Electrochemical oxidation of borohydride at nano-gold-based electrodes: Application in direct borohydride fuel cells
Introduction
Proton exchange membrane electrolyte fuel cells (PEMFCs) based on hydrogen as a fuel have advanced substantially but their successful commercialisation is restricted by safety and storage efficiency of this flammable gas [1], [2], [3], [4]. Therefore, certain liquid-fuels, such as methanol, ethanol, propanol, ethylene glycol, and diethyl ether, have been considered for fuelling PEMFCs directly [5]. Among these, methanol, with a theoretical capacity of 5.06 Ah g−1 and a hydrogen content of 12.8 wt%, is unambiguously the most attractive organic liquid-fuel at present for directly fuelled PEMFCs. Such fuel cells are referred to as direct methanol fuel cells, DMFCs [6], [7], [8]. The low cell voltage and power density due to poisoning of the anode during methanol oxidation and the phenomenon of methanol crossover limits the application of direct methanol fuel cells (DMFCs). One solution to this problem is to explore other promising hydrogen-carrying liquid-fuels, such as sodium borohydride, because of its high energy density and high cell.
NaBH4 oxidises in strong alkaline media at pH greater than 12, to BO2− and water, and generates eight electronsBH4− + 8OH− → BO2− + 6H2O + 8e− (E0 = −1.24 V)
With the oxidation of NaBH4 at the anode, the atmospheric oxygen is electrochemically reduced at the interface between the cathode catalyst and the aqueous electrolyte, and the electrons are consumed2O2 + 4H2O + 8e− → 8OH− (E0 = 0.40 V) for p(OH) = 0 or pH = 14
The coupling of reaction (1) with reaction (2) leads to an overall reaction (3) that is independent of pH.BH4− + 2O2 → BO2− + 2H2O (ECell = 1.64 V)
The theoretical cell voltage is 1.64 V, 0.4 V higher than that of the PEMFC, and the specific capacity and energy density of NaBH4 can reach 5.67 Ah g−1 and 9296 Wh kg−1, respectively. Despite a higher theoretical voltage and faster anodic electro-oxidation rate [9], [10] compared to DMFC, two main obstacles, BH4− crossover and the poor anodic efficiency of BH4− remain to be overcome for energy to be maximised.
The aim of this paper is to report on work to minimise side reactions taking place in a direct borohydride fuel cell through a judicious choice of its different components. To mitigate the chemical hydrolysis of the BH4−, for the anode nano-particulate gold-based materials were selected and fabricated as they were reported to be excellent electrocatalysts for borohydride oxidation and poor catalysts for its hydrolysis [9], [11], [12]. Commercial gold supported on carbon has been evaluated as anode electrocatalyst and its performance compared with QinetiQ's nano-particulate gold-based materials.
As for the cathode, commercially available air-electrodes as well as composite MnO2-based electrodes were explored as the latter was reported to have considerable electrocatalytic activity for oxygen reduction in alkaline medium in presence of BH4− [13]. Membrane electrode assembly (MEA) using the best cathode and anode along with low-cost anionic membrane was fabricated, implemented in QinetiQ's tubular fuel cell design and evaluated at ambient temperature.
Section snippets
Fabrication of composite anode Au/C on carbon cloth or Ni grid
The sodium borohydride oxidation electrocatalyst was gold dispersed over carbon. This supported material was purchased from E-Tek (USA) and consists of 20 wt% of metal loading on carbon Vulcan XC 72 (with a total surface area of ca. 250 m2 g−1). The composite anode was fabricated by weighing appropriate amounts of catalyst and PTFE binder (60 wt% suspension in water from Aldrich) so that the final composite anode consisted of 90 wt% of supported catalyst and 10 wt% PTFE. Deionised water was added to
Characterisation of nano-particulate materials by SEM
Fig. 3a and b shows respectively the SEM pictures of nano-particulate gold and nano-particulate bimetallic Au–Pt deposited onto gold-plated nickel grid. The SEM picture of nano-particulate gold shows agglomeration of gold particles the size of which is around 100 nm, as evidenced by Fig. 3a. Wide voids between clusters of gold particulates are clearly seen which provide channels for ionic transport. As shown in Fig. 3b, the SEM picture of the bimetallic Au–Pt material reveals a similar
Conclusion
Nano-particulate gold-based materials as well as commercial gold supported over carbon were investigated as possible alternative electrocatalysts for the oxidation of borohydride in alkaline medium. Cyclic voltammetry experiments conducted on these materials show very high activity for the nano-particulate materials compared to the commercial E-Tek Au/C-based materials despite a lower loading of gold (0.8 mg cm−2 compared to 1.0 mg cm−2) and a lower interfacial area in the nano-particulate
Acknowledgement
This work was carried out as part of the Weapons and Platform Effectors Domain of the MoD Research Programme.
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