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Unraveling the Beneficial Electrochemistry of IrO/MoO Hybrid as a Highly Stable and Efficient OER Catalyst
Muhammad Tariq, Waqas Qamar Zaman, Wei Sun, Zhenhua Zhou, Yiyi Wu, Li-mei Cao, and Ji Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04266 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018
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Unraveling the Beneficial Electrochemistry of IrO2/MoO3 Hybrid as a Highly Stable
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and Efficient OER Catalyst
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Muhammad Tariqa, Waqas Qamar Zamana, Wei Suna, Zhenhua Zhoua, Yiyi Wua, Li-mei Caoa, Ji Yanga*
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a
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Chemical Processes, School of Resources and Environmental Engineering, East China University of
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Science and Technology, 130 Meilong Road Shanghai 200237, P.R. China
State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on
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* Correspondence: J. Yang, Fax: +86-21-64251668; Tel.: +86-21-64251668; E-mail address:
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[email protected] 12 13 14 15
KEY WORDS: Water electrolysis, Iridium oxide, Electrocatalyst, Mixed oxide composite,
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Molybdenum oxide, Oxygen evolution reaction, Water splitting
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ABSTRACT
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Minimization of noble metal contents along with enhancement in electrochemical properties with
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high durability is a major challenge to be overcomed for commercializing water electrolyzers
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and cheap energy storage devices. Sluggish kinetics of oxygen evolution reaction (OER) within
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the electrolytic cell and high energy demand to form O=O bond have attracted more
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responsiveness to this area. We reported OER beneficial mixed oxide composite of molybdenum
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and iridium oxides by facile hydrothermal method. Adhered IrO2 nanoparticles on MoO3 large
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particles, synergistically possessed robust nature towards harsh acidic water electrolysis as
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compared to alkaline environment. Mass specific OER activity of iridium active centers was
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greatly enhanced by seven folds, twice the current density and was attributed to electronic
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modulation of noble metal. Enhanced surface area and existence of highly oxidative species in
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O-1s spectra of IrO2 and two doublet regions in XPS spectra of molybdenum metal were found,
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accountable for robust performance. Prepared composite possessing only 30% molar fraction of
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noble metal presented the excellent long term stability for 40,000 seconds. Reduction in tafel
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slope from 57 to 77 mV dec-1 for IM-30 and IrO2 respectively had been observed. Conducted
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research will open new avenues for more applications of molybdenum oxides and their
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derivatives for water splitting.
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INTRODUCTION
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Due to increasing depletion rate of carbon based fossil fuels such as natural gas, petroleum and
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coal, there is an urgent need of clean, cost effective, affordable, reliable, most importantly
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renewable and sustainable energy resources in the current scenario as well as for the future
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perspectives.1-2 Electrochemical catalysts, solar energy conversions and rechargeable metal-air
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batteries plays a vital role in the quest of hydrogen based economy. Among numerous advanced
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technologies, electrocatalysis have gained great importance in scientific community. There is
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still research going on from decades to enhance the overall efficiency, activity, stability, storage
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capacity of various storage devices as well as the cost effectiveness of various electrocatalysts.
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Nano electrochemical catalytic materials provide unique properties with reference to electrolytes
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and electrodes in the range of energy storage devices.3-4 Oxygen evolution/water oxidation
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reactions are the major reactions in overall water splitting processes and should be addressed in
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detail for the improvement of electrocatalytic characteristics and long term stability.5 Due to
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sluggish kinetics in oxygen evolution reactions for the formation of O=O bond followed by the
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destruction of O-H bond, high anodic overpotential and scarcity of expensive noble metals,
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commercialization of polymer electrolyte membrane (PEM) water electrolyzers on industrial
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scale have been greatly obstructed. Production of carbon free hydrogen, fabrication of cost
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effective, efficient and stable OER catalyst is of global concern in the field of electrochemistry.6-
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8
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their derivatives via doping or catalyst assistance in the form of mixed metal oxides are two most
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viable approaches in order to achieve the above goals. Formation of mixed oxide composites at
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the expense of lesser precious metal contents lead to the enhancement in activity and stability
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towards acidic and alkaline environment.9-12
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Numerous research articles could be found that molybdenum sulfides, carbides and their
