Self-Supported Ir-FeOOH on Iron Foam for Efficient Oxygen Evolution Reaction

Abstract

Developing high-performance oxygen evolution reaction (OER) electrocatalysts remains a critical challenge for sustainable hydrogen production via water electrolysis. Herein, we present a self-supported atomic iridium-decorated FeOOH nanostructure on iron foam (Ir-FeOOH/IF) by a facile impregnation reduction method. The self-supported Ir-FeOOH/IF electrode integrates the high electrical conductivity and outstanding mass transfer performance of IF. The FeOOH features abundant active sites, while the Ir modification regulated the electronic structure of FeOOH. As a result, the as-prepared Ir-FeOOH/IF catalyst (with the optimized synthesis time) achieves a low overpotential of 145 and 284 mV at current densities of 0.1 and 1 A cm⁻², respectively, and exhibits excellent long-term catalytic stability for 135 h at 0.1 A cm⁻² in a 1 M KOH solution. This work provides a new strategy for the design of low-cost and highly stable OER electrocatalysts.

1. Introduction

Over the past decade, transition metal-based compounds—particularly oxides and hydroxides, followed by sulfides, phosphides, and nitrides—have gained significant research attention as cost-effective alternatives to precious metal electrocatalysts for the oxygen evolution reaction (OER) [1,2,3,4,5,6]. Transition metal-based compounds, particularly Ni/Co-containing systems, have been extensively demonstrated as high-performance OER electrocatalysts [7,8]. This predominance originates from their tunable d-band electronic configurations that optimize the binding energy of oxygen-containing intermediates (*OH, *O, and *OOH) [9,10,11,12], as evidenced by recent studies on NiCoON (η = 247 mV@10 mA cm⁻²) [7] and NiOOH (η = 170 mV@10 mA cm⁻²) [13]. Notably, ligand engineering strategies can further modulate the metal–ligand coordination environments, as demonstrated in Ni(N(CN)₂)₂ derivatives achieving a Tafel slope of 40.8 mV dec⁻¹ [14,15]. Notably, iron (Fe), the second most abundant metal element in the Earth’s crust (5.6%, as shown in Figure S1 in the Supplementary Materials), has attracted substantial attention due to its environmental friendliness, excellent electrical conductivity, and intrinsic catalytic activity [16]. Fe-based catalysts also possess tunable physical and chemical properties, [15,17,18,19], so developing highly active and durable Fe-based electrocatalysts is of profound significance. Among various Fe-based OER catalysts, iron oxyhydroxides (FeOOH) have attracted considerable research interest due to their high catalytic activity and excellent stability [20,21,22]. The catalytic performance of FeOOH can be further enhanced by modulating the metal’s electronic structure by surface atom modification, non-metal doping, core–shell engineering, and interfacial engineering. For example, Amin et al. [23,24,25] reported that optimizing the interfacial interaction between Co₃O₄ and silver/nickel substrates effectively boosts OER kinetic efficiency. Additionally, introducing oxygen vacancies to adjust a metal’s d-band center can further boost catalytic efficiency. Niu et al. [26] developed a simple electrochemical method to create 3D vertically aligned Se-doped FeOOH nanosheets and demonstrated that Se doping optimizes the electrode structure, remarkably enhancing the OER activity of FeOOH. Yu et al. [27] facilitated an S-doped Ni/Fe hydroxide catalyst on nickel foam via a one-step method. The incorporation of S regulates the valence states of Ni and Fe, thereby optimizing the adsorption energy of OER intermediates. As a result, the catalyst exhibits superior OER performance and remarkable durability in both alkaline saline and seawater electrolytes [28,29,30]. Single-atom catalysts (SACs) demonstrate unique advantages in OER, particularly through metal–support electronic interactions that optimize adsorption energetics of oxygen intermediates. The noble metal Ir demonstrates superior OER activity; however, its high cost makes it necessary to achieve atomic-level dispersion [31,32]. Zhao et al. [33] reported an Ir single-atom-decorated (Co, Fe)-OH/ dimethylimidazole catalyst prepared via a simple immersion method, which achieves ultralow overpotentials of 179 mV at 10 mA cm⁻² and 257 mV at 600 mA cm⁻², along with an ultra-small Tafel slope of 24 mV dec⁻¹. Wu et al. [16] reported single Ir atoms anchored to a NiOOH catalyst induced an upward shift in the O 2p band and enhanced the metal–oxygen covalency. The obtained electrode exhibited exceptional electrocatalytic performance, achieving low overpotentials of 142 mV at 10 mA cm⁻² and 308 mV at 1000 mA cm⁻².

