Shape-Control and Electrocatalytic Activity-Enhancement of Pt-Based Bimetallic Nanocrystals

Abstract

Due to the increasing worldwide energy demand and environ-mental concerns, the need for alternative energy sources is growing stronger, and platinum catalysts in fuel cells may help make the technologies a reality. However, the pursuit of highly active Pt-based electrocatalysts continues to be a challenge. Scientists developing electrocatalysts continue to focus on characterizing and directing the construction of nanocrystals and advancing their electrochemical applications. Although chemists have worked on Pt-based bimetallic (Pt-M) preparations in the past, more recent research shows that both shape-controlled Pt-M nanocrystals and the assembly of these nanocrystals into supercrystals are promising new directions. A solution-based synthesis approach is an effective technique for preparing crystallographic facet-directed nanocatalysts. This is aided by careful selection of the metal precursor, capping ligand, reducing agent, and solvent. Incorporating a secondary metal M into the Pt lattice and manipulating the crystal facets on the surface cooperatively alter the electrocatalytic behavior of these Pt-M bimetallic nanocrystals. Specifically, chemists have extensively studied the {111}- and {100}-terminated crystal facets because they show unique atomic arrangement on surfaces, exhibit different catalytic performance, and possess specific resistance to toxic adsorbed carbon monoxide (CO_ads). For catalysts to have maximum efficiency, they need to have resistance to CO_ads and other poisonous carbon-containing intermediates when the catalysts operate under harsh conditions. A necessary design to any synthesis is to clearly understand and utilize the role of each component in order to successfully induce shape-controlled growth. Since chemists began to understand Pt nanocrystal shape-dependent electrocatalytic activity, the main obstacles blocking proton exchange membrane fuel cells are anode poisoning, sluggish kinetics at the cathode, and low activity. In this Account, we discuss the basic concepts in preparation of Pt-M bimetallic nanocrystals, focusing on several immaculate examples of manipulation at the nanoscale. We briefly introduce the prospects for applying Pt-M nanocrystals as electrocatalysts based on the electronic and geometric standpoints. In addition, we discuss several key parameters in the solution-based synthesis approach commonly used to facilitate Pt-M nanocrystals, such as reaction temperature and time, the combination of organic amines and acids, gaseous adsorbates, anionic species, and solvent. Each example features various nanoscale morphologies, such as spheres, cubes, octahedrons, and tetrahedrons. Additionally, we outline and review the superior electrocatalytic performances of the recently developed high-index Pt-M nanostructures. Next, we give examples of the electrocatalytic capabilities from these shape-defined Pt-M architectures by highlighting significant accomplishments in specific systems. Then, using several typical cases, we summarize electrochemical evaluations on the Pt-based shape-/composition-dependent nanocatalysts toward reactions on both the anode and the cathode. Lastly, we provide an outlook of current challenges and promising directions for shape-controlled Pt-M bimetallic electrocatalysts.