Electrochemical Engineering aspects of a direct borohydride fuel cell

Abstract

Fuel cells (FCs) are clean power sources for both large-scale and portable applications, as they provide a viable method to convert the chemical energy of fuel directly into electrical energy. The most developed FC is the H2ǁO2 system, which uses hydrogen as fuel. However, issues have been observed with the use of hydrogen, such as sourcing, the safety of handling and storage problems. Direct borohydride fuel cells (DBFCs) are liquid FCs that address some of these issues. They are composed of both BH4 - fuel, which oxidises at the anode, and hydrogen peroxide (or O2), which reduces at the cathode. Their many advantages, such as their high theoretical specific energy (up to 17 kW h kg-1) and high theoretical cell voltage (up to 3.02 V), have attracted increasing interest. Borohydride is also available in a solid state (sodium borohydride, NaBH4) or as an aqueous electrolyte up to 30 weight percent (wt.%), where it remains unchanged in a strong alkaline solution with a half-life of around 270 days at pH 13.9 (25 o C). Moreover, the final product of the borohydride oxidation reaction (BOR) is metaborate (NaBO2), which is not harmful to the environment and can be reused to produce sodium borohydride again. Finally, borohydride FCs can operate under ambient conditions and in an air-free environment, which makes them more convenient for portable and anaerobic applications (such as in space and underwater). The main challenges to their commercialisation, however, are the selectivity of the anode catalysts and their substrate materials. Many publications have investigated noble metals (e.g. platinum, gold, palladium, etc.) as candidate materials, but none have found an anode catalyst able to meet the needs of both high catalytic activity towards BH4 - oxidation and low activity towards its hydrolysis. Experimentally, Pt catalysts have shown a higher activity rate (by a factor of 10) and lower onset potential of BH4 - oxidation compared to Au. Bimetallic ii electrocatalysts (e.g. Au-Pt, Pt-Ni, Pt-Zn) have also presented considerable cell performance in contrast to single-metal catalysts. As Pt0.75-Ir0.25/Ti and Pt0.25-Ir0.75/Ti anode catalysts have not been studied in DBFCs, this research considers them. Employing 3D electrode materials, such as reticulated metals, is expected to enhance the reactants’ mass transport, avoid channel blocking, fully utilise the active sites, minimise the amount of noble metal required and thus simultaneously reduce electrode costs and increase cell power output. Moreover, optimising the operational conditions (temperature, reactant compositions, background materials, flow rate, etc.) of DBFCs will improve their performance. Therefore, the aim of this research is to investigate approaches to increase the overall performance and efficiency of a DBFC with the following research objectives: investigate the use of Pt-Ir alloys supported by titanium as anode materials for DBFCs; discover the optimal anode geometry among different 3D electrode materials by determining the volumetric mass-transfer coefficient using a limiting-current technique; and find the maximum power density of a DBFC using the optimal electrode through manipulation of operating conditions and electrolyte compositions. Pt0.75-Ir0.25/Ti and Pt0.25-Ir0.75/Ti anode catalysts have been characterised and evaluated, Pt0.75-Ir0.25/Ti being the most effective electrode for direct NaBH4 oxidation. Its reaction parameters – including the activation energy ("# #$$), the charge-transfer coefficient (α), the total number of electrons exchanged (z), the heterogeneous rate constant (ka) of the BH4 - oxidation process and reaction order (β) – were 18.6 kJ mol-1 , 0.59, 5.3, 0.40 cm s-1 and 0.87, respectively. Among 3D electrode materials, the fine mesh showed the best performance. Finally, the operating conditions were discovered to have a significant influence on the cell power density. For example, the cell performance showed a dependence on the operating temperature, that is, the reaction kinetics at the anode and cathode were promoted when it rose. The peak power density also increased dramatically, from 93 to 162 mW cm-2 (~75%), when the temperature elevated from 25 to 65 o C.

Keywords: borohydride oxidation, 3D electrode, iridium, limiting current, mass transport, platinum, porous electrode, operating conditions

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

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