Hybrid as a Highly Stable

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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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ACS Sustainable Chemistry & Engineering

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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]

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KEY WORDS: Water electrolysis, Iridium oxide, Electrocatalyst, Mixed oxide composite,

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Molybdenum oxide, Oxygen evolution reaction, Water splitting

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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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

266

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*,

271

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

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spectra and change in binding energy from 530.9 eV to 530.30 eV for the best prepared

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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

280

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

288

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

296

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

301

interfacial affect eases the electron transportation, which is compulsory for electrocatalysis and

302

provides here the electron transport channel between IrO2 and MoO3 crystal particles, and is

303

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)



Page 18 of 34

313

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

323

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

326

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,


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


336

area of electrode.10 Difference in acidity of electrolyte (0.5M H2SO4 vs 0.1M HClO4) and mass

337

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



Page 20 of 34

350 351

Figure 5. (a) Polarization curves with iR corrected for OER activity of all prepared composites

352

(IrxMo1-xOδ (0.1 ≤ x ≤ 0.5)) in acidic solution (0.1M HClO4) w.r.t catalyst loading of 0.2 mg cm-2

353

on Ti Plate at constant potential of 1.68V vs. RHE. Horizontal dash line representing j= 10 mA

354

cm-2 b-c) indicating the tafel plot and bulk mass activity of IM-30 and bench marking IrO2

355

composites respectively d) stability test of IM-30 at j=10 mA cm-2

356


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


357

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


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



Page 22 of 34

380

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).


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


386 387

Figure 6. (a) Polarization curves with iR corrected for OER activity of three as-prepared

388

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


Page 24 of 34

389

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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412


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


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



435

Page 26 of 34

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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TABLE OF CONTENTS

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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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Hybrid as a Highly Stable

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