Operando spectroscopic observation of dynamic-coupling oxygen on single-atomic iridium catalyst for acidic water oxidation

Uncovering the dynamics of active sites under working state is crucial to realizing increased activity, enhanced stability and reduced cost of oxygen evolution reaction (OER) electrocatalysts in proton exchange membrane electrolytes. Herein, we identify at atomic level a potential-driven dynamic-coupling oxygen on the hetero-nitrogen congured single-atomic Ir sites (HN-Ir NC) during OER working conditions to successfully endow the single-atomic Ir catalyst with an ultrahigh electrochemical acidic-OER activity. Using operando synchrotron radiation infrared and X-ray absorption spectroscopies, we directly observe in the experiment that a dynamic oxygen atom is formed at the Ir site with the O-hetero-Ir-N 4 structure as more electrophilic active center and then effectively promote the generation of the key * OOH intermediates under working potentials, which is exceptionally favourable for the dissociation of H 2 O over Ir sites and resistance to over-oxidation and dissolution of the active sites.The optimal single-atomic HN-Ir NC catalyst delivers a large mass activity of 2860 A g metal−1 and a huge turnover frequency of 5110 h − 1 at a low overpotential of 216 mV (10 mA cm − 2 ), 480 (cid:0) 510 times than that of commercial IrO 2 catalyst. More importantly, the HN-Ir NC catalyst shows no evident deactivation after continuous 100 h OER operation in acidic medium.


Introduction
Polymer electrolyte membrane water electrolysis (PEMEC) is viewed as a clean and promising way to convert electrical energy to hydrogen, playing an important role in modern sustainable energy conversion and storage devices. [1][2][3] Due to the sluggish oxygen evolution reaction (OER) kinetics and harsh acidic environment, the electrode of PEMEC should be catalytically active and highly stable under operating conditions. [3][4][5] Currently, iridium-based metal oxides, such as IrO 2 , have been widely used in water electrolysis. 6,7 Unfortunately, IrO 2 still tends to decompose in strong acidic media ascribed to the loss of active surface area and the changes of iridium species oxidation state under a prolonged operating condition. 8,9 Although signi cant efforts have been devoted to developing various acidic catalysts, the corresponding catalytic mechanisms are still unclear attributed to the complex dynamic reaction system. 10,11 Furthermore, most of operando-tested electrodes are fabricated by simply casting catalyst powder onto substrates with only physical contact, and the detached sample of electrode would affect the signal of operando experiment leading to inauthentic dynamic evolution under OER conditions. Therefore, the development of highly acidic-resistance OER electrocatalysts with an excellent catalytic activity and a stable electrode con guration from the aspect of mechanistic understanding is much anticipated for practical PEMEC applications but remains a considerable challenge. To this end, an atomic-level identi cation of the nature of the active site under working conditions is imperative to design e cient acidic-resistance OER catalysts.
Up to now, enormous efforts have been paid to improve acidic-OER catalysts, such as transition metal substitution, structural engineering, element doping and so on (Co-RuIr, Ir x Cu, IrO x /SrIrO 3 , Ir-STO). [12][13][14][15] However, the easy changes in the oxidation state of the Ir-based sample would cause continuous dissolution of active metal attributed to the presence of manifold Ir-O coordination. 9 Recently, downsizing the expensive IrO x to single-atom iridium provides an e cient pathway to create cost-e cient catalyst and improve its activity and stability because of the strong metal-support interaction. 16 The catalytic activity and stability of single-atom catalysts are highly dependent on the local coordination structure of metal sites, directly related to the interaction between electrons and the geometric structure. The strong metal-support interactions can facilitate large electrons transfer from the metal to neighboring atoms towards high acidic-OER activity. Although various M-N x moieties have been achieved in single-atom catalysts towards e cient electrocatalytic reaction, [17][18][19] inadequate electronic structure activation and unreliable atomic con guration limit the potential of M-N-C catalysts for high e ciency and stable acidic water oxidation. 20,21 Hence, it is urgently necessary to nd an effective way to activate the acidic OER activity and improve the dissolution resistance of ultra-low iridium catalysts via a strong electron coupling between the single-atom active sites and their single-atom anchoring substrate. 22 Moreover, it has been reported that the structure of single-atom catalysts is easily reconstructed to a higher valence state for faster adsorption of reactants to achieve e cient activity in catalytic reactions. This means that the electronic structure of the active center can be dynamically manipulated at low voltages to signi cantly improve the catalytic activity. Therefore, for the single-atom centers embedded into 3D conductive substrate with stable active sites con guration, in-depth understanding of the reaction dynamics of active catalytic center at the atomic scale can provide new insights into the design of acidic-OER catalysts.
