Toward Bifunctional Overall Water Splitting Electrocatalyst: General

Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Toward Bifunctional Overall Water Splitting Electrocatalyst: General Preparation of Transition Metal Phosphide Nanoparticles Decorated N‑Doped Porous Carbon Spheres Konglin Wu,†,‡,§ Zheng Chen,†,‡ Weng-Chon Cheong,†,‡ Shoujie Liu,‡,§ Wei Zhu,⊥ Xing Cao,‡ Kaian Sun,|| Yan Lin,|| Lirong Zheng,# Wensheng Yan,Δ Yuan Pan,*,‡ Dingsheng Wang,‡ Qing Peng,‡ Chen Chen,*,‡ and Yadong Li‡ Downloaded via UNIV OF WINNIPEG on December 21, 2018 at 06:30:06 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

Department of Chemistry, Tsinghua University, Beijing, 100084, China Center of Single-Atom, Clusters and Nanomaterials (CAN), College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, China ⊥ State Key Lab of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China || State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, Shandong 266580, China # Beijing Synchrotron Radiation Facility (NSRF), Chinese Academy of Science, Beijing 100049, China Δ National Synchrotron Radiation Laboratory (NSRL), University of Science and Technology of China, Hefei, Anhui 230029, China §

S Supporting Information *

ABSTRACT: It is very important to explore novel synthesis strategies for constructing highly active and inexpensive electrocatalysts for water-splitting. In present work, a novel and efficient coordination-polymerization-pyrolysis (CPP) strategy was developed to prepare cobalt phosphide nanoparticles modified N-doped porous carbon spheres (CoP@NPCSs) hybrids as a powerful catalyst for overall water-splitting (OWS). It can be found that both the carbonization temperatures and the metal contents affect the electrocatalytic performances. As a result, a device assembled with CoP@NPCSs demonstrates low potential (1.643 V @ 10 mA·cm−2) and good stabilization for OWS. Besides, other transition metal phosphides (TMPs)-based materials also can be synthesized by the CPP approach, evidencing the generality of the CPP strategy. Here, we not only constructs a high-efficiency OWS catalyst, but also broadens the synthetic methodology of TMPs from nanoscale. KEYWORDS: transition metal phosphide, XANES, electrocatalyst, water splitting, general synthesis

A

most of the reported TMPs catalysts were aiming at only HER or OER half reaction. At the same time, these catalysts also had the problems of poor conductivity and lack of active sites. Moreover, TMPs tend to migrate and agglomerate during catalytic process;15,16 therefore, to further improve the electrocatalytic activity of TMPs, a novel strategy for preparing and packaging TMPs nanoparticles (NPs) with a carbon substrate stabilizing TMPs remain an effective approach up to now. In addition, the doping of heteroatom into phosphide,17 sulfide,18,19 or carbon matrix is an efficient way to synergistically accelerate the electrocatalytic process.20 For example, Qiao’s group reported that the HER and OER activity could be enhanced by a singly doped matrix.21−23 Theory calculation also illustrated that the codoping of N and P can activate carbon atoms in a graphene substrate through modulating the

s the major energy resource, traditional fossil fuels bring about environmental issues that have been threatening sustainable development. As an alternative, hydrogen is considered a green and environmentally friendly energy source with minimal ecological impact.1 Water is a huge warehouse of hydrogen, and producing hydrogen from water by electrolysis is a desired method.2,3 There are two important reactions in the overall water splitting (OWS) electrolysis reaction process, namely hydrogen production (HER) and oxygen generation reaction (OER).4 Both are kinetically sluggish and need electrocatalysts to reduce the additional energy expenditure. Usually, the Pt, RuO2, or IrO2 showed the best water splitting electrocatalytic performance, but the scarcity of noble metals makes them unrealistic for practical industrial use.5 Therefore, it is of great significance to explore novel synthetic strategy for constructing high-efficiency and cheap electrocatalysts for OWS.6−9 Transition metal phosphides (TMPs), which were traditionally employed as hydrogenation catalysts,10−14 have been proven to be highly active in HER and OER process. However, © XXXX American Chemical Society

