Traditional NiCo2S4 Phase with Porous ... - ACS Publications

Subscriber access provided by Stockholm University Library

Article

Traditional NiCo2S4 Phase with Porous Nanosheet Array Topology on Carbon Cloth: A Flexible, Versatile and Fabulous Electrocatalyst for Overall Water and Urea Electrolysis Wenxin Zhu, Meirong Ren, Na Hu, Wentao Zhang, Zhengtao Luo, Rong Wang, Jing Wang, Lunjie Huang, yourui suo, and Jianlong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04663 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 12 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

ACS Sustainable Chemistry & Engineering

Traditional NiCo2S4 Phase with Porous Nanosheets Array Topology on Carbon Cloth: A Flexible, Versatile and Fabulous Electrocatalyst for Overall Water and Urea Electrolysis Wenxin Zhu†, Meirong Ren†, Na Hu‡, Wentao Zhang†, Zhengtao Luo†, Rong Wang†, Jing Wang†, Lunjie Huang†, Yourui Suo‡, and Jianlong Wang†,* †

College of Food Science and Engineering, Northwest A&F University, 22 Xinong Road, Yangling 712100, Shaanxi, China Qinghai Key Laboratory of Qinghai-Tibet Plateau Biological Resources, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, 23 Xinning Road, Xining 810008, Qinghai, China

‡

Corresponding author’s E-mail: [email protected]. ABSTRACT: We report on our recent finding here that bimetallic nickel–cobalt sulfide nanosheet arrays on carbon cloth (NiCo2S4 NS/CC) by anion exchange from its NiCo–precursor could act as a flexible, versatile and fabulous electrocatalyst for both the overall water splitting and urea electrolysis in base. The overall water–splitting efficiency of this material is higher than that for reported NiCo2S4 nanowire array on CC with just needing a voltage of 1.66 V to afford 10 mA cm−2. Moreover, this material also shows fabulous catalytic activity and selectivity for urea oxidation reaction (UOR) with an ultralow potential of 0.249 V vs. SCE to obtain 10 mA cm−2, surpassing all reported transition–metal based UOR catalysts. An urea–assisted energy–saving alkaline hydrogen–production system was further built here by replacing anodic oxygen evolution reaction with UOR. This whole urea– electrolysis cell driven by NiCo2S4 NS/CC (+,−) affords 10 mA cm−2 at a relative–low voltage of 1.49 V, which is 210 mV less than that for urea–free counterpart and also comparable to those for other reported catalysts. The amazing catalytic performances should be due to the intrinsic high activity and metallic feature of NiCo2S4 phase, the nanoporous and open–shelled nanosheet topology that exposes more catalytic active sites and accelerates the diffusion of electrolyte and the generated gas bubbles, as well as the spinel structure and mixed valence states of Ni and Co elements in NiCo2S4 phase that offers richer redox reactions. KEYWORDS: nickel–cobalt sulfide, nanosheet array, versatile electrocatalyst, overall water splitting, overall urea electrolysis

INTRODUCTION Constructing a worldwide sustainable energy system and preserving our environment is widely recognized as one of the main crucial and urgent challenges facing humanity today.1,2 Currently, the majority of global energy demand have been always derived from fossil fuels (like coal, petroleum and gas), and with a fast–growing population and expanding industrialization, global energy demand is predicted to continuously rise, accompanied by the increase of carbon dioxide emissions.3 The impetus to reduce the dependence on fossil fuels and exploit distributed energy storage technologies in the future drives us to develop various renewable energy conversion and storage modes such as solar, wind, tidal, nuclear, and hydrogen energies. Among them, hydrogen fuel, with the advantages of non pollution, easy operation, high combustion value with single product of water, as well as facilitation of storage and transportation, has been well identified as the ideal alternative.4 But what is embarrassing is that, traditional H2–evolving mode of steam methane reforming usually accompanies with high–energy heat input,

CO2 emission, and non–high purity product.5 In view of this, one prospective pattern is to develop environmental–friendly and highly–efficient electrolysis technique that could convert water into high–purity hydrogen by coupling with above renewable energies.6,7 Active catalytic electrodes are primarily needed in this energy–conversion system to implement the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) with high electrolysis efficiency and selectivity.6–10 These catalysts can be divided into three catageries: noble–metal based, transition–metal based, and carbon based compounds. For the first, although commercially available Pt and RuO2 are considered as the state–of–the–art HER and OER catalysts, the low natural abundance and prohibitive market price hamper their application on the industrial–scale electrolysis system. For the last, despite the low cost of synthesis, they have relative–poor catalytic activities toward both HER and OER. By contrast, tremendous efforts have been made on exploiting high–performance transition–metal based electrocatalysts, especially the Janus ones that can effectively drive both of these two reactions.11–13

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Of them, considering the catalytic activity of monometallic compounds could be enhanced by introducing secondary transition metals such as Ni, Co, Fe, Cu, Zn, and Mn by virtue of the synergistic effects or tailored electronic and surface properties of the host structure,6,14–20 bimetallic materials particularly for NiCo–based chalcogenides18,21–24 and phosphides19,20,25–27 have been intensively studied recently as bifunctional catalysts for full water splitting with higher efficiency and durability. Traditional NiCo2S4 phase, as the most–studied NiCo–based chalcogenides, has aroused extraordinary interest in supercapacitor, electrocatalysis, and battery due to its conductive nature, spinel structure with multivalent states of both Ni and Co elements, and inner richer redox reactions than corresponding single component sulfides.18,21,28–30 Also note that, other than the modulation of elemental composition, constructional design is also an effective way to improve the electrolysis performance (activity and stability) of catalysts. Till now, to the best of our knowledge, the reported three NiCo2S4–based bifunctional catalysts are all only in the form of nanowire array architecture,18,21,22 which could indeed provide direct channels for efficient electron/ion transport, large surface area and more exposed active sites for reactions. However, this structure is

Page 2 of 12

inclined to collapse in the long–term electrolysis process because of the limited adhesion area on current collectors and excessively high aspect ratio of each nanowire. Thus, regulating NiCo2S4 phase in other topology forms such as porous nanosheet array with much exposed catalytic active domains and structural stability seems to be fascinating for further improving its overall performance. Except for the superior catalytic electrodes, the optimized electrolysis system is another key factor for energy–efficient hydrogen production. Typical electrolysis system always comprises cathodic HER and anodic OER, wherein the sluggish four–proton–coupled reaction kinetics and large theoretical minimum thermodynamic voltage of 1.23 V for OER greatly limit the overall water–splitting efficiency with more energy input (commonly the voltage needed for overall electrolysis at 1.5–1.7 V).31–33 Integrating alternative anodic reactions like electro–oxidation of more readily oxidizable molecules (urea, hydrazine, ethanol, and so on) with lower minimum thermodynamic voltage or faster reaction kinetics than OER into water electrolysis to build a modular configuration has been verified as a valid and feasible means to solve this matter.15,34,35 Of these species, the advantages of low cost and non toxicity, extremely low thermodynamics

Figure 1. Top: Schematic illustration of bare CC, NiCo-precursor, and NiCo2S4 NS/CC. (a) Low- and high-resolution SEM images of (a) bare CC, (b) NiCo-precursor, and (c) NiCo2S4 NS/CC. (d) Single and overlapped elemental mapping images of Ni, Co, and S elements in NiCo2S4 NS/CC. (e,f) TEM and HRTEM images of scratched NiCo2S4 nanosheet. (g) XRD pattern of NiCo2S4 NS/CC.


