Tuning Sulfur Doping for Bifunctional Electrocatalyst with Selectivity

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Letter

Tuning sulfur doping for bifunctional electrocatalyst with selectivity between oxygen and hydrogen evolution Xue Cui, Zhigang Chen, Zhen Wang, Minghai Chen, Xiaohui Guo, and Zhigang Zhao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01186 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Energy Materials

Tuning Sulfur Doping for Bifunctional Electrocatalyst with Selectivity between Oxygen and Hydrogen Evolution Xue Cui,†,‡,Δ Zhigang Chen†,Δ Zhen Wang,† Minghai Chen,† Xiaohui Guo,*,‡ Zhigang Zhao*,†,§ †Key

Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and

Nano-Bionics Chinese Academy of Sciences, Suzhou 215123, China ‡Key

Lab of Synthetic and Natural Functional Molecule Chemistry of Ministry of

Education, the College of Chemistry and Materials Science, Northwest University, Xi'an 710069, P. R. China §Division

of Nanomaterials, Suzhou Institute of Nano-Tech and Nano-Bionics, Nanchang,

Chinese Academy of Sciences, Nanchang 330200, China

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Electricity-driven water splitting to produce hydrogen fuels and oxygen is considered among the most promising technologies for sustainable energy. Despite considerable progress over the last decade in this field, no water-splitting electrocatalyst with selectivity between the oxygen and hydrogen evolution reactions (OER and HER) has been reported so far, to the best of our knowledge. Here, we develop a unique, customizable water-splitting electrocatalyst by systematically modulating the doping of the non-metal element sulfur (S) into cobalt molybdate using thioacetamide (TAA) as a sulfur precursor. The resultant S-doped cobalt molybdate electrocatalysts showed unexpectedly high selectivity, i.e., the OER and HER activities could be freely adjusted through tuning the sulfur doping level. In the case of a high TAA/cobalt molybdate mass ratio of 1.6, the S-doped sample was highly selective for HER, achieving an excellent electrocatalytic activity comparable to those of previously reported state-of-the-art transition-metal (Co, Ni, and Fe)-based HER electrocatalysts. In sharp contrast, when the TAA/cobalt molybdate mass ratio was as low as 0.4, the S-doped sample was remarkably selective for OER, with outstanding catalytic activity that even outperformed the commercial RuO2 catalyst. KEYWORDS: bifunctional electrocatalyst, sulfur doping, alkaline media, oxygen evolution, hydrogen evolution, water splitting


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Severe environmental and societal concerns related to energy use (e.g., acid rain, air pollution, climate change, and fossil fuel depletion) have created a steadily growing demand for sustainable energy technologies.1,2 Electricity-driven water splitting to produce hydrogen fuels and oxygen is considered among the most promising technologies for sustainable energy because it can potentially provide a clean, sustainable, and affordable energy supply.3 Water splitting involves two half-cell reactions: reduction of H+ ions at the cathode (2H+(aq) + 2e→ H2(g)), i.e., the hydrogen evolution reaction (HER),4 and oxidation of water (2H2O(l) → O2(g) + 4H+(aq) + 4e), i.e., the oxygen evolution reaction (OER).5 Both HER and OER require effective electrocatalysts to facilitate the reaction and promote the evolution kinetics, and thus improve the overall efficiency of water splitting.6 Noble-metal-based electrocatalysts (Pt, Ir, Ru, and their alloys) have long been known to possess high activity toward water splitting, but their scarcity and extremely high cost seriously hinder large-scale practical application.7 Therefore, in the past few years, enormous effort has been devoted to the design and synthesis of more economical, non-noble-metal-based water-splitting electrocatalysts with comparable or superior electrochemical performance and stability with respect to



