Alumina-Supported CoPS Nanostructures Derived from LDH as Highly

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Alumina-Supported CoPS Nanostructures Derived from LDH as Highly Active Bifunctional Catalysts for Overall Water-Splitting Tingting Wang, Yu Zhang, Yuanqi Wang, Jieni Zhou, Liqian Wu, Yuan Sun, Xiaobing Xu, Wentao Hou, Xuan Zhou, Youwei Du, and Wei Zhong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01425 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Sustainable Chemistry & Engineering

Alumina-Supported CoPS Nanostructures Derived from LDH as Highly Active Bifunctional Catalysts for Overall Water-Splitting Tingting Wang, Yu Zhang, Yuanqi Wang, Jieni Zhou, Liqian Wu, Yuan Sun, Xiaobing Xu, Wentao Hou, Xuan Zhou, Youwei Du, Wei Zhong*

Collaborative Innovation Center of Advanced Microstructures, National Laboratory of

Solid

State

Microstructures

and

Jiangsu

Provincial

Laboratory

for

NanoTechnology, Nanjing University, No. 22 Hankou Rd., Gulou District, Nanjing 210093, P. R. China

*

Corresponding author e-mail: [email protected]

1

ACS Paragon Plus Environment

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Abstract It is highly desirable to develop the efficient and low-cost electrocatalysts to boost the water splitting reaction, which has been regarded as one of the ideal method for commercial hydrogen production. Herein, the ultrathin amorphous Al2O3 nanosheets decorated with CoPS nanoparticles (size: 4-12 nm) were successfully synthesized using CoAl-layered double hydroxide (CoAl-LDH) as precursor. The as-prepared catalysts exhibit low onset overpotential of 67 mV for hydrogen evolution reaction (HER) and 250 mV for oxygen evolution reaction (OER) together with good stability in alkaline medium. Moreover, employed as the bifunctional electrocatalysts with a two-electrode electrolyzer, CoPS nanostructure also produced a small cell voltage of 1.75 V to attain 10 mA cm-2. The excellent electrocatalytic performance could be attributed not only to the high electron-donating and electron-accepting characters of P2- ligands and Co3+ ions, respectively, but also to the large active surface area and porous structure of sample. This study introduces a new method for synthesizing ultrathin LDH nanosheets, and provides a promising candidate towards highly active electrocatalysts for overall water-splitting.

Keywords:

CoAl-LDH,

Cobalt

phosphosulphide,

Electrocatalyst,

evolution reaction, Oxygen evolution reaction, Water splitting

2


Hydrogen

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Introduction Due to the limited reserves of carbon-based fuels (e.g. oil, ethanol and coal) and the environmentally-friendly demand, considerable efforts have been paid to produce hydrogen (H2), which is regarded as an alternate energy server owing to its zero-emission and renewable properties1-3. Among various methods, water electrolysis reaction (H2O → H2 + 1/2 O2), including a cathodic hydrogen evolution reaction (HER) and an anodic oxygen evolution reaction (OER), is considered as a very promising approach for hydrogen production with high purity4-6. However, it needs efficient electrocatalysts to achieve high current density at a low overpotential for the sluggish kinetics of two-electron transfer in HER, as well as the high energy barrier for breaking the O-H and forming the O-O bond in OER7,8. Generally, the most active electrocatalysts are based on precious metals, such as Pt for HER and Ir/Ru-based catalysts for OER, but the scarcity and the high price of precious metals greatly limit their widespread applications9-11. Be aimed at hereat, numerous non-noble metal materials have been synthesized and used as electrocatalysts, such as MoS212, FeP13, CoS214, NiFeB15, NiMoNx16, Ni1-xCoxSe217. Based on these studies, it is also found that the ternary metal compounds, which containing different metal or non-metal elements, exhibit more excellent catalytic performance than binary metal materials owing to the synergistic effect18. For example, Zhou et al.19 reported that amalgamation of Se into NiP2 could modify the electronic structure of catalyst, and the obtained NiP1.93Se0.07 has more appropriate hydrogen absorption free energy (∆GH*) for atomic hydrogen adsorption. Similar promoting effect has also been found by Han et al.20 in OER, they introduced Zn into Co3O4 lattice to modify the surface structure and increase the density of Oh Co3+ (Co3+ in the octahedral sites, which is confirmed to be the catalytically active site for OER21,22), making the electrocatalytic