Page 3 of 12 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


voltage of 0.37 V, and oxidized conducts being just non–toxic CO2 and N2, enables urea oxidation reaction (UOR) as an ideal candidate for OER.15,36 It is also promising to combine this energy–saving hydrogen production system with the degradation of urea generated from both fertilizer and animal excreta in eutrophic wastewater. To date, some transition– metal oxides–,36 phosphides–,15,37 sulfides–,38,39 and nitrides40– based bifunctional catalysts with mono– or multi–metal component have been already reported for overall urea electrolysis. Of note, the urea–electrolysis efficiency driven by above reported sulfides−based catalysts (> 1.50 V at 10 mA cm–2) is a little inferior to those for phosphides– and nitrides– based ones (< 1.50 V at 10 mA cm−2) due to their relative low HER and ordinary UOR catalytic performances. Therefore, simultaneously boosting the HER and UOR activities of these sulfides via tailing the electronic or structural properties is urgently demanded. Herein, we report on our recent work that bimetallic nickel– cobalt sulfide nanosheet arrays on carbon cloth (NiCo2S4 NS/CC) by anion exchange from its NiCo–precursor could act as a flexible, versatile and fabulous electrocatalyst for both the overall water splitting and urea electrolysis in base. The overall water–splitting efficiency of this material is higher than those for reported NiCo2S4 nanowire array on CC, most of one–component transition–metal sulfide based–, and many other transition–metal based–catalysts with just acquiring a cell voltage of 1.66 V to afford 10 mA cm−2. Moreover, unexpected, this material also presents fabulous catalytic activity, selectivity, and catalytic durability for urea oxidation (UOR) with an ultralow potential of 0.249 V vs. SCE to achieve an anodic current density of 10 mA cm−2, which surpasses all reported transition–metal based UOR catalysts. Considering the aim to enhance the overall water–electrolysis efficiency, an urea–assisted energy–saving alkaline hydrogen– production system was further built here by replacing anodic oxygen evolution reaction with UOR. This whole urea– electrolysis configuration actuated by NiCo2S4 NS/CC (+,−) affords 10 mA cm−2 at a relative–low voltage of 1.45 V, which is 210 mV less than that for urea–free counterpart and also comparable to the performances for other reported catalysts. The fascinating catalytic performances of NiCo2S4 NS/CC could be ascribed to the intrinsic high activity and metallic feature of NiCo2S4 phase, the nanoporous and open–shelled nanosheet topology that exposes more catalytic active sites and accelerates the diffusion of electrolyte and the generated gas bubbles, as well as the spinel structure and mixed valence states of Ni and Co elements in NiCo2S4 phase that offers richer redox reactions. Note that, the overall urea–electrolysis system with NiCo2S4 NS/CC as both the cathode and anode could be well driven by a 1.5 V battery and a 2.0 V solar cell in 1.0 M KOH with 0.33 M urea. The mechanical performance of this material was also tested by deforming this electrode under various bending states in the process of full urea electrolysis.

RESULTS AND DISCUSSION NiCo2S4 NS/CC was synthesized through the following procedure shown in the top portion of Figure 1 (Detailed synthetic procedure in Experimental Section). 1) Carbon cloth was chosen here as the substrate to integrate catalysts on the basis of its 3D, highly conductive, and flexible characteristics

for application in technological devices;41 2) NiCo–precursor was hydrothermally grown on CC; 3) Highly–ordered NiCo2S4 NS/CC was further obtained by a wet–chemistry topotactic conversion from its NiCo–precursor. The changes of optical property from green to purple–black demonstrates the successful conversion of NiCo−precursor to NiCo2S4 NS/CC (Figure S1). Figure 1a reveals the typical low– and high– magnification scanning electron microscopy (SEM) images for bare CC. Low–magnification SEM images of NiCo–precursor on CC suggests the complete coverage of NiCo–precursor in the form of highly–oriented nanoarrays (Figure 1b and S2a). Closer views of such nanoarray configuration (inset of Figure 1b and S2b) demonstrates a smooth and open–shelled nanosheet structure with approximate 31 nm in thickness of NiCo–precursor. Corresponding elemental dispersive X–ray spectrum and mapping images prove the presence and uniform distribution of Ni, Co, and O elements on the NiCo–precursor (Figure S2c and d). After sulfuration conversion, the vertical nanosheet array morphology of NiCo2S4 NS/CC was well sustained and the distance between adjacent layers are around hundreds of nanometres (Figure 1c and S2e). Longitudinal– and cross–sectional SEM images of NiCo2S4 NS/CC show that each unit turns to be more rough and is constituted of closely– connected nanospheres with 25.2 nm in thickness and 1.64 µm in height (inset of Figure 1c, Figure S2f and g). Single and overlapped elemental mapping images (Figure 1d) show the presence and uniform distribution of Ni, Co, and S elements on the NiCo2S4 NS/CC. Transmission electron microscope (TEM) image (Figure 1e) of NiCo2S4 phase scratched from CC further shows a well–defined rough nanosheet morphology. Corresponding high–resolution TEM (HRTEM) image (Figure 1f) reveals this configuration has a well–resolved lattice fringe with inter–planar distance of about 2.83 Å, which could be well indexed to the (311) plane of NiCo2S4.21,22 X–ray diffraction (XRD) pattern for NiCo2S4 NS/CC (Figure 1g) shows that other than the two peaks labelled with " ◆ " are assigned to the CC substrate,42 the diffraction peak positions and intensities of all other peaks accord well with the standard pattern of NiCo2S4 (JCPDS No. 20–0782),21,22 indicative of its high purity and crystallinity. Nitrogen adsorption/desorption isotherm of NiCo2S4 nanosheets scratched from the CC (Figure S3) shows that it has a Brunauer–Emmett–Teller (BET) surface area of about 88.324 m2/g. Barrett–Joyner– Halenda (BJH) pore-size distribution curve (inset) suggests that the NiCo2S4 phase has a porous nanosheet structure with the total pore volume of 0.228 mL/g and average pore radius of 1.818 nm. All these results well support the successful fabrication of porous NiCo2S4 nanosheets array on CC. Surface elemental compositions and valence states for NiCo2S4 NS/CC were characterized by X–ray photoelectron spectroscopy (XPS) analysis. Survey XPS spectrum proves the existence of Ni, Co, and S elements in NiCo2S4 phase after sulfuration conversion (Figure S4a), which is in well agreement with the elemental mapping results. In the high– resolution Ni 2p region spectrum (Figure S4b), two peaks located at 853.3 and 870.7 eV could be ascribed to Ni 2p3/2 and Ni 2p1/2, respectively, which are spin–orbit characteristics of Ni2+, and other two existing binding energies at 856.6 eV for Ni 2p3/2 and 874.8 eV for Ni 2p1/2 are characteristics of Ni3+.18 In the Co 2p region shown in Figure S3c, the binding energies located at 778.8 eV for Co 2p3/2 and 793.7 eV for Co



2p1/2 are characteristics of Co3+, while the binding energies at 781.6 eV for Co 2p3/2 and 797.3 eV for Co 2p1/2 demonstrate the existence of Co2+.43 The peak areas of Ni2+ and Co3+ states calculated from the Ni 2p and Co 2p spectra (Figure S5) are stronger than those for Ni3+ and Co2+, respectively, suggesting that Ni2+ and Co3+ states are predominant in NiCo2S4 phase.18,21 Besides, the S 2p spectrum could be fitted to four subpeaks and one shake–up satellite peak (Figure S4d). The peaks located at 161.5 and 163.1 eV could be assigned to S 2p3/2 and S 2p1/2, respectively. The peak at 163.8 eV is typically characteristic of metal–sulfide bonds,44 and the peak at 162.6 eV could be attributed to the S2− at the surface with low coordination.18,21 Additionally, the relative–broad peak at 168.8 eV corresponds well to surface impurities of typical S– O species, which is due to the oxidation of catalyst exposed in air for a long time. On the basis of above XPS data, the near– surface of this NiCo2S4 phase is affirmed to be composed of Ni2+, Ni3+, Co2+, Co3+, and S2−, which coincides with those for previously reported NiCo2S4–based catalysts.18,21,22,45

Figure 2. (a) Polarization curves and (b) Tafel plots of NiCo2S4 NS/CC, NiCo2O4 NS/CC, and bare CC for HER and OER. (c,d) Multi-current processes of NiCo2S4 NS/CC for HER and OER. The current density started at ±10 mA cm−2 and ended at ±110 mA cm−2, with an increment of ±10 mA cm−2 per 500 s without iR correction (Inset: HER and OER polarization curves for NiCo2S4 NS/CC before and after long-term electrolysis). (e) Polarization curves of overall water splitting for NiCo2S4 NS/CC, NiCo2O4 NS/CC, and bare CC. (f) Chronopotentiometric curve of water electrolysis for NiCo2S4 NS/CC in a two-electrode configuration at a constant 10 mA cm−2 in 1.0 M KOH (Inset: polarization curves for NiCo2S4 NS/CC (+,−) before and after long-term electrolysis).

