platinum metals review · 2016. 1. 28. · cabot vulcan xc72r carbon black. central to the...
TRANSCRIPT
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UK ISSN 0032-1400
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
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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 produc- tion technolou. Despite such advances, there i s still a need for further reduc- tions in the stack cost, through improvements in the performance of the mem- brane 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 effi- ciently 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 mem- brane 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 elec- trolyte 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 require- ment currently limits the temperature of oper- ation 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
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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 cath- ode 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 essen- tial feature of the PEMFC - its ability to gen- erate much higher power densities at lower tem- peratures than other fuel cell types -has opened up the potential of fuel cell technology to pro- vide reliable, low cost power for a new genera- tion of non-polluting automotive engines. It is also being considered for more traditional sta- tionary 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 world- wide. But, for this technology to become widely
Platinum Metals Rev., 1997,41, (3) 103
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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 develop- ment of high volume, low cost manufacturing processes for the stack components. Increased power density, achieved by operating at higher current densities while maintaining cell poten- tials at reasonable levels, will, besides reduc- ing 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 impor- tant, they must be maintained over the operat- ing 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 life- times 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 reduc- ing the costs of the three critical cell compo- nents: 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 poten- tials of 0.75 to 0.65 V, the platinum black load- ing 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 cath- ode. 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 mater- ial, since they combine both activity and sta- bility 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 operat- ing on hydrogedair at substantially lower plat- inum cathode loadings than the 4 mg cm-’ typ- ical 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 struc- ture, 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).
Platinum Metals Rev., 1997,41, (3) 104
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Two basic approaches to electrodes have been developed by these groups.
At LANL thin film catalyst layers compris- ing 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 sur- face is impregnated with the membrane elec- trolyte solution.
Using these approaches LANL and TAMU have reported performances comparable to plat- inum 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 hydro- gen 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 com- patible 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 cat- alyst 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 elec- tronically. 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 vol- ume production and MEAs have been evalu- ated 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 per- formances can be raised on air operation, par- ticularly at higher current densities ( 1 3). In addi- tion, 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 sin- gle 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 elec- trodes 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 elec- trodes prepared in high volume. An electrode manufacturing process based on screen print- ing has been developed (17) and a small pilot plant facility has been established at Johnson
Platinum Metals Rev., 1997, 41, (3) 105
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+ 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 poten- tial for scaling-up as demand rises. Electrodes have been produced from inks comprising car- bon supported platinum catalysts and solutions of the polymer electrolyte. For high volume pro- duction 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 envi- ronment, there is a possibility of interaction between the solvents and the high surface area platinum catalysts, which could result in the pro- duction of organic by-products in the inks and, with high solvent concentrations, possibly com- bustion. 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-’), com- pared with the higher loading unsupported plat- inum 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 cat- alyst 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 plat- inum black MEA. This reduced loading shows considerable progress towards a platinum cat- alyst 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 perfor- mance 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
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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
Platinum Metals Rev., 1997,41, (3) 107
z W
P
z W
P
z W
P
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example the 50 cells in (19). This highlights the excellent degree of consistency achieved in MFL4 production and the efficient control of operat- ing conditions throughout the stack.
