Promotion Of Photoelectrochemical Water Oxidation Of Hematite In Acidic Electrolytes By Iridium Modification

With the constant growth of the global population and the ongoing development of industries,the demand for energy is increasing.Given the environmental issues and energy crisis stemming from the global consumption of fossil fuels,there is an urgent need to develop a sustainable energy system to replace traditional carbon-based fossil fuel energy.Amongst the many chemical fuels,hydrogen energy has great potential for application due to its high energy density,ease of transport and utilization,and the fact that the combustion product(H2O)is environmentally friendly with no greenhouse gas CO2 emissions.In addition,solar energy as a source of energy has already been achieved in several energy directions such as photocatalysis,photothermal synergy and solar cells,and it is believed that photoelectrochemical(PEC)hydrolysis will become an ideal technology for collecting and storing solar energy in the form of hydrogen.Although alkaline hydrolysis has been most widely used for hydrogen production due to its stability and low equipment cost,the hydrogen reduction(HER)reaction rate is two to three orders of magnitude lower than in acidic media.The overpotential for acidic hydrogen production is mainly due to the oxygen evolution reaction(OER)occurring at the photoanode,so the development of efficient and cost-effective OER catalysts under acidic conditions is a major challenge for all hydrolysis technologies.In order to improve the efficiency of the production of PEC hydrogen by water splitting,the development of a photoanode material that can be matched to the photocathode and be stable in acidic solutions has become one of the research priorities.Among the many photoanode materials currently available,hematite(α-Fe2O3)has a suitable band gap size and valence band position,and its elemental content is very large and inexpensive,making it one of the most suitable photoanode materials that can be used for PEC aqueous oxidation.However,the catalytic efficiency is unacceptably poor due to the strong complexation of electron-hole pairs in the bulk phase as well as on the surface,the short carrier lifetime,and the poor kinetics of the OER reaction.To make matters worse,the surface of hematite in acidic media is subject to severe adverse corrosion,destroying the crystal structure and making it impossible to maintain long-term operational stability.Therefore,we have used the most stable iridium-based material currently available for surface loading in acidic environments to accelerate the kinetics of water oxidation while protecting the surface structure of hematite.(1)Fully-dispersed IrOx Atomic Clusters Enable Record Photoelectrochemical Water Oxidation of Hematite in Acidic Media:To avoid price problems associated with high noble metal loading,we drew on the advantages of single-atom catalysis and used the immersion method to load the surface of hematite with fully dispersed atomic clusters consisting of a few atoms,which can fully expose the surface-active atoms for adsorption and conversion of reactants.The resultant photoelectrode exhibited substantially enhanced performance for water oxidation and a world-leading photocurrent of 1.35 mA cm-2 at 1.23 V vs RHE in acidic electrolytes was obtained,which was 2.7 times higher than the titanium-doped hematite photoanode.Theoretical simulations show that the improved performance is associated with modulation of electronic structure,including Fermi energy levels and d-band center,which significantly increases the charge separation efficiency and facilitates the conversion of intermediates from*O to*OOH.A series of electrochemical and structural studies have shown that the introduction of Ir leads to the greatly improved reaction kinetics for PEC water oxidation,with an increase of charge carrier density and the formation of a built-in electric field,facilitating the separation and transportation of the photogenerated carriers.(2)IrOx Nanoparticles Implanted onto Hematite Boosting Water Oxidation Activity and Stability in Acidic Solution:To address the application of hematite in acidic environments,a dense layer of IrOx nanoparticles has been electrodeposited onto the surface of hematite to improve performance while maintaining high stability for use in acidic environments.A stable photocurrent density of 1.71 mA cm-2 was obtained at 1.23 V vs RHE in an acidic electrolyte.Further investigation revealed that the numerous hydroxyl groups on the IrOx surface act as intermediary molecules during the reaction,providing additional active sites.In addition,the loading of IrOx introduces a variety of surface states that act as traps to capture photogenerated holes from hematite and enhance the water oxidation activity by rapidly transferring holes to the surface active sites for participation in the water oxidation reaction under the influence of fast surface charge transfer kinetics as well as low transfer resistance.