Comparative Study of Borohydride Oxidation, at Polycrystalline and Nanostructured Copper Electrodes

Direct borohydride fuel cell (DBFC) is an energy supplier device, which requires a catalyst for the oxidation of the BH₄^- anion; Au nanoparticles supported on carbon substrates has been the best electrode material, followed by Ag [1]. Metals of the group IB show a similar electronic configuration in their valence layer seem to be able to catalyze the borohydride oxidation reaction (BOR). Therefore, metallic copper that belongs to the IB as gold and silver might be a useful-material as catalyst for the BH₄^-. In recent studies it has been shown that Au-Cu bimetallic nanoparticles exhibit  higher catalytic activity for the direct oxidation of BH₄^- than carbon supported pure nanosized Au catalyst [2]. However, up to our knowledge there is not a complete electrochemical study reporting the use of copper nanoparticles as catalyst for BOR in alkaline media. In this work, Cu nanoparticles were synthesized by a chemical approach, using ascorbic acid as a reducing agent as well as stabilizer to control the size of the metallic nanoparticles [3];  subsequently the system has been supported in a vulcan substrate by impregnation followed by a pyrolysis process in order to eliminate organic portion and to prevent the particles agglomeration. Carbon paste electrodes (CPE) were constructed with this composite material. Additionally a Cu foil (99.98%) was used as working polycrystalline electrode to compare the borohydride oxidation with the metallic nanoparticles.

The oxidation of NaBH₄ on copper electrodes was studied by cyclic voltammetry at different scan rates and concentrations of NaBH₄. Figure 1 shows cyclic voltammograms obtained in a 3M NaOH + 1M NaBH₄ aqueous solution of  a polycrystalline Cu electrode (curve a) and copper nanoparticles (curves b and c).  Curve c shows the study in the absence of borohydride. The experiments started at -1.2 V vs. Ag/AgCl towards positive potential values at a potential sweep scan rate of 100 mVs^-1. In these curves, five anodic peaks can be observed, namely peak A1, assigned to the formation of an intermediary due to the hydrolysis of borohydride [4],  peak A2 corresponds to the formation of Cu₂O (Cu I), peak A3 corresponds to subsequent oxidation in CuO (Cu II), peak A4 corresponds to a parallel reaction forming Cu(OH)₂ (Cu II), and finally, peak A5 results from both the formation of Cu (III) and the borohydride oxidation BH₄ [5]. Shift in the potentials associated to the peaks A3, A4 and A5 can be observed when compared the voltammogram on the nanostructurated electrode with the PCE. As far as the borohydride oxidation is concerned, the oxidation starts at +0.3 V vs.Ag/AgCl with copper nanoparticles, while in the PCE the value is +0.4 V. A possible correlation between particle size and reactivity will be explored.

References

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