Triethylenediamine cobalt complex encapsulated in a metal-organic framework cage to prepare a cobalt single-atom catalyst with a high Co-N4 density for an efficient oxygen reduction reaction

Isolated asymmetric CoN4 sites on nitrogen-doped hollow carbons as electrocatalysts for hydrogen evolution reactions in dual pH electrolytes

Atomic transition metal-nitrogen (M-Nx) active sites dispersed on carbon matrix have emerged as highly promising electrocatalysts for energy conversion systems, owing to their maximized metal center utilization and excellent conductivity of the carbon substrate. However, conventional symmetric M-N4 configurations often exhibit limited hydrogen evolution reaction (HER) activity in alkaline due to the inferior water dissociation capacity. In this work, isolated Co single atoms anchored on N-doped hollow carbon substrates (CoSA/NHCs) are successfully fabricated by the pyrolysis of Co2+-containing polypyrrole (Co-PPY) precursor, and the subsequent removal of embedded Co particles. Thanks to the high density of atomically distributed asymmetric Co-N4 moieties, which facilitate efficient water activation andoptimal adsorption of the active hydrogen (H*), the synthesized CoSA/NHCs electrocatalysts show remarkable HER activity and stability in alkaline electrolyte. Besides, they can effectively electrolyze HER in acidic medium, undergoing the Volmer-Heyrovsky pathway. Remarkably, CoSA/NHC-800, pyrolyzed at 800 °C, requires small overpotentials of 230 and 228 mV to achieve the current density of -10 mA cm-2 in 1.0 M KOH and 0.5 M H2SO4 solutions, respectively. Moreover, it displays small Tafel slopes, high Faradic efficiencies (∼100 %) and superior long-term stability. This study paves a strategic approach for designing high-performance single-atom electrocatalysts through asymmetric structural engineering.

Cobalt single-atom catalyst for hydrogen peroxide electrochemical detection in waterlogged foods and living cancer cells

The quantitative detection of hydrogen peroxide (H2O2) in waterlogged foods and living cancer cells is important for food safety and clinical detection. In this study, single-atom cobalt catalysts in polymeric carbon nitride (Co SACs-CN) were synthesized by grinding and pyrolysis. This catalyst was subsequently used to modify a pencil graphite electrode (PGE) for electrochemical detection of H2O2. The electrostatic potential of H2O2 was analyzed using Gaussian and Multiwfn software. The linear range of the prepared electrochemical sensor was 1 - 8000 μM, and the detection limit was 0.31 μM. After 30 days, the current retention rate was 93.4%, which can be used for the electrochemical determination of H2O2 in waterlogged foods. Moreover, the sensor was capable of real-time monitoring of H2O2 release from A549 lung cancer cells. The successful development of this sensor has broadened the application of cobalt-based single-atom nanomaterials in the design of H2O2 sensors and offers a novel alternative for the electrochemical detection of H2O2.

Deciphering the energy storage mechanism of CoS2 nanowire arrays for High-Energy aqueous copper-ion batteries

Transition metal sulfide (TMs) offers ultra-high specific capacity through multi-electron transfer, showing promise for aqueous batteries. However, the poor cycling performance and the uncleared energy storage mechanism are restricted from further development. Herein, CoS2 nanowire arrays grown on carbon cloth (CoS2/CC) are proposed as binder-free and self-supporting electrodes for aqueous copper-ion batteries. The energy storage mechanism is clarified by a series of ex-situ tests: a multi-electron electrode reaction through a three-step reaction of CoS2 → CuS → Cu7S4 → Cu2S. Electrochemical results suggest that the CoS2/CC cathode exhibits excellent long cycle stability (capacity retention of 99.7 % after 1000 cycles at 10 A/g) along with high specific capacity (762.3 mAh g-1 at 1 A/g). The carbon cloth with stable three-dimensional (3D) conductive structure can not only offer high-speed pathways to promote the transfer of electrons but also inhibit the volume change. Meanwhile, CoS2 nanowire arrays with high surface-to-volume ratios can improve wettability of electrolyte and promote redox reactions. Furthermore, an advanced Zn-CoS2/CC hybrid ion aqueous battery reveals an energy density of 724 Wh kg-1 and an output voltage of 1.24 V, providing a promising strategy for the establishment of transition metal sulfide cathode in high-energy aqueous batteries.

Identification of true active sites in N-doped carbon-supported Fe2P nanoparticles toward oxygen reduction reaction

Transition metal-based nanoparticles (NPs) are emerging as potential alternatives to platinum for catalyzing the oxygen reduction reaction (ORR) in zinc-air batteries (ZAB). However, the simultaneous coexistence of single-atom moieties in the preparation of NPs is inevitable, and the structural complexity of catalysts poses a great challenge to identifying the true active site. Herein, by employing in situ and ex situ XAS analysis, we demonstrate the coexistence of single-atom moieties and iron phosphide NPs in the N, P co-doped porous carbon (in short, Fe-N4-Fe2P NPs/NPC), and identify that ORR predominantly proceeds via the atomic-dispersed Fe-N4 sites, while the presence of Fe2P NPs exerts an inhibitory effect by decreasing the site utilization and impeding mass transfer of reactants. The single-atom catalyst Fe-N4/NPC displays a half-wave potential of 0.873 V, surpassing both Fe-N4-Fe2P NPs/NPC (0.858 V) and commercial Pt/C (0.842 V) in alkaline condition. In addition, the ZAB based on Fe-N4/NPC achieves a peak power density of 140.3 mW cm-2, outperforming that of Pt/C-based ZAB (91.8 mW cm-2) and exhibits excellent long-term stability. This study provides insight into the identification of true active sites of supported ORR catalysts and offers an approach for developing highly efficient, nonprecious metal-based catalysts for high-energy-density metal-air batteries.