noble-metal or noble-metal-oxide-based electrocatalysts. However, this remains a considerable challenge.8,9 More effective solutions to the water-splitting problem can be found in nature. In biological systems, the formation of molecular hydrogen is efficiently catalyzed by two major classes of metalloprotein: hydrogenase and nitrogenase, which contain only non-precious transition metals (Fe, Ni, and Mo).10,11 Inspired by biology, researchers have recently paid considerable attention to earth-abundant transition-metal (Co, Ni, and Fe)-based electrocatalysts for water splitting, which offer a diversity of electronic structures and coordination environments.12,13 Specifically, a series of Co-Mo compounds with sulfur doping has been synthesized as catalysts for HER. Markovic et al. obtained a composite CoMoSX structure via the combination of CoSX and MoSX building blocks, showing high HER activities both in alkaline and acidic conditions.14 Metal chalcogenides are believed to also enhance the OER catalytic performance. As suggested by Song et al., advanced OER catalysts derived from metal chalcogenides may benefit from the high surface areas of their nanostructures.15 However, fine-tuning the HER and OER properties of compounds in this family is a highly intricate problem. These compounds tend to be compositionally and structurally complex, and their electrocatalytic activities are usually correlated with various factors at once, such as the structure of the active phases; the covalent contribution to the metal/nonmetal bonds; the coordination environment, d-electron density, and oxidation states of the catalytically active transition metal centers; and the adsorption energies. For example, it was very


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recently found that the doping of tungsten, a 5d transition metal, can modulate the 3d electronic structure of transition-metal oxides, providing optimal adsorption energies for OER intermediates, bringing about excellent OER performance and high electrochemical stability.16 However, the effects of doping on transition-metal (Co, Ni, and Fe)-based electrocatalysts are still not fully understood either theoretically or experimentally. In fact, through novel choices of dopants and their bonding configurations, it may still be possible to accomplish new or improved electrocatalytic properties.17 For example, to the best of our knowledge, there are no reports on the specific design of customizable water-splitting electrocatalysts in which the OER and HER activities of a single material can be freely adjusted. New doping methods may enable such a feature. Herein, we demonstrate for the first time that the electrocatalytic activity of transition-metal-based electrocatalysts such as cobalt molybdate can be bidirectionally modulated by incorporating the non-metal element sulfur. The resultant S-doped cobalt molybdate electrocatalysts showed unexpectedly high selectivity, i.e., the OER and HER activities could be freely adjusted through tuning the sulfur doping level, which is rarely possible for such systems. At a high TAA/cobalt molybdate mass ratio of 1.6, the S-doped sample was highly selective for HER, achieving an excellent HER electrocatalytic activity (onset potential: 87 mV, Tafel slope: 63 mV dec−1), comparable to those of previously reported transition-metal (Co, Ni, and Fe)-based HER electrocatalysts.18,19 In sharp contrast, at a low mass ratio of 0.4, the S-doped sample was remarkably selective for OER, with outstanding catalytic activity that even outperformed



the commercial RuO2 catalyst (overpotential at 10 mA cm−2: 366 mV, Tafel slope: 76 mV dec−1).20 Subsequently, the fundamental molecular origin of the selectivity was investigated on the basis of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and other spectroscopic methods. Our synthetic strategy for the customized electrocatalyst is based on low-temperature sulfurization of a Co-Mo precursor, with different degrees of sulfurization followed by calcination at 350 °C under a flow of argon gas, as schematically depicted in Figure 1a. Briefly, nanoscale cobalt molybdate (CoMoOX) particles are first prepared through a mild hydrothermal method as a precursor for sulfurization.21 Then, to facilitate the sulfurization process, the surfaces of these particles are carefully modified to construct a layer of zeolitic imidazolate framework-67 (ZIF-67) over the CoMoOX by ion exchange with 2-methylimidazole in ethanol. Importantly, the controlled formation of the ZIF-67 layer over the CoMoOX particles is critical to obtain an excellent electrocatalytic performance, as will be shown below by comparing the unmodified CoMoOX and modified CoMoOX@ZIF-67 particles as starting materials for electrocatalysts. Next, the CoMoOX@ZIF-67 particles are further converted to S-doped cobalt molybdate particles by a sulfurization reaction with thioacetamide (TAA) in ethanol followed by calcination at 350 °C under a flow of argon gas.22 The S-doped cobalt molybdate samples with different degrees of sulfurization are denoted as S-x, where x is the mass ratio of TAA and cobalt molybdate (x = 0, 0.2, 0.4, 0.8, 1.6, 2.4). The phase conversion from CoMoOX@ZIF-67 to S-doped cobalt molybdate upon sulfurization was first investigated