To verify the electrocatalytic performance of NiCo2S4 NS/CC for water splitting, electrochemical measurements were made in a home–made three–electrode alkaline electrolyzer (1.0 M KOH) with NiCo2S4 NS/CC as the working electrode, a graphite plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrolysis performances of control samples of

Page 4 of 12

NiCo2O4 NS/CC and bare CC were also tested. Considering the as–measured reaction currents cannot directly reflect the intrinsic activity of catalysts due to the effect of ohmic resistance, an iR correction was applied to all the initial data for further analysis.46 Figure 2a and S6 show the polarization curves of NiCo2S4 NS/CC for HER and OER catalysis in 1.0 M KOH. It could be clearly observed that the NiCo2S4 NS/CC electrode has small overpotentials of 181 and 266 mV for HER, and 240 and 300 mV for OER to reached 10 and 50 mA cm−2 in HER and OER processes, respectively, much superior to NiCo2O4 NS/CC with large overpotentials of 328 mV for HER and 320 mV for OER to afford the same current density, indicating the great enhancement of catalytic activity toward both half–reactions after sulfuration.47 Also note that, both the HER and OER activities of this material are well comparable to those of NiCo2S4 nanowire arrays on CC or Ni foam,18,22 and other newly–reported remarkable transition–metal oxides, chalcogenides, phosphides, and nitrides (Table S1 and S2). In addition, the HER and OER kinetics of this electrode were also studied by comparing their Tafel plots (Figure 2b). The linear portions of Tafel plots are fitted to the Tafel equation: η = b log j + a (1) wherein j is the current density, b is the Tafel slope, and a is the intercept relative to the exchange current density j0).48 Compared to the NiCo2O4 NS/CC (141 mV dec−1 for HER and 95.2 mV dec−1 for OER), the NiCo2S4 NS/CC exhibits smaller Tafel slopes of 130.5 and 90.9 mV dec−1 for HER and OER, respectively. These Tafel slopes are also superior to those for many other reported catalysts like Co/NC (152 mV dec−1 for HER),49 NiCo2S4/CC (141 mV dec−1 for HER),22 Ni1.85Fe0.15P/Ni foam (96 mV dec−1 for OER),50 Ni5Co LDH/Ni foam (91 mV dec−1 for OER),51 and Ni3S2/Ni foam (108 mV dec−1 for OER),52 manifesting its favorable catalytic kinetics for both the HER and OER.53–56 The catalytic stability toward these two reactions of NiCo2S4 NS/CC was also evaluated. Figure 2c and d display the HER and OER multi–step chronopotentiometric curves for NiCo2S4 NS/CC with cathodic and anodic current densities increasing from ±10 to ±100 mA cm−2, respectively. As observed, the potentials promptly level off at the beginning current density and keep constant in the following 500 s and the other steps also give similar phenomenon. By comparing the polarization curves before and after 5000 s electrolysis, negligible activity degradation in HER and OER could be viewed in the insets of Figure 2c and d, indicative of the excellent mass and electron transfer as well as mechanical robustness of this catalytic electrode.31,54 Enlarged views of above chronopotentiometric curves (Figure S7 and S8) show that when current densities alter from one step to another, the response times for both the HER and OER are very short with less than 0.001 and 5 s, respectively, further proving the rapid mass and electron transfer of this catalyst. Moreover, apparently, the response time of OER is at least three order of magnitudes longer than that for HER, which could be well explained by the fact that OER has a more sluggish reaction kinetics compared with HER.55 In view of the high performance of NiCo2S4 NS/CC toward both the HER and OER, we ulteriorly utilized the NiCo2S4 NS/CC as both the anodic and cathodic materials to drive the full water electrolysis using a two–electrode setup (NiCo2S4 NS/CC (+,−)) in 1.0 M KOH to go a step closer to the real application.


Page 5 of 12 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


NiCo2O4 NS/CC and bare CC were also served as both the cathode and anode as controls. As shown in Figure 2e, this NiCo2S4 NS/CC (+,−) pair achieves 10 mA cm−2 at just 1.66 V in 1.0 M KOH, which is much lower than NiCo2O4 NS/CC (+,−) (1.83 V to obtain 10 mA cm−2) and bare CC (+,−) pairs in exactly the same test environment. This exceptional property also compares favorably to those for NiCo2S4/CC (+,−) (1.68 V),22 Ni3S2/Ni foam (+,−) (~1.70 V),52 Ni−CoS2/CC (+,−) (1.66 V),23 NiFe@NiCo2O4/Ni foam (+,−) (1.67 V),8 CoS2/CC (+,−) (1.67 V),56 Ni−Co−S/Cu foam (+,−) (1.67 V),57 Ni5P4/Ni foil (+,−) (1.69 V),58 and some other superior Janus catalysts shown in Table S3. Of note, on the basis of the charge conservation and the equation of I = Q/t (2), if the cathode surface is equal to anode surface, the cathodic and anodic current densities should be equal in amount but opposite in sign. Thus, theoretically, the voltage for overall water electrolysis should be consistent with the potential difference (∆V) between HER and OER at the same current density.59 As shown in Figure S6, the ∆V between HER and OER steady–state polarization curves at 10 mA cm−2 (1.651 V) coincides well with the voltage of 1.66 V needed to afford an overall current density of 10 mA cm−2. Besides, it should be emphasized here that both the full water– and subsequent urea–electrolysis efficiencies were measured from the LSV curves in the reverse scan direction to avoid the impacts of oxidation/capacitive currents, which would otherwise bring an inflated high overall efficiency,60,61 and thus cause an obvious discrepancy between the values of ∆V and voltage to approach to the same current density. Besides, the long−term electrolysis durability of NiCo2S4 NS/CC (+,−) was also probed. As shown in Figure 2f, a starting voltage of 1.68 V was required to deliver 10 mA cm−2 and maintained at around 1.66 V in the process of 20 h electrolysis with nearly no performance degradation for NiCo2S4 NS/CC (+,−) (inset in Figure 2f), demonstrating that this NiCo2S4 NS/CC (+,−) pair could be qualified to act as a stable and high–active Janus catalyst to cater for water electrolysis.

Figure 3. (a) Polarization curves of NiCo2S4 NS/CC in 1.0 M KOH with and without 0.33 M urea with a scan rate of 2 mV s−1. (b) Polarization curves of NiCo2S4 NS/CC, NiCo2O4 NS/CC, and bare CC in 1.0 M KOH with 0.33 M urea with a scan rate of 2 mV s−1. (c) Multi-current process of NiCo2S4 NS/CC for UOR. The current density started at 10 mA cm−2 and ended at 110 mA cm−2, with an increment of 10 mA cm−2 per 500 s without iR correction. (d) Polarization curves for NiCo2S4 NS/CC (+,−) in 1.0 M KOH with and without 0.33 M urea. (e) Polarization curves for NiCo2S4 NS/CC (+,−), NiCo2O4 NS/CC (+,−), and bare CC (+,−) in 1.0 M KOH with 0.33 M urea. (f) Chronopotentiometric curve for NiCo2S4 NS/CC at a constant current density of 10 mA cm−2 (Inset: Polarization curves for NiCo2S4 NS/CC (+,−) before and after over 20 h whole urea electrolysis).