Received: August 31, 2018 Accepted: December 10, 2018 Published: December 10, 2018 A

DOI: 10.1021/acsami.8b14889 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Preparation process for CoP@NPCSs; (b) XRPD pattern (up) and Raman spectrum (down) of CoP@NPCSs; (c−e) morphologies and structures characterization of CoP@NPCSs; (f, g) EDXS mapping of CoP@NPCSs.

valence orbital energy level.24,25 Hence, it is expected to synthesize TMPs NPs decorated in a heteroatom-doped carbon substrate for improving the electrocatalytic performance. Herein, a novel and efficient coordination-polymerizationpyrolysis (CPP) strategy was reported for synthesizing cobalt phosphide nanoparticles decorated N-doped porous carbon spheres (CoP@NPCSs) composites as a robust multifunctional electrocatalyst for OWS. By using this CPP strategy, a series of CoP@NPCSs hybrid catalysts with different carbonization temperatures and molar ratios of metal precursors and dopamine were obtained, and these different conditions could affect the electrocatalytic performances. The optimized CoP@ NPCSs hybrid reveals remarkable electrocatalytic activity and long-time stability. When assembled into a device for OWS, it also exbibits low potential (1.643 V @ 10 mAcm−2) and superior stability (at least 20 h) during electrolysis.

Interestingly, the current CPP method can also be used to synthesizing of Ni2P@NPCSs and FeP@NPCSs hybrids. The preparation process of CoP@NPCSs hybrids is shown in Figure 1a, and the typical synthesis process involved four major steps (detailed information is shown in Supporting Information). In the first step, the cobalt-polydopamine (denoted as Co(II)-PDA) precursors were synthesized by polymerization of dopamine with Co(II) ions. Subsequently, the Co nanoparticles modified N-doped porous carbon spheres (Co@NPCSs) hybrids were obtained by reduction of Co(II)PDA under a H2/Ar atmosphere at 600 °C. The as-prepared Co@NPCSs hybrids were further subjected to oxidation in air at 300 °C to prepare Co3O4@NPCSs hybrids. Finally, the CoP@NPCSs hybrids were obtained by phosphorization of Co3O4@NPCSs hybrids at 300 °C under Ar. Transmission electron microscopy (TEM) image (Figure S1a) exhibited that cobalt nanoparticles (Co NPs) were embedded in carbon B


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Figure 2. XPS spectra for (a) Co 2p, (b) P 2p, and (c) N 1s; (d) C K-edge and N K-edge XANES spectra; (e) XANES and (f) FT of the Co K edge.

elemental mapping results (Figure 1f and 1g) showed that the Co, P, N, and C elements distribute uniformly on the CoP@ NPCSs hybrids. Furthermore, the EDXS spectrum (Figure S4) indicated that atom ratio of Co to P on hybrids was very close to 1:1. Isotherm curves (Figure S5) displayed a type IV feature, indicating both micropores and abundant mesopores.30 The specific surface area of the CoP@NPCSs hybrids was about 25.6 m2 g−1 based on the Brunauer−Emmett−Teller (BET) means. And the CoP@NPCSs hybrids showed a micropore (1.2 nm) and a mesopore (3.8 nm) properties (inset in Figure S5). Large surface area and the abundant mesopores provided the highly exposed active surface and the rapidly mass transfer, which further enhanced the electrocatalytic performance of CoP@NPCSs hybrids.30,31 The CoP@NPCSs hybrids was examined through X-ray photoelectron spectroscopy (XPS), and elements of C, N, Co, and P were found in survey spectrum (Figure S6). Co 2p spectrum exhibited that the obvious peaks around 781.5, 778.6, 783.8, and 785.8 eV (Figure 2a) were indexed to Co 2p3/2.32 Peaks at 798.0 and 803.6 eV for Co 2p1/2 were also observed. Figure 2b indicated that the doublet situated peaks surrounding at 129.8 eV (P 2p3/2) and 130.6 eV (P 2p1/2) were found. Besides, the peak around 134.5 eV for oxidized P species also appeared. Bond energy of Co 2p3/2 in CoP@ NPCSs hybrids had a shift to higher energy compared with Co. Bond energy of P 2p3/2 in CoP@NPCSs hybrids had a shift to lower energy compared with P. On the basis of the above analysis, we concluded that the Co 2p3/2 (778.6 eV) and P 2p3/2 (129.8 eV) were associated with the contributions of Co−P bonds in CoP@NPCSs hybrids,33−36 respectively. Furthermore, the N 1s spectrum (Figure 2c) indicated that there are four types of N, and the content of graphitic-N was 16.85% (400.9 eV), pyrrolic-N 53.9% (399.9 eV), pyridinic-N

