Electrocatalysts current research

E25.17 Electrocatalysts are compounds that are capable of reducing the kinetic barrier for electrochemical reactions (barrier known as overpotential). While platinum is the most efficient electrocatalyst for accelerating oxygen reduction at the fuel cell cathode, it is expensive (recall Section 25.18 Electrocatalysis). Current research is focused on the efficiency of a platinum monolayer by placing it on a stable metal or alloy clusters your book mentions the use of the alloy PtsN. An example would be a platinum monolayer fuel-cell anode electrocatalyst, which consists of ruthenium nanoparticles with a sub-monolayer of platinum. Other areas of research include using tethered metalloporphyrin complexes for oxygen activation and subsequent reduction. 

Gas-phase devices treating dilute components must deal with high concentration overpotentials and subsequent low current densities. Technical solutions to this problem have been found in the case of liquid treatment, as outlined earlier similar solutions may be found for gases. Electrocatalysts for selective oxidation of gaseous components are only now receiving some attention (54). Thus, there appears to be fertile ground for explorative research in these areas. 

A particularly difficult problem of the ORR electrocatalysis is the high loss of potential, which is a substantial source of the decline in the efficiency of fuel cells. As mentioned before, another drawback (from cost point of view) is the high ft loading in cathode or the low mass-activity (current per mass of active metal) in the regular Pt-based electrocatalysts [10, 11]. Thus, the research in the ORR electrocatalysis aims at developing better electrocatalysts in order to reduce the cathode overpotential and the total mass of Pt [12, 13]. 

The effect of the presence of methanol on the ORR activity of commercial Pt/C and as-prepared Pd4Coi/C is presented in Fig 5.8b. As compared to the ORR in pure HCIO4 solution, both catalysts exhibited an increase in overpotential under the same current density in the presence of methanol. For the ORR on Pt/C in methanol-containing solution, the overpotential increases by ca. 200 mV, while only a small negative shift of ca. 15 mV was observed for PdtCoi/C catalyst. In fact, the results obtained by the researchers indicate that the Pd4Coi/C electrocatalyst is very active for the ORR even at a high concentration of methanol. 

Platinum and Pt-based materials are currently the best electrocatalysts for these reactions. The price and the hmited reserves of Pt are the prime obstacles to adequately developing this major field. Extensive research efforts have been devoted to the development of highly active and cost-effective electrocatalysts. This chapter describes recent advances in electrocatalysts for anodic reactions in low-temperature fuel cells that use methanol and ethanol as fuels in acidic media. Special attention is focused on the effort to decrease Pt content in the catalysts. Electrocatalysts employed in alkaline fuel cells are not discussed as they have been adequately covered in Chap. 5. 

Thus far, aU N-doped carbons appear to only be able to reduce oxygen oti graphitic-type nitrogen atoms, and this reduction only leads to the production of H2O2. However, experimentally, it is known that metal-free N-doped carbons are capable of reducing O2 to water with an apparent transfer of 4e, as reported by various research groups covered in this chapter. It is also clear from Fig. 3 in ref. [116], for instance, that similar limiting current densities are reached at the same rotation rate (from 100 to 1,600 rpm) for ORR on a metal-fi ee N-doped carbon, a Fe/N/C or a Pt/C electrocatalyst. 

A particular approach adopted by General Electric In U.S.A. is the solid polymer electrolyte (SPE) cell in which the porous cloth-type separator is replaced by a polymeric ion exchange membrane which is conductive to cations (Figure 5). The particular membrane employed, NAFION, is a perfluorsulphonlc acid pol3nner which is extremely stable in both acid alkaline solution. Appropriate electrocatalysts are coated on each face of the polymer sheet and these are contacted by a metal mesh current collector. Further research is aimed at reducing the cost and improving the electrical efficiency of the system to make it competitive with conventional electrolyzers. 

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