Unmasking active sites in linker coordinated nickel–cobalt hydroxide for the electrocatalytic iodide oxidation reaction

Abstract

Identification of the active sites responsible for both the oxygen evolution reaction (OER) and the iodide oxidation reaction (IOR) is critical for the rational design of high-performance catalysts. However, active sites for the IOR have not been previously explored to the best of our knowledge. Herein, we investigate the active sites of the OER and IOR on a benzoate-coordinated NiCo-hydroxide (BZ-NiCo-H) catalyst. Spectroscopic studies reveal that benzoate coordination modulates the electronic and coordination environment, facilitating the generation of high-valent metal species. BZ-NiCo-H exhibited excellent catalytic activity, delivering 100 mA cm⁻² at 1.42 V vs. RHE potential for the IOR and 1.63 V vs. RHE potential for the OER. In situ Raman spectroscopy coupled with electrochemical analyses identifies Ni⁴⁺ in γ-Ni(O)OH as the OER active site, whereas Ni³⁺ in β-Ni(O)OH functions as the active site for the IOR. Mechanistic investigations demonstrate that benzoate coordination activates lattice oxygen, enabling the OER to proceed via a lattice oxygen mechanism (LOM). Furthermore, in situ electrochemical impedance spectroscopy and kinetic studies show that benzoate coordination significantly enhances charge transfer and lowers kinetic barriers for both reactions. These findings elucidate the distinct active sites for the OER and IOR, highlighting the beneficial role of organic ligand coordination in tuning catalytic performance.

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1. Introduction

In recent years, hybrid water electrolysis has gained significant attention, where the conventional oxygen evolution reaction (OER) at the anode is replaced by anodic oxidation reactions (AORs), involving the oxidation of different organic and inorganic substrates.¹ Researchers have explored various AORs, such as the hydrazine oxidation reaction (HzOR), urea oxidation reaction (UOR), iodide oxidation reaction (IOR), glucose oxidation reaction (GOR), alcohol oxidation reaction (AlOR), amine oxidation reaction (AmOR), and biomass oxidation reaction (BOR), integrated with hydrogen evolution reaction to improve energy efficiency and reduce cell potential.^2,3

Among the various AORs, the IOR has recently gained attention as a promising alternative to the OER, primarily due to its ability to produce high-value added iodate (IO₃⁻) (Fig. 1a and b).^4,5 Iodate is a commercially important compound with diverse applications in the pharmaceutical industry, iodized salt production, and analytical chemistry, and as an oxidizing agent at both laboratory and industrial levels.^6,7 Despite its potential, the IOR has remained largely underexplored compared to other AORs such as HzOR, UOR, GOR, AmOR, and AlOR.^8,9 Only a limited number of studies have explored electrocatalysts for IOR-assisted water electrolysis, and to the best of our knowledge, none have demonstrated high current densities more than 200 mA cm⁻², an important threshold for practical applications.^10,11

Fig. 1 (a) Schematic illustration of the hybrid water electrolysis showing the IOR at the anode and the HER at the cathode; (b) overall water electrolysis and hybrid water electrolysis; and (c) synthesis of BZ-NiCo-H using the hydrothermal method (color code: yellow = Co, green = Ni, red = O, and grey = C).

Electrocatalysts based on nickel have recently been identified as some of the best materials for promoting the OER.^12–14 During the OER, Ni²⁺ sites undergo anodic oxidation to high-valent Ni oxyhydroxide phase, which are generally accepted as the true catalytically active states.^15–17 Therefore, facilitating the generation of such high-valent metal oxyhydroxides is expected to accelerate the overall OER kinetics. Over the last few decades, considerable efforts have been devoted to identify the real active sites for the OER in Ni-based catalysts.^18–23

In contrast, the design of efficient electrocatalysts for the IOR has recently begun to attract attention. A few materials, including MoS₂–FeS₂,²⁴ CoP,²⁵ and CoFe-NC,²⁶ have been investigated as IOR catalysts, and our group has recently reported a strain-engineered V–Fe(O)OH system for this purpose.²⁷ Nevertheless, the precise identification of the catalytically active sites in the Ni-based catalysts for IOR remains underexplored, and the reported catalysts still suffer from limited activity and stability. These challenges highlight the urgent need to uncover the underlying reaction mechanism and pinpoint the true active sites, which would provide a rational basis for designing highly efficient and durable IOR electrocatalysts.