substrate, and the further X-ray powder diffraction (XRPD, Figure S1b) pattern displayed that the particles were Co NPs (JCPDF No. 89−4307). After the Co@NPCSs hybrids were oxidized in air, the XRPD pattern (Figure S2a) revealed that the Co NPs were transformed into Co3O4 NPs. TEM image (Figure S2b) showed that the Co3O4 NPs also embedded in the carbon materials. With further phosphorization process, the Co3 O 4 @NPCSs hybrids were converted into the corresponding CoP@NPCSs hybrids. XRPD pattern (Figure 1b) showed the weak peaks at about 31.6, 36.2, 48.2, and 56.6°, respectively, corresponding to (011), (111), (211), and (301) lattice planes of CoP (JCPDF no. 29−0497). Raman spectrum was a useful technique for studying the structural properties of carbon-based materials. Raman spectra showed two main peaks at about 1337 cm−1 (D) and 1581 cm−1 (G) (Figure 1b), attributing to the vibration of dispersive defectinduced vibrations and sp2-bonded carbon atoms. The ID/IG value of the CoP@NPCSs hybrids was 0.85, which implied that there were more defects in the hybrids. The wide 2D band at about 2700 cm−1 was attributed to the layered carbon structure in CoP@NPCSs hybrids.26−28 With the N dopant, more defects and layered carbon structure were appeared in CoP@NPCSs hybrids. The charge-spin distribution on the sp2conjugated carbon substrate is effectively modified, resulting in excellent intermediate chemisorption and good electron transfer.29 The SEM image (Figure 1c) showed that the CoP@NPCSs hybrids with spherical morphology were obtained, and the corresponding TEM image (Figure 1d) revealed the CoP NPs were dispersed in the carbon spheres. The clear CoP diffraction rings (Figure S3) were found in selected area electron diffraction (SAED). The fringe spacing was ∼2.5 Å from the high resolution TEM (HRTEM) image (Figure 1e) result, which could be indexed to the (111) lattice plane for CoP. Energy-dispersive X-ray spectroscopy (EDXS) C


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Figure 3. (a) LSV curves and (b) Tafel plots for CN, Co@NPCSs, CoP NPs, CoP@NPCSs, and Pt/C for in acidic electrolyte; (c) Nyquist plots and (d) Cdl values of Co@NPCSs, CoP NPs, CoP@NPCSs catalysts in acidic electrolyte; (e) LSV curves and (f) Tafel plots for CN, Co@NPCSs, CoP NPs, CoP@NPCSs, and Pt/C in alkaline electrolyte for HER; (g) LSV curves initial and after 2000 CV cycles of the CoP@NPCSs catalyst in acidic (left) and alkaline electrolyte (right) for HER; (h) I−t curves for CoP@NPCSs in acidic (up) and alkaline electrolyte (down) for HER at a fixed overpotentials of 112 mV and 115 mV.

