Anion-Regulated Selective Generation of Cobalt Sites in Carbon: Toward Superior Bifunctional Electrocatalysis

Nanostructured transition-metal phthalocyanine complexes for catalytic oxygen reduction reaction

Oxygen reduction reaction (ORR) plays a key role in the field of fuel cells. Efficient electrocatalysts for the ORR are important for fuel cells commercialization. Pt and its alloys are main active materials for ORR. However, their high cost and susceptibility to time-dependent drift hinders their applicability. Satisfactory catalytic activity of nanostructured transition metal phthalocyanine complexes (MPc) in ORR through the occurrence of molecular catalysis on the surface of MPc indicates their potential as a replacement material for precious-metal catalysts. Problems of MPc are analyzed on the basis of chemical structure and microstructure characteristics used in oxygen reduction catalysis, and the strategy for controlling the structure of MPc is proposed to improve the catalytic performance of ORR in this review.

Rationally integrated nickel sulfides for lithium storage: S/N co-doped carbon encapsulated NiS/Cu2S with greatly enhanced kinetic property and structural stability

Nickel sulfides are promising anode materials for lithium-ion batteries (LIBs) due to their high theoretical capacities but suffer from the sluggish kinetic process and poor structural stability. Herein, we develop...

Activating bimetallic ZIF-derived polymers using facile steam-etching for the ORR

An α-Fe2O3/Fe@NPC catalyst for the ORR is synthesized via a thermal shock reaction with precursors 1 and 2.

Engineering Co and Ru dual-metal atoms on nitrogen-doped carbon as highly efficient bifunctional oxygen electrocatalysts

Designing dual-metal atoms efficient bifunctional oxygen electrocatalyst by a one-step adsorption and a pyrolysis steps.

A novel and low-cost CuPc@C catalyst derived from the compounds of sunflower straw and copper phthalocyanine pigment for oxygen reduction reaction

A series of carbon and phthalocyanine catalysts were prepared with uniform and stretchable sunflower straw biological materials as the carbon source and inexpensive copper phthalocyanine (CuPc) pigment as a nitrogen doping source by a facile high-temperature carbonization method. This kind of biomass carbon material sunflower straw with abundant pore structure and sponge-like expansion and contraction functions can not only be used as a source of porous carbon in biomass carbon materials, but also as a carbon carrier with high specific surface area to provide nanoparticle adhesion sites. When it was immersed in the copper phthalocyanine pigment solution, more active sites could be exposed, so that CuPc particles could be uniformly doped and distributed on the porous carbon material. As a result, thanks to the doping of nitrogen atoms and the improvement of graphitization degree, the composite catalyst treated at 800 °C (CuPc@C-800) exhibits a porous structure with a 38 mV lower on-set potential and a high stability of 87.4% compared to commercial Pt/C (20%) catalyst. These results demonstrate that CuPc@C series composite catalysts have a splendid electrochemical performance in oxygen reduction reaction catalysts, which can start a new direction for later workers to study combining the properties of biomass carbon material and the phthalocyanine series of catalysts.

A series of CuPc@C composite catalysts were prepared with uniform and stretchable sunflower straw biological materials as the carbon source and inexpensive copper phthalocyanine pigment as a nitrogen doping source.

KOH activation of coal-derived microporous carbons for oxygen reduction and supercapacitors

Due to the dilemma of rapid consumption of fossil fuels and environmental pollution, development of clean, efficient and renewable energy conversion and storage technology has become an urgent need. Supercapacitors and hydrogen-oxygen fuel cells as typical representatives have become the focus of scientific research, in which the electrode materials are of much importance to their improved activity. In this work, a series of porous carbons (PCs) with high specific surface areas were prepared using natural coals as carbon precursors coupled with KOH activation. The effects of the mass ratio of coal and KOH as well as different activation temperatures on the microstructures of the PCs and electrochemical properties were studied in detail. The optimal PC4 (KOH: coal = 4) possessed a high specific surface area (SSA) of 2092 m2 g-1 and a well-developed microporous structure. As the electrocatalyst, it exhibited a positive onset potential of 0.88 V (vs. reversible hydrogen electrode (RHE)) and half-wave potential of 0.78 V (vs. RHE) towards the oxygen reduction reaction (ORR) in an alkaline solution. PC4 also showed the highest specific capacitance of 128 F g-1 at a current density of 0.5 A g-1 among all the samples in this work. The relatively good performance of PC4 resulted from its well-developed microporous structure and large SSA, enabling fast mass transfer of electrolytes.

