An 'active site anchoring' strategy for the preparation of PBO fiber derived carbon catalyst towards an efficient oxygen reduction reaction and zinc-air batteries

In order to promote the wide application of clean energy-fuel cells, it is urgent to develop transition metal-based high-efficiency oxygen reduction reaction (ORR) catalytic materials with a low cost and available rich raw material resources to replace the currently used precious metal platinum-based catalytic materials. Herein, a novel 'active-site-anchoring' strategy was developed to synthesize highly-activated carbon-based ORR catalysts. Firstly, poly(p-phenylene benzobisoxazole) (PBO) fiber with a stable chemical structure was selected as the main precursor, and iron was complexed on its surface, and then poly-dopamine (PDA) was coated on the surface of PBO-Fe to form a PBO-Fe-PDA composite structure. Therefore, carbon-based catalyst PBO-Fe-PDA-900 with abundant Fe2O3 active sites was prepared by anchoring iron sites by PDA after pyrolysis. As a result, the PBO-Fe-PDA-900 catalyst displayed a 30 mV higher half-wave potential (0.86 V) than that of a commercial Pt/C electrocatalyst. Finally, PBO-Fe-PDA-900 was used as a cathode material for zinc-air batteries, showing a high peak power density superior to Pt/C. This work offers new prospects for the design of efficient, non-precious metal-based materials in zinc-air batteries.

Recent advances in zinc-air batteries: self-standing inorganic nanoporous metal films as air cathodes

Zinc-air batteries (ZABs) have promising prospects as next-generation electrochemical energy systems due to their high safety, high power density, environmental friendliness, and low cost. However, the air cathodes used in ZABs still face many challenges, such as the low catalytic activity and poor stability of carbon-based materials at high current density/voltage. To achieve high activity and stability of rechargeable ZABs, chemically and electrochemically stable air cathodes with bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) activity, fast reaction rate with low platinum group metal (PGM) loading or PGM-free materials are required, which are difficult to achieve with common electrocatalysts. Meanwhile, inorganic nanoporous metal films (INMFs) have many advantages as self-standing air cathodes, such as high activity and stability for both the ORR/OER under highly alkaline conditions. The high surface area, three-dimensional channels, and porous structure with controllable crystal growth facet/direction make INMFs an ideal candidate as air cathodes for ZABs. In this review, we first revisit some critical descriptors to assess the performance of ZABs, and recommend the standard test and reported manner. We then summarize the recent progress of low-Pt, low-Pd, and PGM-free-based materials as air cathodes with low/non-PGM loading for rechargeable ZABs. The structure-composition-performance relationship between INMFs and ZABs is discussed in-depth. Finally, we provide our perspectives on the further development of INMFs towards rechargeable ZABs, as well as current issues that need to be addressed. This work will not only attract researchers' attention and guide them to assess and report the performance of ZABs more accurately, but also stimulate more innovative strategies to drive the practical application of INMFS for ZABs and other energy-related technologies.

Markedly boosted peroxymonosulfate- and periodate-based Fenton-like activities of iron clusters on sulfur/nitrogen codoped carbon: Key roles of a sulfur dopant and compared activation mechanisms

Highly dispersed iron nanoclusters on carbon (FeNC@C) hold great promise for wastewater purification in Fenton-like reactions. The microenvironment engineering of central Fe atom is promising to boost the activation capacity of FeNC@C, which is however remains a challenge. This study developed a self-sacrificed templating strategy to S, N-codoped carbon supported Fe nanoclusters (FeNC@SNC) activator and find the key role of sulfur heteroatoms in regulating the electron structure of Fe sites and final activation property. Investigations revealed that the FeNC@SNC composite exhibited unusual bifunctional activity in both peroxymonosulfate (PMS)- and periodate (PI)-based Fenton-like reactions. We also offered insights into the differences between the degradation of organics by the FeNC@SNC/PMS and FeNC@SNC/PI systems. Specifically, under identical conditions, the FeNC@SNC/PMS system delivered a higher oxidation capability and stronger resistance to nontarget matrix constituents, but showed more severe Fe leaching than the FeNC@SNC/PI system. Furthermore, while mediated electron-transfer process was identified as the major route for pollutant decomposition in both systems, the high-valent Fe-oxo species [Fe (IV)] was the auxiliary reactive species found only in the FeNC@SNC/PMS system. Based on these findings, our results provide profound insights into the design of active and durable Fe-based activators toward highly efficient Fenton-like reactions.

