Atomic Heterointerface-Induced Local Charge Distribution and

Subscriber access provided by MIDWESTERN UNIVERSITY

Energy, Environmental, and Catalysis Applications

Atomic Heterointerface-induced Local Charge Distribution and Enhanced Water Adsorption Behavior in Cobalt Phosphide Electrocatalyst for Self-powered Highly Efficient Overall Water Splitting Tao Meng, Jinwen Qin, Dan Xu, and Minhua Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19341 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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.

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 32 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 Applied Materials & Interfaces

Atomic Heterointerface-induced Local Charge Distribution and Enhanced Water Adsorption Behavior in Cobalt Phosphide Electrocatalyst for Self-powered Highly Efficient Overall Water Splitting

Tao Meng, Jinwen Qin, Dan Xu, and Minhua Cao*

Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China. *E-mail: [email protected]

1

ACS Paragon Plus Environment

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

ABSTRACT: Developing economical and highly efficient noble-metal-free electrocatalysts for overall water splitting is an essential precondition for renewable energy conversion. Herein, we highlight atomic heterointerface engineering in constructing highly efficient CoP/Co9S8 electrocatalyst for full water splitting. CoP/Co9S8 hybrid for the first time was prepared by partial homogeneous transformation of in-situ formed Co9S8, in which the atomic heterointerface was formed between CoP and Co9S8. Systematic experiments and theoretical calculations confirm that the as-formed atomic heterointerface can induce local charge distribution in CoP/Co9S8, which can not only accelerate the charge transfer, but also optimize the hydrogen adsorption energy of CoP in favor of the fast transformation of Hads into H2. Meanwhile, the Co9S8 component can also increase the water adsorption capability of CoP/Co9S8. Benefiting from these outstanding advantages, an alkaline electrolyzer based on CoP/Co9S8 as both electrodes achieves a low cell voltage of 1.6 V at an operating current density of 10 mA cm-2, and at the same time it can also be self-powered by a home-assembled Zn-air battery employing the same CoP/Co9S8 as the air electrode for prospectively achieving renewable energy conversion. This work demonstrates the importance of heterostructure engineering in developing noble metal-free catalysts for high-performance water electrolysis.

KEYWORDS: heterointerface; local charge distribution; enhanced water adsorption; cobalt phosphide; overall water splitting

2


Page 2 of 32

Page 3 of 32 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


1. INTRODUCTION Cost-effective electrochemical water splitting powered by renewable energy has been deemed as an effective way to yield hydrogen gas (H2), which is a promising alternative energy carrier of fossil fuels to resolve serious energy and environmental crises.1,2 Generally, water electrolysis simultaneously contains cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), in which HER in basic medium is much more meaningful because of the shortage of the high efficiently acidic OER catalysts.3,4 The HER process in alkaline electrolyte generally undergoes a two-electron-reaction pathway, which follows typical Volmer-Heyrovsky or Volmer-Tafel mechanisms:5,6 H2O + e- → Hads + OH- (Volmer) and H2O + Hads + e- → H2↑ + OH- (Heyrovsky)

(1)

H2O + e- → Hads + OH- (Volmer) and Hads + Hads → H2 ↑ + OH- (Tafel)

(2)

wherein these two pathways both contain the adsorption of water (H2O) molecules followed by the formation of H intermediates (Hads) to yield H2. Clearly, for the whole HER process in basic media, the surface state of the catalyst evolves as following: the initial catalyst-water state, the intermediate catalyst-H-atom state, and the final catalyst-H2 state.4 Therefore, an ideal alkaline HER catalyst, on the one hand, should be featured with strong functions to easily capture the H2O molecules and simultaneously transform them into Hads. Here, it should be noted that the Hads cannot be tightly combined with the catalyst and accordingly subsequent generated H2 can be readily released. On the other hand, excellent conductivity and suitable configuration are another two crucial factors for an excellent HER catalyst since they could boost 3



charge transfer and the exposure of the active sites. Thus, taking into account of all these factors in designing noble metal-free catalysts is highly necessary for realizing highly efficient and cost-effective alkaline HER. Only in this way electrochemical water splitting would become economically viable. Cobalt phosphide (CoP) with high chemical stability and low price was considered as a new candidate electrocatalyst for HER owe to its excellent behaviors in the similar hydrodesulfurization (HDS).7,8 The crystal structure of CoP is composed of edge-sharing PCo6 trigonal prisms and face-sharing CoP6 octahedra (Figures 1a-c), from which it can be seen that CoP features a charge unbalance between Co (δ +) and P (δ−) (Figure 1c). The charge unbalance particularly facilitates the adsorption of H from H2O dissociation on local negatively charged P sites due to the strong electrostatic attraction, while the adjacent Co centers are favour for adsorption of OH− species, both of which are responsible for the intrinsic activity of CoP for HER.6,9 However, the energy barrier for CoP to adsorb the H2O molecules is so large that it is not beneficial for rapidly realizing the initial catalyst-water state of the HER process, hence inhibiting the following reactions and accordingly leading to the CoP catalyst with a fade HER catalytic activity.3 Unfortunately, most of reported woks on CoP-based catalysts mainly focused on optimizing the conductivity as well as increasing the active sites to enhance their HER behavior,10-15 whereas improving the adsorption ability of the catalyst towards H2O has been neglected. Pioneering works have demonstrated that some drawbacks existing in single-component electrochemical catalysts can be solved by common hybridization strategy since the different components in the rationally constructed hybrid materials could play respective roles in the overall multistep HER process. On the other hand, the as-formed heterostructures via the hybridization not only can result in an lopsided charge

4


Page 4 of 32

Page 5 of 32 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


distribution and therefore induce a local interfacial electric field around the interface region due to different band gaps of the involved components, but also can result in structural distortion (i.e., disordered atoms and dangling bonds) because of imperfect lattice match between neighboring layers of the catalyst to further endow it with excellent conductivity and large effective active area (sites).16-18 Notably, theoretical and experimental studies have reported that metal sulfides (i.e., Co9S8 and Ni3S2) possess intrinsic low H2O adsorption energy and excellent conductivity.2,16 Based on this fact, Co9S8 could act as a good coupling candidate for CoP to decrease its energy barrier towards H2O adsorption for realizing its high-performance HER. Motivated by the above considerations, herein, we first preferably choose Co9S8 as the candidate for optimizing CoP to achieve its high-performance overall water splitting. Various advanced characterizations, density functional theory (DFT), as well as systematic experiments display that the as-formed CoP/Co9S8 hybrid can fully utilize the advantages of Co9S8, CoP, and their strongly coupled heterointerface for achieving highly efficient overall water splitting, which was employed a model to deeply disclose the relationship between the atomic interface engineering and its intrinsic water splitting performance. CoP/Co9S8 possesses following several features: (1) the atomic heterointerface-induced local charge distribution in CoP/Co9S8 can optimize the hydrogen adsorption energy of CoP to further favor its fast transformation of Hads into H2; (2) the Co9S8 component can increase the water adsorption capability of CoP/Co9S8; (3) the thus-formed heterointerface in CoP/Co9S8 can significantly increase its conductivity and active surface area. Correspondingly, the as-constructed alkaline electrolyzer via the CoP/Co9S8 can afford a very low cell voltage of 1.6 V to achieve a current density of 10 mA cm-2, and at the same time it

