Investigation of Nickel-Rare Earth Electrodes for Sodium Borohydride Electrooxidation

Sodium borohydride (NaBH₄) is seen as a promising fuel of the future due to its chemical stability, ease in storage and handling, non-toxicity of its oxidation products and possibility of their recycling [1]. Additionally, fuel cells operating with NaBH₄as the anodic fuel and oxygen or hydrogen peroxide as the oxidant, known as direct borohydride fuel cells (DBFCs) or direct borohydride-peroxide fuel cells (DBPFCs), respectively, have higher energy density compared to other types of fuel cells [2].

However, at most electrode materials, parallel with borohydride oxidation reaction (BOR) proceeds borohydride (BH₄^-) hydrolysis reaction that results in lower BOR coulombic efficiency. Consequently, there is a wide search for anode materials for DBFCs that would catalyze only BOR and suppress BH₄^-hydrolysis.

Herein, nickel-samarium (Ni-Sm) and Ni-dysprosium (Ni-Dy) alloys, with different amounts of RE metal (from 5 to 10 at.%), were tested as electrode materials for BH₄^- oxidation in alkaline media. The alloys were prepared by arc-melting starting from the stoichiometricamounts of the parent metals and, subsequently, their microstructure and composition were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy.

Preliminary study of the alloys activity for BOR was performed using cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). CV and CA data obtained made possible evaluation of different reaction parameters including number of electrons exchanged. Influence of temperature on BOR and its parameters was studied in the 25 – 65 ºC range. This investigation further enabled calculation of the activation energy for BOR at the studied alloy electrodes.

EIS measurements were carried out mainly in the potential region of the faradaic processes of BH₄^-oxidation. Resulting capacitance-potential curves for the tested Ni-RE electrodes helped in the interpretation of the electroactivity of the materials for the BOR. Impedance diagrams at different overpotentials in the BOR region indicated the nature of the reaction mechanisms and a possible change of the overall reaction mechanism.

Furthermore, laboratory DBPFCs were constructed employing the studied Ni-RE alloy electrodes as fuel cell anodes. Main cell parameters, namely maximum power density and cell potential were evaluated for temperatures ranging from 25 to 65 ºC.

The obtained parameters were compared to those obtained for DBPFCs employing pure Ni anode, as well as with those employing other Ni-RE anodes [3].

References

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. D.M.F. Santos, P.G. Saturnino, R.F.M. Lobo, C.A.C. Sequeira, Direct borohydride/peroxide fuel cells using Prussian Blue cathodes, J. Power Sources 208 (2012) 131.
3. D.M.F. Santos, B. Šljukić, L. Amaral, D. Macciò, A. Saccone, C.A.C. Sequeira, Nickel and nickel-cerium alloy anodes for direct borohydride fuel cells, J. Electrochem. Soc. 161(5) (2014) F599.