In this work, we fabricated a self-supported Ir atom-modified FeOOH on iron foam (Ir-FeOOH/IF) via a simple two-step method at room temperature. The addition of NaBH₄ induced lots of defects into the in situ generated FeOOH layer, providing abundant anchoring sites for Ir atoms. Moreover, an in situ cation-exchange method allowed for the uniform embedding and firm bonding of Ir atoms on the surface of the FeOOH layer. As a result, the Ir-FeOOH/IF (with optimized synthesis time) catalyst exhibits exceptional long-term catalytic durability and outstanding structural stability, even under a high current density.

2. Results and Discussion

2.1. Structural Characterization of Catalysts

As depicted in Figure 1a, the Ir-FeOOH/IF catalyst was fabricated via two consecutive wet chemical synthesis steps at room temperature. Three-dimensional iron foam (IF) was selected as the substrate due to its superior electrical conductivity, high mechanical strength, porous structure, and its chemical inertness, with a view to corrosion resistance in alkaline environments. Additionally, iron foam also serves as a source of iron for the in situ generation of the FeOOH layer. In the initial step, pristine IF was immersed in a 0.2 M NaBH₄ solution for 60 min. During this process, NaBH₄ hydrolysis generated H₂ bubbles that dynamically modulated the oxidation of IF, culminating in the formation of iron oxyhydroxide nanosheets (FeOOH) rich in atomic defects (Figure S2, Supplementary Materials). These defects, with a hybrid crystalline–amorphous architecture, were intentionally induced by NaBH₄ to optimize subsequent metal anchoring. In the second step, the FeOOH/IF-1 h was immediately transferred to a 0.2 M IrCl₄ solution for 60 min. Residual NaBH₄ sustained a reductive milieu, enabling selective occupation of defect sites—within the FeOOH matrix by Ir⁴⁺ ions, thereby forming stable Ir–O–Fe coordination bonds. This defect-guided strategy achieved atomic-scale dispersion of Ir, which can contribute to the enhancement of catalytic activity and durability. Importantly, to avoid an energy-intensive manufacturing process and to develop potentially scalable production techniques, the entire synthesis was conducted in an open vessel at room temperature. The synergy between NaBH₄-engineered defects and isolated Ir atoms not only enhanced active site density but also tailored the electronic structure of FeOOH, substantially lowering the energy barrier for oxygen-containing intermediates.

Post-synthesis catalyst characterization by scanning electron microscopy (SEM) shows the formation of uniform Ir-FeOOH/IF-1 h/1 h nanosheets (300~500 nm in length), which conformally coated the IF substrate (Figure 1b). Transmission electron microscopy (TEM) images further confirm the ultrathin nanosheet morphology (Figure 1c). The X-ray diffraction patterns show the (110) and (200) crystal planes are attributed to the iron foam, while no diffraction peaks were observed for the FeOOH and Ir crystal structures (Figure S3, Supplementary Materials). This might be due to the relatively lower crystallinity of FeOOH or due to interference from iron foam. We further employed high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS) to characterize the crystal structure and elemental composition of the Ir-FeOOH-1 h/1 h sample. Interestingly, the presence of two lattice fringes were evident in the HRTEM image shown in Figure 1d. It is worth noting that the interplanar spacing of 0.254 and 0.331 nm corresponds to the (211) and (310) crystal planes of FeOOH, confirming the successful construction of FeOOH. EDS elemental mapping shows a uniform distribution of Ir, Fe, and O element, suggesting that Ir atoms were successfully doped into the FeOOH lattice without the formation of an independent phase (Figure 1g). The atomic content of Ir atoms measured by EDS was 0.99 at.% (Figure S4, Supplementary Materials). Additionally, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy images (HAADF-STEM) individually characterized isolated Ir atoms and Ir clusters, which were uniformly dispersed in the FeOOH/IF and marked by orange circles and a yellow rectangle (Figure 1e), respectively.