To address this challenge, we have reported a benzoate-coordinated NiCo-hydroxide (BZ-NiCo-H) that exhibited efficient activity for both the OER and IOR (Fig. 1c). X-ray absorption spectroscopy (XAS) validated that benzoate coordination optimized orbital overlap and modified both the electronic feature and coordination environment, thereby facilitating easier access to high-valent metal sites. The increased interlayer spacing provided the facile diffusion of electrolyte and more accessible active sites. For the IOR, BZ-NiCo-H achieved a remarkable 100 mA cm⁻² at just 1.42 V vs. RHE potential, while for the OER, it required 1.63 V vs. RHE potential to deliver the same current density. The superiority of this system became even more apparent when integrated into a tandem IOR-assisted water electrolysis system, where it achieved 100 mA cm⁻² current density at a cell voltage of 1.92 V, a dramatic reduction from the 2.15 V, typically needed for conventional water splitting. This tandem configuration enhanced energy efficiency by 57%.

Mechanistic studies revealed that benzoate coordination induced a distinct reaction pathway. Interestingly, BZ-NiCo-H showed a LOM pathway governed by proton-decoupled electron transfer (PDET). In situ Raman analysis demonstrated that γ-Ni(O)OH (Ni⁴⁺) was the main active phase for the OER, while β-Ni(O)OH (Ni³⁺) served as the active phase for the IOR in BZ-NiCo-H. Arrhenius kinetic analysis and in situ electrochemical impedance spectroscopy (EIS) suggested that benzoate coordination significantly enhanced charge transfer kinetics and reduced activation energy barriers. These effects collectively contributed to the accelerated reaction rates observed for the OER and IOR. These findings underscore the promise of linker-coordinated hydroxides for sustainable value-added electrochemical transformations.

2. Results and discussion

2.1. Synthesis and characterization of catalysts

A single-step hydrothermal process was used to create the BZ-NiCo-H and the NiCo-H electrocatalysts. The infrared (IR) spectrum of NiCo-H exhibited a peak at 3633 cm⁻¹, corresponding to stretching vibrations of non-hydrogen-bonded O–H groups in β-Ni(OH)₂ (Fig. S1). In contrast, additional peaks were detected in BZ-NiCo-H at 1590 cm⁻¹ and 1403 cm⁻¹, ascribed to the asymmetric and symmetric stretching vibrations of COO⁻, respectively (Fig. S1).^28,29 These characteristic peaks confirm the successful coordination of benzoate linkers with the metal sites.

Powder X-ray diffraction (PXRD) patterns further validated the phase composition of the synthesized materials. The diffraction peaks in BZ-NiCo-H were observed at 5.93°, 11.82°, and 17.70°, corresponding to the (001), (002), and (003) planes of the Ni(OH)(C₆H₅COO) H₂O phase, matching with JCPDS No. 42-1836 (Fig. S2). Additionally, the diffraction peaks in NiCo-H were observed at 19.78°, 33.61°, and 39.09°, attributed to the (001), (100), and (101) planes of the β-Ni(OH)₂ phase (JCPDS No. 14-0177) (Fig. S2). The peak at 5.93° in BZ-NiCo-H corresponded to the d-value of 1.48 nm, which indicated that the linker coordination increased the interlayer spacing.^30,31

The oxidation states of the elements in BZ-NiCo-H and NiCo-H were examined using X-ray photoelectron spectroscopy (XPS). The Ni 2p spectrum of BZ-NiCo-H exhibited peaks at 856.23 eV and 873.88 eV corresponding to Ni 2p_3/2 and Ni 2p_1/2, respectively (Fig. 2a).^29,31–33 The peak for Ni²⁺ was recorded at 856.38 eV. In BZ-NiCo-H, the Ni 2p_3/2 peak exhibited a positive shift of 0.67 eV relative to NiCo-H, indicating the existence of high-valent Ni sites in BZ-NiCo-H. The Co 2p spectrum of BZ-NiCo-H revealed the peaks at 781.37 eV and 796.55 eV, assigned to Co 2p_3/2 and Co 2p_1/2, respectively (Fig. 2b).^31,34–36 The signal at 780.77 eV was attributed to the Co²⁺ species. The signal associated with Co 2p_3/2 of BZ-NiCo-H exhibited a positive binding energy shift of 0.50 eV, relative to NiCo-H. The positive shift indicated the presence of high-valent Co sites in BZ-NiCo-H.

Fig. 2 (a) Ni 2p XPS data of NiCo-H and BZ-NiCo-H; (b) Co 2p XPS data of NiCo-H and BZ-NiCo-H; (c) Ni K-edge XANES spectra of NiCo-H and BZ-NiCo-H compared with NiO and Ni-foil; (d) Co K-edge XANES spectra of NiCo-H and BZ-NiCo-H compared with CoO and Co₃O₄ (inset: Co K pre-edge features); (e) Ni R-space profile of NiCo-H and BZ-NiCo-H; and (f) Co R-space profile of NiCo-H and BZ-NiCo-H.

