Laser Processed Materials For Electrocatalytic And Marine Applications

As the developing world industrializes,humanity needs to produce a more sig-nificant fraction of its energy via renewable resources to alleviate the scarcity of fossil fuels and environmental damage from these fuels'exhaust.However,renewable energy sources tend to be intermittent and require a method to store generated energy for use later.Hydrogen gas is a promising potential fuel be-cause it can be produced from the water with oxygen gas as a by-product,re-sulting in an environmentally-friendly production-consumption cycle with wa-ter as the product upon combustion.This thesis presents a different approach to hydrogen generation from water–using electricity with an earth-abundant metal oxide or hydroxide catalysts and concerns selection,production,and opti-mization of electrocatalytic materials to improve hydrogen efficiency electrolytic production by reducing operational costs.Water splitting consists of two half-reactions:water oxidation and hydrogen evolution.Both reactions rely on highly effective electrocatalysts.High-performance highly effective hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)catalysts are compelling for the conversion of renewable electricity to fuels and feedstocks.Platinum(Pt)and Ir/Ru oxides are the best-known catalysts for HER and OER respectively with the highest performance in the acidic media.The efficient cat-alysts from inexpensive and earth-abundant elements that operate in neutral(hence biocompatible)media could enable low-cost seawater splitting and the realization of bio-upgraded chemical fuels.This dissertation focuses on the in-vestigation of Laser generated electrocatalysts for the overall water splitting.We developed a range of new materials using a scalable,one pot and environmen-tally friendly pulsed laser ablation in liquids(PLAL)approach combined with electrophoretic deposition and ripples or laser-induced periodic surface struc-tures(LIPSS).As produced nanomaterials are used for hydrogen and oxygen generation using overall water splitting in alkaline media.The effects of differ-ent experimental parameters including laser pulse energy and laser pulse width(ns,ps,fs),laser scan speed,the nature of liquid media,time of ablation,and parameters of in-situ or ex-situ applied electric field on size,shape,morphology,composition,and crystalline parameters thoroughly studied using a range of metrological techniques.The nanomaterials are electrophoretically deposited on different substrates used as working electrodes for overall water splitting.Moreover,the catalysts were analyzed in detail by active site identification and mechanistic understanding of the reactions within the scope of each project.The role of the metal species on HER and OER activity was investigated using X-ray photoelectron spectroscopy(XPS).The discussion was further supported by density functional theory(DFT)calculations resulting in structure-activity cor-relations with emphasis on the importance of the nature of the metal.Besides,bimetallic catalysts with optimal hydrogen binding energies were suggested as a promising active catalyst toward HER and OER.This dissertation is an account of five detailed studies on developing highly effective low-cost electrocatalysts for both reactions and includes a preliminary attempt at system integration to build a functional electrochemical water splitting.The electrochemical proper-ties of the fabricated electrocatalysts are tested and following achievements can be concluded.Chapter one describes some general aspects regarding world energy demands and role of hydrogen energy to reduce environmental pollution also give an overview of electrochemical water splitting.Chapter two describes synthesis methods for electrocatalysts including mechanism,structural and electrochemi-cal characterization techniques.In chapter three,we present a single step and fast fabrication of ready-made Ni(OH)₂/NF electrocatalysts for overall water splitting using an entirely envi-ronment friendly and sustainable approach of electric field assisted pulsed laser ablation in liquid for generation of nanoparticles and simultaneous electrode-position of the NPs on Ni foam substrate.The as-fabricated electrocatalyst is found highly efficient and stable in alkaline medium for HER and OER kinetics with remarkably low overpotential of 187 mV(vs.RHE for HER)and 1.66 V(vs.RHE for OER)to reach current density of 20 mA/cm²with HER and OER Tafel slopes of 82 and 74 mV/dec,respectively.The two-electrode electrochemical cell made of the bifunctional Ni(OH)₂/NF electrocatalyst requires overpotential as low as 1.68 V to drive 10 mA/cm²current density over a long period.In chapter four,we introduce a single-step-single-pot ultrafast physical top-down approach of electric-field assisted pulsed laser ablation in liquid for si-multaneous control of sulfur vacancies in MoS_xnanoparticles and their in-situ di-electrophoretic deposition on nickel