Electronic structure control of IrO2 via conjugated polymer support for highly efficient oxygen evolution reaction

In this paper, we report synthesis of novel nanoparticle catalyst of iridium oxide supported on conjugated polymer along with evaluation of activity and durability for oxygen evolution reaction. The IrO2/poly(BIAN-thiophene)/TNT catalyst was prepared from iridium complex and poly(BIAN-thiophene)/TNT by hydrothermal method. The synthesized IrO2/poly(BIAN-thiophene)/TNT catalysts was characterized by scanning electron microscopy, transmission electron microscopy, Fourier transfer-infrared spectroscopy, x-ray photoelectron spectroscopy and electrochemical methods. The average particle size of the IrO2 particles on poly(BIAN-thiophene)/TNT was 2.5 nm. The XPS measurement revealed that Ir complex was completely converted to iridium oxide through hydrothermal treatment. The IrO2/poly(BIAN-thiophene)/TNT catalyst showed sufficient performance for OER activity and durability in acidic condition. Our results indicate that IrO2/poly(BIAN-thiophene)/TNT is one of the prospective candidate catalysts for water splitting.


Introduction
In recent years, diversification of energy sources is required to solve environmental problems such as global warming and fossil fuel depletion.Renewable energy is attracting attention as an energy source with a very low environmental impact.Hydrogen is one such source, and the establishment of a clean production method that does not emit CO 2 is essential.Several hydrogen production processes exist, but we will focus on electrochemical water splitting performed under acidic conditions.A proton exchange membrane water electrolyzers (PEMWE) is one of the electrochemical systems used to split water into oxygen and hydrogen.Hydrogen evolution reaction (HER) occurs as cathode and oxygen evolution reaction (OER) as anode.Pt-based catalysts are widely known for the cathode under acidic conditions [1][2][3][4].However, the OER is a complex reaction involving four electron transfers, resulting in large overpotential and reduced efficiency [5][6][7][8][9][10][11].Therefore, a catalyst that can efficiently generate oxygen at low overpotential is needed.
Iridium oxide is the second most active metal oxide catalyst in the OER and has been widely used as an electrocatalyst for OER [12][13][14][15][16]. Iridium is a noble metal, and the amount of iridium loading must be reduced to keep catalyst costs down.Attempts have been made to improve the catalytic activity and durability of IrO 2 by controlling its morphology and electronic structure.Nørskov et al [17] showed a linear relation between metal oxide d band center and binding energies to O (intermediates) to the surface (ΔE O ) which play a key role in determining the overpotential of the system.Therefore, studies have been conducted on doping various elements such as Cu, Ni, Sr, Mo, and fluorine [18][19][20][21][22].However, it is difficult to prevent metal leaching and to maintain the electronic structure during the reaction.On the other hand, there are approaches to improve the catalytic activity by adjusting the electronic structure of IrO 2 through electron donation from the support material.Our group proposed to change the electronic structure of IrO 2 nanoparticles by strong metal-substrate interactions using electrochemically stable and hetero element-doped carbon substrates [16].When Graphite with 7 wt% nitrogen is loaded with IrO 2 , it reduced the overpotential to 260-270 mV and achieved high stability.
Further precise control of the electronic structure and immobilization of IrO 2 nanoparticles on the substrate is less studied and thus remains open for further research.
To control the electronic structure of iridium oxide, the application of conductive polymers is attractive approach.Among the many conducting polymers, polythiophenes are interesting and have been developed as catalyst support materials.Schrebler et al [23] tailored polythiophene-modified electrodes by electrodeposition of Pt and Pt/Pb metal nanoparticles for the electrooxidation of formic acid.In this study, polythiophene exhibited high electrochemical stability, and polythiophene-containing composites provided an effective alternative support material to conventional carbon support.Several other researchers have also reported that polythiophene is a very stable host polymer matrix [24,25].
In this regard, thiophene-based polymers were investigated as substrates bearing a strong coordination site to control the electronic structure of iridium oxide.Titanium oxide nanotubes were used as a support after coated with BIAN-thiophene.Furthermore, the performance of IrO 2 /poly(BIAN-thiophene)/TNT electrocatalysts for OER activity was evaluated.This study proposes a new strategy to fabricate highly efficient and stable OER electrocatalysts under acidic conditions.