Here, to realize high catalytic activity and long-time durability in acidic medium, we design a new type of single-atomic Ir sites coupling in heterogeneous nitrogen-con gured 3D substrate (HN-Ir NC) as ultralowiridium electrocatalysts via a controllable and novel "electric-driven amino-induced" strategy. At atomic level, the active hetero-Ir-N 4 moieties were spatially con ned in the 3D conductive carbon substrate with a strong interfacial chemical coupling owing to the selective ion-bonding effect in the weak redox units of the PAni and electric-driven NH 2 ( Figure 1a). Interestingly, the operando synchrotron radiation X-ray absorption ne structure (XAFS) spectroscopy reveals that an oxygen was dynamically pre-adsorbed on the Ir site in the form of O-hetero-Ir-N 4 moiety under low driven-potential, which accelerates the transfer of electrons from the metal sites to neighboring atoms under the oxygen traction towards faster reaction kinetics. More importantly, the operando synchrotron radiation infrared (SRIR) and electrochemical impedance spectroscopy (EIS) directly observed H 2 O adsorption under low driven voltage and crucial * OOH intermediate production on the active O-hetero-Ir-N 4 sites because of electrophilic effect of dynamiccoupling oxygen during the acidic-OER process, quickly overcoming the rate-determining step of water oxidation and then boosting an e cient acidic-OER activity and stability. As a result, the developed HN-Ir NC delivers low overpotential of 216 mV to achieve a current density of 10 mA cm -2 for acidic OER with an ultra-high mass activity of 2860 A g metal -1 , and a huge turnover frequency (TOF) of 5110 h -1 , ~480 and 510 times than that of commercial IrO 2 catalyst (6 A g -1 , 10 h -1 ), which endows HN-Ir NC as the best acidic-OER electrocatalyst based on the mass activity reported so far. Furthermore, the HN-Ir NC catalyst could show ~97% Faradaic e ciency for OER and have no obvious attenuation of essential acidic-OER activity in the continuous operation of 100 h in acidic medium.

Results And Discussion
Structure characterization of HN-Ir NC. The ultra-low-iridium electrocatalyst with atomically-dispersed Ir sites coupling in the heterogeneous nitrogen-con gured 3D substrate (HN-Ir NC) was synthesized via a controllable and novel "electric-driven amino-induced" strategy. The polyaniline layer with plenty of surface reductive benzenoid-amine groups was rst electrodeposited onto a 3D carbon substrate, and then functionalized with amino groups by mild heat treatment in ammonia solution. It is noted that the surface reductive benzenoid-amine groups and -NH 2 -derived uncoordinated N site act as the anchoring sites for Ir atoms during ions exchange and pyrolysis processes. Notably, the ultra-low-iridium atoms were chemically coupled with N sites on the 3D substrate to form the active and stable HN-Ir NC catalyst. The To determine the local structure of Ir at the atomic scale, the X-ray absorption near edge spectroscopy (XANES) of C and N K-edges were collected. The C K-edge spectra of PAni-800 ˚C in Figure 2a shows two obvious peaks located at 285.4 and 288.7 eV can be assigned to the 1s→2p excitation of C-C π * and C-N 2 , respectively. 23 After Ir sites coupling onto NC substrate, a new peak was observed at 287.8 eV that can be attributed to the excitation of C-N 1 , suggesting the presence of hetero-nitrogen after ammonia water treatment. The N K-edge in Figure 24,25 This further clearly demonstrates the formation of heterogeneous N con gured with Ir sites for HN-Ir NC. Moreover, the Ir 4f XPS spectra clearly reveals that high-valence Ir is coupled onto 3D substrate in the form of Ir 3+ with hetero-N con guration ( Figure 2d). 26 The atomic structure con guration of Ir site was obtained by the measurements of Ir L 3edge XAFS spectra. As seen from XANES of Ir L 3 -edge in Figure 2e, it can be inferred that the high-valence Ir forms the coordination structure of Ir-N based on the position of the absorption edge and the oscillation frequency. Figure 2f shows one peak located at ~1.6 Å that can be assigned to the Ir-N coordination. 