Layered Phosphate-Incorporated NickelâCobalt Hydrosilicates for

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Layered Phosphate-Incorporated Nickel-Cobalt Hydrosilicates for Highly Efficient Oxygen Evolution Electrocatalysis Ce Qiu, Lunhong Ai, and Jing Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04415 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Sustainable Chemistry & Engineering

Layered Phosphate-Incorporated Nickel-Cobalt Hydrosilicates for Highly Efficient Oxygen Evolution Electrocatalysis Ce Qiu, Lunhong Ai, and Jing Jiang* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, 1# Shida Road, Nanchong 637002, P.R. China

*Corresponding Author E-mail: [email protected] (J. Jiang) Tel/Fax: +86-817-2568081

ACS Paragon Plus Environment

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ABSTRACT

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Design and discovery of highly efficient and cost-effective materials for

electrocatalytic oxygen evolution reaction (OER) based on earth-abundant elements is highly desired but still a great challenge. As generally known, silicon is an element of second abundance in the earth’s crust. We herein chose silicon element for the desgin and fabrication of OER-active layered nickel cobalt hydrosilicates (NCS) and found that phosphate-incorporated strategy can significantly enhance the OER performance of host hydrosilicates. The resulting NCS:P electrocatalyst can generate much larger catalytic current than that of NCS at same applied potential, which is even superior to that of Ni(OH)2 and Co(OH)2 references in 1.0 M KOH. It is expected that structural and composition synergies endow the NCS:P with attractive performance.

KEYWORDS

Water splitting; Oxygen evolution; Hydrosilicates; Electrocatalyst; Layered

structure

INTRODUCTION

With the continuous consumption of traditional fossil fuels and increased energy demands, the energy crisis has become an significant concern over restricting the development of human society.1,2 Fortunately, hydrogen energy has been touted as an ideal fuel in the future due to its high energy density and cleanness.3 The process of hydrogen evolution by electrolysis of water involves hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode.4 However, the key half-reaction of OER is an intrinsically slow reaction that requires an overpotential in substantial excess of its thermodynamic potential (1.23 V vs RHE) to deliver an acceptable current density.5,6 Precious noble-metal-based materials including IrO2 and RuO2 have been utilized as state-of-the-art OER catalysts to lower the activation energy barrier


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and enhance reaction conversion rate, but they are not suitable for large-scale applications because of their scarcity and high costs.7,8 As a result, great efforts have been made to exploit cost-effective alternatives catalysts with high activity and good stability based on earth-abundant elements. Silicon is an element of second abundance in the earth’s crust. As a member of silicates, the hydrosilicates are main existent form of silicon on earth, which are receiving great attention and have been the research subjects in many areas, such as catalysis, adsorption, separation, and energy storage.9-12 In view of their structures (Figure 1a), they are a class of phyllosilicate with a typical layered structure comprised of sheets of corner-shared tetrahedral SiO4 and sheets of edge-shared octahedral MO6. Of special note, without the presence of the polysilicate sheets, their whole structures are fundamentally analogous to OER-active metal hydro(oxy)oxides. Recent research works have been demonstrated that the earth-abundant and environmentally benign hydrosilicates are capable of electrochemically catalyzing OER.13 Despites these advances, the electrocatalytic performances of this material for OER are still low and need a high overpotential to deliver an acceptable current density in alkaline media, because of their poor electrical conductivity. Our recent study has revealed that combination of multiwalled carbon nanotubes with hydrosilicates could greatly enhance the OER performance.14 Nevertheless, more progress is still needed to fully utilize the cheap hydrosilicates for OER. Nonmetallic element incorporation is an effective strategy to tune the electrical conductivity and increase the number of active sites by optimizing the inherent electronic structure of electrocatalysts for the improved electrocatalytic efficiency.15,16 Phosphorus (P) stands out as a highly promising candidate for promoting the performance of host electrocatalysts.17-19 It is anticipated that phosphate incorporation into hydrosilicates could enhance their reactivity and