5



can be self-powered by a home-built Zn-air battery, which air electrode was also assembled by employing the CoP/Co9S8. 2. METHODS Material preparation: Typically, Co(Ac)2·4H2O (0.25 g), thiourea (0.2 g), and F127 (0.8 g) were first added into water medium (20 mL), then keeping continuously stirring for 0.5 h. After that, the as-obtained homogenous solution was freeze drying to harvest the precursor. To prepare the target product (CoP/Co9S8), 0.25 g of the as-obtained precursor and 0.04 g of red phosphorus powder were separately placed in a porcelain boat, and were further treated in a furnace at 600 ºC for 3 h (5 ºC min-1) with H2/Ar atmosphere (H2, 7 vol.%). After being cooled to ambient temperature, the typical CoP/Co9S8 sample was obtained. For comparison, the intermediate sample (Co9S8/CoP-400) was also prepared at 400 °C with other parameters keeping same with those of the typical CoP/Co9S8 sample. CoP/Co9S8-1 and CoP/Co9S8-4 were also prepared by adjusting the thiourea mass to 0.1 and 0.4 g, respectively, while maintaining other parameters same as those of the typical CoP/Co9S8 sample, which is also named as CoP/Co9S8-2. We also prepared a sample (named as CoP/Co9S8-Ar) under Ar atmosphere with other parameters keeping same with those of the typical CoP/Co9S8 sample. The samples prepared above all contain carbon and for comparison, a carbon-free sample (named as CoP/Co9S8-free) was also prepared in the absence of F127 with other parameters keeping same with those of the typical CoP/Co9S8 sample. The bare CoP was prepared under the same parameters as those of the typical CoP/Co9S8 sample only with the equimolar urea to replace the thiourea, while for Co9S8, the above precursor was treated similiarly as that of CoP/Co9S8 only without using the red phosphorus.

6


Page 6 of 32

Page 7 of 32 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


Materials characterizations: The phase was detected by X-ray diffraction (XRD) (Bruker D8, Cu-Kα radiation). (High-resolution) transmission electron microscopy [(HR-)TEM, JEOL JEM-2010] was performed to study the microstructures of the as-obtained products. N2 adsorption-desorption isotherms were used to calculate the specific surface areas (BET method) and the pore-size distribution (BJH model). Raman spectra were performed under a 633 nm wavelength. ESCALAB 250 spectrometer was used to collect the X-ray spectroscopy (XPS) information. The X-ray absorption near edge structure (XANEs) was studied by Synchrotron Radiation Facility in Beijing (Beamlines, 1W1B). Electrochemical measurements: The detail electrochemical tests were similar with those in our previous work15, and all of the test details were also presented in the supporting information. DFT calculations: The theoretical calculations are operated via Viennaab initio simulation package (VASP). The generalized gradient approximation (GGA)-PBE method is used to describe exchange correlation functional and projected augmented wave (PAW) potential is used to describe the interaction between ion-electron. A plane-wave cutoff of 400 eV is used, and a Monkhorst-Pack grid 2×2×1 k-point is used to sample the Brillouin zone. The CoP of Pnma space group and the Co9S8 of Fm-3m space group are selected as the subject. Then their (100) surface is constructed as a slab with 15 Å vacuum in the z-direction. The convergence criteria 10-5 eV is used for the difference between two ion steps. 3. RESULTS AND DISCUSSION CoP/Co9S8 hybrid was synthesized by the solid-phase phosphorization of a precursor (Figure S1a) with atomic-level mixing of cobalt acetate and thiourea induced by freeze-drying technique. During the phosphorization process to 7



(a)

(b)

(d)

(c)

CoP * ■ Co S 9

Co1 δ

+

P1 δ

-

Intensity (a.u.)

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

Page 8 of 32

* **

■ ■

* *

30

8

* * * ■** * ■ ■

*

40 50 2 (degree)

60

70

Figure 1. (a-c) Crystal structure of CoP (blue sphere: Co; pink sphere: P). (d) XRD results of CoP/Co9S8 obtained at 600 °C (red curve) and intermediate Co9S8/CoP prepared at 400 °C (black curve).

form the CoP phase, the reaction between cobalt acetate and thiourea also occurs, resulting in the formation of Co9S8. The X-ray diffraction (XRD) results in Figure 1d (red curve) confirm the coexistence of the CoP phase (JCPDS card no. 29-0497) and the Co9S8 phase (JCPDS card no. 19-0364) in this typical sample obtained at 600 °C. Furthermore, the diffraction peaks of the CoP phase are obviously stronger than those of the Co9S8 phase, suggesting its higher crystallinity. To get a preliminary understanding for the degree of crystallization level of these two phases, a lower phosphorization temperature (400 °C) was also studied and the obtained sample was named as Co9S8/CoP-400. As clearly disclosed by Figure 1d (black curve), the main diffraction peaks for Co9S8/CoP-400 belong to the Co9S8 phase, while the diffraction peaks for CoP are fairly weak. From these two XRD results, we can deduce that the Co9S8 phase is much easier to crystallize than the CoP phase and with the increase of the phosphorization temperature, it is partly transformed into the CoP phase. Moreover, we also investigated the effect of the phosphorization atmosphere on final product. When the H2/Ar atmosphere

8


Page 9 of 32 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


was replaced by pure Ar, the thus-obtained sample is still CoP/Co9S8 hybrid in component (CoP/Co9S8-Ar) but with poorer crystallinity and electric conductivity compared to the sample obtained in H2/Ar atmosphere (Figure S1b,c), indicating that the H2/Ar atmosphere is beneficial for the formation of high-quality CoP/Co9S8 hybrid. The skeleton characteristics of CoP/Co9S8 hybrid were displayed by transmission electron microscopy (TEM) measurements. Clearly, 10-20 nm-sized CoP/Co9S8 nanoparticles (NPs) are uniformly embedded into nanoporous carbon matrix (Figure 2a and Figure S2). Besides, the high-magnification TEM image reveals that each nanoparticle (NP) was surrounded by a carbon layer with a thickness of about 3-5 nm (Figure 2b), which origins mainly from in-situ carbonization of the organic species of F127. The carbon matrix in this hybrid serves as multiple functions, such as prohibiting further growth of the CoP/Co 9S8 NPs (Figure S3), improving the electronic conductivity, and protecting the CoP/Co 9S8 NPs from the erosion of the harsh electrolyte, hence further boosting the electrocatalysis performances (will be discussed below). The high-resolution TEM (HRTEM) images recorded on different CoP/Co9S8 NPs (Figures 2c,d) show two different domains with clearly identified lattice fringe spaces of 0.57 and 0.19 nm, which belong to (111) plane of Co9S8 and (211) plane of CoP, respectively.9,19 And the heterogeneous nanointerface between Co 9S8 and CoP is clearly observed from these two parts. Besides, the elemental mappings and the energy dispersive spectrometer (EDS) spectrum also reveal that these elements of Co, P, S, and C elements exist in CoP/Co9S8 hybrid (Figure S4), and that the S element distributes in the selected area same as the Co and P species

9



(a)

(b)

20 nm

5 nm

(c)

(d)

CoP (211) 0.19 nm

Co9S8 (111) 0.57 nm CoP (211) 0.19 nm

Co9S8 (111) 0.57 nm 2 nm

P

S

C

2 nm

(f)

120

80

dV/dD

Co

3 -1

(e)

0.34 nm

Quantity Absorbed (cm g )

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

Page 10 of 32

0

10 20 30 Pore size (nm)

40

0.0

40

Absorbtion Desorbtion 0.2 0.4 0.6 0.8 Relative Pressure (P/Po)

1.0

Figure 2. (a,b) TEM and (c,d) HRTEM images of CoP/Co 9S8 recorded on different parts. (e) STEM-EDS elemental mapping images of CoP/Co 9S8. (f) N2 adsorption-desorption isotherms of CoP/Co9S8 and corresponding pore size distribution curve (the inset).