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derivatives are used extensively as a stable and active hydrogen evolution reaction (HER)
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catalyst in acidic and alkaline conditions.13-15 However, according to the best of our knowledge,
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there is no significant research work showing the molybdenum oxides and their derivatives as
To reduce the expensive noble metal contents i.e iridium (Ir) and ruthenium (Ru) oxides and
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OER catalysts. Only, very rare work showed the partial utility of Mo oxides in OER
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phenomenon by modified adams’ fusion method.16 So, there exists a gap to study the
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effectiveness of molybdenum oxides and their derivatives as an OER catalyst. Additionally, the
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strong binding interaction of non-noble metal oxides with nanoparticles of noble metal oxides
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have been contemplated as one of the promising approach for effective and efficient utilization
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of noble metals.12,
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support interaction (MMOSI) in rendering robust electrocatalysis towards OER, effect of
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molybdenum oxide as a support material for active IrO2 could be intrigued.7, 18-19 In case of
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mixed metal oxides, increase in Brunauer–Emmett– Teller (BET) surface area, enhancement in
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charge transfer kinetics of electrocatalysts, dispersion of noble metal nanoparticles in the
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solution and presence of different hydroxo species followed by conversion into oxo species
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ensures the improvement in electrochemical properties of the mixed oxide electrocatalysts.12
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Pfeifer, V., et al research work show that high surface area, presence of highly oxidative oxygen
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species O22-/O- and discoveries of electrophilic species OI- and OII- in the iridium based OER
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catalysts favors the enhancement in oxygen evolution to the electrolytic cell.20-21 Formation of
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noble metal rich regions onto the surfaces of non-noble metal oxides are responsible for
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improved conductivity of the materials. All of these OER beneficial consequences from mixed
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oxide composite lie well in line with the early reported research works on use of percolation
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theory for modeling the electrical conductivity of different mixed oxide composites.12, 20, 22-24
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Numerous technological applications of molybdenum oxides and their derivatives attract the
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researcher’s interests in the field of electrochemistry.25-26
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In the present study, we prepared IrxMo1-xOδ (0.1 ≤ x ≤ 0.5) as a mixed metal oxides OER
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catalysts in acidic conditions as confirmed by X- ray diffractory (XRD), Energy dispersive X-ray
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Due to unveiling of influential and critical role of metal/metal-oxide
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spectroscopy (EDS), Scanning electron microscopy (SEM), Transmission electron microscopy
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(TEM), High-angle annular dark-field scanning transmission electron microscopy (HAADF-
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STEM) and Energy dispersive X-ray ( EDX) element mappings images by using well known
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facile hydrothermal method. We found, mixed oxide with only 30% mole fraction of noble metal
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iridium contents as the best composite, showing the outstanding electrochemical catalytic
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characteristics. In-situ X-ray photon electron spectroscopy (XPS) analysis of as-prepared
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composite reveals that iridium and molybdenum were in their highest oxidation states, i.e., +4
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and +6 oxidation states respectively. Enhancement in electrochemical properties and long term
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stability has been observed due to synergistic effect of both metal oxides, establishment of
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excellent conductive network between iridium and molybdenum oxide particles, exposure of
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more active sites of noble metal and difference in Pauling scale electronegativity between Mo
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(2.16) and Ir (2.2) metals followed by electronic modulation in O-1s and Ir-4f spectra of IrO2 as
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confirmed by XPS. It reduces the 70% mole fraction contents of noble metal and revealed five
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times enhancement in BET surface area, two fold increase in current density, seven times
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enhancement in bulk mass specific activity, reduction in overpotential as compared to state of the
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art IrO2 catalyst and excellent stability up to 40,000 seconds under harsh acidic conditions. We
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believe that our conducted research would open the new avenues for molybdenum oxides and
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their derivatives for overall water splitting processes in the field of electrochemistry.
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EXPERIMENTAL SECTION
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Reagents. Ammonium molybdate tetra-hydrated ((NH4)6Mo7O24.4H2O), Iridium chloride tri-
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hydrated (IrCl3.3H2O), hexa-methylene tetramine ((CH2)6N4), ethyl alcohol (C2H5OH), isopropyl
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alcohol (C3H7OH), oxalic acid (C2H2O4) and 5 wt% Nafion were purchased from Aladdin
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industrial corporation. All the chemicals were analytical graded commercially and used without
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any further purification.