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ability improved. However, for real commercial applications, the ideal water splitting should be driven by one material, which can simultaneously catalyze both HER and OER in the same electrolyte to simplify the reaction system, cut the cost and improve the efficiency. To achieve this goal, much research has been devoted to exploring cheaper and bifunctional alternatives in recent years23-26. But so far, there are remaining some significant challenges in the development of bifunctional electrocatalysts, such as complicated preparation process, low yield, poor stability and low catalytic efficiency27,28. Cobalt phosphosulphide (CoPS; space group Pa3, a = 5.422 Å), as the ternary alloy, was identified to be a pyrite compound by F. Hulliger in the 1960s29, but its properties have not been studied until Cabán-Acevedo et al. synthesized CoPS nanostructure successfully30. They confirmed the excellent catalytic performance of CoPS nanostructure toward to HER in acid condition, and even proved to be one of the best HER catalysts reported thus far. Considering the appropriate ∆GH* value and the high density of Co3+ in the octahedral sites, CoPS is expected to be an ideal bifunctional electrocatalysts.

Inspired

by

this,

we

design

and synthesize

alumina-supported CoPS nanostructures with high-yielding, and evaluate its catalytic ability as a bifunctional catalyst. Firstly, ultrathin CoAl-LDH nanosheets with crossed structure feature are prepared using n-butyl alcohol as dispersant. As a precursor, the crossed CoAl-LDH nanosheets guarantee the high surface area and structural stability of final product. Furthermore, the preparation method combined of coprecipitation and hydrothermal can greatly reduce the experimental period while increasing the yield. In the next preparation process, CoPS nanoparticles anchored on porous Al2O3 support (CoPS/Al2O3-n) are directly synthesized by in-situ transformation of CoAl-LDH nanosheets. Such method not only eliminates the need for additional

4


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substrate costs, but also prevents the catalyst agglomeration and deactivation. Finally, linear sweep voltammetry (LSV), cycle voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are carried out to evaluate the performance of CoPS-n as bifunctional electrocatalysts toward both HER and OER. Results and discussion

Figure 1. (a) Powder XRD spectra of the as-synthesized CoAl-LDH precursors, (b) EDS spectrum of the CoAl-LDH-3 with an atomic ratio obtained from Table S1. Because the structure and morphology of CoAl-LDH precursor are directly affecting the size and density of the final catalysts, the structure and phase composition of the precursors were investigated by XRD technique firstly. As shown in Figure 1a, diffraction peaks appearing in the precursors can be well ascribed to Co6Al2(CO3)3(OH)16·4H2O (JCPDS No. 51-0045), and the typical (00l) basal reflections of layered structures clearly demonstrated the successful synthesis of layered double hydroxides (LDHs). Comparing to the sharp and narrow characteristics of diffraction peaks of CoAl-LDH-1, the peaks for CoAl-LDH-2 and CoAl-LDH-3 are significantly broadened, especially for CoAl-LDH-3. This result suggests the layer number of CoAl-LDH-3 is the least, and it may be used as an optimal precursor for the preparation of ideal catalyst. Because few-layered CoAl-LDH can fully contact with thiophosphates (PxSy), making the gas-solid

5



reaction more complete and its large specific surface area ensures high dispersion of the catalyst. The EDS spectrum of CoAl-LDH-3 in Figure 1b further verifies the high purity of the precursor, and it can also obtained the atomic ratio of Co:Al is 18.69:6.41, which is very close to the theoretical value of 3:1.

