High- and low-index faceted metal (Pt, Pd, Ru, Ir, Rh) nanoparticles designed for improved electrocatalytic activity and stability are reviewed.

Single Pt atom catalysts on non-active carbon supports have been key targets for electrochemical reactions because the high exposure of active Pt leads to record-high activities. PtRu alloy catalysts are the most active for the methanol oxidation reaction (MOR) as the Ru atoms decrease CO poisoning of the active Pt. To combine the exceptional activity of single atom Pt catalysts with the bene ts of an active Ru support we must overcome the synthetic challenge of forming single Pt atoms on noble metal nanoparticles. We have developed a concept to grow and spreads Pt islands on faceted Ru branched nanoparticles to make single Pt atom on Ru catalysts. By following the spreading process with in situ TEM, we show that the formation of single atoms is thermodynamically driven by the formation of strong Pt-Ru bonds and a lowering of surface area. The single Pt atom on Ru catalysts successfully limit CO poisoning during MOR to produce record current density and mass activity over time. MainThe methanol oxidation reaction (MOR) is the limiting reaction for the direct methanol fuel cell because CO-poisoning prevents high current densities over time 1 . CO poisoning is one of the most signi cant issues limiting the long-term use of catalysts for reactions such as MOR, ethanol oxidation and formic acid oxidation, where CO intermediates form 2,3 . Pt is the most active MOR catalyst, however CO ads intermediates bind strongly to poison the Pt sites, thus preventing access of methanol to these active sites 4 . CO poisoning occurs by the formation of CO ads bound on top of three Pt atoms in a triangular arrangement 5,6 . As a consequence, single atom catalysts are a promising target to overcome CO poisoning if Pt atoms can be dispersed on a support without formation of these triangular arrangements of Pt atoms.

Controlling the formation of nanosized branched nanoparticles with high uniformity is one of the major challenges in synthesizing nanocatalysts with improved activity and stability. Using a cubic‐core hexagonal‐branch mechanism to form highly monodisperse branched nanoparticles, we vary the length of the nickel branches. Lengthening the nickel branches, with their high coverage of active facets, is shown to improve activity for electrocatalytic oxidation of 5‐hydroxymethylfurfural (HMF), as an example for biomass conversion.

Branched nanoparticles are one of the most promising nanoparticle catalysts as their branch sizes and surfaces can be tuned to enable both high activity and stability. Understanding how the crystallinity and surface facets of branched nanoparticles affect their catalytic performance is vital for further catalyst development. In this work, a synthesis is developed to form highly branched ruthenium (Ru) nanoparticles with control of crystallinity. It is shown that faceted Ru branched nanoparticles have improved stability and activity in the oxygen evolution reaction (OER) compared with polycrystalline Ru nanoparticles. This work achieves a low 180 mV overpotential at 10 mA cm−2 for hours, demonstrating that record‐high stability for Ru nanocrystals can be achieved while retaining high activity for OER. The superior electrocatalytic performance of faceted Ru branched nanoparticles is ascribed to the lower Ru dissolution rate under OER conditions due to low‐index facets on the branch surfaces.

Creating high surface area nanocatalysts that contain stacking faults is a promising strategy to improve catalytic activity. Stacking faults can tune the reactivity of the active sites, leading to improved catalytic performance. The formation of branched metal nanoparticles with control of the stacking fault density is synthetically challenging. In this work, we demonstrate that varying the branch width by altering the size of the seed that the branch grows off is an effective method to precisely tune the stacking fault density in branched Ni nanoparticles. A high density of stacking faults across the Ni branches was found to lower the energy barrier for Ni 2+ /Ni 3+ oxidation and result in enhanced activity for electrocatalytic oxidation of 5-hydroxylmethylfurfural. These results show the ability to synthetically control the stacking fault density in branched nanoparticles as a basis for enhanced catalytic activity.

Producing stable but active materials is one of the enduring challenges in electrocatalysis and other types of catalysis. Producing branched nanoparticles is one potential solution. Controlling the number of branches and branch size of faceted branched nanoparticles is one of the major synthetic challenges to achieve highly active and stable nanocatalysts. Herein, we use a cubic‐core hexagonal‐branch mechanism to synthesize branched Ru nanoparticles with control over the size and number of branches. This structural control is the key to achieving high exposure of active {10–11} facets and optimum number of Ru branches that enables improved catalytic activity for oxygen evolution reaction while maintaining high stability.

Conspectus Branched metal nanoparticles have unique catalytic properties because of their high surface area with multiple branches arranged in an open 3D structure that can interact with reacting species and tailorable branch surfaces that can maximize the exposure of desired catalytically active crystal facets. These exceptional properties have led to the exploration of the roles of branch structural features ranging from the number and dimensions of branches at the larger scales to the atomic-scale arrangement of atoms on precise crystal facets. The fundamental significance of how larger-scale branch structural features and atomic-scale surface faceting influence and control the catalytic properties has been at the forefront of the design of branched nanoparticles for catalysis. Current synthetic advances have enabled the formation of branched nanoparticles with an unprecedented degree of control over structural features down to the atomic scale, which have unlocked opportunities to make improved nanoparticle catalysts. These catalysts have high surface areas with controlled size and surface facets for achieving exceedingly high activity and stability. The synthetic advancement has recently led to the use of branched nanoparticles as ideal substrates that can be decorated with a second active metal in the form of islands and single atoms. These decorated branched nanoparticles have new and highly effective catalytic active sites where both branch metal and decorating metal play essential roles during catalysis. In the opening half of this Account, we critically assess the important structural features of branched nanoparticles that control catalytic properties. We first discuss the role of branch dimensions and the number of branches that can improve the surface area but can also trap gas bubbles. We then investigate the atomic-scale structural features of exposed surface facets, which are critical for enhancing catalytic activity and stability. Well-defined branched nanoparticles have led to a fundamental understanding of how the branch structural features influence the catalytic activity and stability, which we highlight for the oxygen evolution reaction (OER) and biomass oxidation. In discussing recent breakthroughs for branched nanoparticles, we explore the opportunities created by decorated branched nanoparticles and the unique bifunctional active sites that are exposed on the branched nanoparticle surfaces. This class of catalysts is of rapidly growing importance for reactions including the hydrogen evolution reaction (HER) and methanol oxidation reaction (MOR), where two exposed metals are required for efficient catalysis. In the second half of this Account, we explore recent advances in the synthesis of branched nanoparticles and highlight the cubic-core hexagonal-branch growth mechanism that has achieved excellent control of all of the important structural features, including branch dimensions, number of branches, and surface facets. We discuss the slow precursor reduction as an effective strategy for ...

3D interconnected structures can be made with molecular precision or with micrometer size. However, there is no strategy to synthesize 3D structures with dimensions on the scale of tens of nanometers, where many unique properties exist. Here, we bridge this gap by building up nanosized gold cores and nickel branches that are directly connected to create hierarchical nanostructures. The key to this approach is combining cubic crystal–structured cores with hexagonal crystal–structured branches in multiple steps. The dimensions and 3D morphology can be controlled by tuning at each synthetic step. These materials have high surface area, high conductivity, and surfaces that can be chemically modified, which are properties that make them ideal electrocatalyst supports. We illustrate the effectiveness of the 3D nanostructures as electrocatalyst supports by coating with nickel-iron oxyhydroxide to achieve high activity and stability for oxygen evolution reaction. This work introduces a synthetic concept to produce a new type of high-performing electrocatalyst support.