Co9S8–Ni3S2 heterointerfaced nanotubes on Ni foam as highly efficient and flexible bifunctional electrodes for water splitting

Synergistic enhancement of Zn-air battery performance via integration of Ni-doped cobalt sulfide nanoparticles within N, S-doped carbon matrix

Exploring precious metal-free bifunctional electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is essential for the practical application of rechargeable Zn-air battery (ZAB). Herein, Ni-doped Co9S8 nanoparticles embedded in a defect-rich N, S co-doped carbon matrix (d-NixCo9-xS8@NSC) are synthesized via a facile pyrolysis and acid treatment process. The introduction of abundant defects in both the carbon matrix and metal sulfide provides numerous active sites and significantly enhances the electrocatalytic performances for both the ORR and OER. d-NixCo9-xS8@NSC exhibits a superior half-wave potential of 0.841 V vs. RHE for the ORR and delivers a low overpotential of 0.329 V at 10 mA cm-2 for the OER. Additionally, Zn-air secondary battery using d-NixCo9-xS8@NSC as the air cathode displays a higher specific capacity of 734 mAh gZn-1 and a peak power density of 148.03 mW cm-2 compared to those of state-of-the-art Pt/C-RuO2 (673 mAh gZn-1 and 136.9 mW cm-2, respectively). These findings underscore the potential of d-NixCo9-xS8@NSC as a high-performance electrocatalyst for secondary ZABs, offering new perspectives on the design of efficient noble metal-free electrocatalysts for future energy storage and conversion applications.

Strong Interface Coupling Enables Stability of Amorphous Meta-Stable State in CoS/Ni3S2 for Efficient Oxygen Evolution

Rational design of heterostructure catalysts through phase engineering strategy plays a critical role in heightening the electrocatalytic performance of catalysts. Herein, a novel amorphous/crystalline (a/c) heterostructure (a-CoS/Ni3S2) is manufactured by a facile hydrothermal sulfurization method. Strikingly, the interface coupling between amorphous phase (a-CoS) and crystalline phase (Ni3S2) in a-CoS/Ni3S2 is much stronger than that between crystalline phase (c-CoS) and crystalline phase (Ni3S2) in crystalline/crystalline (c/c) heterostructure (c-CoS/Ni3S2) as control sample, which makes the meta-stable amorphous structure more stable. Meanwhile, a-CoS/Ni3S2 has more S vacancies (Sv) than c-CoS/Ni3S2 because of the presence of an amorphous phase. Eventually, for the oxygen evolution reaction (OER), the a-CoS/Ni3S2 exhibits a significantly lower overpotential of 192 mV at 10 mA cm-2 compared to the c-CoS/Ni3S2 (242 mV). An exceptionally low cell voltage of 1.51 V is required to achieve a current density of 50 mA cm-2 for overall water splitting in the assembled cell (a-CoS/Ni3S2 || Pt/C). Theoretical calculations reveal that more charges transfer from a-CoS to Ni3S2 in a-CoS/Ni3S2 than in c-CoS/Ni3S2, which promotes the enhancement of OER activity. This work will bring into play a fabrication strategy of a/c catalysts and the understanding of the catalytic mechanism of a/c heterostructures.

Incorporation of Ag in Co9S8-Ni3S2 for Predominantly Enhanced Electrocatalytic Activities for Oxygen Evolution Reaction: A Combined Experimental and DFT Study

Electrodeposition of abundant metals to fabricate efficient and durable electrodes indicate a viable role in advancing renewable electrochemical energy tools. Herein, we deposit Co9S8-Ag-Ni3S2@NF on nickel foam (NF) to produce Co9S8-Ag-Ni3S2@NF as a exceedingly proficient electrode for oxygen evolution reaction (OER). The electrochemical investigation verifies that the Co9S8-Ag-Ni3S2@NF electrode reveals better electrocatalytic activity to OER because of its nanoflowers' open-pore morphology, reduced overpotential (η10=125 mV), smaller charge transfer resistance, long-term stability, and a synergistic effect between various components, which allows the reactants to be more easily absorbed and subsequently converted into gaseous products during the water electrolysis route. Density functional theory (DFT) calculation as well reveals the introduction of Ag (222) surface into the Co9S8 (440)-Ni3S2 (120) structure increases the electronic density of states (DOS) per unit cell of a system and increases the electrocatalytic activity of OER by considerably lowering the energy barriers of its intermediates. This study provides the innovation of employing trimetallic nanomaterials immobilized on a conductive, continuous porous three-dimensional network formed on a nickel foam (NF) substrate as a highly proficient catalyst for OER.