Figure 2. SEM andTEM images of the (a-c) CoAl-LDH-1, (d-f) CoAl-LDH-2 and (g-i) CoAl-LDH-3 precursors. The SEM and the corresponding TEM images with different magnifications of the three precursors are shown in Figure 2. As observed, CoAl-LDH-1 precursor exhibits flower-like structure with the diameter ranging from 1.0 to 1.5 µm, and each micro-flower is stacked by smooth 2D hexagonal nanosheets (average thickness: 48 nm, Figure 2c). This is the most common morphology of CoAl-LDH with no 6


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surfactant optimized due to the rhombohedral structure of LDHs31. In contrast, both CoAl-LDH-2 and CoAl-LDH-3 precursors are composed of a large number of irregular nanosheets. What is different is the nanosheets that make up CoAl-LDH-2 are mostly tiled, while CoAl-LDH-3 are vertical crossed. This crossed structure can effectively prevent the restack occur in the next synthesis process. In order to gain more information of the precursors, the HRTEM images of CoAl-LDH-2 and CoAl-LDH-3 are presented in Figures 2f and 2i, which reveal their average thicknesses are 15.5 nm and 3.2 nm, corresponding to the number of layer: 36 and 7, respectively. According to the XRD peak (Ps) at 20.5º, it can be calculated that the value of facet spacing (ds, Scheme 1b) is 0.43 nm32, which is further confirm the results obtained from the HRTEM. Given the above, it can be considered that nBuOH as the dispersant play an important role in the synthesis of ultrathin CoAl-LDH nanosheets with crossed structure, and CoAl-LDH-3 are chosen as the ideal precursor to synthesis highly dispersed catalysts for further applications.

Figure 3. (a) XRD patterns, and (b) EDS spectrum of the as-synthesized CoPS/Al2O3-3 catalyst using CoAl-LDH-3 as precursor. After phosphidation and sulphidation, the final product was characterized by XRD to ascertain the information of crystal phase and atomic arrangements. As depicted in Figure 3a, all diffraction peaks can be well indexed to cubic CoPS

7



(JCPDS No. 27-0139). No peaks from impurities such as CoAl-LDH and Co3O4 are detected, indicating the layered structure of CoAl-LDH precursor are completely destroyed with the interlayer anion and water molecules are moved, and the high concentration of PxSy successfully prevent the formation of spinel Co3O4. It is also observed that the width of diffraction peaks broaden obviously, implying the high dispersity of CoPS-3 nanoparticles. Moreover, although there is not any peak about aluminum compounds in XRD pattern, the existence of Al element is still detected by EDS (Figure 3b), and the atomic ratio of Co:Al is 9.22:3.72 (Table S2), which is approximately equal to the atomic ratio of Co:Al in CoAl-LDH-3 precursor mentioned above, suggesting the amorphous Al2O3 is generated during the calcination process33.

Figure 4. XPS spectra of

CoPS/Al2O3-3 catalyst, (a) survey spectrum and high

resolution spectra for (b) Al 2p, (c) O 1s, (d) Co 2p, (e) S 2p and (f) P 2p. Subsequently, the elemental compositions and surface valence states of the CoPS/Al2O3-3 catalysts were studied using XPS. Figure 4b-f exhibit the high resolution spectra of Al 2p, O1s, Co 2p, S 2p and P 2p, respectively. For the Al 2p

8


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core level, the single peak located at 74.6 eV can be assigned to Al3+34, and the peak at 531.7 eV in O 1s spectrum indicates the presence of O2- in the composite35. This result further verified the existence of amorphous Al2O3. In the Figure 4d, two main peaks located at 778.69 and 793.71 eV are assigned to the Co3+ in the octahedral site36, and the other peaks located at 781.53, 785.82, 797.08 and 801.70 eV can be ascribed to Co2+ state37, which is originated from the surface cobalt oxide species caused by the unavoidable oxidation during the washing process. The Co 2p comparison between the CoPS/Al2O3-3 and CoAl-LDH-3 shown in Figure S3 further demonstrates the Co2+ existed in CoPS/Al2O3-3 does not originate from the precursor of CoAl-LDH-3. Regarding the S 2p region, the peaks at 161.8 and 163.1 eV are attributed to S 2p3/2 and S 2p1/2, respectively. Moreover, existence of sulfate components further indicates the CoPS/Al2O3-3 catalysts are slightly oxidized on surface. The P 2p spectrum can be divided into three peaks located at 129.1, 130.7, and 133.9 eV, which correspond to the binding energies of P 2p1/2, P 2p3/2 and P-O in phosphides and phosphorus oxide, respectively38,39. According to the XRD and XPS results, atom arrangement in CoPS crystal structure can be confirmed as shown in Scheme 1b. In this cubic structure, Co atoms occupy the vertices and surface-centers, while the dumbbells with a homogeneous distribution of P2- and S- atoms lie in the center of the edge and body. In contrast to the binary analogues, the electronic structures of such ternary compounds could be modulated by perturbed dianions and thus modified their electrocatalytic properties40.

