Stepwise Understanding on Hydrolysis Formation of the IrO x Nanoparticles as Highly Active Electrocatalyst for Oxygen Evolution Reaction

In this study, we have investigated the synthesis of supported iridium oxide (IrO x ) nanoparticles (NPs) through hydrolysis in a surfactant-free aqueous bath as a possible route for the large-scale production of highly active electrocatalyst for oxygen evolution reaction (OER) in acidic water electrolyzers. The process involves (i) formation of Ir-hydroxides complex from an Ir precursor in basic media followed by (ii) protonation in acidic media to form colloidal hydrated IrO x NPs and (iii) conversion and deposition of IrO x NPs on the surface of carbon or TiN support by probe sonication. The IrO x NPs produced through hydrolysis route form highly stable colloidal solution. Since it is essential to precipitate the catalyst NPs from the colloidal solution for their use in water electrolyzer electrode development, here, we investigate the optimal reaction conditions, e.g., pH, temperature, time, and presence of support, for efficient synthesis of the catalyst NPs. The reaction intermediates formed at different reaction steps are explored to get insights into the chemistry of the process. Under the optimal synthesis conditions, 100% precipitation of IrO x NPs was achieved. Further, the precipitated TiN supported IrO x NPs exhibited high OER activity, superior to that of the commercial benchmark IrO 2 electrocatalyst. The study provides a scalable synthesis route for highly active, low Ir-content OER electrocatalysts for acidic water electrolyzers.


Introduction
The need for water oxidation catalysts that could operate at fast turnover rates and low overpotentials has increased as a result of a recent research focus on water electrolysis and artificial photosynthesis [1][2][3].However, major barriers to the development of the catalysts still hinder their broad use.The oxygen evolution reaction (OER), which occurs at the anode during water electrolysis, proceeds through many proton-coupled electron transfer stages, resulting in poor reaction kinetics [4,5].To improve OER kinetics, great research efforts have been committed to the synthesis of advanced OER catalysts based on, for example, perovskites, spinels, layered structure oxides, metal alloys, clusters, and metallene oxides [6][7][8][9][10][11].The most compatible OER catalysts that can stably perform in a polymer electrolyte membrane (PEM) water electrolyzer (PEMWE) are limited to Ru-and Ir-based electrocatalysts for acidic media [12][13][14][15][16][17][18].However, the scarcity of Ir in earth's upper crust [19] and the high cost hinder the scaled-up manufacturing of the electrocatalysts [20].It is therefore to design and develop innovative ways for producing low-content Ir catalysts with high OER mass activity and high stability under the highly corrosive conditions in acidic electrolyzers [12,13].
Typical materials suitable as the catalyst supports should have high electrical conductivity, a large surface area, and a high corrosion resistance {Mazur et al. 2012 #1} [21,22].The high surface area carbon materials, which are generally used Highlights • Synthesis of supported IrO x nanoparticles through a hydrolysis route was investigated.• The process involves formation of Ir-hydroxides, protonation, and deposition on a support.• The reaction was monitored stepwise wrt pH, temperature, time, and presence of support.• One hundred percent conversion was achieved with IrOx fine particles superior to commercial equivalent.
as supports in the PEM fuel cells, are sensitive to corrosion under the extreme oxidative conditions (acidic environment and oxidative potentials) at the PEMWE anode [23,24].Various attempts have been made to develop highly efficient low Ir content-based catalyst using high surface area supports such as carbon, titanium nitride (TiN), and antimony doped tin oxide (Sb-SnO 2 ) [25][26][27][28][29].However, in the present case, we have used two different catalyst supports, respectively, for the reaction intermediate studies and for the OER activity measurements.Specifically, high surface area carbon is used to study the reaction intermediates, while for the OER activity measurement, nanoparticulate TiN is used as a more realistic support due to its better corrosion resistance.Although TiN is known to be oxidized to TiON and other oxides under OER relevant conditions in acidic media, some studies suggest its high stability in terms of the retained OER activity during the degradation studies.For example, in our previous work [28], it has been observed that the surface oxidation of TiN to TiN x O y under the OER relevant conditions does not hamper its performance as the catalyst support.However, in most cases, complex synthesis processes that might restrict large scale production, have been used.Preparation of the Ir-based catalyst nanoparticles (NPs) of high mass activity through a solution-based hydrolysis method in combination with improved mass utilization of the expensive metal by using high surface area supports would therefore provide better way to achieve scalability and sustainability.Over a century ago, it was reported that IrO x .nH 2 O colloids could be synthesized by alkaline hydrolysis of [IrCl 6 ] 2− , a process that yields blue colloids with particle sizes in the 1-2-nm range [30].Murray and co-workers later demonstrated deposition of [IrCl 6 ] 2− sheets having excellent electrocatalytic activity [31].Further, Zhao et al. [32] have proposed the same process to prepare ligand free IrO x .nH 2 O colloidal NPs by hydrolyzing [IrCl 6 ] 2− .Recently, Mehdipour et al. [33] have utilized similar hydrolyzed process to synthesize IrO x .3H 2 O using colloidal NPs by hydrolyzing [IrCl 6 ] 3− and have further deposited the ligand free IrO x NPs on a Ti substrate through a potentiodynamic deposition route.
Although the reported studies have shown the synthesis of colloidal NPs, preparation of an electrode through their deposition on a substrate is not compatible with the present state-of-the-art methods for fabrication of electrodes for acidic water electrolyzers, which are prepared by coating a catalyst ink of optimal electrocatalyst/solvent compositions directly in the electrolyte membrane [34].Furthermore, the efficiency of the deposition of Ir-based electrocatalysts from the colloidal solution on the substrate has not been generally reported.However, for scalable production of such highly active electrocatalysts for the real-life water electrolyzers, it is essential to obtain the electrocatalyst in powder from, with high efficiency for conversion of the Ir precursor to the desired Ir-based electrocatalyst.
In the present study, we have utilized hydrolysis route to synthesize IrO x NPs using Ir 3+ and [IrCl 6 ] 2− precursors.The colloidal NPs were further deposited on a subtract through a sonication-assisted deposition process.The reaction steps involved during the synthesis of IrO x NPs are investigated using UV-Vis absorption spectroscopy, while the efficiency of deposition of Ir species on the support was determined through determination of Ir concentration in the colloidal solution.Further, the synthesis conditions were optimized for the carbon and TiN supports, respectively.Investigation of the reaction steps involved during the formation of IrO x NPs was examined using UV-Vis absorption spectroscopy.Finally, the mass specific OER activity of the supported IrO x was investigated.