Preparation of titanium dioxide nanotube array
The TiO 2 nanotube array (TNT) was prepared by the anodization method.The Ti chip was hand polished with 200, 600, and 800 grit sandpaper, and the oxide layer was removed by washing the chip in a hydrofluoric acid: nitric acid: DI water (1:3:16 volume ration) mixture.This chip was degreased by washing it in methanol.Ti chip was used as an anode and platinum chip as cathode and Ti chip was anodized at a DC voltage of 50 V for 2.5 h in an aqueous ethylene glycol solution containing 0.5 wt% NH 4 F under ultrasonication.After anodization, TNT was washed with methanol and sintered at 300 °C to eliminate all the organic contaminants.

Preparation of Dimethyl 2,2'-(((1E, 2E)-acenaphthylene-1, 2-diylidene)bis(azaneylylidene))bis-(thiophene-3-carboxylate) (BIAN-thiophene)
The Synthetic scheme of BIAN-thiophene is shown in figure 1.A suspension of acenaphthenequinone (6.6 mmol, 1.20 g) in acetonitrile (60 ml) was prepared and 1 ml of acetic acid was added and dropped into it.This mixture was stirred till all the acenaphthenequinone was dissolved.A solution of methyl 2-aminothiophene-3carboxylate (14.2 mmol, 2.23 g) in acetonitrile was added to the solution and the resulting mixture was stirred under reflux conditions and nitrogen atmosphere for 12 h.After the reaction, the solution was cooled to 0 °C and the product precipitated.The precipitate was washed several times with cold acetonitrile.The yield was 88%.The structural determination of BIAN-thiophene was performed using 1 H NMR spectroscopy on a Bruker Biospin Avance III 400 MHz instrument and FT-IR spectroscopy on a Perkin Elmer Spectrum 100 (figures S1 and S2).

Electropolymerization of thiophene or BIAN-thiophene using TNT as a template
Polymer/TNT was prepared using the three-electrode system by electropolymerization technique.TNT chip, platinum chip and Ag/Ag + were used as the working electrode, the counter electrode and the reference electrode, respectively.Thiophene or BIAN-thiophene (0.1 M) was electropolymerized in acetonitrile with 0.1 M aq HClO 4 .The potential was cycled between −2.0 V to 0 V versus Ag/Ag + at a scan rate of 50 mVs −1 .This cycling was carried out 50 times.After the polymerization, the chip was washed with acetonitrile to remove the monomer and the reaction mixture.Finally, the polymer/TNT was vacuum dried at room temperature.

Formation of the iridium complex
Ir complex was formed by immersing BIAN-thiophene/TNT in DI water into methanol (15:5 volume ration) solution of IrCl 3 •nH 2 O (0.30 g, 1 mmol) for 2 days.

Synthesis of the IrO 2 /polymer composite electrode
The IrO 2 /polymer/TNT composite electrode was synthesized by hydrothermal method.The required amount of H 2 IrCl 6 nH 2 O and polymer/TNT chip were dispersed in 9:1 (vol/vol) ethanol/DI water solution.The mixture was sealed in a Teflon-lined stainless steel autoclave and maintained at 120 °C for 6 h.The composite electrode was washed with 9:1 (vol/vol) ethanol/DI water.Further, the sample was dried at 300 °C for 3 h.

Characterization
1 H NMR spectra were recorded on a Bruker Biospin AVANCE III 400 MHz spectrometer.The FT-IR spectra were recorded using a PerkinElmer 100 FT-IR spectrometer.The spectra were averaged over 100 scans with a resolution of 2 cm −1 in the attenuated total reflection mode.A JASCO V630 spectrophotometer was used to record UV-vis absorption spectra.The morphology of the TNT chip and composite electrode was studied using scanning electron microscopy (SEM) (Hitachi S-4500).The transmission electron microscopy (TEM) measurements were performed to gain insight into the growth of polymer on TNT as well as particle size and distribution of the IrO 2 nps.

