The oxygen evolution reaction (OER) is an important reaction in water splitting. Nickel-iron layered double hydroxide (NiFe LDH) has been considered a potential catalyst for OER. Herein, we report a simple/green approach to improve the oxygen evolution activity of NiFe LDH supported on carbon black (CB). Specifically, ultrasonic exfoliation (UIP500hd, 20 kHz, 500 W) in pure water has been applied to boost the activity of NiFe LDH/CB towards OER. The exfoliated NiFe LDH/CB nanosheets display significantly higher OER activity than their corresponding bulk NiFe LDH/CB in an alkaline solution, with an overpotential of 220 mV at a current density of 10 mA cm-2, which is 60 mV lower than the 280 mV of the bulk NiFe LDH/CB. Notably, it is comparable with the current record overpotential (~200 mV) and superior than other non-precious catalysts. Additionally, after exfoliation, it possesses outstanding kinetics (Tafel slope of 35 mV dec-1) for OER. Besides the improved activity, the exfoliated NiFe LDH/CB has shown a high stability with nearly a constant potential through 12 h electrolysis at a current density of 10 mA cm-2. The characterization analysis reveal that the improved electrochemical performance is closely related to the synergistic effects of the Ni2+ and Fe2+ enriched surface of the exfoliated NiFe LDH/CB, as well as single/few layers structure after exfoliation.

The elevated concentrations of various trace metals beyond existing guideline recommendations in water bodies have promoted research on the development of various electrochemical nanosensors for the trace metals’ early detection. Inspired by the exciting physical and chemical properties of nanomaterials, advanced functional nanocomposites with improved sensitivity, sensitivity and stability, amongst other performance parameters, have been synthesized, characterized, and applied on the detection of various trace metals in water matrices. Nanocomposites have been perceived as a solution to address a critical challenge of distinct nanomaterials that are limited by agglomerations, structure stacking leading to aggregations, low conductivity, and limited porous structure for electrolyte access, amongst others. In the past few years, much effort has been dedicated to the development of various nanocomposites such as; electrochemical nanosensors for the detection of trace metals in water matrices. Herein, the recent progress on the development of nanocomposites classified according to their structure as carbon nanocomposites, metallic nanocomposites, and metal oxide/hydroxide nanocomposites is summarized, alongside their application as electrochemical nanosensors for trace metals detection in water matrices. Some perspectives on the development of smart electrochemical nanosensors are also introduced.