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using X-ray diffraction (XRD) and Raman spectroscopy. As can be seen from the XRD patterns in Figure S1, the peaks of the starting material match well with those of cobalt molybdate (JCPDS No. 14-0086). Upon treatment with 2-methylimidazole, the XRD pattern confirms the existence of the ZIF-67 phase.22 However, for all samples, the characteristic reflections for the ZIF-67 phase completely disappear upon sulfurization and calcination (Figure 1b). The sample calcinated at 350 °C, but without sulfurization, shows three major peaks at 23.3°, 26.5°, and 36.7°, which can be well assigned to the (021), (002), and (400) planes of crystalline monoclinic-phase CoMoO4 (JCPDS No. 21-0868). In sharp contrast, the XRD patterns of the sulfurized samples clearly show some degree of amorphous nature. For example, for the sample S-0.4, all of the above-listed diffraction peaks from the CoMoO4 phase sharply decrease in intensity. The XRD pattern of the sample S-0.8 exhibits only a very weak diffraction peak, probably as a result of its highly polycrystalline structure (see the inset in Figure 1b). For the samples with higher amounts of TAA additive, no diffraction peaks corresponding to crystalline phases can be seen (Figure 1b), indicating that the samples mostly consist of a single amorphous phase. Figure 1c shows the Raman spectra of the S-doped samples with different degrees of sulfurization. The undoped sample shows several distinct bands at 324, 364, 814, 874, and 931 cm−1, which is characteristic of the CoMoO4 phase. The bands at 874 and 931 cm−1 can be attributed to Mo-O-Co stretching vibrations in CoMoO4, while the bands at 324 and 814 cm−1 can be assigned to MoO4 vibrations.23 In contrast, S doping induces the appearance of a new Raman band at 662 cm−1, which



corresponds to Co-S bonds, evidencing the successful doping of S in the CoMoO4 phase.24

Figure 1. (a) Schematic illustration of the synthesis of S-doped cobalt molybdate samples from Co-Mo precursor. (b) XRD patterns and (c) Raman spectra of S-doped cobalt molybdate samples with different degrees of sulfurization. Microstructural examination was performed using TEM and selected-area electron diffraction (SAED). As seen from the TEM image in Figure 2a, the particles in the calcinated sample without sulfurization appear as anisotropic CoMoO4 nanorods with diameter and length of about 150 nm and 2.2 μm, respectively. The SAED pattern reveals that the nanorods are structurally single-crystalline (inset in Figure 2a). High-resolution


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TEM (HRTEM) imaging of the single-crystal CoMoO4 nanorods reveals lattice spacings of 0.33 and 0.38 nm (Figure 2a), corresponding very closely to the d-spacing of the (002) and (021) planes of monoclinic-phase CoMoO4. The sample with a small amount of TAA additive (S-0.4) retains the original rod-like shapes, but the surfaces become rather rough (Figure 2b). Nonetheless, the interiors of the nanorods seem to remain essentially single-crystalline, and the measured spacing (0.33 nm) in the HRTEM image still matches the reported (002) lattice spacing for monoclinic-phase CoMoO4 (Figure 2b). However, closer observation by HRTEM indicates that the nanorods contain large amounts of crystallographic defects that reduce the degree of structural perfection (circled in Figure 2b), which is likely to be responsible for the decrease in crystallinity. With the TAA/cobalt molybdate mass ratio increased to 0.8 (S-0.8 sample), large quantities of nanotubes with very poor crystallinity are unexpectedly obtained. The TEM image clearly elucidates the hollow nature of the nanotubes through the sharp contrast between the granulated shell and the central void space (Figure 2c). The thickness of the nanotube walls is about 20 nm. With further increasing the TAA/cobalt molybdate mass ratio to 1.6 and 2.4 (S-1.6 and S-2.4 samples), the obtained particles retain the structural features of the nanotube shape but become totally amorphous, as shown by the HRTEM images (Figure 2d, e). The hollowness of these sulfurized samples is also confirmed by Brunauer–Emmett–Teller (BET) analysis (Figure S3). The solid-structured CoMoO4 nanorods have a BET surface area of only 8.12 m2g−1, while the hollow-structured sample S-1.6 has a value of 59.49 m2g−1, seven times greater than for its solid-structured



