Development of 2D Based Electrocatalytic Materials for Hydrogen Production

Abstract

The excessive use of non-renewable fossil fuels and subsequent CO2 emissions poses a tremendous threat to the energy security and the environment. Hence, researchers are looking for renewable and green energy sources for longer sustainability. In this regard, electrochemical water splitting is one of the most sustainable and green methods to produce hydrogen; which is considered a green and high calorific value fuel. Electrocatalytic water splitting involves the simultaneous production of hydrogen and oxygen gases at the cathode and anode, respectively. The hydrogen evolution reaction (HER) at the cathode involves a 2e- transfer process, which is influenced by the reduction potential of the electrocatalyst and the simultaneous oxygen evolution reaction (OER) rate. The current state-of-the-art electrocatalyst for the hydrogen evolution reaction (HER) is carbon supported platinum (Pt/C) electrode, which exhibits superior performance with a low overpotential value and Tafel slope. However, due to less abundance and very high cost of platinum, its application is very restricted. Hence, earth-abundant materials have been the focus for the fabrication of efficient HER electrocatalysts.

In this thesis, we chose transition metal sulfides (TMS) mainly the MoS2, WS2, and NbS2 as electrocatalysts, and the titanium-based MXene was adopted as a conducting support to form the supported electrocatalysts. The idea was to intercalate TMSs into the MXene layers which could provide extra reduction potential and channelize the charge transfer in the HER; and also imparts stability to the electrocatalyst against current density deterioration. Hence, first MXene was synthesized from a titanium carbide based bulk MAX phase using a mild etchant by in-situ HF formation. The different class of TMSs viz; WS2, MoS2, and NbS2 were synthesized using hydrothermal, solvothermal and hot-injection methods, respectively. These TMSs were then grown in-situ along with the synthesized MXene, using different concentrations. Hence, series of electrocatalysts were synthesized involving WS2MXene, MoS2-MXene and NbS2-MXene. Various characterization techniques, such as, X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Fourier-Transform Infrared Spectroscopy, and Raman Spectroscopy were adopted to confirm the 2D-2D morphology of the electrocatalysts. The electrocatalysts were found intercalating in between the layers of the conducting support (MXene) resulting in the 2D-2D heterostructure. Among all the synthesized electrocatalysts, the WS2-MXene series showed the better activity and stability for HER; both with Nickel Foam and Platinum bulk electrodes. And 5% WS2-MXene (2D-2D) nanocomposite was found to exhibit superior activity among all towards the HER with a low overpotential value of 60.7 mV @ 10 mAcm-2 and corresponding Tafel value of 211.5 mVdec-1 in 1 M KOH electrolyte. It also exhibited the highest double-layer capacitance (Cdl) calculated from cyclic voltammetry; which indicates enhanced electrochemical surface area and more active sites for HER. It also exhibited great stability for 50 hours of HER reaction.

The results validated the hypothesis that 2D-2D interaction of the TMS-MXene enhanced the reductive properties, charge transfer and stability of the electrocatalysts for HER.

Date of Award	Dec 2022
Original language	American English
Supervisor	AHSAN Qurashi (Supervisor)