Rapid and in-situ synthesis of NiFe LDH nanosheet array using microwave method for efficient electrocatalytic water oxidation

The complex kinetics and high energy barrier of oxygen evolution reaction (OER) seriously restrict the efficiency of hydrogen production in overall water splitting. Layered double hydroxides (LDHs) have reasonable electrocatalytic OER ability, but the resulting performance is unsatisfactory; moreover, their rapid, large-scale, and in situ synthesis remains a great challenge. Compared to the traditional hydrothermal, electrodeposition and co-precipitation methods, microwave synthesis methods have the advantages of cleanliness, high efficiency, and low energy consumption. Importantly, microwave methods can generate many defects to improve the electrocatalytic performance. The microwave-assisted synthesis of LDHs has been reported, but it was limited to powdered nano/micromaterials. The use of microwave methods to grow nano/microarrays in situ on a substrate surface has not been reported. Compared with powders, arrays have multiple advantages, such as favorable electrolyte diffusion, proton/electron transport, and direct use for electrochemical testing without the need for additional binders, so there is an urgent need to develop strategies for the in-situ synthesis of nano/microarrays using microwave methods. Based on these studies, this paper reports a simple and effective microwave synthesis strategy for growing layered double hydroxides arrays in situ on the surface of a carbon cloth (CC), termed as LDH/CC to improve the OER performance. The type of solvent plays a crucial role in the in situ growth of LDHs. Compared to pure water, ethylene glycol, and pentaerythritol aqueous solvents, glycerol has a suitable polarity that can effectively regulate the surface energy of metal ions and the nucleation/migration of metal ions during the synthesis process, promoting the in situ and uniform growth of LDHs on the CC surface. When applied to the electrocatalytic OER, the NiFe LDH/CC catalyst synthesized using glycerol as a solvent exhibited excellent electrocatalytic performance, with a low overpotential of 257 mV (η100), 279 mV (η200), and 331 mV (η400) at 100, 200, and 400 mA cm−2, respectively. These values were superior to those for the catalyst synthesized with ethylene glycol (287, 318, and 386 mV at 100, 200, and 400 mA cm−2, respectively) and pentaerythritol (309, 344, and 406 mV at 100, 200, and 400 mA cm−2, respectively) as a solvent. Moreover, these performances were superior to those of many transition metal hydroxides reported in recent years. Systematic spectroscopic and electrochemical tests have revealed that compared to traditional hydrothermal and electrodeposition methods, the NiFe LDH/CC synthesized using the microwave method shows superior characteristics. The NiFe LDH/CC prepared via microwave synthesis exhibited a uniform distribution, a high oxygen defect content, abundant electrochemical active sites, low interfacial charge transfer resistance, and high intrinsic activity. These features contribute to its outstanding performance in electrocatalytic OER applications. The NiFe LDH/CC catalyst synthesized using the microwave method displayed significantly better electrocatalytic OER performance with overpotentials of only 257 and 331 mV at 100 and 400 mA cm−2, respectively. These values surpassed the performance of catalysts synthesized using hydrothermal (279 and 434 mV at 100 and 400 mA cm−2, respectively) and electrodeposition methods (293 and 380 mV at 100 and 400 mA cm−2, respectively). The NiFe LDH/CC catalyst also exhibited long-term electrocatalytic stability, retaining approximately 95% of its electrocatalytic OER performance after continuous operation for 100 h. The findings highlight the potential of microwave synthesis strategies for enhancing the efficiency of electrocatalytic OER processes. This microwave synthesis strategy is not only suitable for NiFe LDH, but also for in-situ synthesis of NiMn LDH, CoFe LDH, and NiCo LDH, which provides a new avenue for rapid and in-situ synthesis of highly-active LDHs.