27 To further quantify the local coordination structure of Ir, the EXAFS of Ir was additionally tted. The tted results in Supplementary Table 1 show that the coordination number is close to 4 with two short and two long bonds, consisting of Ir-N 1 (1.92 Å) and Ir-N 2 (1.95 Å) and suggesting the hetero-N con guration of atomic-dispersed Ir sites. Therefore, above results clearly verify the formation of hetero-Ir-N 4 moieties coupled onto 3D substrate by coordinating Ir sites with hetero-N ligand for the HN-Ir NC catalyst.
OER performance of HN-Ir NC catalyst. The electrocatalytic OER activities of HN-Ir NC were performed in 0.5 M H 2 SO 4 with a three-electrode system, along with PAni-Ir-800˚C, Ir nanoparticle loading NC catalyst (Ir-NP/NC) and commercial IrO 2 for comparison. Figure 3a shows the linear sweep voltammetry (LSV) curves for the HN-Ir NC and the reference samples. Dramatically, the HN-Ir NC exhibits the most excellent acidic-OER activity, only requiring ultra-low overpotentials of 216 and 292 mV to achieve water oxidation current density of 10 and 100 mA cm -2 , respectively. In contrast, as shown in Figure 3b, the PAni-Ir-800 ˚C and commercial IrO 2 display insu cient acidic-OER activity (320, 412 and 300, 385 mV, respectively, KJ mol -1 ) under the overpotential of 216 mV, which further proves the faster OER kinetics after highvalence Ir single sites integrated onto the carbon substrate with the active hetero-N-coordination.
To further obtain the intrinsic activity of HN-Ir NC, the mass activity and TOF were calculated according to the active Ir sites (Figure 3e). It needs to be specially noted that the mass activity and TOF of HN-Ir NC are about 2860 A g metal -1 and 5110 h -1 at a pretty small overpotential of 216 mV, ~480 and 510 times than that of commercial IrO 2 catalyst (6 A g -1 , 10 h -1 ). Notably, the HN-Ir NC displays ultra-high mass activity of 10900 A g metal -1 and TOF of 18900 h -1 at a representative overpotential of 260 mV. To the best of our knowledge, the mass activity of HN-Ir NC is the highest among the reported acidic-OER electrocatalysts (Supplementary Table 2). With the increase of overpotentials, the mass activity and TOF increase signi cantly, indicating faster OER catalytic kinetics. The details of the oxygen product quanti cation by gas chromatography are presented in Supplementary Fig. 14. It can be seen that the HN-Ir NC offers an excellent four-electron (4e -) OER with Faradaic e ciencies of 95-98% under various potentials in acid medium. Thanks to the excellent acidic OER performance of HN-Ir NC, we have conducted the test of overall water splitting in a two-electrode con guration. The HN-Ir NC and commercial Pt/C were used as the anode and cathode, respectively. It can be drawn from Supplementary Fig. 15 that the input potential of water splitting in practical electrolyzers is about 1.49 V for achieving the current density of 10 mA cm -2 , signi cantly superior to that of standard electrodes (Pt/C vs IrO 2 , 1.58 V). The stability of catalyst is an important performance index for evaluating the real application potential, especially in harsh operating conditions. To assess the durability of HN-Ir NC and reference sample, a chronopotentiometry test at the current density of 10 mA cm -2 was performed. As shown in Figure 3f, essential acidic-OER activity has no apparent attenuation in continuous operation of 100 h for HN-Ir NC, obviously outperforming commercial IrO 2 , which is attributed to the formation of hetero-Ir-N 4 moieties strongly coupled onto 3D substrate ( Supplementary Figs.16, 17). It can e ciently avoid the repeated changes in the Ir oxidation state for faster water oxidation kinetics and strong acid-resistance.