9



Figure 5. (a) SEM, (b) TEM, (c) EDS mapping and (d) HRTEM images of CoPS/Al2O3-3 catalyst. SEM image shown in Figure 5a demonstrates that the CoPS/Al2O3-3 catalysts maintain the sheet-like morphology of the CoAl-LDH-3 precursor after being handled at high temperature of 500ºC. Moreover, it is noteworthy that, compared with the CoPS/Al2O3-3, the phenomena of conglutination and stack are observed in the CoPS/Al2O3-2 product (Figure S1b). Such result fully demonstrates the superiority of the crossed structure of CoAl-LDH-3 precursor. The TEM image under a low magnification indicates the sheet-like morphology of CoPS/Al2O3-3 sample is very uniform. Unlike the smooth surface of CoAl-LDH-3 precursor, some irregular nanoparticles are observed on the surface of ultrathin nanosheets. In order to investigate the element distribution of the nanoparticles and the nanosheets, the EDS mappings of elemental Al, O, Co, P and S are carried out and shown in Figure 5c. As 10


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observed, the distributions of these five elements are distinctly different. Elements of Al and O distribute uniformly across the whole nanosheet support, while elements of Co, P and S show a distinct island distribution on the whole, indicating the nanosheet support is an amorphous Al2O3 and the decorated nanoparticles are composed of Co, P and S. Lattice fringes with a spacing of 0.27 and 0.24 nm we found by HRTEM analysis on the nanoparticles, corresponding to the (200) and (210) plane of cubic CoPS alloy. Moreover, the size of CoPS nanoparticles is measured to be about 4-12 nm (Figure S2). Such 0-2 structure, zero-dimensional (0D) nanoparticles arranged in two-dimensional (2D) support, would be an ideal structure for water splitting, because the small size of nanoparticles guarantees a high active-site density, which is beneficial providing efficient catalytic activity for surface electrochemical reactions, and the ultrathin support can effectively protect the nanoparticles from agglomerating and deactivating in the continuous electrolysis process, contributing to the excellent cyclic stability41.

Scheme 1. (a) Schematic illustration for the fabrication process, (b) crystal structure of CoAl-LDH precursor and CoPS bifunctional catalyst. 11



Scheme 1a illustrates the formation mechanism of ultrathin CoAl-LDH precursor and CoPS catalyst. With the completion of the nucleation (in the co-precipitation reaction) and recrystallization (in the hydrothermal reaction) processes, ultrathin CoAl-LDH nanosheets were successfully prepared. The dispersant of nBuOH is the decisive factor in the synthesis of ideal precursor. On one hand, the selective adsorption of nBuOH on the (001) face of LDH can minimize the surface energy to form and stabilize the thin LDH nanosheets42. On the other hand, interactions among the nBuOH molecules absorbed on the surface can also help form a crossed structure through self-assembly. In the calcination process, the interlayer water and carbonate ions of CoAl-LDH are gradually removed with the increasing temperature. Then, the CoPS nanoparticles are formed and grew under the high concentration of PxSy atmosphere. Through the high-temperature calcination with a fast heating rate of 10°C/min, final products are definitely porous due to the large loss of substances. To verify the existence of pores, BET measurement is carried out at 77 K. As shown in Figure 6, the N2 adsorption/desorption isotherm of CoPS/Al2O3-3 shows a typical type-IV isotherm with a hysteresis loop above P/P0 = 0.9, demonstrating its mesoporous characteristic. The BET surface area and the average pore value are 55.9 m2/g and 0.38 cm3/g, respectively, which are much larger than these of CoPS/Al2O3-1 (15.5 m2/g, 0.07 cm3/g) and CoPS/Al2O3-2 (31.8 m2/g, 0.21 cm3/g). For electrocatalytic water splitting, the higher BET surface area means the lower amount of catalyst loading can lead to high current operations under the same potential43. Moreover, compared to solid materials, porous nanostructures can not only allow electrolyte to infiltration easily into the inside of the catalysts, but also promote the formation and release of bubbles from the catalyst/electrode surface, making the energy conversion more efficient44,45. Therefore, the utilization efficiency of the