Although the overall water–splitting efficiency of NiCo2S4 NS/CC (+,−) in base (1.66 V to yield 10 mA cm−2) compares favorably to the above–mentioned NiCo2S4–based and some of chalcogenide– and phosphide–based Janus catalysts, the needed slightly higher voltage (commonly larger than 1.65 V) still makes this process more energy intensive. By contrast, despite UOR has a more slower reaction kinetics involving a six–electron transfer process than OER with four–electron transfer steps, it has a lower thermodynamic voltage of 0.37 V compared with OER (1.23 V),62 which provides us an ideal anodic alternative toward energy–saving electrolytic hydrogen generation. The catalytic activity of NiCo2S4 NS/CC toward UOR has been studied by linear sweep voltammetry (LSV) in 1.0 M KOH with 0.33 M urea with a scan rate of 1 mV s−1. As observed in Figure 3a, the NiCo2S4 NS/CC electrode shows an exceptional catalytic performance toward UOR with a much lower potential of 0.249 V vs. SCE to attain 10 mA cm−2, compared with the potential of 0.486 V in the absence of urea to get the same current density, which should be assigned to the OER. By contrast, this potential value for UOR is around half of that for OER, indicating that this as–synthesized NiCo2S4 nanosheet species are more active and selective toward urea oxidation. The UOR performances for NiCo2S4 NS/CC, NiCo2O4 NS/CC, and CC were also assessed in 1.0 M KOH with 0.33 Urea (Figure 3b). It could be clearly seen that the NiCo2S4 NS/CC electrode shows a much higher current response and earlier onset potential toward urea in comparison to the NiCo2O4 NS/CC (0.286 V at 10 mA cm−2). Note that the UOR performance of NiCo2S4 NS/CC (0.249 V at 10 mA cm−2) also outperforms all of reported transition–metal based UOR catalysts like MnO2/Ni foam (0.262 V),36 Ni3N/CC (0.282 V),40 Ni2P/CC (0.302 V),37 Zn0.08Co0.92P/Ti mesh (0.42 V),15 MnO2/MnCo2O4/Ni foam (0.262 V),62 Ni(OH)2 NS (0.452 V),63 NiMoS/Ti mesh (0.310 V),38 Se– Ni(OH)2@NiSe/Ni foam (0.30 V),64 CoS2/Ti mesh (0.332 V),39 and other reported UOR catalysts listed in Table S4. Figure S9 presents the LSV curves of NiCo2S4 NS/CC with different scan rates from 1–50 mV s−1 with slight differences of corresponding data re–plotted as the current density at 0.4 V vs. SCE (inset in Figure S8), revealing efficient charge and mass transfer of this electrode in UOR catalytic process.36,65 Figure 3c displays the multi–step chronopotentiometric curve for NiCo2S4 NS/CC toward UOR with the current density increasing from 10–100 mA cm−2 (10 mA cm−2 per 500 s). The potential immediately levels off at the potential of about 0.249 V and keeps constant for the rest 500 s. Similar results could be observed in the subsequent steps. The comparison of UOR polarization curves before and after 5000 s electrolysis proves negligible activity degradation (inset in Figure 3c). All these results indicate the excellent mass and electron transfer



kinetics and mechanical robustness of this catalyst toward UOR. A closer view of the portion of one step rising to another in above curve shows that UOR has a longer response time of 6 s than OER (Figure S10), agreeing well with what mentioned earlier that UOR has a more sluggish kinetics in comparison to OER. Besides, this NiCo2S4 NS/CC electrode could also catalyze well the HER process in 1.0 M KOH with 0.33 M urea and only exhibits a small negative shift of potentials to achieve the same current densities compared with that in 1.0 M KOH, showing that urea has little impact on the HER activity (Figure S11a). And as expected, NiCo2S4 NS/CC has a higher HER activity in comparison to NiCo2O4 NS/CC and CC in 1.0 M KOH with 0.33 M urea (Figure S11b). The catalytic stability for NiCo2S4 NS/CC toward HER in this condition was also studied and proved to be acceptable (Figure S11c).

Figure 4. Optical photographs of overall urea electrolysis devices powered by a 1.5 V battery (a-c) and a 2.0 V solar panel (d-f) with NiCo2S4 NS/CC as both the cathode and anode in 1.0 M KOH with 0.33 M urea. (g-r) Optical photos of the overall urea electrolysis at 15 mA cm−2 driven by the NiCo2S4 NS/CC (+,−) at the bending angles of approximate 0°, 45°, and 90°.

In consideration of the distinguished performances of NiCo2S4 NS/CC for both the UOR and HER, we further fabricated a two–electrode cell utilizing NiCo2S4 NS/CC as both the anode and cathode to verify the feasibility of urea– assisted energy–efficient hydrogen production. It is affirmed here that the NiCo2S4 NS/CC (+,−) pair just needs a small voltage of 1.45 V to yield 10 mA cm−2 in 1.0 M KOH with

Page 6 of 12

urea, which is about 210 mV lower than that for NiCo2S4 NS/CC (+,−) (1.66 V) in 1.0 M KOH to achieve the same current density (Figure 3d). This performance of NiCo2S4 NS/CC (+,−) is not only much superior to those of NiCo2O4 NS/CC (+,−) (1.77 V) and CC (+,−) (Figure 3e), but comparable to the newly–reported bifunctional catalysts such as NiMoS/CC (+,−) (1.59 V),38 CoS2/Ti mesh (+,−) (1.59 V),39 Ni3N/CC (+,−) (1.44 V),40 and MnO2/MnCo2O4/Ni foam (+,−) (1.55 V)62 (Table S5). The chronopotentiometric experiment was then used to evaluate the catalytic durability of NiCo2S4 NS/CC (+,−) working in 1.0 M KOH with 0.33 M urea. As observed in Figure 3f, a voltage of about 1.45 V for NiCo2S4 NS/CC (+,−) could be maintained at 10 mA cm−2 for over 20 hours with little current loss (inset in Figure 3f), suggesting that this NiCo2S4 NS/CC (+,−) pair possesses exceptional catalytic durability toward full urea electrolysis. The high overall water- and urea-electrolysis efficiencies of NiCo2S4 NS/CC could be explained from the experimental and theoretical aspects. (1) More exposed catalytic active sites enabled by the large surface area and nanoporous nanosheet structure. The reaction temperature and Na2S concentration are key parameters in the hydrothermal sulfuration process. The relative-higher reaction temperature and Na2S concentration (150 or 160 oC, 0.1 or 0.2 M Na2S) utilized in both this work and another reported work18 could bring a more porous and rough NiCo2S4 surface with more exposed catalytic active sites. Also, the thickness of synthesized NiCo-precursor nanosheet assisted by HMT is usually smaller than that for the NiCoprecursor nanosheet or nanowire assisted by urea.18,22 This means that after sulfuration conversion, the NiCo2S4 nanosheets in our work will be more possible to possess abundant nanopores and thereby expose more catalytic active sites. SEM image in Figure S2f proves the NiCo2S4 phase is a single-layered closely-connected nanoparticles-consisted film structure, exposing sufficiently the two-sides of one-unit NiCo2S4 nanoparticle. Further BET analysis (Figure S3) also demonstrates that the NiCo2S4 phase has a porous nanosheet structure with abundant nanopores and larger specific surface area. Cyclic voltammetry measurements were ulteriorly adopted to compare the active surface areas for NiCo2S4 NS/CC and NiCo2O4 NS/CC electrodes.31 Figure S12a and b show the cyclic voltammograms (CVs) in a small potential window of 0.10–0.20 V with different scan rates from 5–70 mV s−1, wherein the current response should be only ascribed to the double–layer charging.66,67 The capacitance of NiCo2S4 NS/CC (33.16 mF cm−2) is about four times larger than that of NiCo2O4 NS/CC (8.33 mF cm−2) as presented in Figure S12c, exposing that NiCo2S4 NS/CC has a higher surface roughness which thereby could provide more active sites.66–68 (2) Metallic nature of NiCo2S4 NS/CC with increased conductivity. The electrochemical impedance spectroscopy (EIS) data in Figure S13 clearly exhibit the changes of ionic and electronic resistances before and after sulfuration conversion of this material. The radius of the semicircles of NiCo2S4 NS/CC are smaller than that of NiCo2O4 NS/CC, indicating that the NiCo2S4 NS/CC has a higher charge–transfer rate and thereby a more rapid catalytic kinetics of this catalyst,69 which is consistent with the conclusion that NiCo2S4 is in fact a metal.29 The intercept of X axis in the enlarged figures for NiCo2S4 NS/CC are also smaller than that for NiCo2O4 NS/CC, indicating that the NiCo2S4 NS/CC also has a much lower