Fe based MOF encapsulating triethylenediamine cobalt complex to prepare a FeN3-CoN3 dual-atom catalyst for efficient ORR in Zn-air batteries

Single-atom catalysts show good oxygen reduction reaction (ORR) performance in metal-air battery. However, the symmetric electron distribution results in discontented adsorption energy of ORR intermediates and a lower ORR activity. Herein, Fe-Co dual-atom catalyst with FeN3-CoN3 configuration was prepared by encapsulating nitrogen-rich ion (triethylenediamine cobalt complex, [Co(en)3]3+) in Fe based MOF cage to greatly enhance ORR performance. Due to the confinement effect of the MOF cage, the encapsulated [Co(en)3]3+ is closer to Fe of MOF, thus easily generating FeN3-CoN3 sites. The FeN3-CoN3 sites can break the symmetric electron distribution of single-atom sites, optimizing adsorption energy of oxygen intermediate. Thus, FeCo-NC exhibits extraordinary ORR activity with a high half-wave potential of 0.915 V and 0.789 V in alkaline and acidic electrolyte, respectively, while it was 0.874 V and 0.79 V for Pt/C. The liquid and solid Zn-air batteries with FeCo-NC as cathode show higher peak power density and specific capacity. DFT results indicate that FeN3-CoN3 site can reduce the reaction energy barrier of the rate-determining step resulting in an excellent ORR performance.

Cobalt nanoclusters Deposit on Nitrogen-Doped graphene Sheets as bifunctional electrocatalysts for high performance lithium - Oxygen batteries

Rechargeable lithium-oxygen (Li-O2) batteries are being considered as the next-generation energy storage systems due to their higher theoretical energy density. However, the practical application of Li-O2 batteries is hindered by slow kinetics and the formation of side products during the oxygen reduction and evolution reactions on the cathode. These reactions lead to high overpotentials during charging and discharging. To address these challenges, we propose a simple ultrasonic method for synthesizing cobalt nanoclusters embedded in nitrogen-doped graphene nanosheets (GrZnCo) derived from metal-organic frameworks (MOFs). The resulting material, due to the retention of metallic cobalt structure, exhibits better electronic conductivity. Additionally, the GrZnCo catalyst shows vigorous catalytic activity, which can improve reaction kinetics and suppress side reactions, thus lowering the charging overpotential. We have investigated the impact of different catalyst compositions (GrZnCox; x  = 1, 3, 5) by varying the amounts of cobalt and zinc. The optimum catalyst, GrZnCo3, contains high cobalt-N active components, graphitic-N, pyridinic-N, pyrrolic-N, and abundant defect structures, which enhance the electrochemical performance. The defect-rich GrZnCo3 catalyst enables Li-O2 batteries to achieve a high discharge capacity of 13500 mAh·g-1 at 50 mA·g-1 and a remarkable long-term cycling performance of over 400 cycles at 100 mA·g-1 with a limited capacity of 500 mAh·g-1. This work demonstrates an effective approach to fabricate cost-effective electrocatalysts for various energy storage systems.

Interfacial OOH* mediated Fe(II) regeneration on the single atom Co-N-C catalyst for efficient Fenton-like processes

Fe(II) regeneration is decisive for highly efficient H2O2-based Fenton-like processes, but the role of cobalt-containing reactive sites in promoting Fe(II) regeneration was overlooked. Herein, a single atom Co-N-C catalyst was employed in Fe(II)/H2O2 system to promote the degradation of diverse organic contaminants. The EPR and quenching experiments indicated Co-N-C significantly enhanced the generation of superoxide species, and accelerated hydroxyl radical generation for pollutant degradation. The electrochemical and surface composition analyses demonstrated the enhanced H2O2 activation and Fe(III)/Fe(II) recycling on the catalyst. Furthermore, in-situ Raman characterization with shell-isolated gold nanoparticles was employed to visualize the interfacial reactive intermediates and their time-resolved interaction. The accumulation of interfacial CoOOH* was confirmed when Co-N-C activated H2O2 alone, but it rapidly transformed into FeOOH* upon Fe(II) addition. Besides, the temporal variation of OOH* intermediates and the relative intensity of Co(III)-O and Co(IV)=O peaks depicted the dynamic interaction of reactive intermediates along the H2O2 consumption. With this basis, we proposed a mechanism of interfacial OOH* mediated Fe(II) regeneration, which overcame the kinetical limitation of Fe(II)/H2O2 system. Therefore, this study provided a primary effort to elucidate the overlooked role of interfacial CoOOH* in the Fenton-like processes, which may inspire the design of more efficient catalysts.