The Electrochemical Stability and Calculated Free Energies of PtCr Alloys

Abstract

In this investigation the calculated free energies and electrochemical stability of the normalPtCr system are presented as well as the calculated free energies of the normalPtZr , normalPtTi , and the normalPtV systems. These alloy systems are currently of interest as fuel cell electrodes providing dealloying does not occur and cause degradation of performance. The calculations indicate that the normalPtZr and normalPtCr systems are the most stable with respect to their elements, followed by normalPtTi and finally by the normalPtV system. These results are consistent with the experimental data of previous investigations involving the compositional stability in a fuel cell environment for Pt4normalZr , Pt3normalTi , and Pt3V . The normalPtCr calculations are compared with the experimental electrochemical results of this investigation and are found to support the experimental findings in the low chromium concentration samples but do not support the findings for the higher chromium concentration alloys. Experimental results indicated that the samples of 0–35 atom percent (a/o) Cr were electrochemically very stable although the 25 a/o disordered specimen did undergo some minor changes during cycling. The 50 and 80 a/o Cr samples were quite unstable during cycling, yielding a large surface area increase and a depleted Cr layer. The Cr sample was also quite unstable as seen by the surface pitting which occurred.

Effect of praseodymium oxide and cerium–praseodymium mixed oxide in the Pt electrocatalyst performance for the oxygen reduction reaction in PAFCs

“…1. The electrolyte was H 3 PO 4 (Sigma-Aldrich), purified using the standard procedure described in the literature [16]. A dynamic hydrogen electrode (DHE) was used as the reference electrode.…”

Catalytic Activities of Pt–Metal Alloys on Oxygen Reduction Reaction in Fluorohydrogenate Ionic Liquid

“…Various potential cathode catalysts such as Pt-M (M = Fe, Co, Ni, etc.) alloy catalysts, [12][13][14][15][16][17][18][19][20][21][22][23] Pt core-shell catalysts, 24,25 and Ptfree catalysts 26,27 have been investigated by many research groups. Among these, Pt alloy catalysts were found to exhibit better ORR activities than Pt in acidic solutions.…”

Relating Structural Aspects of Bimetallic Pt3Cr1/C Nanoparticles to Their Electrocatalytic Activity, Stability, and Selectivity in the Oxygen Reduction Reaction

“…The Pt-Cr alloy catalyst has higher stability than other Pt-M alloys used for the ORR, for example, Pt-Cu, Pt-Mn, and Pt-Fe alloys, which are unstable to similar repetitive potential cycling, since the second metal M is leached. [33,57] The free energies of formation, which are an indication of thermodynamic stability, were calculated for various Pt-M alloys, including Pt-Cr, by Glass and Cahen [19] using the Kaufman approach, which is well-known in the areas of phase transformations and thermodynamics. It was found that Pt-Cr alloy has the largest negative free energy of formation.…”

“…The Pt-Cr alloy has also been characterized as a stable catalyst in acidic [18] and oxidizing media at high temperatures under fuel-cell operating conditions. [19] The selectivity towards the ORR, in particular for these Pt-Cr alloy NPs, has been recently tested by Lamy et al [20] In general, the selectivity is principally associated with methanol crossover from anode to cathode side, that is, passage through the DMFC membrane. This is potentially a site at which the ORR performance may be reduced due to occurrence of a mixed potential resulting from competitive reactions, such as oxygen reduction and methanol oxidation.…”

“…The variation of Cr content from 15.43 (EG catalyst) to 26.22 (TD catalyst) leads to small differences in their free energies of formation, that is, from 10.2 to 14.2 kcal g-atom À1 ; these values are still considered to give high stability compared to other bimetallic Pt alloys. [19] …”