Gold-based electro catalysts on titanate nanotubess Support for direct oxidation of borohydride

Abstract

Fuel cells offer a higher specific energy density compared to the traditional batteries and can convert stored chemical energy directly into electricity, not requiring any combustion or mechanical convertor such as turbines. Direct borohydride fuel cells offer higher specific energy output among the existing fuel cell systems. Moreover, sodium borohydride being used in aqueous solution to feed the cell system eases fuel storage and handling, simplifying direct borohydride fuel cell design in terms of engineering perspective. However, there are remaining challenges –mainly arising from anode catalyst development— which prevent commercialization of direct borohydride fuel cells. These are: 1) hydrolysis of borohydride, 2) high cost of anode catalyst due to high dependency on noble metal usage, 3) fuel crossover, and 4) stability of anode catalyst. In this study, titanate nanotubes, highly stable catalyst support in alkaline or acidic medium, was synthesized by wet chemical method. The deposition of gold, nickel, cobalt, and copper onto titanate nanotubes was conducted separately using the ion-exchange deposition-reduction method. The results revealed that the maximum weight percentages of gold, nickel, cobalt, and copper deposited on the titanate nanotubes were 17.28%, 2.11%, 3.65%, and 4.00% respectively. This method allowed for the determination of adsorption isotherms for each metal, which were then used to establish a correlation between catalyst loading and the initial concentration of metal solutions. By utilizing this correlation, gold-metal composite catalysts (Au-Ni/TiNT, Au-Co/TiNT, and Au-Cu/TiNT) were synthesized on the surface of titanate nanotubes at different molar ratios, resulting in a total metal loading of 4% by weight. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) were used to test the borohydride oxidation by these synthesized composited catalysts. The cyclic voltammetry results for the developed catalysts, Au100 TiNT, Au-Ni/TiNT, Au-Co TiNT, and Au-Cu TiNT, show that there is a single oxidation peak during the forward scan, indicating the direct oxidation of borohydride. However, during the reverse scan, an additional oxidation peak is observed, which is believed to be caused by the oxidation of adsorbed intermediates such as BH3OH-. Linear sweep voltammetry used to determine kinetic parameters and diffusion coefficient number for the developed catalysts. Diffusion coefficient number for borohydride ions in 2 M NaBH4 found to be 1.7710-5 cm2 using gold rotating disc electrode. This determined diffusion coefficient number was used in determination of developed catalysts of Au-Ni/TiNT, Au-Co/TiNT, and Au-Cu/TiNT. Transferred electron numbers and apparent rate constant for the developed catalyst were calculated at different potentials. Chronoamperometry was deployed to investigate further catalytic activities and stabilities of developed catalysts. Keywords: borohydride oxidation, gold-nickel composite catalyst, gold-cobalt composite catalyst, gold-copper composite catalyst, titanate nanotubes, TiNT, borohydride fuel cell

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Identifiers

ORCID for Recep Dag: orcid.org/0009-0002-4561-4121

ORCID for Carlos Ponce De Leon Albarran: orcid.org/0000-0002-1907-5913

ORCID for Zheng Jiang: orcid.org/0000-0002-7972-6175

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