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active sites could be greatly improved owing to the porous feature. Based on the above results, we have successfully synthesized the ternary CoPS nanoparticles decorated on amorphous Al2O3 with large specific surface area and porous feature.

Figure 6. N2 adsorption-desorption isotherm of CoPS/Al2O3-3 to show the specific surface area and BJH pore size distribution plot derived from the adsorption branch (inset). The electrocatalytic HER and OER performance of synthesized CoPS/Al2O3-n were investigated in 1 M KOH electrolyte, and the CoPS nanoparticles with no Al2O3 support were also performed for comparison. SEM (Figure S4) and BET (22.6 m2/g, 0.15 cm3/g, Figure S5) results indicate the dispersion of CoPS nanoparticles prepared with Co(OH)2 as a precursor is greatly reduced compared to that supported by ultrathin Al2O3. Figure 7a presents the HER polarization curves obtained by linear sweep voltammetry (LSV) with a scan rate of 5 mV s-1. As displayed, CoPS/Al2O3-3 shows the optimal catalyst performance for HER among the CoPS-based electrodes, indicating the specific surface area and porosity play an important role in its catalytic active. In addition, the HER current density of CoPS/Al2O3-3 increases sharply with the potential increases, and reaches 10, 20 and 100 mA cm-2 at overpotential of -130, -160 and -306 mV, respectively. Such low overpotential possessed by CoPS/Al2O3-3 electrode is better than or comparable to most of the reported transition sulfide and 13



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phosphide HER catalysts in alkaline media46-50. More importantly, although 20% Pt-C shows a lower onset overpotential of -17 mV, when the overpotential increased to -165 mV or even higher, its catalytic ability can be surpassed by that of CoPS/Al2O3-3 electrode due to its poor mass diffusion51-53. From the extrapolation of the linear region of a plot of overpotential against log |J|, we obtained Tafel slope of 53.6 mV dec-1 for CoPS/Al2O3-3, lower than 20% Pt/C (60.4 mV dec-1), suggesting more favorable kinetics of CoPS-3 electrode for HER. It is well known that there are two reaction steps involved in the HER process under alkaline conditions54,55. Volmer reaction: H2O + e- → Hads + OH-

(1)

Tafel reaction: Hads + Hads → H2

(2)

Heyrovsky reaction: H2O + Hads + e- → H2 + OH-

(3)

First, the absorbed hydrogen are formed through the Volmer reaction, and then removed through the Tafel reaction (chemical desorption) or Heyrovsky reaction (electrochemical desorption). Generally, according to the kinetic models for the HER, Tafel slope of about 120, 40 or 30 mV dec-1 would be expected if the Volmer, Heyrovsky, or Tafel step is the rate-determining step, respectively56,57. Therefore, it can be sure that the Volmer-Heyrovsky mechanism takes effect for the HER on CoPS/Al2O3-3 electrode, although the experimental value is somewhat larger than theoretical value because of the influence of bubbles. By extrapolating the linear region of the Tafel plots at the equilibrium potential (i.e., at zero overpotential), exchange current densities (j0,

geometrical)

of 57.8 uA cm-2 was also obtained for

CoPS/Al2O3-3 electrode, indicating the electrons transferring on the interface of CoPS/Al2O3-3 only need a very low activation energy.

