High valence metals engineering strategies of Fe/Co/Ni-based catalysts for boosted OER electrocatalysis

Guaranteeing satisfactory catalytic behavior while ensuring high metal utilization has become the problem that needs to be addressed when designing noble metal based catalysts for electrochemical reactions. Here, the well-dispersed ruthenium (Ru) based clusters with adjacent Ru single atoms (SAs) on layered sodium cobalt oxide (Ru/NC) is demonstrated to be a superb electrocatalyst for alkaline HER. The Ru/NC catalyst manifests an activity increase by a factor of two relative to the commercial Pt/C. Operando characterizations in conjunction with density functional theory (DFT) simulations uncover the origin of the superior activity and establish a structure-performance relationship, that is, under HER condition, the real active species are Ru SAs and metallic Ru clusters supported on the NC substrate. The excellent alkaline HER activity of the Ru/NC catalyst can be understood by a spatially decoupled water dissociation and hydrogen desorption mechanism, where the NC substrate accelerates the water dissociation rate, and the generated H intermediates would then migrate to the Ru SAs or clusters and recombine to have H2 evolution. More importantly, comparing the two forms of Ru sites, it is the Ru cluster that dominates the HER activity. This article is protected by copyright. All rights reserved.

Supported Ruthenium Single-Atom and Clustered Catalysts Outperform Benchmark Pt for Alkaline Hydrogen Evolution.

Lithium-sulfur (Li-S) batteries are supposed to be one of the most potential next-generation batteries owing to their high theoretical capacity and low cost. Nevertheless, the shuttle effect of firm multi-step two-electron reaction between sulfur and lithium in liquid electrolyte makes the capacity much smaller than the theoretical value. Many methods were proposed for inhibiting the shuttle effect of polysulfide, improving corresponding redox kinetics and enhancing the integral performance of Li-S batteries. Here, we will comprehensively and systematically summarize the strategies for inhibiting the shuttle effect from all components of Li-S batteries. First, the electrochemical principles/mechanism and origin of the shuttle effect are described in detail. Moreover, the efficient strategies, including boosting the sulfur conversion rate of sulfur, confining sulfur or lithium polysulfides (LPS) within cathode host, confining LPS in the shield layer, and preventing LPS from contacting the anode, will be discussed to suppress the shuttle effect. Then, recent advances in inhibition of shuttle effect in cathode, electrolyte, separator, and anode with the aforementioned strategies have been summarized to direct the further design of efficient materials for Li-S batteries. Finally, we present prospects for inhibition of the LPS shuttle and potential development directions in Li-S batteries. 

Engineering Strategies for Suppressing the Shuttle Effect in Lithium–Sulfur Batteries

Abstract Electrochemical water splitting represents a promising technology for green hydrogen production. To design advanced electrocatalysts, it is crucial to identify their active sites and interpret the relationship between their structures and performance. Materials extensively studied as electrocatalysts include noble‐metal‐based (e.g., Ru, Ir, and Pt) and non‐noble‐metal‐based (e.g., 3d transition metals) compounds. Recently, advancements in characterization techniques and theoretical calculations have revealed novel and unusual active sites. The present review highlights the latest achievements in the discovery and identification of various unconventional active sites for electrochemical water splitting, with a focus on state‐of‐the‐art strategies for determining true active sites and establishing structure–activity relationships. Furthermore, we discuss the remaining challenges and future perspectives for the development of next‐generation electrocatalysts with unusual active sites. By presenting a fresh perspective on the unconventional reaction sites involved in electrochemical water splitting, this review aims to provide valuable guidance for the future study of electrocatalysts in industrial applications. image 

Advanced electrocatalysts with unusual active sites for electrochemical water splitting

Oxygen evolution reaction (OER) remains a bottleneck for electrocatalytic water‐splitting to generate hydrogen. However, the traditional adsorbed evolution mechanism (AEM) possesses sluggish reaction kinetics due to the scaling relationship, while lattice oxygen mechanism (LOM) triggers an unstable structure due to the escaping of lattice oxygen. Herein, a proof‐of‐concept Fe‐Co(OH)2/Fe2O3 heterostructure is put forward, where Fe‐Co(OH)2 following AEM can complete rapidly deprotonation process while Fe2O3 following LOM can trigger O─O coupling step. Combining the theoretical and experimental investigation confirmed that the redistributed space‐charge of Fe‐Co(OH)2/Fe2O3 junction can optimize synergistically adsorbed and lattice oxygen, the coupling mechanism of AEM and LOM can facilitate synchronously the OER activity and stability. As a result, the Fe‐Co(OH)2/Fe2O3 heterostructure shows excellent OER performance with low overpotential of only 219 and 249 mV to reach a current density of 10 and 100 mA cm−2. Specifically, the Fe‐Co(OH)2/Fe2O3 electrocatalyst maintains excellent long‐term stability for 100 h at a large current density of 100 mA cm−2. This work paves an avenue to break through the limit of the conventional OER mechanism.

