A direct borohydride fuel cell—first demonstrated in the early 1960s—is a type of alkaline fuel cell directly fed by a sodium borohydride or potassium borohydride solution. DBFCs feature a high open circuit voltage (1.64 V), high fuel energy density (9.3 Wh·g_-1 for NaBH₄ and 6.5 Wh·g_-1 for KBH₄), and fast borohydride oxidation reaction (BOR) kinetic. In a DBFC, the cell reaction principle is:

Anode: BH₄^- + 8OH^- → BO₂^- + 8e^-

Cathode: 2O₂ + 4H₂O + 8e^- → 8OH^-

Overall: BH₄^- + 2O₂ → BO₂^- + 2H₂O

The membrane in DBFC serves both for ion transfer and as a separator between anode and cathode compartments.

Nafion membrane, polymer electrolyte membrane (PEM), is indispensable in fuel cells, which provides electric insulator between anode and cathode and prevents reactant crossover from the anode to the cathode. However, crossover is still a serious problem due to the diffusion and osmotic drag that lower the power output by maximum up to 50% in liquid fuel cells. Much research has been conducted on the details of the transport of protons or ions through the polymer matrix and on novel methods of improving its properties, but the crossover has not yet been solved as well as the cost of the PEM4.

The cost of noble metal catalyst is another factor limiting fuel cells commercialisation...Although the performance and stability of Pt-based catalysts have been greatly improved in H2/O₂ PEM fuel cells, its efficiency in DBFCs is still in a low level. The peak power densities ranged at 20–200 mW·cm^-2 when Pt or Pt-based catalysts are used, while they were in ca. 1,000 mW·cm^-2 when in H₂/O₂ PEM fuel cells. Crossover is the main reason for this phenomenon. Pt-based catalysts promote ORR and BOR simultaneously when liquid fuel permeates through PEM and arrives in cathodes, thus cathode efficiency is greatly reduced.

—Yang et al.

The team from Xi’an Jiaotong University fabricated a DBFC with polymer fiber membrane (DBFC-PFM) as a separator replacing the conventional polymer electrolyte membrane (PEM).

They found a peak power density of the DBFC-PFM is 663 mW·cm^-2, an increase by a factor of ca. 1.7 and 3.7 compared with that of DBFCs employing NRE-211 and N-117 membranes. At 0.6 V, the power density of the DBFC-PFM with LaNiO₃ is 640 mW·cm^-2 at 65 °C—a level comparable with the power density of H₂/O₂ PEMFC with the most promising non-noble metal catalyst (750 mW·cm22 at 0.6 V, 80 °C), the researchers noted.

Of the three main obstacles limiting thee commercialization of DBFCs—borohydride hydrolysis, liquid fuel crossover and battery cost—the researchers suggest that they have addressed the last two of those in this structured fuel cell.

The PFM is much cheaper and it is also widely used in Ni-MH batteries as a separator; inexpensive catalysts can be used in both anode and cathode in the DBFC-PFM.

In conventional research, crossover is a serious problem which must be resisted by increasing the thickness of the PEM or developing new PEM. However, increasing the thickness of PEM will increase ohm loss. We solve the problem by a different way that the crossover is allowed. A similar idea of allowing crossover has also been adopted in a swiss-roll liquid-gas mixed-reactant fuel cell. In the situation of allowing crossover, it requires cathodic catalysts decompose O₂ only and are inert to all other ions and reactants. It has been proved that the catalysts used in this work have this ability.

This principle could be extended to other liquid fuel cells, such as direct methanol fuel cell (DMFC)...A peak power density of 64 mW·cm^-2 has been obtained at 60°C. It is the highest value in DMFCs concerning non-Pt-based catalysts to the best of our knowledge. This test provides us a clue that all liquid fuel cells can be studied in the way that crossover is no need to be considered and we just seek cathodic catalysts with selective catalysis for ORR, especially inexpensive metal oxide catalysts.

—Yang et al.

Resources

• Xiaodong Yang, Yongning Liu, Sai Li, Xiaozhu Wei, Li Wang & Yuanzhen Chen (2012). A direct borohydride fuel cell with a polymer fiber membrane and non-noble metal catalysts. Scientific Reports 2, Article number: 567 doi: 10.1038/srep00567

• Jia Ma, Nurul A. Choudhury, Yogeshwar Sahai (2010) A comprehensive review of direct borohydride fuel cells. Renewable and Sustainable Energy Reviews 14, 183–199 doi: 10.1016/j.rser.2009.08.002