Anchoring Single Copper Atoms to Microporous Carbon Spheres as High-Performance Electrocatalyst for Oxygen Reduction Reaction

Although the carbon-supported single-atom (SA) electrocatalysts (SAECs) have emerged as a new form of highly efficient oxygen reduction reaction (ORR) electrocatalysts, the preferable sites of carbon support for anchoring SAs are somewhat elusive. Here, a KOH activation approach is reported to create abundant defects/vacancies on the porous graphitic carbon nanosphere (CNS) with selective adsorption capability toward transition-metal (TM) ions and innovatively utilize the created defects/vacancies to controllably anchor TM–SAs on the activated CNS via TM-Nx coordination bonds. The synthesized TM-based SAECs (TM-SAs@N-CNS, TM: Cu, Fe, Co, and Ni) possess superior ORR electrocatalytic activities. The Cu-SAs@N-CNS demonstrates excellent ORR and oxygen evolution reaction (OER) bifunctional electrocatalytic activities and is successfully applied as a highly efficient air cathode material for the Zn–air battery. Importantly, it is proposed and validated that the N-terminated vacancies on graphitic carbons are the preferable sites to anchor Cu-SAs via a Cu-(N-C2)3(N-C) coordination configuration with an excellent promotional effect toward ORR. This synthetic approach exemplifies the expediency of suitable defects/vacancies creation for the fabrication of high-performance TM-based SAECs, which can be implemented for the synthesis of other carbon-supported SAECs.</p

In Situ Reconstruction to Surface Sulfide Adsorbed Metal Scaffold for Enhanced Electrocatalytic Hydrogen Evolution Activity

Transition-metal-based compounds have been intensively explored as efficient electrocatalysts for hydrogen evolution reaction (HER). Feasible reconstruction to the real active sites, which is yet to be identified, endows the promotion of HER activity. Here, it is reported that the incoming S coordinates and anion vacancies prompt the structural reconstruction of S-doped Co3O4 on carbon cloth (S-Co3O4/CC) during HER. A list of in situ studies reveals that the real active sites for HER are the “metallic surface-adparticles” system embracing metallic Co scaffold and the dilute coverage of S coordinated Coδ+. Reaction mechanism exploration illustrates that interfacial perimeters between the coverage of Co3S4 moieties and metallic Co considerably facilitate the adsorption of H*, improve the kinetics of water dissociation, and consequently promote HER activity. The exemplified sulfide-mediated topotactic transformation strategy is extended to the preparation of S, Fe codoped Ni(OH)2 (S-NiFe/CC) as a bifunctional electrocatalyst. The assembled anion exchange membrane water electrolyzer achieves a current density of 1.0 A cm−2 at 1.72 V, showing excellent capability in catalyzing overall water splitting at ampere level. This study, showing a feasible strategy that enables the facile reconstruction to identify active sites, would inspire the development of efficient electrocatalysts for HER and other electrochemical hydrogenation reaction.</p

Atomically-dispersed Mn-(N-C2)2(O-C2)2 sites on carbon for efficient oxygen reduction reaction

Owing to their ultimate mass-catalytic activity, simple active site configuration and readily tunable electronic structures, transition-metal single atoms (SAs) on carbon support have emerged as a new category of electrocatalysts for oxygen reduction reaction (ORR). Here, we exemplify the use of atomically dispersed Mn with N, O heteroatoms-coordinated Mn sites to enhance alkaline ORR performance. A combined sol-gel/carbonization approach is developed to controllably anchor Mn-SAs onto graphitic carbon nanosheets via a hybridized N, O coordination configuration of Mn-(N-C2)2(O-C2)2 (denoted as Mn-SA@CNSs). The obtained Mn-SA@CNSs exhibits superior alkaline ORR electrocatalytic activity with a half-wave potential of 0.88 V vs. RHE in 0.1 M KOH. The rechargeable Zn–air battery assembled using Mn-SA@CNSs air cathode can readily attain a high power density of 177 mW cm−2 with a narrow voltage gap of 0.76 V at 5 mA cm−2. Density functional theory calculations unveil that altering the coordination environment of Mn-SAs from N-coordinated Mn-(N-C2)4 to N-/O-coordinated Mn-(N-C2)2(O-C2)2 alters the d-band electronic structures and regulates the binding strength of ORR intermediates on Mn-SA sites to dramatically reduce the energy barrier and enhance ORR activity. The exemplified hybridization coordination approach in this work would be applicable to alter the electronic structures of other transition-metal SAs for ORR and other reactions.</p

Ruthenium single-atom modulated Ti3C2Tx MXene for efficient alkaline electrocatalytic hydrogen production

Single-atoms (SAs) supported on various substrates have emerged as a new form of electrocatalysts for hydrogen evolution reaction (HER). The exfoliated MXenes possess rich defects/vacancies and surface oxygen groups, can be favorably utilized to anchor SAs. Here, we take advantage of the exfoliated Ti3C2Tx to anchor Ru-SAs on Ti3C2Tx through a wet-chemistry impregnation process. The obtained RuSA@Ti3C2Tx possesses excellent HER activity, especially under high current densities. Remarkably, RuSA@Ti3C2Tx can readily attain high current densities of 1 and 1.5 A cm−2 at low over potentials of 425.7 and 464.6 mV, respectively, demonstrating its potential for practical applications. The A1g vibration frequency shift of the Raman spectrum is innovatively used to probe the surface -OH coverage on Ti3C2Tx, providing critical information for mechanistic studies. The experimental and theoretical studies reveal that the superior HER electrocatalytic activity of RuSA@Ti3C2Tx results from the Ru-SAs enhanced H2O adsorption and dissociation, and promoted H2 formation.</p