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Synthesis of IrO2-MoO3 Composites. IrO2-MoO3 mixed oxide composites were prepared by
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using well known facile hydrothermal method followed by annealing at 450oC with a dwell time
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of 360 minutes in order to produce the good crystals of the composites. Different stoichiometric
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amounts of iridium chloride tri-hydrated (56.718 mmol L-1) were mixed with ammonium
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molybdate tetra-hydrated (136.69 mmol L-1) followed by the addition of 5 mL hexamethylene
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tetramine, 2 mL anhydrous ethyl alcohol and 10 mL de-ionize water in 40-mL Teflon-lined
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pressure vessels. Then, the mixture was loaded in an oven at 180oC for the time period of 720
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minutes for hydrothermal treatment. After cooling to room temperature, resulting mixture was
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filtered and washed five times with de-ionized water in order to remove unwanted particles
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followed by drying at 80oC for 1 hour. Then, dried rentate was transferred into petry dish
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followed by annealing at 450oC for 360 minutes in order to get excellent crystals of prepared
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composites. In the last, the product was grinded to make fine powders for further use in
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electrochemical measurements and material characterization.
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Material characterization. For
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Nitrogen adsorption-desorption analysis were performed by using Micromeritics Tristar II 3020
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equipment after pretreatment outgas at 200oC for 2 hours followed by the cooling to room
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temperature. The visualization of nature of crystal structure type and the other associated
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properties were investigated by X-ray diffraction (XRD) using a D/max2550 V apparatus with a
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Cu-Kα radiation source (λ=1.5406 Å) with a step size of 0.02o, data were recorded over a range
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of 10o to 80o. The morphologies of the as- prepared composites were perceived by using field
Brunauer–Emmett– Teller (BET) surface areas (m2 g-1),
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emission scanning electron microscope (FESEM) and scanning electron microscope (SEM)
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equipped with a Nova NanoS and JSM-6360LV. To confirm the compositions as in the
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precursors, energy dispersive X-ray (EDX) spectrometer was used with TEAMApollo system. In
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order to examine the transmission electron microscopic (TEM), high resolution transmission
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electron microscopic (HRTEM) images, we use a JEM-2100 transmission electron microscope
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machine and in order to distinguish different particles for High-angle annular dark-field scanning
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transmission electron microscopy (HAADF-STEM) and Energy dispersive X-ray ( EDX)
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element mappings images, we use FEI Talos F200X machine. The surface characteristics and
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oxidation states of the as-prepared composites were determined by X-ray photoelectron
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spectroscopy (XPS) using ESCALAB 250Xi instrument with AL-Kα radiation source followed
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by 0.05 eV as an energy step size for high resolution XPS spectrum. The samples were sputter
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coated with carbon, and all the corresponding spectras were calibrated at a binding energy value
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of 284.6 eV with respect to C-1s spectra. The valance bond spectra (VBS) was received by
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getting from lower binding energy portion (-10 eV to 33 eV) of XPS full spectrum.
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Electrode fabrication. In this study, electrode used for electrochemical measurements were so
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called dimensionally stable anode (DSA) type, fabricated as follows, 6-7 mg of as-prepared
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composites in finally powdered form were taken in vial of 1.5 mL followed by the addition of
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1mL isopropyl alcohol, 0.5 mL deionized water, 5 wt% Nafion (15 µL). The resulting mixture
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was ultrasonicated for 60 minutes in order to form homogeneous ink. Then, 7.5 µL of ink were
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deposited on 0.5 cm × 1.5 cm Ti plates, etched in 10% oxalic acid for almost two hours near its
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bubble point and then washing several times with deionized water. This process was repeated
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five times to get the 0.2 mg cm-2 , mass loading of as prepared composites followed by annealing
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at 80oC for 10 minutes on each cycle in order to stabilization of catalyst powder on the Ti plate.
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Afterwards, all the electrochemical measurements were performed in steady state three-electrode
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type electrolytic cell. The exposed area of working electrode in electrolytic cell was 0.35 cm2
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(0.5 × 0.7cm) as remaining part was insulted leaving a small portion for the connection of the
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wire. Herein, for counter electrode, we used saturated calomel reference electrode (SCE),
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cleaned and polished Pt foil with a conducting area of 1.5 × 1 cm.
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Electrochemical measurements. The obtained electrode potentials from SCE scales were
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converted into the reversible hydrogen electrode (RHE) after calibrating by using following
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equation; E(RHE) = E(SCE)+Ej=0. (1) Furthermore, the overpotential (η) values were calculated
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as follows; η = E(RHE)-1.229-iR.(2) Where “i” is the measured current and “R” is the
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uncompensated ohmic electrolyte resistance measured via high frequency AC impedance
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(PARSTAT 2273) with 10 mV amplitude at open circuit value in 0.1 M HClO4 solution. Before
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collecting the data for linear sweep voltammetry (LSV) and polarization data at different scan
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rates, working electrodes were recycled several times until the overlapping of forward and
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backward lines observed. Bulk mass specific activity (A g-1) was calculated by using current
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density “j” (mA cm-2) divided by electrocatalyst mass loading “m” (0.2 mg cm-2).The tafel plot
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data were recorded by staircase voltammetry method at different potential window (vs. RHE)
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with 10 mV step size (scan rate 0.1 mV sec-1) followed by the current values evaluated at the end
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of each step. All the chemicals were analytically graded and solvent used was deionized water.
