H2O2 Electroreduction at Platinum-Rare Earth (RE = Ce, Sm, Dy, Ho) Cathodes of Direct Borohydride Fuel Cells

Direct borohydride/peroxide fuel cells (DBPFCs) use sodium borohydride (NaBH₄) as an anodic fuel and hydrogen peroxide (H₂O₂) as the oxidant. DBPFCs present higher theoretical cell voltage and higher energy density than most low temperature fuel cells. Additionally, the fact that both fuel and oxidant are liquids at room temperature simplifies their storage, thermal management and internal processing [1]. For these reasons, DBPFCs are promising alternatives as power sources for space, underwater, and specific terrestrial applications where oxygen is not available.

To push forward the DBPFC, it is necessary to develop efficient electrocatalytic materials towards hydrogen peroxide reduction reaction (HPRR), at the lowest possible cost. Platinum (Pt) alloys are known to possess good catalytic activities for HPRR [2]. Recently, it was shown that Pt-rare earth (RE) alloys presented improved catalytic activity for hydrogen evolution reaction, at much lower cost [3]. However, virtually nothing is known about the effects of RE elements on the electrocatalytic behavior of Pt towards the HPRR.

In this study, the HPRR is evaluated in four different Pt-RE intermetallic alloy electrodes, namely Pt-Ce, Pt-Sm, Pt-Dy, and Pt-Ho, all having equiatomic composition. The alloys were prepared by arc melting, starting from the stoichiometric amounts of the two parent metals, and analyzed by XRD and SEM coupled with EDX.

Electrochemical experiments were performed using a standard three-electrode setup, using each of the Pt-RE alloys as a working electrode. The alloys were tested in 2 M NaOH + 0.4 M H₂O₂aqueous electrolytes, at temperatures ranging from 25 ºC to 55 ºC. Cyclic voltammetry (CV), linear scan voltammetry (LSV), chronoamperometry (CA), and chronopotentiometry (CP), were used to investigate the electrodes activity for HPRR reduction in alkaline media. CVs of the alloys presented a well-defined cathodic peak. Plots of peak currents and peak potentials as a function of the potential scan rate allowed calculation of the number of exchanged electrons and of the charge transfer coefficients for the studied temperature range. CA and CP measurements demonstrated good stability of the tested systems.

A small lab DBPFC was assembled using a Pt-RE cathode and main cell parameters (e.g., open circuit voltage, peak power density) were evaluated for temperatures ranging from 25 to 55 ºC. Results show that Pt-RE electrocatalysts show good activity for HPRR at about half of the cost of single Pt electrodes.

1. D.M.F. Santos, C.A.C. Sequeira, Sodium borohydride as a fuel for the future, Renew. Sustain. Energy Rev. 15 (2011) 3980.

2. A.L. Morais, J.R.C. Salgado, B. Šljukić, D.M.F. Santos, C.A.C. Sequeira, Electrochemical behaviour of carbon supported Pt electrocatalysts for H₂O₂reduction, Int. J. Hydrogen Energy 37 (2012) 14143.

3. D.M.F. Santos, C.A.C. Sequeira, D. Macciò, A. Saccone, J.L. Figueiredo, Platinum-rare earth electrodes for hydrogen evolution in alkaline water electrolysis, Int. J. Hydrogen Energy 38 (2013) 3137.