Single‐atom‐alloys (SAAs), as an emerging kind of materials, combine the advantages of alloy and single‐atom catalysts. The full atomic utilization of active sites and well‐defined bonding environments in SAAs lead to superior electrocatalytic performance and give a deep insight into the structural–activity relationship. In this review, the recent advances of SAAs in various electrochemical reactions are highlighted for further designing of highly effective electrocatalysts. This review starts with an introduction to the fundamental concepts of several effects, which influence the electrocatalytic activities. Then the preparation and characterization methods on SAAs are listed and compared. Next, the recent advances of SAAs in hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, alcohol/hydrogen/formic acid oxidation reaction, carbon dioxide reduction reaction, etc. are illustrated and discussed in detail. Finally, the challenges and suggestions for the future development of SAAs in energy‐conversion electrocatalysis are presented.

Recent Advances of Single‐Atom‐Alloy for Energy Electrocatalysis

Near-infrared (NIR) light-driven overall water splitting beyond 800 nm remains a high-priority target yet great challenge. Here we report that efficient utilization of photogenerated electrons in a photosensitized system prepared by site-selective photodeposition of platinum single atoms/clusters (Pt-SACs) on Ni-phytate (PA-Ni)-sensitized polymeric carbon nitride (PCN). The optimal catalyst presents simultaneous hydrogen (H2) and oxygen (O2) evolution with an H2 evolution amount of 1.4 μmol at λ > 800 nm for 24 hours, which its activity was approximately 140 times higher than that of a system without Pt-SAC modification (PA-Ni1.1@PCN). This work represents the first NIR-light responsive photosensitized system for overall water splitting, and may open an avenue for precisely manipulating cocatalyst positions at the atomic level to improve NIR-light-driven overall water splitting via photosensitization.

Pt Atoms/Clusters on Ni-phytate-sensitized Carbon Nitride for Enhanced NIR-light-driven Overall Water Splitting beyond 800 nm.

The precise regulation for the structural properties of nanomaterials at the atomic scale is an effective strategy to develop high-performance catalysts. Herein, a facile dual-regulation approach was developed to successfully synthesize Ru1Ptn single atom alloy (SAA) with atomic Ru dispersed in Pt nanocrystals. High-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure demonstrated that Ru atoms were dispersed in Pt nanocrystals as single atoms. Impressively, the Ru1Ptn-SAA exhibited an ultrahigh specific activity (23.59 mA cm-2) and mass activity (2.805 mA/μg-PtRu) for methanol oxidation reaction (MOR) and exhibited excellent exchange current density activity (1.992 mA cm-2) and mass activity (4.71 mA/μg-PtRu) for hydrogen oxidation reaction (HOR). Density functional theory calculations revealed that the introduction of Ru atoms greatly reduced the reaction free energy for the decomposition of water molecules, which promoted the removal of CO* in the MOR process and adjusted the Gibbs free energy of hydrogen and hydroxyl adsorption to promote the HOR. Our work provided an effective idea for the development of high performance electrocatalysts.

Platinum-Ruthenium Single Atom Alloy as a Bifunctional Electrocatalyst toward Methanol and Hydrogen Oxidation Reactions.

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 Ni2+/Ni3+ 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.

Introducing Stacking Faults into Three-Dimensional Branched Nickel Nanoparticles for Improved Catalytic Activity.

The efficiency of direct methanol fuel cell (DMFC) is largely determined by the activity and durability of methanol oxidation reaction (MOR) catalysts. Herein, we present a CO-resilient MOR catalyst of palladium-tin nano-alloy anchored on Se-doped MXene (PdSn 0.5 /Se-Ti 3 C 2 ) via a progressive one-step electrochemical deposition strategy. MOR mass activity resulting from Pd/Se-Ti 3 C 2 catalyst (1046.2 mA mg -1 ) is over 2-fold larger than that of Pd/Ti 3 C 2 , suggesting that the introduction of Se atoms on MXene might accelerate the reaction kinetics. PdSn 0.5 /Se-Ti 3 C 2 with Se-doping progress of MXene and the cooperated Pd-Sn sites has a superior MOR mass activity (4762.8 mA mg -1 ), outperforming many other reported Pd-based catalysts. Both experimental results and theoretical calculation reveal that boosted electron interaction of metal crystals with Se-doped MXene and optimized distribution of Pd-Sn sites can modulate the d band center, reduce adsorption energies of CO* at Pd site and enhance OH* generation at Sn site, resulting in highly efficient removal of CO intermediates by reaction with neighboring OH species on adjacent Sn sites.

Engineering Support and Distribution of Palladium and Tin on MXene with the Modulation d-Band Center for CO-resilient Methanol Oxidation.

Single Pt atom catalysts are key targets because a high exposure of Pt substantially enhances electrocatalytic activity. In addition, PtRu alloy nanoparticles are the most active catalysts for the methanol oxidation reaction. To combine the exceptional activity of single Pt atom catalysts with an active Ru support we must overcome the synthetic challenge of forming single Pt atoms on noble metal nanoparticles. Here we demonstrate a process that grows and spreads Pt islands on Ru branched nanoparticles to create single-Pt-atom-on-Ru catalysts. By following the spreading process by in situ TEM, we found that the formation of a stable single atom structure is thermodynamically driven by the formation of strong Pt–Ru bonds and the lowering of the surface energy of the Pt islands. The stability of the single-Pt-atom-on-Ru structure and its resilience to CO poisoning result in a high current density and mass activity for the methanol oxidation reaction over time. PtRu nanoparticles are the state-of-the-art catalysts for methanol electrooxidation—the anodic reaction in direct methanol fuel cells. Now, a method of dispersing single Pt atoms over Ru nanoparticles is presented and monitored in situ, thereby boosting the catalytic performance in the methanol oxidation reaction. 

Benjamin K. Miller

Papers

A single-Pt-atom-on-Ru-nanoparticle electrocatalyst for CO-resilient methanol oxidation