Effect of Ni Doping on the MoS2 Structure and Its Hydrogen Evolution Activity in Acid and Alkaline Electrolytes

Dario Mosconi,Doriana Debellis,Laura Calvillo,Gaetano Granozzi,Tomasz Kosmala,Stefano Agnoli,Paul Till,Alessandro Martucci,Denis Garoli

doi:10.3390/surfaces2040039

Abstract

We have investigated three-dimensional (3D) MoS2 nanoarchitectures doped with different amount of Ni to boost the hydrogen evolution reaction (HER) in alkaline environment, where this reaction is normally hindered. As a comparison, the activity in acidic media was also investigated to determine and compare the role of the Ni sites in both media. The doping of MoS2, especially at high loadings, can modify its structural and/or electronic properties, which can also affect the HER activity. The structural and electronic properties of the Ni doped 3D-MoS2 nanoarchitecture were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning and transmission electronic microscopy (SEM; TEM), and X-ray photoemission Spectroscopy (XPS). XPS also allowed us to determine the Ni-based species formed as a function of the dopant loading. The HER activity of the materials was investigated by linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1.0 M KOH. By combining the physicochemical and electrochemical results, we concluded that the Ni sites have a different role in the HER mechanism and kinetics in acidic and in alkaline media. Thus, NiSx species are essential to promote HER in alkaline medium, whereas the Ni-Mo-S ones enhance the HER in acid medium.

Highlights

Nowadays, molecular hydrogen is considered the most promising energy vector for developing a sustainable energy infrastructure based on the efficient interconversion of chemical energy into electricity and vice versa, known as hydrogen economy [1]
We optimized the synthesis of 3D-MoS2 scaffold to maximize the hydrogen evolution reaction (HER) performance of the pure material
This suggests that the metal doping does not affect the morphology, a slightly denser structure is observed when higher Ni loadings are used (Figure S2)

Summary

Introduction

Molecular hydrogen is considered the most promising energy vector for developing a sustainable energy infrastructure based on the efficient interconversion of chemical energy into electricity and vice versa, known as hydrogen economy [1]. The development of new technologies for clean and cost-effective large-scale production of hydrogen is of utmost importance. Electrolyzers based on the water splitting (WS) are, so far, the most promising devices to obtain clean hydrogen through the hydrogen evolution reaction (HER) at the cathode side of an electrochemical cell [2]. This, requires the development and optimization of electrocatalysts, based on cost-effective noncritical raw materials (i.e., noble metal-free), that may guarantee efficiency and durability under operating conditions. As WS entails the oxygen evolution reaction at the anode side, which requires the use of alkaline conditions when catalyzed by non-noble metal catalysts, Surfaces 2019, 2, 531–545; doi:10.3390/surfaces2040039 www.mdpi.com/journal/surfaces. The influence of the pH on the cathodic half-reaction can be portrayed starting from the two accredited mechanisms for HER (i.e., Volmer–Heyrovsky and Volmer–Tafel) that result from the combination of two of the following reactions steps [3,4,5]

Methods

Results

Conclusion