Co9S8-porous carbon spheres as bifunctional electrocatalysts with high activity and stability for oxygen reduction and evolution reactions

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Carbon Nanotube-Coupled Seaweed-like Cobalt Sulfide as a Dual-Functional Catalyst for Overall Water Splitting

Preparation of high-efficiency dual-functional catalysts remains the bottleneck for electrochemical water splitting. To prepare a non-precious metal catalyst with high activity and stability, here, we present a seaweed-like structure consisting of transition-metal sulfide nanoplates self-assembled on carbon nanotube sponge networks (SW-CoS@CNT). By adjusting the key parameters during synthesis (e.g., the loading amount and ratio of Co and S precursors), the microstructure can be tailored in a wide range, and sulfur defects can be introduced into the nanoplates by thermal annealing. The resulting SW-CoS@CNT serves as a freestanding dual-functional catalytic electrode, showing low overpotentials of 105 and 218 mV for the hydrogen evolution reaction and the oxygen evolution reaction, respectively, which are superior to most reported transition-metal-sulfide-based catalysts in alkaline solution. Rational design of this hierarchical biomimetic structure may be useful in developing high-performance electrochemical catalysts in renewable energy and environmental fields.

V. S, Menon RS, Panda SK, Sahu AK. In-situ fabrication of cobalt sulfide decorated N, S co-doped mesoporous carbon and its application as electrocatalyst for efficient oxygen reduction reaction

Designing an efficient electrocatalyst for facile oxygen reduction reaction (ORR) is essential to achieve higher fuel cell performance. Herein, we demonstrate the simple in-situ process to synthesize cobalt sulfide decorated...

Synthesis of Lattice-Contracted Cobalt Disulfide as an Outstanding Oxygen Reduction Reaction Catalyst via Self-assembly Arrangement

Identifying high-performance non-precious metal-based catalysts at the cathode is a major challenge for future practical applications. Herein, a soft-template route through a self-assembly arrangement of sulfur sources was successfully developed, facilitating the anion exchange. In addition, compared with pristine cobalt disulfide synthesized without templates, the cobalt disulfide prepared using the new method presented a lattice shrinking phenomenon due to the hindrance of cobalt hydroxide crystal cell. Based on X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculation, increased occupancy of e_g orbitals was verified for the cobalt disulfide after shrinkage, which was the main factor for enhancing the intrinsic activity of the catalyst. Besides the microscopic morphologic structure, elementary composition, and the valence state of the elements, the possible growth process of the cobalt disulfide was also discussed in detail. As catalyst for the oxygen reduction reaction, CoS₂ showed a similar half-wave potential (0.81 vs. 0.84 V for Pt/C) and higher diffusion-limiting current density (reaching 5.33 vs. 5.19 mA cm^-2 for Pt/C) than a commercial Pt/C catalyst. Hence, our results provide a rational design direction for this type of catalysts.

ZIF-derived "senbei"-like Co9S8/CeO2/Co heterostructural nitrogen-doped carbon nanosheets as bifunctional oxygen electrocatalysts for Zn-air batteries

The rational design and construction of the efficient and robust non-noble metal bifunctional oxygen electrocatalysts is of critical significance due to the attention given to reversible metal-air batteries. In this paper, we report novel two-dimensional "senbei"-like Co9S8/CeO2/Co-NC nitrogen-doped carbon nanosheets (Co9S8/CeO2/Co-NC) derived from a unique 2D Co/Ce bimetallic ZIF. The phase transition from 3D spherical Co-ZIF to 2D Co/Ce-ZIF was achieved through the introduction of Ce ions. Profiting from the successful construction of the unique Co9S8/CeO2 heterostructure and the synergetic effect of two components, the as-prepared Co9S8/CeO2/Co-NC exhibited excellent electro-performance in both the oxygen evolution reaction (Ej=10 = 1.60 V) and oxygen reduction reaction (E1/2 = 0.875 V). Furthermore, when used as a bifunctional air electrode for Zn-air batteries, Co9S8/CeO2/Co-NC reached a high peak power density of ≈164.24 mW cm-2 at a high current density of ≈351 mA cm-2 and displayed an outstanding cycling stability of more than 668 h at 5 mA cm-2. This research provides new guidelines for preparing hybrid materials from cobalt-based sulfide species and CeO2 for electrocatalysis and energy storage or other fields.

Solid-State Ball-Milling of Co3O4 Nano/Microspheres and Carbon Black Endorsed LaMnO3 Perovskite Catalyst for Bifunctional Oxygen Electrocatalysis

Developing a highly stable and non-precious, low-cost, bifunctional electrocatalyst is essential for energy storage and energy conversion devices due to the increasing demand from the consumers. Therefore, the fabrication of a bifunctional electrocatalyst is an emerging focus for the promotion and dissemination of energy storage/conversion devices. Spinel and perovskite transition metal oxides have been widely explored as efficient bifunctional electrocatalysts to replace the noble metals in fuel cell and metal-air batteries. In this work, we developed a bifunctional catalyst for oxygen reduction and oxygen evolution reaction (ORR/OER) study using the mechanochemical route coupling of cobalt oxide nano/microspheres and carbon black particles incorporated lanthanum manganite perovskite (LaMnO3@C-Co3O4) composite. It was synthesized through a simple and less-time consuming solid-state ball-milling method. The synthesized LaMnO3@C-Co3O4 composite was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction spectroscopy, and micro-Raman spectroscopy techniques. The electrocatalysis results showed excellent electrochemical activity towards ORR/OER kinetics using LaMnO3@C-Co3O4 catalyst, as compared with Pt/C, bare LaMnO3@C, and LaMnO3@C-RuO2 catalysts. The observed results suggested that the newly developed LaMnO3@C-Co3O4 electrocatalyst can be used as a potential candidate for air-cathodes in fuel cell and metal-air batteries.