(Figure 2e), again revealing the formation of the atomic heterointerface in CoP/Co9S8. Moreover, CoP/Co9S8 shows type IV N2 sorption isotherms (Figure 2f), indicating the CoP/Co9S8 holds the mesoporous characteristic. And, the pore size distribution curve reveals that the pore sizes of CoP/Co9S8 are mainly in the range of 2-30 nm (the inset in Figure 2f). Accordingly, CoP/Co9S8 holds a large specific Brunauer-Emmett-Teller (BET) surface area of 87.6 m2 g-1 as well as a pore volume of 0.154 cm3 g-1, suggesting its superior textual properties for improving its electrocatalytic performances. The synchrotron radiation technique was first employed to investigate the electronic structure and intrinsic atomic arrangement of the CoP/Co 9S8 10


Page 11 of 32

heterostructure. The Co K-edge absorption spectrum (XANES) spectra (Figure 3a) for CoP/Co9S8 is similar to that of the bare CoP, suggesting that CoP is the major component in the CoP/Co9S8 heterostructure, which matches well with the above XRD result. Moreover, two main excitations (labeled as A and B) were observed in CoP/Co9S8 XANES spectrum, and this phenomenon is consistent well with the reference previously reported.20,21 The peak A located at 7719 eV stands for the excitation of Co1s electron into ligand (i.e., P or S) 3p antibonding states and that this signal intensity (peak A) of CoP/Co9S8 is weaker than that of the bare CoP, indicating an increased Co valence states in CoP/Co9S8 heterostructure due to the electron shift from S to Co. The above result reveals the electron interaction and/or redistribution in the intercoupled atomic heterointerface via the -P-Co-S-Co- model. The peak B (7712 eV) is related to quadrupolar transition of Co 1s electron into unoccupied Co 3d states, 18

Co K-edge 12

CoP/Co9S8

3d

CoP Co9S8

B

6

CoP/Co9S8

(c)

CoP Co9S8

CoP Co9S8

FT(a.u.)

1s

CoP/Co9S8

(b)

0

3

-P-Co-S-CoA

k x(k) (a.u.)

Intensity (a.u.)

(a)

-6

-12

Co foil -18

7720 7740 Energy (eV)

(d)

2

7760

CoP 778.5 sat.

793.6 sat. CoP/Co9S8

775 780 785 790 795 800 805 810 Binding energy (eV)

Intensity (a.u.)

Co 2p1/2 793.8 sat.

6 8 -1 k (Å )

(e)

Co 2p

Co 2p3/2 778.8 sat.

4

P 2p3/2 129.7

10

12

0

CoP 128.8 129.6 132.3

CoP/Co9S8

1

2

3 -1 4 R (Å )

5

128 130 132 134 136 138 140 Binding energy (eV)

6

S 2p

(f)

P 2p P 2p1/2 130.5 133.8

S 2p3/2 161.8 S 2p1/2 163.5

Intensity (a.u.)

7700

Intensity (a.u.)

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


Co9S8 163.1

164.5 CoP/Co9S8

160

162 164 166 168 Binding energy (eV)

170

Figure 3. (a) Co K-edge XANES spectra of CoP/Co9S8, CoP, Co9S8 and Co foil. (b) Co k3χ(k) oscillation curves and (c) corresponding Fourier transformed k3-weighted EXAFS spectra for CoP/Co9S8, CoP and Co9S8. High resolution XPS spectra of Co 2p (d) and P 2p (e) for CoP/Co9S8 and CoP, and S 2p (f) for CoP/Co9S8 and Co9S8.

11



Co K-edges k3χ(k) for CoP/Co9S8 is different from those of CoP and Co9S8 commonly holding a weaker affect than peak A, 21 and therefore we do not take it into consideration. Besides, the Fourier transform (FT) oscillation curve of counterparts (Figure 3b), further suggesting different local atomic arrangement of Co atoms around the coupled heterointerface in CoP/Co 9S8. It is generally believed that the Co atoms around the heterointerface for CoP/Co 9S8 have a particularly complex coordination state due to the imperfect lattice match between neighboring layers and/or the establishment of the -P-Co-S-Co- model, which is fully supported by the following R-space results. All of the samples in the R-space (Figure 3c) have a main peak at around 1.78 Å, which corresponds to Co-P or Co-S bond since Co-P and Co-S have a similar bond length.9,22 However, the peak intensity for CoP/Co9S8 is the strongest, revealing the complicated Co atomic arrangement around the heterointerface. Additionally, the peaks over a range from 3 to 4 Å of CoP/Co9S8 have the features of both the bare CoP and Co9S8, and that the lower intensity of the CoP feature in CoP/Co9S8 suggests the establishment of the coupled atomic heterointerface, thus leading to the structural distortion (i.e., disordered atoms and dangling bonds) to increase the relevant active sites.23 Moreover, X-ray photoelectron spectroscopy (XPS) measurements were also carried out to further reveal the as-formed interaction between CoP and Co 9S8. The survey XPS spectrum demonstrates the CoP/Co 9S8 possesses the Co, P, and S elements besides C and O (Figure S5). Figures 3d,e show Co 2p and P 2p spectra for CoP/Co9S8 and bare CoP. For the bare CoP (Figure S6), the Co 2p3/2 and Co 2p1/2 peaks are located at the binding energies (BEs) of 778.8 and 793.8 eV (Figure 3d), respectively, while the peaks at 129.7, 130.5, and 133.8

12


Page 12 of 32

Page 13 of 32 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


eV can be attributed to P 2p3/2, P 2p1/2 in CoP, and oxidized P species on the surface of CoP, respectively (Figure 3e). Compared with BE of 778.1 eV for metallic Co and 130 eV for elemental phosphorus,9 the BEs of Co 2p3/2 and P 2p3/2 have a positive shift and a negative shift, respectively. This result further confirms that Co and P in the bare CoP have partial positive charge (δ +) and negative charge (δ−), respectively. However, when the Co9S8 component was introduced into CoP to form CoP/Co9S8, the BEs of 2p signals in both Co and P for CoP/Co9S8 both occur negative shift by -0.2~-0.3 eV and -0.9 eV in comparison with those of the CoP, respectively. On the contrary, the BEs of S 2p for CoP/Co9S8 have obvious positive shift in comparison with those of the bare Co9S8 (163.1 and 164.5 eV for CoP/Co9S8; 161.8 and 163.5 eV for Co9S8) (Figure 3f and Figure S7). These changes in the binding energies for CoP/Co9S8 indicate that partial electrons of Co9S8 are transferred to CoP, suggesting the formation of the atomic heterointerface between CoP and Co9S8, agreeing well with the above X-ray absorption spectroscopy (XAS) results. Moreover, the electron transfer between Co 9S8 and CoP would inevitably result in a lopsided charge distribution and accordingly induce a local interfacial electric field around the heterointerface to accelerate ion/electron migration transfer, thus enhancing the electrocatalytic reactions. To verify the influence of the heterointerface effects in CoP/Co9S8 on its electrochemical performances, corresponding electrochemical tests were carried out. The HER catalytic activities of all the as-prepared samples were firstly evaluated in 1.0 M KOH. Clearly, the typical CoP/Co9S8 sample exhibits optimal HER performances (Figures 4 and Figure S8). Specifically, it holds a

