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

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)₄AlC₃ 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_0.67Nb_xAlC₃ (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_1.33Nb₂C₃ s-MXene compared with 95.1 F g^-1 in Mo₂Nb₂C₃ 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.