24.15% (398.4 eV), and weak quaternary N+−O− 5.1% (402.1 eV) (Table S1). To investigate the defect sites in the CoP@NPCSs hybrids, we researched the C and N K-edges. The X-ray absorption near-edge structure (XANES) spectroscopy results (Figure 2d) expounded that the sp2-hybridized carbons of π* and σ* peaks were observed;29,36−41 the σ*-transition (peaks a1 and a2) of pyridinic-N site, the N-3C bridging (peak b) of graphitic-N site, and π*-transition (peak c) of the C−N bond can be observed in N K-edge.29,36−41 XANES spectra for Co K-edge (Figure 2e) demonstrated that the spectral profiles of CoP@ NPCSs hybrids and CoP bulk were different from that of Co foil, Co3O4, and CoO. The valence of Co atom in CoP@ NPCSs hybrids can be confirmed between +2 and +3 by comparing the absorption edge position. The peaks at about 1.7 and 2.5 Å in Fourier-transformed (FT) X-ray absorption fine structure (FT-EXAFS) spectroscopy (Figure 2f) spectra could be deemed to the Co−P bond and Co−Co coordination bond.32 The electrocatalytic activities of CoP@NPCSs hybrids for HER were tested. To explore the optimal synthetic conditions, we first systematically studied the effect of carbonization temperature and molar ratio of metal precursors and dopamine

on the HER activity. Linear sweep voltammetry (LSV) results (Figure S7) show that CoP@NPCSs exhibited the highest HER activity when the carbonization temperature and the molar ratio of metal precursors and dopamine were set to 600 °C and 0.095, respectively. Those results indicated that the sizes of CoP NPs and the contents of carbon and nitrogen in the hybrids were different (Figure S8 and Table S2), which influences the HER electrocatalytic performance. At the same time, the Pt/C (Figure S9), N-doped porous carbon (CN, Figure S10), Co@NPCSs, and CoP NPs (Figure S11) were examined. LSV curves (Figure 3a) displayed that the overpotentials of CN, Co@NPCSs, CoP NPs, CoP@NPCSs, and Pt/C are 538, 453, 198, 112, and 43 mV at 10 mA cm−2. We can find the HER catalytic property of CoP@NPCSs hybrids is higher than the previous reported electrocatalysts (Table S3). The Tafel plots in Figure 3b suggested that the CoP@NPCSs hybrids showed the smaller Tafel slope (70 mV· dec−1) than the CN (170 mV·dec−1), Co@NPCSs (98 mV· dec−1), and CoP NPs (79 mV·dec−1). Those results suggested that the HER mechanism of CoP@NPCSs hybrids followed the Volmer-Heyrovesky pathway.42 Additionally, the superior performance of the CoP@NPCSs hybrids to other catalysts was also reflected by its lowest charge transfer resistance at D


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Figure 4. (a) LSV curves for CN, Co@NPCSs, CoP NPs, CoP@NPCSs, and RuO2 for OER; (b) Tafel plots for CN, Co@NPCSs, CoP NPs, and CoP@NPCSs for OER; (c) Nyquist plots of Co@NPCSs, CoP NPs, and CoP@NPCSs; (d) LSV changes for CoP@NPCSs electrocatalyst initial and after 2000 cycles (inset is the stabilization investigation with an overpotential of 355 mV).

−112 mV vs RHE (Rct = 14.62 Ω) than that of Co@NPCSs (Rct = 314.9 Ω) and CoP NPs (Rct = 786.9 Ω), resulting in the fastest HER reaction kinetics (Figure 3c, Figure S12, and Table S4). Moreover, the cyclic voltammetry (CV) method was used to research the electrochemical active surface area (ECSA) for electrocatalysts.34,43 Here, the double-layer capacitance (Cdl) of the CoP@NPCSs hybrids (Figure S13 and Figure 3d) was 45.6 mF cm−2, it was higher than that of the Co@NPCSs (3.3 mF cm−2), and CoP NPs catalysts (15.2 mF cm−2). We know that the higher ECSA, the better electrocatalytic activity. In 1 M KOH, the CoP@NPCSs hybrids exhibited good catalytic property for HER (Figure 3e). The CoP@NPCSs hybrids showed an overpotential of 115 mV at 10 mA·cm−2, which was lower than the CN (586 mV), Co@NPCSs (450 mV), and CoP NPs (168 mV), respectively. The low Tafel slope of CoP@NPCSs (Figure 3f) suggested that it possesses fast HER kinetics. The CoP@NPCSs hybrids displayed a remarkable HER activity than previous reported catalysts (Table S5), and further demonstrated that this hybrid catalyst could be a potential application material in constructing inexpensive electrocatalyst. Furthermore, the Cdl of the CoP@ NPCSs hybrids (Figure S14) was 10.5 mF cm−2. It was bigger than the 3.28 mF cm−2 for Co@NPCSs and 6.4 mF·cm−2 for CoP NPs catalysts. Above results also suggested that the CoP@NPCSs hybrids revealed the excellent HER property in alkaline electrolyte. As an important performance indicator, the stabilities of CoP@NPCSs hybrids were examined. From the results shown in Figure 3g, the typical CoP@NPCSs hybrids exhibit good catalytic stability for HER. The slight change was revealed in LSV results after 2000 cycles. At a constant overpotential both in acidic and alkaline electrolytes, the