Preparationof IrO x Colloidal Solution
Aqueous solution (conc. 2 mM) of (NH 4 ) 2 IrCl 6 was prepared in milliQ water, adjusted to pH 13 with 2.5 M NaOH to obtain a solution with a light amber hue, held at 90 °C for 30 min, and then cooled down naturally.Further, the solution was placed in an ice bath and subsequently mixed with 3 M HNO 3 to obtain a pH value of 1.The acid condensation was sustained for 90 min in the ice bath.The colloidal solution is quite stable for a long period of time.A similar process was carried out for the precursor IrCl 3 .

Deposition of IrO x NPs on the Support Material
Using a 2.5 M NaOH solution, pH of the acid condensed colloidal solution was adjusted to a value of 5.Then, carbon or TiN was added in the solution with a weight ratio of (20/80: Ir/support).Further, intensive ultrasonication of the above mixture was performed in an ice bath for 10 min at 100 W power using a Hielscher UP200St ultrasonic homogenizer.Thermodynamically, colloidal particles are more stable at ice-cold temperatures, whereas the ultrasonication process assists in depositing these NPs on the TiN surface (IrO x /TiN-20).The product obtained by centrifugation at 5000 rpm, followed by three washes with ultrapure miliQ water.The collected IrO x /TiN-20 sample was dried overnight at 60 °C in a vacuum oven.

