Trinuclear Nickel Catalyst for Water Oxidation - Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

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Metadata	Details
Publication Date	2022-02-14
Journal	CCS Chemistry
Authors	Qi‐Fa Chen, Yao Xiao, Rong‐Zhen Liao, Ming‐Tian Zhang
Institutions	Tsinghua University, Huazhong University of Science and Technology
Citations	26

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE11 Mar 2022Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O-O Bond Formation Qi-Fa Chen, Yao Xiao, Rong-Zhen Liao and Ming-Tian Zhang Qi-Fa Chen Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084 , Yao Xiao Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084 , Rong-Zhen Liao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 and Ming-Tian Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084 https://doi.org/10.31635/ccschem.022.202101668 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail We report a molecular trinuclear nickel (TNC-Ni) catalyst for water oxidation that exhibited high catalytic performance and stability under neutral conditions (pH 7). Electrochemical studies disclosed that cooperation among the three nickel sites plays a vital role in both charge accumulation and O-O bond formation. This TNC-Ni catalyst could accomplish 4e− oxidation of water by involving all three nickel sites and the O-O bond formation was triggered by a charge distribution process from 5 to 5dp via proton-coupled electron transfer. Download figure Download PowerPoint Introduction Water oxidation (2 H2O → O2 + 4 e− + 4 H+) involving multiple protons and electrons transfer is the bottleneck for solar-powered water splitting.1-3 Molecular water oxidation catalysts (WOCs) based on transition metals have attracted extensive attention since the first “blue dimer” catalyst was reported by Meyer and co-workers in 1982.4 Recently, base-metal complexes for water oxidation have undergone rapid development.5-7 Despite the limitation of nickel-based molecular WOCs, compared with others, including Ru,4,8-14 Ir,15,16 Mn,17-21 Fe,22-26 Co,27-29 and Cu,30-37 nickel has been applied extensively in heterogeneous catalytic water oxidation.38-41 Boettcher et al.42 reported that pure NiOx actually exhibited very low activity in electrocatalytic water oxidation unless tiny amounts of Fe3+ were present, indicating that nickel catalysts have a distinctive behavior. Previously reported mononuclear Ni-based catalysts43-46 also disclosed their unique catalytic behaviors in terms of catalytic performance and stability. These findings stimulated us to explore the mystery of Ni-catalyzed water oxidation. According to the heterogeneous NiOx systems47,48 and Ni-O2 chemistry,49-52 multinuclear Ni-complexes are promising candidates for water oxidation because they could avoid charge accumulation only in a single nickel site and promote the formation of O-O bonds via polymetallic cooperation. Previous studies have shown that the oxygen evolution center (OEC) in photosystem II (PSII) is a CaMn4O5 cluster (Figure 1a) that includes a cubic CaMn3O4 and a hanging Mn ion.53 The charge accumulation in the CaMn4O5 cluster could be dispersed on four Mn sites to achieve a formal high oxidation state54 that facilitates the O-O bond formation.55-57 Biomimetic catalysts based on Mn,19,20,58 Fe,25 Co,59 and Cu37,60 have further confirmed the feasibility of polymetallic catalysis. Herein, we developed a molecular trinuclear nickel (TNC-Ni) complex, referring to the active site structure of nickel oxyhydroxides and layered nickelates (Figure 1b). This Ni3 core (Figure 1c) displayed high activity toward electrocatalytic water oxidation at pH 7, and all three nickel sites were involved in the charge accumulation and O-O bond formation. The catalytic performance of this Ni3 complex was 27 times faster than the corresponding binuclear Ni2 complex. Figure 1 | (a) The structure of CaMn4O5 cluster. (b) The fragment of a general nickelate structure: red, oxygen; purple, nickel. (c) The structure of the Ni3O4 investigated in this work: blue, nitrogen. (d) Crystal structure of the TNC-Ni catalyst. Selected bond distances (Å) and angles (deg): Ni1-O1 1.990(7), Ni1-O2 2.074(8), Ni1-O3 2.129(9), Ni1-N1 2.096(6), Ni1-N3 2.142(1), Ni2-O3 1.855(1), Ni2-O4 1.858(8), Ni2-N4 1.869(1), Ni2-N5 1.896(1), Ni3-O1 2.032(7), Ni3-O2 2.046(8), Ni3-O4 2.100(9), Ni3-N6 2.10(1), Ni3-N8 2.118(6). Ni1-O1-Ni3 97.0(3), Ni1-O2-Ni3 93.9(3), Ni1-O3-Ni2 128.2(5), Ni2-O4-Ni3 129.6(4), O3-Ni2-O4 95.1(4), O1-Ni1-O2 84.4(3), O1-Ni3-O2 84.1(3). Download figure Download PowerPoint Experimental Methods Instrumentation Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE III HD 400 spectrometer (Shanghai, China) at 1H = 400 MHz and 13C = 100 MHz. Electrospray ionization high-resolution mass spectrometry (ESI-HRMS) spectra were recorded on a liquid chromatography ion trap/time-of-flight mass spectrometry (LCMS-IT/TOF, Shimadzu, Japan) and Thermo scientific ultimate 3000+ system (Shanghai, China). Field emission scanning electron microscopy (FESEM) images were collected using the Hitachi SU-8010 instrument (Hitachi Ltd., Tokyo, Japan). Dynamic light scattering (DLS) was measured using the SZ-100-Z Nanoparticle Analysis System (HORIBA Scientific, Beijing, China). The X-ray photoelectron spectroscopy (XPS) data were collected on ESCALAB 250Xi (Thermo Fisher, Shanghai, China) photoelectron spectrometer. UV-vis spectra were recorded in phosphate-buffered saline (PBS; 0.1 M, pH 7) using Agilent Cary 8454 or Agilent Cary 60 UV-vis spectrometer (Agilent Technologies Co. Ltd., Beijing, China). Electrochemical experiments All the electrochemical experiments were investigated using CHI-630 electrochemical workstation. Boron-doped diamond (BDD; 0.07 cm2) was used as a working electrode (WE). Pt wire and saturated calomel electrode (SCE; Hg/HgCl2, saturated KCl) were used as a counter electrode (CE) and reference electrode (RE), respectively. The polished BDD electrode was cleaned by multiple cycles of the cyclic voltammetry (CV; 0-2 V vs SCE) in blank PBS (0.1 M, pH 7). Tetrabutylammonium hexafluorophosphate (TBA·PF6) was used as a supporting electrolyte for nonaqueous electrochemical measurements. Oxygen evolution The evolved oxygen was detected by a calibrated Ocean Optics FOXY probe during controlled potential electrolysis (CPE). Fluorine-doped tin oxide (FTO; 1 cm2) electrode was used as WE for bulky electrolysis. After the CPE process, the used FTO electrode was kept for further investigation. Synthesis and structure characterization of catalyst Chan’s trinuclear ligand (H2ahp) was used in this work.61,62 Modified synthetic procedures of the ligand and preparation of (ahp2−)NiII3(μ-OH)22 are summarized in the Supporting Information Schemes S1-S2 and Figures S1-S8. The reaction of 2 equiv Ni(CF3SO3)2 with H2ahp, in the presence of 4 equiv NaOH as a base in CH3OH/H2O (1:1) solution under ambient temperature, induced an instantaneous color change from yellow to pink. This generated tri-nickel complex was recrystallized from CH3OH-diethyl ether. The X-ray crystallography structure of the [(ahp2−)NiII3(μ-OH)2]2+ (Figure 1d) showed that Ni(2) ion had a distorted square-planar N2O2 coordination geometry, Ni(1) and Ni(3) assumed a distorted octahedral N3O3 coordination geometry. This complex was further characterized by XPS and ESI-HRMS, respectively. The ESI-HRMS peak ( Supporting Information Figure S5) at m/z = 267.0544 was assigned to [(ahp2−)NiII3(μ-OH)]3+, [z = 3, m/z = 267.0544 (cal.)], consistent with a complex consisting of three nickel ions. In addition, the reaction between 3 equiv Ni(CF3SO3)2 and 1 equiv H2ahp in the presence of 4 equiv NaOH in CH3OH/H2O (1:1) solution could form a Ni3 complex with μ-Cl bridge (ahp2−)NiII3(μ-OH)(μ-Cl)2], Supporting Information Figure S9), if recrystallizing from acetone-diethyl ether in the presence of 1 equiv KCl. There were no apparent UV-vis absorption differences between [ahp2−]NiII3(μ-OH)(μ-Cl)](CF3SO3)2 and [ahp2−]NiII3(μ-OH)2](CF3SO3)2 in PBS, indicating that [ahp2−]NiII3(μ-OH)(μ-Cl)](CF3SO3)2 could quickly convert to [ahp2−]NiII3(μ-OH)2](CF3SO3)2 in aqueous solution ( Supporting Information Figures S11a and S11b). Results and Discussion Cyclic voltammetry experiments First, the electrochemical behavior of this TNC-Ni