current−time curves (Figure 3h) showed a steady current output for 20 h, respectively. Figure S15 shows the TEM images and XPS spectra of CoP@NPCSs after 20 h electrolysis. After long time electrolysis, the shapes and element compositions of the CoP@NPCSs catalysts almost unchanged, demonstrating good stability because of the strong oxidation resistance of N-doped carbon shell on the surface CoP NPs. The produced H2 measured matched well with the calculated amount in theory under both acidic media and alkaline, suggesting a Faradaic efficiency of ∼100% (Figure S16). The above findings demonstrate that the as-prepared CoP@NPCSs hybrids qualify as an HER catalyst with higher activiy and durability. On the OER process (Figure 4a), the overpotentials of each catalyst were 350 mV for CoP@NPCSs, 1290 mV for CN, 790 mV for Co@NPCSs, 500 mV for CoP NPs, and 370 mV for commercial RuO2 (Figure S17) at 10 mA cm−2, respectively. As-prepared CoP@NPCSs hybrids also revealed higher OER rates, reflected by the lowest Tafel curves (Figure 4b) (103 mV dec−1). Furthermore, the Nyquist plots (Figure 4c and Figure S12) also showed that the CoP@NPCSs hybrids displayed a smaller Rct (65.94 Ω) at 1.585 V vs RHE than the Co@NPCSs and CoP NPs (Table S3 and S6). The above analysis suggested that CoP@NPCSs hybrids are also a commendable OER electrocatalyst compared to previous reported catalysts (Table S7). The stability test (Figure 4d) showed that the LSV curves of CoP@NPCSs hybrids revealed a small displacement after 2000 cycles. The OER activity of CoP@NPCSs hybrids can be maintained at least 20 h (inset in Figure 4d). From the XPS spectra, the Co−P bond was still observed after a long time of testing (Figure S18), which indicated that the N-doped porous E


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Figure 5. (a) LSV curve of the CoP@NPCSs||CoP@NPCSs electrode for OWS (inset photograph is the production H2 in cathode and O2 in anode). (b) Stability test for OWS.

under the actual OER potential, these compounds would develop a thin oxide layer, which act as the real active center. Therefore, the real active species might be CoOOH generated during OER. The generality is important for a novel synthesis methodology. In this present work, the synthesis method is via dopamine oxidation and polymerization under alkaline condition, and the metal ions were coordinated with nitrogen and oxygen atoms. This method offers a green synthesis process, which is readily scalable. We also found that the introduction of the metal ions can greatly increase production, showing that the addition of metal ions was favorable for the formation of dopamine polymer. Here, we extended our CPP strategy to synthesizing other TMPs@NPCSs hybrids. By varying the metal precursors, we successfully synthesized Ni2P@NPCSs (Figures S19a, S20, and S21) and FeP@NPCSs (Figures S19b, S22, and S23) electrocatalysts, further evidencing the universal of CPP method. In conclusion, a novel and efficient CPP tactics for preparing CoP@NPCSs hybrids with porpous carbon sphere-encapsulating nanostructures was developed. Because of the synergism of CoP and N-doped porous carbon spheres (NPCSs) with high conductivity and the big surface area as well as the good stability of NPCSs-encapsulating nanostructures, the resulting CoP@NPCSs electrocatalyst exhibited robust HER and OER performances. When employed as electrocatalysts for electrolysis of water, the CoP@NPCSs revealed a low potential (1.643 V@10 mA cm−2) and a good stabilization (about 20 h) during electrolysis. By extending this method to other nonnoble metals, this CPP strategy may promote the development of a rich variety of TMP-based hybrids for catalytic application.

