Platinum Metals Review
Total Page：16
File Type：pdf, Size：1020Kb
UK ISSN 0032-1400 PLATINUM METALS REVIEW A Quarterly Survey of Research on the Platinum Metals and of Developments in their Application in Industry www.matthey.com and www.platinurn,matthey.com VOL. 46 OCTOBER 2002 NO. 4 Contents Catalysis for Low Temperature Fuel Cells 146 Ey M. P Hogarth and I R. Ralph The Chemistry of the Platinum Group Metals 165 By John Evans Platinum Metals in Biological and Medicinal Chemistry 166 By Matthew D. Hall An Equilibrium in Catalyst Optimisation and Development? 167 ByG. R. Owen Structural Changes and Their Kinetics in Hydrogen-Containing 169 Palladium Systems By V: M. Avdjukhina, A. A. Katsnelson and G. t? Revkevich Polymer-Supported Rhodmm Catalysts Soluble in sc-COz 176 9th International Platinum Symposium 177 By R. G. Cawthorn Recyclable Ruthenium-BINAP Catalysts 180 ACFPower Coatingsm 181 By Paul Williams Electrically Induced Phosphorescence 187 Abstracts 188 New Patents 192 Indexes to Volume 46 195 Communications should be addressed to: The Editor, Susan V. Ashton, Platinum Metals Review, [email protected] Johnson Matthey Public Limited Company, Hatton Garden, London EC1N 8EE Catalysis for Low Temperature Fuel Cells PART 111: CHALLENGES FOR THE DIRECT METHANOL FUEL CELL By M. P. Hogarth and T. R. Ralph Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K. The direct methanol fuel cell (DMFC)is a low temperature fitel cell operating ut temperutures of30 to 130°C. The DMFC is powered by CI liquidfie1 (usually considered essentia1,fortransport uses) and is therefore regarded by some as the idea1,fuel cell system. III this piper, the DMFC is cornpared to the hydrogen-fuelled proton exchange mentbrane firel cell (PEMFC) which was discussed in detail in the Jaiiuaty and July issues. While a typical DMFC is less eficient than a PEMFC, work to improve its performance with new electrocatulyst murerials jor utilisation in the membrane electrode assemblies has proved successfiil. This work is described here and some possible commercial uses jor the DMFC are also considered. Two of the most advanced low temperature developing materials, such as new anode and cath- fuel cells are the proton exchange membrane fuel ode electrocatalysts and new proton conducting cell (PEMFC) and the direct methanol fuel cell polymers, to promote the efficiency of the mem- (DMFC). The DMFC directly consumes liquid fuel brane electrode assemblies (MEiAs) used in the (methanol), while the PEMFC is fuelled by hydro- DMFC stack. Advanced MEA designs have also gen. Operating a fuel cell with liquid fuel is been developed. Since most effort has been direct- considered by some to be essential for transport ed towards increasing the efficiency of the MEA applications - for compatibility with the existing components, the DMFC system itself has petroleum distribution network. The DMFC also remained relatively undeveloped compared to the has some system-related advantages over the PEMFC - particularly for transport use. PEMFC, making it of interest to fuel cell develop- However, interest in producing low tempera- ers. For instance, the DMFC has no need for a fuel ture (< 60°C) ambient-pressure portable DMFC processor (or reformer) to convert a liquid hydro- systems has increased recently. This is because the carbon fuel (gasoline) into a consumable source of power densities now accessible by state-of-the-art hydrogen. This considerably reduces the complex- MEAs may be enough for these systems to ity and cost of the system. The DMFC system does become competitive with leading secondary bat- not require the complex humidification and heat tery technologies. This area could thus become a management hardware modules used in the near-term market oppommity for the DMFC, with PEMFC system: the dute methanol-water mix- transport uses being a longer-term goal, if further tures circulating around the DMFC provide the performance gains can be achieved. necessary humidification and heat management. If it can meet the performance required of a Comparison of PEMFCs and DMFCs commercially viable device, the DMFC system will The PEMFC and DMFC have much in com- be potentially more cost effective than the PEMFC. mon, in particular their MEAs. The MEA and its Performance has been a major problem for the components were described in detail in Part I (1). DMFC: it typically produces only one third of the The MEA of a DMFC usually consists of five lay- PEMFC's power density. Hence, the DMFC com- ers which include gas and liquid diffusion layers, munity has made great efforts to bring the and electrocatalyst layers with a polymeric proton performance closer to that of the PEMFC, and conducting acidic membrane in between (2). The particularly to extend the maximum operating tem- proton conducting membrane acts as an electronic perature. The majority of the work has involved insulator between the electrodes, but