科研之友
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摘要
We studied the borohydride oxidation reaction (BOR) by voltammetry in 0.1 M NaOH/10(-3) M BH(4)(-) on carbon-supported Pt, Ag and alloyed PtAg nanoparticles (here-after denoted as Pt/C, Ag/C and Pt-Ag/C). In order to compare the different electrocatalysts, we measured the BOR kinetic parameters and the number of electrons exchanged per BH(4)(-) anion (faradaic efficiency). The BOR kinetics is much faster for Pt/C than for Ag/C (i(pt) = 0.15, i(Ag) = 3.1 x 10(-4) A cm(-2) at E = -0.65 V vs. NHE at 25 degrees C), but both materials present similar Tafel slope values. The n value involved in the BOR depends on the thickness of the active layer of electrocatalysts. For a "thick layer" (approximately 3 mu m), n is nearly 8 on Pt/C and similar to 4 on Ag/C, whereas n decreases for thinner Pt/C active layers (n similar to 2 for thickness < 1 mu m). These results are in favour of the sequential BH(4)(-) hydrolysis (yielding H(2)) followed by hydrogen oxidation reaction (HOR), or direct sequential BOR on Pt/C, whereas Ag/C promotes direct but incomplete BOR (Ag has no activity regarding hydrogen evolution reaction, HER). The n value close to 8 for the thick Pt/C layer displays the sufficient residence time of the molecules formed (H(2) by heterogeneous hydrolysis or BOR intermediates) within the active layer, which favours the complete HOR and/or BOR. Two PtAg/C nanoparticles alloys have been tested (noted APVES-4C and APVES-E1). They show different behavior: the borohydride oxidation reaction kinetics is faster on APVES-E1 than on APVES-4C (b = 0.15, i(-0.65V) = 0.09 and b = 0.31 V dec(-1), i(-0.65V) = 6.3 x 10(-3) A cm(-2), respectively, at 25 degrees C), but the n values are higher on APVES-4C than APVES-E1 (nearly 8 vs. 3, respectively. at 25 degrees C). These discrepancies probably originate from the heterogeneity of such bimetallic materials, as observed from physicochemical characterizations.
- 出版日期2009-11-1
