Atomically Dispersed p-Block Aluminum-Based Catalysts for Oxygen Reduction Reaction.

The main group metals are commonly perceived as catalytically inert in the context of oxygen reduction reactions (ORR) due to the delocalized valence orbitals. Regulating the local environment and structure of metal center coordinated by nitrogen ligands (M-Nx) is a promising approach to accelerate catalytic dynamics. Herein, we, for the first time, report the atomically dispersed Al catalysts coordinated with N and C atoms for 4-electron ORR. The axial coordinated pyrrolyl N group (No) is constructed in the Al-N4-No moiety to regulate the p-band structure of Al center, effectively steering the local environment and structure of the square planar Al-N4 sites, which typically exhibit too strong interaction with ORR intermediates. The dynamic covalency competition of axial Al-No and Al-O bonding could endow the Al center with moderate hybridization between Al 3p orbital and O 2p orbital, alleviating the binding energy of ORR intermediates. The as-prepared Al-N4-No electrocatalyst exhibits excellent ORR activity, selectivity, and durability, along with the rapid kinetics as demonstrated by in situ Raman spectroscopy. This work offers a fundamental comprehension of the fine regulation on p-band and guides the rational design of main-group metal-based single atom catalysts.

Engineering Co-N-Cr Cross-Interfacial Electron Bridges to Break Activity-Stability Trade-Off for Superdurable Bifunctional Single Atom Oxygen Electrocatalysts.

Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have exhibited encouraging oxygen reduction reaction (ORR) activity. Nevertheless, the insufficient long-term stability remains a widespread concern owing to the inevitable 2-electron byproducts, H2O2. Here, we construct Co-N-Cr cross-interfacial electron bridges (CIEBs) via the interfacial electronic coupling between Cr2O3 and Co-N-C, breaking the activity-stability trade-off. The partially occupied Cr 3d-orbitals of Co-N-Cr CIEBs induce the electron rearrangement of CoN4 sites, lowering the Co-OOH* antibonding orbital occupancy and accelerating the adsorption of intermediates. Consequently, the Co-N-Cr CIEBs suppress the two-electron ORR process and approach the apex of Sabatier volcano plot for four-electron pathway simultaneously. As a proof-of-concept, the Co-N-Cr CIEBs is synthesized by the molten salt template method, exhibiting dominant 4-electron selectively and extremely low H2O2 yield confirmed by Damjanovic kinetic analysis. The Co-N-Cr CIEBs demonstrates impressive bifunctional oxygen catalytic activity (▵E=0.70 V) and breakthrough durability including 100 % current retention after 10 h continuous operation and cycling performance over 1500 h for Zn-air battery. The hybrid interfacial configuration and the understanding of the electronic coupling mechanism reported here could shed new light on the design of superdurable M-N-C catalysts.

Tailoring the d-Orbital Splitting Manner of Single Atomic Sites for Enhanced Oxygen Reduction.

Regulating the electronic states of single atomic sites around the Fermi level remains a major concern for boosting the electrocatalytic oxygen reduction reaction (ORR). Herein, a Fe d-orbital splitting manner modulation strategy by constructing axial coordination on FeN4 sites is presented. Experimental investigations and theoretical calculations reveal that the axial tractions induce the distortion of square-planar field (FeN4 SP), up to the quasi-octahedral coordination (FeN4 O1 OCquasi ), thus leading to the electron rearrangement with a diluted spin polarization. The declined population of unpaired electrons in dz 2 , dx z and dyz states engenders a moderate adsorption of ORR intermediates, thereby reinforcing the intrinsic reaction activity. In situ infrared spectroscopy further demonstrates that the reordering of d-orbital splitting and occupation facilitates the desorption of *OH. The FeN4 O1 OCquasi exhibits a dramatic improvement of kinetic current density and turnover frequency, which are fivefold and tenfold higher than those of FeN4 SP. This work presents a novel understanding on improving the electrocatalytic performance through the orbital-scale manipulation.

d-Orbital Electron Delocalization Realized by Axial Fe4 C Atomic Clusters Delivers High-Performance Fe-N-C Catalysts for Oxygen Reduction Reaction.

