[Paper Review] Engineered MoSe2-based heterostructures for efficient electrochemical hydrogen evolution reaction

Leyla Najafi, Sebastiano Bellani+5 more|arXiv (Cornell University)|Mar 21, 2019

Electrocatalysts for Energy Conversion4 citations

TL;DR

This study develops engineered MoSe2-based heterostructures with graphene or single-wall carbon nanotubes to enhance electrochemical hydrogen evolution reaction (HER) activity. By integrating MoSe2 flakes with carbon nanomaterials and applying thermal annealing and n-butyllithium treatment, the researchers activate basal plane sites via Se-vacancy creation and phase transition, achieving a 4.8-fold increase in exchange current density (0.203 Acm⁻²) and a Tafel slope of 54 mVdec⁻¹.

ABSTRACT

Two-dimensional transition metal-dichalcogenides are emerging as efficient and cost-effective electrocatalysts for hydrogen evolution reaction (HER). However, only the edge sites of their trigonal prismatic phase show HER-electrocatalytic properties, while the basal plane, which is absent of defective/unsaturated sites, is inactive. Here, we tackle the key challenge that is increasing the number of electrocatalytic sites by designing and engineering heterostructures composed of single-/few-layer MoSe2 flakes and carbon nanomaterials (graphene or single-wall carbon nanotubes (SWNTs)) produced by solution processing. The electrochemical coupling between the materials that comprise the heterostructure effectively enhances the HER-electrocatalytic activity of the native MoSe2 flakes. The optimization of the mass loading of MoSe2 flakes and their electrode assembly via monolithic heterostructure stacking provided a cathodic current density of 10mAcm-2 at overpotential of 100mV, a Tafel slope of 63mVdec-1 and an exchange current density (j0) of 0.203 Acm-2. In addition, electrode thermal annealing in a hydrogen environment and chemical bathing in n-butyllithium are exploited to texturize the basal planes of the MoSe2 flakes (through Se-vacancies creation) and to achieve in situ semiconducting-to-metallic phase conversion, respectively, thus they activate new HER-electrocatalytic sites. The as-engineered electrodes show a 4.8-fold enhancement of j0 and a decrease in the Tafel slope to 54mVdec-1.

Motivation & Objective

• To overcome the limited electrocatalytic activity of MoSe2, which is restricted to edge sites, by engineering its basal plane.
• To enhance HER performance through heterostructure integration with carbon nanomaterials (graphene or SWNTs) via solution processing.
• To activate inert basal planes of MoSe2 by creating Se-vacancies and inducing semiconducting-to-metallic phase transition.
• To optimize mass loading and electrode architecture for improved electrochemical performance.
• To achieve high current density at low overpotential with improved Tafel slope and exchange current density.

Proposed method

• Solution-processed heterostructures were fabricated by combining single- or few-layer MoSe2 flakes with graphene or single-wall carbon nanotubes (SWNTs).
• Electrochemical coupling between MoSe2 and carbon nanomaterials enhanced intrinsic HER activity.
• Thermal annealing in a hydrogen atmosphere created Se-vacancies, texturizing the basal planes of MoSe2.
• Chemical treatment with n-butyllithium induced in situ semiconducting-to-metallic phase transition in MoSe2.
• Monolithic heterostructure stacking was used to optimize mass loading and electrode architecture.
• Electrochemical performance was evaluated via linear sweep voltammetry, Tafel analysis, and exchange current density calculation.

Experimental results

Research questions

• RQ1 Can the basal plane of MoSe2 be activated for HER by engineering its surface structure?
• RQ2 How does integration with carbon nanomaterials affect the electrocatalytic activity of MoSe2?
• RQ3 To what extent can Se-vacancy creation and phase transition enhance HER performance in MoSe2-based heterostructures?
• RQ4 What is the optimal mass loading and electrode configuration for maximizing HER activity in MoSe2 heterostructures?
• RQ5 Can the Tafel slope and exchange current density be significantly improved through post-treatment strategies?

Key findings

• The optimized heterostructure achieved a cathodic current density of 10 mAcm⁻² at an overpotential of 100 mV.
• The Tafel slope was reduced to 54 mVdec⁻¹ after engineering treatments, indicating improved reaction kinetics.
• The exchange current density (j₀) increased to 0.203 Acm⁻², representing a 4.8-fold enhancement over pristine MoSe2.
• Se-vacancy creation via hydrogen annealing effectively activated the basal plane of MoSe2.
• n-Butyllithium treatment induced a semiconducting-to-metallic phase transition, further enhancing HER activity.
• Monolithic stacking of heterostructures enabled effective mass loading optimization and improved electrochemical performance.