Dual-Ligand Synergistic Modulation: A Satisfactory Strategy for

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Dual-Ligand Synergistic Modulation: A Satisfactory Strategy for Simultaneously Improving the Activity and Stability of Oxygen Evolution Electrocatalyst lishan Peng, Jun Wang, Yao Nie, Kun Xiong, Yao Wang, Ling Zhang, Ke Chen, Wei Ding, Li Li, and Zidong Wei ACS Catal., Just Accepted Manuscript • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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Dual-Ligand Synergistic Modulation: A Satisfactory Strategy for Simultaneously Improving the Activity and Stability of Oxygen Evolution Electrocatalyst Lishan Peng, Jun Wang, Yao Nie, Kun Xiong, Yao Wang, Ling Zhang, Ke Chen, Wei Ding, Li Li* and Zidong Wei* The State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing Key Laboratory of Chemical Process for Clean Energy and Resource Utilization, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China.

ABSTRACT: The sluggish kinetics of oxygen evolution reaction (OER) is the bottleneck of water electrolysis for hydrogen generation. In developing cost-effective OER materials with a high value of practical application, it is prerequisite to achieve an extreme performance in both activity and stability. Herein, we report a “dual ligand synergistic modulation” strategy to accurately tune the structure of transition-metal materials at atomic level, which finally gets a satisfactory cure for the unity between robust stability and high activity. Remarkably, the elaborately designed S and OH dual-ligand NiCo2(SOH)x catalyst exhibits an excellent OER activity with a very small overpotential of 0.29 V at a current density of 10 mA cm-2, and a strong durability even after 30h accelerated ageing at a large current density of 100 mA cm-2,

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both of which are superior to most of the state-of-the-art OER catalysts so far. The density functional theory (DFT) calculations disclose that the synergy of OH and S ligands on the surface of NiCo2(SOH)x can delicately tune the electronic structure of metal active centers and their chemical environment, which results in an optimal binding energies of the OER intermediates (*OH, *O, and *OOH) and a strengthened binding energy between metal and anion ligands, thus leading to an excellent intrinsically enhanced OER activity and stability respectively. Meanwhile, the special non-magnetism of NiCo2(SOH)x can significantly weaken the resistance of O2 desorption on the catalyst surface and thus facilitating the O2 evolution proceedings.

KEYWORDS: Oxygen evolution, electrocatalysis, dual-ligand modulation, DFT calculation, magnetism

Introduction Efficient and reliable energy supply and storage applications have arguably become one of the most significant challenge in the 21st century.1 Hydrogen, a clean energy carrier with the highest specific energy density, is considered to be the best alternative to the current CO2-emitting fossilfuel-based energy system.2,3 Accordingly, the sustainable generation of hydrogen through water electrolysis using electricity from renewable sources such as wind and solar energy has attracted great attention.4 Fundamentally, the interconversion of water and hydrogen can also be considered as a possible solution to convert electrical energy into a storable, chemical form that in turn can be released upon an electricity shortage.5 To date, most conventional commercial electrolyzers use alkaline electrolytes and operate at relatively modest current densities with cell


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voltages of ~1.8–2.0 V.6 Though the production of purest H2 is the main objective of water electrolysis, the counter-reaction (the oxygen evolution reaction, OER) is the more sluggish one between them and affects the Faradaic efficiency of the electrolytic cell to a greater degree.7 Thus, much effort from the science community has been devoted to searching for a robust OER electrocatalyst with reduced operating over-potential. A major challenge is that the most efficient catalysts for OER strongly depend on rare noble metals8-10, so cost-effective approaches will require the discovery of alternative OER catalysts based on abundant and economic transition metals. Actually, some transition metals (Mn, Fe, Co, Ni)11-14 and their derivatives (oxides, chalcogenides, phosphates, oxy-hydroxides) 15-23 have exhibited impressive activity for the OER in alkaline solution. However, one general problem of (electro)catalysis is the notion introduced by Markovic24 and Otagawa25, that the more active the catalyst is, the lower stability it suffers. This painful conflict is directly reflected by two typical noble metal-free OER catalysts, transition metal hydroxides and sulfides. As for the hydroxides, a good stability is endowed while their activity is limited due to the poor electrical conductivity and hard desorption of *OOH intermediates.20 As for the transition metal sulfides, which can be easily synthesized by fully sulfurating the hydroxides, exhibit a superior activity in catalysis of OER.26,27 Unfortunately, their stability is far from the industrial requirements due to the structure collapse induced by detachment and replacement of S by OH in alkaline solution during continual redox process.17,28,29 In an effort to conquer the obvious defect of the sulfide-based catalysts, some researchers have tried to confine the metal sulfides inside the channels of carbon nanotubes30,31 or layers of graphene.26 Nevertheless, improving the durability via those methods is always at the sacrifice of catalytic activity due to the confining exposure of active sites, and in fact the erosion of integral catalysts has never been inhibited completely.32,33 Given that both the activity and