catalyst ([(ahp2−)NiII3(μ-OH)2]2+) was studied in anhydrous and Ar-saturated acetonitrile (MeCN). Figure 2a (black line) shows that the CV of TNC-Ni using BDD as the WE had two oxidation waves at 0.97 and 1.35 V (vs Fc/Fc+), respectively. Differential pulse voltammetry (DPV) displayed three oxidation peaks at 0.67, 0.96, and 1.32 V (vs Fc/Fc+) (Figure 2a, inset). These waves were attributed to the continuous oxidation of all three nickel ions. Upon adding 10% H2O into the MeCN solution, the current (blue line in Figure 2a) in the MeCN/H2O system was significantly enhanced after the first oxidation peak. This current enhancement is consistent with an electrocatalytic water oxidation process. Figure 2 | (a) CVs of TNC-Ni ([(ahp2−)NiII3(μ-OH)2]2+, 1 mM) in anhydrous MeCN with tetrabutylammonium hexafluorophosphate (TBA·PF6; (0.1 M) as the supporting electrolyte. The scan rate was 100 mV s−1. WE: BDD electrode, CE: Pt electrode, RE: 0.1 M AgNO3/Ag. (Inset: DPV of TNC-Ni (1 mM) in anhydrous MeCN with TBA·PF6 (0.1 M) as the supporting electrolyte.) (b) CVs in PBS (0.1 M, pH 7) without (black line) and with (blue line) TNC-Ni (1 mM). The scan rate was 100 mV s−1. WE: BDD electrode, CE: Pt electrode, RE: Saturated calomel electrode (SCE). (Inset: DPV of TNC-Ni (1 mM) in 0.1 M PBS, pH 7.) Download figure Download PowerPoint The typical CV graph of TNC-Ni in PBS (0.1 M, pH 7) shown in Figure 2b displays two oxidation waves at Ea1 = 1.16 V and Ea2 = 1.40 V versus the normal hydrogen electrode (NHE), respectively. The onset overpotential of TNC-Ni (1.24 V) is ∼340 mV was lower than that with Ni(CF3SO3)2 salt (1.58 V, Supporting Information Figure S14b). The catalytic performance in this tri-nickel system caused by nickel ion was also ruled out, based on the comparison of electrochemical behavior between the TNC-Ni complex and the simple nickel salt. The typical electrochemical behavior of 3 mM Ni(CF3SO3)2 in PBS (0.1 M, pH 7) had a cross-current feature in CV scan and a prominent reduction peak at 1.19 V, corresponding to the reduction of NiOx ( Supporting Information Figure S14b). The normalized current (i/ν1/2) of the second wave increased while the scan rate decreased ( Supporting Information Figure S11c), indicating that the first wave was a diffusion-controlled redox process and the second wave was a catalytic process. The catalytic process could be identified as catalytic water oxidation according to the following facts: First, bubbles were generated on the BDD electrode surface when the CV scanning potential was above 1.40 V versus NHE. Second, oxygen evolution was further confirmed by the CPE experiment (Figure 3a). O2 bubbles appeared on the FTO electrode surface ( Supporting Information Figure S11d), and the evolved O2 was detected with a calibrated Ocean Optics FOXY probe. The generated O2 was negligible in a blank solution without TNC-Ni over time (black line in Figure 3b). In contrast, the dissolved O2 of the solution containing 0.5 mM TNC-Ni increased from 47 to 214 μM during electrolysis (blue line in Figure 3b). Meanwhile, the electrolysis current was maintained at 0.6 mA/cm2 during long-term electrolysis at a TON value of 13, with a Faradaic efficiency of 93 ± 2%, indicating that TNC-Ni sustained acceptable stability during the catalytic process. Figure 3 | (a) CPEs at 1.54 V in PBS (0.1 M, pH 7) without (green line) and with (blue line) TNC-Ni ([(ahp2−)NiII3(μ-OH)2]2+), 0.5 mM, 3 mL) using FTO as working electrode (WE). The black line represents the CPE curve of the used FTO after 1 h CPE in TNC-Ni solution, rinsed with deionized water, and then moved to the PBS without TNC-Ni. WE: 1 cm2 FTO, CE: Pt electrode, RE: SCE. (b) The O2 evolution during the CPE without (blank line) and with (blue line) TNC-Ni (0.5 mM, 10 mL). Download figure Download PowerPoint The confirmation of homogeneous catalysis It was challenging to identify homogeneous or heterogeneous catalysis on a molecular complex for water oxidation. NiOx is easily generated via molecular precatalysts decomposition; thus, responsible for the catalytic performance.63-65 The CPE current shown in Figure 3a increased from 0.1 to 0.6 