Two-Dimensional N,S-Codoped Carbon/Co9S8 Catalysts Derived from Co(OH)2 Nanosheets for Oxygen Reduction Reaction

Strong coordination ability of sulfur with cobalt for facilitating scale-up synthesis of Co9S8 encapsulated S, N co-doped carbon as a trifunctional electrocatalyst for oxygen reduction reaction, oxygen and hydrogen evolution reaction

High activity trifunctional non-noble electrocatalysts, targeting oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER), are rationally designed by integrating the merits of both Co₉S₈ nanoparticles and carbons nanosheets. Herein, Co₉S₈ loaded with S, N co-doped carbon core-shell catalyst (Co₉S₈@SNC) was reasonably designed and synthesized by using the strong coordination effect between Co²⁺ and CS at the molecular level. The significant synergistic effect between the S, N co-doped carbon shell and Co₉S₈ core endows the catalyst with excellent catalytic performance for ORR, HER, and OER reactions. The carbon shell enhances the conductivity of the hybrid material, while the Co₉S₈ core provides the main catalytic active sites. More specifically, the half-wave potential for ORR is 0.846 mV, and the overpotential at 10 mA cm^-2 for OER and HER are 320 mV and 170 mV, respectively. To test its practical application, zinc-air battery assembled by Co₉S₈@SNC shows a high power density of 239 mW cm^-2, excellent rechargeability, and long cyclic stability. This work provides a promising and extensible method to in-situ synthesize core-shell metal sulfides loaded S, N co-doped carbon composites, which can be a promising candidate for electrocatalytic material in energy storage and conversion devices.

Recent Advances in Flexible Zn-Air Batteries: Materials for Electrodes and Electrolytes

Flexible Zn-air batteries (ZABs) draw much attention due to the merits of high energy density, stability, and safety, and show potential applications for wearable devices. However, the development of flexible ZABs with great energy density, high round-trip efficiency, and long cycle life for practical applications is highly restricted by the lack of highly active oxygen catalysts, high ion-conducting solid-state electrolytes, appropriate Zn anodes, and advanced battery configuration. Promising oxygen catalysts should possess both, superior oxygen reduction reaction and oxygen evolution reaction performance and can be directly used as self-supporting cathodes without loading catalysts on support materials such as carbon cloth. In addition, electrolytes play an important role in ZABs; a good electrolyte should be in all-solid state with high ion conductivity. Moreover, for an excellent Zn anode, it is required to stably contact the electrolyte interface during the bending process. Therefore, in this review, recent advances in ZABs are summarized, including: i) the powder and 3D self-supporting oxygen catalysts, ii) the species of solid-state electrolytes, and iii) the rational design of Zn anodes. Finally, the challenges and opportunities of this promising field are presented.

Interface Engineering of CoS/CoO@N-Doped Graphene Nanocomposite for High-Performance Rechargeable Zn-Air Batteries

Low cost and green fabrication of high-performance electrocatalysts with earth-abundant resources for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial for the large-scale application of rechargeable Zn-air batteries (ZABs). In this work, our density functional theory calculations on the electrocatalyst suggest that the rational construction of interfacial structure can induce local charge redistribution, improve the electronic conductivity and enhance the catalyst stability. In order to realize such a structure, we spatially immobilize heterogeneous CoS/CoO nanocrystals onto N-doped graphene to synthesize a bifunctional electrocatalyst (CoS/CoO@NGNs). The optimization of the composition, interfacial structure and conductivity of the electrocatalyst is conducted to achieve bifunctional catalytic activity and deliver outstanding efficiency and stability for both ORR and OER. The aqueous ZAB with the as-prepared CoS/CoO@NGNs cathode displays a high maximum power density of 137.8 mW cm^-2, a specific capacity of 723.9 mAh g^-1 and excellent cycling stability (continuous operating for 100 h) with a high round-trip efficiency. In addition, the assembled quasi-solid-state ZAB also exhibits outstanding mechanical flexibility besides high battery performances, showing great potential for applications in flexible and wearable electronic devices.