Based on the large volume of data now avail- able, it is clear that achieving economical plat- inum 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 gen- eration and are close to the levels required for passenger cars. Indeed, rather than pursuing further cost reductions by lowering the elec- trode 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 auto- motive 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 cat- alyst layer is being utilised. LANL have esti- mated this to be as little as 5 p in their struc- tures (20) and TAMU estimate a 10 pm thick active layer for their electrodes (2 1). While this suggests there is scope for maintaining perfor- mances 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 low- ering platinum loadings on anodes operating with pure hydrogen fuel; anode catalyst load- ings 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 (refor- mate) produced via reforming carbon-contain- ing 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 refor- mate. Carbon dioxide poisoning and dilution effects also reduce performance. But, even using impure hydrogen and with lowered anode plat- inum 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 sup- ported 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 plat- inum 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 elec- trolyte. The stack is operating at a constant cur- rent density of 0.4 A cm-’ under typical sta- tionary power plant testing conditions. The oxidant is air and the reformate fuel contains 70 per cent hydrogen, 25 per cent carbon diox- ide 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
Platinum Metals Rev., 1997, 41, (3) 108
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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 car- bon 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 cat- alyst, 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 over- come, principally because the poisoning does not respond to the air bleed technique. A typ- ical loss is 50 mV at 1 A cm-'. This suggests that while these Pt/Ru anode loadings may be accept- able for stationary applications where the refor- mate fuel is cleaned by the water-gas shift
reaction and selective oxidation, a more poison tolerant anode catalyst may be required for auto- motive 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 tol- erance 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 perfluori- nated 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
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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 experimen- tal Dow XUS-13204.10 membrane in a Ballard Mark V single cell. The electrodes, from Johnson Mattheymallard Power Systems, have low plat- inum loadings. The membrane resistance con- trols the single cell performance in the pseudo- linear region of the plot. The thinner Nafion’ membranes give enhanced performance par- ticularly 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, par- ticularly 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 - mem- brane 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 auto- motive applications.
It is also likely that different membrane elec- trolytes will be employed in automotive and sta- tionary power plants. For stationary applica- tion, 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 mem- brane 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 con- trast, a thinner higher power density membrane, which may be less durable than the perfluori- nated sulfonic acid membranes but have an oper- ating lifetime of 5,000 hours, will probably be used in passenger cars. Steck has reviewed stud- ies on preparing intrinsically lower cost mem- branes, many of which are hydrocarbon based or partially fluorinated (27). Most do not have the necessary stability or the low specific resis- tance 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 auto- motive applications (3, 27). The membrane shows at least equivalent “beginning of life” per- formance to the Dow XUS-13204.10 and Nafion’ 112 membranes in a Ballard Mark V single cell, Figure 6, (27). The Ballard mem- brane has demonstrated this performance for over 4500 hours - almost suficient for an auto- motive 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 elec- trolytes 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 sul- fonic acid membrane electrolyte solution. The PTFE support is claimed to provide mechani- cal 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” elec- trodes 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
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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 spe- cific 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 con- sidered - their replacement offers great poten- tial for reductions in stack size and weight. Two main approaches to fabricating flow field plates are being examined: thin metal plates and com- posite 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 prob- lems 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 poten- tials 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
Platinum Metals Rev., 1997, 41, (3) 111
-
plates fabricated fkom a low cost base metal with a thin protective coating to provide the neces- sary stability to oxidation in the fuel cell envi- ronment. With such substrates it may be possi- ble 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 num- ber of technical challenges remain. Improvements in performance are necessary to increase cell effi- ciency 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 cata- lysts 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 poison- ing, 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 differ- ent suppliers is needed to reduce costs, partic- ularly for automotive use. Membranes with resis- tances 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 manage- ment. 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 suc- cessfully overcome. This view is supported by the increased attention being paid to fuelling and fuel infrastructure issues for vehicle appli- cations. While in the longer term the produc- tion 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 com- mercialisation 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, elec- trodes 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 par- tial oxidation. Heat transfer between the two reactions occurs over microscopic distances within the catalyst, avoiding the need for com- plicated 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 sav- ings 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 elec- tricity or hydrogen by the electrolysis of water (1). Hydrogen is usually photoproduced from water using semiconductors and catalysts, and polymer semiconductors combined with ruthe- nium chloride can produce hydrogen effectively.
Now scientists from the Tokyo Institute of Technology (2) report a photocatalytic system which produces hydrogen from the electroly- sis 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 pro- duced 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 plat- inum 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
Platinum Metals Rev., 1997,41, (3) 113
-
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 sig- nificant 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 mar- kets, such as Thailand and Indonesia, pos- sibly as early as mid-1998. In the meantime the facility produces emission control cat- alysts 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 require- ments 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 cat- alytic converters to be fitted to all new mod- els 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 met- als 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 sen- sors which utilise the luminescence of plat inum metals complexes is discussed.