mA/cm2 for the first 20 min, which indicated that new catalytic-active species (Ni oxide or molecular active intermediate) were formed.43,44,66-73 We carefully examined the stability of our TNC-Ni complex in PBS (0.1 M, pH 7.0) by a series of controlled experiments ( Supporting Information Figures S12-S20). The multiple CVs with FTO electrode ( Supporting Information Figure S12a) and BDD electrode ( Supporting Information Figure S12b) showed that the catalytic currents decreased over the increased cycles, and no significant change of the CV curve was noted. This agreed with a homogeneous electrochemical behavior and different from the typical electrochemical behavior of NiOx64,65 formation by molecular catalyst decomposition. The FTO and BDD electrodes that have been used for multiple CVs experiment with TNC-Ni (1 mM) did not show catalytic performance in blank solution without catalyst (the blue line in Supporting Information Figure S12), which indicated that there was no active colloidal species formation on the electrode surface in the catalytic process. This was further confirmed by electrode surface analysis. The used FTO electrode was analyzed by UV-vis spectrum after 1-h CPE at 1.54 V with 0.5 mM catalyst in PBS (0.1 M, pH 7). There was no additional absorption observed compared with the fresh electrode, and the absorption spectrum of the solution when the CPE was kept constant ( Supporting Information Figure S13), indicating that the solution was stable and no colloidal species were absorbed on the electrode surface during the electrolysis. The FTO electrode after electrolysis with TNC-Ni showed negligible current in blank solution without catalyst during CPE (Figure 3a) and CV tests ( Supporting Information Figure S14a), which further supported the absence of colloidal species formation and TNC-Ni as a homogeneous catalyst. Additionally, SEM, XPS, and DLS analyses on the electrode after electrolysis further indicate that no nickel oxide formation occurred during long-term electrolysis ( Supporting Information Figures S15-S17). The catalytic performance of nickel oxide attracted our attention, so CPE with Ni(CF3SO3)2 as the precursor was applied as a comparison. After electrolysis, the FTO electrode with Ni(CF3SO3)2 exhibited a high current (∼0.9 mA/cm2) in blank solution, which implied that some active nickel oxide species were generated on the FTO surface ( Supporting Information Figure S18). However, the CPE current with Ni(CF3SO3)2 showed an increased period, similar to TNC-Ni. To further check whether nickel oxide would generate from TNC-Ni on FTO surface, multiple cycles of CPE between TNC-Ni and Ni(CF3SO3)2 were conducted: The FTO electrode was taken out from the solution during each CPE (600 s) at 1.54 V, then rinsed with deionized water and returned to the previous solution for next CPE. For the first CPE with Ni(CF3SO3)2 as the catalyst, the current increased with time, while the currents of the second and third cycles showed a large current at the beginning and kept constant for some time, which differed from the behavior of the first cycle ( Supporting Information Figure S19). These results indicated that the active nickel oxide species could catalyze water oxidation and hardly remove the FTO. However, the current with TNC-Ni increased from 0.1 mA/cm2 in each cycle ( Supporting Information Figure S20), implying that no nickel oxide species were deposited on the electrode and that the active species remained the molecular catalyst. Therefore, this was a homogeneous catalytic process, and the TNC-Ni might be converted to a new homogeneous active species rather than NiOx under the catalytic conditions. E-pH diagram and kinetic analysis The electrochemical behavior determined by DPV (Figure 2b, inset) displayed three evident oxidation waves at 1.05, 1.35, and 1.84 V, indicating the successive oxidation of three NiII ions, respectively. Figure 4a shows the E-pH relationship in detail. The first oxidation wave at ∼1.16 V was pH-dependent with a slope of ∼−50 mV/pH, corresponding to a proton-coupled electron transfer (PCET) process, NiII3(μ-OH)(μ-OH2) → NiII2NiIII(μ-OH)2 + e− + H+. The