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stability are indispensable for determining the comprehensive performance of catalysts,24,34 developing a strategy to get an unity between robust stability and high activity rooting from the catalysts’ intrinsic characteristics is a topic of broad scientific significance as well as tremendous commercial value. Herein, we propose a “dual-ligand synergistic modulation” tactics to completely solve the painful compromise between the two conflicted indicators-stability and activity, and further demonstrate its practical application in tailor-making dual-ligand NiCo-sulfhydroxide (NiCo2(Sx·OH2-x)y) catalysts. Specifically, by substituting S ligand into the stable structure of layered hydroxides, we obtain a new class of NiCo2(Sx·OH2-x)y with a single phase and homogenous composition. The elaborately designed dual-ligand catalyst ideally inherits the excellent activity from the conventional transition metal sulfides and the superior stability from the conventional transition metal hydroxides and even exceeds the predecessors. Impressively, the optimized NiCo2(SOH)x with modulated ligand ratio performs a small overpotential of 0.29 V at 10 mA cm-2, and shows no decay after 30h accelerated ageing even at a large current density of 100 mA cm-2, both of which have entered the top of all the transition-metal hydroxides and sulfides reported to date and even exceeded the commercial RuO2 and Ir/C benchmarks (Table S5). The combined experimental and theoretical results further disclose that the synchronously improved activity and stability for OER can be attributed to the discrepant electron-donating character of S and OH ligands. The OH ligands on the surface of NiCo2(SOH)x can attract electrons from the antibonding orbital of M-S bonds to M-O bonds, resulting in a strengthened binding energy between metal and S and thus enhancing the stability. Meanwhile, the synergy between S and OH ligands appropriately modulates the electronic structure of NiCo2(SOH)x, leading to an optimized binding energies of the OER intermediates (*OH, *O, and


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*OOH), therefore facilitating the energetics of oxygen evolution proceedings. Besides, the varied magnetic property induced by the modulation of catalysts’ electronic structure significantly influences the desorption action of paramagnetic O2 from the catalyst surface. The special nonmagnetic NiCo2(SOH)x suffers the lowest O2 desorption resistance and proceeds the smoothest reaction kinetics pathway among NiCo2(SxOH2-x)y catalysts, thus owning the topmost OER activity. Experimental Section Preparation of the Ni, Co-based precursors. All chemical reagents used in this experiment were of analytical grade and were used without any further purification. Prior to the loading of the precursor, the Ni foam (50 mm × 10 mm × 1 mm) was ultrasonically cleaned in a 3.0 M HCl solution for 15 min to remove the surface oxide layer, and then rinsed with DI water and dried in air. Afterwards, 80.0 mL of pink aqueous solution consisting of 25.0 mM Ni(NO3)2•6H2O, 50.0 mM Co(NO3)2•6H2O, 36.0 mM (NH2)2CO and 72.0 mM NH4F was transferred into a Teflonlined stainless steel autoclave, with a treated Ni foam put in it. The autoclave was sealed and subsequently heated to 100 °C for 6 h. When the hydrothermal reaction was over, the samples were washed with DI water and dried in an oven at 60 °C for 12 h. Preparation of the Ni, Co-sulfhydroxides. The sulfuration extent (atomic percentage of S/(S + OH-) × 100%) of NiCo2(OH)x can be tuned by varying the content of sulfide in precursors. Na2S•9H2O (10.0 mM, 20.0 mM, 40.0 mM and 200.0 mM for synthesis of NiCo2(S0.5OH1.5)x, NiCo2(SOH)x, NiCo2(S1.36OH0.64)x and NiCo2S4) was added into DI water (80.0 mL) in a 100.0 mL Teflon liner with stirring to form a homogenous solution. Then, the precursor loaded Ni foam was put into the autoclave and heated in an oven at 160°C for 6 h. After being cooled to room temperature naturally, the Ni foam was removed, washed with DI water and absolute