Page 7 of 12 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


mass-transfer resistance.70 (3) Spinel structure and mixed valence states of Ni and Co elements in NiCo2S4 phase that could offer richer redox reactions than the corresponding single-component sulfides.28 (4) The open-shelled porous nanosheet array topology comprising 3D porous carbon cloth skeleton will largely accelerate the diffusion of electrolyte and the generated gas bubbles. Besides, to verify the structure stability of NiCo2S4 NS/CC under electrolysis operation, the comparison of changes of morphologies and transport resistances between NiCo2S4 NS/CC and NiCo2S4 NW/CC before and after water splitting and urea electrolysis has been provided in Figure S14 and S15. The smaller changes of morphology and transport resistances of NiCo2S4 NS/CC after overall water and urea electrolysis compared with those for NiCo2S4 NW/CC demonstrate that the NiCo2S4 NS/CC has a more stable nanosheet array structure under continuous electrolysis conditions. Considering electrolysis system power–supplied by the commercial battery or solar panel has been well recognized as a portable, sustainable and less energy–output pattern to realize high–efficiency energy conversion,18,71–76 we further built up the 1.5 V battery– and 2.0 V solar panel–driven full urea electrolysis configurations. Figure 4a shows the optical image of 1.5 V battery–driven overall urea electrolysis system, and dynamic video was shown in Movie S1. Closer observations of this system (Figure 4b and c) present clear production of both cathodic H2 and anodic N2/CO2 bubbles. Apparently, the bubbles generated in this setup flow rapider and are larger in size than that in battery–driven overall water splitting, further proving that a higher efficiency of hydrogen production could be obtained by substituting UOR for OER. Figure 4d presents the 2.0 V solar panel–powered overall alkaline urea electrolysis device. The anodic and cathodic NiCo2S4 NS/CC were connected to the two terminals of solar panel. The voltage applied to the cell from the solar panel was monitored by a digital multimeter linked in parallel with the terminals.75,76 Because the used GaAs–based thin film solar cell has high open–circuit voltage and current density, the tunable voltage of about 1.45 V could be readily controlled on the basis of the sunlight irradiation intensity. The enlarged images with different angles (Figure 4e and f) exhibit that obvious bubbles of cathodic H2 and anodic N2/CO2 were occurred on the electrode surfaces. Dynamic video of continuous bubbles evolution at about 1.451 V for overall urea electrolysis (10 mA cm−2) was displayed in Movie S2. Such a solar energy–driven overall urea–electrolysis device is promising to be utilized in portable, less energy–output, and wholesale solar–to–hydrogen generation in future distributed energy storage technologies.18,74-76 Besides, the mechanical property of NiCo2S4 NS/CC (0.5 × 3 cm2) when catalyzing the full urea electrolysis was probed by deforming this electrode under various bending states. Figure 4g–r show the optical photos of the overall urea electrolysis at 15 mA cm−2 driven by the NiCo2S4 NS/CC (+,−) at the bending angles of approximate 0°, 45°, and 90° (dynamic process in Movie S3). Negligible performance degradation in the chronopotentiometric curve was observed, indicative of the superior mechanical ﬂexibility and stability of NiCo2S4 NS/CC toward the urea electrolysis. Also, in the consideration of the non–negligible effect of current collector on the electrolysis performance of NiCo2S4

phase, we also chose Ni foam as substrate to load the NiCo2S4 nanosheet array configuration (NiCo2S4 NS/NF) to compare the catalytic activity between NiCo2S4 NS/NF and NiCo2S4 NS/CC. SEM and elemental mapping images (Figure S16) demonstrate the successful fabrication of NiCo2S4 NS/NF. The water–splitting activity of NiCo2S4 NS/NF was firstly evaluated in 1.0 M KOH. It achieves 50 mA cm−2 at overpotentials of 236 and 265 mV for HER and OER, respectively, lower than those for NiCo2S4 NS/CC and NiCo2O4 NS/NF (Figure S17a). Figure S17b shows the Tafel plots in the linear region of NiCo2S4 NS/NF, NiCo2O4 NS/NF, and bare Ni foam for HER and OER. Obviously, NiCo2S4 NS/NF has the lowest Tafel slopes both for HER and OER. Also, both the HER and OER multi–step chronopotentiometric curves of the NiCo2S4 NS/NF (Figure S17c) depict an impressive durability with fast amperometric responses toward altered operating potentials. The efficiency of full water splitting for NiCo2S4 NS/NF (+,−) was further measured. This pair just needs a small voltage of 1.62 V to approach 10 mA cm−2, which is about 40 mV and 10 mV lower than those for above NiCo2S4 NS/CC (+,−) (1.66 V) and reported NiCo2S4 NW/NF (+,−) (1.63 V),18 implying that the much enhanced overall water–splitting efficiency of NiCo2S4 NS/NF (+,−) (our work) and NiCo2S4 NW/NF (+,−) (reported work) in contrast to those of NiCo2S4 NS/CC (+,−) (our work) and NiCo2S4 NW/CC (+,−) (reported work),22 might mainly originate from the enlarged surface area and native good catalytic activity of Ni foam.77 The long–term durability of NiCo2S4 NS/NF (+,−) was probed by chronopotentiometric stripping analysis at fixed 10 mA cm−2 over 20 h with almost no activity loss (Figure S17d). Moreover, the UOR and overall urea electrolysis performances of NiCo2S4 NS/NF were explored. For UOR, NiCo2S4 NS/NF drives 50 mA cm−2 with just 0.308 V vs. SCE in 1.0 M KOH with 0.33 M urea, much better than those of NiCo2S4 NS/CC and corresponding NiCo2O4 NS/NF (Figure S18a and b). Multi–step chronopotentiometric curves for HER and UOR were also recorded to verify the catalytic stability of NiCo2S4 NS/NF (Figure S18c). Moreover, this NiCo2S4 NS/NF (+,−) pair just needs a very small voltage of 1.42 V to afford 10 mA cm−2 and maintains it for over 20 h electrolysis in 1.0 M KOH with 0.33 M urea (Figure S18d). All above experimental results prove that Ni foam could not only offer an enlarged and conductive skeleton for the loading of NiCo2S4, but also benefit for the overall performance of this monolithic NiCo2S4 NS/NF on account of its inherent catalytic activity.77

CONCLUSION In conclusion, traditional NiCo2S4 phase with novel nanosheet array topology on carbon cloth (NiCo2S4 NS/CC) converted by wet–chemistry sulfurization of its NiCo2O4 precursor could be qualified as a high–active and durable non–noble–metal versatile catalyst for both overall water and urea electrolysis in base. Its efficiency toward full water splitting (1.66 V at 10 mA cm−2) is better than previously–reported NiCo2S4 nanowire arrays on CC, and most of sulfide–based bifunctional catalysts. Moreover, surprisingly, we found this material shows fabulous catalytic activity, selectivity, and catalytic durability toward electrolytic urea oxidation (UOR) with an ultralow potential of 0.249 V vs. SCE to afford an anodic current density of 10 mA cm−2, which surpasses those



of all reported transition metal–based UOR catalysts. In view of this incredible finding and the aim to enhance the overall efficiency of water electrolysis, we further designed an urea– mediated less energy–intensive hydrogen production system by replacing anodic oxygen evolution reaction with UOR in 1.0 M KOH with 0.33 M urea. This complete urea–electrolysis cell driven by NiCo2S4 NS/CC (+,−) offers 10 mA cm−2 at a relative–low voltage of 1.45 V, which is 210 mV less than that for urea–free counterpart and also comes up to the performances of other reported catalytic materials. The extraordinary catalytic performances of NiCo2S4 NS/CC should be determined by the native excellent activity and metallic features of NiCo2S4 species as well as the nanoporous, open–shelled, and 3D nanosheet topology. This work not only exhibits a morphology–regulated NiCo2S4–based catalyst with high performance toward the overall water splitting, but also fortuitously seeks out a state–of–the–art anodic material for urea oxidation as well as provides new opportunities in utilizing bimetallic sulfides for simultaneous realization of energy–saving urea–assisted electrolytic production of hydrogen fuels and purification of urea–rich wastewater for applications.

(6)

(7)

(8)

(9)

(10)

(11)

ASSOCIATED CONTENT Supporting Information Experimental section; Optical photographs; SEM images; EDX spectrum and mapping images; XPS spectra; Electrochemical data; Movie S1-S3; Table S1-S5. This material is available free of charge via the Internet at http://pubs.acs.org.

