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 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-SnO2) [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 TiNxOy 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 IrOx.nH2O colloids could be synthesized by alkaline hydrolysis of [IrCl6]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 [IrCl6]2− sheets having excellent electrocatalytic activity [31]. Further, Zhao et al. [32] have proposed the same process to prepare ligand free IrOx.nH2O colloidal NPs by hydrolyzing [IrCl6]2−. Recently, Mehdipour et al. [33] have utilized similar hydrolyzed process to synthesize IrOx.3H2O using colloidal NPs by hydrolyzing [IrCl6]3− and have further deposited the ligand free IrOx 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 IrOx NPs using Ir3+ and [IrCl6]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 IrOx 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 IrOx NPs was examined using UV–Vis absorption spectroscopy. Finally, the mass specific OER activity of the supported IrOx was investigated.

Materials and Methods

Materials

Analytical grade chemicals such as iridium trichloride hydrate (IrCl3nH2O) and diammonium hexacholoiridate (NH4)2IrCl6 were used as the Ir-precursors, while titanium nitride (99.99% TiN, Alroko®) or Ketjenblack EC-600JD (KB600) acted as the catalyst support. Other chemicals such as sodium hydroxide (NaOH), nitric acid (HNO3 65 v/v%), and hydrochloric acid (HCl 37 v/v%) were purchased from Sigma-Aldrich. For electrochemical measurements, 0.1 M HClO4, prepared by diluting high purity HClO4 (TRACESELECT Ultra, Fluka) was used. As a solvent, ultrapure water (milliQ; resistivity 18.2 MΩ cm at 25 °C) was used for all aqueous solution preparations. Commercially available IrO2 powder purchased from Alfa Aeser was utilized as the benchmark OER electrocatalyst for activity comparisons.

Preparationof IrOx Colloidal Solution

Aqueous solution (conc. 2 mM) of (NH4)2IrCl6 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 HNO3 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 IrCl3.

Deposition of IrOx 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 (IrOx/TiN-20). The product obtained by centrifugation at 5000 rpm, followed by three washes with ultrapure miliQ water. The collected IrOx/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 = 1.5418 Ǻ), 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 HClO4 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 positive-going 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.

Results and Discussion

Hydrolysis of [NH4]2IrCl6 and IrCl3

As shown in the UV–Vis spectra of Fig. 1a, the [IrCl6]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.

Fig. 1
figure 1

a UV–Visible spectra and b photographs of the samples at different stages during synthesis of IrOx NPs using (NH4)2IrCl6 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 IrCl3 as the Ir-precursor are shown respectively in c and d. The spectra are recorded after 10 × dilution of the sample with ultrapure water. Photographs in b and d correspond to the Ir-precursor solution (left), the intermediate sample at pH 13 (90 °C, 20 min) (center) and the solution after reaction with HNO3 (right)

Full size image

Further, as shown in Fig. 1c, the IrCl3 precursor solution undergoes the hydrolysis process in a similar manner. The absorption peaks at ~ 330 nm and ~ 390 nm for the IrCl3 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 Ir3+ ions. The absorption peak for this complex appears similar to that for the monomeric [Ir(OH)6]2− complex from Ir4+ 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 HNO3 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 (NH4)2IrCl6 and IrCl3 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 IrOx 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., (NH4)2IrCl6 and IrCl3 are favorable for the preparation of IrOx 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 Ci is the Ir concentration of the initial solution ((NH4)2IrCl6 or IrCl3 solution in water), while Cf is the final Ir concentration in the centrifuged supernatant.

$$\%{\text{conversion}}=\frac{{C}_{i}-{C}_{f}}{{C}_{i}}\times 100$$
(1)

The estimated results are summarized in Table 1, which reveal that the IrCl3 precursor is a better choice for the IrOx synthesis as it shows a > 3 times higher %conversion as compared to that of the (NH4)2IrCl6 precursor.

Table 1 Estimation of Ir conversion using XRF after coating of IrOx colloidal NPs on the surface of carbon using probe sonication
Full size table

The H+ concentration plays a crucial role in the formation and precipitation of the stable IrOx 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 HNO3 (≤ 3 M). Noticeably, the debilitated absorption at 580 nm indicates partial conversion of the [Ir(OH)6]2− complex to the IrOx 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.

Fig. 2
figure 2

a, b UV–Visible spectra of acid condensed IrOx colloidal solution at different HNO3 concentrations (suffix:- Ref.: No Ir species present; IrOx: Ir-species present), c UV–Visible spectra of supernatant (suffix: S) liquid after colloidal IrOx NPs deposited on the surface of carbon support in the form of IrOx, and d photograph of supernatant liquids after certain time lag

Full size image

Further, higher than 3 M HNO3 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 HNO3, respectively, were used for the protonation; thus, their final HNO3 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 HNO3 which is characteristic peak of hydrated IrOx 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.

Further, Fig. 2c shows the UV–Vis spectra of the supernatant solutions obtained after centrifugal separation of the IrOx NPs deposited on the carbon support via acid condensation followed by probe sonication. Noticeably, the characteristic absorption peak of IrOx colloidal NPs (580 nm) disappears for 3 M HNO3, whereas the peak obtained below 310 nm is due to the presence of HNO3 in the supernatant. An HNO3 concentration of 3 M is found to be sufficient as the UV–Vis spectrum of the supernatant appears similar to that of the HNO3 background, which indicates complete conversion of the Ir species to IrOx 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 HNO3. The obtained results suggest higher proton concentration makes the IrOx NPs more stable in the colloidal form. Further, Fig. 2d shows a photograph of the supernatant liquids from the samples treated at different HNO3 concentrations. The solution color appears darker with increasing acid concentrations, which, as per the conversion efficiency, suggests increasing amounts of the IrOx NPs in the supernatant.