Operando EIS and SRIR analysis. EIS is an effective electrochemical measurement method to obtain the adsorption and desorption kinetics information of the reactants on the electrode surface. To intuitively obtain the evolution of the reactive species H 2 O or key intermediate * OOH on active sites, operando EIS tests were performed at different applied voltages to further understand the electrochemical reaction kinetics process. 28 As seen from the Nyquist plots in Figure 4a  Importantly, these Nyquist plots should rst need to be tted by equivalent circuit diagrams to access to the key tting parameters R CT and C DL (Supplementary Fig. 18). R CT and C DL can be used to represent the H 2 O/ * OOH ions adsorption resistance and pseudocapacitance on the catalyst surface. It can be seen from the changes of tting results for R ct in Figure 4c, the R ct of HN-Ir NC is lower than that of PAni-Ir-800 ˚C, suggesting a faster adsorption kinetics of key intermediates * OOH during acidic OER process (Supplementary Table 3, 4). Notably, the R ct decreases faster than that of PAni-Ir-800˚C within 1.1 V, which further reveals that oxygen-species are rapidly adsorbed at lower drivenvoltages to produce key intermediates * OOH accumulated over the active hetero-Ir-N 4 moieties.
To effectively identify the reaction intermediates and access to the catalytic reaction mechanism, operando SRIR measurements, which is highly sensitive to oxygen-containing intermediates, was performed with a home-made electrochemical SRIR cell. 29,30 The SRIR results of HN-Ir NC were obtained at different applied voltages in Figure 4d. acidic-OER process. 30 In comparison, operando SRIR measurements were performed for PAni-Ir-800˚C at typical voltages of 1.30 V (representing initial potential) and 1.45 V (representing reaction potential). Figure 4e shows that no obvious absorption bands were observed in the vibration frequencies of 1300-600 cm -1 for PAni-Ir-800˚C under potential of 1.45 V (Supplementary Figs.19). This phenomenon indicates that high-valence Ir sites coupled onto 3D substrate with hetero-N con guration by strong coupling substrate effect can pre-adsorb an oxygen to form stable and highly active O-hetero-Ir-N 4 structure, obviously accelerating the 4-electron kinetics process and promoting the formation of key OER reaction intermediates. To clarify the dynamic relationships between Ir-O and intermediates * OOH, the peak intensity enhancements of the absorption bands at 784 and 1055 cm -1 were plotted as a function of applied potential in Figure 4f. Notably, along with the increased applied potentials, the vibration band intensity of key * OOH for HN-Ir NC catalyst increases obviously higher than that of PAni-Ir-800˚C. Most importantly, the dynamic-coupling of oxygen occurs at 1.25 V, clearly before the appearance of the new vibration band of key * OOH for HN-Ir NC during OER working conditions. Above results clearly reveal that the potential-driven dynamic-coupling of oxygen over the Ir single sites was observed, which would in turn evidently promote the generation of the key *OOH intermediates over the O-hetero-Ir-N 4 moieties towards an e cient and fast 4e -OER process in acidic medium.
