Porous carbon electrodes have emerged as important cathode materials for metal-air battery systems. However, most approaches for fabricating porous carbon electrodes from biomass are highly energy inefficient as they require the breaking down of the biomass and its subsequent reconstitution into powder-like carbon. Here, enzymes are explored to effectively hydrolyze the partial cellulose in bulk raw wood to form a large number of nanopores, which helps to maximally expose the inner parts of the raw wood to sufficiently dope nitrogen onto the carbon skeletons during the subsequent pyrolysis process. The resulting carbons exhibit excellent catalytic activity with respect to the oxygen reduction and oxygen evolution reactions. As-fabricated cellulose-digested, carbonized wood plates are mechanically strong, have high conductivity, and contain a crosslinked network and natural ion-transport channels and can be employed directly as metal-free electrodes without carbon paper, polymer binders, or carbon black. When used as metal-free cathodes in zinc-air batteries, they result in a specific capacity of 801 mA h g-1 and an energy density of 955 W h kg-1 with the long-term stability of the batteries being as high as 110 h. This work paves the way for the ready conversion of abundant biomass into high-value engineering products for energy-related applications.

Hierarchically Porous Carbon Plates Derived from Wood as Bifunctional ORR/OER Electrodes

FeCo/FeCoP encapsulated in N, Mn-codoped three-dimensional fluffy porous carbon nanostructures as highly efficient bifunctional electrocatalyst with multi-components synergistic catalysis for ultra-stable rechargeable Zn-air batteries.

FeCo/FeCoP encapsulated in N, Mn-codoped three-dimensional fluffy porous carbon nanostructures as highly efficient bifunctional electrocatalyst with multi-components synergistic catalysis for ultra-stable rechargeable Zn-air batteries

Motivated by the increased demand for energy storage technologies with maximum power density and safety, zinc–air batteries have drawn extensive attention. However, developing a competent and robust bifunctional oxygen reduction and oxygen evolution reaction (ORR/OER) catalyst and studying the underlying active-site mechanisms with the aim of optimizing the electrochemical process remains challenging. In this work, defective/graphitic synergy in heteroatom-interlinked-triggered asphaltene (D/G-HASP) is explored via Raman spectroscopy, density functional theory (DFT), and other characterization approaches, in which reactions involving the oxygen-containing intermediates on the carbon substrate can be accelerated by the electron deficit triggered by heteroatoms, thus enhancing the oxygen reaction performance. As estimated, D/G-HASP reveals a superior half-wave potential (E1/2 = 840 mV) and the lowest overpotential (η10 = 310 mV) towards the ORR and OER compared to its other counterparts. Remarkably, zinc–air batteries primed with such an air-electrode show outstanding reversibility and stability. This effort hence delivers insightful understanding of the synergistic reaction principles of heteroatom-interlinked-triggering nanomaterials when used in zinc–air batteries.

Defective/graphitic synergy in a heteroatom-interlinked-triggered metal-free electrocatalyst for high-performance rechargeable zinc–air batteries

A heterogeneous catalyst is a backbone of modern sustainable green industries; and understanding the relationship between its structure and properties is the key for its advancement. Recently, many upscaling synthesis strategies for the development of a variety of respectable control atomically precise heterogeneous catalysts are reported and explored for various important applications in catalysis for energy and environmental remediation. Precise atomic-scale control of catalysts has allowed to significantly increase activity, selectivity, and in some cases stability. This approach has proved to be relevant in various energy and environmental related technologies such as fuel cell, chemical reactors for organic synthesis, and environmental remediation. Therefore, this review aims to critically analyze the recent progress on single-atom catalysts (SACs) application in oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and chemical and/or electrochemical organic transformations. Finally, opportunities that may open up in the future are summarized, along with suggesting new applications for possible exploitation of SACs.

Single-Atom Catalysts: A Sustainable Pathway for the Advanced Catalytic Applications

As one of the most promising alternatives for future energy systems, the rechargeable Zn–air battery (ZAB) has attracted extensive attention due to its extraordinarily high theoretical specific energy density. However, several obstacles restrict its practical application. One challenge is the sluggish kinetics of oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER) in the discharging and charging processes of ZABs. In addition, when using unifunctional ORR or OER electrocatalysts as air electrodes, like noble metal catalysts (Pt/C or Ru/IrO2), there are the disadvantages of high cost and poor stability. Therefore, rational design of non-noble metal bifunctional ORR/OER electrocatalysts with high activity and stability is essential for the development of ZABs. In this review, we discuss the latest developments of non-noble metal bifunctional ORR/OER electrocatalysts for ZABs. Firstly, the related reaction mechanisms of ORR and OER are introduced. Then, the latest developments of bifunctional ORR/OER materials for ZABs are discussed in detail from three aspects: (i) MOF-based catalysts, including pristine MOFs and their derivatives; (ii) metal-free-based carbon catalysts, including heteroatom-doped carbon and defective carbon; (iii) metal-based catalysts, including metal–nitrogen–carbon materials (such as metals/alloys, single-atom) and metal compound materials. Finally, some challenges and outlooks for the optimal design of bifunctional air electrodes for rechargeable ZABs with high activity and ultra-long lifetime are put forward.

Bifunctional electrocatalysts for Zn–air batteries: recent developments and future perspectives

Energetic MOF-derived cobalt/iron nitrides embedded into N, S-codoped carbon nanotubes as superior bifunctional oxygen catalysts for Zn–air batteries

In this work, inspired by a water-assisted three-dimensional supramolecular structure 1, we use a mixed-ligand strategy to form a 3D pillared-layered matrix by the introduction of linear ligands to compete against the water molecules. The resulting analogue microporous MOFs of 2-H, 2-F and 2-N, decorated with different functional groups, similarly show the CO2 uptake. Thanks to the negligible N2 adsorption capacity, enhanced selective adsorption towards CO2 is achieved in compound 2-N. That is, we present here an alternative plan for the high CO2 selective adsorption performance. In addition, the structure stability and moderate affinity for CO2 of these microporous MOFs endow them with excellent reusability.

Molecular engineering in a family of pillared-layered metal-organic frameworks for tuning gas adsorption behavior.

The facile exfoliation of a two-dimensional metal-organic nanosheet of {[Co(HL)(H2O)(Py)3/4]·1/2H2O·DMF}n [1-Py; H3L = 5-(1H-pyrazol-4-yl)isophthalic acid and Py = pyridine] was achieved, via a molecular scalpel strategy, by weakening intermolecular forces between adjacent layers. The resulting 1-Py/KB40 (KB = Ketjen black) shows an increased oxygen evolution reaction (OER) performance with an overpotential of 370 mV at a current density of 10 mA cm-2 and a Tafel slope of 58 mV dec-1. This work sheds light on the structure-morphology-reactivity relationship of such materials in OER.

Shuai Liang

Papers

Bifunctional electrocatalysts for Zn–air batteries: recent developments and future perspectives

Energetic MOF-derived cobalt/iron nitrides embedded into N, S-codoped carbon nanotubes as superior bifunctional oxygen catalysts for Zn–air batteries

Molecular engineering in a family of pillared-layered metal-organic frameworks for tuning gas adsorption behavior.

From Hydrogen Bond to van der Waals Force: Molecular Scalpel Strategy to Exfoliate a Two-Dimensional Metal-Organic Nanosheet.