr

) P

olym

er

Abb

revi

atio

n M

olec

ular

for

mul

a (M

olec

ular

wei

ght)

S

olub

ility

pa

ram

eter

, 6,

(J cm

-3)1

n A,

IO

-~

B, 1

0"

b,

pO2(

S =

YZ),

torr

17.9

6.

53

6.30

4.

94

92

0.4

Cel

lulo

se

acet

ate

buty

rate

CA

B

(30.000)

17.9

0.

01 15

7.

49

0.65

5 14

6 0.

4 C

ellu

lose

ac

etat

e CA

r

1

(30,000)

~

209

1.6

PVAc

19

.1

3.07

4.

34

7.31

(-

CH

LH(O

LCH

3j)n

(1

13,0

00)

(-CH

2CH

(CsH

s)-)n

(4

5.00

0)

Pol

y(vi

ny1

acet

ate)

Pol

ysty

rene

~

245

PS

18.5

2.

53

5.93

11

.5

2.27

6.

29

16.3

30

0 1.

3 P

MM

A

18.8

(-C

H2C

(CH

3)(C

O2C

H,)-

)n

(1 20

,000

) (-C

H,C

(CH

3)(C

OzC

H,)-

)n

(33,

800)

(95.

000)

(-C

HzC

H(C

1)-)n

Pol

y( m

ethy

l rn

etha

cryl

ate)

Pol

y( m

ethy

l rn

etha

cryl

ate)

Pol

y(vi

ny1

chlo

ride)

~

18.8

2.

47

2.72

8.

09

308

PM

MA

i 1.

3

1.7

PVC

19

.2

1.53

1.

90

4.15

45

7

~ iser

l Fo

r CA

B

R=

-CO

CH

3 or -

CO

CH

,CH

EH

, fo

r C

A R

= -C

OC

H2 o

r H. A6 =

Z4p

olyr

ner)

- 6

(pli

- N N Platinum Metals Rev., 1997,41, (3)

Fig. 5 Structures of the major platinum and palladium porphyrins used to create the new range of very sensitive optical oxygen sensors; see Table V

Porphyrin

c Octaethyl porphyrin ketone

(OEPK)

M

Octaethyl porphyrin (OEP) Tetraphenyl porphyrin (TPP)

COOH <

HOOC

COOH

SO+I

Coproporphyrin (CPP) Tetra (p-sultophenyl) porphyri n (TSPP)

this lack of any short-lived fluorescence is an advantage over many other long-lived phos- phorescent species, such as polycyclic aromat- ics, since it reduces the problems associated with background interference. Most of the platinum metals porphyrin optical oxygen sensors reported to-date are shown in Table V; unless stated other- wise the encapsulating medium is homogeneous, that is no plasticisers are included. It can be seen that the non-quenched, natural, lifetimes, to, of the platinum and palladium porphyrins differ by about an order of magnitude and, as a result, any such pair of porphyrin probes allows a considerable range of oxygen concentrations to be covered.

Unlike the ruthenium(1I) diimine complexes, the excited states of the platinum and palladium

porphyrins are much longer lived, typically many tens of microseconds for platinum porphyrins and many hundreds of microseconds for palladium porphyrins; see the lifetime data in Tables I and V. As a consequence, when the platinum metals porphyrins are incorporated into the same polymer media used for the ruthenium diimine optical oxygen sensors, the resultant sensors are more oxygen sensitive. This can be seen by com- paring the pOz(S = %) values for the ruthenium(I1) complexes listed in Tables I1 and 111 with those for the platinum and palladium porphyrins, listed in Table V. For example, the most oxygen sensitive of the ruthenium@) complexes, [R~(dpp)?(Ph~B-)~], when incorporated into non-plasticised PVC, produces a very oxygen insensitive film with pOz(S = %) > 1000 tom,

Platinum Metah Rev., 1997, 41, (3 ) 123

Pro

be

To.

Mem

issi

on).

Med

ium

pO

2lS

= 'h),

Com

men

ts

ms

nm &

(4J

l/Kw

.

Pd-C

PP

Pd-C

PP

Ref

.

Pd-C

PP

Pd-C

PP

0.53

5 3.

57

7.2

27.1

49.2

56.9

Pt-O

EPK

Pt-O

EPK

30

31

A li

fetim

e st

udy,

incl

udin

g an

inve

stig

atio

n of

the

effe

cts

of te

mpe

ratu

re, a

gein

g an

d po

ssib

le in

terfe

rant

s. s

uch

as n

itrou

s ox

ide

and

carb

on d

ioxi

de. T

his

silic

one

rubb

er is

a c

lear

. one

com

pone

nt,

acet

ic a

cid

rele

asin

g pr

epol

ymer

, with

an

unsp

ecifi

ed a

mou

nt o

f sili

ca fi

ller

32

33

The

~/K

w

valu

e he

re w

as c

alcu

late

d fr

om T~ =

61

.4~

s

and

T~~

~ = 1

6.3

~~

. Th

e Pt

-OEP

K is

tens

of t

imes

mor

e ph

otos

tabl

e th

an P

t-OEP

Pt-O

EPK

Als

o en

caps

ulat

ed in

WC

alth

ough

the

film

s ar

e no

t as

sens

itive

Alth

ough

a K

W va

lue

of 0

.169

tori

-' is

repo

rted

in T

able

I of

this

ref.

it is

10

times

too

big,

as

show

n by

the

Ste

rn-V

olm

er p

lot (

Fig.