In order to ascertain the chemical composition, oxidation state, and electronic interactions between FeOOH and Ir atoms, XPS analysis was performed. As illustrated in Figure 2a, the full-scan XPS spectra demonstrate the presence of the Fe, C, and O elements in the FeOOH/IF-1 h and Ir-FeOOH/IF-1 h/1 h samples. Due to the low Ir content, the characteristic peaks are not obvious in the full XPS spectrum of Ir-FeOOH/IF-1 h/1 h. The high-resolution Ir 4f XPS spectrum of Ir-FeOOH/IF-1 h/1 h is divided into four peaks at 60.89/63.86 and 62.27/65.23 eV, which are ascribed to Ir⁰ and Ir⁴⁺ states (Figure 2b), respectively, indicating the successful integration of Ir. Notably, during the synthesis of Ir-FeOOH sample, Ir atoms substitute for Fe atoms and form bonds with bridging oxygen atoms, leading to the formation of an Ir-O-Fe structure [34]. Thus, the valence of Ir is not limited to zero but other oxidation states are also present. Additionally, the +4 oxidation state of Ir is also likely attributable to its surface exposure to atmospheric oxygen. The high-resolution Fe 2p XPS peak fitting of Ir-FeOOH/IF-1 h/1 h demonstrates the characteristic peaks of 2p_1/2 and 2p_3/2 at 724.74, 721.10, 713.05, and 710.65 eV, respectively, which suggests that the elemental Fe coexists in the form of +2 and +3 valences (Figure 2c) [35]. Compared with the FeOOH/IF-1 h, the main Fe 2p XPS peaks in Ir-FeOOH/IF-1 h/1 h shift to higher binding energies, indicating that Ir atoms induce the local electron redistribution in FeOOH and optimize its electronic structure [15,36]. As illustrated in Figure 2d, the high-resolution O 1s XPS spectra of FeOOH/IF-1 h and Ir-FeOOH/IF-1 h/1 h demonstrate clear distinctions. For Ir-FeOOH/IF-1 h/1 h, the three peaks observed at 530.19, 531.66, and 532.14 eV could be ascribed to the lattice oxygen (M-O), hydroxyl oxygen (M-OH), and adsorbed oxygen in physically/chemically adsorbed water, respectively. After introducing Ir atoms, the O 1s XPS peak of Ir-FeOOH/IF-1 h/1 h significantly shifts toward the higher binding energies and shows higher lattice oxygen M-O concentrations in comparison with FeOOH/IF-1 h, which is attributed to the anchoring of Ir atoms on the surface of FeOOH/IF-1 h, suggesting the formation of abundant Ir-O-Fe motifs, which could enhance the interfacial charge transfer and local electron redistribution [37,38].

