A DBFC is an electrochemical device that generates electrical energy by electro-oxidation of borohydride ion (BH4-) and electro-reduction of an oxidant. Usually, a DBFC employs an alkaline solution of sodium borohydride (NaBH4) as the fuel, and oxygen or hydrogen peroxide as the oxidant. DBFCs are considered attractive energy suppliers because of their high electrochemical activity, open circuit potential, energy storage capacity, and power performance at ambient temperature. The key component of a DBFC is membrane-electrode-assembly (MEA). To facilitate the successful commercialization of DBFCs, high-performance and cost-effective MEA must be developed. This research attempts to develop an effective MEA of a DBFC using low-cost materials.
An active single fuel cell system was set up and optimized, and this provides platform to investigate MEA performance. Borohydride ions undergo electro-oxidation readily on non-precious electrocatalyst materials, which provides a solution to reduce DBFC cost. Ni-based composite was employed as anode electrocatalyst. High power performance and reasonable stability were achieved by a DBFC with the prepared Ni-based composite anode catalyst. Different co-catalysts (palladium on carbon and platinum on carbon), anode substrates (carbon paper and Ni foam) were employed and compared. Ni foam was found to be an effective anode substrate which facilitates mass transport and extends electrochemical active area.
An important constituent of an electrochemical energy conversion or storage device is electrode binder. Polymers, particularly Nafion® ionomer, are generally employed as electrode binders in various types of fuel cells. To replace expensive Nafion® material, alternative electrode binders were developed based on polyvinyl alcohol (PVA) and chitosan. Both PVA and chitosan are low-cost materials, and chitosan is derived from a natural abundant biopolymer. Both PVA and chitosan chemical hydrogel were prepared and found to be efficient as electrode binder in DBFCs in terms of electrode stability and yielding high electrochemical performance.
In addition, membrane electrolytes based on PVA and chitosan were developed. A PVA chemical hydrogel membrane was prepared and employed in a DBFC using oxygen as oxidant, and this cell achieved a peak power density a little higher than that using Nafion® membranes. A chitosan hydrogel membrane was prepared by covalently cross-linking chitosan with glutaraldehyde homogenously. A chitosan membrane was also heterogeneously modified with sulfuric acid/sulfate, phosphate, or triphosphate. A chitosan-based DBFC gave significantly superior power performance and comparable stability and efficiency to a Nafion® -based DBFC.
Various studies have demonstrated that a DBFC is able to give high power performance with low-cost MEA materials. The use of Ni-based anode, polymer chemical hydrogel electrode binder and membrane electrolyte based on PVA or chitosan would reduce cost of DBFC and thus may help its commercialization.