Large-scale defect-rich iron/nitrogen co-doped graphene-based materials as the excellent bifunctional electrocatalyst for liquid and flexible all-solid-state zinc-air batteries

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

Low-temperature molten salt synthesis and catalytic mechanism of CoS2/NC as an advanced bifunctional electrocatalyst

The development of productive and sustainable bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) plays an important role in the commercial evolution of metal-air batteries. In this paper, a low-temperature molten salt template method was adopted to synthesize the composite of CoS₂ and nitrogen-doped carbon (CoS₂/NC) without the protection of inert gas. The structural characterization studies show that the specific surface area (SSA) and crystal growth kinetics are increased and effectively improved, respectively, by the composite of CoS₂ and NC. The as-synthesized CoS₂/NC composite demonstrates outstanding bifunctional catalytic activity in alkaline electrolytes and exhibits a half-wave potential (E_1/2) of 0.854 V (vs. RHE) and an overpotential of only 220 mV for the OER at a current density of 10 mA cm^-2 (η₁₀). Simultaneously, CoS₂/NC also exhibits excellent electrochemical stability. Additionally, density functional theory (DFT) calculations have manifested that the synergistic effect of CoS₂ and NC results in a remarkable enhancement in the bifunctional catalytic performance of the composite materials. This study offers a new pathway and theoretical guidance for the fabrication of efficient bifunctional electrocatalysts.

Precisely manipulated π-π stacking of catalytic exfoliated iron polyphthalocyanine/reduced graphene oxide hybrid via pyrolysis-free path

Metal macrocycles with well-defined molecular structures are ideal platforms for the in-depth study of electrochemical oxygen reduction reaction (ORR). Structural integrity of metal macrocycles is vital but remain challenging since the commonly used high-temperature pyrolysis would cause severe structure damage and unidentifiable active sites. Herein, we propose a pyrolysis-free strategy to precisely manipulate the exfoliated 2D iron polyphthalocyanine (FePPc) anchored on reduced graphene oxide (rGO) via π-π stacking using facile high-energy ball milling. A delocalized electron shift caused by π-π interaction is firstly found to be the mechanism of facilitating the remarkable ORR activity of this hybrid catalyst. The optimal FePPc@rGO-HE achieves superior half-wave potential (0.90 V) than 20 % Pt/C. This study offers a new insight in designing stable and high-performance metal macrocycle catalysts with well-defined active sites.

Solvent environment engineering to synthesize FeNC nanocubes with densely Fe-Nx sites as oxygen reduction catalysts for Zn-air battery

Zeolitic imidazole framework (ZIF)-derived iron-nitrogen-carbon (FeNC) materials are expected to be high-efficiency catalysts for oxygen reduction reaction (ORR). However, increasing the density of active sites while avoiding metal accumulation still faces significant challenges. Herein, solvent environment engineering is used to synthesize the FeNC containing dense Fe-N_x moieties by adjusting the solvent during the ZIF precursor synthesis process. Compared with methanol and water/methanol, the aqueous media can provide a more moderate Fe content for the ZIF precursor, which facilitates the construction of high-density Fe-N_x sites and prevent the appearance of iron-based nanoparticles during pyrolysis. Therefore, the FeNC(C) nanocubes synthesized in an aqueous media have the highest single atom Fe loading (0.6 at%) among the prepared samples, which presents excellent oxygen reduction properties and durability under alkaline and acidic conditions. The advantage of FeNC(C) is proven in Zn-air batteries, with outstanding performance and long-term stability.

Starch-based porous carbon microsphere composited NiCo2O4 nanoflower as bifunctional electrocatalyst for zinc-air battery

It is significant to explore and design outstanding bifunctional oxygen electrocatalysts to promote the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in zinc-air batteries. Herein, a novel porous carbon microspheres (CMS₂) modified by NiCo₂O₄ nanoflower (CMS₂-NiCo₂O₄) has been prepared as an ORR and OER catalyst. The hierarchical porous structure of CMS provides high conductivity and abundant active sites for ORR, whereas the synergistic effect of NiCo₂O₄ nanosheets and a small amount of FeZn oxides act as the positive phase for OER. The efficient oxygen catalytic activity is gained by creating a coupling interface between NiCo₂O₄ and CMS. The optimized CMS₂-NiCo₂O₄ shows a half-wave potential of 0.82 V toward ORR and an overpotential of 392 mV toward OER. Particularly, CMS₂-NiCo₂O₄ also exhibits an excellent peak power density (175.5 mW cm^-2) as a catalyst for zinc-air batteries, which is superior to the commercial Pt/C + RuO₂ catalyst (120.5 mW cm^-2), and it also demonstrates a remarkable stability even after the charge-discharge cycles of 167 h. The prepared CMS₂-NiCo₂O₄ is promising for the application of the bimetallic oxide catalyst for zinc-air battery.