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ethanol several times, and dried in an oven at 60 °C for 2 h. The estimated loading amount of active material on the Ni foam is approximately 0.7 mg/cm2. During this process, the color of the treated electrode turned from purple to black as illustrated in Figure S1. Since the partial sulfuration treatment of NiCo2(OH)x is accompanied by a moderate ion exchange reaction (MOH + S2-↔ MSOH + 2e−, M=Ni, Co…), almost no morphology changes were observed in the scanning electron microscopy (SEM) images after sulfuration. Preparation of the commercial RuO2/Ni electrode. 10 mg of the commercial RuO2 catalyst and 10 µL of 5 wt% Nafion solution were dispersed in 490µL of water/ethanol (49:50 v/v) mixed solvent, followed by ultrasonic blending at least 30 min. Then 10 µL of the ink was dropped onto a Ni foam and spread evenly to achieve a Ru loading of 1.0 mg/cm2. Finally, the electrode was subjected to drying at 60 °C and was denoted as RuO2/Ni, which contained 0.75 mg/cm2 of Ru. Characterization and electrochemical measurements. The surface morphologies and microstructures of the catalysts were analyzed using X-ray diffraction (XRD-6000, Shimadzu), X-ray photoelectron spectroscopy (XPS, PHI 550 ESCA/SAM), field-emission scanning electron microscopy (FE-SEM, JSM-7800, Japan), and energy dispersive X-ray spectra (EDS, OXFORD Link-ISIS-300). Electrochemical measurements were conducted in a three-electrode cell system using an Electrochemical Workstation (CHI660D, Shanghai Chenhua Device Company, China). Sizable and shapeable electrodes were prepared by tailoring the Ni foam, and the obtained NiCo2(SxOH2-x)y grown on Ni foam can be directly used as the working electrode (1 cm2) without employing extra substrates (e.g., glassy-carbon electrode) or binders (e.g., Nafion). A Pt foil in a parallel orientation to the working electrode was used as the counter electrode, and an Hg/HgO electrode was used as the reference electrode. The catalytic performance of the prepared electrodes for OER was systematically investigated in a 1.0 M NaOH electrolyte. The catalytic


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behaviors of the different materials were studied and compared using LSV and CV at a scan rate of 10 mV/s and 50 mV/s, respectively, and the stability was studied using an amperometric technique. All of the electrochemical measurement data were corrected using a resistance drop of ~ 1.0 Ω, and all the potentials mentioned in this work were converted to values in reference to a reversible hydrogen electrode (RHE). Theoretical Calculations. Calculations were carried out using the Vienna ab initio simulation program (VASP). The ion-core interactions were presented using the projector–augmented wave (PAW) method. Plane wave basis sets were used to expand the wave functions of the valence electrons. The energy cutoff for the plane wave basis set was 520 eV. Electronic states were computed using a 3 × 3 × 1 k-point grid in the first Brillouin zone. The Perdew-Burke-Ernzerh (PBE) functional was used within the Generalized Gradient Approximation (GGA) to describe non-local exchange and correlation effects. Structural optimizations were performed by minimizing the forces on all the atoms to less than 0.05 eV·Å-1. Completely sulfurized Co3S4 with a spinel structure was modeled using a supercell containing 24 cobalt atoms and 32 oxygen atoms. Co(OH)2 with a layered structure was modeled using two layers of edge sharing octahedral [Co(OH)6], and the partially sulfurized Co3(SOH)x was constructed via a partial substitution of OH groups with S atoms in Co(OH)2. OER process. The possible OER pathways on these transition metal compounds are listed in Supplementary equation (1) to (4) OER steps: H2O + * → *OH + H+ + e- (1) *OH → *O + H+ + e-

