Polynary transition-metal atom catalysts are promising to supersede platinum (Pt)-based catalysts for oxygen reduction reaction (ORR). Regulating the local configuration of atomic catalysts is the key to catalyst performance enhancement. Different from the previously reported single-atom or dual-atom configurations, a new type of ternary-atom catalyst, which consists of atomically dispersed, nitrogen-coordinated Co-Co dimers, and Fe single sites (i.e., Co2 -N6 and Fe-N4 structures) that are coanchored on highly graphitized carbon supports is developed. This unique atomic ORR catalyst outperforms the catalysts with only Co2 -N6 or Fe-N4 sites in both alkaline and acid conditions. Density functional theory calculations clearly unravels the synergistic effect of the Co2 -N6 and Fe-N4 sites, which can induce higher filling degree of Fe-d orbitals and favors the binding capability to *OH intermediates (the rate determining step). This ternary-atom catalyst may be a promising alternative to Pt to drive the cathodic ORR in zinc-air batteries.

Atomically Dispersed Co2 -N6 and Fe-N4 Costructures Boost Oxygen Reduction Reaction in Both Alkaline and Acidic Media.

Developing a highly efficient, easy-to-fabricate and non-noble metal electrocatalyst is vital for the oxygen reduction reaction (ORR). Herein, we fabricate a single Fe site catalyst Fe1/d-CN by developing a double-layer MOF strategy. The Fe1/d-CN catalyst shows an excellent ORR activity in the pH-universal range, especially in alkaline electrolytes with a record-level half-wave potential of 0.950 V, exceeding those of almost all the reported non-noble electrocatalysts and the commercial Pt/C catalyst (0.863 V). Besides its exceptional ORR activity in alkaline electrolytes, it also exhibits comparable ORR activity to the commercial Pt/C catalyst in acidic and neutral electrolytes. We speculate that the sources of the excellent pH-universal ORR performance can be attributed to the regulation of the electronic structure of Fe centres and the excellent electron/proton transport capability that comes from the rich defects and hierarchical porous features of the Fe1/d-CN catalyst. More excitingly, the catalyst possesses remarkable durability, and exhibits a negligible decrease after 30k accelerated durability test (ADT) cycles in pH-universal media. The flexible quasi-solid-state alkaline rechargeable Zn–air batteries with the Fe1/d-CN catalyst as the cathode show superior performance such as a high OCV of 1.50 V and peak power density of 78.0 mW cm−2. Furthermore, the flexible quasi-solid-state neutral rechargeable Zn–air batteries also exhibited remarkable performance.

A pH-universal ORR catalyst with single-atom iron sites derived from a double-layer MOF for superior flexible quasi-solid-state rechargeable Zn–air batteries

The rechargeable Zn-air batteries as an environmentally friendly sustainable energy technology have been extensively studied. However, it is still a challenge to develop non-noble metal bifunctional catalysts with high oxygen reduction as well as oxygen evolution reaction (ORR and OER) activity and superior durability, which limit the large-scale application of rechargeable Zn-air batteries. Herein, we synthesized an ultrastable FeCo bifunctional oxygen electrocatalyst on Se-doped CNTs (FeCo/Se-CNT) via a gravity guided chemical vapor deposition (CVD) strategy. The catalyst exhibits excellent ORR (E1/2 = 0.9 V) and OER (overpotential at 10 mA cm-2 = 340 mV) properties simultaneously, surpassing commercial Pt/C and RuO2/C catalysts. More importantly, the catalyst shows an unordinary stability, that is, is no obvious decrease after 30K cycles accelerated durability test for ORR and OER processes. The small potential gap (0.75 V) represents superior bifunctional ORR and OER activities of the FeCo/Se-CNT catalyst. The FeCo/Se-CNT catalyst possesses outstanding electrochemical performance for the rechargeable liquid and flexible all-solid-state Zn-air batteries, for example, a high open circuit voltage (OCV) and peak power density of 1.543 and 1.405 V and 173.4 and 37.5 mW cm-2, respectively.

Ultrastable FeCo Bifunctional Electrocatalyst on Se-Doped CNTs for Liquid and Flexible All-Solid-State Rechargeable Zn-Air Batteries.

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications.

Quasi-solid-state Zn-air batteries are usually limited to relatively low-rate ability (<10 mA cm-2), which is caused in part by sluggish oxygen electrocatalysis and unstable electrochemical interfaces. Here we present a high-rate and robust quasi-solid-state Zn-air battery enabled by atomically dispersed cobalt sites anchored on wrinkled nitrogen doped graphene as the air cathode and a polyacrylamide organohydrogel electrolyte with its hydrogen-bond network modified by the addition of dimethyl sulfoxide. This design enables a cycling current density of 100 mA cm-2 over 50 h at 25 °C. A low-temperature cycling stability of over 300 h (at 0.5 mA cm-2) with over 90% capacity retention at -60 °C and a broad temperature adaptability (-60 to 60 °C) are also demonstrated. 

https://www.nature.com/articles/s41467-022-31383-4.pdf

Quasi-solid-state Zn-air batteries with an atomically dispersed cobalt electrocatalyst and organohydrogel electrolyte

Owing to the energy crisis and environmental pollution, developing efficient and robust electrochemical energy storage (or conversion) systems is urgently needed but still very challenging. Next-generation electrochemical energy storage and conversion devices, mainly including fuel cells, metal-air batteries, metal-sulfur batteries, and metal-ion batteries, have been viewed as promising candidates for future large-scale energy applications. All these systems are operated through one type of chemical conversion mechanism, which is currently limited by poor reaction kinetics. Single atom catalysts (SACs) perform maximum atom efficiency and well-defined active sites. They have been employed as electrode components to enhance the redox kinetics and adjust the interactions at the reaction interface, boosting device performance. In this Review, we briefly summarize the related background knowledge, motivation and working principle toward next-generation electrochemical energy storage (or conversion) devices, including fuel cells, Zn-air batteries, Al-air batteries, Li-air batteries, Li-CO2 batteries, Li-S batteries, and Na-S batteries. While pointing out the remaining challenges in each system, we clarify the importance of SACs to solve these development bottlenecks. Then, we further explore the working principle and current progress of SACs in various device systems. Finally, future opportunities and perspectives of SACs in next-generation electrochemical energy storage and conversion devices are discussed.

Yueyang Li

Papers

Single Atom Catalysts for Fuel Cells and Rechargeable Batteries: Principles, Advances, and Opportunities