Mo-Doped Zn, Co Zeolitic Imidazolate Framework-Derived Co9S8 Quantum Dots and MoS2 Embedded in Three-Dimensional Nitrogen-Doped Carbon Nanoflake Arrays as an Efficient Trifunctional Electrocatalysts for the Oxygen Reduction Reaction, Oxygen Evolution Reaction, and Hydrogen Evolution Reaction

Herein, we first propose a facile strategy to synthesize Co₉S₈ and MoS₂ nanocrystals embedded in porous carbon nanoflake arrays supported on carbon nanofibers (Co₉S₈-MoS₂/N-CNAs@CNFs) by the pyrolysis of Mo-doped Zn, Co zeolitic imidazolate framework grown on carbon nanofibers and subsequent sulfuration. The electrocatalyst shows high and stable electrocatalytic performance, with a half-wave potential of 0.82 V for oxygen reduction reaction and an overpotential at 10 mA cm^-2 for oxygen evolution reaction (0.34 V) and hydrogen evolution reaction (0.163 V), which outperform the metal-organic framework-derived transition metal sulfide catalysts reported so far. Furthermore, the Co₉S₈-MoS₂@N-CNAs@CNFs are employed as an air cathode in a liquid-state and all-solid-state zinc-air battery, presenting high power densities of 222 and 96 mW cm^-2, respectively. Such excellent catalytic activities are mainly owing to the unique three-dimensional structure and chemical compositions, optimal electronic conductivity, adequate surface area, and the abundance of active sites. Thus, this work provides an important method for designing other metal-organic framework-derived three-dimensional structural sulfide quantum dot multifunctional electrocatalysts for wider application in highly efficient catalysis and energy storage.

Thin metal organic layer derived Co/Co9S8/N,S co-doped carbon nanosheets synthesized by the space confinement effect of montmorillonite for oxygen electrocatalysis

Co/Co₉S₈ anchored to N,S co-doped carbon nanosheets resulting T-CCSNC electrocatalysts show excellent ORR and OER catalytic activity and stability.

Fe–N4 engineering of S and N co-doped hierarchical porous carbon-based electrocatalysts for enhanced oxygen reduction in Zn–air batteries

A highly active yet stable electrocatalyst was prepared by in situ formed template-assisting method.

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.

N-Doped porous carbon nanosheets decorated with graphitized carbon layer encapsulated Co9S8 nanoparticles: an efficient bifunctional electrocatalyst for the OER and ORR

Electrocatalysts with highly effective activity for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are considered to be the key factor of renewable energy technologies. Herein, we designed and synthesized bi-functional electrocatalyst Co9S8/C nanosheets (NSs) by reducing the Co3O4/C NS precursor with sodium sulfide. From the structural characterization, Co9S8 is anchored on the graphitized carbon NSs homogeneously. According to the electrochemical measurements, the Co9S8/C NSs show excellent electrocatalytic activity towards both the ORR (Eonset = 0.892 V vs. RHE and E1/2 is 0.778 V vs. RHE) and OER (Eonset = 1.50 V vs. RHE and the overpotential is 0.434 V vs. RHE at 10 mA cm-2) in 0.1 M KOH solution. In the ORR, Co9S8/C catalyzes through a 4-electron process like the commercial Pt-based catalyst. In addition, the Co9S8/C NSs show great long-term stability. The Co9S8/C NSs show excellent catalytic performance due to the rich active sites and two-dimensional sheet structure. The developed strategy offers an effective method for preparing bi-functional electrocatalysts for both the OER and ORR.

Coordination-Assisted Polymerization of Mesoporous Cobalt Sulfide/Heteroatom (N,S)-Doped Double-Layered Carbon Tubes as an Efficient Bifunctional Oxygen Electrocatalyst

It is a critical challenge to construct efficient precious-metal-free bifunctional oxygen electrocatalysts for fuel cell and metal-air batteries via structural and component engineering. Herein, a one-dimensional mesoporous double-layered tubular structure, where Co₉S₈ nanocrystals are incorporated into nitrogen, sulfur codoped carbon, is successfully synthesized via the coordinated-assisted polymerization and sacrificial template methods. The double-layered tubular structure provides for a large electrochemically active surface area and promotes fast mass transfer. Cobalt oxides/oxyhydroxides, which are evolved from the sulfides during the catalytic processes, as the main active sites efficiently catalyze the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), in cooperation with the Co-N-C and heteroatom-induced active sites. Hence, it demonstrates excellent bifunctional electrocatalytic activity with the overvoltage between the OER potential at 10 mA cm^-2 ( E₁₀) and ORR half-wave potential ( E_1/2) of 0.707 V, which is superior to most of precious-metal-free bifunctional oxygen electrocatalysts reported recently, as well as the state-of-art Pt/C and RuO₂ catalysts.