(2)

*O + H2O → *OOH + H+ + e-

(3)


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*OOH → * + O2 + H+ + e- (4) We applied a method previously developed for modeling the thermochemistry of electrochemical reactions based on density functional calculations. In this method, Gibbs free energy was used as a descriptor to evaluate whether reactions go through spontaneously. Specifically, Gibbs free energy can be obtained by adding corrections including entopic (TS) and zero-point energy (ZPE) to calculated DFT energy, so that ΔG = ∆EDFT + ∆ZPE – T∆S – eU (5) Where the EDFT is the calculated DFT reaction energy, ΔZPE is the change in ZPE calculated from the vibrational frequencies and Δ S is the change in the entropy referring to thermodynamics databases. The electrode potential are adopted with respect to the reversible hydrogen electrode, which makes the standard electrochemical potential of electron involved in reaction (Ge) equal to -eU, and the standard electrochemical potential of the proton (GH+) equal to that of the hydrogen atom in gaseous H2 (1/2GH2). Considering that the triplet state of the O2 molecule is poorly described in the current DFT scheme, the free energy of the O2 molecule was derived according to GO2 = 2GH2O -2GH2 + 4.92. The adsorption free energy of various oxygenated species could be described by equations below: ∆G*OH = G*OH + 1/2GH2 – G* – GH2O ∆G*O = G*O + GH2 – G* – GH2O

(6)

(7)

∆G*OOH = G*OOH + 3/2GH2 – G* – 2GH2O (8) ∆G*O2 = G*O2 + 2GH2 – G* – 2GH2O (9) Then, the reaction free energy of eq. (6)-(9) can be determined by adsorption free energy of those oxygen-containing intermediates, namely: ∆G1 = ∆G*OH – eU (10)


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∆G2 = ∆G*O – ∆G*OH – eU (11) ∆G3 = ∆G*OOH – ∆G*O – eU

(12)

∆G4 = 4.92 – ∆G*OOH – eU (13) As the reaction free energy of whole OER process is confirmed, the free energy diagram on different catalysts can be plotted against reaction route by assigning the absolute free energy of first stage of OER to zero. Desulfurizing process. The reaction energy for the displacement of S by OH ligand of Co3S4 and Co3(SOH)x was calculated using the following equations: ∆E1 = E(Co(OH)2) + 4/3E(S) – 1/3E(Co3S4) – E(OH)

(14)

∆E2 = E(Co(OH)2) + E(S) – E(Co3(SOH)x) – E(OH)

(15)

Where E(Co3S4), E(Co3(SOH)x) and E(Co(OH)2) represent the total energy of unit cell of Co3S4, Co3(SOH)x and Co(OH)2, and E(S) and E(OH) represent total energy of S atom and OH group in vacuum condition. A more positive reaction energy indicates a higher endothermic energy, leading to more difficulty of the displacement reaction. To maintain the stoichiometry, one OH group will replace four thirds S atoms for Co3S4, while only one S atom for Co3(SOH)x. Calculations show that half-sulfurated Co3(SOH)x has a more positive reaction energy of 1.788 eV than full-sulfurated Co3S4 (0.391 eV), implying the half-sulfurated Co3(SOH)x is more difficult to be transformed into Co(OH)2, thus leading to higher stability during the OER process. Results and Discussion To evaluate whether the dual ligand synergistic modulation can indeed ameliorate the catalysts’ performances, the free energy diagram of the OER process on catalysts were calculated


