Developments of Highly Efficient Electrocatalytic 2D Nonlayered Materials via Ionic Layer Epitaxy Strategy

Many traditional high-performance electrocatalysts, such as precious metals (e.g., Pt, Pb) and metal oxides (e.g., RuO₂, IrO₂), have a nonlayered crystal structure with intrinsic isotropic chemical bonds in three dimensions. Creating 2D morphology from these nonlayered catalytic materials may offer higher impacts on catalyst design by exposing massive surface dangling bonds and active defects. However, compared with van der Waals solids, 2D nonlayered materials synthesis requires the control of kinetics to break the crystal symmetry fostering 2D anisotropy in crystal growth and stabilizing the structures far away from thermodynamic equilibrium. Here, we introduce a solution-based 2D nonlayered materials synthesis technology that distinguishes itself from others with the capability of unit-cell-level thickness control called the ionic layer epitaxy (ILE) technique. In this technology, an ionic surfactant monolayer is used as a soft template at the air-water interface to guide the growth of 2D materials; here, it was found that the control of the surfactant type and packing density in the monolayer brings up a new capability for the unit-cell-level precision thickness control and vacancy concentration tuning of 2D nonlayered material in ILE synthesis, which may enable a more comprehensively quantitative study on 2D electrocatalysis. ILE was recently developed as an effective strategy to synthesize 2D nonlayered materials, such as Pd, La₂O₃, and CoO, and we demonstrated that these are promising electrocatalysts. Owing to the ultrasmall thickness, high crystallinity, and exposure of highly active FCC (100) crystal facets, the 2 nm quasi-square shaped Pd nanosheets (NSs) exhibited a mass oxidation current as high as 1132.6 mA/mg which is more than 30 times higher catalytic activity in formic acid oxidation compared to the commercial Pd black.¹ This offered a promising approach toward rational design and synthesis of ultrathin metallic 2D NSs with enhanced functionality. The ultrathin rare-earth metal-based oxide, La₂O₃ NSs demonstrated a significantly improved oxygen evolution reactions (OER) performance.² Due to the ultrasmall thickness, the as-synthesized 2.27 nm La₂O₃ NS exhibits a current density of 10 mA cm^–2 and a mass activity as high as 6666.7 A g⁻¹ at an overpotential of 310 mV, which is three orders of magnitude higher than benchmark OER electrocatalysts, such as commercial IrO₂ and RuO₂. This presents a sustainable way for the development of highly efficient electrocatalysts with largely reduced mass loading of precious elements. Furthermore, 2.5 nm quadrangle-shaped bimetallic Co-Mo-O NSs with a 2:1 Co-to-Mo ratio exhibited an enhanced OER catalytic activity with 3 orders of magnitude higher mass activity compared to commercial IrO₂ and RuO₂ in an alkaline environment, delivered an even lower overpotential of 273 mV at 10 mA cm^–2, with a mass activity of 5472 A g^–1 and a superior turnover frequency of 1.47 s^–1 (η = 330 mV) due to its ultrasmall thickness.<br/>The ultrathin NSs synthesized via ILE techniques demonstrated a promising strategy for the development of substantial highly efficient 2D electrocatalyst materials that the remarkable mass activity associated with the unique ultrathin structure is particularly valuable for preserving rare and precious catalyst elements that have high costs and low supplies.<br/><br/>REFERENCES<br/>1. Yin, X. <i>et al.</i> <i>Chemistry of Materials</i> <b>30</b>, 3308-3314, doi:10.1021/acs.chemmater.8b00575 (2018).<br/>2. Yan, G. <i>et al.</i> <i>Nano-Micro Letters</i> <b>12</b>, 49, doi:10.1007/s40820-020-0387-5 (2020).<br/>3. Zhao, Y. <i>et al.</i> <i>ACS Energy Letters</i> <b>6</b>, 3367-3375, doi:10.1021/acsenergylett.1c01302 (2021).