An organic-inorganic heterojunction electrocatalyst for highly efficient urea oxidation

Constructing a p-n heterojunction allows the modulation of the interfacial electronic structure and boosts electron transfer, leading to enhanced electrocatalytic performance. Herein, an organic-inorganic heterojunction electrocatalyst made of Ni(OH)2, a triazine-based covalent organic framework (COF), and multi-walled carbon nanotubes (CNTs), denoted as Ni(OH)2/COF/CNT, is reported to exhibit significantly enhanced electrocatalytic performance for urea oxidation in comparison with its single- and double-component counterparts (Ni(OH)2, COF/CNT, Ni(OH)2/CNT, and Ni(OH)2/COF). As revealed by its band structure analysis, the constructed Ni(OH)2-COF (p-n) heterojunction enables a built-in electric field to boost charge transfer across the heterointerface (from the inorganic Ni(OH)2 to the organic COF), resulting in significantly improved catalytic activity. The mass activity is 363 A gNi(OH)2-1 at 1.5 V (vs. reversible hydrogen electrode, RHE) for the Ni(OH)2/COF/CNT heterojunction electrocatalyst, showing 322 and 72% enhancement in comparison with Ni(OH)2 and Ni(OH)2/CNT, respectively. Moreover, Ni(OH)2/COF/CNT exhibits a considerable turnover frequency (TOF, 0.11 s-1 at 1.5 V), large reaction rate constant (k = 1.9 x 106 cm3 mol-1 s-1), high coulombic efficiency (similar to 98%), and meritorious catalytic stability (24 hours at 20 mA cm-2) for urea oxidation. The present work enriches the design strategies for developing advanced electrocatalysts. By constructing a Ni(OH)2-COF (p-n) heterojunction, a built-in electric field can be induced to boost charge transfer across the heterointerface, thereby facilitating the generation of Ni3+ active sites for enhanced urea oxidation performance.