Coupling Adsorbed Evolution and Lattice Oxygen Mechanism in Fe‐Co(OH)2/Fe2O3 Heterostructure for Enhanced Electrochemical Water Oxidation

The application of renewable energy conversion devices is considered as one of the effective ways to alleviate the energy shortage and environmental pollution. Designing electrocatalysts with excellent performance and affordable price is promising to accelerate the reaction process and large‐scale application. At present, ruthenium (Ru)‐based nanomaterials have shown similar catalytic activity but superior price demand compared to commercial Pt/C. This undoubtedly makes Ru‐based nanomaterials a perfect candidate to replace advanced Pt catalysts. Significant progress is made in the rational design of Ru‐based electrocatalysts, but an in‐depth understanding of the engineering strategies and induced effects is still at an early stage. This review summarizes the modification strategies for enhancing the catalytic activity of Ru, including surface structure, metal element, nonmetal element, size, bimetallic oxides, and heterostructure engineering strategies. Then the induced electronic modulation effects generated by the intramolecular and intermolecular of the Ru‐based nanomaterials are elucidated. Further, the application progress of engineered Ru‐based nanomaterials for hydrogen and oxygen conversion reactions is highlighted, and the correlations of engineering strategies, catalytic activity, and reaction pathways are elaborated. Finally, challenges and prospects are presented for the future development and practical application of Ru‐based nanomaterials.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/aenm.202202119

Recent Advances in Engineered Ru‐Based Electrocatalysts for the Hydrogen/Oxygen Conversion Reactions

Electrocatalysts for highly efficient oxygen reduction reaction (ORR) are crucial for the energy conversion and storage devices. Single-atom catalysts with maximized metal utilization and altered electronic structure are the most promising alternatives to replace current benchmark precious metals. However, the atomic level understanding of the functional role for each species at the anchoring sites is still unclear and poorly elucidated. Herein, we report Fe single atom catalysts with the sulfur and oxygen functional groups near the atomically dispersed metal centers (Fe1/NSOC) for highly efficient ORR. The Fe1/NSOC delivers a half-wave potential of 0.92 V vs. RHE, which is much better than those of commercial Pt/C (0.88 V), Fe single atoms on N-doped carbon (Fe1/NC, 0.89 V ) and most reported nonprecious metal catalysts. The spectroscopic measurements reveal that the presence of sulfur group induces the formation of epoxy groups near the FeN4S2 centers, which not only modulate the electronic structure of Fe single atoms but also participate the catalytic process to improve the kinetics. The density functional theory calculations demonstrate the existence of sulfur and epoxy group engineer the charges of Fe reactive center and facilitate the reductive release of OH* (rate-limiting step), thus boosting the overall oxygen reduction efficiency.

Epoxy-rich Fe Single Atom Sites Boost Oxygen Reduction Electrocatalysis.

Electrochemical Carbon Dioxide Reduction to Ethylene: From Mechanistic Understanding to Catalyst Surface Engineering

Developing efficient and robust catalysts to replace Pt group metals for the oxygen reduction reaction (ORR) is conducive to achieving highly efficient energy conversion. Here, we develop a general ion exchange strategy to construct highly efficient ORR catalysts consisting of various atomically dispersed metal atoms anchored on N-doped porous carbon (M-SAs/NC) to investigate the atomic structure-catalytic activity relationship. The structure characterization results demonstrated that the achieved atomic structure varied due to the presence of different metal centers. Mn-SAs/NC consists of MnN3O1 centers, in which the Mn single atoms are stabilized by 3 N and 1 O. In contrast, the center metals in Fe-/Co-/Cu single-atom catalysts are coordinated by merely N atoms. Mn-SAs/NC delivers superior performance for the ORR with a half-wave potential (E1/2) of 0.91 V vs. RHE in 0.1 M KOH solution, outperforming that of the commercial Pt/C catalyst and the control Fe-/Co-/Cu single-atom catalysts. Furthermore, Mn-SAs/NC also shows excellent methanol tolerance and stability up to 5000 cycles. Density functional theory (DFT) calculations reveal that Mn single atom catalysts with MnN3O1 centers contributed to the superior ORR performance with lower energy barriers and optimized adsorption capacity of intermediates. These findings provide insights into the design and development of specific coordinated structures of atomically dispersed catalysts to facilitate the practical applications of energy conversion.

Atomically dispersed Mn atoms coordinated with N and O within an N-doped porous carbon framework for boosted oxygen reduction catalysis.

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