Operando XAFS analysis. To further access to the evolutionary nature of the active Ir single sites under working conditions, operando XAFS measurement ( Supplementary Fig. 20) was carried out in a homemade cell by using three-electrode standard electrochemical workstation. 32 The XANES spectra under different potentials in Figure 5a show that the intensity of white-line of HN-Ir NC increases gradually with the increase of voltage, accompanied by a slight positive-shift. This indicates that more electron vacant states of Ir sites were observed under voltage-driven, where more electrons move from Ir to nearby atoms and adsorb oxygen-species to active electronic surface states of Ir centers. This result suggests that the traction of pre-adsorbed-oxygen on the Ir site under potential-driven obviously optimized the electronic structure of hetero-Ir-N 4 moieties for faster reaction kinetics. To further clarify the evolution of the local coordination structure of Ir sites under applied potential, the FT Ir-L 3 edge EXAFS were shown in Figure   5b, where a dominant peak of ~1.6 Å can be attributed to the coordination structure of metallic and nonmetallic bonding (Ir-N/O). Interestingly, the peak intensity increased signi cantly at potential of 1.25 V and further enhanced with increased potential applied onto the electrode, suggesting a local structural self-optimization and adsorption of reactive species under OER working conditions. Quantitatively, the Ir L 3 -edge EXAFS tting results (Figure 5c, Supplementary Fig. 21 and Table 5 the key * OOH intermediates on the activated O-hetero-Ir-N 4 moieties by electrophilic effect, which is consistent with the operando SRIR results. The operando SRIR and XAFS results jointly reveal that a potential-driven dynamic oxygen-coupling was achieved under OER working conditions, which promotes the generation of key * OOH intermediates and then accelerates the four-electron reaction kinetics towards an e cient OER in acidic environments (Figure 5d).

Discussion
In summary, a highly-e cient and durable single-atomic HN-Ir NC catalyst with strong coupling hetero-Ir-N 4 moieties was developed by a controllable and novel "electric-driven amino-induced" strategy. Dynamic mechanism studies by using the advanced operando XAFS and SRIR techniques indicated that an oxygen is dynamically coupled onto the Ir site and then a key *OOH is produced by the electrophilic effect of dynamic-coupling oxygen over O-hetero-Ir-N 4 moieties under working potentials, which greatly accelerates the 4-electron reaction kinetics towards an e cient acidic-OER activity and stability. The asobtained single-atomic HN-Ir NC catalyst delivers an ultra-low overpotential of 216 mV at 10 mA cm -2 , reaching an ultra-high mass activity of 2860 A g -1 and a huge turnover frequency (TOF) of 5110 h -1 , which endows HN-Ir NC as the best acidic-OER electrocatalyst based on the mass activity and TOF reported so far. Hence, this discovery of active sites evolution under working conditions can provide a coordination-engineered strategy for designing advanced acidic-OER electrocatalysts.

Methods
Synthesis of the HN-Ir NC. The HN-Ir NC was fabricated in typical three well-designed steps, consist of surface functionalization of 3D carbon paper, surface amino-treatment and ion-exchange, hightemperature pyrolysis. Firstly, the 3D carbon paper is activated and cleaned by calcining it at 500℃ under air for 2h, and then washed with concentrated nitric acid and deionized water. Next, the polyaniline is electrodeposited on the 3D substrate in a three-electrode system. Speci cally, pre-treated carbon paper, carbon rod, and saturated calomel electrode (SCE) were employed as working, counter, and referenced electrodes respectively, and a mixed solution of aniline (10 ml), concentrated nitric acid (12 ml) and deionized water (78 ml) was used as electrolyte. The working electrode was rst subjected to anodic treatment at an oxidative constant voltage of 0. surface benzenoid-amine and amino groups in surface polyaniline can easily couple with Ir complex ions, rmly anchoring Ir ions with heterogeneous nitrogen on the surface of carbon paper. Finally, the obtained 3D carbon paper with rich Ir atoms was transferred to a tubular furnace for high temperature annealing in an argon atmosphere, where the furnace was maintained at 800°C for 3h to obtained the HN-Ir NC catalyst.
Synthesis of the PAni-Ir-800°C. The general process of synthesis is similar to that of the preparation of HN-Ir NC. It is noted that in the process of substrate surface functionalization, PAni was not treated with concentrated ammonia water, and the nal catalyst did not form hetero-nitrogen coordination and it was called PAni-Ir-800°C.