Furthermore, the O 1s spectrum of BZ-NiCo-H revealed two signals for surface –OH groups at 531.86 eV and metal–oxygen bond at 529.46 eV (Fig. S3).^37,38 The peak of the –OH group in O 1s XPS of NiCo–H was observed at a binding energy of 531.2 eV, whereas in the case of BZ-NiCo-H it was seen at 531.86 eV, reflecting a negative shift of 0.84 eV in NiCo-H compared to BZ-NiCo-H (Fig. S3). The XPS study established that linker coordination altered the coordination and electronic features of BZ-NiCo-H.

The coordination and electronic properties of the catalysts were thoroughly investigated using X-ray absorption spectroscopy (XAS) studies. NiCo-H and BZ-NiCo-H exhibited distinct coordination and electronic characteristics, particularly in terms of metal oxidation states and coordination environments. The Ni K-edge XANES spectra of both NiCo-H and BZ-NiCo-H displayed a prominent absorption peak (∼8350 eV), comparable to that of the NiO standard, indicating the presence of the Ni²⁺ state, coordinated in an octahedral environment (Fig. 2c).^39,40 The pre-edge features of NiCo-H and BZ-NiCo-H observed at around 8335 eV were attributed to metallic Ni, arising from the underlying Ni foam.

Notably, the main absorption peak of BZ-NiCo-H exhibited a higher intensity than that of NiCo-H, implying an increased oxidation state of Ni, associated with low-spin Ni³⁺ in BZ-NiCo-H. In BZ-NiCo-H, the low-spin Ni³⁺ species exhibited an e¹_g configuration, inducing a Jahn–Teller distortion within the catalyst lattice. Such structural distortion promotes favorable adsorption–desorption of reaction intermediates, thereby enhancing the overall catalytic activity. For the Co K-edge XANES, both BZ-NiCo-H and NiCo-H displayed characteristic features indicative of Co²⁺ in an octahedral coordination environment, closely resembling the CoO reference (Fig. 2d).^41,42 Moreover, the absorption edge positions for both catalysts were significantly higher and closely aligned with CoO, further confirming that Co predominantly exists as Co²⁺. Additionally, the Co K-edge XANES spectra displayed a pre-edge feature at ∼7710 eV, with BZ-NiCo-H showing a more intense pre-edge peak than NiCo-H (Fig. 2d inset). This was attributable to increased Co 3d-4p orbital mixing, suggesting the greater degree of structural distortion in BZ-NiCo-H. Furthermore, the observed shift of the main absorption peak to higher energy in BZ-NiCo-H relative to NiCo-H validated an elevated oxidation state of Co, reflecting the changes in the coordination environment due to the linker coordination.

The Ni R-space profile of NiCo-H exhibited two distinct peaks located at approximately 1.50 Å and 2.49 Å, corroborated with Ni–O and Ni–Ni/Co coordination, respectively (Fig. 2e).^43,44 A noticeable shortening in the distance of the Ni–O bond was observed in BZ-NiCo-H compared to NiCo-H. This bond contraction was associated with an increased oxidation state of Ni in BZ-NiCo-H. The increased peak intensity of the Ni-O shell in BZ-NiCo-H compared to NiCo-H suggested a higher coordination number around the Ni atom, which could be due to the stronger coordination effect induced by the benzoate linker. The above observation further indicated that linker coordination changed the electronic structure of Ni sites.

To further explore the structural differences, the Co R-space profile was analysed. In the profile, two primary features at around 1.45 Å and 2.83 Å were associated with the Co–O and Co–Co/Ni coordination shells, respectively (Fig. 2f).^45,46 For BZ-NiCo-H, the bond distance in the Co–O first shell was reduced compared to the NiCo-H. This bond contraction was attributed to the coordination of the benzoate linker, which increased the crystal field strength and stabilized the low-spin Co³⁺ state, as also supported by the main absorption feature in Fig. 2d. In addition, the decreased peak intensity of the Co–O shell in BZ-NiCo-H reflected local structural distortion, consistent with the enhanced pre-edge peak intensity observed in the XANES spectra.

The average oxidation state of Ni in BZ-NiCo-H and NiCo-H was estimated by establishing a linear relationship between the first derivative peak energy and the nominal oxidation states of Ni standard samples. Based on the analytical results, the average Ni oxidation states in BZ-NiCo-H was estimated to be +2.60, which was higher than that in NiCo-H (+2.23) (Fig. S4). Correspondingly, the average oxidation state of Co was also derived from the Co K-edge XANES using the same first derivative approach. The results indicated that the Co average oxidation state in BZ-NiCo-H (+2.31) was higher than that in NiCo-H (+2.09) (Fig. S4). The XAS analysis clearly demonstrated that benzoate linker coordination promoted the facile access of high-valent Ni and Co sites, to improve the anodic oxidation activity.

