Ultrathin two-dimensional medium-entropy alloy as a highly efficient and stable electrocatalyst for oxygen evolution reaction

The development of highly active, durable and low-cost electrocatalysts is crucial and urgent for electrocatalytic hydrogen production. Ultrathin two-dimensional (2D) nanomaterials exhibit an extremely large specific surface area, making them one of the most desirable electrocatalyst morphologies. Meanwhile, the medium-entropy alloy (MEA) has compositional tunability and entropy-driven structural stability, making it ideal electrocatalyst candidate. In this work, MEA MoCoNi with ultrathin 2D morphology was successfully developed using a facile ionic layer epitaxial (ILE) method. Ultrathin 2D MEA MoCoNi showed excellent oxygen evolution reaction (OER) electrocatalytic performance with a low overpotential of 167 mV at the current density of 10 mA cm-2 and a small Tafel slope of 33.2 mV dec-1. At the overpotential of 167 mV, ultrathin 2D MEA MoCoNi exhibited an ultrahigh mass activity of 3359.6 A g-1, which is 3 orders of magnitude higher than that of the commercial noble metal RuO2 (1.15 A g-1). This excellent electrocatalytic performance was attributed to the synergy of multiple active metals induced medium entropy as well as the ultrathin thickness which greatly shortened charge transfer distance and thus significantly promoted charge transfer. Due to natural entropy-stabilizing effect, ultrathin 2D MEA MoCoNi still maintained 90% of the initial current after 134-hours continuous OER electrocatalytic test, showing impressive electrocatalytic stability. This study opens up new avenues for developing high-performance and low cost electrocatalyst materials by creating medium-entropy alloys with ultrathin two-dimensional morphology.