Due to its widespread importance, the quan- titative 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 sen- sors, 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, supe- rior techniques for oxygen detection, and opti- cal oxygen sensors represent one of the new hopefuls in this area (2,3). Optical oxygen sen- sors 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 emit- ted 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 domi- nated 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 sen- sitivity than earlier ones based on [Ru(bpy)3IZ'.
In an homogeneous medium, such as an aque- ous 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 (life- times) of the luminescence in the absence and presence, respectively, of oxygen at partial pres- sure, pOz, and IGv is the Stern-Volmer constant. The Stern-Volmer constant depends directly upon the rate constant for the diffusion of oxy- gen, the solubility of oxygen and the natural life- time 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 com- mercial 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 advan- tage 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 vari- ation in the emission spectrum of [R~(bpy)~]" dissolved in water under atmospheres of nitro- gen, air and oxygen.
In most of the luminescent oxygen sensors developed to-date, where the lumophore is incor- porated 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.
Platinum Metals Rev., 1997, 41, (3) 116
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 mod- els 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 oxy- gen 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 com- plex is encapsulated in silicone rubber. In the silicone rubber medium the Stern-Volmer plot is curved and less steep, indicating that it is gen- erally 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 homo- geneous 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 sen- sors, the variation in luminescence as a func- tion 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 ruthe- nium(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 sol- uble 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 sol- vent evaporates (8). Such sensors still have prob- lems, such as dye leaching and fogging, espe- cially 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 bet- ter 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 lumi- nescent state in the absence of oxygen, to; long lived excited states favour the creation of sen- sitive oxygen sensors. [b] The rate of diffusion and solubility of oxy- gen 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 lumi- nescent 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 lumi- nescent probe dye. Thus, sensors of low sensi- tivity (pOz(S = %) = 377 torr) and high sensi- tivity (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 encap- sulating media which have different values for factor [b]. This is most dramatically illustrated
Platinum Metals Rev., 1997, 41, (3) 118
-
.- 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 encapsu- lated 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 encapsu- lating 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 interfer- ence 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 char- acteristics of the final cured encapsulating medium determine the overall sensitivity of the resul- tant 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 methacry- late (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 encap- sulating medium. However, these three poly- mers are also readily plasticised, with such species as tributyl phosphate (TBP) and dioctyl phthalate, to create films which have much bet- ter oxygen diffusionlsolubility characteristics than the pure polymer (28). As a result, plasti- cisation of a polymer encapsulating medium, such as CA, PMMA or PVC, with TBP for example, allows the sensitivity of an optical oxy- gen 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 sensitiv- ity of the optical oxygen film increases with plas- ticisation, 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, cel- lulose 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 gen- eral, 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 sen- sitivity 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 parame- ters are expected to improve with plasticisation.
Optical Oxygen Sensors Based on Platinum Metals Porphyrins
Much of the early work on optical oxygen sen- sors 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 ruthe- nium(I1) diimine complexes, when encapsu- lated 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 pack- aging; 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 oxy- gen 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 pack- aging is intact. Other possible areas of appli- cation 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 lumines- cent 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 plat- inum metals porphyrin structures used as lumi- nescent 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 por- phyrins 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 homoge- neous, that is no plasticisers are included. It can be seen that the non-quenched, natural, life- times, to, of the platinum and palladium por- phyrins 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 pal- ladium 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 ruthe- nium(I1) complexes listed in Tables I1 and 111 with those for the platinum and palladium por- phyrins, 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 sen- sitive as @u(dpp)i2’] in silicone rubber, or highly plasticised Ph4MA (Table 11).
Interestingly, the platinum and palladium por- phyrins do not usually display non-linear, down- ward-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 asso- ciated with the platinum and palladium por- phyrin 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