Loading...

Investigation of Electrocatalytic Behavior of Some Transition Metal Oxides, Sulfides and Phosphides Nanostructures Toward Electrochemical Water Splitting

Shamloofard, Maryam | 2022

150 Viewed
  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55645 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Shahrokhian, Saeed
  7. Abstract:
  8. Energy and the environment issues are among the most important concerns of the present age. Global demand for energy is growing rapidly and energy demand is projected to double in the coming years. However, today most of the energy consumption comes from fossil fuels, which have limited and unstable reserves and are among the sources of environmental pollution. Hence, the production and conversion of clean energy from renewable sources is considered as an intelligent solution to significantly reduce dependence on fossil fuels and protect the environment. Fuel cells, metal-air batteries, and water electrolysis are among the simplest, most efficient, and most reliable technologies among several energy conversion systems controlled by electrochemical reactions. Water electrolysis has attracted a lot of attention due to the production of hydrogen and oxygen. The process of water electrolysis has a long history, but researchers have always made great efforts to improve the performance of this process. Due to the advancement of technology and the emergence of nanotechnologies and the possibility of synthesizing various nanostructures, efforts have always been made to overcome the great limitation of this method, i.e. its high cost due to the use of expensive catalysts. On the other hand, although the main purpose of water electrolysis is to produce pure hydrogen, the reaction of the electrode against it, that is, the oxygen evolution reaction (OER), cannot be ignored. Therefore, the development of effective and inexpensive electrocatalysts for water electrolysis, especially the water oxidation process, due to the more complex reaction, is essential to improve the process efficiency to obtain clean and stable fuels.In the first part, a practical strategy for preparing an efficient tri-metal electrocatalyst (NiCoZn) with a new flower-like three-dimensional structure for oxygen evolution reaction (OER) was presented. For this purpose, first, the Zn-Co flower-like structure was synthesized through a one-step process without the need for any additional methods. The main advantage of this method was that it was single-stage, which saved material consumption and, most importantly, avoided the use of hazardous and harmful solvents. After that, heat treatment was applied on the prepared Zn-Co under air atmosphere to prepare the ZnCo2O4 compound. Then, electrodeposition of NiCo-LDH was performed on ZnCo2O4 by chronoamprometry method and the catalytic properties of these compounds were investigated. Also, to investigate the effect of zinc metal on the electrocatalytic behavior of the synthesized structure, the Co3O4 compound was prepared in a similar way and its catalytic behavior in improving the performance of OER was compared with previous compounds. Among the studied samples, NiCo-LDH/ZnCo2O4 demonstrated the best electrocatalytic performance with the overpotential of 260 mV at the current density of 10 mA cm−2 and a Tafel slope of 62 mV dec−1 with long-term catalytic stability in an alkaline electrolyte. Its outstanding electrocatalytic performances are endowed from porous structure of NiCo-LDH/ZnCo2O4 catalyst, highly exposed active sites, accelerated mass and electron transport, and the synergistic effect of multiple hybrid components.In the second part, an easy method for designing and synthesizing Co-Mo-P/Co-Zn-S catalyst was introduced, consisting of preparation of the star-like Co-Zn-S and subsequent electrodeposition of Co-Mo-P on the Co-Zn-S star-like structure. The Co-Mo-P/Co-Zn-S catalyst was an efficient electrocatalyst for both HER and OER processes in alkaline electrolyte. In this section, first, a Co-Zn star-like precursor was synthesized using hydrothermal method, and then, to study the effect of sulfidation, the heat treatment in air and argon gas was applied on this precursor and the electrocatalytic behavior of the samples were compared with Co-Zn-S for OER and HER. Then, to investigate the effect of molybdenum on the structure of Co-Mo-P, compounds with different amounts of molybdenum were synthesized on the surface of a glassy carbon electrode by corneoamprometry. In the next step, Co-Mo-P was electrodeposited on Co-Zn-S with the amount of molybdenum optimized by corneoamprometry method and the Co-Mo-P/Co-Zn-S catalyst was prepared. The results of this section showed that the Co-Mo-P/Co-Zn-S catalyst need the low overpotentials of 273 mV and 273 mV at the current density of 10 mA cm−2 for OER and HER respectively. Also, the Tafel slopes of the Co-Mo-P/Co-Zn-S were 61.7 mV dec−1 and 42 mV dec−1 for OER and HER respectively. The excellent electrocatalytic performance of this compound for OER and HER processes is due to the good synergistic effects between the various hybrid components, which results in the production of abundant active catalytic sites and accelerates mass and electron transfer. This study demonstrated an effective and efficient approach to the rational design and synthesis of bifunctional catalysts with three-dimensional porous metals-based structure as inexpensive, highly efficient electrocatalysts for energy conversion reactions.In the third part, a simple one-step synthetic method was used to prepare different catalysts with different morphologies. In this section, different amounts of NH4F were used to control the morphology and electrochemical properties of the samples and then the electrocatalytic behavior of the prepared catalysts was evaluated in OER and HER processes. The obtained results showed that MnCo (7.5) catalyst exhibits suitable bifunctional behavior for OER and HER. The MnCo catalyst (7.5) showed the low overpotentials of 240 mV and 319 mV (versus RHE) at the current densities of 10 mA cm−2 and 100 mA cm−2 respectively, in the OER process. Also, in the HER process, the catalyst showed the low overpotentials of 82 mV and 216 mV (versus RHE) at the current densities of 10 mA cm−2 and 100 mA cm−2 respectively. Also, the electrochemical behavior of MnCo (7.5) was evaluated in the overall water splitting in a two-electrode system, which showed the cell voltage of 1.6 V and 1.85 V at the current densities of 10 mA cm−2 and 100 mA cm−2 respectively. The results showed that the presence of the different amounts of NH4F can control the morphology and electrochemical properties of the catalysts. Also, the presence of F- in the structure of the prepared catalysts can increase the polarity of the chemical bonds and thus the ability of the catalysts to adsorb hydroxide and other oxygen intermediates in the OER process. According to the obtained results, MnCo (7.5) with rich active sites showed superior electrocatalytic activity for OER and HER compared to other catalysts
  9. Keywords:
  10. Electrocatalysts ; Water Electrolysis ; Linear Sweep Voltammetry ; Hydrogen Evolution Reaction ; Nickel/Cobalt Layered Double Hydroxide ; Cobalt-Zinc Star-Like Structure ; Oxygen Evolution Reaction (OER)

 Digital Object List

 Bookmark

...see more