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to analyze the surface features of the synthesized materials. SEM images of BZ-NiCo-H revealed the growth of long nanowires across the nickel foam, while NiCo-H exhibited nanosheet-like morphology (Fig. S5). The TEM analysis of BZ-NiCo-H further confirms the nanowire morphology, while HR-TEM displayed a stripe spacing of 1.25 nm, suggesting the microporous structure (Fig. 3a and b).⁴⁷ These findings suggested that the coordination of benzoate linkers led to a structural reorganization of the material to form 1D nanowires, which had micropores facilitating the electrolyte diffusion. The existence of Ni, Co, O, and C in BZ-NiCo-H and Ni, Co, and O in NiCo-H was validated by energy-dispersive X-ray spectroscopy (EDX) and associated elemental mapping, demonstrating their elemental composition (Fig. S6–S9).

Fig. 3 (a) TEM image of BZ-NiCo-H; and (b) HRTEM image showing d-spacing of 1.25 nm (SAED inset).

2.2. Electrochemical OER performance

The oxygen evolution reactivity of the catalysts was thoroughly evaluated in a 1.0 M KOH electrolyte. Among the two, BZ-NiCo-H exhibited superior electrocatalytic performance, achieving a current density of 100 mA cm⁻² at an impressively low potential of 1.63 V vs. RHE (Fig. 4a). In contrast, NiCo-H required a higher potential of 1.66 V vs. RHE to reach the same current density. BZ-NiCo-H exhibited superior OER performance compared to RuO₂ and other recently reported electrocatalysts. (Fig. S10 and Table S1). This enhancement in catalytic efficiency for BZ-NiCo-H is attributed to the presence of high-valent Ni⁴⁺ species, which are known to promote O–O bond creation.

Fig. 4 (a) OER and IOR activities of BZ-NiCo-H and NiCo-H; (b) pH-dependent reaction order of BZ-NiCo-H and NiCo-H for the OER determined at 1.70 V vs. RHE; (c) Arrhenius plot of BZ-NiCo-H and NiCo-H for the OER at 1.70 V vs. RHE; and (d) Arrhenius plot of BZ-NiCo-H and NiCo-H for the IOR at 1.70 V vs. RHE.

To further elucidate the underlying mechanism of the OER, the pH-dependence of OER activity was evaluated at different pH values (13.50, 13.68, 13.80, and 13.90) (Fig. S11).^48–50 Notably, BZ-NiCo-H exhibited a larger sensitivity to pH changes compared to NiCo-H. The correlation between current density and pH provides critical insight into the involvement of protons in the OER when referenced against the RHE scale. The evaluated ρ^RHE value for BZ-NiCo-H was 0.75, substantially lower than that of NiCo-H (0.52) (Fig. 4b).

A ρ^RHE value close to 1 indicates that the OER activity of the catalyst is highly influenced by pH. This suggests that BZ-NiCo-H likely operated via the lattice oxygen mechanism (LOM), where proton and electron transfers occurred in a decoupled manner. In comparison, NiCo-H displayed a lower ρ^RHE and a weaker pH dependence, pointing toward the adsorbate evolution mechanism (AEM).

Further mechanistic distinction was achieved using tetramethylammonium (TMA⁺) cations as mechanistic probes. In the LOM pathway, peroxo intermediates interact with TMA⁺, leading to a decline in OER activity (Fig. S12).^48–50 NiCo-H exhibited minimal variation in activity in the presence of TMA⁺, corroborating an AEM-type mechanism. In contrast, BZ-NiCo-H showed a marked decrease in catalytic performance in TMAOH, indicating significant interaction between TMA⁺ and reactive peroxo intermediates, and thus validating its LOM behavior. These results collectively confirmed that benzoate coordination in BZ-NiCo-H modulated the OER pathway, favoring LOM.

To further explore the pH dependence of OER thermodynamics, Pourbaix diagram was constructed at a low current density (10 mA cm⁻²) (Fig. S13).^50,51 The slope derived for BZ-NiCo-H was 118.3 mV pH⁻¹, while NiCo-H exhibited a slope of 105.5 mV pH⁻¹. According to the Nernst equation, a slope of ∼59 mV pH⁻¹ corresponds to a 1 : 1H⁺/e⁻ stoichiometry, whereas a slope near 118 mV pH⁻¹ implies a 2H⁺/2e⁻ transfer process. Thus, the 118.3 mV pH⁻¹ slope for BZ-NiCo-H suggested the involvement of a two-proton–two electron transfer.

To demonstrate the superior OER kinetics of BZ-NiCo-H relative to its counterpart NiCo-H, a series of temperature-dependent electrochemical measurements were performed (Fig. S14).^52–54 LSV was conducted over a range of temperatures, revealing a consistent enhancement in OER activity as the temperature increased, an expected outcome given the thermally activated nature of electrochemical reactions.

