Controlled construction of bimetallic nanostructures with a well-defined heterophase is of great significance for developing highly efficient nanocatalysts and investigating the structure-dependent catalytic performance. Here, a wet-chemical synthesis method is used to prepare Au@Pd core−shell nanorods with a unique fcc-2H-fcc heterophase (fcc: face-centered cubic; 2H: hexagonal close-packed with a stacking sequence of "AB"). The obtained fcc-2H-fcc heterophase Au@Pd core−shell nanorods exhibit superior electrocatalytic ethanol oxidation performance with a mass activity as high as 6.82 A mg Pd −1 , which is 2.44, 6.96, and 6.43 times those of 2H-Pd nanoparticles, fcc-Pd nanoparticles, and commercial Pd/C, respectively. The operando infrared reflection absorption spectroscopy reveals a C2 pathway with fast reaction kinetics for the ethanol oxidation on the prepared heterophase Au@Pd nanorods. Our experimental results together with density functional theory calculations indicate that the enhanced performance of heterophase Au@Pd nanorods can be attributed to the unconventional 2H phase, the 2H/fcc phase boundary, and the lattice expansion of the Pd shell. Moreover, the heterophase Au@Pd nanorods can also serve as an efficient catalyst for the electrochemical oxidation of methanol, ethylene glycol, and glycerol. Our work in the area of phase engineering of nanomaterials (PENs) opens the way for developing highperformance electrocatalysts toward future practical applications.

Crystal phase engineering of noble-metal-based alloy nanomaterials paves a new way to the rational synthesis of high-performance catalysts for various applications. However, the controlled preparation of noble-metal-based alloy nanomaterials with unconventional crystal phases still remains a great challenge due to their thermodynamically unstable nature. Herein, we develop a robust and general seeded method to synthesize PdCu alloy nanomaterials with unconventional hexagonal close-packed (hcp, 2H type) phase and also tunable Cu contents. Moreover, galvanic replacement of Cu by Pt can be further conducted to prepare unconventional trimetallic 2H-PdCuPt nanomaterials. Impressively, 2H-Pd67Cu33 nanoparticles possess a high mass activity of 0.87 A mg–1 Pd at 0.9 V (vs reversible hydrogen electrode (RHE)) in electrochemical oxygen reduction reaction (ORR) under alkaline condition, which is 2.5 times that of the conventional face-centered cubic (fcc) Pd69Cu31 counterpart, revealing the important role of crystal phase on determining the ORR performance. After the incorporation of Pt, the obtained 2H-Pd71Cu22Pt7 catalyst shows a significantly enhanced mass activity of 1.92 A mg–1 Pd+Pt at 0.9 V (vs RHE), which is 19.2 and 8.7 times those of commercial Pt/C and Pd/C, placing it among the best reported Pd-based ORR electrocatalysts under alkaline conditions.

nanomaterials (PEN), [2] tuning the crystal phase of noble-metal heterostructures paves a new avenue to efficiently tailor their properties and functions. In particular, noble-metal heterostructures with unconventional heterophases, which are composed of two or more phases, can show great application potentials owing to the presence of phase boundaries and the synergistic effect between different phases. [1f,2a,3] Seeded epitaxial growth represents an efficient approach to prepare heterophase noble-metal heterostructures with desired compositions and morphologies. For instance, the 4H/face-centered cubic (fcc) heterophase Au@Pd core-shell nanorods [3a] and 4H/fcc heterophase Au-Ru hybrid nanostructures [1f ] with random 4H and fcc phase distributions were obtained by using the pre-synthesized 4H/fcc Au nanomaterials as templates, which exhibited excellent catalytic performance towards the electrochemical ethanol oxidation reaction and hydrogen evolution reaction (HER), respectively. However, it still remains challenging to rationally design and synthesize heterophase noblemetal nanostructures with well-controlled phase distribution. Very recently, our group reported the phase-selective epitaxial growth of 2H-and fcc-Au nanostructures on unconventional 2H-Pd seeds to prepare the fcc-2H-fcc heterophase Pd@Au core-shell nanorods for highly efficient electrocatalytic carbon dioxide (CO 2 ) reduction. [3b] Such unique epitaxial growth route could open a new gateway towards the With the development of phase engineering of nanomaterials (PEN), construction of noble-metal heterostructures with unconventional crystal phases, including heterophases, has been proposed as an attractive approach toward the rational design of highly efficient catalysts. However, it still remains challenging to realize the controlled preparation of such unconventional-phase noble-metal heterostructures and explore their crystal-phase-dependent applications. Here, various Pd@Ir core-shell nanostructures are synthesized with unconventional fcc-2H-fcc heterophase (2H: hexagonal close-packed; fcc: face-centered cubic) through a wet-chemical seeded method. As a result, heterophase Pd 66 @Ir 34 nanoparticles, Pd 45 @Ir 55 multibranched nanodendrites, and Pd 68 @Ir 22 Co 10 trimetallic nanoparticles are obtained via the phaseselective epitaxial growth of fcc-2H-fcc-heterophase Ir-based nanostructures on 2H-Pd seeds. Importantly, the heterophase Pd 45 @Ir 55 nanodendrites exhibit excellent catalytic performance toward electrochemical hydrogen evolution reaction (HER) under acidic conditions. An overpotential of only 11.0 mV is required to achieve a current density of 10 mA cm −2 on Pd 45 @Ir 55 nanodendrites, which is lower than those of the conventional fcc-Pd 47 @Ir 53 counterparts, commercial Ir/C and Pt/C. This work not only demonstrates an appealing route to synthesize novel heterophase nanomaterials for promising applications in the emerging field of PEN, but also highlights the significant role of the crystal phase in determining their cata...

Currently, the oxygen evolution reaction (OER) plays a key role in the industrial application of renewable electrochemical technologies. Thus, developing electrocatalysts with high performance and sufficient stability for the OER is urgently pursued. Although perovskite oxides have provided numerous degrees of freedom for enhancing the electrocatalytic activity due to their diversity and flexibility, their investigation for the OER is mostly limited to pseudocubic structures. In this study, a complex perovskite oxide, Ba0.9Sr0.1Co0.8Fe0.1Ir0.1O3−δ (BSCFI-91), with a six-layer hexagonal (6H) structure, is synthesized first, displaying higher OER activity. Based on parent Ba0.9Sr0.1Co0.8Fe0.2O3−δ (BSCF-91), BSCFI-91 is obtained by replacing iron (Fe) with low-level iridium (Ir) doping and produces a current density of 10 mA cm–2 at a low overpotential of 300 mV, a small Tafel slope of 61.2 mV dec–1, and good stability up to 10 h in a 1.0 M KOH electrolyte. The dramatically enhanced OER performance is achieved by optimizing cobalt (Co) valence and highly oxidative oxygen species based on the hexagonal structure. The Ir incorporation facilitated the oxygen (O) p band center approaching to the Fermi level, indicating that BSCFI-91 could be a candidate in the electrocatalyst application. Moreover, this study opens up a new way to design efficient perovskite oxides for OER catalysis in terms of hexagonal crystal structures and composition modulation strategy.

Oxygen evolution reaction (OER) electrocatalysts with high efficiency and durability are urgently needed by energy conversion and storage of clean energy. Herein, we report the finding in the development of...