Nano-Scale Engineering of Heterojunction for Alkaline Water Electrolysis

Alkaline water electrolysis is promising for low-cost and scalable hydrogen production. Renewable energy-driven alkaline water electrolysis requires highly effective electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, the most active electrocatalysts show orders of magnitude lower performance in alkaline electrolytes than that in acidic ones. To improve such catalysts, heterojunction engineering has been exploited as the most efficient strategy to overcome the activity limitations of the single component in the catalyst. In this review, the basic knowledge of alkaline water electrolysis and the catalytic mechanisms of heterojunctions are introduced. In the HER mechanisms, the ensemble effect emphasizes the multi-sites of different components to accelerate the various intermedium reactions, while the electronic effect refers to the d-band center theory associated with the adsorption and desorption energies of the intermediate products and catalyst. For the OER with multi-electron transfer, a scaling relation was established: the free energy difference between HOO* and HO* is 3.2 eV, which can be overcome by electrocatalysts with heterojunctions. The development of electrocatalysts with heterojunctions are summarized. Typically, Ni(OH)2/Pt, Ni/NiN3 and MoP/MoS2 are HER electrocatalysts, while Ir/Co(OH)2, NiFe(OH)x/FeS and Co9S8/Ni3S2 are OER ones. Last but not the least, the trend of future research is discussed, from an industry perspective, in terms of decreasing the number of noble metals, achieving more stable heterojunctions for longer service, adopting new craft technologies such as 3D printing and exploring revolutionary alternate alkaline water electrolysis.

A novel sacrificial solvent method to synthesize self-supporting Co9S8/Ni3S2 heterostructure catalyst for efficient oxygen evolution reaction

Synthesizing catalysts for efficient oxygen evolution reaction (OER) with lower cost and simpler design is of significant importance to achieve sustainable hydrogen production. In this work, we propose a novel "sacrificial solvent method" for the first time. Dicobalt octacarbonyl (Co2(CO)8), dimethyl sulfoxide (DMSO), and Ni foam (NF) were used as the raw materials in the solvothermal process. DMSO played the role of both the sacrificial solvent and the sulfur source. Through the self-consumption of DMSO, we finally obtained the Co9S8/Ni3S2 heterostructure supported on the NF (Co9S8/Ni3S2@NF) in one step. The Co9S8/Ni3S2@NF catalyst exhibited excellent OER activity in alkaline environment, with an overpotential of only 264 mV at a current density of 20 mA cm-2, a low Tafel slope of 68.28 mV dec-1 and maintained its current density after 20 h of constant potential testing. This work introduces a new method for synthesizing metal sulfide catalysts using DMSO as a sacrificial solvent. It provides broader opportunities for the development of more efficient and sustainable catalysts for energy conversion and storage.

Polysulfide induced synthesis of a MoS2 self-supporting electrode with wide-layer-spacing for efficient electrocatalytic water splitting

Efficient non-noble metal bifunctional electrocatalysts can increase the conversion rate of electric energy in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Herein, a ball & sheet MoS2/Ni3S2 composite with wide-layer-spacing and high 1T-rich MoS2 is assembled on nickel foam (NF) via a two-step solvothermal method with polymeric sulfur (S-r-DIB) as the sulfur source. The obtained material serves as both the cathode and the anode toward overall water splitting in an alkaline electrolyte. The results proved that the interpenetration of MoS2/Ni3S2-p with a ball and sheet structure increased the material active surface area and exposed more catalytic active sites, which contributed to the penetration of solution and the transfer of charge/hydrion. Meanwhile, two different semiconductors of MoS2 and Ni3S2 along with the presence of ample active sulfur edge sites and few-layer, wide-layer-spacing structures of MoS2 lead to an outstanding electrocatalytic activity. In particular, the electrodes of MoS2/Ni3S2-p only need a battery voltage of 1.55 V at 10 mA cm-2. The bifunctional electrocatalyst MoS2/Ni3S2-p also shows excellent stability at large current densities during the electrochemical test.

Phase-Controllable Growth Nix Py Modified CdS@Ni3 S2 Electrodes for Efficient Electrocatalytic and Enhanced Photoassisted Electrocatalytic Overall Water Splitting

The rational design and construction of cost-effective nickel-based phosphide or sulfide (photo)electrocatalysts for hydrogen production from water splitting has sparked a huge investigation surge in recent years. Whereas, nickel phosphides (Ni_x P_y ) possess more than ten stoichiometric compositions with different crystalline. Constructing Ni_x P_y with well crystalline and revealing their intrinsic catalytic mechanism at atomic/molecular levels remains a great challenge. Herein, an easy-to-follow phase-controllable phosphating strategy is first proposed to prepare well crystalline Ni_x P_y (Ni₃ P and Ni₁₂ P₅ ) modified CdS@Ni₃ S₂ heterojunction electrocatalysts. It is found that Ni₃ P modified CdS@Ni₃ S₂ (CdS@Ni₃ S₂ /Ni₃ P) exhibits remarkable stability and bifunctional electrocatalytic activities in both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory results suggest that P-Ni sites and P sites in CdS@Ni₃ S₂ /Ni₃ P, respectively, serve as OER and HER active sites during electrocatalytic water splitting processes. Moreover, benefiting from the advantageous photocatalyst@electrocatalyst core@shell structure, CdS@Ni₃ S₂ /Ni₃ P delivers an advantaged photoassisted electrocatalytic water splitting property. The champion electrical to hydrogen and solar to hydrogen energy conversion efficiencies of CdS@Ni₃ S₂ /Ni₃ P, respectively, reach 93.35% and 4.65%. This work will provide a general guidance for synergistically using solar energy and electric energy for large-scale H₂ production from water splitting.