To quantitatively evaluate the temperature effect on reaction kinetics, the LSV data were analyzed using the Arrhenius equation.^52–54 The activation energies (E_a) for the OER were extracted by plotting the logarithm of current density against the reciprocal of temperature at a constant potential. BZ-NiCo-H possessed a significantly smaller activation energy of 14.01 kJ mol⁻¹, compared to NiCo-H (18.68 kJ mol⁻¹), confirming the enhanced kinetic favorability of the benzoate-coordinated catalyst (Fig. 4c). The reduced activation energy reflects a lower enthalpic barrier for the OER, which is attributed to the electronic modulation introduced by linker coordination.

To investigate how benzoate coordination into NiCo-H enhanced catalytic activity and to evaluate the interfacial catalytic dynamics, in situ electrochemical impedance spectroscopy was performed across a range of applied potentials (Fig. 5a, b and S15).^52–54 The Bode analysis revealed that different frequency domains corresponded to distinct electrochemical processes: high-frequency responses related to the activation of catalytic sites, while low-frequency signals corresponded to faradaic processes associated with the OER. Compared to NiCo-H, BZ-NiCo-H displayed a lower phase angle in the high-frequency region, indicating more effective activation of the catalyst to create high-valent metal sites (Fig. 5a and b).

Fig. 5 (a) Bode plots of BZ-NiCo-H at different potentials for the OER; (b) Bode plots of NiCo-H at different potentials for the OER; (c) Bode plots of BZ-NiCo-H at different potentials for the IOR; and (d) Bode plots of NiCo-H at different potentials for the IOR.

With increasing applied potential, the low-frequency phase angle decreased. For BZ-NiCo-H, the Bode phase plots showed the onset of the OER at approximately 1.50 V vs. RHE, which was lower than the 1.55 V required for NiCo-H, reflecting a reduced energy barrier for reaction initiation (Fig. 5a and b).^52–54 The onset potential of both the catalysts was also corroborated with the LSV profiles. This analysis further confirmed that benzoate coordination facilitated easier accessibility to Ni³⁺/Ni⁴⁺ sites at lower potentials, thereby enabling the OER to be initiated at a reduced onset potential. Beyond the onset, BZ-NiCo-H exhibited a steeper drop in low-frequency phase angle, signifying faster OER kinetics.

BZ-NiCo-H also demonstrated a lower R_ct (2.81 Ω) compared to NiCo-H (4.48 Ω) at 1.55 V vs. RHE, implying faster charge transfer properties in BZ-NiCo-H (Fig. S15). Additionally, the admittance at the lowest frequency, linked to long-range charge mobility, was higher for BZ-NiCo-H (0.19 S) than NiCo-H (0.16 S), confirming improved charge transport (Fig. S15). Analysis of charge-transfer resistance (R_ct) and phase angle beyond the OER onset further validated the advantage of benzoate coordination. BZ-NiCo-H exhibited markedly lower R_ct values and reduced phase angles, highlighting its superior charge-transfer efficiency and faster reaction kinetics relative to NiCo-H, underscoring the beneficial role of benzoate coordination in boosting electrocatalytic performance.

2.3. Electrochemical IOR performance

Encouraged by the outstanding OER activity, the electrochemical IOR performance was also evaluated in 1.0 M KOH having 0.33 M KI. BZ-NiCo-H demonstrated excellent IOR activity, reaching 100 mA cm⁻² current density at 1.42 V vs. RHE potential, whereas NiCo-H required a higher potential of 1.48 V vs. RHE for the identical current density (Fig. 4a). In comparison with RuO₂ and other reported electrocatalysts, BZ-NiCo-H exhibited superior activity towards the IOR (Fig. S16 and Table S2). Notably, BZ-NiCo-H achieved a higher current density of 300 mA cm⁻² at 1.50 V vs. RHE potential lower than the onset of the OER, highlighting its catalytic selectivity and potential for scalable oxidation reactions.

To validate the enhanced IOR kinetics of BZ-NiCo-H compared to the unmodified NiCo-H, temperature-dependent electrochemical analyses were carried out (Fig. S17).^52–54 LSV was performed across various temperatures, consistently showing improved IOR performance with rising temperature, an outcome aligned with the thermally driven nature of electrochemical processes. To quantify the influence of temperature on reaction kinetics, the LSV results were further interpreted using the Arrhenius equations.^52–54 BZ-NiCo-H demonstrated a significantly smaller activation energy of 6.71 kJ mol⁻¹ than NiCo-H (11.97 kJ mol⁻¹), indicating superior IOR kinetics (Fig. 4d). This reduction in E_a suggested that benzoate functionalization effectively tuned the electronic structure of the catalyst, facilitating more favorable reaction kinetics.