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since oxygen evolution activity has been demonstrated to be strongly correlated with the chemisorption energy of OER intermediates.35,36 Single metal compound models including Co(OH)x, Co3(SOH)x and Co3S4, and bimetal compound model (NiCo2(SOH)x) were constructed to decipher the contribution of the ligand and the metal on the OER activity of catalysts. The stable surfaces of each catalyst were chosen and all possible adsorption sites on them were taken into consideration. The optimized structures of the above models with intermediates adsorbed as well as the elementary reactions of OER in an alkaline environment are presented in figure 1. Figure 1a displays that the Gibbs free energy barrier of potential-determining step (∆Gpds) of the discussed models decrease in the order of Co-OH (0.702 eV) > Co-S (0.519 eV) > S-Co-OH (0.408 eV)> S-NiCo-OH (0.385 eV). The OER onsetpotentials (Uoneset) shown in figure 1b also decrease in the same order. These results imply that the intrinsic OER catalytic activities of the above four models follow the order of Co-OH < Co-S < S-Co-OH < S-NiCo-OH. Compared to the Co-OH and Co-S with single ligand, the ∆Gpds of S-Co-OH with dual ligand sharply decreases by 0.294 eV and 0.111 eV, respectively. The potential-determining step (PDS) for Co-OH and S-Co-OH is the *OOH formation, and the PDS for Co-S is the *O formation. Besides, all the elementary reaction steps in S-Co-OH system become spontaneous when over 1.64 V, namely, its OER onsetpotential is 1.64 V (Figure 1b), which is superior than that of CoOH and Co-S (their onsetpotential is 1.93 V and 1.75 V, respectively, Figure 1b), and even lower than the best catalysts, RuO2, (its theoretically identified overpotential is ∼1.65 V36). These results reveal the superiority of dual-ligand modulated S-Co-OH catalyst compared to its singleligand metal counterparts (Figure 1c). Correspondingly, the binding energies of the OER intermediates on catalytic surface are optimized by the dual-ligand synergistic modulation, which are beneficial for improving the catalytic activity. The calculated adsorption free energy of *OH


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for Co-S is 0.483 eV and the adsorption free energy of *O for Co-OH system is 1.980 eV. While by virtue of the synergistic modulation of OH and S ligands, the adsorption free energy of *OH and *O for S-Co-OH increase to 0.767 eV and 2.261 eV, respectively (Table S1), indicating that the formation of *O and *OOH on S-Co-OH system becomes more facile than that on single ligand Co-S and Co-OH system. Compared with single metal model, the value of ∆Gpds and Uoneset for the bimetal model SNiCo-OH further decrease (Figure 1a, b), revealing the incorporation of Ni into the dual-ligand S-Co-OH can improve its OER activity. The Gibbs free energy of *O and *OOH formation for S-NiCo-OH are 0.385 eV and 0.301 eV, respectively. Because the energy difference of *O and*OOH formation for S-NiCo-OH is below the error value of DFT calculation (below 0.1 eV), the OER process on S-NiCo-OH is jointly controlled by the *O and*OOH formation. Comparing the contribution of ligand and metal on the catalytic activity, it can be discovered that the dual ligand plays a more prominent role in enhancing the OER activity. Thus, an improved electrocatalytic activity based on bimetal dual ligand system is expected in practical OER process. As the physical-chemical and catalytic properties of NiCo-sulfhydroxides can be directed and modulated by controlling the chemical composition of ligands, a series of dual-ligand NiCo2(SxOH2-x)y

catalysts

(including

NiCo2(OH)x,

NiCo2(S0.5OH1.5)x,

NiCo2(SOH)x,

NiCo2(S1.36OH0.64)x, and NiCo2S4) were constructed to further reveal the advantages of dual ligand synergistic modulation. As shown in figure S2a,b and figure S3, the nanoflower-like structure of NiCo2(SOH)x is composed by a large number of ∼100 nm nanowires directing outside, which is purposely used to enhance the mechanical stability and increase the amount of accessible active sites for electrolyte splitting. The X-ray diffraction (XRD) pattern of the