A multi-interfacial FeOOH@NiCo2O4 heterojunction as a highly efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic water decomposition is the key to sustainable energy, and the design and synthesis of cost-effective electrocatalysts is the main objective of electrocatalytic water splitting. In this paper, multi-interfacial FeOOH@NiCo2O4 hybrid nanoflowers are prepared through a two-step hydrothermal reaction. In such heterostructures, NiCo2O4 nanoflowers are coated with a layer of FeOOH nanoparticles. In addition, the obtained electrocatalyst could provide abundant electroactive sites and the formation of FeOOH@NiCo2O4 nanointerfaces can also improve the charge transfer rate. As a result, under the HER and OER conditions, the prepared catalysts show an outstanding electrocatalytic performance. Moreover, in a two-electrode water splitting system, the FeOOH@NiCo2O4 heterostructure, as a dual-function electrocatalyst, needs a cell voltage of only 1.58 V at a current density of 10 mA cm-2. This study provides a facile and feasible method to construct different kinds of heterostructures as bifunctional electrocatalysts with multiple interfaces by a simple hydrothermal method.

Efficient Oxygen Electrocatalyst for Zn-Air Batteries: Carbon Dots and Co9S8 Nanoparticles in a N,S-Codoped Carbon Matrix

Non-noble metal-based bifunctional electrocatalysts for both oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) are an essential component of high-performance rechargeable Zn-air batteries (ZABs). Herein, we report a novel and simple method for preparing Co₉S₈ nanoparticles embedded in N and S codoped carbon materials with aid of carbon dots (CDs). CDs play a key role in distributing Co₉S₈ nanoparticles in the matrix uniformly and enhancing the specific surface area and the electric conductivity simultaneously. The obtained Co₉S₈/CD@NSC exhibits an excellent ORR and OER bifunctional catalytic activity and a great long-term durability, with a half-wave potential of 0.84 V versus reversible hydrogen electrode (RHE) for the ORR and a low potential of 1.62 V versus RHE at 10 mA cm^-2, which outperform the popular Pt/C and RuO₂ commercial catalysts. Moreover, the Co₉S₈/CD@NSC catalyst also displays a superior activity and cycling stability as a cathode material in ZABs, which is far better than Pt/C + RuO₂ mixture catalysts. Such a ZAB shows a low charge/discharge voltage gap of 0.62 V and great cycling stability over 125 h at 10 mA cm^-2.

Cobalt sulfide/N,S-codoped defect-rich carbon nanotubes hybrid as an excellent bi-functional oxygen electrocatalyst

The design of high-efficient and durable bi-functional oxygen electrocatalysts is still a great challenge. In this work, novel Co₉S₈ nanoparticles/N,S-codoped defect-rich carbon nanotubes (Co₉S₈/N,S-CNTs) were fabricated by an ingenious template method. CdS nanowires, as a sacrificial template, can be removed simultaneously during the carbonization process without additional post-treatments. The large BET surface area (661.2 m² g^-1) and pore volume (1.49 cm³ g^-1) of Co₉S₈/N,S-CNTs could largely enhance the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities by facilitating the mass transportation and exposing more active sites. The abundant defects produced by the evaporation of Cd and S doping could provide more active sites for ORR and OER. Coupling with Co₉S₈, Co₉S₈/N,S-CNTs possesses more defects in the carbon skeleton, better electron conductivity, and larger effective electrochemical area. Co₉S₈/N,S-CNTs not only performs excellent ORR activity with a half-wave potential of 0.821 V but also owns RuO₂-like OER activity in alkaline solution. The potential difference (ΔE) between ORR and OER is as low as 0.78 V in 0.10 M KOH. The excellent bi-functional performance enables the potential to be utilized in fuel cells and metal-air batteries.