(12)

AUTHOR INFORMATION

(13)

Corresponding Author *

E-mail: [email protected].

(14)

Author Contributions ⊥ W.Z. and M.R. contributed equally to this work. Notes The authors declare no competing financial interest. (15)

ACKNOWLEDGMENTS This work was financed by the National Natural Science Foundation of China (21675127), the Fundamental Research Funds for the Northwest A&F University of China (2014YB093, 2452015257), and the Development Project of Qinghai Key Laboratory (2017-ZJ-Y10).

(16)

REFERENCES (1) (2)

(3)

(4)

(5)

Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972−974, DOI: 10.1126/science.1103197. Chu S.; Majumdar, A. Opportunities and Challenges for a Sustainable Energy Future. Nature 2012, 488, 294−303, DOI: 10.1038/nature11475. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998, DOI: 10.1126/science.aad4998. Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332−337, DOI: 10.1038/35104599. Cook, T. R.; Dogutan, D. K.; Reece, S. K.; Suren-dranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and

(17)

(18)

(19)

Page 8 of 12

Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474−6502, DOI: 10.1021/cr100246c. Tang, C.; Zhang, R.; Lu, W.; He, L.; Jiang, X.; Asiri, A. M.; Sun, X. Fe-Doped CoP Nanoarray: A Monolithic Multifunctional Catalyst for Highly Efficient Hydrogen Generation. Adv. Mater. 2017, 29, 1602441, DOI: 10.1002/adma.201602441. Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z. SelfTemplating Synthesis of Hollow Co3O4 Microtube Arrays for Highly Efficient Water Electrolysis. Angew. Chem. Int. Ed. 2017, 56, 1324−1328, DOI: 10.1002/anie.201610413. Xiao, C.; Li, Y.; Lu, X.; Zhao, C. Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting. Adv. Funct. Mater. 2016, 26, 3515−3523, DOI: 10.1002/adfm.201505302. Duan, J.; Chen, S.; Zhao, C. Ultrathin Metal-Organic Framework Array for Efficient Electrocatalytic Water Splitting. Nat. Commun. 2017, 8, 15341, DOI: 10.1038/ncomms15341. Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Grätzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-Abundant Catalysts. Science 2014, 345, 1593−1596, DOI: 10.1126/science.1258307. Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148−5180, DOI: 10.1039/c4cs00448e. Yan, Y.; Xia, B. Y.; Zhao, B.; Wang, X. A Review on Noble-Metal-Free Bifunctional Heterogeneous Catalysts for Overall Electrochemical Water Splitting. J. Mater. Chem. A 2016, 4, 17587−17603, DOI: 10.1039/C6TA08075H. Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X. Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv. Mater. 2016, 28, 215−230, DOI: 10.1002/adma.201502696. Song, J.; Zhu, C.; Xu, B. Z.; Fu, S.; Engelhard, M. H.; Ye, R.; Du, D.; Beckman, S. P.; Lin, Y. Bimetallic CobaltBased Phosphide Zeolitic Imidazolate Framework: CoPx Phase-Dependent Electrical Conductivity and Hydrogen Atom Adsorption Energy for Efficient Overall Water Splitting. Adv. Energy Mater. 2017, 7, 1601555, DOI: 10.1002/aenm.201601555. Liu, T.; Liu, D.; Qu, F.; Wang, D.; Zhang, L.; Ge, R.; Hao, S.; Ma, Y.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X. Enhanced Electrocatalysis for Energy-Efficient Hydrogen Production over CoP Catalyst with Nonelectroactive Zn as a Promoter. Adv. Energy Mater. 2017, 7, 1700020, DOI: 10.1002/aenm.201700020. Tang, C.; Gan, L.; Zhang, R.; Lu, W.; Jiang, X.; Asiri, A. M.; Sun, X.; Wang, J.; Chen, L. Ternary FexCo1–xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-Like Activity: Experimental and Theoretical Insight. Nano Lett. 2016, 16, 6617−6621, DOI: 10.1021/acs.nanolett.6b03332. Liu, T.; Ma, X.; Liu, D.; Hao, S.; Du, G.; Ma, Y.; Asiri, A. M.; Sun, X.; Chen, L. Mn Doping of CoP Nanosheets Array: An Efficient Electrocatalyst for Hydrogen Evolution Reaction with Enhanced Activity at All pH Values. ACS Catal. 2017, 7, 98−102, DOI: 10.1021/acscatal.6b02849. Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661−4672, DOI: 10.1002/adfm.201600566. Liang, H.; Gandi, A. N.; Anjum, D. H.; Wang, X.; Schwingenschlögl, U.; Alshareef, H. N. Plasma-Assisted


Page 9 of 12 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

(20)

(21)

(22)

(23)

(24)

(25)

(26)

(27)

(28)

(29)

(30)

(31)

(32)

(33)


Synthesis of NiCoP for Efficient Overall Water Splitting. Nano Lett. 2016, 16, 7718−7725, DOI: 10.1021/acs.nanolett.6b03803. Yan, L.; Cao, L.; Dai, P.; Gu, X.; Liu, D.; Li, L.; Wang, Y.; Zhao, X. Metal-Organic Frameworks Derived Nanotube of Nickel-Cobalt Bimetal Phosphides as Highly Efficient Electrocatalysts for Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1703455, DOI: 10.1002/adfm.201703455. Xue, Y.; Zuo, Z.; Li, Y.; Liu, H.; Li, Y. GraphdiyneSupported NiCo2S4 Nanowires: A Highly Active and Stable 3D Bifunctional Electrode Material. Small 2017, 13, 1700936, DOI: 10.1002/smll.201700936. Liu, D.; Lu, Q.; Luo, Y.; Sun, X.; Asiri, A. M. NiCo2S4 Nanowires Array as an Efficient Bifunctional Electrocatalyst for full Water Splitting with Superior Activity. Nanoscale 2015, 7, 15122−15126, DOI: 10.1039/c5nr04064g. Fang, W.; Liu, D.; Lu, Q.; Sun, X.; Asiri, A. M. Nickel Promoted Cobalt Disulfide Nanowire Array Supported on Carbon Cloth: An Efficient and Stable Bifunctional Electrocatalyst for Full Water Splitting. Electrochem. Commun. 2016, 63, 60−64, DOI: 10.1016/j.elecom.2015.10.010. Xu, X.; Liang, H.; Ming, F.; Qi, Z.; Xie, Y.; Wang, Z. Prussian Blue Analogues Derived Penroseite (Ni,Co)Se2 Nanocages Anchored on 3D Graphene Aerogel for Efficient Water Splitting. ACS Catal. 2017, 7, 6394−6399. DOI: 10.1021/acscatal.7b02079. Du, C.; Yang, L.; Yang, F.; Cheng, G.; Luo, W. Nest-Like NiCoP for Highly Efficient Overall Water Splitting. ACS Catal. 2017, 7, 4131−4137, DOI: 10.1021/acscatal.7b00662. Li, Y.; Zhang, H.; Jiang, M.; Kuang, Y.; Sun, X.; Duan, X. Ternary NiCoP Nanosheet Arrays: An Excellent Bifunctional Catalyst for Alkaline Overall Water Splitting. Nano Res. 2016, 9, 2251−2259, DOI: 10.1007/s12274-0161112-z. Zhang, X.; Huang, L.; Wang, Q.; Dong, S. Transformation of Homobimetallic MOFs into Nickel-Cobalt Phosphide/Nitrogen-Doped Carbon Polyhedral Nanocages for Efficient Oxygen Evolution Electrocatalysis. J. Mater. Chem. A 2017, 5, 18839−18844, DOI: 10.1039/c7ta06272a. Xiao, J.; Wan, L.; Yang, S.; Xiao, F.; Wang, S. Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for HighPerformance Pseudocapacitors. Nano Lett. 2014, 14, 831−838, DOI: 10.1021/nl404199v. Xia, C.; Li, P.; Gandi, A. N.; Schwingenschlögl, U.; Alshareef, H. N. Is NiCo2S4 Really a Semiconductor? Chem. Mater. 2015, 27, 6482−6485, DOI: 10.1021/acs.chemmater.5b01843. Zou, R.; Zhang, Z.; Yuen, M. F.; Sun, M.; Hu, J.; Lee, C.S.; Zhang, W. Three-Dimensional-Networked NiCo2S4 Nanosheet Array/Carbon Cloth Anodes for HighPerformance Lithium-Ion Batteries. NPG Asia Mater. 2015, 7, e195, DOI: 10.1038/am.2015.63. Lu, X.; Zhao, C. Electrodeposition of Hierarchically Structured Three-Dimensional Nickel-Iron Electrodes for Efficient Oxygen Evolution at High Current Densities. Nat. Commun. 2015, 6, 6616, DOI: 10.1038/ncomms7616. Xu, X.; Song, F.; Hu, X. A Nickel Iron Diselenide-Derived Efficient Oxygen-Evolution Catalyst. Nat. Commun. 2016, 7, 12324, DOI: 10.1038/ncomms12324. Zhou, H.; Yu, F.; Sun, J.; He, R.; Chen, S.; Chu, C.-W.; Ren, Z. Highly Active Catalyst Derived from a 3D Foam of Fe(PO3)2/Ni2P for Extremely Efficient Water Oxidation. PNAS 2017, 114, 5607−5611, DOI: 10.1073/pnas.1701562114.