Unveiling the role of defects in iron oxyhydroxide for oxygen evolution

Development of earth-abundant and robust oxygen evolution reaction (OER) catalysts is imperative for cost-effective hydrogen production via water electrolysis. Herein, we report ultrafine iron (oxy)hydroxide nanoparticles with average particle size of 2.6 nm and abundant surface defects homogeneously supported on oleum-treated graphite (FeO_x(n)@HG-T), providing abundant active sites for the OER. The optimal FeO_x(0.03)@HG-110 exhibits high electrocatalytic OER activity and excellent stability. Electrochemical testing results and theoretical calculations reveal that the outstanding OER activity of FeO_x(0.03)@HG-110 is due to its stronger charge transfer ability and lower OER energy barrier than defect-free FeO_x nanoparticles. This work demonstrates that the OER performance of oxyhydroxide-based electrocatalysts can be improved by surface defect engineering.

Manipulating the electrocatalytic activity of sulfur cathode via distinct cobalt sulfides as sulfur host materials in lithium-sulfur batteries

For the better development of lithium-sulfur (Li-S) batteries, it is necessary to fabricate sulfur hosts with cheap, rapid sulfur reaction dynamic and inhibiting the shuttling effect of lithium polysulfides (LiPSs). Herein, four hollow cubic materials with two kinds of nitrogen-doped carbon derived from Prussian blue analogues (PBA) precursor, Co₉S₈/MnS/NC@NC-400, CoS₂/MnS/NC@NC-500, CoS_1.097/MnS/NC@NC-600 and CoS_1.097/MnS/NC@NC-700, are reported when the vulcanization temperatures are regulated at 400 °C, 500 °C, 600 °C and 700 °C, respectively. Among them, Co₉S₈/MnS/NC@NC-400, CoS₂/MnS/NC@NC-500 and CoS_1.097/MnS/NC@NC-600 have the similar hollow cubic structure, which can physically confine the LiPSs's shuttle, however, the Co vacancies of CoS_1.097 in the CoS_1.097/MnS/NC@NC-600 can promote the rearrangement of surface electrons, which is beneficial to the diffusion of Li⁺/e^-, improving the electrochemical reaction kinetics. As for the CoS_1.097/MnS/NC@NC-700 with the same substance but almost collapsed structure, the CoS_1.097/MnS/NC@NC-600 can accommodate the volume expansion of sulfur conversion. In the four sulfur-host materials, the CoS_1.097/MnS/NC@NC-600 not only displays the outstanding adsorption ability on LiPSs, but also presents the best electrocatalytic activity in the Li₂S potentiostatic deposition experiments and active sulfur reduction/oxidation conversion reactions, greatly promoting the electrochemical performances of Li-S batteries. The S@CoS_1.097/MnS/NC@NC-600 cathode can deliver 1010.2 mA h g^-1 at 0.5 C and maintain 651.1 mA h g^-1 after 200 cycles. In addition, the in-situ X-ray diffraction (in-situ XRD) test reveals that the sulfur conversion mechanism is the processes of the α-S₈ → Li₂S → β-S₈ (first cycle), then β-S₈ ↔ Li₂S during the subsequent cycles. Based on the fundamental understanding of the design and preparation of Co_xS_y/MnS/NC@NC hosts with the desired adsorption and catalysis functions, the work can provide new insights and reveal the defect-engineering to develop the advanced Li-S batteries.

Introducing mesoporous-silica-protected calcination for improving the electrochemical performance of Cu@Fe-N-C composite in oxygen reduction reaction and supercapacitor applications

The application of zeolite imidazolate framework (ZIF)-based nanomaterials for oxygen reduction reaction (ORR) and supercapacitor (SC) is drastically confined by their quick aggregation and irreversible fusion of metal nanoparticles. Herein,...