An ultrathin 2D NiCo-LDH nanosheet decorated NH2-UiO-66 MOF-nanocomposite with exceptional chemical stability for electrocatalytic water splitting

Abstract

Utilization of bifunctional high-efficiency non-precious electrocatalysts for stable and effective water splitting is crucial to the growth of the clean energy industry. Topologically, the predetermined ordered structures of metal–organic frameworks (MOFs) can be contrived through the judicious assembly of a tailor-made synthesis strategy of layered double hydroxide (LDH) films. Aiming at NiCo-LDH@NH₂-UiO-66 as a model system, for the first time, we examine the 2-methyl imidazole-induced ultrathin 2D NiCo-LDH nanosheet arrays in NH₂-UiO-66 as an effective bifunctional electrocatalytic system for overall H₂O splitting with marvellous performance and robustness in alkaline environments. The progressively tuned NiCo-LDH@NH₂-UiO-66 catalyst demands overpotential values of 296 and 224 mV to deliver a current density of 50 mA cm⁻² for the O₂ evolution reaction (OER) and H₂ evolution reaction (HER) in 1 M KOH aqueous solution, respectively. Tafel studies also revealed favorable reaction kinetics during electrochemical processes. The NiCo-LDH@NH₂-UiO-66 bifunctional electrode displayed superior activity exhibiting a voltage of 1.65 V at a benchmarking current density of 10 mA cm⁻² towards overall water splitting. Importantly, the NiCo-LDH@NH₂-UiO-66 electrode shows an excellent specific capacitance of 0.00364 mF cm⁻² with remarkable durability of the capacitor after 1000 cycles. To compare with the experimental result, we have performed density functional theory (DFT)-based calculation to estimate the HER and OER activity of the NiCo-LDH@NH₂-UiO-66 heterostructure. From the HER free energy profile and Bader charge analysis, we have confirmed that the presence of NH₂-UiO-66 helps in H₂ production with 0.10 eV free energy of H₂ adsorption (G_H*). From the OER free energy profile, the estimated overpotential (η) is about 0.96 eV, which confirms that the electrochemical reaction towards the OER is also possible on the NiCo-LDH@NH₂-UiO-66 structure. This study will bestow a beneficial blueprint for the utilization of an effective, durable, and economical MOF-based bifunctional catalytic system for overall water splitting.