A high power density paper-based zinc-air battery with a hollow channel structure

In light of the surging research interest in disposable electronics, great demands have been imposed on compact power sources. Herein, a paper-based zinc-air battery that takes advantage of a hollow channel structure is reported. Unlike conventional paper-based metal-air batteries and fuel cells that tightly immobilize the electrode on the paper channel, a hollow channel layer containing potassium hydroxide solution electrolyte is sandwiched between the electrodes and paper channel layer. This novel zinc-air battery is capable of delivering a peak power density of 102 mW cm-2, surpassing state-of-the-art paper-based power sources. The superior power density originates from the boosted electrochemically active surface area of the cathode, which enhances the oxygen reduction reaction kinetics.

Fe3C cluster-promoted single-atom Fe, N doped carbon for oxygen-reduction reaction

A key challenge in carrying out an efficient oxygen reduction reaction (ORR) is the design of a highly efficient electrocatalyst that must have fast kinetics, low cost and high stability for use in an energy-conversion device (e.g. metal-air batteries). Herein, we developed a platinum-free ORR electrocatalyst with a high surface area and pore volume via a molten salt method along with subsequent KOH activation. The activation treatment not only increases the surface area to 940.8 m2 g-1 by generating lots of pores, but also promotes the formation of uniform Fe3C nanoclusters within the atomic dispersed Fe-Nx carbon matrix in the final material (A-FeNC). A-FeNC displays excellent activity and long-term stability for the ORR in alkaline media, and shows a greater half-wave potential (0.85 V) and faster kinetics toward four-electron ORR as compared to those of 20 wt% Pt/C (0.83 V). As a cathode catalyst for the Zn-air battery, A-FeNC presents a peak power density of 102.2 mW cm-2, higher than that of the Pt/C constructed Zn-air battery (57.2 mW cm-2). The superior ORR catalytic performance of A-FeNC is ascribed to the increased exposure of active sites, active single-atom Fe-N-C centers, and enhancement by Fe3C nanoclusters.

Single iron atoms coordinated to g-C3N4 on hierarchical porous N-doped carbon polyhedra as a high-performance electrocatalyst for the oxygen reduction reaction

Isolated single Fe atoms coordinated to g-C₃N₄ on hierarchical porous N-doped carbon polyhedra show superior electrocatalytic performance for oxygen reduction under alkaline conditions.

Principle understanding towards synthesizing Fe/N decorated carbon catalysts with pyridinic-N enriched and agglomeration-free features for lithium–oxygen batteries

Metal-N-decorated carbon catalysts are cheap and effective alternatives for replacing the high-priced Pt-based ones in activating the reduction of oxygen for metal–air or fuel cells. The preparation of such heterogeneous catalysts often requires complex synthesis processes, including harsh acid treatment, secondary pyrolysis processes, etching, etc., to make the heteroatoms evenly dispersed in the carbon substrates to obtain enhanced activities. Through combined experimental characterizations, we found that by precise control of the precursors added, a Fe/N uniformly distributed, agglomeration-free Fe/N decorated Super-P carbon material (FNDSP) can be easily obtained by a one-pot synthesis process with distinctly higher pyridinic-N content and elevated catalytic activity. An insight into this phenomenon was carefully demonstrated and also verified in Li–O₂ batteries, which delivered a high discharging platform of 2.85 V and can be fully discharged with a capacity of 5811.5 mA h g_{carbon+catalyst}⁻¹ at the cut-off voltage of 2.5 V by the low-cost Super-P modified catalyst.

Synthesizing a pyridinic-N enriched and agglomeration-free Fe/N-decorated carbon catalyst for lithium–oxygen batteries.

Highly dispersed γ-Fe2O3 embedded in nitrogen doped carbon for the efficient oxygen reduction reaction

A sacrificial template strategy is developed to synthesize highly dispersed γ-Fe₂O₃ embedded in porous N-doped carbon. The as-synthesized catalyst exhibits high ORR performance and presents a power density of 112 mW cm⁻² in zinc–air battery.

Achieving nitrogen-doped carbon/MnO2 nanocomposites for catalyzing the oxygen reduction reaction

N-Doped carbon/MnO₂ nanocomposites generated via pyrolyzing polypyrrole/MnO₂ show excellent catalytic performance towards the oxygen reduction reaction owing to the synergistic effect between N-doped carbon and MnO₂.