Intrinsic Catalytic Activity for the Alkaline Hydrogen Evolution of Layer-Expanded MoS₂ Functionalized with Nanoscale Ni and Co Sulfides

Abstract

The hydrogen evolution reaction (HER) under alkaline conditions is subject to significant kinetic limitations even with the most active platinum-based catalysts, while more affordable non-noble-metal-based catalytic materials present further challenges in terms of activity and durability in operation. To improve on these aspects, we present a new microwave-assisted synthetic route to fabricate sulfides of nickel and cobalt integrated into a layer expanded molybdenum sulfide (NiS x /MoS 2 LE and CoS x /MoS 2 LE ), which efficiently catalyze H 2 evolution in 1 M KOH. The use of the microwave-synthesis conditions enables the formation of nanoscale Ni and Co sulfides distributed homogeneously within the highly disordered layered molybdenum sulfide, as established using a comprehensive suite of physical methods. Synthesis of FeS x /MoS 2 LE is also presented, but the resulting material did not exhibit promising properties. Electrocatalytic tests reveal higher activity of the Ni-based catalyst as compared to CoS x /MoS 2 LE and especially unmodified MoS 2 LE . The performance of NiS x / MoS 2 LE at a HER overpotential of 0.15 V at ambient temperature and 60 °C corresponds to specific H 2 evolution rates of 28 ± 4 and 58 ± 10 A g −1 , respectively. Analysis of the electrokinetic data indicates that the exchange current density of the HER per an electrochemically active surface area of the sulfide-based materials is not high (∼0.001 mA cm −2 at ambient temperature), and that the high performance per unit mass observed here is supported by the well-developed surface area of the material (corresponding to a specific capacitance of ∼71 F g −1 ). A similar conclusion likely applies to many nickel and cobalt sulfide-based alkaline hydrogen evolution catalysts reported previously. Durability in operation of NiS x /MoS 2 LE and CoS x /MoS 2 LE is also demonstrated, in particular through a 2-week-long two-electrode water splitting test.

Understanding the Role of (W, Mo, Sb) Dopants in the Catalyst Evolution and Activity Enhancement of Co3O4 during Water Electrolysis via In Situ Spectroelectrochemical Techniques

“…Given the similarity in phase transformation (and surface chemistry) during OER and HER for both pristine and doped cobalt oxides, (pseudo)capacitance (C p ) measurements were conducted to study the electrochemically active surface area (Figure S13, Supporting Information), i.e., number of active sites variation. [9,[50][51][52][53] It was found that introducing the W dopant increases the C p by nearly 1.5 times, from 0.44 ± 0.01 to 0.65 ± 0.01 mF cm −2 (Figure S13c, Supporting Information). Likewise, we also analyzed the density of electrochemically active cobalt sites (Γ) by applying Faraday's law to the changes associated with the Co voltammetric reduction peaks by assuming one electron transfer.…”

“…The H 2 -or O 2 -saturated electrolyte was used for HER and OER measurements, respectively. [52,60] The EIS was recorded at potentials where no significant faradaic processes occur (typically, 0.02-0.07 V vs RHE) with a frequency range of 100 kHz to 0.1 Hz, amplitude 0.010 V. [61] Mott-Schottky was measured at a frequency of 1 kHz at an applied potential window ranging from −1.2 to 0 V versus RHE. Different scan rates were used in the cyclic voltammetry measurement at the potential window of 1.05 to 1.10 V versus RHE to obtain the electrochemical capacitance current for the evaluation of the relative electrochemically active surface area.…”

“…The H 2 ‐ or O 2 ‐saturated electrolyte was used for HER and OER measurements, respectively. [ 52,60 ]…”

Phosphorus-Induced One-Step Synthesis of NiCo2S4 Electrode Material for Efficient Hydrazine-Assisted Hydrogen Production

“…Transition-metal sulfides have gained significant interest in recent years due to their potential applications in energy storage and catalysis . Nickel cobalt-based sulfides, in particular, are found to be efficient materials in electrocatalyzing a variety of reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), urea oxidation reaction (UOR), and hydrazine oxidation reaction (HzOR), owing to their high electrical conductivity and efficiency. − In particular, the NiCo 2 S 4 spinel nanostructure (NS) is interesting because the Ni and Co combination has a more favorable synergistic effect due to the formation of a single phase . However, NiCo 2 S 4 is mostly prepared by multistep synthesis and requires high temperatures to stabilize this phase.…”

Microwave‐Assisted Synthesis of Cobalt‐Based Selenides as Catalyst Precursors for the Alkaline Water Oxidation

“…This can be addressed through the synthesis of high-surface-area CoSe n precursors, which is sometimes challenging to achieve with the conventional synthetic methods, especially those involving treatment at elevated temperatures. A method that has proven to produce highly dispersed nanomaterials of various compositions, including chalcogenides for the catalysis of water electrolysis, [28,38,39] is the microwave-assisted synthesis. This approach is well known to generate materials with enhanced electrochemically active surface area and to enable fine control over the morphology of the products.…”