A networked iron and nitrogen-doped ZIF-8/MWCNTs heterostructure for oxygen reduction reaction.

Zeolitic Imidazolate Frameworks-8 (ZIF-8) is commonly used as an ideal precursor for non-noble metal catalysts because of its high specific surface area, ultra-high porosity, and N-rich content. Upon pyrolyzing ZIF-8 at 900 °C in Ar, the resulting material, referred to as Z8, displayed good activity toward the oxygen reduction reaction (ORR). Then the ZIF-8 was mixed with various conductive carbon materials, such as multiwall carbon nanotubes (MWCNTs), Acetylene black (ACET), Vulcan XC-72R (XC-72R), and Ketjenblack EC-600JD (EC-600JD), to form Z8 composites. The Z8/MWCNTs composite exhibited enhanced ORR activity owing to its network structure, meso-/microporous hierarchical porous structure, improved electrical conductivity, and graphitization. Subsequently, iron and nitrogen co-doping is achieved through the pyrolysis of a mixture comprising Fe, N precursor, and ZIF-8/MWCNTs, which is denoted as FeN-Z8/MWCNTs. The intrinsically high electrical conductivity of MWCNTs facilitated efficient electron transfer during the ORR, while the meso-/microporous hierarchical porous structure and network structure of Fe, N co-doped ZIF-8/MWCNTs promoted oxygen transport. The presence of Fe-containing species in the catalyst acted as activity centers for ORR. This strategy of preparing Z8 composites and modifying them with Fe, N co-doping offers an insightful approach to designing cost-effective electrocatalysts.

Effect of the metal-support interaction in platinum anchoring on heteroatom-doped graphene for enhanced oxygen reduction reaction.

Three kinds of Pt anchoring on heteroatom-doped graphene were synthesised and their effects on catalytic performance were discussed. The introduction of N and P into graphene is helpful to decrease the Pt particle size with a homogeneous distribution and favor the electronic configuration for the ORR.

Metal-Organic Frameworks (MOFs) Derived Materials Used in Zn-Air Battery.

It is necessary to develop new energy technologies because of serious environmental problems. As one of the most promising electrochemical energy conversion and storage devices, the Zn-air battery has attracted extensive research in recent years due to the advantages of abundant resources, low price, high energy density, and high reduction potential. However, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of Zn-air battery during discharge and charge have complicated multi-electron transfer processes with slow reaction kinetics. It is important to develop efficient and stable oxygen electrocatalysts. At present, single-function catalysts such as Pt/C, RuO2, and IrO2 are regarded as the benchmark catalysts for ORR and OER, respectively. However, the large-scale application of Zn-air battery is limited by the few sources of the precious metal catalysts, as well as their high costs, and poor long-term stability. Therefore, designing bifunctional electrocatalysts with excellent activity and stability using resource-rich non-noble metals is the key to improving ORR/OER reaction kinetics and promoting the commercial application of the Zn-air battery. Metal-organic framework (MOF) is a kind of porous crystal material composed of metal ions/clusters connected by organic ligands, which has the characteristics of adjustable porosity, highly ordered pore structure, low crystal density, and large specific surface area. MOFs and their derivatives show remarkable performance in promoting oxygen reaction, and are a promising candidate material for oxygen electrocatalysts. Herein, this review summarizes the latest progress in advanced MOF-derived materials such as oxygen electrocatalysts in a Zn-air battery. Firstly, the composition and working principle of the Zn-air battery are introduced. Then, the related reaction mechanism of ORR/OER is briefly described. After that, the latest developments in ORR/OER electrocatalysts for Zn-air batteries are introduced in detail from two aspects: (i) non-precious metal catalysts (NPMC) derived from MOF materials, including single transition metals and bimetallic catalysts with Co, Fe, Mn, Cu, etc.; (ii) metal-free catalysts derived from MOF materials, including heteroatom-doped MOF materials and MOF/graphene oxide (GO) composite materials. At the end of the paper, we also put forward the challenges and prospects of designing bifunctional oxygen electrocatalysts with high activity and stability derived from MOF materials for Zn-air battery.

Phosphorus-Doped Graphene Electrocatalysts for Oxygen Reduction Reaction.

Developing cheap and earth-abundant electrocatalysts with high activity and stability for oxygen reduction reactions (ORRs) is highly desired for the commercial implementation of fuel cells and metal-air batteries. Tremendous efforts have been made on doped-graphene catalysts. However, the progress of phosphorus-doped graphene (P-graphene) for ORRs has rarely been summarized until now. This review focuses on the recent development of P-graphene-based materials, including the various synthesis methods, ORR performance, and ORR mechanism. The applications of single phosphorus atom-doped graphene, phosphorus, nitrogen-codoped graphene (P, N-graphene), as well as phosphorus, multi-atoms codoped graphene (P, X-graphene) as catalysts, supporting materials, and coating materials for ORR are discussed thoroughly. Additionally, the current issues and perspectives for the development of P-graphene materials are proposed.

N, P-Codoped Graphene Dots Supported on N-Doped 3D Graphene as Metal-Free Catalysts for Oxygen Reduction.

Nitrogen and phosphorus-codoped graphene dots supported on nitrogen-doped three-dimensional graphene (N, P-GDs/N-3DG) have been synthesized by a facile freeze-annealing process. On the surface of the 3D interconnected porous structure, the N, P-GDs are uniformly dispersed. The as-prepared N, P-GDs/N-3DG material served as a metal-free catalyst for oxygen reduction reaction (ORR) in an alkaline medium and evaluated by a rotating ring-disk electrode. The N, P-GDs/N-3DG catalyst exhibits excellent ORR activity, which is comparable to that of the commercial Pt/C catalyst. Furthermore, it exhibits a higher tolerance to methanol and better stability than the Pt/C. This enhanced electrochemical catalytic performance can be ascribed to the presence of abundant functional groups and edge defects. This study indicates that P-N bonded structures play a vital role as the active sites in ORR.

Tailorable PC71 BM Isomers: Using the Most Prevalent Electron Acceptor to Obtain High-Performance Polymer Solar Cells.

Despite being widely used as electron acceptor in polymer solar cells, commercially available PC71 BM (phenyl-C71 -butyric acid methyl ester) usually has a "random" composition of mixed regioisomers or stereoisomers. Here PC71 BM has been isolated into three typical isomers, α-, ß1 - and ß2 -PC71 BM, to establish the isomer-dependent photovoltaic performance on changing the ternary composition of α-, ß1 - and ß2 -PC71 BM. Mixing the isomers in a ratio of α/ß1 /ß2 =8:1:1 resulted in the best power conversion efficiency (PCE) of 7.67 % for the polymer solar cells with PTB7:PC71 BM as photoactive layer (PTB7=poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]). The three typical PC71 BM isomers, even though sharing similar LUMO energy levels and light absorption, render starkly different photovoltaic performances with average-performing PCE of 1.28-7.44 % due to diverse self-aggregation of individual or mixed PC71 BM isomers in the otherwise same polymer solar cells.