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RESULTS AND DISCUSSION
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Iridium based molybdenum mixed oxide composites are described as IM-x% where IM
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representing the iridium and molybdenum oxides respectively while x showing the molar %
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contents of iridium in the solution. The structure of the as-prepared composites were first
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characterized by scanning electron microscope (SEM). SEM images of as-prepared composites
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are shown in figure 1 & S1. In these micrographs, small particles were assigned to IrO2 crystals
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while plate like large particles were assigned to MoO3 crystals as shown in figure S1(a-b).
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Formation of IrO2 nanoparticles on the surfaces of molybdenum oxide crystals are supposed to
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be mixed oxides composites as shown in figure 1(a-b) and S1 (c-f) in line with the previously
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reported different articles.7,
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microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray
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photoelectron spectroscopy (XPS), X-ray diffractometry (XRD) and energy dispersive
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spectroscopy (EDS) analysis were executed. Figure 1c shows the TEM image of IM-30,
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signifying the mixed oxide crystals, adhering of IrO2 nanoparticles on the surface of plate like
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large MoO3 particles. While, figure 1d reveals the HRTEM image of IM-30 (insert shows the
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inverse fast fourier transform (FFT) pattern of (110) phases), lattice fringe spacing of both metal
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oxides ensuring the presence of both metal oxides which is in line with XRD analysis, EDS
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spectra, HAADF-STEM and elemental mapping as described below.17 HRTEM images of IrO2
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and IM-30, describing the lattice fringe spacing of both metal oxides of different phases are
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shown in figure S2 (a-b).
12, 27
In order to prove this hypothesis, transmission electron
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Figure 1. (a-b) SEM images of IM-30 at different magnifications (c-d) TEM images of IM-30.
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Inset in d shows the inverse fast fourier transform (FFT) pattern of (110) phases and d-spacing of
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(110) planes of Ir and Mo oxides in IM-30 composite.
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The X-ray diffraction (XRD) analysis of IM-30 elucidated us the mixed oxide crystals of MoO3
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and IrO2 as shown in figure 2a confirming the well agreement with the previous reported
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works.12, 28 Different phases were identified coinciding with JCPDS cards Nos. 15-0870 and 85-
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2407 for IrO2 and MoO3 composites respectively. Different planes of MoO3 at diffraction angles
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(2ϴ), 12.710, 23.020, 25.830 and 27.410, indexed as (001), (100), (002) and (011) respectively
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overlapped with less intense IrO2 diffraction patterns in IM-30 composite as described in figure
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2a. Three planes of pure IrO2 crystals indexed as (101), (200) and (201), at diffraction angles of
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34.710, 39.860 and 54.020 respectively coincided with different peaks in IM-30 composite as
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shown in figure 2a proving the existence of both metal oxides in IM-30 as also revealed by TEM
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and SEM images discussed above. XRD patterns of all prepared composites of IrxMo1-xOδ (0.1 ≤
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x ≤ 0.5) depicted the mixed oxide crystals of IrO2 and MoO3 as shown in figure S3. Energy
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dispersive spectra (EDS) of IM-30 further validated the presence of Ir and Mo metals as
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presented in figure 2b.17 Furthermore, energy dispersive X-ray spectroscopy (EDS) spectra of all
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as-prepared composites of IrxMo1-xOδ (0.1 ≤ x ≤ 0.5) is shown in supplementary figure S4
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showing increase in the intensity of peaks for iridium and decrease for the molybdenum which
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was in consistent to precursor amounts. Results obtained from nitrogen adsorption-desorption
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isotherms showing the BET surface areas (m2 g-1) of prepared composites were, 11.53, 3.32 and
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54.30 m2 g-1 for IrO2, MoO3 and IM-30 respectively as shown in table S1. Five times increased
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in surface area of IM-30 as compared to state of the art IrO2 catalyst and sixteen times increase in
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surface area as compared to non-noble metal oxide MoO3 prepared under the same conditions
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ensured the improvement in electrochemical properties of IM-30 composite as discussed later.