second oxidation wave at ∼1.40 V was pH-independent, which was a single electron transfer process, NiII2NiIII(μ-OH)2 → NiIINiIII2(μ-OH)2 + e−. The third oxidation wave at ∼1.84 V was pH-dependent with a slope of −31 mV/pH, indicating a PCET process involving 1H+/2e− transfer process, NiIINiIII2(μ-OH)2 → NiIII2NiIV(μ-OH)(μ-O) + 2e− + H+. The NiIII2NiIV(μ-OH)(μ-O) should be the in situ active species toward water oxidation, according to the oxygen evolution experiments discussed above. Figure 4 | (a) E-pH diagram of TNC-Ni (1 mM) in PBS (0.1 M, pH 6.57-9.39) (the potentials were determined by differential pulse voltammetry (DPV) method). (b) CVs of TNC-Ni complex in PBS (0.1 M, pH 7) with different catalyst concentrations. The scan rate was 100 mV s−1. WE: BDD electrode, CE: Pt electrode, RE: SCE. Inset: Plot of the diffusion currents (id) at ∼1.16 V, the catalytic currents (icat) at ∼1.40 V, and the catalytic currents (icat) at ∼1.84 V vs catalyst concentration ([TNC-Ni]), respectively. Download figure Download PowerPoint To elucidate the catalytic kinetics for this TNC-Ni catalyst, we further explored the relationship between the catalytic current and the different catalyst concentrations (Figure 4b). The catalytic currents in CV varied linearly with catalyst concentrations (Figure 4b, inset), which indicated that the catalytic process only involved a single TNC-Ni molecule rather than a dimer species. Accordingly, the relationship between the catalytic current icat and catalyst concentrations should obey the eq. 1, in which ncat (=4) is the number of the transferred electrons during the catalytic cycle, F (=96485.3 C mol−1) is the Faraday constant, A is the surface area of the WE (0.07 cm2 for BDD electrode used in this work), [Cat.] is the concentration of the TNC-Ni catalyst, Dcat is the diffusion coefficient of the catalyst, and kcat is the catalytic rate constant of the catalyst. i cat = n cat F A [ Cat . ] D cat 1 2 k cat 1 2 (1) The normalized CVs (i/ν1/2) versus scan rates ν showed that the first irreversible oxidation wave at ∼1.16 V was a diffusion-controlled wave ( Supporting Information Figure S11c), which is in accordance with the Randles-Sevcik eq. 2,74 where α (=0.5) is the transfer coefficient of the catalyst, nd (=1) is the number of transferred electrons in this diffusion process, R is the universal gas constant, and T is the absolute temperature. i d = 0.496 α 1 2 n d F A [ Cat . ] ( F v n d D cat / RT ) 1 2 (2) An equation containing icat and id is obtained by dividing eq. 1 by eq. 2. The kcat for catalytic water oxidation could be calculated using the following equation: i cat i d = 0.359 n cat n d 3 / 2 k cat / α ν (3) The normalized CVs (i/ν1/2) show that the normalized currents increased with the decreasing scan rates, which indicates a catalytic process. By plotting the icat/id versus 1/ν1/2, kcat was calculated as 0.54 s−1 by eq. 3 (Figure 5a). The involvement of three nickel sites in the water oxidation process was validated by comparing the performance between Ni3 complex and Ni2 complex. For this purpose, we used the same ligand to assemble the Ni2 complex characterized by HRMS and elemental analysis (the synthetic details and characterization are listed in the Supporting Information Figure S10). The electrochemical properties of the Ni2 complex were carried out under the same conditions as the Ni3 complex (Figure 5b). The Ni2 complex showed much lower current intensity, and the normalized CV current (i/ν1/2) was weakly dependent on the scan rate, indicating that the Ni2 complex was not active toward water oxidation (kcat = 0.02 s−1 shown in Supporting Information Figures S21-S23). This finding disclosed a key role played by the Ni3 core in the TNC-Ni triad in the O-O bond formation. Figure 5 | (a) Plot of the catalytic currents (at 1.84 V) icat/id vs ν−1/2 for TNC-Ni complex. (b) The CV graphs of Ni3 (1 mM, TNC-Ni) and Ni2 (1 mM) complex in PBS (0.1 M, pH 7), respectively. Download figure Download PowerPoint Density function theory calculations Density functional theory calculations at the were to elucidate the reaction of water oxidation by this TNC-Ni complex details