13



0.4

-2

J (mA cm )

-10 -20 CoP/Co9S8

-30

CoP Co9S8

-40

CoP C9S8

0.3

-1

V 88 m

0.1

CoP/Co9S8-2

0

-20

-2

CoP CoP/Co9S8-1

(d) J (mA cm )

-2

-2

0

-10

-30

-40

Co9S8 -0.6

-10

-40 -0.3

-60

-0.4 -0.2 Potential (V vs. RHE)

st

-20

1 cycle th 1000 cycle th 3000 cycle

-30

CoP/Co9S8-4

-40

-1

c V de -1 67 m dec 56 mV

-0.3 0.0 0.3 0.6 0.9 1.2 1.5 -2 Log [J (mA cm )]

0.0

(c)

-20

dec

0.0 -0.4 -0.3 -0.2 -0.1 Potential (V vs. RHE)

J (mA cm )

0

-1

dec mV 110

Pt/C

0.2

Pt/C -50 -0.5

(b)

CoP/C9S8

(a) Overpotential (V)

0

J (mA cm )

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

Page 14 of 32

0

2

-0.2 -0.1 0.0 Potential (V vs. RHE)

4 6 Time (h)

8

10

Figure 4. (a) Polarization curves and (b) Tafel slopes of CoP/Co9S8, CoP, Co9S8, and Pt/C in alkaline solution for HER. (c) Polarization curves of series of CoP/Co9S8. (d) Chronoamperometric response of CoP/Co9S8 tested at -0.155 V vs. RHE, and polarization curves before and after 3000 cycles (the inset).

small onset potential (vs. RHE) of -80 mV at 1 mA cm-2 (Figure 4a), which is far smaller than those of bare CoP (-105 mV) as well as Co9S8 (-135 mV), clearly revealing that the hybridization based on CoP and Co9S8 can significantly improve the HER catalytic activity. Although the commercial Pt/C displays much lower onset potential (-44 mV), its current density (J) drops below that of CoP/Co9S8 after the potential of -194 mV, at which position CoP/Co9S8 holds a better activity. Moreover, the overpotential (at 10 mA cm-2) for CoP/Co9S8 is only 155 mV, far outperforming those of CoP (208 mV), Co9S8 (269 mV), carbon matrix (506 mV), CoP/Co9S8-free (240 mV) (Figure S9) and many other HER catalysts in the same test conditions, for example, CoP@BCN (215 mV),24 CoP/CC (205 mV),25 Co9S8@NOSC (235 mV),26 and Co-NRCNTs (370 mV).27 A detailed comparison of series reported HER electrocatalysts in 1.0 M KOH is listed in Table S1, which can further reveal the excellent activity of CoP/Co9S8.

14


Page 15 of 32 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


The HER reaction kinetics is further investigated by Tafel plots. As shown in Figure 4b, the fitted value for CoP/Co9S8 is 67 mV dec-1, which is lower than those of CoP (88 mV dec-1), Co9S8 (110 mV dec-1), and even Pt/C (56 mV dec-1), indicating that CoP/Co9S8 possesses a rapid reaction kinetics. The Tafel value of CoP/Co9S8 also indicates that its HER process undergoes a Volmer-Heyrovesky pathway,28 which agrees well with the above result that the CoP/Co9S8 holds a larger catalytic J at the higher bases than Pt/C (Figure 4a). Moreover, the exchange current density (j0) for CoP/Co9S8 is 0.05 mA cm-2 (Figure S10), which outperforms those of most previously reported HER electrocatalysts (Table S1). The small Tafel slope and large j0 for CoP/Co9S8 manifest that CoP/Co9S8 has a favorable HER reaction kinetics corresponding to its high catalytic activity. Moreover, the HER LSVs for series CoP/Co9S8 prepared by adjusting the amount of thiourea aiming at regulating of the two phases were also studied, and all of the CoP/Co9S8 have a better HER activity than these of bare CoP and Co9S8 (Figure 4c). These results clearly reveal that the establishment of the atomic heterointerface can significantly boost the kinetics behaviors and increase the effective HER activity of the CoP/Co9S8 catalyst, which are responsible for its outstanding HER performance. As we all know, it is of exceptional importance with the aim of developing the high-performance electrocatalysts suitable for a broad PH window of working electrolytes and therefore the HER behavior of CoP/Co9S8 in acid medium was also investigated. As testing in 0.5 M H2SO4 (Figure S11a), the overpotential at 10 mA cm-2 for CoP/Co9S8 is 123 mV, and this value is superior to those of the bare CoP (185 mV), and Co9S8 (214 mV). Also, this activity of CoP/Co9S8 in acid medium is considered to be fairly good among most reported HER

15



electrocatalysts (Table S1), especially viewing their polyfunctionality in both acidic and alkaline mediums. Besides, CoP/Co9S8 also holds a small Tafel slope of 65 mV dec-1, a large j0 of 0.125 mA cm-2, and a robust stability (Figures S11b-d), again revealing its excellent HER performance in acid medium. Moreover, long-term durability of CoP/Co9S8 was also performed to further evaluate its HER electrocatalytic performance. The chronoamperometric response for CoP/Co9S8 at -0.155 V displays a J of 10 mA cm-2 within 10 h (Figure 4d), demonstrating its remarkable HER stability in the alkaline medium. Moreover, the excellent stability of CoP/Co 9S8 was further assessed by 3000 continuous cycles and only slight current attenuation is observed after this long process (the inset in Figure 4d), again confirming its highly stable performance. Moreover, to get in-depth understanding of the catalysis behavior of CoP/Co9S8 during the HER process, the ex-situ XPS and HR-TEM analyses were employed after the chronoamperometric test. The high-resolution XPS spectra of Co 2p, P 2p, and S 2p as well as HR-TEM result for CoP/Co9S8 after the HER catalytic process nearly keep well with those of the fresh catalyst, indicating no obvious changes in component and structure (Figures S12). This fantastic result further confirms that CoP/Co9S8 as a HER catalyst possesses the robust character, which is responsible for its high activity and excellent stability. On the basis of the HER process for CoP/Co 9S8 in alkaline electrolyte described above, it is clear that CoP/Co9S8 possesses superior activity and stability during the HER process. Therefore, it is highly necessary to elucidate the inherent reason of the high HER performance of CoP/Co 9S8 so that the construction strategy of the catalyst used in current study can be interpreted and

16


Page 16 of 32

(a)

Absorption energy (eV)

-0.12

Co9S8

CoP

0.9

(b)

