PEMFC Tolerance

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Mukeijee S, Urian RC, Lee SJ, TicianeUi EA, McBeen J. 2004. Electrocatalysis of CO tolerance by carbon-supported PtMo electrocatalysts in PEMFCs. J Electrochem Soc 151 A1094-A1103. 

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

The PEMFCs require expensive polymer membrane (e.g., Nation ), and operate at a low temperature (e.g., 80°C). Although low temperature reduced the cost of material, the heat generated at low temperatures is more difficult to remove. Alternate proton conducting membranes (e.g., inorganic polymer composites) that will operate at a high temperature (e.g., 200°C) are required. The expensive platinum catalyst used for electrochemical reactions can be poisoned by even trace amounts of carbon monoxide in the hydrogen fuel stream. Hence, a more tolerant catalyst material needs to be developed. 

A typical problem to fuel cells operating at low temperatures comes from the catalyst, which can be damaged (or poisoned ) by the presence of CO or C02 and needs to be replaced AFC and PEMFC are rather intolerant to C02 and CO, while PAFC is moderately tolerant to CO and MCFC and SOFC are fully tolerant to CO. 

Besides the PEMFC being developed for vehicle propulsion, SOFC are being considered for APU applications in vehicles, since they operate at very high temperatures and therefore require long start-up times (an hour or more). In APU applications, the fuel cell can be left running most of the time, or could be started far in advance of an anticipated stop. The principal attraction of SOFCs is their high tolerance to hydrocarbon fuels. The heat of the SOFC can be used in the air-conditioning unit, either as heat or as cold. 

The strategy towards CO tolerance has therefore been changed, towards the development of proton conducting polymers suitable for high-temperature operation of the PEMFC, i.e., 120 °C and higher. It is already demonstrated [68] that at this temperature, 1000 ppm CO leads to only minor loss of performance. The high temperature operation will be further addressed in the final section of this chapter. 

S. C. Ball and D. Thompsett. Material Research Society Symposium Proceedings (2003), 756 (Solid State lonics-2002), 353-364 Ultra CO tolerant PtMo/PtRu anodes for PEMFCs... 

CO contamination is widely documented in the literature and recognized as a serious issue in the investigation of PEMFCs a decline in PEMFC performance is very often due to deactivation of the Pt anode catalyst, caused by traces of CO. Many attempts [14, 41, 42] have been made to use the EIS method to understand the mechanisms of CO poisoning and CO tolerance, by feeding an anode gas mixture of CO and H2. Examples of such studies will be discussed in detail in Chapter 6. 

Kim JD, Park YT, Kobayashi K, Nagai M, Kunimatsu M (2001) Characterization of CO tolerance of PEMFC by ac impedance spectroscopy. Solid state ionics diffusion and... 

PEM fuel cells use a solid proton-conducting polymer as the electrolyte at 50-125 °C. The cathode catalysts are based on Pt alone, but because of the required tolerance to CO a combination of Pt and Ru is preferred for the anode [8]. For low-temperature (80 °C) polymer membrane fuel cells (PEMFC) colloidal Pt/Ru catalysts are currently under broad investigation. These have also been proposed for use in the direct methanol fuel cells (DMFC) or in PEMFC, which are fed with CO-contaminated hydrogen produced in on-board methanol reformers. The ultimate dispersion state of the metals is essential for CO-tolerant PEMFC, and truly alloyed Pt/Ru colloid particles of less than 2-nm size seem to fulfill these requirements [4a,b,d,8a,c,66j. Alternatively, bimetallic Pt/Ru PEM catalysts have been developed for the same purpose, where nonalloyed Pt nanoparticles <2nm and Ru particles <1 nm are dispersed on the carbon support [8c]. From the results it can be concluded that a Pt/Ru interface is essential for the CO tolerance of the catalyst regardless of whether the precious metals are alloyed. For the manufacture of DMFC catalysts, in... 

The material of PtRu alloy exhibits good properties for CO tolerance in polymer electrolyte membrane fuel cells (PEMFC) [68] and has been studied extensively in recent years [69]. Particular interest has been focused on the application of the PtRu alloy materials as anodes in methanol fuel cells (MFC) for electric vehicles [70]. The most convenient way to alter the surface composition of a PtRu alloy is to employ the electrochemical co-deposition method in the preparation of the alloy. Richcharz and co-workers have studied the surface composition of a series of PtRu alloys using X-ray photoelectron spectroscopy (XPS) and low-energy ion spectroscopy (LFIS)... 

The overall objective is to operate PEMFCs at lOO-MO C to improve CO tolerance, mitigate water and thermal management challenges and reduce membrane cost. The basic approach is to develop a composite membrane consisting of mechanical support and high-temperature proton conduction phases. In order to improve cathode performance, modification of cathode formulation and structure is on-going. Promising solid superacids are incorporated into the cathode. 

In June 2001 we initiated this project to explore the possibilities of decreasing the Pt loading in Pt-Ru catalysts for H2/CO oxidation in the polymer electrolyte membrane fuel cells (PEMFCs). We have demonstrated a new method for the preparation of the Pt-Ru catalysts involving spontaneous deposition of Pt on Ru nanoparticles that we explored first with single crystal Ru surfaces. The resulting catalysts have a high CO tolerance with considerably lower Pt loading than the commercial catalysts. 

PA. Adcock, S. Pacheco, E. Brosha, T. Zawodzinski, and F. Uribe, "Maximization of CO Tolerance of PEMFC Systems Using Reconfigured Anodes". To be presented at the Electrochemical Society Meeting, Salt Lake City, UT (Fall 2002). 

F. Uribe and T. Zawodzinski, "PEMFC Reconfigured Anodes for Enhancing CO Tolerance with Air Bleed", 201st Electrochemical Society Meeting, Philadelphia (2002). Abstract No. 806. 

The phosphoric acid fuel cell (PAFC) has a quite similar construction and components as the PEMFC the electrolyte is liquid phosphoric acid in an inert matrix. The operation temperature of 200°C avoids formation of liquid water and improves CO tolerance of the electrocatalyst. For the catalyst properties, the same requirements are valid as for the PEMFC - nanoparticles with a high surface area and a good dispersion on the carbon carrier material are required. The application of PAFC typically is the combined heat and power supply in the 200-kW power range. 

The electrocatalysts for PAFC and PEMFC are quite similar the increase in operation temperature from 80°C being typical for the PEMFC to 200°C for PAFC is not so significant that the use of expensive noble metals could be avoided. The CO tolerance of the PAFC anode is much better with a tolerable level of approximately 1 vol% of CO compared with 10 - maximum lOOppm in short transients for the PEMFC. Thus, there were a lot of parallels in the catalyst development, and PEMFC developers could make use of the insight gained by PAFC development and vice versa. Therefore, the following chapter refers to the PEMFC but uses results being generated for PAFC. 

One of the drawbacks of the DMFC is that the low-temperature oxidation of methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which typically means that a larger quantity of expensive platinum catalyst is required than in conventional PEMFCs. In addition, the anode has a limited carbon monoxide tolerance. Further, the overall effrdency is smaller than for a PEMFC. 

---