Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional Electrocatalysts for Overall Water Splitting and Zn-Air Batteries

Porous Co9S8/Nitrogen, Sulfur-Doped Carbon@Mo2C Dual Catalyst for Efficient Water Splitting

The exploration of highly efficient and stable bifunctional electrocatalysts for overall water splitting is currently of extreme interest for the efficient conversion of sustainable energy sources. Herein, we provide an earth-abundant, low-cost, and highly efficient bifunctional electrocatalyst composed of cobalt sulfide (Co₉S₈) and molybdenum carbide (Mo₂C) nanoparticles anchored to metal-organic frameworks (MOFs)-derived nitrogen, sulfur-codoped graphitic carbon (Co₉S₈-NSC@Mo₂C). The new composite mode of the electrocatalyst was realized through simple pyrolysis processes. The composite electrocatalyst shows outstanding hydrogen evolution reaction (HER) performance and excellent stability over the entire pH range. For example, it has a lower overpotential of 74, 89, and 121 mV with the Tafel slopes of 69.3, 86.7, and 106.4 mV dec^-1 to achieve a current density of 10 mA cm^-2 in 0.5 M H₂SO₄, 1.0 M KOH, and 1.0 M phosphate-buffered saline solutions, respectively. Moreover, it shows a small overpotential of 293 mV with a Tafel slope of 59.7 mV dec^-1 to reach 10 mA cm^-2 for the oxygen evolution reaction (OER) in 1.0 M KOH. The significantly enhanced HER and OER activities of Co₉S₈-NSC@Mo₂C are mainly attributable to the electron transfer from Co to Mo₂C, resulting in a lower Mo valence and a higher Co valence in Co₉S₈-NSC@Mo₂C. Furthermore, using the Co₉S₈-NSC@Mo₂C bifunctional electrocatalyst as both the anode for the OER and the cathode for the HER for overall water splitting, a cell voltage of only 1.61 V is needed to derive a current density of 10 mA cm^-2. This interesting work offers a general method for designing and fabricating highly efficient and stable non-noble electrocatalysts for promising energy conversion.

Two-Dimensional MoS2 Confined Co(OH)2 Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes

The development of abundant and cheap electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. However, to achieve low-cost HER electrocatalysis, especially in alkaline media, is still a big challenge due to the sluggish water dissociation kinetics as well as the poor long-term stability of catalysts. In this paper we report the design and synthesis of a two-dimensional (2D) MoS₂ confined Co(OH)₂ nanoparticle electrocatalyst, which accelerates water dissociation and exhibits good durability in alkaline solutions, leading to significant improvement in HER performance. A two-step method was used to synthesize the electrocatalyst, starting with the lithium intercalation of exfoliated MoS₂ nanosheets followed by Co²⁺ exchange in alkaline media to form MoS₂ intercalated with Co(OH)₂ nanoparticles (denoted Co-Ex-MoS₂), which was fully characterized by spectroscopic studies. Electrochemical tests indicated that the electrocatalyst exhibits superior HER activity and excellent stability, with an onset overpotential and Tafel slope as low as 15 mV and 53 mV dec^-1, respectively, which are among the best values reported so far for the Pt-free HER in alkaline media. Furthermore, density functional theory calculations show that the cojoint roles of Co(OH)₂ nanoparticles and MoS₂ nanosheets result in the excellent activity of the Co-Ex-MoS₂ electrocatalyst, and the good stability is attributed to the confinement of the Co(OH)₂ nanoparticles. This work provides an imporant strategy for designing HER electrocatalysts in alkaline solutions, and can, in principle, be expanded to other materials besides the Co(OH)₂ and MoS₂ used here.