Fe-N-C catalyst for oxygen reduction reaction (ORR) has been considered as the most promising nonprecious metal catalyst due to its comparable catalytic performance to Pt in proton exchange membrane fuel cells (PEMFCs). The active centers of Fe-pyrrolic N4 have been proven to be extremely active for ORR. However, forming a stable Fe-pyrrolic N4 structure is a huge challenge. Here, a Cyan-Fe-N-C catalyst with Fe-pyrrolic N4 as the intrinsic active center is constructed with the help of axial Fe4 C atomic clusters, which shows a half-wave potential of up to 0.836 V (vs. RHE) in the acid environment. More remarkably, it delivers a high power density of 870 and 478 mW cm-2 at 1.0 bar in H2 -O2 and H2 -Air fuel cells, respectively. According to theoretical calculation and in situ spectroscopy, the axial Fe4 C can provide strong electronic perturbation to Fe-N4 active centers, leading to the d-orbital electron delocalization of Fe and forming the Fe-pyrrolic N4 bond with high charge distribution, which stabilizes the Fe-pyrrolic N4 structure and optimizes the OH* adsorption during the catalytic process. This work proposes a new strategy to adjust the electronic structure of single-atom catalysts based on the strong interaction between single atoms and atomic clusters.

Fe-N-C catalysts decorated with oxygen vacancies-rich CeOx to increase oxygen reduction performance for Zn-air batteries.

Platinum group metal (PGM)-free catalysts represented by nitrogen and iron co-doped carbon (Fe-N-C) catalysts are desirable and critical for metal-air batteries, but challenges still exist in performance and stability. Here, cerium oxides (CeOx) are incorporated into a two-dimensional Fe-N-C catalyst (FeNC-Ce-950) via a host-guest strategy. The Ce4+/Ce3+ redox system creates a large number of oxygen vacancies for rapid O2 adsorption to accelerate the kinetics of oxygen reduction reaction (ORR). Consequently, the as-synthesized FeNC-Ce-950 catalyst exhibits a half-wave potential (E1/2) of 0.921 V and negligible decay (<2 mV for ΔE1/2) after 5,000 accelerated durability cycles, significantly outperforming most of ORR catalysts reported in recent years and precious metal counterparts. When applied in a zinc-air battery, it demonstrates a peak power density of 175 mW cm-2 and a specific capacity of 757 mAh gZn-1. This study also provides a reference for the exploration of Fe-N-C catalysts decorated with variable valence metal oxides.

Low-Loading Sub-3 nm PtCo Nanoparticles Supported on Co-N-C with Dual Effect for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells.

Developing low-loading Pt-based catalysts possessing glorious catalytic performance can accelerate oxygen reduction reaction (ORR) and hence significantly advance the commercialization of proton exchange membrane fuel cells. In this report, we propose a hybrid catalyst that consists of low-loading sub-3 nm PtCo intermetallic nanoparticles carried on Co-N-C (PtCo/Co-N-C) via the microwave-assisted polyol procedure and subsequent heat treatment. Atomically dispersed Co atoms embedded in the Co-N-C carriers diffuse into the lattice of Pt, thus forming ultrasmall PtCo intermetallic nanoparticles. Owing to the dual effect of the enhanced metal-support interaction and alloy effect, as-fabricated PtCo/Co-N-C catalysts deliver an extraordinary performance, achieving a half-wave potential of 0.921 V, a mass activity of 0.700 A mgPt-1@0.9 V, and brilliant durability in the acidic medium. The fuel cell employing PtCo/Co-N-C as the cathode catalyst with an ultralow Pt loading of 0.05 mg cm-2 exhibits an impressive peak power density of 0.700 W cm-2, higher than that of commercial Pt/C under the same condition. Furthermore, the enhanced intrinsic ORR activity and stability are imputed to the downshifted d-band center and the strengthened metal-support interaction, as revealed by density functional theory calculations. This report affords a facile tactic to fabricate Pt-based alloy composite catalysts, which is also applicable to other alloy catalysts.

Coupling fine Pt nanoparticles and Co-Nx moiety as a synergistic bi-active site catalyst for oxygen reduction reaction in acid media.