Fusiform-Like Copper(II)-Based Metal-Organic Framework through Relief Hypoxia and GSH-Depletion Co-Enhanced Starvation and Chemodynamic Synergetic Cancer Therapy

The therapeutic effect of traditional chemodynamic therapy (CDT) agents is severely restricted by their weakly acidic pH and glutathione (GSH) overexpression in the tumor microenvironment. Here, fusiform-like copper(II)-based tetrakis(4-carboxy phenyl)porphyrin (TCPP) nanoscale metal-organic frameworks (nMOFs) were designed and constructed for the first time (named PCN-224(Cu)-GOD@MnO₂). The coated MnO₂ layer can not only avoid conjugation of glucose oxidase (GOD) to damage normal cells but also catalyzes the generation of O₂ from H₂O₂ to enhance the oxidation of glucose (Glu) by GOD, which also provides abundant H₂O₂ for the subsequent Cu⁺-based Fenton-like reaction. Meanwhile, the Cu²⁺ chelated to the TCPP ligand is converted to Cu⁺ by the excess GSH in the tumor, which reduces the tumor antioxidant activity to improve the CDT effect. Next, the Cu⁺ reacts with the plentiful H₂O₂ by enzyme catalysis to produce a toxic hydroxyl radical (^•OH), and singlet oxygen (¹O₂) is synchronously generated from combination with Cu⁺, O₂, and H₂O via the Russell mechanism. Furthermore, the nanoplatform can be used for both TCPP-based in vivo fluorescence imaging and Mn²⁺-induced T₁-weighted magnetic resonance imaging. In conclusion, fusiform-like PCN-224(Cu)-GOD@MnO₂ nMOFs facilitate the therapeutic efficiency of chemodynamic and starvation therapy via combination with relief hypoxia and GSH depletion after acting as an accurate imaging guide.

Graphitic Carbon Nitride (g-C3N4)-Derived Bamboo-Like Carbon Nanotubes/Co Nanoparticles Hybrids for Highly Efficient Electrocatalytic Oxygen Reduction

The oxygen reduction reaction (ORR) is an extremely important reaction in many renewable energy-related devices. The sluggish kinetics of the ORR limits the development of many fuel cells. Design and synthesis of highly efficient nonprecious electrocatalysts are of vital importance for electrochemical reduction of oxygen. Herein, we develop a graphitic carbon nitride (g-C₃N₄)-derived bamboo-like carbon nanotubes/carbon-wrapped Co nanoparticles (BCNT/Co) electrocatalyst by a simple high-temperature pyrolysis and acid-leaching method. The catalytic performance of the as-designed electrocatalyst toward ORR outperforms the commercial Pt/C catalyst in alkaline solution. The onset potential of nonprecious BCNT/Co-800 catalyst was 1.12 V. The half-wave potential was 0.881 V. The result was superior to that of commercial Pt/C (0.827 V vs RHE). The Co nanoparticles, bamboo-like carbon nanotubes, defects, and Co-N_x active sites all result in the remarkable ORR activity, stability, and great methanol tolerance.

Hierarchical microsphere assembled by nanoplates embedded with MoS2 and (NiFe)S x nanoparticles as low-cost electrocatalyst for hydrogen evolution reaction

The development of low-cost electrocatalysts with high performance is important to provide sustainable hydrogen energy. In this work, via one-step sulfuration of [Formula: see text] intercalated NiFe-layered double hydroxide (abbr. NiFe-MoO₄-LDH), hierarchical microspheres are assembled by intersecting nanoplates (15-30 nm in thickness) which are then decorated with MoS₂ and (NiFe)S _x nanoparticles (∼25 nm in size). The NiFe-MoO₄-LDH is synthesized beforehand by a one-pot hydrothermal reaction. Under sulfuration at 300 °C, 400 °C and 600 °C, the NiFe-MoO₄-LDH transforms into multi-metal sulfides of NiFeMoS-T (T is applied temperature). During sulfuration, the confinement effect of LDH limits the growth of metal sulfides, causing formation of nanoparticles of MoS₂ and (NiFe)S _x to expose more catalytic active sites. In an alkaline medium, NiFeMoS-400 depicts superior performance for hydrogen evolution reaction (HER), giving an overpotential of 210 mV at 10 mA cm^-2. A Tafel slope of 88 mV dec^-1 indicates a mixed Volmer-Heyrovsky rate-determining step. The electrode also maintains long-term electrochemical durability during 15 h electrolysis at 25 mA cm^-2. The NiFe-MoO₄-LDH precursor owns three metal elements (Ni, Fe and Mo), which ensure the formation of polymetallic sulfides, and maximum utilization of the LDH layer and interlayer metals contributes to the optimal electrocatalytic activity. The NiFeMoS nanoassembly is a potential low-cost and high-efficiency electrocatalyst.