-0.11

(c) M-H2O

M + H2 O

-0.09 -0.08

-1

50 mV s

Pt -0.3 Co9S8

H2 O

H*

1/2 H2

-2

m Fc 3m

3. =2

C dl

0.5

-120

CoP/Co9S8 CoP

-60

-2

m mF c =12.3

C dl

0.0 0.92

(f)

CoP

-Z'' (ohm)

-2

-2

CoP/Co9S8

(e)

1.5

1.0

-1

10 mV s

0.88 0.89 0.90 0.91 Potential (V vs. RHE)

P (CoP)

-180

-1 -2 0.87

S (Co9S8)

0.3 0.0

-0.06

1 0

Co (Co9S8)

-0.07

(J1-J0)/2 (mA cm )

(d)

Co (CoP)

0.6

-0.10

CoP

2

J (mA cm )

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


△ GH*(eV)

Page 17 of 32

0

10

20 30 40 50 -1 Scan rate (mV s )

0 60

0

60

120 Z' (ohm)

180

Figure 5. (a) The optimized stable structures of Co 9S8 and CoP with adsorbed H 2O [the atom color map is: blue (Co), pink (P), yellow (S), white (H), red (O)]. (b) Calculated water adsorption energy and (c) HER free-energy diagram for CoP and Co9S8. (d) CVs for CoP/Co9S8 measured at different scan rates from 10 to 50 m Vs-1 in 1.0 M KOH. (e) Plots of the current density at 0.9 V (V vs. RHE) vs. the scan rate for CoP/Co9S8 and CoP. (f) EIS spectra of CoP/Co9S8 and CoP at -0.2 V and the inset is the equivalent circuit diagram.

understood properly. As we all know, for most non-precious metal catalysts, the Volmer step (H2O + e- → H* + OH-) in alkaline media often is the rate-determining step because the water activation process is usually sluggish. Generally, the water adsorption energy is utilized to assess the energy barrier of a catalyst towards water activation. Thus, the water adsorption energies on CoP and Co9S8 catalysts were firstly calculated. Figure 5a shows the optimized stable structures of Co9S8 and CoP with adsorbed H2O. Clearly, for CoP, it is the Co atom that bonds to the O atom of H2O, while for Co9S8, two S atoms bond to two H atoms, respectively. In view of this combination mode, Co 9S8 shows lower water adsorption energy (-0.092 eV) than CoP (-0.07 eV) (Figure 5b). These results suggest that the existence of Co9S8 could expedite the formation of the initial catalyst-water state. Here, it should be noted that the constructed heterointerfaces between Co9S8 and CoP have the advantages of the 17



easier water-adsorption on Co9S8 and the faster transformation of Hads into H2 on the linked CoP species, which has been confirmed by calculating the H* adsorption free energy (ΔGH*) on the catalyst. As well known, the ΔGH* is an important descriptor to estimate the HER catalytic ability of the electrocatalyst, and an ideal one should possess a value close to zero. Clearly, the calculated ΔGH* values for the Co (CoP) site and P (CoP) site of CoP are 0.73 and -0.002 eV, respectively, while those for the Co (Co 9S8) site and S (Co9S8) site of Co9S8 are 0.49 and 0.338 eV (Figure 5c and Figure S13), revealing that CoP holds a higher HER activity with faster transformation of Hads into H2 than Co9S8. This can be assigned to the charge imbalance of Co (δ +) and P (δ−) in CoP, which particularly facilitates the adsorption of H from H2O dissociation on local negatively charged P sites due to the strong electrostatic attraction, while the adjacent Co centers are favor for adsorption of OH− species, both of which are responsible for the intrinsic activity of CoP for HER.6 At the same time, the electron transfer from Co9S8 to CoP

(accurately the P sites) resulting from the

heterostructure in CoP/Co9S8 (as confirmed by XANEs and XPS), charges the P sites of CoP with much more negative charges to significantly enhance the transformation of P-Hads into H2. Besides above two significant factors, the atomic heterogeneous interface in CoP/Co9S8 constructed by two disparate crystals (CoP and Co9S8) can also result in structural distortion (i.e., disordered atoms and dangling bonds) due to imperfect lattice match between neighboring layers, which would contribute to high electrochemical surface area for relevant adsorption and catalysis reactions. This result can be supported by the tested electrochemical double-layer capacitance (Cdl), which collected by cyclic voltammogram (CV) results

18


Page 18 of 32

Page 19 of 32 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


(Figure 5d and Figure S14), since the electrochemical surface area is positively correlated with the Cdl of the catalyst. As revealed by Figure 5e, CoP/Co9S8 holds a larger Cdl (23.3 mF cm-2) than the bare CoP (12.3 mF cm-2), indicating more adsorption and catalytic sites available. Aparts from these advantages above, CoP/Co9S8 electrode also displays the fast HER kinetics supported

by

electrochemical

impedance

spectroscopy

(EIS).

Clearly,

CoP/Co9S8 holds smaller charge-transfer resistance (Rct) than that of bare CoP (Figure 5f), suggesting that CoP/Co9S8 has a faster charge-transfer ability during this HER process. The rapid electron/ion-transport capacity for CoP/Co9S8 can also contribute to a better HER performance, and therefore we further investigated the electronic structures of these catalysts by the following theoretical calculations and experimental measurements. The electron transfer from Co9S8 to CoP (Figure 6a) can lead to a lopsided charge distribution between Co9S8 and CoP, which induces a local interfacial electric field (Figure 6b) and therefore can promote the charge transfer (supported by the much lower Rct impedance in Figure 5f) when the HER happens. Meanwhile, to investigate the electronic information of CoP/Co9S8, ultraviolet photoelectron spectroscopy (UPS) was used to measure the work function (Φ) of CoP/Co9S8 in depth. Φ corresponds to the energy difference between EF and vacuum level to reflect the dynamics of electrons on the surface of the samples, and it can be obtained by equation of Φ = hν − Eonset,18 (hν is 40.0 eV and stands for the incident photon energy, while, Eonset represents the onset level associated with secondary edge) (Figure 6c). By the UPS spectra of CoP/Co9S8 and CoP, the Φ values are determined to be 7.9 eV for CoP/Co9S8 and 8.1 eV for CoP, respectively, indicating that CoP/Co 9S8 possesses better

19



(a)

(c)

(d) Density of State (DOS)

31.9

CoP

Eonset

32.1 34

(b)

Eonset

Intensity (a.u.)