2.2. Electrocatalytic Performances of Ir-FeOOH/IF

In order to determine the optimal synthesis conditions for FeOOH catalysts, the effect of impregnation time on their catalytic activity was systematically investigated using the three-electrode system in the 1.0 M KOH solution. From the linear sweep voltammogram (LSV) curves, the electrocatalytic OER of FeOOH on IF gets better with the increase in impregnation time. As the impregnation time arrives at 1 h, the FeOOH/IF shows the best electrocatalytic performance. The FeOOH/IF-1 h sample delivers very low overpotentials of 220, 308, and 480 mV at current densities of 0.01, 0.1, and 0.5 A cm⁻², respectively (Figure 3a,b). For a better understanding of the OER performance, Tafel plots were drawn from their corresponding LSV curves. The Tafel slope of the FeOOH/IF-1 h catalyst was determined to be 55.7 mV dec⁻¹, which is significantly lower than those of FeOOH/IF-15 min (67.9 mV dec⁻¹), FeOOH/IF-30 min (62.4 mV dec⁻¹), FeOOH/IF-45 min (69.4 mV dec⁻¹), FeOOH/IF-3 h (61.3 mV dec⁻¹), FeOOH/IF-5 h (61.9 mV dec⁻¹), and IF (72.6 mV dec⁻¹), indicating much faster OER kinetics for the FeOOH/IF-1 h. Thus, the FeOOH/IF-1 h catalyst was chosen for the subsequent deposition of Ir atoms due to its superior catalytic performance (Figure 1e). The electrochemical impedance spectroscopy (EIS) tests show that the charge transfer resistance (R_ct) of FeOOH/IF-1 h is only 2.25 Ω, which is lower than those of the control samples by one order of magnitude (IF Blank: 56.78 Ω, FeOOH/IF-15 min: 35.72 Ω, FeOOH/IF-30 min: 22.24 Ω, FeOOH/IF-45 min: 16.91 Ω, FeOOH/IF-3 h: 5.5 Ω, FeOOH/IF-5 h: 2.49 Ω), indicating significant improvement in the interfacial charge transfer efficiency (Figure 3c). To evaluate the intrinsic activities of the as-prepared electrocatalysts, the electrochemical active surface area (ECSA) of an electrocatalyst was measured by a double-layer capacitance (C_dl) method, obtained from cyclic voltammetry curves in a non-Faradaic potential range at different scan rates. The double-layer capacitance (C_dl) value of FeOOH/IF-1 h was determined to be 28.71 mF cm⁻² (Figure 3d and Figure S6, Supplementary Materials), which is also significantly higher than those of IF (16.35 mF cm⁻²) and other control samples with different impregnation times, implying that FeOOH/IF-1 h has the largest electrochemically active surface area and exposes more active sites. Obviously, the FeOOH catalysts prepared by 1 h of impregnation show the best performance in terms of reaction kinetics, charge transfer efficiency, and active site density. Further, Tafel slopes were calculated by linearly fitting the polarization curves to investigate the OER kinetics. The Tafel slope of FeOOH-1 h was determined to be 55.7 mV dec⁻¹, which is significantly smaller than the value of the compared catalysts, indicating that FeOOH-1 h has superior OER kinetics. Thus, the FeOOH/IF-1 h catalyst was selected for the subsequent deposition of Ir atoms.