Structural Characterizations
UV-visible spectroscopy of the solutions at different stages of the hydrolysis reaction was using a Perkin Elmer Lambda 900 UV/VIS spectrophotometer over the wavelength range between 250 and 800 nm.The spectral resolution and the integration time for the measurements were 1 nm and 100 ms, respectively.Solvent without Ir precursor was used as the reference sample.X-ray diffraction (XRD) patterns of samples were obtained using a Rigaku Miniflex 600 X-ray diffractometer equipped with a Cu K radiation (wavelength = 1.5418Ǻ), where a rate of 3°/min for diffraction angles (2θ) ranging between 10 and 90° was used.Further, transmission electron microscope (TEM) images of the optimal sample were recorded using a JEOL JEM-2010F TEM.

Electrochemical Measurements
Electrochemical measurements were performed using a conventional three-electrode setup, which included a Pt wire counter electrode, an Ag/AgCl (double junction, sat.KCl) reference electrode and a glassy carbon (GC) rotating disc electrode (RDE) coated with the catalyst being studied as the working electrode.The catalyst coating on the working electrode was realized by drop casting 10 μL of a catalyst ink prepared by ultrasonic mixing (1 min; Hielscher UP200St ultrasonic homogenizer) of the catalyst powder (10 mg) in 5 mL of a stock solution (2-propanol, ultrapure water and 5 wt% Nafion solution (D521, ion power) in volumetric ratios of 20:78.6:1.4).The catalyst ink on GC RDE was dried under ambient conditions at a constant rotation speed of 700 rpm.Accurate determination of Ir-loading on the GC RDE was made using an X-ray fluorescence (XRF) analyzer (Thermo Scientific Niton XL3t GOLDD +) [35].
The potentials measured against the Ag/AgCl reference electrode were reported with respect to a reversible hydrogen electrode (RHE).The electrochemical measurements were performed on a ZahnerIM6e electrochemical workstation.The OER performance of the catalyst was assessed using cyclic voltammetry (CV) in an Ar-saturated 0.1 M HClO 4 electrolyte.The electrode surface was initially activated by potentiodynamic cycling for 20 cycles between 1.00 and 1.70 V at a scan rate of 100 mV s −1 .Following activation, the initial OER performance was observed through cyclic voltammetry (CV) for 2 cycles within the potential range of 1.00 to 1.65 V at a scan rate of 10 mV s −1 , with the positivegoing portion of the second cycle being used for the OER activity assessment.During the activation and initial OER activity measurement, the working electrode was kept under a constant rotation of 1600 rpm.

Hydrolysis of [NH 4 ] 2 IrCl 6 and IrCl 3
As shown in the UV-Vis spectra of Fig. 1a, the [IrCl 6 ] 2− complex exhibits two broad UV-Vis absorption peaks centered at ~ 430 and ~ 500 nm, which disappear immediately on addition of NaOH with appearance of a new peak at ~ 329 nm.Therefore, in present case, formation of monomeric [Ir(OH) 6 ] 2− complex is observed just after addition of NaOH in the solution as the strong absorption peak appeared at 329 nm [36].Further, on holding the solution at 90 °C, the peak shifts to a lower wavelength of ~ 313 nm, signifying the formation of the polymeric [Ir(OH) 6 ] 2− .The visual colors of the sample at different stages can be seen in the optical photographs of Fig. 1b.
Further, as shown in Fig. 1c, the IrCl 3 precursor solution undergoes the hydrolysis process in a similar manner.The absorption peaks at ~ 330 nm and ~ 390 nm for the IrCl 3 precursor [37] disappear on addition of NaOH and a new peak appears at ~ 329 nm.After holding the solution at 90 °C, the peak becomes intense and shifts to 313 nm due to formation of the monomer complex [Ir(OH) 6 ] 3− [33,38], which is possible due to the precursor containing Ir 3+ ions.The absorption peak for this complex appears similar to that for the monomeric [Ir(OH) 6 ] 2− complex from Ir 4+ precursor [32,33].This similarity in both precursors might be due to the fact that both the monomer complexes possess similar octahedral structures.