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pristine NiCo2(OH)x (Figure S4, pink line) matches the typical crystalline structure of a mixture of β-Co(OH)2 (JCPDS-03-0177) and Ni(OH) (JCPDS-74-1057).15 After partially substituting OH by S ligands, the NiCo2(SOH)x still preserves the pristine layered structure and accordingly, its XRD pattern is almost the same as that of NiCo2(OH)x, which is consistent with the theoretically simulated results (Figure S4, red line). Figure S5 presents the X-ray photoelectron spectroscopy (XPS) results of catalysts with different sulfuration degree and we can get the information about the valence states and surface content of each element. As sulfur has a lower electronegativity than oxygen,37 the binding energies of 2p 3/2 and 2p 1/2 peaks of both Ni and Co in S substituted NiCo2(OH)x are shifted to lower values for about 1 eV compared to those of pure NiCo2(OH)x . The energy dispersive spectrometer (EDS, Figure S2c) elemental mappings of NiCo2(SOH)x reveal that the Ni, Co, S and O are uniformly distributed over the marked detection range of the 3D constructed

catalyst, and approximately half of the OH groups in the

NiCo2(OH)x are replaced by S2- (Figure S7), which agrees well with the XPS results (Figure S6). The OER catalytic performance of the NiCo2(OH)x, NiCo2(SOH)x and NiCo2S4 electrodes was evaluated by steady-state electrochemistry measurements in a 1.0 M NaOH aqueous solution using a typical three-electrode cell setup (see Methods in Supporting Information). As shows in figure 2a, the NiCo2(OH)x exhibits poor OER response with a very high onset potential at ~1.53 V, which is consistent with the result observed previously.16 In sharp contrast, the good OER catalytic activities of NiCo2(SOH)x are evidenced by their low onset potentials and high current densities, which is even better than those of NiCo2S4. Besides, the change law of the OER activities of the single metal Co3(SxOH1-x)y is the same as that of NiCo2(SxOH1-x)y (Figure S9). These indicate that the dual ligand synergistic modulation is indeed effective in enhancing catalysts’ OER activity. For comparison , the activity of a state-of-the-art OER electrode based


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on RuO2 nanoparticles, a benchmark catalyst for OER,38-40 was also evaluated and it can be seen that its OER activity is even inferior to that of the NiCo2(SOH)x in terms of both the onset potential and the current density . Moreover, the OER activities of the electrodes, regardless of the bare Ni substrate, or Ni substrate modified with Ni3(SOH)x or Co3(SOH)x individually, are lower than that of NiCo2(SOH)x (Figure S8, S9), suggesting a synergistic effect between Ni and Co for an enhanced OER activity. We further investigated the catalytic performance of NiCo2(SxOH2-x)y with different sulfuration extent. As shown in figure S9 and Table S4, the catalytic activity of the NiCo2(SOH)x reaches the top when the atomic ratio of S and OH is 1:1. The Tafel slope, an indicator of electrode performance, is the smaller, the better. From the extrapolation of the linear region of a plot of overpotential versus log J (Figure 2b), we obtained Tafel slope of 47 mV/decade for NiCo2(SOH)x, which is much lower than that of RuO2 (82 mV/decade). Such a low Tafel slope value suggests favorable reaction kinetics of NiCo2(SOH)x. Besides, the Tafel slope of 47 mV/decade for NiCo2(SOH)x indicates that NiCo2(SOH)x proceeds a RDS involving the electron transfer between 2nd and 3rd, which is well consistent with DFT predicted PDS. In addition, the linear dependence of Tafel region extends much more into a high current density region for the NiCo2(SOH)x catalyst, suggesting a lower mass transport resistance and a higher catalytic activity of NiCo2(SOH)x at high current densities, which is ascribed to the untrammeled desorption and escape of O2 bubbles from catalyst surface.41 Further exploration of the symmetry between the upper and lower PDOS fields of three models (Figure 2c) suggests catalysts with different ligands exhibit different magnetic properties, which further influence their OER process vitally. The paramagnetic O2 product generated on the surface of magnetic catalyst will excite a magnetic force, which makes O2 itself to be attracted to the magnetic catalyst surface,42,43 and thus leads the desorption of O2 from catalyst surfaces to be