counterpart. The chemical compositions of the undoped and sulfurized samples were further examined using inductively coupled plasma mass spectroscopy (ICP-MS) and X-ray photoelectron spectroscopy (XPS) (Table S1). XPS is a surface-sensitive technique used to analyze the surface composition, while the bulk composition was determined by ICP-MS. Both analytical techniques show that the amount of S continuously increases as the amount of added TAA increases, while the relative content of O decreases, further confirming the incorporation of S. Furthermore, the S content is higher in the XPS results measured from the surface than in the ICP-AES results measured from the bulk. This can be attributed to the diffusion-controlled sulfurization process in our synthesis. In the S doping step, the outer shells of the cobalt molybdate nanoparticles are sulfurized first, after which the S2− ions migrate inward through the shells to sulfurize the cobalt molybdate in the interior of the nanoparticles. This eventually leads to the formation of hollow S-doped cobalt molybdate nanoparticles.


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Figure 2. TEM images, SAED patterns, and HRTEM images of undoped and S-doped cobalt molybdate samples with different degrees of sulfurization. (a) S-0, (b) S-0.4, (c) S-0.8, (d) S-1.6, (e) S-2.4. The S-doped cobalt molybdate electrocatalysts derived from CoMoOX@ZIF-67 with different amounts of TAA additive were electrochemically characterized for their HER and OER performance in 1 M KOH solution with a typical three-electrode setup at room temperature (for details see the supporting information). The use of alkaline media for HER and OER is strongly preferred in industrial contexts due to its advantages in material stability and corrosion resistance.25 All of the reported potentials from the electrochemical test have been iR-corrected. For the HER, the iR-corrected polarization curves and Tafel plots for the samples with different TAA levels are shown in Figure 3a, b, respectively. The undoped sample shows sluggish HER kinetics with an onset potential around 180 mV and a Tafel slope of 118 mV per decade. In contrast, the HER activities



of all of the S-doped samples are significantly improved. The dependence of HER activity on the amount of TAA is shown in Figure 3e. The HER activity firstly increases with increasing sulfur doping concentration, reaches the maximum value at a high TAA/cobalt molybdate mass ratio of 1.6, then drops sharply with further increasing amounts of TAA. At the optimal TAA/cobalt molybdate ratio of 1.6, the S-doped sample exhibits an onset potential of just 87 mV, and a greatly decreased Tafel slope of 63 mV per decade. The HER performance of this sample is highly competitive with previously reported transition-metal (Co, Ni, and Fe)-based HER electrocatalysts in the literature (Table S2). The electrocatalytic activities of the same samples were next assessed for OER in the same electrolyte using the same electrode configuration. From the polarization curves (Figure 3d), it is evident that the electrocatalyst with a low TAA/cobalt molybdate of 0.4 exhibits the best OER activity, achieving a cathodic current density of 10 mA cm−2 at only 320 mV. Note that this overpotential is even lower than that of the commercial RuO2 electrocatalyst (366 mV),20 highlighting the much enhanced OER activity of the cobalt molybdate sample with a low TAA additive content. The Tafel plots of these S-doped cobalt molybdate samples are illustrated in Figure 3f. The corresponding Tafel slopes of the various samples are in the range of 60–90 mV per decade, showing the fast kinetics of the OER. The sample with a low TAA/cobalt molybdate of 0.4 achieves the optimal Tafel slope of 60 mV per decade, lower than the reported 76 mV per decade for the commercial RuO2 electrocatalyst (Table S3). Therefore, OER shows a significantly different trend


