Ruthenium crossover

P. Piela, C. Eickes, E. Brosha, F. Garzon, and P. Zelenay, Ruthenium Crossover in Direct Methanol Fuel Cell with Pt-Ru Black Anode, J. Electrochem. Soc., 151, A2053 (2004). 

Other property that could be relevant for the durability of DAFC is the ruthenium crossover, that is, the dissolution of Ru from anode catalysts containing Ru (a typical anode catalyst in DMFC) and its re-precipitation in the cathode [14]. The water and methanol permeability was expected to have an effect on Ru crossover, but the only study available on this seems to indicate that differences is not the case [15]. 

Piela P, Eickes C, Brosha E, Garzon F, Zelenay P (2004) Ruthenium crossover in direct methanol fuel cell with Pt-Ru black anode. J Electrochem Soc 15LA2053-A2059... 

DMFC performance loss due to catalyst degradation has been attributed to several factors a decrement of the electrochemically active surface area (ECSA) of the platinum electrocatalyst supported on a high-surface-area carbon, a loss of cathode activity towards the ORR by surface oxide formation, and ruthenium crossover [83, 85, 116, 117]. 

Ruthenium crossover is a DMFC specific mechanism of degradation, due to the use of Ru/Pt alloys as catalyst in the anode, which is considered one of the most active catalyst for the MOR. However, the limited stability of the ruthenium in the alloy (or in the nonalloyed fraction of the catalyst) lead to crossover of ruthenium from the anode to the cathode. The presence of Ru in the cathode of DMFC was determined by different authors resorting to CO stripping scans of the cathode [124], X-ray fluorescence [124], EDXS [122, 125] and XRAS [126]. 

Choi JH, Kim YS, Bashyan R, Zelenay P (2006) Ruthenium crossover in DMFCs operating with different proton conducting membranes. ECS Trans 1 437 45... 

Figure 13.10. DMFC performance losses caused by average (left) and extreme (right) contamination of the cathode by ruthenium crossover. The polarization plot for a DMFC with a Pt-Ru cathode, instead of a Pt cathode, is shown for reference in the left-hand graph. Cell temperature 75 °C [64]. (Reproduced by permission of ECS—The Electrochemical Society, from Zelenay P. Performance durability of direct methanol fuel cells.)...

Figure 23.14. Impact of ruthenium on oxygen reduction performance (a) CO stripping scans for the cathode and anode, (b) steady-state anode polarization plots before and alter contamination of the eathode, (c) H2-air steady-state polarization curves, and (d) DMFC steady-state polarization curves. Methanol concentration 0.3 M, anode potential during contamination 1.3 V vs. hydrogen counter/quasi-reference electrode, cell temperature 75 °C [65]. (Reprinted by permission of ECS— The Electrochemical Society, from Piela P, Eickes C, Brosha E, Garzon F, Zelenaya P. Ruthenium crossover in direct methanol fuel cell with Pt-Ru black anode.)...

Several approaches have been used to limit ruthenium crossover in DMFCs, including (1) high-temperature cure of the anode catalyst by hot-pressing (Piela et al. 2004), (2) preleach of the loose ruthenium phase in an aqueous solution of inorganic acid (Piela et al. 2004), (3) use of low-permeability manbranes (Choi et al. 2005), and (4) electrochemical removal of mobile ruthenium (Choi et al. 2005). Although not entirely preventing ruthenium contamination of the cathode, all these approaches lead to incremental lowering of currentless ruthenium contamination. 

Of the two electrodes, the anode, although made of a less stable Pt-Ru alloy, suffers lower performance loss over time than the cathode. The anode is, however, the source of mobile ruthenium species, either neutral or ionic, capable of permeating the proton-conducting membrane (ruthenium crossover) and depositing at the cathode, leading to a decrease in ORR activity of the platinum catalyst. 

Cathode catalyst oxidation is another source of a major DMFC performance loss that is associated with electrocatalytic properties of platinum in aqueous media. Unlike the performance loss caused by ruthenium crossover, the loss due to the surface oxide (hydroxide) fonnation can be easily reversed via reduction, for example, by breaking the flow of air to the cathode. 

Several methods have been developed over the years to mitigate performance losses in operating DMFCs. Some of the methods, such as cathode catalyst reduction to regain active sites lost owing to platinum oxidation, or several methods aimed at minimizing ruthenium crossover (including acid leach of mobile ruthenium species) have been quite successful. 

Johnston, C.M., Choi, J., Kim, YS. and Zelenay, P. (2(X)6) Towards understanding ruthenium crossover effects the oxygen reduction reaction on Ru-modified platinum surfaces. 209th Electrochemical Society meeting, Denver, Colorado, May 07-May 12, Abs. no. 1123. 

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