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realize the increased OER performance due to the different valence electrons and electronegativity between P and Si. Furthermore, bimetallic compounds have been reported to present enhanced OER performance over their single counterparts due to their rich redox reaction sites.20,21 Herein, we present a facile strategy to incorporate phosphate into layered nickel cobalt hydrosilicates (NCS:P) and demonstrate their dramatically enhanced OER activity compared with the host hydrosilicates ((Ni,Co)3Si2O5(OH)4: NCS). This work would provide a new perspective for enhancing OER performances of transition-metal hydrosilicates or other compounds in electrochemical water splitting. RESULTS AND DISCUSSION The synthesis procedure and formation process of the NCS:P. are schematically illustrated in Figure 1b. The well-defined silica spheres were firstly synthesized by a sol-gel method, and then served as a silicon source to prepare NCS under hydrothermal condition. In this process, the SiO2 spheres are first attacked by OH- released from the hydrolysis of ammonia to generate silicate ions and then reacted with metal ions to in situ form metal silicate shell on their outer surfaces.22,23 The inner SiO2 core is gradually consumed as the reaction is prolonged, finally resulting in the formation of the hollow structured NCS. To further fabricate the hierarchical hollow structured NCS:P, the NCS was then used as a precursor and subjected to thermal treatment in the presence of NaH2PO2 under nitrogen atmosphere (see Experimental section for details, Supporting Information).


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Figure 1. Schematic illustration of formation of NCS:P. The morphology and microstructure of the as-prepared NCS:P were imaged and analyzed by scanning electron microscopy (SEM). Figure 2a displays that the NCS:P presents a typical flower-like spherical profiles with a diameter of about 800 nm. The close observation (Figure 2b) reveals that the NCS:P is actually built from the ultrathin nanosheets with a thickness of about 5 nm. These nanosheets stack together in a staggered way, resulting in a large amount of voids on the surface of the spheres. Interestingly, the morphology of the NCS:P perfectly retains that of parent NCS (Figure S1, Supporting Information), suggesting the P-incorporation did not affect the morphology of NCS. The detailed microstructures of NCS:P were further determined by transmission electron microscopy (TEM). Figure 2c is a general image of the NCS:P by TEM view, which confirms the sphere-shaped NCS:P and is consistent with SEM observation. More specifically, it also distinctly depicts hollow structured feature of the NCS:P, which evidenced by the clear contrast between the dark edge and the bright center of the spheres. Also, each sphere has a shell thickness of about 65 nm and seems to be closely interconnected together, appearing a unique chain-spherical structure. Figure 2d obviously displays that the surface of the spheres is constructed by lots of nanosheets with random growth directions. Figure 2e and Figure S2 are


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typical high-resolution TEM (HRTEM) images of the NCS:P, which indicate the clear ripples and corrugations, confirming the ultrathin characteristic of nanosheets. Moreover, a lamellar spacing of the vertical nanosheets is approximately 1.1 nm, which is about three folds of the lattice spacing of (002) plane for Co3Si2O5(OH)4 or (004) plane of Ni3Si2O5(OH)4, consistent with previous observations on the microstructures of NCS.14,24 Meanwhile, high-angle annular dark-field scanning TEM (HAADF-STEM) image in shown Figure 2f evidences clearly visible hollow structures with the inner voids and outer shells. The corresponding HAADF-STEM elemental mapping (Figure 2f) suggests the coexistence of Ni, Co, O, P and Si and their even distributions in the NCS:P.


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Figure 2. SEM (a,b), TEM (c,d), HRTEM (e) and HAADF-STEM elemental mapping images of the NCS:P. Crystalline phase structures of the NCS and NCS:P were studied by X-ray diffraction (XRD). Figure 3a shows the typical XRD patterns of the NCS and NCS:P. The diffraction peaks of parent NCS are readily indexed to orthorhombic Ni3Si2O5(OH)4 (JCPDS Card No. 49-1859) and Co3Si2O5(OH)4 (JCPDS Card No. 21-0872), and closely match the XRD patterns of NCS reported previously.14,25 The XRD pattern of NCS:P are almost the same as that of NCS, which


