Intermetallic Compounds: Liquid‐Phase Synthesis and Electrocatalytic Applications

Abstract

Characterized by long-range atomic ordering, welldefined stoichiometry, and controlled crystal structure, intermetallics have attracted increasing attention in the area of chemical synthesis and catalytic applications. Liquid-phase synthesis of intermetallics has arisen as the promising methodology due to its precise control over size, shape, and resistance toward sintering compared with the traditional metallurgy. This short review tends to provide perspectives on the liquid-phase synthesis of intermetallics in terms of both thermodynamics and methodology, as well as its applications in various catalytic reactions. Specifically, basic thermodynamics and kinetics in the synthesis of intermetallics will be first discussed, followed by discussing the main factors that will affect the formation of intermetallics during synthesis. The application of intermetallics in electrocatalysis will be demonstrated case by case at last. We conclude the review with perspectives on the future developments with respect to both synthesis and catalytic applications.

Fundamental understanding of the synthesis of well-defined supported non-noble metal intermetallic compound nanoparticles

“…The performance of the supported IMC nanoparticles in catalytic, electronic, or sensing applications is affected by or directly dependent upon the compositional stability of the particle bulk, 19,21,[152][153][154][155] surface composition, 64,80,[156][157][158][159] and morphology of the particle. 91,[160][161][162] Control of particle surface composition is also paramount in heterogeneous catalysis and sensing applications. Because the as-reduced Ni + Ga, CoGa, and Ni + In IMC nanoparticles exhibited average particle size distributions around 2-5 nm, their bulk and surface composition stability may be less than ideal for Fig.…”

“…58,80 Low-temperature liquid phase methods such as coreduction, sol-gel, solvothermal, and hydrothermal synthesis that produce IMC colloids have been developed and employ strong reducing agents to drive precursor reduction and IMC formation. [81][82][83][84][85][86][87][88][89][90][91] However, this approach can easily lead to kinetically trapped species and mixed-phase materials since the reducing agent cannot provide enough vibrational energy for structural relaxation of IMC crystallites. Likewise, highly reactive reducing and capping agents can strongly influence the particle surface composition and may drive the formation of surface compositions that include elements from the reagents when they are removed at elevated temperatures.…”

Microwave Heating of Nanocrystals for Rapid, Low-Aggregation Intermetallic Phase Transformations

“…Intermetallic nanocrystals (NCs) are one of the most studied classes of materials for electrocatalysis − due to their high electrochemical stability − at lower precious metal loadings. ,,,− Pt–Co intermetallic face-centered-tetragonal (fct) phase (L1 0 ) NCs have been demonstrated as viable electrocatalysts for the oxygen reduction reaction (ORR). ,− The ability to perform the ORR with high electrochemical stability while decreasing the Pt loading is of key importance for proton exchange membrane fuel cells (PEMFCs), ,,− with the capability of providing a source of clean energy able to compete with fossil fuels . To make PEMFCs economically viable, there is a need to optimize the activity of the catalyst per unit Pt, , also known as the mass activity.…”

“…We use near-monodisperse Cu-doped PtCo 2 NCs in the intermetallic L1 0 phase as a model system. Copper-doping is known to reduce the barrier associated with the phase transformation. ,, The L1 0 phase is desirable because of simultaneously higher electrochemical stability and lower cobalt leaching during catalysis than the random alloy (A1) phase. − ,− ,,,, Typically, as-synthesized A1 face-centered-cubic (fcc) phase NCs are transformed into intermetallic L1 0 NCs by a thermal treatment that involves temperatures exceeding 400 °C over several hours. ,,,,, During this process, NC aggregation, or sintering, occurs, leading to a decrease in the electrochemically active surface area (ECSA) and resulting in a decrease in mass activity for the ORR. ,,, Several literature reports have attempted to prevent NCs from aggregating during the thermal treatment by anchoring the NCs to the substrate, ,, or by synthesizing the NCs directly in the intermetallic phase. ,,− Other methods have demonstrated a monodisperse dispersion of intermetallic NCs; however, these require a specialized technique such as Joule heating or electrochemical dealloying. ,− So far, no method has shown a complete phase transformation with minimized aggregation.…”

“…9,23,24 The L1 0 phase is desirable because of simultaneously higher electrochemical stability and lower cobalt leaching during catalysis than the random alloy (A1) phase. [1][2][3][6][7][8]13,15,19,25 Typically, as-synthesized A1 face-centered-cubic (fcc) phase NCs are transformed into intermetallic L1 0 NCs by a thermal treatment that involves temperatures exceeding 400 °C over several hours. 1,2,6,13,26,27 During this process, NC aggregation, or sintering, occurs, leading to a decrease in the electrochemically active surface area (ECSA) and resulting in a decrease in mass activity for the ORR.…”