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Figure 7. (a) Polarization curves of the 20% Pt/C and CoPS-based electrodes, (b) the corresponding Tafel plots for the data presented in (a), (c) CV curves of the CoPS/Al2O3-3 electrode with different scan rates, (d) the capacitive currents at 0.25 V against the scan rates and (e) Nyquist plots for CoPS-based catalysts, and (f) long-term stability measurements for CoPS/Al2O3-3 and CoPS with no Al2O3 support. Considering the fact that, the determination of electrochemically active surface area (ECSA) is difficult due to the unknown capacitive behavior of the support58. The double-layer capacitance (Cdl), proportional to the EACS is calculated. Figure 7c and the Figure S6 show the CV curves of the CoPS-based electrode at different sweep scan rates (20-200 mV s-1) in the potential range of 0.2-0.3 V vs RHE. As observed, all the curves display pseudo-capacitive characteristics without any obvious Faradaic peaks, indicating all measured current in this potential region can be assumed to double-layer charging59. By plotting the ∆j/2 at a given potential of 0.25 V (vs. RHE) against the scan rates, the values of Cdl are calculated. As shown in Figure 7d, the Cdl value for the CPS/Al2O3-3 electrode is about 3.46 mF cm-2, 23.1 times higher than that of CoPS nanoparticles with no Al2O3 support. The high Cdl values, which might

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be attributed to the porous structure, small size and high dispersion, means the efficient mass diffusion and charge transport capability for CoPS/Al2O3-3. The HER kinetics at the electrode/electrolyte interface of CoPS electrodes are examined by the electrochemical impedance spectroscopy (EIS) over the frequency range from 1,000,000 Hz to 0.01 Hz in 1M KOH solution at selected overpotential of -100 mV. As displayed in Figure 7e and Figure S7, a small semicircle in the high frequency region and a large semicircle in the low frequency can be observed in the Nyquist plots. The model, consisting of the solution resistance (Rs) in series with two parallel CPE-R circuits connected (proposed by Chen and Lasia60), is commonly used to illustrate the electrochemical response of the two semicircles. The semicircle at high frequency is related to the surface porosity of electrode, and the other at low frequency is associated with the charge-transfer process61,62. By fitting the experimental data, charge-transfer resistance (Rct) of 5847, 9552, 3591 and 109 Ω are obtained for CoPS, CoPS/Al2O3-1, CoPS/Al2O3-2 and CoPS/Al2O3-3 electrodes, respectively, which are in accordance with the tendency of LSVs and Tafel, further evidencing that the superior activity of CoPS/Al2O3-3 is attributed to the faster catalytic kinetics. The significant reduction in Rct may attribute to the porous feature and crossed structure of CoPS/Al2O3-3 electrode, which can increase the contact of the active sites with the electrolyte and short ion transport pathways, leading to a significant acceleration of the interfacial electrocatalytic reactions. Equivalent circuit

Sample

Rs (Ω)

R1 (Ω)

Rct (Ω)

CoPS

3.87

6.74

5847

CoPS/Al2O3-1

3.18

8.92

9552

CoPS/Al2O3-2

3.90

6.86

3591

CoPS/Al2O3-3

4.46

5.89

109

Table 1. Equivalent circuit and the kinetic parameters of the HER obtained from AC