Electrochemical measurements
All the electrochemical measurements including linear sweep voltammetry (LSV), chronoamperometry (stability test) and chronoamperometry (oxygen evolution test), were conducted at room temperature and performed using a potentiostat (HSV-110, Hokuto Denko Co.) under 0.5 M H 2 SO 4 as an electrolyte.LSV and chronoamperometry were performed using a three-electrode system.The electrocatalyst and Pt mesh were used as working and counter electrodes, respectively.The reference electrode was 3.0 M Ag/AgCl.The durability test of the electrocatalyst was performed by chronoamperometry (CP).The CP study employed a two-electrode system.The electrocatalyst and Pt mesh were used as working and counter electrodes, respectively.The oxygen concentrations in the cell after 15 and 30 min were quantitatively analyzed by a Shimadzu GC system (GC-8A).

Result and discussion
SEM micrographs of TNT and poly(BIAN-thiophene) are shown in figure 2. Figure 2(a) shows the tubular morphology of TNT with a diameter of 140 nm.Figures 2(b) and (c) are micrographs of the poly(BIANthiophene)/TNT composite.Figure 2(c) shows that pores on surface of TNTs were filled, confirming the growth of the polymer.Furthermore, the pores of the TNT were also maintained and indicated a high specific surface area.The elemental composition after hydrothermal synthesis analyzed using energy dispersive x-ray Figure 3(a) is a TEM micrograph of IrO 2 nanoparticles decorated on poly(BIAN-thiophene)/TNT.The deposited polymer was observed on the TNT, and the IrO 2 nanoparticles loaded were observed as black spots.Figure 3(b) shows that the IrO 2 nanoparticles are homogeneously distributed over the entire surface of poly (BIAN-thiophene)/TNT.The average size of IrO 2 nanoparticles was 2.5 ± 0.3 nm (figure 3(c)).On the other hand, the particle sizes of commercial IrO 2 powders and Ir metal powders without supported substrates was 1 ∼ 2 orders of magnitude larger than those of IrO 2 nanoparticles on poly(BIAN-thiophene)/TNT [12].Since specific surface area is proportional to the inverse of particle size, it is suggested that commercial IrO 2 powders and Ir metal powders have specific surface area of 1 ∼ 2 orders of magnitude lower than that of IrO 2 nanoparticles.
Figure 3(d) shows a high-resolution TEM image of IrO 2 /poly(BIAN-thiophene)/TNT, in which lattice fringes of IrO 2 nanoparticles are observed.The average lattice spacing of the IrO 2 particles was 0.226 nm, corresponding to the (020) plane of the IrO 2 rutile structure [26].On the other hand, the average lattice spacing of commercial IrO 2 and Ir metal powders observed in high-resolution TEM images is 0.304 nm and 0.345 nm, respectively, corresponding to the (110) plane of the IrO 2 rutile structure and the (100) plane of the Ir facecentered cubic structure [12].TEM results indicate that the hydrothermal method successfully formed catalysts with homogeneously distributed IrO 2 nanoparticles on poly(BIAN-thiophene)/TNT.The Ir 4f peaks on IrO 2 /polythiophene/TNT have Ir 4f 5/2 and 4f 7/2 peaks at 64.1eV and 61.1eV, and at 66.0eV and 63.0eV, respectively on a bent background baseline.On the other hand, IrO 2 /poly(BIAN-thiophene)/TNT has peaks of Ir 4f 5/2 and 4f 7/2 at 63.9 eV and 60.9 eV, respectively, and satellite peaks at 66.8 eV and 62.9 eV, respectively, on a bent background baseline.The binding energy of Ir 4f 7/2 of IrO 2 /poly(BIAN-thiophene)/TNT compared to unloaded IrO 2 was shifted by 0.9 eV [27].The lower binding energy of IrO 2 /poly(BIAN-thiophene)/TNT is attributed to the higher electron density of Ir by anchoring of Ir nanoparticles to the polymer due to the coordination power of the N atom.Such electronic interactions are generally referred to as strong metalsubstrate interactions (SMSI) and are one of the important factors that determine both stability and electrocatalytic activity of the catalyst.In addition, the shift of Ir 4f peak is associated with an increase in Ir 4+ sites.Increased Ir 4+ sites results in OH adsorption, H dissociation, and O-O bond formation, which are responsible for OER activity [28].IrO 2 supported on poly(BIAN-thiophene)/TNT via formation of Ir complexes was expected to show excellent catalytic performance [29].
Figures 5(a) and (b) show the LSV and Tafel plots of each electrocatalyst.The current density at 1.65 V versus RHE for IrO 2 /poly(BIAN-thiophene)/TNT was 58.3 mAcm −2 , which was approximately 50 times higher than that of the IrO 2 powder catalyst [30].The overpotential at a current density of 10 mA m −2 was found to be 330 mV for the IrO 2 /polythiophene/TNT electrode and 260 mV for the IrO 2 /poly(BIAN-thiophene)/TNT electrode, respectively.IrO 2 /poly(BIAN-thiophene)/TNT showed 10-70 mV lower overpotential at current density of 10 mAcm −2 compared to other IrO 2 catalysts reported (table S1).The Tafel plots was a straight line because the current density increased exponentially with increasing voltages.The smaller the slope of Tafel plots, the larger the rise of current density and the higher the catalytic activity [31,32].The slope of IrO 2 /poly(BIAN-thiophene)/TNT was 97 mVdec −1 , which is lower than other samples, indicating higher activity of this electrocatalyst (IrO 2 /polythiophene/TNT:157 mVdec −1 , IrO 2 /TNT:182 mVdec −1 ).The amount of oxygen evolution after 15 and 30 min is shown in figure 5(c).The oxygen evolution of IrO 2 /poly(BIAN-thiophene)/TNT after 30 min was 269 μmol, which was approximately 2.2 times larger than that of IrO 2 /TNT.Therefore, a catalyst prepared using conjugated polymer composite was found to be extremely active and efficient in generating oxygen.The durability of the catalyst was investigated by chronopotentiometry (CP).The potentials at every 5 h are shown in figure 5(d).The potential of IrO 2 /poly(BIAN-thiophene)/TNT based cell after 5 h was 1.68 V.Even after 100 h, the increase was only by 50 mV.The potential remained unchanged for 40 h, indicating that the material retains highly durable.This should be because the Ir nanoparticles were fixed to the polymer due to coordination power of N atoms in poly(BIANthiophene), which suppressed the dissolution of Ir.
Figure 6 shows the XPS spectra of IrO 2 /poly(BIAN-thiophene)/TNT before and after the stability test.The binding energy of Ir 4f 7/2 after the stability test was 60.9 eV (figure 6(b)).Since no change occurred in the binding energy of Ir 4f 7/2 before and after the stability test, it was demonstrated that the oxidation state of Ir was maintained, and the catalytic activity was also retained.