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from HER with respect to sulfur content: while high sulfur doping levels are favorable for HER, the OER activity is optimized at a much lower doping level. For comparison, the electrocatalytic activities of S-doped samples derived from CoMoOX, instead of CoMoOX@ZIF-67, toward HER and OER were also evaluated in 1 M KOH solution (Figure S4). These Co-Mo oxide-derived samples show very poor capability for catalyzing both HER and OER, which highlights the competitive advantages of CoMoOX@ZIF-67 as the starting material in our experiments. To further investigate the possible correlation between

electrochemical

surface

area

(ECSA)

and

the electrocatalytic performance of our S-doped cobalt molybdate electrocatalysts, the electrochemical double-layer capacitances (CdI) for both HER and OER catalysts have been measured in the non-Faraday range of 0.1-0.3 V (HER) and 1.12-1.25 V (OER), respectively. It has been reported that CdI can be used to represent the ECSA of the electrocatalysts, which serves as an estimation of the effective surface area at the solid-liquid interface for electrochemical reactions. Usually, the larger the ECSA of catalyst is, the more the relative active sites for electrocatalytic reactions will be.26, 27 As shown in Figure S5 and S6, the S-1.6 sample shows the largest CdI value of 28.3 mF/cm2 in the non-Faraday potential range for HER, while the S-0.4 sample exhibits a maximal value up to 68.6 mF/cm2 for OER process, both indicating the existence of large amounts of active sites for their corresponding electrocatalytic reaction (HER/OER).28,29 Meanwhile, As shown in Figure S7, the Nyquist plots of the S-0, S-0.4 and S-1.6 sample are obtained by electrochemical impedance spectroscopy (EIS), in which the semicirclar



diameters of S-0.4 and S-1.6 are obviously smaller than that of S-0 sample, suggesting that the S-0.4 and S-1.6 possesses a remarkably lower charge transfer impendence, which can be beneficial to the electrocatalytic performance.30 Durability is another important criterion in evaluating electrocatalysts for commercial applications. To this end, the durability of the S-doped cobalt molybdate electrocatalysts for HER and OER was evaluated by chronopotentiometric measurement in 1 M KOH solution at a current density of 10 mA cm−2. As is clear from Figure S8, the current density remains highly stable, without obvious degradation over 12 h, corroborating the good durability of the device for HER and OER. Judging from the above results, it can be concluded that our S-doping cobalt molybdate samples are found to be efficient catalysts for the HER/OER with high activity and durability among the (Co, Fe, Ni)-based catalysts.31-34


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Figure 3. (a, c) HER and OER polarization curves of S-0, S-0.4, S-0.8, S-1.6, S-2.4 at 5 mV s−1 in 1 M KOH. All of the results are corrected for iR losses. The corresponding Tafel plots after iR correction are shown in (b) and (d), respectively. (e, f) Dependence of the HER and OER activities of S-doped cobalt molybdate samples on the sulfur doping level. To investigate the mechanism of the customizable electrocatalytic performances of the S-doped cobalt molybdate samples, XPS was performed. In the Mo 3d region in Figure



4a, five peaks were resolved by peak fitting. The doublet with binding energies of 228.6 and 231.8 eV is attributed to Mo4+, and the other doublet at 232.4 and 235.4 eV corresponds to Mo6+, revealing the coexistence of +4 and +6 oxidation states of Mo in the sample.18 Finally, the small peak at 225.9 eV can be assigned to the S 2s state, which agrees well with previous reports.35 The S 2p core-level spectrum is shown in Figure 4b. The spectrum contains three main peaks at 161.4, 162.6, and 163.5 eV, which are assigned to terminal S2− for the MoS2 site, Co-S, and bridging S22− for the MoS3 site, respectively.18 The other two discrete peaks at 168.8 and 169.6 eV in the S 2p spectrum are indexed to sulfate and polythionate, respectively, originating from partly-oxidized sulfur on the electrocatalyst surface.36,39 Both bridging S22− and terminal S2− have been proposed to act as active sites in HER. Thus, the ratio of the XPS fitted-peak areas of terminal S2− and bridging S22− was analyzed as a function of sulfur doping level to investigate which S environment was mainly responsible for HER in our sample (Figure 4d). The samples with lower terminal S2−/bridging S22− ratios, i.e., S-0, S-0.2, S-0.4, S-0.8 and S-2.4, show strikingly inferior HER performances. In sharp contrast, the sample S-1.6, having the highest percentage of terminal S22−, yields the best HER performance (Figure 4d). Therefore, the major HER active sites in our samples must be terminal S2− rather than bridging S22−.


