Three-dimensional Ni 4 O 4 -cubane metal-organic framework as a high-performance electrocatalyst for urea oxidation.

The urea oxidation reaction (UOR) is considered to be a replacement of the sluggish anodic oxygen evolution reaction (OER) in overall water-splitting. A three-dimensional (3D) nickel-containing metal-organic framework {[Ni II 2 (pdaa)(OH) 2 (H 2 O)] n (MOF 1) (where, H 2 pdaa = 1,4-phenylene diacetic acid) was investigated as a robust and highly efficient electrocatalyst for the UOR. MOF 1 comprised 1D nickel(ii) chains crosslinked through Ni 4 O 4 cubane units to form a 3D extended network. Dangling Ni⋯OH - groups were exposed in the MOF 1 structure, and could act as active catalytic centers for the UOR. MOF 1 required a very small onset potential of 1.18 V for urea oxidation in KOH (1 M) and urea (0.33 M) and had a low Tafel slope of 38.8 mV dec -1 (in contrast to 1.84 V for the oxygen evolution reaction). The overpotential required to attain a catalytic current density of 10 mA cm -2 was 1.24 V, which is much lower than that for many materials. Controlled potential electrolysis, powder X-ray diffraction, and X-ray photoelectron spectroscopy affirmed the physicochemical integrity of the catalyst over a 17 h test reaction. This work not only addresses the problem of urea contamination, it also helps to utilize it in an energy-conversion process.