foam for preparation of ready-to-use MoS_x/NiF electrocatalysts on support in a few minutes for highly efficient over-all water splitting.The applied electric field efficiently controls S active sites in the electrocatalysts and consequently HER and OER performances.Both experi-mental and density functional theory calculations results show that a MoS_x/NiF electrocatalyst produced at a higher electric field possesses a larger density of S vacancy sites with a lower Gibbs free energy(?GH^*)for H^*adsorption.Con-sequently,it exhibits the high cathodic current density of 150 mA/cm²at an ultra-low overpotential of?310 mV with the Tafel slope as low as 66 mV/dec.Conversely,an electrocatalyst produced at a lower electric field exhibiting lower S vacancy sites is found more suitable for oxygen evolution reaction with the Tafel slope as low as 71 mV/dec.Importantly,when two MoS_x/NiF electrocata-lysts are assembled in a two-electrode cell,it requires only 1.63 V of potential to drive 10 mA/cm²of current density with excellent stability.In chapter five,we developed an innovative strategy to control electrocatalytic and morphological properties of CuMoO₄nanostructures for a range of appli-cations.In a typical procedure,a pulsed picosecond laser beam focused on MoS₂target surface at solid-liquid interface.Two parallel copper electrodes were mounted around the laser produced plasma(LPP)plume.The applied electric field across electrodes was varied to size and morphology of the MoS₂NPs the density of Cu ions that is being doped into MoS₂NPs.Pure MoS₂NPs at zero external electric field,and pure CuO nanostructures without laser ablation were also produced for comparison.The structural,compositional,and morpho-logical properties of as produced nanostructures were investigated using XRD,Raman,EDAX,and XPS techniques.Finally,the produced samples were tested for electrochemical water splitting.In chapter six,we report?45%reduction in the required electrical power to achieve a hydrogen generation rate of?3×10¹⁶molecules cm^-2s^-1)(current density:10 mA/cm²)via localization of electric field through laser-induced peri-odic surface structuring(LIPSS)of nickel foam(NF)electrodes.Finite element model is used to simulate the spatial distribution of electric field and under-stand roles of the LIPSS geometric parameters and size of nanoparticles on the surface of LIPSS in the electric field enhancement.Furthermore,the optimized LIPSS pattern electrodes are used as cathode and anode substrates to support electrocatalyst powders and localize electric field at the same time.Pt/C,the model HER electrocatalyst,powder loaded on the LIPSS patterned NF electrode demonstrated?40.4%reduction in the required electrical potential to drive10 mA/cm²of current density in hydrogen evolution reaction and 270%higher current density at 558 mV potential over a similar pristine NF substrate.Sim-ilarly,RuO₂powder,the model OER electrocatalyst,loaded on the LIPSS pat-terned NF substrate demonstrated 6.1%reduction in the?₁₀overpotential and211%enhancement in the current density at 1.98 V applied potential.Impor-tantly,when two LIPSS electrodes were assembled simultaneously as anode and cathode,it requires 330 mV of lower electric potential over a similar cell made of pristine NF electrodes to drive 10 mA/cm².Moreover,the capability of LIPSS patterned electrodes to operate at significantly reduced electric potentials are also demonstrated in a range of electrolytes including acidic,alkaline,neutral,and seawater.This work demonstrates a physical and versatile approach of elec-trode surface patterning to boost electrocatalytic fuel generation performance and can be applied to any metal and semiconductor catalysts for a range of electrochemical reactions.In chapter seven,we present a comparative study of the corrosion protection,performance,and durability of three different methods(fs-laser structuring,chemical etching,and hydrothermal)fabricated Al alloy against sea water.The corrosion-resistance of the hydrophobic/superhydrophobic surface fabricated on bare Al alloy were measured using polarization and electrochemical impedance spectroscopy(EIS)techniques,we found that the fs-laser structured superhy-drophobic surface significantly enhanced the corrosion resistance as compared to the other two methods.The fs-laser structured sample exhibit the corro-sion inhibition efficiency(CIE)of 99.9%in seawater.Moreover,Our sample showing good long-term stability in seawater.These two functionalities,namely hydrophobicity and corrosion resistance,together with the facile processing per-formed directly onto the bare Al alloy surface,without the need to deposit any coating,opens the way for the laser-based production of high-performance Al components for a variety of applications.Taking in consideration the results obtained and reported in this dissertation can be used as a roadmap for future catalyst optimization,particularly apply-ing these procedures to a high-performance catalyst.