carbon spheres decorated with TMPs were conducive to the stability of the electrocatalysts. Additionally, a two-electrode OWS device using CoP@ NPCSs||CoP@NPCSs at both anode and cathode was assembled. During the OWS process, the cathode and anode show a mass of bubbles (Video S1). The obtained device only requires a potential of 1.643 V for OWS at 10 mA cm−2 (Figure 5a). As-obtained CoP@NPCSs electrocatalyst is approximate to the previously reported OWS catalysts (Table S8). Moreover, the long-time electrolysis for OWS at 1.643 V was tested (Figure 5b). After 20 h of continuous measuring, the current density was maintained at about 10 mA cm−2. Above results indicated that the excellent activity and durability render CoP@NPCSs electrocatalyst a hopeful alternative to noble metal electrocatalysts for OWS in practical applications. The high catalytic property of CoP@NPCSs may be due to the following factors. First, for HER, CoP NPs are the major catalytic active sites, in which the Co serve as a hydride receptor, whereas the P act as a proton receptor.36 The intrinsic catalytic activity strongly depends on the positively charged Co site (δ+) and negatively charged P site (δ−).11,30,44 From the XPS and XANES, it can be concluded that the charge of Co in CoP@NPCSs between +2 and +3, and the charge of P in CoP@NPCSs is lower than zero. The change of electronic structures for both Co and P atoms is beneficial for the HER. Second, the CoP NPs are coated with carbon layers, which can effectively inhibit the agglomeration of NPs in the actual electrocatalytic process. Because of the size effect, NPs with large sizes usually exhibit a low catalytic activity. Here, the coated carbon shell on NPs play an important and positive roles.45 When the TMP NPs put on a piece of armor (namely coating with carbon layers) which can be used to enhance the combat effectiveness of the TMPs NPs. Moreover, the presence of carbon layers greatly improved the conductivity, and further promoted the electron transfer.46 At the same time, the micropores and mesopores in the carbon coating also effectively improved the mass transfer. These aspects can greatly enhance the electrocatalytic performance of TMPs. In addition, due to the aggregation of dopamine, the skeleton of carbon materials is doped with a large amount of N atoms. After pyrolysis, many defects exist in the carbon substrates, which also contributes to the electrocatalytic properties. Therefore, the introduction of carbon layers into the TMPs display a positive role. For alkaline OER, according to the previous reports,47 the TMP actually is a precatalyst, because

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14889. Experimental Section, Figure S1−S23, and Tables S1− S8 (PDF)

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Video S1 (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected] (Y. P.). *E-mail:[email protected] (C. C.). F


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Wensheng Yan: 0000-0001-6297-4589 Dingsheng Wang: 0000-0003-0074-7633 Chen Chen: 0000-0001-5902-3037 Yadong Li: 0000-0003-1544-1127 Author Contributions †

K.W., Z.C., and W.-C.C. contributed equally to this work. K.W., Z.C., W.-C.C., and Y.P. designed and carried out the synthesis and characterizations of samples, tested the electrochemical performance, analyzed the data, and wrote the manuscript. W.Z. and X.C. modified the illustrations. K.S. tested the Faradaic efficiency. Y.L. revised and embellished the paper. S.L., L.Z., and W.Y. performed XAFS measurement and analyzed the data. C.C. supervised the project. All the authors commented on the manuscript and have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thanks the National Key R&D Program of China (2017YFA0700101, 2016YFA0202801), the National Natural Science Foundation of China (21890383, 21573119, 21590792, 21890383, 21501004), China Postdoctoral Science Foundation (2017M610076, 2018T110089), and Beijing Natural Science Foundation (2184104) for supporting our work. We thank the Beijing Synchrotron Radiation Facility (BSRF) and National Synchrotron Radiation Laboratory (NSRL) for the help in characterizations.

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