Using the FeOOH/IF-1 h as support, the atomic Ir is loaded via an impregnation reduction method. Figure 4a shows the LSV curves for the Ir-FeOOH/IF-1 h with different immersion times. Similarly, the OER catalytic activity of the Ir-FeOOH/IF-1 h continuously improves with immersion times. Obviously, when impregnation time is 1 h, the Ir-FeOOH/IF-1 h/1 h catalyst shows the best OER catalytic activity, with very low overpotentials of 145, 211, 245, and 284 mV at current densities of 0.01, 0.1, 0.5, and 1 A cm⁻², respectively (Figure 4a,d). The charge transfer for different catalysts was analyzed using EIS spectra. As shown in Figure 4b, the R_ct value of Ir-FeOOH/IF-1 h/1 h is 0.33 Ω, which is lower than those of Ir-FeOOH/IF-1 h/15 min (2.25 Ω), Ir-FeOOH/IF-1 h/30 min (1.25 Ω), Ir-FeOOH/IF-1 h/45 min (0.81 Ω), Ir-FeOOH/IF-1 h/3 h (0.48 Ω), and Ir-FeOOH/IF-1 h/5 h (0.66 Ω), implying the fastest charge transfer rate in the electrochemical OER process for the Ir-FeOOH/IF-1 h/1 h catalyst. The Tafel slope of Ir-FeOOH/IF-1 h/1 h was determined to be 49.4 mV dec⁻¹, which is smaller than those of Ir-FeOOH/IF-1 h/15 min (83.4 mV dec⁻¹), Ir-FeOOH/IF-1 h/30 min (70.7 mV dec⁻¹), Ir-FeOOH/IF-1 h/45 min (61.9 mV dec⁻¹), Ir-FeOOH/IF-1 h/3 h (54.2 mV dec⁻¹), and Ir-FeOOH/IF-1 h/5 h (53.6 mV dec⁻¹), indicating its faster reaction dynamics (Figure 4c). Figure 4f and Figure S7 show the C_dl value for the Ir-FeOOH/IF-1 h/1 h sample is 24.33 mF cm⁻², which is higher than those of Ir-FeOOH/IF-1 h/15 min (10.67 mF cm⁻²), Ir-FeOOH/IF-1 h/30 min (12.90 mF cm⁻²), Ir-FeOOH/IF-1 h/45 min (13.96 mF cm⁻²), Ir-FeOOH/IF-1 h/3 h (21.47 mF cm⁻²), and Ir-FeOOH/IF-1 h/5 h (20.84 mF cm⁻²), suggesting the Ir-FeOOH/IF-1 h/1 h has the largest electrochemical active surface area. To further evaluate the long-term stability of the Ir-FeOOH/IF-1 h/1 h catalyst, a chronoamperometry test was performed at a constant potential of 1.43 V for 135 h (Figure 4e). The current density of the Ir-FeOOH/IF-1 h/1 h catalyst decreased from an initial value of 101.7 mA cm⁻² to 93.45 mA cm⁻², representing only an 8.1% reduction. Figure S9 shows a multi-potential step chronoamperometry test under various applied potentials ranging from 1.45 V to 1.48 V, in accordance with the previous reported method [39]. The current density of Ir-FeOOH/IF-1 h/1 h remained nearly constant in the equal potential section, demonstrating its outstanding durability. As shown in Figure S10, the LSV curve after the long-term stability increases 20 mV compared to the initial curve at a current density of 0.5 A cm^-2. These results indicate that the developed Ir-FeOOH/IF-1 h/1 h exhibited superior OER activity and good durability in an alkaline solution. The H₂ and O₂ products are collected using a drainage method under constant potential of 1.6 V. The measured amount of oxygen was consistent with the theoretical oxygen output, and the Faradaic efficiency (FE) of oxygen is close to 100% (Figure 4g and Figure S8, Supplementary Materials), indicating excellent charge transfer ability of Ir-FeOOH/IF-1 h/1 h catalyst.

3. Materials and Methods

3.1. Materials

Iridium (IV) chloride hydrate (IrCl₄·xH₂O, 99.9%) was purchased from Shanghai Adamas Reagent Co., Ltd. (Shanghai, China). Sodium borohydride (NaBH₄, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Iron foam was procured from Kunshan Lone Xuan Drill Electronics Technology Co., Ltd. (Suzhou, China).

3.2. Synthesis of FeOOH/IF

Iron hydroxide (FeOOH) nanosheets were in-situ grown on iron foam (IF) through a spontaneous reaction between IF and a NaBH₄ solution at room temperature (RT, 25 °C). Firstly, the iron foam (IF) was cut into dimensions of 1 cm × 2 cm. Then, the IF was cleaned sequentially with ethanol and deionized water, each for 15 min. Subsequently, the IF was immersed in a 0.2 M NaBH₄ aqueous solution and maintained at room temperature for 1 h. The other catalyst samples were also prepared with different reduction times (15 min, 30 min, 45 min, 3 h, and 5 h), which were, respectively, named FeOOH/IF-15 min, FeOOH/IF-30 min, FeOOH/IF-45 min, FeOOH/IF-3 h, and FeOOH/IF-5 h.

3.3. Synthesis of Ir-FeOOH/IF

The FeOOH/IF-1 h sample was taken out of the NaBH₄ solution and subsequently immersed in 0.2 mg mL⁻¹ of IrCl₄ aqueous solution for 1 h. Then, the obtained Ir-FeOOH/IF was rinsed with deionized water and ethanol, then dried under vacuum at 60 °C for 2 h. We also prepared control group samples with different reduction times (15 min, 30 min, 45 min, 3 h, and 5 h), which were, respectively, named Ir-FeOOH/IF-1 h/15 min, Ir-FeOOH/IF-1 h/30 min, Ir-FeOOH/IF-1 h/45 min, Ir-FeOOH/IF-1 h/1 h, Ir-FeOOH/IF-1 h/3 h, and Ir-FeOOH/IF-1 h/5 h.