Acid Condensation
On addition of HNO 3 in the hydrolyzed solutions after ice bath cooling, the color of the light blue solutions gradually changes to deep blue, as depicted in Fig. 1b, d, respectively corresponding to the (NH 4 ) 2 IrCl 6 and IrCl 3 Ir-precursors.The final blue color becomes darker over time.The prominent absorption band at ~ 313 nm associated with [Ir(OH) 6 ] 2− or [Ir(OH) 6 ] 3− disappears rapidly.The observed color change has been attributed to the acid-condensation reaction, leading to formation of IrO x NPs [32].UV-Vis spectra of the acid-condensed samples show a broad absorption peak at 580 nm, attributed to a consistent progression of spectral line with hydrolysis and rapid acid condensation to form O-Ir-O linkages [32,33,39].
According to the observed absorption spectra, both precursors, i.e., (NH 4 ) 2 IrCl 6 and IrCl 3 are favorable for the preparation of IrO x NPs in colloidal form.However, it is not worthy if the NPs are not separated out in the pure form (solid powder) in order to utilize them as the OER electrocatalyst in PEMWEs.Therefore, here, we have attempted to separate the colloidal NPs from the solution using high surface area carbon (KB600) support via probe sonication.The Ir conversion efficiency was evaluated using XRF measurements of the Ir concentrations in the initial solution and in the final supernatant.In order to estimate the conversion efficiency (%conversion), following equation was used, where C i is the Ir concentration of the initial solution ((NH 4 ) 2 IrCl 6 or IrCl 3 solution in water), while C f is the final Ir concentration in the centrifuged supernatant. (1) The estimated results are summarized in Table 1, which reveal that the IrCl 3 precursor is a better choice for the IrO x synthesis as it shows a > 3 times higher %conversion as compared to that of the (NH 4 ) 2 IrCl 6 precursor.
The H + concentration plays a crucial role in the formation and precipitation of the stable IrO x NPs through acid condensation.To explore the effect of H + concentration on the %conversion, acid condensation was performed at different acid concentrations.Figure 2a shows the UV-Vis spectra of the colloidal solutions after the acid condensation process using different concentrations of HNO 3 (≤ 3 M).Noticeably, the debilitated absorption at 580 nm indicates partial conversion of the [Ir(OH) 6 ] 2− complex to the IrO x colloidal NPs.On the other hand, the absorption peak observed at 313 nm with slightly lower intensity compared to that for the hydrolyzed solution (as shown in Fig. 1c) for acid concentrations < 3 M is a strong evidence of partial conversion, probably due to insufficient proton concentration to convert all of the [Ir(OH) 6 ] 2− to O-Ir-O.Further, Fig. 2c shows the UV-Vis spectra of the supernatant solutions obtained after centrifugal separation of the IrO x NPs deposited on the carbon support via acid condensation followed by probe sonication.Noticeably, the characteristic absorption peak of IrO x colloidal NPs (580 nm) disappears for 3 M HNO 3 , whereas the peak obtained below 310 nm is due to the presence of HNO 3 in the supernatant.An HNO 3 concentration of 3 M is found to be sufficient as the UV-Vis spectrum of the supernatant appears similar to that of the HNO 3 background, which indicates complete conversion of the Ir species to IrO x and hence no absorption peak correspond to the Ir species appears in the spectrum.Table 2 depicts the XRF measurements associated with the initial and final amounts of Ir in the supernatant, revealing highest %conversion in case of 3 M HNO 3 .The obtained results suggest higher proton concentration makes the IrO x NPs more stable in the colloidal form.Further, Fig. 2d shows a photograph of the supernatant liquids from the samples treated at different HNO 3 concentrations.The solution color appears darker with increasing acid concentrations, which, as per the conversion efficiency, suggests increasing amounts of the IrO x NPs in the supernatant.

