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Effect of Additives on the Direct Oxidation of Sodium Borohydride

Author: LeiShu
Tutor: ChenChangGuo
School: Chongqing University
Course: Physical and chemical
Keywords: Sodium borohydride Direct oxidation Hydrolysis Crossover Additives
CLC: TM911.4
Type: Master's thesis
Year: 2009
Downloads: 106
Quote: 6
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Abstract


Direct sodium borohydride fuel cell (DBFC) is greatly concerned because of its higher theoretical energy density (9.3Wh/g) , theoretical open circuit voltage (1.64V) and theoretical conversion efficiencies(0.91), and will gradually become an important portable chemical power.In recent years, DBFC development faces two main technique obstacles: In the first place, sodium borohydride(NaBH4) carries out direct oxidation and hydrolysis reaction in the same time, so the anodic open-circuit potentials were essentially mixed potentials, the fuel efficiency and energy conversion efficiency decreased. Secondly, borohydride ions (BH4– ) crossover through the membrane and the electrochemical oxidization of BH4– on cathode carries out, so the number of electrons of the reaction can not generate 8e-. Therefore, it is great significance problems of DBFC commercially available development how to inhibit of BH4– catalytic hydrolysis reaction and crossover.In this dissertation, the effect of experiment conditions on direct oxidation of sodium borohydride alkaline solution is studied by linear scan voltammetry and cyclic voltammetry. The results have shown that adjusting the C (NaOH)/C (NaBH4) value,sodium borohydride direct oxidation was less affected on the bubbles generated by the hydrolysis and electrochemical reaction can be fully carried out on Pt flake electrode with the range of temperature from 293.15 K to 313.15K.Secondly, number of electrons exchanged of sodium borohydride direct oxidation depended on the anode catalyst. Experimental results show that: when using platinum as catalyst, there are two consecutive oxidation reactions of NaBH4 in alkaline solution occurs.The direct electrochemical oxidization of BH4– on Pt near -0.4V, which is response to a 7.4e- process, while the latter electrooxidation of intermediates BH3OH? from the catalytic hydrolysis of NaBH4 yields a 2.4e- process.When using nickel as catalyst, as compared with platinum, NaBH4 electrochemical oxidation potential more negative, but the formation of oxidized nickel film on surface in alkaline solution, on which the catalysis activity of NaBH4 is depressed and the number of electrons transfer decreased.The cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry are used for the study on influences of different additives in electrolyte for NaBH4 electrochemical oxidization behavior on Pt. The results have shown that the different additives with the unshared pair electrons are studied.With adding appropriate concentration of additive C minimizes the surface coverage of BH4– on Pt, an inhibitor of catalytic hydrolysis and the crossover, the catalysis activity of NaBH4 is improved and the number of the reactions on the electrode is promoted .In addition, there are some advantages to dcrease poisoning for metallic catalysts compare to TU, the charge transfer resistance is decreased and efficiency increased.

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CLC: > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Chemical power sources,batteries, fuel cells > Fuel cell
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