Fabricating high-efficiency catalysts of Pt nanoparticles coupled with single-atom sites (MNC) attracts intensive attention to accelerate the oxygen reduction reaction (ORR). Here we rationally design the low-Pt hybrid catalyst containing fine Pt nanoparticles coupled with Co-Nx moieties via a microwave-assisted heating process. The well-dispersed Pt nanoparticles are anchored by CoNC supports because of the metal-support interaction. Furthermore, the Co-Nx moiety acts as an electron donor to regulate the electronic structure of Pt through the electron synergistic effect, moderating the adsorption energy of oxygen intermediates on Pt sites, and then increasing the intrinsic activity of Pt. In addition, the overflow effect from CoNC to Pt facilitates a nearly four-electron process and enhances the kinetics of ORR. In acid media, the optimized 10% Pt/CoNC hybrid catalysts with Pt nanoparticles size (2.18 nm) exhibit improved ORR activity and robust durability, delivering a half-wave potential (E1/2) of 0.886 V and negligible loss after accelerated durability test, exceeding the best-in-class commercial Pt/C. The finding of the synergy between CoNC supports and Pt nanoparticles offers a novel ideation to construct various low-loading Pt-based hybrid catalysts.

3D Nano-heterostructure of ZnMn2O4@Graphene-Carbon Microtubes for High-Performance Li-Ion Capacitors.

Heterostructures show great potential in energy storage due to their multipurpose structures and function. Recently, two-dimensional (2D) graphene has been widely regarded as an excellent substrate for active materials due to its large specific surface area and superior electrical conductivity. However, it is prone to self-aggregation during charging and discharging, which limits its electrochemical performance. To address the graphene agglomeration problem, we interspersed polypyrrole carbon nanotubes between the graphene cavities and designed three-dimensional (3D)-heterostructures of ZnMn2O4@rGO-polypyrrole carbon nanotubes (ZMO@G-PNTs), which demonstrated a high rate and cyclic stability in lithium-ion capacitors (LICs). Furthermore, the 3D porous structure provided more surface capacity contribution than 2D graphene, ultimately resulting in a better stability (333 mAh g-1 after 1000 cycles at 1 A g-1) and high rate capacity (208 mAh g-1 at 5 A g-1). Also, the mechanism of performance difference between ZMO@G-PNTs and ZMO@G was investigated in detail. Moreover, LICs built from ZMO@G-PNTs as an anode and activated carbon as a cathode showed an energy density of 149.3 Wh kg-1 and a power density of 15 kW kg-1 and cycling stability with a capacity retention of 61.5% after 9000 cycles.

Evaluation of long-term biocompatibility and osteogenic differentiation of graphene nanosheet doped calcium phosphate-chitosan AZ91D composites.

To improve the biodegradable performance and osteoinductivity of magnesium (Mg) alloy, a novel graphene nanosheet (GNS) doped micro-arc oxidized-AZ91D (MAO-AZ91D) based calcium phosphate (CaP)-chitosan (CS) (GNS-CaP-CS/AZ91D) composite was fabricated. Their long-term in vitro biocompatibility, osteoinductivity and the related signaling pathway were evaluated. The electrochemical test, Mg concentration and SEM results showed that the corrosion rate of GNS-CaP-CS/AZ91D became much slower, whose corrosion degree with immersion time being 90 d was similar to that of AZ91D being 16 d. Good biocompatibility and non-cytotoxicity were observed during the whole immersion process of GNS-CaP-CS/AZ91D. Extracts from GNS-CaP-CS/AZ91D especially in the early stage (16 days and before) could significantly enhance alkaline phosphatase (ALP) activity, calcium mineral deposition and osteoblast-related genes expression of human bone marrow mesenchymal stem cells (hBMSCs). GNS released from GNS-CaP-CS/AZ91D composites might serve a critical role to induce osteogenic differentiation through ERK/MAPK pathway in the long-term immersion process, while, PI3K/Akt signaling only significantly in the early stage. Moreover, Mg element could promote osteogenic differentiation through ERK/MAPK pathway and the maximum concentration was about the range between 200 and 500 ppm.