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After 30% molar concentration of iridium oxide, the surface area started decreasing due to
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increasing number of large size particles of molybdenum oxide crystals coincided with the
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previously reported works.12, 29-30
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Figure 2. XRD patterns of different as-prepared composites (a) for IrO2, IM-30, and MoO3 (b)
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EDS spectra for IM-30
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In order to clearly distinguish the iridium and molybdenum oxide particles, HAADF-STEM and
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EDX elemental mapping have been shown in figure 3 and figure S5. Figure 3(a-b) representing
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the HAADF-STEM image of IM-30 at different positions and different magnifications. Small
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bright particles are assigned to IrO2 and large particles are assigned to molybdenum particles as
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discussed and shown in SEM and TEM images above. In order to make this hypothesis stronger,
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EDX-elemental mapping is shown in figure 3 (c-e). Figure 3 c further clarifying the iridium rich
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regions on the surface of molybdenum large particles along with the figure 3 (d-e) showing the
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colors of individual iridium and molybdenum metals which is in line with the literature.12, 28, 31
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Moreover, figure S5 also ensures the formation of small bright particles on the surface of large
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particles of molybdenum oxide followed by the EDX elemental mapping at different positions.
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Figure 3. HAADF-STEM and EDX element mappings at different positions and magnifications
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for IM-30
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High resolution X-ray photoelectron spectroscopy (XPS) of Mo-3d core level spectra shows the
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dual oxidation states of the Mo as shown in figure 4a. Due to several contribution of oxidation
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states, Mo-3d region could be de-convoluted according to oxidation states and each one
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composed of doublet region between 3d3/2 and 3d5/2 peaks in IM-30 composite. There were two
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doublet regions of Mo5+(3d5/2), Mo5+(3d3/2), Mo6+(3d5/2) and Mo6+(3d3/2) peaks, located at
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binding energy values of 232.20, 235.34, 233.13 and 236.25 eV respectively, suggesting the
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presence of hydroxyl groups or loss of oxygen atom from MoO3 crystals to the electrolytic cell
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prepared by hydrothermal route.11, 32 Two major peaks appearing at 233.13 eV and 236.25 eV
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were assigned to Mo6+ species, showing that molybdenum metal was in its highest oxidation
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state in IM-30 composite that is well coincided with previously reported works.33-36 Formation of
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MoO3 crystals are further confirmed by O/Mo ratio which was 3.3, calculated from the XPS
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analysis which is coinciding with the stoichiometric ratio and also validated by early reported
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research works.37 High resolution XPS spectra of Mo-3d for IrxMo1-xOδ (0.1 ≤ x ≤ 0.5) as-
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prepared composites and corresponding pure oxide are shown in figure S6. It can be clearly seen
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the doublet regions in mixed oxide composites as compared to pure MoO3. High-resolution
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spectra of Ir-4f depicts the change in binding energy for Ir4+(4f7/2) from 62.44 eV to 62.15 eV
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and for Ir4+(4f5/2) from 65.37 eV to 65.14 eV as shown in figure 4b, confirming the electronic
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structure modulation of noble metal spectra in highly active and stable prepared composite, i.e,
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IM-30 as coincided with previously reported works.
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are further confirmed by O/Mo ratio which was 3.3, calculated from the XPS analysis which is
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coinciding with the stoichiometric ratio and also verified by XRD results discussed previously.
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High resolution XPS spectra of Ir-4f for IrxMo1-xOδ (0.1 ≤ x ≤ 0.5) prepared composites and pure
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IrO2 are shown in figure S7. Due to Pauling scale electronegativity difference of Ir (2.20) and
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Mo (2.16), partial polarization among Mo-O-Ir and shifting of electron density towards the
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iridium results in enhanced conversion of oxy-hydroxyl species to oxygen on the surface of
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molybdenum oxide crystals responsible for long term stability and activity as described by
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earlier reported work
9, 39
9-10, 21, 24, 38-39
Formation of MoO3 crystals
. Due to partial support of non-noble metal oxide crystals i.e, MoO3,
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enhanced BET surface area of mixed oxide crystals and spillover of highly oxidative species (O*,
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OH*,OOH*) from noble metal oxides as shown in figure 4c to the electrolytic cell confirms the
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highly improvement in electrochemical properties of IM-30 with respect to state of the art IrO2
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catalyst as described in figure 5. Figure 4c shows the disappearance of minor peaks in O-1s
274
spectra and change in binding energy from 530.9 eV to 530.30 eV for the best prepared
275
composite i.e. IM-30 as compared to pure IrO2. It might have occurred due to the presence of
276
oxygen defects in O-1s spectra of IrO2 results in partial polarization of molybdenum atom with
277
adsorbed oxygen in IrO2
278
IrO2 as reported previously, electrophilic attack on these species results in spillover of oxygen
279
evolution to the electrolytic cell followed by the improvement of electrochemical nature of
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mixed oxide crystals as shown in figure 5 that is coincided with previous reported research
281
works.20, 24, 40
23
or presence of OI- and OII- electrophilic species in O-1s spectra of
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Figure 4. High resolution XPS spectra of the (a) Mo-3d in IM-30 as-prepared composite and (b-
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c) showing the spectra of Ir-4f and O-1s in IM-30 & IrO2 respectively. All the values were
285
calibrated at 284.6 eV with respect to C-1s spectra.
