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Ni-Fe layered double hydroxide nanosheets supported on exfoliated graphite for efficient urea oxidation in direct urea fuel cells.

Urea-rich waste water can cause serious eutrophication problem to the water environment. On the other hand, urea is a potential fuel with high energy density, which can be effectively utilized by direct urea fuel cell. In this work, exfoliated graphite with high surface area and electrical conductivity is obtained by microwave irradiation, which is used to support the Ni-Fe layered double hydroxide, leading to a highly-efficient and low-cost urea oxidation catalyst (Ni-Fe LDH/EG). Compared with commercial RuO 2 , the as-prepared Ni-Fe LDH/EG exhibits a lower onset potential of 1.25 V vs RHE as well as a lower Tafel slope of 44 mV dec -1 . The catalyst durability is also proved to be excellent. The optimized Ni-Fe molar ratio is confirmed to be 3:1, while the most suitable catalyst to EG ratio is 3:50. When applied in a dual-electrolyte direct urea fuel cell, the peak power density reaches 12 mW cm -2 , and the long-term discharge is also stable with negligible voltage loss at 10 mA cm -2 for 3 hours. Such a low-cost and efficient urea oxidation catalyst can be widely utilized in future direct urea fuel cells, which achieve waste water treatment and renewable electricity generation at the same time.