Identifying the Lewis Base Chemistry in Preventing the Deposition of Metal Oxides on Ketone-Enriched Carbon Cathodes for Highly Durable Metal-Air Batteries

Se-functionalized metal-free catalysts rich in sp3-hybridized carbon enable efficient oxygen electroreduction

Carbon-based metal-free materials are emerging as leading candidates to replace noble-metal catalysts in the oxygen reduction reaction (ORR). Herein, we introduce a facile secondary carbonation technique for fabricating Se and N co-doped metal-free catalysts using a zeolite imidazole framework (ZIF-8) as the precursor. The optimal electrocatalyst, designated SeNC-900, exhibited good ORR performance under both alkaline and acidic conditions, with half-wave potentials of 0.864 V and 0.731 V (vs. RHE), respectively. Density functional theory (DFT) calculations reveal that the enhanced activity of SeNC-900 originates from Se doping, which triggers an increase in the intrinsic defects of sp3-hybridized C. Concurrently, the sp3-hybridized C, in concert with Se dopant, modulates the electronic structure of the active C atoms. This work not only underscores the significance of tuning the electronic structure to boost catalytic performance by enriching intrinsic defects but also presents a fresh insight into the effect of heteroatom doping on carbon-based materials for electrocatalysis.

Ionic Covalent Organic Framework Membrane as Active Separator for Highly Reversible Zinc-Sulfur Battery

Zinc-sulfur (Zn-S) batteries exhibit a high theoretical energy density, nontoxicity, and cost-effectiveness, demonstrating significant potential for integration into large-scale energy storage systems. However, the phenomenon of polysulfide (including dissolved S8 and Sx2-) shuttling is a major issue that results in rapid capacity decay and a short lifespan, limiting the practical performance of sulfur-based batteries. Herein, we fabricated an ionic covalent organic framework (iCOF) membrane as an active separator for the Zn-S battery. Sulfonic acid groups were introduced to the COF membrane, providing abundant negative charge sites in its pore wall. By combining size sieving and charge interaction between the polysulfide and pore wall, the iCOF membrane inhibited the crossover of polysulfides to the Zn metal anode without affecting the transport of metal ions. The Zn-S battery with the iCOF membrane as the separator shows a high-performance and low attenuation rate of 0.05% per cycle over 300 cycles at 2.5 A g-1. This study emphasizes the significance of separator design in enhancing Zn-S batteries and showcases the potential of functionalized framework materials for the development of high-performance energy storage systems.

Activating doped graphene surface by cobalt-rich sulfide encapsulation toward oxygen reduction electrocatalysis

Similar to proton exchange membrane fuel cell, anion-exchange membrane fuel cell is also a significant energy conversion device for achieving the utilization of clean hydrogen energy. However, the cathodic alkaline oxygen reduction reaction (ORR) is kinetically not favored and usually requires platinum-group metal (PGM) catalysts such as Pt/C to reduce the overpotential. The major challenge in using PGM-free catalysts for ORR is their low efficiency and poor stability, which urgently demands new concepts and strategies to address this issue. Herein, we controllably manufactured a N, S-co doped graphene encapsulating uniform cobalt-rich sulfides (Co8FeS8@NSG) by a universal synthesis strategy. After encapsulation, electron transfer from the encapsulated cobalt-rich sulfides to the doped graphene was greatly promoted, which effectively optimizes the electronic structure of the doped graphene, thereby enhancing the ORR activity of the doped graphene surface. Consequently, the Co8FeS8@NSG exhibits enhanced ORR activity with a higher half-wave potential of 0.868 V (versus reversible hydrogen electrode, vs. RHE) when compared with pure NSG (0.765 V vs. RHE). Density functional theory calculations further confirm that the construction of interface for NSG encapsulating cobalt-rich sulfides could conspicuously elevate the ORR activity through slightly positively-charged C active site and thus simultaneously enhancing electronic conductivity.

Enhanced utilization of active sites of Fe/N/C catalysts by pore-in-pore structures for ultrahigh mass activity

Carbon material doped with nitrogen and transition metal is a kind of promising candidate of the platinum for oxygen reduction reaction (ORR) process due to its low cost, efficiency and stability. Here we demonstrate an original type of Fe/N/C catalyst based on pore-in-pore structures (P-P Fe/N/C), showing one of the highest oxygen reduction reaction performances among all reported Fe/N/C-type catalysts (onset potential of 0.995 V, half-wave potential of 0.881 V vs. RHE with a relatively low mass loading of 0.32 mg cm^-2 and long-term durability (97% relative current in 60 000 s operation) in alkaline media. Such outstanding performances can be ascribed to the efficient active sites activated by the encapsulated atomic and subnanoscale iron, and great exposure of these active sites due to the unique pore-in-pore hierarchical construction. Once assembled in lithium-O₂ batteries, a specific capacity of 7250 mA h g^-1 at 70 mA g^-1 can be obtained by the P-P Fe/N/C catalyst. Moreover, upon cycling, the P-P Fe/N/C electrode can be cycled 150 times with no capacity loss, which is much longer than six cycles of pure Super P air electrode. These results evidently reveal the developed Fe/N/C catalyst holds great promise to serve as an alternative to the conventional Pt-based noble metal catalysts.

Atomic Metal Vacancy Modulation of Single-Atom Dispersed Co/N/C for Highly Efficient and Stable Air Cathode

Metal-air batteries have received great attention as a new energy supply for next-generation electronic devices. However, their widespread application is still hindered by several challenges including sluggish kinetics of the cathodic reactions and undesirable stability of the air cathode due to the possible deposition of the discharge product. Herein, we propose an atomic metal vacancy modulation of a single-atom dispersed Co/N/C cathode to provide the zinc-air battery with both reduced overpotential and enhanced stability. As illustrated by theoretical calculations and electrochemical measurements, deliberate introduction of metal vacancies would modulate the electronic structure and contribute to enhanced catalytic activity, affording the catalyst with a half-wave potential of 0.89 V versus reversible hydrogen electrode and an overall oxygen electrode potential gap of 0.72 V. Moreover, abundant pyridinic-N groups are exposed due to the removal of metal centers, generating strong Lewis basicity to effectively prevent the access of negatively charged zincate ions and realize the nondeposition of ZnO on the air cathode. Rechargeable zinc-air battery assembled with such an air cathode delivers superior cyclic performance with low discharge/charge overpotential and negligible plateau gap increase of only 0.05 V for 1000 cycles. Flexible all-solid-state battery demonstrates robust durability of over 35 h and excellent flexibility to light-up a light-emitting diode (LED) rose, indicating its potential feasibility as a flexible and safe power source for modern life.