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means that the P-incorporation under low temperature would not lead to its clearly structural change. The broad peak at about 12o (Figure S3, Supporting Information) indicates their layered structures with a d-spacing of 0.74 nm. X-ray photoelectron spectroscopy (XPS) were typically performed to analyze the electronic states of the NCS and NCS:P. The survey XPS spectra (Figure 3b) suggests the NCS is composed of Si, O, Ni, and Co elements and the presence of P element in NCS:P, confirming effective incorporation of P into NCS. As shown in Figure 3c, the high-resolution Co 2p XPS spectra present main two Co 2p peaks with the binding energies at 781 eV (Co 2p3/2) and 796 eV (Co 2p1/2). The corresponding satellite peaks appear around 5 eV above them, indicating characteristic of the Co2+ state.26 Notably, in comparison with NCS, the P-incorporation results in the positive shift of Co 2p binding energy, which can be ascribed to the higher electronegativity of P (2.19) than Si (1.90).27 The high-resolution Ni 2p XPS spectra (Figure 3d) exhibit two main peaks at binding energies of about 856 eV (Ni 2p3/2) and 874 eV (Ni 2p1/2), along with a prominent satellite peaks at 862 and 879 eV, indicative of feature of Ni2+.28 Compared with pure NCS, the corresponding peaks also exhibit a positive shift, further implying the P-incorporation modify the local electronic structure of NCS.29,30 The highresolution O 1s XPS spectrum of NCS (Figure 3e) can be well deconvoluted into two peaks at binding energies of 530.3 and 531.6 eV, which are assigned to metal-oxygen and Si-O bond in NCS, respectively.14 In addition to these two peaks, the prominent peak at binding energy of 532.9 eV is deconvoluted in the spectrum of NCS:P, revealing the appearance of P-O bond of phosphate.31 More specifically, NCS:P presents a single peaks centered at binding energy of 133.1 eV, which can be assigned with phosphate and confirm the existence of P−O bond, while there is no observation on P signal in NCS. It is specially noted that the absence of characteristic of P-Co (located below 130 eV) confirms that there is no any phosphides existed in NCS:P.32


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Furthermore, the FTIR spectrum (Figure S4, Supporting Information) of NCS:P also confirms the presence of phosphate in the NCS:P.

Figure 3. XRD patterns (a), survey (b), Co 2p (c), Ni 2p (d), O 1s (e) and P 2p (f) XPS spectra of the NCS and NCS:P.


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To investigate the electrocatalytic OER performances of the NCS:P, the samples were dropcasted onto the glassy carbon electrode and then used as working electrode in a three-electrode system for electrochemical tests. The linear sweep voltammetry (LSV) was recorded to evaluate the OER performance of samples. Figure 4a shows the polarization curves of NCS and NCS:P measured in 1.0 M KOH, along with typical metal hydroxides, such as Co(OH)2 and Ni(OH)2, and commercial RuO2 as references. Although the OER performance of NCS:P is lower than that of commercial RuO2, it still presents the significantly enhanced activity compared with the bare NCS and is superior to that of Co(OH)2 and Ni(OH)2, evidencing by its relatively earlier onset potential and larger catalytic current at the same overpotential. This result confirms the phosphate incorporation is an effective strategy for greatly enhancing OER activity of NCS. It is worth mentioning that the catalytic current density generated by NCS:P at potential of 1.65 V vs RHE is five times as high as that of NCS. To optimize the OER activity and obtain the details on effect of phosphate incorporation, we further synthesize different NCS:P by varying the calcination temperatures. The obtained samples designated as NCS:P-X, where X means the calcination temperatures. These synthesized NCS:P-X samples exhibits a similar crystalline structure (Figure S5, Supporting Information) and microstructures (Figure S6, Supporting Information). Figure 4b gives the effect of calcination temperatures on the OER activity of NCS:P. As expected, all the NCS:P samples exhibit the remarkably enhanced OER activity, which are superior to that of bare NCS. Although activity index reported in literature is an important criterion to evaluate the OER activity of an electrocatalyst,33 we here used another common criterion to evaluate the OER activity, namely, the potential required for the current density of 10 mA cm-2. The overpotential (η) of NCS:P-275 required to deliver a current density of 10 mA·cm−2 is ~394 mV (Figure 4c), which is superior to that of NCS:P-250 (~430 mV),


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NCS:P-300 (~401 mV) and NCS (~472 mV). Moreover, its OER performance is better than the OER-active hydrosilicate electrocatalysts reported ever13,14 and compares favorably to that of the other Ni-based transition metal compounds.34-36 The outstanding catalytic activity of NCS:P-275 for OER is confirmed by examining the Tafel slopes according to the Tafel equation: η = b log j + a

(1)

where η is overpotential, j is the current density, and b is the Tafel slope. As shown in Figure 4d, the smallest Tafel slope (67 mV dec-2) is observed for the NCS:P-275 sample, indicating its more rapid rate for catalyzing OER. Figure S7 depicts the cyclic voltammogram (CV) measurement on redox behaviors of the various catalysts in alkaline medium in the potential region before the OER. All of the catalysts show a quasi-reversible redox behavior, associated with the redox reaction:14 (NiIICoII)3Si2O5(OH)4 + 2OH- ↔ (NiIIICoIII)3Si2O5(OH)6 + 2e-