Conclusion
IrO 2 /poly(BIAN-thiophene)/TNT electrocatalysts for OER were prepared by electrochemical polymerization and simple hydrothermal treatment.The designed poly(BIAN-thiophene) was used as a substrate to support IrO 2 nanoparticles, and the N atoms present in poly(BIAN-thiophene) were used as nucleation sites to enhance the specific surface area of IrO 2 .XPS measurements revealed the formation of strong electronic interactions between the nanoparticles and poly(BIAN-thiophene). Furthermore, electrochemical studies of IrO 2  nanoparticles supported on poly(BIAN-thiophene)/TNT substrates prepared by hydrothermal method showed that they exhibit high OER activity along with high durability.In summary, the increase in the specific surface area of IrO 2 is attributed to the reduction of its particle size by using poly(BIAN-thiophene), which is electronically conductive substrate.It is also suggested that the enhanced anchoring interaction between the substrate and the nanoparticles is responsible for the high OER activity and durability under acidic conditions.These results indicate that IrO 2 /poly(BIAN-thiophene)/TNT is a promising OER catalyst and is expected to be applied as an anode in PEMWE.

Figure 5 .
Figure 5. (a) Linear sweep voltammograms of each cell.(b) Tafel plots of OER on each cell reproduced from LSV curve.(c) The amount of oxygen evolution after 15 and 30 min (d) Time dependence of open circuit potential.