Transition Metal Sulfides and Carbides Co-Catalyzed with Metal-Organic Frameworks for Enhanced Electrochemical Water Splitting

Abstract

In recent times, there has been considerable curiosity towards the exploration of sustainable and green energy reserves as a means of addressing the challenges posed by rising global demand for energy, eco-pollution, and exhaustion of fossil fuel resources. Hydrogen, in particular, has emerged as an encouraging green fuel source. As it offers limitless potential as it proposes carbon free solution fuel with high energy density. The water-splitting process can produce hydrogen from the amplest resource on the Earth, i.e., water, with almost negligible environmental impact. The process comprises of oxygen evolution reaction and hydrogen evolution reaction. However, the kinetics for these reactions are sluggish. It becomes essential to use a catalyst to overcome the strong bonds within a water molecule for the process to work efficiently. Generally, catalysts involving noble metals are utilized for electrochemical water-splitting process. Still, their usage is restricted because of their high cost, limited availability, and poor durability, which hinders the scalability of the process. Besides catalyst composition, morphology is an important reason that influences activity of the catalyst for watersplitting. Therefore, optimizing catalyst material and structure is paramount for an efficient water-splitting process. Mo, W, and Cd, which are readily accessible transition metals, provide promising substitutes for noble metal-based catalysts. They are renowned for their widespread availability and exceptional performance, coupled with remarkable stability. Metal-organic frameworks are coordinated, porous crystalline materials that possess distinctive catalytic, and electrical properties. These MOFs and materials derived from them have demonstrated exceptional efficacy as catalysts for the electrochemical process of water-splitting. This dissertation accounts for six detailed studies on developing highly effective low-cost hybrid electrocatalysts composed of transition metals (Mo, W, Cd) carbides and sulfides with metal-organic frameworks (UiO-66, MIL-101(Fe), Prussian Blue). Hybrid substances exhibit enhanced electrocatalytic performance owing to the modified structures and synergistic effects generated by their combination. Solvothermal, hightemperature heat treatment and sonication synthesis approaches were used to fabricate pure and hybrid catalysts. The catalysts that were synthesized underwent comprehensive xiii characterization through techniques including X-ray Diffraction (XRD), Scanning Electron Microscopy/Transmission Electron Microscopy (SEM/TEM), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) adsorption, and Energy Dispersive X-Ray Analysis (EDX). Furthermore, the electrochemical performance of these catalysts was assessed using a range of methods, including linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), stability tests, and evaluation of the overall cell potential. A detailed study describes the synthesis and investigation of a UiO-66/MoS2 hybrid catalyst prepared using two different synthesis methods. The optimized hybrid demonstrated superior electrocatalytic activity for hydrogen generation, needing an “overpotential” of 129 mV to attain 10.0 mA/cm2 of “current density” and a Tafel value of 51 mV/dec. Furthermore, at 10.0 mA/cm2 it exhibited an “overpotential” of 180 mV for Oxygen evolution with a Tafel slope of 64 mV/dec. The catalyst was also stable over prolonged periods, with only a slight reduction in activity. Another study on UiO66/Mo2C hybrid catalyst synthesized with different compositions is also detailed. Mo2C/UiO-66 hybrids offer improved catalytic activity compared to pure compounds in an alkaline environment for water-splitting. The optimized Mo2C/UiO-66 hybrid, comprising 50% of each component, exhibited the most efficient catalytic performance for hydrogen and OERs. It resulted in a minimal “overpotential” of 174.1 mV to achieve a “current density” of 10.0 mA/cm2 and a 147 mV/dec Tafel plot value for HER. Similarly, for OER, it delivered a low activation “overpotential” of approximately 180 mV to achieve a “current density” of 20.0 mA/cm2 and a Tafel plot value of 134 mV/dec. Hybrids composed of Tungsten sulfide with UiO-66 in detail are reported. An active electrocatalyst comprising WS2 and UiO-66 was constructed with different compositions. The optimized WS2/UiO-66 catalyst demonstrates remarkable OER & HER activity in basic electrolytes, with a low “overpotential” of 121 mV for HER and 220 mV for OER, achieving a “current density” of 10.0 mA/cm2 . In the next set of experiments, hybrids of Tungsten Carbide with UiO-66 in different compositions have been prepared and explained. It was noticed that the optimized hybrid with twice the weight percent of WC showed better electrocatalytic activity with a low “overpotential” of 104 mV and 152 mV for HER & OER to achieve 10.0 mA/cm2 of “current density”. Hybrids of cadmium sulfide with MIL- xiv 101(Fe) were studied for HER. In an alkaline environment, these hybrids presented enhanced reactivity for the HER compared to pure compounds. In particular, a hybrid with CdS three times MIL-101(Fe) displayed a small “overpotential” of 108 mV at 10.0 mA/cm2 and a low Tafel value of 47 mV/dec. It was stable for 24 h and 1000 cycles with a minute decline in activity. The other study explored a bifunctional electrode composed of cadmium sulfide and Prussian blue nanorod heterostructures (PBNP/CdS). It has been optimized to exhibit significantly reduced “overpotential”s of 126 mV and 181 mV at current densities of 10.0 mA/cm2 and 20.0 mA/cm2 , respectively, for HER. However, for OER, it displays “overpotential”s of 250 mV and 316 mV at current densities of 10.0 mA/cm2 and 20.0 mA/cm2 , respectively. The aim of this study is to develop bifunctional electrocatalysts that can efficiently facilitate the water-splitting process. The catalyst design involves combining two distinct components, each with crucial roles in achieving the desired outcome, resulting in enhanced activity. The results of this Ph.D. research provide valuable insights into the design and development of effective and competent electrocatalysts for sustainable energy applications.