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much harder. Interestingly, the co-existence of S and OH groups controllably tunethe chemical/electronic environments of their neighboring metal atoms and the magnetic moment of optimal Co3(SOH)x is completely offset due to the antiparallel electron spin of the metal atoms. The bigger O2 adsorption free energy in antiferromagnetic Co3(SOH)x compared to that in magnetic Co(OH)2 and Co3S4 (Table S1) suggests that the Co3(SOH)x undergoes a smoother path of O2 desorption. In order to accurately evaluate the difficulty degree of O2 desorption from catalysts at real OER situation, the desorption capabilities of O2 on thoes three catalysts were further probed using a temperature programmed desorption (TPD) method.44 As shown in figure 2d, the peak temperature of Oads desorption for the non-magnetic NiCo2(SOH)x down-shifts by 114 °C and 60 °C relative to that for the magnetic NiCo2(OH)x and NiCo2S4, respectively, evidencing that O2 desorption on the non-magnetic NiCo2(SOH)x surface is much easier . Stability is an indispensable concern for a catalyst. A catalyst may be meaningless if it is not stable in the long-term OER operations. Impressively, NiCo2(SOH)x maintains almost 100% of its initial catalytic activity over a 30 h accelerated aging test (ADT, Figure 3a and Figure S11) with a current density of ~100 mA cm-2 and 10 mA cm-2. In sharp contrast, there is a > 50% activity loss in the fully sulfurated NiCo2S4. The XPS survey spectra (Figure S12) and high resolution XPS spectra of S 2p (Figure 3c, e) show that there is only 8% sulfur loss on the NiCo2(SOH)x surface after a lengthy OER process while the sulfur loss for NiCo2S is up to 85%. The great sulfur loss in the case of NiCo2S4 indicates an irreparable structural collapse (Figure S13) accompanied by a severe activity decay (Figure 3a, d). Besides, almost no changes are observed in the XRD and XPS of NiCo2(SOH)x (Figure S15, S16) before and after 30 h ADT tests and the valences of the metal active sites still remain their original state (Table S3), indicating that the chemical properties of the catalyst, i.e., structure, valence and so on, are not


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changed. The high stability of NiCo2(SOH)x was also confirmed by comparison of the polarization curves (Figure 3b) measured before and after 1000 CV cycles. Figure 3b shows that the performance of NiCo2(SOH)x for OER catalysis improves after 1000 CV cycles while the performance of NiCo2S4 decreases significantly. Moreover, the stability of the single metal compounds (Ni3(SOH)x and Co3(SOH)x) exhibits the same rule as that of the bimetallic NiCo2(SOH)x (Figure S17), implying that it is the co-existence of the anions of S and OH groups that strongly affects the catalytic stability. As the instability of Co3S4 or NiCo2S4 is mainly due to their structural collapse induced by the detachment and replacement of S with OH and self-oxidation during a continual redox process,17,45 it is necessary to analyze the desulfurizing process. In terms of the thermodynamic principle evaluated by DFT calculations (Figure 3d,f), the desulfurizing process of halfsulfurated Co3(SOH)x has a more positive reaction energy (∆E2 = 1.79 eV) than that of fullsulfurated Co3S4 (∆E1 = 0.39 eV), which means that the desulfurization in Co3(SOH)x is harder than that in Co3S4. In addition, the calculated bond length also suggests that NiCo2(SOH)x is more stable than NiCo2S4, as the M-S bond length in NiCo2(SOH)x (dM-S = 2.16 Å) is shorter compared to that in NiCo2S4 (dM-S = 2.30 Å) (Table S2). In fact, the stronger electronegativity of the OH ligands compared to S ligands renders OH as an electron-attracting reagent, which can impell electrons from the antibonding orbital of the M-S bond to that of the M-O bond (Figure S18) and therefor making the M-S bond become shorter and more stable in NiCo2(SOH)x than that in NiCo2S4, and meanwhile making the M-O bond become slightly longer in NiCo2(SOH)x (dM-O = 2.08 Å) than that in NiCo2(OH)x, (dM-O= 2.04 Å). Such an electron transfer inside the NiCo2(SOH)x enhances the binding energy between the metal and S, consequently strengthening