In situ EIS was employed to gain further insights into the IOR using both the synthesized catalysts (Fig. 5c, d and S18).^52–54 The obtained Bode spectra revealed two distinct electrochemical processes. The low-frequency domain was attributed to the IOR, while the high-frequency region corresponded to the intrinsic electrochemical reconstruction dynamics to generate catalytically active phases. The pronounced variation and reduced phase angle with increasing potential in the high-frequency region suggested that BZ-NiCo-H underwent a more rapid reconstruction process to generate the active phase compared to NiCo-H (Fig. 5c and d). Furthermore, in the low-frequency region, BZ-NiCo-H exhibited a lower phase angle than NiCo-H for the IOR process, suggesting that BZ-NiCo-H enabled more efficient charge transfer and faster reaction kinetics during the IOR. Compared with the OER, the decrease in phase angle within the low-frequency region during the IOR indicated a lower onset potential and more favorable reaction kinetics for the IOR relative to the OER in both catalysts.

2.4. Two-electrode system performance

Taking advantage of its remarkable activity for the IOR and OER, BZ-NiCo-H was employed as a bifunctional catalyst. In a two-electrode H-type electrochemical setup, 1.0 M KOH was filled in both compartments for overall water electrolysis. For substituting the OER with the IOR, 0.33 M KI was added in 1.0 M KOH to assess the performance. The LSV curves of BZ-NiCo-H highlight the difference in performance between overall water electrolysis and IOR-assisted water electrolysis (Fig. S19).

To reach a current density of 100 mA cm⁻², a cell voltage of 2.15 V was required for overall water electrolysis, whereas IOR-assisted water electrolysis achieved the same current density at a lower voltage of 1.92 V, demonstrating improved overall cell efficiency (Fig. S19). This approach enhanced energy efficiency by 57%, lowering the energy consumption from 4.90 kWh m⁻³ H₂ of conventional water electrolysis to 2.08 kWh m⁻³ H₂ of IOR-assisted water electrolysis (Fig. S20).²⁷

2.5. Active site identification for the OER and IOR

To evaluate active sites for the OER, in situ Raman spectroscopy was performed (Fig. 6a). When 1.1 V vs. RHE potential was applied, two characteristic bands of the β-Ni(OH)₂ phase appeared at 450 and 530 cm⁻¹.^55–59 Increasing the potential up to 1.3 V vs. RHE, the intensity of these bands also increased. When the potential reached 1.4 V vs. RHE, the intensity of these bands was diminished and progressively shifted to 464 and 550 cm⁻¹ at 1.5 V, corresponding to the E_g bending mode [δ(Ni–O)] and A_1g stretching mode [ν(Ni–O)] in the active Ni(O)OH phase, respectively (Fig. 6a).^60–63 Notably, the intensity of these Ni(O)OH bands also increased as the potential increased up to 1.6 V vs. RHE. Ni(OH)₂ underwent electrochemical reconstruction between 1.4 and 1.5 V to create an active Ni(O)OH phase. The CV profile of BZ-NiCo-H also showed the oxidation peak for the Ni²⁺/Ni³⁺ transition at 1.45 V vs. RHE. The activation of lattice hydroxyl groups (Ni²⁺–OH to Ni³⁺–O) initiates the creation of the active Ni(O)OH phase through a dehydrogenation process once the potential was increased from 1.4 V vs. RHE.

Fig. 6 (a) In situ Raman data of BZ-NiCo-H for the OER; (b) in situ Raman data of BZ-NiCo-H for the IOR; (c) CV profiles of BZ-NiCo-H for the IOR and OER; (d) chronoamperometric i–t profile for the OER and IOR using BZ-NiCo-H; and (e) schematic illustration showing the active phases for the OER and IOR in the BZ-NiCo-H catalyst as determined from in situ Raman and electrochemical analysis.

A key observation lies in the distinct differences in the δ(Ni–O) to ν(Ni–O) intensity ratio (denoted as I_δ/ν) in the catalyst, which reflect variations in the active catalyst phase.^60–63 Typically, Ni(O)OH can crystallize in β or γ phases, with γ-Ni(O)OH, exhibiting a comparatively weaker ν(Ni–O) band and a higher I_δ/ν, due to its more open, disordered framework. BZ-NiCo-H showed a higher I_δ/ν value at 1.5 and 1.6 V vs. RHE, suggesting γ-Ni(O)OH as the active phase for the OER.^60–63 Literature reports indicate that γ-Ni(O)OH is enriched with highly oxidized Ni⁴⁺ species, corresponding to an average Ni oxidation state of about +3.7.^60–63 This analysis indicates Ni⁴⁺ in the γ-Ni(O)OH phase as the active site for the OER. In an alkaline electrolyte, γ-Ni(O)OH generally promotes O–O coupling via producing a NiOO⁻ intermediate on Ni⁴⁺, characteristic of the LOM pathway.^60–63 This characteristic Raman band of NiOO⁻ was observed at 1060 cm⁻¹, indicating a LOM pathway for the OER, which further supported the pH-dependent and TMAOH experiments.⁶⁴