Three-Dimensional Graphene-Supported Ni3Fe/Co9S8 Composites: Rational Design and Active for Oxygen Reversible Electrocatalysis

The development of low-cost and efficient electrocatalysts with a bicomponent active surface for reversible oxygen electrode reactions is highly desirable and challenging. Herein, we develop an effective calcination-hydrothermal approach to fabricate graphene aerogel-anchored Ni₃Fe-Co₉S₈ bifunctional electrocatalyst (Ni₃Fe-Co₉S₈/rGO). The mutually beneficial Ni₃Fe-Co₉S₈ bifunctional active components efficiently balance the performance of oxygen reduction and oxygen evolution reactions (ORR/OER), in which Co₉S₈ promotes the ORR and Ni₃Fe facilitates the OER. This balance behavior has an obvious advantage over that of monocomponent Ni₃Fe/rGO and Co₉S₈/rGO catalysts. Meanwhile, the additional synergy between porous rGO aerogels and Ni₃Fe-Co₉S₈ endows the composite with more exposed active sites, faster electrons/ions transport rate, and better structural stability. Benefiting from the reasonable material selection and structural design, the Ni₃Fe-Co₉S₈/rGO exhibits not only outstanding ORR activity with the high onset- and half-wave potentials ( E_onset = 0.91 V and E_1/2 = 0.80 V) but also satisfactory OER activity with a low overpotential at 10 mA cm^-2 (0.39 V). Moreover, rechargeable Zn-air cells equipped with Ni₃Fe-Co₉S₈/rGO exhibit excellent rechargeability and a fast dynamic response.

In Situ Coupling Strategy for Anchoring Monodisperse Co9S8 Nanoparticles on S and N Dual-Doped Graphene as a Bifunctional Electrocatalyst for Rechargeable Zn-Air Battery

An in situ coupling strategy to prepare Co₉S₈/S and N dual-doped graphene composite (Co₉S₈/NSG) has been proposed. The key point of this strategy is the function-oriented design of organic compounds. Herein, cobalt porphyrin derivatives with sulfo groups are employed as not only the coupling agents to form and anchor Co₉S₈ on the graphene in situ, but also the heteroatom-doped agent to generate S and N dual-doped graphene. The tight coupling of multiple active sites endows the composite materials with fast electrochemical kinetics and excellent stability for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The obtained electrocatalyst exhibits better activity parameter (ΔE = 0.82 V) and smaller Tafel slope (47.7 mV dec^-1 for ORR and 69.2 mV dec^-1 for OER) than commercially available Pt/C and RuO₂. Most importantly, as electrocatalyst for rechargeable Zn-air battery, Co₉S₈/NSG displays low charge-discharge voltage gap and outstanding long-term cycle stability over 138 h compared to Pt/C-RuO₂. To further broaden its application scope, a homemade all-solid-state Zn-air battery is also prepared, which displays good charge-discharge performance and cycle performance. The function-oriented design of N₄-metallomacrocycle derivatives might open new avenues to strategic construction of high-performance and long-life multifunctional electrocatalysts for wider electrochemical energy applications.

One-step MOF-derived Co/Co9S8 nanoparticles embedded in nitrogen, sulfur and oxygen ternary-doped porous carbon: an efficient electrocatalyst for overall water splitting

Co/Co₉S₈ nanoparticles encapsulated in a N, S, and O ternary-doped carbon matrix were synthesized utilizing a Co-NSOMOF as a single precursor, and they exhibited excellent bifunctional electrocatalytic activity for the OER and HER.

Fe1−xS/nitrogen and sulfur Co-doped carbon composite derived from a nanosized metal–organic framework for high-performance lithium-ion batteries

Fe_1−xS/NSC composite was prepared as an anode for LIBs by a one-step pyrolysis approach, and it exhibited outstanding cycle stability and rate performance.

Recent Advances of Cobalt-Based Electrocatalysts for Oxygen Electrode Reactions and Hydrogen Evolution Reaction

This review summarizes recent progress in the development of cobalt-based catalytic centers as the most potentially useful alternatives to noble metal-based electrocatalysts (Pt-, Ir-, and Ru-based) towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in acid and alkaline media. A series of cobalt-based high-performance electrocatalysts have been designed and synthesized including cobalt oxides/chalcogenides, Co–Nx/C, Co-layered double hydroxides (LDH), and Co–metal-organic frameworks (MOFs). The strategies of controllable synthesis, the structural properties, ligand effect, defects, oxygen vacancies, and support materials are thoroughly discussed as a function of the electrocatalytic performance of cobalt-based electrocatalysts. Finally, prospects for the design of novel, efficient cobalt-based materials, for large-scale application and opportunities, are encouraged.