(2)

It is interesting that the current densities of NCS:P samples are significantly larger than that of bare NCS, indicating the phosphate-incorporation can greatly increase the active surface area. To elucidate the OER reaction kinetics and charge transfer process, AC electrochemical impedance spectroscopy (EIS) is performed at potential of 1.643 V vs RHE. Typical Nyquist plots of the NCS and NCS:P samples are presented in Figure 4e. The corresponding Nyquist plots are well fitted to a two-time constant equivalent circuit model (inset in Figure 4e), containing a solution resistance (Rs), a resistance related to surface porosity (Rp), and a charge transfer resistance (Rct) for the electrochemical OER. The Rct value (~45 Ω) of the NCS:P-275 is much smaller than that of the NCS:P-250 (~85 Ω), NCS:P-300 (~54 Ω) and NCS (~135 Ω), suggesting the more efficient charge transport of the NCS:P-275 during the electrochemical OER process. Given the similar morphology between NCS:P samples, the electrochemically active surface area (ECSA)


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were also estimated from the electrochemical double-layer capacitance (Cdl) test, since the Cdl is linearly proportional to the ECSA. As shown in Figure 4f, the linear plots derived from the cyclic voltammograms collected in the non-Faradaic region (Figure S8, Supporting Information) give a Cdl value of 3.3 mF cm-2 for NCS:P-275, which is 2.5 times that of NCS (1.3 mF cm-2). According to our previous work,37 the ECSA values for the NCS:P-275 and NCS are calculated to be 0.95 and 0.38 cm2, respectively. We further normalized the current density of the NCS:P275 and NCS by ECSA, and the result is shown in Figure S9. Clearly, the NCS:P-275 still exhibits the better OER activity than that of NCS. In addition to the activity, the OER long-term stability is still important concern on OER electrocatalysts. The chronopotentiometric curve at 10 mA cm-2 and 50 mA cm-2 shown in Figure S10 suggest that NCS:P-275 can maintain a good catalytic activity for 12000 s of continuous operation.


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Figure 4. (a) Polarization curves of NCS, NCS:P, Ni(OH)2 and Co(OH)2 at a scan rate of 5 mV s-1 in 1.0 M KOH solution. (b) Polarization curves, (c) Tafel plots, (d) comparison on overpotential at 10 mA cm-2, (e) Nyquist plots and (f) capacitive currents at 1.13 V against scan rates of NCS:P samples obtained at different temperatures. The excellent OER performance of NCS:P is mainly based on the merits of unique structures and incorporated phosphate. First, the NCS as a typical layered compound bearing edge-shared


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octahedral MO6, which are analogous to the layered metal hydro(oxy)oxides, thus working as OER-active centers in NCS:P. Second, the hollow and ultrathin nanosheets of NCS:P engender more exposed active sites, opened ion diffusion channels, and shortened electron transfer paths during OER process. Third, the phosphate with strong nucleophilic property can introduce additional reactive sites to NCS with greatly enhanced surface reactivity, which is critical to improve the interfacial proton dynamics and the catalytic activity.38 CONCLUSIONS In summary, we developed a facile strategy to realize the phosphate incorporation to the NCS and achieve the greatly enhanced performance for electrocatalytic OER in alkaline medium. The as-prepared NCS:P yielded much larger catalytic current than that of host NCS, Ni(OH)2 and Co(OH)2 references in 1.0 M KOH. This investigation showed the promise of NCS:P as a lowcost and highly efficient electrocatalyst for electrochemical water splitting. Our work would open up a new way to introduce phosphate to improve OER performance of transition metal compounds. ASSOCIATED CONTENT Supporting Information: Experimental section, SEM images, HRTEM image, FTIR spectra, XRD patterns, CVs, LSV normalized by ECSA and chronopotentiometry curves. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (21771147 and 51572227), Sichuan Youth Science and Technology Foundation (2013JQ0012), Major Cultivating Foundation of Education Department of Sichuan Province (17CZ0036), Meritocracy Research Funds of CWNU (17YC007 and 17YC017) and Innovative Research Team of CWNU (CXTD2017-1). REFERENCES (1)

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This study demonstrates the dramatically enhanced OER activity of the Ni-Co hydrosilicates by incorporating phosphate.


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