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

33

Page 20 of 32

CoP/Co9S8

32 31 30 29 Binding energy (eV)

28

20 CoP/Co9S8 CoP 10

0 -12

-9

-6 -3 Energy (eV)

0

3

(e)

Figure 6. (a) The schematic diagram for the charge transfer between Co 9S8 and CoP. (b) The schematic diagram of the induced interface electric field and corresponding crystal structures of Co9S8 and CoP. (c) The UPS spectra for CoP/Co 9S8 and the bare CoP. (d) The DOS spectra of as-obtained CoP/Co9S8 and CoP. (e) The schematic catalytic mechanism using the CoP/Co9S8 catalyst.

electrical conductivity. Moreover, the density of states (DOS) of the CoP/Co 9S8 and bare CoP were also calculated (Figure 6d). Clearly, the as-prepared CoP/Co9S8 displays a strong signal at around 1.0 eV in comparison with the bare CoP, which is very close to the Fermi level (EF), revealing that the establishment of the coupled atomic heterointerface between CoP and Co 9S8 obviously improves the conductivity of CoP/Co9S8. The high electrical conductivity of CoP/Co9S8 may benefit from the electric field effect caused by the closely contact atomic heterointerface between CoP and Co 9S8. Briefly, the electronic structrure of CoP was successfully tuned via hybrizing with Co9S8, which is also 20


Page 21 of 32 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


an important aspect that is responsible for the outstanding HER behavior of CoP/Co9S8. Based on the above theoretical and experimental studies, benefiting from the advantages of constructed heterointerfaces between Co9S8 and CoP such as the easier water-adsorption process on Co9S8, faster transformation of Hads into H2 on the adjacent CoP species, improved electron transport capacity between the coupled heterointerface, and large effective active area, the CoP/Co9S8 catalyst, is expected to promote not only the water adsorption to accelerate the Volmer step, but also the Heyrovsky step to rapidly transfer P-Hads into H2, hence affording a better HER electrocatalytic activity than the bare CoP counterpart (Figure 6e). Afterwards, the oxygen evolution reaction (OER) behavior for CoP/Co9S8 was also evaluated in alkaline electrolyte. The responsive current collected from CoP/Co9S8 shows a sharp start potential at ~1.50 V, after which the anode current climbs rapidly (Figure 7a), indicating its outstanding OER activity. The operating potential at the J of 10 mA cm-2 for CoP/Co9S8 is only 1.55 V, which can catch up with that of IrO2/C (1.55 V), and is much lower than those of CoP (1.60 V), Co9S8 (1.62 V), and other series reported electrocatalyst, for instance, IrO2/C (1.60 V, 0.1 M KOH),29 CoP/rGO (1.57 V, 1.0 M KOH),30 and CoS2 (1.61 V, 0.1 M KOH).23 The fitted Tafel slope for CoP/Co9S8 is only 42 mV dec-1, which is much smaller than those of CoP (76 mV dec-1), Co9S8 (84 mV dec-1) and IrO2/C (73 mV dec-1) (Figure 7b), indicating the excellent OER kinetics of CoP/Co9S8 catalyst. The fast OER kinetics corresponding to the high catalytic activity for CoP/Co9S8 may benefit from the advantages of the constructed heterointerfaces. On the one hand, the initial catalyst-water state can easily happen on Co9S8 to

21



expedite the whole water splitting process of CoP/Co9S8. On the other hand, the constructed heterointerfaces between Co9S8 and CoP can lead to a local lopsided charge distribution and structural distortion (i.e., disordered atoms and dangling bonds) to further promote the charge transfer and enlarge the high electrochemical surface area for relevant adsorption and catalysis reactions, and therefore the transmission of electrons and the OH-intermedium can be enhanced. To further investigate the OER reaction mechanism of CoP/Co 9S8, the rotating ring disk electrode (RRDE) was employed. The results indicate and it

60

0.6

CoP/Co9S8

40

CoP Co9S8

30 20

IrO2/C

10

-2

10 mA cm 0 1.3 1.4 1.5 1.6 Potential (V vs. RHE) 10

CoP Co9S8

0.5 0.4

-1

ec

-1

dec mV 6 7

1.7

0

8

-1

ec 42-1mV d dec V 73 m 0.0 0.5 1.0 -2 1.5 2.0 Log[J (mA cm )]

0.3

10

(b) Vd 84 m

IrO2/C

(c)

(d) Idisk = 0

4 OH

_

O2+ 2 H2O

O2-saturated 1.0 M KOH

Iring (μA)

-10

6 4

CoP/Co9S8

(a) Overpotential (V)

-2

J (mA cm )

50

Iring (μA)

-20 -30

2

-40

0

-50 1.2 1.4 1.6 1.8 Potential (V vs. RHE)

Idisk = 200 μA 0 5 10 15 0 5 10 15 20 25 30 Time (s)

100

(f) 80 -2

J (mA cm )

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

Page 22 of 32

60 40 20 0 1.2

st

1 th 500 th 1000 th 3000 1.4 1.6 Potential (V vs. RHE)

1.8

Figure 7. (a) Polarization curves and (b) Tafel slopes of CoP/Co9S8, CoP, Co9S8, and IrO2/C for OER in 1.0 M KOH. Ring current collected on an RRDE in 1.0 M KOH with O2-saturated (ring potential: 1.50 V) (c) and N2-saturated (ring potential: 0.40 V) (d). (e) Chronoamperometric measurement under potential at 10.0 mA cm -2. (f) Polarization curves of CoP/Co9S8 before and after 3000 cycles.

22


Page 23 of 32 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


can be identified as a four electron-dominated reaction pathway with negligible peroxide intermediates and a high Faradaic efficiency of 96.3% (Figures 7c,d). Furthermore, CoP/Co9S8 displays excellent stability. Specifically, for the chronoamperometric response, the anodic current attenuation within 10 h of CoP/Co9S8 is only 10.0%, whereas that is 3 times larger (35.0%) for IrO 2/C (Figure 7e), suggesting that CoP/Co9S8 has higher stability than IrO2/C. Besides, the polarization curve of the CoP/Co 9S8 electrode after 3000 cycles kept well with the original one (Figure 7f), again confirming that CoP/Co9S8 can maintain excellent catalytic stability. After the OER test, the XPS and HR-TEM measurements were performed to check the chemical and structural evolution of the CoP/Co8S9 catalyst. As shown in Figures S15, it can be clearly seen that that CoP/Co9S8 was in-situ transferred partly into CoOOH and Co3O4, which may act as the OER active sites, in good agreement with the previous reports.6,9 Encouraged by the excellent HER and OER performances, the as-obtained CoP/Co9S8 coated on Ni foam substrate can be employed as both the anode and cathode to assemble an alkaline electrolyzer for overall water splitting. Clearly, CoP/Co9S8 electrodes display a catalytic current when the cell voltage achieves at a small value of 1.40 V, follow by the response current rapidly climbing to 10 mA cm-2 only at 1.6 V, which corresponding to an overall overpotential of 370 mV for full water splitting (Figure 8a). It should be emphasized here that the full water splitting activity of CoP/Co9S8 is outstanding among the Pt/C-IrO2/C, Pt/C-Pt/C electrodes, and some reported electrocatalysts in 1.0 M KOH, for examples, CoP/NCNHP (1.64 V),9 CoP-MNA (1.62V),6 and NiS2/CoS2-ONWs (1.768 V)31 (Figure 8b).

