Rare‐Earth (R) In‐Plane Ordering in Novel (Mo, R, Nb)4AlC3 Quinary o‐MAX Nanolaminates and their 2D Derivatives

Abstract Nanolamellar transition metal carbides are gaining increasing attentions because of the promising application in energy storage of their 2D derivatives. There are in‐plane and out‐of‐plane atomic ordered occupations, which is thought to only be formed in separated systems due to totally different origins and crystallographic structure. In present work, starting from (Mo, Nb) 4 AlC 3 o‐ MAX phase where out‐of‐plane ordered occupation is experimentally and theoretically proved for Mo/Nb atoms, rare‐earth elements ( R = Y, Gd‐Tm, Lu) are introduced, and the novel Mo 3.33‐ x R 0.67 Nb x AlC 3 ( x = 1, 1.25, 1.5, 1.75, 2, 2.25, and 2.5) super‐ordered ( s ‐) MAX phase is synthesized, where R is ordered at the outer layer in the strict stoichiometry meanwhile Mo/Nb maintains the out‐of‐plane ordered occupation. By R introduction, s ‐MAX is easier to be delaminated to obtain the s ‐MXene with the topochemical ordered vacancies, leading into the enhanced supercapacitance of 114.9 F g −1 in Mo 1.33 Nb 2 C 3 s ‐MXene compared with 95.1 F g −1 in Mo 2 Nb 2 C 3 o ‐MXene. By Pt anchoring, very low overpotential of 22 mV at a current density of 10 mA cm −2 is achieved for HER applications. This study demonstrates a novel variety of s ‐MAX phase and seeks to inspire further exploration of the ordered MAX and MXene families.