2(b

), Re

f. 34

) of t

he o

rigin

al d

ata

for t

he

film

-w

e th

ank

the

auth

ors

for c

onfir

min

g th

is. T

he v

alue

for

l/Ksv

repo

rted

here

w

as c

alcu

late

d fr

om th

e la

tter d

ata

A v

ery

curv

ed S

tern

-Vol

mer

plo

t, su

gges

ted

to b

e du

e to

dye

agg

rega

tion

Pd-O

EPK

34

35

Pc-O

EPK

Pd-

OF

PK

the

oxyg

en s

ensi

tivity

by

a fa

ctor

of 3

.6

ka fo

r ox

ygen

= 3

.8 x

lo9 d

m3 m

ol"

s I; a

dditi

on o

f bov

ine

seru

m a

lbum

in lo

wer

s th

e ox

ygen

sen

sitiv

ity b

y a

fact

or o

f 17

The

silic

one

rubb

er is

a c

lear

, one

-par

t po

lym

er. w

ith n

o fil

ler:

all s

enso

rs c

over

ed b

y a

Teflo

n" m

embr

ane.

All

Ste

rn-V

olm

er p

lots

sho

w d

ownw

ard

curv

atur

e; h

alog

enat

ion

impr

oves

pho

tost

abili

ty o

f po

rphy

rin s

enso

rs

air

= 23

/.IS

The

l/Ksv

valu

e w

as c

alcu

late

d fr

om th

e S

tern

-Vol

mer

plo

t (Fi

g. 6

) in

the

refe

renc

e in

w

hich

the

x-ax

is (

P(a

ir}} is

alm

ost c

erta

inly

mis

labe

lled

GI

kPa.

whe

re it

sho

uld

be

Pd-T

PP

38

32

39

(32.

33,'3

9)

0.40

66

7 (0

.2)1

6 w

ater

0.

80

667

(0.2

)"

silic

one

rubb

er

RTV

118(

GE)

1.

06

667 (0.2)'6

PS

0.91

66

7 (0

.2)"

P

MM

A

0.06

1 76

0 (0

.1 )'

PS

0.06

1 75

9 (0

.12)

PS

790

(0.0

1)

Ara

chid

ic a

cid

Lano

mui

r-

32

5.6

685

89.3

2.58

Pd-T

SPP

I 1 .o

I

702a

nd76

3 I

wat

er

I 0.

45

I ka

foro

xyge

n =

1.3

x10'

drn3

mol

~:s

' I

36

Pd-T

SPP

I 0.5

I

698,

685

I wat

er

I 0.

40

I ko

for o

xyge

n =

2.9

x lo

9 dm

' m

ol '

s-'; a

dditi

on o

f bo

vine

ser

um a

lbum

in lo

wer

s 37

Pd-C

PP

R-T

DC

PP

Pt

-TFM

PP

Pt-B

r6TM

P

Pd-O

EP

Pt-O

EP

Pt-O

EP

0.53

66

7 w

ater

0.

29

Sili

cone

ru

bher

R

lV 73

2 0.

99

670

(0.2

)

53.9

I I

I I

I 0-

100

kPa.

giv

ing

a 1/

Ks~

valu

e of

54

torr,

con

sist

ent w

ith la

ter w

ork

by th

e au

thor

(32)

I

CPP

, cop

ropo

rphy

rin.

OEP

K: o

ctae

thyl

porp

hyrin

keto

ne.

TPP

: tet

raph

enylp

orph

yrin

. TS

PP

: tet

raki

s(4-

sulfo

nato

phen

yl) p

orph

yrin

. TD

CPP

: m

eset

etra

l2.6

~dic

hlor

ophe

nylIp

orph

yrin

: TF

MPP

: mes

etet

ra(3

.5~b

is(t

rifl

uoro

~eth

yl)p

heny

l)po

rphy

~n: Br.T

MP:

m

eset

etra

rnes

ityl-p

octa

bror

nopo

rphy

rin:

OE

P: o

ctae

thyl

por

phyr

in

Tabl

e V

Pla

tinum

and

Pal

ladi

um P

orph

yrin

Bas

ed O

ptic

al O

xyge

n Se

nsor

s

Platinum Metals Rev., 1997,41, (3)

Table 111. On the other hand, when platinum and palladium octaethylporphyrin ketones (Pt- OEPK and Pd-OEPK) are incorporated into non-plasticised PVC the oxygen sensors are much more sensitive, pOz(S = %) are 685 and 89.3 torr, respectively, see Table V. Indeed, Pd- OEPK in non-plasticised PVC is almost as sensitive as @u(dpp)i2’] in silicone rubber, or highly plasticised Ph4MA (Table 11).

Interestingly, the platinum and palladium porphyrins do not usually display non-linear, downward-curving Stern-Volmer plots; thus the pOz (S = %) values reported in Table V have been calculated using Equation (iii); whereas those in Table I1 have been calculated using Equation (iv). Presumably, linear Stern-Volmer plots associated with the platinum and palladium porphyrin sensors indicate that the platinum and palladium porphyrins largely locate in the hydrophobic parts of the polymer, whereas the ruthenium diimine complexes can be found in different regions of the film, which thus exhibits multisite quenching behaviour. Optical oxygen sensors based on platinum and palladium octaethylporphyrins do show s

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