3.4. Instruments and Characterization

The surface morphology of the catalyst was characterized using a scanning electron microscope (SEM, JEOL JSM-7800F, JEOL Ltd., Shanghai, China). The internal fine structure of the materials was examined by transmission electron microscopy (TEM, FEI TF20, UCLA, Los Angeles, CA, USA) with energy-dispersive X-ray spectroscopy (EDS, ESCALABMK II, IRDQ, Montreal, QC, Canada). The crystal structure was analyzed using X-ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan), while the chemical states of surface elements were determined by X-ray photoelectron spectroscopy (XPS, Thermo K-Alpha, Thermo Scientific, Waltham, MA, USA). Electrochemical properties were evaluated using an electrochemical workstation (CHI 760e, Shanghai Chenhua Instrument Co., Ltd. Shanghai, China), and stability tests under high current densities were conducted using a Henghui DC power supply (PLD-3010) (Shenzhen, China).

3.5. Catalyst Characterization and Electrochemical Testing

The electrocatalytic performance of all the above samples was tested on an electrochemical workstation (CHI 760e, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). Electrochemical measurements were conducted at room temperature in a standard three-electrode system, where the as-prepared catalyst, Hg/HgO electrode, and Pt sheet electrode served as the working electrode, reference electrode, and counter electrode, respectively. Prior to testing, oxygen gas was continuously delivered into the electrolyzer to obtain O₂-saturated 1 M KOH aqueous solution. All potentials reported in our work were converted to the reversible hydrogen electrode (RHE) scale according to the following: E (vs. RHE) = E (vs. Hg/HgO) + 0.059 × pH + 0.098 V (herein pH = 14), with an iR drop compensation of 85%. Linear scanning voltammetry (LSV) was performed at a scanning rate of 5 mV s⁻¹. The overpotential (η) was calculated according to the following: η = E (vs. RHE)-1.23 V. The Tafel slope was obtained by fitting the experimental data to the equation η = a + b × log |j|, where b is the Tafel slope, η is the overpotential, and j is the current density (mA cm⁻²). The electrochemical active surface area (ECSA) was determined using the double-layer capacitance (C_dl) method: the double-layer charging current was measured and plotted as a function of scan rates (20–100 mV s⁻¹) within a potential range of 1.1–1.2 V (vs. RHE). Electrochemical impedance spectroscopy (EIS) was performed at a potential corresponding to a current density of 10 mA cm⁻² in the frequency range of 100 kHz to 0.01 Hz with an amplitude of 5 mV. Long-term stability was evaluated via chronoamperometry (i-t) at a constant current density of 100 mA cm⁻² for 135 h continuously.

4. Conclusions

In summary, we fabricated an atomic Ir-decorated FeOOH catalyst on iron foam via a two-step facile impregnation–reduction method using NaBH₄ as the reductant. The as-obtained Ir-FeOOH/IF-1 h/1 h catalyst exhibits excellent water oxidation performance, achieving low overpotentials of 254 mV at a current density of 500 mA cm⁻² and 284 mV at 1 A cm⁻² in 1.0 M KOH electrolyte at 25 °C. The enhanced conductivity and stability of the Ir-FeOOH/IF-1 h/1 h catalyst are attributed to the modulation of the electronic structure of Fe sites by Ir atoms. The R_ct analysis reveals that the OER kinetics were accelerated and the energy barrier decreased after Ir deposition on the FeOOH layer. Furthermore, this catalyst demonstrates excellent long-term durability, maintaining stable OER performance for 135 h at a current density of 100 mA cm⁻². This work presents a simple in situ synthesis method for highly effective Ir-FeOOH catalysts, thereby paving the way for cost-effective and scalable water electrolysis approaches.