Single-Atomic Co-N4 Sites with CrCo Nanoparticles for Metal-Air Battery-Driven Hydrogen Evolution.

Designing highly active and robust earth abundant trifunctional electrocatalysts for energy storage and conversion applications remain an enormous challenge. Herein, we report a trifunctional electrocatalyst (CrCo/CoN4@CNT-5), synthesized at low calcination temperature (550 °C), which consists of Co-N4 single atom and CrCo alloy nanoparticles and exhibits outstanding electrocatalytic performance for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. The catalyst is able to deliver a current density of 10 mA cm-2 in an alkaline electrolytic cell at a very low cell voltage of ∼1.60 V. When the catalyst is equipped in a liquid rechargeable Zn-air battery, it endowed a high open-circuit voltage with excellent cycling durability and outperformed the commercial Pt/C+IrO2 catalytic system. Furthermore, the Zn-air battery powered self-driven water splitting system is displayed using CrCo/CoN4@CNT-5 as sole trifunctional catalyst, delivering a high H2 evolution rate of 168 µmol h-1. Theoretical calculations reveal synergistic interaction between Co-N4 active sites and CrCo nanoparticles, favoring the Gibbs free energy for H2 evolution. The presence of Cr not only enhances the H2O adsorption and dissociation but also tunes the electronic property of CrCo nanoparticles to provide optimized hydrogen binding capacity to Co-N4 sites, thus giving rise to accelerated H2 evolution kinetics. This work highlights the importance of the presence of small quantity of Cr in enhancing the electrocatalytic activity as well as robustness of single-atom catalyst and suggests the design of the multifunctional robust electrocatalysts for long-term H2 evolution application.

Dual Single-Atomic Co-Mn Sites in Metal-Organic-Framework-Derived N-Doped Nanoporous Carbon for Electrochemical Oxygen Reduction.

Synthesizing dual single-atom catalysts (DSACs) with atomically isolated metal pairs is a challenging task but can be an effective way to enhance the performance for electrochemical oxygen reduction reaction (ORR). Herein, well-defined DSACs of Co-Mn, stabilized in N-doped porous carbon polyhedra (named CoMn/NC), are synthesized using high-temperature pyrolysis of a Co/Mn-doped zeolitic imidazolate framework. The atomically isolated Co-Mn site in CoMn/NC is recognized by combining microscopic as well as spectroscopic techniques. CoMn/NC exhibited excellent ORR activities in alkaline (E1/2 = 0.89 V) as well as in acidic (E1/2 = 0.82 V) electrolytes with long-term durability and enhanced methanol tolerance. Density functional theory (DFT) suggests that the Co-Mn site is efficiently activating the O-O bond via bridging adsorption, decisive for the 4e- oxygen reduction process. Though the Co-Mn sites favor O2 activation via the dissociative ORR mechanism, stronger adsorption of the intermediates in the dissociative path degrades the overall ORR activity. Our DFT studies conclude that the ORR on an Co-Mn site mainly occurs via bridging side-on O2 adsorption following thermodynamically and kinetically favorable associative mechanistic pathways with a lower overpotential and activation barrier. CoMn/NC performed excellently as a cathode in a proton exchange membrane (PEM) fuel cell and rechargeable Zn-air battery with high peak power densities of 970 and 176 mW cm-2, respectively. This work provides the guidelines for the rational design and synthesis of nonprecious DSACs for enhancing the ORR activity as well as the robustness of DSACs and suggests a design of multifunctional robust electrocatalysts for energy storage and conversion devices.

Coupling Single-Ni-Atom with Ni-Co Alloy Nanoparticle for Synergistically Enhanced Oxygen Reduction Reaction.

Developing nonprecious metal-based oxygen reduction reaction (ORR) electrocatalysts with superior activity and durability is crucial for commercializing proton-exchange membrane (PEM) fuel cells. Herein, we report a metal-organic framework (MOF)-derived unique N-doped hollow carbon structure (NiCo/hNC), comprising of atomically dispersed single-Ni-atom (NiN4) and small NiCo alloy nanoparticles (NPs), for highly efficient and durable ORR catalysis in both alkaline and acidic electrolytes. Density functional theory (DFT) calculations reveal the strong coupling between NiN4 and NiCo NPs, favoring the direct 4e- transfer ORR process by lengthening the adsorbed O-O bond. Moreover, NiCo/hNC as a cathode electrode in PEM fuel cells delivered a stable performance. Our findings not only furnish the fundamental understanding of the structure-activity relationship but also shed light on designing advanced ORR catalysts.

Immobilizing a homogeneous manganese catalyst into MOF pores for α-alkylation of methylene ketones with alcohols.

Herein, for the first time, a metal-organic framework (MOF) is reported as a catalyst for α-alkylation of ketones with alcohols. Using an encapsulation strategy via nano-confinement of a homogeneous Mn-phenanthroline complex into MOF pores, functionalized branched ketones were selectively produced. Mechanistic investigations and deuterium labelling experiments validated the utilization of the borrowing hydrogen strategy. The formation of extra Lewis acid sites, defects, and pore enhancement during catalysis helped in achieving higher activity and selectivity.