286 287
For the measurement of electrochemical properties of the as-prepared composites, steady state
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standard three electrode type electrolytic cell was used in 0.1M HClO4 solution. Further, detail
289
for electrode fabrication and electrochemical measurements had been discussed in experimental
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section. Linear sweep voltammetry (LSV) study demonstrated the superior activity of IM-30
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composite as compared to pure IrO2 and MoO3 catalysts in acidic media towards OER. The
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synergistic effect of both metal oxides exhibiting the interfaces in IrO2/MoO3 were one of the
293
main factor leading to enhancement of the electrochemical properties and long term stability.41-42
294
Prominent increase in BET surface area of IM-30 as compared to pure IrO2 and MoO3 as listed
295
in table S1 is also responsible for improvement in electrochemical properties of the mixed oxide
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composite.28, 43 Oakton, Emma. et al. prepare the IrO2-TiO2 mixed oxide composite and concludes
297
that 40% content of noble metal shows the best composite for electrochemical performance.12 It
298
may happen due to high conductivity of iridium metal, establishment of excellent conductive
299
network between iridium and molybdenum oxide particles, exposure of more active sites of
300
noble metal and a rationale for this phenomenon provided by percolation theory.28, 44 This unique
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interfacial affect eases the electron transportation, which is compulsory for electrocatalysis and
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provides here the electron transport channel between IrO2 and MoO3 crystal particles, and is
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speculated as the possible hypothesis for synergistic affect along with the electronic modulation
304
of O-1s, Ir-4f and Mo-3d spectras of in IM-30 composite shown in figure 4 as well as other
305
prepared composites as shown in figure S6, S7.
306
curve of IM-30, MoO3 and IrO2 composites. IM-30 composite shows the current density of 25.2
307
mA cm-2 which is almost two times higher than the benchmarking IrO2 catalyst (j =11.7 mA cm-2)
308
due to the electronic modulation and presence of highly oxidative oxygen species (O*,
309
OH*,OOH*) or OI- and OII- electrophilic species as discussed earlier by XPS shown in figure 4.
310
Due to the formation of excellent conductive path, enhancements in BET surface areas of all
311
mixed oxide composites and maximum exposure of active sites of the noble metal makes the IM-
312
30 best composite among all other prepared composites as shown in figure S8. From figure S8,
42, 45-46
Figure 5a shows the current density (j)
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IM-10 and IM-50 shows the less activity as compared to pure iridium oxide due to less
314
synergistic effect, poor conductive network, weak electronic transport channel and small
315
percolation limit as described in the literature.28, 42, 47 On the other hand, activity of pure MoO3
316
was recorded as zero under the same experimental conditions. Indeed, pyrolization of ammonium
317
molybdate tetra-hydrated ((NH4)6Mo7O24.4H2O) and hexa-methylene tetramine (CH2)6N4 slowly
318
generates the OH- and MoO42- ions simultaneously followed by the formation of small fragments
319
of metal molybdate and hydroxides.
