Borohydride Electro-Oxidation on Ni-Based Electrocatalysts: Investigation of Electrocatalytic Activity and Hydrolysis Process

Direct borohydride fuel cells (DBFCs) or borohydride batteries are promising devices since they present high energy density and involve easy handling and storage of the fuel (1). Sodium borohydride can be used directly or dissolved in an alkaline electrolyte (2), which enable the use of non-noble metals as catalysts. However, the development of such devices is hampered by the low efficiency BH₄^- electro-oxidation, because of the parallel occurrence of BH₄^- hydrolysis, leading to borates and hydrogen evolution (3). Therefore, the development of electrocatalysts that allow minimization of the hydrogen release is of paramount importance for improving the viability of such electrochemical systems. In this context, this work aims at investigating the electrocatalytic activity and the extent of BH₄^- hydrolysis at different nickel-based materials (Ni powder, NiO/C and LaNi_4.7Sn_0.2Cu_0.1).

The NiO/C catalysts were prepared by a thermal decomposition method (4, 5) with 33 wt.% and 66 wt.% of metal on Vulcan carbon (XC-72R). The metal hydride alloy was prepared from high-purity metals by the arc melting technique under an inert gas atmosphere. The electrochemical cell used was jacketed to control de reaction temperature at 25°C. The working electrode was prepared depositing 10 mg of an ink composed by 50 mg of catalyst, 50 mg of carbon, 0.984 mL of water, 0.200 mL of iso-propanol and 0.516 mL of Nafion (5 – 10 % of water), on carbon paper (Toray). To improve the overall conductivity and the catalyst adhesion, the composites were hot-pressed for 5 min at 5 bar and 125oC. An Hg/HgO electrode was used as a reference electrode and Au wire served as counter electrode. Before and after the electrochemical tests, the electrocatalysts were characterized by XRD, XPS and TEM. Cyclic voltammetry (CVs) was performed in 1 M NaOH electrolyte in the absence and in the presence of 0.5 M sodium borohydride. Differential electrochemical mass spectrometry (DEMS) has been employed to evaluate the hydrogen formation during the borohydride oxidation. 

CV results with the electrocatalysts composed by nickel powder and carbon-supported nickel nanoparticles have shown high BH₄^- oxidation currents, with an onset potential located at ca. -0,4V vs Hg/HgO. Chronoamperometric results presented a stable current profile. Furthermore, the results have shown that the exposition of the electrode to 0.5 M borohydride solution leads to significant hydrolysis. Interesting, for pure nickel particles (nickel powder), the hydrogen release resulting from BH₄^- hydrolysis ceased after 3 hours, when the electrode was polarized above the BOR on set potential. On the other hand, the results for LaNi_4.7Sn_0.2Cu_0.1 have shown that the hydrogen release initiates only after several hours of BH₄^- oxidation, but it suffered deactivation during prolonged polarization measurements. These results for the pure Ni catalysts evidence that the surface phase produced during the BOR, becomes inactive for the BH₄^- hydrolysis, but remained active for the BOR. This would increase the overall faradaic efficiency. For the alloyed material, the occurrence of a segregation phenomenon of the Ni atoms to the surface may initiate the catalysis of the hydrolysis process.

References

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