RESUMO
A free-standing catalyst electrode for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) in a urea electrolysis cell was synthesized by electroplating a Ni-Fe alloy onto carbon felt, followed by phosphidation (P-NiFe@CF). The prepared P-NiFe@CF catalyst consisted of Ni5P4, NiP2, and FeP with 3D flower-like P-NiFe architecture on CF. P-NiFe@CF exhibited excellent electrocatalytic activity for the UOR (demanding only 1.39 V (vs. RHE) to achieve 200 mA cm-2), and for the HER with a low overpotential of 0.023 V (vs. RHE) at 10 mA cm-2, indicating its feasibility as a bifunctional catalyst electrode for urea electrolysis. A urea electrolysis cell with P-NiFe@CF as both the free-standing anode and cathode generated a current density of 10 mA cm-2 at a cell potential of 1.37 V (vs. RHE), which is considerably lower than that of water electrolysis, and also lower than previously reported values. The results indicate that the P-NiFe@CF catalyst electrodes can be used as free-standing bifunctional electrodes for urea electrolyzers.
RESUMO
Low-cost Ni-based catalysts have been widely used for urea oxidation in direct urea fuel cells. However, they suffer from issues such as high overpotential, poor stability, and low activity. Herein, we demonstrate the synthesis of a highly porous nanostructured Ni-Co@C catalyst for efficient electrooxidation of urea, via the calcination of Co-doped Ni-based metal-organic framework (Ni/Co-MOF). The porosity of the MOF-derived particles is considerably higher than the Ni/Co-MOF precursor. Furthermore, the Co doping at 30 mol% significantly increases the peak current density and reduces the overpotential of the electro-oxidation of urea. A urea/H2O2 fuel cell with Ni0.7Co0.3@C as the anode exhibits maximum power density of 3.4 and 20.0 mW cm-2 with 0.5 M urea in 5 M KOH as anolyte at 25 and 80 °C, respectively. Thus, this work suggests that the highly porous Ni-Co@C catalysts derived from MOF templates can be used for urea oxidation and as efficient anode materials for urea-based fuel cells.
RESUMO
Highly porous self-assembled nanostructured Ni@C and NiO@C were synthesized via calcination of a Ni-based metal-organic framework. The morphology, structure, and composition of as synthesized Ni@C and NiO@C were characterized by SEM, FIB-SEM, TEM, and XRD. The electro-catalytic activity of the Ni@C and NiO@C catalysts towards urea oxidation was investigated using cyclic voltammetry. It was found that the Ni@C had a higher residual carbon content and a higher specific surface area than NiO@C, thus exhibiting an enhanced electrochemical performance for urea oxidation. A direct urea fuel cell with Ni@C as an anode catalyst featured an excellent maximum power density of 13.8 mW cm-2 with 0.33 M urea solution in 1 M KOH as fuel and humidified air as oxidant at 50 °C, additionally showing excellent stability during continuous 20-h operation. Thus, this work showed that the highly porous carbon-supported Ni catalysts derived from Ni-based metal-organic framework can be used for urea oxidation and as an efficient anode material for urea fuel cells.