Similarly, in situ Raman spectroscopy was conducted to determine whether Ni³⁺ or Ni⁴⁺ species participates in the IOR process (Fig. 6b). Similar to the OER, two distinct Raman bands characteristic of the β-Ni(OH)₂ phase appeared at 450 and 530 cm⁻¹ from 1.1 V to 1.3 V vs. RHE.^55–59 As the potential increased to 1.4 V, these bands gradually shifted to 462 and 543 cm⁻¹, corresponding to the E_g bending mode and the A_1g stretching mode of Ni(O)OH, respectively.^60–63Furthermore, the intensity of these bands increased as the potential was increased up to 1.6 V vs. RHE. Interestingly, the I_δ/ν ratio was observed to be 0.95 and 0.92 at 1.50 V and 1.60 V vs. RHE, respectively, indicating preferential creation of β-Ni(O)OH with dominant Ni³⁺ sites as the active phase for the IOR.^60–63 Additionally, at the potential of 1.4 V vs. RHE, where β-Ni(O)OH was generated, a new vibrational band emerged at 757 cm⁻¹, attributed to the surface adsorbed IO₃⁻.^65–67 As the β-Ni(O)OH bands intensified with an increase in potential, the IO₃⁻ band also enhanced , indicating that Ni³⁺ species played a dominant role in catalyzing the oxidation of iodide to IO₃⁻.

Furthermore, CV analysis revealed a sequential oxidation of Ni²⁺ [Ni(OH)₂] to Ni³⁺ [Ni(O)OH] during anodic scanning in 1.0 M KOH.⁶⁸ Additionally, the disappearance of the Ni²⁺/Ni³⁺ anodic peak in CV after KI addition supports the consumption of Ni³⁺ during the IOR (Fig. 6c). Interaction of iodide ions with β-Ni(O)OH involving Ni³⁺ as the true active sites was validated by chronoamperometry (Fig. 6d). Upon the addition of KI, a sharp current increase (∼150 mA cm⁻² from ∼14 mA cm⁻²) was observed, indicating an immediate redox interaction between I⁻ and Ni(O)OH. In situ Raman spectroscopy and CV studies unequivocally identified Ni³⁺ as the active site facilitating the IOR, while Ni⁴⁺ was determined to be the catalytically active species for the OER (Fig. 6e).

To elucidate the active sites in NiCo-H, in situ Raman spectroscopy was performed. At a lower potential of 1.3 and 1.4 V vs. RHE, no band was observed while at the potential of 1.5 and 1.6 V vs. RHE, two characteristic bands at 471 and 556 cm⁻¹ were detected, corresponding to the E_g bending mode [δ(Ni–O)] and A_1g stretching mode [ν(Ni–O)] of the γ-Ni(O)OH phase, respectively (Fig. S21).^60–63 This analysis indicates the generation of the γ-Ni(O)OH phase under an anodic potential, which acts as the key active site for the OER.

During the OER, Ni²⁺/Ni³⁺ species in Ni(OH)₂/NiOOH undergo further oxidation to form highly oxidized Ni⁴⁺ [γ-Ni(O)OH]. It acts as the true active center for the OER, because its higher oxidation state facilitates lattice oxygen activation and deprotonation of OH intermediates. The formation of Ni⁴⁺ increases the electrophilicity of surface oxygen, promoting key OER steps such as M–O bond breaking, O–O coupling, and *OOH stabilization. Therefore, the presence of Ni⁴⁺ correlates with enhanced OER kinetics and lowers the overpotential. In contrast, the IOR operates at considerably lower potentials, where Ni²⁺ is oxidized only to Ni³⁺ [β-Ni(O)OH] but does not reach the Ni⁴⁺ [γ-Ni(O)OH]. The Ni³⁺/Ni²⁺ redox couple aligns well with the thermodynamics of iodide oxidation, making Ni³⁺ an effective redox mediator for the IOR. Ni³⁺ accepts electrons from iodide and is subsequently reduced back to Ni²⁺, allowing for rapid and reversible cycling of the catalyst and promoting the oxidation of iodide to iodate.