Toward Efficient Carbon and Water Cycles: Emerging Opportunities with Single-Site Catalysts Made of 3d Transition Metals

Advances in the chemical and electrochemical transformation of carbon and water are vital for delivering affordable and environmentally friendly energy sources and chemicals. Central to this challenge is the performance of materials. Traditionally, noble metal particles or metal complexes have been used as catalysts for many reactions. Recently, 3d transition-metal single-site catalysts (3dTM-SSCs) have emerged as potentially transformational candidates for the next-generation high-performance noble-metal-free catalysts. Designing catalysts at the molecular level can lead to a more efficient utilization of metal atoms and at the same time enhance catalytic performance under harsh reaction conditions. Despite this promise, several fundamental issues remain, in particular the structural evolution of 3dTM-SSCs during the synthesis, the molecular-level insights into the structure of the active sites, catalytic mechanisms, and the long-term cycling stability. Here, the material chemistries that facilitate the 3dTM-SSCs generation through a controlled pyrolytic synthesis are discussed, with focus on elucidating the underlying performance descriptors that can tune the catalytic properties in various critical reactions in carbon and water cycles. The current challenges and possible solutions for improving these novel catalytic materials are also highlighted.

Atomically Dispersed Binary Co-Ni Sites in Nitrogen-Doped Hollow Carbon Nanocubes for Reversible Oxygen Reduction and Evolution

With the inspiration of developing bifunctional electrode materials for reversible oxygen electrocatalysis, one strategy of heteroatom doping is proposed to fabricate dual metal single-atom catalysts. However, the identification and mechanism functions of polynary single-atom structures remain elusive. Atomically dispersed binary Co-Ni sites embedded in N-doped hollow carbon nanocubes (denoted as CoNi-SAs/NC) are synthesized via proposed pyrolysis of dopamine-coated metal-organic frameworks. The atomically isolated bimetallic configuration in CoNi-SAs/NC is identified by combining microscopic and spectroscopic techniques. When employing as oxygen electrocatalysts in alkaline medium, the resultant CoNi-SAs/NC hybrid manifests outstanding catalytic performance for bifunctional oxygen reduction/evolution reactions, boosting the realistic rechargeable zinc-air batteries with high efficiency, low overpotential, and robust reversibility, superior to other counterparts and state-of-the-art precious-metal catalysts. Theoretical computations based on density functional theory demonstrate that the homogenously dispersed single atoms and the synergistic effect of neighboring Co-Ni dual metal center can optimize the adsorption/desorption features and decrease the overall reaction barriers, eventually promoting the reversible oxygen electrocatalysis. This work not only sheds light on the controlled synthesis of atomically isolated advanced materials, but also provides deeper understanding on the structure-performance relationships of nanocatalysts with multiple active sites for various catalytic applications.

Atomically Transition Metals on Self-Supported Porous Carbon Flake Arrays as Binder-Free Air Cathode for Wearable Zinc-Air Batteries

Metal single-atom materials with their high atom utilization efficiency and unique electronic structures usually show remarkable catalytic performances in many crucial chemical reactions. Herein, a facile and easily scalable "impregnation-carbonization-acidification" strategy for fabricating a class of single-atom-anchored (including cobalt and nickel single atoms) monolith as superior binder-free electrocatalysts for developing high-performance wearable Zn-air batteries is reported. The as-prepared single atoms, supported by N-doped carbon flake arrays grown on carbon nanofibers assembly (M SA@NCF/CNF), demonstrate the dual characteristics of excellent catalytic activity (reversible oxygen overpotential of 0.75 V) and high stability, owing to the greatly improved active sites' accessibility and optimized single-sites/pore-structures correlations. Furthermore, wearable Zn-air battery based on Co SA@NCF/CNF air electrode displays superior stability under deformation, satisfactory energy storage capacity, and good practicality to be utilized as an integrated battery system. Theoretical calculations reveal a mechanism for the promotion of the catalytic performances on single atomic sites by lowering the overall oxygen reduction/evolution reaction barriers comparing to metal cluster co-existing configuration. These findings provide a facile strategy for constructing free-standing single-atom materials as well as the engineering of high-performance binder-free catalytic electrodes.