Table 2 Effect of acid concentration on the deposition efficiency (%conversion) of the IrOx colloidal NPs on the carbon support
Full size table

pH Adjustment

Further, to improve the %conversion of IrOx NPs on the surface of carbon, the pH of the colloidal solution attained to 5 by adding NaOH in order to destabilize the IrOx 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 IrOx NPs, which further improves the anchoring of IrOx on the surface of carbon during sonication. Figure 3 shows the UV–Vis spectra of the samples before and after centrifugal separation of the IrOx NPs. Interestingly, complete conversion of IrOx 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.

Fig. 3
figure 3

UV–Visible spectra of acid condensed IrOx 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 HNO3 for reference background

Full size image

The hydrolysis step was initiated by forming \({\left[Ir{\left(OH\right)}_{6}\right]}^{3-}\) complex under alkaline condition, as given in Eq. (2).

$${{\text{IrCl}}}_{3}+{6{\text{OH}}}^{-}\to {\left[{\text{Ir}}{\left({\text{OH}}\right)}_{6}\right]}^{3-}+{3{\text{Cl}}}^{-}$$
(2)

In the second step, the protonation by rapid condensation under acidic media leads to formation of hydrated \({{\text{IrO}}}_{{\text{x}}}\) colloidal NPs, as shown in Eq. (3).

$${\left[{\text{Ir}}{\left({\text{OH}}\right)}_{6}\right]}^{3-}+{{\text{nH}}}^{+}\to {{\text{IrO}}}_{{\text{x}}}\bullet {{\text{nH}}}_{2}{\text{O}}$$
(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 IrOx NPs as no significant change in the Ir concentration in the synthesis bath 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 IrOx NPs. Thus, no additional analyses of the products obtained under these synthesis conditions were conducted.

Effect of Acid Type

The influence of specific acid (HNO3 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 HNO3 and HCl, respectively. In addition, Table 3 summarizes the %conversion of Ir in form of IrOx deposited on the carbon support after the acid condensation using HNO3 or HCl followed by increase of pH value from 1 to 5 using NaOH. It is clear that the acid condensation by HNO3 leads to complete deposition of the IrOx NPs on the support carbon, while condensation performed using HCl showed poor conversion. This observation could be explained by the facts that (i) HNO3 acts as an oxidizing agent and (ii) Ir3+ and Ir4+ form stable complexes with Cl-, which are stable under acidic conditions. Here, HNO3 leads to formation of IrOx NPs through conversion of the Ir3+ to Ir4+ species during acid condensation. Furthermore, HNO3 may oxidize the carbon support, providing surface functional groups as the anchoring points for the deposition of the colloidal IrOx NPs. On the other hand, absorption peaks similar to those from IrCl3 are observed after acid condensation using HCl, suggesting backward reaction to form IrCl3 after acid condensation and hence resulting in poor deposition efficiency on the carbon support.

Fig. 4
figure 4

UV–Visible spectral progression during hydrolysis, acid condensation, and sonication process using a HNO3 and b HCl for acid condensation

Full size image
Table 3 Ir %conversion on the surface of carbon support using HNO3 and HCl for acid condensation
Full size table

OER Activity Study

In the present study, we have optimized all the IrOx conversion in terms of formation and deposition of IrOx on KB600, the high surface area carbon support. However, carbon-based 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 IrOx on the support by sonication at a pH of 5 was used to synthesize the IrOx/TiN catalyst with the Ir loading on TiN being 20 wt%. In terms of %conversion of Ir to IrOx deposited on the support, the TiN support shows results similar to those obtained with carbon support.

TEM images of the synthesized IrOx/TiN catalyst are shown in Fig. 5a, b, which reveal the TiN support particles of ~ 10 nm coated with fine IrOx NPs (~ 1 nm). The higher magnification TEM image of Fig. 5b exhibits uniform dispersion of the IrOx NPs on the TiN support. Further, the IrOx/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 IrOx compound in the catalysts, which suggests an amorphous nature of the synthesized IrOx. All the observed peaks correspond to the TiN support phase.

Fig. 5
figure 5

a, b TEM images of the synthesized IrOx/TiN electrocatalyst, c XRD patterns of bare TiN support and the IrOx/TiN sample, and d OER polarization curves for the IrOx/TiN catalyst along with the IrO2 commercial samples (inset: polarization curve with logarithmic scale for y-axis)

Full size image

OER performance of the as prepared IrOx/TiN and a commercial IrO2 (supplier: alfa Aesar) catalysts were examined by comparing their polarization curves in Ar-saturated 0.1 M HClO4 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 IrOx/TiN catalyst shows 2 times higher performance comparable with the standard commercial IrO2 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 (IrOx/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.

Table 4 Comparative OER activity with previous reported electrocatalysts
Full size table

Conclusions

The reaction steps involved in the formation of hydrated iridium oxide/hydroxide NPs have been systematically investigated. The stable colloids containing ligand-free IrOx particles were prepared in a two-step approach; at first, [Ir(OH)6]2- was created by hydrolyzing IrCl3 3H2O in an alkaline solution. Subsequently, IrOx particles are produced via acid treatment and the aging process, which oxidize the polymerized [Ir(OH)6]2- monomers. Further as prepared IrOx colloidal NPs decorated on the surface of carbon or TiN support using probe sonication process in the form of IrOx. 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 IrOx 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.