Electronic modulation and proton transfer by iron and borate co-doping for synergistically triggering the oxygen evolution reaction on a hollow NiO bipyramidal prism

Designing an Earth-abundant and inexpensive electrocatalyst to drive the oxygen evolution reaction (OER) for high-purity hydrogen production is of great importance. Herein, the cation (iron) and anion (borate) co-doping strategy was proposed to effectively trigger the OER performance on a low-cost NiO material. The optimal hollow Fe/B_i-NiO bipyramidal prism shows superior OER performance, and displays a low overpotential (261 mV) at 10 mA cm^-2, accompanied by a low Tafel slope (46 mV dec^-1), excellent intrinsic activity and robust stability. The overall alkaline water splitting using Fe/B_i-NiO/NF as an anode affords low cell voltages of 1.50 and 1.63 V at 10 and 100 mA cm^-2, and operates steadily at a high current density of 100 mA cm^-2 for 55 h without decay. The excellent electrocatalytic activity could be ascribed to the hollow structure to shorten the mass transfer pathway, the electronic modulation by Fe doping, the increased accessible electroactive sites created by oxygen vacancies through borate doping, and the formation of BO₃^3--OH^- to accelerate the deprotonation of OH_ads.

Enhancing electrocatalytic water splitting by surface defect engineering in two-dimensional electrocatalysts

Overall electrocatalytic water splitting can efficiently and sustainably produce clean hydrogen energy to alleviate the global energy crisis and environmental pollution. Two-dimensional (2D) materials with a unique band structure and surface conformation have emerged as promising electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, the intrinsic activities of primitive 2D materials in the catalytic process are still inferior to those of noble metal-based electrocatalysts. Surface defect engineering can modulate the electronic structure of 2D materials and induce new physicochemical properties, promoting their electrocatalytic performance. Herein, this minireview focuses on some recent developments in surface defect engineering, including the contribution of active sites, the derivation of the heterogeneous interface, and the anchoring of active substances, which provides an effective way to further optimize 2D electrocatalysts for water splitting. Furthermore, the typical morphological characteristics, catalytic activity, stability and catalytic mechanism of these 2D electrocatalysts are introduced. We believe that this minireview will help design more efficient and economical electrocatalysts for overall water splitting.

Chinese hydrangea lantern-like Co9S8@MoS2 composites with enhanced lithium-ion battery properties

Chinese hydrangea lantern-like Co₉S₈@MoS₂ composites are prepared by a facile solvothermal method. Ultra-thin MoS₂ nanosheets as the shells grow tightly and uniformly on the surface of the Co₉S₈ core. Due to their unique hierarchical core-shell structure and novel morphology, the composites show excellent electrochemical performance as the anode materials of lithium-ion batteries. They can deliver reversible discharge capacities of around 1298, 1150, 1089, 1018 and 941 mA h g^-1 at the current densities of 0.1, 0.5, 1, 1.5 and 2.0 A g^-1, respectively. Moreover, the Co₉S₈@MoS₂ composites can still maintain a discharge capacity of 1048 mA h g^-1 after 300 cycles at a current density of 1.0 A g^-1.

Three dimensional Ni3S2 nanorod arrays as multifunctional electrodes for electrochemical energy storage and conversion applications

The increasing demand for energy and environmental protection has stimulated intensive interest in fundamental research and practical applications. Nickel dichalcogenides (Ni3S2, NiS, Ni3Se2, NiSe, etc.) are promising materials for high-performance electrochemical energy storage and conversion applications. Herein, 3D Ni3S2 nanorod arrays are fabricated on Ni foam by a facile solvothermal route. The optimized Ni3S2/Ni foam electrode displays an areal capacity of 1602 µA h cm-2 at 5 mA cm-2, excellent rate capability and cycling stability. Besides, 3D Ni3S2 nanorod arrays as electrode materials exhibit outstanding performances for the overall water splitting reaction. In particular, the 3D Ni3S2 nanorod array electrode is shown to be a high-performance water electrolyzer with a cell voltage of 1.63 V at a current density of 10 mA cm-2 for overall water splitting. Therefore, the results demonstrate a promising multifunctional 3D electrode material for electrochemical energy storage and conversion applications.

Uncovering the role of Ag in layer-alternating Ni3S2/Ag/Ni3S2 as an electrocatalyst with enhanced OER performance

It is increasingly important to develop an efficient OER catalyst that can provide high current density at low overpotentials to improve water splitting efficiency.