Low-Temperature Cathodes

General strategies to develop low-temperature cathode materials include the following  

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Fig. 14.21 (a) Polarization resistance (partly determined by the catalytic properties of the electrode) of cathode materials, as measured by impedance spectroscopy of symmetrical cells, (b) The area enclosed by the box (bottom left) represents the target area for low-temperature cathode development. 

G. Schoemakers, B. Rietveld, P.V. Aravind, Development of low temperature cathode materials, Proceedings of 7th European SOFC Forum, Luzern, Switzerland, 2006. European Fuel Cell Forum, Oberrohrdorf, Switzerland (2006). 

It is very important to develop a high performance cathode catalyst, because a sluggish ORR causes a large overpotential at low temperatures. With respect to the total performance of activity and stability, the cathode catalyst material is limited to Pt or its alloys at present. In acidic media such as Nation electrolyte or aqueous acid solutions, four-electron reduction is dominant at Pt-based electrodes ... 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Ralph TR, Hogarth MP. 2002a. Catalysis for low temperature fuel cells. Part I The cathode challenges. Platinum Metals Rev 46 3-14. 

In addition to their proven capacity to catalyze a highly efficient and rapid reduction of O2 under ambient conditions (e.g., cytochrome c oxidase, the enzyme that catalyzes the reduction of >90% of O2 consumed by a mammal, captures >80% of the free energy of ORR at a turnover frequency of >50 O2 molecules per second per site), metalloporphyrins are attractive candidates for Pt-free cathodes. Probably the major impetus for a search for Pt-free cathodic catalysts for low temperature fuel cells is... 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

The effects of transfer of atoms by tunneling may play an essential role in a number of phenomena involving the transfer of atoms and atomic groups in the condensed phase. One may expect that these effects may exist not only in the proton transfer reactions considered above but also in such processes as the diffusion of hydrogen atoms and other light ions (e.g., Li+) in liquids, tunnel inversion and isomerization in some molecules, quantum diffusion of defects and light atoms in the electrode at cathodic incorporation of the ions, ion transfer across the liquid/solid interface, and low-temperature chemical reactions. 

Zhou, F., Zhao, X., Zheng, H., Shen, T. and Tang, C. (2005) Low-temperature refluxing synthesis of nanosized LiMn204 cathode materials. Chemistry Letters, 34, 1270-1271. 

Pereira-Ramos, J.-P., Electrochemical properties of cathodic materials synthesized by low-temperature techniques, J. Power Sources 54, 120-126(1995). 

In addition to bilayered anode and cathode functional layer and current collector/sup-port layer combinations, bilayered electrolyte structures are commonly fabricated, particularly for low-temperature operation below 700°C, by a variety of processing methods. Bilayered electrolytes are used for several purposes ... 

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