pH Adjustment
Further, to improve the %conversion of IrO x NPs on the surface of carbon, the pH of the colloidal solution attained to 5 by adding NaOH in order to destabilize the IrO x NPs.The  protonation of the anionic clusters leads to their aggregation and condensation with protonated [Ir(OH) 6 ] 2− .The addition of base can partially reverse the condensation that results in the formation of IrO x NPs, which further improves the anchoring of IrO x on the surface of carbon during sonication.Figure 3 shows the UV-Vis spectra of the samples before and after centrifugal separation of the IrO x NPs.Interestingly, complete conversion of IrO x is obtained on the support carbon as the supernatant shows the spectrum similar to that of the solvent.
The possible reaction kinetics is as follows.
The hydrolysis step was initiated by forming Ir(OH) 6 3− complex under alkaline condition, as given in Eq. ( 2).
In the second step, the protonation by rapid condensation under acidic media leads to formation of hydrated IrO x col- loidal NPs, as shown in Eq. ( 3).Furthermore, during the initial optimization, it was found that direct pH adjustment to a value of 5 does not promote the formation of the product, i.e., the IrO x NPs as no significant change in the Ir concentration in the synthesis bath (2) was observed under these conditions.Again, pH adjustment to 5 after formation of the hydroxyl complex of Ir leads to a poor conversion of 29% Ir in form of the IrO x NPs.Thus, no additional analyses of the products obtained under these synthesis conditions were conducted.

Effect of Acid Type
The influence of specific acid (HNO 3 or HCl) used for NPs condensation was critically analyzed using UV-Vis test at each step of the reaction to find out the best option for deposition of the NPs on the carbon support.Figure 4a, b illustrates the UV-Vis spectra for the steps involved in the reactions using HNO 3 and HCl, respectively.addition, Table 3 summarizes the %conversion of Ir in form of IrO x deposited on the carbon support after the acid condensation using HNO 3 or HCl followed by increase of pH value from 1 to 5 using NaOH.It is clear that the acid condensation by HNO 3 leads to complete deposition of the IrO x NPs on the support carbon, while condensation performed using HCl showed poor conversion.This observation could be explained by the facts that (i) HNO 3 acts as an oxidizing agent and (ii) Ir 3+ and Ir 4+ form stable complexes with Cl-, which are stable under acidic conditions.Here, HNO 3 leads to formation of IrO x NPs through conversion of the Ir 3+ to Ir 4+ species during acid condensation.Furthermore, HNO 3 may oxidize the carbon support, providing surface functional groups as the anchoring points for the deposition of the colloidal IrO x NPs.On the other hand, absorption peaks similar to those from IrCl 3 are observed after acid condensation using HCl, suggesting backward reaction to form IrCl 3 after acid condensation and hence resulting in poor deposition efficiency on the carbon support.

OER Activity Study
In the present study, we have optimized all the IrO x conversion in terms of formation and deposition of IrO x on KB600, the high surface area carbon support.However, carbonbased materials are intrinsically unstable at the PEMWE anode.Therefore, we have applied an alternative support of TiN which is quite stable even in highly oxidizing or corrosive conditions at PEMWE anodes.Here, using the optimal synthesis process that involves hydrolysis at a pH of 13, acid condensation in an ice bath at a pH of 1 and deposition of the  IrO x on the support by sonication at a pH of 5 was used to synthesize the IrO x /TiN catalyst with the Ir loading on TiN being 20 wt%.In terms of %conversion of Ir to IrO x deposited on the support, the TiN support shows results similar to those obtained with carbon support.TEM images of the synthesized IrO x /TiN catalyst are shown in Fig. 5a, b, which reveal the TiN support particles of ~ 10 nm coated with fine IrO x NPs (~ 1 nm).The higher magnification TEM image of Fig. 5b exhibits uniform dispersion of the IrO x NPs on the TiN support.Further, the IrO x /TiN catalyst was examined using X-ray diffraction in order to know the crystal structure and phase formation (Fig. 5c).However, there are no obvious peaks related to Ir or IrO x compound in the catalysts, which suggests an amorphous nature of the synthesized IrO x .All the observed peaks correspond to the TiN support phase.
OER performance of the as prepared IrO x /TiN and a commercial IrO 2 (supplier: alfa Aesar) catalysts were examined by comparing their polarization curves in Arsaturated 0.1 M HClO 4 solution at the electrode rotation rate of 1600 rpm (scan rate: 10 mV/s).Values of the Ir loading normalized currents at 1.65 V were used to compare the OER activities of the electrocatalysts being investigated.Noticeably, the mass normalized activity of the IrO x /TiN catalyst shows 2 times higher performance comparable with the standard commercial IrO 2 catalyst as shown in Fig. 5d.This suggest the present approach to be suitable for scalable production of such OER electrocatalysts with high mass activity and hence low Ir loading.Table 4 illustrates comparative OER activity of the present electrocatalyst (IrO x /TiN) with previous reported literature [26,[40][41][42][43][44][45][46][47].Furthermore, XRF determination of Ir loading on the GC RDE before and after the OER activity measurements suggests no significant loss of Ir from the electrode during the electrochemical study.

