Electrocatalysis Mechanism and Structure-Activity Relationship of Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Electrocatalytic Reactions.

Atomically dispersed metal-nitrogen-carbon catalysts (M-N-C) have been widely used in the field of energy conversion, which has already attracted a huge amount of attention. Due to their unsaturated d-band electronic structure of the center atoms, M-N-C catalysts can be applied in different electrocatalytic reactions by adjusting their own microscopic electronic structures to achieve the optimization of the structure-activity relationship. Consequently, it is of great significance for the revelation of electrocatalytic mechanism and structure-activity relationship of M-N-C catalysts. Thus, this review first introduces the relative research methods, including in situ/operando characterization techniques and theoretical calculation methods. Furthermore, clarifying the electrocatalytic mechanism and structure-activity relationship of M-N-C catalysts in different electrochemical energy conversion reactions is focused. Moreover, the future research directions are pointed out based on the discussion. This review will provide good guidance to systematically study the catalytic mechanism of single-atom catalysts and reasonably design the single-atom catalysts.

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.

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.

How to appropriately assess the oxygen reduction reaction activity of platinum group metal catalysts with rotating disk electrode.

The sluggish oxygen reduction reaction (ORR) has becoming the bottleneck of largescale implementation of proton exchange membrane fuel cells. However, when it comes to the ORR activity assessing of platinum group metals (PGMs) with rotating disk electrode, the corresponding potential conversion vs. reversible hydrogen electrode, test protocols, and activity calculation processes are still in chaos in many published literatures. In this work, two standard calculation processes for PGM ORR activities are demonstrated, followed by a specification for the usage of reference electrodes. Then a 4-fold discrepancy in ORR activities obtained via different test protocols is found for the same Pt/C, and an average adsorption model and the "coverage effects" are proposed to illustrate the hysteresis loop between negative and positive-going ORR polarization plots. Finally, four motions over appropriate assessment of PGM ORR activity are emphasized, hoping to bring a fair communication platform for researchers from different groups.

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.

A sponge-templated sandwich-like cobalt-embedded nitrogen-doped carbon polyhedron/graphene composite as a highly efficient catalyst for Zn-air batteries.

Non-noble metal materials are regarded as the most promising catalysts for the oxygen reduction reaction (ORR) to overcome the inherent defects of Pt-based catalysts, like high cost, limited availability and insufficient stability. Here, we fabricate sandwich-like Co encapsulated nitrogen doped carbon polyhedron/graphene (s-Co@NCP/rGO) via a facile and scalable strategy by loading Co-based zeolitic imidazolate framework (ZIF-67) and graphene oxide (GO) layers individually on a polyurethane (PU) sponge template. The 3D sandwich structure is maintained with the assistance of the sponge template, which promotes the uniform dispersion of ZIF-67-derived Co embedded nitrogen doped carbon polyhedra (Co@NCP) and prevents the graphene plates from agglomerating during the annealing process. The final product demonstrates considerable catalytic performance for the ORR with a half-wave potential of 0.85 V, preferable stability and increased poisoning tolerance by comparison to 20 wt% Pt/C, which stems from the 3D sandwich-like structure, N/Co-doping effect, large accessible surface area and hierarchical porous structures. The excellent ORR performance of the catalysts means that they can be utilised in a Zn-air battery as cathode catalysts. During such a demonstration, s-Co@NCP/rGO shows a high open-circuit voltage of 1.466 V, remarkable long-term durability and an outstanding peak power density of 186 mV cm-2, which shows its high potential as a prospective alternative for widespread practical application in the field of non-noble metal ORR catalysts.

Pseudocapacitive Crystalline MnCo2O4.5 and Amorphous MnCo2S4 Core/Shell Heterostructure with Graphene for High-Performance K-Ion Hybrid Capacitors.

Potassium-ion capacitors (KICs) have received a surge of interest because of their higher reserves and lower costs of potassium than lithium. However, the cycle performance and capacity of potassium devices have been reported to be unsatisfactory. Herein, a unique crystalline MnCo2O4.5 and amorphous MnCo2S4 core/shell nanoscale flower structure grown on graphene (MCO@MCS@rGO) was synthesized by a two-step hydrothermal process and demonstrated in KICs. The MCO@MCS@rGO exhibits improved electrical conductivity and excellent structural integrity during the charging and discharging process. The reasons could be attributed to the cavity structure of MCO, the mechanical buffer and high electrolyte diffusion rate of MCS, and the auxiliary effect of graphene. The electrical conductivity of MCO@MCS shows a specific capacity of 272.3 mA h g-1 after 400 cycles at 1 A g-1 and a capacity of 125.6 mA h g-1 at 2 A g-1. Besides, the MCO@MCS@rGO and high-surface-area activated carbon in KICs exhibit a relative energy density of 85.3 W h kg-1 and a power density of 9000 W kg-1 and outstanding cycling stability with a capacity retention of 76.6% after 5000 cycles. Moreover, the reaction mechanism of MCO@MCS@rGO in the K-ion cell was investigated systematically using X-ray diffraction and transmission electron microscopy, providing guidance on the further development of pseudocapacitive materials.