(34)

(35)

(36)

(37)

(38)

(39)

(40)

(41)

(42)

(43)

(44)

(45)

(46)

Tang, C.; Zhang, R.; Lu, W.; Wang, Z.; Liu, D.; Hao, S.; Du, G.; Asiri, A. M.; Sun, X. Energy-Saving Electrolytic Hydrogen Generation: Ni2P Nanoarray as a HighPerformance Non-Noble-Metal Electrocatalyst. Angew. Chem. 2017, 129, 860−864, DOI: 10.1002/anie.201608899. Dai, L.; Qin, Q.; Zhao, X.; Xu, C.; Hu, C.; Mo, S.; Wang, Y. O.; Lin, S.; Tang, Z.; Zheng, N. Electrochemical Partial Reforming of Ethanol into Ethyl Acetate Using Ultrathin Co3O4 Nanosheets as a Highly Selective Anode Catalyst. ACS Cent. Sci. 2016, 2, 538−544, DOI: 10.1021/acscentsci.6b00164. Chen, S.; Duan, J.; Vasileff, A.; Qiao, S. Size Fractionation of Two-Dimensional Sub-Nanometer Thin Manganese Dioxide Crystals towards Superior Urea Electrocatalytic Conversion. Angew. Chem. Int. Ed. 2016, 55, 3804−3808, DOI: 10.1002/anie.201600387. Liu, D.; Liu, T.; Zhang, L.; Qu, F.; Du, G.; Asiri, A. M.; Sun, X. High-Performance Urea Electrolysis towards Less Energy-Intensive Electrochemical Hydrogen Production Using a Bifunctional Catalyst Electrode. J. Mater. Chem. A 2017, 5, 3208−3213, DOI: 10.1039/c6ta11127k Wang, X.; Wang, J.; Sun, X.; Wei, S.; Cui, L.; Yang, W.; Liu, J. Hierarchical Coral-Like NiMoS Nanohybrids as Highly Efficient Bifunctional Electrocatalysts for Overall Urea Electrolysis. Nano Res. 2018, 11, 988−996, DOI: 10.1007/s12274-017-1711-3. Wei, S.; Wang, X.; Wang, J.; Sun, X.; Cui, L.; Yang, W.; Zheng, Y.; Liu, J. CoS2 Nanoneedle Array on Ti Mesh: A Stable and Efficient Bifunctional Electrocatalyst for UreaAssisted Electrolytic Hydrogen Production. Electrochim. Acta 2017, 246, 776−782, DOI: 10.1016/j.electacta.2017.06.068. Liu, Q.; Xie, L.; Qu, F.; Liu, Z.; Du, G.; Asiri, A. M.; Sun, X. A Porous Ni3N Nanosheet Array as a High-Performance Non-Noble-Metal Catalyst for Urea-Assisted Electrochemical Hydrogen Production. Inorg. Chem. Front. 2017, 4, 1120−1124, DOI: 10.1039/C7QI00185A. Ma, T.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. PhosphorusDoped Graphitic Carbon Nitrides Grown In Situ on Carbon-Fiber Paper: Flexible and Reversible Oxygen Electrodes. Angew. Chem. Int. Ed. 2015, 54, 4646−4650, DOI: 10.1002/anie.201411125. Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0–14. J. Am. Chem. Soc. 2014, 136, 7587−7590, DOI: 10.1021/ja503372r. Yang, B.; Yu, L.; Yan, H.; Sun, Y.; Hu, S.; Yuan, Y.; Liu, L.; Wang, J. Fabrication of Urchin-Like NiCo2(CO3)1.5(OH)3@NiCo2S4 on Ni Foam by an IonExchange Route and Application to Asymmetrical Supercapacitors. J. Mater. Chem. A 2015, 3, 13308−13316, DOI: 10.1039/c5ta02684a. Peng, Z.; Jia, D.; Al-Enizi, A. M.; Elzatahry, A. A.; Zheng, G. From Water Oxidation to Reduction: Homologous NiCo Based Nanowires as Complementary Water Splitting Electrocatalysts. Adv. Energy Mater. 2015, 5, 1402031, DOI: 10.1002/aenm.201570050. Jiang, J.; Yan, C.; Zhao, X.; Luo, H.; Xue, Z.; Mu, T. A PEGylated Deep Eutectic Solvent for Controllable Solvothermal Synthesis of Porous NiCo2S4 for Efficient Oxygen Evolution Reaction. Green Chem. 2017, 19, 3023−3031, DOI: 10.1039/c7gc01012e. Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. Closely Interconnected Network of Molybdenum Phosphide Nanoparticles: A Highly Efficient Electrocatalyst for Generating Hydrogen from Water. Adv. Mater. 2014, 26, 5702−5707, DOI: 10.1002/adma.201401692.



(47)

(48)

(49)

(50)

(51)

(52)

(53)

(54)

(55)

(56)

(57)

(58)

(59)

(60)

Hod, I.; Deria, P.; Bury, W.; Mondloch, J. E.; Kung, C. W.; So, M.; Sampson, M. D.; Peters, A. W.; Kubiak, C. P.; Farha, O. K.; Hupp, J. T. A Porous Proton-Relaying MetalOrganic Framework Material that Accelerates Electrochemical Hydrogen Evolution. Nat. Commun. 2015, 6, 8304, DOI: 10.1038/ncomms9304. Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 1529−1541, DOI: 10.1039/C5CS00434A. Wang, A.; Lin, J.; Xu, H.; Tong, Y.; Li, G. Ni2P-CoP Hybrid Nanosheet Arrays Supported on Carbon Cloth as an Efficient Flexible Cathode for Hydrogen Evolution. J. Mater. Chem. A 2016, 4, 16992−16999, DOI: 10.1039/C6TA07704H. Wang, P.; Pu, Z.; Li, Y.; Wu, L.; Tu, Z.; Jiang, M.; Kou, Z.; Amiinu, I. S.; Mu, S. Iron-Doped Nickel Phosphide Nanosheet Arrays: An Efficient Bifunctional Electrocatalyst for Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 26001−26007, DOI: 10.1021/acsami.7b06305. Zhang, Y.; Shao, Q.; Pi, Y.; Guo, J.; Huang, X. A CostEfficient Bifunctional Ultrathin Nanosheets Array for Electrochemical Overall Water Splitting. Small 2017, 13, 1700355, DOI: 10.1002/smll.201700355. Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-Index Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14023−14026, DOI: 10.1021/jacs.5b08186. Xing, Z.; Gan, L.; Wang, J.; Yang, X. Experimental and Theoretical Insights into Sustained Water Splitting with an Electrodeposited Nanoporous Nickel Hydroxide@Nickel Film as an Electrocatalyst. J. Mater. Chem. A 2017, 5, 7744−7748, DOI: 10.1039/c7ta01907f. Tang, C.; Chen, N.; Pu, Z.; Xing, W.; Sun, X. NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew. Chem. Int. Ed. 2015, 54, 9351−9355, DOI: 10.1002/ange.201503407. Zhu, W.; Yue, X.; Zhang, W.; Yu, S.; Zhang, Y.; Wang, J.; Wang, J. Nickel Sulfide Microsphere Film on Ni Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Commun. 2016, 52, 1486−1489, DOI: 10.1039/c5cc08064a. Guan, C.; Liu, X.; Elshahawy, A. M.; Zhang, H.; Wu, H.; Pennycook, S. J.; Wang, J. Metal-Organic Framework Derived Hollow CoS2 Nanotube Arrays: An Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Nanoscale Horiz. 2017, 2, 342−348, DOI: 10.1039/c7nh00079k. Liu, T.; Sun, X.; Asiri, A. M.; He, Y. One-Step Electrodeposition of Ni-Co-S Nanosheets Film as a Bifunctional Electrocatalyst for Efficient Water Splitting. Int. J. Hydrogen Energy 2016, 41, 7264−7269, DOI: 10.1016/j.ijhydene.2016.03.111. Ledendecker, M.; Calderón, S. K.; Papp, C.; Steinrück, H. P.; Antonietti, M.; Shalom, M. The Synthesis of Nanostructured Ni5P4 Films and their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting. Angew. Chem. 2015, 127, 12538−12542, DOI: 10.1002/ange.201502438. Jin, Y.; Wang, H.; Li, J.; Yue, X.; Han, Y.; Shen, P. K.; Cui, Y. Porous MoO2 Nanosheets as Non-Noble Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Mater. 2016, 28, 3785−3790, DOI: 10.1002/adma.201506314. Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional Non-Noble Metal Oxide Nanoparticle Electrocatalysts through Lithium-Induced Con-