23



Pt/C 1.4 1.6 1.8 Cell voltage (V)

1600

400

CoP/Co9S8 Pt/C

300

1200 200 800 100

400 0

0 0

100

(f)

200 300 400 -2 J (mA cm )

H2 + O 2

400

300

500

-S C oS 2 F P O A P rk SC NO P-MN Ni 2 NCNH S @M SHG LDH/N CTs/Co Se@N CoS 2s wo / @ Thi o 9S 8@ e Co Co 9 8 CP/ Co 0.85 NiS 2 C CoP NiF NA 0CoP-M 2.5

(e)

(d)

CoP/Co9S8

-1

2.0

CoP Co9S8

-2

(c)

H2 O

J (mA cm )

2000

(b)

200

2.0

Power density (mW cm ) Cell voltage (V)

-2

10 mA cm

-2

20

2

rO

/C

40

Overpotentials (mV)

CoP /Co 9S

60

0 1.2

Cell voltage (mV)

500

8

(a)

Pt /C -I

-2

J (mA cm )

80

Cell Voltage (mV)

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

Page 24 of 32

1.5 1.0 CoP/Co9S8

0.5

-2

Pt/C -3 -4

Pt/C

-5

0.0 0

10

20 30 40 -2 J (mA cm )

50

(g)

60

(h)

0.2

0.4 0.6 0.8 1.0 Potential (V vs. Ag/AgCl)

H2

O2

Figure 8. (a) Polarization curves based on an alkaline electrolyzer using CoP/Co9S8//CoP/Co9S8, Pt/C//IrO2/C and Pt/C//Pt/C as both the electrodes in 1.0 M N2-saturated KOH and the inset in Figure 8a is the digital photograph of the two electrodes of the electrolyzer. (b) The overpotentials of full water splitting for various reported catalysts. (c) Power densities of the CoP/Co9S8 and commercial Pt/C cathodes in primary Zn–air battery. (d) Charge- discharge curves of the rechargeable Zn–air battery. (e) LSVs of CoP/Co9S8, CoP, Co9S8 and Pt/C in O2-saturated 0.1 M KOH. (f) Cycling test at 10 mA cm-2 for the rechargeable Zn-air battery. (g) Photographs for open-circuit voltage of the home-made Zn–air battery tested by a multimeter, and (h) the self-powered overall water electrolysis device.

By utilizing renewable energy (solar and wind energy) to efficiently catalyze the water splitting is a promising way to generate H2 and O2 products, thus achieving renewable energy conversion. The rechargeable Zn-air batteries can work as a type of safety devices to store the renewable energy. Hence, a home-made rechargeable Zn-air battery was constructed by employing CoP/Co9S8 as the air cathode, and displays the highest power density of 238.1 mW cm-2 (Figure 8c and Figure S16). Moreover, CoP/Co9S8 exhibits a 24


Page 25 of 32 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


charging voltage (OER) that is superior to that of the commercial Pt/C catalyst, while for the discharging process [oxygen reduction reaction (ORR)], CoP/Co9S8 can come up to the Pt/C (Figure 8d). This is consistent with the results obtained by using the three-electrode system, in which CoP/Co9S8 can not only catalyze OER, but also stably catalyze the reverse ORR through a four-electron pathway (Figure 8e and Figure S17). Furthermore, CoP/Co9S8 displays a outstanding cycle stability as no obvious voltage change was observed during the 210 cycles (35 h), while, the Pt/C displays an obvious voltages increase in both the discharge and charge processes even after only 120 cycles (Figure 8f). More importantly, this home-made Zn-air battery affords a large open circuit voltage of 1.56 V (Figure 8g), which can well power the device to efficiently catalyze the overall water splitting. And a large amount of H2 and O2 were clearly observed on the electrodes, reflecting the high HER and OER activities of CoP/Co9S8 (Figure 8h and Video S1). 4. CONCLUSION In summary, we have developed the CoP/Co 9S8 heterostructure as the electrocatalyst for high-performance overall water splitting for the first time. The resultant CoP/Co9S8 heterostructure manifests the synergistic effect of all the Co9S8, CoP, and their strongly coupled heterointerface, which endow CoP/Co9S8 with low water adsorption energy, fast transformation of H ads into H2, high conductivity for electron transfer and large effective active surface area for electrochemical reactions, thus leading to highly efficient full water splitting performance. Particularly, a basic electrolyzer employing CoP/Co9S8 as both the anode and cathode can be operated at a low cell voltage of 1.6 V to transfer a current density of 10 mA cm-2, and it can also be efficiently 25



self-powered by a Zn-air battery with CoP/Co9S8 as the air cathode for prospectively achieving renewable energy conversion. The designing and understanding of the heterostructure engineering provides a favorable direction for developing promising energy-related materials. AUTHOR INFORMATION Conflicts of interest There are no conflicts of interest to declare. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21872008 and 21471016) and the 111 Project (B07012). The authors would like to thank the Analysis & Testing Center of Beijing Institute of Technology for performing FESEM and TEM measurements. ASSOCIATED CONTENT Supporting Information. Experimental section, details about the additional XRD, Raman, XPS, N2 adsorption/desorption results, and electrochemical performances of relevant samples. References (1) Wu, H. B.; Xia, B. Y.; Yu, L.; Yu, X. Y.; Lou, X. W. Porous Molybdenum Carbide Nano-Octahedrons Synthesized via Confined Carburization in Metal-Organic Frameworks for Efficient Hydrogen Production. Nat. Commun. 2015, 6, 6512. (2) Feng, J. X.; Wu, J. Q.; Tong, Y. X.; Li, G. R. Efficient Hydrogen Evolution on Cu Nanodots-Decorated Ni3S2 Nanotubes by Optimizing Atomic Hydrogen Adsorption and Desorption. J. Am. Chem. Soc. 2018, 140, 610-617. (3) Xu, K.; Cheng, H.; Lv, H.; Wang, J.; Liu, L.; Liu, S.; Wu, X.; Chu, W.; Wu, C.; Xie, Y. Controllable Surface Reorganization Engineering on Cobalt Phosphide

26


Page 26 of 32

Page 27 of 32 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


Nanowire Arrays for Efficient Alkaline Hydrogen Evolution Reaction. Adv. Mater. 2018, 30, 1703322. (4) Chen, P.; Xu, K.; Tao, S.; Zhou, T.; Tong, Y.; Ding, H.; Zhang, L.; Chu, W.; Wu, C.; Xie, Y.: Phase-Transformation Engineering in Cobalt Diselenide Realizing Enhanced Catalytic Activity for Hydrogen Evolution in an Alkaline Medium. Adv. Mater. 2016, 28, 7527-7532. (5) Skúlason, E.; Tripkovic, V.; Björketun, M. E.; Gudmundsdóttir, S.; Karlberg, G.; Rossmeisl, J.; Bligaard, T.; Jónsson, H.; Nørskov, J. K. Modeling the Electrochemical Hydrogen Oxidation and Evolution Reactions on the Basis of Density Functional Theory Calculations. J. Phys. Chem. C 2010, 114, 18182-18197. (6) Zhu, Y. P.; Liu, Y. P.; Ren, T. Z.; Yuan, Z. Y. Self-Supported Cobalt Phosphide Mesoporous Nanorod Arrays: A Flexible and Bifunctional Electrode for Highly Active Electrocatalytic Water Reduction and Oxidation. Adv. Funct. Mater. 2015, 25, 7337-7347. (7) 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. (8) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 126, 6828-6832. (9) Pan, Y.; Sun, K.; Liu, S.; Cao, X.; Wu, K.; Cheong, W. C.; Chen, Z.; Wang, Y.; Li, Y.; Liu, Y.; Wang, D.; Peng, Q.; Chen, C.; Li, Y. Core-Shell ZIF-8@ZIF-67-Derived CoP Nanoparticle-Embedded N-Doped Carbon Nanotube Hollow Polyhedron for Efficient Overall Water Splitting. J. Am. Chem. Soc. 2018, 140, 2610-2618.