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Figure 4. Core-level XPS spectra of (a) Mo 3d, (b) S 2p, and (c) Co 2p for S-doped cobalt molybdate samples with different degrees of sulfurization. (d) Changes in the HER current densities at −0.2 V and the S2−/S22− molar ratios as a function of the doping amount of sulfur. (e) Changes in the OER current densities at 1.58 V and the Co3+/Co2+ molar ratios as a function of the doping amount of sulfur.



In contrast, for OER, Co sites (Co3+) have recently been proposed to serve as the active sites.29 However, the heterogeneity of OER electrocatalysts makes identification of the true OER active site very challenging, and several different chemical states, including Co3+, Co2+, and a mixed state of Co2+/Co3+, have been pinpointed as the true OER active site in previous studies.16,38 To interpret the trends of the OER activity of our samples, the Co 2p XPS spectra of the various S-doped cobalt molybdate samples were recorded. As shown in Figure 4c, the Co 2p core-level spectra are deconvoluted into several fitted peaks: the peaks at 782.6 and 798.4 eV are from the Co2+ in Co-O,16 the peaks at 780.8 and 796.9 eV are from the Co3+ in Co-O with satellite peaks at 786.6 and 803.3 eV, and the peaks at 778.5 and 793.6 eV are from the Co3+ in Co-S.39 In other words, similarly to Mo, both +2 and +3 oxidation states of Co are present. Moreover, the integrated peak areas for Co2+ and Co3+ suggest that the Co3+/Co2+ molar ratio initially increases (relative to the undoped sample) with the addition of small amounts of TAA, then continuously decreases upon further increasing the TAA/cobalt molybdate ratio from 0.4 to 1.6 (Figure 4e). The Co3+/Co2+ molar ratio is maximized at a low TAA/cobalt molybdate ratio of 0.4. Interestingly, the change in the Co3+/Co2+ molar ratio as a function of the amount of TAA additive (corresponding to the S-doping level) is, by and large, in agreement with the trends in the OER activity, if the sample with a TAA/cobalt molybdate ratio of 2.4 is excluded. These results imply that the higher oxidation state, Co3+, dominates the OER in our S-doped cobalt molybdate samples, while Co2+ is relatively inactive. However, at extremely high sulfur doping levels (such as S-2.4), excess sulfur leads to the formation


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of long-chain polythionate complexes, which block the active sites of the electrocatalyst surface, and sharply lower the OER activity of S-doped cobalt molybdate.37 The reason for the change in the amount of Co3+ remains to be further explored; however, it may be related to the partial formation of an oxide layer on the surface of the catalyst, as evident from the increasing percentage of Co-O bonding in Co 2p spectra (Table S4).15 As a matter of fact, recent references propose that the formed sulfide-oxide interface helps for better carrier transportation, thus enhancing the efficiency of OER.40-42 The favorable role of ZIF-67 in improving the electrocatalytic activity is also illustrated by the XPS analysis (Figure S4). Compared with the samples derived from CoMoOX, both the HER and OER active sites (terminal S2− and Co3+, respectively) are significantly more numerous in the samples derived from CoMoOX@ZIF-67, which improves the electrocatalytic ability. The enhanced S doping, leading to the increased density of active sites, is possibly caused by the facilitated ion-exchange of S2− in the lattice of ZIF-67 due to its open crystal structure.22 Finally, a two-electrode alkaline electrolyzer for overall water splitting was constructed using S-doped cobalt molybdate samples with low and high S doping concentration as anode and cathode, respectively. As shown in Figure 5a, the S-modified electrolyzer achieves rapid water splitting with current densities of 10 and 20 mA cm−2 at small cell voltages of 1.57 V and 1.63 V, respectively. The water-splitting performance of the new device is superior to many other state-of-the-art water-splitting electrolyzers. For example, the required potential for a current density of 10 mA cm−2 for our