Boosting the ORR performance of modified carbon black via C-O bonds

In the research into oxygen reduction reaction (ORR) catalysts that are applicable to proton exchange membrane fuel cells (PEMFCs), many efforts have been made over a long time period to increase the catalytic activity and reduce the cost. Conductive carbon black is a type of load material widely used in industry. We have developed a cheap carbonization method using Co2+, Zn2+ and 2-methylimidazole (2-MI) on the surface of carbon black. The modified carbon black (MCB) catalyst with high ORR activity has a large diffusion-limited current density (MCB-3 6.18 mA cm-2), a half-wave potential (MCB-3 0.858 V), and no obvious decay after 20 000 cyclic voltammetry cycles. The characterization and controlled experiment results show that the metal content in the MCB is very low, even though it cannot be detected using extended X-ray absorption fine structure spectroscopy (EXAFS), and its ORR activity may be related to the formation of C-O bonds on the surface during the modification process. Subsequent density functional theory calculation results also support this idea. Through the simple modification of carbon black, a catalyst with excellent performance and low price can be obtained. At the same time, the study of the active site of the C-O bond will also provide new ideas for the study of ORR catalysts.

Carbon-Rich Nanomaterials: Fascinating Hydrogen and Oxygen Electrocatalysts

Hydrogen energy is commonly considered as a clean and sustainable alternative to the traditional fossil fuels. Toward universal utilization of hydrogen energy, developing high-efficiency, low-cost, and sustainable energy conversion technologies, especially water-splitting electrolyzers and fuel cells, is of paramount significance. In order to enhance the energy conversion efficiency of the water-splitting electrolyzers and fuel cells, earth-abundant and stable electrocatalysts are essential for accelerating the sluggish kinetics of hydrogen and oxygen reactions. In the past decade, carbon-rich nanomaterials have emerged as a promising class of hydrogen and oxygen electrocatalysts. Here, the development and electrocatalytic activity of various carbon-rich materials, including metal-free carbon, conjugated porous polymers, graphdiyne, covalent organic frameworks (COFs), atomic-metal-doped carbon, as well as metal-organic frameworks (MOFs), are demonstrated. In particular, the correlations between their porous nanostructures/electronic structures of active centers and electrocatalytic performances are emphatically discussed. Therefore, this review article guides the rational design and synthesis of high-performance, metal-free, and noble-metal-free carbon-rich electrocatalysts and eventually advances the rapid development of water-splitting electrolyzers and fuel cells toward practical applications.

A Universal Method to Engineer Metal Oxide-Metal-Carbon Interface for Highly Efficient Oxygen Reduction

Oxygen is the most abundant element in the Earth's crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes and energy converting/storage systems. Developing techniques toward high-efficiency ORR remains highly desired and a challenge. Here, we report a N-doped carbon (NC) encapsulated CeO₂/Co interfacial hollow structure (CeO₂-Co-NC) via a generalized strategy for largely increased oxygen species adsorption and improved ORR activities. First, the metallic Co nanoparticles not only provide high conductivity but also serve as electron donors to largely create oxygen vacancies in CeO₂. Second, the outer carbon layer can effectively protect cobalt from oxidation and dissociation in alkaline media and as well imparts its higher ORR activity. In the meanwhile, the electronic interactions between CeO₂ and Co in the CeO₂/Co interface are unveiled theoretically by density functional theory calculations to justify the increased oxygen absorption for ORR activity improvement. The reported CeO₂-Co-NC hollow nanospheres not only exhibit decent ORR performance with a high onset potential (922 mV vs RHE), half-wave potential (797 mV vs RHE), and small Tafel slope (60 mV dec^-1) comparable to those of the state-of-the-art Pt/C catalysts but also possess long-term stability with a negative shift of only 7 mV of the half-wave potential after 2000 cycles and strong tolerance against methanol. This work represents a solid step toward high-efficient oxygen reduction.

A review of anion-regulated multi-anion transition metal compounds for oxygen evolution electrocatalysis

Recent advances in the anion regulation on multi-anion transition metal compounds as electrocatalysts for oxygen evolution reaction are reviewed.