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impedance measurements in 1 M KOH. In order to further investigate the intrinsic activity of each electrode, the current densities were normalized by BET surface area and active sites, and expressed in terms of specific activity (SA, Figure S8a) and turnover frequency (TOF, Figure S8b), respectively. At η = -300 mV, the SA and TOF of CoPS/Al2O3-3 are found to be -0.612 mA cm-2 and -0.180 s-1, respectively, outperforming CoPS (-0.039 mA cm-2 , -0.010 s-1), CoPS/Al2O3-1 (-0.040 mA cm-2, -0.009 s-1) and CoPS/Al2O3-2 (-0.107 mA cm-2, -0.022 s-1). This result indicates that the CoPS/Al2O3-3 has the optimal intrinsic activity compared with other three electrodes, and expected to be a promising alternative for highly active nonprecious metal electrocatalysts for HER. Considering the durability of HER electrocatalysts is another key element for their practical application in daily life except for the high activity, the long-term stability of CoPS/Al2O3-3 electrode is measured at a current density of 30 mA cm-2 for 16 h. As shown in Figure 7f, comparing to the large loss of CoPS nanoparticles with no Al2O3 support (74.6%), the decrement of CoPS/Al2O3-3 (12.3%) in current density is insignificant. Furthermore, it can also observed from the TEM images exhibited in Figure S9 that CoPS/Al2O3-3 still retain its initial morphology after HER, and no obvious agglomeration like CoPS nanoparticles with any Al2O3 support is found. Therefore, it can be inferred that the long-term catalytic durability of CoPS/Al2O3-3 electrode in alkaline environment benefits from the support of amorphous Al2O3, which can not only protect the CoPS nanoparticles from deactivation, but also hold the 0-2 structure of catalysts rely on its good stability in alkaline63-65.

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Figure 8. (a) Oxygen evolution reaction LSV curves and (b) Tafel plots of IrO2 and CoPS-based catalysts in 1 M KOH and (c) long-term stability measurements for CoPS/Al2O3-3 and CoPS with no Al2O3 support. The test for OER performance of CoPS-based electrodes was performed in 1 M KOH saturated with O2, and the electrocatalytic activity of IrO2 was also measured for comparison. It can be obtained in Figure 8a that the onset potential for OER on CoPS/Al2O3-3 electrode is 1.48 V, correspond to the onset overpotential of 250 mV (the thermodynamic OER potential (H2O/O2 = 1.23 V) is used as reference), which is slight higher than that of the IrO2 electrode 1.46 V. However, the current density of CoPS/Al2O3-3 electrode is higher than that of IrO2 when the potential reaches to 1.63 V and higher, suggesting the excellent electrocatalytic performance of CoPS/Al2O3-3 for OER. Linear fitting of the Tafel plots is also performed to evaluate the reaction kinetics. For CoPS, CoPS/Al2O3-1, CoPS/Al2O3-2 and IrO2 electrodes, the Tafel slops are 94.5, 78.2, 75.5, 68.4 and 75.2 mV dec-1, respectively. Furthermore, at the overpotential of 350 mV, the SA and TOF for CoPS/Al2O3-3 are 0.060 mA cm-2 and 0.009 s-1 (Figure S10), respectively, much higher than those of CoPS (0.021 mA cm-2, 0.003 s-1), CoPS/Al2O3-1 (0.009 mA cm-2, 0.001 s-1) and CoPS/Al2O3-2 (0.031 mA cm-2, 0.003 s-1). The best OER performance of CoPS/Al2O3-3 may due to its porous and cross structure, which is beneficial for mass and electron transfer. In addition, when the CoPS/Al2O3-3 is used as the catalyst, the current density loss is only 17.0%

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after 16 h electrochemical testing, and the initial morphology is still maintained (Figure S11), implying its good stability for OER in alkaline media. Finally, XPS spectra of CoPS/Al2O3-3 before and after the electrochemical measurements are examined to understand the HER and OER mechanism. The Co 2p spectra shown in Figure S12a reveal the proportion of Co3+ is decreased during the OER process, indicating the surface of the catalyst is partially converted to form an oxide layer. The S 2p and P 2p spectra further confirmed the oxidation of CoPS/Al2O3-3, especially for P 2p, the peak located at 129.2 eV almost disappeared. According to the previous reports10,66, the interface introduced by the surface oxide layer, is beneficial to enhance the OH- adsorption, which can further promote the OER catalytic activity. For HER, the surface composition of the CoPS/Al2O3-3 does not change obviously after the long-term tests. It is noteworthy that, due to the good stability of Al2O3 support in alkaline, no significant change in position and strength of the peaks located in Al 2p and O 1s regions are found after HER or OER.

