Effective Iridium Utilization On Combining With Non-noble Transition Metals For Efficient Electrocatalytic Water Splitting

A green environment in today's world of high pace industrialization is a subtle challenge for current generation.To cope with the energy demands of rapidly increasing population the conventional resources are being extremely utilized.These resources,like fossil fuels,after burning release hazardous pollutants in air.The resulting contaminated air then constitutes upon harmful particulates and greenhouse gases that enter into atmosphere,and cause severe adverse health effects for all the inhabitants.Therefore,to avoid environmental pollution,efforts are required for lessening the dependency on fossil fuels and to elevate energy harvesting from renewables.The proficient energy harvesting from renewables centers on the role of hydrogen in energy sector.Hydrogen energy is sustainable and can provide an environment friendly pathway to contribute towards future demand of energy supply and storage.It possesses the ability to cope with low and high energy generation periods from renewables via water electrolyzers and fuel cells,respectively.Unfortunately,the electrolytic hydrogen production is momentously inhibited by the sluggish oxygen evolution reaction(OER)at anode,which is the key half reaction for producing H2 fuel.OER is a complex process associated with the loss of four electrons while liberation of an O2 molecule,which requires potential much higher than the standard reaction potential(V=1.23 volts).Due to this surplus energy is consumed during electrochemical processes and it also affects the wide effectuation of renewable technologies.Currently,iridium based oxides,especially IrO2,are the only state-of-the-materials that withstand these problems for electrocatalytic hydrogen production.Unfortunately,iridium is very precious and expensive earth element;therefore,effective utilization of iridium remains a challenge to be overwhelmed for essentially demoting the dependency on fossil fuels and for promoting renewable technologies as primary energy source.In this doctoral study,we have reduced the noble iridium content by different routes on coupling with cheaper transition metals.All of these approaches present breakthrough advancements for the mass specific utilization of noble iridium and are briefed as below.(1)We prepared nickel and cobalt codoped rutile iridium based oxide and exemplified this approach as an extension to the currently known approach for doping IrO2.The synthesized material during study was both experimentally and computationally verified to possess greater flexibility for codoping.Solid solution with tremendously reduced iridium content was found intrinsically more robust than the pure IrO2.The superb rise in OER activity was found to originate due to widening of Ir-5d bands as responsible for improving the OER reaction kinetics.The codoped IrO2 reflected an overpotential of only 285 mV at a current density of 10 mA.cm-2,which is appreciably lower than 320 mV and 330 mV for individually doping cobalt and nickel,respectively.This approach reduces 50 atomic%of iridium than in IrO2 by substituting with cheaper transition metals.(2)Recently,some theoretical and experimental studies showed that IrOx possesses a change in electronic configuration at Fermi level in correspondence to different structural motifs.In this study we identified NiCo2O4 as a potential host structure for iridium that provides OER beneficial electronic manipulation.The novel existence of iridium in spinel host structure,as an OER catalyst,was duly confirmed from X-ray diffraction analysis and X-ray absorption spectroscopic analysis that depicted shortening of Ir-Ir bond length for neighboring IrO6 octahedral units.Specifically,composite comprising 20 mol%iridium in NiCo2-xO4 rendered acid stable 15 folds enhancement in mass specific OER activity relative to IrO2 resulting in substantial reduction of iridium content.Moreover,80 atomic%of iridium is reduced than in IrO2 by incorporating iridium into spinel structure.(3)In this study,we hydrothermally synthesized a mixed oxide comprising upon phases of Co3O4 and IrO2 that presented a core shell morphology.IrO2 being anchored at shell side robustly countered the acidic environment,while shielding the Co3O4 nanorods as core material,at a very minimal expense of 7%atomic iridium.A durable 20 folds enhanced mass specific activity was observed for the best mixed composite in relative to IrO2.The exceptional performance was due to synergistic effect of metal cations on account of their electronic modulations as revealed from XPS analysis and due to one dimensional nanorod shaped assembling of substrate particles.The opted route in this study enabled reduction of 93 atomic%iridium as compared to pure IrO2 marking as best approach for effective utilization of noble iridium.