allows protons PhfimmMetals Rev., 2002,46, (4), 146164 146 I Fig. I The perjhnance losses seen in a typical DMFC MEA operuting with dilute MeOH and air at SOT,compared to those in a PEMFC. The PEMFC is operating with pure hydrogen. A list of furtors affecting the eflciencies of both .fuel cells is on the right in the Figure to migrate efficiently from the anode to the cath- tice, the cell voltage in both fuel cells is much less ode. The membrane also functions as a physical than this, see Figure 1. For example, at a current barrier to prevent mixing of the reactants. In ad&- density of 500 mA cm”, the cell voltage is typical- tion, a soluble form of the membrane mated is ly around 0.75 V for the PEMFC (1) and 0.4 V for used to impregnate the electrocatalyst layers to the DMFC (3). Therefore, the power density and extend the membrane interface. This provides a efficiency are considerably hgher in the PEMFC proton conducting pathway. (61 per cent) than in the DMFC (34 per cent). While the structures of the MEAs used in the PEMFC and DMFC are similar, the performance The Effect of Poor Kinetics of each is very different. A comparison of the per- Both types of fuel cell are limited by the poor formance of the two fuel cells and the factors electrochemical activiy of their cutbode-r, for rea- which limit their efficiencies is shown in Figure 1. sons described in Part I (1). This reduces the cell The DMFC has a maximum thermodynamic voltage of both by up to 0.4 V at 500 mA voltage of 1.18 V at 25”C, dehned by its anode and However, unlike the PEMFC (when operated cathode half-cell reactions: with pure hydrogen), the DMFC anode is also h- ited by poor electrochemical activity (kinetic loss [8] Anode reaction: CH30H + H20 = COz + 6H’ + 6e- in Figure 1). This can account for a further loss in E, = 0.046 v (i) cell voltage of more than 0.3 V at 500 mA cm-’ (at Cathode reaction: 3/202 + 6H’ + 6e- = 3H20 900C). E”, = 1.23 V (ii) To increase both the anode and cathode activi- ties in the DMFC, the electrocatalysts employed Cell reaction: CH3OH + HzO + 3/202 = COZ+ 3H20 are unsupported (with high Pt loadmgs of Encd= 1.18 V (ii) usually typically 5 to 10 mg Pt cm-’ for each electrode) In comparison, the PEMFC has a maximum rather than the carbon-supported electrocatalysts thermodynamic voltage of 1.23 V at 25°C. In prac- used in the PEMFC. This Pt loading is too high Phtinnm MefaLr h.,2002,46, (4) 147 for commercial exploitation of the DMFC (but it methanol concentrations used in the DMFC are does of course dramatically increase the power low, the anode structure has to be designed to allow densities attainable by the MEA). By contrast, typ- both efficient diffusion of the liquid fuel into the ical PEMFC electrodes are carbon-supported electrocatalyst layer and effective removal of the electrocatalysts, loaded at 0.2 to 0.5 mg Pt cm”. product carbon dioxide (COz). Correct design of the anode electrode spucture is very important for Fuel Crossover limiting anode mass iransportlosses ([GI in Figure 1). Another critical effect, which reduces the effi- ciency of the DMFC, is fuel crossover (methanol Anode Electrocatalyst Limitations ~JJOVW[4] in Figure 1). Methanol and water readi- Although the electrooxidation of methanol is ly diffuse through all the commercially available thermodynamically driven (by the negative Gibbs polymeric membrane electrolytes (such as Nafion), free energy change, AG, in the fuel cell), in prac- and significant quantities of methanol and particu- tice, the rate of methanol electrooxidation is larly water pass from the anode to the cathode. severely limited by poor reaction kinetics. To This reduces the cathode efficiency in two ways. increase the efficiency of the anode reaction, it is First, any methanol that comes into contact with necessary to understand the reaction mechanism. the cathode electrocatalyst will reduce the efficien- Indeed, there are now probably over 100 published cy of the oxygen reduction reaction by a compet- papers that deal with identifying the nature and ing electrochemical process - known as the mixed rate limiting steps of this reaction (7). potential effect. Second, the cathode structure The most likely reaction scheme to describe the becomes waterlogged or flooded, and is no longer methanol electrooxidation process is shown in an efficient structure for gas diffusion (mars trans- Figure 2 (Steps i to viii). Only Pt-based electrocat- port loss, [3] in Figure 1). Both these effects can alysts display the necessary reactivity and stability reduce the cell voltage by a further 0.2 to 0.3 V, in the acidic environment of the DMFC. particularly when practical air flows are used. Spectroscopic studies on polycrystalline Pt have In practice, the effects of methanol crossover shown that methanol is electrosorbed in a complex can be reduced to a large extent by careful design process analogous to dehydrogenation.