Superaerophobic graphene nano-hills for direct hydrazine fuel cells

Hydrazine fuel-cell technology holds great promise for clean energy, not only because of the greater energy density of hydrazine compared to hydrogen but also due to its safer handling owing to its liquid state. However, current technologies involve the use of precious metals (such as platinum) for hydrazine oxidation, which hinders the further application of hydrazine fuel-cell technologies. In addition, little attention has been devoted to the management of gas, which tends to become stuck on the surface of the electrode, producing overall poor electrode efficiencies. In this study, we utilized a nano-hill morphology of vertical graphene, which efficiently resolves the issue of the accumulation of gas bubbles on the electrode surface by providing a nano-rough-edged surface that acts as a superaerophobic electrode. The growth of the vertical graphene nano-hills was achieved and optimized by a scalable plasma-enhanced chemical vapor deposition method. The resulting metal-free graphene-based electrode showed the lowest onset potential (−0.42 V vs saturated calomel electrode) and the highest current density of all the carbon-based materials reported previously for hydrazine oxidation. Graphene electrodes can eliminate bubbles and hence boost the performance of high-energy-density fuel cells based on hydrazine. Conventional fuel cells generate electricity by oxidizing hydrogen, but the inorganic alkaline liquid hydrazine (N2H4) promises higher energy densities and is thus being considered for new types of fuel cells. However, oxidization of hydrazine creates nitrogen which builds up as bubbles on the electrode and reduces the reaction efficiency. Now, Seung-Hyun Chun of Sejong University in Korea and co-workers have shown that the nanostructured surface of a vertical graphene electrode prevents these reaction-blocking bubbles from forming due to its ‘superaerophobic’ nature. By exploiting this effect, they created a hydrazine fuel cell that has the highest current density yet attained using a carbon-based material. The researchers anticipate that nanorough surfaces could be used in other electrocatalytic applications. Nano-rough surface offered by vertical graphene with nano-hill type morphology of corresponding peaks and valleys. These nano-hills result in the formation of smaller sized nitrogen bubbles when utilized for the oxidation of hydrazine. Due to their low adhesion force on the electrode surface, these bubbles leave the surface quickly and hence render the superaerophobic property to vertical graphene nano-hills.