electrolyzer is much lower than that for the Ir/C//Ir/C (1.90 V), Pt/C//Pt/C (1.75 V), Ni5P4/Ni foil//Ni5P4/Ni foil (1.70 V), and Ni(OH)2/NiSe2//Ni(OH)2/NiSe (1.78 V) electrodes, and even outperforms the traditional benchmark combination catalyst, Ir/C (anode)//Pt/C cathode (1.62 V).43,44 Due to the low water-splitting voltages, our electrolyzer can be easily powered by two 1.5-volt batteries at room temperature. Bubbles of hydrogen and oxygen are clearly seen to be evolved from the cathode and anode when connecting the batteries with the electrolyzer (the insert in Figure 5b). Moreover, a chronoamperometric test was performed to study the durability of the constructed electrolyzer. The chronopotentiometry (i-t curve) at 1.57 V reveals that the overall water-splitting performance of the electrolyzer is stable for at least 30,000 s, demonstrating its good stability (Figure 5b).

Figure 5. Overall water-splitting performance. (a) Polarization curves of overall water splitting for S-1.6/S-0.4 hybrid electrode at a scan rate of 5 mV s−1 in 1 M aqueous KOH. (b) Chronopotentiometry test (i-t curves) of water electrolysis for the hybrid electrode at 1.57 V. Insert: photograph of two-electrode water electrolysis device.


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In summary, a novel electrocatalyst with high selectivity between oxygen and hydrogen evolution was developed by systematically doping the non-metal element sulfur into cobalt molybdate. High-level doping of sulfur was demonstrated to substantially increase the number of terminal S22− sites, resulting in excellent hydrogen evolution performance with a small Tafel slope of 67 mV dec−1. Contrastingly, low-level doping of sulfur was found to result in more Co3+ sites, making the sample highly active for oxygen evolution (320 mV at 1 mA cm−2; Tafel slope 60 mV dec−1). Exploiting the unique selectivity of the electrocatalyst, a two-electrode alkaline electrolyzer for overall water splitting was constructed using S-doped cobalt molybdate samples with low and high S doping concentrations as the two electrodes, which generated a current density of 20 mA cm−2 at 1.63 V. This work may shed new light on the rational design of novel electrocatalysts with high selectivity between oxygen and hydrogen evolution.

■ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI. Experimental details; XRD patterns of cobalt molybdate and CoMoOX@ZIF-67; TEM images,

SAED

patterns

and

HRTEM

images

of

S-0.4

and

S-1.6;

N2

adsorption-desorption isotherms of S-0, S-0.4, S-0.8, S-1.6, S-2.4; Electrocatalytic performances of S-doped samples derived from CoMoOx and CoMoOx@ZIF-67; Cyclic



voltammetry curves of S-0, S-0.4, S-0.8, S-1.6, S-2.4 at scan rates from 20 to 100 mV·s−1 during the HER process; Cyclic voltammetry curves of S-0, S-0.4, S-0.8, S-1.6, S-2.4 at scan rates from 20 to 100 mV·s−1 during the OER process; Nyquist plots of S-doped cobalt molybdate and undoped cobalt molybdate; Chronopotentiometry curves of the samples S-0.4 and S-1.6 under current density of 10 and -10 mA cm-2, respectively; ICP-MS and XPS analysis of different S-doped cobalt molydbate samples for atomic percentage; Comparison of HER activity data among various catalysts in 1 M KOH solution. (η: overpotential at the current density of 10 mA cm-2); Comparison of OER activity data among various catalysts in 1 M KOH solution. (η: overpotential at the current density of 10 mA cm-2). ■AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected], [email protected] ORCID Zhigang Zhao: 0000-0002-9327-9893 Author Contributions ΔX.

C. and Z. G. C. contributed equally to this work

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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Table of Contents graphic

Controlled synthesis of S-doped cobalt molybdate with high activity and selectivity for oxygen and hydrogen evolution.


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Tuning Sulfur Doping for Bifunctional Electrocatalyst with Selectivity

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