Highly Dispersed Cu in Hierarchical Hollow NiCo₂O₄/NiO Nanorods as an Advanced Oxygen Evolution Electrocatalyst

Water splitting is a green and sustainable method for hydrogen production. However, developing efficient nonprecious metal electrocatalysts for the oxygen evolution reaction (OER) remains challenging. Here, an electrocatalyst of Cu-doped hollow NiCo₂O₄/NiO composed of short nanorods (denoted as Cu@LQ-500) was synthesized by controlled thermal treatment of a layered double hydroxide (LDH) precursor. This precursor─hollow NiCo LDH with tetrakis(4-carboxyphenyl)-copper porphyrin (Cu-TCPP) dispersed on its surface, in pores, and within cavities─was obtained via ion etching and impregnation using zeolite imidazole framework-67 (ZIF-67) as a template. The resulting Cu@LQ-500 demonstrated excellent OER activity in an alkaline medium, requiring an overpotential of only 178 mV to achieve a current density of 10 mA cm^-2, which surpasses most reported catalysts. This performance is attributed to the interaction between Cu-TCPP and NiCo LDH during pyrolysis. This interaction resulted in highly dispersed Cu while concurrently enhancing the specific surface area and expanding the mesopore dimensions. The highly dispersed Cu further enables profound modulation of the electronic structure in the NiCo LDH, thereby promoting the formation of more active centers. Density functional theory (DFT) calculations reveal that the highly dispersed of Cu dopants significantly modulate the electronic structure of the material. This modulation not only increases the Co³⁺/Ni³⁺content but also optimizes adsorption via the downshift of the Co/Ni d-band center, collectively enhancing the electrocatalytic performance. This work provides a pathway for preparing highly efficient OER electrocatalysts for water splitting.