Synthesis of Ir-NP/NC. The general process of synthesis is similar to that of the preparation of PAni-Ir-800°C. It is noted that in the process of the adsorption of Ir precursor, it is necessary to increase the metal feeding amount to 0.18 mmol/L, and then the carbonization temperature was increased to 1000°C. The obvious Ir nanoparticles were obtained over the 3D carbon paper and it was called Ir-NP/NC.

Calculation of Roughness Factor (RF)
The F represents Faraday constant (96485 c cm -1 ). NA represents Avogadro constant (6.02*10 23 ) We estimated the number of the active sites as the total number of the surface sites of Ir for the catalysts, which might underestimate the real TOF.
Mass activity (A g metal -1 ) values were calculated from the electrocatalyst loading m and the measured current density j at overpotential of 216 and 260 mV: Operando EIS measurements. Operando characterization tandem electrochemical operation were carried out specify potential for 10 min to obtain the surface chemical composition and structural information of materials. EIS tests were performed at different potentials in the frequency range of 0.01-100000 Hz.
Operando SRIR measurements. Operando SRIR measurements were performed at the infrared beamline BL01B of the National Synchrotron Radiation Laboratory (NSRL, China) by using home-made top-plate cell, which the infrared transmission window of cell is made of ZnSe crystal with low light absorption (cut-off energy of ~625 cm −1 ). The synchrotron radiation IR testing device consists of two carefullydesigned parts, including an IR spectrometer (Bruker 66 v/s) with a KBr beam splitter and various detectors (herein, a liquid-nitrogen-cooled MCT detector was used) and an IR microscope (Bruker Hyperion 3000) with a 16x objective, which can provide infrared spectroscopy measurements over a broad range of 15-4000 cm -1 and a high spectral resolution of 0.25 cm -1 . Attentively, during the operando SRIR measurement, infrared signal is very sensitive to water molecules and is easily disturbed. By pressing the electrode and window to control the thickness of the water lm in the micron-scale, the infrared signal-to-noise ratio (SNR) is improved. To obtain the information of catalyst surface and further improve the signal quality of SRIR spectra under working condition, the re ection mode of infrared light with vertical incidence is adopted in the test. Bedsides, each high-resolution infrared absorption spectrum with resolution of 2 cm −1 was obtained by averaging 514 scans. To avoid the signal difference caused by sample falling off, a constant potential was applied to the catalyst electrode for 20 min, and then all infrared spectral acquisitions were carried. The background spectrum of the catalyst electrode was acquired at an open-circuit voltage before each systemic OER measurement, and the measured potential ranges of the OER were 1.0-1.42 V. Noticeably, in order to show a higher signal-to-noise ratio of the data, the curves were smoothed through 100 points in the later stage of data processing.
Operando XAFS measurements. The operando XAFS measurements of Ir L 3 -edge were carried out at the 1W1B station in the Beijing Synchrotron Radiation Facility (BSRF), China. The storage ring of BSRF was operated at 2.5 GeV with a maximum current of 250 mA. The beam from the bending magnet was monochromatized utilizing a Si (111) double-crystal monochromator and further detuning of 15% to remove higher harmonics. The electrochemical operando XAFS tests were performed by home-made cell in 0.5 M H 2 SO 4 electroyte. The XAFS spectra were collected through the uorescence mode with 19element solid state detector. The HN-Ir NC catalyst on the 3D carbon paper was cut into 1*2 cm 2 and then sealed in a cell by Kapton lm. In order to obtain the dynamic evolution information of the active site during the electrochemical reaction, a series of representative voltages (1.25~1.45 V) were applied to the electrode. During the collection of XAFS measurements, the position of the absorption edge (E 0 ) was calibrated using standard sample of Ir, and all XAFS data were collected during one period of beam time.

Declarations
Data availability All the data reported in this paper are available from the corresponding author upon request.