320
rich active sites for the successful OER processes, high valance metal Mo6+ plays the decisive
321
rule and electronic modulation of iridium metal results in synergistic effect of both metals oxides
322
along with the improvement in electrochemical properties and long term stability. The potential
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required at current density value of j= 10 mA cm-2 is a very important parameter due to its close
324
relevancy with solar fuel synthesis and different energy storage devices. At this current density
325
value, reduction in 110 mV potential for IM-30 as compared to bench marking IrO2 catalyst
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makes it more valuable suitability for the aforementioned applications. Polarization curves with
327
iR corrected for OER activity of all prepared composites (IrxMo1-xOδ (0.1 ≤ x ≤ 0.5)) in acidic
328
solution (0.1M HClO4) w.r.t catalyst loading of 0.2 mg cm-2 on Ti Plate at constant potential of
329
1.68V vs. RHE is shown in figure S8. Figure 5b shows the tafel slope, 57 mV dec-1 and 77 mV
330
dec-1 for IM-30 & IrO2 as-prepared composites respectively. This reduction in tafel data ensures
331
the increasing rate of oxygen evolution in the electrolytic cell which confirms the improved
332
catalytic activity and long term stability of IM-30 composite. Many investigations on IrO2
333
electrocatalyst shows the tafel slope around 60-70 mV dec-1 which is lower than our current
334
study. This is because of that tafel plot data mainly depends upon the particle size of the
335
composite, nature of electrolyte, mass loading of catalyst, and type of precursors and effective
48-49
Activation of classical oxy (hydroxide) OER catalysts,
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area of electrode.10 Difference in acidity of electrolyte (0.5M H2SO4 vs 0.1M HClO4) and mass
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loading of (4-6 mg cm-1 and 0.2 mg cm-1) as well as using hexamethylene tetramine (HMT) in
338
the precursor, results in enhancement of tafel slope for IrO2 in this present study.
339
In figure 5c, it is clearly observed that IM-30 shows the seven times enhancement of bulk mass
340
activity for iridium active centers as compared to state of the art IrO2 catalyst prepared under the
341
same experimental conditions. It illustrated this superb mass specific activity along with 70%
342
mole fraction reduction of precious iridium metal contents as compared to IrO2 catalyst and
343
uncompromised stability for 40, 000 seconds as shown in figure 5d under harsh acidic conditions
344
at an overpotential of 450 mV. Polarization curve of IM-30 shows the negligible drop in OER
345
activity of best performed composite before and after the chronopotentiometric test as linear
346
sweep voltammetry (LSV) curves almost overlapped as shown in figure S9. While, the stability
347
test curves of individual molybdenum and iridium oxides in acidic environment are shown in
348
figure S10. It clearly shows highly unstable behavior in acidic environment as in line with the
349
literature.50
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Figure 5. (a) Polarization curves with iR corrected for OER activity of all prepared composites
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(IrxMo1-xOδ (0.1 ≤ x ≤ 0.5)) in acidic solution (0.1M HClO4) w.r.t catalyst loading of 0.2 mg cm-2
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on Ti Plate at constant potential of 1.68V vs. RHE. Horizontal dash line representing j= 10 mA
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cm-2 b-c) indicating the tafel plot and bulk mass activity of IM-30 and bench marking IrO2
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composites respectively d) stability test of IM-30 at j=10 mA cm-2
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The electrochemical performance of all as-prepared composites are tested in basic solution
358
(0.1M NaOH solution) and all the electrochemical properties are reported in figure 6 and figure
359
S11. Figure 6a representing the values of current densities as 22.21, 11.47 and 0.49 mg cm-2 for
360
IM-30, pure IrO2 and pure MoO3 respectively at constant potential of 1.73V.The potential
361
required to achieve the current density value of 10 mg cm-2 are 360 mV and 470 mV for IM-30
362
and pure IrO2 respectively. This increase in activity and reduction in tafel slope from 470 mV to
363
360 mV of mixed oxide composite might have occurred due to enhancement in BET surface area
364
as compared to pure iridium and molybdenum oxides as shown in table S1, establishment of
365
excellent conductive network between iridium and molybdenum oxide particles, exposure of
366
more active sites of noble metal and electronic modulation of Ir-4f, O-1s and Mo-3d spectras as
367
discussed above. Bulk mass specific activity of iridium metal active centers was greatly
368
increased by five times for IM-30 composite as compared to pure iridium oxide51 due to increase
369
in activity of the composite as shown in figure 6b. Activities of pure iridium and molybdenum
370
oxides were recorded as much lower than that of IM-30 as shown in figure 6a. Due to the
371
synergistic effect42 of both pure oxides could be one of the factor for the enhancement of
372
activity of IM-30. Polarization curves with iR corrected for OER activity of all prepared
373
composites (IrxMo1-xOδ (0.1 ≤ x ≤ 0.5)) in basic solution (0.1M NaOH) w.r.t catalyst loading of
374
0.2 mg cm-2 on Ti Plate at constant potential of 1.73V vs. RHE is shown in figure S11.The
375
activities of IM-10 and IM-50 are lower than the activity of pure iridium oxide due to the less
376
synergistic effect, poor conductive network, weak electronic transport channel and small
377
percolation limit, less amount of noble metal as described in the literature.28, 42, 47 The increasing
378
activity of composites continues up to 30% noble metal contents then starts decreasing due to the
379
less conductivity of the mixed oxide composites and decreasing the BET surface areas of the
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composites as coinciding with early reported research works.12, 28 The stability curves of the
381
pure IrO2 and IM-30 is shown in figure 6(c-d) and they are not stable in basic media in line with
382
early reported research works.50, 52 The polarization curves for pure iridium oxide and IM-30 did
383
not show any consistency/overlap of curves conducted before and after the stability test further
384
ensuring the instability of the iridium oxide and IM-30 composites in basic media as shown in
385
figure 6 (e-f).