2.6. Analysis of value-added products

The formation of iodate at the anode, as the oxidation product during the IOR process, was verified using Raman spectroscopy, which displayed a distinct vibrational band corresponding to iodate (Fig. S22 and S23). At an applied potential of 1.60 V vs. RHE, BZ-NiCo-H demonstrated a high iodate generation rate of 0.97 mmol cm⁻² h⁻¹ with a faradaic efficiency of 98.9% (Fig. S24). Under the same experimental conditions, NiCo-H produced iodate at a significantly lower rate of 0.79 mmol cm⁻² h⁻¹ (Fig. S24). To further explore the influence of potential, the IOR was examined at 1.50 V and 1.40 V vs. RHE. At 1.50 V, BZ-NiCo-H exhibited a production rate of 0.60 mmol cm⁻² h⁻¹, which decreased to 0.35 mmol cm⁻² h⁻¹ at 1.40 V (Fig. S25), highlighting the potential-dependent nature of iodate formation. Conclusively, the potential increment increased the iodate production rate. The electrocatalytic stability of BZ-NiCo-H was thoroughly investigated for the IOR. During the IOR, BZ-NiCo-H exhibited stable current density for 24 h during the chronoamperometric (CA) test, reflecting its excellent operational durability (Fig. S26). This analysis confirmed its robust and durable nature for the IOR.

2.7. Electrochemical properties

Further electrochemical evaluations were carried out to explore the factor behind the enhanced catalytic activity of the materials. The number of active Ni³⁺ sites in BZ-NiCo-H and NiCo-H was estimated by integrating the Ni³⁺/Ni²⁺ redox peaks (Fig. S27).⁶⁹ BZ-NiCo-H exhibited 47.12 × 10¹⁷ active sites, surpassing NiCo-H, which showed 31.62 × 10¹⁷ active sites. This suggests that BZ-NiCo-H provides a greater number of electrochemically accessible Ni sites, contributing to its superior catalytic activity. Additional insights were gained through double-layer capacitance (C_dl) measurements in the non-faradaic region (Fig. S28). BZ-NiCo-H displayed a C_dl of 1.72 mF cm⁻², surpassing the 0.89 mF cm⁻² measured for NiCo-H, indicating a larger electrochemical surface area (ECSA).

2.8. Post-catalytic characterization

Literature studies have revealed that electrocatalysts undergo the reconstruction process when subjected to an anodic potential during the electrochemical reaction.^8,9,37,38 Therefore, XPS and TEM analysis were carried out to investigate the electronic and morphological changes after catalysis. The Ni 2p XPS spectrum of BZ-NiCo-H exhibited two prominent peaks, corresponding to Ni 2p_3/2 and Ni 2p_1/2. The deconvolution of the Ni 2p_3/2 peak revealed two signals for the Ni²⁺ and Ni³⁺ species.^70,71 The Ni 2p_3/2 peak is shifted positively by 0.22 eV compared to fresh BZ-NiCo-H. The presence of Ni³⁺ suggested the formation of the Ni(O)OH active catalyst due to anodic oxidation (Fig. S29). Similarly, the Co 2p spectrum revealed the signals associated with mixed-valence Co³⁺ and Co²⁺ species (Fig. S30).^72–76 The XPS of O 1s displayed two distinct peaks at 529.01 eV and 530.92 eV, attributed to the metal–oxygen bond and surface –OH groups, respectively (Fig. S31). TEM analysis revealed that the morphology of BZ-NiCo-H was completely disrupted during the catalytic process, with nanowires transforming into ultrathin nanosheets (Fig. S32).

3. Conclusions

In this study, we explored the active site identification for the OER and IOR using a BZ-NiCo-H catalyst. This study establishes that benzoate coordination in NiCo-hydroxide plays a pivotal role in tailoring the electronic environment and exposing high-valent metal sites, thereby enabling superior activity toward both the OER and IOR. Through in situ spectroscopic and electrochemical analyses, we identified Ni⁴⁺ [γ-Ni(O)OH] as the active site for the OER and Ni³⁺ [β-Ni(O)OH] as the active site for the IOR. The benzoate coordination facilitated the activation of lattice oxygen to enable the LOM pathway for the OER, while also accelerating charge–transfer processes and lowering the kinetic barrier for both reactions. These insights not only provide the first direct evidence of active-site identification of the Ni-based catalysts for the IOR but also highlight the potential of molecular coordination strategies to engineer highly efficient electrocatalysts.

Author contributions

Ayusie Goyal carried out the synthesis, characterization, and data analysis. Labham Singh performed TEM. Anamika Yadav and Pragya Arora were involved in product identification. Trin-Hai Bin and Chung-Li Dong carried out XAS. The planning, designing, and editing were performed by Apparao Draksharapu and Baghendra Singh. The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: catalyst synthesis, characterization, and optimization of the catalytic reactions. See DOI: https://doi.org/10.1039/d5ta08429f.

Acknowledgements

AD acknowledges SERB, India (CRG/2023/001112), and BS thanks DST, Govt of India for the INSPIRE Faculty Fellowship (DST/INSPIRE/04/2024/000772). AG expresses her gratitude to IIT Kanpur for the junior research fellowship.

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