were listed in Supporting Information Figures and The calculations showed that a water molecule could to both and Ni3 of this Ni3 complex to form 1 charge of Figure and this process was by The of 1 was calculated to be that 1 is the stable form at pH The on and Ni3 in 1 were calculated to be while the on Ni2 was ( Supporting Information Figure the presence of two high NiII and a square-planar low NiII In an aqueous solution, 1 a PCET to form 2 charge of Supporting Information Figure two The on Ni3 decreased from in 1 to in 2 ( Supporting Information indicating that the electron from Ni3 and a from the water Meanwhile, the feature of the Ni3O4 core was The bond of Ni3-O1 and Ni3-O2 decreased from to and from to respectively. These were than of Ni1-O1 and Ni1-O2 in consistent with the formation of at the Ni3 site ( Supporting Information Figure oxidation of 2 to 3 charge of Supporting Information Figure The on Ni2 increased from to ( Supporting Information which indicated Ni2 was the site for this process. 4 charge of was generated from 3 a PCET process, where the on O2 was ( Supporting Information Figure Therefore, this process could be as the oxidation of the to generate a during the oxidation of 4 to 5 the charge of Supporting Information Figure a ion with was by an electron transfer from to the and an additional electron from to the Meanwhile, the Ni1-O3 and bond both decreased significantly from to and from to (Figure and Supporting Information Figure respectively. The potentials for four oxidation at pH were calculated to be 1.32 V V V and V Figure respectively. The of the first two oxidation agreed with the results (Figure where the for the first two E-pH were and mV/pH, respectively. However, a single peak was observed for the of 3 to form 5 in the CV with an oxidation process involving two electrons and The calculated potential from this process was V at pH 7, and the slope for the E-pH curve was mV/pH, which with the value of −31 the calculated potentials of and V were in with the of and 1.84 V from the CV Figure | diagram for water oxidation by TNC-Ni complex at the Download figure Download PowerPoint further of 5 to 5dp charge of Figure and Supporting Information Figure with two oxygen this process, an electron transfer occurred from of the to Ni2 to a by the change of on Ni2 from in 5 to in Supporting Information Meanwhile, the bond of Ni3-O1 and Ni1-O1 decreased from to and from to (Figure and Supporting Information Figure respectively. the two bond distances in 5dp were very for and the and bonds showed a significantly in a structure for The on and the in 5dp are and ( Supporting Information respectively. These results implied that in 5dp could be a ion and Ni3 as a ion Figure | of key and transition for the O-O bond including and are in while on key are indicated in The details are in the Supporting Download figure Download PowerPoint The above calculations showed that the O-O bond formation was triggered by which to be a The and were and in respectively. The structure of 5dp (Figure 7) could be as a low = with the = a low NiII = and a low = O-O bond was by the of the and the ( Figure 7). The for was calculated to be to 5dp in the ( Supporting Information Figure while the a lower of (Figure had only of which to the O-O bond where the O-O was ( Supporting Information Figure and the on and Ni3 were and (Figure 7). This O-O bond formation was very similar to the O-O bond by and A Figure and Supporting Information Figure was to via oxidation of V) with electron in Ni2 to generate the of a of two water and the could catalyst 2 for the next water oxidation CV experiments were also in ( Supporting Information Figure A kinetic = of indicated that there was no bond involved in the O-O bond formation. The electrochemical behaviors of TNC-Ni with different concentrations (the of = M) were carried out ( Supporting Information Figure and there no was These results further supported the O-O bond formation via the of two rather than the water on the high O-O bond formation have also been The of a

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DOI: https://doi.org/10.31635/ccschem.022.202101668