Figure 9. LSV curves and Tafel plots of the investigated electrode catalysts for (a, b) HER and (c, d) OER in 1 M KOH, respectively. The above results fully demonstrate the structural advantages of the catalyst prepared with CoAl-LDH-3 as the precursor. Then, we prepared CoP/Al2O3-3 and CoS2/Al2O3-3 as control samples to investigate the advantages of ternary metal compounds as catalysts. It can be clearly seen from the Figure S13 that the CoP/Al2O3-3 and CoS2/Al2O3 have the same characteristics in morphology as that of

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CoPS/Al2O3-3, and no impurity peaks are observed in their XRD patterns. This result not only proves the repeatability of the synthesis method we reported, but also ensures the accuracy of subsequent tests. Figure 9 shows the LSV curves and corresponding Tafel plots of CoP/Al2O3-3, CoS2/Al2O3-3 and CoPS/Al2O3-3 for HER and OER. As expected, CoPS/Al2O3-3 displays more excellent catalytic performance. Moreover, the XPS results shown in Figure S14 further indicate the superior catalytic performance of CoPS/Al2O3-3 can be attributed exclusively to its ternary structure under the same morphology. The reasons can be summarized as the following: (1) the high concentration of electron-donating P2- can promote the formation of Hads in HER process and make the reaction more efficient; (2) once spontaneous hydrogen adsorption at open P sites, the ∆GH* at the adjacent Co sites will become comparable to that of Pt30; (3) the more valence 3d electron orbits and higher electron-accepting characteristics of Co3+ are facilitate the four elemental steps in OER.

Figure 10. LSV curves using Pt/C//IrO2 and CoPS/Al2O3-3//CoPS/Al2O3-3 as two electrodes in 1 M KOH with the scan rate of 2 mV s-1. Encouraged by the above results, a two-electrode system was assembled and the CoPS/Al2O3-3 catalysts were served as both cathode and anode for overall water splitting. It can be observed from the Figure 10 that a small cell voltage of 1.75 V is 20


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needed to afford a current density of 10 mA cm-2 for CoPS/Al2O3-3// CoPS/Al2O3-3, which is slightly higher than that of Pt/C//IrO2. This result demonstrates that the CoPS/Al2O3-3 samples can be used as the bifunctional catalyst to boost the water splitting reaction. Conclusions In summary, porous amorphous Al2O3 nanosheets anchoring CoPS nanoparticles with the particle size ranging from 4 to 12 nm are synthesized via a common wet-chemical method followed by phosphidation/sulphidation process. The results from the XRD, XPS and EDS mapping indicate that the CoPS nanoparticles decorated on Al2O3 are indeed a distinctive ternary alloy phase. In-depth studies show that the as-prepared sample exhibits highly catalytic activity in catalyzing both HER (onset overpotential, -67 mV; Tafel slope, 53.6 mV dec-1) and OER (onset overpotential, 250 mV; Tafel slope, 68.4 mV dec-1), and the main reason contains two points in the following. 1) The high electron-donating character of P2- ligands makes CoPS have a comparatively ideal energy for atomic hydrogen absorb on their surface, resulting in an excellent HER performance; and 2) the high number of accessible redox active Oh Co3+ sites effectively promotes the OER activity. Beyond these, the substrate of porous Al2O3 also plays an important role in the process of electrochemical water splitting. On one hand, protects CoPS nanoparticles from agglomeration effectively, making more active sites exposed. On the other hand, guarantees the long-term durability of catalysts owing to its good stability in alkaline. Thus, the catalysts can show good performance as anode and cathode in overall-water splitting cell. It is also believed that the synthetic method using LDH as precursor could be expanded to design other multi-element metallic materials, and applied in various energy and environmental science fields.

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Supporting Information Experimental section; Figures of SEM, TEM and HRTEM images; N2 adsorption-desorption isotherms; electrochemical data; tables of EDS data. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11474151 and No. 11774156), the National Key Project for Basic Research (Grant No. 2012CB932304), and PAPD, People’s Republic of China. References 1.

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Graphical abstract

Synopsis The excellent activity and stability of alumina-supported CoPS nanostructures provide an economical catalyst to obtain clean hydrogen energy through large-scale electrochemical water splitting.

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Alumina-Supported CoPS Nanostructures Derived from LDH as Highly

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