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the immobilization of the sulfur ions and giving the NiCo2(SOH)x catalyst excellent stability during long-term work. Conclusions In summary, we report a “dual ligand synergistic modulation” tactics to completely solve the painful unity between the conflicted stability and activity issues of transition metal catalysts for OER (Figure 4). Combined experimental studies with theoretical calculations, we have clearly identified the crucial roles of dual ligand synergistic modulation in engineering the catalysts’ electronic structure, and finally improved catalytic activity and stability synchronously. In detail, the synergy between S atoms and OH groups appropriately optimize the binding energies of the OER intermediates (*OH, *O, and *OOH) on the catalytic surface and the desorption energy of O2 product from the catalytic surface, which is beneficial for improving the energetics and kinetics of the OER. Moreover, the OH ligands on the surface of NiCo2(Sx·OH2-x)y can attract electrons from the antibonding orbital of M-S bonds to M-O bonds, resulting in a strengthened binding energy between metal-S and thus achieving a much improved stability. The catalytic performance of NiCo2(SOH)x is stable and superior to that of reported for metal chalcogenides and metal hydroxides, and among the most active transition metal-based OER catalysts reported so far. This study provides a new, effective guideline for designing and fabricating new catalysts by accurately tuning the catalysts’ electronic structure to enhance the electrocatalytic activity and stability of transition metal compounds.


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Figure 1. Design of the dual-ligand M(SOH)x catalyst and the results from DFT calculations. (a, b) The reaction steps and corresponding free energy diagram of the OER process on the pristine Co–OH, S-Co–OH, S-NiCo–OH and Co–S surfaces at their minimum OER potentials; (c) The theoretical results of the three OER catalysts.


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Figure 2. OER performance of NiCo-sulfhydroxide catalysts: (a) OER polarization curves; (b) Tafel plots of Ni, RuO2, NiCo2S4, NiCo2(OH)x, and NiCo2(SOH)x; (c) Calculated PDOS diagrams for Co(OH)2, Co3(SOH)x and Co3S4 and the corresponding magnetic moments of O2 excited via the magnetization effect of the catalysts; (d) TPD of O atoms adsorbed on NiCo2(OH)x, NiCo2(SOH)x and NiCo2S4.


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Figure 3. Stability enhancement of NiCo-sulfhydroxide catalysts: (a) Chronoamperometric curves of NiCo2(SOH)x and NiCo2S4 at a current density of 100mA/cm2; (b) 1st and 1,000th polarization curves of NiCo2(SOH)x and NiCo2S4; the corresponded high resolution S 2p XPS spectra of (c) NiCo2(SOH)x and (e) NiCo2S4 before and after ageing. (d, f) Total energy change during the desulfuration process of S2- in Co3S4 (upper) and Co3(SOH)x (bottom) replaced by OH-. The non-spontaneous process of Co3(SOH)x with a more positive energy change discloses a more thermodynamic stable structure of Co3(SOH)x compared to Co3S4.


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Figure 4. The ultimate unity of OER performance of NiCo2(SxOH2-x)y. Here, Co, S, O and H atoms are shown in blue, yellow, red and white, respectively.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supporting Figures 1-19 Supporting Tables 1-5 Supporting References S1-S13


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AUTHOR INFORMATION Corresponding Author *Corresponding author. E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest ACKNOWLEDGMENTS This research work was financially sponsored by National Natural Science Foundation of China (Grant No.: 91534205, 21436003 and 21576032). REFERENCES 1.

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BRIEFS. The “dual-ligand synergistic modulation” tactics is reported to accurately tune the electronic structure of transition-metal materials in atomic level, which completely solve its painful unity between the conflicted stability and activity for OER. The elaborately designed NiCo2(SOH)x exhibits excellent OER activity and strong durability simultaneously, both of which are superior to most of the state-of-the-art OER catalysts so far. TOC figure


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Dual-Ligand Synergistic Modulation: A Satisfactory Strategy for

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