Conclusions
The reaction steps involved in the formation of hydrated iridium oxide/hydroxide NPs have been systematically investigated.The stable colloids containing ligand-free IrO x particles were prepared in a two-step approach; at first, [Ir(OH) 6 ] 2-was created by hydrolyzing IrCl 3 3H 2 O in an alkaline solution.Subsequently, IrO x particles are produced via acid treatment and the aging process, which oxidize the polymerized [Ir(OH) 6 ] 2-monomers.Further as prepared IrO x colloidal NPs decorated on the surface of carbon or TiN support using probe sonication process in the form of IrO x .Besides, XRF measurement was found to be better tool to examine actual conversion of Ir species on the surface of carbon or TiN support comparing with UV-Vis absorption analysis.However chemical changes could be effectively analyzed using UV-Vis absorption spectroscopy.Complete conversion of Ir, in terms of formation of IrO x and its deposition on the support from the reaction bath, is realized by adjusting the pH to an optimal value of 5 after acid condensation at pH 1.The approach may be developed further for scalable production of the low Ir content and supported Ir electrocatalysts for PEMWE anodes.

Fig. 1 a
Fig. 1 a UV-Visible spectra and b photographs of the samples at different stages during synthesis of IrO x NPs using (NH 4 ) 2 IrCl 6 as the Ir-precursor (black: Ir precursor solution in water; red: Ir solution + NaOH, before heating at 90 °C, magenta: Ir solution + NaOH after heating at 90 °C; green: solution after acid condensation).UV-Vis spectra and photographs of the reaction intermediates using IrCl 3

Further, higher than 3 M
HNO 3 concentrations (6 and 12 M) were compared, and corresponding UV-Vis spectra are shown in the Fig. 2b.Notably, identical volumes of 3 M, 6 M, and 12 M HNO 3 , respectively, were used for the protonation; thus, their final HNO 3 concentrations in the reaction baths were obtained to be ~ 0.2 M, ~ 0.4 M, and ~ 0.8 M, respectively.The UV-Vis absorption spectra illustrate strong absorption peak at 580 nm for 3 M HNO 3 which is characteristic peak of hydrated IrO x colloidal NPs.For higher acid concentrations of 6 M and 12 M, a small blue shift of 20 nm can be observed, which may suggest strong O-Ir-O bond interaction.

Fig. 2 a
Fig. 2 a, b UV-Visible spectra of acid condensed IrO x colloidal solution at different HNO 3 concentrations (suffix:-Ref.:No Ir species present; IrO x : Ir-species present), c UV-Visible spectra of supernatant (suffix: S) liquid after colloidal IrO x NPs deposited on the surface of carbon support in the form of IrO x , and d photograph of supernatant liquids after certain time lag

( 3 )Fig. 3 3 Fig. 4
Fig.3UV-Visible spectra of acid condensed IrO x colloidal solution after adjusting pH-5 using NaOH along with the supernatant liquids after precipitation and sonication with carbon support; UV-Vis spectra of 3 M HNO 3 for reference background

Fig. 5 a
Fig. 5 a, b TEM images of the synthesized IrO x /TiN electrocatalyst, c XRD patterns of bare TiN support and the IrO x /TiN sample, and d OER polarization curves for the IrO x /TiN catalyst along with the IrO 2 commercial samples (inset: polarization curve with logarithmic scale for y-axis)

Table 3
Ir %conversion on the surface of carbon support using HNO 3 and HCl for acid condensation