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Investigation of the use of sodium borohydride for fuel cells

Investigation of the use of sodium borohydride for fuel cells
Investigation of the use of sodium borohydride for fuel cells
The use of NaBH4 for fuel cells offers a promising alternative to incumbent electrical power generation technologies. The predicted high energy density (9.3 kW h kg-1) of the direct borohydride fuel cell (DBFC) and its capacity to release 8e- per molecule converts it to a potential substitute for the H2/O2 system. Sodium borohydride, with 10.6 wt. % hydrogen content, can also generate H2 gas to be fed into a traditional H2/O2 fuel cell in an indirect borohydride fuel cell (IBFC). However, there are fundamental aspects of the DBFC and the IBFC that need to be addressed to achieve their optimal performance. The hydrolysis of borohydride is the key factor in both cases, being the undesired parallel reaction that occurs during the borohydride oxidation in the DBFC, and the main reaction taking place and to be promoted in the IBFC. The competition between BH4- oxidation and its hydrolysis depends on the electrode material, electrolyte composition and operation conditions, such as the temperature.

In this work, two approaches to the use of NaBH4 for fuel cells are considered. Catalysts such as Pd-Ir alloy on microbifrous carbon, gold coated reticulated vitreous carbon (RVC), planar gold and dispersed nanoparticulate gold supported on carbon (Au/C), were tested for the direct borohydride oxidation while Pd-Ir alloy, Pt nanoparticles on carbon paper and Pd deposited on granular carbon (Pd/C) were evaluated to generate H2 from NaBH4 for use in an IBFC. The gold coated RVC electrodes demonstrated good activity towards the borohydride oxidation, increasing the kinetic rate constants and the current density with the thickness of the coating and the porosity grades. The Pd-Ir alloy was also catalytic towards the DBFC, with current densities between 100 and 200 mA cm-2 but with low H2 generation rates (< 0.1 cm3 min-1). As computational methods could play a prominent role in the design and characterisation of DBFCs, density functional theory (DFT) was used to investigate the reaction mechanism of borohydride oxidation at Pd-Ir surfaces. This work also studies the use of surfactants, including Triton X-100, Zonyl FSO, S-228M, sodium dodecyl sulphate and FC4430, during the direct oxidation of borohydride ions using a planar gold and Au/C electrodes. The addition of 0.001 wt. % Triton X-100 to the alkaline borohydride solution decreased the H2 generation by 23 % at the Au/C electrode, while the borohydride oxidation remained unaffected. In contrast, in the H2 generator, the Pd/C catalyst showed an excellent activity towards the borohydride hydrolysis, obtaining a maximum rate of 8 × 103 cm3 min-1 gmetal-1 during 120 minutes using 4 mol dm-3 NaBH4 in 350 cm3 distilled water and 15 g of catalyst. This is the highest H2 generation rate reported in a laboratory scale reactor using borohydride and a Pd base catalyst.
Keywords: borohydride oxidation and decomposition, catalysis, DBFC, IBFC, hydrogen generation, hydrolysis inhibition, kinetic rate constant.
borohydride oxidation and decomposition, catalysis, DBFC, IBFC, hydrogen generation, hydrolysis inhibition, kinetic rate constant
Merino Jimenez, Irene
b7e7de25-13c8-4475-b98f-ceffe060e32a
Merino Jimenez, Irene
b7e7de25-13c8-4475-b98f-ceffe060e32a
Ponce De Leon Albarran, Carlos
508a312e-75ff-4bcb-9151-dacc424d755c

Merino Jimenez, Irene (2013) Investigation of the use of sodium borohydride for fuel cells. University of Southampton, Engineering Sciences, Doctoral Thesis, 296pp.

Record type: Thesis (Doctoral)

Abstract

The use of NaBH4 for fuel cells offers a promising alternative to incumbent electrical power generation technologies. The predicted high energy density (9.3 kW h kg-1) of the direct borohydride fuel cell (DBFC) and its capacity to release 8e- per molecule converts it to a potential substitute for the H2/O2 system. Sodium borohydride, with 10.6 wt. % hydrogen content, can also generate H2 gas to be fed into a traditional H2/O2 fuel cell in an indirect borohydride fuel cell (IBFC). However, there are fundamental aspects of the DBFC and the IBFC that need to be addressed to achieve their optimal performance. The hydrolysis of borohydride is the key factor in both cases, being the undesired parallel reaction that occurs during the borohydride oxidation in the DBFC, and the main reaction taking place and to be promoted in the IBFC. The competition between BH4- oxidation and its hydrolysis depends on the electrode material, electrolyte composition and operation conditions, such as the temperature.

In this work, two approaches to the use of NaBH4 for fuel cells are considered. Catalysts such as Pd-Ir alloy on microbifrous carbon, gold coated reticulated vitreous carbon (RVC), planar gold and dispersed nanoparticulate gold supported on carbon (Au/C), were tested for the direct borohydride oxidation while Pd-Ir alloy, Pt nanoparticles on carbon paper and Pd deposited on granular carbon (Pd/C) were evaluated to generate H2 from NaBH4 for use in an IBFC. The gold coated RVC electrodes demonstrated good activity towards the borohydride oxidation, increasing the kinetic rate constants and the current density with the thickness of the coating and the porosity grades. The Pd-Ir alloy was also catalytic towards the DBFC, with current densities between 100 and 200 mA cm-2 but with low H2 generation rates (< 0.1 cm3 min-1). As computational methods could play a prominent role in the design and characterisation of DBFCs, density functional theory (DFT) was used to investigate the reaction mechanism of borohydride oxidation at Pd-Ir surfaces. This work also studies the use of surfactants, including Triton X-100, Zonyl FSO, S-228M, sodium dodecyl sulphate and FC4430, during the direct oxidation of borohydride ions using a planar gold and Au/C electrodes. The addition of 0.001 wt. % Triton X-100 to the alkaline borohydride solution decreased the H2 generation by 23 % at the Au/C electrode, while the borohydride oxidation remained unaffected. In contrast, in the H2 generator, the Pd/C catalyst showed an excellent activity towards the borohydride hydrolysis, obtaining a maximum rate of 8 × 103 cm3 min-1 gmetal-1 during 120 minutes using 4 mol dm-3 NaBH4 in 350 cm3 distilled water and 15 g of catalyst. This is the highest H2 generation rate reported in a laboratory scale reactor using borohydride and a Pd base catalyst.
Keywords: borohydride oxidation and decomposition, catalysis, DBFC, IBFC, hydrogen generation, hydrolysis inhibition, kinetic rate constant.

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More information

Published date: November 2013
Keywords: borohydride oxidation and decomposition, catalysis, DBFC, IBFC, hydrogen generation, hydrolysis inhibition, kinetic rate constant
Organisations: University of Southampton, Engineering Science Unit

Identifiers

Local EPrints ID: 363760
URI: http://eprints.soton.ac.uk/id/eprint/363760
PURE UUID: bd061b99-ac4f-4ebd-9b89-efb77a8e2268
ORCID for Carlos Ponce De Leon Albarran: ORCID iD orcid.org/0000-0002-1907-5913

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Date deposited: 09 Apr 2014 11:19
Last modified: 06 Jun 2018 12:42

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Author: Irene Merino Jimenez

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