Advanced non-noble materials in bifunctional catalysts for ORR and OER toward aqueous metal-air batteries.

The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.

Enhancing Na-Ion Storage at Subzero Temperature via Interlayer Confinement of Sn2.

Sluggish kinetics and limited reversible capacity present two major challenges for layered titanates to achieve satisfactory sodium-ion storage performance at subzero-temperatures (subzero-T). To facilitate sodiation dynamics and improve reversible capacity, we proposed an additive-free anode with Sn(II) located between layers. Sn-5s in interlayer-confining Sn(II), which has a larger negative charge, will hybridize with O-2p to trigger charge redistribution, thereby enhancing electronic conductivity. H-titanates with an open framework are designed to stabilize Sn(II) and restrain subsequent volume expansion, which could potentially surpass the capacity limitation of titanate-based materials via a joint alloying-intercalation reaction with high reversibility. Moreover, the generation of conductive Na14Sn4 and the expansion of interlayer spacing resulting from the interlayered alloying reaction are beneficial for charge transfer. These effects synergistically endow the modified sample with a considerably lower activation energy and a 3-fold increase in diffusion. Consequently, the designed anode delivers excellent subzero-T adaptability when compared to the unmodified sample, maintaining capacity retention of 91% after 1200 cycles at -20 °C and 90% after 850 cycles at -30 °C.

Phosphotungstic acid immobilized nanofibers-Nafion composite membrane with low vanadium permeability and high selectivity for vanadium redox flow battery.

A novel composite vanadium-blocking proton exchange membrane for vanadium redox flow battery (VRB) was designed and constructed by immobilizing phosphotungstic acid (PWA) on Nano Kevlar Fibers (NKFs) via a solution-casting method. The proton/vanadium selectivity of the composite membrane is greatly improved by incorporating the complex formed by NKFs and PWA into the Nafion matrix. Simple tuning of the complex doping quantity results in different composite membranes with superior vanadium barrier properties (namely, a minimum vanadium permeability of 2.46 × 10-7 cm2 min-1). This coupled with proton conductivities reaching 0.061 S cm-1 at room temperature indicates a proton/vanadium selectivity of 2.48 × 105 S min cm-3, which is 6.3 times higher than that of recast Nafion (0.34 × 105 S min cm-3). When tested in a VRB, the performance of the single cell assembled with the composite membrane greatly outperforms that with recast Nafion at current densities ranging from 40 to 100 mA cm-2.

Cobalt and Nitrogen Codoped Carbon Nanosheets Templated from NaCl as Efficient Oxygen Reduction Electrocatalysts.

The oxygen reduction reaction (ORR) in a cathode is an essential component of many electrochemical energy storage and conversion systems. Two-dimensional materials are beneficial for electron conduction and mass transport with high density, showing prominent electrochemical catalytic performance towards the ORR. Herein, a simple NaCl-assisted method to synthesize cobalt-nitrogen-doped carbon materials (CoNC), which present prominent performance towards the ORR in alkaline media, is described. The utilization of the NaCl template endows the product with a large specific surface area of 556.4 m2 g-1 , as well as good dispersion of cobalt nanoparticles. CoNC-800@NaCl (800 indicates the calcination temperature in °C) displays an excellent onset potential of 0.94 V (vs. a reversible hydrogen electrode), which is close to that of commercial Pt/C. Additionally, CoNC-800@NaCl also exhibits better long-term durability and methanol tolerance than that of Pt/C. The high-performance CoNC-800@NaCl catalyst provides a hopeful alternative to noble-metal catalysts for the ORR in practical applications.

3D Hierarchical Pt-Nitrogen-Doped-Graphene-Carbonized Commercially Available Sponge as a Superior Electrocatalyst for Low-Temperature Fuel Cells.

Three-dimensional hierarchical nitrogen-doped graphene (3D-NG) frameworks were successfully fabricated through a feasible solution dip-coating method with commercially available sponges as the initial backbone. A spongy template can help hinder the graphene plates restacking in the period of the annealing process. The Pt/3D-NG catalyst was synthesized employing a polyol reduction process. The resultant Pt/3D-NG exhibits 2.3 times higher activity for methanol electro-oxidation along with the improvement in stability as compared with Pt/G owing to their favorable features including large specific surface area, high pore volume, high N doping level, and the homogeneous dispersion of Pt nanoparticles. Besides, Pt/3D-NG also presents high oxygen reduction reaction (ORR) performance in acid media when compared with Pt/3D-G and Pt/G. This work raises a valid solution for the fabrication of 3D functional freestanding graphene-based composites for a variety of applications in fuel cell catalysis, energy storage, and conversion.