(61)

(62)

(63)

(64)

(65)

(66)

(67)

(68)

(69)

(70)

(71)

(72)

(73)

(74)

Page 10 of 12

version for Overall Water Splitting. Nat. Commun. 2015, 6, 7261, DOI: 10.1038/ncomms8261. Liu, W.; Du, K.; Liu, L.; Zhang, J.; Zhu, Z.; Shao, Y.; Li, M. One-Step Electroreductively Deposited Iron-Cobalt Composite Films as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. Nano Energy 2017, 38, 576−584, DOI: 10.1016/j.nanoen.2016.11.047. Xiao, C.; Li, S.; Zhang, X.; MacFarlane, D. R. MnO2/MnCo2O4/Ni Heterostructure with Quadruple Hierarchy: A Bifunctional Electrode Architecture for Overall Urea Oxidation. J. Mater. Chem. A 2017, 5, 7825−7832, DOI: 10.1039/c7ta00980a. Zhu, X.; Dou, X.; Dai, J.; An, X.; Guo, Y.; Zhang, L.; Tao, S.; Zhao, J.; Chu, W.; Zeng, X. C.; Wu, C.; Xie, Y. Metallic Nickel Hydroxide Nanosheets Give Superior Electrocatalytic Oxidation of Urea for Fuel Cells. Angew. Chem. Int. Ed. 2016, 55, 12465−12469, DOI: 10.1002/anie.201606313. Tang, C.; Zhao, Z. L.; Chen, J.; Li, B.; Chen, L.; Li, C. M. Se-Ni(OH)2-Shelled Vertically Oriented NiSe Nanowires as a Superior Electrocatalyst toward Urea Oxidation Reaction of Fuel Cells. Electrochim. Acta 2017, 248, 243−249, DOI: 10.1016/j.electacta.2017.06.159. Chen, S.; Duan, J.; Jaroniec, M.; Qiao, S. ThreeDimensional N-Doped Graphene Hydrogel/NiCo Double Hydroxide Electrocatalysts for Highly Efficient Oxygen Evolution. Angew. Chem. Int. Ed. 2013, 52, 13567−13570, DOI: 10.1002/anie.201306166. Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. SelfSupported Cu3P Nanowire Arrays as an Integrated HighPerformance Three-Dimensional Cathode for Generating Hydrogen from Water. Angew. Chem. Int. Ed. 2014, 53, 9577−9581, DOI: 10.1002/anie.201403842. Zhu, W.; Liu, L.; Yue, Z.; Zhang, W.; Yue, X.; Wang, J.; Yu, S.; Wang, L.; Wang, J. Au Promoted Nickel-Iron Layered Double Hydroxide Nanoarrays: A Modular Catalyst Enabling High-Performance Oxygen Evolution. ACS Appl. Mater. Interfaces 2017, 9, 19807−19814, DOI: 10.1021/acsami.7b03033. Swesi, A. T.; Masud, J.; Nath, M. Nickel Selenide as a High-Efficiency Catalyst for Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 1771−1782, DOI: 10.1039/C5EE02463C. Zhu, W.; Tang, C.; Liu, D.; Wang, J.; Asiri, A. M.; Sun, X. A Self-Standing Nanoporous MoP2 Nanosheet Array: An Advanced pH-Universal Catalytic Electrode for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2016, 4, 7169−7173, DOI: 10.1039/c6ta01328g. Feng, J.; Ye, S.; Xu, H.; Tong, Y.; G. Li. Design and Synthesis of FeOOH/CeO2 Heterolayered Nanotube Electrocatalysts for the Oxygen Evolution Reaction. Adv. Mater. 2016, 28, 4698−4703, DOI: 10.1002/adma.201600054. Yu, L.; Zhou, H.; Sun, J.; Qin, F.; Yu, F.; Bao, J.; Yu, Y.; Chen, S.; Ren, Z. Cu Nanowires Shelled with NiFe Layered Double Hydroxide Nanosheets as Bifunctional Electrocatalysts for Overall Water Splitting. Energy Environ. Sci. 2017, 10, 1820−1827, DOI: 10.1039/c7ee01571b. Luo, J.; Vermaas, D. A.; Bi, D.; Hagfeldt, A.; Smith, W. A.; Grätzel, M. Bipolar Membrane-Assisted Solar Water Splitting in Optimal pH. Adv. Energy Mater. 2016, 6, 1600100, DOI: 10.1002/aenm.201600100. Zhao, X.; Dai, L.; Qin, Q.; Pei, F.; Hu, C.; Zheng, N. SelfSupported 3D PdCu Alloy Nanosheets as a Bifunctional Catalyst for Electrochemical Reforming of Ethanol. Small 2017, 13, 1602970, DOI: 10.1002/smll.201602970. Jia, Y.; Zhang, L.; Gao, G.; Chen, H.; Wang, B.; Zhou, J.; Soo, M. T.; Hong, M.; Yan, X.; Qian, G.; Zou, J.; Du, A.;


Page 11 of 12 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

(75)

(76)

(77)


Ya, X. A Heterostructure Coupling of Exfoliated Ni-Fe Hydroxide Nanosheet and Defective Graphene as a Bifunctional Electrocatalyst for Overall Water Splitting. Adv. Mater. 2017, 29, 1700017, DOI: 10.1002/adma.201700017. Sivanantham, A.; Shanmugam, S. Nickel Selenide Supported on Nickel Foam as an Efficient and Durable Non-Precious Electrocatalyst for the Alkaline Water Electrolysis. Appl. Catal. B-Environ. 2017, 203, 485−493, DOI: 10.1016/j.apcatb.2016.10.050. Zhu, W.; Yue, Z.; Zhang, W.; Hu, N.; Luo, Z.; Ren, M.; Xu, Z.; Wei, Z.; Suo, Y.; Wang, J. Wet-Chemistry Topotactic Synthesis of Bimetallic Iron-Nickel Sulfide Nanoarrays: An Advanced and Versatile Catalyst for Energy Efficient Overall Water and Urea Electrolysis. J. Mater. Chem. A 2018, DOI: org/10.1039/C7TA10584C. Zhu, W.; Zhang, R.; Qu, F.; Asiri, A. M.; Sun, X. Design and Application of Foams for Electrocatalysis. ChemCatChem 2017, 9, 1721−1743, DOI: 10.1002/cctc.201601607.



Page 12 of 12

For Table of Contents Use Only

Traditional NiCo2S4 phase with porous nanosheets array topology on carbon cloth (NiCo2S4 NS/CC) could act as a promising and versatile water- and urea-electrolysis electrode for sustainable hydrogen production and environmental remediation.


Traditional NiCo2S4 Phase with Porous ... - ACS Publications

Recommend Documents