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Figure 6. (a) Polarization curves with iR corrected for OER activity of three as-prepared
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composites in basic solution (0.1M NaOH) w.r.t catalyst loading of 0.2 mg cm-2 on Ti Plate at 23 ACS Paragon Plus Environment
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constant potential of 1.73V vs. RHE. Horizontal dash line representing the value of current
390
density j= 10 mA cm-2 (b) bulk mass activity of all prepared composites (IrxMo1-xOδ (0.1 ≤ x ≤
391
0.5)) (c-d) indicating the durability test of pure IrO2 and IM-30 composites respectively (e-f)
392
Polarization curves with iR corrected for OER activity of pure IrO2 and IM-30 composites
393
respectively before and after the stability test in 0.1M NaOH solution.
394 395
Overall, in this current study, we presented the superb electrochemical performance and long
396
term stability of hydrothermally prepared MoO3 and IrO2 mixed oxide composite in acidic
397
environment as compared to alkaline environment. Adhered IrO2 nano sized particles on the
398
MoO3 plate like large particles were duly confirmed by SEM and TEM images, HAADF-STEM
399
and EDX elemental mapping along with XRD analysis and EDS spectra. Due to the synergistic
400
effect of Mo and Ir oxides, establishment of excellent conductive network between iridium and
401
molybdenum oxide particles, exposure of more active sites of noble metal, electronic modulation
402
along with the MMOSI and presence of different electrophilic species and spillover of highly
403
oxidative oxygen species to the electrolytic cell results in improvement of electrochemical
404
properties of IM-30 as described earlier in figure 5. We believe that this work would further
405
enhance the utilization of molybdenum oxide and their derivatives in the field of
406
electrochemistry with excellent reduction of noble metal contents tremendously improving the
407
intrinsic activity of mixed metal composites.
408 409 410 411
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CONCLUSIONS
413 414
In summary, novel IrO2/MoO3 mixed oxide composite is prepared for beneficial OER
415
phenomenon first time ever by facile hydrothermal route in acidic environment and alkaline
416
environment with significant reduction of 70% mole fraction of noble metal contents. Mixed
417
oxide composite, IM-30 shows excellent stability and activity in acidic environment. Interfacial
418
support interactions and enhanced surface area of Mo and Ir mixed oxides, establishment of
419
excellent conductive network between iridium and molybdenum oxide particles, exposure of
420
more active sites of noble metal, spillover of highly oxidative species to the electrolytic cell and
421
electronic modulation of noble metal lead to the exhibition of excellent improvement in
422
electrochemical characteristics and long term durability of mixed oxide composite. Tafel slope
423
was observed as 57 mV dec-1 in relative to 77 mV dec-1 for IM-30 and IrO2 composites
424
respectively prepared under the same conditions. Best prepared composite displayed, two times
425
improvement in current density along with the seven fold increase in bulk mass specific activity
426
as compared to state of the art IrO2 catalyst. Moreover, it showed no significant loss in potential
427
even after a continuous OER process in dissipating 10 mA cm-2 for 40,000 seconds. We believe
428
that our conducted research would open the new avenue for the applicability of molybdenum
429
oxides and their derivatives in the field of electrochemistry which will be extremely helpful in
430
overall water splitting processes in future.
431 432 433 434
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ACKNOWLEDGMENTS
436 437
This research is based on work supported by the National Natural Science Foundation of China
438
(51778229). We would like to thank beamline BL14W1 (Shanghai Synchrotron Radiation
439
Facility) for providing the beam time.
440 441
SUPPORTING INFORMATION
442 443
SEM &TEM images, HAADF-STEM and EDX elemental mapping images, XRD patterns, EDS
444
spectras, BET surface areas, XPS spectras of Mo and Ir, polarization curves and stability test of
445
all as-prepared composites.
446 447
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SYNOPSIS: A novel IrO2/MoO3 mixed oxide composite with enhanced efficiency of oxygen
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evolution reaction and stability under acidic environment.
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