Proton durability improvement

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

The stability and durability of Pt alloys, especially those involving a >d transition metal, are the major hurdles preventing them from commercial fuel cell applications. "" The transition metals in these alloys are not thermodynamically stable and may leach out in the acidic PEM fuel cell environment. Transition metal atoms at the surface of the alloy particles leach out faster than those under the surface of Pt atom layers." The metal cations of the leaching products can replace the protons of ionomers in the membrane and lead to reduced ionic conductivity, which in turn increases the resistance loss and activation overpotential loss. Gasteiger et al. showed that preleached Pt alloys displayed improved chemical stability and reduced ORR overpotential loss (in the mass transport region), but their long-term stability has not been demonstrated. " These alloys experienced rapid activity loss after a few hundred hours of fuel cell tests, which was attributed to changes in their surface composition and structure." ... 

To allow PEM fuel cells to operate under the hotter, drier conditions required for widespread use in applications such as automobiles, new materials are needed. These include new electrolytes with higher proton conductivity and improved durability at low relative humidity (RH) and at higher temperatures. New electrodes that can provide adequate performance with less water are needed. 

The ideal additive would enhance proton conductivity and stabihty. One demonstration of this was in a composite PFSA membrane using Pt nanoparticles supported on titania or silica [63]. The composite membranes when employed in MEAs demonstrated unhumidifled fuel cell performance comparable to that of a similar humidified fuel cell. Whether adding Pt to the membrane will help durability or hurt it is still a matter of some debate [64, 65]. Unfortunately, it is not commercially feasible at this time to add additional Pt to the MEA and so this approach while novel is not practical. The HPAs are known peroxide decomposition catalysts and so these inorganic oxides have been demonstrated to improve performance and decompose peroxide in fuel cells and if they could be immobilized would present a practical solution to this problem [66]. 

Many structures of TiOj and substoichiometric Ti02 exist which seem suitable for PEMFC catalyst supports. The added benefit of proton conductivity and improved durability in the catalyst support is advantageous over traditional carbon black supports however, the performance of Pt/TiOj catalysts are lacking according to pubhshed research. More directed research with these types of catalyst supports might prove effective for high durability, low Nafion-loaded MEA applications. 

---