Electrochemical Synthesis of Nanostructured Ordered Intermetallic Materials under Ambient Conditions.

ConspectusThe enhanced catalytic properties of alloy nanostructures have made them a focus of extensive research in the field of catalysis. Alloy nanostructures can be classified into two types: disordered alloys (also known as solid solutions) and ordered intermetallics. The latter are of particular interest as they possess long-range atomic scale ordering, which leads to well-defined active sites that can be used to accurately assess structure-property relationships and their impact on (electro)catalytic performance.While many ordered intermetallics (OICs) have been synthesized and evaluated as electrocatalysts, there is still a lack of understanding on how the local structure of atoms controls their catalytic performance. Ordered intermetallics are difficult to synthesize and often require high-temperature annealing for the atoms to equilibrate into ordered structures. High temperature processing results in aggregated structures (usually >30 nm) and/or contamination from the support, which can decrease their performance and preclude these materials from being used as model systems for elucidating insight into structure and electrochemical properties. Therefore, alternative methods are required to enable more efficient atomic ordering while maintaining some level of morphological control.This Account delves into the potential of electrochemical methods as a practical alternative for synthesizing ordered intermetallics at lower temperatures. Specifically, it explores the viability of electrochemical dealloying and electrochemical deposition to synthesize Pd-Bi and Cu-Zn intermetallics at room temperature and atmospheric pressure. These methods have proven useful in synthesizing phases that are typically inaccessible under ambient conditions. The high homologous temperatures at which these materials are synthesized provide the necessary atomic mobility required for equilibration and formation of ordered phases, thus making the direct synthesis of ordered intermetallic materials at room temperature by electrochemical means a reality.Beyond synthesis, the electrocatalytic performance of these intermetallics was assessed for the oxygen reduction reaction (ORR), which is an important process employed in fuel cells. The OICs displayed increased performance with respect to commercial Pd/C and Pt/C benchmarks because of lower coverages of spectator species. Furthermore, these materials exhibited improved methanol tolerance.This Account provides valuable insights into the electrochemical synthesis of ordered intermetallics and their potential use as highly effective catalysts for electrocatalytic reactions. By using electrochemical methods, it is possible to obtain ordered intermetallics with unique atomic arrangements and tailored properties, which can be optimized for specific catalytic applications. With further research, electrochemical synthesis methods may enable the development of new and improved ordered intermetallics with even higher catalytic activity and selectivity, making them ideal candidates for use in a wide range of industrial processes. Furthermore, the ability to access intermetallics under milder conditions may accelerate the ability to use these materials as model systems for revealing fundamental insight into electrocatalyst structure and function.

Sonoelectrosynthesis of monodisperse metal nanoparticles.

Traditional colloidal syntheses of metal nanoparticles (NPs) are highly sensitive to the selection of and quality of chemical reducing agents and metal precursors. To address these challenges, we demonstrate the complete sonoelectrochemical synthesis of monodisperse metal NPs starting from bulk metal, using Cu as a model system. Electrochemical syntheses of NPs are of great interest as the oxidation and reduction processes that account for product formation can occur directly at the anode and cathode, respectively. This ability has the potential to improve reproducibility by simplifying the chemical pathway to NPs, with electrosyntheses often also providing unique kinetic pathways toward green product formation. Herein, ultrasound is coupled with electrosynthesis to clean the electrode surface, dispersing the NPs produced at the electrode into solution. We were able to shift the size distribution to form monodispersed metal NPs through control of applied potential (Vapplied) and ultrasonic pulses. The synthesis begins with electrooxidation of bulk Cu metal to directly dissolve metal ions into a microemulsion system. This step is followed by sonoelectroreduction of the ions, which facilitates the formation of dispersible, monodisperse Cu NPs with diameters <10 nm. The size distribution can be controlled by adjusting the Vapplied, pulse intensity, and pulse sequence implemented during sonoelectroreduction. We view this technique as a scalable method to synthesize metal NPs from bulk metal without chemical reducing agents.

Versatile Tools for Understanding Electrosynthetic Mechanisms.

Electrosynthesis is a popular, green alternative to traditional organic methods. Understanding the mechanisms is not trivial yet is necessary to optimize reaction processes. To this end, a multitude of analytical tools is available to identify and quantitate reaction products and intermediates. The first portion of this review serves as a guide that underscores electrosynthesis fundamentals, including instrumentation, electrode selection, impacts of electrolyte and solvent, cell configuration, and methods of electrosynthesis. Next, the broad base of analytical techniques that aid in mechanism elucidation are covered in detail. These methods are divided into electrochemical, spectroscopic, chromatographic, microscopic, and computational. Technique selection is dependent on predicted reaction pathways and electrogenerated intermediates. Often, a combination of techniques must be utilized to ensure accuracy of the proposed model. To conclude, future prospects that aim to enhance the field are discussed.