Boosting Oxygen Reduction of Single Iron Active Sites via Geometric and Electronic Engineering: Nitrogen and Phosphorus Dual Coordination

Active Sites in Single-Atom Fe-Nx-C Nanosheets for Selective Electrochemical Dechlorination of 1,2-Dichloroethane to Ethylene

Electrochemical dechlorination of 1,2-dichloroethane (DCE) is one of the prospective and economic strategies for the preparation of high-value ethylene. However, the exploration of advanced electrocatalysts with high reactivity and selectivity and the identification of their active sites are still a challenge. Herein, a single-atom (SA) Fe-N_x-C nanosheet with the presence of a highly efficient Fe-N₄ coordination pattern is reported. The as-prepared single-atom electrocatalyst exhibits a higher reactivity and ethylene selectivity for DCE dechlorination reaction than those of the commercially adopted 20% Pt-C catalyst. By a combination of experiments and theoretical calculations, the atomically dispersed Fe center in the Fe-N₄ structure was unveiled to be the dominating active site for electrochemical production of ethylene. Our work would offer an approach for the rational development of SA materials and supply crucial insight into the mechanism of ethylene production through the DCE dechlorination reaction.

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) plays significant roles in electrochemical energy storage and conversion systems as well as clean synthesis of fine chemicals. However, the ORR process shows sluggish kinetics and requires platinum-group noble metal catalysts to accelerate the reaction. The high cost, rare reservation, and unsatisfied durability significantly impede large-scale commercialization of platinum-based catalysts. Single-atom electrocatalysts (SAECs) featuring with well-defined structure, high intrinsic activity, and maximum atom efficiency have emerged as a novel field in electrocatalytic science since it is promising to substitute expensive platinum-group noble metal catalysts. However, finely fabricating SAECs with uniform and highly dense active sites, fully maximizing the utilization efficiency of active sites, and maintaining the atomically isolated sites as single-atom centers under harsh electrocatalytic conditions remain urgent challenges. In this review, we summarized recent advances of SAECs in synthesis, characterization, oxygen reduction reaction (ORR) performance, and applications in ORR-related H₂O₂ production, metal-air batteries, and low-temperature fuel cells. Relevant progress on tailoring the coordination structure of isolated metal centers by doping other metals or ligands, enriching the concentration of single-atom sites by increasing metal loadings, and engineering the porosity and electronic structure of the support by optimizing the mass and electron transport are also reviewed. Moreover, general strategies to synthesize SAECs with high metal loadings on practical scale are highlighted, the deep learning algorithm for rational design of SAECs is introduced, and theoretical understanding of active-site structures of SAECs is discussed as well. Perspectives on future directions and remaining challenges of SAECs are presented.

Engineering unsymmetrically coordinated Cu-S1N3 single atom sites with enhanced oxygen reduction activity

Atomic interface regulation is thought to be an efficient method to adjust the performance of single atom catalysts. Herein, a practical strategy was reported to rationally design single copper atoms coordinated with both sulfur and nitrogen atoms in metal-organic framework derived hierarchically porous carbon (S-Cu-ISA/SNC). The atomic interface configuration of the copper site in S-Cu-ISA/SNC is detected to be an unsymmetrically arranged Cu-S1N3 moiety. The catalyst exhibits excellent oxygen reduction reaction activity with a half-wave potential of 0.918 V vs. RHE. Additionally, through in situ X-ray absorption fine structure tests, we discover that the low-valent Cuprous-S1N3 moiety acts as an active center during the oxygen reduction process. Our discovery provides a universal scheme for the controllable synthesis and performance regulation of single metal atom catalysts toward energy applications.

Pollen-derived porous carbon decorated with cobalt/iron sulfide hybrids as cathode catalysts for flexible all-solid-state rechargeable Zn-air batteries

The development of flexible all-solid-state rechargeable Zn-air batteries (FS-ZABs) for wearable applications faces challenges from the balance between performance and flexibility of the battery; efficient cathode catalyst and reasonable electrode construction design are key factors. Herein, a low-cost pollen derived N,S co-doped porous carbon decorated with Co9S8/Fe3S4 nanoparticle hybrids (Co-Fe-S@NSRPC) has been synthesized. Owing to the active Co9S8/Fe3S4 nanoparticles, N,S co-doping, and large specific area of the pollen derived porous carbon matrix, the Co-Fe-S@NSRPC composite exhibits an excellent bifunctional catalytic activity with a small potential gap (ΔE = 0.80 V) between the half-wave potential for the ORR (0.80 V) and the potential at 10 mA cm-2 for the OER (1.60 V), and endows a liquid Zn-air battery with a high power density of 138 mW cm-2, a larger specific capacity of 891 mA h g-1 and a stable rechargeability of up to 331 cycles. Based on the Co-Fe-S@NSRPC cathode catalyst, a 2D coplanar FS-ZAB has been fabricated with specially designed parallel narrow strip electrodes alternately arrayed on a polyacrylamide polyacrylic acid copolymer hydrogel solid electrolyte. The presented FS-ZAB exhibits excellent battery performance with high open-circuit-voltage (1.415 V), competitive peak power density (78 mW cm-2), large specific capacity (785 mA h g-1) and stable rechargeability (150 cycles), offers robust flexibility to maintain stable charge/discharge capacity under different bending deformations, and provides convenient coplanar integrability to realize parallel or series connection of multiple cells in a relatively small area.

Engineering a metal-organic framework derived Mn-N4-C x S y atomic interface for highly efficient oxygen reduction reaction

Atomic interface engineering is an effective pathway to regulate the performance of single metal atom catalysts for electrochemical reactions in energy applications. Herein, we construct a sulfur modified Mn-N-C single atom catalyst through a metal-organic framework derived atomic interface strategy, which exhibits outstanding ORR activity with a half-wave potential of 0.916 V vs. RHE in alkaline media. Moreover, operando X-ray absorption spectroscopy analysis indicates that the isolated bond-length extending the low-valence Mn-N4-C x S y moiety serves as an active site during the ORR process. These findings suggest a promising method for the advancement of single atom catalysis.

Chemical Synthesis of Single Atomic Site Catalysts

Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chemical synthesis. The recent emergence of single atomic site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the maximum efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.

Metallic single-atoms confined in carbon nanomaterials for the electrocatalysis of oxygen reduction, oxygen evolution, and hydrogen evolution reactions

Recent advances of single-atom-based carbon nanomaterials for the ORR, OER, HER, and bifunctional electrocatalysis are covered in this review article.