27



(10) Zhou, L.; Shao, M.; Li, J.; Jiang, S.; Wei, M.; Duan, X. Two-Dimensional Ultrathin Arrays of CoP: Electronic Modulation toward High Performance Overall Water Splitting. Nano Energy 2017, 41, 583-590. (11) Xin, Y.; Kan, X.; Gan, L. Y.; Zhang, Z. Heterogeneous Bimetallic Phosphide/Sulfide Nanocomposite for Efficient Solar-Energy-Driven Overall Water Splitting. ACS Nano 2017, 11, 10303-10312. (12) Xu, K.; Ding, H.; Zhang, M.; Chen, M.; Hao, Z.; Zhang, L.; Wu, C.; Xie, Y. Regulating Water-Reduction Kinetics in Cobalt Phosphide for Enhancing HER Catalytic Activity in Alkaline Solution. Adv. Mater. 2017, 29, 1606980. (13) Li, X. Z.; Fang, Y. Y.; Li, F.; Tian, M.; Long, X. F.; Jin. J.; Ma, J. T. Ultrafine Co2P Nanoparticles Encapsulated in Nitrogen and Phosphorus Dual-Doped Porous Carbon Nanosheet/Carbon Nanotube Hybrids: High-Performance Bifunctional Electrocatalysts for Overall Water Splitting. J. Mater. Chem. A. 2016, 4, 15501-15510. (14) Hu, Y. P.; Li, F.; Long, Y.; Yang, H. D.; Gao, L. L.; Long, X. F.; Hu, H. G.; Xu, N.; Jin. J.; Ma, J. T. Ultrafine CoPS Nanoparticles Encapsulated in N, P, and S Tri-Doped Porous Carbon as an Efficient Bifunctional Water Splitting Electrocatalyst in both Acid and Alkaline Solutions. J. Mater. Chem. A. 2018, 6, 10433-10440. (15) Meng, T.; Hao, Y. N.; Zheng, L. R.; Cao, M. H. Organophosphoric Acid-Derived CoP Quantum Dots@S,N-Codoped Graphite Carbon as a Trifunctional Electrocatalyst for Overall Water Splitting and Zn-Air Batteries. Nanoscale. 2018, 10, 14613-14626. (16) Hou, Y.; Qiu, M.; Nam, G.; Kim, M. G.; Zhang, T.; Liu, K.; Zhuang, X.; Cho, J.; Yuan, C.; Feng, X. Integrated Hierarchical Cobalt Sulfide/Nickel Selenide Hybrid Nanosheets as an Efficient Three-dimensional Electrode for Electrochemical and Photoelectrochemical Water Splitting. Nano Lett. 2017, 17, 4202-4209. (17) Zhou, X.; Liu, Y.; Ju, H.; Pan, B.; Zhu, J.; Ding, T.; Wang, C.; Yang, Q. Design

28


Page 28 of 32

Page 29 of 32 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


and Epitaxial Growth of MoSe2-NiSe Vertical Heteronanostructures with Electronic Modulation for Enhanced Hydrogen Evolution Reaction. Chem. Mater. 2016, 28, 1838-1846. (18) Yang, J.; Wang, C.; Ju, H.; Sun, Y.; Xing, S.; Zhu, J.; Yang, Q. Integrated Quasiplane Heteronanostructures of MoSe2/Bi2Se3 Hexagonal Nanosheets: Synergetic Electrocatalytic Water Splitting and Enhanced Supercapacitor Performance. Adv. Funct. Mater. 2017, 27, 1703864. (19) Dong, Q.; Zhang, Y.; Dai, Z.; Wang, P.; Zhao, M.; Shao, J.; Huang, W.; Dong, X. Graphene as an Intermediary for Enhancing the Electron Transfer Rate: A Free-Standing Ni3S2@Graphene@Co9S8 Electrocatalytic Electrode for Oxygen Evolution Reaction. Nano Research 2018, 11, 1389-1398. (20) Blanchard, P. E. R.; Grosvenor, A. P.; Cavell, R. G.; Mar, A. Effects of Metal Substitution in Transition-Metal Phosphides (Ni1-xM[Prime or Minute]x)2P (M[Prime or Minute]=Cr, Fe, Co) Studied by X-Ray Photoelectron and Absorption Spectroscopy. J. Mater. Chem. 2009, 19, 6015-6022. (21) Grosvenor, A. P.; Cavell, R. G.; Mar, A. Next-Nearest Neighbour Contributions to P 2p3/2 X-Ray Photoelectron Binding Energy Shifts of Mixed Transition-Metal Phosphides M1-xM′xP with the MnP-Type Structure. J. Solid State Chem. 2007, 180, 2702-2712. (22) Lin, Y.; Yang, L.; Zhang, Y.; Jiang, H.; Xiao, Z.; Wu, C.; Zhang, G.; Jiang, J.; Song, L. Defective Carbon-CoP Nanoparticles Hybrids with Interfacial Charges Polarization for Efficient Bifunctional Oxygen Electrocatalysis. Adv. Energy Mater. 2018, 8, 1703623. (23) Sun, Y.; Gao, S.; Lei, F.; Xie, Y. Atomically-Thin Two-Dimensional Sheets for Understanding Active Sites in Catalysis. Chem. Soc. Rev. 2015, 44, 623-636.

29



(24) Tabassum, H.; Guo, W.; Meng, W.; Mahmood, A.; Zhao, R.; Wang, Q.; Zou, R. Metal-Organic Frameworks Derived Cobalt Phosphide Architecture Encapsulated into B/N Co-Doped Graphene Nanotubes for All pH Value Electrochemical Hydrogen Evolution. Adv. Energy Mater. 2017, 7, 1601671. (25) 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. (26) Huang, S.; Meng, Y.; He, S.; Goswami, A.; Wu, Q.; Li, J.; Tong, S.; Asefa, T.; Wu, M. N-, O-, and S-Tridoped Carbon-Encapsulated Co9S8 Nanomaterials: Efficient Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1606585. (27) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmeková, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem., Int. Ed. 2014, 126, 4461-4465. (28) Yan, Y.; Thia, L.; Xia, B. Y.; Ge, X.; Liu, Z.; Fisher, A.; Wang, X. Construction of Efficient 3D Gas Evolution Electrocatalyst for Hydrogen Evolution: Porous FeP Nanowire Arrays on Graphene Sheets. Adv. Sci. 2015, 2, 1500120. (29) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-Doped Carbon Nanomaterials as Non-Metal Electrocatalysts for Water Oxidation. Nat. Commun. 2013, 4, 2390. (30) Jiao, L.; Zhou, Y. X.; Jiang, H. L. Metal-Organic Framework-Based CoP/Reduced Graphene Oxide: High-Performance Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Sci. 2016, 7, 1690-1695. (31) Yin, J.; Li, Y.; Lv, F.; Lu, M.; Sun, K.; Wang, W.; Wang, L.; Cheng, F.; Li, Y.; Xi, P.; Guo, S. Oxygen Vacancies Dominated NiS2/CoS2 Interface Porous Nanowires for

30


Page 30 of 32

Page 31 of 32 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


Portable Zn-Air Batteries Driven Water Splitting Devices. Adv. Mater. 2017, 29, 1704681.

31



The table of contents entry

32


Page 32 of 32

Atomic Heterointerface-Induced Local Charge Distribution and

Recommend Documents