Enrooted-Type Metal-Support Interaction Boosting Oxygen Evolution Reaction in Acidic Media.

Supported metal catalysts with appropriate metal-support interactions (MSIs) hold a great promise for heterogeneous catalysis. However, ensuring tight immobilization of metal clusters/nanoparticles on the support while maximizing the exposure of surface active sites remains a huge challenge. Herein, we report an Ir/WO3 catalyst with a new enrooted-type MSI in which Ir clusters are, unprecedentedly, atomically enrooted into the WO3 lattice. The enrooted Ir atoms decrease the electron density of the constructed interface compared to the adhered (root-free) type, thereby achieving appropriate adsorption toward oxygen intermediates, ultimately leading to high activity and stability for oxygen evolution in acidic media. Importantly, this work provides a new enrooted-type supported metal catalyst, which endows suitable MSI and maximizes the exposure of surface active sites in contrast to the conventional adhered, embedded, and encapsulated types.

Regulating Morphological Features of Nickel Single-Atom Catalysts for Selective and Enhanced Electroreduction of CO2.

Single-atom catalysts (SACs) are of interest for chemical transformations of significant energy and environmental relevance because of the envisioned efficient use of active sites and the flexibility in tuning their coordination environment. Future advancement in this vein hinges upon the ability to further increase the number and accessibility of active sites in addition to fine-tuning their chemical environment. In this work, a Ni SAC is reported with a unique hierarchical hollow structure (Ni/HH) that allows increased accessibility of the active sites. The successful obtainment of such a uniquely structured catalyst was enabled by the judiciously chosen solvent mixtures for the preparation of the precursor whose hierarchical feature is maintained during the subsequent pyrolysis and etching of the pyrolysis product. Comparative catalytic and mechanistic studies with reference to three closely related but more compact Ni SACs established the superior performance of Ni/HH for selective electroreduction of CO2 to CO. Experimental analyses by in situ attenuated total reflection surface-enhanced infrared spectroscopy reveal that it is the facilitated formation of the *COOH intermediate in the rate-determining step that leads to the enhanced reaction kinetics and the overall catalytic performance.

Linking Enhanced Kinetics of Electrocatalytic Oxygen Reduction Reaction with Increased Utilization of Active Sites in a Hierarchical Single-Atom Catalyst.

Single-atom catalysts (SACs) are of tremendous current research due to maximized use of metal atoms and enhanced activity and selectivity for a great variety of chemical reactions. Hierarchically structured SACs have been explored to further increase the number and accessibility of active sites to realize the full potentials of SACs. However, though plausible-sounding, these supposed advantages of hierarchically structured SACs are largely untested. The assumed enhancing effects on the formation of intermediates on and the overall reaction kinetics remain largely unknown. Herein is reported a Fe-SAC with a hierarchical hollow structure (Fe/HH) that showed excellent activity in oxygen reduction reaction and proton exchange membrane fuel cell. Comparative experimental and computational studies with respect to Fe/SS-the counterpart of Fe/HH with a compact primary structure-reveal a significantly increased number of active sites and their utilization in Fe/HH as reflected by the facilitated formation of the rate-determining-step intermediate Fe-OOH*. This work thus establishes unambiguously the connection between the increased utilization of active sites and the enhanced kinetics of the electrocatalytic reduction of oxygen.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis.

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Heterostructured FeNi hydroxide for effective electrocatalytic oxygen evolution.

Hydrogen production technology by water splitting has been heralded as an effective means to alleviate the envisioned energy crisis. However, the overall efficiency of water splitting is limited by the effectiveness of the anodic oxygen evolution reaction (OER) due to the high energy barrier of the 4e- process. The key to addressing this challenge is the development of high-performing catalysts. Transition-metal hydroxides with high intrinsic activity and stability have been widely studied for this purpose. Herein, we report a gelatin-induced structure-directing strategy for the preparation of a butterfly-like FeNi/Ni heterostructure (FeNi/Ni HS) with excellent catalytic performance. The electronic interactions between Ni2+ and Fe3+ are evident both in the mixed-metal "torso" region and at the "torso/wing" interface with increasing Ni3+ as a result of electron transfer from Ni2+ to Fe3+ mediated by the oxo bridge. The amount of Ni3+ also increases in the "wings", which is believed to be a consequence of charge balancing between Ni and O ions due to the presence of Ni vacancies upon formation of the heterostructure. The high-valence Ni3+ with enhanced Lewis acidity helps strengthen the binding with OH- to afford oxygen-containing intermediates, thus accelerating the OER process. Direct evidence of FeNi/Ni HS facilitating the formation of the Ni-OOH intermediate was provided by in situ Raman studies; the intermediate was produced at lower oxidation potentials than when Ni2(CO3)(OH)2 was used as the reference. The Co congener (FeCo/Co HS), prepared in a similar fashion, also showed excellent catalytic performance.

Study on the Structure-Activity Relationship Between Single-Atom, Cluster and Nanoparticle Catalysts in a Hierarchical Structure for the Oxygen Reduction Reaction.

Literature reports have shown that in primary structures, single-atom catalysts exhibit better performance than cluster and nanoparticles due to their maximum atom utilization and the fine-tuning of the electronic structure of the active sites. Hierarchical structures have recently been extensively studied because of increased active sites and orderliness of channels significantly improves the catalytic performance compare to primary structures especially in nanoparticles, however, the different sized effect of catalysts research has not been reported. Herein, a unique hollow double-shell structure (a distinct cavity-containing) is used as a hierarchical model to study the possible difference between single atom, cluster, and nanoparticle and to establish the corresponding structure-activity relationship. Three Co catalysts are prepared: single atoms (Co-Catalyst-1), clusters (Co-Catalyst-2, 0.5-1 nm), and nanoparticles (Co-Catalyst-3, ≈5 nm) and their oxygen-reduction capacity is evaluated. The unique electronic interactions, the strong electron-withdrawing ability of N in Co-N4 (Co-Catalyst-1), attract electrons from the electrode to Co, specifically by expediting the generation and transformation of the rate-determining step intermediates *OOH. The variant spatial structure which is caused by Co atom aggregation, and led to surface area, pore size, and carbon disorder, is a distinct, therefore significant variation in mass and charge transport efficiency, and activities.