Electrodeposited Pt/Rare Earth Metals As Catalysts for the Oxygen Reduction Reaction

The main problem arising at the cathode in proton exchange membrane fuel cells is the voltage drop due to the sluggish oxygen reduction reaction (ORR) kinetics. The limiting state-of-the-art operating potential is 0.7 V which is far from the equilibrium potential of 1.2 V [1]. Reducing the Pt loading without compromising fuel cell performance is an effective strategy to meet the cost requirements for fuel cell commercialization. One possibility is alloying Pt with other metals. Some Pt-early transition metal alloys and Pt-rare earth alloys (e.g. Pt₃Y, Pt₅Gd, Pt₅La and Pt₅Ce), which exhibit a high activity towards the ORR as well as a high stability in acidic media, are promising candidates for cathode catalysis [2-5]. However, the electrochemical preparation of rare earth metals and especially Pt_xM alloys (M = La, Gd, Y, Sc) is not well examined [6, 7]. The main challenge during the electrochemical deposition of these metals results from their negative deposition potentials (La³⁺ / La ~ -2.379 V, Y³⁺ / Y ~ -2.372 V, Gd³⁺ / Gd ~ -2.279 V vs. SHE [8]), whereas the standard potential of Pt²⁺/ Pt is 1.188 V vs. SHE, which is nearly 3.5 V more positive. Therefore, the deposition of rare earth metals from aqueous solutions is prevented by the decomposition of water at a more positive potential. Therefore, the deposition of these metals and their alloys with Pt have been tried in ionic liquids which are stable over a wide potential window at different substrates. However, the deposition processes in ionic liquids still are not fully understood, and deposits obtained often cannot be dissolved reversibly. The focus of this work lies in the discussion of alternative ways to produce nanoparticles of these metals and their alloys with Pt. Non-aqueous solvents able to dissolve the rare earth metal precursors and stable over a wide potential window were selected. Boron-doped Diamond (BDD)  was chosen as a substrate as it is a rather inert electrode material with a wide electrochemical potential window in aqueous as well as non-aqueous media [9]. The deposition of the materials from these solvents are discussed based on electrochemical methods , the results from surface analytical techniques and ex-situ scanning probe techniques as well as electrocatalytic measurements for the ORR.

References

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